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We investigate a strain-induced topological phase transition in the ferromagnetic Janus monolayer MnSbBiS 2 Te 2 using first-principles calculations. The electronic, magnetic, and topological properties are studied under biaxial strain within the range of -8 to +8%. The ground state of monolayer MnSbBiS 2 Te 2 is metallic with an out-of-plane magnetic easy axis. A band gap is opened when a compressive strain between -4% and -7% is applied. We observe a topological phase transition at a biaxial strain of -5%, where the material becomes a Chern insulator exhibiting a quantum anomalous hall (QAH) effect. We find that biaxial strain and spin-orbit coupling (SOC) are responsible for the topological phase transition in MnSbBiS 2 Te 2 . In addition, we find that biaxial strain can alter the direction of the magnetic easy axis of MnSbBiS 2 Te 2 . The Curie temperature is calculated using the Heisenberg model and is found to be 24 K. This study could pave the way to the design of topological materials with potential applications in spintronics, quantum computing, and dissipationless electronics.

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/SiO 2 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., (© 2023 The Authors. Advanced Science published by Wiley-VCH GmbH.)

Polarons are entities of excess electrons dressed with local response of lattices, whose atomic-scale characterization is essential for understanding the many body physics arising from the electron-lattice entanglement, yet difficult to achieve. Here, using scanning tunneling microscopy and spectroscopy (STM/STS), we show the visualization and manipulation of single polarons in monolayer CoCl 2 , that are grown on HOPG substrate via molecular beam epitaxy. Two types of polarons are identified, both inducing upward local band bending, but exhibiting distinct appearances, lattice occupations and polaronic states. First principles calculations unveil origin of polarons that are stabilized by cooperative electron-electron and electron-phonon interactions. Both types of polarons can be created, moved, erased, and moreover interconverted individually by the STM tip, as driven by tip electric field and inelastic electron tunneling effect. This finding identifies the rich category of polarons in CoCl 2 and their feasibility of precise control unprecedently, which can be generalized to other transition metal halides., (© 2023. The Author(s).)

Understanding the dynamics of charge transfer at vertical heterostructures of transition metal dichalcogenide monolayers is fundamentally important for future technological applications, given the unique feature of van der Waals interactions at the interface. Here, we employ time-dependent density functional theory formalism combined with molecular dynamics to investigate photoexcited electrons and holes in the type-I MoS 2 /PtSe 2 van der Waals heterobilayer. While type-I junctions have been traditionally viewed as being ineffective in photocarrier separation, we show that here a different mechanism from type-II is at play, which effectively separates photoelectrons from photoholes. The key is the phonon bottleneck, arising from the characteristically different dynamic band alignments in the valence and conduction bands, respectively, which only affects the transfer of holes but not electrons. The disparity between electron and hole transfer rates offers a new direction for effective control of charge separation at interfaces.

Realizing van der Waals (vdW) epitaxy in the 1980s represents a breakthrough that circumvents the stringent lattice matching and processing compatibility requirements in conventional covalent heteroepitaxy. However, due to the weak vdW interactions, there is little control over film qualities by the substrate. Typically, discrete domains with a spread of misorientation angles are formed, limiting the applicability of vdW epitaxy. Here, the epitaxial growth of monocrystalline, covalent Cr 5 Te 8 2D crystals on monolayer vdW WSe 2 by chemical vapor deposition is reported, driven by interfacial dative bond formation. The lattice of Cr 5 Te 8 , with a lateral dimension of a few tens of micrometers, is fully commensurate with that of WSe 2 via 3 × 3 (Cr 5 Te 8 )/7 × 7 (WSe 2 ) supercell matching, forming a single-crystalline moiré superlattice. This work establishes a conceptually distinct paradigm of thin-film epitaxy, termed "dative epitaxy", which takes full advantage of covalent epitaxy with chemical bonding for fixing the atomic registry and crystal orientation, while circumventing its stringent lattice matching and processing compatibility requirements; conversely, it ensures the full flexibility of vdW epitaxy, while avoiding its poor orientation control. Cr 5 Te 8 2D crystals grown by dative epitaxy exhibit square magnetic hysteresis, suggesting minimized interfacial defects that can serve as pinning sites., (© 2022 Wiley-VCH GmbH.)

The photocatalytic CO 2 reduction reaction is a sustainable route to the direct conversion of greenhouse gases into chemicals without additional energy consumption. Given the vast amount of greenhouse gas, numerous efforts have been devoted to developing inorganic photocatalysts, e.g., titanium dioxide (TiO 2 ), due to their stability, low cost, and environmentally friendly properties. However, a more efficient TiO 2 photocatalyst without noble metals is highly desirable for CO 2 reduction, and it is both difficult and urgent to produce selectively valuable compounds. Here, a novel "single-atom site at the atomic step" strategy is developed by anchoring a single tungsten (W) atom site with oxygen-coordination at the intrinsic steps of classic TiO 2 nanoparticles. The composition of active sites for CO 2 reduction can be controlled by tuning the additional W 5+ to form W 5+ -O-Ti 3+ sites, resulting in both significant CO 2 reduction efficiency with 60.6 μmol g - 1 h - 1 and selectivity for methane (CH 4 ) over carbon monoxide (CO), which exceeds those of pristine TiO 2 by more than one order of magnitude. The mechanism relies on the accurate control of the single-atom sites at step with 22.8% coverage of surface sites and the subsequent excellent electron-hole separation along with the favorable adsorption-desorption of intermediates at the sites., (© 2022 Wiley-VCH GmbH.)

