Density Of Unoccupied States Research Articles

Carbon K edge spectra of functionalized molybdenum-based MXenes, Mo2CT2 (T = F, OH and O), from first-principles calculations

In this study, the effect of the termination groups (T = F, OH, O) on the energy loss near edge structure (ELNES) of carbon K edge in Mo2C MXene at orientation-independent conditions has been investigated using first-principles calculations based on the full-potential linearized augmented plane wave (FP-LAPW) method. The results show that within the YS-PBE0 functional, the Mo2CF2 is a semiconductor with an indirect band gap of 0.723 eV. For Mo2CO2, the indirect band gap increases to 0.17 eV within the screened hybrid functional. The calculation results of ELNES spectra with the affection of core-hole show that, in comparison to pristine Mo2C, as a fingerprint of termination groups, the spectral structures in Mo2CT2 are reproduced at higher energies. Moreover, the spectral features of Mo2CT2 are sensitive to the chemical nature and the location of the T groups on the pristine Mo2C MXene surface. When going from T = O to T = F and, further, to T = OH, the energy separation between the main peaks increases, which is a sign of decreasing the Mo-C bond length, respectively, from T = O to T = F and to T = OH. The comparison of ELNES spectra and the unoccupied densities of states (DOS) reveal that, the origin of the first structure at the carbon K edge of Mo2CT2 is mostly result from electron transition to pz state, while in pristine Mo2C, mainly due to the transition to px + py state. Other structures at higher energies mainly arise from electron transitions to px + py state and partially to pz state. The spectral decomposition of the ELNES into in-plane (l′ = 1, m′ = ± 1) and out-of-plane (l′ = 1, m′ = 0) components also confirms these results. Generally, in both Mo2C and Mo2CT2, the contribution of in-plane element in most of the structures is more considerable.

Understanding the role of structural distortions on the transport properties of Ar ion irradiated SrTiO3 thin films: X-ray absorption investigation

We report the evolution of structural distortions on the pulsed laser deposited SrTiO3 (STO) thin films irradiated by 1 MeV Ar ion beam and their response in the electrical and thermoelectric properties correlated with their electronic structures. The structural studies reveal a modification in the crystalline nature resulting in the synergistic enhancement of resistivity and the Seebeck coefficient due to ion irradiation-induced oxygen vacancies and augment the power factor to ∼17 μW m−1K−2, which is 2.5 times higher than the pristine STO. An in-depth analysis of the conduction mechanism using various theoretical models implies the role of band conduction and Mott-variable range hopping. Further, the changes in the local electronic structures are reflected in the variation in the Ti–O bond length and the density of unoccupied states that alter the spectral intensities and chemical shifts and are responsible for the observed enhancement in the thermoelectric properties. The above results indicate the significance of the ion beams to tune the electrical and thermoelectric properties.

Efficient high-temperature electromagnetic wave absorption enabled by structuring binary porous SiC with multiple interfaces

Introducing enhanced interface polarization would be an effective way to adjust the dielectric behavior with temperature and prepare high-temperature electromagnetic (EM) wave absorbers. Herein, carbon nanotube (CNT)-derived binary porous SiC with multiple interfaces was successfully fabricated by structuring SiC nanowire networks in porous SiC skeleton. The amount of secondary SiC nanowires and electronic structures of the as-received binary SiC were changed by different heat temperatures. The positively correlated relationship of the ID/IG value of Raman spectrum and unoccupied density of states in the C K-edge was manifested by structuring SiC nanowire networks. The synergism of CNT-derived SiC nanowires and SiC skeleton improved the formation of multiple interfaces and propagation path. These enhanced dielectric properties could induce high-temperature EM wave absorption performance of the fabricated binary porous SiC. For instance, the minimum reflection loss (RLmin) of −47.0 dB was achieved at 1.8 mm thickness in frequency range of 9.3–12.4 GHz. The optimum RL could reach −51.0 dB at 8.6 GHz, when the temperature increased to 600 °C. Meanwhile, structuring SiC nanowire networks substantially increased the compressive strength. This multi-function of the CNT-derived binary SiC indicated that they were outstanding materials with potential applications in EM wave absorption at high temperatures.

