Electronics based on two-dimensional materials

Applied Physics Letters

The development of photonic integrated circuits would benefit from a wider selection of materials that can strongly control near-infrared (NIR) light. Transition metal dichalcogenides (TMDs) have been explored extensively for visible spectrum optoelectronics; the NIR properties of these layered materials have been less-studied. The measurement of optical constants is the foremost step to qualify TMDs for use in NIR photonics. Here, we measure the complex optical constants for select sulfide TMDs (bulk crystals of MoS 2 , TiS 2, and ZrS 2 ) via spectroscopic ellipsometry in the visible-to-NIR range. We find that the presence of native oxide layers (measured by transmission electron microscopy) significantly modifies the observed optical constants and need to be modeled to extract actual optical constants. We support our measurements with density functional theory calculations and further predict large refractive index contrast between different phases. We further propose that TMDs could find use as photonic phase-change materials, by designing alloys that are thermodynamically adjacent to phase boundaries between competing crystal structures, to realize martensitic (i.e., displacive, order-order) switching.

Advanced Materials Interfaces

npj 2D Materials and Applications

Platinum diselenide (PtSe 2) is an exciting new member of the two-dimensional (2D) transition metal dichalcogenide (TMD) family. It has a semimetal to semiconductor transition when approaching monolayer thickness and has already shown significant potential for use in device applications. Notably, PtSe 2 can be grown at low temperature making it potentially suitable for industrial usage. Here, we address thickness-dependent transport properties and investigate electrical contacts to PtSe 2 , a crucial and universal element of TMD-based electronic devices. PtSe 2 films have been synthesized at various thicknesses and structured to allow contact engineering and the accurate extraction of electrical properties. Contact resistivity and sheet resistance extracted from transmission line method (TLM) measurements are compared for different contact metals and different PtSe 2 film thicknesses. Furthermore, the transition from semimetal to semiconductor in PtSe 2 has been indirectly verified by electrical characterization in field-effect devices. Finally, the influence of edge contacts at the metal-PtSe 2 interface has been studied by nanostructuring the contact area using electron beam lithography. By increasing the edge contact length, the contact resistivity was improved by up to 70% compared to devices with conventional top contacts. The results presented here represent crucial steps toward realizing high-performance nanoelectronic devices based on group-10 TMDs.

Nanomaterials (Basel, Switzerland), 2017

Among the various two-dimensional semiconductors, indium selenide has recently triggered the interest of scientific community, due to its band gap matching the visible region of the electromagnetic spectrum, with subsequent potential applications in optoelectronics and especially in photodetection. In this feature article, we discuss the main issues in the synthesis, the ambient stability and the application capabilities of this novel class of two-dimensional semiconductors, by evidencing open challenges and pitfalls. In particular, we evidence how the growth of single crystals with reduced amount of Se vacancies is crucial in the road map for the exploitation of indium selenide in technology through ambient-stable nanodevices with outstanding values of both mobility of charge carriers and ON/OFF ratio. The surface chemical reactivity of the InSe surface, as well as applications in the fields of broadband photodetection, flexible electronics and solar energy conversion are also disc...

Sensors

In this study, electrical characteristics of MoTe2 field-effect transistors (FETs) are investigated as a function of channel thickness. The conductivity type in FETs, fabricated from exfoliated MoTe2 crystals, switched from p-type to ambipolar to n-type conduction with increasing MoTe2 channel thickness from 10.6 nm to 56.7 nm. This change in flake-thickness-dependent conducting behavior of MoTe2 FETs can be attributed to modulation of the Schottky barrier height and related bandgap alignment. Change in polarity as a function of channel thickness variation is also used for ammonia (NH3) sensing, which confirms the p- and n-type behavior of MoTe2 devices.

Applied Physics Letters, 2015

We report on the fabrication and characterization of synthesized multiwall MoS 2 nanotube (NT) and nanoribbon (NR) field-effect transistors (FETs). The MoS 2 NTs and NRs were grown by chemical transport, using iodine as a transport agent. Raman spectroscopy confirms the material as unambiguously MoS 2 in NT, NR, and flake forms. Transmission electron microscopy was used to observe cross sections of the devices after electrical measurements and these were used in the interpretation of the electrical measurements allowing estimation of the current density. The NT and NR FETs demonstrate n-type behavior, with ON/OFF current ratios exceeding 10 3 , and with current densities of 1.02 µA/µm, and 0.79 µA/µm at V DS = 0.3 V and V BG = 1 V, respectively. Photocurrent measurements conducted on a MoS 2 NT FET, revealed short-circuit photocurrent of tens of nanoamps under an excitation optical power of 78 W and 488 nm wavelength, which corresponds to a responsivity of 460 A/W. A long channel transistor model was used to model the common-source characteristics of MoS 2 NT and NR FETs and was shown to be consistent with the measured data.

Scientific reports, 2015

Tungsten ditelluride (WTe2) is a transition metal dichalcogenide (TMD) with physical and electronic properties that make it attractive for a variety of electronic applications. Although WTe2 has been studied for decades, its structure and electronic properties have only recently been correctly described. We experimentally and theoretically investigate the structure, dynamics and electronic properties of WTe2, and verify that WTe2 has its minimum energy configuration in a distorted 1T structure (Td structure), which results in metallic-like transport. Our findings unambiguously confirm the metallic nature of WTe2, introduce new information about the Raman modes of Td-WTe2, and demonstrate that Td-WTe2 is readily oxidized via environmental exposure. Finally, these findings confirm that, in its thermodynamically favored Td form, the utilization of WTe2 in electronic device architectures such as field effect transistors may need to be reevaluated.

Annalen der Physik

The exploitation of the excellent intrinsic electronic properties of graphene for device applications is hampered by a large contact resistance between the metal and graphene. The formation of edge contacts rather than top contacts is one of the most promising solutions for realizing low ohmic contacts. In this paper the fabrication and characterization of edge contacts to large area CVD-grown monolayer graphene by means of optical lithography using CMOS compatible metals, i.e. Nickel and Aluminum is reported. Extraction of the contact resistance by Transfer Line Method (TLM) as well as the direct measurement using Kelvin Probe Force Microscopy demonstrates a very low width specific contact resistance down to 130 Ωµm. The contact resistance is found to be stable for annealing temperatures up to 150°C enabling further device processing. Using this contact scheme for edge contacts, a field effect transistor based on CVD graphene with a high transconductance of 0.63 mS/µm at 1 V bias voltage is fabricated. Graphene Flagship (contract No. 696656). D.N. and W.M.

Nature communications, 2017

The coupling of an electromagnetic plane wave to a thin conductor depends on the sheet conductance of the material: a poor conductor interacts weakly with the incoming light, allowing the majority of the radiation to pass; a good conductor also does not absorb, reflecting the wave almost entirely. For suspended films, the transition from transmitter to reflector occurs when the sheet resistance is approximately the characteristic impedance of free space (Z = 377 Ω). Near this point, the interaction is maximized, and the conductor absorbs strongly. Here we show that monolayer graphene, a tunable conductor, can be electrically modified to reach this transition, thereby achieving the maximum absorptive coupling across a broad range of frequencies in terahertz (THz) band. This property to be transparent or absorbing of an electromagnetic wave based on tunable electronic properties (rather than geometric structure) is expected to have numerous applications in mm wave and THz components a...

International Journal of Molecular Sciences

In recent times, food safety has become a topic of debate as the foodborne diseases triggered by chemical and biological contaminants affect human health and the food industry’s profits. Though conventional analytical instrumentation-based food sensors are available, the consumers did not appreciate them because of the drawbacks of complexity, greater number of analysis steps, expensive enzymes, and lack of portability. Hence, designing easy-to-use tests for the rapid analysis of food contaminants has become essential in the food industry. Under this context, electrochemical biosensors have received attention among researchers as they bear the advantages of operational simplicity, portability, stability, easy miniaturization, and low cost. Two-dimensional (2D) nanomaterials have a larger surface area to volume compared to other dimensional nanomaterials. Hence, researchers nowadays are inclined to develop 2D nanomaterials-based electrochemical biosensors to significantly improve the...

Physical Review B

Two-dimensional transition-metal dichalcogendes M X2 (es. MoS2, WS2, MoSe2,. . .) are among the most promising materials for bandgap engineering. Widely studied in these compounds, by means of ab-initio techniques, is the possibility of tuning the direct-indirect gap character by means of in-plane strain. In such kind of calculations however the lattice degrees of freedom are assumed to be classical and frozen. In this paper we investigate in details the dependence of the bandgap character (direct vs. indirect) on the out-of-plane distance h between the two chalcogen planes in each M X2 unit. Using DFT calculations, we show that the bandgap character is indeed highly sensitive on the parameter h, in monolayer as well as in bilayer and bulk compounds, permitting for instance the switching from indirect to direct gap and from indirect to direct gap in monolayer systems. This scenario is furthermore analyzed in the presence of quantum lattice fluctuation induced by the zero-point motion. On the basis of a quantum analysis, we argue that the directindirect bandgap transitions induced by the out-of-plane strain as well by the in-plane strain can be regarded more as continuous crossovers rather than as real sharp transitions. The consequences on the physical observables are discussed.

Advanced Functional Materials

The high field transport characteristics of nanostructured transistors based on layered materials are not only important from a device physics perspective but also for possible applications in next generation electronics. With the growing promise of layered materials as replacements to conventional silicon technology, we study here the high current density properties of the layered material titanium trisulfide (TiS3). We observe high breakdown current densities up to 1.7•10 6 A cm-2 in TiS3 nanoribbon-based field-effect transistors which are among the highest found in semiconducting nanomaterials. Investigating the mechanisms responsible for current breakdown, we perform a thermogravimetric analysis of bulk TiS3 and compare the results with density functional theory (DFT) and Kinetic Monte Carlo calculations. We conclude that oxidation of TiS3 and subsequent desorption of sulfur atoms plays an important role in the electrical breakdown of the material in ambient conditions. Our results show that TiS3 is an attractive material for high power applications and lend insight to the thermal and defect activated mechanisms responsible for electrical breakdown in nanostructured devices.

Nanomaterials

The discovery of layered materials, including transition metal dichalcogenides (TMD), gives rise to a variety of novel nanoelectronic devices, including fast switching field-effect transistors (FET), assembled heterostructures, flexible electronics, etc. Molybdenum disulfide (MoS2), a transition metal dichalcogenides semiconductor, is considered an auspicious candidate for the post-silicon era due to its outstanding chemical and thermal stability. We present a Kelvin probe force microscopy (KPFM) study of a MoS2 FET device, showing direct evidence for pinch-off formation in the channel by in situ monitoring of the electrostatic potential distribution along the conducting channel of the transistor. In addition, we present a systematic comparison between a monolayer MoS2 FET and a few-layer MoS2 FET regarding gating effects, electric field distribution, depletion region, and pinch-off formation in such devices.

Nanomaterials

Understanding the nature of the barrier height in a two-dimensional semiconductor/metal interface is an important step for embedding layered materials in future electronic devices. We present direct measurement of the Schottky barrier height and its lowering in the transition metal dichalcogenide (TMD)/metal interface of a field effect transistor. It is found that the barrier height at the gold/ single-layer molybdenum disulfide (MoS2) interfaces decreases with increasing drain voltage, and this lowering reaches 0.5–1 V We also show that increase of the gate voltage induces additional barrier lowering.

RSC Adv.

High quality luminescent nanosheets of MoS2 interfaced with the amphiphilic protein Vmh2.

IETE Journal of Research, 2016

We study the effect of surface adsorption of 27 different adatoms on the electronic and magnetic properties of monolayer black phosphorus using density functional theory. Choosing a few representative elements from each group, ranging from alkali metals (group I) to halogens (group VII), we calculate the band structure, density of states, magnetic moment and effective mass for the energetically most stable location of the adatom on monolayer phosphorene. We predict that group I metals (Li, Na, K), and group III adatoms (Al, Ga, In) are effective in enhancing the n-type mobile carrier density, with group III adatoms resulting in lower effective mass of the electrons, and thus higher mobilities. Furthermore we find that the adatoms of transition metals Ti and Fe, produce a finite magnetic moment (1.87 and 2.31 µB) in monolayer phosphorene, with different band gap and electronic effective masses (and thus mobilities), which approximately differ by a factor of 10 for spin up and spin down electrons opening up the possibility for exploring spintronic applications.

Scientific reports, 2016

As scaling of conventional silicon-based electronics is reaching its ultimate limit, considerable effort has been devoted to find new materials and new device concepts that could ultimately outperform standard silicon transistors. In this perspective two-dimensional transition metal dichalcogenides, such as MoS2 and WSe2, have recently attracted considerable interest thanks to their electrical properties. Here, we report the first experimental demonstration of a doping-free, polarity-controllable device fabricated on few-layer WSe2. We show how modulation of the Schottky barriers at drain and source by a separate gate, named program gate, can enable the selection of the carriers injected in the channel, and achieved controllable polarity behaviour with ON/OFF current ratios >10(6) for both electrons and holes conduction. Polarity-controlled WSe2 transistors enable the design of compact logic gates, leading to higher computational densities in 2D-flatronics.

npj 2D Materials and Applications

Two-dimensional materials have gained immense attention for technological applications owing to their characteristic properties. MXene is one of the fast-growing family of 2D materials that exhibits remarkable physiochemical properties that cater numerous applications in the field of energy and storage. This review comprises the significant advancement in the field of 2D MXene and discusses the evolution of the design, synthetic strategies, and stability. In addition to illuminating the state-of-the-art applications, we discuss the challenges and limitations that preclude the scientific fraternity from realizing functional MXene with controlled structures and properties for renewable clean energy conversion and storage applications.

npj 2D Materials and Applications

Layered transition metal dichalcogenides (TMDs) are commonly classified as quasi-two-dimensional materials, meaning that their electronic structure closely resembles that of an individual layer, which results in resistivity anisotropies reaching thousands. Here, we show that this rule does not hold for 1T-TaS2—a compound with the richest phase diagram among TMDs. Although the onset of charge density wave order makes the in-plane conduction non-metallic, we reveal that the out-of-plane charge transport is metallic and the resistivity anisotropy is close to one. We support our findings with ab initio calculations predicting a pronounced quasi-one-dimensional character of the electronic structure. Consequently, we interpret the highly debated metal-insulator transition in 1T-TaS2 as a quasi-one-dimensional instability, contrary to the long-standing Mott localisation picture. In a broader context, these findings are relevant for the newly born field of van der Waals heterostructures, wh...

Advanced Materials

The pervasiveness of information technologies in all aspects of our daily lives has brought us to an impressive generation of data, which need to be stored and accessed very quickly. Non-volatile memories (NVMs), with their ever-growing capacity and speed, are making inroads into highcapacity storage to replace hard disk drives, fuelling the rapid expansion of the global storage-class memory market. As silicon-based flash memories are approaching their fundamental limit, vertical stacking of multiple memory-cell layers (i.e. 3D integration), as well as innovative device concepts and novel materials are being intensively investigated for the development of new NVMs. In this context, emerging two-dimensional (2D) materials, such as graphene, transition metal dichalcogenides (TMDs) and black phosphorous (BP), offer a host of outstanding physical and chemical properties, which could both improve existing memory technologies and enable the nextgeneration of low-cost, flexible and wearable information-storage devices. In this review article, we provide an overview on the exploitation of graphene and related 2D materials (GRMs) in different types of NVM cells, including resistive random access, flash, magnetic and phase-change memories. We discuss in depth the physical and chemical mechanisms underlying the non-volatile switching of GRM-based memory devices developed at the laboratory scale in the last decade. Although at this stage most of the proof-of-concept devices developed in academia do not compete with state-of-theart market products, a number of promising technological advancements have emerged, particularly within the area of low-cost and flexible electronics, which could become the focus of considerable development efforts in the forthcoming years. Here, the most relevant material properties and device structures are analysed, emphasizing both opportunities and challenges towards the realization of practical NVM devices that exploit the unique properties of GRMs.

Nature Communications

Power dissipation is a fundamental issue for future chip-based electronics. As promising channel materials, two-dimensional semiconductors show excellent capabilities of scaling dimensions and reducing off-state currents. However, field-effect transistors based on two-dimensional materials are still confronted with the fundamental thermionic limitation of the subthreshold swing of 60 mV decade−1 at room temperature. Here, we present an atomic threshold-switching field-effect transistor constructed by integrating a metal filamentary threshold switch with a two-dimensional MoS2 channel, and obtain abrupt steepness in the turn-on characteristics and 4.5 mV decade−1 subthreshold swing (over five decades). This is achieved by using the negative differential resistance effect from the threshold switch to induce an internal voltage amplification across the MoS2 channel. Notably, in such devices, the simultaneous achievement of efficient electrostatics, very small sub-thermionic subthreshol...

