X-ray Photoemission Spectroscopy Spectra Research Articles

Magnetic properties and large cryogenic magnetocaloric effects in the RE2SiO5 (RE = Gd, Dy, Ho, and Er) silicates

We herein investigated four rare earth (RE)-based oxides, the RE2SiO5 (RE = Gd, Dy, Ho, and Er) silicates, regarding their structural and cryogenic magnetic properties. All the RE2SiO5 silicates are found to crystallized in a monoclinic structure. X-ray photoemission spectroscopy spectra illustrate that RE, Si and O elements in RE2SiO5 silicates are present as RE3+, Si4+, and O2−, respectively. The Dy2SiO5 reveals a first-order magnetic transition around 4.5 K. The magnetic phase transition temperatures are probably below 2 K for the RE2SiO5 (RE = Gd, Ho, and Er) silicates. Large cryogenic magnetocaloric effects (MCE) and good magnetocaloric performances were observed in the RE2SiO5 silicates. The MCE parameters in terms of the maximum magnetic entropy changes, the temperature-averaged entropy change (5 K-lift) and refrigerant capacity under a magnetic field change of 0–5 T are determined to be 35.5, 30.4 J/kgK, and 215.7 J/kg for Gd2SiO5, to be 13.3, 12.9 J/kgK, and 190.5 J/kg for Dy2SiO5, to be 15.3, 15.1 J/kgK, and 274.1 J/kg for Ho2SiO5 and to be 17.9, 16.5 J/kgK, and 181.14 J/kg for Er2SiO5, respectively. These derived values of MCE parameters of the RE2SiO5 silicates, especially for Gd2SiO5, are at similarly high level with or better than those of the updated candidate materials, making them also of potential for practical magnetic refrigeration applications.

Differences in the Ti4+/Ti3+ Redox Reactions between Li4Ti5O12 and LiTi2(PO4)3 Observed By Soft X-Ray Spectroscopy

Clarification of the redox reaction in electrode materials for Li-ion batteries is essential to obtain strategies to improve the performances such as charge-discharge capacity, voltage and cyclability. We focus on the difference in voltage, i.e., redox potential between Li4Ti5O12 (LTO) and LiTi2(PO4)3 (LTP). Spinel-type LTO with a redox potential around 1.5 V vs Li/Li+ is one of the anode materials [1], while the main redox potential of LTP (NASICON-type structure) is around 2.4 V vs Li/Li+ [2]. Although the difference in redox potential is as large as ~0.9 V, it has been thought to be that Ti4+/Ti3+ redox couple is identical for both LTO and LTP. Here, we demonstrate the electronic-structure analysis of LTO and LTP by using soft X-ray absorption spectroscopy (XAS) and soft X-ray photoemission spectroscopy (XPS). Operando Ti L 2,3-edge XAS by use of an all-solid-state cell [3,4] successfully revealed reversible Ti4+/Ti3+ redox reactions by discharge/charge for both materials, but the XAS line shapes were different. By analyzing the spectra using multiplet calculation [5], differences in the crystal-field splitting for the TiO6 octahedron and O 2p -> Ti 3d charge-transfer (CT) effect were found. The different CT effect was also confirmed by Ti 2p-3d resonant XPS measurements for the initial-state LTO and LTP samples. Moreover, the difference in redox potential is mostly explained by the energy levels of the pre-edge structures in the O K-edge XAS calibrated with the O 1s XPS spectra. In the presentation, the understandings of the spectroscopic results will be discussed in detail.

Microstructural Characterization of the Sol-Gel-Derived Sr2CrReO6 Powders and Their Magnetic and Electrical Transport Properties.