Unveiling spin-dependent unoccupied electronic states of Co2MnGe (Ga) film via Ge (Ga) L2,3 absorption spectroscopy

X-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) spectroscopy were applied at the Ge (Ga) ${L}_{2,3}$ edge to unravel the spin-resolved unoccupied electronic states of ${\mathrm{Co}}_{2}\mathrm{Mn}\mathrm{Ge}$ (Ga). Complicated spectral features were observed in both XAS and XMCD spectra. For their interpretation, we compared the experimental XAS and XMCD spectra with the calculated Ge (Ga) $4s$ and $4d$ orbital partial density of states. The comparison enabled a qualitative explanation of the XMCD spectra as the difference between the majority- and minority-spin unoccupied density of states summed over the $4s$ and $4d$ orbitals. Our finding provides a new approach to uncover the spin-split partial density of states above the Fermi level.

Valence Alignment of Mixed Ni-Fe Hydroxide Electrocatalysts through Preferential Templating on Graphene Edges for Enhanced Oxygen Evolution.

Engineering the metal-carbon heterointerface has become an increasingly important route toward achieving cost-effective and high-performing electrocatalysts. The specific properties of graphene edge sites, such as the high available density of states and extended unpaired π-bonding, make it a promising candidate to tune the electronic properties of metal catalysts. However, to date, understanding and leveraging graphene edge-metal catalysts for improved electrocatalytic performance remains largely elusive. Herein, edge-rich vertical graphene (er-VG) was synthesized and used as a catalyst support for Ni-Fe hydroxides for the oxygen evolution reaction (OER). The hybrid Ni-Fe/er-VG catalyst exhibits excellent OER performance with a mass current of 4051 A g-1 (at overpotential η = 300 mV) and turnover frequency (TOF) of 4.8 s-1 (η = 400 mV), outperforming Ni-Fe deposited on pristine VG and other metal foam supports. Angle-dependent X-ray absorption spectroscopy shows that the edge-rich VG support can preferentially template Fe-O units with a specific valence orbital alignment interacting with the unoccupied density of states on the graphene edges. This graphene edge-metal interaction was shown to facilitate the formation of undersaturated and strained Fe-sites with high valence states, while promoting the formation of redox-activated Ni species, thus improving OER performance. These findings demonstrate rational design of the graphene edge-metal interface in electrocatalysts which can be used for various energy conversion and chemical synthesis reactions.

Propagation of Low-Energy Electrons and the Density of Unoccupied States in Ultrathin TCNQ Layers on the Oxidized Silicon Surface

The formation of unoccupied electronic states and the boundary potential barrier during thermal deposition of tetracyanoquinodimethane (TCNQ) films to 7 nm in thickness on a (SiO2)n-Si surface has been studied. The electronic characteristics of the surface under study are measured by the total current spectroscopy (TCS) using a testing electron beam with energies from 5 to 20 eV above the Fermi level. The formation of a boundary potential barrier in the (SiO2)n-Si/TCNQ structure is accompanied by an increase in the work function of the surface from 4.2 ± 0.1 to 4.7 ± 0.1 eV. Using the results of the TCS experiments, the DOUS dependences of the TCNQ films have been built. To analyze the experimental DOUS dependences, the orbital energies of the TCNQ molecules are calculated by the density functional theory (DFT) at the B3LYP/6-31G(d) level with subsequent correction and the inclusion of the polarization energy of the condensed medium. In the energy range indicated above, DOUS of the TCNQ films has four main maxima. The DOUS maximum at energy 7.0 eV above EF is predominantly formed by π* orbitals. Three DOUS maxima in the energy range from 8.0 to 20 eV above EF are formed by approximately the same amount of the π* and σ* orbitals.

