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Two-dimensional (2D) materials with excellent photocatalytic properties and unique piezoelectric response have attracted great attention. However, these characters are rare for traditional 2D structures. With an intrinsic electric field, the Janus 2D materials show great promise in photocatalytic and out-of-plane piezoelectric applications. Herein, we show that Janus In2X2X' (X and X' = S, Se, and Te) monolayers are desirable in the overall water splitting and piezoelectric devices. Comprehensive investigations reveal that the band gaps of these Janus monolayers are from 0.34 to 2.27 eV. With proper band edge positions, strong solar absorption, fast transfer and efficient separation of carriers, and high solar to hydrogen (STH) efficiencies (reaching 37.70%), eight members of them stand out. Besides, the electrons and holes have sufficient driving forces in the process of redox reaction. The piezoelectric response for in- and out-of-plane is superior for all monolayers. These compelling features make them suitable for photocatalysts, sensors, actuators, and energy conversion devices.

Recently, Janus two-dimensional (2D) materials have received considerable interest due to their intrinsic vertical dipole, and hence they have great potential in photocatalytic and piezoelectric applications. Here, a new series of Janus 2D structures MM′XX′ (M, M′ = Ga, In; X, X′ = S, Se, Te) are investigated by means of first-principles calculations. It is found that 2D Janus MM′XX′ exhibit high dynamical stability and have band gaps in the range of 0.89–2.03 eV. Most outstandingly, these MM′XX′ monolayers exhibit appropriate band edge positions, strong light absorption (1 × 104 cm−1) in the visible light region, high energy conversion efficiencies (up to 18.51%), effective spatial separation and fast transfer of carriers (at least 103 cm2 V−1 s−1), which make them promising candidates for photocatalytic water splitting (except InGaSTe which has a small band gap of 0.89 eV). What is more, the in-plane piezoelectric coefficients of these MM′XX′ monolayers (2.62–6.21 pm V−1) are comparable to those of the common bulk materials such as α-quartz (2.3 pm V−1), wurtzite GaN (3.1 pm V−1) and AlN (5.1 pm V−1), and the out-of-plane piezoelectric coefficients (0.28–0.41 pm V−1) are higher than those of the Janus MXY (M = Mo, W; X, Y = S, Se, Te) monolayers (0.007–0.030 pm V−1). Our findings reveal the potential applications of these monolayers as efficient photocatalysts and piezoelectric materials.

Two-dimensional (2D) Janus materials with large exciton binding energies have attracted enormous attention for their novel photoelectric properties. Here, the quasiparticle (QP) band structures and...

Two-dimensional (2D) materials with excellent photocatalytic properties and unique piezoelectric response have attracted great attention. However, these characters are rare for traditional 2D structures. With an intrinsic electric field, the Janus 2D materials show great promise in photocatalytic and out-of-plane piezoelectric applications. Herein, we show that Janus In

The van der Waals heterostructure (vdWH) has attracted widespread attention as a unique structure for future electronic and optoelectronic devices. In this paper, we constructed the ZnO-SeMoS and ZnO-SMoSe vdWHs and systematically investigated their electronic structures and band alignments considering vertical strain and external electric field effects. It is found that the ZnO-SeMoS and ZnO-SMoSe vdWHs both exhibit type-II band alignment with indirect band gaps of 1.31 and 0.63 eV respectively, depending on the interface characteristics. What's more, the band alignment of these two heterostructures can be tuned to type I or type III, and their band gap can be modified to direct feature by applying vertical strain and an electric field. The results reveal that ZnO/MoSSe vdWHs have promising potential in multi-functional nanodevices, and provide a way to modify the electronic properties of Janus-based heterojunctions using interface characteristics.

By using the transfer matrix method, we theoretically investigated Rashba electron's spin transport in a straight waveguide with a stub which has a smooth boundary. It is proved that the two spin states propagate independently. The conductance of the waveguide has been calculated. Conductance quantization is common in this structure when we change the Fermi energy and the width of the stub. If the shape of the stub converges toward the limit–a quadrate stub, the conductance will reduce distinctly due to reflection.