The double-perovskite (DP) Sr2CrReO6 (SCRO) oxide has gained much attention due to its high Curie temperature (∼635 K), high spin polarization, and strong spin-orbit coupling, which provides promising potential for room-temperature spintronic devices. In this work, we report on microstructures of a set of sol-gel-derived SCRO DP powders and their magnetic and electrical transport properties. The SCRO powders crystallize into a tetragonal crystal structure (space group of I4/m). X-ray photoemission spectroscopy spectra verify that the rhenium ions possess variable valences (Re4+ and Re6+) in the SFRO powders while chromium ions are presented as Cr3+. Ferrimagnetic behavior was observed in the SFRO powders at 2 K, and the saturation magnetization was evaluated to be 0.72 μB/f.u. and the coercive field to be 7.54 kOe. The Curie temperature was derived from susceptibility measurements to be 656 K at 1 kOe. Such ferrimagnetic behavior stems from the Cr3-Re4+(Re6+) super exchange interaction via intervening oxygen. Electrical transport measurements revealed that the SFRO ceramic grains were semiconducting and the electrical transport process was governed by the small polarons hopping with variable ranges. The hopping paths for these small polarons are provided by the hetero-valent Re ions in the SCRO ceramics. Negative magnetoresistance (MR) was observed in the SCRO ceramics, and the plot of MR vs magnetic field (H) exhibited a butterfly-like shape. The MR (2 K, 6 T) was measured to be -5.3%, due to the intergranular magneto-tunneling effect. The present results demonstrate a unique combination of high-temperature ferrimagnetism and intrinsic semiconducting nature of the sol-gel-derived SCRO oxides, which are highly attractive for oxide spintronics.

Coordination Chemistry of Uranyl Ions with Surface-Immobilized Peptides: An XPS Study.

The coordination chemistry of uranyl ions with surface immobilized peptides was studied using X-ray photoemission spectroscopy (XPS). All the peptides in the study were modified using a six-carbon alkanethiol as a linker on a gold substrate with methylene blue as the redox label. The X-ray photoemission spectra reveal that each modified peptide interacts differently with the uranyl ion. For all the modified peptides, the XPS spectra were taken in both the absence and presence of the uranium, and their comparison reveals that the interaction depends on the chemical group present in the peptides. The XPS results show that, among all the modified peptides in the current study, the (arginine)9 (R9) modified peptide showed the largest response to uranium. In the order of response to uranium, the second largest response was shown by the modified (arginine)6 (R6) peptide followed by the modified (lysine)6 (K6) peptide. Other modified peptides, (alanine)6 (A6), (glutamic acid)6 (E6) and (serine)6 (S6), did not show any response to uranium.

Impact of atomic defects in the electronic states of FeSeS superconducting crystals

The electronic properties of Fe-based superconductors are drastically affected by deformations on their crystal structure introduced by doping and pressure. Here we study single crystals of FeSeS x and reveal that local crystal deformations such as atomic-scale defects impact the spectral shape of the electronic core level states of the material. By means of scanning tunneling microscopy we image S-doping induced defects as well as diluted dumbbell defects associated with Fe vacancies. We have access to the electronic structure of the samples by means of x-ray photoemission spectroscopy (XPS) and show that the spectral shape of the Se core levels can only be adequately described by considering a principal plus a minor component of the electronic states. We find this result for both pure and S-doped samples, irrespective that in the latter case the material presents extra crystal defects associated to doping with S atoms. We argue that the second component in our XPS spectra is associated with the ubiquitous dumbbell defects in FeSe that are known to entail a significant modification of the electronic clouds of surrounding atoms.

Bulk and surface electronic structure of MnPSe3 revealed by photoemission and x-ray absorption spectroscopy

Bulk and surface electronic states of ${\mathrm{MnPSe}}_{3}$ have been investigated by means of x-ray photoemission spectroscopy (XPS), x-ray absorption spectroscopy (XAS), and theoretical calculations. In Mn $2p$ XPS, the main peak is accompanied by a charge-transfer satellite which is commonly observed in various Mn chalcogenides. The intensity of the charge-transfer satellite is considerably reduced by oxidization of the surface. The multiplet structure of Mn $2p$ XAS indicates the high spin ${\mathrm{Mn}}^{2+}$ configuration. The cluster model analyses of the XPS and XAS spectra indicate that the Se $4p$-to-Mn $3d$ charge-transfer energy $\mathrm{\ensuremath{\Delta}}$ is negative providing the strong $p\ensuremath{-}d$ hybridization. The magnetic interaction between the third nearest neighboring Mn spins is enhanced by the smallness of $\mathrm{\ensuremath{\Delta}}$ and the unique molecular orbitals of the ${\mathrm{P}}_{2}{\mathrm{Se}}_{6}$ cluster.

