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.

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.

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.

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.

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.

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.

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.

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.

A general strategy for the functionalization of two-dimensional metal chalcogenides.

Two-dimensional (2D) metal chalcogenides (MC) such as MoS2 have been recognized as promising materials for near future applications. However, general strategies to functionalize them are still scarce, while the nature of functionalization still remains unclear. Herein, we demonstrate a simple and universal functionalization route through complexation reaction between the amino-containing organic agents and MCs. Degrees of functionalization are tunable by adjusting the organic group types and ratios. No further defects are introduced and the functionalized 2D MCs are dispersible in corresponding typical solvents. Both experimental results and geometry optimization calculations indicate that the grafting of functional groups through the coordination effect truly exist, while the surface properties and resulting photoelectric properties of 2D MCs are greatly altered. More intriguingly, our proposed functionalization process is demonstrated to be universal and can be applied to different MCs, thus opening new avenues for the application of 2D MCs.

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.

Insight into the hydrogen evolution reaction of nickel dichalcogenide nanosheets: activities related to non-metal ligands.

Transition metal dichalcogenides, MX2 (M = Fe, Co, Ni, X = S, Se, Te), have been proven to be promising substitutes for noble metals in hydrogen evolution reactions (HERs). However, forthright comparisons of metal sulfides, metal selenides, and metal tellurides are rarely conducted, let alone the mechanism of the important role of their non-metal ligands. In this paper, we report the pilot study of a controllable method for the preparation of a series of NiX2 (X = S, Se, Te) nanosheets via a facile anion-exchange reaction. Consequently, the HER activities and stabilities of NiS2, NiSe2, and NiTe2 nanosheets were tested in both acid and alkaline solutions. The required overpotentials to reach 10 mA cm-2 in 0.5 M H2SO4 for NiS2, NiSe2, and NiTe2 were 213, 156, and 276 mV, respectively. The best performance of NiSe2 was also confirmed in 1 M KOH. Besides NiS2 and NiTe2 nanosheets, the HER properties of NiSe2 nanosheets are superior to most of the available nickel catalysts. Interestingly, the results from electrochemical measurements were found to be fully consistent with the data based on density function theory calculation. Among various factors that might influence the HER activities of nickel dichalcogenides, the free energies of hydrogen adsorption and conductivities have played important roles.

Bottom-up Approach Design, Band Structure, and Lithium Storage Properties of Atomically Thin γ-FeOOH Nanosheets.

As a novel class of soft matter, two-dimensional (2D) atomic nanosheet-like crystals have attracted much attention for energy storage devices due to the fact that nearly all of the atoms can be exposed to the electrolyte and involved in redox reactions. Herein, atomically thin γ-FeOOH nanosheets with a thickness of ∼1.5 nm are synthesized in a high yield, and the band and electronic structures of the γ-FeOOH nanosheet are revealed using density-functional theory calculations for the first time. The rationally designed γ-FeOOH@rGO composites with a heterostacking structure are used as an anode material for lithium-ion batteries (LIBs). A high reversible capacity over 850 mAh g(-1) after 100 cycles at 200 mA g(-1) is obtained with excellent rate capability. The remarkable performance is attributed to the ultrathin nature of γ-FeOOH nanosheets and 2D heterostacking structure, which provide the minimized Li(+) diffusion length and buffer zone for volume change. Further investigation on the Li storage electrochemical mechanism of γ-FeOOH@rGO indicates that the charge-discharge processes include both conversion reaction and capacitive behavior. This synergistic effect of conversion reaction and capacitive behavior originating from 2D heterostacking structure casts new light on the development of high-energy anode materials.

Band Gap Characters and Ferromagnetic/Antiferromagnetic Coupling in Group-IV Monolayers Tuned by Chemical Species and Hydrogen Adsorption Configurations.

