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Transition metal carbides have attracted considerable attention and are widely used in machining tools, hard coatings and aerospace components, owing to their excellent mechanical and thermal properties. The Zr–C system is a typical refractory and hard transition-metal carbide, and its structural integrity and stability under extreme conditions are critical for practical applications. Here, a computational study focusing on the structural stability and crystal evolution pattern of Zr2C under ambient and high-pressure conditions was performed using a particle-swarm optimization algorithm, in combination with first-principles calculations. The calculations identified seven viable stable or metastable crystalline phases of Zr2C, exhibiting Fd 3 m, R 3 m, Cmcm, Cmca, Pbcn, Pnma and I4/mcm symmetries; further, a series of structural phase transitions were determined as the pressure increased: Fd 3 m → R 3 m → Cmcm → Cmca. In addition, the mechanical and dynamical stabilities of these phases were verified, and their structural properties were investigated. Overall, this work reveals valuable information concerning the structural, mechanical and electronic properties of Zr2C, providing key insights into the mechanisms underlying its crystal evolution behavior.

Quasi-two dimensional perovskites have emerged as candidates of high-performance materials for various optoelectronic applications due to the unique excitonic properties in their multilayer structures.

Quasi-2D Ruddlesden-Popper perovskites attract great attention as an optical gain media in lasing applications due to their excellent optoelectronic properties. Herein, a novel quasi-2D Ruddlesden-Popper perovskite based on 2-thiophenemethylammonium (ThMA) is synthesized by a facile solution-processed method. In addition, an anti-solvent treatment method is proposed to tune the phase distribution, and preferential orientation of quasi-2D (ThMA)2Cs n -1Pb n Br3n+1 thin films. The large-n-dominated narrow domain distribution improves the energy transfer efficiency from small-n to large-n phases. Also, the highly oriented nanocrystals facilitate the efficient Förster energy transfer, beneficial for the carrier population transfer. Furthermore, a green amplified spontaneous emission with a low threshold of 13.92 µJ/cm2 is obtained and a single-mode vertical-cavity laser with an 0.4 nm linewidth emission is fabricated. These findings provide insights into the design of the domain distribution to realize low-threshold multicolor continuous-wave or electrically driven quasi-2D perovskites laser.

Intramolecular exciton dissociation is critical for high efficient mobile charge carrier generations in organic solar cells. Yet despite much attention, the effects of π bridges on exciton dissociation dynamics in donor–π–acceptor (D-π-A) alternating conjugated polymers remain still unclear. Here, using a combination of femtosecond time-resolved transient absorption (TA) spectroscopy and steady-state spectroscopy, we track ultrafast intramolecular exciton relaxation dynamics in three D-π-A alternating conjugated polymers which were synthesized by Qin's group and named HSD-A, HSD-B, HSD-C. It is found that the addition of thiophene unit as π bridges will lead to the red shift of steady-state absorption spectrum. Importantly, we reveal the existence of a new intramolecular exciton dissociation pathway mediated by a bridge-specific charge transfer (CT′) state with the TA fingerprint peak at 1200 nm in π-bridged HSD-B and HSD-C. This CT′ state results in higher electron capture rates for HSD-B and HSD-C as compared to HSD-A. Depending on the proportion of CT′ state and nongeminate recombination are important step for the understanding of high power conversion efficiencies in HSD-B than in HSD-C. We propose that this bridge-specific exciton dissociation pathway plays an important role in ultrafast intramolecular exciton dissociation of organic photovoltaic material D-π-A alternating conjugated polymers.

Recently the kagome compounds have inspired enormous interest and made some great progress such as in the field of superconductivity and topology. Here we predict a different kagome magnesium triboride (MgB3) superconductor with a calculated Tc ~12.2 K and Tc ~15.4 K by external stress, the potentially highest among the reported diverse kagome-type superconductors. We reveal its various exotic physical properties including the van Hove singularity, flat-band, multiple Dirac points, and nontrivial topology. The system can be described by a two-band model with highly anisotropic superconducting gaps on Fermi surfaces. Its topological and nodal superconducting nature is unveiled by a recently developed symmetry indicators method. Our results suggest that MgB3 can be a new platform to study exotic physics in the kagome structure, and pave a way to seek for more superconductors and topological materials with XY3-type kagome lattice., Comment: 6 pages, 4 figures

