Probing a Defect-Site-Specific Electronic Orbital in Graphene with Single-Atom Sensitivity.

Visualization of individual electronic states ascribed to specific unoccupied orbitals at the atomic scale can reveal fundamental information about chemical bonding, but it is challenging since bonding often results in only subtle variations in the whole density of states. Here, we utilize atomic-resolution energy-loss near-edge fine structure (ELNES) spectroscopy to map out the electronic states attributed to specific unoccupied p_{z} orbital around a fourfold coordinated silicon point defect in graphene, which is further supported by theoretical calculations. Our results illustrate the power of atomic-resolution ELNES towards the probing of defect-site-specific electronic orbitals in monolayer crystals, providing insights into understanding the effect of chemical bonding on the local properties of defects in solids.

Ultrastable Zinc Anode by Simultaneously Manipulating Solvation Sheath and Inducing Oriented Deposition with PEG Stability Promoter.

Aqueous zinc-ion batteries are a low-cost and safe energy storage system, but suffer from detrimental side reactions and Zn dendrites due to the strong interactions between Zn2+ and water molecules in the electrolytes, and random Zn2+ deposition on the anode surface. Here, an electrolyte involving a dual-functional additive of polyethylene glycol (PEG) to bypass these issues is reported. The electrolyte can not only tailor the solvation sheath of Zn2+ but also enable favorably oriented deposition of Zn2+ on the anode surface. The dendrite-free Zn anode in Zn//Zn cells is obtained with high Columbic efficiency (98.8%) and long cycling lifespan (1500 h), six times longer than that of electrolyte without PEG at 0.25 mA cm-2 . What is more, the excellent cycling stability of the prepared batteries (Zn//V2 O5 ·1.6 H2 O) suggests that the developed tailoring strategy may propel a promising pathway for stabilizing Zn metal anodes.

Depolarization of few-layer III-V and II-VI materials through symmetric rumpling.

It is generally believed that few-layer films of wurtzite materials remove the destabilizing dipole by converting to a flat hexagonal structure. However, using first-principles calculations, we demonstrate that contrary to the existing consensus these few-layer hexagonal films exhibit a small symmetric rumpling and are not perfectly flat. We then perform a systematic study of the rumpling behavior of a range of few-layer III-V and II-VI films. The symmetric rumpled configuration enables such films to cancel out the dipole and thereby to avoid the polar instability. This stabilization mechanism is quite distinct from those known for bulk and few-layer polar materials. Compared to the perfectly flat films, the rumpled films exhibit lower electrostatic potential energy, lower total energy, higher bonding strength, and thus greater stability and larger band gaps. We also discuss the relationship between rumpling behavior, interlayer interactions, and ionicity through electrostatic analysis.

Anomalous Redox Features Induced by Strong Covalency in Layered NaTi1-y Vy S2 Cathodes for Na-Ion Batteries.

The rising demand for energy density of cathodes means the need to raise the voltage or capacity of cathodes. Transition metal (TM) doping has been employed to enhance the electrochemical properties in multiple aspects. The redox voltage of doped cathodes usually falls in between the voltage of undoped layered cathodes. However, we found anomalous redox features in NaTi1-y Vy S2 . The first discharge platform potential (2.4 V) is significantly higher than that of undoped NaTiS2 and NaVS2 (both around 2.2 V), and the energy density is raised by 15 %. We speculate that the anomalous voltage is mainly attributed to the strong hybridization in the Ti-V-S system. Ti3+ and V3+ undergo charge transfer and form a more stable Ti (t2g 0 eg 0 ) and V (t2g 3 eg 0 ) electronic configuration. Our results indicate that higher voltage of cathode materials could be achieved by strong TM-ligand covalency, and this conclusion provides possible opportunities to explore high voltage materials for future layered cathodes.

Tunable electronic properties and Schottky barrier in a graphene/WSe2 heterostructure under out-of-plane strain and an electric field.

Tuning the electrical transport behavior and reducing the Schottky barrier height of nanoelectronic devices remain a great challenge. To solve this issue, the electronic properties and Schottky barrier of the graphene/WSe2 heterostructure are investigated by the first-principles method under out-of-plane strain and an electric field. Our results show that the WSe2 monolayer and graphene could form a stable van der Waals heterostructure and the intrinsic electronic properties are well preserved. Furthermore, a transformation of a Schottky contact from the n-type to p-type occurs at d = 3.87 Å and E = +0.06 V Å-1. In addition, an ohmic contact is formed with E = -0.50, ±0.60 V Å-1. Lastly, the effective masses of electrons and holes are calculated to be 0.057m0 and -0.055m0 at the equilibrium state, respectively, indicating that the heterostructure has a high carrier mobility. Our research will provide promising approaches for the future design and development of graphene/WSe2 nano-field effect transistors.

