Chemical and electronic structure of the heavily intermixed (Cd,Zn)S:Ga/CuSbS2 interface.

The interface formation and chemical and electronic structure of the (Cd,Zn)S:Ga/CuSbS2 thin-film solar cell heterojunction were studied using hard X-ray photoelectron spectroscopy (HAXPES) of the bare absorber and a buffer/absorber sample set for which the buffer thickness was varied between 1 and 50 nm. We find a heavily intermixed interface, involving Cu, Zn, and Cd as well as significant Ga and Cu profiles in the buffer. The valence band (VB) offset at the buffer/absorber interface was derived as (-1.3 ± 0.1) eV, which must be considered an upper bound as the Cu diffused into the buffer might form a Cu-derived VB maximum located closer to the Fermi level. The estimated conduction band minimum was 'cliff'-like; a situation made more severe considering the Cu-deficiency found for the CuSbS2 surface. The complex interface structure's effect on the performance of (Cd,Zn)S:Ga/CuSbS2-based solar cells and its limitation is discussed together with possible mitigation strategies.

Local electronic structure of the peptide bond probed by resonant inelastic soft X-ray scattering.

The local valence orbital structure of solid glycine, diglycine, and triglycine is studied using soft X-ray emission spectroscopy (XES), resonant inelastic soft X-ray scattering (RIXS) maps, and spectra calculations based on density-functional theory. Using a building block approach, the contributions of the different functional groups of the peptides are separated. Cuts through the RIXS maps furthermore allow monitoring selective excitations of the amino and peptide functional units, leading to a modification of the currently established assignment of spectral contributions. The results thus paint a new-and-improved picture of the peptide bond, enhance the understanding of larger molecules with peptide bonds, and simplify the investigation of such molecules in aqueous environment.

Site-specific electronic structure of imidazole and imidazolium in aqueous solutions.

The occupied and unoccupied electronic structure of imidazole (C3N2H4) and imidazolium (C3N2H5+) in aqueous solutions is studied by X-ray emission spectroscopy (XES) and resonant inelastic soft X-ray scattering (RIXS). Both systems show distinct RIXS fingerprints with strong resonant effects. A comparison with calculated X-ray emission spectra of isolated imidazole and imidazolium suggests only a small influence of hydrogen bonding in the aqueous solution on the electronic structure of imidazole and imidazolium, and allows the attribution of specific spectral features to the non-equivalent nitrogen and carbon atoms in the molecules. In the case of nitrogen, this can also be achieved by site-selective resonant excitation. Furthermore, we find spectator shifts and symmetry selectivity in the RIXS spectra, as well as indications for rapid proton dynamics on the femtosecond timescale of the RIXS process, and derive the HOMO-LUMO gaps for the two molecules in aqueous solution.

X-ray Emission Spectroscopy of Proteinogenic Amino Acids at All Relevant Absorption Edges.

Nonresonant N K, O K, C K, and S L2,3 X-ray emission spectra of the 20 most common proteinogenic amino acids in their solid zwitterionic form are reported. They represent a comprehensive database that can serve as a reliable basis for the X-ray absorption spectroscopy (XES) studies of peptides and proteins. At the most important N and O K edges, clear similarities and differences between the spectra of certain amino acids are observed and associated with the specific chemical structure of these molecules and their functional groups. Analysis of these spectra allows the generation of spectral fingerprints of the protonated amino group, the deprotonated carboxylic group, and, using a building block approach, the specific nitrogen- and oxygen-containing functional groups in the side chains of the amino acids. Some of these fingerprints are compared to the spectra of reference compounds with the respective functional groups; they exhibit reasonable similarity, underlining the validity of the spectral fingerprint approach. The C K and S L2,3 XES spectra are found to be specific for each amino acid, in accordance with the different local environments of the involved C and S atoms, respectively.

Isotope Effects in the Resonant Inelastic Soft X-ray Scattering Maps of Gas-Phase Methanol.