Achieving stable high-magnetism light-element structures at nanoscale is vital to the field of magnetism, which has traditionally been ruled by transition-metal elements with localized d or f electrons. By first-principles calculations, we show that superatoms made of pure earth-abundant light elements (i.e., boron and nitrogen) exhibit desired magnetic properties that rival those of rare-earth elements, and the magnetism is dictated entirely by Hund's maximum spin rule. Importantly, the chemical and structural stabilities of the superatoms are not jeopardized by its high spins and are in fact better than those of transition-metal-element-embedded clusters. Our work thus establishes the basic principles for designing novel light-element, high-stability, and high-moment magnetic superatoms.

Molecular linkers, such as cysteine, are used to stabilize colloidal quantum dots (QDs) and anchor them. Despite the typically large molecular HOMO/LUMO gap of linkers, they can increase the quantum yield and provide an effective charge-transfer channel. Through first-principles calculations, we investigate the ligand binding and the implications for charge transfer using a prototypical CdSe-Cysteine-MoS 2 three-way heterostructure. We find that the deprotonated ligand interacts with both sides of the heterostructure, which allows for successful self-passivation of the cysteine ligand molecule and the formation of dative bonds with a greatly reduced molecular gap compared with the gas phase. This leads to the formation of a charge-transfer state that is delocalized across the ligand and can directly assist electron transfer from the conduction band of colloidal CdSe QDs to the underlying MoS 2 substrate, which is a mechanism that could extend far beyond 0D-2D hybrid systems.

Two-dimensional (2D) monolayer-bilayer (ML-BL) lateral junctions (LJs) have recently attracted attention due to their straightforward synthesis and resulting clean interface. Such systems consist of an extended ML with a secondary layer present only over half of the system, leading to an interface that is associated with the terminating edge of the secondary half layer. Our first-principles calculations reveal that the edges of the half layer completely lack reconstruction in the presence of unintentional dopants, in this case, Re. This observation is in startling contrast to the known physics of three-dimensional (3D) semiconductor surfaces where reconstruction has been widely observed. Herein, the electrostatics of the reduced dimensionality allows for greater separation between compensating defects, enabling dopants to remotely passivate edge states without needing to directly participate in the chemistry.

Lead iodide (PbI 2 ) as a layered material has emerged as an excellent candidate for optoelectronics in the visible and ultraviolet regime. Micrometer-sized flakes synthesized by mechanical exfoliation from bulk crystals or by physical vapor deposition have shown a plethora of applications from low-threshold lasing at room temperature to high-performance photodetectors with large responsivity and faster response. However, large-area centimeter-sized growth of epitaxial thin films of PbI 2 with well-controlled orientation has been challenging. Additionally, the nature of grain boundaries in epitaxial thin films of PbI 2 remains elusive. Here, we use mica as a model substrate to unravel the growth mechanism of large-area epitaxial PbI 2 thin films. The partial growth leading to uncoalesced domains reveals the existence of inversion domain boundaries in epitaxial PbI 2 thin films on mica. Combining the experimental results with first-principles calculations, we also develop an understanding of the thermodynamic and kinetic factors that govern the growth mechanism, which paves the way for the synthesis of high-quality large-area PbI 2 on other substrates and heterostructures of PbI 2 on single-crystalline graphene. The ability to reproducibly synthesize high-quality large-area thin films with precise control over orientation and tunable optical properties could open up unique and hitherto unavailable opportunities for the use of PbI 2 and its heterostructures in optoelectronics, twistronics, substrate engineering, and strain engineering.

Exploring two-dimensional (2D) van der Waals (vdW) systems is at the forefront of materials of physics. Here, through molecular beam epitaxy on graphene-covered SiC(0001), we report successful growth of AlSb in the double-layer honeycomb (DLHC) structure, a 2D vdW material which has no direct analogue to its 3D bulk and is predicted to be kinetically stable when freestanding. The structural morphology and electronic structure of the experimental 2D AlSb are characterized with spectroscopic imaging scanning tunneling microscopy and cross-sectional imaging scanning transmission electron microscopy, which compare well to the proposed DLHC structure. The 2D AlSb exhibits a band gap of 0.93 eV versus the predicted 1.06 eV, which is substantially smaller than the 1.6 eV of bulk. We also attempt the less-stable InSb DLHC structure; however, it grows into bulk islands instead. The successful growth of a DLHC material here demonstrates the feasibility for the realization of a large family of 2D DLHC traditional semiconductors with characteristic excitonic, topological, and electronic properties.

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 Sb 2 Te 3 , 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 SiO 2 (001) surfaces or hexagonal Boron Nitride, the monolayer Sb 2 Te 3 can reach SET recrystallization in 0.54 ns or even as fast as 0.12 ns, but with unpassivated SiO 2 (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., Competing Interests: The authors declare no conflict of interest., (© 2021 The Authors. Advanced Science published by Wiley‐VCH GmbH.)

