Core Electronic Structure Research Articles

Optimization and characterization of III–V surface cleaning

Achieving clean surfaces is a major and challenging requirement for the study of surfaces and surface reactions. Nondestructive cleaning is a crucial step in semiconductor manufacturing, growth of materials, and processing. We use photoemission spectroscopy (PES) to systematically study the core and valence band electronic structure of various chemical treatments of InP(100), GaAs(100), and GaN(0001). These surfaces undergo wet chemical cleaning of H2SO2/H2O2/H2O followed by thermal heating. In order to achieve the necessary surface sensitivity and spectral resolution, synchrotron radiation in the energy range of 60–1000 eV is used for PES. In tuning the sulfuric acid based chemistry, we achieve oxygen free GaAs and InP surfaces, as shown in our valence band and core level PES analysis. Furthermore, core level PES shows oxygen coverage of the GaN surface is reduced to less than 0.1 monolayer (ML). The carbon coverage is also reduced dramatically for the III–V surfaces, <10% ML for InP and GaAs and approximately 1% ML for GaN. The chemical reactions and species at different cleaning stages are determined and cleaning mechanisms are proposed. Our study shows that material similarities do not imply exact correlation to the chemical cleaning properties among III–V materials.

Spectroscopic and computational studies on [(PhTt(tBu))2Ni2(mu-O)2]: nature of the bis-mu-oxo (Ni3+)2 diamond core.

Spectroscopic and density functional theory (DFT) electronic structure computational studies on a binuclear bis-mu-oxo bridged (Ni(3+))(2) complex, [(PhTt(t(Bu))(2)Ni(2)(mu-O)(2)] (1), where (PhTt(t(Bu)) represents phenyl-tris((tert-butylthio)methyl)borate, are presented and discussed. These studies afford a detailed description of the Ni(2)O(2) core electronic structure in bis-mu-oxo (Ni(3+))(2) dimers and provide insight into the possible role of the (PhTt(t(Bu)) thioether ligand in the formation of 1 from a Ni(1+) precursor by O(2) activation. From a normal coordinate analysis of resonance Raman data, a value of k(Ni)(-)(O) = 2.64 mdyn/A is obtained for the Ni-O stretch force constant for 1. This value is smaller than k(Cu)(-)(O) = 2.82-2.90 mdyn/A obtained for bis-mu-oxo (Cu(3+))(2) dimers possessing nitrogen donor ligands, indicating a reduced metal-oxo bond strength in 1. Electronic absorption and magnetic circular dichroism spectroscopic techniques permit identification of several O-->Ni and S-->Ni charge transfer (CT) transitions that are assigned on the basis of DFT calculations. The dominant O-->Ni CT transition of 1 occurs at 17 700 cm(-)(1), red-shifted by approximately 7000 cm(-)(1) relative to the corresponding transition in bis-mu-oxo (Ni(3+))(2) dimers with nitrogen donor ligands. This red-shift along with the relatively low value of k(Ni)(-)(O) are due primarily to the presence of the thioether ligands in 1 that greatly affect the compositions of the Ni(2)O(2) core MOs. This unique property of the thioether ligand likely contributes to the reactivity of the Ni center in the precursor [(PhTt(t(Bu))Ni(1+)CO] toward O(2). DFT computations reveal that conversion of a hypothetical side-on peroxo (Ni(2+))(2) dimer, [(PhTt(t(Bu))(2)Ni(2)(mu-eta(2):eta(2)-O(2))], to the bis-mu-oxo (Ni(3+))(2) dimer 1 is energetically favorable by 32 kcal/mol and occurs without a significant activation energy barrier (DeltaH++) = 2 kcal/mol).

Role of the Coordinating Histidine in Altering the Mixed Valency of Cu A: An Electron Nuclear Double Resonance-Electron Paramagnetic Resonance Investigation

