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Hexagonal boron nitride (h-BN) nanosheets are grown at room temperature by pulsed laser deposition that exhibits remarkable functional properties, creating a scenario for “h-BN on demand” under a frugal thermal budget, essential for nanotechnology.

The surface functionalization of diamond has been extensively studied through a variety of techniques, such as oxidation. Several oxygen groups have been correspondingly detected on the oxidized diamond, such as COC (ester), CO (ketonic), and COH (hydroxyl). However, the composition and relative concentration of these groups on diamond surfaces can be affected by the type of oxygenation treatment and the diamond surface quality. To investigate the stability of the oxygenated groups at specific diamond surfaces, we evaluated through fully atomistic reactive molecular mechanics (FARMM) simulations, using the ReaxFF force field, the formation energies of CO, COC, and COH groups on pristine and defective diamond surfaces (110), (111), and (311). According to our findings, the COH group has the lowest formation energy on a perfect (110) surface, while the COC is favored on a defective surface. As for the (111) surface, the COC group is the most stable for both pristine and defective surfaces. Similarly, COC group is also the most stable one on the defective/perfect (311) surface. In this way, our results suggest that if in a diamond film the (110) surface is the major exposed facet, the most adsorbed oxygen group could be either COH or COC, in which the COC would depend on the level of surface defects., 2 Figures, 1 Table, 6 pages

It is known that surface terminations contribute significantly to diamond properties. As one of the most commonly studied types, oxygen-terminated diamond surface possesses a positive electron affinity (PEA) and hydrophilicity, making it suitable for electronic device fabrication and bioapplications. Various oxidation methods have been reported on diamond, but a systematic comparison is still lacking. Herein, we present a comparative study on the oxidation of microcrystalline diamond powder (DP) and polycrystalline diamond film (PCD) by wet chemical treatments, including various acid mixtures, as well as by dry processes, including O2 plasma and UV ozone. X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared spectroscopy (FTIR) results reveal that H2SO4/HNO3 5:1 at 360 °C demonstrates the best oxidation performance on DP and PCD compared to other wet chemical methods, while the PCD treated by O2 plasma exhibits the highest oxygen content among all the treated samples. Besides, the concentration of carbon-oxygen groups on diamond surfaces is found largely determined by oxidation methods. These experimental results can be attributed to the corresponding oxidation mechanism of different treatments and agree well with theoretical simulations. Finally, we investigated the sp2 bonded carbon concentration and surface roughness of oxidized PCD, suggesting the optimized diamond surface cleaning conditions.

Wide and ultrawide-bandgap semiconductors lie at the heart of next-generation high-power, high-frequency electronics. Here, we report the growth of ultrawide-bandgap boron nitride (BN) thin films on wide-bandgap gallium nitride (GaN) by pulsed laser deposition. Comprehensive spectroscopic (core level and valence band XPS, FTIR, Raman) and microscopic (AFM and STEM) characterizations confirm the growth of BN thin films on GaN. Optically, we observed that BN/GaN heterostructure is second-harmonic generation active. Moreover, we fabricated the BN/GaN heterostructure-based Schottky diode that demonstrates rectifying characteristics, lower turn-on voltage, and an improved breakdown capability (234 V) as compared to GaN (168 V), owing to the higher breakdown electrical field of BN. Our approach is an early step towards bridging the gap between wide and ultrawide-bandgap materials for potential optoelectronics as well as next-generation high-power electronics., 5 Figures, Published in Applied Physics Letters

We report on the effects of surface-acoustic phonon scattering on the charge transport behavior of diamond based FET devices. Motivated by the promising role of diamond in the realization of high power and high frequency electronic devices, the present work is focused on detailed formulation of relaxation times due to the hole-surface-acoustic phonon scattering, which appears to have been an overlooked scattering mechanism important to diamond-based devices. The matrix element, scattering rates and relaxation times have been calculated by taking into account, for the first time Rayleigh waves near the surface. This is achieved by quantizing the Rayleigh waves and using the corresponding acoustic phonon to calculate the Fermi golden rule based scattering rate of holes in the two-dimensional hole gas. The results show that the scattering of holes with surface acoustic Rayleigh waves reduced relative to scattering from bulk 3D acoustic phonons. Moreover, the mobilities are found to be higher than those based on the theory for 3D acoustic phonons. The results reveal significant insights to diamond based electronics having acoustic phonons Rayleigh waves thus opening new research endeavors.

