Tunable valley polarization effect and second-order topological state in monolayer FeClSH.

In two-dimensional (2D) materials, breaking the inversion symmetry plays an important role in valleytronics. Ferrovalley (FV) materials can achieve spontaneous valley polarization (VP) without additional modulation due to the magnetic exchange interaction and strong spin-orbit coupling. Using first-principles calculations, we predict a new 2D material, Janus FeClSH, which exhibits a large spontaneous VP. This monolayer is a perfect FV material, where the valence band maximum and conduction band minimum are located at the K/K' point. A large VP of 102.95 meV is spontaneously generated for the case of out-of-plane magnetization. Additionally, we propose that the irradiating circularly polarized light can be used to realize VP for the case of in-plane magnetization. Remarkably, a triangular nanoflake of FeClSH with armchair edges can show nontrivial corner states, exhibiting a second-order topological insulator (SOTI) state. The VP effect and SOTI state are tunable with the Hubbard U parameter, making the FeClSH monolayer promising for the study of the coupling between VP and SOTI.

Electronic Structures of Penta-SiC2 and g-SiC3 Nanoribbons: A First-Principles Study.

The dimensions of nanoribbons have a significant impact on their material properties. In the fields of optoelectronics and spintronics, one-dimensional nanoribbons exhibit distinct advantages due to their low-dimensional and quantum restrictions. Novel structures can be formed by combining silicon and carbon at different stoichiometric ratios. Using density functional theory, we thoroughly explored the electronic structure properties of two kinds of silicon-carbon nanoribbons (penta-SiC2 and g-SiC3 nanoribbons) with different widths and edge conditions. Our study reveals that the electronic properties of penta-SiC2 and g-SiC3 nanoribbons are closely related to their width and orientation. Specifically, one type of penta-SiC2 nanoribbons exhibits antiferromagnetic semiconductor characteristics, two types of penta-SiC2 nanoribbons have moderate band gaps, and the band gap of armchair g-SiC3 nanoribbons oscillates in three dimensions with the width of the nanoribbon. Notably, zigzag g-SiC3 nanoribbons exhibit excellent conductivity, high theoretical capacity (1421 mA h g-1), moderate open circuit voltage (0.27 V), and low diffusion barriers (0.09 eV), making them a promising candidate for high storage capacity electrode material in lithium-ion batteries. Our analysis provides a theoretical basis for exploring the potential of these nanoribbons in electronic and optoelectronic devices as well as high-performance batteries.

High Interfacial Thermal Stability of Flexible Flake-Structured Aluminum Thin-Film Electrodes for Bi2Te3-Based Thermoelectric Devices.

Environmental thermal energy harvesting based on thermoelectric devices is greatly significant to the advancement of next-generation self-powered wearable electronic devices. However, the rigid electrodes and interface diffusion of electrodes/thermoelectric materials would lead to the wearable discomfort and performance degradation of the thermoelectric device. Here, a flake-structured Al thin-film electrode with high conductivity and excellent reliability is prepared by regulating the microstructure and crystallinity of the films. The as-prepared Al thin film not only maintains its robustness after 1000 bending cycles but also does not delaminate from the substrate when subjected to the 3M tape test, exhibiting excellent flexibility and adhesion to substrate. By comparing with the annealed interface of the double-layer Cu/Bi2Te3 film, the interface of the heat-treated Al/Bi2Te3 film has almost no element diffusion, demonstrating high interfacial thermal stability. Moreover, a thermoelectric temperature sensor based on the Al thin-film electrode is prepared. The sensitivity of the annealed sensor is still linear, and it can stably monitor the temperature variation, showing high reliability. This discovery could provide a facile and effective strategy to achieving highly reliable thermoelectric devices and flexible electronic devices without any additional diffusion barriers.

Construction of organic-inorganic "chelate" adsorption sites on metal oxide semiconductor for room temperature NO2 sensing.