Experimental evidence for fractional valence of La in LaAl2 : Electronic structure from x-ray photoelectron spectroscopy and band structure calculations

We confirmed the fractional filling of the $4f$ shell of La in ${\mathrm{LaAl}}_{2}$ by x-ray photoemission spectroscopy (XPS) and ab initio calculations. The La $3d$ core-level XPS spectra were analyzed by use of the Gunnarsson-Sch\"onhammer theoretical model, which allowed us to determine the valency of La smaller than 3 ($v=2.9$). A similar occupation of the La $4f$ shell results from the band structure calculations. In real unstable La atom charge is transferred in the mixing process from its outer $5d6s$ electron shells to the local $4f$ state, according to $4{f}^{\ensuremath{\nu}}(5{d}^{1\ensuremath{-}\ensuremath{\nu}}6{s}^{2})$, where $0<\ensuremath{\nu}\ensuremath{\ll}1$. The calculated density of states fit well the valence band XPS spectra. We have shown that the fractional valence of La plays a key role in electric transport properties of ${\mathrm{LaAl}}_{2}$ and the fractional filling of the $4f$ shell is really at the root of the resistivity anomalies either at the high or low temperatures.

Magnetic phase transition and magneto-elastic coupling in Fe1+xCr2-xSe4 (x = 0.0 - 0.50)

A comprehensive study of the structural and magnetic properties of the series Fe1+xCr2-xSe4 (with x = 0.0, 0.10, 0.25, and 0.50) is carried out by magnetization, x-ray photoemission spectroscopy (XPS), and temperature dependent x-ray diffraction measurements. The parent compound (x = 0.0) undergoes a paramagnetic to antiferromagnetic (AFM) transition at TN ∼ 220 K. As x increases, the XPS spectra indicate that the concentration of Fe3+ increases, which leads to an increase in effective moment and the evolution of ferrimagnetic (FRI) ground state through an intermediate state where both AFM and FRI phases co-exist. The compound with x = 0.50 exhibits a complete FRI ground state (TC ∼ 210 K) and shows a dip in the magnetic susceptibility at a temperature Tdip ∼ 170 K. The magnetocaloric effect curves also exhibit anomalies across these temperatures (TN and Tdip). The magnetic ordering in this series of compounds is accompanied by a distortion in the crystal lattice, as evident from the anisotropic variation of lattice parameters.

Copper-Content Dependent Structural and Electrical Properties of CZTS Films Formed by “Green” Colloidal Nanocrystals

Thin films of colloidal CZTS nanocrystals (NCs) synthesized using a “green” approach in water with a variation of the copper-to-tin ratio are investigated by Raman scattering, mid-infrared (molecular vibrations) and near-infrared (free carrier) absorption, X-ray photoemission spectroscopy (XPS), electrical conductivity, and conductive atomic force microscopy (cAFM). We determined the effect of the actual Cu content on the phonon spectra, electrical conductivity, and spectral parameters of the plasmon band. An increase in the electrical conductivity of the NC films upon annealing at 220 °C is explained by three factors: formation of a CuxS nanophase at the CZTS NC surface, partial removal of ligands, and improved structural perfection. The presence of the CuxS phase is concluded to be the determinant factor for the CZTS NC film conductivity. CuxS can be reliably detected based on the analysis of the modified Auger parameter of copper, derived from XPS data and corroborated by Raman spectroscopy data. Partial removal of the ligand is concluded from the agreement of the core-level XPS and vibrational IR spectra. The degree of lattice perfection can be conveniently assessed from the Raman data as well. Further important information derived from a combination of photoelectron and optical data is the work function, ionization potential, and electron affinity of the NC films.