Owing to its novel electronic and magnetic properties, two-dimensional CrI 3 has great potential in the application of spintronic devices. However, as an inevitable line defect, the properties of the edges of CrI 3 remain elusive. Here, via first-principles calculations with spin-orbit coupling, we investigated the thermodynamic stabilities, electronic and magnetic properties of thirteen CrI 3 edges with different structures. We showed that zigzag edges are more stable than armchair edges, and a CrI 3 nanoribbon can be either metallic or insulating depending on its chemical growth conditions. The edge stability and associated electronic properties can be understood in terms of the octahedron ligand field and electron counting model. In most cases, both the magnetic moment and Curie temperature can be enhanced by edges, which are in startle contrast to the surfaces of three-dimensional ferromagnetic materials, where a magnetic dead layer is often observed.

The platinum single-atom-catalyst is verified as a very successful route to approach the size limit of Pt catalysts, while how to further improve the catalytic efficiency of Pt is a fundamental scientific question and is challenging because the size issue of Pt is approached at the ultimate ceiling as single atoms. Here, a new route for further improving Pt catalytic efficiency by cobalt (Co) and Pt dual-single-atoms on titanium dioxide (TiO 2 ) surfaces, which contains a fraction of nonbonding oxygen-coordinated Co-O-Pt dimers, is reported. These Co-Pt dimer sites originate from loading high-density Pt single-atoms and Co single-atoms, with them anchoring randomly on the TiO 2 substrate. This dual-single-atom catalyst yields 13.4% dimer sites and exhibits an ultrahigh and stable photocatalytic activity with a rate of 43.467 mmol g -1 h -1 and external quantum efficiency of ≈83.4% at 365 nm. This activity far exceeds those of equal amounts of Pt single-atom and typical Pt clustered catalysts by 1.92 and 1.64 times, respectively. The enhancement mechanism relies on the oxygen-coordinated Co-O-Pt dimer coupling, which can mutually optimize the electronic states of both Pt and Co sites to weaken H* binding. Namely, the "mute" Co single-atom is activated by Pt single-atom and the activity of the Pt atom is further enhanced through the dimer interaction. This strategy of nonbonding interactive dimer sites and the oxygen-mediated catalytic mechanisms provide emerging rich opportunities for greatly improving the catalytic efficiency and developing novel catalysts with creating new electronic states., (© 2021 Wiley-VCH GmbH.)

The emergence of two-dimensional (2D) materials launched a fascinating frontier of flatland electronics. Most crystalline atomic layer materials are based on layered van der Waals materials with weak interlayer bonding, which naturally leads to thermodynamically stable monolayers. We report the synthesis of a 2D insulator composed of a single atomic sheet of honeycomb structure BeO (h-BeO), although its bulk counterpart has a wurtzite structure. The h-BeO is grown by molecular beam epitaxy (MBE) on Ag(111) thin films that are also epitaxially grown on Si(111) wafers. Using scanning tunneling microscopy and spectroscopy (STM/S), the honeycomb BeO lattice constant is determined to be 2.65 Å with an insulating band gap of 6 eV. Our low-energy electron diffraction measurements indicate that the h-BeO forms a continuous layer with good crystallinity at the millimeter scale. Moiré pattern analysis shows the BeO honeycomb structure maintains long-range phase coherence in atomic registry even across Ag steps. We find that the interaction between the h-BeO layer and the Ag(111) substrate is weak by using STS and complementary density functional theory calculations. We not only demonstrate the feasibility of growing h-BeO monolayers by MBE, but also illustrate that the large-scale growth, weak substrate interactions, and long-range crystallinity make h-BeO an attractive candidate for future technological applications. More significantly, the ability to create a stable single-crystalline atomic sheet without a bulk layered counterpart is an intriguing approach to tailoring 2D electronic materials.

Semiconducting monolayers of a 2D material are able to concatenate multiple interesting properties into a single component. Here, by combining opto-mechanical and electronic measurements, we demonstrate the presence of a partial 2H-1T' phase transition in a suspended 2D monolayer membrane of MoS 2 . Electronic transport shows unexpected memristive properties in the MoS 2 membrane, in the absence of any external dopants. A strong mechanical softening of the membrane is measured concurrently and may only be related to the 2H-1T' phase transition, which imposes a 3% directional elongation of the topological 1T' phase with respect to the semiconducting 2H. We note that only a few percent 2H-1T' phase switching is sufficient to observe measurable memristive effects. Our experimental results combined with first-principles total energy calculations indicate that sulfur vacancy diffusion plays a key role in the initial nucleation of the phase transition. Our study clearly shows that nanomechanics represents an ultrasensitive technique to probe the crystal phase transition in 2D materials or thin membranes. Finally, a better control of the microscopic mechanisms responsible for the observed memristive effect in MoS 2 is important for the implementation of future devices.

