Papers by Camilla Coletti
ACS Nano, 2021
We demonstrate a graphene−MoS 2 architecture integrating multiple field-effect transistors (FETs)... more We demonstrate a graphene−MoS 2 architecture integrating multiple field-effect transistors (FETs), and we independently probe and correlate the conducting properties of van der Waals coupled graphene−MoS 2 contacts with those of the MoS 2 channels. Devices are fabricated starting from high-quality single-crystal monolayers grown by chemical vapor deposition. The heterojunction was investigated by scanning Raman and photoluminescence spectroscopies. Moreover, transconductance curves of MoS 2 are compared with the current−voltage characteristics of graphene contact stripes, revealing a significant suppression of transport on the n-side of the transconductance curve. On the basis of ab initio modeling, the effect is understood in terms of trapping by sulfur vacancies, which counterintuitively depends on the field effect, even though the graphene contact layer is positioned between the backgate and the MoS 2 channel.
Optically enabled graphene-based transmitter for Gbit/s links at 93 GHz carrier frequency
Photonics in Switching and Computing 2021
Journal of Physics: Materials
Monolayer tungsten disulfide (WS2) has recently attracted large interest as a promising material ... more Monolayer tungsten disulfide (WS2) has recently attracted large interest as a promising material for advanced electronic and optoelectronic devices such as photodetectors, modulators, and sensors. Since these devices can be integrated in a silicon (Si) chip via back-end-of-line (BEOL) processes, the stability of monolayer WS2 in BEOL fabrication conditions should be studied. In this work, the thermal stability of monolayer single-crystal WS2 at typical BEOL conditions is investigated; namely (i) heating temperature of 300 °C, (ii) pressures in the medium-(10-3 mbar) and high-(10-8 mbar) vacuum range; (iii) heating times from 30 minutes to 20 hours. Structural, optical and chemical analyses of WS2 are performed via scanning electron microscopy (SEM), Raman spectroscopy, photoluminescence (PL) and X-ray photoelectron spectroscopy (XPS). It is found that monolayer single-crystal WS2 is intrinsically stable at these temperature and pressures, even after 20 hours of thermal treatment. The thermal stability of WS2 is also preserved after exposure to low-current electron beam (12 pA) or low-fluence laser (0.9 mJ/m 2), while higher laser fluencies cause photo-activated degradation upon thermal treatment. These results are instrumental to define fabrication and in-line monitoring procedures that allow the integration of WS2 in device fabrication flows without compromising the material quality.
In this work we have critically reviewed the processes in high-temperature sublimation growth of ... more In this work we have critically reviewed the processes in high-temperature sublimation growth of graphene in Ar atmosphere using enclosed graphite crucible. Special focus is put on buffer layer formation and free charge carrier properties of monolayer graphene and quasi-freestanding monolayer garphene on 4H-SiC. We show that by introducing Ar at different temperatures, TAr one can shift to higher temperatures the formation of the buffer layer for both n-type and semi-insulating substrates. A scenario explaining the observed suppresed formation of buffer layer at higher TAr is proposed and discussed. Increased TAr is also shown to reduce the sp3 hybridization content and defect densities in the buffer layer on n-type conductive substrates. Growth on semi-insulating substrates results in ordered buffer layer with significantly improved structural properties, for which TAr plays only a minor role. The free charge density and mobility parameters of monolayer graphene and quasi-freestand...
Tuning $WS_2$ photoluminescence using polymeric micro-actuators in a van der Waals heterostructure
arXiv: Mesoscale and Nanoscale Physics, 2019
The control of the local strain profile in 2D materials offers an invaluable tool for tailoring e... more The control of the local strain profile in 2D materials offers an invaluable tool for tailoring electronic and photonic properties of solid-state devices. In this paper, we demonstrate that strain in $WS_2$ based van der Waals heterostructures can be modulated on a local scale by means of polymeric micrometric actuators implemented by electron beam lithography directly on the growth substrate. Thanks to the underlying material structure, i.e. graphene on SiC, $WS_2$ flakes are found to slide with negligible friction, thus circumventing the technical challenges of other approaches requiring layer transfer and production of suspended membranes. As proof of concept, in our experiment we show strain-induced local modulation of the excitonic photoluminescence of such non-suspended $WS_2$ monolayers under application of custom strain profiles.
