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A real-space method using generating integers is used to determine possible commensurate lattice Moiré patterns for a bilayer of two equal hexagonal lattices, which can in principle be extended to lattice mismatched bilayers. These Moiré patterns can be classified by a pair of relatively prime integers (n , m) , wherein a rotation θ (n , m) of the top hexagonal lattice maps its lattice vector (n , m) to (m , n) of the bottom lattice. Within this formulation, the area of the commensurate supercell is proportional to (n 2 + m 2 + n m) and the number of coincident lattice sites per supercell is given by (n − m) 2. Taking bilayer boron nitride (BN) as an example, we present how to systematically generate Moiré patterns and explore the differences in local chemistry in the interstitial region by impurity intercalation. Systematic calculations of the properties of intercalated 3d transition metals were performed in an h-BN (4 , 3) bilayer, corresponding to a rotation of 9.43 degrees. These calculations reveal that local symmetry in the intercalated region significantly affect the energetics and magnetization of the intercalated species. These results highlight that Moiré pattern physics is not limited to optoelectronic/electronic phenomena, such as interfacial exciton formation or magic angle superconductivity, but it also produces chemical and magnetic atomic site selectivity, which may play important roles in adsorption, catalysis, or quantum information. [ABSTRACT FROM AUTHOR]

Disorder‐induced electronic localization is responsible for the OFF state of the Ovonic threshold switching (OTS) device, which is an indispensable component in the present 3D‐crossbar‐architecture phase‐change memory circuit. However, the atomic mechanism of the OTS device, especially the role of thermal effect, remains a long‐term open question. Recent researches suggest that the working current of the OTS ON‐state is often large enough to melt the adjacent phase‐change memory material in the OTS+PCM devices. Thus, Joule heating‐induced atomic/electronic structure transition in OTS materials must be seriously considered. Taking the typical OTS material GeSe as an example, first‐principles calculations reveal an unexpected electronic delocalization induced by the enhanced structural disorder upon solid‐liquid transition. Meanwhile, as the temperature rises, the band gap decreases or even closes, leading to an increase in carrier concentration. Therefore, the melting filament with high conductivity could be responsible for the holding‐ON state of OTS materials. The results in this study may provide possible explanations for some puzzles of OTS devices, such as the reasons for holding‐ON state and limited endurance. [ABSTRACT FROM AUTHOR]

Intrinsic silicon (Si) is forbidden for infrared (IR) sensing at the communication wavelength like 1.31 or 1.55 µm due to the well‐known bandgap limitation. In this work, an unexpected physical picture of using argon (Ar) is identified, which is usually inert to the surrounding chemical environment and used as a protective agent in semiconductor processing, to overcome the IR‐sensing‐forbidden problem in Si. Here, it is shown by an analysis of a dynamic secondary ion mass spectrometer that such a Si, when exposed to laser pulse in Ar gas, can contain a very high dose of Ar up to 1020 cm−3 even after 1300 days. First‐principles calculations, molecular dynamics, and Hall effect measurements reveal that, due to both steric and dynamic repulsions by Ar orbitals to Si dangling bonds, the Ar‐filled‐vacancy produces a much wider defect band inside the gap, which is not only responsible for strong infrared absorption, but also causes a significant increase in n‐type conductivity, both in line with experiments. The study proves that originally inert elements in fact can act as active impurities in semiconductors for advanced applications, which updates the current knowledge of defect physics. [ABSTRACT FROM AUTHOR]

Chalcogenide perovskites have emerged as promising semiconductor materials due to their appealing properties, including tunable bandgaps, high absorption coefficients, reasonable carrier lifetimes and mobilities, excellent chemical stability, and environmentally benign nature. However, beyond the well‐studied BaZrS3, reports on chalcogenide perovskite thin films with diverse compositions are scarce. In this study, the realization of four different types of chalcogenide perovskite thin films with controlled phases, through CS2 annealing of amorphous chalcogenide precursor films deposited by pulsed laser deposition (PLD), is reported. This achievement is guided by a thorough theoretical investigation of the phase stability of chalcogenide perovskites. Upon crystallization in the distorted perovskite phase, all materials exhibit photoluminescence (PL) with peak positions in the visible range, consistent with their expected bandgap values. However, the full‐width‐at‐half‐maximum (FWHM) of the PL spectra varies significantly across these materials, ranging from 99 meV for SrHfS3 to 231 meV for BaHfS3. The difference is attributed to the difference in kinetic barriers between local structural motifs for the Sr and Ba compounds. The findings underscore the promise of chalcogenide perovskite thin films as an alternative to traditional halide perovskites for optoelectronic applications, while highlighting the challenges in optimizing their synthesis and performance. [ABSTRACT FROM AUTHOR]

We investigated band structures of α-graphyne and its derivative two-dimensional carbon compounds (2DCCs) via tight-binding approximations with 'two-site' and 'all-atom' models. The renormalized 'two-site' model captures the band-gap features of α-graphyne and 2DCCs. This model suggests ways of tuning the band gaps of graphynes, namely, by adding adatoms or substituting the vertex sp2 carbons with heteroatom. Because the 'two-site' model cannot accurately reproduce first-principles results over a large range of wave vectors, we derived an 'all-atom' model, which includes all pz orbitals in a unit cell. All atom tight-binding calculations show improved performances in describing the DFT band structures, and reveal that the flat bands in DFT band structures are mainly ascribed to the pz orbitals of the edge carbons. The results will help to uncover the underlying mechanisms of the band features of graphyne and 2DCCs and to design other graphyne- or graphdiyne-based 2DCCs for applications in the future. [ABSTRACT FROM AUTHOR]