Carrier dynamics across the interface of heterostructures have important technological, photovoltaic, and catalytic implications. Using first-principles time-dependent density functional theory, we have systematically investigated the charge transfer of excited carriers from CdS to MoS 2 and found that two interdependent mechanisms are responsible for the transfer, one slow and one fast. While the slower process may be attributed to typical electron-phonon coupling, the interfacial dipole resulting from this transfer enables a fast secondary process involving a level crossing of the excited carrier state in CdS with receiving states in MoS 2 . An analysis based on the interfacial binding energy reveals that the Cd-terminated (001) interface is by far the most energetically favorable, which in addition to its calculated fast resonant electron transfer suggests it is a good candidate to explain the experimentally observed charge transfer between CdS and MoS 2 .

BaZrS 3 , a prototypical chalcogenide perovskite, has been shown to possess a direct band gap, an exceptionally strong near band edge light absorption, and good carrier transport. Coupled with its great stability, nontoxicity with earth-abundant elements, it is thus a promising candidate for thin film solar cells. However, its reported band gap in the range of 1.7-1.8 eV is larger than the optimal value required to reach the Shockley-Queisser limit of a single-junction solar cell. Here, we report the synthesis of Ba(Zr 1- x Ti x )S 3 perovskite compounds with a reduced band gap. It is found that Ti-alloying is extremely effective in band gap reduction of BaZrS 3 : a mere 4 atom % alloying decreases the band gap from 1.78 to 1.51 eV, resulting in a theoretical maximum power conversion efficiency of 32%. Higher Ti-alloying concentration is found to destabilize the distorted chalcogenide perovskite phase., Competing Interests: The authors declare no competing financial interest., (Copyright © 2020 American Chemical Society.)

Critical topological phases, possessing flat bands, provide a platform to study unique topological properties and transport phenomena under a many-body effect. Here, we propose that critical nodal points and nodal lines or rings can be found in Kagome lattices. After the C3 rotation symmetry of a single-layer Kagome lattice is eliminated, a quadratic nodal point splits into two critical nodal points. When the layered Kagome lattices are stacked into a three-dimensional (3D) structure, critical nodal lines or rings can be generated by tuning the interlayer coupling. Furthermore, we use Kagome graphene as an example to identify that these critical phases could be obtained in real materials. We also discuss flat-band-induced ferromagnetism. It is found that the flat band splits into two spin-polarized bands by hole-doping, and as a result the Dirac-type critical phases evolve into Weyl-type phases.

While various excitonic insulators have been studied in the literature, due to the perceived too-small spin splitting, spin-triplet excitonic insulator is rare. In two-dimensional systems such as a semihydrogenated graphene (known as graphone), however, it is possible, as revealed by first-principles calculations coupled with Bethe-Salpeter equation. The critical temperature, given by an effective Hamiltonian, is 11.5 K. While detecting excitonic insulators is still a daunting challenge, the condensation of triplet excitons will result in spin superfluidity, which can be directly measured by a transport experiment. Nonlocal dielectric screening also leads to an unexpected phenomenon, namely, an indirect-to-direct transition crossover between single-particle band and exciton dispersion in the semihydrogenated graphene, which offers yet another test by experiment.

Parity-time symmetry plays an essential role for the formation of Dirac states in Dirac semimetals. So far, all of the experimentally identified topologically nontrivial Dirac semimetals (DSMs) possess both parity and time reversal symmetry. The realization of magnetic topological DSMs remains a major issue in topological material research. Here, combining angle-resolved photoemission spectroscopy with density functional theory calculations, it is ascertained that band inversion induces a topologically nontrivial ground state in EuCd 2 As 2 . As a result, ideal magnetic Dirac fermions with simplest double cone structure near the Fermi level emerge in the antiferromagnetic (AFM) phase. The magnetic order breaks time reversal symmetry, but preserves inversion symmetry. The double degeneracy of the Dirac bands is protected by a combination of inversion, time-reversal, and an additional translation operation. Moreover, the calculations show that a deviation of the magnetic moments from the c-axis leads to the breaking of C3 rotation symmetry, and thus, a small bandgap opens at the Dirac point in the bulk. In this case, the system hosts a novel state containing three different types of topological insulator: axion insulator, AFM topological crystalline insulator (TCI), and higher order topological insulator. The results provide an enlarged platform for the quest of topological Dirac fermions in a magnetic system., (© 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Metal oxides, as one of the mostly abundant and widely utilized materials, are extensively investigated and applied in environmental remediation and protection, and in energy conversion and storage. Most of these diverse applications are the result of a large diversity of the electronic states of metal oxides. Noticeably, however, many metal oxides present obstacles for applications in catalysis, mainly due to the lack of efficient active sites with desired electronic states. Here, the fabrication of single-tungsten-atom-oxide (STAO) is demonstrated, in which the metal oxide's volume reaches its minimum as a unit cell. The catalytic mechanism in the STAO is determined by a new single-site physics mechanism, named as quasi-atom physics. The photogenerated electron transfer process is enabled by an electron in the spin-up channel excited from the highest occupied molecular orbital to the lowest unoccupied molecular orbital +1 state, which can only occur in STAO with W 5+ . STAO results in a record-high and stable sunlight photocatalytic degradation rate of 0.24 s -1 , which exceeds the rates of available photocatalysts by two orders of magnitude. The fabrication of STAO and its unique quasi-atom photocatalytic mechanism lays new ground for achieving novel physical and chemical properties using single-metal-atom oxides (SMAO)., (© 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

As an intensively studied electrode material for secondary batteries, TiS 2 is known to exhibit high electrical conductivity without extrinsic doping. However, the origin of this high conductivity, either being a semimetal or a heavily self-doped semiconductor, has been debated for several decades. Here, combining quasi-particle GW calculations, density functional theory (DFT) study on intrinsic defects, and scanning tunneling microscopy/spectroscopy (STM/STS) measurements, we conclude that stoichiometric TiS 2 is a semiconductor with an indirect band gap of about 0.5 eV. The high conductivity of TiS 2 is therefore caused by heavy self-doping. Our DFT results suggest that the dominant donor defect that is responsible for the self-doping under thermal equilibrium is Ti interstitial, which is corroborated by our STM/STS measurements.