Wereport on the fabrication and measurement of hydrogen-terminated diamond field-effect transistors (FETs) incorporating V2O5 as a surface acceptor material to induce transfer doping. Comparing a range of gate lengths down to 50 nm, we observe inversely scaling peak output current and transconductance. Devices exhibited a peak drain current of ~700 mA/mm and a peak transconductance of ~150 mS/mm, some of the highest reported thus far for a diamond metal semiconductor FET (MESFET). Reduced sheet resistance of the diamond surface after V2O5 deposition was verified by four probe measurement. These results show great potential for improvement of diamond FET devices through scaling of critical dimensions and adoption of robust transition metal oxides such as V2O5.

Two-dimensional van der Waals (2D-vdW) layered hexagonal boron nitride (h-BN) has gained tremendous research interest over recent years due to its unconventional domain growth morphology, fascinating properties and application potentials as an excellent dielectric layer for 2D-based nano-electronics. However, the unidirectional domain growth of h-BN thin films directly on insulating substrates remains significantly challenging because of high-bonding anisotropicity and complex growth kinetics than the conventional thin films growth, thus resulting in the formation of randomly oriented domains morphology, and hindering its usefulness in integrated nano-devices. Here, ultra-wide bandgap h-BN thin films are grown directly on low-miscut atomically smooth highly insulating c-plane sapphire substrates (without using any metal catalytic layer) by pulsed laser deposition, showing remarkable unidirectional triangular-shape domains morphology. This unidirectional domain growth is attributed to the step-edge guided nucleation caused by reducing the film-substrate interfacial symmetry and energy, thereby breaking the degeneracy of nucleation sites of random domains, as revealed by the density functional theory (DFT) calculations. Through extensive characterizations, we further demonstrate the excellent single crystal-like functional properties of films. Our findings might pave the way for feasible large-area direct growth of electronic-quality h-BN thin films on insulating substrates for high-performance 2D-electronics, and in addition would be beneficial for hetero engineering of 2D-vdW materials with emergent phenomena., Comment: 39 pages, 7 figures, Revised accepted version of the article

Description, Diamond defects, previously applied in measurement of electromagnetic fields and temperature, can now be used to sense rotations., We demonstrate the operation of a rotation sensor based on the nitrogen-14 (14N) nuclear spins intrinsic to nitrogen-vacancy (NV) color centers in diamond. The sensor uses optical polarization and readout of the nuclei and a radio-frequency double-quantum pulse protocol that monitors 14N nuclear spin precession. This measurement protocol suppresses the sensitivity to temperature variations in the 14N quadrupole splitting, and it does not require microwave pulses resonant with the NV electron spin transitions. The device was tested on a rotation platform and demonstrated a sensitivity of 4.7°/s (13 mHz/Hz), with a bias stability of 0.4 °/s (1.1 mHz).

Hexagonal boron nitride (h-BN) has emerged as a strong candidate for two-dimensional (2D) material owing to its exciting optoelectrical properties combined with mechanical robustness, thermal stability, and chemical inertness. Super-thin h-BN layers have gained significant attention from the scientific community for many applications, including nanoelectronics, photonics, biomedical, anti-corrosion, and catalysis, among others. This review provides a systematic elaboration of the structural, electrical, mechanical, optical, and thermal properties of h-BN followed by a comprehensive account of state-of-the-art synthesis strategies for 2D h-BN, including chemical exfoliation, chemical, and physical vapor deposition, and other methods that have been successfully developed in recent years. It further elaborates a wide variety of processing routes developed for doping, substitution, functionalization, and combination with other materials to form heterostructures. Based on the extraordinary properties and thermal-mechanical-chemical stability of 2D h-BN, various potential applications of these structures are described.

Hydrogenated diamond has been regarded as a promising material in electronic device applications, especially in field-effect transistors (FETs). However, the quality of diamond hydrogenation has not yet been established, nor has the specific orientation that would provide the optimum hydrogen coverage. In addition, most theoretical work in the literature use models with 100% hydrogenated diamond surfaces to study electronic properties, which is far from the experimentally observed hydrogen coverage. In this work, we have carried out a detailed study using fully atomistic reactive molecular dynamics (MD) simulations on low indices diamond surfaces i.e. (001), (013), (110), (113) and (111) to evaluate the quality and hydrogenation thresholds on different diamond surfaces and their possible effects on electronic properties. Our simulation results indicate that the 100% surface hydrogenation in these surfaces is hard to achieve because of the steric repulsion between the terminated hydrogen atoms. Among all the considered surfaces, the (001), (110), and (113) surfaces incorporate a larger number of hydrogen atoms and passivate the surface dangling bonds. Our results on hydrogen stability also suggest that these surfaces with optimum hydrogen coverage are robust under extreme conditions and could provide homogeneous p-type surface conductivity in the diamond surfaces, a key requirement for high-field, high-frequency device applications., Comment: 24 pages

