Core-level Photoemission Spectroscopy Research Articles

A systematic spectroscopic study of the FePc–Si interfaces

A systematic spectroscopic study on the interfaces between the organic molecular semiconductor and the (110) phase of the doped polysilicon has been carried out using valence and core-level photoemission spectroscopy. All photoemission spectra (UPS/XPS) exhibited shifts in the positions of the peaks following the deposition of the organic films. A set of clearly resolved valence band features is found associated to the dispersion of the molecular orbitals. The nonexistence of Fermi edge in the FePc films is thought to account for its semiconducting nature. The thickness dependent change in the work function of the substrate is noticed and a minute amount of charge transfer is found. The asymmetric C1s peak is resolved into three components, reflecting photoemission from multiple sites within the organic molecule. A larger shift in the benzene carbon (Cb) as compared to the pyrrole carbon (Cp) peak suggests that more charge transfers from the former carbon to the substrate. The observed negative sign of the sizable interface dipole potential indicates that the guest molecule and the hosting substrate have donor and acceptor character, respectively. Furthermore, the dipole width, i.e. d=1.35Å, is determined on the basis of the measured quantities and basic physics.

One-Dimensional Corrugation of the h-BN Monolayer on Fe(110)

We report on a new nanopatterned structure represented by a single atomic layer of hexagonal boron nitride (h-BN) forming long periodic waves on the Fe(110) surface. The growth process and the structure of this system are characterized by X-ray absorption (XAS), core-level photoemission spectroscopy (CL PES), low-energy electron microscopy (LEEM), microbeam low-energy electron diffraction (μLEED), and scanning tunneling microscopy (STM). The h-BN monolayer on Fe(110) is periodically corrugated in a wavy fashion with an astonishing degree of long-range order, periodicity of 2.6 nm, and the corrugation amplitude of ∼0.8 Å. The wavy pattern results from a strong chemical bonding between h-BN and Fe in combination with a lattice mismatch in either [111] or [111] direction of the Fe(110) surface. Two primary orientations of h-BN on Fe(110) can be observed corresponding to the possible directions of lattice match between h-BN and Fe(110), with approximately equal area of the boron nitride domains of each orientation.

Electronic charges and electric potential at LaAlO3/SrTiO3interfaces studied by core-level photoemission spectroscopy

We studied LaAlO${}_{3}$/SrTiO${}_{3}$ interfaces for varying LaAlO${}_{3}$ thickness by core-level photoemission spectroscopy. In Ti $2p$ spectra for conducting ``$n$-type'' interfaces, Ti${}^{3+}$ signals appeared, which were absent for insulating ``$p$-type'' interfaces. The intensity of the Ti${}^{3+}$ signals was significantly higher than that expected for the transport carrier densities, indicating that part of the carriers at the interfaces are localized. The Ti${}^{3+}$ signals increased with LaAlO${}_{3}$ thickness, but started well below the critical thickness of 4 unit cells for metallic transport. Core-level shifts with LaAlO${}_{3}$ thickness were much smaller than predicted by the polar catastrophe model. We attribute these observations to surface defects and/or adsorbates providing charges to the interface even below the critical thickness.

Variation of Coverage-Dependent Attachment of Multifunctional Groups in Alanine and Leucine to the Ge(100)-2×1 Surface: Bonding Configuration and Adsorption Stability

The coverage-dependent attachment of multifunctional groups included in alanine and leucine molecules adsorbed to the Ge(100)-2×1 surface was investigated and compared using core-level photoemission spectroscopy (CLPES) and density functional theory (DFT) calculations. The bonding configuration, stability, and adsorption energies were evaluated for two different coverage levels. In both molecules, the core-level spectra at a low coverage indicated that both the carboxyl and amine groups participated in the bonding with the Ge(100) surface by “O–H dissociated and N-dative bonded structure”. This is consistent with the DFT calculation results showing that such adsorption geometry is the most stable and follows the minimum reaction pathway. However, at high coverage level, an additional adsorption geometry of “O–H dissociation bonded structure” appeared possibly to minimize the steric hindrance between adsorbed molecules.

Evidence of Chemical Functionalized Molecules Adsorbed on the Interface Region of Epitaxial Graphene: Interface Roughness and Modification of the Electronic Properties

We investigate the variation of the electronic properties and morphological surface change of epitaxial graphene (EG) by chemically functionalizing the graphene layer with a surface-adsorbed organic free radical, 2,2,6,6-tetramethyl-1-piperridinyloxy (TEMPO) using scanning tunneling microscopy (STM), low energy electron diffraction (LEED), and core-level photoemission spectroscopy (CLPES). STM and LEED images revealed that the adsorption of TEMPO occurred at the interface regions, observed as surface roughness in STM images and haziness in the (6(31/2) × 6(31/2))R30° LEED patterns. TEMPO molecules adsorbed onto EG through the NO radical group, as confirmed by the binding energies of the N 1s and O 1s core-level spectra. Changes in the work function, as measured by secondary electron edge, indicated that the adsorption of NO free radicals onto EG acts as an n-type dopant.