Electron Energy Loss Structures of Terminated Scandium and Hafnium MXene Monolayers from First-Principles Calculations

In this study, the energy loss near edge structure (ELNES) of two-dimensional transition metal carbides and nitrides, namely ‘MXenes’, has been calculated in the density functional theory using the full-potential linearized augmented plane wave method. Calculations of band structure demonstrate that, in agreement with other studies, although MAX phases (bulks) and pristine MXenes (monolayers) are all metallic, some functionalized MXenes are semiconductors. For the first time, we have calculated the ELNES spectra of carbon and nitrogen K-edges in MAX M2AlX phases, pristine M2X MXenes and functionalized MXenes M2XT2 (M = Sc, Hf; X = C, N; T = F, OH, O) at magic angle conditions. Compared to the M2AlX bulks, the differences of energy between the main peaks in carbon and nitrogen K edges of M2X monolayers increase due to the slightly smaller bond lengths that can be used as a fingerprint for monolayers. The structures in ELNES spectra are sensitive to the chemical nature and the location of the T groups on the MXene surface. The energy position of the first main peak and the separation between the main peaks in the ELNES spectra of M2XT2 increase from T = O to T = OH and to T = F surface groups, respectivelyshows that the M-X bond length decreases from T = O to T = OH and to T = F. This short bond in M2XF2 leads to enhanced M-X interaction. The main features in carbon and nitrogen K edges correspond to the peaks in the unoccupied densities of states (DOS), namely σ* and π* states; however, the contribution of σ* states is dominant. Furthermore, for accurate insight into the excitonic effects (electron–hole coupling) in core edges, we have used the solving of the equation of motion of the two-particle Green’s function, the Bethe–Salpeter equation (BSE) to determine these effects on the K x-ray absorption near edge structure (XANES) of semiconductors Sc2CF2, Sc2C(OH)2, Sc2CO2 and Hf2CO2 MXenes. Calculations have been performed in two different levels of theory, with and without considering the excitonic effects. The excitonic effects considerably result in shifting the spectral features to the lower energies and changing in the intensity and overall shapes of the absorption spectra.

Nonlinear optical properties of Rh–Pd and Rh–Pt solid-solution alloy nanoparticles prepared by a laser-induced nucleation method in aqueous solution

The nonlinear optical properties of Rh–Pd and Rh–Pt solid-solution alloy nanoparticles (NPs) were experimentally investigated by means of the z-scan technique. The open aperture (OA) measurement showed a reverse saturable behavior, whereas the closed aperture (CA) measurement showed a peak- and valley-shape. Both the Rh–Pd and Rh–Pt NPs exhibited a positive nonlinear optical index of refraction at 800 nm relating to the self-focusing phenomenon. In addition, the nonlinear absorption of the Rh–Pt NPs (4.39 × 10−12 cm/W) was higher than that of the Rh–Pd NPs (1.63 × 10−12 cm/W) due to the small interval between the occupied and unoccupied density of states (DOS) of Rh–Pt than Rh–Pd. The nonlinear responses of the Rh–Pd and Rh–Pt NPs was attributed to the hot electron contribution and the reverse saturation of intraband and interband transitions.

Separate measurement of the 5f5/2 and 5f7/2 unoccupied density of states of UO2

Coupling High Energy Resolution Fluorescence Detection (HERFD) with electric dipole selection rules, the uranium 5f5/2 and 5f7/2 Unoccupied Densities of States (UDOS) have been determined in UF4, UO2 and UCd11. Striking changes were observed between UF4 (5f2) to UCd11 (5f3), consistent with the Intermediate Coupling Model. Results for UO2 were confirmed by Bremstrahlung Isochromat Spectroscopy (BIS). This new approach can be a powerful tool for shedding light on the hidden mysteries of Actinides 5f-electrons properties.

Facile synthesis and enhanced luminescence behavior of ZnO:Reduced graphene oxide(rGO) hybrid nanostructures

The structural, electronic and luminescence behavior of ZnO nanorods and ZnO-reduced graphene oxide (ZnO:rGO) hybrid nanostructures have been studied using X-diffraction (XRD), Raman scattering, X-ray absorption near edge structure (XANES) spectroscopy and photoluminescence (PL) studies. XRD and Raman analyses reveal the formation of hybrid nanostructures composed of ZnO and rGO. The morphology and elemental composition of the grown nanostructures were investigated using FESEM and EDX measurements. The hybridization of rGO with ZnO provides an additional path way for the transfer of charge from the defect states of ZnO to rGO as evidenced from the increase in unoccupied density of states in the O-K edge XANES spectra and the enhanced UV, blue emissions as well as the suppressed visible emission in the PL spectra, respectively. Our systematic experimental investigations demonstrate the hybridization of rGO with ZnO is an effective approach to tailor the optical properties of ZnO and offers the prospective way for the fabrication of novel optoelectronic devices.

