Density Of Unoccupied Electronic States Research Articles

Density of Unoccupied Electronic States of the Ultrathin Layers of Dibromo-Bianthracene on the Surface of Layer-by-Layer Grown ZnO

Density of Unoccupied Electronic States of the Ultrathin Layers of Dibromo-Bianthracene on the Surface of Layer-by-Layer Grown ZnO

Antimony substituted lanthanum orthoniobate proton conductor – Structure and electronic properties

AbstractX‐ray and neutron diffraction have been utilized to analyze the crystalline and electronic structure of lanthanum orthoniobate substituted by antimony. Using X‐ray absorption spectroscopy and photoelectron spectroscopy, changes in the electronic structure of the material upon substitution have been analyzed. The structural transition temperature between fergusonite and scheelite phases for 30 mol% antimony substitution was found to be 15°C. Based on the neutron data, the oxygen nonstoichiometry was found to be relatively low. Moreover no influence on the position of the valence band maximum was observed. The influence of the protonation on the electronic structure of constituent oxides has been studied. Absorption data show that the incorporation of protonic defects into the lanthanum orthoniobate structure leads to changes in lanthanum electronic structure and a decrease in the density of unoccupied electronic states.

Effect of Structural Disorders on the Li Storage Capacity of Graphene Nanomaterials: A First-Principles Study.

We employed first-principles calculations to investigate the effect of structural disorders on the Li storage capacity of graphene nanomaterials. Our calculations first revealed that the Li storage capacity of a graphene monolayer does not necessarily increase with the size of a C vacancy created but is largely determined by the local geometry of the defect sites. Our electronic structure analysis further revealed that the enhanced Li storage capacity by the C vacancy defect is mainly attributed to the increased number of the unoccupied electronic density of states lying near the Fermi level, which can be substantially increased by raising the number of bond rotations within the vacancy sites. Furthermore, it was also found that the Li storage capacity of graphene can be effectively enhanced by increasing the degree of local ring disorders without the presence of any vacancy defect. The amorphous graphene structure was shown to possess a relatively higher Li storage capacity compared to pristine graphene, primarily owing to the presence of many nonhexagonal rings randomly distributed in the graphene lattice. These nonhexagonal rings can create many electron-deficient regions on the graphene surface to effectively accommodate more electrons from Li, thereby substantially enhancing the Li storage capacity of graphene nanomaterials.

EXAFS as a probe of actinide oxide formation in the tender X-ray regime

Recent developments in Resonant Inelastic X-Ray Scattering Experiments have vastly improved the resolution in X-ray Spectroscopies such as X-ray Absorption. However, the use of Tender X-rays (∼3 keV) and the reactivity of actinides such as uranium have given rise to new questions about bulk and surface sensitivity, what constitutes a surface and how to characterize the formation of an oxide. It will be shown that the in situ electron scattering features that are part of the RIXS spectrum can be used to quantify the uranium oxide formation and determine cleanliness, independently of the near edge features that provide information about the unoccupied electronic density of states.

Плотность свободных электронных состояний полупроводниковых пленок молекул нафталина и дифенилфталида, модифицированных электроактивными функциональными группами

The results on comparing the peak structure of the density of unoccupied electronic states (DOUS) of ultrathin films of naphthalene anhydride-1,4,5,8-teracabonic acid (NTCDA) and naphthalene-1,8-dicarboxylic acid anhydride (NDCA) and of two types of phthalide-based films: 3,3-bis (phenyl) phthalide (DPP) and 3,3-bis (phenyl) phthalide-4 ′, 4′-dicarboxylic acid (DPP-DCA) are presented. The measurements of the structure of the unoccupied electronic states in the energy range from 5 eV to 20 eV above the Fermi level of the films studied having thickness of 8–10 nm were conducted using the total current spectroscopy (TCS) technique. Analysis of the experimental results was conducted using the model total current spectra and DOUS dependences generated using the calculated orbital energies of the studied molecules by means of the density functional theory (DFT) method at the B3LYP/6-31G(d) level. The difference in the DOUS spectra of NTCDA and NDCA films is characterized by the shift of the main DOUS maxima of the NTCDA film to lower energies by about 1 eV at energies less than 12.5 eV, and at higher energies the DOUS maxima are shifted by 1.5-2 eV. The energy positions of the maxima of the total current spectra of the DPP-DCA and DPP films practically do not change when using various substrates: highly ordered pyrolytic graphite (HOPG) and layer-by-layer deposited CdS. The relative intensities of the maxima differ when using different substrates. The characteristic shift of the maxima of the total current spectra of DPP-DCA films is about 1 eV at energies less than 12.5 eV above the Fermi level and 1.5-2 eV and at higher energies, compared with the position of the corresponding maxima of the DPP films.