Phase‐change random‐access memory (PCRAM) devices suffer from pronounced resistance drift originating from considerable structural relaxation of phase‐change materials (PCMs), which hinders current developments of high‐capacity memory and high‐parallelism computing that both need reliable multibit programming. This work realizes that compositional simplification and geometrical miniaturization of traditional GeSbTe‐like PCMs are feasible routes to suppress relaxation. While to date, the aging mechanisms of the simplest PCM, Sb, at nanoscale, have not yet been unveiled. Here, this work demonstrates that in an optimal thickness of only 4 nm, the thin Sb film can enable a precise multilevel programming with ultralow resistance drift coefficients, in a regime of ≈10−4–10−3. This advancement is mainly owed to the slightly changed Peierls distortion in Sb and the less‐distorted octahedral‐like atomic configurations across the Sb/SiO2 interfaces. This work highlights a new indispensable approach, interfacial regulation of nanoscale PCMs, for pursuing ultimately reliable resistance control in aggressively‐miniaturized PCRAM devices, to boost the storage and computing efficiencies substantially. [ABSTRACT FROM AUTHOR]

Improving electronic structure calculations for practical and technologically important materials has been a never-ending pursue. This is especially true for transition and post-transition metal oxides for which the current first-principles approaches still suffer various drawbacks. Here, we present a hierarchical-hybrid functional approach built on the use of pseudopotentials. The key is to introduce different amounts of exact exchange to core and valence electrons. It allows for treating the delocalization errors of sp and d electrons differently, which have been known to be an important source of error for the band structure. Using wurtzite ZnO as a prototype, we show that the approach is successful in simultaneously reproducing the bandgap and d-band position. Importantly, the same approach, without having to change the hybrid mixing parameters from those of Zn, works reasonably well for other binary 3d transition and post-transition metal oxides across board. Our findings thus point out a new direction of systematically improving the exchange functional in first-principles calculations. [ABSTRACT FROM AUTHOR]

[GeTe] m -[Sb 2 Te 3 ] n superlattice (GST-SL) is a promising material candidate to solve the critical problem of high-power consumption for phase-change memory technology. However, the switching mechanism is still under strong debate during the last decade. A key controversial question is that whether melting in GST-SL is possible. In this work, a large-scale machine learning interatomic potential (MLIP) molecular dynamics (MD) simulation with a one-order-of-magnitude larger atom number than that of conventional density functional theory (DFT) MD captures a unique partial melting behavior in GST-SL, where a sparse-nucleation-and-growth governed melting behavior is triggered by the flipping of atoms into van der Waals (vdW) gaps and forming cation-in-vdW-gap (CiV) defect as starting point, and then is slowly developed via propagating liquid-crystalline interfaces. In great contrast, the traditional melting of rock salt (RS)-GST occurs very fast due to instant homogeneous nucleation from high-density intrinsic vacancies distributed randomly in its cubic lattice. Therefore, the melting regions in GST-SL can be spatially localized in form of partial melting and are readily controlled. Moreover, the atomic distribution after melting in GST-SL is more chemical ordering than that in RS-GST. By the large-scale MLIP-MD, the present study provides a critical atomic insight into GST-SL phase-change behaviors that conventional DFT-MDs hardly achieve. This partial-melting nature in GST-SL helps explain the long-term-confused working principle of GST-SL-based phase-change memory, which will accelerate its application in the big-data era. [Display omitted] [ABSTRACT FROM AUTHOR]

Phase change memory has been considered as the next generation in non-volatile electronic data storage. The property modulation of such materials by the doping of rare-earth elements has drawn a lot of attention, which motivates us to search for the optimal dopants and reveal the underlying mechanisms. Here, we investigate the role of Gd as a dopant in Ge2Sb2Te5, which exhibits higher crystalline resistance and better thermal stability and antioxidant capacity than the undoped counterpart. Moreover, Gd dopants suppress both the processes of phase transition and grain growth. The crystalline structure remains unchanged with Gd dopants and vacancies are randomly distributed. Furthermore, the bonding mechanism was theoretically investigated. In the amorphous state, Gd atoms modify the local structures around Ge, Sb, and Te atoms. The large coordination number of Gd and the "Gd–Te distorted pentagonal bipyramidal-like" structure can be attributed to the good thermal stability. These microscopic findings figure out some of the key issues about the bonding mechanism, electrical properties, and crystallization behaviors of Gd doped phase change memory materials, which could be useful for storage devices. [ABSTRACT FROM AUTHOR]

