Core-level Photoemission Spectroscopy Spectra Research Articles

Inter-row Adsorption Configuration and Stability of Threonine Adsorbed on the Ge(100) Surfaces

E-mail: easyscan@sookmyung.ac.krReceived December 7, 2012, Accepted January 8, 2013The adsorption structures of threonine on the Ge(100) surface were investigated using core-level photoemissionspectroscopy (CLPES) in conjunction with density functional theory (DFT) calculations. CLPES measurementswere performed to identify the experimentally preferred adsorption structure. The preferred structure indicatedthe relative reactivities of the carboxyl and hydroxymethyl groups as electron donors to the Ge(100) surfaceduring adsorption. The core-level C 1s, N 1s, and O 1s CLPES spectra indicated that the carboxyl oxygencompeted more strongly with the hydroxymethyl oxygen during the adsorption reaction. Three among sixpossible adsorption structures were identified as energetically favorable using DFT calculation methods thatconsidered the inter- and intra-bonding configurations upon adsorption onto the Ge(100) surface. Thesestructures were O-H dissociated N dative inter bonding, O-H dissociated N dative intra bonding, O-H dissociationbonding. One of the adsorption structures: O-H dissociated N dative inter bonding was predicted to be stablein light of the transition state energies. We thus confirmed that the most favorable adsorption structure is theO-H dissociated N dative-inter bonding structure using CLPES and DFT calculation.Key Words : Threonine, Ge(100), Inter-row adsorption configuration, DFT calculation, CLPESIntroductionStudies of the reactions between organic molecules andsemiconductor surfaces have become increasingly importantover the past few years. Technologies that rely on interactionsbetween organic materials and semiconductor technologiesare being developed for molecular electronics, biosensor,and nanotechnology applications.

Coverage‐Dependent Variation of Adsorption Configurations of Methionine on Ge(100)

The adsorption configurations of methionine molecules on the Ge(100) surface have been studied by using DFT calculations, core-level photoemission spectroscopy (CLPES), and low-energy electron diffraction (LEED) to scrutinize the adsorption structure as a function of coverage. At first, we obtained two important and stable structures. One is the most stable structure between these structures described as an "O-H dissociated-N dative-S dative-bonded structure" and the other is a less stable adsorption structure of these indicating an "O-H dissociated-S dative-bonded structure" by using DFT calculations. We also performed CLPES to clarify our DFT calculation results. Through the spectral analysis of the S 2p, C 1s, N 1s, and O 1s core-level spectra, we acquired the reasonable results that also revealed quite different bonding configurations depending on the methionine coverage. At low coverage (ca. 0.30 ML), a single type of sulfur and charged nitrogen peaks, which indicate an "O-H dissociated-N dative-S dative-bonded structure", were observed. On the other hand, two types of sulfur peaks with thiol formation and two nitrogen peaks with neutralized and charged characteristics were monitored at a higher coverage (0.60 ML and above), which can be described as an "O-H dissociated-S dative-bonded structure". Hence, we can clearly demonstrate that our results obtained from CLPES spectra and DFT calculations are matched well with each other. Moreover, we additionally confirmed that the relative population of the two types of thiols and amines being included in methionine in between half monolayer induces a surface reorientation in the ordering from 2×1 to 1×1 employing LEED. This interesting variation of the methionine adsorbed on the Ge(100) surface by coverage dependence will be precisely discussed by using DFT calculations, CLPES, and LEED.

Analysis of x‐ray irradiation effect in high‐k gate dielectrics by time‐dependent photoemission spectroscopy using synchrotron radiation

AbstractWe have developed a method to precisely determine the band offsets for hafnium oxide (HfO2), hafnium silicate (HfSiO) and nitrided hafnium silicate (HfSiON) films on Si by elimination of x‐ray irradiation effects with time‐dependent photoemission spectroscopy (PES) and x‐ray absorption spectroscopy. The core‐level PES spectra shift during PES measurements, which is explained as the dielectric films charging due to photoemission by the x‐ray irradiation. For elimination of the x‐ray irradiation effect on the determination of the band offsets, time‐dependent photoemission spectroscopy was performed. The precisely determined valence band offsets for HfO2, HfSiO and HfSiON are 3.2, 3.4, and 1.9 eV, respectively. On the other hand, the conduction band offsets are almost the same values of about 1.3 eV. Thus, the elimination of x‐ray irradiation effect enables to precisely determine the band offsets, leading to the accurate estimation of gate leakage current. Copyright © 2008 John Wiley & Sons, Ltd.