The electronic structure of gas-phase methanol molecules (H3COH, H3COD, and D3COD) at atmospheric pressure was investigated using resonant inelastic soft X-ray scattering (RIXS) at the O K and C K edges. We observe strong changes of the relative emission intensities of all valence orbitals as a function of excitation energy, which can be related to the symmetries of the involved orbitals causing an angularly anisotropic RIXS intensity. Furthermore, all observed emission lines are subject to strong spectator shifts of up to -0.9 eV at the O K edge and up to -0.3 eV at the C K edge. At the lowest O K resonance, we find clear evidence for dissociation of the methanol molecule on the time scale of the RIXS process, which is illustrated by comparing X-ray emission spectra of regular and deuterated methanol.

The complex interface chemistry of thin-film silicon/zinc oxide solar cell structures.

The interface between solid-phase crystallized phosphorous-doped polycrystalline silicon (poly-Si(n(+))) and aluminum-doped zinc oxide (ZnO:Al) was investigated using spatially resolved photoelectron emission microscopy. We find the accumulation of aluminum in the proximity of the interface. Based on a detailed photoemission line analysis, we also suggest the formation of an interface species. Silicon suboxide and/or dehydrated hemimorphite have been identified as likely candidates. For each scenario a detailed chemical reaction pathway is suggested. The chemical instability of the poly-Si(n(+))/ZnO:Al interface is explained by the fact that SiO2 is more stable than ZnO and/or that H2 is released from the initially deposited a-Si:H during the crystallization process. As a result, Zn (a deep acceptor in silicon) is "liberated" close to the silicon/zinc oxide interface presenting the inherent risk of forming deep defects in the silicon absorber. These could act as recombination centers and thus limit the performance of silicon/zinc oxide based solar cells. Based on this insight some recommendations with respect to solar cell design, material selection, and process parameters are given for further knowledge-based thin-film silicon device optimization.

"Building block picture" of the electronic structure of aqueous cysteine derived from resonant inelastic soft X-ray scattering.

The electronic structure of the amino acid L-cysteine in an aqueous environment was studied using resonant inelastic soft X-ray scattering (RIXS) in a 2D map representation and analyzed in the framework of a "building block" approach. The element selectivity of RIXS allows a local investigation of the electronic structure of the three functional groups of cysteine, namely, the carboxyl, amino, and thiol groups, by measuring at the O K, N K, and S L2,3 edges, respectively. Variation of the pH value allows an investigation of molecules with protonated and deprotonated functional groups, which can then be compared with simple reference molecules that represent the isolated functional groups. We find that such building blocks can provide an excellent description of X-ray emission spectroscopy (XES) and RIXS spectra, but only if all nearest-neighbor atoms are included. This finding is analogous to the building block principle commonly used in X-ray absorption spectroscopy. The building blocks show a distinct spectral character (fingerprint) and allow a comprehensive interpretation of the cysteine spectra. This simple approach opens the path to investigate the electronic structure of more complex biological molecules in aqueous solutions using XES and RIXS.

Setup for in situ investigation of gases and gas/solid interfaces by soft x-ray emission and absorption spectroscopy.

We present a novel gas cell designed to study the electronic structure of gases and gas/solid interfaces using soft x-ray emission and absorption spectroscopies. In this cell, the sample gas is separated from the vacuum of the analysis chamber by a thin window membrane, allowing in situ measurements under atmospheric pressure. The temperature of the gas can be regulated from room temperature up to approximately 600 °C. To avoid beam damage, a constant mass flow can be maintained to continuously refresh the gaseous sample. Furthermore, the gas cell provides space for solid-state samples, allowing to study the gas/solid interface for surface catalytic reactions at elevated temperatures. To demonstrate the capabilities of the cell, we have investigated a TiO2 sample behind a mixture of N2 and He gas at atmospheric pressure.

Non-equivalent carbon atoms in the resonant inelastic soft X-ray scattering map of cysteine.

Resonant inelastic soft x-ray scattering (RIXS) was used to study the electronic structure of solid cysteine films. A RIXS map approach, i.e., plotting the x-ray emission intensity as a function of excitation and emission energy, allows us to separate the contributions of the three chemically non-equivalent carbon atoms in cysteine. In particular, we can identify orbitals localized near the photoexcited atoms, as well as orbitals that are delocalized over the entire molecule.