The unique electronic structure of diamond and its excellent thermal properties allow a broad range of possible applications; from electron sources to RF electronics. However, knowledge of the surface energy levels is essential to produce efficient, high-quality devices. We investigate the valence band position and resulting negative electron affinity for hydrogen terminated diamond under ambient, low vacuum and ultra-high vacuum (UHV) conditions. There was a -0.5 eV change in valence band position causing a negative electron affinity shift from -1.1 eV under UHV to -0.6 eV in ambient pressure. We compare the photoemission current under each environment to predict the ability of the sample to be used as an electron source. The maximum emission was observed when the sample displayed the largest negative affinity. A scanning photoemission measurement is demonstrated to highlight the superior photoemission yield from the hydrogen terminated diamond surface compared to the stainless steel contact. A scanning Kelvin probe measurement is shown to illustrate a method of analyzing the contact potential difference across the diamond surface. Within high-power RF electronics, devices are likely to be operating at increased temperatures so knowledge of the impact of temperature on the energy levels is important. We study the valence band and Fermi level positions for hydrogen terminated diamond from room temperature (300K) to 425K under low and UHV conditions. The Fermi level moved below the valence band edge at increased temperature, illustrating the effect of the 2D hole gas at the surface. We also analyzed the photoemission characteristics and found an increase in yield with increasing temperature. The measurement techniques used to evaluate the energy levels of diamond: photoemission spectroscopy and Kelvin probe measurements, in ambient and vacuum, allow analysis to be completed in minutes. This offers an initial analysis alternative to elucidate more information and predict performance prior to the more time-consuming full device manufacture and characterization.

In this report, the thermal performance of a hydrogen (H)-terminated diamond field-effect transistor (FET) is investigated using Raman spectroscopy and electrothermal device modeling. First, the thermal conductivity (κdiamond) of the active diamond channel was determined by measuring the temperature rise of transmission line measurement structures under various heat flux conditions using nanoparticle-assisted Raman thermometry. Using this approach, κdiamond was estimated to be 1860 W/m K with a 95% confidence interval ranging from 1610 to 2120 W/m K. In conjunction with measured electrical output characteristics, this κ was used as an input parameter for an electrothermal device model of an H-terminated diamond FET. The simulated thermal response showed good agreement with surface temperature measurements acquired using nanoparticle-assisted Raman thermometry. These diamond-based structures were highly efficient at dissipating heat from the active device channel with measured device thermal resistances as low as ∼1 mm K/W. Using the calibrated electrothermal device model, the diamond FET was able to operate at a very high power density of 40 W/mm with a simulated temperature rise of ∼33 K. Finally, the thermal resistance of these lateral diamond FETs was compared to lateral transistor structures based on other ultrawide bandgap materials (Al0.70Ga0.30N, β-Ga2O3) and wide bandgap GaN for benchmarking. These results indicate that the thermal resistance of diamond-based lateral transistors can be up to ∼10× lower than GaN-based devices and ∼50× lower than other UWBG devices.

Charge carrier trapping in diamond surface conduction field effect transistors (FETs) has been analyzed. For these devices two methods were used to obtain a negative electron affinity diamond surface; either plasma hydrogenation or annealing in an H2 environment. In both cases the Al2O3 gate dielectric can trap both electrons and holes in deep energy levels with emission timescales of seconds, while the diamond – Al2O3 interface traps exhibit much shorter time scales in the microsecond range. Capacitance-Voltage (CV) analysis indicates that these interface traps exhibit acceptor-like characteristics. Correlation with CV based free hole density measurements indicates that the conductance based interface trap analysis provides a method to quantify surface characteristics that lead to surface conduction in hydrogenated diamond where atmospheric adsorbates provide the acceptor states for transfer doping of the surface.

Diamond is a promising wide band-gap semiconductor material for use in devices; therefore a thorough understanding of the surface electronic structure is important. The Kelvin Probe (KP), Surface Photovoltage / Surface Photovoltage Spectroscopy (SPV/SPS) and Ambient Pressure Photoemission Spectroscopy (APS) techniques are commonly applied to traditional and organic semiconductor materials. The application of these techniques to synthetic and natural diamond samples provides some challenges: surface charge on the samples and atypical capacitive interaction with the KP tip. In this study, measurements using a combination of KP, SPV/SPS and APS techniques are taken of samples of natural and synthetic diamond samples to investigate their surface electronic structure and compare their different properties. These techniques are all non-contact and non-destructive. The Fermi Level position of the diamond samples was found to vary, typically between 4.3 – 4.9 eV, depending on the light illumination. For example, when a natural diamond sample was illuminated with 400 nm light from a 150W Quartz Tungsten Halogen light source, there was a surface photovoltage response of ∼250 mV. The oxygen terminated synthetic diamond sample required near continuous illumination at low visible wavelengths in order to retain sufficient conductivity to allow measurement with the Kelvin Probe. By contrast, the natural diamond samples measured showed good conductivity in the layers underneath the top surface. In summary, the KP, SPV/SPS and APS measurement techniques provided some interesting information on the diamond samples and an initial investigation of their surface electronic states is performed.

