Structure Responsible for the Superconducting State in La3Ni2O7 at High-Pressure and Low-Temperature Conditions.

Very recently, a new superconductor with Tc = 80 K has been reported in nickelate (La3Ni2O7) at around 15-40 GPa conditions (Nature, 621, 493, 2023), which is the second type of unconventional superconductor, besides cuprates, with Tc above liquid nitrogen temperature. However, the phase diagram plotted in this report was mostly based on the transport measurement under low-temperature and high-pressure conditions, and the assumed corresponding X-ray diffraction (XRD) results were carried out at room temperature. This encouraged us to carry out in situ high-pressure and low-temperature synchrotron XRD experiments to determine which phase is responsible for the high Tc state. In addition to the phase transition from the orthorhombic Amam structure to the orthorhombic Fmmm structure, a tetragonal phase with the space group of I4/mmm was discovered when the sample was compressed to around 19 GPa at 40 K where the superconductivity takes place in La3Ni2O7. The calculations based on this tetragonal structure reveal that the electronic states that approached the Fermi energy were mainly dominated by the eg orbitals (3dz2 and 3dx2-y2) of Ni atoms, which are located in the oxygen octahedral crystal field. The correlation between Tc and this structural evolution, especially Ni-O octahedra regularity and the in-plane Ni-O-Ni bonding angles, is analyzed. This work sheds new light to identify what is the most likely phase responsible for superconductivity in double-layered nickelate.

Phase evolution for oxidizing bismuth selenide.

The novel Bi2O2Se, produced by the oxidation of the layered Bi2Se3, has been considered as one of the most promising candidates for the next-generation electronics owing to its high carrier mobility and air-stability. In this work, by using crystal structure prediction and first-principles calculations, we report the phase transformations from the hexagonal Bi2Se3to the monoclinic Bi2OSe2, and then to the tetragonal Bi2O2Se with the gradual oxidization. Owing to the difference in electronegativity between selenium (Se) and oxygen (O), the oxidation process is accompanied by an increase in bond ionicity. Our results shed light on the phenomena occurring in the interaction between the precursors Bi2Se3and O2and have a potential contribution to the application of optoelectronic devices. The intermediate Bi2OSe2with calculated band gap of 1.01 eV, may be a candidate for photovoltaic application in future.

Electronic Tuning in WSe2/Au via van der Waals Interface Twisting and Intercalation.

The transition metal dichalcogenide (TMD)-metal interfaces constitute an active part of TMD-based electronic devices with optimized performances. Despite their decisive role, current strategies for nanoscale electronic tuning remain limited. Here, we demonstrate electronic tuning in the WSe2/Au interface by twist engineering, capable of modulating the WSe2 carrier doping from an intrinsic p-type to n-type. Scanning tunneling microscope/spectroscopy gives direct evidence of enhanced interfacial interaction induced doping in WSe2 as the twist angle with respect to the topmost (100) lattice of gold changing from 15 to 0°. Taking advantage of the strong coupling interface achieved this way, we have moved a step further to realize an n-p-n-type WSe2 homojunction. The intrinsic doping of WSe2 can be recovered by germanium intercalation. Density functional theory calculations confirm that twist angle and intercalation-dependent charge transfer related doping are involved in our observations. Our work offers ways for electronically tuning the metal-2D semiconductor interface.

Predicted stable high-pressure phases of copper-nitrogen compounds.

The nitrogen-rich compounds are promising candidates for high-energy-density applications, owing to the large difference in the bonding energy between triple and single/double nitrogen bonds. The exploration of stable copper-nitrogen (Cu-N) compounds with high-energy-density has been challenging for a long time. Recently, through a combination of high temperatures and pressures, a new copper diazenide compound (P63/mmc-CuN2) has been synthesized (Binnset al2019J. Phys. Chem. Lett.101109-1114). But the pressure-composition phase diagram of Cu-N compounds at different temperatures is still highly unclear. Here, by combining first-principles calculations with crystal structure prediction method, the Cu-N compounds with different stoichiometric ratios were searched within the pressure range of 0-150 GPa. Four Cu-N compounds are predicted to be thermodynamically stable at high pressures,Pnnm-CuN2, two CuN3compounds with theP-1 space group (named as I-CuN3and II-CuN3) andP21/m-CuN5containing cyclo-N5-. Finite temperature effects (vibrational energies) play a key role in stabilizing experimentally synthesizedP63/mmc-CuN2at ∼55 GPa, compared to our predictedPnnm-CuN2. These new Cu-N compounds show great promise for potential applications as high-energy-density materials with the energy densities of 1.57-2.74 kJ g-1.

From Two-, One-, to Zero-Dimensional Vacancies: A Densification Pattern for a Typcial Transition-Metal Dichalcogenide of TiSe2.