Efficient light harvesting devices need to combine strong absorption in the visible spectral rang... more Efficient light harvesting devices need to combine strong absorption in the visible spectral range with efficient ultrafast charge separation. These features commonly occur in novel ultimately thin van der Waals heterostructures with type II band alignment. Recently, ultrafast charge separation was also observed in monolayer WS2/graphene heterostructures with type I band alignment. Here we use time- and angle-resolved photoemission spectroscopy to show that ultrafast charge separation also occurs at the interface between bilayer WS2 and graphene indicating that the indirect band gap of bilayer WS2 does not affect the charge transfer to the graphene layer. The microscopic insights gained in the present study will turn out to be useful for the design of novel optoelectronic devices.
50Gb/S CVD Graphene-Insulator-Graphene Electro-Absorption Modulator on Si Waveguide
We demonstrate a non-return to zero optical modulation at 50Gb/s with a CVD grown Graphene-Insula... more We demonstrate a non-return to zero optical modulation at 50Gb/s with a CVD grown Graphene-Insulator-Graphene electro-absorption modulator integrated on a Si photonic waveguide. The device exhibits a 3dB electro-optical bandwidth of 30GHz. © 2019 The Author(s)
Applied Physics Letters
This paper was selected as an Editor's Pick ARTICLES YOU MAY BE INTERESTED IN Characterization of... more This paper was selected as an Editor's Pick ARTICLES YOU MAY BE INTERESTED IN Characterization of GaON as a surface reinforcement layer of p-GaN in Schottky-type p-GaN gate HEMTs Applied Physics Letters 119, 053503 (2021);
Nano Letters
Floquet theory has spawned many exciting possibilities for electronic structure control with ligh... more Floquet theory has spawned many exciting possibilities for electronic structure control with light, with enormous potential for future applications. The experimental demonstration in solids, however, remains largely unrealized. In particular, the influence of scattering on the formation of Floquet− Bloch states remains poorly understood. Here we combine timeand angle-resolved photoemission spectroscopy with time-dependent density functional theory and a two-level model with relaxation to investigate the survival of Floquet−Bloch states in the presence of scattering. We find that Floquet−Bloch states will be destroyed if scatteringactivated by electronic excitationsprevents the Bloch electrons from following the driving field coherently. The two-level model also shows that Floquet−Bloch states reappear at high field intensities where energy exchange with the driving field dominates over energy dissipation to the bath. Our results clearly indicate the importance of long scattering times combined with strong driving fields for the successful realization of various Floquet phenomena.
Black Phosphorus n-Type Doping by Cu: A Microscopic Surface Investigation
The Journal of Physical Chemistry C
ACS Nano
Graphene and related materials can lead to disruptive advances in next-generation photonics and o... more Graphene and related materials can lead to disruptive advances in next-generation photonics and optoelectronics. The challenge is to devise growth, transfer and fabrication protocols providing high (≥5000 cm 2 V −1 s −1) mobility devices with reliable performance at the wafer scale. Here, we present a flow for the integration of graphene in photonics circuits. This relies on chemical vapor deposition (CVD) of single layer graphene (SLG) matrices comprising up to ∼12000 individual single crystals, grown to match the geometrical configuration of the devices in the photonic circuit. This is followed by a transfer approach which guarantees coverage over ∼80% of the device area, and integrity for up to 150 mm wafers, with room temperature mobility ∼5000 cm 2 V −1 s −1. We use this process flow to demonstrate double SLG electro-absorption modulators with modulation efficiency ∼0.25, 0.45, 0.75, 1 dB V −1 for device lengths ∼30, 60, 90, 120 μm. The data rate is up to 20 Gbps. Encapsulation with single-layer hexagonal boron nitride (hBN) is used to protect SLG during plasma-enhanced CVD of Si 3 N 4 , ensuring reproducible device performance. The processes are compatible with full automation. This paves the way for large scale production of graphenebased photonic devices.