Metal oxide semiconductors (MOS) have been extensively studied for gas sensing due to their excellent chemical stability and adjustable electronic properties. However, there is still a lack of ingenious design strategies to achieve customizable gas detection in complex environments. Herein, a novel and scalable strategy of constructing organic-inorganic "chelate" adsorption sites is proposed to promote the affinity of MOS sensing materials to target molecules. Specifically, 3-aminopropyltriethoxysilane (APTES)-functionalized reduced graphene oxide (rGO) was decorated on In2O3 tubes (AG/Inx), and its NO2 sensing performance was studied. As a result, the optimal AG/Inx shows boosted room-temperature NO2 response, and its response to 1 ppm NO2 is 4.8 times that of In2O3. More attractively, the optimal AG/Inx exhibits good selectivity, as well as outstanding detection ability (Rg/Ra = 1.6) for low concentration NO2 (20 ppb). Experimental results suggest that APTES-rGO not only acts as the electron acceptor to accelerate charge transfer, but also enhances NO2 adsorption. Further theoretical calculations reveal that NO2 is simultaneously adsorbed at rGO and APTES via a flexible "chelate" mechanism. The multidentate adsorption configuration remarkably strengthens the NO2-host interaction, which is conducive to improving sensing performance. This work may inspire the material design of a new generation high-performance gas sensors.

Enormous-sulfur-content cathode and excellent electrochemical performance of Li-S battery accouched by surface engineering of Ni-doped WS2@rGO nanohybrid as a modified separator.

The poor conductivity of sulfur, the lithium polysulfide's shuttle effect, and the lithium dendrite problem still impede the practical application of lithium-sulfur (Li-S) batteries. In this work, the ultrathin nickel-doped tungsten sulfide anchored on reduced graphene oxide (Ni-WS2@rGO) is developed as a new modified separator in the Li-S battery. The surface engineering of Ni-WS2@rGO could enhance the cell conductivity and afford abundant chemical anchoring sites for lithium polysulfides (LiPSs) adsorption, which is convinced by the high adsorption energy and the elongate SS bond given using density-functional theory (DFT) calculation. Concurrently, the Ni-WS2@rGO as a modified separator could effectively catalyze the conversion of LiPSs during the charging/discharging process. The Li-S cell with Ni-WS2@rGO modified separator achieves a high initial capacity of 1160.8 mA h g-1 at the current density of 0.2C with a high-sulfur-content cathode up to 80 wt%, and a retained capacity of 450.7 mA h g-1 over 500 cycles at 1C, showing an efficient preventing polysulfides shuttle to the anode while having no influence on Li+ ion transference across the decorating separator. The strategy adopted in this work would afford an effective pathway to construct an advanced functional separator for practical high-energy-density Li-S batteries.

Two-dimensional XC6-enes (X = Ge, Sn, Pb) with moderate band gaps, biaxial negative Poisson's ratios, and high carrier mobility.

Graphene-based analogs and derivatives provide numerous routes to achieve unconventional properties and potential applications. Particularly, two-dimensional (2D) binary materials of group-IV elements are drawing increasing interest. In this work, we proposed the design of three 2D graphene-based materials, namely, XC6-enes (X = Ge, Sn, or Pb), based on first-principles calculations. These new materials possess intriguing properties superior to graphene, such as biaxial negative Poisson's ratio (NPR), moderate bandgap, and high carrier mobility. These XC6-enes comprise sp2 carbon and sp3 X (X = Ge, Sn, Pb) atoms with hexagonal and pentagonal units by doping graphene with X atoms. The stability and plausibility of these 2D materials are verified from formation energies, phonon spectra, ab initio molecular dynamic simulations, and elastic constants. The incorporation of X atoms leads to highly anisotropic mechanical properties along with NPR due to the unique tetrahedral structure and hat-shaped configuration. In the equilibrium state, all the XC6-enes are moderate-band-gap semiconductors. The carrier mobilities of the XC6-enes were highly anisotropic (∼104 cm-2 V-1 s-1 along the [010]-direction). Such outstanding properties make the 2D frameworks promising for application in novel electronic and micromechanical devices.

Metallic 2H-Tantalum Selenide Nanomaterials as Saturable Absorber for Dual-Wavelength Q-Switched Fiber Laser.

A novel 2H-phase transition metal dichalcogenide (TMD)-tantalum selenide (TaSe2) with metallic bandgap structure is a potential photoelectric material. A band structure simulation of TaSe2 via ab initio method indicated its metallic property. An effective multilayered TaSe2 saturable absorber (SA) was fabricated using liquid-phase exfoliation and optically driven deposition. The prepared 2H-TaSe2 SA was successfully used for a dual-wavelength Q-switched fiber laser with the minimum pulse width of 2.95 µs and the maximum peak power of 64 W. The repetition rate of the maximum pulse energy of 89.9 kHz was at the level of 188.9 nJ. The metallic 2H-TaSe2 with satisfactory saturable absorbing capability is a promising candidate for pulsed laser applications.

