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Terahertz spectroscopy is a perfect tool to investigate the electronic intraband conductivity of graphene, but a phenom-enological model (Drude-Smith) is often needed to describe disorder. By studying the THz response of isotropically strained polycrystalline graphene and using a fully atomistic computational approach to fit the results, we demonstrate here the connection between the Drude-Smith parameters and the microscopic behavior. Importantly, we clearly show that the strain-induced changes in the conductivity originate mainly from the increased separation between the single-crystal grains, leading to enchanced localization of the plasmon excitations. Only at the lowest strain values explored, a behavior consistent with the deformation of the individual grains can instead be observed.

We present a micrometer-scale, on-chip integrated, plasmonic enhanced graphene photodetector (GPD) for telecom wavelengths operating at zero dark current. The GPD is designed to directly generate a photovoltage by the photothermoelectric effect. It is made of chemical vapor deposited single layer graphene, and has an external responsivity ∼12.2 V/W with a 3 dB bandwidth ∼42 GHz. We utilize Au split-gates to electrostatically create a p-n-junction and simultaneously guide a surface plasmon polariton gap-mode. This increases the light-graphene interaction and optical absorption and results in an increased electronic temperature and steeper temperature gradient across the GPD channel. This paves the way to compact, on-chip integrated, power-efficient graphene based photodetectors for receivers in tele- and datacom modules.

The electronic and optical properties of graphene can be precisely tuned by generating deterministic arrangements of strain features. In this paper, we report the formation of widespread and controlled buckling delamination of monolayer graphene deposited on hexagonal boron-nitride promoted by a significant squeezing of the graphene flake and induced by polymeric micro-actuators. The flexibility of this method offers a promising technique to create arbitrary buckling geometries and arrays of wrinkles which could also be subjected to iterative folding-unfolding cycles. Further development of this method could pave the way to tune the properties of several kinds of other two-dimensional materials, such as transition metal dichalcogenides, by tailoring their surface topography.

Optoelectronics is a valuable solution to scale up wireless links frequency to sub-THz in the next generation antenna systems and networks. We propose an innovative, low-power consumption, small footprint building block for 5G and 6G new radio wireless transmission allowing broadband capacity (e.g., 10–100 Gb/s per link and beyond). We pioneer the first wireless datalink based on graphene, reaching setup limited sub-THz carrier frequency and multi-Gbit/s data rate using a graphene-based integrated optoelectronic mixer capable of mixing an optically generated ~ 100 GHz reference oscillator with a baseband electrical signal. We report > 96GHz optoelectronic bandwidth and − 44 dB upconversion efficiency with a footprint significantly smaller than those of state-of-the-art photonic transmitters (i.e., 2). This paves the way to the development of novel arrayed-antennas for millimeter-wave technology relying on a new approach which takes advantage of optics. These results have been achieved thanks to an integrated-photonic technology based on wafer-scale high-mobility graphene.

Excitons dominate the light absorption and re-emission spectra of monolayer transition-metal dichalcogenides (TMD). Microscopic investigations of the excitonic response in TMD almost invariably extract information from the radiative recombination step, which only constitutes one part of the picture. Here, by exploiting imaging spectroscopic ellipsometry (ISE), we investigate the spatial dependence of the dielectric function of chemical vapor deposition (CVD)-grown WS2 flakes with a microscopic lateral resolution, thus providing information about the spatially varying, exciton-induced light absorption in the monolayer WS2. Comparing the ISE results with imaging photoluminescence spectroscopy data, the presence of several correlated features was observed, along with the unexpected existence of a few uncorrelated characteristics. The latter demonstrates that the exciton-induced absorption and emission features are not always proportional at the microscopic scale. Microstructural modulations across the flakes, having a different influence on the absorption and re-emission of light, are deemed responsible for the effect.

