Transition Metal Dichalcogenides

En este trabajo se estudia la cinética de ordenamiento de un copolímero dibloque confinado a un cascarón esférico. El proceso de separación de fases es estudiado en cascarones de diferentes radios mediante el modelo de Ginzburg-Landau dependiente ...

Emergent phenomena driven by electronic reconstructions in oxide heterostructures have been intensively discussed. However, the role of these phenomena in shaping the electronic properties in van der Waals heterointerfaces has hitherto not been established. By reducing the material thickness and forming a heterointerface, we find two types of charge-ordering transitions in monolayer VSe on graphene substrates. Angle-resolved photoemission spectroscopy (ARPES) uncovers that Fermi-surface nesting becomes perfect in ML VSe. Renormalization-group analysis confirms that imperfect nesting in three dimensions universally flows into perfect nesting in two dimensions. As a result, the charge-density wave-transition temperature is dramatically enhanced to a value of 350 K compared to the 105 K in bulk VSe. More interestingly, ARPES and scanning tunneling microscopy measurements confirm an unexpected metal-insulator transition at 135 K that is driven by lattice distortions. The heterointerface...

USA -We present the results for the RKKY interaction in monolayer and bilayer graphene in terms of Meijer-G functions for the undoped, doped and biased cases. The results are obtained from the linear-response expression for susceptibility written in terms of the integral over Green's functions and using Dirac bands. The salient features of the large-distance behavior in each case will be discussed. For instance, for doped monolayer graphene, the interaction falls off as R -2 and oscillates as the product of two terms, one being a {1 + cos[(K -K ′ ).R]}-like interference term from both Dirac cones and the second term scaled by Fermi momentum k F . For doped and unbiased bilayer graphene, the interaction decays as For the gated bilayer graphene, k F must be replaced by another scaled momentum k U , which depends on Fermi energy, gate voltage and the interlayer hopping energy, allowing the possibility of tuning of the interaction by gate voltage. References: M.

Scanning tunneling microscopy (STM) and atomic force microscopy (AFM) have been used to characterize the structural and electronic properties of single-crystal MoS,. nickel-substituted MoS, (Ni, MO, ~, S2), and chalcogenide-substituted MO& (MO& \ Ch,, Ch G Se. Te) at the atomic level. Images of Ni,, , Mo,,,S? demonstrate that nickel substitution causes localized changes in the electronic states. although the structure of the surface sulfur layer is unchanged compared with MoS,. Investigations of MoS, ,5Se,,2, also show that within the detection limits of AFM selenium substitution does not perturb the sulfur surface structure. and STM data further indicate that the substituted selenium is electronically delocalized. In contrast. AFM studies of MoS, ,,Te,, 25 show that tellurium substitution produces atomic-sized structural protrusions that may modify significantly the tribological properties of MoS?. In addition, we demonstrate that material wear can be characterized on an atomic scale by AFM. These studies indicate that the microscopic origin of material wear as well as the local structure and electronic properties should be considered to develop further models of friction and wear in metal dichalcogenide materials.

A single graphene layer is superior many ways preferably in electronic devices application. However, mild modification of the graphene network could open a new potential to the ultrathin graphene membrane. Moreover, recent studies demonstrated that a few simple techniques could generate and control the nanopores size on single layer graphene sheet simultaneously. This review paper will discuss all potential techniques that are capable to generate nanopores structure on the pristine single layer graphene network.

Aquí tienes un resumen del libro Advanced Two-Dimensional Nanomaterials for Environmental and Sensing Applications:
Este libro explora los avances recientes en materiales bidimensionales (2D) y su aplicación en la remediación ambiental y la detección de contaminantes. A lo largo de 17 capítulos, expertos analizan cómo la evolución industrial ha llevado a la generación de desechos y contaminantes que afectan el ecosistema, destacando la necesidad de tecnologías avanzadas para su tratamiento.
El contenido se divide en tres secciones:
- Fundamentos de los materiales 2D (Capítulos 1–3): Se analizan sus propiedades estructurales, métodos de síntesis y caracterización.
- Detección y monitoreo de contaminantes (Capítulos 4–7): Se estudia el uso de materiales 2D para identificar y medir contaminantes en gases, líquidos y sólidos.
- Aplicaciones en la remediación ambiental (Capítulos 8–17): Se presentan diversas estrategias para eliminar contaminantes volátiles, elementos tóxicos radiactivos y residuos orgánicos y metálicos en aire, suelo y agua.
El libro también profundiza en materiales emergentes como semiconductores de óxido metálico, grafeno y nitruro de carbono, MXenes y estructuras híbridas. Su enfoque multidisciplinario lo convierte en una referencia clave para estudiantes, académicos e investigadores en ciencia de materiales y tecnologías ambientales.

