Large-area graphene films are best synthesized via chemical vapour and/or solid deposition methods at elevated temperatures (~1,000 °C) on polycrystalline metal surfaces and later transferred onto other substrates for device applications. Here we report a new method for the synthesis of graphene films directly on sio 2 /si substrates, even plastics and glass at close to room temperature (25-160 °C). In contrast to other approaches, where graphene is deposited on top of a metal substrate, our method invokes diffusion of carbon through a diffusion couple made up of carbon-nickel/substrate to form graphene underneath the nickel film at the nickelsubstrate interface. The resulting graphene layers exhibit tunable structural and optoelectronic properties by nickel grain boundary engineering and show micrometre-sized grains on sio 2 surfaces and nanometre-sized grains on plastic and glass surfaces. The ability to synthesize graphene directly on non-conducting substrates at low temperatures opens up new possibilities for the fabrication of multiple nanoelectronic devices.

The growth of high quality epitaxial beta-gallium oxide (β-Ga2O3) using a compound source by molecular beam epitaxy has been demonstrated on c-plane sapphire (Al2O3) substrates. The compound source provides oxidized gallium molecules in addition to oxygen when heated from an iridium crucible in a high temperature effusion cell enabling a lower heat of formation for the growth of Ga2O3, resulting in a more efficient growth process. This source also enabled the growth of crystalline β-Ga2O3 without the need for additional oxygen. The influence of the substrate temperatures on the crystal structure and quality, chemical bonding, surface morphology, and optical properties has been systematically evaluated by x-ray diffraction, scanning transmission electron microscopy, x-ray photoelectron spectroscopy, atomic force microscopy, spectroscopic ellipsometry, and UV-vis spectroscopy. Under optimized growth conditions, all films exhibited pure 2¯01  oriented β-Ga2O3 thin films with six-fold rotational symmetry when grown on a sapphire substrate. The thin films demonstrated significant absorption in the deep-ultraviolet (UV) region with an optical bandgap around 5.0 eV and a refractive index of 1.9. A deep-UV photodetector fabricated on the high quality β-Ga2O3 thin film exhibits high resistance and small dark current (4.25 nA) with expected photoresponse for 254 nm UV light irradiation suggesting that the material grown using the compound source is a potential candidate for deep-ultraviolet photodetectors.

The Poisson's ratio is a fundamental measure of the elastic-deformation behaviour of materials. Although negative Poisson's ratios are theoretically possible, they were believed to be rare in nature. In particular, while some studies have focused on finding or producing materials with a negative Poisson's ratio in bulk form, there has been no such study for nanoscale materials. Here we provide numerical and theoretical evidence that negative Poisson's ratios are found in several nanoscale metal plates under finite strains. Furthermore, under the same conditions of crystal orientation and loading direction, materials with a positive Poisson's ratio in bulk form can display a negative Poisson's ratio when the material's thickness approaches the nanometer scale. We show that this behaviour originates from a unique surface effect that induces a finite compressive stress inside the nanoplates, and from a phase transformation that causes the Poisson's ratio to depend strongly on the amount of stretch.

The covalently bonded in-plane heterostructure (HS) of monolayer transition-metal dichalcogenides (TMDCs) possesses huge potential for high-speed electronic devices in terms of valleytronics. In this study, high-quality monolayer MoSe-WSe lateral HSs are grown by pulsed-laser-deposition-assisted selenization method. The sharp interface of the lateral HS is verified by morphological and optical characterizations. Intriguingly, photoluminescence spectra acquired from the interface show rather clear signatures of pristine MoSe and WSe with no intermediate energy peak related to intralayer excitonic matter or formation of MoWSe alloys, thereby confirming the sharp interface. Furthermore, the discrete nature of laterally attached TMDC monolayers, each with doubly degenerated but nonequivalent energy valleys marked by (K, K') for MoSe and (K, K') for WSe in k space, allows simultaneous control of the four valleys within the excitation area without any crosstalk effect over the interface. As an example, K and K valleys or K' and K' valleys are simultaneously polarized by controlling the helicity of circularly polarized optical pumping, where the maximum degree of polarization is achieved at their respective band edges. The current work provides the growth mechanism of laterally sharp HSs and highlights their potential use in valleytronics.

The van der Waals epitaxy of functional materials provides an interesting and efficient way to manipulate the electrical properties of various hybrid two-dimensional (2D) systems. Here we show the controlled epitaxial assembly of semiconducting one-dimensional (1D) atomic chains, AuCN, on graphene and investigate the electrical properties of 1D/2D van der Waals heterostructures. AuCN nanowire assembly is tuned by different growth conditions, although the epitaxial alignment between AuCN chains and graphene remains unchanged. The switching of the preferred nanowire growth axis indicates that diffusion kinetics affects the nanowire formation process. Semiconducting AuCN chains endow the 1D/2D hybrid system with a strong responsivity to photons with an energy above 2.7 eV, which is consistent with the bandgap of AuCN. A large UV response (responsivity ∼104 A/W) was observed under illumination using 3.1 eV (400 nm) photons. Our study clearly demonstrates that 1D chain-structured semiconductors can play a crucial role as a component in multifunctional van der Waals heterostructures.