The influence of relative humidity upon Cu(In,Ga)Se2 thin-film surface chemistry: An X-ray photoelectron spectroscopy study

We performed a comprehensive study of Cu(In,Ga)Se2 (CIGS) surface reactivity in air, focusing on the evolution of the surface chemistry via X-ray photoemission spectroscopy (XPS). By using the different transitions (photopeaks and X-ray Auger electron spectroscopy transitions) available on XPS spectra, in order to probe different surface thicknesses, complementary chemical information is provided to investigate the surface reactivity mechanism of CIGS surface and especially highlight the importance of the relative humidity rate of the atmosphere. Indeed, by maintaining a relative humidity of 20%, we demonstrate that this degradation can be limited, even almost stopped, showing that the degradation mechanism is not only related to O2 interaction but more importantly, to the thin water layer, inherent to relative humidity, formed upon surfaces.

Revisit of XPS Studies of Supersonic O2 Molecular Adsorption on Cu(111): Copper Oxides.

We report the X-ray photoemission spectroscopy (XPS) characterization of the bulk Cu2O(111) surface and “8” and “29” oxide structures on Cu(111) prepared using a 0.5 eV O2 supersonic molecular beam. We propose a new structural model for the “8” oxide structure and also confirm the previously proposed model for the “29” oxide structure on Cu(111), based on the O 1s XPS spectra. The detection angle dependence of the O 1s spectra supports that the nanopyramidal model is more preferable for the (√3 × √3)R30° Cu2O(111). We also report electronic excitations that O 1s electrons suffer.

Electronic properties of DNA-related molecules containing a bromine atom

Purpose To clarify the radiosensitization mechanism masking the Auger effect of the cells possessing brominated DNA, the electronic properties of DNA-related molecules containing Br were investigated by X-ray spectroscopy and specific heat measurement. Materials and Methods X-ray absorption near-edge structure (XANES) and X-ray photoemission spectroscopy (XPS) were used to measure the electronic properties of the nucleotides with and without Br. We determined the specific heat of 5-bromouracil crystals with thymine as a reference molecule at low temperatures of 3–48 K to calculate the microscopic state numbers. Results Obtained XANES and XPS spectra indicated that both the lowest unoccupied molecular orbital (LUMO) and the core-levels were not affected by the Br incorporation. The state numbers of 5-bromouracil calculated from the specific heats obtained around 25 K was about 1.5 times larger than that for thymine below 20 K, although the numbers were almost the same below 5 K. Discussion Our results suggest that the Br atom may not contribute substantially to the LUMO and core-level electronic states of the molecule, but rather to the microscopic states related to the excitation of lattice vibrations, which may be involved in valence electronic states.

Influence of Ag doping on room-temperature TCR of La0.67Sr0.33−xAgxMnO3 polycrystalline ceramics

La0.67Sr0.33−xAgxMnO3 (LSAMO, x = 0.00, 0.15, 0.18, 0.19, 0.20, 0.21, and 0.24) polycrystalline ceramics are prepared by the normative sol–gel method at a sintering temperature of 1450 °C for 12 h. The chemical composition, surface morphology, crystal structure, and electrical transport properties of the specimens are systematically studied by means of X-ray powder diffraction (XRD), field emission scanning electron microscopy (FESEM), energy-dispersive spectrometry (EDS), X-ray photoemission spectroscopy (XPS), and four-probe method (ρ-T). XRD spectra reveal that all ceramics specimens crystallized in rhombohedron perovskite structure belonging to space group of $${\text{R}}\stackrel{\mathrm{-}}{3}{\text{c}}$$ . FESEM displays that the grain size first increased and then decreased with the increase of Ag doping. Large grain size could improve the electrical properties of LSAMO ceramics. The element mapping and EDS results show that specimens contain La, Sr, Ag, Mn, and O elements without any impurity. The presence of Ag element has confirmed by XPS spectra, corresponding to XRD and EDS results. By adjusting the Ag content, a wide metal–insulator transition temperature (TMI) in the range from 369.9 K (x = 0.00) to 299.5 K (x = 0.24) has achieved. At x = 0.19, the resistivity temperature coefficient (TCR) value reaches 16.19% K−1 meanwhile the peak temperature of the TCR is 297.43 K (room temperature). The results indicate that Ag doping has a significant changing on the double exchange (DE) effect and plays an important role in improving the electrical transport properties of LSAMO ceramics. Visibly, the addition of Ag as stoichiometric ratio achieves a large TCR at room temperature, which could provide favorable conditions for application in advanced uncooled infrared detectors.