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.

First-principles calculations reveal an unusual electronic state (dubbed as half excitonic insulator) in monolayer 1T-MX_{2} (M=Co, Ni and X=Cl, Br). Its one spin channel has a many-body ground state due to excitonic instability, while the other is characterized by a conventional band insulator gap. This disparity arises from a competition between the band gap and exciton binding energy, which exhibits a spin dependence due to different orbital occupations. Such a state can be identified by optical absorption measurements and angle-resolved photoemission spectroscopy. Our theory not only provides new insights for the study of exciton condensation in magnetic materials but also suggests that strongly correlated materials could be fertile candidates for excitonic insulators.

We show that non-equilibrium dynamics plays a central role in the photoinduced 2H-to-1T' phase transition of MoTe 2 . The phase transition is initiated by a local ordering of Te vacancies, followed by a 1T' structural change in the original 2H lattice. The local 1T' region serves as a seed to gather more vacancies into ordering and subsequently induces a further growth of the 1T' phase. Remarkably, this process is controlled by photogenerated excited carriers as they enhance vacancy diffusion, increase the speed of vacancy ordering, and are hence vital to the 1T' phase transition. This mechanism can be contrasted to the current model requiring a collective sliding of a whole Te atomic layer, which is thermodynamically highly unlikely. By uncovering the key roles of photoexcitations, our results may have important implications for finely controlling phase transitions in transition metal dichalcogenides.

The photosynthetic protein complex, photosystem II (PSII), conducts the light-driven water-splitting reaction with unrivaled efficiency. Proton-coupled electron transfer (PCET) reactions at the redox-active tyrosine residues are thought to play a critical role in the water-splitting chemistry. Addressing the fundamental question as to why the tyrosine residue, YZ, is kinetically competent in comparison to a symmetrically placed tyrosine residue, YD, is important for the elucidation of the mechanism of PCET in the water-splitting reaction of PSII. Here, using all-quantum-mechanical calculations we study PCET at the YZ and YD residues of PSII. We find that when YZ is in its protein matrix under physiological conditions, the HOMO of YZ constitutes the HOMO of the whole system. In contrast, the HOMO of YD is buried under the electronic states localized elsewhere in the protein matrix and PCET at YD requires the transfer of the phenolic proton, which elevates the HOMO of YD to become the HOMO of the whole system. This leads to the oxidation of YD, albeit on a slower timescale. Our study reveals that the key differences between the electronic structure of YZ and YD are primarily determined by the protonation state of the respective hydrogen-bonding partners, D1-His190 and D2-His189, or more generally by the H-bonding network of the protein matrix.

Phase transition is a fundamental physical phenomenon that has been widely studied both theoretically and experimentally. According to the Landau theory, the coexistence of high- and low-temperature phases is thermodynamically impossible during a second-order phase transition in a bulk single crystal. Here, the coexistence of two (α and β) phases in wedge-shaped nanosized single-crystal Cu 2 Se over a large temperature range are demonstrated. By considering the surface free-energy difference between the two phases and the shape effect, a thermodynamic model is established, which explicitly explains their coexistence. Intriguingly, it is found that with a precise control of the heating temperature, the phase boundary can be manipulated at atomic level. These discoveries extend the understanding of phase transitions to the nanoscale and shed light on rational manipulation of phase transitions in nanomaterials., (© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The built-in potential is of central importance to the understanding of many interfacial phenomena because it determines the band alignment at the interface. Despite its importance, its exact sign and magnitude have generally been recognized as ill-defined quantities for more than half a century. Here, we provide a common energy reference of bulk matter which leads to an unambiguous definition of the built-in potential and innate (i.e., bulk) band alignment. Further, we find that the built-in potential is explicitly determined by the bulk properties of the constituent materials when the system is in electronic equilibrium, while the interface plays a role only in the absence of equilibrium. Our quantitative theory enables a unified description of a variety of important properties of interfaces, ranging from work functions to Schottky barriers in electronic devices.

