Facile synthesis of hierarchical core-shell carbon@ZnIn2S4 composite for boosted photothermal-assisted photocatalytic H2 production.

Against the backdrop of energy shortage, hydrogen energy has attracted much attention as a green and clean energy source. In order to explore efficient hydrogen production pathways, we designed a composite photocatalyst with carbon-based core-shell photothermal-assisted photocatalytic system (Carbon@ZnIn2S4, denoted as C@ZIS). The well-designed catalyst C@ZIS composites demonstrated a photocatalytic hydrogen precipitation rate of 2.97 mmol g-1 h-1 even in the absence of the noble metal Pt co-catalyst. The incorporation of carbon-based core-shell photocatalysts into a photocatalytic reaction significantly affects the activity of the reaction by triggering a photothermal effect in the reaction solution. The results of the physicochemical experiments demonstrated that the carbon spheres in C@ZIS composite system could provide a greater number of active sites, thereby accelerating the electron transfer and separation efficiency, and thus enhancing the photocatalytic activity. The study presents an efficacious design concept for the development of efficacious carbon-based core-shell photothermal-assisted photocatalysts, which is anticipated to facilitate the efficient conversion of solar energy to hydrogen energy.

Tunable Multistate Ferroelectricity of Unit-Cell-Thick BaTiO3 Revived by a Ferroelectric SnS Monolayer via Interfacial Sliding.

Stabilization of multiple polarization states at the atomic scale is pivotal for realizing high-density memory devices beyond prevailing bistable ferroelectric architectures. Here, we show that two-dimensional ferroelectric SnS or GeSe is able to revive and stabilize the ferroelectric order of three-dimensional ferroelectric BaTiO3, even when the latter is thinned to one unit cell in thickness. The underlying mechanism for overcoming the conventional detrimental critical thickness effect is attributed to facile interfacial inversion symmetry breaking by robust in-plane polarization of SnS or GeSe. Furthermore, when invoking interlayer sliding, we can stabilize multiple polarization states and achieve efficient interstate switching in the heterostructures, accompanied by dynamical ferroelectric skyrmionic excitations. When invoking sliding and twisting, the moiré domains exhibit nontrivial polar vortexes, which can be laterally displaced via different sliding schemes. These findings provide an intuitive avenue for simultaneously overcoming the standing critical thickness issue in bulk ferroelectrics and weak polarization issue in sliding ferroelectricity.

Abundant Edge Active Sites-Modified High-Crystalline g-C3N5 for Hydrogen Peroxide Production from Pure-Water via a Quasi-Homogeneous Photocatalytic Process.

Ultrathin carbon nitride pioneered a paradigm that facilitates effective charge separation and acceleration of rapid charge migration. Nevertheless, the dissociation process confronts a disruption owing to the proclivity of carbon nitride to reaggregate, thereby impeding the optimal utilization of active sites. In response to this exigency, the adoption of a synthesis methodology featuring alkaline potassium salt-assisted molten salt synthesis is advocated in this work, aiming to craft a nitrogenated graphitic carbon nitride (g-C3N5) photocatalyst characterized by thin layer and hydrophilicity, which not only amplifies the degree of crystallization of g-C3N5 but also introduces a plethora of abundant edge active sites, engendering a quasi-homogeneous photocatalytic system. Under visible light irradiation, the ultra-high H2O2 production rate of this modified high-crystalline g-C3N5 in pure water attains 151.14 µm h-1. This groundbreaking study offers a novel perspective for the innovative design of highly efficient photocatalysts with a quasi-homogeneous photocatalytic system.

Topological Nodal-Point Superconductivity in Two-Dimensional Ferroelectric Hybrid Perovskites.

Two-dimensional (2D) hybrid organic-inorganic perovskites (HOIPs) with enhanced stability, high tunability, and strong spin-orbit coupling have shown great potential in vast applications. Here, we extend the already rich functionality of 2D HOIPs to a new territory, realizing topological superconductivity and Majorana modes for fault-tolerant quantum computation. Especially, we predict that room-temperature ferroelectric BA2PbCl4 (BA for benzylammonium) exhibits topological nodal-point superconductivity (NSC) and gapless Majorana modes on selected edges and ferroelectric domain walls when proximity-coupled to an s-wave superconductor and an in-plane Zeeman field, attractive for experimental verification and application. Since NSC is protected by spatial symmetry of 2D HOIPs, we envision more exotic topological superconducting states to be found in this class of materials due to their diverse noncentrosymmetric space groups, which may open a new avenue in the fields of HOIPs and topological superconductivity.

