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Recently, there has been a rapidly growing interest in two-dimensional (2D) transition metal chalcogenide monolayers (MLs) due to their unique magnetic and electronic properties. By using an evolutionary algorithm and first-principles calculations, we report the discovery of a previously unexplored, chemically, energetically, and thermodynamically stable 2D antiferromagnetic (AFM) CrSe ML with a N\'eel temperature higher than room temperature. Remarkably, we predict an electric field-controllable metal-insulator transition (MIT) in a van der Waals (vdW) heterostructure comprised of CrSe ML and ferroelectric Sc2CO2. This tunable transition in CrSe/Sc2CO2 heterostructure is attributed to the change in the band alignment between CrSe and Sc2CO2 caused by the ferroelectric polarization reversal in Sc2CO2. Our findings suggest that 2D AFM CrSe ML has important potential applications in AFM spintronics, particularly in the gate voltage conducting channel., Comment: 13 Pages, 4 Figures, published at Applied Physics Letters

A novel single-atom catalyst of Fe adsorbed on χ

Stacking faults and the partial dislocations associated with them in a cold-rolled Mg–3wt%Bi alloy have been systematically characterized using transmission electron microscopy and high-angle annular dark-field scanning transmission electron microscopy. Intrinsic stacking faults I2 and I1 are both detected. The I2 fault results from dissociation of a basal 〈a〉 dislocation into two Shockley partial dislocations and exists in both matrix and {10 1 ¯ 2} tension twins. Most of the I1 faults are from dissociation of 〈c + a〉 dislocations, but a small fraction has a structure similar to that of growth I1, and they might result from condensation of vacancies, interstitial precipitation, or coalescence of 〈c + a〉 ribbons. The I1 faults are distributed densely in {10 1 ¯ 2} tension twins but sparsely in the magnesium matrix. They are either bounded by Frank partials or linked to twin boundaries. Unexpectedly, Shockley partials are often found lying at the edge of steps within the I1 faults. Three cases are categorized in terms of the number and the sign of the Shockley partials located in each single I1 fault: one Shockley partial, two Shockley partials having the same sign, and two Shockley partials having opposite signs. The fault bounded by two Shockley partials is explicitly I1 but not I2, which is different from the dissociation of the basal 〈a〉 dislocation. Based on the geometric analysis, two possible mechanisms are proposed to explain the origin of Shockley partials formed in the I1 faults. The mechanism in which basal dislocations react with the bounding Frank partial of I1 is more likely to operate, and this mechanism is supported by molecular dynamics simulations.

Several enhanced sampling methods, such as umbrella sampling or metadynamics, rely on the identification of an appropriate set of collective variables. Recently two methods have been proposed to alleviate the task of determining efficient collective variables. One is based on linear discriminant analysis; the other is based on a variational approach to conformational dynamics and uses time-lagged independent component analysis. In this paper, we compare the performance of these two approaches in the study of the homogeneous crystallization of two simple metals. We focus on Na and Al and search for the most efficient collective variables that can be expressed as a linear combination of X-ray diffraction peak intensities. We find that the performances of the two methods are very similar. Wherever the different metastable states are well-separated, the method based on linear discriminant analysis, based on its harmonic version, is to be preferred because simpler to implement and less computationally demanding. The variational approach, however, has the potential to discover the existence of different metastable states., The Journal of Chemical Physics, 150 (9), ISSN:0021-9606, ISSN:1089-7690

Two-dimensional (2D) magnetic materials have been experimentally recognized recently, however, the Curie temperatures (T C) of known 2D systems are quite low. Generally, magnetic systems can be seen as constituent magnetic elements providing spins and the non-magnetic elements providing frameworks to host the magnetic elements. Short bond lengths between the magnetic and non-magnetic elements would be beneficial for strong magnetic interactions and thus high T C. Based on this, we propose to combine the magnetic element Cr and the non-magnetic element boron to design novel 2D magnetic systems. Using our self-developed software package IM2ODE, we design a series of chromium-boride based 2D magnetic materials. Nine stable magnetic systems are identified. Among them, we find that CrB4-I, CrB4-II and CrB5-I with common structural units [CrB8] are ferromagnetic metals with estimated T C of 270 K, 120 K and 110 K, respectively. On the other hand, five CrB3 phases with structural units [Cr2B12] are antiferromagnetic metals. Additionally, we also find one antiferromagnetic semiconductor CrB2-I. Our work may open new directions for identifying 2D magnetic systems with high T C.