High-resolution soft X-ray photoemission spectroscopy of spinel-type compound CuIr2S4

Abstract The electronic structure of spinel-type chalcogenide CuIr 2 S 4 , which exhibits a metal–insulator transition, has been studied by photoemission spectroscopy (PES). The valence-band PES spectra indicate a gap formation below the transition temperature T MI . The line shape of the Ir 4f core-level PES spectra shows a dramatic change across T MI , which may be associated with the variation of the Ir 5d electron states.

Step Edges as Reservoirs of Ag Adatom Gas on a Si(111) Surface

The Si(111)√3×√3-Ag surface, prepared under various annealing conditions, was systematically investigated by scanning tunneling microscopy (STM) and reflection-high-energy electron diffraction (RHEED). Although the RHEED patterns were seemingly the same for all cases, the STM images showed a clear difference in surface morphology; step edges were roundish and decorated by random adatom adsorption when annealed below 600°C, while steps were straight with periodic protrusions along them by higher-temperature and prolonged annealing. Such distinctions may be the origin for different results in core-level photoemission spectroscopy (CLPES); the step edges may act as a reservoir of on-terrace adatom gas of Ag that makes the CLPES spectra broaden for samples with insufficient annealing, while the sufficient annealing removes the reservoir and adatom gas, resulting in very sharp CLPES spectra.

Two-Dimensional Surface Adatom of Gas Phase and Core-Level Photoemission Spectroscopy

Si(111)√3×√3-Ag surfaces, prepared under various annealing conditions, was systematically investigated by scanning tunneling microscopy (STM) and reflection-high-energy electron diffraction (RHEED). Although the RHEED patterns were seemingly the same for all cases, the STM images showed clear difference in surface morphology and local structures at steps; step edges were roundish and decorated by random adsorption of clusters when annealed below 600oC. However, the steps were straight with periodic protrusions along them by higher-temperature and prolonged annealing, due to a formation of very narrow domains of 6×1-Ag structure at step edges. Such distinctions may be the origin for different results in core-level photoemission spectroscopy (CLPES). Since it is known that two-dimensional adatom gas (2 DAG) phase of excess Ag atoms exits on terraces, which changes the band bending in the substrate, the step edges (and clusters adsorbed there) may play as reservoirs of the 2 DAG. For the samples without sufficient annealing, the concentration of 2 DAG and the resulting band bending are inhomogenous over the sample surfaces, which make the CLPES spectra broaden. Sufficient annealing removes the reservoirs and 2 DAG, resulting in a homogenous band bending over the surfaces to get very sharp CLPES spectra.

Adsorption and photon-stimulated desorption of CCl4 on an Al(111) surface investigated with synchrotron radiation

Adsorption and desorption of CCl4 molecules on an Al(111) surface at 90 K are characterized with photoemission spectroscopy (PES) and photon-stimulated ion desorption (PSID) techniques following valence-level and core-level excitations. Results of valence-level and Cl(2p) core-level PES spectra indicate that CCl4 dissociates partially upon adsorption on an Al(111) surface at submonolayer coverage and that molecular CCl4 adsorbs to form multilayers at large exposures. The dissociation upon adsorption of CCl4 on an Al surface at 90 K is likely mediated by the charge-transfer process. The Cl+ desorption threshold at ∼18.5 eV in valence-level PSID spectra may originate from the 5t2→7a1* (C–Cl antibonding orbital) transition consistent with the Menzel–Gomer–Redhead (MGR) mechanism. The total-electron yield (TEY) spectrum and the Cl+ PSID spectrum of solid CCl4 following the Cl L-edge excitation are clearly dissimilar. The enhanced desorption yield of Cl+ ions is detected at the Cl 2p→7a1* excitation, compared to the Cl 2p→8t2* and Cl 2p→Rydberg state excitations. Cl(2p) core-level excitations yield much greater desorption of ions compared with direct valence-band excitation. Based on resonant photoemission spectra, core resonant excitations decay predominantly via spectator Auger transitions, whereas shape resonance excitation is followed by normal Auger decay. Enhanced Cl+ ion desorption from solid CCl4 following Cl 2p→7a1* excitation is interpreted in terms of the rapid desorption via a repulsive surface which is directly related to spectator electrons localized in antibonding orbitals.