By using the transfer matrix method, we investigated spin transport in some straight structures in presence of the Rashba spin-orbit interaction. It is proved that the interference of two spin states is the same as that in one-dimensional Datta-Das spin field-effect transistor. The conductance of these structures has been calculated. Conductance quantization is common in these waveguides when we change the Fermi energy and the width of the waveguide. Using a periodic system of quadrate stubs and changing the Fermi energy, a nearly square-wave conductance can be obtained in some regions of the Fermi energy.

Doping is a key technology in modern semiconductor industry. With reduced size of semiconductor devices, quantum effect becomes significant, and the classic theory for device design will no longer be valid. Thus the traditional doping technology is facing a great challenge. On the other hand, the third generation wide-gap semiconductors like GaN, ZnO and TiO2 have great application potential in optoelectronics and microelectronics due to their wide band gap, direct gap character and good stability. However, there is a doping bottleneck on the p-type doping of these materials. In the field of application of semiconductor technology, doping mechanism in photocatalysis, spintronic materials, and novel two-dimensional (2D) semiconductors are also the frontier of research. The research in this project focuses on the above key topics. The primary discoveries include the follows: (i) Based on first-principles band structure calculations and analysis of the band-edge wave function characters, we proposed that passivated (Mo+C)-doped TiO2 is a strong candidate for hydrogen production through water splitting. The results received highly attention from the experts in the field of photocatalysis, and the feasibility was confirmed by recent experimental studies. (ii) We discovered that the presence of spontaneous magnetization in nitrides and oxides with sufficient holes is an intrinsic property of these first-row d0 semiconductors. The origin and enhancement of such hole-induced ferromagnetism is studied, which suggested new possibility of preparing non-magnetically doped spintronic semiconductor devices. (iii) We systematically studied the doping mechanism and bottleneck of donor and acceptor impurities in quantum dots and quantum wires. Due to the effect of quantum confinement effect, the energy of the conduction band minimum (CBM) increases whereas that of the valence and maximum (VBM) decreases. We found that defect formation energy and transition energy level increase when the size of the quantum dot (QD) decreases, leading to the self-purification effect and degrade the performance of devices. Focused on this problem, we proposed the approach of overcoming the doping bottleneck in nanostructures, and developed a model to describe the quantum Stark effect in hole impurity state of quantum dots. These works promoted the research in the application of nano-devices. (iv) For the first time, the band alignment between two-dimensional transition-metal dichalcogenides MX2 (M=Mo, W; X=S, Se, Te) is calculated. We predicted the type-II band alignment in MoX2/WX2 heterostructures. In addition, direct-indirect gap transition and semiconductor-metal transition in strained MoS2 is observed. Moreover, we studied the mechanical, optical properties and band gap modulation of graphyne, a new allotrope of carbon, and discovered that Ca-doped graphyne had high capacity of hydrogen storage. The project has attracted great attentions, and the related publications are highly cited by researchers all over the world. These works bring better understanding of semiconductor doping theory, and have important scientific value for the device design and property prediction of new generation semiconductors and nano-devices.

Magnetic tunneling junctions (MTJs) have atomic thickness due to the use of two-dimensional (2D) materials. Combining density functional theory with non-equilibrium Green's function formalism, we systematically investigate the structural and magnetic properties of CrX3/h-BN/CrX3 (X = Br, I) MTJs, as well as their spin-dependent transport characteristics. Through calculation of the transmission spectrum, the large tunneling magnetoresistance (TMR) effect was observed in these MTJs. Moreover, their conductance based on two-dimensional materials was greatly improved over that of traditional MTJs. The transmission mechanism was analyzed using the symmetry of the orbit and the eigenstates of the transmitted electrons. We also discuss the problem of Schottky contact between different metal electrodes and devices. Our results suggest that MTJs based on two-dimensional ferromagnets are feasible.

Magnetic tunnel junctions (MTJs) have attracted tremendous interests recently because of their potential application in magnetoresistive random access high-density memory and magnetic sensor. However, the performance of them is far from satisfying due to the various problems exist in tunnel barrier materials. Here, we propose to use two-dimensional (2D) Bi 2 O 2 Se material, which is advantageous in reducing the MTJ size, as the tunnel barrier, and demonstrate that it is able to generate very large tunnel magnetoresistance (TMR) when integrated with CoFe electrodes. The underlying mechanism is elaborated by analyzing the band structures, electron transmission and interface properties. These results provide important guidance for designing high-density and high-performance MTJs.