Comparative theoretical and experimental study of the radiation-induced decomposition of glycine.

The radiation-induced decomposition of glycine is studied using a combination of near-edge X-ray absorption fine structure (NEXAFS) measurements and DFT calculations. The measured spectra show strong dose- or time-dependent effects consistent with a complex, multistep decomposition. Principal component analysis was used to determine the number of distinct molecules that were needed to explain the observed changes in the measured spectra, and the emerging absorption features are assigned to various product molecules through comparison with simulated spectra of several model compounds. It is clear from the experiment that the major effect of soft X-ray irradiation is the fragmentation of the molecule, primarily at the carbonyl sites. Peptide formation is shown to occur under irradiation; a condensation reaction initiated by the removal of a carbonyl oxygen is the proposed mechanism. This study utilizes a novel approach to the study of radiation damage that can occur during measurements and suggests that it may be possible to use simulated model spectra to correct for these effects in measured spectra.

Substituent effects in the iron 2p and carbon 1s edge near-edge X-ray absorption fine structure (NEXAFS) spectroscopy of ferrocene compounds.

The iron 2p and carbon 1s near-edge X-ray absorption fine structure (NEXAFS) spectra of substituted ferrocene compounds (Fe(Cp-(CH3)5)2, Fe(Cp)(Cp-COOH), Fe(Cp-COOH)2, and Fe(Cp-COCH3)2) are reported and are interpreted with the aid of extended Hückel molecular orbital (EHMO) theory and density functional theory (DFT). Significant substituent effects are observed in both the Fe 2p and C 1s NEXAFS spectra. These effects can be related to the electron donating/withdrawing properties of the cyclopentadienyl ligands and their substituents as well as the presence of pi* conjugation between the cyclopentadienyl ligand and unsaturated substituents.

Experimental and theoretical investigation of the electronic structure of 5-fluorouracil compounds.

We present a comparison between experimental and theoretical X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES) of 5-fluorouracil compounds, with an emphasis on the effects of the inclusion of nickel in the structure. By focusing on the 1s thresholds of carbon, nitrogen, oxygen, and fluorine it was possible to provide a complete picture of the occupied and unoccupied partial density of states of the 5-fluorouracil systems. Spectra calculated using density functional theory are compared to experimental results. Most experimental results agree well with our theoretical calculations for the XAS and XES of the compounds. All spectral features are assigned. Our results reveal that the nickel in the compound is coordinated with the nitrogen sites of the 5-fluorouracil ligands.

Combined X-ray absorption spectroscopy and density functional theory examination of ferrocene-labeled peptides.

A combination of soft X-ray absorption spectroscopy (XAS) measurements and StoBe density functional theory (DFT) calculations has been used to study the electronic structures of the ferrocene-labeled peptides Fc-Pro(n)-OBz (n = 1-4). Excellent agreement between the measured and the simulated data is observed in all cases, and the origin of all major spectral features was assigned. The breaking of the degeneracy of the ferrocene 3e(2u)-like unoccupied molecular orbital under the influence of a substituent attached to a Cp ring was observed experimentally. The influence of the bonding environment on the O 1s and N 1s XAS spectra was examined. A corrected assignment of one of the major features in the Fe 2p XAS spectra of ferrocene is proposed and supported by the DFT simulations, as well as the measured spectra.

An x-ray emission and density functional theory study of the electronic structure of Zn(1-x)Mn(x)S.

Mn 3d electronic states in the dilute magnetic semiconductor Zn(1-x)Mn(x)S (x = 0.1-0.3) are studied using soft x-ray emission (XES) measurements and density functional theory (DFT). Mn L(2,3) emission spectra of Zn(1-x)Mn(x)S (x = 0.1-0.3) suggest that the Mn impurities do not form clusters in the host ZnS lattice, in agreement with previous models. A shift in the position of a Mn L(3) XES feature suggests a change in the nature of the hybridization between the Mn 3d(3/2) and S 3p states as a function of x. Our DFT calculations reproduce the weak interatomic exchange interaction, as well as the strong intra-atomic exchange splitting that is expected from observations of Zeeman splitting in such materials.