Liujiang Zhou

New Family of Quantum Spin Hall Insulators in Two-dimensional Transition-Metal Halide with Large Nontrivial Band Gaps.

Topological insulators (TIs) are promising for achieving dissipationless transport devices due to the robust gapless states inside the insulating bulk gap. However, currently realized two-dimensional (2D) TIs, quantum spin Hall (QSH) insulators, suffer from ultrahigh vacuum and extremely low temperature. Thus, seeking for desirable QSH insulators with high feasibility of experimental preparation and large nontrivial gap is of great importance for wide applications in spintronics. On the basis of the first-principles calculations, we predict a novel family of 2D QSH insulators in transition-metal halide MX (M = Zr, Hf; X = Cl, Br, and I) monolayers, especially, which is the first case based on transition-metal halide-based QSH insulators. MX family has the large nontrivial gaps of 0.12-0.4 eV, comparable with bismuth (111) bilayer (0.2 eV), stanene (0.3 eV), and larger than ZrTe5 (0.1 eV) monolayers and graphene-based sandwiched heterstructures (30-70 meV). Their corresponding 3D bulk materials are weak topological insulators from stacking QSH layers, and some of bulk compounds have already been synthesized in experiment. The mechanism for 2D QSH effect in this system originates from a novel d-d band inversion, significantly different from conventional band inversion between s-p, p-p, or d-p orbitals. The realization of pure layered MX monolayers may be prepared by exfoliation from their 3D bulk phases, thus holding great promise for nanoscale device applications and stimulating further efforts on transition metal-based QSH materials.

Predicting Single-Layer Technetium Dichalcogenides (TcX2, X = S, Se) with Promising Applications in Photovoltaics and Photocatalysis

One of the least known compounds among transition metal dichalcogenides (TMDCs) is the layered triclinic technetium dichalcogenides (TcX2, X = S, Se). In this work, we systematically study the structural, mechanical, electronic, and optical properties of TcS2 and TcSe2 monolayers based on density functional theory (DFT). We find that TcS2 and TcSe2 can be easily exfoliated in a monolayer form because their formation and cleavage energy are analogous to those of other experimentally realized TMDCs monolayer. By using a hybrid DFT functional, the TcS2 and TcSe2 monolayers are calculated to be indirect semiconductors with band gaps of 1.91 and 1.69 eV, respectively. However, bilayer TcS2 exhibits direct-bandgap character, and both TcS2 and TcSe2 monolayers can be tuned from semiconductor to metal under effective tensile/compressive strains. Calculations of visible light absorption indicate that 2D TcS2 and TcSe2 generally possess better capability of harvesting sunlight compared to single-layer MoS2 and ReSe2, implying their potential as excellent light-absorbers. Most interestingly, we have discovered that the TcSe2 monolayer is an excellent photocatalyst for splitting water into hydrogen due to the perfect fit of band edge positions with respect to the water reduction and oxidation potentials. Our predictions expand the two-dimensional (2D) family of TMDCs, and the remarkable electronic/optical properties of monolayer TcS2 and TcSe2 will place them among the most promising 2D TMDCs for renewable energy application in the future.

Computational Dissection of Two-Dimensional Rectangular Titanium Mononitride TiN: Auxetics and Promises for Photocatalysis

Recently, two-dimensional (2D) transition-metal nitrides have triggered an enormous interest for their tunable mechanical, optoelectronic, and magnetic properties, significantly enriching the family of 2D materials. Here, by using a broad range of first-principles calculations, we report a systematic study of 2D rectangular materials of titanium mononitride (TiN), exhibiting high energetic and thermal stability due to in-plane d-p orbital hybridization and synergetic out-of-plane electronic delocalization. The rectangular TiN monolayer also possesses enhanced auxeticity and ferroelasticity with an alternating order of Possions Ratios, stemming from the competitive interactions of intra- and inter- Ti-N chains. Such TiN nanosystem is a n-type metallic conductor with specific tunable pseudogaps. Halogenation of TiN monolayer downshifts the Fermi level, achieving the optical energy gap up to 1.85 eV for TiNCl(Br) sheet. Overall, observed electronic features suggest that the two materials are potential photocatalysts for water splitting application. These results extend emerging phenomena in a rich family 2D transition-metal-based materials and hint for a new platform for the next-generation functional nanomaterials.