Carbon is a versatile element that has allotropes with both sp2 (graphene) and sp3 (diamond) bonding. However, none of the allotropes can be used as light-absorber materials in solar cells due to either too large or too small band gap. Here, we propose a novel concept that enables a tunable band gap of carbon phases with sp2 carbon atoms within a sp3 carbon structure. The tunability is due to the quantum confinement effect. By embedding the sp2 atoms within the sp3 structure, we can design new carbon allotropes with ideal optical properties for optoelectronic applications. Five carbon allotropes incorporated this structural feature were identified by combining this new concept with our freshly developed multi-objective inverse band structure design approach. They all have proper band gaps for optical absorption, and the simulated photovoltaic efficiency of C10-C is even higher than conventional absorber materials such as GaAs, which indicates that C10-C with mixed sp2 sp3 hybridization may have potential application as light-absorber material in electronic and optoelectronic devices.

We use inverse design of materials by multi-objective differential evolution (IM2ODE) method to globally search the most stable configurations of Au n , Au n - 1 Pt, Au n - 2 Pt 2 (n = 2–9) clusters. Combining with first-principles calculations, based on the density functional theory (DFT), we find that all of the ground state structures of clusters prefer to keep low spin multiplicity and form planar structures. Pt atom tends to occupy the most highly coordinated position. Especially Au 6 Pt and Au 4 Pt 2 clusters exhibit highly relative stability and have a closed electronic shell based on the spherial jellium model, so we conclude that Au 6 Pt and Au 4 Pt 2 should be magic number clusters. Mulliken occupation analysis of Pt and Au atoms indicate that a competition exits between electronegativity of atoms and valence electron configuration of atoms in small Au n Pt m clusters.

A new design strategy for polyimides/carbon nanotube networks is reported, aiming to passivate the hydrogen-evolution mechanism on the molecular structures of electrodes, thus substantially boosting their aqueous energy-storage capabilities. The intrinsic sluggish hydrogen-evolution activity of polyimides is further passivated via Li(+) association during battery charging, leading to a much wider voltage window and exceptional energy-storage capability.

Despite the recent progress of developing graphitic carbon nitride (g-C3N4) as a metal-free photocatalyst, the synthesis of nanostructured g-C3N4 has still remained a complicated and time-consuming approach from its bulk powder, which substantially limits its photoelectrochemical (PEC) applications as well as the potential to form composites with other semiconductors. Different from the labor-intensive methods used before, such as exfoliation or assistant templates, herein, we developed a facile method to synthesize graphitic C3N4 quantum dots (g-CNQDs) directly grown on TiO2 nanowire arrays via a one-step quasi-chemical vapor deposition (CVD) process in a homemade system. The as-synthesized g-CNQDs uniformly covered over the surface of TiO2 nanowires and exhibited attractive photoluminescence (PL) properties. In addition, compared to pristine TiO2, the heterojunction of g-CNQD-decorated TiO2 nanowires showed a substantially enhanced PEC photocurrent density of 3.40 mA/cm2 at 0 V of applied potential vs Ag/AgCl under simulated solar light (300 mW/cm2) and excellent stability with ?82% of the photocurrent retained after over 10 h of continuous testing, attributed to the quantum and sensitization effects of g-CNQDs. Density functional theory calculations were further carried out to illustrate the synergistic effect of TiO2 and g-CNQD. Our method suggests that a variety of g-CNQD-based composites with other semiconductor nanowires can be synthesized for energy applications. 2016 American Chemical Society. We thank the following funding agencies for supporting this work: the National Key Basic Research Program of China (2013CB934104), the Natural Science Foundation of China (21322311, 21473038, 21471034), the Science and Technology Commission of Shanghai Municipality (14JC1490500), the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning, and the Collaborative Innovation Center of Chemistry for Energy Materials (2011-iChem). The authors extend their sincere appreciation to the Deanship of Scientific Research at King Saud University for funding the Prolific Research group (PRG-1436-14). Scopus

