Ultrathin epitaxial Bi film growth on 2D HfTe2template.

Among ultrathin monoelemental two-dimensional (2D) materials, bismuthene, the single layer of heavier group-VΑ element bismuth (Bi), has been predicted to have large non trivial gap. Here, we demonstrate the growth of Bi films by molecular beam epitaxy on 2D-HfTe2template. At the initial stage of Bi deposition (1-2 bilayers, BL), both the pseudocubic Bi(110) and the hexagonal Bi(111) phases are formed. When reaching 3 BL Bi, a transformation to pure hexagonal Bi(111) occurs. The electronic band structure of 3 BL Bi(111) films was measured by angle-resolved photoemission spectroscopy showing very good matching with the density functional theory band structure calculations of 3 BL free standing Bi(111). The grown Bi(111) thin film was capped with a protective Al2O3layer and its stability under ambient conditions, necessary for practical applications and device fabrication, was confirmed by x-ray photoelectron spectroscopy and Raman spectroscopy.

Direct versus reverse vertical two-dimensional Mo2C/graphene heterostructures for enhanced hydrogen evolution reaction electrocatalysis.

Mo2C/graphene heterostructures prepared by chemical vapor deposition have demonstrated excellent electrocatalytic activity in a hydrogen evolution reaction (HER). This is attributed to the high catalytic activity of Mo2C while the high electrical conductivity of graphene facilitates charge transfer. In the as-grown direct vertical order, graphene is placed above the Mo2C film. This reduces the catalytic activity of the heterostructure, since graphene in chemically inert. Here, a simple transfer method is proposed that results in the reverse order deposition of the heterostructure on the electrode. This method places graphene at the interface between Mo2C and the electrode, enhancing charge transfer between them, which results in an overpotential of 440 mV at 10 mA cm-2 and corresponds to ∼65 mV overpotential reduction as compared to the direct heterostructure. At the same time, when a direct Cu/Mo2C/graphene junction with a Cu catalyst substrate is used as a working electrode, the improvement of the heterostructure HER activity is observed which is manifested in an overpotential of 275 mV at 10 mA cm-2 with a correspondent ∼230 mV reduction. All above performances are accompanied with excellent endurance.

Advanced Photocatalysts Based on Reduced Nanographene Oxide-TiO2 Photonic Crystal Films.

Surface functionalization of TiO2 inverse opals by graphene oxide nanocolloids (nanoGO) presents a promising modification for the development of advanced photocatalysts that combine slow photon-assisted light harvesting, surface area, and mass transport of macroporous photonic structures with the enhanced adsorption capability, surface reactivity, and charge separation of GO nanosheets. In this work, post-thermal reduction of nanoGO-TiO2 inverse opals was investigated in order to explore the role of interfacial electron transfer vs. pollutant adsorption and improve their photocatalytic activity. Photonic band gap-engineered TiO2 inverse opals were fabricated by the coassembly technique and were functionalized by GO nanosheets and reduced under He at 200 and 500 °C. Comparative performance evaluation of the nanoGO-TiO2 films on methylene blue photodegradation under UV-VIS and visible light showed that thermal reduction at 200 °C, in synergy with slow photon effects, improved the photocatalytic reaction rate despite the loss of nanoGO and oxygen functional groups, pointing to enhanced charge separation. This was further supported by photoluminescence spectroscopy and salicylic acid UV-VIS photodegradation, where, in the absence of photonic effects, the photocatalytic activity increased, confirming that fine-tuning of interfacial coupling between TiO2 and reduced nanoGO is a key factor for the development of highly efficient photocatalytic films.

Massless Dirac Fermions in ZrTe2 Semimetal Grown on InAs(111) by van der Waals Epitaxy.

Single and few layers of the two-dimensional (2D) semimetal ZrTe2 are grown by molecular beam epitaxy on InAs(111)/Si(111) substrates. Excellent rotational commensurability, van der Waals gap at the interface and moiré pattern are observed indicating good registry between the ZrTe2 epilayer and the substrate through weak van der Waals forces. The electronic band structure imaged by angle resolved photoelectron spectroscopy shows that valence and conduction bands cross at the Fermi level exhibiting abrupt linear dispersions. The latter indicates massless Dirac Fermions which are maintained down to the 2D limit suggesting that single-layer ZrTe2 could be considered as the electronic analogue of graphene.

Epitaxial 2D SnSe2/ 2D WSe2 van der Waals Heterostructures.

van der Waals heterostructures of 2D semiconductor materials can be used to realize a number of (opto)electronic devices including tunneling field effect devices (TFETs). It is shown in this work that high quality SnSe2/WSe2 vdW heterostructure can be grown by molecular beam epitaxy on AlN(0001)/Si(111) substrates using a Bi2Se3 buffer layer. A valence band offset of 0.8 eV matches the energy gap of SnSe2 in such a way that the VB edge of WSe2 and the CB edge of SnSe2 are lined up, making this materials combination suitable for (nearly) broken gap TFETs.

Epitaxial 2D MoSe2 (HfSe2) Semiconductor/2D TaSe2 Metal van der Waals Heterostructures.

Molecular beam epitaxy of 2D metal TaSe2/2D MoSe2 (HfSe2) semiconductor heterostructures on epi-AlN(0001)/Si(111) substrates is reported. Electron diffraction reveals an in-plane orientation indicative of van der Waals epitaxy, whereas electronic band imaging supported by first-principles calculations and X-ray photoelectron spectroscopy indicate the presence of a dominant trigonal prismatic 2H-TaSe2 phase and a minor contribution from octahedrally coordinated TaSe2, which is present in TaSe2/AlN and TaSe2/HfSe2/AlN but notably absent in the TaSe2/MoSe2/AlN, indicating superior structural quality of TaSe2 grown on MoSe2. Apart from its structural and chemical compatibility with the selenide semiconductors, TaSe2 has a workfunction of 5.5 eV as measured by ultraviolet photoelectron spectroscopy, which matches very well with the semiconductor workfunctions, implying that epi-TaSe2 can be used for low-resistivity contacts to MoSe2 and HfSe2.

Observation of surface Dirac cone in high-quality ultrathin epitaxial Bi2Se3 topological insulator on AlN(0001) dielectric.

Bi2Se3 topological insulators (TIs) are grown on AlN(0001)/Si(111) substrates by molecular beam epitaxy. In a one-step growth at optimum temperature of 300 °C, Bi2Se3 bonds strongly with AlN without forming interfacial reaction layers. This produces high epitaxial quality Bi2Se3 single crystals with a perfect registry with the substrate and abrupt interfaces, allowing thickness scaling down to three quintuple layers (QL) without jeopardizing film quality. It is found by angle-resolved photoelectron spectroscopy that, remarkably, Bi2Se3 films maintain the 3D TI properties at very low thickness of 3QL (∼2.88 nm), exhibiting top surface gapless metallic states in the form of a Dirac cone.