Emergent, Non-Aging, Extendable, and Rechargeable Exchange Bias in 2D Fe3 GeTe2 Homostructures Induced by Moderate Pressuring.

As a crucial concept in magnetism and spintronics, exchange bias (ExB) measures the asymmetry in the hysteresis loop of a pinned ferromagnet (FM)/antiferromagnet (AFM) interface. Previous studies are mainly focused on FM/AFM heterostructures composed of conventional bulk materials, whose complex interfaces prohibit precise control and full understanding of the phenomenon. Here, the enabling power of 2D magnets is exploited to demonstrate the emergence, non-aging, extendability, and rechargeability of ExB in van der Waals Fe3 GeTe2 homostructures, upon moderate pressuring. The emergence of the ExB is attributed to a local stress-induced FM-to-AFM transition, as validated using first-principles calculations, and confirmed in magneto-optical Kerr effect and second harmonic generation measurements. It is also observed that, negligible ExB aging before the training effect suddenly takes place through avalanching, pronounced delay of the avalanche via timed pressure repetition (extendability), ExB recovery in the post-training sample upon refreshed pressuring (rechargeability), and demonstrate its versatile tunability. These striking findings offer unprecedented insights into the underlying principles of ExB and its training, with immense technological applications in sight.

Photoinduced Rippling of Two-Dimensional Hexagonal Nitride Monolayers.

Inducing structural changes and deformation using noninvasive methods, such as ultrafast laser technology, is an attractive route to multiple optomechanical and optoelectronic applications. Here, we show how photon excitation could accumulate in-plane stress and induce long-wavelength ripples in two-dimensional (2D) materials. Numerical results based on first-principles calculations and a continuum model predict that long-range nanoscale rippling could emerge under photon excitation in hexagonal nitride single atomic sheets. The photosoftened transverse acoustic mode dominates the out-of-plane distortion of the sheet, and the resultant rippling pattern strongly depends on the boundary condition. We reveal that the wavelength and height of the ripple scale as I-1/3 and I1/6, respectively, where I is the incident light energy flux. Our findings based on multiscale theory and simulations elucidate the interplay between carrier excitation, phonon dispersion, and long-range mechanical deformations, which could find potential usage in flexible electronics and electromechanical devices.

Converting a Monolayered NbSe2 into an Ising Superconductor with Nontrivial Band Topology via Physical or Chemical Pressuring.

Two-dimensional transition metal dichalcogenides possessing superconductivity and strong spin-orbit coupling exhibit high in-plane upper critical fields due to Ising pairing. Yet to date, whether such systems can become topological Ising superconductors remains to be materialized. Here we show that monolayered NbSe2 can be converted into Ising superconductors with nontrivial band topology via physical or chemical pressuring. Using first-principles calculations, we first demonstrate that a hydrostatic pressure higher than 2.5 GPa can induce a p-d band inversion, rendering nontrivial band topology to NbSe2. We then illustrate that Te-doping can function as chemical pressuring in inducing nontrivial topology in NbSe2-xTex with x ≥ 0.8, due to a larger atomic radius and stronger spin-orbit coupling of Te. We also evaluate the upper critical fields within both approaches, confirming the enhanced Ising superconductivity nature, as experimentally observed. Our findings may prove to be instrumental in material realization of topological Ising superconductivity in two-dimensional systems.

Coexistence of Superconductivity and Nontrivial Band Topology in Monolayered Cobalt Pnictides on SrTiO3.

As an intrinsically layered material, FeSe has been extensively explored for potentially revealing the underlying mechanisms of high transition temperature (high-Tc) superconductivity and realizing topological superconductivity and Majorana zero modes. Here we use first-principles approaches to identify that the cobalt pnictides of CoX (X = As, Sb, Bi), none of which is a layered material in bulk form. Nevertheless, all can be stabilized as monolayered systems either in freestanding form or supported on the SrTiO3(001) substrate. We further show that each of the cobalt pnictides may potentially harbor high-Tc superconductivity beyond the Cu- and Fe-based superconducting families, and the underlying mechanism is inherently tied to their isovalency nature with the FeSe monolayer. Most strikingly, each of the monolayered CoX's on SrTiO3 is shown to be topologically nontrivial, and our findings provide promising new platforms for realizing topological superconductors in the two-dimensional limit.

