Ultraviolet Photoelectron Spectra Research Articles

Studies of electronic structure across a quantum phase transition in CeRhSb1\u2212xSnx

We study an electronic structure of CeRhSb1−xSnx system, which displays quantum critical transition from a Kondo insulator to a non-Fermi liquid at x = 0.13. We provide ultraviolet photoelectron spectra of valence band obtained at 12.5 K. A coherent peak at the Fermi level is not present in the data, but a signal related to 4f17∕2 final state is detected. Spectral intensity at the Fermi edge has a general tendency to grow with Sn content. Theoretical calculations of band structure are realized with full-potential local-orbital minimum-basis code using scalar relativistic and full relativistic approach. The calculations reveal a depletion of density of states at the Fermi level for CeRhSb. This gap is shifted above the Fermi energy with increasing Sn content and thus a rise of density of states at the Fermi level is reflected in the calculations. It agrees with metallic properties of compounds with larger x. The calculations also yield another important effect of Sn substitution. Band structure is displaced in a direction corresponding to hole doping, although with deviations from a rigid band shift scenario. Lifshitz transitions modify a topology of the Fermi surface a few times and a number of bands crossing the Fermi level increases.Graphical abstract

Conformational distortion in solution processable PVK:TcTa blends and the effect on extra warm white organic phosphorescent light emitting diodes

Abstract Organic semiconductors generally display significant conformational flexibility which results in significant energetic disorder. In this research, ultraviolet photoelectron spectra are compared with the Gaussian density of states obtained from the density functional theory on assembly of molecules optimized using molecular dynamic simulations. It was found out that spin-coated organic semiconductor, tris(4-carbazoyl-9-ylphenyl)amine (TcTa), displays significant conformational distortion from its minimum geometry with significant Gaussian broadening. Adding poly(vinylcarbazole) (PVK) into TcTa results in increased face to face stacking of carbazole moieties from PVK as evident from increased red-shifted of absorption of PVK. Despite reduction of energy barrier upon addition of TcTa into PVK, the current efficiency of the extra warm white organic light emitting diodes showed limited improvement probably from the increased triplet quenchers from the carbazole dimers.

Effect of oxygen vacancies on the resistive switching behavior of hexagonal WO3 nanowire

The concentration of oxygen vacancies in hexagonal WO3 nanowires has been well regulated after being reduced in H2 environment at different temperatures. The current-voltage characteristics based on the Au/WO3 nanowire/Au device reveal that the conduction mechanism is converted from a bulk-limited type (space-charge-limited conduction) to an electrode-limited type (Schottky emission) with the increase of oxygen vacancy concentration and bias voltage. Ultraviolet photoelectron spectra of WO3 nanowires indicate that the defect states of oxygen vacancy approach the Fermi level gradually as the concentration of oxygen vacancies increasing. These defect states are always localized owing to the disordered distribution of oxygen vacancies, and then the space-charge-limited conduction and the transport of electrons are suppressed gradually. Under larger bias voltage, more localized states will be filled, which enhances the space-charge-limited conduction in WO3 nanowire. In the meantime, it will enhance the height of the interfacial barrier and turn the Ohmic contact into Schottky type. Therefore, it is possible to regulate the resistive switching behaviors of WO3 nanowire by adjusting the concentration of oxygen vacancies and even bias sweeping range.

Valence electronic structure of [EMIM][B(CN)4]: ion-pair vs. bulk description

The ultraviolet photoelectron spectrum of the [EMIM][B(CN)4] ionic liquid was recorded and analyzed. Together with different ab initio calculation methods, detailed insight into the electronic structure of this simple room temperature ionic liquid is possible. The ion-pair approximation to the liquid electronic structure was not sufficient. Therefore bulk ab initio calculations were performed on a proposed crystal structure. The modelling of bulk electronic spectra is able to explain the experimental electronic structure of the ionic liquid. Most notably, the dispersion corrected PBE calculation (PBE-D3BJ) showed good agreement with the experimental UPS spectrum. The spectra simulated by the B97-D and the BLYP-D3(BJ) functionals were also in agreement with the experimental data. The LDA approximation only provided qualitative agreement while the optB88-vdW and CX-vdW functionals were not good. However, it will be shown that many requirements have to be met in order to accurately describe the electronic structure of this ionic liquid.