Lead halide perovskites have emerged as promising photovoltaic (PV) materials owing to their superior optoelectronic properties. However, they suffer from poor stability and potential toxicity. Here, computational screening with experimental synthesis is combined to explore stable, lead‐free, and defect‐tolerant PV materials. Heavy cations with lone‐pair electrons and mixed anions of chalcogens and halogens as a descriptor for simultaneous realization of defect tolerance and high stability are adopted. Together with the criteria of possessing direct band gap and optimal gap value, the inorganic material database is screened and CuBiSCl2 in the post‐perovskite structure is identified with an ideal band gap of 1.37 eV. The electronic structure and defect calculations suggest its defect‐tolerant characteristics. By optical absorption measurement, its band gap is confirmed to be ≈1.44 eV, with strong absorption near the band edge. The material is stable against thermal decomposition up to 300 °C and can survive from 25 days of storage at ambient conditions with 60% relative humidity. Prototype solar cells are fabricated and demonstrate an open circuit voltage of 1.09 V and a power conversion efficiency of 1.00%. With the excellent properties above, CuBiSCl2 is proposed to be a promising candidate for PV application. [ABSTRACT FROM AUTHOR]

High-temperature phases of solids are often dynamically stable only. First-principles study of point defects in such solids at 0 K is prohibited by their static instability, which results in random structures of the defect-containing supercell so that the total energy of the supercell is randomly affected by structural distortions far away from the defect. Taking cubic perovskite α -CsPbI3 as an example, we first present the problem incurred by the static instability and then propose an approach based on molecular dynamics to carry out ensemble average for tackling the problem. Within affordable simulation time, we obtain converged defect ionization energies, which are unattainable by a standard approach and allow us to evaluate its defect tolerance property. Our work paves the way for studying defects in statically unstable solids. [ABSTRACT FROM AUTHOR]

The interactions between neutral AlX( I) (X = Al, C, N and P) nanoparticles and DNA nucleobases, namely adenine (A), thymine (T), guanine (G) and cytosine (C), as well as the Watson−Crick base pairs (BPs) AT and GC, were investigated by means of density functional theory computations. The AlX clusters can tightly bind to DNA bases and BPs to form stable complexes with negative binding Gibbs free energies at room temperature, and considerable charge transfers occur between the bases/BPs and the AlX clusters. These strong interactions, which are also expected for larger Al nanoparticles, may have potentially adverse impacts on the structure and stability of DNA and thus cause its dysfunction. [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

First-principles calculations reveal a Mexican-hat potential energy surface (PES) for two-dimensional (2D) In2Se3. This unique PES leads to a pseudo-centrosymmetric paraelectric β phase that resolves the current controversy between theory and experiment. We further show that while the α-to-β (ferroelectric-to-paraelectric) phase transition is fast and coherent, assisted by an in-plane shear phonon mode, a random distribution of the atoms in the trough of the PES acts as an entropy barrier against the reverse β-to-α transition. This will be the origin of the speed limitation of current In2Se3 ferroelectric devices. However, if one orders the β phase (due to the formation of in-plane ferroelectric domains), the reverse transition can take place within tens of picoseconds in the presence of a perpendicular electric field. Finally, the Mexican-hat PES is a general feature for the entire family of 2D III2-VI3 materials. Our finding offers a critical physical picture in controlling the ultrafast reversible phase transition in 2D In2Se3 and other III2-VI3 materials, which will benefit their practical industrial development for advanced ferroelectric devices. [ABSTRACT FROM AUTHOR]

One central task of developing nonvolatile phase change memory (PCM) is to improve its scalability for high‐density data integration. In this work, by first‐principles molecular dynamics, to date the thinnest PCM material possible (0.8 nm), namely, a monolayer Sb2Te3, is proposed. Importantly, its SET (crystallization) process is a fast one‐step transition from amorphous to hexagonal phase without the usual intermediate cubic phase. An increased spatial localization of electrons due to geometrical confinement is found to be beneficial for keeping the data nonvolatile in the amorphous phase at the 2D limit. The substrate and superstrate can be utilized to control the phase change behavior: e.g., with passivated SiO2 (001) surfaces or hexagonal Boron Nitride, the monolayer Sb2Te3 can reach SET recrystallization in 0.54 ns or even as fast as 0.12 ns, but with unpassivated SiO2 (001), this would not be possible. Besides, working with small volume PCM materials is also a natural way to lower power consumption. Therefore, the proposed PCM working process at the 2D limit will be an important potential strategy of scaling the current PCM materials for ultrahigh‐density data storage. [ABSTRACT FROM AUTHOR]

One central task of developing nonvolatile phase change memory (PCM) is to improve its scalability for high‐density data integration. In this work, by first‐principles molecular dynamics, to date the thinnest PCM material possible (0.8 nm), namely, a monolayer Sb2Te3, is proposed. Importantly, its SET (crystallization) process is a fast one‐step transition from amorphous to hexagonal phase without the usual intermediate cubic phase. An increased spatial localization of electrons due to geometrical confinement is found to be beneficial for keeping the data nonvolatile in the amorphous phase at the 2D limit. The substrate and superstrate can be utilized to control the phase change behavior: e.g., with passivated SiO2 (001) surfaces or hexagonal Boron Nitride, the monolayer Sb2Te3 can reach SET recrystallization in 0.54 ns or even as fast as 0.12 ns, but with unpassivated SiO2 (001), this would not be possible. Besides, working with small volume PCM materials is also a natural way to lower power consumption. Therefore, the proposed PCM working process at the 2D limit will be an important potential strategy of scaling the current PCM materials for ultrahigh‐density data storage. [ABSTRACT FROM AUTHOR]