Physical Review B

We report a combined experimental and theoretical study of the PdSe 2−x Te x system. With increasing Te fraction, structural evolutions, first from an orthorhombic phase (space group Pbca) to a monoclinic phase (space group C2/c) and then to a trigonal phase (space group P-3m1), are observed accompanied with clearly distinct electrical transport behavior. The new monoclinic phase (C2/c) belongs to the very rare Verbeekite polymorphism and is discovered within a narrow range of Te composition (0.3 x 0.8). Electronic calculations and detailed transport analysis of the Verbeekite polymorphic PdSe 1.3 Te 0.7 phase are presented. In the trigonal phase region, superconductivity with enhanced critical temperature is also observed within a narrow range of Te content (1.0 x 1.2). The rich phase diagram, new polymorphic structure, and abnormally enhanced superconductivity could further stimulate interest to explore new types of polymorphs and investigate their transport and electronic properties in the family of transition metal dichalcogenides, which are of significant interest.

Physical Review B

The coexistence of ferroelectricity and spin-orbit coupling in noncentrosymmetric systems may allow for a nonvolatile control of spin textures in the momentum space by tuning the ferroelectric polarization. Based on first-principles calculations, supplemented with k • p analysis, we report the emergence of the reversible spin textures in the two-dimensional (2D) GaXY (X = Se, Te; Y = Cl, Br, I) monolayer compounds, a new class of 2D materials exhibiting in-plane ferroelectricity. We find that due to the large in-plane ferroelectric polarization, unidirectional out-of-plane spin textures are induced in the topmost valence band having giant spin splitting. Importantly, such out-of-plane spin textures, which can host a long-lived helical spin mode known as a persistent spin helix, can be fully reversed by switching the direction of the in-plane ferroelectric polarization. We show that the application of an external in-plane electric field oriented oppositely to the in-plane ferroelectric polarization direction is an effective method to flip the orientation of the out-of-plane spin textures. Thus, our findings can open avenues for interplay between the unidirectional out-of-plane spin textures and the in-plane ferroelectricity in 2D materials, which is useful for efficient and nonvolatile spintronic devices.

Scientific reports, 2017

Two-dimensional semiconducting materials of the transition-metal-dichalcogenide family, such as MoS2 and WSe2, have been intensively investigated in the past few years, and are considered as viable candidates for next-generation electronic devices. In this paper, for the first time, we study scaling trends and evaluate the performances of polarity-controllable devices realized with undoped mono- and bi-layer 2D materials. Using ballistic self-consistent quantum simulations, it is shown that, with the suitable channel material, such polarity-controllable technology can scale down to 5 nm gate lengths, while showing performances comparable to the ones of unipolar, physically-doped 2D electronic devices.

Journal of Applied Physics

This study examines the magnetic interactions between spatially-variable manganese and chromium trimers substituted into a graphene superlattice. Using density functional theory, we calculate the electronic band structure and magnetic populations for the determination of the electronic and magnetic properties of the system. To explore the super-exchange coupling between the transition-metal atoms, we establish the magnetic magnetic ground states through a comparison of multiple magnetic and spatial configurations. Through an analysis of the electronic and magnetic properties, we conclude that the presence of transition-metal atoms can induce a distinct magnetic moment in the surrounding carbon atoms as well as produce an RKKY-like super-exchange coupling. It hoped that these simulations can lead to the realization of spintronic applications in graphene through electronic control of the magnetic clusters.

ACS Nano

Metal halides are a class of layered materials with promising electronic and magnetic properties persisting down to the two-dimensional limit. While most recent studies focused on the trihalide components of this family, the rather unexplored metal dihalides are also van der Waals layered systems with distinctive magnetic properties. Here we show that the dihalide NiBr 2 grows epitaxially on a Au(111) substrate and exhibits semiconducting and magnetic behavior starting from a single layer. Through a combination of a low-temperature scanningtunneling microscopy, low-energy electron diffraction, X-ray photoelectron spectroscopy, and photoemission electron microscopy, we identify two competing layer structures of NiBr 2 coexisting at the interface and a stoichiometrically pure layer-bylayer growth beyond. Interestingly, X-ray absorption spectroscopy measurements revealed a magnetically ordered state below 27 K with in-plane magnetic anisotropy and zero-remanence in the single layer of NiBr 2 /Au(111), which we attribute to a noncollinear magnetic structure. The combination of such two-dimensional magnetic order with the semiconducting behavior down to the 2D limit offers the attractive perspective of using these films as ultrathin crystalline barriers in tunneling junctions and low-dimensional devices.

Communications Physics

Two-dimensional (2D) materials such as graphene have become the focus of extensive research efforts in condensed matter physics. They provide opportunities for both fundamental research and applications across a wide range of industries. Ideally, characterization of graphene requires non-invasive techniques with single-atomic-layer thickness resolution and nanometer lateral resolution. Moreover, commercial application of graphene requires fast and large-area scanning capability. We demonstrate the optimized balance of image resolution and acquisition time of non-invasive confocal laser scanning microscopy (CLSM), rendering it an indispensable tool for rapid analysis of mass-produced graphene. It is powerful for analysis of 1-5 layers of exfoliated graphene on Si/SiO 2 , and allows us to distinguish the interfacial layer and 1-3 layers of epitaxial graphene on SiC substrates. Furthermore, CLSM shows excellent correlation with conventional optical microscopy, atomic force microscopy, Kelvin probe force microscopy, conductive atomic force microscopy, scanning electron microscopy and Raman mapping.

RSC Adv.

During the past few years, two-dimensional (2D) layered materials have emerged as the most fundamental building blocks of a wide variety of optoelectronic devices.

RSC Adv.

Herein, we report the effect of passivation layer composition on thermal stability as measured by Raman spectra of a phosphorene/Al-doped ZnO (AZO) heterostructure.

C

In this paper, carbon thin films were grown using the plasma-enhanced atomic layer deposition (PE-ALD). Methane (CH4) was used as the carbon precursor to grow the carbon thin film. The grown film was analyzed by the high-resolution transmission electron micrograph (TEM), X-ray photoelectron spectroscopy (XPS) analysis, and Raman spectrum analysis. The analyses show that the PE-ALD-grown carbon film has an amorphous structure. It was found that the existence of defective sites (nanoscale holes or cracks) on the substrate of copper foil could facilitate the formation of nanolayered carbon films. The mechanism for the formation of nanolayered carbon film in the nanoscale holes was discussed. This finding could be used for the controlled growth of nanolayered carbon films or other two-dimensional nanomaterials while combining with modern nanopatterning techniques.

Nanomaterials

The interest in graphene-based electronics is due to graphene’s great carrier mobility, atomic thickness, resistance to radiation, and tolerance to extreme temperatures. These characteristics enable the development of extremely miniaturized high-performing electronic devices for next-generation radiofrequency (RF) communication systems. The main building block of graphene-based electronics is the graphene-field effect transistor (GFET). An important issue hindering the diffusion of GFET-based circuits on a commercial level is the repeatability of the fabrication process, which affects the uncertainty of both the device geometry and the graphene quality. Concerning the GFET geometrical parameters, it is well known that the channel length is the main factor that determines the high-frequency limitations of a field-effect transistor, and is therefore the parameter that should be better controlled during the fabrication. Nevertheless, other parameters are affected by a fabrication-relat...

Small (Weinheim an der Bergstrasse, Germany), 2018

The biotransformation and biological impact of few layer graphene (FLG) and graphene oxide (GO) are studied, following ingestion as exposure route. An in vitro digestion assay based on a standardized operating procedure (SOP) is exploited. The assay simulates the human ingestion of nanomaterials during their dynamic passage through the different environments of the gastrointestinal tract (salivary, gastric, intestinal). Physical-chemical changes of FLG and GO during digestion are assessed by Raman spectroscopy. Moreover, the effect of chronic exposure to digested nanomaterials on integrity and functionality of an in vitro model of intestinal barrier is also determined according to a second SOP. These results show a modulation of the aggregation state of FLG and GO nanoflakes after experiencing the complex environments of the different digestive compartments. In particular, chemical doping effects are observed due to FLG and GO interaction with digestive juice components. No structur...

Microscopy and Microanalysis

2021

We propose and investigate the intrinsically thinnest transistor concept: a monolayer ballistic heterojunction bipolar transistor based on a lateral heterostructure of transition metal dichalcogenides. The device is intrinsically thinner than a Field Effect Transistor because it does not need a top or bottom gate, since transport is controlled by the electrochemical potential of the base electrode. As typical of bipolar transistors, the collector current undergoes a tenfold increase for each 60 mV increase of the base voltage over several orders of magnitude at room temperature, without sophisticated optimization of the electrostatics. We present a detailed investigation based on self-consistent simulations of electrostatics and quantum transport for both electron and holes of a pnp device using MoS2 for the 10-nm base and WSe2 for emitter and collector. Our three-terminal device simulations confirm the working principle and a large current modulation ION/IOFF ∼ 108 for ∆VEB = 0.5 V...

Nanotechnology, 2020

Black arsenic (BAs) is an elemental two-dimensional semiconductor that is promising for a wide range of electronic and photonic applications. The narrow bandgap and symmetric band structure suggest that ambipolar (both n- and p-type) transport should be observable, however, only p-type transport has been experimentally studied to date. Here, we demonstrate and characterize ambipolar transport in exfoliated BAs field effect transistors. In the thickest flakes (~ 80 nm), maximum currents, Imax, up to 60 μA/μm and 90 μA/μm are achieved for hole and electron conduction, respectively. Room-temperature hole (electron) mobilities up to 150 cm2/Vs (175 cm2/Vs) were obtained, with temperature-dependence consistent with a phonon-scattering mechanism. The Schottky barrier height for Ni contacts to BAs was also extracted from the temperature-dependent measurements. Imaxfor both n- and p-type conductivity was found to decrease with reduced thickness, while the ratio of Imaxto the minimum current...

ChemEngineering

Reduced graphene oxide has certain unique qualities that make them versatile for a myriad of applications. Unlike graphene oxide, reduced graphene oxide is a conductive material and well suited for use in electrically conductive materials, such as solar cell devices. In this study, we report on the synthesis of graphene oxide as well as the fabrication and characterization of dye-sensitized solar cells with a photoanode which is an amalgam of reduced graphene oxide and titanium dioxide. The synthesized reduced graphene oxide and the corresponding photoanode were fully characterized using Ultraviolet-visible, Fourier transform infrared (FTIR), and Raman Spectrometry. The morphology of the sample was assessed using Atomic Force Microscopy, Field Emission Scanning Electron Microscopy, Transmission Electron Microscopy, and Energy Dispersive X-ray Spectroscopy. The photovoltaic characteristics were determined by photocurrent and photo-voltage measurements of the fabricated solar cells. T...

Green Electronics, 2018

In this chapter, we report the nano-heat transport in metal-oxide-semiconductor field effect transistor (MOSFET). We propose a ballistic-diffusive model (BDE) to inquire the thermal stability of nanoscale MOSFET's. To study the mechanism of scattering in the interface oxide-semiconductor, we have included the specularity parameter defined as the probability of reflection at boundary. In addition, we have studied the effective thermal conductivity (ETC) in nanofilms we found that ETC depend with the size of nanomaterial. The finite element method (FEM) is used to resolve the results for a 10 nm channel length. The results prove that our proposed model is close to those results obtained by the Boltzmann transport equation (BTE).

DAE SOLID STATE PHYSICS SYMPOSIUM 2018

Sodium-ion battery has shown hope in the field of future rechargeable battery technology due to its low-cost and abundance. Among the existing anode materials, layered transition metal dichalcogenides have the potential for sodium-ion batteries due to their interesting properties such as specific capacity, structural stability, and accessible layered structure. Among the existing 2D layered materials Molybdenum ditelluride (MoTe 2) has the ability for sodiumion storage properties due to its comparative more interlayer spacing. In this study, the MoTe 2 polycrystalline powder sample has been synthesized by a solid-state route and the crystallographic and morphological analysis have been carried out by Synchrotron X-ray diffraction (SXRD), X-ray absorption near edge structure (XANES), FE-SEM, EDS etc. SXRD patterns have revealed the well crystalline structure of the material having the hexagonal structure. Further, X-ray absorption near structure (XANES) was conducted to study the electronic nature of MoTe 2 cycled electrodes to understand the needed electrochemical mechanism. The initial discharge capacity of the MoTe 2 powder showed an excellent capacity of Na-ion storage 326 mA h g −1 at a current density of 1.0 A g −1 and continued to 40 cycles with 264 mA h g −1 capacity. The obtained results showed the well-developed two-dimensional layers of MoTe 2 have the unique capability to deliver superior sodium-ion storage ability for its utility as an anode material for sodium-ion battery.

Advanced Functional Materials

MXenes, a young family of two-dimensional (2D) transition metal carbides/nitrides, show great potential in electrochemical energy storage applications. Herein, a high performance ultra-thin flexible solid-state supercapacitor is demonstrated based on a Mo1.33C MXene with vacancyordering in an aligned layer structure MXene/PEDOT:PSS composite film post-treated with concentrated H2SO4. The flexible solid-state supercapacitor delivers a maximum capacitance of 568 F cm-3 , an ultrahigh energy density of 33.2 mWh cm-3 and a power density of 19470 mW cm-3. The Mo1.33C MXene/PEDOT:PSS composite film shows a reduction in resistance upon H2SO4 treatment, a higher capacitance (1310 F cm-3) and improved rate-capabilities than both pristine Mo1.33C MXene and the non-treated Mo1.33C/PEDOT:PSS composite films. The enhanced capacitance and stability are attributed to the synergistic effect of increased interlayer

Nanoscale Research Letters, 2020

Molybdenum dioxide (MoO 2 ) a kind of semi-metal material shows many unique properties, such as high melting point, good thermal stability, large surface area-to-volume ratio, high-density surface unsaturated atoms, and excellent conductivity. There is a strong connection between structural type and optoelectronic properties of 2D nanosheet. Herein, the rectangular and hexagonal types of thin and thick MoO 2 2D nanosheets were successfully prepared from MoO 3 powder using two-zone chemical vapor deposition (CVD) with changing the experimental parameters, and these fabricated nanosheets displayed different colors under bright-field microscope, possess margins and smooth surface. The thickness of the blue hexagonal and rectangular MoO 2 nanosheets are ~ 25 nm and ~ 30 nm, respectively, while typical thickness of orange-colored nanosheet is around ~ 100 nm. Comparative analysis and investigations were carried out, and mix-crystal phases were indentified in thick MoO 2 as main matrix th...

Nanomaterials

The international technology roadmap of semiconductors (ITRS) is approaching the historical end point and we observe that the semiconductor industry is driving complementary metal oxide semiconductor (CMOS) further towards unknown zones. Today’s transistors with 3D structure and integrated advanced strain engineering differ radically from the original planar 2D ones due to the scaling down of the gate and source/drain regions according to Moore’s law. This article presents a review of new architectures, simulation methods, and process technology for nano-scale transistors on the approach to the end of ITRS technology. The discussions cover innovative methods, challenges and difficulties in device processing, as well as new metrology techniques that may appear in the near future.

ACS Applied Materials & Interfaces

Interface of transition metal dichalcogenide (TMDC) and high-k dielectric transition metal oxides (TMO) had triggerred umpteen discourses due to the indubitable impact of TMO in reducing the contact resistances and restraining the Fermi-level pinning for the metal-TMDC contacts. In the present work, we focus on the unresolved tumults of large-area TMDC/TMO interfaces, grown by adopting different techniques. Here, on a pulsed laser deposited (PLD) MoS 2 thin film, a layer of TiO 2 is grown by using both atomic layer deposition (ALD) and PLD. These two different techniques emanate TiO 2 layers with different crystalline properties, thicknesses and interfacial morphologies, subsequently influencing the electronic and optical properties of the interfaces. In contrast to the earlier reports of n-type doping for exfoliated MoS 2 /TiO 2 interfaces, large-area MoS 2 /Anatase-TiO 2 films had demonstrated a ptype doping of the underneath MoS 2 , irrespective of the adopted deposition technique. In addition, they manifest a boost in the extent of p-type doping with increasing thickness of TiO 2 , as emerged after analyzing the core-level shifts of the X-ray photoelectron spectra (XPS). Density functional analysis of the MoS 2 /Anatase-TiO 2 interfaces, for pristine and in presence of a wide range of interfacial defects, could explain the interdependence of doping and the terminating atomic-surface of TiO 2 on MoS 2. The theoretical results resemble well with the realistic scenario of large-area growths after incorporating minimization of surfacestrain via mutual rotation of the constituent layers. The optical properties of the interface, encompassing the photoluminescence (PL), transient absorption and z-scan two-photon absorption indicate the presence of defect-induced localized mid-gap levels in MoS 2 /TiO 2 (PLD), resulting quenched exciton signals. On the contrary, the relatively defect-free interface in MoS 2 /TiO 2 (ALD) demonstrates a clear presence of both A and B excitons of MoS 2. This outcome corroborates with the first-principles observation of the presence of localized traps at the interfacial band-structure in presence of the point defects. From the investigation of optical properties, we indicate that MoS 2 /TiO 2 (PLD) interface may act as a promising saturable absorber, having a significant non-linear response for the sub-bandgap excitations. Moreover, MoS 2 /TiO 2 (PLD) interface had resulted a better photo-transport. A potential application of MoS 2 /TiO 2 (PLD) is demonstrated by the fabrication of a p-type photo-transistor with the ionic-gel top gate. This endeavor to analyse and understand the MoS 2 /TiO 2 interface establishes the prospectives of large-area interfaces in the field of optics and optoelectronics.