This project aims to develop solid-state gyroscopes based on ensembles of negatively charged nitrogen-vacancy (NV) centers in diamond [1], [2]. The NV center is a defect formed in diamond by one substitutional nitrogen atom and an adjacent vacancy. The NV- center features a ground state with electronic spin $\mathrm{S}=1$ , which can be initialized, manipulated, and detected via convenient optical, microwave and radiofrequency transitions (Fig. 1). Nuclear spins are appealing in the context of gyroscopes because they have much smaller gyromagnetic ratios than that of the electron (by a factor of about 1000), reducing the requirements on static magnetic-field stability and homogeneity. The lifetime of nuclear spin-polarization is much longer than for electron spins. Recent work [3] has shown that it is possible to achieve high, $\sim$ 98%, polarization of $^{14}\mathrm{N}$ or $^{15}\mathrm{N}$ spins in diamond using excited-state level-anticrossing induced by hyperfine coupling of the nitrogen nucleus in the NV center with the electrons of the defect.

When designing semiconductor heterostructures, it is expected that epitaxial alignment will facilitate low-defect interfaces and efficient vertical transport. Here, we report lattice-matched epitaxial growth of molybdenum disulfide (MoS2) directly on gallium nitride (GaN), resulting in high-quality, unstrained, single-layer MoS2 with strict registry to the GaN lattice. These results present a promising path toward the implementation of high-performance electronic devices based on 2D/3D vertical heterostructures, where each of the 3D and 2D semiconductors is both a template for subsequent epitaxial growth and an active component of the device. The MoS2 monolayer triangles average 1 μm along each side, with monolayer blankets (merged triangles) exhibiting properties similar to that of single-crystal MoS2 sheets. Photoluminescence, Raman, atomic force microscopy, and X-ray photoelectron spectroscopy analyses identified monolayer MoS2 with a prominent 20-fold enhancement of photoluminescence in the center regions of larger triangles. The MoS2/GaN structures are shown to electrically conduct in the out-of-plane direction, confirming the potential of directly synthesized 2D/3D semiconductor heterostructures for vertical current flow. Finally, we estimate a MoS2/GaN contact resistivity to be less than 4 Ω·cm(2) and current spreading in the MoS2 monolayer of approximately 1 μm in diameter.

We report a first-principles study of the structural and electronic properties of two-dimensional (2D) layer/hydrogen-terminated diamond (100) heterostructures. Both the 2D layers exhibit weak van-der-Waals (vdW) interactions and develop rippled configurations with the H-diamond (100) substrate to compensate for the induced strain. The adhesion energy of the hexagonal boron nitride (hBN) layer is slightly higher, and it exhibits a higher degree of rippling compared to the graphene layer. A charge transfer analysis reveals a small amount of charge transfer from the H-diamond (100) surface to the 2D layers, and most of the transferred charge was found to be confined within the vdW gap. In the graphene/H-diamond (100) heterostructure, the semi-metallic characteristic of the graphene layer is preserved. On the other hand, the hBN/H-diamond (100) heterostructure shows semiconducting characteristics with an indirect bandgap of 3.55 eV, where the hBN layer forms a Type-II band alignment with the H-diamond (100) surface. The resultant conduction band offset and valence band offset are 0.10 eV and 1.38 eV, respectively. A thin layer of hBN offers a defect-free interface with the H-diamond (100) surface and provides a layer-dependent tunability of electronic properties and band alignment for surface-doped diamond field effect transistors.

In recent years, large investments into the research of semiconducting two-dimensional (2D) materials such as graphene and transition metal dichalcogenides (TMDs) have elucidated interesting device related physical phenomena such as valleytronics [1], 2D superconductivity [2], 2D excitonic effects [3] and vertical tunneling [4]. TMDs offer layer-dependent chemical tunability of electronic and optoelectronic properties governed by interlayer van der Waals (vdW) forces [5]. Because of their layered nature, these low-dimensional materials can be combined to form multifunctional heterostructure materials exhibiting entirely new physical systems offering new degrees of flexibility in designing electronics, optoelectronics and other novel devices [6], [7]. In the last couple of years, the focus in the 2D materials research have shifted from exploration of proof-of-concept devices using mechanically exfoliated materials to more advanced device processing using high-quality large-scale growth based on advanced scalable vdW-epitaxy techniques such as powder vapor deposition (PVD) and chemical vapor deposition (CVD).