In this work, we show that remote heteroepitaxy can be achieved when Cu thin film is grown on single crystal, monolayer graphene buffered sapphire(0001) substrate via a thermal evaporation process. X-ray diffraction and electron backscatter diffraction data show that the epitaxy process forms a prevailing Cu crystal domain, which is remotely registered in-plane to the sapphire crystal lattice below the monolayer graphene, with the (111) out-of-plane orientation. As a poor metal with zero density of states at its Fermi level, monolayer graphene cannot totally screen out the stronger charge transfer/metallic interactions between Cu and substrate atoms. The primary Cu domain thus has good crystal quality as manifested by a narrow crystal misorientation distribution. On the other hand, we show that graphene interface imperfections, such as bilayers/multilayers, wrinkles and interface contaminations, can effectively weaken the atomic interactions between Cu and sapphire. This results in a second Cu domain, which directly grows on and follows the graphene hexagonal lattice symmetry and orientation. Because of the weak van der Waals interaction between Cu and graphene, this domain has inferior crystal quality. The results are further confirmed using graphene buffered spinel(111) substrate, which indicates that this remote epitaxial behavior is not unique to the Cu/sapphire system.

Strong Coulomb interactions in single-layer transition metal dichalcogenides (TMDs) result in the emergence of strongly bound excitons, trions, and biexcitons. These excitonic complexes possess the valley degree of freedom, which can be exploited for quantum optoelectronics. However, in contrast to the good understanding of the exciton and trion properties, the binding energy of the biexciton remains elusive, with theoretical calculations and experimental studies reporting discrepant results. In this work, we resolve the conflict by employing low-temperature photoluminescence spectroscopy to identify the biexciton state in BN-encapsulated single-layer WSe 2 . The biexciton state only exists in charge-neutral WSe 2 , which is realized through the control of efficient electrostatic gating. In the lightly electron-doped WSe 2 , one free electron binds to a biexciton and forms the trion-exciton complex. Improved understanding of the biexciton and trion-exciton complexes paves the way for exploiting the many-body physics in TMDs for novel optoelectronics applications.

Two-dimensional (2D) B-C-N alloys have recently attracted much attention but unfortunately, Chemical Vapor Deposition (CVD) B-C-N alloys typically phase separate. In spite of that, our analysis of the B-C-N alloy fabricated by electron-beam irradiation suggests that non-phase-separated B-C-N may in fact exist with a carbon concentration up to 14 at%. While this analysis points to a new way to overcome the phase-separation in 2D B-C-N, by first-principles calculations, we show that these B-C-N alloys are made of motifs with even numbers of carbon atoms, in particular, dimers or six-fold rings (in a molecule-like form), embedded in a 2D BN network. Moreover, by tuning the carbon concentration, the band gap of the B-C-N alloys can be reduced by 35% from that of BN. Due to a strong overlap of the wavefunctions at the conduction band and valance band edges, the non-phase-separated B-C-N alloys maintain the strong optical absorption of BN.

A flatband representing a highly degenerate and dispersionless manifold state of electrons may offer unique opportunities for the emergence of exotic quantum phases. To date, definitive experimental demonstrations of flatbands remain to be accomplished in realistic materials. Here, we present the first experimental observation of a striking flatband near the Fermi level in the layered Fe_{3}Sn_{2} crystal consisting of two Fe kagome lattices separated by a Sn spacing layer. The band flatness is attributed to the local destructive interferences of Bloch wave functions within the kagome lattices, as confirmed through theoretical calculations and modelings. We also establish high-temperature ferromagnetic ordering in the system and interpret the observed collective phenomenon as a consequence of the synergetic effect of electron correlation and the peculiar lattice geometry. Specifically, local spin moments formed by intramolecular exchange interaction are ferromagnetically coupled through a unique network of the hexagonal units in the kagome lattice. Our findings have important implications to exploit emergent flat-band physics in special lattice geometries.

InSe, a newly rediscovered two-dimensional (2D) semiconductor, possesses superior electrical and optical properties as a direct-band-gap semiconductor with high mobility from bulk to atomically thin layers and is drastically different from transition-metal dichalcogenides, in which the direct band gap only exists at the single-layer limit. However, absorption in InSe is mostly dominated by an out-of-plane dipole contribution, which results in the limited absorption of normally incident light that can only excite the in-plane dipole at resonance. To address this challenge, we have explored a unique geometric ridge state of the 2D flake without compromising the sample quality. We observed the enhanced absorption at the ridge over a broad range of excitation frequencies from photocurrent and photoluminescence (PL) measurements. In addition, we have discovered new PL peaks at low temperatures due to defect states on the ridge, which can be as much as ∼60 times stronger than the intrinsic PL peak of InSe. Interestingly, the PL of the defects is highly tunable through an external electrical field, which can be attributed to the Stark effect of the localized defects. InSe ridges thus provide new avenues for manipulating light-matter interactions and defect engineering that are vitally crucial for novel optoelectronic devices based on 2D semiconductors.

First-principles calculations within density functional theory reveal the preferred structures of red phosphorus in the two-dimensional (2D) limit to be porous with intriguing structural, electronic, and chemical properties. These few-atomic-layer structures are stabilized as novel 2D clathrates with tunable pore sizes and varying semiconducting band gaps, labelled as V-Hex, P-Monoclinic, P-Hex, and V-Tetr in descending energetic stabilities. The cohesive energies of the 2D clathrates are all substantially higher than that of white phosphorus. More strikingly, the V-Hex structure is energetically as stable as black phosphorene, but possesses distinctly superior chemical stability when exposed to O 2 due to the presence of a much higher activation barrier against chemisorption. We also exploit the salient properties of these 2D clathrates for their important application potentials, including serving as effective elemental photocatalysts for visible-light-driven water splitting, and as a new class of sieves for molecular separation and DNA sequencing.