Two-dimensional transition metal dichalcogenides with outstanding properties open up a new way to develop optoelectronic devices such as phototransistors and light-emitting diodes. Heterostructure with light-harvesting materials can produce many photogenerated carriers via charge and/or energy transfer. In this paper, the ultrafast dynamics of charge transfer in zero-dimensional CsPbBr3 quantum dot/two-dimensional MoS2 van der Waals heterostructures are investigated through femtosecond time-resolved transient absorption spectroscopy. Hole and electron transfers in the ps and fs magnitude at the interfaces between MoS2 and CsPbBr3 are observed by modulating pump wavelengths of the pump-probe configurations. Our study highlights the opportunities for realizing the exciton devices based on quantum dot/two-dimensional semiconductor heterostructures.

The superconductivities and topological properties of ${\mathrm{MgB}}_{2}$-type diborides are investigated by means of first-principles calculations with different exchange-correlation functionals. Functionals with the van der Waals (vdW) correction (such as OptB88-vdW) may predict critical temperature (${T}_{c}$) comparable with experimental results for several ${\mathrm{MgB}}_{2}$-type superconductors, particularly for the pristine ${\mathrm{MgB}}_{2}$ (39.3 vs 39 K). Interestingly, the spin-fluctuation is found to play a significant role in the superconducting behavior of diborides with transition metal elements, and their ${T}_{c}$ can be enhanced monotonically by applying tensile strains. Furthermore, Dirac surface states of $\mathrm{Ta}\phantom{\rule{0.16em}{0ex}}{\mathrm{B}}_{2}$ and ${\mathrm{NbB}}_{2}$ are revealed, suggesting their potential use as topological superconducting materials. This paper provides a useful guideline for ab initio studies of superconductivities and topological properties of vdW layered materials.

The electrochemical properties of V2C and V2CT2 (T = O, S) MXenes with and without vacancy as anode materials for Na-ion and Li-ion batteries, have been studied using first-principles calculation. The present results indicate that the adsorption strength of Li-ion and Na-ion on V2CS2 are less than that of O-functionalized, together with a lower diffusion barrier. Simultaneously, V2CS2 monolayer exhibits lower open-circuit voltage (OCV) values of 0.72 and 0.49 V for Li- and Na-ion, respectively. Interestingly, the presence of atomic vanadium vacancy on V2CS2 monolayer exerts more prominent effects on enhancing adsorption strength than that of carbon vacancy for Li-ion and Na-ion, but with an exception for the diffusion of Li-ion and Na-ion on V2CS2 monolayer. The finding suggests that the V2CS2 monolayer is expected to be a potential candidate as anode material for Li-ion and Na-ion battery due to its lower open-circuit voltages and diffusion barriers.

The gas sensing performance of monolayer Hf2CO2 was evaluated by means of first-principles calculation. Present results indicate that Hf2CO2 sheet possesses high selectivity and remarkable sensitivity toward NH3 molecule, by virtue of the moderate adsorption energy, sharply enhanced electronic conductivity beyond a specific bias voltage and appreciable work function change, as well as good stability, short recovery time. Moreover, the interaction of NH3 with Hf2CO2 can be enhanced by pre-adsorbed H2O molecule via forming hydrogen bonds between the adsorbates, while pre-covered O2 molecule can barely affect the adsorption of NH3 molecule. Particularly, NH3 capture/release on monolayer Hf2CO2 is predicted to be realized by applying external strains and/or electric fields. These appealing features signify monolayer Hf2CO2 as promising nanomaterials for gaseous NH3 detection and storage.

The electronic properties and electrochemical performance of double-metal MXenes: TiNbC and TiNbCT2 (T = O, F and OH) as anode materials for Li-ion batteries (LIBs) have been studied to understand the effects of Nb(Ti)-layer inserting in Ti2C (Nb2C) monolayer and terminated functional-group (T = O, F, and OH) on the electrochemical performance. The results demonstrate that all the TiNbC and TiNbCT2 monolayers are metallic conductivity, while the adsorption of lithium-ion on TiNbC(OH)2 is unstable, and TiNbCF2 is unfavorable as anode material for LIBs due to the F-groups and Li-ions forming ring structure. Specifically, TiNbC and TiNbCO2 nanosheets exhibit such distinctive features as moderate adsorption energies and quite low diffusion barriers of lithium, theoretical storage capacities comparable to Ti2C monolayer, which manifest the double-metal TiNbC and TiNbCO2 monolayers as excellent anode materials for LIBs.