Phase change memory (PCM) is an emerging non‐volatile data storage technology concerned by the semiconductor industry. To improve the performances, previous efforts have mainly focused on partially replacing or doping elements in the flagship Ge‐Sb‐Te (GST) alloy based on experimental "trial‐and‐error" methods. Here, the current largest scale PCM materials searching is reported, starting with 124 515 candidate materials, using a rational high‐throughput screening strategy consisting of criteria related to PCM characteristics. In the results, there are 158 candidates screened for PCM materials, of which ≈68% are not employed. By further analyses, including cohesive energy, bond angle analyses, and Born effective charge, there are 52 materials with properties similar to the GST system, including Ge2Bi2Te5, GeAs4Te7, GeAs2Te4, so on and other candidates that have not been reported, such as TlBiTe2, TlSbTe2, CdPb3Se4, etc. Compared with GST, materials with close cohesive energy include AgBiTe2, TlSbTe2, As2Te3, TlBiTe2, etc., indicating possible low power consumption. Through further melt‐quenching molecular dynamic calculation and structural/electronic analyses, Ge2Bi2Te5, CdPb3Se4, MnBi2Te4, and TlBiTe2 are found suitable for optical/electrical PCM applications, which further verifies the effectiveness of this strategy. The present study will accelerate the exploration and development of advanced PCM materials for current and future big‐data applications. [ABSTRACT FROM AUTHOR]

Transparent conducting materials (TCMs) have been widely used in optoelectronic applications such as touchscreens, flat panel displays and thin film solar cells. These applications of TCMs are currently dominated by n-type doped oxides. High-performance p-type TCMs are still lacking due to their low hole mobility or p-type doping bottleneck, which impedes efficient device design and novel applications such as transparent electronics. Here, based on first-principles calculations, we propose chalcogenide perovskite YScS3 as a promising p-type TCM. According to our calculations, its optical absorption onset is above 3 eV, which allows transparency to visible light. Its hole conductivity effective mass is 0.48m0, which is among the smallest in p-type TCMs, suggesting enhanced hole mobility. It could be doped to p-type by group-II elements on cation sites, all of which yield shallow acceptors. Combining these properties, YScS3 holds great promise to enhancing the performance of p-type TCMs toward their n-type counterparts. [ABSTRACT FROM AUTHOR]

Carbon has numerous one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D) allotropic structures. The study of carbon materials has long been a major focus of material sciences and condensed matter physics. Recently, a number of topological semi-metallic carbon allotropes with vastly different topological phases have been predicted from first-principles, showing exceptionally clean and robust topological properties near the Fermi surfaces. In this review, we provide a brief summary of the development of carbon allotropes from 1D to 3D. Then, we discuss topological properties of pure carbon materials, despite their exceedingly small spin–orbit coupling. Instead, the nearest-neighbor p z orbit interaction has been identified as the origin for the non-trivial topological properties, which vary due to different p z connectivity:​ zigzag, armchair, or pentagonal, as well as the interactions among further neighboring atoms. Next, we consider possible expansions of the topological properties of carbon materials to other light-element materials such as boron. Finally, we discuss future prospects in light of experimental feasibility and the recent surge in pursue of exotic carbon physics due to twisted graphene, which may also be viewed as a subset of the topological physics involving sp 2 /p z carbon. [ABSTRACT FROM AUTHOR]

Organic–inorganic halide perovskites are intrinsically unstable when exposed to moisture and/or light. Additionally, the presence of lead in many perovskites raises toxicity concerns. Herein, a thin film of barium zirconium sulfide (BaZrS3), a lead‐free chalcogenide perovskite, is reported. Photoluminescence and X‐ray diffraction measurements show that BaZrS3 is far more stable than methylammonium lead iodide (MAPbI3) in moist environments. Moisture‐ and light‐induced degradations in BaZrS3 and MAPbI3 are compared by using simulations and calculations based on density functional theory. The simulations reveal drastically slower degradation in BaZrS3 due to two factors—weak interaction with water and very low rates of ion migration. BaZrS3 photodetecting devices with photoresponsivity of ≈46.5 mA W−1 are also reported. The devices retain ≈60% of their initial photoresponse after 4 weeks under ambient conditions. Similar MAPbI3 devices degrade rapidly and show a ≈95% decrease in photoresponsivity in just 4 days. The findings establish the superior stability of BaZrS3 and strengthen the case for its use in optoelectronics. New possibilities for thermoelectric energy conversion using these materials are also demonstrated. [ABSTRACT FROM AUTHOR]

Prompted by the very recent claim that the volleyball-shaped B80 fullerene [X. Wang, Phys. Rev. B 82, 153409 (2010)] is lower in energy than the B80 buckyball [N. G. Szwacki, A. Sadrzadeh, and B. I. Yakobson, Phys. Rev. Lett. 98, 166804 (2007)] and core-shell structure [J. Zhao, L. Wang, F. Li, and Z. Chen, J. Phys. Chem. A 114, 9969 (2010)], and inspired by the most recent finding of another core-shell isomer as the lowest energy B80 isomer [S. De, A. Willand, M. Amsler, P. Pochet, L. Genovese, and S. Goedecher, Phys. Rev. Lett. 106, 225502 (2011)], we carefully evaluated the performance of the density functional methods in the energetics of boron clusters and confirmed that the core-shell construction (stuffed fullerene) is thermodynamically the most favorable structural pattern for B80. Our global minimum search showed that both B101 and B103 also prefer a core-shell structure and that B103 can reach the complete core-shell configuration. We called for great attention to the theoretical community when using density functionals to investigate boron-related nanomaterials. [ABSTRACT FROM AUTHOR]