One-side semihydrogenated monolayers of carbon, silicon, germanium, and their binary compounds with different configurations of hydrogen atoms are investigated by density functional theory. Among three considered configurations, zigzag, other than the most studied chair configuration, is energetically the most favorable structure of one-side semihydrogenation. Upon semihydrogenation, the semimetallic silicene, germanene, and SiGe become semiconductors, while the band gap in semiconducting SiC and GeC is reduced. Semihydrogenated silicene, germanene, SiGe, and GeC with chair configuration are found to be ferromagnetic semiconductors. For semihydrogenated SiC, it is ferromagnetic when all hydrogen atoms bond with silicon atoms, while an antiferromagnetic coupling is predicted when all hydrogen atoms bond with carbon atoms. The effect of interatomic distance between two neighboring magnetic atoms to the ferromagnetic or antiferromagnetic coupling is studied. For comparison, properties of one-side and both-side fully hydrogenated group-IV monolayers are also calculated. All fully hydrogenated group-IV monolayers are nonmagnetic semiconductors with band gaps larger than those of their semihydrogenated counterparts.

ELNES for boron, carbon, and nitrogen K-edges with different chemical environments in layered materials studied by density functional theory.

Electron energy-loss near-edge fine structures (ELNES) were calculated for graphene, doped graphene, a hexagonal BN monolayer, and a hexagonal BC2N layer using an ab initio pseudopotential plane wave method including the core-hole effect. Spectral features that can be used to distinguish different chemical environments are identified. The spectral features are closely related to the atomic species and arrangement. The connection between chemical environments and fine structures is discussed.

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.

Metal core bonding motifs of monodisperse icosahedral Au13 and larger Au monolayer-protected clusters as revealed by X-ray absorption spectroscopy and transmission electron microscopy.

The atomic metal core structures of the subnanometer clusters Au13[PPh3]4[S(CH2)11CH3]2Cl2 (1) and Au13[PPh3]4[S(CH2)11CH3]4 (2) were characterized using advanced methods of electron microscopy and X-ray absorption spectroscopy. The number of gold atoms in the cores of these two clusters was determined quantitatively using high-angle annular dark field scanning transmission electron microscopy. Multiple-scattering-path analyses of extended X-ray absorption fine structure (EXAFS) spectra suggest that the Au metal cores of each of these complexes adopt an icosahedral structure with a relaxation of the icosahedral strain. Data from microscopy and spectroscopy studies extended to larger thiolate-protected gold clusters showing a broader distribution in nanoparticle core sizes (183 +/- 116 Au atoms) reveal a bulklike fcc structure. These results further support a model for the monolayer-protected clusters (MPCs) in which the thiolate ligands bond preferentially at 3-fold atomic sites on the nanoparticle surface, establishing an average composition for the MPC of Au180[S(CH2)11CH3]40. Results from EXAFS measurements of a gold(I) dodecanethiolate polymer are presented that offer an alternative explanation for observations in previous reports that were interpreted as indicating Au MPC structures consisting of a Au core, Au2S shell, and thiolate monolayer.

Sub-nanometer Au monolayer-protected clusters exhibiting molecule-like electronic behavior: quantitative high-angle annular dark-field scanning transmission electron microscopy and electrochemical characterization of clusters with precise atomic stoichiometry.

The synthesis and characterization of the clusters Au13[PPh3]4[S(CH2)11CH3]2Cl2 (1) and Au13[PPh3]4[S(CH2)11CH3]4 (2) are described. These mixed-ligand, sub-nanometer clusters, prepared via exchange of dodecanethiol onto phosphine-halide gold clusters, show enhanced stability relative to the parent. The characterization of these clusters features the precise determination of the number of gold atoms in the cluster cores using high-angle annular dark-field scanning transmission electron microscopy, allowing the assignment of 13 gold atoms (+/-3 atoms) to the composition of both cluster molecules. Electrochemical and optical measurements reveal discrete molecular orbital levels and apparent energy gaps of 1.6-1.7 eV for the two cluster molecules. The electrochemical measurements further indicate that the Au13[PPh3]4[S(CH2)11CH3]2Cl2 cluster undergoes an overall two-electron reduction. The electrochemical and spectroscopic properties of the two Au13 cluster molecules are compared with those of a secondary synthetic product, which proved to be larger Au thiolate-derivatized monolayer-protected clusters with an average core of Au180. The latter shows behavior fully consistent with the adoption of metallic-like properties.