Intramolecular exciton dissociation is critical for high efficient mobile charge carrier generations in organic solar cells. Yet despite much attention, the effects of π bridges on exciton dissociation dynamics in Donor-π-Acceptor (D-π-A) alternating conjugated polymers remain still unclear. Here, using a combination of femtosecond time-resolved transient absorption (TA) spectroscopy and steady-state spectroscopy, we track ultrafast intramolecular exciton relaxation dynamics in three D-π-A alternating conjugated polymers which were synthesized by Qin's group and named HSD-A, HSD-B, HSD-C. It is found that the addition of thiophene unit as π bridges will lead to the red shift of steady-state absorption spectrum. Importantly, we reveal the existence of a new intramolecular exciton dissociation pathway mediated by a bridge-specific charge transfer (CT') state with the TA fingerprint peak at 1200 nm in π-bridged HSD-B and HSD-C. This CT' state results in higher electron capture rates for HSD-B and HSD-C as compared to HSD-A. Depending on the proportion of CT' state and nongeminate recombination are important step for the understanding of high power conversion efficiencies in HSD-B than in HSD-C. We propose that this bridge-specific exciton dissociation pathway plays an important role in ultrafast intramolecular exciton dissociation of organic photovoltaic material D-π-A alternating conjugated polymers.

Performing density functional theory calculations, we have studied the adsorption of nanoparticle Con (n = 2, 4, 6) on graphene sheets and Co4 stands out for the most favorable stability. Furthermore, the catalytic activity of Co4-supported on a variety of graphene sheets was explored toward ammonia dehydrogenation. Present results demonstrate that the existence of C-vacancy in graphene considerably enhances the interaction of NHx (x = 3–0) with Co4 cluster, and effectively facilitates H abstraction from ammonia. Specifically, Co4 anchored on monovacancy graphene sheet outperforms the others for its superb catalytic activity in NH3 decomposition, giving rise to the intermediates NH and/or N bound with the nanocomposite. In contrast, doping of N or B atom in graphene hinders the N H bond cleaving of NH3. The higher catalytic activity of Co4 supported over the vacancy graphene is correlated well with the lower d-band center value relative to the Fermi level, consistent with the literature emphasizing on the importance of support to metal catalysts.

The hydrogen storage behaviors of the nitride-based MXenes Ti2N have been investigated by performing density functional calculations. It is observed that H2-molecules could be adsorbed on and desorbed from Ti2N monolayer with suitable adsorption strength under ambient conditions. Moreover, the stability and hydrogen adsorption behaviors are analyzed by computing adsorption energy, charge population and ab initio molecular dynamic simulations. The Kubas-type interaction of the H2-molecules with Ti2N monolayer is investigated according to the charge transfer and electronic projected density of states. Specifically, it is found that the maximum hydrogen storage capacity of Ti2N reaches up to 8.555 wt%, in which the reversible hydrogen storage capacity is of 3.422 wt%. In addition, OH-group-decorated Ti2N has great advantages on hydrogen storage, and the reversible hydrogen storage capacity is of 2.656 wt%. Therefore, the 2D Ti2N MXenes are expected for a potential candidate as reversible hydrogen storage materials under ambient conditions.

By merging a b i n i t i o calculations and particle-swarm optimization algorithm, we have predicted two new phases of thorium dichalcogenides ThX2 (X = S, Se and Te) in the F m 3 ¯ m and I 4 / m m m symmetry. The F m 3 ¯ m phase is proved to be the ground-state phase of ThS2 and ThSe2. The calculated enthalpies indicate that the F m 3 ¯ m to P n m a phase transition pressures are about 2.3 GPa and 0.35 GPa for ThS2 and ThSe2, respectively; and the P n m a to I 4 / m m m phase transition pressures are about 37 GPa, 17 GPa and 2 GPa for ThS2, ThSe2 and ThTe2, respectively. The phonon dispersion curves and elastic constants suggest that all of the F m 3 ¯ m , P n m a and I 4 / m m m phases are dynamically and mechanically stable. The electronic calculations show that a pressure-induced semiconductor to metal transitions of these three compounds will occur following the P n m a to I 4 / m m m phase transition. Our calculated bulk modulus B 0 and elastic constants C i j of the various phases of these compounds show that the materials become softer one after another from ThS2 to ThSe2 and then to ThTe2.