Topological Band Engineering of Lieb Lattice in Phthalocyanine-Based Metal-Organic Frameworks.

Topological properties of the Lieb lattice, i.e., the edge-centered square lattice, have been extensively studied and are, however, mostly based on theoretical models without identifying real material systems. Here, based on tight-binding and first-principles calculations, we demonstrate the Lieb-lattice features of the experimentally synthesized phthalocyanine-based metal-organic framework (MPc-MOF), which holds various intriguing topological phase transitions through band engineering. First, we show that the MPc-MOFs indeed have a peculiar Lieb band structure with 1/3 filling, which has been overlooked because of its unconventional band structure deviating from the ideal Lieb band. The intrinsic MPc-MOF presents a trivial insulating state, with its gap size determined by the on-site energy difference (ΔE) between the corner and edge-center sites. Through either chemical substitution or physical strain engineering, one can tune ΔE to close the gap and achieve a topological phase transition. Specifically, upon closing the gap, topological semimetallic/insulating states emerge from nonmagnetic MPc-MOFs, while magnetic semimetal/Chern insulator states arise from magnetic MPc-MOFs, respectively. Our discovery greatly enriches our understanding of the Lieb lattice and provides a guideline for experimental observation of the Lieb-lattice-based topological states.

Unidirectional Spin-Orbit Interaction Induced by the Line Defect in Monolayer Transition Metal Dichalcogenides for High-Performance Devices.

Spin-orbit (SO) interaction is an indispensable element in the field of spintronics for effectively manipulating the spin of carriers. However, in crystalline solids, the momentum-dependent SO effective magnetic field generally results in spin randomization by a process known as the Dyakonov-Perel spin relaxation, leading to the loss of spin information. To overcome this obstacle, the persistent spin helix (PSH) state with a unidirectional SO field was proposed but difficult to achieve in real materials. Here, on the basis of first-principles calculations and tight-binding model analysis, we report for the first time a unidirectional SO field in monolayer transition metal dichalcogenides (TMDs, MX2, M = Mo, W; and X = S, Se) induced by two parallel chalcogen vacancy lines. By changing the relative positions of the two vacancy lines, the direction of the SO field can be tuned from x to y. Moreover, using k·p perturbation theory and group theory analysis, we demonstrate that the emerging unidirectional SO field is subject to both the structural symmetry and 1D nature of such defects engineered in 2D TMDs. In particular, through transport calculations, we confirm that the predicted SO states carry highly coherent spin current. Our findings shed new light on creating PSH states for high-performance spintronic devices.

Electron-nuclear hyperfine coupling in quantum kagome antiferromagnets from first-principles calculation and a reflection of the defect effect.

The discovery of ideal spin-1/2 kagome antiferromagnets Herbertsmithite and Zn-doped Barlowite represents a breakthrough in the quest for quantum spin liquids (QSLs), and nuclear magnetic resonance (NMR) spectroscopy plays a prominent role in revealing the quantum paramagnetism in these compounds. However, interpretation of NMR data that is often masked by defects can be controversial. Here, we show that the most significant interaction strength for NMR, i.e. the hyperfine coupling (HFC) strength, can be reasonably reproduced by first-principles calculations for these proposed QSLs. Applying this method to a supercell containing Cu-Zn defects enables us to map out the variation and distribution of HFC at different nuclear sites. This predictive power is expected to bridge the missing link in the analysis of the low-temperature NMR data.

Physical Properties and Photovoltaic Application of Semiconducting Pd2Se3 Monolayer.

Palladium selenides have attracted considerable attention because of their intriguing properties and wide applications. Motivated by the successful synthesis of Pd2Se3 monolayer (Lin et al., Phys. Rev. Lett., 2017, 119, 016101), here we systematically study its physical properties and device applications using state-of-the-art first principles calculations. We demonstrate that the Pd2Se3 monolayer has a desirable quasi-direct band gap (1.39 eV) for light absorption, a high electron mobility (140.4 cm²V-1s-1) and strong optical absorption (~105 cm-1) in the visible solar spectrum, showing a great potential for absorber material in ultrathin photovoltaic devices. Furthermore, its bandgap can be tuned by applying biaxial strain, changing from indirect to direct. Equally important, replacing Se with S results in a stable Pd2S3 monolayer that can form a type-II heterostructure with the Pd2Se3 monolayer by vertically stacking them together. The power conversion efficiency (PCE) of the heterostructure-based solar cell reaches 20%, higher than that of MoS2/MoSe2 solar cell. Our study would motivate experimental efforts in achieving Pd2Se3 monolayer-based heterostructures for new efficient photovoltaic devices.