Initial results for diamond RF transistor technology are presented. Field Effect Transistors (FETs) were fabricated with gate lengths $(\mathbf{L}_{\mathrm{g}})$ ranging from $\mathbf{4}\pmb{\mu}\mathbf{m}$ to 50nm. The FETs have total gate width $(\mathbf{W}_{\mathrm{g}})$ of 40 or $\mathbf{120}\pmb{\mu}\mathbf{m}. \mathbf{L}_{\mathbf{g}}=\mathbf{100\ nm}$ devices show DC drain current $\mathbf{I}_{\mathbf{D}}=\mathbf{600\ mA}/\mathbf{mm} (>\mathbf{V}_{\mathbf{GS}}=-\mathbf{3V},\mathbf{V}_{\mathbf{DS}}=-\mathbf{10V}$ ) with transconductance $\mathbf{g}_{\mathbf{m}}=\mathbf{140mS}/\mathbf{mm} (>\mathbf{V}_{\mathbf{GS}}=-\mathbf{0.3V},\mathbf{V}_{\mathbf{DS}}=-\mathbf{4V}$ ). Small signal S-parameters were measured to evaluate the high-frequency performance of the diamond FETs. Extrinsic $\mathbf{f}_{\mathbf{t}}$ and $\mathbf{f}_{\mathbf{max}}$ were measured to be 41GHz and 44GHz, respectively. Load pull measurements were used to characterize the devices under large signal excitation. The $\mathbf{L}_{\mathbf{g}}=\mathbf{200nm},\ \mathbf{W}_{\mathbf{g}}=\mathbf{40}\pmb{\mu}\mathbf{m}$ device, tested at 2GHz, shows peak efficiency of 30.5% at $\mathbf{V}_{\mathbf{DS}}=-\mathbf{5V}$ . Both peak gain of 19.5dB and peak output power density of 0.66W/mm were achieved at $\mathbf{V}_{\mathbf{DS}}=- \mathbf{30V}$ , Biasing the device at $\mathbf{V}_{\mathbf{DS}}=-\mathbf{15V}$ provides a trade off point for the large signal parameters - gain of ~15dB, efficiency of ~20%, and output power of ~0.5W/mm.

Current status of the GeTe phase change research at ARL is presented. The work covers determination of the power handling limits, both in ON and OFF condition, for state-of-the-art PCS technology. Maximum ON-state power handling, after 50 conditioning cycles, of 33dBm is reported. Experimental evidence of 75% reduction in the minimum energy to crystallize is presented. The effects of the applied external electric field are discussed. Also, initial results are shown for theoretical model of GeTe crystalization.

In dimensionally confined multilayer heterostructures, phonons that are joint modes of the materials composed of these heterostructures are known to exist over specific frequency ranges. These modes, known as interface phonons, can exhibit phonon-electron interactions that are enhanced as the thicknesses of the layers of the heterostructure are reduced in size to about 10 nm or less. These modes have been shown to be important in phonon engineering and have been applied in optoelectronic and electronic devices, primarily for semiconductor heterostructures with underlying cubic lattices, with few studies existing for heterostructures based on wurtzite III-nitride. Motivated by applications of interface modes in ternary-based nitride structures, such as heat transport, this paper presents generalized expressions for the phonon-electron Frohlich interactions as well as the dispersion relations for these joint modes for the technologically important case of III-nitride materials. Frequency conditions are found to restrict the existence of interface modes as illustrated through several structures.

Understanding of nickel (Ni) grain size, distribution, and structure are critical parameters in a sputter-deposited Ni catalyst for achieving the desired number of graphene layers [1] grown by atmospheric pressure chemical vapor deposition (APCVD). The size and distribution of grains can be controlled by variations in sputtering parameters, but the final crystal structure and defects are not apparent until after the high temperature annealing. We analyzed the x-ray diffraction patterns in the Ni catalyst to determine effect of thermal annealing on the Ni grain size, orientation, and structural defects. Experiments have shown that in-situ sputter-deposited Ni films at 250 °C are highly oriented in the direction [111] that produced the high yield of graphene films with desired number of layers. Low defect density in a sputtered nickel (Ni) catalyst is a necessary ingredient for achieving precision number of graphene layers. These sputtering parameters can accelerate or postpone the final preferred orientation of the Ni film. A sputter temperature of 250 °C achieved complete transformation from polycrystalline film to the preferred [111] orientated film.