The Raman spectra of several locally stable structures of the graphene oxide (GO) have been simulated by ab initio calculations. Compared to graphite, the G band of GO is broadened and blueshifted due to the emergence of a series of new Raman peaks. The Raman intensities and positions of the D and G bands depend sensitively on the local atomic configurations. In addition to the normal epoxy and hydroxyl groups, other oxidation groups such as epoxy pairs are also studied. Epoxy pairs induce large blueshift of G band with respect to that of the graphite. [ABSTRACT FROM AUTHOR]

Abstract: Ferroelectricity is usually found in compound materials composed by different elements. Here, based on first‐principles calculations, spontaneous electric polarization and ferroelectricity in 2D elemental group‐V (As, Sb, and Bi) monolayer with the puckered lattice structure similar to phosphorene is revealed. These are the first example of elemental ferroelectric materials. The polarization is due to the spontaneous lattice distortion with atomic layer buckling and has quite sizable values, comparable or even larger than that recently found in 2D monolayer compound SnTe. Interestingly, for Bi monolayer, apart from the ferroelectric phase, it is found that it can also host an antiferroelectric phase. The Curie temperatures of these elemental materials can be higher than room temperature, making them promising for realizing ultrathin ferroelectric devices of broad interest. A general model is constructed to understand and search for 2D ferroelectric and antiferroelectric materials in future studies. [ABSTRACT FROM AUTHOR]

Phase-change memory (PCM) material is the promising material system for nonvolatile-memory technology. Performance optimization of PCM device urgently requires the deeper clarification of its material “Gene”. In this study, through first-principles calculations, p -orbital-aligned atom chains are identified to play important roles in governing optoelectronic reflectivity in amorphous Ge 2 Sb 2 Te 5 . These atom chains make the electronic state of the amorphous Ge 2 Sb 2 Te 5 hold strong electron-polarized components, thereby governing the optical property. The present study offers a new understanding of “Gene” for PCM materials which benefit the material design and the performance improvement of PCM devices. [ABSTRACT FROM AUTHOR]

GeSbTe alloys have the ability of rapidly transforming between amorphous and crystalline phases. Therefore, they can be used in the non-volatile phase change memory. Recently, a vacancy-ordered cubic Ge 2 Sb 2 Te 5 ( VOC GST) phase change material where the vacancies are highly ordered in the (111) plane, has been experimentally demonstrated by STEM. However, studies are mainly on the structural characterization, rather than on the phase change behavior and possible applications of the VOC GST. Here, using first-principles molecular dynamic simulations, we study the melt-quenched amorphization process and its possible applications. We find that the VOC GST exhibits a quasi-two-dimensional amorphization process that is triggered by the diffusion of Ge atoms but not others. A partial amorphous ( P-amor ) phase is obtained, which can act as an intermediate state between the pure amorphous and pure crystalline phases for possible ternary-state data storage. [ABSTRACT FROM AUTHOR]

Single crystal CdTe films are desirable for optoelectronic device applications. An important strategy of creating films with high crystallinity is through epitaxial growth on a proper single crystal substrate. We report the metalorganic chemical vapor deposition of epitaxial CdTe films on the CdS/mica substrate. The epitaxial CdS film was grown on a mica surface by thermal evaporation. Due to the weak van der Waals forces, epitaxy is achieved despite the very large interface lattice mismatch between CdS and mica (∼21–55%). The surface morphology of mica, CdS and CdTe were quantified by atomic force microscopy. The near surface structures, orientations and texture of CdTe and CdS films were characterized by the unique reflection high-energy electron diffraction surface pole figure technique. The interfaces of CdTe and CdS films and mica were characterized by X-ray pole figure technique and transmission electron microscopy. The out-of-plane and in-plane epitaxy of the heteroepitaxial films stack are determined to be CdTe(111)//CdS(0001)//mica(001) and [ 1 ¯ 2 1 ¯ ] CdTe //[ 1 ¯ 100] CdS //[010] mica , respectively. The measured photoluminescence (PL), time resolved PL, photoresponse, and Hall mobility of the CdTe/CdS/mica indicate quality films. The use of van der Waals surface to grow epitaxial CdTe/CdS films offers an alternative strategy towards infrared imaging and solar cell applications. [ABSTRACT FROM AUTHOR]

Spontaneous construction of graphene on boron-doped diamond (111) surface has been found in first-principles calculations. Boron-doping-induced surface reconstruction is the mechanism responsible for the diamond-to-graphene phase transition. Furthermore, the effect on the surface is unexpectedly observed at any doping depth down to the 7th layer, and a low concentration of substituent boron – only ¼ – is effective for graphene formation. Amazingly, when boron atoms are incorporated into the fifth layer, the direct optimization of the 1 × 1 surface automatically gives rise to a graphene structure from the first bilayer with no energy barrier, indicating that the formation of graphene is spontaneous. These findings provide an alternative strategy for graphene synthesis on wide-gap insulators with high thermal conductivity. [ABSTRACT FROM AUTHOR]