Density of Electronic States in the Conduction Band of Ultrathin Films of Naphthalenedicarboxylic Anhydride and Naphthalenetetracarboxylic Dianhydride on the Surface of Oxidized Silicon

The results of examination of the electronic structure of the conduction band of naphthalenedicarboxylic anhydride (NDCA) films in the process of their deposition on the surface of oxidized silicon are presented. These results were obtained using total current spectroscopy (TCS) in the energy range from 5 to 20 eV above the Fermi level. The energy position of the primary maxima of the density of unoccupied states (DOUS) of an NDCA film was determined based on the experimental TCS data and calculated data and compared with the position of the DOUS maxima of a naphthalenetetracarboxylic dianhydride (NTCDA) film. The theoretical analysis involved calculating the energies and the spatial distribution of orbitals of the molecules under study at the B3LYP/6-31G(d) DFT (density functional theory) level and correcting the obtained energies in accordance with the procedure that was proven effective in earlier studies of the conduction band of films of small conjugated organic molecules. It was found that the DOUS maxima of the NTCDA film in the studied energy interval from 5 to 20 eV above the Fermi level are shifted toward lower electron energies by 1–2 eV relative to the corresponding DOUS maxima of the NDCA film Subdivision of the Ufa Federal Research Centre of the.

Плотность электронных состояний в зоне проводимости сверхтонких пленок дикарбоксильно замещенного и тетракарбоксильно замещенного нафталина на поверхности окисленного кремния

AbstractThe results of examination of the electronic structure of the conduction band of naphthalenedicarboxylic anhydride (NDCA) films in the process of their deposition on the surface of oxidized silicon are presented. These results were obtained using total current spectroscopy (TCS) in the energy range from 5 to 20 eV above the Fermi level. The energy position of the primary maxima of the density of unoccupied states (DOUS) of an NDCA film was determined based on the experimental TCS data and calculated data and compared with the position of the DOUS maxima of a naphthalenetetracarboxylic dianhydride (NTCDA) film. The theoretical analysis involved calculating the energies and the spatial distribution of orbitals of the molecules under study at the B3LYP/6-31G(d) DFT (density functional theory) level and correcting the obtained energies in accordance with the procedure that was proven effective in earlier studies of the conduction band of films of small conjugated organic molecules. It was found that the DOUS maxima of the NTCDA film in the studied energy interval from 5 to 20 eV above the Fermi level are shifted toward lower electron energies by 1–2 eV relative to the corresponding DOUS maxima of the NDCA film Subdivision of the Ufa Federal Research Centre of the .

Enhanced Nonenzymatic Glucose-Sensing Properties of Electrodeposited NiCo2O4-Pd Nanosheets: Experimental and DFT Investigations.

Here, we report the facile synthesis of NiCo2O4 (NCO) and NiCo2O4-Pd (NCO-Pd) nanosheets by the electrodeposition method. We observed enhanced glucose-sensing performance of NCO-Pd nanosheets as compared to bare NCO nanosheets. The sensitivity of the pure NCO nanosheets is 27.5 μA μM-1 cm-2, whereas NCO-Pd nanosheets exhibit sensitivity of 40.03 μA μM-1 cm-2. Density functional theory simulations have been performed to qualitatively support our experimental observations by investigating the interactions and charge-transfer mechanism of glucose on NiCo2O4 and Pd-doped NiCo2O4 through demonstration of partial density of states and charge density distributions. The presence of occupied and unoccupied density of states near the Fermi level implies that both Ni and Co ions in NiCo2O4 can act as communicating media to transfer the charge from glucose by participating in the redox reactions. The higher binding energy of glucose and more charge transfer from glucose to Pd-doped NiCo2O4 compared with bare NiCo2O4 infer that Pd-doped NiCo2O4 possesses superior charge-transfer kinetics, which supports the higher glucose-sensing performance.