We present a study on the growth and characterization of high-quality single-layer MoS 2 with a single orientation, i.e. without the presence of mirror domains. This single orientation of the MoS 2 layer is established by means of x-ray photoelectron diffraction. The high quality is evidenced by combining scanning tunneling microscopy with x-ray photoelectron spectroscopy measurements. Spin-and angle-resolved photoemission experiments performed on the sample revealed complete spin-polarization of the valence band states near the K and -K points of the Brillouin zone. These findings open up the possibility to exploit the spin and valley degrees of freedom for encoding and processing information in devices that are based on epitaxially grown materials.

In scanning tunneling microscopy (STM) conductance curves, the superconducting gap of cuprates is sometimes accompanied by small sub-gap structures at very low energy. This was documented early on near vortex cores and later at zero magnetic field. Using mean-field toy models of coexisting d-wave superconductivity (dSC), d -form factor density wave (dFF-DW), and extended s-wave pair density wave (s PDW), we find agreement with this phenomenon, with s PDW playing a critical role. We explore the high variability of the gap structure with changes in band structure and density wave (DW) wave vector, thus explaining why sub-gap structures may not be a universal feature in cuprates. In the absence of nesting, non-superconducting results never show signs of pseudogap, even for large density waves magnitudes, therefore reinforcing the idea of a distinct origin for the pseudogap, beyond mean-field theory. Therefore, we also briefly consider the effect of DWs on a pre-existing pseudogap.

Scalable growth is essential for graphene-based applications. Recent development has enabled the achievement of the scalability by use of chemical vapor deposition (CVD) at 1000°C with copper as a catalyst and methane as a precursor gas. Here we report our observation of early stage of graphene growth based on an ethylenebased CVD method, capable of reducing the growth temperature to 770°C for monolayer graphene growth on copper. We track the early stages of slow growth under low ethylene flow rate and observe the graphene domain evolution by varying the temperature and growth time. Temperature-dependence of graphene domain density gives an apparent activation energy of 1.0 eV for nucleation.

In this study, we report the newly engineered functional polymeric vesicles as the robust nanostructured materials for the recognition and quantification of transition metal cations i.e. Hg(II) and Cr(III) as the naked eye recognition architectures in aqueous solution. The as prepared visual detector was synthesized by the combination of a hyperbranched poly(3-ethyl-3-oxetanemethanol)-star-poly(ethylene oxide) (HBPO-star-PEO, HSP) functionalized with rhodamine-2-amino-5-bromopyrimidine based chemosensor (RBP). The nanostructured vesicular sensor was regarded as HSP-RBP sensor. The naked eye detection owes from the spriolactam ring opening of the rhodamine based sensor (RBP) which was grafted onto the multiarm hyperbranched amphiphilic polymer. This newly synthesized sensor can facilely self-assemble into vesicles with enriched RBP groups on the outer surface of amphiphilic polymer. As a result of the introduction of these analytes (Hg 2+ /Cr 3+ ) the structural changes of RBP takes place and a transition in color change from colorless solution to pink colored solution takes place. This transition of color can clearly be seen with bare eye. Therefore, the term visual detection can be coined for the vesicular sensor. Furthermore, the limit of detection for Hg 2+ and Cr 3+ was calculated as low as 1.2 and 2.9 μM respectively. The striking feature of the newly prepared sensor, is its extraordinary ability for the recognition and quantification of Hg 2+ /Cr 3+ with outstanding selectivity in raw water samples.

In this study, we report the newly engineered functional polymeric vesicles as the robust nanostructured materials for the recognition and quantification of transition metal cations i.e. Hg(II) and Cr(III) as the naked eye recognition architectures in aqueous solution. The as prepared visual detector was synthesized by the combination of a hyperbranched poly(3-ethyl-3-oxetanemethanol)-star-poly(ethylene oxide) (HBPO-star-PEO, HSP) functionalized with rhodamine-2-amino-5-bromopyrimidine based chemosensor (RBP). The nanostructured vesicular sensor was regarded as HSP-RBP sensor. The naked eye detection owes from the spriolactam ring opening of the rhodamine based sensor (RBP) which was grafted onto the multiarm hyperbranched amphiphilic polymer. This newly synthesized sensor can facilely self-assemble into vesicles with enriched RBP groups on the outer surface of amphiphilic polymer. As a result of the introduction of these analytes (Hg 2+ /Cr 3+ ) the structural changes of RBP takes place and a transition in color change from colorless solution to pink colored solution takes place. This transition of color can clearly be seen with bare eye. Therefore, the term visual detection can be coined for the vesicular sensor. Furthermore, the limit of detection for Hg 2+ and Cr 3+ was calculated as low as 1.2 and 2.9 μM respectively. The striking feature of the newly prepared sensor, is its extraordinary ability for the recognition and quantification of Hg 2+ /Cr 3+ with outstanding selectivity in raw water samples.