Sulfur-doped anatase TiO 2 nanoparticles with sulfur atoms occupying oxygen sites were realized by low-temperature hydrothermal oxidation of titanium disulfide (TiS 2 ) with deionized water as the solvent. S doping can effectively narrow the bandgap of TiO 2 due to the fact that S 3 p level is higher than O 2p orbital. The large surface area caused by this doping method induces higher photocatalytic activity in the visible regime compared with S-TiO 2 oxidized at high temperature. The low-temperature hydrothermal oxidation of TiS 2 in pH neutral solvent is an environmental-friendly method to obtain S-TiO 2 with high photocatalytic activity, which can be extended to other nonmetal doping of TiO 2 or other semiconductors for bandgap engineering. [ABSTRACT FROM AUTHOR]

The repercussions of an inaccurate account of electronic states near the Fermi level by hybrid functionals in predicting several important metallic properties are investigated. The difficulties include a vanishing or severely suppressed density of states (DOS) at E F , significantly widened valence bandwidth, greatly enhanced electron–phonon (el–ph) deformation potentials, and an overestimate of magnetic moment in transition metals. The erroneously enhanced el–ph coupling calculated by hybrid functionals may lead to a false prediction of lattice instability. The main culprit of the problem comes from the simplistic treatment of the exchange functional rooted in the original Fock exchange energy. The use of a short-ranged Coulomb interaction alleviates some of the drawbacks but the fundamental issues remain unchanged. [ABSTRACT FROM AUTHOR]

The Dirac bands and flat bands are difficult to coexist because they represent two extreme ends of electronic properties. However, in this paper, we propose a carbon-Kagome-lattice (CKL) family based on first-principles calculations, and the coexistence of Dirac and flat bands are observed in this series of three-dimensional carbon structures. The flat bands are originated from the orbital interactions of the Kagome lattices, while the Dirac bands are related to the carbon zigzag chains. A tight-binding model is used to explain the various band structures in different CKLs. The coexistence of flat and Dirac bands around the Fermi level implies that CKL structures maybe can serve as superconductors. In addition, electronic properties of the thinnest CKL slabs, only consisting of benzene rings, are studied. Flat bands are found in the band spectra of the two-dimensional structures, and split into spin-up and spin-down bands because of strong correlated effect in the case of hole doping. [ABSTRACT FROM AUTHOR]

Cube-textured Ni(001) foils have been considered as a viable alternative substrate to grow high quality functional films for large area optoelectronic devices. In this work, we report the heteroepitaxial growth of CaF 2 (001) films on cube-textured Ni(001) foils at 350–600 °C with in-plane orientation of CaF 2 [110]//Ni[100] and CaF 2 [ 1 - 10 ]//Ni[010] with 45 ° rotation respect to the Ni(001) substrate. Unlike CaF 2 (111)/Ni(001) films where there exist four independent rotational domains with rotational domain boundaries, CaF 2 (001)/Ni(001) contains no rotational domains or rotational domain boundaries. This makes CaF 2 (001)/Ni(001) films better candidates as templates for the growth of high quality functional semiconductors. We also demonstrate that Ge(001) film with no rotational domains and with a grain size of ~ 50 μm similar to that of the Ni substrate can be grown on the CaF 2 (001) buffered Ni substrate. [ABSTRACT FROM AUTHOR]

Ge 1 Cu 2 Te 3 is an important candidate for high-temperature phase change memory due to the fine amorphous stability. Yet, the basic bonding chemistry for its high-temperature application is still not completely clear. In this work, a new bonding mechanism for its amorphous and crystalline phases is proposed and demonstrated by first-principles calculations. Compared to the tetrahedral environment distributed evenly in crystalline form, Cu atoms in the amorphous state tend to be accumulated as trigonal clusters. For the crystalline phase, a bonding configuration of nonequivalent sp 3 hybridization with Te lone-pair electrons is proposed without Cu d electron participation. In the amorphous phase, however, a significant bonding reconfiguration of Cu d electrons occurs due to the isolation of the Te lone-pair electrons. Therefore, the notable contrast in the Cu atomic and electronic structures between the crystalline and amorphous phases results in an obvious phase transition barrier for high-temperature storage. The mechanism presented in this study serves as a reference for other transition-metal alloyed phase-change materials. [ABSTRACT FROM AUTHOR]

The physical and chemical behaviorsof materials used in energystorage devices, such as lithium-ion batteries (LIBs), are mainlycontrolled by an electrochemical process, which normally involvesinsertion/extraction of ions into/from a host lattice with a concurrentflow of electrons to compensate charge balance. The fundamental physicsand chemistry governing the behavior of materials in response to theions insertion/extraction is not known. Herein, a combination of insitu lithiation experiments and large-scale ab initio molecular dynamicssimulations are performed to explore the mechanisms of the electrochemicallydriven solid-state amorphization in Li–Si systems. We findthat local electron-rich condition governs the electrochemically drivensolid-state amorphization of Li–Si alloys. This discovery providesthe fundamental explanation of why lithium insertion in semiconductorand insulators leads to amorphization, whereas in metals, it leadsto a crystalline alloy. The present work correlates electrochemicallydriven reactions with ion insertion, electron transfer, lattice stability,and phase equilibrium. [ABSTRACT FROM AUTHOR]