Synthesis of Honeycomb-Structured Beryllium Oxide via Graphene Liquid Cells.

Using high-resolution transmission electron microscopy and electron energy-loss spectroscopy, we show that beryllium oxide crystallizes in the planar hexagonal structure in a graphene liquid cell by a wet-chemistry approach. These liquid cells can feature van-der-Waals pressures up to 1 GPa, producing a miniaturized high-pressure container for the crystallization in solution. The thickness of as-received crystals is beyond the thermodynamic ultra-thin limit above which the wurtzite phase is energetically more favorable according to the theoretical prediction. The crystallization of the planar phase is ascribed to the near-free-standing condition afforded by the graphene surface. Our calculations show that the energy barrier of the phase transition is responsible for the observed thickness beyond the previously predicted limit. These findings open a new door for exploring aqueous-solution approaches of more metal-oxide semiconductors with exotic phase structures and properties in graphene-encapsulated confined cells.

Layer-Dependent Chemically Induced Phase Transition of Two-Dimensional MoS2.

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) with layered structures provide a unique platform for exploring the effect of number of layers on their fundamental properties. However, the thickness scaling effect on the chemical properties of these materials remains unexplored. Here, we explored the chemically induced phase transition of 2D molybdenum disulfide (MoS2) from both experimental and theoretical aspects and observed that the critical electron injection concentration and the duration required for the phase transition of 2D MoS2 increased with decreasing number of layers. We further revealed that the observed dependence originated from the layer-dependent density of states of 2H-MoS2, which results in decreasing phase stability for 2H-MoS2 with increasing number of layers upon electron doping. Also, the much larger energy barrier for the phase transition of monolayer MoS2 induces the longer reaction time required for monolayer MoS2 as compared to multilayer MoS2. The layer-dependent phase transition of 2D MoS2 allows for the chemical construction of semiconducting-metallic heterophase junctions and, subsequently, the fabrications of rectifying diodes and all 2D field effect transistors and thus opens a new avenue for building ultrathin electronic devices. In addition, these new findings elucidate how electronic structures affect the chemical properties of 2D TMDCs and, therefore, shed new light on the controllable chemical modulations of these emerging materials.

A honeycomb-like monolayer of HfO2 and the calculation of static dielectric constant eliminating the effect of vacuum spacing.

A novel dielectric material of monolayer 1T-HfO2 has been investigated using first-principles calculations. The stability of 1T-HfO2 has been proved by both phonon dispersions and ab initio molecular dynamics calculations, although its 2H structural counterpart is dynamically unstable. 1T-HfO2 monolayer can be cleaved from the (111) facet of cubic HfO2. It is found that 1T-HfO2 has a large band gap of 6.73 eV, exceeding the band gaps of h-BN (5.97 eV) and bulk HfO2 (5.7 eV). From the microscopic perspective of dielectric polarization, we provide an explanation for the dependence of the dielectric constant directly calculated from the supercell of a two-dimensional (2D) system on the variable vacuum spacing, and we thus obtain a rational method for accurately evaluating the dielectric constants of 2D materials based on the calculated value obtained from a supercell to meet periodic conditions. Our derivation can be verified by the data fitting of a series of calculations with different vacuum spacings. The static dielectric constants of 1T-HfO2 along the in-plane and out-of-plane directions are 27.35 and 4.80, respectively, higher than those of monolayer h-BN. The large band gap and high dielectric constant make 1T-HfO2 a promising candidate as a dielectric layer in 2D field-effect transistors and heterojunctions.

Momentum and thickness dependent excitonic and plasmonic properties of 2D h-BN and MoS2 restored from supercell calculations.

