Unoccupied States Research Articles

The evolution of local structure of Mo6S8 during Li+ electrochemical storage studied by in-situ tender X-ray absorption spectroscopy

Chevrel phase Mo6S8 has attracted great interest because of its capability to accommodate different types of small cations through reversible topotactic redox reactions. However, the intercalation mechanism of Mo6S8 is still far from being fully understood owing to the complexity of the crystal structure and limitation of the probe tools. In our study, the reaction mechanism of Li ​+ ​intercalation into Mo6S8 was studied as a model by in-situ tender X-ray absorption spectroscopy (itXAS) in the viewpoint of local structure. According to the isobestic points of the 1st derivative, the discharge process is divided into five regions while the charge process is divided into four regions, where most of the regions are assigned to the two-phase transition reactions. The intensity change of the two spectroscopic features of the 1st derivative is found to be correlated, implying that the intercluster bond length directly affects the localization of the unoccupied states of the intracluster bond. The asymmetric evolution of the intensity of a’ and b’ as well as the asymmetric partition of reaction regions demonstrate the asymmetry of the reaction path in discharge and charge process. The results help to clarify the remaining debates on the phase transitions. Besides, itXAS is demonstrated to be a powerful tool to study the reaction mechanism of Mo6S8, which is complementary to NMR, XRD and so on.

Second-order phase transition of silicon from a band insulator to metal induced by strong magnetic fields

We present the second-order phase transition from a band insulator to metal that is induced by a strong magnetic field. The magnetic-field dependences of the magnetization and energy band gap of a crystalline silicon immersed in a magnetic field are investigated by means of the nonperturbative magnetic-field-containing relativistic tight-binding approximation method (2018 Phys. Rev. B 97 195135). It is shown that the energy band gap disappears at the critical magnetic field of 2.22 × 104 (T). At the critical magnetic field, the magnetic-field dependence of the magnetization exhibits a kink behavior, which means that this phenomenon is the second-order phase transition from a band insulator to metal. It is found that in strong magnetic fields above the critical magnetic field, namely in the metallic phase, the oscillation of the magnetization appears. It is shown that this magnetic oscillation comes from the magnetic energy bands in the magnetic Brillouin zone that change from the occupied states to unoccupied states or vice vasa.

Conduction band tuning by strengthening s-p hybridization of novel layered oxyhalide Bi4SbO8Cl for efficient visible-light photocatalytic water splitting

Bismuth-based Sillen–Aurivillius oxyhalide Bi4MO8Cl are promising visible-light photocatalysts. The element M of [MO4]3- perovskite layer in the Bi4MO8Cl is always a transition element. Due to d orbitals in transition element weakening the Bi 6s and O 2p orbitals hybridization, the bandgap energy would be decreased and the CBM would be lowered. Thus, Bi4MO8Cl always shows feeble reducibility for photocatalytic hydrogen generation. In this paper, we choose the main group element Sb substitute for transition element M in the Sillen–Aurivillius Bi4MO8Cl. Due to the s-p-d hybridization in Bi4NbO8Cl being replaced by only s-p hybridization in Bi4SbO8Cl, the strengthened Bi 6s and O 2p orbitals hybridization would eventually lead to the enlarged energy bandgap and elevated CBM. The bandgap of Bi4SbO8Cl was about 2.53 eV with the CBM estimated about −0.33 eV. Because of the high carrier mobility and strong reducibility brought by the dispersed Bi 6s states in the whole unoccupied states and the raised position of Bi 6p states in the CBM and linearity Sb–O–Sb, Bi4SbO8Cl shows enhanced photocatalytic H2 generation compared to Bi4NbO8Cl under the visible-light irradiation.

Improving Catalytic Activity of "Janus" MoSSe Based on Surface Interface Regulation.