The binuclear Cu A site engineered into Pseudomonas aeruginosa azurin has provided a Cu A-azurin with a well-defined crystal structure and a CuSSCu core having two equatorial histidine ligands, His120 and His46. The mutations His120Asn and His120Gly were made at the equatorial His120 ligand to understand the histidine-related modulation to Cu A, notably to the valence delocalization over the CuSSCu core. For these His120 mutants Q-band electron nuclear double resonance (ENDOR) and multifrequency electron paramagnetic resonance (EPR) (X, C, and S-band), all carried out under comparable cryogenic conditions, have provided markedly different electronic measures of the mutation-induced change. Q-band ENDOR of cysteine C β protons, of weakly dipolar-coupled protons, and of the remaining His46 nitrogen ligand provided hyperfine couplings that were like those of other binuclear mixed-valence Cu A systems and were essentially unperturbed by the mutation at His120. The ENDOR findings imply that the Cu A core electronic structure remains unchanged by the His120 mutation. On the other hand, multifrequency EPR indicated that the H120N and H120G mutations had changed the EPR hyperfine signature from a 7-line to a 4-line pattern, consistent with trapped-valence, Type 1 mononuclear copper. The multifrequency EPR data imply that the electron spin had become localized on one copper by the His120 mutation. To reconcile the EPR and ENDOR findings for the His120 mutants requires that either: if valence localization to one copper has occurred, the spin density on the cysteine sulfurs and the remaining histidine (His46) must remain as it was for a delocalized binuclear Cu A center, or if valence delocalization persists, the hyperfine coupling for one copper must markedly diminish while the overall spin distribution on the CuSSCu core is preserved.

Surface electronic structure of GaAs(311)A studied by angle-resolved photoelectron spectroscopy

The valence and core electronic structure of the sputtered and annealed 1 × 1 periodic GaAs(311)A surface has been studied by angle-resolved photoelectron spectroscopy. Five surface bands are identified and their dispersions along high symmetry lines in the surface Brillouin zone are mapped out. While electron-counting indicates a metallic surface, the experiment show no evidence for partly occupied surface bands. Analysis of the core level spectra reveals one As and two Ga surface shifted components. The results are discussed in terms of surface geometry models.

Core hole screening and electronic structure information

Understanding the nature of core hole screening is fundamental for interpretation of the results of electron spectroscopies of metals and alloys. The idea of complete screening of the core hole explains a large amount of XPS and high resolution Auger (XAES) data. Here, we address the question of the extent to which the complete screening model can be used to extract quantitative information about the electronic structure of metals and their alloys from measured Auger energies. Starting from general considerations, analysis of Auger parameter data illuminates the screening process throughout the 4 d and 5 d series, including the consequences of closing the d valence shell; the observed systematics are faithfully reproduced, quantitatively as well as qualitatively, in the 3 d series as well; and parameters of a simple model consistent with complete screening which are extracted from the Auger parameter data are applicable to analysis of XPS and Auger shifts in alloys. Similar treatment of core level Auger shift data of the noble metal component in bimetallic alloys indicates the relation between the measured shifts, the relative Fermi energy (a usually unknown quantity involved in binding energy shifts) and the charge transfer. Consideration of various alloys (AuPd, CuPd, AgPd, AuCu, AuAg, and CuPt) implies a very small charge transfer and a unique determination of the relative Fermi energy. Use of noble metals as “impurities” in trimetallic alloys bears out these conclusions regarding charge transfer and Fermi energies.

Core expansion and electronic structure of the porphyrin in the neutral pH form of copper cytochrome c

Core expansion and electronic structure of the porphyrin in the neutral pH form of copper cytochrome c

Chemical bonding at the polyimide surface

Surface spectroscopy techniques (UPS and XPS) have been combined with theoretical calculations to investigate the chemical bonding characteristics of the high-temperature polymer polyimide. Both valence and core electronic structure of the intrinsic (‘‘clean’’) polyimide surface as observed in UPS and XPS are well described by the results of the self-consistent-field molecular orbital calculations. Even though the polyimide is already fully cured to complete the imidization reaction and form the final molecular structure of the polymer chains, the valence electronic structure of such as-prepared surfaces changes significantly upon in situ annealing at temperatures above ∼195 °C but well below those which decompose the polymer (≥400 °C). This annealing causes desorption of gas phase H2O and produces the intrinsic polyimide surface. The sorption/desorption of H2O is reversible and thermally activated, and it seems to play an important role in the chemical properties of the polyimide surface. These changes upon annealing and H2O exposure appear primarily in the valence electronic structure as observed by UPS, but corresponding alterations of the core level spectra seen by XPS are considerably smaller.

Core electron ionization energies and electronic structure of Ti(NO 3) 4 and Cu(NO 3) 2

Gas phase X-ray photoelectron spectra of Ti(NO 3) 4 and Cu(NO 3) 2 are reported and discussed in terms of the molecular charge distributions. No measurable splitting is observed between the 1 s ionization energies of the chemically distinct oxygen atoms in either molecule. Ab initio calculations for Cu(NO 3) 2 suggest that this is due in large measure to differential orbital relaxation occurring upon core electron ionization.