The comprehension and manipulation of the propagation characteristics of elementary excitations, such as excitons and plasmons, play a crucial role in tailoring the optical properties of low-dimensional materials. To this end, investigations into the momentum (q) dispersions of excitons and plasmons in confined geometry are required fundamentally. Due to advancements in momentum-resolved spectroscopy techniques, research on the q-dependent excitons or plasmons in low-dimensional materials is beginning to emerge. However, previous simulations of low-dimensional systems are adversely affected by the artificial vacuum spacing employed in the supercell approximation. Furthermore, the significance of layer thickness in determining the excitonic and plasmonic characteristics of two-dimensional (2D) materials remains largely unexplored in the context of finite q. Therefore, an extensive investigation into the momentum and thickness dependent behaviours of both excitons and plasmons in 2D materials, which are free of the influence of vacuum spacing, is lacking at present. In this article, we develop a restoration procedure to eliminate the influence of vacuum spacing, and obtain a comprehensive picture of momentum and layer thickness dependent excitonic and plasmonic properties of 2D hexagonal boron nitride (h-BN) and molybdenum disulphide (MoS2). Our restored simulations are not only found to be in excellent agreement with available experiments, but also elucidate the roles of momentum and layer thickness in the excitonic and plasmonic properties of 2D h-BN and MoS2. We further unveil the dimensionality effect on the dispersion characteristics of excitons and plasmons in h-BN and MoS2. Our contribution will hopefully promote the understanding of the elementary excitations propagating in low-dimensional materials and pave the way for next-generation nanophotonic and optoelectronic devices.

Addressing the Conflict between Mobility and Stability in Oxide Thin-film Transistors.

Amorphous oxide semiconductor thin-film transistors (AOS TFTs) are ever-increasingly utilized in displays. However, to bring high mobility and excellent stability together is a daunting challenge. Here, the carrier transport/relaxation bilayer stacked AOS TFTs are investigated to solve the mobility-stability conflict. The charge transport layer (CTL) is made of amorphous In-rich InSnZnO, which favors big average effective coordination number for all cations and more edge-shared structures for better charge transport. Praseodymium-doped InSnZnO is used as the charge relaxation layer (CRL), which substantially shortens the photoelectron lifetime as revealed by femtosecond transient absorption spectroscopy. The CTL and CRL with the thickness suitable for industrial production respectively afford minute potential barrier fluctuation for charge transport and fast relaxation for photo-generated carriers, resulting in transistors with an ultrahigh mobility (75.5 cm2 V-1 s-1 ) and small negative-bias-illumination-stress/positive-bias-temperature-stress voltage shifts (-1.64/0.76 V). The design concept provides a promising route to address the mobility-stability conflict for high-end displays.

Exciton Emissions in Bilayer WSe2 Tuned by the Ferroelectric Polymer.

In this work, a hybrid structure of multilayer transition-metal dichalcogenides (TMDs) and a ferroelectric polymer is designed to achieve passive control of optical properties in situ. The electrical polarization in the ferroelectric poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) polymer can regulate the photoluminescence (PL) in bilayer WSe2. The total PL emission intensity is substantially suppressed or enhanced under large gate voltage in bilayer WSe2. This is because electrons transfer between the conduction band K valley and the conduction band Λ valley by the electrostatic field in the P(VDF-TrFE) polymer. This electron transfer further adjusts the proportion of direct and indirect excitons and, in turn, changes the overall optical radiation efficiency. We also illustrate that the engineered PL originates from the external electric-field-dependent transferred electron effect. The theoretical result matches the experimental data well. This work demonstrates a device platform in which passive regulation is achieved using 2D TMDs modulated by polarized ferroelectric materials.

Anionic Redox Regulated via Metal-Ligand Combinations in Layered Sulfides.

The increasing demand for energy storage is calling for improvements in cathode performance. In traditional layered cathodes, the higher energy of the metal 3d over the O 2p orbital results in one-band cationic redox; capacity solely from cations cannot meet the needs for higher energy density. Emerging anionic redox chemistry is promising to access higher capacity. In recent studies, the low-lying O nonbonding 2p orbital was designed to activate one-band oxygen redox, but they are still accompanied by reversibility problems like oxygen loss, irreversible cation migration, and voltage decay. Herein, by regulating the metal-ligand energy level, both extra capacities provided by anionic redox and highly reversible anionic redox process are realized in NaCr1- y Vy S2 system. The simultaneous cationic and anionic redox of Cr/V and S is observed by in situ X-ray absorption near edge structure (XANES). Under high d-p hybridization, the strong covalent interaction stabilizes the holes on the anions, prevents irreversible dimerization and cation migration, and restrains voltage hysteresis and voltage decay. The work provides a fundamental understanding of highly reversible anionic redox in layered compounds, and demonstrates the feasibility of anionic redox chemistry based on hybridized bands with d-p covalence.

A method to restore the intrinsic dielectric functions of 2D materials in periodic calculations.