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 time- and 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 scattering-activated by electronic excitations-prevents 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. This work was supported by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) through CRC 925 (Project 170620586), CRC 1277 (Project 314695032), and the Cluster of Excellence “CUI: Advanced Imaging of Matter”. Further, the work received funding from the European Research Council (Starting Grant 851280 and Advanced Grant 694097), the European Union Graphene Flagship under Grant Agreement Nos. 785219 and 881603, Grupos Consolidados (IT1249-19), and JSPS KAKENHI (Grant Number JP20K14382). The Flatiron Institute is a division of the Simons Foundation

In this work, we demonstrate the application of terahertz-optical Hall effect (THz-OHE) to determine directionally dependent free charge carrier properties of ambient-doped monolayer and quasi-freestanding-bilayer epitaxial graphene on 4H-SiC(0001). Directionally independent free hole mobility parameters are found for the monolayer graphene. In contrast, anisotropic hole mobility parameters with a lower mobility in direction perpendicular to the SiC surface steps and higher along the steps in quasifree-standing-bilayer graphene are determined for the first time. A combination of THz-OHE, nanoscale microscopy and optical spectroscopy techniques are used to investigate the origin of the anisotropy. Different defect densities and different number of graphene layers on the step edges and terraces are ruled out as possible causes. Scattering mechanisms related to doping variations at the step edges and terraces as a result of different interaction with the substrate and environment are discussed and also excluded. It is suggested that the step edges introduce intrinsic scattering in quasi-free-standing-bilayer graphene, that is manifested as a result of the higher ratio between mean free path and average terrace width parameters. The suggested scenario allows to reconcile existing differences in the literature regarding the anisotropic electrical transport in epitaxial graphene. (C) 2020 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license. Funding Agencies|Swedish Research Council VRSwedish Research Council [2016-00889, 2016-05362]; Swedish Foundation for Strategic ResearchSwedish Foundation for Strategic Research [FL120181, RIF14-055, GMT14-0077, RMA15024]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Link_oping University, Faculty Grant SFO Mat LiU [2009-00971]; National Science FoundationNational Science Foundation (NSF) [DMR 1808715]; Air Force Office of Scientific ResearchUnited States Department of DefenseAir Force Office of Scientific Research (AFOSR) [FA9550-18-1-0360]; Nebraska Materials Research Science and Engineering Center [DMR 1420645]; European Unions Horizon 2020 research and innovation program [696656, 785219]; University of Nebraska Foundation; J. A. Woollam Foundation

Terahertz radiation is extremely suitable for various imaging applications. In real life, these range from food or pharmaceutical quality control to illegal materials or human security control inspections. Despite its great potential, the wide usage and commercialization of terahertz imaging systems are still limited by the lack of compact technologies. The emerging graphene-based devices can efficiently contribute to fill this gap, offering higher versatility, scalability, and superior electronic properties compared to conventional semiconductors. In this work, we study a new scheme for realizing a multielement terahertz sensor, which is capable of multipixel parallel detection. The array consists of linearly distributed antenna-coupled graphene field-effect transistors, which are realized by exploiting a deterministic growth by chemical vapor deposition of single-crystal graphene. This novel growth technique ensures high material quality and offers large adaptability to different electronic device architectures. Relatively uniform terahertz detection performances (with a maximum homogeneity degree of 80 $\%$ ) were obtained with a maximum responsivity of the order of 1 V/W and an estimated response time in the picosecond scale. These detectors have demonstrated to fulfill several main requirements for image sensors (pixel uniformity, operability, and scalability), becoming very promising candidates for the realization of commercial high-resolution room-temperature terahertz cameras.