A promising approach for high speed and high power electronics is to integrate two-dimensional (2D) materials with conventional electronic components such as bulk (3D) semiconductors and metals. In this study we explore a basic integration step of inserting a single monolayer MoS2 (1L-MoS2) inside a Au/p-GaN junction and elucidate how it impacts the structural and electrical properties of the junction. Epitaxial 1L-MoS2 in the form of triangle domains are grown by powder vaporization on a p-doped GaN substrate, and the Au capping layer is deposited by evaporation. Transmission electron microscopy (TEM) of the van der Waals interface indicates that 1L-MoS2 remained distinct and intact between the Au and GaN and that the Au is epitaxial to GaN only when the 1L-MoS2 is present. Quantitative TEM analyses of the van der Waals interfaces are performed and yielded the atomic plane spacings in the heterojunction. Electrical characterization of the all-epitaxial, vertical Au/1L-MoS2/p-GaN heterojunctions enables the derivations of Schottky barrier heights (SBH) and drawing of the band alignment diagram. Notably, 1L-MoS2 appears to be electronically semi-transparent, and thus can be considered as a modifier to the Au contact rather than an independent semiconductor component forming a pn-junction. The I–V analysis and our first principles calculation indicated Fermi level pinning and substantial band bending in GaN at the interface. Lastly, we illustrate how the depletion regions are formed in a bipolar junction with an ultrathin monolayer component using the calculated distribution of the charge density across the Au/1L-MoS2/GaN junction.

Sputter-prepared nickel (Ni) films can lose more than half their starting thickness due to evaporation in hydrogen (H2) annealing environments. The loss rate of the sputtered Ni films during the chemical vapor deposition growth of graphene has not been reported earlier. The evaporation rate of sputtered Ni film with the amorphous, mixed, preferred ⟨111⟩ texture was experimentally determined to be 20, 11, and 6 nm/m, respectively. An increase of argon mixture in H2 was found to reduce pitting defects in the films during annealing. The quality of grown graphene on top of the Ni improved when the growth temperature was raised from 900 to 1000 °C, as monitored by Raman spectroscopy. More importantly, loss in the starting Ni film thickness can inhibit the growth of graphene layers. By maintaining the growth of the graphene to two layers or less, a high optical transparency of 95% or better can be achieved.

Tungsten diselenide (WSe2) is a two-dimensional material that is of interest for next-generation electronic and optoelectronic devices due to its direct bandgap of 1.65 eV in the monolayer form and excellent transport properties. However, technologies based on this 2D material cannot be realized without a scalable synthesis process. Here, we demonstrate the first scalable synthesis of large-area, mono and few-layer WSe2 via metal-organic chemical vapor deposition using tungsten hexacarbonyl (W(CO)6) and dimethylselenium ((CH3)2Se). In addition to being intrinsically scalable, this technique allows for the precise control of the vapor-phase chemistry, which is unobtainable using more traditional oxide vaporization routes. We show that temperature, pressure, Se:W ratio, and substrate choice have a strong impact on the ensuing atomic layer structure, with optimized conditions yielding8 μm size domains. Raman spectroscopy, atomic force microscopy (AFM), and cross-sectional transmission electron microscopy (TEM) confirm crystalline monoto-multilayer WSe2 is achievable. Finally, TEM and vertical current/voltage transport provide evidence that a pristine van der Waals gap exists in WSe2/graphene heterostructures.

Integration of two-dimensional (2D) and conventional (3D) semiconductors can lead to the formation of vertical heterojunctions with valuable electronic and optoelectronic properties. Regardless of the growth stacking mechanism implemented so far, the quality of the formed heterojunctions is susceptible to defects and contaminations mainly due to the complication involved in the transfer process. We utilize an approach that aims to eliminate the transfer process and achieve epitaxial vertical heterojunctions with low defect interfaces necessary for efficient vertical transport. Monolayers of MoS2 of approximately 2 μm domains are grown epitaxially by powder vaporization on GaN substrates forming a vertical 2D/3D heterojunction. Cross-sectional transmission electron microscopy (XTEM) is employed to analyze the in-plane lattice constants and van der Waals (vdW) gap between the 2D and 3D semiconductor crystals. The extracted in-plane lattice mismatch between monolayer MoS2 and GaN is only 1.2% which correspon...