Due to the growing needs of thermal management in modern electronics, high thermal conductive polymer composites are increasingly demanded. Boron nitride nanosheet (BNNS) was prepared through molten hydroxide assisted liquid exfoliation of hexagonal boron nitride (h-BN) powder and used as thermally conductive filler. The poly(vinylidene fluoride) (PVDF)/BNNS films were obtained through solution blend and hot pressing. With only 4 wt% BNNS, the in-plane thermal conductivity of PVDF/BNNS composite achieved 4.69 W/mK, with a thermal conductivity enhancement of 2297% compared to neat PVDF. However, the through-plane thermal conductivity of the composites is only 0.23 W/mK, which shows a high thermal conductive anisotropy over 20. The thermal conductive anisotropy and the high in-plane thermal conductivity can be attributed to the formation of thermally conductive network in PVDF matrix. Thus, the BNNS reinforced PVDF films are promising for use as an efficient heat spreader for electronic cooling applications.

The gas sensing properties of single-layer GeS have been studied by performing density functional theory (DFT) calculations, and the present results demonstrate that monolayer GeS exhibits high selectivity and sensitivity toward NO2 molecules and short recovery time, coupled with a strong anisotropic transport feature. Moreover, applying biaxial strains and electric fields could effectively modulate the capture and release of gaseous NO2. Further calculations indicate that oxygen atoms and water molecules decorated on GeS hardly affect the superb selectivity toward NO2 against the other gases. The presence of an S-vacancy on the GeS monolayer greatly promotes the adsorption of NO2, facilitating molecular dissociation. The underlying mechanism is also elaborated at the level of electronic structures. The present findings make GeS monolayers potential materials for NO2 gas sensors and capturers in future applications.

The electronic properties, dynamic stabilities and mechanical properties of ZrC, HfC, and their solid solutions were determined using first-principles calculations. The crystal structures and elastic moduli of Zr x Hf 1 − x C compounds were investigated under different transition metal atom solutions, and linear changes were observed in cell volumes, bond lengths and elastic properties with increasing hafnium atom content. The obtained phonon spectra and elastic constants show that all the compounds are dynamically and mechanically stable because they satisfy the corresponding stability criteria. The solid solutions of ZrC and HfC did not affect their electronic and mechanical properties. They are still metals, and their mechanical properties are comparable to those of ZrC and HfC. Moreover, the corresponding properties can be adjusted to some extent by changing the solid-solution ratio of the transition metal atoms. This provides a considerable advantage and convenience for the practical application of such materials.

We investigate the electronic transport properties of several nanotube heterojunctions which are composed of carbon and boron nitride nanotubes (i.e., CNTs and BNNTs), using the non-equilibrium Green's function method combined with the density functional theory. The results reveal that, the pristine (n, n) and (n, 0) CNTs are metal and present the linear I–V curves, and the BNNTs are semiconductors and little current can be propagated through. While when the (n, n)CNT and (n, n)BNNT are combined into a heterojunction, it displays a perfect rectifying effect. When different types of CNTs (i.e., (n, 0) and (n, n) CNTs) are combined into a heterojunction, its I–V curve still presents a linear behavior. Interestingly, however, if the outside surface of the (n, n) CNT is fluorinated to tune its electronic structures, it will display a perfect rectifying effect. These proposed nanotube heterojunctions have a significant potential value in the field of nano rectifiers.

By means of first-principles calculation, we explored the physical properties of substituting Al with Si atom in layered ternary carbide Ti 3 AlC 2 . Present results revealed that the Ti 3 (Al 1- x Si x )C 2 ( x = 0.0–1.0) compounds under study are thermodynamically stable brittle materials, and the mechanical properties can be improved to some extent by Si-doping. Moreover, all the compounds show metallic character with a strong Ti C bonding and possess a rather stiff lattice and good thermal conductivity. It is notable that these compounds show highly anisotropic at photon energies lower than 18.0 eV. Specifically, all the Ti 3 (Al 1- x Si x )C 2 compounds exhibit high reflectivity, nearly reaching the maximum of ∼98%, in the photon energy range of 0–12.5 eV. Therefore, we expect that Si-doped Ti 3 (Al 1- x Si x )C 2 solid solutions are possibly a good candidate for coating materials in ultraviolet region.