Growth of large-scale patterned, wrinkle-free graphene and the gentle transfer technique without further damage are most important requirements for the practical use of graphene. Here we report the growth of wrinkle-free, strictly uniform monolayer graphene films by chemical vapor deposition on a platinum (Pt) substrate with texture-controlled giant grains and the thermal-assisted transfer of large-scale patterned graphene onto arbitrary substrates. The designed Pt surfaces with limited numbers of grain boundaries and improved surface perfectness as well as small thermal expansion coefficient difference to graphene provide a venue for uniform growth of monolayer graphene with wrinkle-free characteristic. The thermal-assisted transfer technique allows the complete transfer of large-scale patterned graphene films onto arbitrary substrates without any ripples, tears, or folds. The transferred graphene shows high crystalline quality with an average carrier mobility of ∼ 5500 cm(2) V(-1) s(-1) at room temperature. Furthermore, this transfer technique shows a high tolerance to variations in types and morphologies of underlying substrates.

Today, state-of-the-art III-Ns technology has been focused on the growth of c-plane nitrides by metal-organic chemical vapor deposition (MOCVD) using a conventional two-step growth process. Here we show that the use of graphene as a coating layer allows the one-step growth of heteroepitaxial GaN films on sapphire in a MOCVD reactor, simplifying the GaN growth process. It is found that the graphene coating improves the wetting between GaN and sapphire, and, with as little as ~0.6 nm of graphene coating, the overgrown GaN layer on sapphire becomes continuous and flat. With increasing thickness of the graphene coating, the structural and optical properties of one-step grown GaN films gradually transition towards those of GaN films grown by a conventional two-step growth method. The InGaN/GaN multiple quantum well structure grown on a GaN/graphene/sapphire heterosystem shows a high internal quantum efficiency, allowing the use of one-step grown GaN films as 'pseudo-substrates' in optoelectronic devices. The introduction of graphene as a coating layer provides an atomic playground for metal adatoms and simplifies the III-Ns growth process, making it potentially very useful as a means to grow other heteroepitaxial films on arbitrary substrates with lattice and thermal mismatch.

High-temperature-processing-induced double-stacking-fault 3C-SiC inclusions in 4H SiC were studied with ballistic electron emission microscopy in ultrahigh vacuum. Distinctive quantum well structures corresponding to individual inclusions were found and the quantum well two-dimensional conduction band minimum was determined to be approximately 0.53Ϯ0.06 eV below the conduction band minimum of bulk 4H SiC. Macroscopic diode I-V measurements indicate no significant evidence of metal/semiconductor interface state variation across the inclusions. Quantum well ͑QW͒ structures in semiconductor materials have played an important role in the fabrication of semiconductor lasers and other devices. Most QW structures are fabricated by changing the chemical composition of epitaxial layers during growth. However, the polytypism of SiC has made possible a unique type of ''structure-only'' QW ͑with no change in composition, density, or nearest-neighbor stacking across the QW boundaries͒ involving thin layers of cubic 3C SiC ͑which has the smallest band gap among SiC polytypes͒ embedded in a higher band gap SiC host. 1 Recently, evidence of self-forming 3C inclusions in hexagonal SiC due to stacking fault formation along the ͑1000͒ hexagonal basal plane 2 has been observed in 4H-and 6H-SiC p-n diodes after high current operation, 3,4 and also in 4H-SiC materials with heavily n-type epilayers 5 or substrates 6,7 after high temperature processing. Observations of reduced-energy luminescence from these transformed materials and theoretical calculations of the band structure led to the proposal that the 3C inclusions in 4H and 6H SiC behave as electron QW's. 5,6,8 -10 In this Rapid Communication, we report high-resolution electronic characterization of individual 3C inclusions ͑of the ''double stacking fault'' type 5-7 ͒ that intersect a metalcoated 4H-SiC wafer surface using ballistic electron emission microscopy ͑BEEM͒. 11 The results directly verify the QW nature of the inclusions. We find that the QW states are propagating two-dimensional ͑2D͒ states with a 2D conduction band minimum ͑CBM͒ energy 0.53Ϯ0.06 eV lower than the 4H-SiC bulk CBM. Microscopic BEEM measurements as well as macroscopic diode I-V measurements show no evidence of significant metal/semiconductor ͑M/S͒ interface state variation across the inclusions. BEEM has previously been shown to be a powerful tool to investigate planar resonant tunneling structures located close to the M/S interface, 12 and our ''cross-sectional BEEM'' further extends its capability to probe propagating states in individual QW's.The original 4H-SiC samples of this study were 35 mm diameter wafers purchased from Cree, Inc. and were processed and first studied using electrical, optical, and structural methods at Arizona State University. 6,13 They had a 2 m lightly n-type N-doped ͓(1 -1.5)ϫ10 17 cm Ϫ3 ͔ epilayer on a heavily n-type N-doped (ϳ3ϫ10 19 cm Ϫ3 ) Si-face substrate with an 8°surface miscut from the basal plane. After being thermally oxidized at 1150°C for 90 min in dry oxygen,...