Facet effect on the photoelectrochemical performance of a WO3/BiVO4 heterojunction photoanode

Different WO3 facets have different surface energies and electronic structures, and exhibit different water oxidation abilities and photocatalytic performance as a result. Because of the material’s limited photoresponse region, loading a narrow bandgap material on WO3 is a generally known method for improving photo-harvesting. In this paper, we have synthesized WO3 films with different crystal facet ratios. After loading BiVO4 on these WO3 films, we measured the photoelectrochemical (PEC) performance to investigate the effects of WO3 facet choice on the heterojunction film electrode’s performance. We found that a high-intensity ratio of the (002) WO3 facet in X-ray diffraction (XRD) leads to a more negative onset potential and higher photocurrents in a lower potential region. The ultraviolet photoelectron spectra show a lower work function for the 002-dominant WO3 film compared to other WO3 films, which may result in a higher quasi-fermi level for the heterojunction electrode. Based on the XRD results, the high-intensity ratio of the (002) WO3 facet preferentially exposes the (020) BiVO4 facet, which may be a reason for the better charge extraction observed at low applied potential and high faradic efficiency on PEC water splitting. Together, this results in a high hole injection efficiency for 002-dominant WO3/BiVO4 films compared with WO3/BiVO4 films favoring other WO3 facet ratios.

Thiourea Interfacial Modification for Highly Efficient Planar Perovskite Solar Cells

Interfacial engineering is an efficient strategy for enhancing the performance of perovskite photovoltaic cells. Here, we present a novel method to modify the surface of TiO2 with thiourea. X-ray photoelectron spectroscopy indicates that thiourea treatment inactivates the TiO2 surface groups. Kelvin probe force microscopy and ultraviolet photoelectron spectra demonstrate an energy level downshift of the modified TiO2. Photoluminescence spectra reveal the reduced interface charge recombination and an improved charge transportation/extraction. Tafel and Nyquist plots elucidate a decrease of trap-state density and an increase of conductivity of the modified TiO2. Hence, the planar perovskite solar cell based on the TiO2 electron transport layer (ETL) modified with 0.1 M thiourea yields an efficiency of 19.18%, which is 7.5% larger than that of the device based on the pristine TiO2 ETL. The simple procedure and the significant performance improvement render this method promising.

Mapping Valence Band and Interface Electronic Structure Changes during the Oxidation of Mo to MoO3 via MoO2 and MoO3 Reduction to MoO2: A NAPPES Study

Tuning the surface energetics, especially work function (ϕ) of the materials, is of a great deal of interest for a wide range of surface- and interface-based devices and applications. How the ϕ of a solid surface changes under the reaction conditions is of paramount interest to the chemists, particularly in the areas of surface-dependent phenomena such as, catalysis and electrochemistry. In the present study, by using the valence band and core-level photoelectron spectroscopy, surface electronic changes from Mo to MoO3 via MoO2 was studied under relevant near-ambient pressure (NAP) and high temperature conditions. A very significant change in ϕ from Mo to MoO3 was observed and it is well corroborated with the changes in gas-phase vibrational features of O2 in both near-ambient pressure ultraviolet photoelectron spectra (NAPUPS) as well in NAP X-ray photoelectron spectroscopy. Reversible changes in the electronic structure is observed when MoO3 was reduced in H2 to MoO2. On the basis of the extent of oxida...

Multivariate analysis to evaluate indium behavior at the copper phthalocyanine thin film

In this paper, we demonstrate the application of multivariate chemometric analysis to enhance information available from photoelectron spectra for the metal-organic interface: indium on copper phthalocyanine (CuPc). The previous studies conducted by Aristov et al. [Phys. Rev. B 72, 165318 (2005)] and Ivanco et al. [Phys. Rev. B 81, 115325 (2010)] reported inconsistent and contradictory findings about the chemical reaction between In and CuPc at the initial stages of interface formation. The study of changes in ultraviolet photoelectron spectra (UPS) by principal component analysis (PCA) provides evidence for the coexistence of two types of indium phases, “reactive” and “non-reactive,” whose behavior is of importance for devices based on multilayer metal-organic semiconductors. The key information about the metal-organic chemical reaction has been gained from a PCA biplot. This characteristic graph simplifies interpretation of the UPS spectra and enables to estimate the number of pronounced peaks, their energetic positions, and the strength of correlation between them. To detect one of the indium phases, not clearly observed in the experimental spectra, we propose a new concept based on PCA methodology for an enhancement of the UPS spectra resolution. Moreover, we confirm a co-existence of different indium phases by the multivariate curve resolution alternating least square (MCR) analysis on the experimental data. We have found satisfactory agreement between the identified model and experimental data.