The employment of two-dimensional materials, as growth substrates or buffer layers, enables the epitaxial growth of layered materials with different crystalline symmetries with a preferential crystalline orientation and the synthesis of heterostructures with a large lattice constant mismatch. In this work, we employ single crystalline graphene to modify the sulfurization dynamics of copper foil for the deterministic synthesis of large-area Cu9S5 crystals. Molecular dynamics simulations using the Reax force-field are used to mimic the sulfurization process of a series of different atomistic systems specifically built to understand the role of graphene during the sulphur atom attack over the Cu(111) surface. Cu9S5 flakes show a flat morphology with an average lateral size of hundreds of micrometers. Cu9S5 presents a direct band-gap of 2.5 eV evaluated with light absorption and light emission spectroscopies. Electrical characterization shows that the Cu9S5 crystals present high p-type doping with a hole mobility of 2 cm(2) V-1 s(-1).

Van der Waals heterostructures have attracted increasing interest, owing to the combined benefits of their constituents. These hybrid nanostructures can be realized via epitaxial growth, which offers a promising approach for the controlled synthesis of the desired crystal phase and the interface between van der Waals layers. Here, the epitaxial growth of a continuous molybdenum disulfide (MoS2) film on large-area graphene, which was directly grown on a sapphire substrate, is reported. Interestingly, the grain size of MoS2 grown on graphene increases, whereas that of MoS2 grown on SiO2 decreases with an increasing amount of hydrogen in the chemical vapor deposition reactor. In addition, to achieve the same quality, MoS2 grown on graphene requires a much lower growth temperature (400 °C) than that grown on SiO2 (580 °C). The MoS2/graphene heterostructure that was epitaxially grown on a transparent platform was investigated to explore its photosensing properties and was found to exhibit inverse photoresponse with highly uniform photoresponsivity in the photodetector pixels fabricated across a full wafer. The MoS2/graphene heterostructure exhibited ultrahigh photoresponsivity (4.3 × 104 A W–1) upon exposure to visible light of a wide range of wavelengths, confirming the growth of a high-quality MoS2/graphene heterostructure with a clean interface.

Two-dimensional semiconductors are gaining increasing interest for their potential application in several fields. In particular, when combined with graphene into vertical van der Waals heterostructures, they have demonstrated unique properties, such as large spin-orbit coupling at the valence band maximum and ultrafast charge transfer. An understanding of the WS2 stability on epitaxial graphene in ambient conditions is crucial for the development of potential applications. In this work, we study the environmental aging related degradation of WS2 monolayers directly synthesized on top of epitaxial graphene. We experimentally demonstrate that the oxidation of the WS2 monolayers on epitaxial graphene starts from the flake edges, being attributed to a local high concentration of defects. The oxidation leads to the complete deterioration of the semiconducting material in less than 120 days. In addition, we demonstrate that the oxidation mechanism is accelerated on epitaxial graphene in comparison to that on insulating sapphire substrate. First-principles theoretical analysis reveals that the oxidation mechanism is strongly promoted by the presence of sulfur vacancies and is further accelerated by transfer of electron density from the substrate, whereas it is slowed down by depletion of electron density from the support. Our exhaustive approach sheds light on the oxidative process promoted by defects and not self-limited to the oxidation of the monolayer edges.

Two-dimensional (2D) materials have received extensive research attentions over the past two decades due to their intriguing physical properties (such as the ultrahigh mobility and strong light–matter interaction at atomic thickness) and a broad range of potential applications (especially in the fields of electronics and optoelectronics). The growth of single-crystal 2D materials is the prerequisite to realize 2D-based high-performance applications. In this review, we aim to provide an in-depth analysis of the state-of-the-art technology for the growth and applications of 2D materials, with particular emphasis on single crystals. We first summarize the major growth strategies for monolayer 2D single crystals. Following that, we discuss the growth of multilayer single crystals, including the control of thickness, stacking sequence, and heterostructure composition. Then we highlight the exploration of 2D single crystals in electronic and optoelectronic devices. Finally, a perspective is given to outline the research opportunities and the remaining challenges in this field.