Topological Electride Y2C.

Two-dimensional (2D) electrides are layered ionic crystals in which anionic electrons are confined in the interlayer space. Here, we report a discovery of nontrivial [Formula: see text] topology in the electronic structures of 2D electride Y2C. Based on first-principles calculations, we found a topological [Formula: see text] invariant of (1; 111) for the bulk band and topologically protected surface states in the surfaces of Y2C, signifying its nontrivial electronic topology. We suggest a spin-resolved angle-resolved photoemission spectroscopy (ARPES) measurement to detect the unique helical spin texture of the spin-polarized topological surface state, which will provide characteristic evidence for the nontrivial electronic topology of Y2C. Furthermore, the coexistence of 2D surface electride states and topological surface state enables us to explain the outstanding discrepancy between the recent ARPES experiments and theoretical calculations. Our findings establish a preliminary link between the electride in chemistry and the band topology in condensed-matter physics, which are expected to inspire further interdisciplinary research between these fields.

Stabilizing benzene-like planar N6 rings to form a single atomic honeycomb BeN3 sheet with high carrier mobility.

It is a longstanding quest to use the planar N6 ring as a structural unit to build stable atomic sheets. However, unlike C6H6, the neutral N6 ring is unstable due to the strong repulsion of the lone-pair of electrons. Using first-principles calculations and the global structure search method, we show that the N6 unit can be stabilized by the linkage of Be atoms, forming a h-BeN3 honeycomb monolayer, in which the geometry and the π-molecular orbitals of the N6 rings are well kept. This sheet is not only energetically, dynamically and thermally stable, but also can withstand high temperatures up to 1000 K. Band structure calculation combined with a group theory analysis and a tight-binding model uncover that h-BeN3 has a π-band dominated band structure with an indirect band gap of 1.67 eV. While it possesses a direct band gap of 2.07 eV at the Γ point lying in the photon energy region of visual light, its interband dipole transition is symmetrically allowed so that electrons can be excited by photons free of phonons. Based on deformation potential theory, a systematic study of the transport properties reveals that the h-BeN3 sheet possesses a high carrier mobility of ∼103 cm2 V-1 s-1, superior to the extensively studied transition metal dichalcogenide monolayers. We further demonstrate that this sheet can be rolled up into either zigzag or armchair nanotubes. These nanotubes are also dynamically stable, and are all direct band gap semiconductors with carrier mobility comparable to that of their 2D counterparts, regardless of their chirality and diameter. The robust stability and novel electronic and transport properties of the h-BeN3 sheet and its tubular derivatives endow them with great potential for applications in nanoelectronic devices.

Atomic scale electronic structure of the ferromagnetic semiconductor Cr2Ge2Te6.

Cr2Ge2Te6 is an intrinsic ferromagnetic semiconductor with van der Waals type layered structure, thus represents a promising material for novel electronic and spintronic devices. Here we combine scanning tunneling microscopy and first-principles calculations to investigate the electronic structure of Cr2Ge2Te6. Tunneling spectroscopy reveals a surprising large energy level shift and change of energy gap size across the ferromagnetic to paramagnetic phase transition, as well as a peculiar double-peak electronic state on the Cr-site defect. These features can be quantitatively explained by density functional theory calculations, which uncover a close relationship between the electronic structure and magnetic order. These findings shed important new lights on the microscopic electronic structure and origin of magnetic order in Cr2Ge2Te6.

Topological insulating states in 2D transition metal dichalcogenides induced by defects and strain.

First-principles calculations and extensive analyses reveal that the H phase of two-dimensional (2D) transition metal dichalcogenides (TMDs) can be tuned to topological insulators by introducing square-octagon (4-8) defects and by applying equi-biaxial tensile strain simultaneously. The 2D structure composed of hexagonal rings with 4-8 defects, named sho-TMD, is dynamically and thermally stable. The critical equi-biaxial tensile strain for the topological phase transition is 4%, 6%, and 4% for sho-MoS2, sho-MoSe2 and sho-WS2, respectively, and the corresponding nontrivial band gap induced by the spin-orbit coupling is 2, 8, and 22 meV, implying the possibility of observing the helical conducting edge states that are free of backscattering in experiment. It is equally interesting that the size of the energy band gap of the H-phase can be flexibly tuned by changing the concentration of 4-8 defects while the feature of the quasi-direct band gap semiconductor remains. These findings add additional traits to the TMD family, and provide a new strategy for engineering the electronic structure and the band topology of 2D TMDs for applications in nanoelectronics and spintronics.