A perpendicular electric field breaks the layer symmetry of Bernal-stacked bilayer graphene, resulting in the opening of a band gap and a modification of the effective mass of the charge carriers. Using scanning tunneling microscopy and spectroscopy, we examine standing waves in the local density of states of bilayer graphene formed by scattering from a bilayer/trilayer boundary. The quasiparticle interference properties are controlled by the bilayer graphene band structure, allowing a direct local probe of the evolution of the band structure of bilayer graphene as a function of electric field. We extract the Slonczewski-Weiss-McClure model tight binding parameters as $\gamma_0 = 3.1$ eV, $\gamma_1 = 0.39$ eV, and $\gamma_4 = 0.22$ eV., Comment: 12 pages, 4 figures

Monolayer molybdenum disulfide (MoS2) has attracted tremendous attention due to its promising applications in high-performance field-effect transistors, phototransistors, spintronic devices and nonlinear optics. The enhanced photoluminescence effect in monolayer MoS2 was discovered and, as a strong tool, was employed for strain and defect analysis in MoS2. Recently, large-size monolayer MoS2 has been produced by chemical vapour deposition, but has not yet been fully explored. Here we systematically characterize chemical vapour deposition-grown MoS2 by photoluminescence spectroscopy and mapping and demonstrate non-uniform strain in single-crystalline monolayer MoS2 and strain-induced bandgap engineering. We also evaluate the effective strain transferred from polymer substrates to MoS2 by three-dimensional finite element analysis. Furthermore, our work demonstrates that photoluminescence mapping can be used as a non-contact approach for quick identification of grain boundaries in MoS2.

We have studied temperature-dependent (77–300 K) electrical characteristics and low-frequency noise (LFN) in chemical vapor deposited (CVD) single-layer molybdenum disulfide (MoS2) based back-gated field-effect transistors (FETs). Electrical characterization and LFN measurements were conducted on MoS2 FETs with Al2O3 top-surface passivation. We also studied the effect of top-surface passivation etching on the electrical characteristics of the device. Significant decrease in channel current and transconductance was observed in these devices after the Al2O3 passivation etching. For passivated devices, the two-terminal resistance variation with temperature showed a good fit to the activation energy model, whereas for the etched devices the trend indicated a hopping transport mechanism. A significant increase in the normalized drain current noise power spectral density (PSD) was observed after the etching of the top passivation layer. The observed channel current noise was explained using a standard unified model incorporating carrier number fluctuation and correlated surface mobility fluctuation mechanisms. Detailed analysis of the gate-referred noise voltage PSD indicated the presence of different trapping states in passivated devices when compared to the etched devices. Etched devices showed weak temperature dependence of the channel current noise, whereas passivated devices exhibited near-linear temperature dependence.

The crystal structure of a material plays an important role in determining its electronic properties. Changing from one crystal structure to another involves a phase transition that is usually controlled by a state variable such as temperature or pressure. In the case of trilayer graphene, there are two common stacking configurations (Bernal and rhombohedral) that exhibit very different electronic properties. In graphene flakes with both stacking configurations, the region between them consists of a localized strain soliton where the carbon atoms of one graphene layer shift by the carbon-carbon bond distance. Here we show the ability to move this strain soliton with a perpendicular electric field and hence control the stacking configuration of trilayer graphene with only an external voltage. Moreover, we find that the free-energy difference between the two stacking configurations scales quadratically with electric field, and thus rhombohedral stacking is favoured as the electric field increases. This ability to control the stacking order in graphene opens the way to new devices that combine structural and electrical properties., M.Y. and B.J.L. were supported by the US Army Research Laboratory and the US Army Research Office under contract/grant number W911NF-09-1-0333. J.I-J.W. was partially supported by a Taiwan Merit Scholarship TMS-094-1-A-001. J.I-J.W and P.J-H. have been primarily supported by the US DOE, BES Office, Division of Materials Sciences and Engineering under Award DE-SC0001819. Early fabrication feasibility studies were supported by NSF Career Award No. DMR-0845287 and the ONR GATE MURI. This work made use of the MRSEC Shared Experimental Facilities supported by NSF under award No. DMR-0819762 and of Harvard’s CNS, supported by NSF under grant No. ECS-0335765. A.G.B. was supported by the US Army Research Laboratory (ARL) Director’s Strategic Initiative program on interfaces in stacked 2D atomic layered materials. P.S-J. received financial support from the Spanish Ministry of Economy (MINECO) through Grant no. FIS2011-23713, the European Research Council Advanced Grant (contract 290846) and from the European Commission under the Graphene Flagship (contract CNECT-ICT-604391).

The large family of layered transition-metal dichalcogenides is widely believed to constitute a second family of two-dimensional (2D) semiconducting materials that can be used to create novel devices that complement those based on graphene. In many cases these materials have shown a transition from an indirect band gap in the bulk to a direct band gap in monolayer systems. In this work we experimentally show that folding a 1$H$ molybdenum disulfide (MoS${}_{2}$) layer results in a turbostratic stack with enhanced photoluminescence quantum yield and a significant shift to the blue by \ensuremath{\sim}90 meV. This is in contrast to the expected 2$H$-MoS${}_{2}$ band-structure characteristics, which include an indirect gap and quenched photoluminescence. We present a theoretical explanation for the origin of this behavior in terms of exciton screening.