Preparation of low cost catalysts for proton exchange membrane fuel cell

Nitrogen-doped reduced graphene oxide (NG) with high nitrogen level was synthesized by a facile pyrolysis. NG has been getting attention because of its high catalytic activity toward the oxygen reduction reaction (ORR) and reduce cost. The synthesis of NG used graphene oxide (GO) and urea as a N-precursor were dissolved in ethanol. Then the mixture was evaporated by ultrasonic bath for 30 min. The mixture was slurry then was transferred to tube furnace and pyrolyzed at 300°C and 800°C (NG300 and NG800) with heating rate of 2.5 °C/min in N2 atmosphere for 30 min. The morphology and structure of nitrogen doped graphene oxide were investigated by Scanning Electron Microscopy (SEM) and X-ray photoemission spectroscopy (XPS). The XPS spectra of NG indicated that NG300 had the highest intensity of N1S peak among others. Mass of nitrogen of NG300 and NG800 were evaluated and had about 15.5%wt and 6.6%wt, respectively. Furthermore, N spectra at high-resolution was analysed and de-convoluted to three N chemical states of pyridinic-N, pyrrolic-N, and graphitic-N. The electrochemical properties of NG were determined by cyclic voltammetry (CV) and Linear sweep voltammetry (LSV). From the results shown that NG800 catalyst yielded highest electrochemical activity particularly for oxygen reduction reaction (ORR) over GO and NG300. Thus, N atoms doped into the graphene were responsible for the ORR catalytic activity resulting from doping N atoms and provided more density of active sites and conductivity. Moreover, NG can be applied as a supporting material for Non-precious metal group catalysts of fuel cell.

Optical properties and theoretical study of Mn doped ZnAl2O4 nanoparticles with spinel structure

Zn1-xMnxAl2O4 (x = 0, 0.10, 0.20, 0.30 and 0.40) nanoparticles were synthesized by hydrothermal method, and the effects of Mn doping on the microstructure, morphology, binding energy and optical property of the samples were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HRTEM), energy dispersive X-ray spectroscopy (EDX), x-ray photoemission spectroscopy (XPS), ultraviolet–visible (UV–Vis) spectroscopy, photoluminescence spectra (PL) and fourier transform infrared spectroscopy (FT-IR). And the energy band structure of various defects in ZnAl2O4 structure were calculated by means of the first-principles calculations. The experimental results show that Zn1-xMnxAl2O4 nanoparticles possess cubic spinel structure without other impurity phases. The morphology of the samples exhibits irregular spherical or ellipsoid particles with uniform particle size. EDX Mapping display that Zn, Al, O and Mn elements are uniformly distributed without any enrichment phenomenon throughout the sample. Mn ions successfully used as doping agent replaced Zn2+ and entered ZnAl2O4 matrix. Combined with the calculated results of the first principle calculations, the substitution of Mn introduced new energy levels, which level make the intensity of PL spectra decrease and occur quenching phenomenon. XPS spectra demonstrate that the doped Mn ions mainly occupy the tetrahedral sites. UV–vis spectra indicate with the increase of Mn ion concentration, the band gap of the doped sample decrease with the increase of Mn content and exhibit red shift. FT-IR show that all Mn2+ ions substituted for Zn2+ ions without changing ZnAl2O4 spinel structure.