Electron stimulated ring opening in diphenylphthalide dicarboxylic acid: Its likely role in the unique properties of phthalide-based materials.

The electronic properties of diphenylphthalide dicarboxylic acid (DPDA) are studied under gas-phase conditions using dissociative electron attachment spectroscopy and in the condensed environment by means of total current spectroscopy. The experimental features are assigned with the support of density functional theory calculations of the energies of the lowest-lying anion states to describe both resonances responsible for low-energy (0-15 eV) electron attachment to the isolated molecule and the maxima in the density of unoccupied electronic states in the condensed ultrathin (up to 10 nm) films. Resonance electron attachment to DPDA is found to be followed by the opening of the γ-lactone ring in the molecular negative ions, an unusual mechanism leading to their stabilization. A similar mechanism is expected to be responsible for the unique properties of phthalide-based materials in the condensed state.

Density of unoccupied electronic states of vapor-deposited films of dioctyl-substituted and diphenyl-substituted perylenedicarboximides

The results of the investigation of the density of unoccupied electronic states (DOUS) in the energy range from 5 to 20 eV above the Fermi level (E F) in dioctyl-substituted perylenedicarboximide (PTCDI-C8) and diphenyl-substituted perylenedicarboximide (PTCDI-Ph) ultrathin films have been presented. The experimental results have been obtained from measurements of the secondary low-energy electron current with the use of the total current spectroscopy (TCS) technique. A theoretical analysis has been performed, including the density functional theory calculation of the energies and spatial distribution of the orbitals of the molecules under investigation and the subsequent scaling of the calculated orbital energies according to the procedure well-proven previously in studies of small conjugated organic molecules. It has been found that, for each of the two types of the studied films, at energies below 8 eV above the Fermi level E F, there are two main maxima of the density of unoccupied electronic states predominantly formed by the π*-orbitals of the molecules. The higher-lying maxima have essentially a σ*-character. The influence of dioctyl- and diphenyl-substituent groups on the density of unoccupied electronic states has been analyzed in comparison with the results obtained for the studied types of films. In the case of the π*-maxima, the relative shift has been observed at an energy of approximately 1 eV. In the region of σ*-electronic states, there is a small transformation of the structure of the maxima.

Structure of vacant electronic states of an oxidized germanium surface upon deposition of perylene tetracarboxylic dianhydride films

This paper presents the results of the investigation of the interface potential barrier and vacant electronic states in the energy range of 5 to 20 eV above the Fermi level (EF) in the deposition of perylene tetracarboxylic dianhydride (PTCDA) films on the oxidized germanium surface ((GeO2)Ge). The concentration of oxide on the (GeO2)Ge surface was determined by X-ray photoelectron spectroscopy. In the experiments, we used the recording of the reflection of a test low-energy electron beam from the surface, implemented in the mode of total current spectroscopy. The theoretical analysis involves the calculation of the energy and spatial distribution of the orbitals of PTCDA molecules by the density functional theory (DFT) using B3LYP functional with the basis 6-31G(d), followed by the scaling of the calculated values of the orbital energy according to the procedure well-proven in the studies of small organic conjugated molecules. The pattern of changes in the fine structure of the total current spectra with increasing thickness of the PTCDA coating on the (GeO2)Ge surface to 6 nm was studied. At energies below 9 eV above EF, there is a maximum of the density of unoccupied electron states in the PTCDA film, formed mainly by π* molecular orbitals. The higher density maxima of unoccupied states are of σ* nature. The formation of the interface potential barrier in the deposition of PTCDA at the (GeO2)Ge surface is accompanied by an increase in the work function of the surface, Evac–EF, from 4.6 ± 0.1 to 4.9 ± 0.1 eV. This occurs when the PTCDA coating thickness increases to 3 nm, and upon further deposition of PTCDA, the work function of the surface does not change, which corresponds to the model of formation of a limited polarization layer in the deposited organic film.