Sampling complex free energy surfaces is one of the main challenges of modern atomistic simulation methods. The presence of kinetic bottlenecks in such surfaces often renders a direct approach useless. A popular strategy is to identify a small number of key collective variables and to introduce a bias potential that is able to favor their fluctuations in order to accelerate sampling. Here we propose to use machine learning techniques in conjunction with the recent variationally enhanced sampling method [Valsson and Parrinello, Physical Review Letters 2014] to determine such potential. This is achieved by expressing the bias as a neural network. The parameters are determined in a reinforcement learning scheme aimed at minimizing the variationally enhanced sampling functional. This required the development of a new and more efficient minimization technique. The expressivity of neural networks allows accelerating sampling in systems with rapidly varying free energy surfaces, removing boundary effects artifacts, and making one more step towards being able to handle several collective variables.

The band offset between different semiconductors is an important physical quantity determining carrier transport properties near the interface in heterostructure devices. Computation of the natural band offset is a longstanding challenge. We propose an intermediate-phase method to predict the natural band offset between two structures with different symmetry, for which the superlattice model cannot be directly constructed. With this method and the intermediate phases obtained by our searching algorithm, we successfully calculate the natural band offsets for two representative systems: (i) zinc-blende CdTe and wurtzite CdS and (ii) diamond and graphite. The calculation shows that the valence band maximum (VBM) of zinc-blende CdTe lies 0.71 eV above that of wurtzite CdS, close to the result 0.76 eV obtained by the three-step method. For the natural band offset between diamond and graphite which could not be computed reliably with any superlattice methods, our calculation shows that the Fermi level of graphite lies 1.51 eV above the VBM of diamond using an intermediate phase. This method, under the assumption that the transitivity rule is valid, can be used to calculate the band offsets between any semiconductors with different symmetry on condition that the intermediate phase is reasonably designed.

Two-dimensional (2D) semiconductors can be very useful for novel electronic and optoelectronic applications because of their good material properties. However, all current 2D materials have shortcomings that limit their performance. As a result, new 2D materials are highly desirable. Using atomic transmutation and differential evolution global optimization methods, we identified two group IV-VI 2D materials, Pma2-SiS and silicene sulfide. Pma2-SiS is found to be both chemically, energetically, and thermally stable. Most importantly, Pma2-SiS has shown good electronic and optoelectronic properties, including direct bandgaps suitable for solar cells, good mobility for nanoelectronics, good flexibility of property tuning by layer control and applied strain, and good air stability as well. Therefore, Pma2-SiS is expected to be a promising 2D material in the field of 2D electronics and optoelectronics. The designing principles demonstrated in identifying these two tantalizing examples have great potential to accelerate the finding of new functional 2D materials.

Hydrogen peroxide (H2O2) is an important molecular messenger for cellular signal transduction. The capability of direct probing of H2O2 in complex biological systems can offer potential for elucidating its manifold roles in living systems. Here we report the fabrication of three-dimensional (3D) WS2 nanosheet networks with flower-like morphologies on a variety of conducting substrates. The semiconducting WS2 nanosheets with largely exposed edge sites on flexible carbon fibers enable abundant catalytically active sites, excellent charge transfer, and high permeability to chemicals and biomaterials. Thus, the 3D WS2-based nano-bio-interface exhibits a wide detection range, high sensitivity and rapid response time for H2O2, and is capable of visualizing endogenous H2O2 produced in living RAW 264.7 macrophage cells and neurons. First-principles calculations further demonstrate that the enhanced sensitivity of probing H2O2 is attributed to the efficient and spontaneous H2O2 adsorption on WS2 nanosheet edge sites. The combined features of 3D WS2 nanosheet networks suggest attractive new opportunities for exploring the physiological roles of reactive oxygen species like H2O2 in living systems.