The monolayer Janus MoSSe is considered to be a promising catalytic material due to its unique asymmetric structure. In order to improve its catalytic performance for hydrogen evolution reactions (HERs) and oxygen evolution reactions (OERs), many attempts have been made. In this work, a series of transition metal (TM) atoms were anchored on the Janus MoSSe surface to screen effective TM single-atom catalysts for HERs and OERs through density functional theory (DFT) calculations. Fe@MoSSe presents excellent HERs performance and Ni@MoSSe presents excellent catalytic performance for OERs with extremely low over-potential of 0.32 V. The enhanced activity is attributed to the modest energy level of the d band center of the transition metal atom, and the transition metal atom improves the conductivity of the original MoSSe and offers unoccupied states near the Fermi level. At the same time, the anchoring of transition metal atoms redistributes the charge in the MoSSe system, which effectively regulates the electronic structure of the material itself. The strain calculation shows that the activity of the catalyst can be improved by reasonably adjusting the value of the applied strain.

Theoretical investigations of oxygen vacancy effects in nickel-doped zirconia from ab initio XANES spectroscopy at the oxygen K-edge.

In this study, we present theoretical X-ray absorption near-edge structure (XANES) spectra at the K-edge of oxygen in zirconia containing Ni dopant atoms and O vacancies at varying concentrations. Specifically, our model system consist of a supercell composed of a zirconia (ZrO2) matrix containing two nickel dopants (2Ni), which substitute two Zr atoms at a finite separation. We found the 2Ni atoms to be most stable in a ferromagnetic configuration in the absence of oxygen vacancies. In this system, each Ni atom is surrounded by two shells of O with tetrahedral geometry, in a similar way as in bulk cubic zirconia. The oxygen K-edge XANES spectrum of this configuration shows a pre-edge peak, which is attributable to dipole transitions from O 1s to O 2p states that are hybridized with unoccupied Ni 3d states. The intensity of this pre-edge peak, however, reduces upon the introduction of a single vacancy in the 2Ni-doped zirconia matrix. The corresponding ground state remains ferromagnetic, while one of the nickel atoms adopts a trigonal bipyramidal geometry, and the other one remains in a tetrahedral geometry. Furthermore, the introduction of two vacancies in the 2Ni-doped zirconia results in the two Ni atoms having distorted octahedral and trigonal bipyramidal geometries and being coupled antiferromagnetically in the ground state. Additionally, the oxygen K-edge XANES spectrum shows a further decrease in the intensity of the pre-edge peak, compared to the case of a single vacancy. Thus, the changes in the intensity of the pre-edge peak evidence a major structural change in the local environment around nickel atoms and, by extension, in the zirconia matrix. This change is due to the structural disorder induced by the 2Ni dopants and the O vacancies. Furthermore, the analysis of the XANES signatures shows that the oxidation state of nickel atoms changes with the introduction of oxygen vacancies. Our study therefore shows a possibility to control the oxidation state and magnetic order in a typical diluted magnetic oxide. Such a finding may be crucial for spintronics-related applications.

Effect of PbPc on electron structure and carrier dynamics of black phosphorus

Using lead phthalocyanine (PbPc) as surface doping material on black phosphorous (BP) we observe enhanced photo-excited carriers in the PbPc/BP heterostructure. The interfacial energy level alignment is investigated with ultra violet photoemission spectroscopy (UPS) and x-ray photoemission spectroscopy (XPS). The heterojunction is type I with gap of BP nested in that of PbPc, facilitating confinement of electrons and holes in BP. Ultrafast time-resolved two-photon photoemission (TR-2PPE) spectroscopy is used to study the influence of PbPc on the photo excited unoccupied electronic states and the dynamics of the relaxation processes. Monolayer PbPc can greatly increase the pump excited hot electrons and the 2 photon emission of BP. The enhanced population in the intermediate states is attributed to the straddling of the band alignment which benefits the photo excited electrons in PbPc transferring to BP. Density functional theory calculations supported the interface dipole and charge redistribution. Our results provide a fundamental understanding of the excellent opto-electrical response of PbPc/BP interface of promising application in the high efficient photo detectors.

Tackling Disorder in γ-Ga2 O3.