Absorption edges of black phosphorus: A comparative analysis

We have studied the excited states of black phosphorus through the analysis of K and L absorption edges of black phosphorus with energy‐filtered transmission electron microscopy. Energy‐loss spectra exhibit several peaks reflecting features in the density of conduction‐band states at corresponding energies above the vacuum level. We have reproduced the experimental electron energy‐loss near‐edge structures by means of ab initio self‐consistent real‐space multiple‐scattering theory. The comparison of the calculated symmetry‐projected density of unoccupied states and electron energy loss spectra allowed us to assign the spectral features to transitions to specific electronic states.

Electron Spectroscopy Study of Li[Ni,Co,Mn]O2/Electrolyte Interface: Electronic Structure, Interface Composition, and Device Implications

In recent years, there have been significant efforts to understand the role of the electronic structure of redox active materials according to their performance and thermodynamic stability in electrochemical storage devices and to develop novel materials with higher energy density and higher power. It is generally recognized that transition metal compounds used as a positive electrode determine the specific capacity and the energy density of rechargeable batteries, while the charge transfer resistance at the electrolyte–electrode interface plays a key role in delivering the power of the electrochemical cell. In the present work, we study the stability of LixNi0.2Co0.7Mn0.1O2 thin films through the evolution of the occupied and unoccupied density of states as a function of the charging state of the electrode as well as the physicochemical conditions influencing the ionic transport across the electrode–electrolyte interface. A comprehensive experimental quasi in situ approach has been applied by using synchrotron X-ray photoelectron spectroscopy (SXPS) and O K-edge and Co, Ni, Mn L-edges XANES. Our experimental data demonstrate the change of the Fermi level position with Li+ removal and Ni2+ → Ni4+ and Co3+ → Co4+ changes of oxidation state for the charge compensation in the bulk of the material. As is evidenced by the experimentally determined energy band diagram of Lix≤1.0Ni0.2Co0.7Mn0.1O2 vs the evolution of the Fermi level, no hole transfer to the O2p bands is observed up to a charging state of 4.8 V, which evidences the thermodynamic stability of Lix≤1.0Ni0.2Co0.7Mn0.1O2 under high charging voltage in contrast to LiCoO2. A very thin solid electrolyte interface layer (less than 30 Å thickness) on the Lix≤1.0Ni0.2Co0.7Mn0.1O2 film is formed in a decomposition reaction of the electrolyte also involving the transition metal oxide. The enhanced concentration of lithium in the interface layer correlates evidently with the electron transfer to the transition metal sites changing their electronic configuration. It is concluded that Lix≤1.0Ni0.2Co0.7Mn0.1O2 can serve as a high energy density cathode material, but the delivery of high power, which is a critical parameter for an electric vehicle, is strongly influenced by the physicochemical conditions at the solid electrolyte interface, which can suppress Li+ diffusion or even block the Li+ paths across the interface.

Hard and soft x-rays XAS characterization of charge ordered LuFe2O4

This work presents a thorough characterization of the mixed valence compound LuFe2O4 by means of x-ray absorption spectroscopy (XAS). Polarized XAS measurements at the Fe K-edge, Fe L2,3-edges and O K-edge have been carried out to validate the bimodal Fe+2/Fe+3 charge ordering (CO) proposed to give rise to ferroelectricity at TCO ≈ 320 K and also to study the anisotropy and temperature dependence of the Fe local structure. Our results discard a bimodal CO below TCO and agree with the presence of a Fe+2.5±δ distribution with δ ≤ 0.25. Unexpectedly, the strong anisotropy of the hexagonal crystallographic structure is not reflected in either the Fe (4p, 3d) or the O (2p) density of unoccupied states. Finally, the so- called CO transition is originated by the ordering of local distortions as it is revealed by the temperature evolution of the XAS spectra.