Via in situ TEM tensile tests onsingle crystalline copper nanowireswith an advanced tensile device, we report here a crystalline-liquid-rubber-like(CRYS-LIQUE-R) behavior in fracturing crystalline metallic nanowires.A retractable strain of the fractured crystalline Cu nanowires canapproach over 35%. This astonishing CRYS-LIQUE-R behavior of the fracturinghighly strained single crystalline Cu nanowires originates from aninstant release of the stored ultralarge elastic energy in the crystallinenanowires. The release of the ultralarge elastic energy was estimatedto generate a huge reverse stress as high as ∼10 GPa. The effectivediffusion coefficient (Deff) increasedsharply due to the consequent pressure gradient. In addition, dueto the release of ultrahigh elastic energy, the estimated concomitanttemperature increase was estimated as high as 0.6 Tm (Tm is the meltingpoint of nanocrystalline Cu) on the fractured tip of the nanowires.These factors greatly enhanced the atomic diffusion process. Moleculardynamic simulations revealed that the very high reverse stress triggereddislocation nucleation and exhaustion. [ABSTRACT FROM AUTHOR]

We present a direct atom-by-atomchemical identification of thenanostructures and defects of topological insulators (TIs) with astate-of-the-art atomic mapping technology. Combining this techniqueand density function theory calculations, we identify and explainthe layer chemistry evolution of Bi2Te3–xSexternary TIs. We alsoreveal a long neglected but crucially important extended defect foundto be universally present in Bi2Te3films, theseven-layer Bi3Te4nanolamella acceptors. Intriguingly,this defect is found to locally pull down the conduction band, leadingto local n-type conductivity, despite being an acceptor which pinsthe Fermi energy near the valence band maximum. This nanolamella mayexplain inconsistencies in measured conduction type as well as openup a new route to manipulate bulk carrier concentration. Our workmay pave the way to more thoroughly understand and tailor the natureof the bulk, as well as secure controllable bulk states for futureapplications in quantum computing and dissipationless devices. [ABSTRACT FROM AUTHOR]

Abstract: Here we establish a fundamental principle to open a relatively large band gap for graphene by hydrogenation. Specifically, the large band gap can be obtained when the number of nonmagnetic sp2-substructures on graphene basal plane is maximized. The principle indicates unequivocally what additive patterning should be used to attain the largest band gap for a given addition coverage. According to this principle, the graphene band gap can be continuously tuned from 0 to 5.2eV. These findings may open a door to create graphene-based nanosystems with desired band gaps, and will become a significant foundation for graphene nanotechnology applications. [Copyright &y& Elsevier]

Abstract: Lattice parameters and lattice energies of twelve selected molecular crystals are computed by using density functional theory (DFT) with different treatments of dispersion correction, including the local atomic potential (LAP) and three popular DFT-D methods. Inclusion of LAPs improves the description of intermolecular interactions in molecular crystals over standard DFT calculations. The DFT+LAP approach provides accurate structural parameters and lattice energies that are comparable to the PBE-Grimme scheme. Our results suggest that the DFT+LAP approach is a promising alternative for efficient and accurate quantum simulations on molecular crystals and other systems involving noncovalent interactions. [Copyright &y& Elsevier]

The intriguing electronic and magnetic properties of fully and partially hydrogenated boron nitride nanoribbons (BNNRs) were investigated by means of first-principles computations. Independent of ribbon width, fully hydrogenated armchair BNNRs are nonmagnetic semiconductors, while the zigzag counterparts are magnetic and metallic. The partially hydrogenated zigzag BNNRs (using hydrogenated BNNRs and pristine BNNRs as building units) exhibit diverse electronic and magnetic properties: they are nonmagnetic semiconductors when the percentage of hydrogenated BNNR blocks is minor, while a semiconductor-halt-metal-metal transition occurs, accompanied by a nonmagnetic-magnetic transfer, when the hydrogenated part is dominant. Although the half-metallic property is not robust when the hydrogenation ratio is large, this behavior is sustained for partially hydrogenated zigzag BNNRs with a smaller degree of hydrogenation. Thus, controlling the hydrogenation ratio can precisely modulate the electronic and magnetic properties of zigzag BNNRs, which endows BN nanomaterials many potential applications in the novel integrated functional nanodevices. [ABSTRACT FROM AUTHOR]