In the areas of condensed matter physics, geoscience, material science, and inorganic chemistry, how the crystal structures evolve under an external field such as high-pressure is a fundamental question. By taking TiSe2 as the case, we investigate the phase transformations of the layered transition-metal dichalcogenides (TMDs) under high-pressure. The ambient 6-fold P-3m1 TiSe2 undergoes a transformation into the monoclinic 8-fold coordinated C2/m phase at 15 GPa and then into the hexagonal 9-fold Fe2P-type structure at 34 GPa. The above phase transitions can be unitedly described as the evolution of the vacancies: from a layered structure with two-dimensional (2D) vacancies to the structure with one-dimensional (1D) and zero-dimensional (0D) vacancies. The proposed densification model of TiSe2 reveals the processes how the symmetry breaking phase of spatial chemical bonding restores the symmetry under the isotropic external pressure.

Metallic Graphene Nanoribbons.

Isolated graphene nanoribbons (GNRs) usually have energy gaps, which scale with their widths, owing to the lateral quantum confinement effect of GNRs. The absence of metallic GNRs limits their applications in device interconnects or being one-dimensional physics platform to research amazing properties based on metallicity. A recent study published in Science provided a novel method to produce metallic GNRs by inserting a symmetric superlattice into other semiconductive GNRs. This finding will broader the applications of GNRs both in nanoelectronics and fundamental science.

The recurrence of dense face-centered cubic cesium.

At ambient conditions, alkali metal cesium (Cs) owns a body-centered cubic phase, and this phase will transform to a face-centered cubic (fcc) phase at a pressure of 2.3 GPa. Under stronger compression, Cs will transform to oC84, tI4, oC16, and double hexagonal close-packed (dhcp) phases in sequence. Here, using first-principles structure searching prediction and total-energy calculation, we report that the Cs will re-transform to the fcc phase as the post-dhcp phase above 180 GPa. The transition state calculations suggest that the phase transition takes place by overcoming an energy barrier (144 meV/atom at 200 GPa) and finishes within a volume collapse of 0.3%. The electronic states at Fermi level are derived mainly from d electrons and there is a large overlap between inner core electrons, making the high-pressure fcc Cs distinguished from the first one at low pressure. The same phase transition also occurs in potassium and rubidium but with higher pressures.

Pressure induced superconductive 10-fold coordinated TaS2: a first-principles study.

By crystal structure prediction and first-principles calculations, we researched the structure transformation and electronic states for a typical transition metal dichalcogenides (TMDs): 1T-TaS2 under hydrostatic pressure. The layered 1T-TaS2 will first transform to an 8-fold monoclinic C2/m phase and then to a 10-fold coordinated tetragonal I4/mmm phase which has 3D covalent bond network linked in space. Our calculations suggest that the lone pair electrons of S in 1T-TaS2, which keep the stablity of the layered structure, will be activated by pressure and participate the chemical bonding with Ta, to form the high-pressure C2/m and I4/mmm phases. Additionally, collective electronic states of superconductivity also retains in the I4/mmm phase and the critical transition temperature of superconductivity is 9 K at 100 GPa.

Flat Boron: A New Cousin of Graphene.

The mechanical exfoliation of graphene from graphite provides the cornerstone for the synthesis of other 2D materials with layered bulk structures, such as hexagonal boron nitride, transition metal dichalcogenides, black phosphorus, and so on. However, the experimental production of 2D flat boron is challenging because bulk boron has very complex spatial structures and a rich variety of chemical properties. Therefore, the realization of 2D flat boron marks a milestone for the synthesis of 2D materials without layered bulk structures. The historical efforts in this field, particularly the most recent experimental progress, such as the growth of 2D flat boron on a metal substrate by chemical vapor deposition and molecular beam epitaxy, or liquid exfoliation from bulk boron, are described.

Correlated High-Pressure Phase Sequence of VO2 under Strong Compression.

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 (MO2) 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 VO2. 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 MO2 to give away the ionicity in exchange for covalency under pressure.

Improved NO2 Gas Sensing Properties of Graphene Oxide Reduced by Two-beam-laser Interference.

We report on the fabrication of a NO2 gas sensor from room-temperature reduction of graphene oxide(GO) via two-beam-laser interference (TBLI). The method of TBLI gives the distribution of periodic dissociation energies for oxygen functional groups, which are capable to reduce the graphene oxide to hierarchical graphene nanostructures, which holds great promise for gaseous molecular adsorption. The fabricated reduced graphene oxide(RGO) sensor enhanced sensing response in NO2 and accelerated response/recovery rates. It is seen that, for 20 ppm NO2, the response (Ra/Rg) of the sensor based on RGO hierarchical nanostructures is 1.27, which is higher than that of GO (1.06) and thermal reduced RGO (1.04). The response time and recovery time of the sensor based on laser reduced RGO are 10 s and 7 s, which are much shorter than those of GO (34 s and 45 s), indicating that the sensing performances for NO2 sensor at room temperature have been enhanced by introduction of nanostructures. This mask-free and large-area approach to the production of hierarchical graphene micro-nanostructures, could lead to the implementation of future graphene-based sensors.