Hexagonal boron nitride (hBN), sometimes referred to as white graphene, receives growing interest in the scientific community, especially when combined into van der Waals (vdW) homo- and heterostacks, in which novel and interesting phenomena may arise. hBN is also commonly used in combination with two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDCs). The realization of hBN-encapsulated TMDC homo- and heterostacks can indeed offer opportunities to investigate and compare TMDC excitonic properties in various stacking configurations. In this work, we investigate the optical response at the micrometric scale of mono- and homo-bilayer WS2 grown by chemical vapor deposition and encapsulated between two single layers of hBN. Imaging spectroscopic ellipsometry is exploited to extract the local dielectric functions across one single WS2 flake and detect the evolution of excitonic spectral features from monolayer to bilayer regions. Exciton energies undergo a redshift by passing from hBN-encapsulated single layer to homo-bilayer WS2, as also confirmed by photoluminescence spectra. Our results can provide a reference for the study of the dielectric properties of more complex systems where hBN is combined with other 2D vdW materials into heterostructures and are stimulating towards the investigation of the optical response of other technologically-relevant heterostacks.

Nanomaterials can be game-changers in the arena of sustainable energy production because they may enable highly efficient thermoelectric energy conversion and harvesting. For this purpose, doped thin film oxides have been proven to be promising systems for achieving high thermoelectric performances. In this work, the design, realization, and experimental investigation of the thermo electric properties exhibited by a set of five Al:ZnO thin films with thicknesses of 300 nm and Al doping levels ranging from 2 to 8 at.% are described. Using a multi-technique approach, the main structural and morphological features of the grown thin films are addressed, as well as the electrical and thermoelectrical transport properties. The results show that the samples exhibited a Seebeck coefficient absolute value in the range of 22–33 µV/K, assuming their maximum doping level was 8 at.%, while the samples’ resistivity was decreased below 2 × 10−3 Ohm·cm with a doping level of 3 at.%. The findings shine light on the perspectives of the applications of the metal ZnO thin film technology for thermoelectrics., This research was funded by MIUR-PON, scholarship PhD program “Physics and nano sciences”021/22.“Nanotecnologie per la termoelettricità e l’energy harvesting” (Azione IV.5 “Dottorati su tematiche green”) and by project QUANTEP, INFN CSN5. VB was supported by the project “Brosynano”MICINN-FEDER (PID2019-106820RB-C22). C.T. acknowledges the funding from FCT/PIDDAC through the Strategic Funds project reference UIDB/04650/2020-2023. A.M., G.P. acknowledge support by the Italian Ministry of University and Research through PRIN UTFROM N. 20178PZCB5. This work was also partially funded under the National Recovery and Resilience Plan (NRRP), Mission 04 Component 2 Investment 1.5 – NextGenerationEU, Call for tender n. 3277 dated 30/12/2021. Award Number: 0001052 dated 23/06/2022.

We study surface charge transfer doping of exfoliated black phosphorus (bP) flakes by copper using scanning tunneling microscopy (STM) and spectroscopy (STS) at room temperature. The tunneling spectra reveal a gap in correspondence of Cu islands, which is tentatively attributed to Coulomb blockade phenomena. Moreover, using line spectroscopic measurements across small copper islands, we exploit the potential of the local investigation, showing that the n-type doping effect of copper on bP is short-ranged. These experimental results are substantiated by first-principles simulations, which quantify the role of cluster size for an effective n-type doping of bP and show an electronic decoupling of the topmost bP layer from the underlying layers driven by the copper cluster, consistent with the Coulomb blockade interpretation. Our results provide novel understanding--difficult to retrieve by transport measurements--of the doping of bP by copper, which appears promising for the implementation of ultrasharp p-n junctions in bP.