The recent discovery of ferromagnetic single-layer CrI 3 creates ample opportunities for studying the fundamental properties and the spintronic applications of atomically thin magnets. Through first-principles calculations and model Hamiltonian simulations, here we build for the first time a substantial magnetic phase diagram under lateral strain and charge doping, the two factors that are easily modulated in single-layer CrI 3 via substrate and gating controls. We demonstrate that both lateral strain and charge doping efficiently change the coupling between the local spins and thus have unexpected effects on the magnetic properties of CrI 3 . In particular, the strain tunes the magnetic order and anisotropy: a compressive strain leads to a phase transition from a ferromagnetic insulator to an antiferromagnetic insulator, while a tensile strain can flip the magnetic orientation from off-plane to in-plane. Furthermore, we find that the phase transition under compressive strain is insensitive to charge doping, whereas the phase transition under tensile strain is modulated by electron doping significantly. Our predicted magnetic phase diagram and rationalized analysis indicate the single-layer CrI 3 to be an ideal system to harness both basic magnetic physics and building blocks for magnetoelastic applications.

Silicon and graphene are two wonder materials, and their hybrid heterostructures are expected to be very interesting fundamentally and practically. In the present study, by adopting fast dry transfer and ultra-high vacuum annealing, atomically flat monolayer graphene is successfully prepared on the chemically active Si(111) substrate. More importantly, the graphene overlayer largely maintains its intrinsic electronic properties, as validated by the results of the energy-dependent electronic transparency, Dirac point observation and quantum coherence characteristics, and further confirmed by first-principles calculations. The intrinsic properties of graphene are retained up to 1030 K. The system of atomically flat and thermally stable graphene on a chemically active silicon surface with preserved inherent characteristics renders the graphene/silicon hybrid a promising system in the design of high-performance devices and the exploitation of interfacial topological quantum effects.

Time-dependent density-functional theory molecular dynamics reveals an unexpected effect of optical excitation in the experimentally observed rhombohedral-to-cubic transition of GeTe. The excitation induces coherent forces along [001], which may be attributed to the unique energy landscape of Peierls-distorted solids. The forces drive the A_{1g} optical phonon mode in which Ge and Te move out of phase. Upon damping of the A_{1g} mode, phase transition takes place, which involves no atomic diffusion, defect formation, or the nucleation and growth of the cubic phase.

Two-dimensional (2D) junction devices have recently attracted considerable attention. Here, we show that most 2D junction structures, whether vertical or lateral, act as a lateral monolayer-bilayer-monolayer junction in their operation. In particular, a vertical structure cannot function as a vertical junction as having been widely believed in the literature. Due to a larger electrostatic screening, the bilayer region in the junction always has a smaller bandgap than its monolayer counterpart. As a result, a potential well, aside from the usual potential barrier, will form universally in the bilayer region to affect the hole or electron quantum transport in the form of transmission or reflection. Taking black phosphorus as an example, our calculations using a non-equilibrium Green function combined with density functional theory show a distinct oscillation in the transmission coefficient in a two-electrode prototypical device, and the results can be qualitatively understood using a simple quantum well model.

Understanding how the structures of a crystal behave under compression is a fundamental issue both for condensed matter physics and for geoscience. Traditional description of a crystal as the stacking of a unit cell with special symmetry has gained much success on the analysis of physical properties. Unfortunately, it is hard to reveal the relationship between the compressed phases. Taking the family of metal dioxides (MO 2 ) as an example, the structural evolution, subject to fixed chemical formula and highly confined space, often appears as a set of random and uncorrelated events. Here we provide an alternative way to treat the crystal as the stacking of the coordination polyhedron and then discover a unified structure transition pattern, in our case VO 2 . X-ray diffraction (XRD) experiments and first-principles calculations show that the coordination increase happens only at one apex of the V-centered octahedron in an orderly fashion, leaving the base plane and the other apex topologically intact. The polyhedron evolves toward increasing their sharing, indicating a general rule for the chemical bonds of MO 2 to give away the ionicity in exchange for covalency under pressure.

Based on high-throughput density functional theory calculations, we investigated the adsorption characteristics of various elements across the Periodic Table on few-layer black phosphorus (BP). Using the criterion that the ratio of adsorption energy ( E ads ) to bulk cohesive energy ( E coh ) is greater than one ( E ads / E coh > 1), we selected fifteen elements. The adsorption of these elements on few-layer BPs could significantly shift their conduction-band minimum (CBM) downward, suggesting the possibility of preventing the few-layer BPs from oxidation if the CBM can be shifted below the O 2 /O 2 - redox potential. Our study offers an efficient approach to overcoming the technical barrier in the practical application of few-layer BPs by enhancing its ambient stability via surface modification., Competing Interests: The authors declare no competing financial interest., (This journal is © The Royal Society of Chemistry.)

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.