A three-dimensional (3D) mesoporous Fe2O3-CdS nanopyramid heterostructure is developed for solar-driven, real-time, and selective photoelectrochemical sensing of Cu(2+) in the living cells. Fabrication of the mesoporous Fe2O3 nanopyramids is realized by an interfacial aligned growth and self-assembly process, based on the van der drift model and subsequent selective in situ growth of CdS nanocrystals. The as-prepared mesoporous Fe2O3-CdS heterostructures achieve significant enhancement (∼3-fold) in the photocurrent density compared to pristine mesoporous Fe2O3, which is attributed to the unique mesoporous heterostructures with multiple features including excellent flexibility, high surface area (∼87 m(2)/g), and large pore size (∼20 nm), enabling the PEC performance enhancement by facilitating ion transport and providing more active electrochemical reaction sites. In addition, the introduction of Cu(2+) enables the activation of quenching the charge transfer efficiency, thus leading to sensitive photoelectrochemical recording of Cu(2+) level in buffer and cellular environments. Furthermore, real-time monitoring (∼0.5 nM) of Cu(2+) released from apoptotic HeLa cell is performed using the as-prepared 3D mesoporous Fe2O3-CdS sensor, suggesting the capability of studying the nanomaterial-cell interfaces and illuminating the role of Cu(2+) as trace element.

The direct production of branched semiconductor arrays with highly ordered orientation has proven to be a considerable challenge over the last two decades. Here we report a mesoporous interfacial atomic rearrangement (MIAR) method to directly produce highly crystalline, finger-like branched iron oxide nanoarrays from the mesoporous nanopyramids. This method has excellent versatility and flexibility for heteroatom doping of metallic elements, including Sn, Bi, Mn, Fe, Co, Ni, Cu, Zn, and W, in which the mesoporous nanopyramids first absorb guest-doping molecules into the mesoporous channels and then convert the mesoporous pyramids into branching artificial nanofingers. The crystalline structure can provide more optoelectronic active sites of the nanofingers by interfacial atomic rearrangements of doping molecules and mesopore channels at the porous solid-solid interface. As a proof-of-concept, the Sn-doped Fe2O3 artificial nanofingers (ANFs) exhibit a high photocurrent density of ∼1.26 mA/cm(2), ∼5.25-fold of the pristine mesoporous Fe2O3 nanopyramid arrays. Furthermore, with surface chemical functionalization, the Sn-doped ANF biointerfaces allow nanomolar level recognition of metabolism-related biomolecules (∼5 nm for glutathione). This MIAR method suggests a new growth means of branched mesostructures, with enhanced optoelectronic applications.

Inverse design is a promising approach in the realm of material science for finding structures with desired property. We developed a new package with novel algorithm for inverse design named as IM2ODE (inverse design of Materials by Multi-Objective Differential Evolution). The target properties of concern include the optical and electronic-structure properties of semiconductors, hardness of crystals, etc. IM2ODE can easily predict the atomic configurations with desired properties for three dimensional structure, interface and cluster, even complex defect in solid. Tests have been run on multiple systems and it has been proved that IM2ODE is highly efficient and reliable, which can be applied widely.

The $\mathrm{P}{\mathrm{b}}_{\mathrm{I}}$ antisite was reported to be a possible deep-level recombination-center defect in $\mathrm{C}{\mathrm{H}}_{3}\mathrm{N}{\mathrm{H}}_{3}\mathrm{Pb}{\mathrm{I}}_{3}$ solar cells with a concentration higher than ${10}^{15}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3}$ under I-poor conditions. However, whether it is really an effective nonradiative recombination center and limits the photovoltaic efficiency depends also on its cross sections for capturing the electron or hole carriers, which is difficult to determine in both experiment and theory. Here we use a recently developed method to overcome the high computational cost of electron-phonon coupling calculation and implement it in the standard first-principles code quantum espresso so that we can calculate the carrier-capture cross sections effectively for the point defects in complicated semiconductors, such as $\mathrm{C}{\mathrm{H}}_{3}\mathrm{N}{\mathrm{H}}_{3}\mathrm{Pb}{\mathrm{I}}_{3}$. The calculation showed that both the hole- and the electron-capture cross sections and capture rates of $\mathrm{P}{\mathrm{b}}_{\mathrm{I}}$ are large relative to those of the point defects in conventional semiconductors, such as Si or GaP, which is attributed mainly to the heavy Pb-I atoms and the soft phonon modes in this Pb halide. Nonradiative recombination increases the thermal energy by exciting the phonons in the soft Pb-I lattice, whereas the contribution of the organic $\mathrm{C}{\mathrm{H}}_{3}\mathrm{N}{\mathrm{H}}_{3}$ group is negligible. Since $\mathrm{P}{\mathrm{b}}_{\mathrm{I}}$ has a higher concentration under I-poor conditions, especially when the semiconductor is $p$-type, our results suggest that the minority-carrier lifetime and thus the photovoltaic efficiency should be very limited in $p$-type $\mathrm{C}{\mathrm{H}}_{3}\mathrm{N}{\mathrm{H}}_{3}\mathrm{Pb}{\mathrm{I}}_{3}$ and $p$-type doping should be avoided for fabricating high-efficiency $\mathrm{C}{\mathrm{H}}_{3}\mathrm{N}{\mathrm{H}}_{3}\mathrm{Pb}{\mathrm{I}}_{3}$ solar cells under I-poor conditions. Similar calculations can be used for studying the influence of various defects on the photovoltaic performance of other organic-inorganic hybrid and inorganic halide perovskites, shedding light on the design of high-efficiency solar cells.