Optical properties of Cr doped ZnAl2O4 nanoparticles with Spinel structure synthesized by hydrothermal method

Zn1−xCrxAl2O4 (x = 0, 0.01, 0.03, 0.05 and 0.07) nanoparticles were synthesized by hydrothermal method and heat treatment technology, and the effects of Chromium doping ratios on the microstructure, morphology, element distribution, binding energy and optical property of the samples were characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), x-ray energy dispersive spectroscopy (XEDS), x-ray photoemission spectroscopy (XPS), ultraviolet-visible (UV–vis) spectroscopy, photoluminescence spectra (PL) and fourier transform infrared spectroscopy (FT-IR). The experimental results show that Zn1−xCrxAl2O4 nanoparticles possess cubic spinel structure without other impurity phases. The average crystallite size decrease and the lattice parameter increase with the increase of Cr doping concentration. The morphology of the samples exhibits irregular spherical or ellipsoid particles with uniform particle size. XEDS spectra display Cr3+ successfully used as doping agent replaced Zn2+ and entered ZnAl2O4 matrix. XPS spectra demonstrate that the doped Cr ions mainly occupy the tetrahedral sites in Zn0.95Cr0.05Al2O4 sample. UV–vis spectra indicate the band gap of the doped sample decrease and exhibit red shift with the increase of Cr ion concentration. The intensity of FT-IR spectra for Zn1−xCrxAl2O4 samples decreases slowly and occurs red shift. PL spectra show the intensity decrease and occur luminescence quenching phenomenon for Cr doped sample.

Electronic States and Transport Phenomena of Pt Nanoparticle Catalysts Supported on Nb-Doped SnO2 for Polymer Electrolyte Fuel Cells.

Semiconducting oxide nanoparticles are strongly influenced by surface-adsorbed molecules and tend to generate an insulating depletion layer. The interface between a noble metal and a semiconducting oxide constructs a Schottky barrier, interrupting the electron transport. In the case of a Pt catalyst supported on the semiconducting oxide Nb-doped SnO2 with a fused-aggregate network structure (Pt/Nb-SnO2) for polymer electrolyte fuel cells, the electronic conductivity increased abruptly with increasing Pt loading, going from 10-4 to 10-2 S cm-1. The Pt X-ray photoemission spectroscopy (XPS) spectra at low Pt loading amount exhibited higher binding energy than that of pristine Pt metal. The peak shift for the Pt XPS spectra was larger than that of the Pt hard X-ray photoemission spectroscopy (HAXPES) spectra. For all of the spectra, the peaks approached the binding energy of pristine Pt metal with increasing Pt loading. The Sn XPS spectral peak proved the existence of Sn metal with increasing Pt loading, and the peak intensity was larger than that for HAXPES. These spectroscopic results, together with the scanning transmission electron microscopy with energy dispersive X-ray spectroscopy (STEM-EDX) spectra, proved that a PtSn alloy was deposited at the interface between Pt and Nb-SnO2 as a result of the sintering procedure under dilute hydrogen atmosphere. Both Nb spectra indicated that the oxidation state of Nb was +5 and thus that the Nb cation acts as an n-type dopant of SnO2. We conclude that the PtSn alloy at the interface between Pt and Nb-SnO2 relieved the effect of the Schottky barrier, enhanced the carrier donation from Pt to Nb-SnO2, and improved the electronic transport phenomena of Pt/Nb-SnO2.