The encapsulation of few-layer transition metal dichalcogenide (TMD) structures in hexagonal boron nitride (h-BN) is known to improve their optical properties, which is of crucial importance for their applications. In order to study the effect of encapsulation on interlayer interactions in few-layer TMDs the low-energy Raman scattering spectrum of bi- and trilayer MoTe$_2$ is investigated. Three breathing modes are observed in the spectra of these few-layer MoTe$_2$ structures deposited on/encapsulated in h-BN as compared to one breathing mode for the flakes deposited on an SiO$_2$/Si substrate. Conversely, the shear mode is not affected by changing the substrate. We relate the emerged structure of breathing modes to the interaction of MoTe$_2$ with the h-BN substrate. The interaction slightly affects the energy of the main breathing mode and contributes to the combination modes due to interlayer and layer-substrate interactions. We also show that the h-BN substrate determines the R...

Water oxidation or oxygen evolution reaction (OER) in electrochemical cells is considered to be a major bottleneck in the way of hydrogen production by electro-synthesis, mainly due to a sluggish kinetics that characterizes the OER steps. Layered transition metal dichalcogenides, such as WSe2 , are emerging as promising non-precious electrocatalysts for water splitting due to their excellent activity and stability. This paper aims to shed light on the (100) WSe 2-aqueous interface in catalyzing the slow kinetics of the OER in the context of water splitting electro-catalysis. We employ state-of-the-art DFT-metadynamics to explore reaction mechanisms, activation free energies, and catalytic sites. This study reveals an energetically preferred water-assisted OER mechanism, where proton transfer is facilitated by the surrounding aqueous environment. Our findings not only provide insights into the OER process but also offer a design strategy for optimizing WSe2-based catalysts and a modeling protocol for future DFT-based OER investigations.

The electron-phonon coupling strength in the spin-split valence band maximum of single-layer MoS 2 is studied using angle-resolved photoemission spectroscopy and density functional theorybased calculations. Values of the electron-phonon coupling parameter λ are obtained by measuring the linewidth of the spin-split bands as a function of temperature and fitting the data points using a Debye model. The experimental values of λ for the upper and lower spin-split bands at K are found to be 0.05 and 0.32, respectively, in excellent agreement with the calculated values for a free-standing single-layer MoS 2 . The results are discussed in the context of spin and phase-space restricted scattering channels, as reported earlier for single-layer WS 2 on Au(111). The fact that the absolute valence band maximum in single-layer MoS 2 at K is almost degenerate with the local valence band maximum at Γ can potentially be used to tune the strength of the electron-phonon interaction in this material.

for an Invited Paper for the MAR14 Meeting of the American Physical Society Physical foundations and future perspectives of the epitaxial silicene ALESSANDRO MOLLE, CNR-IMM, Laboratorio MDM Silicene, a graphene-like Si monolayer, has been so far a fascinating theoretical hypothesis with no experimental counterpart as due to the natural sp 3 hybridization of Si bonding. Artificially forcing the silicene lattice was firstly made possible in the epitaxial growth of a Si monolayer on Ag(111) substrates . Here it is shown through an in situ scanning tunnelling microscopy investigation [3] that unlike graphene, non-trivial atomistic arrangements (reconstructions) can coexist in multiphase silicene nanosheets as due to the balance between planar and buckled bonding . This structural complexity is kinetic in character as it can be governed by tuning the thermal condition during growth or in a post-growth stage, and it is expected to bring absolutely peculiar physical properties . Silicene is intrinsically limited by its intimate chemical instability. An ad hoc engineered Al 2 O 3 encapsulation is proposed for ex situ investigations such as Raman spectroscopy . Supported by ab initio calculations, the measured Raman spectrum of the silicene phases is consistent with a prevailing a sp 2 hybridization, and it also evidences a reconstruction dependent resonant/non-resonant behavior . To integrate silicene in transistor structures, decoupling from the metallic templates is highly desired. New directions in this respect are outlined which include silicene deposition on cleavable substrates (e.g. Ag(111) films on mica), and the van der Waals growth of Si nanosheets on layered templates (such as MoS 2 layers). Perspectives on the silicene "portability" for a device-oriented exploitation will be discussed.