Tunable electron property induced by B-doping in g-C3N4.

Graphitic carbon nitrides are a research hotspot of two-dimensional (2D) materials, which attract more and more attention from researchers. Topological properties are a focus in graphitic carbon nitrides materials. Using first-principles calculations, we modified the g-C3N4 (formed by tri-s-triazine) by B atoms, proposing a novel two-dimensional monolayer, g-C6N7B, which showed excellent stability verified by positive phono modes, molecular dynamic simulations and mechanical criteria. The valence band and conduction band touch at the Γ point. Interestingly, g-C6N7B is topologically nontrivial, because the valance and conduction band can be gapped by the spin-orbit coupling (SOC) effect associated with robust gapless edge states. Additionally, molecular dynamic simulations indicate that g-C6N7B will still maintain good geometry structure when the temperature is as high as 1500 K. The flexibility of g-C6N7B is confirmed by its elastic constants and Young's moduli. This work opens an avenue for graphitic carbon nitride materials with topological properties.

Approaching high-performance of ordered structure Sb2Te3 film via unique angular intraplanar grain boundaries.

In this paper, we present an innovative electric-field-assisted magnetron-sputtering deposition method for films preparation. By grain boundary-engineering, we successeful obtained the ordered Sb2Te3 film with greatly high figure of merit via controlling external electric field. It has been found that the electric field can induce the change in the angle of intraplanar grain boundaries between (0 1 5) and (0 1 5) planes, which leads to the enhanced holes mobility and maintained low thermal conductivity. The energy filtering takes place at the angular intraplanar grain boundaries. At room temperature, a high ZT value of 1.75 can be achieved in the deposited Sb2Te3 film under 30 V external electric field. This is a very promising approach that the electric field induced deposition can develop high-performance Sb2Te3-based thermoelectric films.

Two-Dimensional Square-A2B (A = Cu, Ag, Au, and B = S, Se): Auxetic Semiconductors with High Carrier Mobilities and Unusually Low Lattice Thermal Conductivities.

Using evolutionary structure search combined with ab initio theory, we investigate the electronic, thermal, and mechanical properties of two-dimensional (2D) A2B (A = Cu, Ag, Au, and B = S, Se) auxetic semiconductors. Two types of structures are found to have low energy, namely, s(I/II)-A2B, which have direct bandgaps in the range 1.09-2.60 eV and high electron mobilities. Among these semiconductors, Cu2B and Ag2B have light holes with 2 orders of magnitude larger mobility than the heavy holes, up to 9.51 × 104 cm2 V-1 s-1, giving the possibility of achieving highly anisotropic hole transport with the application of a uniaxial strain. Due to the ionic bonding nature, s-A2B structures have unusually low lattice thermal conductivities down to 1.5 W m-1 K-1 at 300 K, which are quite promising for new generation thermoelectric devices. Besides, s-A2B structures show extraordinary flexibility with ultralow Young's moduli (down to 20 N/m), which are lower than most previously reported 2D materials. Moreover, under strain along the diagonal direction, five of the structures have in-plane negative Poisson's ratios.

Synthesis of 1-(ß-coumarinyl)-1-(ß-indolyl)trifluoroethanols through regioselective Friedel-Crafts alkylation of indoles with ß-(trifluoroacetyl)coumarins catalyzed by Sc(OTf)3.

A highly efficient Friedel-Crafts alkylation of indole derivatives with ß-(trifluoroacetyl)coumarins using Sc(OTf)3 as a catalyst has been developed, which gives regioselective 1,2-adducts to afford 1-(ß-coumarinyl)-1-(ß-indolyl)trifluoroethanols. A series of tertiary trifluoroethanols containing different indole and coumarin groups were synthesized in moderate to excellent yields (up to 95%) in the presence of 5 mol% catalyst in a short time (only 2 minutes at least). A mechanism of the reaction, in which the trace amount of water plays the role of proton transfer in catalyzing circulation was proposed and confirmed.