Ga2 O3 and its polymorphs are attracting increasing attention. The rich structural space of polymorphic oxide systems such as Ga2 O3 offers potential for electronic structure engineering, which is of particular interest for a range of applications, such as power electronics. γ-Ga2 O3 presents a particular challenge across synthesis, characterization, and theory due to its inherent disorder and resulting complex structure-electronic-structure relationship. Here, density functional theory is used in combination with a machine-learning approach to screen nearly one million potential structures, thereby developing a robust atomistic model of the γ-phase. Theoretical results are compared with surface and bulk sensitive soft and hard X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, spectroscopic ellipsometry, and photoluminescence excitation spectroscopy experiments representative of the occupied and unoccupied states of γ-Ga2 O3 . The first onset of strong absorption at room temperature is found at 5.1eV from spectroscopic ellipsometry, which agrees well with the excitation maximum at 5.17eV obtained by photoluminescence excitation spectroscopy, where the latter shifts to 5.33eV at 5K. This work presents a leap forward in the treatment of complex, disordered oxides and is a crucial step toward exploring how their electronic structure can be understood in terms of local coordination and overall structure.

Band structural and absorption characteristics of antimonene/bismuthene monolayer heterojunction calculated by first-principles.

The band gap of lateral heterojunctions (LHSs) can be continuously tuned by changing the widths of their components. In this work, Sb/Bi LHSs based on monolayer Sb and Bi atoms with armchair and zigzag interfaces are constructed, respectively. It exhibits an atom’s number in planner-dependent tunable band gap and near-infrared range absorption characteristics. They are systematically studied by first-principles calculations. The widths are represented by the number (n) of Sb or Bi atom chains. When n increases from 2 to 8, the bandgaps of armchair Sbn/Bin LHSs decrease from 0.89 to 0.67 eV, and the band gaps of zigzag Sbn/Bin LHSs decrease from 0.92 to 0.76 eV. The partial density of states spectra indicate that the occupied states of the valence band are mainly provided by the Bi 6p orbitals. Additionally, the unoccupied states of the conduction band are always provided by the Sb 5p orbitals and Bi 6p orbitals. For Sbn/Bin LHSs, the absorption edge along XX and YY directions move toward the long wavelength direction. These results provide an approach for the applications of two-dimensional materials in near-infrared devices.

Giant intrinsic spin Hall effect in layered material Pt2B

Spin Hall effect (SHE) is highly promising for future spintronic applications. Using ab initio calculations , we predict that the hexagonal layered material Pt 2 B exhibits a giant intrinsic SHE with an intrinsic spin Hall conductivity (SHC) of 2159 ( ħ / e ) S/cm, which can reach its maximum value of 2274 ( ħ / e ) S/cm at 0.04 eV above the intrinsic Fermi level. The large SHC mainly comes from the contributions of the two occupied bands below the Fermi level, and the energy difference between the occupied and unoccupied states determines the energy position of the maximal SHC. Pt 2 B’s spin Hall angle (SHA) is of the same order as pure metal Pt. Our finds suggest that Pt 2 B is a fascinating material for producing pure spin current. • Pt 2 B could be a ideal system for achieving large SHC. It has an intrinsic SHC σ x y z as large as 2159 ( ħ / e ) S/cm. • The large SHC mainly come from the contribution of the two occupied bands below the Fermi level, and the energy difference between occupied and unoccupied states determines the energy position of maximal SHC. • The bands around the Fermi level do not have the SOC-induced anti-cross. Therefore, the mechanism of the large SHC of Pt 2 B is different from many previous reported materials.

Unoccupied topological surface state in MnBi2Te4

The unoccupied part of the band structure in the magnetic topological insulator MnBi$_2$Te$_4$ is studied by first-principles calculations. We find a second, unoccupied topological surface state with similar electronic structure to the celebrated occupied topological surface state. This state is energetically located approximate $1.6$ eV above the occupied Dirac surface state around $\Gamma$ point, which permit it to be directly observed by the two-photon angle-resolved photoemission spectroscopy. We propose a unified effective model for the occupied and unoccupied surface states. Due to the direct optical coupling between these two surface states, we further propose two optical effects to detect the unoccupied surface state. One is the polar Kerr effect in odd layer from nonvanishing ac Hall conductance $\sigma_{xy}(\omega)$, and the other is higher-order terahertz-sideband generation in even layer, where the non-vanishining Berry curvature of the unoccupied surface state is directly observed from the giant Faraday rotation of optical emission.