The theoretical study of grain boundaries (GBs) in polycrystalline semiconductors is currently stalemated by their complicated nature, which is difficult to extract from any direct experimental characterization. Usually, coincidence-site-lattice models are constructed simply by aligning two symmetric planes ignoring various possible reconstructions. Here, we propose a general self-passivation rule to determine the low-energy GB reconstruction and find new configurations for the CdTe \ensuremath{\Sigma}3 (112) GBs. First-principles calculations show that it has lower formation energies than the prototype GBs adopted widely in previous studies. Surprisingly, the reconstructed GBs show self-passivated electronic properties without deep-level states in the band gap. Based on the reconstructed configurations, we revisited the influence of $\mathrm{CdC}{\mathrm{l}}_{2}$ post-treatment on the CdTe GBs and found that the addition of both Cd and Cl atoms in the GB improves the photovoltaic properties by promoting self-passivation and inducing $n$-type levels, respectively. The present study provides a new route for further studies of GBs in covalent polycrystalline semiconductors and highlights that previous studies on the GBs of multinary semiconductors, which are based on the unreconstructed prototype GB models, should be revisited.

Aquaglycero-aquaporins (agAQPs) are the structural foundation of rapid water transport and they appear to participate in cancer proliferation and malignancy. AQP3 expression is increased and AQP9 expression is decreased in hepatocellular carcinoma (HCC) compared to normal liver, which suggests their possible use as targets for cancer treatment. AQP-based modifiers, such as Auphen and dibutyryladenosine 3', 5'-cyclic monophosphate (dbcAMP), might be used to treat several diseases and as chemical tools for assessing the functions of AQPs in biological systems. We investigated the effects of both Auphen on AQP3 and dbcAMP on AQP9 in SMMC-7721 cells. We used western blotting, real-time quantitative polymerase chain reaction (qPCR) and immunohistochemistry to evaluate changes in AQP3 and AQP9 expression in SMMC-7721 cells after culturing with Auphen and dbcAMP, respectively. We also determined the proliferation of SMMC-7721 cells. We found that compared to HL-7702 (L02) liver cells, Auphen increased AQP3 expression in tumor cells, whereas dbcAMP decreased expression of AQP9 in these cells. Also, high concentrations of Auphen and dbcAMP inhibited proliferation of SMMC-7721 cells in vitro. Auphen and dbcAMP may inhibit HCC development and could be considered targets for HCC diagnosis and therapy.

The ability to release, as electrical energy, potential energy stored at the water:carbon interface is attractive, since water is abundant and available. However, many previous reports of such energy converters rely on either flowing water or specially designed ionic aqueous solutions. These requirements restrict practical application, particularly in environments with quiescent water. Here, a carbon-based chemical-to-electricity device that transfers the chemical energy to electrical form when coming into contact with quiescent deionized water is reported. The device is built using carbon nanotube yarns, oxygen content of which is modulated using oxygen plasma-treatment. When immersed in water, the device discharges electricity with a power density that exceeds 700 mW m-2 , one order of magnitude higher than the best previously published result. X-ray absorption and density functional theory studies support a mechanism of operation that relies on the polarization of sp2 hybridized carbon atoms. The devices are incorporated into a flexible fabric for powering personal electronic devices.