Tetragonal bismuth bilayer: a stable and robust quantum spin hall insulator

Topological insulators (TIs) exhibit novel physics with great promise for new devices, but considerable challenges remain to identify TIs with high structural stability and large nontrivial band gap suitable for practical applications. Here we predict by first-principles calculations a two-dimensional (2D) TI, also known as a quantum spin Hall (QSH) insulator, in a tetragonal bismuth bilayer (TB-Bi) structure that is dynamically and thermally stable based on phonon calculations and finite-temperature molecular dynamics simulations. Density functional theory and tight-binding calculations reveal a band inversion among the Bi-p orbits driven by the strong intrinsic spin-orbit coupling, producing a large nontrivial band gap, which can be effectively tuned by moderate strains. The helical gapless edge states exhibit a linear dispersion with a high Fermi velocity comparable to that of graphene, and the QSH phase remains robust on a NaCl substrate. These remarkable properties place TB-Bi among the most promising 2D TIs for high-speed spintronic devices, and the present results provide insights into the intriguing QSH phenomenon in this new Bi structure and offer guidance for its implementation in potential applications.

Novel Excitonic Solar Cells in Phosphorene–TiO2 Heterostructures with Extraordinary Charge Separation Efficiency

Constructing van der Waals heterostructures is an efficient approach to modulate the electronic structure, to advance the charge separation efficiency, and thus to optimize the optoelectronic property. Here, we theoretically investigated the phosphorene interfaced with TiO2(110) surface (1L-BP/TiO2) with a type-II band alignment, showing enhanced photoactivity. The 1L-BP/TiO2 excitonic solar cell (XSC) based on the 1L-BP/TiO2 exhibits large built-in potential and high power conversion efficiency (PCE), dozens of times higher than conventional solar cells, comparable to MoS2/WS2 XSC. The nonadiabatic molecular dynamics simulation shows the ultrafast electron transfer time of 6.1 fs, and slow electron-hole recombination of 0.58 ps, yielding >98% internal quantum efficiency for charge separation, further guaranteeing the practical PCE. Moreover, doping in phosphorene has a tunability on built-in potential, charge transfer, light absorbance, as well as electron dynamics, which greatly helps to optimize the optoelectronic efficiency of a XSC.∗To whom correspondence should be addressed †Bremen Center for Computational Materials Science, University of Bremen, Am Falturm 1, 28359 Bremen, Germany ‡These authors contributed equally to this work ¶Beijing National Laboratory for Condensed Matter Physics, and Institute of Physics, Chinese Academy of Sciences, Beijing, 100190, P. R. China §Department of Materials Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, China ‖School of Chemistry, Physics and Mechanical Engineering Faculty, Queensland University of Technology, Garden Point Campus, QLD 4001, Brisbane, Australia ⊥Collaborative Innovation Center of Quantum Matter, Beijing 100190, China 1 ar X iv :1 51 2. 01 67 5v 1 [ co nd -m at .m tr lsc i] 5 D ec 2 01 5 Phosphorene, a new elemental two dimensional (2D) material recently isolated by mechanical exfoliation, holds the feature of a direct band gap of around 2.0 eV, overcoming graphene’s weaknesses (zero band gap) to realize the potential application in optoelectronic devices. Constructing van der Waals heterostructures is an efficient approach to modulate the band structure, to advance the charge separation efficiency, and thus to optimize the optoelectronic properties. Here, we theoretically investigated three type-II heterostructures based on perfect phosphorene and its doped monolayers interfaced with TiO2(110) surface. Doping in phosphorene has a tunability on built-in potential, charge transfer, light absorbance, as well as electron dynamics, which helps to optimize the light absorption efficiency. Three excitonic solar cells (XSCs) based on the phosphorene−TiO2 heterojunctions have been proposed, which exhibit high power conversion efficiencies dozens of times higher than conventional solar cells, comparable to MoS2/WS2 XSC. The nonadiabatic molecular dynamics within the time-dependent density functional theory framework shows ultrafast electron transfer time of 6.1−10.8 fs, and slow electron−hole recombination of 0.58−1.08 ps, yielding > 98% quantum efficiency for charge separation, further guaranteeing the practical power conversion efficiencies in XSC.