Ultrafast nonradiative transition pathways in photo-excited pyrazine: Ab initio analysis of time-resolved vacuum ultraviolet photoelectron spectrum

The internal conversion of photo-excited pyrazine, which occurs rapidly on a time scale of about 20 fs, has long been considered to proceed via a conical intersection between the optically bright S2 (1B2u, ππ∗) and dark S1 (1B3u, nπ∗) states. Since 2008, several theoretical studies have raised the possibility that other dark states S3 (1Au, nπ∗) and S4 (1B2g, nπ∗) may participate dominantly in the early stage of the nonradiative decay of S2. To clarify this issue, being motivated by the recent pump–probe experiment by Horio et al. [J. Chem. Phys. 145 (2016) 044306], we calculated vacuum ultraviolet photoelectron spectra for ionization from each of the four excited states. Comparison was made with the measured time-resolved photoelectron spectrum exhibiting a temporally varying multi-band structure. We confirmed no contribution of S3 or S4 and thus the validity of the conventional two-state (S2 → S1) picture for ultrafast nonradiative transition in pyrazine.

Boron chelate complexes: X-ray and UV photoelectron spectra and electronic structure

Published data on the electronic structure of boron chelates are summarized for the first time. Ultraviolet photoelectron spectra of vapors, X-ray photoelectron spectra of molecular crystals, and results of modeling within the framework of the density functional theory are analyzed. Data on the effect of substituents on the electronic structure of complexes are systematized.

Adsorption of Ultrathin Ethylene Carbonate Films on Pristine and Lithiated Graphite and Their Interaction with Li.

Aiming at a better understanding of the solid-electrolyte interphase formation in Li-ion batteries, we have investigated the interaction of ultrathin films of ethylene carbonate (EC), which is a key solvent of battery electrolytes, with pristine and lithiated highly oriented pyrolytic graphite (HOPG) and with postdeposited Li. Employing X-ray and ultraviolet photoelectron spectroscopy as well as Fourier transform infrared spectroscopy under ultrahigh-vacuum conditions, in combination with density functional theory (DFT)-based calculations, we find that EC adsorbs molecularly intact on pristine HOPG in the entire temperature range between 80 K and desorption at 200 K. Features in the ultraviolet photoelectron spectra could be related to the molecular orbitals of EC obtained from DFT calculations, and a similar adsorption/desorption behavior is obtained also on lithiated HOPG. In contrast, stepwise postdeposition of ∼0.5 and one monolayer of Li0 on a preadsorbed EC adlayer leads not only to stabilization of Li+/Liδ+ at the surface, possibly as adsorbed Li+(EC) n species, but also to EC decomposition, forming products such as Li2CO3, ROCO2Li (CH2OCO2Li)2, and Li2O. Consequences on the electronic surface properties and on the stabilization of the resulting adlayer are discussed. Upon annealing up to room temperature, some residual Li-containing decomposition products remain on the surface, which is considered as the initial stage of the solid|electrolyte interphase formation at the electrode|electrolyte interface.

Ultra-trace (parts per million-ppm) W6+ dopant ions induced anatase to rutile transition (ART) of phase pure anatase TiO2 nanoparticles for highly efficient visible light-active photocatalytic degradation of organic pollutants

A series of novel photocatalysts were synthesized by incorporating tungsten (W6+) ions in to the lattice of pure anatase TiO2 nanoparticles with ultra-trace concentrations (ppm) such as 10 ppm, 50 ppm, 90 ppm and 120 ppm using a simple one-step hydrothermal method under identical conditions. X-ray diffraction (XRD) studies, revealed a spectacular structural phase transformation from anatase to rutile on doping and associated morphological changes are seen by scanning electron microscopy (SEM). The phase transformation is promoted with the increase of W concentration up to 50 ppm and inhibited thereafter. Microstructural lattice distortions due to the presence of W6+ ions are monitored by high-resolution transmission electron microscopy (HR-TEM). The valence band X-ray photoelectron spectra (XPS), Ultraviolet photoelectron spectra (UPS) and UV–Vis absorption studies confirm the reduction of band gap for W doped TiO2 mixed phase nanoparticles. The doping causes an enhanced visible light absorption. Room temperature photoluminescence (PL) spectroscopy ensures an oxygen vacancy mediated phase transformation. The photocatalytic performances of the W doped TiO2 catalysts are evaluated for the degradation of Rhodamine B (RhB), Methylene blue (MB) and Methyl orange (MO) aqueous solutions under visible-light irradiation. The efficiency of photocatalytic degradation of the dyes gradually increased with the content of W6+ ions (till 50 ppm) and then decreased which is consistent with the structural and optical studies. The control experiments, showed possible oxidative species, are hydroxyl (·OH-) and oxygen (·O2-) radicals while the earlier plays a dominant role. The resuability tests performed for five cycles demonstrate high stability and reusability of the photocatalysts. A possible synergistic mechanism based on the impurity energy levels of W6+ ions, oxygen vacancies and anatase-rutile heterojunction for the enhancement of the photocatalytic activity under visible-light irradiation is proposed.