Hydrogen spillover and storage for single-site metal catalysts, including single-atom catalysts (SACs) and single nanocluster catalysts, have been elucidated for various supports but remain poorly understood for inert carbon supports. Here, we use synchrotron radiation-based methods to investigate the role of single-site Ti catalysts on graphene for hydrogen spillover and storage. Our in-situ angle-resolved photoemission spectra results demonstrate a bandgap opening and the X-ray absorption spectra reveal the formation of C-H bonds, both indicating the partial graphene hydrogenation. With increasing Ti deposition and H2 exposure, the Ti atoms tend to aggregate to form nanocluster catalysts and yield 13.5% sp3-hybridized carbon atoms corresponding to a hydrogen-storage capacity of 1.11 wt% (excluding the weight of the Ti nanoclusters [1]). Our results demonstrate how a simple spillover process at Ti SACs can lead to covalent hydrogen bonding on graphene, thereby providing a strategy for a rational design of carbon-supported single-site catalysts.

Graphene grown via chemical vapour deposition (CVD) on copper foil has emerged as a high-quality, scalable material, that can be easily integrated on technologically relevant platforms to develop promising applications in the fields of optoelectronics and photonics. Most of these applications require low-contaminated high-mobility graphene (i.e., approaching 10 000 $cm^2 V^{-1} s^{-1}$) at room temperature) to reduce device losses and implement compact device design. To date, these mobility values are only obtained when suspending or encapsulating graphene. Here, we demonstrate a rapid, facile, and scalable cleaning process, that yields high-mobility graphene directly on the most common technologically relevant substrate: silicon dioxide on silicon (SiO$_2$/Si). Atomic force microscopy (AFM) and spatially-resolved X-ray photoelectron spectroscopy (XPS) demonstrate that this approach is instrumental to rapidly eliminate most of the polymeric residues which remain on graphene after transfer and fabrication and that have adverse effects on its electrical properties. Raman measurements show a significant reduction of graphene doping and strain. Transport measurements of 50 Hall bars (HBs) yield hole mobility ${\mu}_h$ up to 9000 $cm^2 V^{-1} s^{-1}$ and electron mobility ${\mu}_e$ up to 8000 $cm^2 V^{-1} s^{-1}$, with average values ${\mu}_h$ 7500 $cm^2 V^{-1} s^{-1}$ and ${\mu}_e$ 6300 $cm^2 V^{-1} s^{-1}$. The carrier mobility of ultraclean graphene reach values nearly double of that measured in graphene HBs processed with acetone cleaning, which is the method widely adopted in the field. Notably, these mobility values are obtained over large-scale and without encapsulation, thus paving the way to the adoption of graphene in optoelectronics and photonics., Comment: 20 pages, 11 figures

We demonstrate a graphene-MoS2 architecture integrating multiple field-effect transistors (FETs), and we independently probe and correlate the conducting properties of van der Waals coupled graphene-MoS2 contacts with those of the MoS2 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 MoS2 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 MoS2 channel.

We demonstrate a graphene–MoS2 architecture integrating multiple field-effect transistors (FETs), and we independently probe and correlate the conducting properties of van der Waals coupled graphene–MoS2 contacts with those of the MoS2 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 MoS2 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 MoS2 channel.

Excitons dominate the light absorption and re-emission spectra of monolayer transition-metal dichalcogenides (TMD). Microscopic investigations of the excitonic response in TMD almost invariably extract information from the radiative recombination step, which only constitutes one part of the picture. Here, by exploiting imaging spectroscopic ellipsometry (ISE), we investigate the spatial dependence of the dielectric function of chemical vapor deposition (CVD)-grown WS2 flakes with a microscopic lateral resolution, thus providing information about the spatially varying, exciton-induced light absorption in the monolayer WS2. Comparing the ISE results with imaging photoluminescence spectroscopy data, the presence of several correlated features was observed, along with the unexpected existence of a few uncorrelated characteristics. The latter demonstrates that the exciton-induced absorption and emission features are not always proportional at the microscopic scale. Microstructural modulations across the flakes, having a different influence on the absorption and re-emission of light, are deemed responsible for the effect.