2021

The present work is based on the computational study of MoS2 monolayer and effect of tensile strain on its atomic level structure. The bandgap for MoS2 monolayer, defected MoS2 monolayer and Silicon-doped monolayer are 1.82 eV (direct bandgap), 0.04 (indirect bandgap) and 1.25 eV (indirect bandgap), respectively. The impact of tensile strain (0-0.7 %) on the bandgap and effective mass of charge carriers of these MoS2 structures has been investigated. The bandgap decrease of 5.76 %, 31.86 % and 6.03 % has been observed in the three structures for biaxial strain while the impact of uniaxial strain is quite low. The impact of higher temperature on the bandgap under biaxial tensile strain has been also analyzed in this paper. These observations are extremely important for 2D material-based research for electronic applications.

Advanced Functional Materials

Memristive devices have drawn considerable research attention due to their potential applications in non-volatile memory and neuromorphic computing. The combination of resistive switching devices with light-responsive materials is considered a novel way to integrate optical information with electrical circuitry. On the other hand, 2D materials have attracted substantial consideration thank to their unique crystal structure, as reflected in their chemical and physical properties. Although not the major focus, van der Waals solids were proven to be potential candidates in memristive devices. In this scheme, the majority of the resistive switching devices were implemented on planar flakes, obtained by mechanical exfoliation. Here we utilize a facile and robust methodology to grow large-scale vertically aligned MoS2 (VA-MoS2) films on standard silicon substrates. Memristive devices with the structure silver/VA-MoS2/Si are shown to have low set-ON voltages (<0.5V), large-retention times (>2x10 4 s) and high thermal stability (up to 350 °C). The proposed memristive device also exhibits long term potentiation / depression (LTP/LTD) and photo-active memory states. The large-scale fabrication, together with the low operating voltages, high thermal stability, light-responsive behaviour and long-term potentiation/depression, makes this approach very appealing for real-life non-volatile memory applications.

Physical Review B, 2015

The phase space for graphene's minimum conductivity σmin is mapped out using Landauer theory modified for scattering using Fermi's Golden Rule, as well as the Non-Equilibrium Green's Function (NEGF) simulation with a Monte Carlo sampling over impurity distributions. The resulting 'fan diagram' spans the range from ballistic to diffusive over varying aspect ratios (W/L), and bears several surprises. The device aspect ratio determines how much tunneling (between contacts) is allowed and becomes the dominant factor for the evolution of σmin from ballistic to diffusive regime. We find an increasing (for W/L > 1) or decreasing (W/L < 1) trend in σmin vs. impurity density, all converging around 128q 2 /π 3 h ∼ 4q 2 /h at the dirty limit. In the diffusive limit, the conductivity quasi-saturates due to the precise cancellation between the increase in conducting modes from charge puddles vs the reduction in average transmission from scattering at the Dirac Point. In the clean ballistic limit, the calculated conductivity of the lowest mode shows a surprising absence of Fabry-Pérot oscillations, unlike other materials including bilayer graphene. We argue that the lack of oscillations even at low temperature is a signature of Klein tunneling.

2D Materials, 2016

The thermoelectric figures of merit of pristine two-dimensional materials are predicted to be significantly less than unity, making them uncompetitive as thermoelectric materials. Here we elucidate a new strategy that overcomes this limitation by creating multi-layer nanoribbons of two different materials and allowing thermal and electrical currents to flow perpendicular to their planes. To demonstrate this enhancement of thermoelectric efficiency ZT, we analyse the thermoelectric performance of monolayer molybdenum disulphide (MoS 2) sandwiched between two graphene monolayers and demonstrate that the cross-plane (CP) ZT is significantly enhanced compared with the pristine parent materials. For the parent monolayer of MoS 2 , we find that ZT can be as high as approximately 0.3, whereas monolayer graphene has a negligibly small ZT. In contrast for the graphene/MoS 2 /graphene heterostructure, we find that the CP ZT can be as large as 2.8. One contribution to this enhancement is a reduction of the thermal conductance of the van der Waals heterostructure compared with the parent materials, caused by a combination of boundary scattering at the MoS 2 /graphene interface which suppresses the phonons transmission and the lower Debye frequency of monolayer MoS 2 , which filters phonons from the monolayer graphene. A second contribution is an increase in the electrical conductance and Seebeck coefficient associated with molybdenum atoms at the edges of the nanoribbons.

Nanomaterials

The low on-current and direct source-to-drain tunneling (DSDT) issues are the main drawbacks in the ultrascaled tunneling field-effect transistors based on carbon nanotube and ribbons. In this article, the performance of nanoscale junctionless carbon nanotube tunneling field-effect transistors (JL CNTTFETs) is greatly improved by using the synergy of electrostatic and chemical doping engineering. The computational investigation is conducted via a quantum simulation approach, which solves self-consistently the Poisson equation and the non-equilibrium Green’s function (NEGF) formalism in the ballistic limit. The proposed high-performance JL CNTTFET is endowed with a particular doping approach in the aim of shrinking the band-to-band tunneling (BTBT) window and dilating the direct source-to-drain tunneling window, while keeping the junctionless paradigm. The obtained improvements include the on-current, off-current, ambipolar behavior, leakage current, I60 metric, subthreshold swing, c...

Nanoscale Horiz.

Control of the electronic properties of 2D transition material carbides by replacing the outer Ti layers in Ti3C2Tx with Mo, creating Mo2TiC2Tx.

Beilstein Journal of Nanotechnology

Background: Applications of two-dimensional (2D) materials in electronic devices require the development of appropriate measuring methods for determining their typical semiconductor parameters, i.e., mobility and carrier lifetime. Among these methods, contactless techniques and mobility extraction methods based on field-effect measurements are of great importance. Results: Here we show a contactless method for determining these parameters in 2D semiconductors that is based on the photomagnetoelectric (PME) effect (also known as the photoelectromagnetic effect). We present calculated dependences of the PME magnetic moment, evoked in 2D Corbino configuration, on the magnetic field as well as on the intensity and spatial distribution of illumination. The theoretical predictions agree with the results of the contactless investigations performed on non-suspended single-layer graphene. We use the contactless PME method for determining the dependence of carrier mobility on the concentratio...

Physical Review B

A trigonal phase existing only as small patches on chemically exfoliated few layer, thermodynamically stable 1H phase of MoS2 is believed to influence critically properties of MoS2 based devices. This phase has been most often attributed to the metallic 1T phase. We investigate the electronic structure of chemically exfoliated MoS2 few layered systems using spatially resolved (≤120 nm resolution) photoemission spectroscopy and Raman spectroscopy in conjunction with state-of-the-art electronic structure calculations. On the basis of these results, we establish that the ground state of this phase is a small gap (~90 meV) semiconductor in contrast to most claims in the literature; we also identify the specific trigonal (1Tʹ) structure it has among many suggested ones.

Electronics, 2015

Two-dimensional materials have attracted great scientific attention due to their unusual and fascinating properties for use in electronics, spintronics, photovoltaics, medicine, composites, etc. Graphene, transition metal dichalcogenides such as MoS2, phosphorene, etc., which belong to the family of two-dimensional materials, have shown great promise for gas sensing applications due to their high surface-to-volume ratio, low noise and sensitivity of electronic properties to the changes in the surroundings. Two-dimensional nanostructured semiconducting metal oxide based gas sensors have also been recognized as successful gas detection devices. This review aims to provide the latest advancements in the field of gas sensors based on various two-dimensional materials with the main focus on sensor performance metrics such as sensitivity, specificity, detection limit, response time, and reversibility. Both experimental and theoretical studies on the gas sensing properties of graphene and other two-dimensional materials beyond graphene are also discussed. The article concludes with the current challenges and future prospects for two-dimensional materials in gas sensor applications.

Nano Letters

Sustainability has become a critical concern in the semiconductor industry as hazardous wastes released during the manufacturing process of semiconductor devices have an adverse impact on human beings and the environment. The use of hazardous solvents in existing fabrication processes also restricts the use of polymer substrates because of their low chemical resistance to such solvents. Here, we demonstrate an environmentally friendly mechanical, bilayer lithography that uses just water for development and lift-off. We show that we are able to create arbitrary patterns achieving resolution down to 310 nm. We then demonstrate the use of this technique to create functional devices by fabricating a MoS 2 photodetector on a polyethylene terephthalate (PET) substrate with measured response times down to 42 ms.

The European Physical Journal B

The surface mode of the Birmingham Cluster Genetic Algorithm (S-BCGA), which performs an unbiased global optimisation search for clusters adsorbed on a surface, has been employed for the global optimisation of noble metal pentamers on an MgO(1 0 0) substrate. The effect of element identity and alloying in surface-bound neutral subnanometre particles is calculated by energetic analysis of all compositions of supported 5-atom PdAu and PdPt clusters. Our results show that the binding strengths of the component elements to the surface are in the order Pt > Pd > Au. In addition, alloying Pd with Au and Pt is favorable for this size since excess energy calculations show a preference for bimetallic clusters for both cases. Furthermore, the electronic behaviour, which is intermediate between molecular systems and bulk metals allows tuning of the characteristics of particles in the subnanometre size range. The adsorption of CO and O2 probe molecules are also modelled and it is found that CO and O2 adsorption leads to a weakening of the cluster-surface interaction.

Scientific reports, 2016

Scaling transistors&#39; dimensions has been the thrust for the semiconductor industry in the last four decades. However, scaling channel lengths beyond 10 nm has become exceptionally challenging due to the direct tunneling between source and drain which degrades gate control, switching functionality, and worsens power dissipation. Fortunately, the emergence of novel classes of materials with exotic properties in recent times has opened up new avenues in device design. Here, we show that by using channel materials with an anisotropic effective mass, the channel can be scaled down to 1 nm and still provide an excellent switching performance in phosphorene nanoribbon MOSFETs. To solve power consumption challenge besides dimension scaling in conventional transistors, a novel tunnel transistor is proposed which takes advantage of anisotropic mass in both ON- and OFF-state of the operation. Full-band atomistic quantum transport simulations of phosphorene nanoribbon MOSFETs and TFETs base...

Scientific Reports

Unlike MoS2 ultra-thin films, where solution-based single source precursor synthesis for electronic applications has been widely studied, growing uniform and large area few-layer WS2 films using this approach has been more challenging. Here, we report a method for growth of few-layer WS2 that results in continuous and uniform films over centimetre scale. The method is based on the thermolysis of spin coated ammonium tetrathiotungstate ((NH4)2WS4) films by two-step high temperature annealing without additional sulphurization. This facile and scalable growth method solves previously encountered film uniformity issues. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) were used to confirm the few-layer nature of WS2 films. Raman and X-Ray photoelectron spectroscopy (XPS) revealed that the synthesized few-layer WS2 films are highly crystalline and stoichiometric. Finally, WS2 films as-deposited on SiO2/Si substrates were used to fabricate a backgated Field Effect ...

Scientific Reports

We introduce a simple criterion to identify two-dimensional (2D) materials based on the comparison between experimental lattice constants and lattice constants mainly obtained from Materials-Project (MP) density functional theory (DFT) calculation repository. Specifically, if the relative difference between the two lattice constants for a specific material is greater than or equal to 5%, we predict them to be good candidates for 2D materials. We have predicted at least 1356 such 2D materials. For all the systems satisfying our criterion, we manually create single layer systems and calculate their energetics, structural, electronic, and elastic properties for both the bulk and the single layer cases. Currently the database consists of 1012 bulk and 430 single layer materials, of which 371 systems are common to bulk and single layer. The rest of calculations are underway. To validate our criterion, we calculated the exfoliation energy of the suggested layered materials, and we found that in 88.9% of the cases the currently accepted criterion for exfoliation was satisfied. Also, using molybdenum telluride as a test case, we performed X-ray diffraction and Raman scattering experiments to benchmark our calculations and understand their applicability and limitations. The data is publicly available at the website http:// www.ctcms.nist.gov/~knc6/JVASP.html. Two-dimensional (2D) materials 1, 2 have great potential in sub-micron level electronics 3 , flexible and tunable electronics 4 , superconductivity 5 , photo-voltaic 6 , water purification 7 , sensors 8 , thermal management 9 , ethanol distillation and energy storage 10 , medicine 11 , quantum dots 12, 13 and composites 14-16. Despite a huge upsurge in graphene and other 2D materials research, search for novel 2D materials and their systematic comparison of properties is still in development phase 14, 17, 18. A consistent, high throughput investigation of such materials using materials modelling tool such as density functional theory 19-21 would allow better understanding of the nature and properties of 2D materials 22. One of the successful works in the field of making 2D material repository was initiated by Ding et al. 23 , who focused on investigating MX 2 (M = Mo, Nb, W, Ta; X = S, Se, Te) monolayers for structural, vibronic and electronic properties using local density approximation (LDA) and semi-local generalized gradient approximation (GGA) with Perdew-Burke-Ernzerhof (PBE) functionals 24. They also calculated hybrid functional (HSE) and many-body method based GW bandgaps of the materials 25. However, one of the major limitations of this work is that they only considered honeycomb H structures for layered materials, while other structures have also been identified experimentally 26. Another important database for 2D materials was developed by Bjorkman et al. 27. They used DFT to characterize 2D materials with interlayer binding energies in the range of 10-20 meV/Å 2 , except those with significant covalent bonds. Their work shows that PBE strongly under-binds these materials, which can lead up to 20% overestimation of interlayer distance. Moreover, LDA is clearly shown to over bind materials. While authors' focus is to study the structural properties of 2D materials with different functionals, they investigate bulk 2D materials only, instead of finite-number-of-layer cases, which are of much more technological importance. Later on, Lesbegue et al. 28 used crystal packing fraction to identify bulk 2D materials from Inorganic Crystal Structure Database (ICSD) database 29. They computed magnetic properties and PBE-based band structures for 46 candidate 2D materials. Although they provide criteria for selection of many 2D materials, they do not calculate properties for single/multi-layer materials. In addition, they performed all their calculations

Scientific Data

We introduce the systematic database of scanning tunneling microscope (STM) images obtained using density functional theory (DFT) for two-dimensional (2D) materials, calculated using the Tersoff-Hamann method. It currently contains data for 716 exfoliable 2D materials. Examples of the five possible Bravais lattice types for 2D materials and their Fourier-transforms are discussed. All the computational STM images generated in this work are made available on the JARVIS-STM website (https://jarvis.nist.gov/jarvisstm). We find excellent qualitative agreement between the computational and experimental STM images for selected materials. As a first example application of this database, we train a convolution neural network model to identify the Bravais lattice from the STM images. We believe the model can aid high-throughput experimental data analysis. These computational STM images can directly aid the identification of phases, analyzing defects and lattice-distortions in experimental STM...

Handbook of Graphene, 2019

Graphene/MoS 2 heterostructures are formed by combining the nanosheets of graphene and monolayer MoS 2. The electronic features of both constituent monolayers are rather well-preserved in the resultant heterostructure due to the weak van der Waals interaction between the layers. However, the proximity of MoS 2 induces strong spin orbit coupling effect of strength ∼1 meV in graphene, which is nearly three orders of magnitude larger than the intrinsic spin orbit coupling of pristine graphene. This opens a bandgap in graphene and further causes anticrossings of the spin-nondegenerate bands near the Dirac point. Lattice incommensurate graphene/MoS 2 heterostructure exhibits interesting moiré patterns which have been observed in experiments. The electronic bandstructure of heterostructure is very sensitive to biaxial strain and interlayer twist. Although the Dirac cone of graphene remains intact and no charge-transfer between graphene and MoS 2 layers occurs at ambient conditions, a strain-induced charge-transfer can be realized in graphene/MoS 2 heterostructure. Application of a gate voltage reveals the occurrence of a topological phase transition in graphene/MoS 2 heterostructure. In this chapter, we discuss the crystal structure, interlayer effects, electronic structure, spin states, and effects due to strain and substrate proximity on the electronic properties of graphene/MoS 2 heterostructure. We further present an overview of the distinct topological quantum phases of graphene/MoS 2 heterostructure and review the recent advancements in this field.