It is of great fundamental and practical interest to develop effective means of modulating the magnetic hystereses of magnetic materials and their heterostructures. A notable example is the exchange bias (EB) effect between an antiferromagnet or ferrimagnet and a ferromagnet, which has been widely employed to manipulate magnetic anisotropy in spintronic devices and artificial magnets. Here, we report the design, synthesis and characterization of a synthetic perpendicularly-magnetized ferrimagnet based on [Mn2.9Ga/Co2MnSi]n superlattices, which attains thermal stability above 400 K and a coercive field up to 45 kOe through a mechanism of magnetic compensation. The structure is incorporated into a prototype Heusler alloy and MgO barrier based magnetic tunnel junction, which demonstrates high dynamic range linear field responses and an unusual in-plane EB effect. With increasing temperature, the coercive field reaches beyond 70 kOe at 400 K in this device due to the increasing degree of magnetic moment compensation in the superlattice. The results demonstrate that the compensation mechanism can be utilized to achieve simultaneous thermal robustness and high coercivity in realistic spintronic devices.

Magnetic tunneling junctions (MTJs) have atomic thickness due to the use of two-dimensional (2D) materials. Combining density functional theory with non-equilibrium Green's function formalism, we systematically investigate the structural and magnetic properties of CrX

Because tetragonal structured MnGa alloy has intrinsic (not interface induced) giant perpendicular magnetic anisotropy (PMA), ultra-low damping constant and high spin polarization, it is predicted to be a kind of suitable magnetic electrode candidate in the perpendicular magnetic tunnel junction (p-MTJ) for high density spin transfer torque magnetic random access memory (STT-MRAM) applications. However, p-MTJs with both bottom and top MnGa electrodes have not been achieved yet, since high quality perpendicular magnetic MnGa films can hardly be obtained on the MgO barrier due to large lattice mismatch and surface energy difference between them. Here, a MnGa-based fully p-MTJ with the structure of MnGa/Co2MnSi/MgO/Co2MnSi/MnGa is investigated. As a result, the multilayer is with high crystalline quality, and both the top and bottom MnGa electrodes show well PMA. Meanwhile, a distinct tunneling magnetoresistance (TMR) ratio of 65% at 10 K is achieved. Ultrathin Co2MnSi films are used to optimize the interface quality between MnGa and MgO barrier. A strong antiferromagnetic coupling in MnGa/Co2MnSi bilayer is confirmed with the interfacial exchange coupling constant of −5erg/cm2. This work proposes a novel p-MTJ structure for the future STT-MRAM progress.

We have systematically investigated the electronic structures, magnetic properties and strain effects of quaternary Heusler compounds FeMnCrZ (Z = P, As, Sb, Bi, Se, Te) by using the self-consistent full-potential linearized-augmented plane-wave (FPLAPW) method with the generalized gradient approximation (GGA) and the local density approximation (LDA) + U methods based on the density functional theory (DFT). By analyzing the electronic structures of these six compounds within GGA, we have found that the FeMnCrAs, FeMnCrSb, and FeMnCrTe alloys show excellent half-metallic ferrimagnetism, and the FeMnCrP and FeMnCrBi compounds present near half-metallicity. By using the LDA + U method, we found that FeMnCrP, FeMnCrAs and FeMnCrTe are half-metals. The total spin magnetic moments for all the half-metals satisfy the Slater-Pauling 24 electron-rule, confirming the existence of the half-metallic gap. The estimated Curie temperatures of all compounds exceed the room temperature, making the half-metal compounds promising for spintronic applications. Finally, the effects of uniform and tetragonal strain have been investigated. The half-metallicity can be either enhanced, reduced or destroyed by a uniform strain. A ferrimagnetism to ferromagnetism phase transition can be induced by a uniform lattice expansion within the LDA + U method. The effects of the tetragonal distortion turn out to be negligible for these compounds.