Nano-α-Fe2O3 enhanced photocatalytic degradation of diethyl phthalate ester by citric Acid/UV (300–400 nm): A mechanism study

Nano-α-Fe2O3 can accelerate the degradation of diethyl phthalate ester (DEP) in the presence of citric acid under UV (300–400 nm) irradiation, but the underlying mechanism remains unclear. In this study, based on ultraviolet photoelectron spectra (UPS) and density functional theory (DFT) analyses, it was found that thermodynamically, a lower bandgap energy (Ebandgap = ECB-EVB < 5.8 eV) of the complex of CA-α-Fe2O3 predicted its higher catalytic ability than citric acid (Ebandgap = ELUMO-EHOMO = 7.0 eV). X-ray photoelectron spectroscopy (XPS) study indicated that citric acid was chemically adsorbed on the surface of α-Fe2O3 through its carboxyl group by filling the surface oxygen vacancies of α-Fe2O3 (disappearance of Fe2p3/2 peak at 711.5 eV). UV light induced a succession of reactions which resulted in heterogeneous photo-Fenton-like reactions that produced ·OH for DEP degradation. The dissolved oxygen was proposed to be the initial oxidant forming the reactive oxygen species. The solution pH, citric acid and α-Fe2O3 concentration substantially influenced DEP degradation. The method in this study can be further applied to study other organic acids and metal oxides catalysts.

Stable Organic Radicals as Hole Injection Dopants for Efficient Optoelectronics.

Precursors of reactive organic radicals have been widely used as n-dopants in electron-transporting materials to improve electron conductivity and enhance electron injection. However, the utilization of organic radicals in hole counterparts has been ignored. In this work, stable organic radicals have been proved for the first time to be efficient dopants to enhance hole injection. From the absorbance spectra and the ultraviolet photoelectron spectra, we could observe an efficient electron transfer between the organic radical, (4-N-carbazolyl-2,6-dichlorophenyl)bis(2,4,6-trichlorophenyl)methyl (TTM-1Cz), and the widely used hole injection material, 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HAT-CN). When the unpaired electron of TTM-1Cz is transferred to HAT-CN, it would be oxidized to a TTM-1Cz cation with a newly formed lowest unoccupied molecular orbital which is quite close to the highest occupied molecular orbital (HOMO) of the hole-transporting material (HTM). In this way, the TTM-1Cz cation would promote the electron extraction from the HOMO of the HTM and improve hole injection. Using TTM-1Cz-doped HAT-CN as the hole injection layer, efficient organic light-emitting diodes with extremely low voltages can be attained.

Valence electronic structure of [EMIM][BF4] ionic liquid: photoemission and DFT+D study.

The ultraviolet photoelectron spectrum (UPS) of the [EMIM][BF4] ionic liquid was recorded and analyzed. Together with the gas-phase UPS spectrum of the [EMIM][BF4] vapor and ab initio calculation methods, detailed insight into the electronic structure of this simple ionic liquid is possible. The low binding energy tail in the UPS spectrum is about 7.4 eV, in agreement with previous estimations of the HOMO–LUMO gap of the [EMIM][BF4] ion-pair. The bulk ab initio calculations are able to explain most of the features in the spectrum. However, DFT consistently lacks accuracy in the description of the top of the valence band. The dispersion corrected PBE calculation (PBE-D3) did offer very good agreement with the experimental structure, but the recently-developed vdW-DF functionals C09, optPBE, optB88 and CX were found to offer the best agreement in terms of the electronic structure.