Electron energy loss spectrum of graphane from first-principles calculations

In this study, the energy loss near edge structure (ELNES) of carbon atoms in chair and tricycle conformers of hydrogenated graphene, namely ‘graphane’, has been calculated in the density functional theory using FP-LAPW method, and then, it has been compared with that of graphite and graphene. Using ELNES from chair conformer, the carbon K-edge was found to have a few main features including electron transition from 1s orbital of carbon atom to π*, σ*, and a hybridization of these two states. The first feature in tricycle conformer, however, has contributions of both π* and σ* states. The comparison of ELNES and the unoccupied density of states in each structure also justifies this. The energy difference between π* and σ* features of graphane conformers was decreased relative to it in graphite and graphene. Since the inclusion of core-holes and super-cells is essential for accurate reproduction of features in graphite and graphene, it may be essential as well for the ELNES spectra of graphane conformers.

Nonrigid Band Behavior of the Electronic Structure of LiCoO2 Thin Film during Electrochemical Li Deintercalation

In this study, a comprehensive experimental in situ analysis of the evolution of the occupied and unoccupied density of states as a function of the charging state of the Lix≤1CoO2 films has been done by using synchrotron X-ray photoelectron spectroscopy (SXPS), X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and O K- and Co L3,2-edges XANES. Our experimental data demonstrate the change of the Fermi level position and the Co3d–O2p hybridization under the Li+ removal and provide the evidence for the involvement of the oxygen states in the charge compensation. Thus, the rigid band model fails to describe the observed changes of the electronic structure. The Co site is involved in a Co3+ → Co4+ oxidation at the period of the Li deintercalation (x ∼ 0.5), while the electronic configuration at the oxygen site is stable up to 4.2 V. Further lowering of the Fermi level promoted by Li+ extraction leads to a deviation of the electronic density of states due to structural distor...

High energy resolution off-resonant spectroscopy for x-ray absorption spectra free of self-absorption effects.

X-ray emission spectra recorded in the off-resonant regime carry information on the density of unoccupied states. It is known that by employing the Kramers-Heisenberg formalism, the high energy resolution off-resonant spectroscopy (HEROS) is equivalent to the x-ray absorption spectroscopy (XAS) technique and provides the same electronic state information. Moreover, in the present Letter we demonstrate that the shape of HEROS spectra is not modified by self-absorption effects. Therefore, in contrast to the fluorescence-based XAS techniques, the recorded shape of the spectra is independent of the sample concentration or thickness. The HEROS may thus be used as an experimental technique when precise information about specific absorption features and their strengths is crucial for chemical speciation or theoretical evaluation.

Conduction-band electronic structure of 1T-TaS2revealed by angle-resolved inverse-photoemission spectroscopy

We have investigated the conduction-band electronic structure of 1$T$-TaS${}_{2}$ in the incommensurate (IC) and commensurate (C) charge-density-wave (CDW) phases by means of angle-resolved inverse-photoemission spectroscopy (ARIPES) along the $\ensuremath{\Gamma}\phantom{\rule{0.28em}{0ex}}(A)--M\phantom{\rule{0.28em}{0ex}}(L)$ direction. In the ARIPES spectra for the ICCDW phase, a continuous and dispersive Ta 5$d$ band is clearly observed near the Fermi level (${E}_{\mathrm{F}}$). For the CCDW phase, on the other hand, the Ta 5$d$ band splits up and down into two manifolds around the $M\phantom{\rule{0.28em}{0ex}}(L)$ point, reflecting formation of the $\sqrt{13}\ifmmode\times\else\texttimes\fi{}\sqrt{13}$ superstructure of the Ta plane. In addition, the CCDW-ARIPES spectrum around the $M\phantom{\rule{0.28em}{0ex}}(L)$ points exhibits a shoulder structure at 0.7 eV above ${E}_{\mathrm{F}}$, ascribed to the upper Hubbard band folded from the $\ensuremath{\Gamma}\phantom{\rule{0.28em}{0ex}}(A)$ point. We have also observed an energy gap of $\ensuremath{\sim}$100 meV in the unoccupied density of states in the CCDW phase.