Two-dimensional (2D) magnetic crystals have attracted great attention due to their emerging new physical phenomena. They provide ideal platforms to study the fundamental physics of magnetism in low dimensions. In this research, magnetic tunneling junctions (MTJs) based on XSe2 (X=Mn, V) with room-temperature ferromagnetism were studied using first-principles calculations. A large tunneling magnetoresistance (TMR) of 725.07% was obtained in the MTJs based on monolayer MnSe2. Several schemes were proposed to improve the TMR of these devices. Moreover, the results of our non-equilibrium transport calculations showed that the large TMR was maintained in these devices under a finite bias. The transmission spectrum was analyzed according to the orbital components and the electronic structure of the monolayer XSe2 (X=Mn, V). The results in this paper demonstrated that the MTJs based on a 2D ferromagnet with room-temperature ferromagnetism exhibited reliable performance. Therefore, such devices show the possibility for potential applications in spintronics.

ZnMg and NbCl5 were intercalated in graphite and the presence of such molecules between the graphene sheets results in n- and p-type doping, respectively. The doping effect is confirmed by Hall and Raman measurements and the intercalation process is monitored by scanning tunneling microscopy. After intercalation the carrier concentration increase almost an order of magnitude and reaches values as high as 10 19 and 10 18 cm � 3 for pand n-type doping, respectively. For higher intercalation times, the intercalated graphite turns back to be as ordered as pristine one as evidenced by the reduction in the D peak in Raman measurements. Intercalation compounds show remarkable stability allowing us to permanently tune the physical properties of few-layer graphite. Our study has provided a new route to produce stable and functional graphite intercalation compounds and the results can be applied to other graphitic structures such as few-layer graphene on SiC.

First-principles calculations have been performed to study Au-decorated silicene (Au/silicene) as a high-activity catalyst for CO oxidation. The high binding strength of the Au/silicene system and the high diffusion-energy barrier of Au adsorbates, as well as the assisted Coulomb repulsion effect, jointly prevent the formation of Au clusters. Au/silicene transfers many more electrons to O2 than to CO, thus facilitating CO oxidation first by the Langmuir–Hinshelwood (LH) mechanism (CO + O2 → OOCO → CO2 + O) and then by Eley–Rideal (ER) mechanism (CO + O → CO2). The two reaction processes have quite low catalytic energy barriers of 0.34 and 0.32 eV, respectively. The underlying mechanism of high catalytic oxidation of CO can be attributed to electronic-state hybridization among Au d orbitals and CO and O2 2π* antibonding states around the Fermi energy. These findings enrich the applications of Si-based materials to the high-activity catalytic field.

The electronic structures of self-assembled InAs 1− x N x /GaAs nanostructures from quantum lens to quantum rings (QRs) are calculated using the 10-band k.p method and the valence force field (VFF) method. With the variation of shape of the nanostructure and nitrogen (N) content, it shows that the N and the strains can significantly affect the energy levels especially the conduction band because the N resonant state has repulsion interaction with the conduction band due to the band anticrossing (BAC). The structures with N and greater height have smaller transition energy, and the structures with N have greater optical gain due to its overwhelming greater value of factor f c + f v − 1 . After analyzing the shape effect, we suggested that the nanostructures with volcano shape are preferred because the maximum optical gain occurs for quantum volcano. With our simulation result, researchers could select quantum dots (QDs) structures to design laser with better performance.

A method to calculate the band alignment between materials with different structures using passivated quantum dots is proposed. By using this method, the rutile/anatase titanium dioxide band offset is determined. The valence band maxima of anatase and rutile are close to each other, whereas the conduction band minimum of anatase is found to be about 0.2 eV higher than that of rutile, which is in agreement with the experimental fact that anatase has higher photocatalytic activity. The reliability of this method is further tested on several semiconductors, and reasonable results are obtained for most cases.

Using first-principles calculations, we investigated the electronic and magnetic properties of Mn-doped, Fe-doped, and Co-doped diamond. It is found that the Mn-, Fe-, and Co-doped diamond are stabilized in ferromagnetic configurations. The origins of the magnetic ordering are explained successfully by the phenomenological band coupling model based on the p–d and d–d level repulsions between the dopant ions and host elements. According to Heisenberg model, high Curie temperature may be expected for Mn-, Fe-, and Co-doped diamond if there are no native defects or other impurities.