Local metal and deuterium ordering in the deuterated ZrTiNi C14 Laves phase

Local structure of the hexagonal C14 Laves phase in an annealed approximately equiatomic ZrTiNi alloy was studied before and after deuteration using neutron total scattering. Rietveld refinements of the P6 3 / mmc AB 2 model demonstrate that the A-sites are shared by Zr and Ti, whereas the two non-equivalent B-sites are occupied by a mixture of Ti and Ni. Reverse Monte Carlo refinements using a joint fit of the neutron total scattering data in real and reciprocal spaces revealed significant short-range ordering of Ti and Ni on the B-sites. The refined Ti–Ni nearest-neighbor distances are appreciably shorter than the corresponding Ti–Ti and Ni–Ni distances. Nevertheless, the differences in the effective sizes of chemically distinct tetrahedral interstices (i.e. [Zr 2Ti 2], [Zr 2TiNi], [Zr 2Ni 2], etc.), which are important for hydrogen absorption, remain rather small. Rietveld refinements of the structure of the deuterated sample TiZrNiD 2.14 confirmed that deuterium occupies various [A 2B 2] tetrahedral interstices. RMC refinements suggested a strong preferential occupation of deuterium in the [Zr 2Ti 2] and [Zr 2TiNi] tetrahedra whereas the [Zr 2Ni 2] sites remained nearly empty. These results support earlier models which predicted preferential occupancy by hydrogen in sites coordinated by metals that form the most stable binary hydrides. The deuterium atoms in the [Zr 2TiNi] tetrahedra are displaced toward Ni. X-ray absorption near-edge structure measured for Ti, Zr, and Ni K-edges demonstrated that deuteration is accompanied by the reduction in the density of unoccupied electronic states (just above the Fermi level) associated with Ti and Zr.

Microwave-Assisted Synthesis of a Core–Shell MWCNT/GONR Heterostructure for the Electrochemical Detection of Ascorbic Acid, Dopamine, and Uric Acid

In this study, graphene oxide nanoribbons (GONRs) were synthesized from the facile unzipping of multiwalled carbon nanotubes (MWCNTs) with the help of microwave energy. A core-shell MWCNT/GONR-modified glassy carbon (MWCNT/GONR/GC) electrode was used to electrochemically detect ascorbic acid (AA), dopamine (DA), and uric acid (UA). In cyclic voltammograms, the MWCNT/GONR/GC electrode was found to outperform the MWCNT- and graphene-modified GC electrodes in terms of peak current. For the simultaneous sensing of three analytes, well-separated voltammetric peaks were obtained using a MWCNT/GONR/GC electrode in differential pulse voltammetry measurements. The corresponding peak separations were 229.9 mV (AA to DA), 126.7 mV (DA to UA), and 356.6 mV (AA to UA). This excellent electrochemical performance can be attributed to the unique electronic structure of MWCNTs/GONRs: a high density of unoccupied electronic states above the Fermi level and enriched oxygen-based functionality at the edge of the graphene-like structures, as revealed by X-ray absorption near-edge structure spectroscopy, obtained using scanning transmission X-ray microscopy.

Electronic Structure of Warm Dense Copper Studied by Ultrafast X-Ray Absorption Spectroscopy

We use time-resolved x-ray absorption spectroscopy to investigate the unoccupied electronic density of states of warm dense copper that is produced isochorically through the absorption of an ultrafast optical pulse. The temperature of the superheated electron-hole plasma, which ranges from 4000 to 10 000 K, was determined by comparing the measured x-ray absorption spectrum with a simulation. The electronic structure of warm dense copper is adequately described with the high temperature electronic density of state calculated by the density functional theory. The dynamics of the electron temperature is consistent with a two-temperature model, while a temperature-dependent electron-phonon coupling parameter is necessary.

Interface doping of conjugated organic films by means of diffusion of atomic components from the surfaces of semiconductors and of metal oxides

The paper reports the results on the interface formation of 5–10 nm thick conjugated layers of Cu-phthalocyanine (CuPc) with a number of solid surfaces: polycrystalline Au, (SiO 2) n–Si, ZnO ( 0 0 0 1 ¯ ) , Si(1 0 0), Ge(1 1 1), CdS ( 0 0 0 1 ¯ ) and GaAs(1 0 0). The results were obtained using Auger electron spectroscopy (AES) and low-energy target current electron spectroscopy (TCS). The organic overlayers were thermally deposited in situ in UHV onto substrate surfaces. The island-like organic deposits were excluded from the analysis so that only uniform organic deposits were considered. In the cases of polycrystalline Au, Si(1 0 0) and Ge(1 1 1) substrates the AES peaks of the substrate material attenuated down to the zero noise level upon the increase of the CuPc film thickness of 8–10 nm. The peaks corresponding to oxygen atoms in the case of SiO 2 substrate, and to atoms from the ZnO, GaAs and CdS substrates were clearly registered in the AES spectra of the 8–10 nm thick CuPc deposits. The relative concentration of the substrate atomic components diffused into the film was different from their relative concentration at the pure substrate surface. The concentration of the substrate dopant atoms in the CuPc film was estimated as one atom per one CuPc molecule. Using the target current electron spectroscopy, it was shown that the substrate atoms admixed in the CuPc film account for the appearance of a new peak in the density of unoccupied electronic states. Formation of intermediate TCS spectra until the CuPc deposit reaches 2–3 nm was observed in the cases of GaAs(1 0 0), ZnO ( 0 0 0 1 ¯ ) , Ge(1 1 1) surfaces. The intermediate spectra show a less pronounced peak structure different from the one typical for the CuPc films. It was suggested that the intermediate layer was formed by the CuPc molecules fully or partially decomposed due to the interaction with the relatively reactive semiconductor surfaces.