Previous calculations of the dielectric and optical properties of 2D materials often overlooked or circumvented the influence of vacuum spacing introduced in periodic calculations, which gave rise to mispredictions of the intrinsic properties of 2D materials or merely qualitative results. We first elucidated the relationship between the vacuum spacing and the dielectric and optical properties of 2D materials in periodic calculations, and then formulated an effective method to accurately predict the dielectric and optical properties of 2D materials by restoring the intrinsic dielectric functions of 2D materials independent of the additional vacuum spacing. As examples, the intrinsic dielectric and optical properties of ultrathin hexagonal boron nitride (h-BN) and molybdenum sulphide (MoS2) from a monolayer to a pentalayer, including dielectric functions, optical absorption coefficients, refraction indexes, reflectivities, extinction coefficients, and energy loss functions, have been calculated by our method. Our calculations reveal that the out-of-plane optical dielectric constants, static refraction indexes, and static reflectivities of 2D h-BN and MoS2 increase as the number of layers increases, while the in-plane counterparts remain unchanged. The excitonic frequency-dependent optical properties of h-BN and MoS2 from a monolayer to bulk are also calculated by solving the Bethe-Salpeter equation and they show strong anisotropy. The present method shows better agreement with the experimental results compared to previous calculations and demonstrates enormous potential to investigate the dielectric and optical properties of other 2D materials extensively and quantitatively.

Superior-Performance Aqueous Zinc-Ion Batteries Based on the In Situ Growth of MnO2 Nanosheets on V2CTX MXene.

Mn-based aqueous zinc-ion batteries (ZIBs) are promising candidate for large-scale rechargeable energy storage because of easy fabrication, low cost, and high safety. Nevertheless, the commercial application of Mn-based cathode is hindered by the challenging issues of low rate capability and poor cyclability. Herein, a manganese-vanadium hybrid, K-V2C@MnO2 cathode, featured with MnO2 nanosheets uniformly formed on a V2CTX MXene surface, is elaborately designed and synthesized by metal-cation intercalation and following in situ growth strategy. Benefiting from the hybrid structure with high conductivity, abundant active sites, and the synergistic reaction of Mn2+ electrodeposition and inhibited structural damage of MnO2, K-V2C@MnO2 shows excellent electrochemical performance for aqueous ZIBs. Specifically, it presents the high specific capacity of 408.1 mAh g-1 at 0.3 A g-1 and maintains the specific capacity of 119.2 mAh g-1 at a high current density of 10 A g-1 in a long-term cycle of up to 10000 cycles. It is superior to almost all reported Mn-based cathodes for ZIBs in the aqueous electrolyte. The superior electrochemical performance suggests that the Mn-based cathode materials designed in this work can be a rational approach to be applied for high-performance ZIBs cathodes.

Peculiar bond characters of fivefold coordinated octet compound crystals.

The present work exemplifies complementary perspectives offered by the band and bond pictures of solids, with an emphasis on the chemical intuition pertaining to the latter, especially in the presence of interfaces. The modern computational method of constructing a unique set of maximally localized Wannier functions from delocalized band states imparts new interpretations to the familiar concept of chemical bonds in the context of crystalline solids. By bridging the band and bond pictures using advanced computational tools, we reveal for the first time the unusual bond characters of a long-predicted fivefold coordinated structure of binary octet compounds A N B8-N consisting of AA' stacked planar AB honeycombs. While the isolated monolayer retains the familiar p z -π bonding in a honeycomb framework as in graphene and hexagonal boron nitride, the bulk foregoes in-plane π bonding and embraces out-of-plane ⋯A-B-A-B⋯ chain bonding via overlapping p z orbitals. Not only does the chemical intuition gained by invoking the bond picture clarify the chemical nature of the fivefold coordination, but it also facilely explains a salient discrepancy in theoretical predictions in otherwise sound ample experimental evidence in the form of epitaxial thin films, paving the way towards rational synthesis of such thin films for optoelectronic applications. On the other hand, we show that the conduction band minimum, important in determining the electrical and optical properties, is a distinctly extended state that can only be properly described within the band picture.

Core-level spectroscopy calculation and the plane wave pseudopotential method.

A plane wave based method for the calculation of core-level spectra is presented. We provide details of the implementation of the method in the pseudopotential density functional code CASTEP, including technical issues concerning the calculations, and discuss the applicability and accuracy of the method. A number of examples are provided for comparing the results to both experiment and other density functional theory techniques.

First-principles calculation of spectral features, chemical shift and absolute threshold of ELNES and XANES using a plane wave pseudopotential method.