Triply degenerate points (TDPs) in band structure of a crystal can generate novel TDP fermions without high-energy counterparts. Although identifying ideal TDP semimetals, which host clean TDP fermions around the Fermi level (E_{F}) without coexisting with other quasiparticles, is critical to explore the intrinsic properties of this new fermion, it is still a big challenge and has not been achieved up to now. Here, we disclose an effective approach to search for ideal TDP semimetals via selective band crossing between antibonding s and bonding p orbitals along a line in the momentum space with C_{3v} symmetry. Applying this approach, we have successfully identified the NaCu_{3}Te_{2} family of compounds to be ideal TDP semimetals, where two, and only two, pairs of TDPs are located around the E_{F}. Moreover, we demonstrate a fundamental mechanism to modulate energy splitting between a pair of TDPs, and we illustrate the intrinsic features of TDP Fermi arcs in these ideal TDP semimetals.

Twin nucleation in a face-centered cubic crystal is believed to be accomplished through the formation of twinning partial dislocations on consecutive atomic planes. Twinning should thus be highly unfavorable in face-centered cubic metals with high twin-fault energy barriers, such as Al, Ni, and Pt, but instead is often observed. Here, we report an in situ atomic-scale observation of twin nucleation in nanocrystalline Pt. Unlike the classical twinning route, deformation twinning initiated through the formation of two stacking faults separated by a single atomic layer, and proceeded with the emission of a partial dislocation in between these two stacking faults. Through this route, a three-layer twin was nucleated without a mandatory layer-by-layer twinning process. This route is facilitated by grain boundaries, abundant in nanocrystalline metals, that promote the nucleation of separated but closely spaced partial dislocations, thus enabling an effective bypassing of the high twin-fault energy barrier.

Femtosecond lasers (fs) can cause a disparity between electronic and lattice temperatures in the very short period after irradiation. In this relatively cool lattice regime, the material properties can differ drastically from those under thermal equilibrium. In particular, first-principles calculations reveal two general mechanical effects on semiconductors. Firstly, the excitation can induce a negative pressure on the lattice, causing a >10% expansion, even for superhard diamond. Secondly, it induces inhomogeneous local forces on the atoms, for both perfect and distorted lattices. In the case of phase-change-memory for Ge 2 Sb 2 Te 5 and GeTe alloys, such random forces cause a simultaneous phase transition from crystalline to amorphous, which enables faster data writing. These excitation effects are further supported by the time-dependent density functional theory. This work could be an important step in advancing fs laser techniques for the atomic-level control of structures, rather than relying on traditional melting or ablation approaches which often apply to much larger and non-atomic scales.

High-temperature vapor phase epitaxy (VPE) has been proved ubiquitously powerful in enabling high-performance electro-optic devices in III-V semiconductor field. A typical example is the successful growth of p-type GaN by VPE for blue light-emitting diodes. VPE excels as it controls film defects such as point/interface defects and grain boundary, thanks to its high-temperature processing condition and controllable deposition rate. For the first time, single-crystalline high-temperature VPE halide perovskite thin film has been demonstrated-a unique platform on unveiling previously uncovered carrier dynamics in inorganic halide perovskites. Toward wafer-scale epitaxial and grain boundary-free film is grown with alkali halides as substrates. It is shown the metal alkali halides could be used as universal substrates for VPE growth of perovskite due to their similar material chemistry and lattice constant. With VPE, hot photoluminescence and nanosecond photo-Dember effect are revealed in inorganic halide perovskite. These two phenomena suggest that inorganic halide perovskite could be as compelling as its organic-inorganic counterpart regarding optoelectronic properties and help explain the long carrier lifetime in halide perovskite. The findings suggest a new avenue on developing high-quality large-scale single-crystalline halide perovskite films requiring precise control of defects and morphology., (© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Carbon, the basic building block of our universe, enjoys a vast number of allotropic structures. Owing to its bonding characteristic, most carbon allotropes possess the motif of hexagonal rings. Here, with first-principles calculations, we discover a new metastable three-dimensional carbon allotrope entirely composed of pentagon rings. The unique structure of this Pentagon Carbon leads to extraordinary electronic properties, making it a cornucopia of emergent topological fermions. Under lattice strain, Pentagon Carbon exhibits topological phase transitions, generating a series of novel quasiparticles, from isospin-1 triplet fermions to triply degenerate fermions and further to Hopf-link Weyl-loop fermions. Its Landau level spectrum also exhibits distinct features, including a huge number of almost degenerate chiral Landau bands, implying pronounced magneto-transport signals. Our work not only discovers a remarkable carbon allotrope with highly rare structural motifs, it also reveals a fascinating hierarchical particle genesis with novel topological fermions beyond the Dirac and Weyl paradigm.

A semi-Dirac semimetal is a material that exhibits linear band dispersion in one direction and quadratic band dispersion in the orthogonal direction and, therefore, hosts massless and massive fermions at the same point in the momentum space. While a number of interesting physical properties have been predicted in semi-Dirac semimetals, it has been rare to realize such materials in condensed matter. Based on the fact that some honeycomb materials are easily oxidized or chemically absorb other atoms, here, we theoretically propose an approach of modifying their band structures by covalent addition of group-VI elements and strain engineering. We predict a silicene oxide with the chemical formula of Si 2 O to be a candidate semi-Dirac semimetal. Our approach is backed by the analysis and understanding of the effect of p-orbital frustration on the band structure of graphene-like materials.