Electronic properties of the surface of perylene tetracarboxylic acid dianhydride film upon deposition of the ultrathin conjugated layers of Pyronine B

Abstract Ultrathin conjugated layers of Pyronine B were thermally deposited in UHV on the surface of perylene tetracarboxylic acid dianhydride (PTCDA) film. The structure of unoccupied electron states located 5–20 eV above the Fermi level ( E F ) and the surface potential were monitored during the Pyronine B overlayer deposition, using an incident beam of low-energy electrons according to the total current electron spectroscopy (TCS) method. Electronic work function of the PTCDA surface changed from 4.9 ± 0.1 eV, during the Pyronine B deposition due to the change of the contents of the surface layer, until it reached a stable value 4.6 ± 0.1 eV at the Pyronine B film thickness 8–10 nm. The interface dipole corresponds to electron transfer from the Pyronine B overlayer to the PTCDA surface and the polarization in the Pyronine B overlayer was found confined within approximately 1 nm near the interfaces. The edges of major bands of density of unoccupied electronic states (DOUS) of PTCDA substrate and of the Pyronine B overlayer were unaffected by the process of the interface formation. The major TCS spectral features of the Pyronine B film corresponding to the DOUS band edges were identified and the assignment of the π * , σ * (C–C) and σ * (C C) character was suggested.

Formation and electron properties of the interface between organic (TPD) and inorganic (ZnO) materials

The initial stage of formation of an organic coating of N,N′-bis(3-methylphenyl)-N,N′-diphenylbenzidine (TPD) on a crystalline (ZnO) surface in the course of thermal deposition in high vacuum has been studied using the low-energy total current spectroscopy technique. A change in the work function and the density of unoccupied electron states in an energy interval of 0–20 eV above the vacuum level was traced as the organic coating thickness increased up to 8 nm. The energy positions of the bands of unoccupied electron states in the TPD film have been determined, including π* band (7–9 eV above the Fermi level), σ1* band (10–12 eV), σ2* band (14–16 and 18–20 eV).

Effect of chemical disorder on NiMnSb investigated by appearance potential spectroscopy: an experimental and theoretical study

The half-Heusler alloy NiMnSb is one of the local-moment ferromagnets with unique properties for future applications. Band structure calculations exclusively predict majority bands at the Fermi level, thus indicating 100% spin polarization there. For the development of functional materials, chemical disorder has to be considered quantitatively. The magnetization, spin polarization and electronic structure are expected to be sensitive to structural and stoichiometric changes. We studied the influence of chemical disorder on the unoccupied electronic density of states of NiMnSb(0 0 1) by the use of the ab initio coherent potential approximation method and results are discussed along with corresponding measurements.

X-ray Raman Scattering Study of Aligned Polyfluorene

We present a non-resonant inelastic x-ray scattering study at the carbon K-edge on aligned poly[9,9-bis(2-ethylhexyl)-fluorene-2,7-diyl] and show that the x-ray Raman scattering technique can be used as a practical alternative to x-ray absorption measurements. We demonstrate that this novel method can be applied to studies on aligned $\pi$-conjugated polymers complementing diffraction and optical studies. Combining the experimental data and a very recently proposed theoretical scheme we demonstrate a unique property of x-ray Raman scattering by performing the symmetry decomposition on the density of unoccupied electronic states into $s$- and $p$-type symmetry contributions.