Physical Review Applied, 2018

Two-Dimensional Transition Metal Dichalcogenides (2D-TMDCs) such as MoS 2 , MoSe 2 , WS 2 , and WSe 2 with van der Waal's type interlayer coupling is being widely explored as channel materials in a Schottky Barrier Field Effect Transistor (SB-FET) configuration. While their excellent electrostatic control and high ON/OFF ratios have been identified, a clear correlation between electronic transport and the lowfrequency noise with different atomic layer thickness is missing. For multi-layer channels in MoS 2 FETs, the effects of interlayer coupling resistance on device conductance and mobility have been studied, but no systematic study included interlayer effects in consideration of the intrinsic (channel) and extrinsic (total device) noise behavior. Here we report the 1/f noise properties in MoSe 2 FETs with varying channel thickness (3 to 40 atomic layers). Contributions of channel vs. access/contact regions were extracted from current-voltage (transport) and 1/f noise measurements. The measured noise amplitude shows a direct crossover from channel-to contact-dominated noise as the gate voltage is increased. The results can be interpreted in terms of a Hooge relationship associated with the channel noise, a transition region, and a saturated high-gate voltage regime whose characteristics are determined by a voltage-independent conductance and noise source associated with the metallurgical contact and the interlayer resistance. Both the channel Hooge coefficient and the channel/access noise amplitude decrease with increasing channel thickness over the range of 3 to 15 atomic layers, with the former remaining approximately

Physical Review E, 2022

Recent experimental utilization of liquid substrate in the production of two-dimensional crystals, such as graphene, together with a general interest in amorphous materials, raises the following question: is it beneficial to use a liquid substrate to optimize amorphous material production? Inspired by epitaxial growth, we use a two-dimensional coarse-grained model of interacting particles to show that introducing a motion for the substrate atoms improves the self-assembly process of particles that move on top of the substrate. We find that a specific amount of substrate liquidity (for a given sample temperature) is needed to achieve optimal self-assembly. Our results illustrate the opportunities that the combination of different degrees of freedom provides to the self-assembly processes.

Nature, 2019

The development of silicon semiconductor technology has produced breakthroughs in electronics-from the microprocessor in the late 1960s to early 1970s, to automation, computers and smartphones-by downscaling the physical size of devices and wires to the nanometre regime. Now, graphene and related two-dimensional (2D) materials offer prospects of unprecedented advances in device performance at the atomic limit, and a synergistic combination of 2D materials with silicon chips promises a heterogeneous platform to deliver massively enhanced potential based on silicon technology. Integration is achieved via three-dimensional monolithic construction of multifunctional high-rise 2D silicon chips, enabling enhanced performance by exploiting the vertical direction and the functional diversification of the silicon platform for applications in opto-electronics and sensing. Here we review the opportunities, progress and challenges of integrating atomically thin materials with silicon-based nanosystems, and also consider the prospects for computational and non-computational applications.

ACS Nano, 2021

The current quest for two-dimensional transition metal carbides and nitrides (MXenes) has been to circumvent the slow, hazardous, and laborious multistep synthesis procedures associated with conventional chemical MAX phase exfoliation. Here, we demonstrate a one-step synthesis method with local Ti 3 AlC 2 MAX to Ti 3 C 2 T z MXene conversion on the order of milliseconds, facilitated by proton production through solution dissociation under megahertz frequency acoustic excitation. These protons combined with fluorine ions from LiF to selectively etch the MAX phase into MXene, whose delamination is aided by the acoustic forcing. These results have important implications for the future applicability of MXenes, which crucially depend on the development of more efficient synthesis procedures. For proof-of-concept, we show that flexible electrodes fabricated by this method exhibit comparable electrochemical performance to that previously reported.

Advanced materials (Deerfield Beach, Fla.), 2017

Manipulating the anisotropy in 2D nanosheets is a promising way to tune or trigger functional properties at the nanoscale. Here, a novel approach is presented to introduce a one-directional anisotropy in MoS2 nanosheets via chemical vapor deposition (CVD) onto rippled patterns prepared on ion-sputtered SiO2 /Si substrates. The optoelectronic properties of MoS2 are dramatically affected by the rippled MoS2 morphology both at the macro- and the nanoscale. In particular, strongly anisotropic phonon modes are observed depending on the polarization orientation with respect to the ripple axis. Moreover, the rippled morphology induces localization of strain and charge doping at the nanoscale, thus causing substantial redshifts of the phonon mode frequencies and a topography-dependent modulation of the MoS2 workfunction, respectively. This study paves the way to a controllable tuning of the anisotropy via substrate pattern engineering in CVD-grown 2D nanosheets.

Physical Review B, 2019

We present a symmetry-adapted real-space formulation of Kohn-Sham density functional theory for cylindrical geometries and apply it to the study of large X (X=C, Si, Ge, Sn) nanotubes. Specifically, starting from the Kohn-Sham equations posed on all of space, we reduce the problem to the fundamental domain by incorporating cyclic and periodic symmetries present in the angular and axial directions of the cylinder, respectively. We develop a high-order finite-difference parallel implementation of this formulation, and verify its accuracy against established planewave and realspace codes. Using this implementation, we study the band structure and bending properties of X nanotubes and Xene sheets, respectively. Specifically, we first show that zigzag and armchair X nanotubes with radii in the range 1 to 5 nm are semiconducting, other than the armchair and zigzag type III carbon variants, for which we find a vanishingly small bandgap, indicative of metallic behavior. In particular, we find an inverse linear dependence of the bandgap with respect to the radius for all nanotubes, other than the armchair and zigzag type III carbon variants, for which we find an inverse quadratic dependence. Next, we exploit the connection between cyclic symmetry and uniform bending deformations to calculate the bending moduli of Xene sheets in both zigzag and armchair directions, while considering radii of curvature up to 5 nm. We find Kirchhoff-Love type bending behavior for all sheets, with graphene and stanene possessing the largest and smallest moduli, respectively. In addition, other than graphene, the sheets demonstrate significant anisotropy, with larger bending moduli along the armchair direction. Finally, we demonstrate that the proposed approach has very good parallel scaling and is highly efficient, enabling ab initio simulations of unprecedented size for systems with a high degree of cyclic symmetry. In particular, we show that even micron-sized nanotubes can be simulated with modest computational effort. Overall, the current work opens an avenue for the efficient ab-initio study of 1D nanostructures with large radii as well as 1D/2D nanostructures under uniform bending.

Applied Sciences, 2020

Herein, we present a facile synthesis route for the mesoporous silica nanoflakes on two types of templates and evaluate their potential as potential drug delivery systems. Silica materials are attractive due to their biocompatibility, low cytotoxicity, high surface area, and tunable pores. In addition, they can be multifunctionalized. These properties were used to create multifunctional drug delivery systems combining folic acid as a target molecule and methotrexate (MTX) as an anticancer drug. The silica nanoflakes were formed using graphene oxide and double-layered hydroxide as templates, respectively. After the removal of matrices, the silica flakes were functionalized by folic acid and loaded with methotrexate. The differences in drug release performance and structural stability were analyzed with respect to the detailed physicochemical characterization of the produced silica nanoflakes.

Advanced Materials Interfaces, 2019

the implementation of new materials, apart from silicon, to accomplish the ambitious technological aims. [4-6] A promising approach to supersede Si electronic devices is the implementation of spintronic components, like in ultrathin CrX 3 layers (X = Cl, Br, or I), that use the electron spins as information memory instead of charge carriers. [7-11] Electrically insulating chromium(III) halides CrX 3 (X = Cl, Br, I) exhibit strongly anisotropic magnetic properties, even in a single layer. [12] By reason of the Cr 3+ ions are in 3d 3 electronic configuration with total spin 3/2 that form isotypic honeycomb nets, Huang et al. finally reported on the first experimental proof using CrI 3 (bulk T C = 61 K), which showed that the respective monolayer (T C = 45 K) and trilayer are ferromagnetic, while the bilayer couples antiferromagnetically. [13,14] The CrX 3 anisotropy in atomically thin layers makes them intriguing candidates for sensing applications or future magnetoelectronics. Recently, Zhang et al. reported on similar investigation of monolayer CrBr 3. [15] Moreover, the efficiency of CrCl 3 , used as catalyst material, could be enhanced due to nanoscaling and an enlarged surface-to-volume ratio, similar to a recently shown approach of TiCl 3. [16,17] However, the preparation of highly crystalline CrX 3 nanolayers down to the monolayer limit is still an experimental challenge. The commonly employed state-of-the-art exfoliation technique proceeding from prior synthesized bulk CrX 3 platelets lacks due to absolute nonreproducibility. Ideally, in trigonal CrBr 3 each The experimental observation of intrinsic ferromagnetism in single layered chromium trihalides CrX 3 (X = Cl, Br, I) has gained outstanding attention recently due to their possible implementation in spintronic devices. However, the reproducible preparation of highly crystalline chromium(III) halide nanolayers without stacking faults is still an experimental challenge. As chromium trihalides consist of adjacent layers with weak interlayer coupling, the preparation of ultrathin CrX 3 nanosheets directly on substrates via vapor transport proves as an advantageous synthesis technique. It is demonstrated that vapor growth of ultrathin highly crystalline CrX 3 micro-and nanosheets succeeds directly on yttrium stabilized zirconia substrates in a one-step process via chemical vapor transport (CVT) in temperature gradients of 100 K (600 °C → 500 °C for CrCl 3 and 650 °C → 550 °C for CrBr 3 or CrI 3) without a need for subsequent delamination. Due to simulation results, optimization of synthesis conditions is realized and phase pure CrX 3 nanosheets with thicknesses ≤25 nm are obtained via short term CVT. The nanosheets morphology, crystallinity, and phase purity are analyzed by several techniques, including microscopy, diffraction, and spectroscopy. Furthermore, a potential subsequent delamination technique is demonstrated to give fast access to CrX 3 monolayers using the example of CrCl 3 .

Nanomaterials, 2016

Two-dimensional (2D) semiconductors such as transition metal dichalcogenides (TMDCs) and black phosphorous have drawn tremendous attention as an emerging optical material due to their unique and remarkable optical properties. In addition, the ability to create the atomically-controlled van der Waals (vdW) heterostructures enables realizing novel optoelectronic devices that are distinct from conventional bulk counterparts. In this short review, we first present the atomic and electronic structures of 2D semiconducting TMDCs and their exceptional optical properties, and further discuss the fabrication and distinctive features of vdW heterostructures assembled from different kinds of 2D materials with various physical properties. We then focus on reviewing the recent progress on the fabrication of 2D semiconductor optoelectronic devices based on vdW heterostructures including photodetectors, solar cells, and light-emitting devices. Finally, we highlight the perspectives and challenges of optoelectronics based on 2D semiconductor heterostructures.

Advanced Electronic Materials, 2019

research for electronic devices. The critical size of field effect transistors (FETs) is aggressively scaled down to the nano meter regime to acquire higher chip density, higher speed, and lower power consumption. [3,4] Among the low-dimensional materials, 2D layered materials [5] such as graphene, [6] black phosphorus (BP), [7-9] and transition metal dichalcogenides (TMDCs) [10] have been widely investigated for their potential development in future electronics and optoelectronics technology due to their unique physical, chemical, optical, and electronic properties. As carrier transport is confined in plane, 2D layered materials are considered as promising candidates for high-mobility "tunnel-electronic" devices. In addition, their flexibility and tunable band gaps promote the potential deployment in future electronics, photonic and optoelectronics technologies. [11,12] While n-type FETs represented by InGaZnO [13]-based thinfilm transistors (TFTs) gradually mature and step forward toward practical use, p-type FETs clearly need further development. Recently, researchers have paid particular attention to the intrinsic p-type tin monochalcogenides (SnX) (X = O, S, Se, and Te) [14,15] with the same crystal structure as black phosphorus. Tin monosulfide (SnS) has already attracted attention as a solar cell material owing to the good absorption across the electromagnetic spectrum with the proper optical properties exhibiting narrow band gaps (1.3-1.5 eV [direct], 1.0-1.1 eV [indirect] and a good absorption coefficient that is larger than 10 4 cm −1 in the visible region. [16] SnS has both low-temperature stable α-SnS (Pnma-D 2h 16 Layered metal monochalcogenides have attracted significant interest in the 2D family since they show different unique properties from their bulk counterparts. The comprehensive synthesis, characterization, and optoelectrical applications of 2D-layered tin monosulfide (SnS) grown by pulsed laser deposition are reported. Few-layer SnS-based field-effect transistors (FETs) and photodetectors are fabricated on Si/SiO 2 substrates. The premium 2D SnS FETs yield an on/off ratio of 3.41 × 10 6 , a subthreshold swing of 180 mV dec −1 , and a field effect mobility (µ FE) of 1.48 cm 2 V −1 s −1 in a 14-monolayer SnS device. The layered SnS photodetectors show a broad photoresponse from ultraviolet to near-infrared (365-820 nm). In addition, the SnS phototransistors present an improved detectivity of 9.78 × 10 10 cm 2 Hz 1/2 W −1 and rapid response constants of 60 ms for grow-time constant τ g and 10 ms for decay-time constant τ d under extremely weak 365 nm illumination. This study sheds light on layer-dependent optoelectronic properties of 2D SnS that promise to be important in next-generation 2D optoelectronic devices.

Chemistry – A European Journal, 2020

One of the applications of graphene in which its scalable production is of utmost importance is the development of polymer composites. Among the techniques used to produce graphene flakes, the liquid phase exfoliation (LPE) of graphite stands out due to its versatility and scalability. However, the solvents suitable for the LPE process are generally toxic and have a high boiling point, making the processing challenging. The use of low boiling point solvents could be convenient for the processing, due to the easiness of their removal. In this study, we propose the use of poly methyl methacrylate (PMMA) as a stabilizing agent for the production of graphene flakes in a low boiling point solvent, i.e., acetone. The graphene dispersions produced in the mixture acetone-PMMA increases the concentration by +175 %, and contain a higher percentage of few-layer graphene flakes (< 5 layers), e.g., +60 %, compared to the dispersions prepared in acetone. The as-produced graphene dispersions are used to develop graphene/acrylonitrile-butadienestyrene composites. The mechanical properties of the pristine polymer are improved, i.e., + 22 % in the Young's modulus, by adding 0.01 wt. % of graphene flakes. Moreover, a decrease of ~ 20 % in the oxygen permeability is obtained using 0.1 wt. % of graphene flakes filler, compared to the unloaded matrix.

APL Materials, 2021

Paper published as part of the special topic on Emerging Materials for Spin-Charge Interconversion ARTICLES YOU MAY BE INTERESTED IN Chirality-selective easily adjustable spin current from uniaxial antiferromagnets

Advanced Biosystems, 2018

The interface of biological components with semiconductors is a growing field with numerous applications. For example, the interfaces can be used to sense and modulate the electrical activity of single cells and tissues. From the materials point of view, silicon is the ideal option for such studies due to its controlled chemical synthesis, scalable lithography for functional devices, excellent electronic and optical properties, biocompatibility and biodegradability. Recent advances in this area are pushing the bio-interfaces from the tissue and organ level to the single cell and subcellular regimes. In this progress report, we will describe some fundamental studies focusing on miniaturizing the bioelectric and biomechanical interfaces. Additionally, many of our highlighted examples involve freestanding silicon-based nanoscale systems, in addition to substrate-bound structures or devices; the former offers new promise for basic research and clinical application. In this report, we will describe recent developments in the interfacing of neuronal and cardiac cells and their networks. Moreover, we will briefly discuss the incorporation of semiconductor nanostructures for interfacing non-excitable cells in applications such as probing intracellular force dynamics and drug delivery. Finally, we will suggest several directions for future exploration.

Small (Weinheim an der Bergstrasse, Germany), 2017

Despite substantial progress in the science and technology of 2D nanomaterials, facile fabrication of ultrathin 2D metals remains challenging. Herein, an efficient hot-pressing method is developed to fabricate free-standing ultrathin Bi nanosheets from Bi nanoparticles. Highly crystalline Bi nanosheets with thickness as low as ≈2 nm and area of more than several micrometers are successfully fabricated on silicon substrates. The ultrathin Bi nanosheets exhibit morphology and structural dependent enhanced broad range photoemission in visible region of spectrum. Our cost-effective hot-pressing strategy may open an insight for production, application, and deficient fundamental understanding of other 2D semimetals/metalloids and noble metals.

