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Combined with first-principles calculations and semiclassical Boltzmann transport theory, Janus {\alpha}-STe2 and {\alpha}-SeTe2 monolayers are investigated systematically. Janus {\alpha}-STe2 and {\alpha}-SeTe2 monolayers are indirect semiconductors with band gaps of 1.20 and 0.96 eV. It is found they possess ultrahigh figure of merit (ZT) values of 3.9 and 4.4 at 500 K, much higher than that of the pristine {\alpha}-Te monolayer (2.8). The higher ZT originates from Janus structures reduce lattice thermal conductivities remarkably compared with pristine {\alpha}-Te monolayer. The much higher phonon anharmonicity in Janus monolayers leads to significant lower lattice thermal conductivity. It is also found electronic thermal conductivity can play an important role in thermoelectric efficiency for the materials with quite low lattice thermal conductivity. This work suggests the potential applications of Janus {\alpha}-STe2 and {\alpha}-SeTe2 monolayers as thermoelectric materials and highlights Janus structure as an effective way to enhance thermoelectric performance.

Charge doping is an effective way to induce metal-insulate transition (MIT) in correlated materials for many important utilizations, which is however practically limited by problem of low stability. In this study, we have achieved pronounced phase modulation and stabilized the metallic state of monoclinic vanadium dioxide (VO2) at room temperature, via a novel electron-proton co-doping mechanism driven by surface absorption of self-assembled L-ascorbic acid (AA) molecules. The ionized AA- species in solution donate effective electrons to the adsorbed VO2 surface, which then electrostatically attract surrounding protons to penetrate, and eventually results in stable hydrogen-doped metallic VO2. The variations of phase and electronic structures as well as the electron occupancy of V-3d/O-2p hybrid orbitals were examined by synchrotron characterizations and first-principle theoretical simulations, which explain the formation of stable metallic state. Importantly, the adsorbed molecules protect hydrogen dopants from escaping out of lattice and thereby stabilize the metallic phase for VO2. Such an electron-proton co-doping mechanism driven by suitable molecules absorption would open a new door for engineering properties of correlated oxide materials., Comment: 25 pages, 5 figures

VO2 material is promising for developing energy-saving "smart window", owing to its thermochromic property induced by metal-insulator transition (MIT). However, its practical application is greatly limited by the relatively high critical transition temperature (~68oC), low luminous transmittance (<60%) and poor solar energy regulation ability (<15%). Here we developed a reversible and non-volatile electric-field control on the MIT of monoclinic VO2 film. With a solid electrolyte layer assisted gating treatment, we modulated the insertion/extraction of hydrogens into/from VO2 lattice at room temperature, causing tri-state phase transitions accompanied with controllable transmission adjustment. The dramatic increase of visible/infrared transmittance during the phase transition from the metallic (lightly H-doping) to insulating (heavily H-doping) phase leads to an increased solar energy regulation ability up to 26.5%, while keep 70.8% visible-luminous transmittance. These results beat all previous records and even exceeded the theoretical limit for traditional VO2 smart window, removing intrinsic disadvantages of VO2 for energy-saving utilizations. Our findings not only demonstrated an electric-field controlled phase modulation strategy, but also open the door for high-performance VO2-based smart window applications., Comment: 19 pages, 5 figures

Hydrogenation is an effective way to tune material property1-5. Traditional techniques for doping hydrogen atoms into solid materials are very costly due to the need for noble metal catalysis and high-temperature/pressure annealing treatment or even high energy proton implantation in vacuum condition5-8. Acid solution contains plenty of freely-wandering protons, but it is difficult to act as a proton source for doping, since the protons always cause corrosions by destroying solid lattices before residing into them. Here we achieve a facile way to hydrogenate monoclinic vanadium dioxide (VO2) with protons in acid solution by attaching suitable metal to it. Considering the Schottky contact at the metal/VO2 interface, electrons flow from metal to VO2 due to workfunction difference and simultaneously attract free protons in acid solution to penetrate, forming the hydrogens dopants inside VO2 lattice. This metal-acid treatment constitutes an electron-proton co-doping strategy, which not only protects the VO2 lattice from corrosion, but also causes pronounced insulator-to-metal transitions. In addition, the metal-acid induced hydrogen doping behavior shows a ripple effect, and it can spread contagiously up to wafer-size area (>2 inch) even triggered by a tiny metal particle attachment (~1.0mm). This will stimulate a new way of simple and cost-effective atomic doping technique for some other oxide materials., Comment: 15 pages, 4 figures

We report modulation of a reversible phase transition in VO2 films by hydrogen doping. A metallic phase and a new insulating phase are successively observed at room temperature as the doping concentration increases. It is suggested that the polarized charges from doped hydrogens play an important role. These charges gradually occupy V3d-O2p hybridized orbitals and consequently modulate the filling of the VO2 crystal conduction band-edge states, which eventually evolve into new valence band-edge states. This demonstrates the exceptional sensitivity of VO2 electronic properties to electron concentration and orbital occupancy, providing key information for the phase transition mechanism., Comment: 16 pages, 4 figures