Nature Communications
Paper is the ideal substrate for the development of flexible and environmentally sustainable ubiq... more Paper is the ideal substrate for the development of flexible and environmentally sustainable ubiquitous electronic systems, which, combined with two-dimensional materials, could be exploited in many Internet-of-Things applications, ranging from wearable electronics to smart packaging. Here we report high-performance MoS2 field-effect transistors on paper fabricated with a “channel array” approach, combining the advantages of two large-area techniques: chemical vapor deposition and inkjet-printing. The first allows the pre-deposition of a pattern of MoS2; the second, the printing of dielectric layers, contacts, and connections to complete transistors and circuits fabrication. Average ION/IOFF of 8 × 103 (up to 5 × 104) and mobility of 5.5 cm2 V−1 s−1 (up to 26 cm2 V−1 s−1) are obtained. Fully functional integrated circuits of digital and analog building blocks, such as logic gates and current mirrors, are demonstrated, highlighting the potential of this approach for ubiquitous electr...
Combining scanning tunneling microscopy and angle-resolved photoemission spectroscopy, we demonst... more Combining scanning tunneling microscopy and angle-resolved photoemission spectroscopy, we demonstrate how to tune the doping of epitaxial graphene from p to n by exploiting the structural changes that occur spontaneously on the Ge surface upon thermal annealing. Furthermore, using first principle calculations we build a model that successfully reproduces the experimental observations. Since the ability to modify graphene electronic properties is of fundamental importance when it comes to applications, our results provide an important contribution towards the integration of graphene with conventional semiconductors.
Journal of Applied Physics
Actuation of thin polymeric films via electron irradiation is a promising avenue to realize devic... more Actuation of thin polymeric films via electron irradiation is a promising avenue to realize devices based on strain engineered two dimensional (2D) materials. Complex strain profiles demand a deep understanding of the mechanics of the polymeric layer under electron irradiation; in this article we report a detailed investigation on electron-induced stress on poly-methyl-methacrylate (PMMA) thin film material. After an assessment of stress values using a method based on dielectric cantilevers, we directly investigate the lateral shrinkage of PMMA patterns on epitaxial graphene, which reveals a universal behavior, independent of the electron acceleration energy. By knowing the stress-strain curve, we finally estimate an effective Young's modulus of PMMA on top of graphene which is a relevant parameter for PMMA based electronbeam lithography and strain engineering applications.
Effect of Chemical Vapor Deposition WS2 on Viability and Differentiation of SH-SY5Y Cells
Frontiers in Neuroscience
In recent years, transition metal dichalcogenides have been attracting an increasing interest in ... more In recent years, transition metal dichalcogenides have been attracting an increasing interest in the biomedical field, thus implying the need of a deeper understanding of their impact on cell behavior. In this study we investigate tungsten disulfide (WS2) grown via chemical vapor deposition (CVD) on a transparent substrate (sapphire) as a platform for neural-like cell culture. We culture SH-SY5Y human neuroblastoma cells on WS2, using graphene, sapphire and standard culture well as controls. The quality, thickness and homogeneity of the materials is analyzed using atomic force microscopy and Raman spectroscopy. The cytocompatibility of CVD WS2 is investigated for the first time by cell viability and differentiation assessment on SH-SY5Y cells. We find that cells differentiated on WS2, displaying a viability and neurite length comparable with the controls. These findings shine light on the possibility of using WS2 as a cytocompatible material for interfacing neural cells.