Spectral features, chemical shifts, and absolute thresholds of electron energy loss near-edge structure (ELNES) and x-ray absorption near-edge structure (XANES) for selected compounds, i.e. TiO(2) (rutile), TiO(2) (anatase), SrTiO(3), Ti(2)O(3), Al(2)O(3), AlN and ß-Ga(2)O(3), were calculated by a plane wave pseudopotential method. Experimental ELNES/XANES of those compounds were well reproduced when an excited pseudopotential, which includes a core hole, was used. In addition to the spectral features, it was found that chemical shifts among different compounds were also reproduced by correcting the contribution of the excited pseudopotentials to the energy of the core orbital.

Structures and characteristics of atomically thin ZrO2 from monolayer to bilayer and two-dimensional ZrO2-MoS2 heterojunction.

The understanding of the structural stability and properties of dielectric materials at the ultrathin level is becoming increasingly important as the size of microelectronic devices decreases. The structures and properties of ultrathin ZrO2 (monolayer and bilayer) have been investigated by ab initio calculations. The calculation of enthalpies of formation and phonon dispersion demonstrates the stability of both monolayer and bilayer ZrO2 adopting a honeycomb-like structure similar to 1T-MoS2. Moreover, the 1T-ZrO2 monolayer or bilayer may be fabricated by the cleavage from the (111) facet of non-layered cubic ZrO2. Moreover, the contraction of in-plane lattice constants in monolayer and bilayer ZrO2 as compared to the corresponding slab in cubic ZrO2 is consistent with the reported experimental observation. The electronic band gaps calculated from the GW method show that both the monolayer and bilayer ZrO2 have large band gaps, reaching 7.51 and 6.82 eV, respectively, which are larger than those of all the bulk phases of ZrO2. The static dielectric constants of both monolayer ZrO2 (ε ‖ = 33.34, ε ⊥ = 5.58) and bilayer ZrO2 (ε ‖ = 33.86, ε ⊥ = 8.93) are larger than those of monolayer h-BN (ε ‖ = 6.82, ε ⊥ = 3.29) and a strong correlation between the out-of-plane dielectric constant and the layer thickness in ultrathin ZrO2 can be observed. Hence, 1T-ZrO2 is a promising candidate in 2D FETs and heterojunctions due to the high dielectric constant, good thermodynamic stability, and large band gap for applications. The interfacial properties and band edge offset of the ZrO2-MoS2 heterojunction are investigated herein, and we show that the electronic states near the VBM and CBM are dominated by the contributions from monolayer MoS2, and the interface with monolayer ZrO2 will significantly decrease the band gap of the monolayer MoS2.

Development of electron energy-loss spectroscopy for nanoscience.

Electron energy-loss spectroscopy (EELS) has been well established in providing the composition and chemical bonding information of materials, particularly for light elements. Its potential for structural determination has long been known but has yet to be fully explored. With the convergence of rapid development in computing power and improvement in the efficiency of the material specific electronic structure simulation, plus the recent breakthrough in the development of C(s)-corrected electron microscopy, the reconstruction of the local three dimensional structure of nanomaterial using EELS in conjunction with advanced structural imaging and diffraction techniques is becoming increasingly feasible. In this paper, we will review from our own examples the progress in EELS instrumentation, methods and simulation to illustrate the progress that has been made. They include the density-function-theory-based ab initio spectroscopic simulation for standard-less fingerprint applications for metastable polymorph identification, magic angle electron energy-loss spectroscopy as well as recent results from the dual-detectors EELS system which allows the energy instability of the spectrometer to be analyzed in real-time and eventually compensated on-line.

Novel Ultrafine Fibrous Poly(tetrafluoroethylene) Hollow Fiber Membrane Fabricated by Electrospinning.

Novel poly(tetrafluoroethylene) (PTFE) hollow fiber membranes were successfully fabricated by electrospinning, with ultrafine fibrous PTFE membranes as separation layers, while a porous glassfiber braided tube served as the supporting matrix. During this process, PTFE/poly(vinylalcohol) (PVA) ultrafine fibrous membranes were electrospun while covering the porous glassfiber braided tube; then, the nascent PTFE/PVA hollow fiber membrane was obtained. In the following sintering process, the spinning carrier PVA decomposed; meanwhile, the ultrafine fibrous PTFE membrane shrank inward so as to further integrate with the supporting matrix. Therefore, the ultrafine fibrous PTFE membranes had excellent interface bonding strength with the supporting matrix. Moreover, the obtained ultrafine fibrous PTFE hollow fiber membrane exhibited superior performances in terms of strong hydrophobicity (CA > 140°), high porosity (>70%), and sharp pore size distribution. The comprehensive properties indicated that the ultrafine fibrous PTFE hollow fiber membranes could have potentially useful applications in membrane contactors (MC), especially membrane distillation (MD) in harsh water environments.