2D Amphiphilic Amorphous photoreactors, with two components of rich-sulfur MoS 2+x and deficient-oxygen MoO 3-y to selectively chemisorb and activate N 2 and H 2 O molecules, produce ammonia by gas-phase nitrogen fixation at room temperature and normal pressure. [Display omitted] • 2D amorphous nature of photocatalysts provides ubiquitous active sites. • Selective chemisorption and activation of N 2 and H 2 O molecules on amphiphilic surface. • Gas-phase nitrogen fixation at room temperature and normal pressure. • Low-energy cost solution-based synthesis is facile and readily up-scalable. The gas-phase Haber–Bosch process has been the main industrial source of ammonia over the last century. However, its reliance on high temperature and high pressure for enormous energy input emits greenhouse gas to air and is associated with the depletion of fossil fuels. The need of the hour is to replace this method with environmentally sustainable processes, among which, photocatalytic nitrogen reduction has attracted much attention. In this work, we report a low cost, scalable manufacturing of gas-phase photoreactors for nitrogen reduction under ambient conditions based on 2D amorphous molybdenum oxysulfides, i.e. MoS 2+x -MoO 3-y composites. They show excellent nitrogen reduction efficiencies of ∼141 μmol/g/h with remarkably stable performance. The amphiphilic MoS 2+x and MoO 3-y composites selectively chemisorb and activate N 2 and H 2 O molecules, respectively, and enable multiple photodriven redox reactions towards NH 3 evolution. Such photoreactors for ambient ammonia synthesis provides a potentially feasible route towards next-generation gas-phase industrial ammonia production. [ABSTRACT FROM AUTHOR]

We review the concept of stochasticity—i.e., unpredictable or uncontrolled fluctuations in structure, chemistry, or kinetic processes—in materials. We first define six broad classes of stochasticity: equilibrium (thermodynamic) fluctuations; structural/compositional fluctuations; kinetic fluctuations; frustration and degeneracy; imprecision in measurements; and stochasticity in modeling and simulation. In this review, we focus on the first four classes that are inherent to materials phenomena. We next develop a mathematical framework for describing materials stochasticity and then show how it can be broadly applied to these four materials-related stochastic classes. In subsequent sections, we describe structural and compositional fluctuations at small length scales that modify material properties and behavior at larger length scales; systems with engineered fluctuations, concentrating primarily on composite materials; systems in which stochasticity is developed through nucleation and kinetic phenomena; and configurations in which constraints in a given system prevent it from attaining its ground state and cause it to attain several, equally likely (degenerate) states. We next describe how stochasticity in these processes results in variations in physical properties and how these variations are then accentuated by—or amplify—stochasticity in processing and manufacturing procedures. In summary, the origins of materials stochasticity, the degree to which it can be predicted and/or controlled, and the possibility of using stochastic descriptions of materials structure, properties, and processing as a new degree of freedom in materials design are described. [ABSTRACT FROM AUTHOR]

SnO 2 is one of the most interesting oxide semiconductors due to its wide band gap, good transparency, high thermal/chemical resistances, and low cost. As-grown SnO 2 usually exhibits n -type conductivity with high carrier concentrations. Similar to the case of ZnO, it is difficult to dope SnO 2 into p -type. Consequently, applications of SnO 2 for optoelectronic as well as electronic devices are limited. In principle, substitution of group-III elements, including Al, Ga, and In, for Sn atom in SnO 2 could give a hole carrier resulting in a p -type conductivity. Based on the HSE functional calculations (Varley et al., 2009) [13], it has been reported that these dopants are shallow acceptor defects. However, the calculations with PBE0 functional (Scanlon and Watson, 2012) [14] showed that these dopants are deep acceptors. In this work, Ga-doped SnO 2 are revisited by using HSE functional. We found that Ga Sn defect is indeed a deep acceptor indicating that Ga Sn can serve only as a compensating acceptor defect in SnO 2 . Hoping to make the acceptor level shallower, we further study the effect of (compressive) strain on the acceptor level associated with Ga Sn defect by alloying SnO 2 with Si atoms (Si x Sn 1− x O 2 alloy where x ~ 0.17). Our results showed that even with the application of strain, the acceptor level remains too deep to be useful. [ABSTRACT FROM AUTHOR]

Highly directional-dependent metal-insulator transition is observed in epitaxial double perovskite LaBaCo2O5.5+δ films. The film exhibit metallic along [100], but remain semiconducting along [010] under application of a magnetic field parallel to the surface of the film. The physical origin for the properties is identified as in-plane tensile strain arising from oxygen vacancies. First-principle calculations suggested the tensile strain drastically alters the band gap, and the vanishing gap opens up [100] conduction channels for Fermi-surface electrons. Our observation of strain-induced highly directional-dependent metal-insulator transition may open up new dimension for multifunctional devices. [ABSTRACT FROM AUTHOR]

Giant optical transmittance changes of over 300% in wide wavelength range from 500 nm to 2500 nm were observed in LaBaCo2O5.5+δ thin films annealed in air and ethanol ambient, respectively. The reduction process induces high density of ordered oxygen vacancies and the formation of LaBaCo2O5.5 (δ = 0) structure evidenced by aberration-corrected transmission electron microscopy. Moreover, the first-principles calculations reveal the origin and mechanism of optical transmittance enhancement in LaBaCo2O5.5 (δ = 0), which exhibits quite different energy band structure compared to that of LaBaCo2O6 (δ = 0.5). The discrepancy of energy band structure was thought to be the direct reason for the enhancement of optical transmission in reducing ambient. Hence, LaBaCo2O5.5+δ thin films show great prospect for applications on optical gas sensors in reducing/oxidizing atmosphere. [ABSTRACT FROM AUTHOR]

The title semiconductor with an indirect band gap of 0.45 eV is synthesized from a stoichiometric mixture of Tl, Sn, Sb2Se3, and Se (carbon-coated quartz tube, 530 °C, 10 h). [ABSTRACT FROM AUTHOR]