The synthesis of monolayer transition-metal dichalcogenides (TMDCs) by conventional hot-walled CVD remains critical issues of large-scale, spatial homogeneity and quality [1]. Here we demonstrate a coldwall CVD (CW-CVD) system with advanced functions for the growth of high-quality wafer-scale homogeneous monolayer molybdenum disulphide (MoS2) on various substrates [2]. This automatic system can precisely control the temperatures of multizone, pressure and gaseous flow rate with individual precursors of Mo(CO)6 and sulfur. In addition, the system has high flexibility to be upgraded for extra functions such as plasma source, multi-precursor supply and load-lock system. The Raman spectrum of as-synthesized MoS2 on 2-inch sapphire wafer shows the significant E2g and A1g peaks with 19cm-1 inter-distance, and the photoluminescence (PL) spectrum shows an apparent peak at 650 nm. Both spectra reveal the monolayer and high quality feature of CW-CVD synthesized MoS2. References [1] Lee, Y.-H....

vakia, J. FEDOR, M. IAVARONE, Temple University, Philadelphia -We investigate atomic scale scanning tunneling microscopy and spectroscopy in Cu x TiSe 2 single crystals at low temperatures. We map the CDW and superconducting phase diagram as a function of copper doping. STM measurements reveal coexistence of chiral charge density wave and superconductivity. In case of optimally doped and overdoped cases we find that the amplitude of charge density wave modulation is strongly suppressed with respect to strongly underdoped case (x < 0.06) with the chiral domain size remaining approximately the same. Superconductivity exhibits BCS character at variety of dopings with 2∆/kT c ∼ 3.6 ÷ 3.7 indicating an intermediate coupling strength. Application of the external magnetic field introduces the Abrikosov vortex lattice that is weakly pinned. The size of the vortex core extracted from vortex images corresponds to the one extracted from the magnetization measurements. Our results suggest that, if charge density wave quantum critical point exist, it should be well above the optimal copper concentration of x=0.08.

Layered materials with the noncentrosymmetric stacking order are attracting increasing interest due to the presence of ferroelectric polarization, which is dictated by weak interlayer hybridization of atomic orbitals. In this study, we use the density functional theory modeling to systematically build a library of van der Waals chalcogenides that exhibit substantial ferroelectric polarization. For the most promising materials, we also analyze the pressure dependence of the ferroelectric effect and charge accumulation of photo-induced electrons and holes on surfaces and at internal twin boundaries in thin films of such materials.

We study the Ruderman-Kittel-Kasuya-Yosida interaction between two magnetic impurities connected to the edges of zigzag-terminated transition-metal dichalcogenide triangular flakes. When the impurities lie on the edges of the flake, the effective exchange configuration is helical, showing a sizable noncollinear Dzyaloshinskii-Moriya interaction. We analyze the interaction decay exponent for doping levels inside the band gap of the infinite layer, corresponding to edge states for the flake. The exponents show a sub-2D behavior for different band fillings, with a decay that is slower than quadratic. Remarkably, the noncollinear component can feature positive exponents, signaling an interaction that grows with the impurity separation. This effect can be attributed to the presence of an envelope with a wavelength longer than the size of the flake.

We study the Ruderman-Kittel-Kasuya-Yosida interaction between magnetic impurities embedded in p-doped transition metal dichalcogenide triangular flakes. The role of underlying symmetries is exposed by analyzing the interaction as a function of impurity separation along zigzag and armchair trajectories, in specific parts of the sample. The large spin-orbit coupling in these materials produces strongly anisotropic interactions, including a Dzyaloshinskii-Moriya component that can be sizable and tunable. We consider impurities hybridized to different orbitals of the host transition-metal and identify specific characteristics for onsite and hollow site adsorption. In the onsite case, the different components of the interaction have similar magnitude, while for the hollow site, the Ising component dominates. We also study the dependence of the interaction with the level of hole doping, which supplies a further degree of tunability. Our results could provide ways of controlling helical long range spin order in magnetic impurity arrays embedded in these materials.

We study the Ruderman-Kittel-Kasuya-Yosida effective exchange interaction between magnetic impurities embedded on the edges of transition-metal dichalcogenide flakes, using a three-orbital tight-binding model. Electronic states lying midgap of the bulk structure have strong onedimensional (1D) character, localized on the edges of the crystallite. This results in exchange interactions with 1/r (or slower) decay with distance r, similar to other 1D systems. Most interestingly, however, the strong spin-orbit interaction in these materials results in sizable non-collinear Dzyaloshinskii-Moriya interactions between impurities, comparable in size to the usual Ising and in-plane components. Varying the relevant Fermi energy by doping or gating may allow one to modulate the effective interactions, controlling the possible helical ground state configurations of multiple impurities.

The absorption spectra of 2D semiconductors are dominated by excitons with binding energy of several hundreds of meV. Nevertheless, even single layers show an appreciable photovoltaic effect and work as the active material in high sensitivity photodetectors, thus indicating some degree of free charge carrier photogeneration. Here, we perform ultrafast transient absorption spectroscopy on monolayer MoS2 in a field-effect transistor configuration. We show that even a moderate in-plane electric field of a few kVcm -1 can significantly increase the yield of charge carriers from photogenerated hot electron-hole pairs.