Graphene has been proposed to be integrated with Si Photonics because of its very high mobility, fast carrier dynamics and ultra-broadband optical properties. High speed graphene photodetectors have been demonstrated so far, however the most are based on the photo-bolometric or photo-conductive effect. These devices are characterized by large dark current, in the order of milli-Amperes. Photothermal effect (PTE) photodetectors can be used in voltage detection mode with no dark current, it is ultra fast and it operates near zero-bias. Graphene PTE-based photodetectors have been reported so far but high-speed optical telecommunication signal detection has not been shown yet. Here, we report on a graphene PTE-based photodetector on SOI waveguide. Thanks to the optimized design we show a direct detection of 105Gb/s non-return to zero (NRZ) and 120Gb/s 4-level pulse amplitude modulation (PAM) optical signals.

Out of the different structural phases of molybdenum ditelluride (MoTe2), the distorted octahedral 1T′ possesses great interest for fundamental physics and is a promising candidate for the implementation of innovative devices such as topological transistors. Indeed, 1T′-MoTe2 is a semimetal with superconductivity, which has been predicted to be a Weyl semimetal and a quantum spin Hall insulator in bulk and monolayer form, respectively. Large instability of monolayer 1T′-MoTe2 in environmental conditions, however, has made its investigation extremely challenging so far. In this work, we demonstrate homogeneous growth of large single-crystal (up to 500 μm) monolayer 1T′-MoTe2via chemical vapor deposition (CVD) and its stabilization in air with a scalable encapsulation approach. The encapsulant is obtained by electrochemically delaminating CVD hexagonal boron nitride (hBN) from copper foil, and it is applied on the freshly grown 1T′-MoTe2via a top-down dry lamination step. The structural and electrical properties of encapsulated 1T′-MoTe2 have been monitored over several months to assess the degree of degradation of the material. We find that when encapsulated with hBN, the lifetime of monolayer 1T′-MoTe2 successfully increases from a few minutes to more than a month. Furthermore, the encapsulated monolayer can be subjected to transfer, device processing, and heating and cooling cycles without degradation of its properties. The potential of this scalable heterostack is confirmed by the observation of signatures of low-temperature phase transition in monolayer 1T′-MoTe2 by both Raman spectroscopy and electrical measurements. The growth and encapsulation methods reported in this work can be employed for further fundamental studies of this enticing material as well as facilitate the technological development of monolayer 1T′-MoTe2.

In this work we have critically reviewed the processes in high-temperature sublimation growth of graphene in Ar atmosphere using closed graphite crucible. Special focus is put on buffer layer formation and free charge carrier properties of monolayer graphene and quasi-freestanding monolayer graphene on 4H–SiC. We show that by introducing Ar at higher temperatures, TAr, one can shift the formation of the buffer layer to higher temperatures for both n-type and semi-insulating substrates. A scenario explaining the observed suppressed 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-freestanding monolayer graphene with different TAr and different environmental treatment conditions are determined by contactless terahertz optical Hall effect. An efficient annealing of donors on and near the SiC surface is suggested to take place for intrinsic monolayer graphene grown at 2000 ∘C, and which is found to be independent of TAr. Higher TAr leads to higher free charge carrier mobility parameters in both intrinsically n-type and ambient p-type doped monolayer graphene. TAr is also found to have a profound effect on the free hole parameters of quasi-freestanding monolayer graphene. These findings are discussed in view of interface and buffer layer properties in order to construct a comprehensive picture of high-temperature sublimation growth and provide guidance for growth parameters optimization depending on the targeted graphene application.

Graphene metrology needs to keep up with the fast pace of developments in graphene growth and transfer. Terahertz time-domain spectroscopy (THz-TDS) is a non-contact, fast, and non-destructive characterization technique for mapping the electrical properties of graphene. Here we show several case studies of graphene characterization on a range of different substrates that highlight the versatility of THz-TDS measurements and its relevance for process optimization in graphene production scenarios.