Electronic properties of a zinc oxide surface modified by ultra-thin layers of conjugated organic molecules

Thin films of Cu-phthalocyanine (CuPc), perylene and 3,4,9,10-perylenetetracarboxylic acid dianhydride (PTCDA) were thermally deposited in situ in UHV on ZnO(0 0 0 −1) surface. The surface potential, the surface work function Φ and the structure of density of unoccupied electron states (DOUS) located 5–25 eV above the Fermi level ( E F) were monitored during the film deposition, using an incident beam of low-energy electrons according to the total current electron spectroscopy (TCS) method. Auger electron spectroscopy (AES) was used to monitor atomic composition of the surfaces under study. The deposition of the perylene and PTCDA films resulted in formation of the DOUS structure typical for these films while the deposition of CuPc films of less than 2 nm thickness resulted in formation of an intermediate DOUS structure which was replaced by a stable DOUS of CuPc along with a further increase of the deposit thickness. Migration of the substrate atoms into the intermediate layer was observed. The electronic work function Φ of the organic films changed during the film deposition until it reached stable values at a film thickness of 8–10 nm. Width of the interface dipole charge transfer layer and formation of intermediate TCS spectra during the film deposition are selected as relevant parameters for further comparison of the features of the interfaces studied.

Organic–organic interfaces and unoccupied electronic states of thin films of perylene and naphthalene derivatives

Thin films of N, N′-Bis(benzyl)-3,4,9,10-perylenetetracarboxylic diimide (BPTCDI, Fig. 1b) and N, N′-Bis(benzyl)-1,4,5,8-naphthalenetetracarboxylic diimide (BNTCDI, Fig. 1d) were thermally deposited in UHV on 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA, Fig. 1a) and 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTCDA, Fig. 1c) film surfaces, respectively, in order to form organic–organic interfaces so that molecules constituting the interfacing layers differ by the substituent group. The surface potential and the density of unoccupied electron states (DOUS) located 5–25 eV above the Fermi level (E F) were measured during the film deposition using an incident beam of low-energy electrons according to the total current electron spectroscopy (TCS) method. Analysis of the TCS data allowed us to assign the π( band located 5–7.5 eV above E F for all the four films under study and the higher located σ* 1 and σ* 2 bands and the splitting within them. In order to perform the analysis the molecules were hypothetically divided into benzene-like, conjugated and non-conjugated fragments that may individually contribute to the peaks in the DOUS bands. It was shown that a non-conjugated fragment would serve for decreasing of the energy corresponding to the σ* 1 and σ* 2 bands and the sub-bands within them while an addition of a benzene-like fragment would do the opposite. The BPTCDI/PTCDA and BNTCDI/NTCDA interfaces were found non-reacted and a 4.1±0.1 eV work function value for both BPTCDI and BNTCDI films was determined, which is about 0.25 eV lower than the work functions of the PTCDA and the NTCDA films.

Unoccupied electronic band structure of conjugated molecular films interfacing polycrystalline gold surface

Thin films of Cu-phthalocyanine (CuPc) and of tri-oligo(phenylene–vinylene) end terminated by di-butyl-thiole (tOPV) were thermally deposited in situ in UHV on polycrystalline Au substrate surfaces. The surface potential, the surface work function and the structure of density of unoccupied electron states (DOUS) located 5–25 eV above the Fermi level ( E F) were monitored during the film deposition, using an incident beam of low-energy electrons according to the total current electron spectroscopy (TCS) method. Auger electron spectroscopy (AES) was used to monitor atomic composition of the surfaces under study. The deposition of the CuPc films resulted in formation of the DOUS structure typical for these films while the deposition of tOPV films of less than 2 nm thickness resulted in formation of an intermediate DOUS structure which was replaced by a stable DOUS of tOPV along with a further increase of the deposit thickness. The electronic work function of the CuPc and the tOPV films changed during the film deposition until it reached a stable value of 4.4 ± 0.1 and 4.3 ± 0.1 eV, respectively, at a film thickness of 8–10 nm. The width of the interface dipole layer in the CuPc and the tOPV films interfacing with the Au surface did not exceed 2 nm. Analysis of the TCS data allowed us to assign the major DOUS features of the CuPc and tOPV films under study.

Thickness dependence of the LUMO position for phthalocyanines on hydrogen passivated silicon (1 1 1)

Inverse photoemission spectroscopy (IPES) was employed to study the density of unoccupied electronic states of fluorinated and non-fluorinated cooper phthalocyanine layers deposited onto hydrogen passivated Si(1 1 1) substrates. For the non-fluorinated cooper phthalocyanine (CuPc) the lowest unoccupied molecular orbital (LUMO) is found to shift gradually towards the Fermi level with increasing film thickness. The shift amounts to 400 meV and appears for film thicknesses between one monolayer and 10 nm. This finding complements previous results obtained using ultraviolet photoemission spectroscopy where the highest occupied molecular orbital (HOMO) was found to shift as a function of film thickness. Fluorinated cooper phthalocyanine (F 16CuPc) shows the opposite behaviour, that is the distance between LUMO and Fermi level is increasing by 1.2 eV.