Interest in two-dimensional (2D) van der Waals materials has grown rapidly across multiple scientific and engineering disciplines in recent years. However, ferroelectricity, the presence of a spontaneous electric polarization, which is important in many practical applications, has rarely been reported in such materials so far. Here we employ first-principles calculations to discover a branch of the 2D materials family, based on In2Se3 and other III2-VI3 van der Waals materials, that exhibits room-temperature ferroelectricity with reversible spontaneous electric polarization in both out-of-plane and in-plane orientations. The device potential of these 2D ferroelectric materials is further demonstrated using the examples of van der Waals heterostructures of In2Se3/graphene, exhibiting a tunable Schottky barrier, and In2Se3/WSe2, showing a significant band gap reduction in the combined system. These findings promise to substantially broaden the tunability of van der Waals heterostructures for a wide range of applications.

. We find that in bilayer MoS 2 the circularly polarized photoluminescence can be continuously tuned from −15% to 15% as a function of gate voltage, whereas in structurally non-centrosymmetric monolayer MoS 2 the photoluminescence polarization is gate independent. The observations are well explained as resulting from the continuous variation of orbital magnetic moments between positive and negative values through symmetry control.The Dirac-valley degree of freedom has recently been considered for new modes of electronic and photonic device operation 4,5,[9][10][11][12][13][14][15][16][17] following the arrival of atomically thin two-dimensional (2D) electronic systems 6,7,18,19 (Fig. 1a). In this context, phenomena such as valley polarization and anomalous valley-and spin-Hall effects have been discussed for the +K and −K Dirac valleys at opposite corners of the Brillouin zone in hexagonal systems [9][10][11][12]15 . The realization of these effects hinges on achieving control of valley contrast, that is, of properties that differ between the two valleys, in particular the magnetic moment (m) and Berry curvature ( ). Time-reversal symmetry dictates that each pseudovector, m as well as , has the same magnitude but opposite sign in the two valleys, whereas inversion symmetry requires them to have the same sign. Therefore, a necessary condition for valley-contrasting m and is inversion symmetry breaking 4 . Monolayer MoS 2 lacks structural inversion symmetry (Fig. 1a), and thus m and are non-zero, having equal magnitude but opposite signs in the two ±K valleys owing to timereversal symmetry. One direct consequence of non-zero m is valley-contrasting optical dichroism 5,8,9 , whereby charge carriers in the two valleys can be selectively excited by circularly polarized optical fields [9][10][11] . This effect permits optical generation of valley polarization, as recently demonstrated using polarized

Enriching the functionality of ferroelectric materials with visible-light sensitivity and multiaxial switching capability would open up new opportunities for their applications in advanced information storage with diverse signal manipulation functions. We report experimental observations of robust intralayer ferroelectricity in two-dimensional (2D) van der Waals layered α-InSe ultrathin flakes at room temperature. Distinct from other 2D and conventional ferroelectrics, InSe exhibits intrinsically intercorrelated out-of-plane and in-plane polarization, where the reversal of the out-of-plane polarization by a vertical electric field also induces the rotation of the in-plane polarization. On the basis of the in-plane switchable diode effect and the narrow bandgap (∼1.3 eV) of ferroelectric InSe, a prototypical nonvolatile memory device, which can be manipulated both by electric field and visible light illumination, is demonstrated for advancing data storage technologies.

Topological insulators are characterized by a non-trivial band topology driven by the spin-orbit coupling. To fully explore the fundamental science and application of topological insulators, material realization is indispensable. Here we predict, based on tight-binding modelling and first-principles calculations, that bilayers of perovskite-type transition-metal oxides grown along the [111] crystallographic axis are potential candidates for two-dimensional topological insulators. The topological band structure of these materials can be fine-tuned by changing dopant ions, substrates and external gate voltages. We predict that LaAuo 3 bilayers have a topologically non-trivial energy gap of about 0.15 eV, which is sufficiently large to realize the quantum spin Hall effect at room temperature. Intriguing phenomena, such as fractional quantum Hall effect, associated with the nearly flat topologically non-trivial bands found in e g systems are also discussed.

We have fabricated ultrathin lead films on silicon substrates with atomic-scale control of the thickness over a macroscopic area. We observed oscillatory behavior of the superconducting transition temperature when the film thickness was increased by one atomic layer at a time. This oscillating behavior was shown to be a manifestation of the Fabry-Perot interference modes of electron de Broglie waves (quantum well states) in the films, which modulate the electron density of states near the Fermi level and the electron-phonon coupling, which are the two factors that control superconductivity transitions. This result suggests the possibility of modifying superconductivity and other physical properties of a thin film by exploiting well-controlled and thickness-dependent quantum size effects.

Unearthing an ideal model for disclosing the role of defect sites in solar CO reduction remains a great challenge. Here, freestanding gram-scale single-unit-cell o-BiVO layers are successfully synthesized for the first time. Positron annihilation spectrometry and X-ray fluorescence unveil their distinct vanadium vacancy concentrations. Density functional calculations reveal that the introduction of vanadium vacancies brings a new defect level and higher hole concentration near Fermi level, resulting in increased photoabsorption and superior electronic conductivity. The higher surface photovoltage intensity of single-unit-cell o-BiVO layers with rich vanadium vacancies ensures their higher carriers separation efficiency, further confirmed by the increased carriers lifetime from 74.5 to 143.6 ns revealed by time-resolved fluorescence emission decay spectra. As a result, single-unit-cell o-BiVO layers with rich vanadium vacancies exhibit a high methanol formation rate up to 398.3 μmol g h and an apparent quantum efficiency of 5.96% at 350 nm, much larger than that of single-unit-cell o-BiVO layers with poor vanadium vacancies, and also the former's catalytic activity proceeds without deactivation even after 96 h. This highly efficient and spectrally stable CO photoconversion performances hold great promise for practical implementation of solar fuel production.

Using first-principles calculations within density functional theory, we explore the feasibility of converting ternary half-Heusler compounds into a new class of three-dimensional topological insulators (3DTI). We demonstrate that the electronic structure of unstrained LaPtBi as a prototype system exhibits a distinct band-inversion feature. The 3DTI phase is realized by applying a uniaxial strain along the [001] direction, which opens a band gap while preserving the inverted band order. A definitive proof of the strained LaPtBi as a 3DTI is provided by directly calculating the topological Z2 invariants in systems without inversion symmetry. We discuss the implications of the present study to other half-Heusler compounds as 3DTI, which, together with the magnetic and superconducting properties of these materials, may provide a rich platform for novel quantum phenomena.

Using first-principles calculations within density functional theory, we investigate the electronic and chemical properties of a single-layer MoS(2) adsorbed on Ir(111), Pd(111), or Ru(0001), three representative transition metal substrates having varying work functions but each with minimal lattice mismatch with the MoS(2) overlayer. We find that, for each of the metal substrates, the contact nature is of Schottky-barrier type, and the dependence of the barrier height on the work function exhibits a partial Fermi-level pinning picture. Using hydrogen adsorption as a testing example, we further demonstrate that the introduction of a metal substrate can substantially alter the chemical reactivity of the adsorbed MoS(2) layer. The enhanced binding of hydrogen, by as much as ~0.4 eV, is attributed in part to a stronger H-S coupling enabled by the transferred charge from the substrate to the MoS(2) overlayer, and in part to a stronger MoS(2)-metal interface by the hydrogen adsorption. These findings may prove to be instrumental in future design of MoS(2)-based electronics, as well as in exploring novel catalysts for hydrogen production and related chemical processes.