Doped graphenes as anodes with large capacity for lithium-ion batteries

To understand the chemical doping effect on the lithium (Li) storage of graphene, we have performed first-principles calculations to study the adsorption and diffusion of Li adatoms on boron (B)- and nitrogen (N)-doped graphenes, which include individual and paired B (and N) dopants in graphene. Our results show that the adsorption of a Li adatom on B-doped graphene is exothermic, in contrast to a more endothermic adsorption on graphitic N doped graphene. Particularly on the meta B–B pair doped graphene the adsorption energy of a Li adatom is lowered by about 1.84 eV with respect to that on the un-doped graphene. On B-doped graphene, Li adatoms would tend to migrate toward the B-doped region and unfavorably migrate outward. Based on the stable structures of multiple Li adatoms on graphene, which are identified by globally searching with the assistance of the particle swarm optimization (PSO) algorithm, we have analysed the Li adsorption and the electronic structures of graphene substrates via the states-filling model. The B-doped graphenes would achieve a maximum Li capacity of Li6.84B2C70, demonstrating a significant enhancement of Li capacity for graphene as the anode material of LIBs.

Prediction of the quantum spin Hall effect in monolayers of transition-metal carbides MC (M. =. Ti, Zr, Hf)

We report the existence of the quantum spin Hall effect (QSHE) in monolayers of transition-metal carbides MC (M = Zr, Hf). Under ambient conditions, the ZrC monolayer exhibits QSHE with an energy gap of 54 meV, in which topological helical edge states exist. Enhanced d xy −d xy interaction induces band inversion, resulting in nontrivial topological features. By applying in-plane strain, the HfC monolayer can be tuned from a trivial insulator to a quantum spin Hall insulator with an energy gap of 170 meV, three times that of the ZrC monolayer. The strong stability of MC monolayers provides a new platform for QSHE and spintronic applications.

Two-dimensional rectangular tantalum carbide halides TaCX (X = Cl, Br, I): novel large-gap quantum spin Hall insulators

Quantum spin Hall (QSH) insulates exist in special two-dimensional (2D) semiconductors, possessing the quantized spin-Hall conductance that are topologically protected from backscattering. Based on the first-principles calculations, we predict a novel family of QSH insulators in 2D tantalum carbide halides TaCX (X = Cl, Br, and I) with unique rectangular lattice and large direct energy gaps. The mechanism for 2D QSH effect originates from an intrinsic d−d band inversion in the process of chemical bonding. Further, stain and intrinsic electric field can be used to tune the electronic structure and enhance the energy gap. TaCX nanoribbon, which has the single-Dirac-cone edge states crossing the bulk band gap, exhibits a linear dispersion with a high Fermi velocity comparable to that of graphene. These 2D materials with considerable nontrivial gaps promise great application potential in the new generation of dissipationless electronics and spintronics.

High-mobility anisotropic transport in few-layer γ-B28 films

Recent reports of successful synthesis of atomically thin boron films have raised great prospects of discovering novel electronic and transport properties in a new type of 2D materials. Here we show by first-principles calculations that monolayer and bilayer γ-B28 films are intrinsically metallic while the thicker films possess intriguing electronic states that exhibit moderate to large bandgaps in all the interior layers but are nearly gapless at the surface. Remarkably, these surface electronic states are tunable by strain, allowing the outermost layer to transition between a semimetal and a narrow-gap semiconductor. Moreover, these surface states almost exclusively occupy a wide energy range around the Fermi level, thus dominating the electronic transport in γ-B28 films. The dispersions of the surface electronic bands are direction sensitive, and with hole injection producing anisotropic and very high carrier mobility up to 104 cm2 V-1 s-1. Surprisingly, surface passivation can open a sizable bandgap, which offers an additional avenue for effective band engineering and explains the experimental observation of a large bandgap in the synthesized film. These results make few-layer γ-B28 films desirable candidate materials for catalysis and electronics applications.

Two-dimensional hexagonal M3C2 (M = Zn, Cd and Hg) monolayers: novel quantum spin Hall insulators and Dirac cone materials

The intriguing Dirac cones in honeycomb graphene have motivated the search for novel two-dimensional (2D) Dirac materials. Based on density functional theory and the global particle-swarm optimization method, herein, we predict a new family of 2D materials in honeycomb transition-metal carbides M3C2 (M = Zn, Cd and Hg) with intrinsic Dirac cones. The M3C2 monolayer is a kinetically stable state with a linear geometry (CMC), which to date has not been observed in other transition-metal-based 2D materials. The intrinsic Dirac cones in the Zn3C2, Cd3C2 and Hg3C2 monolayers arise from p–d band hybridizations. Importantly, the Hg3C2 monolayer is a room-temperature 2D topological insulator with a sizable energy gap of 44.3 meV. When an external strain is applied, additional phases with node-line semimetal states emerge in the M3C2 monolayer. These novel stable transition-metal–carbon-framework materials hold great promise for 2D electronic device applications.