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We study optical transitions in indirect excitons in transition metal dichalcogenide (TMDC) double layers separated by an integer number of hexagonal boron nitride (h-BN) monolayers. By solving the Schrödinger equation with the Keldysh potential for an indirect exciton, we obtain eigenfunctions and eigenenergies for the ground and excited states and study their dependence on the interlayer separation, controlled by varying the number of h-BN monolayers. The oscillator strength, optical absorption coefficient, and optical absorption factor, the fraction of incoming photons absorbed in the double layer, are evaluated and studied as a function of the interlayer separation. Using input parameters from the existing literature which give the largest and the smallest indirect exciton binding energy, we provide upper and lower bounds on all quantities presented.

Two-dimensional materials can be combined by placing individual layers on top of each other, so that they are bound only by their van der Waals interaction. The sequence of layers can be chosen arbitrarily, enabling an essentially atomic-level control of the material and thereby a wide choice of properties along one dimension. However, simultaneous control over the structure in the in-plane directions is so far still rather limited. Here, we combine spatially controlled modifications of 2D materials, using focused electron irradiation or electron beam induced etching, with the layer-by-layer assembly of van der Waals heterostructures. A novel assembly process makes it possible to structure each layer with an arbitrary pattern prior to the assembly into the heterostructure. Moreover, it enables a stacking of the layers with accurate lateral alignment, with an accuracy of currently 10nm, under observation in an electron microscope. Together, this enables the fabrication of almost arbitrary 3D structures with highest spatial resolution.

We investigate the interface of single layer graphene, molybdenum disulfide lateral heterostructures using scanning tunneling microscopy (STM). Samples are fabricated using chemical vapor deposition to deposit graphene, photolithography to pattern graphene and metal-organic chemical vapor deposition to grow molybdenum disulfide in patterned areas. The lateral junction of the two materials allows investigation of structural and electronic properties at the interface of the two materials, an interface usually buried in conventional stacked heterostructures. STM is used to image the stitching of the two materials with nanoscale resolution. STM is also used to perform local spectroscopy, probing the local density of states on an atomic scale across the junction. Interesting phenomena such as the charge transfer and atomic bonding are investigated. The spatially changing chemical potential between the two materials is also examined at different gate voltages.

Monolayer transitional metal dichalcogenides (TMDs) MX 2 (M = Mo, W, Ti etc; X = S, Se, Te) are a new platform for exploring new electronic and optical phenomena and functionality. However, much remains to be understood about their chemical and local electronic properties when taken to the monolayer limit. We will discuss a scanning tunneling microscopy (STM) study on exfoliated single-layer MoSe 2 using a 4-probe STM system. The ability to carry out scanning electron microscopy (SEM) in our system allows us to easily locate and measure single-layer MoSe 2 flakes that are mechanically exfoliated on a SiO 2 /Si substrate and are only a few micrometers in lateral size. Using a combination of imaging and spectroscopy, we will discuss the chemical purity and nature of defect states in this monolayer material. Using a electrostatic back gate, we will describe measurements of the single-particle electronic bandgap as a function of the chemical potential.

Strain Engineering of Transition Metal Dichalcogenides ALI DADGAR, ABHAY PASUPATHY, IRVING HERMAN, DENNIS WANG, Columbia University, KYUNGNAM KANG, EUI-HYEOK YANG, Stevens Institute of Technology -The application of strain to materials can cause changes to bandwidth, effective masses, degeneracies and even structural phases. In the case of the transition metal dichalcogenide (TMD) semiconductors, small strain (around 1 percent) is expected to change band gaps and mobilities, while larger strains are expected to cause phase changes from the triangular 2H phase to orthorhombic 1T' phases. We will describe experimental techniques to apply small and large (around 10 percent) strains to one or few layer samples of the TMD semiconductors, and describe the effect of the strain using optical (Raman, photoluminescence) and cryogenic transport techniques.