Toxic gaseous SO2 adsorption on O-terminated M2CO2 (M = Sc, Hf, Zr, and Ti) monolayers has been studied by means of first-principle calculations. It is demonstrated that monolayer Sc2CO2 is most preferred for the SO2 molecule with suitable adsorption strength and apparent charge transfer. Moreover, the electronic conductivity of Sc2CO2 displays a sharp increase after the adsorption of SO2. In particular, the adsorption strength of SO2 on Sc2CO2 can be further enhanced or weakened by applying tensile strains or controlling external electronic fields, which is greatly desirable to realize the capture or reversible release of toxic SO2 gas. These distinctive features endow the Sc2CO2 monolayer with high selectivity and sensitivity as a potential candidate for SO2 gas sensor, as well as efficient control for gaseous SO2 capture.

Previous methods for the evaluation of the exfoliation of two-dimensional (2D) layered materials have drawbacks in computational efficiency and are unable to describe cases with semi-infinite substrates. Based on a Green's function surface (GFS) model, here we develop a new approach to efficiently determine the tendency of exfoliation of 2D materials from their bulk crystals or semi-infinite substrates. By constructing appropriate surface configurations, we may calculate the exfoliation energy more precisely and quickly than the traditional way with the slab model. Furthermore, the GFS approach can provide angle-resolved photoemission spectroscopy (ARPES) of surface systems for direct comparison with experimental data. Our findings indicate that the GFS approach is powerful for studies of 2D materials and various surface problems., 6 pages, 5 figures

Several layered transition metal borides can now be realized by a simple and general fabrication method (Fokwa et al 2018 Adv. Mater. 30 1704181), inspiring our interest to transition metal borides monolayer. Herein, we predict a new two-dimensional (2D) transition metal diboride MoB

By using first-principles calculation, we study the properties of h-BN/BC3 heterostructure and the effects of external electric fields and strains on its electronic and optical properties. It is found that the semiconducting h-BN/BC3 has good dynamical stability and ultrahigh stiffness, enhanced electron mobility, and well-preserved electronic band structure as the BC3 monolayer. Meanwhile, its electronic band structure is slightly modified by an external electric field. In contrast, applying an external strain can mildly modulate the electronic band structure of h-BN/BC3 and the optical property exhibits an apparent redshift under a compressive strain relative to the pristine one. These findings show that the h-BN/BC3 hybrid can be designed as optoelectronic device with moderately strain-tunable electronic and optical properties.

Using the first-principles calculation, we have studied the mechanical and thermal properties of the ceramic material Ti-alloyed Nb 4 AlC 3 solid solutions. Calculated phase stability, elastic, mechanical properties indicate that all the (Nb 1− x Ti x ) 4 AlC 3 ( x increasing from 0 to 1.0 in a step of 0.125) solid solutions are thermodynamically, elastically stable and brittle materials. Moreover, lattice thermal conductivity and other related calculations predict these compounds have a relative high Debye temperature and melting point. This implies that the (Nb 1− x Ti x ) 4 AlC 3 compounds possess a rather stiff lattice and good thermal conductivity. Particularly, the analysis of electronic structures and mechanical property calculations consistently indicate that the (Nb 0.875 Ti 0.125 ) 4 AlC 3 compound possess the highest hardness among the compounds considered. As expected, the mechanical properties of Nb 4 AlC 3 can be improved to some extent by doping with Ti, in accordance with experiments.

Using first-principles calculation, we have studied the adsorption of AsH3 molecule on Hf2CO2 monolayer and a physisorption nature is shown with a moderate adsorption energy. This binding strength is hardly affected by pre-adsorbed H2O molecules and other functional group (OH, F) doped in the sheet, while the presence of oxygen vacancy on Hf2CO2 monolayer greatly promotes the AsH3 adsorption. Notably, applying external electric fields can realize the reversible storage of toxic AsH3 molecule by effectively modulating its adsorption strength with Hf2CO2 monolayer via dipole polarization. Moreover, monolayer Hf2CO2 shows an excellent adsorption capability and a steady binding strength toward AsH3 molecules, as well as a good stability at room temperature. These features highlight the promise of Hf2CO2 monolayer as an appealing candidate of toxic arsine adsorbents at room temperature.