Microelectronic Engineering, 2020

Two-dimensional (2D) semiconducting TMDCs materials have recently gained much attention and are considered as promising materials with a high surface-to-volume ratio for electronics. The performance of devices greatly affected by environmental factors. For attaining the high performance of the device, environmental issues must be addressed. Here, we have demonstrated an n-type doping effect from DUV + N2 treatment to overcome the environmental influences and to enhance the performance of MoTe2 FET. After the n-type doping effect from DUV + N2 treatment mobility, charge carrier density, and ION/IOFF ratio increased up to 62.4 cm2/Vs, 3 × 1012 cm−2 and 107 respectively. The negative shift of threshold voltage (Vth) and Raman peaks towards the lower wavenumber confirms the n-type doping effect in the MoTe2 FET from DUV + N2 treatment. By using this method, we can change and control the polarity of the MoTe2 FET by changing the doping time. First-principles calculation on the study of s...

Nano Research

Anti-perovskites A3SnO (A = Ca, Sr, and Ba) are an important class of materials due to the emergence of Dirac cones and tiny mass gaps in their band structures originating from an intricate interplay of crystal symmetry, spin-orbit coupling, and band overlap. This provides an exciting playground for modulating their electronic properties in the two-dimensional (2D) limit. Herein, we employ first-principles density functional theory (DFT) calculations by combining dispersion-corrected SCAN + rVV10 and mBJ functionals for a comprehensive side-by-side comparison of the structural, thermodynamic, dynamical, mechanical, electronic, and thermoelectric properties of bulk and monolayer (one unit cell thick) A3SnO anti-perovskites. Our results show that 2D monolayers derived from bulk A3SnO anti-perovskites are structurally and energetically stable. Moreover, Rashba-type splitting in the electronic structure of Ca3SnO and Sr3SnO monolayers is observed owing to strong spin-orbit coupling and ...

Chemical Society reviews, 2016

Hybrid nanostructures composed of graphene or other two-dimensional (2D) nanomaterials and plasmonic metal components have been extensively studied. The unusual properties of 2D materials are associated with their atomically thin thickness and 2D morphology, and many impressive structures enable the metal nanomaterials to establish various interesting hybrid nanostructures with outstanding plasmonic properties. In addition, the hybrid nanostructures display unique optical characteristics that are derived from the close conjunction of plasmonic optical effects and the unique physicochemical properties of 2D materials. More importantly, the hybrid nanostructures show several plasmonic electrical effects including an improved photogeneration rate, efficient carrier transfer, and a plasmon-induced &quot;hot carrier&quot;, playing a significant role in enhancing device performance. They have been widely studied for plasmon-enhanced optical signals, photocatalysis, photodetectors (PDs), a...

Angewandte Chemie (International ed. in English), 2018

MXenes-two-dimensional (2D) compounds generated from layered bulk materials, have attracted significant attention in energy-related fields. However, most previous MXenes synthesis methods inevitably involve hydrofluoric acid, which is highly corrosive and harmful to the performance of lithium-ion batteries and supercapacitors. Here, we report an alkali-assisted hydrothermal method to prepare a typical MXene-Ti3C2Tx (T= -OH, -O). This route is inspired from a Bayer process that is widely used in bauxite refining. The entire process is totally free of fluorine and yields multilayer Ti3C2Tx with ~92 wt.% in purity (via 27.5 M NaOH, 270°C). Without the -F terminations, the resulting Ti3C2Tx film electrode (~52 μm in thickness, ~1.63 g cm-3 in density) is 314 F g-1 via gravimetric capacitance at 2 mV s-1 in 1 M H2SO4. This surpasses (by ~214%) that of the multilayer Ti3C2Tx prepared via HF treatments. This fluorine-free method also provides an alkali-etching strategy for exploring new MX...

Nature Electronics, 2019

Nano Research, 2019

The morphology and structural stability of metal/2D semiconductor interfaces strongly affect the performance of 2D electronic devices and synergistic catalysis. However, the structural evolution at the interfaces has not been well explored particularly at atomic resolution. In this work, we study the structural evolution of Au nanoparticles (NPs) on few-layer MoS 2 by high resolution transmission electron microscope (HRTEM) and quantitative high-angle annular dark field scanning TEM. It is found that in the transition of Au from nanoparticles to dendrites, a dynamically epitaxial alignment between Au and MoS 2 lattices is formed, and Moiré patterns can be directly observed in HRTEM images due to the mismatch between Au and MoS 2 lattices. This epitaxial alignment can occur in ambient conditions, and can also be accelerated by the irradiation of high-energy electron beam. In situ observation clearly reveals the rotation of Au NPs, the atom migration inside Au NPs, and the transfer of Au atoms between neighboring Au NPs, finally leading to the formation of epitaxially aligned Au dendrites on MoS 2. The structural evolution of metal/2D semiconductor interfaces at atomic scale can provide valuable information for the design and fabrication of the metal/2D semiconductor nano-devices with desired physical and chemical performances.

Applied Physics Letters, 2019

The contact resistance at two-dimensional graphene/MoS 2 lateral heterojunctions is theoretically studied, using first-principles simulations based on density functional theory and the nonequilibrium Green's function method. The computed contact resistance lies in the range of 10 2 to 10 4 X lm, depending on the contact edge symmetry (armchair or zigzag) and termination (Mo and/or S terminated). This large variation in the contact resistance arises from the variation in the interface barrier height, which is sensitive to the presence of polar C-Mo bonds or sulfur dangling bonds at the interface. These results highlight that the control of the edge symmetry and/or edge termination is crucial to achieve a low contact resistance (in the range of a few hundred ohms micrometer) at graphene/MoS 2 lateral heterojunctions for 2D materialbased field-effect devices.

Advanced Functional Materials, 2018

Liquid metal reaction media provides exciting new avenues for synthesizing low dimensional materials. Here the synthesis of atomically thin sheets and nanofibers of boehmite (γ-AlOOH) and their transformation into cubic alumina (γ-Al 2 O 3) via annealing is explored. The sheets are as thin as one orthorhombic boehmite unit cell. The addition of aluminum into a room temperature alloy of gallium, followed by exposing the melt to either liquid water or water vapor, allows growing either two-dimensional sheets or one-dimensional fibers, respectively. The isolated oxide hydroxides feature large surface areas, with the sheet-morphologies also showing a high Young's modulus. The method is green, since the liquid metal solvent can be fully reused. The ultrathin boehmite sheets are found suitable for the development of free-standing membrane filters that enable excellent separation of heavy metal ions and oil from aqueous solutions at extraordinary filtrate flux. The developed liquid metal-based synthesis process offers a sustainable, green and rapid method for synthesizing nano-morphologies of metal oxides which are challenging to obtain by conventional This article is protected by copyright. All rights reserved. 3 methods. The process is both sustainable and scalable and may be explored for the creation of other types of metal oxide compounds.

Biosensors

Two-dimensional transition metal dichalcogenides (2D TMDs) have gained considerable attention due to their distinctive properties and broad range of possible applications. One of the most widely studied transition metal dichalcogenides is molybdenum disulfide (MoS2). The 2D MoS2 nanosheets have unique and complementary properties to those of graphene, rendering them ideal electrode materials that could potentially lead to significant benefits in many electrochemical applications. These properties include tunable bandgaps, large surface areas, relatively high electron mobilities, and good optical and catalytic characteristics. Although the use of 2D MoS2 nanosheets offers several advantages and excellent properties, surface functionalization of 2D MoS2 is a potential route for further enhancing their properties and adding extra functionalities to the surface of the fabricated sensor. The functionalization of the material with various metal and metal oxide nanostructures has a signifi...

Advanced Materials, 2017

Physical Review B, 2019

Grain boundaries in two-dimensional transition metal dichalcogenides can strongly affect the transport properties by reducing the electron mobility or allowing gap conduction through extended grain boundary states. Here, by combining advanced modeling tools-density-functional-theorycalibrated tight-binding Hamiltonians and Green's function techniques-we investigate transport along and across mirror twin grain boundaries in MoS2. Our results show that the grain boundary conductive channels are strongly affected by sulfur vacancies, while short-range Anderson disorder has a moderate impact, which we quantitatively analyze, and long-range disorder has a very weak effect. As for transport across the grain boundaries, the system conductance turns out to be less than half that for the pristine system, and the spin-orbit coupling and intervalley scattering are found to play an important role. Our findings are beneficial to the understanding and the prediction of the impact of mirror twin grain boundaries in the transport phenomena, and could be of help in designing electronic devices based on transition metal dichalcogenides.

Physical Review B, 2019

Energy-saving spintronics are believed to be implementable on the systems hosting persistent spin helix (PSH) since they support an extraordinarily long spin lifetime of carriers. However, achieving the PSH requires a unidirectional spin configuration in the momentum space, which is practically non-trivial due to the stringent conditions for fine-tuning the Rashba and Dresselhaus spin-orbit couplings. Here, we predict that the PSH can be intrinsically achieved on a two-dimensional (2D) group-IV monochalcogenide M X monolayer, a new class of the noncentrosymmetric 2D materials having in-plane ferroelctricity. Due to the C 2v point group symmetry in the M X monolayer, a unidirectional spin configuration is preserved in the out-of-plane direction and thus maintains the PSH that is similar to the [110] Dresselhaus model in the [110]-oriented quantum well. Our first-principle calculations on various M X (M : Sn, Ge; X: S, Se, Te) monolayers confirmed that such typical spin configuration is observed, in particular, at near the valence band maximum where a sizable spin splitting and a substantially small wavelength of the spin polarization are achieved. Importantly, we observe reversible out-of-plane spin orientation under opposite in-plane ferroelectric polarization, indicating that an electrically controllable PSH for spintronic applications is plausible.

Physical Review B, 2019

Finding a new class of materials supporting a long spin lifetime is essential in development of energy-saving spintronics, which is achievable by using a persistent spin helix (PSH) materials. However, for spintronic devices, the PSH states with large spin splitting is required for operation at room temperature. By employing first-principles calculations, we show that the PSH states with large spin splitting are achieved in the SnSe monolayer (ML) functionalized by a substitutional halogen impurity. We find the PSH states in the Fermi level where k-space Fermi surface is characterized by the shifted two loops, dominated by out-of-plane spin orientations. We clarify the PSH states in term of an effective k • p Hamiltonian obtained from symmetry consideration. Finally, large spin-orbit strength in the PSH states with a substantially small wavelength are found, rendering that this system is promising for the development of an efficient and high-density scalable spintronic devices operating at room temperatures.

Applied Physics Letters, 2017

We have measured both the current-voltage (I SD-V GS) and capacitance-voltage (C-V GS) characteristics of a MoS 2 − LiNbO 3 eld eect transistor. From the measured capacitance we calculate the electron surface density and show that its gate voltage dependence follows the theoretical prediction resulting from the twodimensional free electron model. This model allows us to t the measured I SD-V GS characteristics over the entire range of V GS. Combining this experimental result with the measured current-voltage characteristics, we determine the eld eect mobility as a function of gate voltage. We show that for our device this improved combined approach yields signicantly smaller values (more than a factor of 4) of the electron mobility than the conventional analysis of the current-voltage characteristics only.

Scientific reports, 2017

We investigate the emission from confined excitons in the structure of a single-monolayer-thick quasi-two-dimensional (quasi-2D) InxGa1-xN layer inserted in GaN matrix. This quasi-2D InGaN layer was successfully achieved by molecular beam epitaxy (MBE), and an excellent in-plane uniformity in this layer was confirmed by cathodoluminescence mapping study. The carrier dynamics have also been investigated by time-resolved and excitation-power-dependent photoluminescence, proving that the recombination occurs via confined excitons within the ultrathin quasi-2D InGaN layer even at high temperature up to ~220 K due to the enhanced exciton binding energy. This work indicates that such structure affords an interesting opportunity for developing high-performance photonic devices.

Journal of the Optical Society of America B, 2018

In this article, we propose a class of high-performance and low-loss waveguide by exploiting the hybridization of the plasmon-phonon modes through the coupling graphene with hexagonal boron nitride (hBN). It is found that inserting an ultra-thin hBN layer with hyperbolicity behavior between graphene and a thin low-index substrate leads to squeezing a light into hBN, enabling a low-loss light propagation as compared with a traditional graphene/substrate plasmonic waveguide. Furthermore, the results show that by choosing appropriate values for physical parameters of the proposed waveguide as well as the chemical potential of graphene, a low-loss and high-performance optical plasmon-phonon mode in the mid-infrared range can be achieved simultaneously. In particular, by increasing the chemical potential from 0.2 to 1 eV, 12-and 6-fold enhancement for propagation length (Lm) and figure of merit, respectively, is achieved with the optimized proposed waveguide. The results presented here are very helpful for designing low-loss nanophotonic devices.

ACS Applied Materials &amp; Interfaces

Adherence of metal oxides to graphene is of fundamental significance to graphene nanoelectronic and spintronic interfaces. Titanium oxide and aluminum oxide are two widely used tunnel barriers in such devices, which offer optimum interface resistance and distinct interface conditions that govern transport parameters and device performance. Here, we reveal a fundamental difference in how these metal oxides interface with graphene through electrical transport measurements and Raman and photoelectron spectroscopies, combined with ab initio electronic structure calculations of such interfaces. While both oxide layers cause surface charge transfer induced p-type doping in graphene, in sharp contrast to TiO x , the AlO x /graphene interface shows the presence of appreciable sp 3 defects. Electronic structure calculations disclose that significant p-type doping occurs due to a combination of sp 3 bonds formed between C and O atoms at the interface and possible slightly off-stoichiometric defects of the aluminum oxide layer. Furthermore, the sp 3 hybridization at the AlO x /graphene interface leads to distinct magnetic moments of unsaturated bonds, which not only explicates the widely observed low spin-lifetimes in AlO x barrier graphene spintronic devices but also suggests possibilities for new hybrid resistive switching and spin valves.

Nanoscale Advances, 2020

The role of organic promoters is clarified in the growth mechanism of MoS2.

Nanomaterials (Basel, Switzerland), 2018

Low-dimensional layered transition metal dichalcogenides (TMDs) have recently emerged as an important fundamental research material because of their unique structural, physical and chemical properties. These novel properties make these TMDs a suitable candidate in numerous potential applications. In this review, we briefly summarize the properties of low-dimensional TMDs, and then focus on the various methods used in their preparation. The use of TMDs in electronic devices, optoelectronic devices, electrocatalysts, biosystems, and hydrogen storage is also explored. The cutting-edge future development probabilities of these materials and numerous research challenges are also outlined in this review.

Nanomaterials

In this article, ultrascaled junctionless (JL) field-effect phototransistors based on carbon nanotube/nanoribbons with sub-10 nm photogate lengths were computationally assessed using a rigorous quantum simulation. This latter self-consistently solves the Poisson equation with the mode space (MS) non-equilibrium Green’s function (NEGF) formalism in the ballistic limit. The adopted photosensing principle is based on the light-induced photovoltage, which alters the electrostatics of the carbon-based junctionless nano-phototransistors. The investigations included the photovoltage behavior, the I-V characteristics, the potential profile, the energy-position-resolved electron density, and the photosensitivity. In addition, the subthreshold swing–photosensitivity dependence as a function of change in carbon nanotube (graphene nanoribbon) diameter (width) was thoroughly analyzed while considering the electronic proprieties and the quantum physics in carbon nanotube/nanoribbon-based channels...

Nano Research, 2017

Producing monolayers and few-layers in high yield with environment-stability is still a challenge in hafnium disulphide (HfS 2), which is a layered two-dimensional material of group-IV transition metal dichalcogenides, to reveal its unlocked electronic and optoelectronic applications. For the first time, to the best of our knowledge, we demonstrate a simple and cost-effective method to grow layered belt-like nano-crystals of HfS 2 with surprisingly large interlayer spacing followed by its chemical exfoliation. Various microscopic and spectroscopic techniques reveal these as-grown crystals exfoliate into single or few layers in some minutes using solvent assisted ultrasonification method in N-Cyclohexyl-2-pyrrolidone. The exfoliated nanosheets of HfS 2 exhibit an indirect bandgap of 1.3 eV with high stability against ambient degradation. Further, we demonstrate that these nanosheets holds potential for electronic applications by fabricating field-effect transistors based on few layered HfS 2 exhibiting field-effect mobility of 0.95 cm 2 /V-s with a high current modulation ratio (Ion/Ioff) of 10 4 in ambient. The method is scalable and has potential significance for both academy and industry.