First-Principles Study of the Optical Properties of Zinc Antimonide Using the mBJ Approximation

This computational study focused on the optical properties of zinc antimonide ZnSb. It relates to the complex dielectric function ε (ω), the refractive index n (ω), the extinction function k (ω), the optical conductivity σ (ω), the reflectivity R(ω), the absorption coefficient α (ω) and the energy loss spectrum L(ω). These properties are calculated and discussed for a growing energy of the incident electromagnetic radiation ranging from 0 to 14 eV, comprising infrared, visible and ultraviolet regions. All these properties are obtained using the Full Potential Linearized Augmented Plane Wave (FP-LAPW), by solving Kohn-Sham equations. This method based on Density Functional Theory (DFT), implemented in Wien2k simulation package. This compound is already used in photo-optical applications, it is for this reason that we interested in the calculation of its optical properties according to the energy of the incident photons, in order to open up for it other use ways. Since the zinc antimonide ZnSb is a semiconductor, its optical properties are investigated using Generalized Gradient Approximation plus modified Becke–Johnson as the exchange correlation (GGA-mBJ). Our calculations are performed by considering only the interband transition of electrons between the occupied states in valence band and unoccupied conduction band states along high symmetry points in Brillouin zone. In addition, the relations of the optical properties to these transitions are discussed in detail. We have also verified the Penn’s model by showing the inverse relationship between the static real part of dielectric function ε1(0) and the optical band gap Eg. The results obtained are compared with other results existing in the literature.

Surface charge-transfer doping a quantum-confined silver monolayer beneath epitaxial graphene

Recently the graphene/SiC interface has emerged as a versatile platform for the epitaxy of otherwise unstable, monoelemental, two-dimensional (2D) layers via intercalation. Intrinsically capped into a van der Waals heterostructure with overhead graphene, they compose a new class of quantum materials with striking properties contrasting their parent bulk crystals. Intercalated silver presents a prototypical example where 2D quantum confinement and inversion symmetry breaking entail a metal-to-semiconductor transition. However, little is known about the associated unoccupied states, and control of the Fermi level position across the bandgap would be desirable. Here, we n-type dope a graphene/2D-Ag/SiC heterostack via in situ potassium deposition and probe its band structure by means of synchrotron-based angle-resolved photoelectron spectroscopy. While the induced carrier densities on the order of $10^{14}$ cm$^{-2}$ are not yet sufficient to reach the onset of the silver conduction band, the band alignment of graphene changes relative to the rigidly shifting Ag valence band and substrate core levels. We further demonstrate an ordered potassium adlayer ($2\times 2$ relative to graphene) with free-electron-like dispersion, suppressing plasmaron quasiparticles in graphene via enhanced metalization of the heterostack. Our results establish surface charge-transfer doping as an efficient handle to modify band alignment and electronic properties of a van der Waals heterostructure assembled from graphene and a novel type of monolayered quantum material.

Resonant inelastic soft x-ray scattering on LaPt2Si2

X-ray absorption and resonant inelastic x-ray scattering spectra of LaPt2Si2 single crystal at the Si 2p and La 4d edges are presented. The data are interpreted in terms of density functional theory, showing that the Si spectra can be described in terms of Si s and d local partial density of states (LPDOS), and the La spectra are due to quasi-atomic local 4f excitations. Calculations show that Pt d-LPDOS dominates the occupied states, and a sharp localized La f state is found in the unoccupied states, in line with the observations.

Ab Initio Investigation of Covalently Immobilized Cobalt-Centered Metal–Organic Catalysts for CO2 Reduction: The Effect of the Substrate on the Reaction Energetics