Exploring long-wave infrared transmitting materials with AxBy form: First-principles gene-like studies.

Long-wave infrared (8-12 µm) transmitting materials play critical roles in space science and electronic science. However, the paradox between their mechanical strength and infrared transmitting performance seriously prohibits their applications in harsh external environment. From the experimental view, searching a good window material compatible with both properties is a vast trail-and-error engineering project, which is not readily achieved efficiently. In this work, we propose a very simple and efficient method to explore potential infrared window materials with suitable mechanical property by first-principles gene-like searching. Two hundred and fifty-three potential materials are evaluated to find their bulk modulus (for mechanical performance) and phonon vibrational frequency (for optical performance). Seven new potential candidates are selected, namely TiSe, TiS, MgS, CdF2, HgF2, CdO, and SrO. Especially, the performances of TiS and CdF2 can be comparable to that of the most popular commercial ZnS at high temperature. Finally, we propose possible ranges of infrared transmission for halogen, chalcogen and nitrogen compounds respectively to guide further exploration. The present strategy to explore IR window materials can significantly speed up the new development progress. The same idea can be used for other material rapid searching towards special functions and applications.

Boron based two-dimensional crystals: theoretical design, realization proposal and applications.

The successful realization of free-standing graphene and the various applications of its exotic properties have spurred tremendous research interest for two-dimensional (2D) layered materials. Besides graphene, many other 2D materials have been successfully produced by experiment, such as silicene, monolayer MoS2, few-layer black phosphorus and so on. As a neighbor of carbon in the periodic table, element boron is interesting and many researchers have contributed their efforts to realize boron related 2D structures. These structures may be significant both in fundamental science and future technical applications in nanoelectronics and nanodevices. In this review, we summarize the recent developments of 2D boron based materials. The theoretical design, possible experimental realization strategies and their potential technical applications are presented and discussed. Also, the current challenges and prospects of this area are discussed.

Determination of formation and ionization energies of charged defects in two-dimensional materials.

We present a simple and efficient approach to evaluate the formation energy and, in particular, the ionization energy (IE) of charged defects in two-dimensional (2D) systems using the supercell approximation. So far, first-principles results for such systems can scatter widely due to the divergence of the Coulomb energy with vacuum dimension, denoted here as L_{z}. Numerous attempts have been made in the past to fix the problem under various approximations. Here, we show that the problem can be resolved without any such assumption, and a converged IE can be obtained by an extrapolation of the asymptotic IE expression at large L_{z} (with a fixed lateral area S) back to the value at L_{z}=0. Application to defects in monolayer boron nitride reveal that defects in 2D systems can be unexpectedly deep, much deeper than the bulk.

A novel two-dimensional MgB6 crystal: metal-layer stabilized boron kagome lattice.

Based on first-principles calculations, we designed for the first time a boron-kagome-based two-dimensional MgB6 crystal, in which two boron kagome layers sandwich a triangular magnesium layer. The two-dimensional lattice is metallic with several bands across the Fermi level, and among them a Dirac point appears at the K point of the first Brillouin zone. This metal-stabilized boron kagome system displays electron-phonon coupling, with a superconductivity critical transition temperature of 4.7 K, and thus it is another possible superconducting Mg-B compound besides MgB2. Furthermore, the proposed 2D MgB6 can also be used for hydrogen storage after decoration with Ca. Up to five H2 molecules can be attracted by one Ca with an average binding energy of 0.225 eV. The unique properties of 2D MgB6 will spur broad interest in nanoscience and technology.

Slide fastener reduction of graphene-oxide edges by calcium: insight from ab initio molecular dynamics.

The reduction of graphene oxide can be used as a simple way to produce graphene on a large scale. However, the numerous edges produced by the oxidation of graphite seriously degrade the quality of the graphene and its carrier transport property. In this work, the reduction of oxygen-passivated graphene edges and the subsequent linking of separated graphene sheets by calcium are investigated by using first-principles calculations. The calculations show that calcium can effectively remove the oxygen groups from two adjacent edges. The joining point of the edges serves as the starting point of the reduction and facilitates the reaction. Once the oxygen groups are removed, the crack is sutured. If the joining point is lacking, it becomes difficult to zip the separated fragments. A general electron-reduction model and a random atom-reduction model are suggested for these two situations. The present study sheds light on the reduction of graphene-oxide edges by using reactive metals to give large-sized graphene through a simple chemical reaction.