Photoelectron spectra and electronic structure of aza-boron-dipyridomethene derivatives

The electronic structure of three aza-boron-dipyridomethene derivatives containing different hydrocarbon groups at the boron atom is studied by ultraviolet photoelectron spectroscopy and calculations at the density functional theory level. According to the experimental and theoretical data, the higher occupied molecular orbitals of anthracene, acridine, and the studied complexes are of the same character. For the three studied compounds, the effect of alkyl and phenyl substituents on the electronic structure is determined. The parameters of the electronic structure of aza-boron-dipyridomethene (phenyl groups at the boron atom) and its β-diketonate analogue are compared. It is shown that in an energy range up to 11 eV the calculated results correlate with the ultraviolet photoelectron spectra.

The trade-off between electrochromic stability and contrast of a thiophene—Quinoxaline copolymer

The stability of organic electrochromic devices is a crucial issue for their applications. However, until now the degradation mechanism of electrochromic materials are still not fully understood especially for electrochromic conjugated polymers (ECPs). To improve device stability, intensive investigation on the degradation mechanism of ECPs is urgently needed. Here we report our study on the electrochromic degradation in a thiophene–quinoxaline copolymer: poly [2,3-bis-(3-octyloxyphenyl)quinoxaline-5,8-diyl-alt-thiophene-2,5-diyl] (TQ1). The results of X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectra (UPS) and UV–vis transmission spectra reveal that there are three main factors during the electrochromic degradation of TQ1. The first one is anion (ClO4−) irreversibly deep trapped, while the second is peroxidation of the thiophene group in TQ1. Both factors reduce the conductivity and electrochromism of TQ1. The third is structural relaxation resulting lager conjugated system of TQ1 molecules in film, which is gradually developed during 400 cycling of CV at a narrow potential range (0–1V). When a potential range 0–0.7V is applied, all three factors are prohibited, no electrochromism degradation is observed anymore, although the contrast becomes smaller. Our investigation systematically discloses the degradation mechanism during the electrochemistry processing of a ECP (TQ1), demonstrating the significance of trade-off between the electrochromic stability and contrast of the ECP.

Interface Engineering of Perovskite Solar Cells with Air Plasma Treatment for Improved Performance.

For high-efficiency perovskite solar cells (PSCs), interface engineering becomes critical for carrier collection from the active perovskite material to the transport layer. To enhance the power conversion efficiency (PCE), herein we demonstrate a novel method named surface plasma treatment on a mesoporous TiO2 electron-transport layer (ETL) to improve electron extraction and transport properties at the perovskite/TiO2 interface. According to the XPS results, the plasma treatment induced a partial reduction of Ti4+ to Ti3+ within the TiO2 lattice and increased the concentration of oxygen vacancies on the TiO2 surface. Ultraviolet photoelectron spectra (UPS) show that the Fermi level of TiO2 upshifts about 0.2 eV which may effectively promote carrier separation and transfer at the perovskite/TiO2 interface. In addition, these created donor levels of Ti3+ and oxygen vacancies donate extra electrons, increasing the conductivities of TiO2 films and which could further promote transport. The time-resolved photoluminescence spectra (TRPL) confirm that the decay time decreases dramatically from 656 ns to 235 ns after 90 s plasma treatment, which indicates a more efficient electron-transfer process. Based on all the above-mentioned results, a remarkable enhancement in cell efficiency was obtained, such that the average efficiency was improved from 11.5 % to 14.3 % under AM 1.5G irradiation (100 mW cm-2 ).

Efficiency of chemometric methodology for characterization of In/CuPc films on HOPG and InSb based on ultraviolet photoelectron spectra

Efficiency of chemometric methodology for characterization of In/CuPc films on HOPG and InSb based on ultraviolet photoelectron spectra

Valence band photoelectron spectra of [EMIM][BF4] ionic liquid vapor: Evidences of electronic relaxation

Experimental ultraviolet photoelectron spectra of [EMIM][BF4] molecules were recorded and analyzed. The low binding energy tail of the HOMO peak in the UPS spectrum is about 7.5eV, in agreement with previous estimations of the liquid phase HOMO-LUMO gap. These gas-phase spectra reveal detailed information about the electronic structure of the molecules of this common ionic liquid and enable comparison with ab initio calculation methods like density functional theory (DFT) and Møller–Plesset perturbation theory (MP2). The ab initio calculations are able to explain most of the features in the spectra.