Recently, experimental studies of covalently immobilized CO2 reduction organometallic catalysts have reported remarkable activity; however, the reason for this and the underlying mechanisms are not currently understood. To advance our understanding of such systems, we perform ab initio calculations investigating how covalent immobilization of such molecular catalysts and the substrate geometry affect the reaction pathways. We address this as realistically as possible by doing a survey of possible structures including common surface defects and nanotubes, as well as linkers. In particular, we study covalently immobilized cobalt-centered phthalocyanine (CoPc) and tetraphenylporphyrin (CoTPP), as used in CO2 electroreduction reactions (CO2ERR), immobilized on pristine and defective graphene and single-walled carbon nanotubes (SWCNTs), with and without linkers. The bonding energies of the CoPc and CoTPP catalysts to the different substrates are found to be consistently stronger on the SWCNTs and on defect sites, suggesting that such structures will be the anchoring sites for these immobilized molecular catalysts. Covalently immobilized CoPc and CoTPP catalysts show improved CO2ERR pathway performance compared to their homogeneous analogues. Favorable reaction pathways are found for upright bonded CoPc and CoTPP on a Stone–Wales defect and an octagon–pentagon line defect, respectively, in graphene. CoPc immobilized via a pyridine linker is found to have the most favorable reaction pathway due to strong exothermic behavior of CO2 adsorption and COOH formation while having a weak endothermic final step for CO desorption, consistent with its excellent experimental performance. We attribute this to unoccupied dz2 states just above the Fermi level. On comparing the calculated free energy reaction pathways with corresponding available experimental results of catalyst performance, we find consistent agreement. The present study provides a new detailed understanding into the covalent immobilization and function of these organometallic catalysts and the role that the charge, defects, and structure have on the free energy reaction pathways for CO2ERR.

Oxygen-plasma-assisted formaldehyde adsorption mechanism of SnO2 electrospun fibers

Chemisorbed oxygen acts a crucial role in the redox reaction of semiconductor gas sensors, and which is of great significance for improving gas sensing performance. In this study, an oxygen-plasma-assisted technology is presented to enhance the chemisorbed oxygen for improving the formaldehyde sensing performance of SnO2 electropun fiber. An inductively coupled plasma device was used for oxygen plasma treatment of SnO2 electrospun fibers. The surface of SnO2 electrospun fibers was bombarded with high-energy oxygen plasma for facilitating the chemisorption of electronegative oxygen molecules on the SnO2 (110) surface to obtain an oxygen-rich structure. Oxygen-plasma-assisted SnO2 electrospun fibers exhibited excellent formaldehyde sensing performance. The formaldehyde adsorption mechanism of oxygen-rich SnO2 was investigated using density functional theory. After oxygen plasma modification, the adsorption energy and the charge transfer number of formaldehyde to SnO2 were increased significantly. And an unoccupied electronic state appeared in the SnO2 band structure, which could enhance the formaldehyde adsorption ability of SnO2. The gas sensing test revealed that plasma-treated SnO2 electrospun fibers exhibited excellent gas sensing properties to formaldehyde, low operating temperature, high response sensitivity, and considerable cross-selectivity. Thus, plasma modification is a simple and effective method to improve the gas sensing performance of sensors.

Influence of structural, electronic and optical properties of boron and nitrogen doping in tetragonal PbTiO3: insight from first-principles

The structural, electronic and optical properties of intrinsic, B- and N-doped tetragonal PbTiO3 are investigated by first-principles calculations. After doping with B (N) atom, the stability of the system decreases, but it is still thermodynamically stable. B-doped (N-doped) PbTiO3 appear new B-2p (N-2p) impurity bands near Fermi level, reducing the bandwidth from 2.02eV to 0.56eV (0.43eV). Imaginary part of dielectric function has the largest peak at the photon energy of 5.81 eV, which originates from the transition of electrons from the occupied O-2p VB states to the unoccupied Ti-3d CB states. The static refractive index of B- and N-doped PbTiO3 are larger than that of PbTiO3. B-doped (N-doped) PbTiO3 can increase the light absorption capacity of near-infrared wavelengths and visible light, and energy loss in the visible light range is very small, indicating that Band N-doping are expected to improve the optical properties of PbTiO3.