Optical spectroscopy in high magnetic fields B ≤ 65 T is used to reveal the very different nature of carriers in monolayer and bulk transition metal dichalcogenides. In monolayer WSe 2 , the exciton emission shifts linearly with the magnetic field and exhibits a splitting which originates from the magnetic field induced valley splitting. The monolayer data can be described using a single particle picture with a Dirac-like Hamiltonian for massive Dirac fermions, with

We report efficient nonradiative energy transfer (NRET) from core−shell, semiconducting quantum dots to adjacent two-dimensional sheets of graphene and MoS 2 of single-and few-layer thickness. We observe quenching of the photoluminescence (PL) from individual quantum dots and enhanced PL decay rates in time-resolved PL, corresponding to energy transfer rates of 1−10 ns −1. Our measurements reveal contrasting trends in the NRET rate from the quantum dot to the van der Waals material as a function of thickness. The rate increases significantly with increasing layer thickness of graphene, but decreases with increasing thickness of MoS 2 layers. A classical electromagnetic theory accounts for both the trends and absolute rates observed for the NRET. The countervailing trends arise from the competition between screening and absorption of the electric field of the quantum dot dipole inside the acceptor layers. We extend our analysis to predict the type of NRET behavior for the near-field coupling of a chromophore to a range of semiconducting and metallic thin film materials.

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The structural, optoelectronic, and elastic properties of Carbon doped Calcium Selenide binary and ternary semiconductor alloys with the general form of Ca(1-x)CxSe,
0 ≤ x ≤ 1 in B1 and B3 phases have been scrutinized adopting augmented plane wave plus local orbital methods within density functional theory applying different
approximations like the Local density approximation (LDA), Wu-Cohen-Generalized Gradients Approximation (WC-GGA) and modified Becke-Johnson (mBJ).
Structural properties summarize the crystal structure, relevant atomic positions, lattice constant, Bulk modulus B0, minimum energy E0, and minimum volume V0.
Elastic constants confirm the structure stability of the B1 and B3 phases. The band gap was modified from a wider for appropriate optoelectronic devices application
to narrow that enhanced the range of applicability of these binary and ternary semiconductor compounds for optoelectronic device applications. Whereas optical
properties which include the study of zero frequency limit of static dielectric constant ε0(ω) with its real and imaginary parts, static refractive index n(0), optical
reflectivity R(ω), optical absorption coefficient α(ω), optical conductivity σ(ω), and loss function L(ω) studied comprehensively to get real electrical and optical
properties of these materials and give semiconductor market best alternative candidate which shows its effectiveness in the visible, ultraviolet and infrared region of
the electromagnetic spectrum. Obtained results compared together and with the available experimental along with theoretical work on these binary and ternary
Carbon-doped Alkaline earth Selenide.

2D transition metal dichalcogenides (TMDs) have received widespread interest by virtue of their excellent electrical, optical, and electrochemical characteristics. Recent studies on TMDs have revealed their versatile utilization as electrocatalysts, supercapacitors, battery materials, and sensors, etc. In this study, MoS2 nanosheets are successfully assembled on the porous VS2 (P‐VS2) scaffold to form a MoS2/VS2 heterostructure. Their gas‐sensing features, such as sensitivity and selectivity, are investigated by using a quartz crystal microbalance (QCM) technique. The QCM results and density functional theory (DFT) calculations reveal the impressive affinity of the MoS2/VS2 heterostructure sensor toward ammonia with a higher adsorption uptake than the pristine MoS2 or P‐VS2 sensor. Furthermore, the adsorption kinetics of the MoS2/VS2 heterostructure sensor toward ammonia follow the pseudo‐first‐order kinetics model. The excellent sensing features of the MoS2/VS2 heterostructure rend...

We report efficient nonradiative energy transfer (NRET) from core−shell, semiconducting quantum dots to adjacent two-dimensional sheets of graphene and MoS 2 of single-and few-layer thickness. We observe quenching of the photoluminescence (PL) from individual quantum dots and enhanced PL decay rates in time-resolved PL, corresponding to energy transfer rates of 1−10 ns −1. Our measurements reveal contrasting trends in the NRET rate from the quantum dot to the van der Waals material as a function of thickness. The rate increases significantly with increasing layer thickness of graphene, but decreases with increasing thickness of MoS 2 layers. A classical electromagnetic theory accounts for both the trends and absolute rates observed for the NRET. The countervailing trends arise from the competition between screening and absorption of the electric field of the quantum dot dipole inside the acceptor layers. We extend our analysis to predict the type of NRET behavior for the near-field coupling of a chromophore to a range of semiconducting and metallic thin film materials.

Submitted for the MAR16 Meeting of The American Physical Society Orbital and anisotropy effects on the itinerant exchange interaction in 3D Dirac semimetals SERGIO ULLOA, Ohio University, DIEGO MASTROGIUSEPPE, Instituto de Fisica Rosario, NANCY SANDLER, Ohio University-Dirac semimetals are new materials that can be considered analogues of graphene in three dimensions. Their band structure exhibits robust Dirac points that are protected by crystalline symmetry, and strong spin-orbit interaction. These unusual properties suggest that magnetic impurities may reveal exotic behavior with potential technological importance. In metallic hosts, magnetic impurities interact through the electron gas via the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction that depends strongly on the band structure of the material. We report on the RKKY interaction in 3D Dirac semimetals, such as Na 3 Bi and Cd 3 As 2 [1, 2]. We discuss asymptotic expressions for the interaction corresponding to settings with magnetic impurities at different distances and relative angle with respect to high symmetry directions on the lattice. We show that the Fermi velocity anisotropy produces a strong renormalization of the magnitude of the interaction, and a correction to the frequency of oscillation in real space. Hybridization of the impurities to different conduction electron orbitals results in interesting anisotropic interactions which can generate spiral spin structures in doped samples. [1] Z.