Role of Ti-Ru interaction in SrTi0.5Ru0.5O3 : Physical properties, x-ray spectroscopy, and cluster model calculations

We have performed a systematic study of the physical properties and electronic structure of ${\mathrm{SrTiO}}_{3}, {\mathrm{SrTi}}_{0.5}{\mathrm{Ru}}_{0.5}{\mathrm{O}}_{3}$, and ${\mathrm{SrRuO}}_{3}$. For the mixed compound, the temperature dependence of the magnetization is consistent with the occurrence of $\mathrm{Ru}Ti$ and $\mathrm{Ti}Ru$ defects. Despite being a semiconductor, the behavior of the electrical resistivity as a function of temperature is compatible with the emergence of small metallic regions richer in Ru concentration, a feature supported by the finite spectral weight at the Fermi level observed in the valence-band x-ray photoemission spectroscopy spectrum. The x-ray photoemission and absorption spectra of the ${\mathrm{SrTi}}_{0.5}{\mathrm{Ru}}_{0.5}{\mathrm{O}}_{3}$ compound were simulated by double cluster model calculations, which include the ${\mathrm{TiO}}_{6}\ensuremath{-}{\mathrm{RuO}}_{6}$ interaction, and also by the linear combination of single cluster calculations for ${\mathrm{SrTiO}}_{3}$ and ${\mathrm{SrRuO}}_{3}$. The results indicate that the interaction between different octahedra may give rise to distinct peak characters, depending on the experimental spectrum being calculated. We argue that these effects are only captured with the explicit inclusion of the Ti-Ru interaction.

Band Alignments, Band Gap, Core Levels, and Valence Band States in Cu3BiS3 for Photovoltaics.

The earth-abundant semiconductor Cu3BiS3 (CBS) exhibits promising photovoltaic properties and is often considered analogous to the solar absorbers copper indium gallium diselenide (CIGS) and copper zinc tin sulfide (CZTS) despite few device reports. The extent to which this is justifiable is explored via a thorough X-ray photoemission spectroscopy (XPS) analysis: spanning core levels, ionization potential, work function, surface contamination, cleaning, band alignment, and valence-band density of states. The XPS analysis overcomes and addresses the shortcomings of prior XPS studies of this material. Temperature-dependent absorption spectra determine a 1.2 eV direct band gap at room temperature; the widely reported 1.4-1.5 eV band gap is attributed to weak transitions from the low density of states of the topmost valence band previously being undetected. Density functional theory HSE06 + SOC calculations determine the band structure, optical transitions, and well-fitted absorption and Raman spectra. Valence band XPS spectra and model calculations find the CBS bonding to be superficially similar to CIGS and CZTS, but the Bi3+ cations (and formally occupied Bi 6s orbital) have fundamental impacts: giving a low ionization potential (4.98 eV), suggesting that the CdS window layer favored for CIGS and CZTS gives detrimental band alignment and should be rejected in favor of a better aligned material in order for CBS devices to progress.

Surface analysis and surface doping of graphene on indium-tin-oxide

Surface doped indium tin oxide (ITO)/graphene systems as a hole transporting transparent front electrode in electro-optical applications are potential alternatives to conventional ITO with wet chemically prepared polymers.In this paper we investigate, firstly, chemical impurities of chemical vapor deposition prepared graphene transferred onto glass/ITO substrates under ambient conditions using x-ray photoemission spectroscopy (XPS) at the oxygen and carbon peak positions. For each of the two elements the intensity contribution of the graphene layer could be extracted from a mathematical simulation of the respective XPS spectra. Impurities from the supporting poly(methyl methacrylate) layer for the transfer were identified. Secondly, we study effects of graphene surface-doping on the work function of such transparent front electrodes using ultraviolet photoemission spectroscopy data. As doping material we have deposited ultra-thin layers of low-cost electron acceptors, iron chloride, vanadium oxide, molybdenum oxide (MoOx) or tetracyano-2,3,5,6-tetrafluoroquinodimethane in order to increase its initial low surface work function of Φ = 4.3 eV up to Φ = 6.3 eV.Finally, we fabricated under vacuum conditions a bulk heterojunction organic photovoltaic device based on a Zn-phtalocyanine:C60 absorber layer thermally deposited on an ITO/graphene front electrode, which has been electronically adjusted to the Zn-phthalocyanine donor (Φ = 5.2 eV) by applying the matching dopant MoOx. The current-voltage characteristics of the device shows improved open circuit voltage and fill factor, hence improved power conversion efficiency compared to that with pristine ITO/graphene.