Promotion of Overall Water Splitting Activity Over a Wide pH Range by Interfacial Electrical Effects of Metallic NiCo-nitrides Nanoparticle/NiCo2O4 Nanoflake/graphite Fibers.

Many efforts have been made to develop bifunctional electrocatalysts to facilitate overall water splitting. Here, a fibrous bifunctional 3D electrocatalyst is reported for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) with high performance. The remarkable electrochemical performance is attributed of the catalysts to a number of factors: the metallic character of the three components (i.e., Ni3N, CoN, and NiCo2O4); the electronic structure, nanoflake-nanosphere network with abundant electroactive sites, and the electric field effect at the interfaces between different components. The oxide-nitride/graphite fibers have the lowest overpotential requirements of 71 and 183 mV at 10 mA cm-2 for HER and OER in alkaline medium, respectively. These values are comparable to those of commercial Pt/C (20 wt%) and RuO2. The electrodes also show a response to HER and OER in both neutral and acid media. Furthermore, the 3D structure can be highlighted by all-round electrodes for overall water splitting. The calculations on the changes in electrons transfer and the Femi level from oxides to oxides/nitrides reveal that the observed superb electrocatalytic performance can be attributed to the presence of Ni3N and CoN derived from the in situ nitridation of NiCo2O4.

Valley-selective circular dichroism and high carrier mobility of graphene-like BC6N.

The two-dimensional (2D) hybrid structures of boron nitride (BN) and graphene with properties superior to the individuals are long desired. In this work, we demonstrate theoretically that this goal can be reached in a new graphene-like borocarbonitride (g-BC6N) whose domain has been synthesized in recent experiments. It has a direct band gap of 1.833 eV and a high carrier mobility comparable to that of black phosphorene. The inversion symmetry breaking in g-BC6N leads to a pair of inequivalent valleys with opposite Berry curvatures in the vicinities of the vertices (K and K') of the Brillouin zone. The coexistence of valley-selective circular dichroism and high carrier mobility in g-BNC6 is beneficial to realize the valley Hall effect. We also propose a tight-binding (TB) model to describe the intrinsic features of this type of lattice, revealing a new class of 2D valleytronic materials.

Silicene and germanene on InSe substrates: structures and tunable electronic properties.

Using first-principles calculations, we show that the recently synthesized two-dimensional (2D) van der Waals layered material indium selenide (InSe) nanosheets can serve as a suitable substrate for silicene and germanene, which form commensurate and stable silicene/InSe (Si/InSe) and germanene/InSe (Ge/InSe) heterolayers (HLs). The buckled honeycomb geometries and Dirac-cone-like band structures of silicene and germanene are well preserved in these HLs. The interaction between silicene (or germanene) and the InSe substrate opens up a band gap of 141 meV (or 149 meV) at the Dirac points, while electron effective masses (EEM) remain as small as 0.059 and 0.067 times the free-electron mass (m0). The band gap and the EEM of the HLs can be further modulated effectively by applying an external electric field or strain. These features are attributed to the built-in electric field due to the interlayer charge transfer of the HLs which breaks the equivalence of the two sublattices of silicene and germanene. Multilayer (ML) InSe substrates have also been considered. We also proposed a parallel plate capacitor model to describe the interaction between silicene (or germanene) and the InSe substrate as well as the electronic band structure modification in response to an external field. This work is expected to offer an ideal substrate material for the growth of silicene and germanene and a promising van der Waals (vdW) layered heterostructure for electronic devices.

Novel Conductive Metal-Organic Framework for a High-Performance Lithium-Sulfur Battery Host: 2D Cu-Benzenehexathial (BHT).

Despite the high theoretical capacity of lithium-sulfur (Li-S) batteries, their commercialization is severely hindered by low cycle stability and low efficiency, stemming from the dissolution and diffusion of lithium polysulfides (LiPSs) in the electrolyte. In this study, we propose a novel two-dimensional conductive metal-organic framework, namely, Cu-benzenehexathial (BHT), as a promising sulfur host material for high-performance Li-S batteries. The conductivity of Cu-BHT eliminates the insulating nature of most S-based electrodes. The dissolution of LiPSs into the electrolyte is largely prevented by the strong interaction between Cu-BHT and LiPSs. In addition, orientated deposition of Li2S on Cu-BHT facilitates the kinetics of the LiPS redox reaction. Therefore, the use of Cu-BHT for Li-S battery cathodes is expected to suppress the LiPS shuttle effect and to improve the overall performance, which is ideal for practical application of Li-S batteries.