The structural stabilities and crystal evolution behaviors of the hyper stoichiometric compound ZrC2 (carbon rich; C/Zr > 1.0) are studied under ambient and high pressure conditions using first-principles calculations in combination with the particle-swarm optimization algorithm. Six viable structures of ZrC2 in P21/c, Cmmm, Cmc21, P42/nmc, Immm and P6/mmm symmetries are identified. These structures are dynamically stable as their phonon spectra have no imaginary modes at zero pressure or at the selected high-pressure points. Among them, the P21/c phase represents the ground state structure, whereas P21/c, P42/nmc, Immm and P6/mmm phases are part of the phase transition series. The phase order and critical pressures of the phase transition are determined to be approximately 300 GPa according to the equation of states and enthalpy. Furthermore, the mechanical and electronic properties are investigated. The P21/c and Cmc21 phases display a semi-metal nature, whereas the P42/nmc, Immm, P6/mmm and Cmmm phases exhibit a metallic nature. Moreover, the present study reveals considerable information regarding the structural, mechanical and electronic properties of ZrC2, thereby providing key insights into its material properties and evaluating its behavior in practical applications.

The intrinsic spin-dependent transport properties of two types of lateral VS2|MoS2 heterojunctions are systematically investigated using first-principles calculations, and their various nanodevices with novel properties are designed. The lateral VS2|MoS2 heterojunction diodes show a perfect rectifying effect and are promising for the applications of Schottky diodes. A large spin-polarization ratio is observed for the A-type device and pure spin-mediated current is then realized. The gate voltage significantly tunes the current and rectification ratio of their field-effect transistors (FETs). In addition, they all have sensitive photoresponse to blue light, and could be used as photodetector and photovoltaic device. Moreover, they generate the effective thermally-driven current when a temperature gratitude appears between the two terminals, suggesting them as potential thermoelectric materials. Hence, the lateral VS2|MoS2 heterojunctions show a multifunctional nature and have various potential applications in spintronics, optoelectronics, and spin caloritronics.

Using first-principles calculations, the thermodynamic stability, mechanical and optical properties of the recently synthesized (Ti1−xNb x )3AlC2 (x = 0–1.0) solid solutions are studied. The present results revealed that all the (Ti1−xNb x )3AlC2 compounds are thermodynamically and elastically stable brittle materials, and the mechanical properties can be improved to some extent by Nb-doping. Moreover, the temperature dependence of lattice thermal conductivity for all the considered compounds are studied firstly and the minimum lattice thermal conductivities are also obtained. Specifically, calculated optical properties demonstrate that Nb-doped (Ti1−xNb x )3AlC2 solid solutions are possibly a good candidate materials in photoelectric applications.

Step-like zigzag graphene nanoribbons (ZGNRs) with different step widths are designed, and their electronic transport properties are investigated by using the non-equilibrium Green’s function method combined with the density functional theory. The results reveal that one with a small step structure can exhibit better conductive capability and interesting negative differential resistance (NDR) behavior under negative applied biases. More importantly, with the increase of step width, these step-like ZGNR nanojunctions present valuable rectification effects, and show a rule that the rectification ratio increases with increasing the step width. It is also shown that the rectification effect can be further inversed and enhanced through introducing a defect around the step. Transmission spectra, densities of states, energy barriers, transmission eigenstates, and transmission pathways are analyzed subsequently to understand the electronic transport properties of these step-like ZGNR devices, which can be used in developing nanoscale NDR devices and rectifiers.

Spin-dependent electronic transport properties of the zigzag silicon carbon nanoribbon (Z-SiCNR) are studied by employing the non-equilibrium Green's function method in the framework of density functional theory. It is found that the Z-SiCNR exhibits a variety of exotic physical properties. While the Z-SiCNR in the metallic FM state presents spin filtering and current-limited effects, it is shown that the abnormal oscillation of spin-polarized currents with spin polarization as high as 100% under a certain bias voltage emerges in the half-metallic AFM state. The results demonstrate that tuning the spin state of the zigzag SiC nanoribbon provides a possible avenue to design next generation spin nanodevices with novel functionalities.