manybody Kondo screening of an impurity's magnetic moment by electrons in a host material is exquisitely sensitive to the local density of states sampled by the impurity. Such sensitivity is greatly augmented in materials with strong spin-orbit interaction, where an exponential enhancement of the characteristic Kondo temperature is possible, even in the presence of weak Zeeman fields 2. In graphene and transition metal dichalcogenides (TMDs), strain fields produced by folds and bubbles can also strongly modify the density of states near these features and change which host degrees of freedom are affected by an impurity level. To elucidate these effects, we use renormalization-group methods to solve an Anderson model for a magnetic impurity in a strained layered material, allowing comparison between the Kondo physics in graphene and TMDs, which have very different spin-orbit interactions. The strain fields, reflecting also the underlying lattice symmetry of the host material, are found to modulate the competition between screening channels of different angular momenta and to alter the spin correlation functions associated with the Kondo state.

Submitted for the MAR14 Meeting of The American Physical Society RKKY interaction in MoS 2 1 DIEGO MASTROGIUSEPPE, NANCY SANDLER, SERGIO ULLOA, Ohio University-MoS 2 belongs to a family of layered compounds-the transition metal dichalcogenides-that are attracting increasing attention in the solid state community due to their very rich phase diagram. In particular, the semiconducting ones in their 2D form, are of particular interest in the search for a new generation of devices in nanoelectronics and nanophotonics. The hexagonal lattice allows one to describe the low-energy physics with a massive Dirac equation around the K and K ′ points. Moreover, the presence of a large intrinsic spin-orbit interaction due to the presence of transition metal atoms, leads to a valley-dependent splitting of the states of an otherwise spin-degenerate valence spectrum. We study the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between two magnetic impurities in the direct band gap semiconducting single-layer MoS 2 , focusing in the p-doped case. Going beyond a recent study [1], we include the effects of the spin-degenerate valence bands at the center of the Brillouin zone, relevant for energies close to the valence band maximum. The easy experimental tunability of the carrier concentration by electrical or chemical means, makes possible the study of the carrier-mediated spin-spin interaction at different fillings.

Submitted for the MAR12 Meeting of The American Physical Society Sorting Category: 12.1.2 (T) Kondo Effect in Bilayer Graphene 1 DIEGO MAS-TROGIUSEPPE, SERGIO ULLOA, NANCY SANDLER, Ohio University-Because of the linear dependence of the density of states (near Dirac points), the physics of localized magnetic moments on graphene exhibits unique characteristics [1]. From the experiments by Mattos et al [2], where a possible observation of the 2-channel Kondo effect was reported, to recent studies on vacancies [3], the role of local moments on graphene remains poorly understood. The technical difficulties to determine the nature of the origin of the local moment add to the complexity of the problem. To gain insight into this problem, we have undertaken a study of a bilayer graphene system with Bernal stacking and an intercalated magnetic impurity. We model the system with a multiband Anderson impurity model and obtain the effective Kondo Hamiltonian via a Schrieffer-Wolff transformation. Although several conducting channels couple to the impurity, the standard 1-channel Kondo regime is recovered at low temperatures. The effective Kondo exchange couplings depend on the interlayer hopping giving rise to tunable Kondo temperatures.

Graphene growth on dielectric substrates has potential to enable new kinds of devices and applications. We explore graphene growth by direct deposition of carbon on different dielectric substrates in a MBE environment. Here we consider h-BN and sapphire substrates. The quality of fabricated graphene layers depends on growth conditions such as carbon deposition rate, substrate temperature and total amount of deposited carbon. Characterizations by spatially resolved Raman spectra and AFM images suggest the formation of high quality graphene. On h-BN substrates, single layer growth occurs as nano-domains. On sapphire, large area growth happens with monolayer thickness fluctuations. These results are consistent with a van der Waals growth mode of graphene on dielectric substrates.