ACS Applied Materials &amp; Interfaces, 2020

Metal contacts play a fundamental role in nanoscale devices. In this work, Schottky metal contacts in monolayer molybdenum disulfide (MoS 2) field-effect transistors are investigated under electron beam irradiation. It is shown that the exposure of Ti/Au source/drain electrodes to an electron beam reduces the contact resistance and improves the transistor performance. The electron beam conditioning of contacts is permanent, while the irradiation of the channel can produce transient effects. It is demonstrated that irradiation lowers the Schottky barrier at the contacts because of thermally induced atom diffusion and interfacial reactions. The simulation of electron paths in the device reveals that most of the beam energy is absorbed in the metal contacts. The study demonstrates that electron beam irradiation can be effectively used for contact improvement through local annealing.

MRS Advances, 2018

ABSTRACTTransition metal dichalcogenides are 2D structures with remarkable electronic, chemical, optical and mechanical properties. Monolayer and crystal properties of these structures have been extensively investigated, but a detailed understanding of the properties of their few-layer structures are still missing. In this work we investigated the mechanical differences between monolayer and multilayer WSe2 and MoSe2, through fully atomistic molecular dynamics simulations (MD). It was observed that single layer WSe2/MoSe2 deposited on silicon substrates have larger friction coefficients than 2, 3 and 4 layered structures. For all considered cases it is always easier to peel off and/or to fracture MoSe2 structures. These results suggest that the interactions between first layer and substrate are stronger than interlayer interactions themselves. Similar findings have been reported for other nanomaterials and it has been speculated whether this is a universal-like behavior for 2D layer...

Oxford Open Materials Science, 2020

Understanding and optimizing the properties of photoactive two-dimensional (2D) Van der Waals solids is crucial for developing optoelectronics applications. The main goal of this work is to present a detailed investigation of layer dependent photoconductive behavior of indium selenide (InSe)-based field-effect transistors (FETs). InSe-based FETs with five different channel thicknesses (t, 20 nm &lt; t &lt; 100 nm) were investigated with a continuous laser source of λ = 658 nm (1.88 eV) over a wide range of illumination power (Peff) of 22.8 nW &lt; P &lt; 1.29 μW. All the devices studied showed signatures of photogating; however, our investigations suggest that the photoresponsivities are strongly dependent on the thickness of the conductive channel. A correlation between the field-effect mobility (µFE) values (as a function of channel thickness, t) and photoresponsivity (R) indicates that in general R increases with increasing µFE (decreasing t) and vice versa. Maximum responsivitie...

Advanced Electronic Materials, 2017

Two-dimensional (2D) thin materials have been extensively studied in the past ten years due to their promising transport properties and potential technological applications, especially in the field of nanoelectronics [1-3]. Graphene has many extraordinary properties [4,5] , but it does not possess a band gap, needed in conventional field-effect transistors (FETs). However, a plethora of 2D crystals exists [6-8] , many of which are intrinsic semiconductors that have been

Scientific Reports, 2017

The performance of devices and systems based on two-dimensional material systems depends critically on the quality of the contacts between 2D material and metal. A low contact resistance is an imperative requirement to consider graphene as a candidate material for electronic and optoelectronic devices. Unfortunately, measurements of contact resistance in the literature do not provide a consistent picture, due to limitations of current graphene technology, and to incomplete understanding of influencing factors. Here we show that the contact resistance is intrinsically dependent on graphene sheet resistance and on the chemistry of the graphene-metal interface. We present a physical model of the contacts based on ab-initio simulations and extensive experiments carried out on a large variety of samples with different graphene-metal contacts. Our model explains the spread in experimental results as due to uncontrolled graphene doping and suggests ways to engineer contact resistance. We a...

Nanomaterials

Optical measurements under externally applied stresses allow us to study the materials’ electronic structure by comparing the pressure evolution of optical peaks obtained from experiments and theoretical calculations. We examine the stress-induced changes in electronic structure for the thermodynamically stable 1T polytype of selected MX2 compounds (M=Hf, Zr, Sn; X=S, Se), using the density functional theory. We demonstrate that considered 1T-MX2 materials are semiconducting with indirect character of the band gap, irrespective to the employed pressure as predicted using modified Becke–Johnson potential. We determine energies of direct interband transitions between bands extrema and in band-nesting regions close to Fermi level. Generally, the studied transitions are optically active, exhibiting in-plane polarization of light. Finally, we quantify their energy trends under external hydrostatic, uniaxial, and biaxial stresses by determining the linear pressure coefficients. Generally,...

physica status solidi (b), 2015

We present in-situ Raman measurements of laser-induced oxidation in exfoliated single-layer graphene. By using high-power laser irradiation, we can selectively and in a controlled way initiate the oxidation process and investigate its evolution over time. Our results show that the laserinduced oxidation process is divided into two separate stages, namely tensile strain due to heating and subsequent p-type doping due to oxygen binding. We discuss the temporal evolution of the D/G-mode ratio during oxidation and explain the unexpected steady decrease of the defect-induced D mode at long irradiation times. Our results provide a deeper understanding of the oxidation process in single-layer graphene and demonstrate the possibility of sub-µm patterning of graphene by an optical method.

Scientific Reports, 2017

We have investigated the electronic response of single crystals of indium selenide by means of angle-resolved photoemission spectroscopy, electron energy loss spectroscopy and density functional theory. The loss spectrum of indium selenide shows the direct free exciton at ~1.3 eV and several other peaks, which do not exhibit dispersion with the momentum. The joint analysis of the experimental band structure and the density of states indicates that spectral features in the loss function are strictly related to single-particle transitions. These excitations cannot be considered as fully coherent plasmons and they are damped even in the optical limit, i.e. for small momenta. The comparison of the calculated symmetry-projected density of states with electron energy loss spectra enables the assignment of the spectral features to transitions between specific electronic states. Furthermore, the effects of ambient gases on the band structure and on the loss function have been probed.

PLOS ONE, 2019

The study of two-dimensional (2D) materials is a rapidly growing area within nanomaterial research. However, the high equipment costs, which include the processing systems necessary for creating these materials, can be a barrier to entry for some researchers interested in studying these novel materials. Such process systems include those used for chemical vapor deposition. This article presents the first open-source design for an automated chemical vapor deposition system that can be built for less than a third of the cost for a similar commercial system. Our design can be easily customized and expanded on, depending upon the needs of the user. With a process chamber built as described, we demonstrate that a variety of 2D nanomaterials and their heterostructures can be grown via chemical vapor deposition. Specifically, our experimental results demonstrate the capability of this open-source design in producing high quality, 2D nanomaterials such as graphene and tungsten disulfide, which are at the forefront of research in emerging semiconductor devices, sensors, and energy storage applications.

Applied Physics Letters, 2016

Physical Review B, 2017

The electronic conductance of graphene-based bilayer flake systems reveal different quantum interference effects, such as Fabry-Pérot resonances and sharp Fano antiresonances on account of competing electronic paths through the device. These properties may be exploited to obtain spinpolarized currents when the same nanostructure is deposited above a ferromagnetic insulator. Here we study how the spin-dependent conductance is affected when a time-dependent gate potential is applied to the bilayer flake. Following a Tien-Gordon formalism we explore how to modulate the transport properties of such systems via appropriate choices of the ac-field gate parameters. The presence of the oscillating field opens the possibility of tuning the original antiresonances for a large set of field parameters. We show that interference patterns can be partially or fully removed by the time-dependent gate voltage. The results are reflected in the corresponding weighted spin polarization which can reach maximum values for a given spin component. We found that differential conductance maps as functions of bias and gate potentials show interference patterns for different ac-field parameter configurations. The proposed bilayer graphene flake systems may be used as a frequency detector in the THz range.

ChemistrySelect, 2019

The main graphene-family materials used in biomedicine include graphene, graphene oxide, and reduced graphene oxide. They have been well documented for their distinctive microstructure and excellent physicochemical properties. Although they also have some shortcomings, such as slow biodegradation and dose-dependent biotoxicity, these problems can be solved by using them in conjunction with other biomaterials. Silk fibroin (SF), a natural polymer material with many advantageous properties, has been used widely in biomedicine. However, SF requires modification by other biomaterials to improve its mechanical properties and control its degradation rate for further use. In recent years, the combined application of graphene-family materials and SF has increased gradually, and the effects of it on cell behavior have been preliminary investigated, such as promoting cell adhesion, proliferation, and differentiation. Further studies of the combined use have been conducted in biomedicine, including tissue engineering, biosensor, and other biomedical usage. In this paper, this combined application has been outlined and summarized, and some envisioned challenges and future perspectives have been mentioned. We hope that this review will provide some reference and inspiration for the combined application of graphene-family materials and SF in biomedicine in the future.

RSC Advances

Quantum confined ultrathin nanosheets of In2S3 were synthesized from a new structurally characterized molecular precursor. The prototype photoelectrochemical cell based on the material exhibited high photostability and photoresponsivity.

Advanced materials (Deerfield Beach, Fla.), 2016

An ultra-clean method to directly transfer a large area MoS2 film from the original growth substrate to a flexible substrate by using epoxy glue has been developed. The transferred film is observed to be free of wrinkles and cracks and to be as smooth as the film synthesized on the original substrate.

Nature Electronics, 2019

Two-dimensional semiconductors could be used as a channel material in low-power transistors, but the deposition of high-quality, ultrathin high-κ dielectrics on such materials has proved challenging. In particular, atomic layer deposition typically leads to non-uniform nucleation and island formation, creating a porous dielectric layer that suffers from current leakage, particularly when the equivalent oxide thickness is small. Here, we report the atomic layer deposition of high-κ gate dielectrics on twodimensional semiconductors using a monolayer molecular crystal as a seeding layer. The approach can be used to grow dielectrics with an equivalent oxide thickness of 1 nm on graphene, molybdenum disulfide (MoS 2) and tungsten diselenide (WSe 2). Compared with dielectrics created using established methods, our dielectrics exhibit a reduced roughness, density of interface states and leakage current, as well as an improved breakdown field. With the technique, we fabricate graphene radio-frequency transistors that operate at 60 GHz, and MoS 2 and WSe 2 complementary metal-oxide-semiconductor transistors with a supply voltage of 0.8 V and subthreshold swing down to 60 mV dec −1. We also create MoS 2 transistors with a channel length of 20 nm, which exhibit an on/off ratio of over 10 7 .

Nanomaterials

The thermal-assisted exfoliation phenomena of boehmite particles under moderate heating rates were examined. The exfoliation that generated flakes of 5–6 nm in thickness can be achieved because of the perfect cleavage on the boehmite particles that are stripped when thermal treatments bring about dehydration and γ-Al2O3 formation in sequential phase transformation of boehmite. Examinations of the exfoliation effects were carried out on calcined boehmite single crystal particles, which were about 500 nm in diameter, and obtained at three heating rates 0.5, 1.0, and 2.0 °C/min with the heating schedules. The TEM techniques, BET-N2 measurements, XRD-Scherrer equation, and AFM images were employed in the examination. That the BET values increased as increasing of exfoliated flakes reflected two stages of exfoliation. In the beginning stage, during which the BET values were &lt;40 m2/g, the exfoliation resulted from the stress produced by dehydration. In the second stage, the increased r...

physica status solidi (b), 2018

van der Waals heterostructures of (TMDL=1/BNL=1-4/TMDL=1/BNL=1-4), [TMD = MoS2, WS2, and ReS2] are grown on c-plane sapphire substrate by pulsed laser deposition under slow kinetic condition. The heterostructure systems show strong emission around 2.3 eV and subsidiary peaks around 2.8, 1.9, 1.7 and 1.5 eV. BN and TMDs forms type-I heterojunction and the emission peaks observed are explained in terms of various band to band recombination processes and considering relative orientation of Brillouin Zones. The emission peak around 2.3eV is promising for solar and photovoltaic application. The observation is almost similar for three different heterostructure systems.

Advanced Electronic Materials, 2016

Applied Sciences, 2019

Atom-thick two-dimensional materials usually possess unique properties compared to their bulk counterparts. Their properties are significantly affected by defects, which could be uncontrollably introduced by irradiation. The effects of electromagnetic irradiation and particle irradiation on 2H MoS 2 two-dimensional nanolayers are reviewed in this paper, covering heavy ions, protons, electrons, gamma rays, X-rays, ultraviolet light, terahertz, and infrared irradiation. Various defects in MoS 2 layers were created by the defect engineering. Here we focus on their influence on the structural, electronic, catalytic, and magnetic performance of the 2D materials. Additionally, irradiation-induced doping is discussed and involved.

Frontiers in Materials, 2021

The essential properties of monolayer HfX2 (X = S, Se, or Te) are fully explored by first-principles calculations. The optimal lattice symmetries, sublattice buckling, electronic energy spectra, and density of states are systematically investigated. Monolayer HfS2, HfSe2, and HfTe2, respectively, belong to middle-gap semiconductor, narrow-gap one and semimetal, with various energy dispersions. Moreover, the van Hove singularities (vHs) mainly arise from the band-edge states, and their special structures in the density of states strongly depend on their two or three-dimensional structures and the critical points in the energy-wave-vector space. The above-mentioned theoretical predictions are attributed to the multi-orbital hybridizations of [dx2−y2, dxy, dyz, dzx, dz2]–[s, px, py, pz] in the Hf-X chemical bonds. The diversified physical phenomena clearly indicate a high potential for applications, as observed in MoS2-related emergent materials ions.

Scientific Reports

In the last decade, research on 2D materials has expanded massively due to the popularity of graphene. Although the chemical engineering of two-dimensional elemental materials as well as heterostructures has been extensively pursued, the fundamental understanding of the synthesis of 2D materials is not yet complete. Structural parameters, such as buckling or the interface structure of a 2D material to the substrate directly affect its electronic characteristics. In order to proceed the understanding of the element-specific growth and the associated ability of tuning material properties of two-dimensional materials, we performed a study on the structural evolution of the promising 2D material germanene on Ag(111). This study provides a survey of germanium formations at different layer thicknesses right up to the arising of quasi-freestanding germanene. Using powerful surface analysis tools like low-energy electron diffraction, x-ray photoelectron spectroscopy, and x-ray photoelectron...

Scientific Reports, 2019

We implement a logic switch by using a graphene acoustoelectric transducer at room temperature. We operate two pairs of inter-digital transducers (IDTs) to launch surface acoustic waves (SAWs) on a LiNbO3 substrate and utilize graphene as a channel material to sustain acoustoelectric current Iae induced by SAWs. By cooperatively tuning the input power on the IDTs, we can manipulate the propagation direction of Iae such that the measured Iae can be deliberately controlled to be positive, negative, or even zero. We define the zero-crossing Iae as $${I}_{ae}^{off}$$ I a e o f f , and then demonstrate that Iae can be switched with a ratio $${I}_{ae}^{on}/{I}_{ae}^{off}\, \sim \,{10}^{4}$$ I a e o n / I a e o f f ~ 10 4 at a rate up to few tens kHz. Our device with an accessible operation scheme provides a means to convert incoming acoustic waves modulated by digitized data sequence onto electric signals with frequency band suitable for digital audio modulation. Consequently, it could po...

Journal of Applied Physics, 2018

Black phosphorus (BP) is receiving significant attention because of its direct 0.4-1.5 eV layerdependent band gap and high mobility. Because BP devices rely on exfoliation from bulk crystals, there is a need to understand native impurities and defects in the source material. In particular, samples are typically p-doped, but the source of the doping is not well understood. Here, we use scanning tunneling microscopy and spectroscopy to compare atomic defects of BP samples from two commercial sources. Even though the sources produced crystals with an order of magnitude difference in impurity atoms, we observed a similar defect density and level of p-doping. We attribute these defects to phosphorus vacancies and provide evidence that they are the source of the p-doping. We also compare these native defects to those induced by air exposure and show they are distinct and likely more important for control of electronic structure. These results indicate that impurities in BP play a minor role compared to vacancies, which are prevalent in commercially-available materials, and call for better control of vacancy defects.

Inorganics, 2022

An attempt was made, using computational methods, to understand whether the intermolecular interactions in the dimers of molybdenum dichalcogenides MoCh2 (Ch = chalcogen, element of group 16, especially S, Se and Te) and similar mixed-chalcogenide derivatives resemble the room temperature experimentally observed interactions in the interfacial regions of molybdenites and their other mixed-chalcogen derivatives. To this end, MP2(Full)/def2-TVZPPD level electronic structure calculations on nine dimer systems, including (MoCh2)2 and (MoChCh′2)2 (Ch, Ch′ = S, Se and Te), were carried out not only to demonstrate the energetic stability of these systems in the gas phase, but also to reproduce the intermolecular geometrical properties that resemble the interfacial geometries of 2D layered MoCh2 systems reported in the crystalline phase. Among the six DFT functionals (single and double hybrids) benchmarked against MP2(full), it was found that the double hybrid functional B2PLYPD3 has some a...