The growth of large-area bilayer graphene has been of technological importance for graphene electronics. The successful application of graphene bilayers critically relies on the precise control of the stacking orientation, which determines both electronic and vibrational properties of the bilayer system. Toward this goal, an effective characterization method is critically needed to allow researchers to easily distinguish the bilayer stacking orientation (i.e., AB stacked or turbostratic). In this work, we developed such a method to provide facile identification of the stacking orientation by isotope labeling. Raman spectroscopy of these isotopically labeled bilayer samples shows a clear signature associated with AB stacking between layers, enabling rapid differentiation between turbostratic and AB-stacked bilayer regions. Using this method, we were able to characterize the stacking orientation in bilayer graphene grown through Low Pressure Chemical Vapor Deposition (LPCVD) with enclosed Cu foils, achieving almost 70% AB-stacked bilayer graphene. Furthermore, by combining surface sensitive fluorination with such hybrid (12)C/(13)C bilayer samples, we are able to identify that the second layer grows underneath the first-grown layer, which is similar to a recently reported observation.

Contactless high-frequency information about nanowires is obtained by placing patterned sheets of oriented wires on a dielectric insert that partially fills a rectangular waveguide. Exact expressions for the fields within the guide are derived by treating the nanowire sheet within the partially-filled guide as an impedance boundary condition at the air-dielectric interface. Because the partially filled guide mixes TM and TE modes of an empty rectangular guide in a specific way, the S-parameters of a segment of such a guide are sensitive to the presence of the coated air-dielectric interface, whose surface impedance can be de-embedded from the measurements. Effects of disorder in wire orientation and location on the surface can be included in the surface impedance description and the de-embedding procedure.

Leveraging nanoscale field-effect transistors (FETs) in integrated circuits depends heavily on its transfer characteristics and low-frequency noise (LFN) properties. Here, we report the transfer characteristics and LFN in FETs fabricated with molybdenum disulfide (MoS2) with different layer (L) counts. 4L to 6L devices showed highest ION-IOFF ratio (≈108) whereas LFN was maximum for 1L device with normalized power spectral density (PSD) ≈1.5 × 10−5 Hz−1. For devices with L ≈ 6, PSD was minimum (≈2 × 10−8 Hz−1). Further, LFN for single and few layer devices satisfied carrier number fluctuation (CNF) model in both weak and strong accumulation regimes while thicker devices followed Hooge's mobility fluctuation model in the weak accumulation regime and CNF model in strong accumulation regime, respectively. Transfer-characteristics and LFN experimental data are explained with the help of model incorporating Thomas-Fermi charge screening and inter-layer resistance coupling.

We present a combined experimental and computational study of two-dimensional molybdenum disulfde (MoS2) and the effect of temperature on the frequency shifts of the Raman-active E2g and A1g modes in the monolayer. While both peaks show an expected red-shift with increasing temperature, the frequency shift is larger for the A1g more than for the E2g mode. This is in contrast to previously reported bulk behavior, in which the E2g mode shows a larger frequency shift with temperature. The temperature dependence of these phonon shifts is attributed to the anharmonic contributions to the ionic interaction potential in the two-dimensional system., Comment: 4 pages, 4 figures

Molybdenum disulfide (MoS2) field effect transistors (FET) were fabricated on atomically smooth large-area single layers grown by chemical vapor deposition. The layer qualities and physical properties were characterized using high-resolution Raman and photoluminescence spectroscopy, scanning electron microscopy, and atomic force microscopy. Electronic performance of the FET devices was measured using field effect mobility measurements as a function of temperature. The back-gated devices had mobilities of 6.0 cm2/V s at 300 K without a high-κ dielectric overcoat and increased to 16.1 cm2/V s with a high-κ dielectric overcoat. In addition the devices show on/off ratios ranging from 105 to 109.

Large-area chemical vapor deposited graphene/boron-nitride (G/BN) thin films are co-transferred layer-by-layer to silicon-di-Oxide (SiO2) substrates, and transistors are constructed and examined. Raman spectra and high resolution transmission electron microscopy imaging show films of high quality. The graphene/boron-nitride/SiO2 devices have a significantly increased peak electron/hole mobility of 3400/2200 cm2/Vs with a reduced effective doping density over reference graphene/SiO2 devices. The mobility dependence as a function of carrier density is compared with a physically based empirical model and is in agreement with the improvements due to a consistent reduction in the substrate induced phonon and impurity scattering and an improvement in the overall surface quality owed to the boron-nitride interlayer that separates the graphene from the SiO2. Large-area G/BN thin films are promising for future high performance thin film electronic devices.