Two-dimensional (2D) materials have emerged as promising candidates for next-generation electronic and optoelectronic applications. Yet, only a few dozen 2D materials have been successfully synthesized or exfoliated. Here, we search for 2D materials that can be easily exfoliated from their parent compounds. Starting from 108,423 unique, experimentally known 3D compounds, we identify a subset of 5,619 compounds that appear layered according to robust geometric and bonding criteria. High-throughput calculations using van der Waals density functional theory, validated against experimental structural data and calculated random phase approximation binding energies, further allowed the identification of 1,825 compounds that are either easily or potentially exfoliable. In particular, the subset of 1,036 easily exfoliable cases provides novel structural prototypes and simple ternary compounds as well as a large portfolio of materials to search from for optimal properties. For a subset of 258 compounds, we explore vibrational, electronic, magnetic and topological properties, identifying 56 ferromagnetic and antiferromagnetic systems, including half-metals and half-semiconductors.

Two-dimensional materials from high-throughput computational exfoliation of experimentally known compounds

CuO nanostructures: Synthesis, characterization, growth mechanisms, fundamental properties, and applications

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. The development of devices based on 2D materials beyond graphene benefits from identifying compounds with diverse functional properties. Here, the authors predict computationally that 2D In2Se3and related materials are room temperature ferroelectrics with both in- and out-of-plane polarization.

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Prediction of intrinsic two-dimensional ferroelectrics in In 2 Se 3 and other III 2 -VI 3 van der Waals materials.

The effect of defects on electron–hole separation is not always clear and is sometimes contradictory. Herein, we initially built clear models of two-dimensional atomic layers with tunable defect concentrations, and hence directly disclose the defect type and distribution at atomic level. As a prototype, defective one-unit-cell ZnIn2S4 atomic layers are successfully synthesized for the first time. Aberration-corrected scanning transmission electron microscopy directly manifests their distinct zinc vacancy concentrations, confirmed by positron annihilation spectrometry and electron spin resonance analysis. Density-functional calculations reveal that the presence of zinc vacancies ensures higher charge density and efficient carrier transport, verified by ultrafast photogenerated electron transfer time of ∼15 ps from the conduction band of ZnIn2S4 to the trap states. Ultrafast transient absorption spectroscopy manifests the higher zinc vacancy concentration that allows for ∼1.7-fold increase in average recove...

Defect-Mediated Electron–Hole Separation in One-Unit-Cell ZnIn2S4 Layers for Boosted Solar-Driven CO2 Reduction

Atomically thin group-VIB transition metal dichalcogenides (TMDs) have recently emerged as a new class of two-dimensional (2D) semiconductors with extraordinary properties including the direct band gap in the visible frequency range, the pronounced spin–orbit coupling, the ultra-strong Coulomb interaction, and the rich physics associated with the valley degree of freedom. These 2D TMDs exhibit great potential for device applications and have attracted vast interest for the exploration of new physics. 2D TMDs have complex electronic structures which underlie their physical properties. Here we review the bulk electronic structures of these new 2D materials as well as the theoretical models developed at different levels, along which we sort out the understanding of the origins of a variety of properties observed or predicted.

Electronic structures and theoretical modelling of two-dimensional group-VIB transition metal dichalcogenides.

A combined experimental and theoretical study is reported on the vibrational properties of tenorite CuO and paramelaconite Cu4O3. The optically active modes have been measured by Raman scattering and infrared absorption spectroscopy. First-principles calculations have been carried out with the LDA+U approach to account for strong electron correlation in the copper oxides. The vibrational properties have been computed ab initio using the so-called direct method. Excellent agreement is found between the measured Raman and infrared peak positions and the calculated phonon frequencies at the Brillouin zone center, which allows the assignment of all prominent peaks of the Cu4O3 spectra. Through a detailed analysis of the displacement eigenvectors, it is shown that a close relationship exists between the Raman modes of CuO and Cu4O3.

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Vibrational Properties of CuO and Cu4O3 from First-Principles Calculations, and Raman and Infrared Spectroscopy

We use first-principle calculations to investigate the electronic structure of InSe and In2Se3. The interlayer binding energy is found to be in the same range as for other 2D systems, and the monolayers are found to be dynamically stable, which suggest the possibility to obtain them as isolated layers. The GW approximation including spin-orbit is used to obtain the bandgaps, which are in the range relevant for application in electronics. Also, it is shown that an electric field perpendicular to the layers can induce a semiconductor to metal transition in this family of compounds.

Two-Dimensional Indium Selenides Compounds: An Ab Initio Study.

By means of first-principles GW calculations, we have studied the electronic structure properties of MX2 (M = Mo, W; X = S, Se, Te) bilayers, including hybrid structures of MX2 building blocks. The effect of spin-orbit coupling on the electronic structure and the effect of van der Waals interaction on the geometry were taken into account. All the homogeneous bilayers are identified as indirect band-gap materials, with an increase of the band gap when Mo is changed to W, and a decrease of the band gap when the atomic number of X is increased. The same behavior is also observed for hybrid bilayers with common chalcogen atoms, while bilayers with common metal atoms have a direct band gap. Finally, it is shown that due to their particular band alignment, some heterobilayers enable electron-hole separation, which is of interest for solar cell applications.

Electronic structure of two-dimensional transition metal dichalcogenide bilayers from ab initio theory

New light is shed on the previously known perovskite material, Cs2 Au2 I6 , as a potential active material for high-efficiency thin-film Pb-free photovoltaic cells. First-principles calculations demonstrate that Cs2 Au2 I6 has an optimal band gap that is close to the Shockley-Queisser value. The band gap size is governed by intermediate band formation. Charge disproportionation on Au makes Cs2 Au2 I6 a double-perovskite material, although it is stoichiometrically a single perovskite. In contrast to most previously discussed double perovskites, Cs2 Au2 I6 has a direct-band-gap feature, and optical simulation predicts that a very thin layer of active material is sufficient to achieve a high photoconversion efficiency using a polycrystalline film layer. The already confirmed synthesizability of this material, coupled with the state-of-the-art multiscale simulations connecting from the material to the device, strongly suggests that Cs2 Au2 I6 will serve as the active material in highly efficient, nontoxic, and thin-film perovskite solar cells in the very near future.

Mixed Valence Perovskite Cs2Au2I6: A Potential Material for Thin-Film Pb-Free Photovoltaic Cells with Ultrahigh Efficiency

We report the observation of coherent lattice vibrations in mono- and few-layer WSe2 in the time domain, which were obtained by performing time-resolved transmission measurements. Upon the excitation of ultrashort pulses with the energy resonant to that of A excitons, coherent oscillations of the A1g optical phonon and longitudinal acoustic phonon at the M point of the Brillouin zone (LA(M)) were impulsively generated in monolayer WSe2. In multilayer WSe2 flakes, the interlayer breathing mode (B1) is found to be sensitive to the number of layers, demonstrating its usefulness in characterizing layered transition metal dichalcogenide materials. On the basis of temperature-dependent measurements, we find that the A1g optical phonon mode decays into two acoustic phonons through the anharmonic decay process.

L. Debbichi

Papers

Vibrational Properties of CuO and Cu4O3 from First-Principles Calculations, and Raman and Infrared Spectroscopy

Two-Dimensional Indium Selenides Compounds: An Ab Initio Study.

Electronic structure of two-dimensional transition metal dichalcogenide bilayers from ab initio theory

Mixed Valence Perovskite Cs2Au2I6: A Potential Material for Thin-Film Pb-Free Photovoltaic Cells with Ultrahigh Efficiency

Coherent Lattice Vibrations in Mono- and Few-Layer WSe2.