RSC Advances

The preparation of borophene flakes by ball milling is shown in the following schematic.

Nanotechnology, 2018

Two-dimensional electron gas (2DEG) is crucial in condensed matter physics and is present on the surface of liquid helium and at the interface of semiconductors. Monolayer MoS2 of 2D materials also contains 2DEG in an atomic layer as field effect transistor (FET) ultrathin channel. In this study, we synthesized double triangular MoS2 through a chemical vapor deposition method to obtain grain boundaries for forming a ripple structure in FET channel. When the temperature was higher than approximately 175 K, the temperature dependence of the electron mobility μ was consistent with those in previous experiments and theoretical predictions. When the temperature was lower than approximately 175 K, the mobility behavior decreased with the temperature; this finding was also consistent with that of the previous experiments. We are the first research group to explain the decreasing mobility behavior by using the Wigner crystal phase and to discover the temperature independence of ripplon-limited mobility behavior at lower temperatures. Although these mobility behaviors have been studied on the surface of liquid helium through theories and experiments, they have not previously analyzed in 2D materials and semiconductors. We are the first research group to report the similar temperature-dependent mobility behavior of the surface of liquid helium and the monolayer MoS2.

Physical Review B

The negative piezoelectric coefficient in ferroelectric polymers has been widely studied. However, the same in the two-dimensional (2D) materials is scarcely reported, which stresses the need for its thorough investigation. Here, the 2D van der Waals (vdW) heterobilayers (vdWHs) of the dialkali metal monochalcogenide M 2 X (M = Na, K, Rb, or Cs; and X = O, S, Se, or Te) monolayers show negative out-of-plane piezoelectricity. Out of all the explored heterobilayers of the family, the maximum piezoelectric strain coefficient in the out-of-plane direction d 33 = −39 pm V −1 is found in the Na 2 Te/Cs 2 S system. This strong piezoelectric coefficient arises from the notable electrostatic potential energy difference and band offset between the constituent monolayers. The small out-of-plane Young's modulus (Y ∼ 11 N m −1), originating from the weak vdW interlayer bonding relative to the stronger intralayer bonding, accounts for the anomalous negative piezoelectricity in the heterostructure. The combination of minute out-of-plane (Y =∼ 11 N m −1) and in-plane (Y =∼ 23 N m −1) Young's moduli with small bending modulus (8.83 eV) and negative piezoelectric coefficient underlines the suitability of the proposed vdWH for stretchable and flexible piezotronic devices. Moreover, external perturbations are found to modify the properties of the system. The vertical compressive strain widens the band gap by elevating the conduction band minimum in the heterostructure. An externally applied electric field of low strength (−0.33 and +0.31 V/Å) is found to change the semiconducting heterostructure to metallic upon the closure of the band gap, which is attributed to the dropping of nearly free electron gas (NFEG) states to the Fermi level. A significant enhancement in electrical conductivity has been brought up by the NFEG states upon reaching the Fermi level, which inspires the use of the heterostructure in a nonresistive charge carrier transport application in electronics.

Applied Materials Today, 2020

One of the most intriguing properties of layered materials is their ability to form inherently ultra-thin atomically sharp vertical interfaces and hybrid layered compounds, in moderate environment conditions, which are ideal platforms for both, scientific research and many applications. Here, we present the selective van der Waals epitaxial formation of β-In 2 Se 3 on transition metal dichalcogenides (TMDCs). This is achieved in a two-step chemical vapor deposition (CVD) process, in which first, the monolayer TMDC, is synthesized and then the β-In 2 Se 3 is grown on top of it. The thickness of the second phase can be controlled by the growth conditions, while the crystal size is dictated by the MoS 2 single-crystal domain size. High-resolution transmission electron microscope (HRTEM) studies reveal a clean and sharp interface, while selected area diffraction (SAED) demonstrates a clear registry between both phases. The hybrid layered-compound exhibit better electrical transport than the intrinsic indium-selenide layer. The high crystallinity of In 2 Se 3 grown on MoS 2 yield fast response among the CVD-derived In 2 Se 3-based photodetectors with rise/fall times of 4/7 ms, photoresponsivity of up to ~23 A/W and specific detectivity of ~5 × 10 11. Our methodology allows high quality thin In 2 Se 3 layers to be formed via van der Waals epitaxy on TMDCs, forming complex vertical heterostructures for optoelectronic applications.

Chemistry of Materials, 2021

The properties of metal-semiconductor junctions are often unpredictable because of non-ideal interfacial structures, such as interfacial defects or chemical reactions introduced at junctions. Black phosphorus (BP), an elemental two-dimensional (2D) semiconducting crystal, possesses the puckered atomic structure with high chemical reactivity, and the establishment of a realistic atomic-scale picture of BP's interface toward metallic contact has remained elusive. Here we examine the interfacial structures and properties of physically-deposited metals of various kinds on BP. We find that Au, Ag, and Bi form single-crystalline films with (110) orientation through guided van der Waals epitaxy. Transmission electron microscopy and X-ray photoelectron spectroscopy confirm that atomically sharp van der Waals metal-BP interfaces forms with exceptional rotational alignment. Under a weak metal-BP interaction regime, the BP's puckered structure play an essential role in the adatom assembly process and can lead to the formation of a single crystal, which is supported by our theoretical analysis and calculations. The experimental survey also demonstrates that the BP-metal junctions can exhibit various types of interfacial structures depending on metals, such as the formation of polycrystalline microstructure or metal phosphides. This study provides a guideline for obtaining a realistic view on metal-2D semiconductor interfacial structures, especially for atomically puckered 2D crystals. Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Schematic of metal/BP sample fabrication process, AFM topography images, TEM/STEM characterization data of samples, interfacial binding energy calculation and structure relaxation of Au on BP, extra XPS spectra of Au-BP, summarized the lattice mismatch between BP and investigated metals, and calculated Gibbs free energies of metal phosphides at 298 K.

Chemistry of Materials, 2021

Boosting growth of MoSe2 with Na2SeO3. An enhanced CVD growth of continuous MoSe2 film was achieved by adding Na2SeO3 to Na2MoO4. This work provides a general strategy for large-area growth of 2D transition metal dichalcogenides using mixed molten salts.

Crystals

The electronic and optical properties of finite GaS nanoribbons are investigated using density functional theory calculations. The effect of size, edge termination, and chemical modification by doping and edge passivation are taken into account. The dynamical stability is confirmed by the positive vibration frequency from infrared spectra; further, the positive binding energies ensure the stable formation of the considered nanoribbons. Accurate control of the energy gap has been achieved. For instance, in armchair nanoribbons, energy gaps ranging from ~ 1 to 4 eV were obtained in varying sizes. Moreover, the energy gap can be increased by up to 5.98 eV through edge passivation with F-atoms or decreased to 0.98 eV through doping with Si-atoms. The density of states shows that the occupied molecular orbitals are dominated by S-atoms orbitals, while unoccupied ones are mostly contributed to by Ga orbitals. Thus, S-atoms will be the electron donor sites, and Ga-atoms will be the electro...

Physical Review B, 2015

We report that the hydrogenation of a single crystal 2H-MoS 2 induces a novel-intermediate phase between 2H and 1T phases on its surface, i.e., the large-area, uniform, robust, and surface array of atomic stripes through the intralayer atomic-plane gliding. The total energy calculations confirm that the hydrogenation-induced atomic stripes are energetically most stable on the MoS 2 surface between the semiconducting 2H and metallic 1T phase. Furthermore, the electronic states associated with the hydrogen ions, which is bonded to sulfur anions on both sides of the MoS 2 surface layer, appear in the vicinity of the Fermi level (E F) and reduces the band gap. This is promising in developing the monolayer-based field-effect transistor or vanishing the Schottky barrier for practical applications.

Advanced Materials, 2020

Two-dimensional (2D) semiconductors are promising material candidates for nextgeneration nanoelectronics. However, there are fundamental challenges related to their metalsemiconductor (MS) contacts which limit the performance potential for practical device applications. In this work, we exploit 2D monolayer hexagonal boron nitride (h-BN) as an ultrathin decorating layer to form a metal-insulator-semiconductor (MIS) contact, and demonstrate a novel diode-like selective enhancement of the carrier transport through it. Compared to the conventional MS contact, the MIS contact dominated by both thermionic emission and quantum tunneling can significantly reduce the contact resistance and boost the electron transport from the semiconductor to the metal, but has negligible effects on the electron transport oppositely. We also investigate the negative barrier height with the concept of carrier collection barrier, and show its critical role at the drain end for determining the overall transistor performance.

RSC Advances, 2015

Direct epitaxial growth of WS2 isolated crystals and WS2 continuous films onto epitaxial- and CVD-graphene providing a homogeneous and narrow PL peak.

npj 2D Materials and Applications

Inspired by massive parallelism, an increase in internet-of-things devices, robust computation, and Big-data, the upsurge research in building multi-bit mem-transistors is ever-augmenting with different materials, mechanisms, and state-of-the-art architectures. Herein, we demonstrate monolayer WS2-based functional mem-transistor devices which address nonvolatility and synaptic operations at high temperature. The ionotronic memory devices based on WS2 exhibit reverse hysteresis with memory windows larger than 25 V, and extinction ratio greater than 106. The mem-transistors show stable retention and endurance greater than 100 sweep cycles and 400 pulse cycles in addition to 6-bit (64 distinct nonvolatile storage levels) pulse-programmable memory features ranging over six orders of current magnitudes (10−12–10−6 A). The origin of the multi-bit states is attributed to the carrier dynamics under electrostatic doping fluctuations induced by mobile ions, which is illustrated by employing a...

Physical Review Applied, 2015

We report highly linearly polarized remote luminescence that emerges at the cleaved edges of nanoscale gallium selenide slabs tens of micrometers away from the optical excitation spot. The remote-edge luminescence (REL) measured in the reflection geometry has a degree of linear polarization above 0.90, with polarization orientation pointing toward the photoexcitation spot. The REL is dominated by an index-guided optical mode that is linearly polarized along the crystalline c-axis. This luminescence is from out-of-plane dipoles that are converted from in-plane dipoles through a spin-flip process at the excitation spot.

Nano Research, 2017

published as an ASAP article. Please note that technical editing may introduce minor changes to the manuscript text and/or graphics which may affect the content, and all legal disclaimers that apply to the journal pertain. In no event shall TUP be held responsible for errors or consequences arising from the use of any information contained in these "Just Accepted" manuscripts. To cite this manuscript please use its Digital Object Identifier (DOI®), which is identical for all formats of publication.

Physical Review Applied, 2020

The ideal diode is a theoretical concept that completely conducts the electric current under forward bias without any loss and that behaves like a perfect insulator under reverse bias. However, real diodes have a junction barrier that electrons have to overcome and thus they have a threshold voltage VT , which must be supplied to the diode to turn it on. This threshold voltage gives rise to power dissipation in the form of heat and hence is an undesirable feature. In this work, based on half-metallic magnets (HMMs) and spin-gapless semiconductors (SGSs) we propose a diode concept that does not have a junction barrier and the operation principle of which relies on the spin-dependent transport properties of the HMM and SGS materials. We show that the HMM and SGS materials form an Ohmic contact under any finite forward bias, while for a reverse bias the current is blocked due to spin-dependent filtering of the electrons. Thus, the HMM-SGS junctions act as a diode with zero threshold voltage VT , and linear current-voltage (I-V) characteristics as well as an infinite on:off ratio at zero temperature. However, at finite temperatures, non-spin-flip thermally excited high-energy electrons as well as low-energy spin-flip excitations can give rise to a leakage current and thus reduce the on:off ratio under a reverse bias. Furthermore, a zero threshold voltage allows one to detect extremely weak signals and due to the Ohmic HMM-SGS contact, the proposed diode has a much higher current drive capability and low resistance, which is advantageous compared to conventional semiconductor diodes. We employ the nonequilibrium Greens function method combined with density-functional theory to demonstrate the linear I-V characteristics of the proposed diode based on two-dimensional half-metallic Fe/MoS2 and spin-gapless semiconducting VS2 planar heterojunctions.

Nanoscale, 2015

Vertical graphene-based device concepts that rely on quantum mechanical tunneling are intensely being discussed in the literature for applications in electronics and optoelectronics. In this work, the carrier transport mechanisms in semiconductor-insulator-graphene (SIG) capacitors are investigated with respect to their suitability as electron emitters in vertical graphene base transistors (GBTs). Several dielectric materials as tunnel barriers are compared, including dielectric double layers. Using bilayer dielectrics, we experimentally demonstrate significant improvements in the electron injection current by promoting Fowler-Nordheim tunneling (FNT) and step tunneling (ST) while suppressing defect mediated carrier transport. High injected tunneling current densities approaching 10 3 A cm −2 (limited by series resistance), and excellent current-voltage nonlinearity and asymmetry are achieved using a 1 nm thick high quality dielectric, thulium silicate (TmSiO), as the first insulator layer, and titanium dioxide (TiO 2) as a high electron affinity second layer insulator. We also confirm the feasibility and effectiveness of our approach in a full GBT structure which shows dramatic improvement in the collector on-state current density with respect to the previously reported GBTs. The device design and the fabrication scheme have been selected with future CMOS process compatibility in mind. This work proposes a bilayer tunnel barrier approach as a promising candidate to be used in high performance vertical graphene-based tunneling devices.

Nanomaterials

In recent decades, two-dimensional materials (2D) such as graphene, black and blue phosphorenes, transition metal dichalcogenides (e.g., WS2 and MoS2), and h-BN have received illustrious consideration due to their promising properties. Increasingly, nanomaterial thermal properties have become a topic of research. Since nanodevices have to constantly be further miniaturized, thermal dissipation at the nanoscale has become one of the key issues in the nanotechnology field. Different techniques have been developed to measure the thermal conductivity of nanomaterials. A brief review of 2D material developments, thermal conductivity concepts, simulation methods, and recent research in heat conduction measurements is presented. Finally, recent research progress is summarized in this article.

Physical Review Materials, 2020

Identification of new two-dimensional (2D) materials with magnetic properties has received strong research attention in the development of advanced spinbased devices. By means of first-principles calculations, we investigate the stability, electronic properties and the hole-doping-induced magnetic properties of novel metal oxide (MO, M = Ga, In) monolayers. They are intrinsically non-magnetic stable semiconductors, with high energetic, vibrational and thermal stability. Hole doping can switch them from non-magnetic to ferromagnetic and turn them into half-metals over a wide range of hole densities. Monte Carlo simulations predict that the highest Curie temperature of the GaO monolayer can reach ~125 K. Our results indicate that monolayer MO could be eligible candidate materials for novel 2D spintronic devices.

Crystals, 2018

Graphene is a kind of typical two-dimensional material consisting of pure carbon element. The unique material shows many interesting properties which are dependent on crystallographic orientations. Therefore, it is critical to determine their crystallographic orientations when their orientation-dependent properties are investigated. Raman spectroscopy has been developed recently to determine crystallographic orientations of two-dimensional materials and has become one of the most powerful tools to characterize graphene nondestructively. This paper summarizes basic aspects of Raman spectroscopy in crystallographic orientation of graphene nanosheets, determination principles, the determination methods, and the latest achievements in the related studies.

Metals and Materials International, 2020

In the present study, ultrasonic and thermophysical behaviour of thermoelectric hexagonal structured HfX 2 (X = S, Se) compounds have been analysed by the theoretical evaluation of second and third order elastic constants (SOECs and TOECs) using many body interaction potential model. The computed SOECs have been used to determine the temperature dependent specific heat, thermal energy density, elastic coupling constants, Grüneisen parameters, ultrasonic velocities, Debye average velocity, ultrasonic attenuation, and thermal relaxation time. We have observed that the temperature dependent ultrasonic attenuation and thermal relaxation time for HfX 2 are mainly affected by thermal conductivity. The elastic properties of the material are compared with existing data in literature for the validation of the work. The obtained values of elastic stiffness are found very large in comparison to that of other material from transition metal dichalcogenides of group IVB indicating better mechanical properties than the same group materials. Calculated elastic, thermal, and ultrasonic properties are correlated for better characterization of the materials which are useful for thermoelectric and energy transport applications.

npj 2D Materials and Applications

Recent research on two-dimensional (2D) transition metal dichalcogenides (TMDCs) has led to remarkable discoveries of fundamental phenomena and to device applications with technological potential. Large-scale TMDCs grown by chemical vapor deposition (CVD) are now available at continuously improving quality, but native defects and natural degradation in these materials still present significant challenges. Spectral hysteresis in gate-biased photoluminescence (PL) measurements of WSe2 further revealed long-term trapping issues of charge carriers in intrinsic defect states. To address these issues, we apply here a two-step treatment with organic molecules, demonstrating the “healing” of native defects in CVD-grown WSe2 and WS2 by substituting atomic sulfur into chalcogen vacancies. We uncover that the adsorption of thiols provides only partial defect passivation, even for high adsorption quality, and that thiol adsorption is fundamentally limited in eliminating charge traps. However, a...