Promising half-metallicity in ductile NbF3: a first-principles prediction.

Materials with half-metallicity are long desired in spintronics. Using first-principles calculations, we predicted that the already-synthesized NbF3 crystal is a promising half-metal with a large exchange splitting and stable ferromagnetism. The mechanical stability, ductility and softness of the NbF3 crystal were confirmed by its elastic constants and moduli. The Curie temperature (TC = 120 K) estimated from the Monte Carlo simulations based on the 3D Ising model is above the liquid nitrogen temperature (78 K). The ferromagnetism and half-metallicity can be preserved on the surfaces of NbF3. The NbOF2 formed by substituting F with O atoms, however, has an antiferromagnetic ground state and a normal metallic band structure. This work opens an avenue for half-metallic materials and may find applications in spintronic devices.

Negative Poisson's ratio and high-mobility transport anisotropy in SiC6 siligraphene.

The diverse forms of silicon carbides lead to versatile properties, but an auxetic allotrope at zero pressure has never been reported. Here, using first-principles calculations we propose a two-dimensional (2D) auxetic silicon carbide material, namely SiC6 siligraphene. The plausibility of the SiC6 siligraphene is verified by the low formation energy, positive phonon spectrum and high mechanical stability. The unique framework of sp2 carbon and sp3 silicon atoms leads to unusual in-plane negative Poisson's ratios and electronic properties superior to both graphene and silicene. SiC6 siligraphene possesses a natural band gap of 0.73 eV and a high carrier mobility. The theoretical mobility in the order of 104 cm2 V-1 s-1 for electrons along the [1[combining macron]10] direction is comparable to the hole mobility in black phosphorene, whereas the hole transport along the [110] direction is blocked. Both the electronic band structure and carrier mobility of the SiC6 siligraphene can be tuned by applying external strain. A possible synthetic route is also proposed. The exotic properties make SiC6 siligraphene a versatile and promising 2D material for applications in nanomechanics and nanoelectronics.

Zr2Si: an antiferromagnetic Dirac MXene.

MXenes, which constitute a kind of graphene-like material, have been intensively investigated due to their applications in future nanoelectronics technology. These MXenes are either metallic or semiconducting, whereas Dirac cones similar to graphene have rarely been reported. Using first-principles calculations, we proposed a new MXene, namely Zr2Si, whose antiferromagnetic (AFM) ground state exhibited in these calculations anisotropic Dirac cones with Fermi velocities comparable to that in graphene. The Dirac spectrum here was determined to arise mainly from the dx2-y2 and dz2 orbitals of Zr atoms. Additionally, the Dirac cones can be gapped when taking the spin-orbit coupling (SOC) and Coulomb repulsive interaction (U) into account, which opens an avenue for using the Zr2Si MXene for electronics applications.

Electronic structures of rutile (011)(2 × 1) surfaces: A many-body perturbation theory study.

Using the GW method within many-body perturbation theory, we investigate the electronic properties of the rutile (011) surfaces with different reconstruction patterns. We find that keeping the Ti:O ratio on the reconstructedsurface to 1:2 enlarges the bandgap of the rutile (011) surface to ca. 4.0 eV. Increasing the content of O atoms in the surface can turn rutile into a semi-metal. For some surfaces, it is important to apply self-consistent GW calculation to get the correct charge distributions for the frontier orbitals, which are relevant to the photocatalytic behavior of TiO2.

Strain-Modulated Electronic Structure and Infrared Light Adsorption in Palladium Diselenide Monolayer.

Two-dimensional (2D) transition-metal dichalcogenides (TMDs) exhibit intriguing properties for both fundamental research and potential application in fields ranging from electronic devices to catalysis. Based on first-principles calculations, we proposed a stable form of palladium diselenide (PdSe2) monolayer that can be synthesized by selenizing Pd(111) surface. It has a moderate band gap of about 1.10 eV, a small in-plane stiffness, and electron mobility larger than that of monolayer black phosphorus by more than one order. Additionally, tensile strain can modulate the band gap of PdSe2 monolayer and consequently enhance the infrared light adsorption ability. These interesting properties are quite promising for application in electronic and optoelectronic devices.