Despite the recent progress on two-dimensional multilayer materials (2DMM) with weak interlayer interactions, the investigation on 2DMM with strong interlayer interactions is far from its sufficiency. Here we report on first-principles calculations that clarify the structural evolution and optoelectronic properties of such a 2DMM, multilayer silicene. With our newly developed global optimization algorithm, we discover the existence of rich dynamically stable multilayer silicene phases, the stability of which is closely related to the extent of sp3 hybridization that can be evaluated by the average bonds and effective bond angles. The stable Si(111) surface structures are obtained when the silicene thickness gets up to four, showing the critical thickness for the structural evolution. We also find that the multilayer silicene with pi-bonded surfaces present outstanding optoelectronic properties for the solar cells and optical fiber communications due to the incorporation of sp2-type bonds in the sp3-type bonds dominated system. This study is helpful to complete the picture of structure and related property evolution of 2DMM with strong interlayer interactions., 4 figures

In a semimetal, both electrons and holes contribute to the density of states at the Fermi level. The small band overlaps and multiband effects engender novel electronic properties. We show that a moderate hydrostatic pressure effectively suppresses the band gap in the elemental semiconductor black phosphorus. An electronic topological transition takes place at approximately 1.2 GPa, above which black phosphorus evolves into a semimetal state that is characterized by a colossal positive magnetoresistance and a nonlinear field dependence of Hall resistivity. The Shubnikov-de Haas oscillations detected in magnetic field reveal the complex Fermi surface topology of the semimetallic phase. In particular, we find a nontrivial Berry phase in one Fermi surface that emerges in the semimetal state, as evidence of a Dirac-like dispersion. The observed semimetallic behavior greatly enriches the material property of black phosphorus and sets the stage for the exploration of novel electronic states in this material.

Band offsets between different semiconductors are important parameters that determine the electronic transport properties near the interface in the heterostructure devices. The computation of the natural band offset is a well-known challenge. In this paper, we propose a new method, which is called the three-step method, to accurately predict the natural band offset. Compared to previous methods, the present method is more direct and can be easily applied to systems with larger lattice mismatch and to systems with lower symmetry. Using the present method, we successfully calculate the natural band offset between the inorganic halide perovskites $AB{X}_{3}\phantom{\rule{0.28em}{0ex}}(A=\mathrm{Cs};\phantom{\rule{0.28em}{0ex}}B=\mathrm{Sn},\phantom{\rule{0.28em}{0ex}}\mathrm{Pb};\phantom{\rule{0.28em}{0ex}}X=\mathrm{Cl},\phantom{\rule{0.28em}{0ex}}\mathrm{B},\phantom{\rule{0.28em}{0ex}}\mathrm{I})$ in the cubic and orthorhombic phase. We show that the valence band maximum shifts down as the atomic number of the $X$ site ion increases, while the valence band maximum shifts up as $B$ site ion varies from Sn to Pb or as the compound transforms from the cubic phase to the orthorhombic phase. It is found that the band gap differences between these compounds can be attributed primarily to the valence band offsets, with a much smaller contribution from the conduction band offsets.

The organic-inorganic hybrid perovskite CH3NH3PbI3 has attracted significant interest for its high performance in converting solar light into electrical power with an efficiency exceeding 20%. Unfortunately, chemical stability is one major challenge in the development of the CH3NH3PbI3 solar cells. It was commonly assumed that moisture or oxygen in the environment causes the poor stability of hybrid halide perovskites, however, here we show from the first-principles calculations that the room-temperature tetragonal phase of CH3NH3PbI3 is thermodynamically unstable with respect to the phase separation into CH3NH3I + PbI2, i.e., the disproportionation is exothermic, independent of the humidity or oxygen in the atmosphere. When the structure is distorted to the low-temperature orthorhombic phase, the energetic cost of separation increases, but remains small. Contributions from vibrational and configurational entropy at room temperature have been considered, but the instability of CH3NH3PbI3 is unchanged. When I is replaced by Br or Cl, Pb by Sn, or the organic cation CH3NH3 by inorganic Cs, the perovskites become more stable and do not phase-separate spontaneously. Our study highlights that the poor chemical stability is intrinsic to CH3NH3PbI3 and suggests that element-substitution may solve the chemical stability problem in hybrid halide perovskite solar cells., Comment: 11 pages, 3 figures