Phosphorus K4 Crystal: A New Stable Allotrope.

The intriguing properties of phosphorene motivate scientists to further explore the structures and properties of phosphorus materials. Here, we report a new allotrope named K4 phosphorus composed of three-coordinated phosphorus atoms in non-layered structure which is not only dynamically and mechanically stable, but also possesses thermal stability comparable to that of the orthorhombic black phosphorus (A17). Due to its unique configuration, K4 phosphorus exhibits exceptional properties: it possesses a band gap of 1.54 eV which is much larger than that of black phosphorus (0.30 eV), and it is stiffer than black phosphorus. The band gap of the newly predicted phase can be effectively tuned by appling hydrostastic pressure. In addition, K4 phosphorus exibits a good light absorption in visible and near ultraviolet region. These findings add additional features to the phosphorus family with new potential applications in nanoelectronics and nanomechanics.

Bonding-restricted structure search for novel 2D materials with dispersed C2 dimers.

Currently, the available algorithms for unbiased structure searches are primarily atom-based, where atoms are manipulated as the elementary units, and energy is used as the target function without any restrictions on the bonding of atoms. In fact, in many cases such as nanostructure-assembled materials, the structural units are nanoclusters. We report a study of a bonding-restricted structure search method based on the particle swarm optimization (PSO) for finding the stable structures of two-dimensional (2D) materials containing dispersed C2 dimers rather than individual C atoms. The C2 dimer can be considered as a prototype of nanoclusters. Taking Si-C, B-C and Ti-C systems as test cases, our method combined with density functional theory and phonon calculations uncover new ground state geometrical structures for SiC2, Si2C2, BC2, B2C2, TiC2, and Ti2C2 sheets and their low-lying energy allotropes, as well as their electronic structures. Equally important, this method can be applied to other complex systems even containing f elements and other molecular dimers such as S2, N2, B2 and Si2, where the complex orbital orientations require extensive search for finding the optimal orientations to maximize the bonding with the dimers, predicting new 2D materials beyond MXenes (a family of transition metal carbides or nitrides) and dichalcogenide monolayers.

Tuning the electronic and mechanical properties of penta-graphene via hydrogenation and fluorination.

Penta-graphene has recently been proposed as a new allotrope of carbon composed of pure pentagons, and displays many novel properties going beyond graphene [Zhang et al., Proc. Natl. Acad. Sci. U. S. A., 2015, 112, 2372]. To further explore the property modulations, we have carried out a theoretical investigation of the hydrogenated and fluorinated penta-graphene sheets. Our first-principles calculations reveal that hydrogenation and fluorination can effectively tune the electronic and mechanical properties of penta-graphene: turning the sheet from semiconducting to insulating; changing the Poisson's ratio from negative to positive, and reducing the Young's modulus. Moreover, the band gaps of the hydrogenated and fluorinated penta-graphene sheets are larger than those of fully hydrogenated and fluorinated graphene by 0.37 and 0.04 eV, respectively. The phonon dispersions and ab initio molecular dynamics simulations confirm that the surface modified penta-graphene sheets are dynamically and thermally stable, and show that the hydrogenated penta-graphene has more Raman-active modes with higher frequencies as compared to the fluorinated penta-graphene.

Thermal exfoliation of stoichiometric single-layer silica from the stishovite phase: insight from first-principles calculations.

Mechanical cleavage, chemical intercalation and chemical vapor deposition are the main methods that are currently used to synthesize nanosheets or monolayers. Here, we propose a new strategy, thermal exfoliation for the fabrication of silica monolayers. Using a variety of state-of-the-art theoretical calculations we show that a stoichiometric single-layer silica with a tetragonal lattice, T-silica, can be thermally exfoliated from the stishovite phase in a clean environment at room temperature. The resulting single-layer silica is dynamically, thermally, and mechanically stable with exceptional properties, including a large band gap of 7.2 eV, an unusual negative Poisson's ratio, a giant Stark effect, and a high breakdown voltage. Moreover, other analogous structures like single-layer GeO2 can also be obtained by thermal exfoliation of its bulk phase. Our findings are expected to motivate experimental efforts on developing new techniques for the synthesis of monolayer materials.