Science Advances
We use time- and angle-resolved photoemission spectroscopy (tr-ARPES) to investigate ultrafast ch... more We use time- and angle-resolved photoemission spectroscopy (tr-ARPES) to investigate ultrafast charge transfer in an epitaxial heterostructure made of monolayer WS2 and graphene. This heterostructure combines the benefits of a direct-gap semiconductor with strong spin-orbit coupling and strong light-matter interaction with those of a semimetal hosting massless carriers with extremely high mobility and long spin lifetimes. We find that, after photoexcitation at resonance to the A-exciton in WS2, the photoexcited holes rapidly transfer into the graphene layer while the photoexcited electrons remain in the WS2 layer. The resulting charge-separated transient state is found to have a lifetime of ∼1 ps. We attribute our findings to differences in scattering phase space caused by the relative alignment of WS2 and graphene bands as revealed by high-resolution ARPES. In combination with spin-selective optical excitation, the investigated WS2/graphene heterostructure might provide a platform ...
Nanomaterials
In this paper, we present a study of tungsten disulfide (WS2) two-dimensional (2D) crystals, grow... more In this paper, we present a study of tungsten disulfide (WS2) two-dimensional (2D) crystals, grown on epitaxial Graphene. In particular, we have employed scanning electron microscopy (SEM) and µRaman spectroscopy combined with multifunctional scanning probe microscopy (SPM), operating in peak force–quantitative nano mechanical (PF-QNM), ultrasonic force microscopy (UFM) and electrostatic force microscopy (EFM) modes. This comparative approach provides a wealth of useful complementary information and allows one to cross-analyze on the nanoscale the morphological, mechanical, and electrostatic properties of the 2D heterostructures analyzed. Herein, we show that PF-QNM can accurately map surface properties, such as morphology and adhesion, and that UFM is exceptionally sensitive to a broader range of elastic properties, helping to uncover subsurface features located at the buried interfaces. All these data can be correlated with the local electrostatic properties obtained via EFM mappi...
Nature Communications
The synthesis of two-dimensional (2D) transition metals has attracted growing attention for both ... more The synthesis of two-dimensional (2D) transition metals has attracted growing attention for both fundamental and application-oriented investigations, such as 2D magnetism, nanoplasmonics and non-linear optics. However, the large-area synthesis of this class of materials in a single-layer form poses non-trivial difficulties. Here we present the synthesis of a large-area 2D gold layer, stabilized in between silicon carbide and monolayer graphene. We show that the 2D-Au ML is a semiconductor with the valence band maximum 50 meV below the Fermi level. The graphene and gold layers are largely non-interacting, thereby defining a class of van der Waals heterostructure. The 2D-Au bands, exhibit a 225 meV spin-orbit splitting along the $$\overline {{\mathrm{\Gamma }}{\mathrm{K}}}$$ΓK¯ direction, making it appealing for spin-related applications. By tuning the amount of gold at the SiC/graphene interface, we induce a semiconductor to metal transition in the 2D-Au, which has not yet been obser...
Scientific Reports
Metamaterials have recently established a new paradigm for enhanced light absorption in state-of-... more Metamaterials have recently established a new paradigm for enhanced light absorption in state-of-the-art photodetectors. Here, we demonstrate broadband, highly efficient, polarization-insensitive, and gate-tunable photodetection at room temperature in a novel metadevice based on gold/graphene Sierpinski carpet plasmonic fractals. We observed an unprecedented internal quantum efficiency up to 100% from the near-infrared to the visible range with an upper bound of optical detectivity of 1011 Jones and a gain up to 106, which is a fingerprint of multiple hot carriers photogenerated in graphene. Also, we show a 100-fold enhanced photodetection due to highly focused (up to a record factor of |E/E0| ≈ 20 for graphene) electromagnetic fields induced by electrically tunable multimodal plasmons, spatially localized in self-similar fashion on the metasurface. Our findings give direct insight into the physical processes governing graphene plasmonic fractal metamaterials. The proposed structure...
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Papers by Camilla Coletti