We analyzed the transport properties in normal metal and unconventional superconductor junctions, like cuprates and Fe-based superconductors. We find that the crossed Andreev reflection (CAR) probability depends on the symmetry of the order parameter in the superconductor. We analyzed the differential conductance in GS and GG ′ S unconventional systems (G (′) : graphene (graphene's film) in normal state and S: superconductor). For GS the conductance shows a different behavior for armchair and zigzag boundaries, for non perfect transparencies. The differential conductance in a GG ′ S junction shows resonances with voltage and Klein's paradox. The resonances are due to quasibound states appearing in G ′ , while Klein paradox is relevant when G ′ is an insulator barrier and we show that is possible with zigzag and armchair interfaces. For a graphene pnp junction, we find that the system is similar to a Veselago lens, where an electron from G L region can be focused as an electron or a hole in G R region with negative reflection index. Finally, the CAR probability analysis on the different systems in this work may be useful to generate entangled electrons using superconductors.

The effects of electromagnetic and particle irradiation on twodimensional materials (2DMs) are discussed in this review. Radiation creates defects that impact the structure and electronic performance of materials. Determining the impact of these defects is important for developing 2DM-based devices for use in high-radiation environments, such as space or nuclear reactors. As such, most experimental studies have been focused on determining total ionizing dose damage to 2DMs and devices. Total dose experiments using X-rays, gamma rays, electrons, protons, and heavy ions are summarized in this review. We briefly discuss the possibility of investigating single event effects in 2DMs based on initial ion beam irradiation experiments and the development of 2DM-based integrated circuits. Additionally, beneficial uses of irradiation such as ion implantation to dope materials or electron-beam and helium-beam etching to shape materials have begun to be used on 2DMs and are reviewed as well. For non-ionizing radiation, such as low-energy photons, we review the literature on 2DM-based photo-detection from terahertz to UV. The majority of photo-detecting devices operate in the visible and UV range, and for this reason they are the focus of this review. However, we review the progress in developing 2DMs for detecting infrared and terahertz radiation.

thin two-dimensional materials of transition metal dichalcogenides have recently attracted great interest due to their diverse electronic properties and potential applications. Upon cooling to low temperature, some materials exhibit interesting phenomena of collective electronic states such as superconductivity and charge density waves. While charge density waves in bulk materials of transition metal dichalcogenides have been extensively studied in the past few decades, the understanding of this collective electronic state in materials with reduced dimensionality is still in its infancy. Here, we report in-situ ultra-low temperature scanning tunneling microscopy and spectroscopy measurements on VSe 2 thin films synthesized by molecular beam epitaxy. We observed an unconventional charge density wave which does not follow previous reports of hexagonal symmetry of VSe 2. Spectroscopy results will be discussed in relation to other characterizations using electrical transport and transmission electron microscopy.

We study the Ruderman-Kittel-Kasuya-Yosida interaction between two magnetic impurities connected to the edges of zigzag-terminated transition-metal dichalcogenide triangular flakes. When the impurities lie on the edges of the flake, the effective exchange configuration is helical, showing a sizable noncollinear Dzyaloshinskii-Moriya interaction. We analyze the interaction decay exponent for doping levels inside the band gap of the infinite layer, corresponding to edge states for the flake. The exponents show a sub-2D behavior for different band fillings, with a decay that is slower than quadratic. Remarkably, the noncollinear component can feature positive exponents, signaling an interaction that grows with the impurity separation. This effect can be attributed to the presence of an envelope with a wavelength longer than the size of the flake.

We study the Ruderman-Kittel-Kasuya-Yosida interaction between magnetic impurities embedded in p-doped transition metal dichalcogenide triangular flakes. The role of underlying symmetries is exposed by analyzing the interaction as a function of impurity separation along zigzag and armchair trajectories, in specific parts of the sample. The large spin-orbit coupling in these materials produces strongly anisotropic interactions, including a Dzyaloshinskii-Moriya component that can be sizable and tunable. We consider impurities hybridized to different orbitals of the host transition-metal and identify specific characteristics for onsite and hollow site adsorption. In the onsite case, the different components of the interaction have similar magnitude, while for the hollow site, the Ising component dominates. We also study the dependence of the interaction with the level of hole doping, which supplies a further degree of tunability. Our results could provide ways of controlling helical long range spin order in magnetic impurity arrays embedded in these materials.

We study the Ruderman-Kittel-Kasuya-Yosida effective exchange interaction between magnetic impurities embedded on the edges of transition-metal dichalcogenide flakes, using a three-orbital tight-binding model. Electronic states lying midgap of the bulk structure have strong onedimensional (1D) character, localized on the edges of the crystallite. This results in exchange interactions with 1/r (or slower) decay with distance r, similar to other 1D systems. Most interestingly, however, the strong spin-orbit interaction in these materials results in sizable non-collinear Dzyaloshinskii-Moriya interactions between impurities, comparable in size to the usual Ising and in-plane components. Varying the relevant Fermi energy by doping or gating may allow one to modulate the effective interactions, controlling the possible helical ground state configurations of multiple impurities.