The interaction of hydrogen with graphene-supported Co4 was studied by first-principles calculation, with a particular focus on the role of support defects in modulating the adsorption properties of hydrogen. It is shown that point defects in graphene can effectively improve the capacity of Co4 for hydrogen storage by modifying the interactions between metal cluster and supports, but exert different effects. By virtue of excellent structural stability and favorable hydrogen uptake, Co4 decorated on defective graphene with dopant N accompanying with C-vacancy stand out as promising hydrogen storage media, and such a better performance is more likely relating to a lower d-band center of supported-Co4. This work explores the possibility of Co4-decorated defective graphene sheets as hydrogen storage materials through introducing the appropriate defects in support.

Several layered transition metal borides can now be realized by a simple and general fabrication method [Fokwa et al.,Adv. Mater. 2018, 30, 1704181],inspiring our interest to transition metal borides monolayer. Here, we predict a new two-dimensional (2D) transition metal diboride MoB2 monolayer (ML) and study its intrinsic mechanical, thermal, electronic, and transport properties. The MoB2 ML has isotropic mechanic properties along the zigzag and armchair directions with a large Young's stiffness, and has an ultralow room-temperature thermal conductivity. The Mo atoms dominate the metallic nature of MoB2 ML. It shows an obvious electrical anisotropy and a current-limiting behavior. Our findings suggest that MoB2 ML is a promising multifunctional material used in ultrathin high-strength mechanical materials, heat insulating materials, electrical-anisotropy-based materials, and current limiters. It is helpful for the experimentalists to further prepare and utilize the transition metal diboride 2D materials., 7 figures

We investigate the charge transport properties of zigzag boron nitride nanoribbons (ZBNNRs) with various hydrogen passivations by employing density functional theory (DFT) combined with the non-equilibrium Green's function (NEGF) formalism. The calculated results reveal that the ZBNNR-based devices exhibit negative differential resistance (NDR) characteristics except those models whose both edges are passivated, due to the mechanism in which the overlap of bands near the Fermi level between the left and right electrodes gets smaller or disappears under a high bias. The NDR characteristics of the perfect ZBNNRs with one or two bare edges are weakly dependent on their widths. This is one intrinsic NDR characteristic of the ZBNNR-based devices, including some defective structures. The intuitive electronic current channels are plotted and analyzed to better understand the charge transport mechanisms. Our results suggest that the ZBNNR-based structures could be favorable candidates for preparing nanoscale NDR devices.

We investigate the electronic transport properties of four types of lateral graphene/h-BN nanoribbon heterojunctions using the non-equilibrium Green's function method in combination with the density functional theory. The results show that the heterojunction displays an interesting rectifying effect when the interface has a left-right type structure, while a pronounced negative differential resistance (NDR) effect when the interface has an up-down type structure. Moreover, when the interface of the heterojunction has a left-bank or right-bank type structure, it presents the rectifying (with a larger rectification ratio) and NDR effects. This work is helpful to further construct and prepare a nanodevice based on the graphene/h-BN heterojunction materials according to the proposed structures.

High pressure phase stability and physical properties of bulk ternary-layered carbide Ti4SiC3 compound are studied through ab initio calculations. Present results indicate that the thermodynamic stability and elastic stability can be maintained up to about 200 GPa for this compound. And it is predicted to be a quasi-brittle ceramic and possesses good thermal conductivity due to a relative high Debye temperature (849.7 K). Moreover, the analysis of optical properties suggests that Ti4SiC3can be used as good coating materials in the ultraviolet region. These findings identify the compound of Ti4SiC3 as a material with excellent mechanical, ceramic and elastic isotropic properties for future applications.

Using first-principles methods, we investigate the electronic transport properties of zigzag MoS2 nanoribbons (Z-MoS2NRs). The current?voltage (I?V) curves of Z-MoS2NRs show a negative differential resistive (NDR) effect, and are independent of nanoribbon width. The current flowing through the nanoribbon is mainly along the Mo-edge, with two different local current channels (Mo?????Mo hop current and S?????Mo?????S bond current). The current will be suppressed when introducing a Mo vacancy-defect at the Mo-edge under low biases?while, under high biases, the current through the defected Z-MoS2NRs will increase a little, due to the other S-edge channel being opened.