First-principles calculations on the diffusion and electronic properties of CuI doped by cation and anion

CuI has attracted much attention for the optoelectronic devices due to the wide band gap of 3.1 eV (p-type). Previous theoretical and experimental results demonstrate doping successfully tunes stability of the intrinsic defects in CuI. We study the diffusion and electronic properties of CuI doped with Li, Na, Mg, F, Cl, and Br using first-principles calculations. Importantly, the low diffusion energy barriers (<0.6 eV) enable uniform distribution of the dopants. Br, Cl, and F at the I site reduce the band gap to 1.08, 1.06, and 1.00 eV as compared to 1.09 eV for undoped CuI. The anion at the interstitial site introduces some unoccupied states at 0.06 eV above the valence band maximum, enhancing the p-type conductivities.

Nitrogen Decoration of Basal-Plane Dislocations in 4H - SiC

Basal-plane dislocations (BPDs) pose a great challenge to the reliability of bipolar power devices based on the 4H silicon carbide (4H-SiC). It is well established that heavy nitrogen (N) doping promotes the conversion of BPDs to threading edge dislocations (TEDs) and improves the reliability of 4H-SiC-based bipolar power devices. However, the interaction between N and BPDs, and the effect of N on the electronic properties of BPDs are still ambiguous, which significantly hinder the understanding on the electron-transport mechanism of 4H-SiC-based bipolar power devices. Combining molten-alkali etching and the Kelvin probe force microscopy (KPFM) analysis, we demonstrate that BPDs create acceptor-like states in undoped 4H-SiC, while acting as donors in N-doped 4H-SiC. First-principles calculations verify that BPDs create occupied defect states above the valence band maximum (VBM) and unoccupied defect states under the conduction-band minimum (CBM) of undoped 4H-SiC. The electron transfer from the defect states of intrinsic defects and native impurities to the unoccupied defect states of BPDs gives rise to the acceptor-like behavior of BPDs in undoped 4H-SiC. Defect formation energies indicate that N atoms can spontaneously decorate BPDs during the N doping of 4H-SiC. The binding between N and BPD is strong against decomposition. The accumulation of N dopants at the core of BPDs results in the accumulation of donor-like states at the core of BPDs in N-doped 4H-SiC. This work not only enriches the understanding on the electronic behavior of BPDs in N-doped 4H-SiC, but also helps understand the electron transport mechanism of 4H-SiC-based bipolar power devices.

Abatement of Nitrogen Oxides via Selective Catalytic Reduction over Ce1-W1 Atom-Pair Sites.

Environmentally benign CeO2-WO3/TiO2 catalysts are promising alternatives to commercial toxic V2O5-WO3/TiO2 for controlling NOx emission via selective catalytic reduction (SCR), but the insufficient catalytic activity of CeO2-WO3/TiO2 catalysts is one of the obstacles in their applications because of a lack of an in-depth understanding of the CeO2-WO3 interactions. Herein, we design a Ce1-W1/TiO2 model catalyst by anchoring Ce1-W1 atom pairs on anatase TiO2(001) to investigate the synergy between Ce and W in SCR. A series of characterizations combined with density functional theory calculations and in situ diffuse-reflectance infrared Fourier-transform experiments reveal that there exists a strong electronic interaction within Ce1-W1 atom pairs, leading to a much better SCR performance of Ce1-W1/TiO2 compared with that of Ce1/TiO2 and W1/TiO2. The Ce1-W1 synergy not only shifts down the lowest unoccupied states of Ce1 near the Fermi level, thus enhancing the abilities in adsorbing and oxidizing NH3 but also makes the frontier orbital electrons of W1 delocalized, thus accelerating the activation of O2. The deep insight of the Ce-W synergy may assist in the design and development of efficient catalysts with an SCR activity as high as or even higher than V2O5-WO3/TiO2.

Impact of molecular distribution on unoccupied image potential states of PbPc/HOPG surfaces

Two-photon photoemission (2PPE) spectroscopy revealed modified image potential states (IPSs) of sub-monolayer lead phthalocyanine (PbPc) films on graphite (HOPG). Angle-resolved (AR) 2PPE for room temperature films showed unique energy/momentum maps in which the inside of the parabolic dispersion curve of IPS on HOPG area is filled up to a specific energy. For cooled films, AR maps showed two parabolic curves originating from IPSs on HOPG areas and PbPc islands. The changes are explained with simple models of electron scattering by molecular islands. IPS feature is an indicator of nano-scale superstructures, which are not easily assessed by other experiments.