Applied Sciences, 2020

The high contact resistance at metal/two-dimensional (2D) semiconductor junctions is a major issue for the integration of 2D materials in nanoelectronic devices. We review here recent theoretical results on the contact resistance at lateral heterojunctions between graphene or 1T-MoS2 with 2H-MoS2 monolayers. The transport properties at these junctions are computed using density functional theory and the non-equilibrium Green’s function method. The contact resistance is found to strongly depend on the edge contact symmetry/termination at graphene/2H-MoS2 contacts, varying between about 2 × 102 and 2 × 104 Ω∙μm. This large variation is correlated to the presence or absence of dangling bond defects and/or polar bonds at the interface. On the other hand, the large computed contact resistance at pristine 1T/2H-MoS2 junctions, in the range of 3–4 × 104 Ω.μm, is related to the large electron energy barrier (about 0.8 eV) at the interface. The functionalization of the metallic 1T-MoS2 conta...

Journal of Materials Science, 2020

In this study, we have examined the geometrical, electronic and optical properties of penta-PdX 2 (X = As,P) using density functional calculation. The electronic structure calculations show that the penta-PdAs 2 and PdP 2 are semiconductors with direct band gaps of 0.34 eV and 0.30 eV, respectively. The dynamical stability of penta-PdX 2 monolayer is proved by the absence of imaginary frequencies in the phonon dispersion curve. By applying a biaxial strain (for PdAs 2 :-6% to ? 6% and for PdP 2 :-5.5% to ? 5.5%) on the monolayer, the effective mass and band edges are tuned effectively. Remarkably, the range of penta-PdX 2 carrier mobility was obtained in an extremely high order of 10 5 cm 2 V-1 s-1 for holes and 10 4 cm 2 V-1 s-1 for electrons. The optical properties of penta-PdX 2 were also strained-tunable and exhibit outstanding absorption of infrared, visible and ultraviolet light. More importantly, the band edges alignment has tunable with implemented external electric field (V/Å) along the z-direction. Our work would stimulate the fabrication of penta-PdX 2 monolayer, and it is envisioned that it is an appropriate future candidate for optoelectronic and ultra-fast electronic applications.

Journal of Applied Physics, 2020

Digital holography has found applications in many walks of life, from medicine to metrology, due to its ability to measure complex fields. Here, we use the power of digital holography to quantitatively image two-dimensional Transition Metal Dichalcogenides (TMDs) such as MoS2 and WS2 placed on a SiO2/Si substrate and determine their complex refractive indices or layer thicknesses. By considering the different refractive indices of the TMDs as they are thinned down from bulk to monolayers and by holographically capturing both the amplitude and the phase of reflected light, single atomic layers of TMDs, about 0.7 nm thick, can be resolved. Using holography, we also predict the number of layers contained within a thick TMD flake, which shows agreement with results obtained using Atomic Force Microscopy (AFM). A Bland–Altman analysis was performed to compare our experimental results with the standard AFM measurements, yielding a limit of agreement &lt;5 nm for samples with thicknesses r...

Research Square (Research Square), 2023

The demand for high-performance thin-lm transistors (TFTs) has increased signi cantly due to the increasing functionalities of electronic devices, such as displays, sensors, and computing platforms. The requirements for TFTs have also become more stringent because future electronic products necessitate denser device arrays, lower power consumption, higher mechanical exibility, and lower-temperature processing without compromising their performance. To meet these demands, two-dimensional (2D) semiconductors are an ideal solution due to their excellent scalability, transferability, atomically thin thickness, and relatively high carrier mobility. Nevertheless, studies on 2D materials have been limited to small laboratory-scale demonstrations, focusing on proof-of-concept devices with single-crystalline 2D lms. In this study, we present industrialization strategies speci cally designed for polycrystalline MoS 2 TFTs on a 200-mm wafer scale. We achieved nearly 100% device yield across the wafer by processing it in one of the Samsung's 200-mm fabrication facilities. We nd that the metal-semiconductor junction in polycrystalline 2D MoS 2 is fundamentally different from that in its single-crystalline counterpart. Thus, we redesigned the process ow to nearly eliminate the Schottky barrier height at the MoS 2 -metal contact, yielding excellent FET performance equivalent to that of state-of-the-art FETs fabricated from singlecrystalline akes.

Nanoscale, 2017

Hybrid van der Waals (vdW) heterostructures composed of two-dimensional (2D) layered materials and self-assembled organic molecules are promising systems for electronic and optoelectronic applications with enhanced properties and performance.

Applied Physics Letters, 2018

SnSe2 is currently considered a potential two-dimensional material that can form a near-broken gap heterojunction in a tunnel field-effect transistor due to its large electron affinity which is experimentally confirmed in this letter. With the results from internal photoemission and angle-resolved photoemission spectroscopy performed on Al/Al2O3/SnSe2/GaAs and SnSe2/GaAs test structures where SnSe2 is grown on GaAs by molecular beam epitaxy, we ascertain a (5.2 ± 0.1) eV electron affinity of SnSe2. The band offset from the SnSe2 Fermi level to the Al2O3 conduction band minimum is found to be (3.3 ± 0.05) eV and SnSe2 is seen to have a high level of intrinsic electron (n-type) doping with the Fermi level positioned at about 0.2 eV above its conduction band minimum. It is concluded that the electron affinity of SnSe2 is larger than that of most semiconductors and can be combined with other appropriate semiconductors to form near broken-gap heterojunctions for the tunnel field-effect t...

Optical and Quantum Electronics, 2023

Poly(3,4-ethylenedioxythiophene) (PEDOT) is the most successful conductive polymer. 1 PEDOT films with high conductivity (∼ 700 S/cm) are easily accessible through in situ polymerization of 3,4-ethylenedioxythiophene (EDOT). The PEDOT complex with poly(styrene sulfonic acid) (PSS) that is dispersed in an aqueous medium is commercially available and from moderately to highly conductive PEDOT-PSS films can also be easily prepared from the water dispersion. These PEDOT films are stable under ambient conditions in the positively oxidized (p-doped) conductive states and have transparent properties. Because of these unique properties, PEDOTs are used for various applications, such as antistatic coatings, cathodes in capacitors, through-hole plating, OLED's, OFET's, photovoltaics, and electrochromic films. 1 As an analogue of PED-OT, poly(3,4-propylenedioxythiophene)s (PProDOTs) were also prepared. 2À5 One advantage in using a propylene bridge instead of an ethylene bridge is that solubilizing side chains can be introduced in a symmetrical manner when two same units are substituted at the center of the propylene bridge. The p-doped states of these polymers are also transparent, and the solution-processable electrochromic films have also been developed. 2,4

Applied Surface Science, 2021

Density functional theory (DFT) calculations have been performed to investigate adsorption and incorporation of arsenic on the boron phosphide (1 1 1) surface. We considered different arsenic coverages. Dimers (1/2 ML), trimers (¾ ML), and atomic wires (1 ML) are formed upon increasing As coverage. However, at full monolayer, the most relevant result is the hexagonal boron phosphide (h-BP) monolayer formation, a 2D graphene-like structure. The h-BP monolayer phonon dispersion shows only positive frequencies indicating dynamical stability. At the same time, this suggests the possible h-BP monolayer exfoliation (desorption energy of only 0.26 eV/ 1x1) because it is weakly attached to the substrate by van der Waals interactions, as demonstrated by the noncovalent interaction index analysis. Electronic band structures indicate that this 2D layer may support direct transitions, making it useful for applications in electronic devices. The density of states and projected density of states complement the electronic properties analysis. We encourage the experimental realization of h-BP by As incorporation on BP surfaces.

Physical Chemistry Chemical Physics, 2019

Advanced Energy Materials, 2017

The successful isolation of phosphorene (atomic layer thick black phosphorus) in 2014 has currently aroused the interest of 2D material researchers. In this review, first, the fundamentals of phosphorus allotropes, phosphorene, and black phosphorus, are briefly introduced, along with their structures, properties, and synthesis methods. Second, the readers are presented with an overview of their energy applications. Particularly in electrochemical energy storage, the large interlayer spacing (0.53 nm) in phosphorene allows the intercalation/deintercalation of larger ions as compared to its graphene counterpart. Therefore, phosphorene may possess greater potential for high electrochemical performance. In addition, the status of lithium ion batteries as well as secondary sodium ion batteries is reviewed. Next, each application for energy generation, conversion, and storage is described in detail with milestones as well as the challenges. These emerging applications include supercapacit...

Science China Information Sciences, 2019

Two-dimensional transition metal oxychlorides (MOCl, M = Fe, Cr, V, Ti, Sc) with the metaloxygen plane sandwiched by two layers of chloride ions possess many exotic physical properties. Nevertheless, it is of great challenge to grow two-dimensional single-crystal MOCl because polyvalent nature of transition metal elements usually gives rise to mixed oxyhalides compounds with distinct physical properties. Here, we take VOCl as an example to present a solution for synthesizing 2D freestanding MOCl with various thicknesses through chemical vapor deposition (CVD) method. The single crystal and elementary composition as well as elements ratio of as-grown samples have been characterized through measurements of X-ray diffraction, X-ray photoelectron spectroscopy and energy-dispersive spectroscopy, respectively. Furthermore, we demonstrate that 2D VOCl-based memristive devices show low power consumption and excellent device reliability due to the layered-structure and electrically insulating properties of 2D VOCl flakes. Besides, we utilize the feature of multilevel resistive switching that memristive devices exhibit to emulate depression and potentiation of synaptic plasticity. This method developed in this study may open up a new avenue for the growth of 2D MOCl with single crystal and pave the way for high-performance electronic applications.

Science advances, 2017

The success of silicon as a dominant semiconductor technology has been enabled by its moderate band gap (1.1 eV), permitting low-voltage operation at reduced leakage current, and the existence of SiO2 as a high-quality &quot;native&quot; insulator. In contrast, other mainstream semiconductors lack stable oxides and must rely on deposited insulators, presenting numerous compatibility challenges. We demonstrate that layered two-dimensional (2D) semiconductors HfSe2 and ZrSe2 have band gaps of 0.9 to 1.2 eV (bulk to monolayer) and technologically desirable &quot;high-κ&quot; native dielectrics HfO2 and ZrO2, respectively. We use spectroscopic and computational studies to elucidate their electronic band structure and then fabricate air-stable transistors down to three-layer thickness with careful processing and dielectric encapsulation. Electronic measurements reveal promising performance (on/off ratio &gt; 10(6); on current, ~30 μA/μm), with native oxides reducing the effects of interf...

Journal of Materials Chemistry C, 2021

Engineering physicochemical properties of two-dimensional transition metal dichalcogenide (2D-TMD) materials by surface manipulation is essential to enabling their practical and large-scale application. This is especially challenging for colloidal 2D-TMDs that are plagued by the unintentional formation of structural defects during the synthetic procedure. However, the available methods to manage surface states of 2D-TMDs in solution phase are still limited, hampering the production of high-quality colloidal 2D-TMD inks. Here, we demonstrate an efficient solution-phase strategy to passivate surface defect states of colloidal WS 2 nanoflakes with halide ligands, which results in the activation of their photoluminescence emission. Photophysical investigation and density functional theory calculations suggest that halide atoms enable the suppression of non-radiative recombination through the elimination of deep gap trap states and the introduction of localized states in the energy band structure from which excitons can efficiently recombine. Importantly, halide passivated WS 2 nanoflakes retain colloidal stability and photoluminescence emission after several weeks of storage in ambient atmosphere, corroborating the potential of developed WS 2 inks thereof.

Journal of Applied Physics, 2017

The quest for new materials with extraordinary electronic, magnetic, and optical properties leads to the synthesis of 2D nitrogenated microporous materials with the hole diameter of 1.16 nm. We computationally study the evolution of the energy bandgaps, optical, and transport properties with the following substituents: hydrogen, fluorine, chlorine, and iodine. We find that such a small perturbation by these atoms has a tremendous impact on the electronic properties of these materials. Indeed, the direct energy bandgaps can be tuned from 1.64 to 0.96 eV by the substituents from hydrogen to iodine. The optical gaps demonstrate similar dependence. From the transport properties, we calculate the effective masses of π-conjugated microporous polymers and find that the conduction electron effective masses are insensitive to halogen substituents while for some low-lying energy valence bands the effective masses can be drastically increased from 0.71 to 2.98 me and 0.28 to 0.58 me for the he...

ACS Nano, 2021

Single-and multi-walled molybdenum disulfide (MoS2) nanotubes have been coaxially grown on small diameter boron nitride nanotubes (BNNTs) which were synthesized from heteronanotubes by removing single-walled carbon nanotubes (SWCNTs), and systematically investigated by optical spectroscopy. The strong photoluminescence (PL) from single-walled MoS2 nanotubes supported by core BNNTs is observed in this work, which evidences a direct band gap structure for single-walled MoS2 nanotubes with around 6 -7 nm in diameter. The observation is consistent with our DFT results that the single-walled MoS2 nanotube changes from an indirect-gap to a direct-gap semiconductor when the diameter of a nanotube is more than around 5 nm. On the other hand, when there are SWCNTs inside the heteronanotubes of BNNTs and MoS2 nanotubes, the PL signal is considerably quenched. The charge transfer and energy transfer between SWCNTs and single-walled MoS2 nanotubes were examined through characterizations by PL, XPS, and Raman spectroscopy. Unlike the single-walled MoS2 nanotubes, multi-walled MoS2 nanotubes do not emit light. Single-and multi-walled MoS2 nanotubes exhibit different Raman features in both resonant and non-resonant Raman spectra. The method of assembling heteronanotubes using BNNTs as templates provides an efficient approach for exploring the electronic and optical properties of other transition metal dichalcogenide nanotubes.

Nanoscale, 2021

In this review we highlight the recent progress in 2DM growth on LMCat, which in combination with in situ characterization presents a viable and large-scale sustainable direction that has the prospect of achieving defect-free 2D materials.

MRS Advances, 2021

Transition metal dichalcogenides have shown great potential for next-generation electronic and optoelectronic devices. However, native oxidation remains a major issue in achieving their long-term stability, especially for Zr-containing materials such as ZrS2. Here, we develop a first principles-informed reactive forcefield for Zr/O/S to study oxidation dynamics of ZrS2. Simulation results reveal anisotropic oxidation rates between (210) and (001) surfaces. The oxidation rate is highly dependent on the initial adsorption of oxygen molecules on the surface. Simulation results also provide reaction mechanism for native oxide formation with atomistic details. Graphic Abstract

Chemical Communications, 2021

Magnetic anisotropy of cobalt in the LDH crystallites is caused by clusters with honeycomb-type cation coordination of Co2+.

Energy Technology

ACS applied energy materials, 2020

Currently, different metal sulfides (NiS, Co 9 S 8 , FeS 2 , and CuS) have been extensively studied as alternative electrodes for rechargeable batteries that can satisfy the performance requirements for more powerful energy supply and storage technologies for various applications and industries. Among them, copper sulfides have gained significant attention as a promising electrode material in rechargeable metal-ion (Li, Mg, Na, and Al) batteries. A wide range of synthesis routes and methods have been implemented in order to prepare various stoichiometry Cu x S (1 ≤ x ≤ 2) micro-/nanostructured materials with excellent electrochemical properties. Since the bulk microsized electrode materials have almost reached their performance limits for energy devices, the introduction of nanoscale Cu x S composites is now in high demand. This review focuses on the influence of the material morphology and dimensions on their performance in secondary batteries. The structures of Cu x S materials from zero-dimensional (0D) to 3D and their preparation are discussed. The primary purpose of this work is to provide an overview of the unique electrochemical and physical properties of particular structure and dimensionality which can promote these materials' application in the energy storage field. Along with this, this work summarizes the information on various synthesis strategies and how they can manage the morphologies of Cu x S micro-/nanocomposites. In the current fast technologically advancing society, the development of the most economically profitable and efficient synthesis routes is especially encouraged and required, and this aspect is also commented on in this review.

Nanoscale Research Letters, 2016

In the present paper, we show tungsten diselenide (WSe 2 ) devices that can be tuned to operate as n-type and p-type field-effect transistors (FETs) as well as band-to-band tunnel transistors on the same flake. Source, channel, and drain areas of the WSe 2 flake are adjusted, using buried triple-gate substrates with three independently controllable gates. The device characteristics found in the tunnel transistor configuration are determined by the particular geometry of the buried triple-gate structure, consistent with a simple estimation of the expected off-state behavior.