Electro-chemical deposition of zinc oxide nanostructures by using two electrodes

One of the most viable ways to grow nanostructures is electro deposition. However, most electrodeposited samples are obtained by three-electrode electrochemical cell. We successfully use a much simpler two-electrode cell to grow different ZnO nanostructures from common chemical reagents. Concentration, pH of the electrolytes and growth parameters like potentials at the electrodes, are tailored to allow fast growth without complexity. Morphology and surface roughness are investigated by Scanning Electron and Air Force Microscopy (SEM and AFM) respectively, crystal structure by X-Ray Diffraction measurements (XRD) and ZnO stoichiometry by core level photoemission spectroscopy (XPS).

Enhanced Deseleniumization of Selenophene Molecules Adsorbed on Si(100)-2 × 1 Surface

We report the bonding structure of the selenophene molecules adsorbed on the Si(100)-2 × 1 surface at 300 K, and its evolution upon annealing investigated by adopting core-level photoemission spectroscopy, near-edge X-ray absorption fine structure (NEXAFS), and ab initio calculations. The Si 2p, C 1s, Se 3d core-level spectra measured at two temperatures, 300 and 350 K, are consistently interpreted in terms of the two major structures suggested by theory, a twisted (T) 2,5-dihydroselenophene (T-DHS) and a T-deseleniumization where the selenium atom is dissociated from the selenophene ring. We find a significantly enhanced deseleniumization of selenophene molecules by mild thermal annealing indicating that these two equally abundant structures at 300 K become a single uniform phase of the T-deseleniumization structures at 350 K by overcoming a relatively low dissociation energy barrier between the two structures. In addition, we obtain an average tilt angle of a selenophene ring at 300 K from our NEXAFS spectra α∼53 ± 3°, which represents an ensemble average of the tilt angles, α = 15° of the T-DHS and 75° of the T-deseleniumization.

Failure of potassium dimer formation on the β-SiC(100)-c (4 × 2) surface

Using ab initio molecular dynamics, we study the formation of dimers on the β-SiC(100)-c(4 × 2) surface. According to previously published experimental results of Derycke et al. [Appl. Phys. Lett. 88(2), 022105 (2006)] (using scanning tunneling spectroscopy, core-level photoemission spectroscopy, and low-energy electron diffraction) the (100)-c(4 × 2) surface is highly prone to form potassium dimers. In their work, potassium atoms fill adjacent well-defined pedestal sites when there are lower coverages of potassium deposited onto the surface (∼0.16 monolayers). Contrary to the work by Derycke et al., we find that potassium does not prefer to form dimers on the (100)-c(4 × 2) surface, but rather potassium tends to reconstruct (after annealing simulations) by choosing distant pedestal sites in order to minimize the overlap of the ionic radii of the potassium atoms.

단결정 그라핀 위에서의 퓨란의 고리모양 형성

본 연구 그룹은 6H-SiC (0001)에서 성장시킨 그라핀 층에 흡착된 퓨란(furan)의 고리 형성과 전자적 성질을 원자 힘 현미경(Atomic Force Microscope : AFM), C K-edge에 대한 Near Edge X-ray Absorption Fine Structure (NEXAFS) 스펙트럼, 핵심부 준위 광전자 분광스펙트럼(Core-level Photoemission Spectroscopy : CLPES)을 이용하여 연구했다. 우리는 그라핀위에 흡착된 퓨란 분자들이 화학적 도핑이 가능한 산소기의 홀 전자쌍을 통하여 그라핀의 특성을 조절할 수 있는 화학적 기능화에 이용될 수 있다는 것을 알아냈다. 또한, 퓨란이 자발적으로 세 가지의 다른 결합 구조들 중 하나로 고리를 형성한다는 것과 그라핀 위에 퓨란에 의해 형성된 고리의 전자적 성질들이 AFM, NEXAFS, CLPES를 이용하여 각각 설명될 수 있다는 것을 보여주었다. The ring formation and electronic properties of furan adsorbed on graphene layers grown on 6H-SiC (0001) has been investigated using atomic force microscopy (AFM), near edge X-ray absorption fine structure (NEXAFS) spectra for the C K-edge, and core level photoemission spectroscopy (CLPES). Moreover, we observed that furan molecules adsorbed on graphene could be used for chemical functionalization via the lone pair electrons of the oxygen group, allowing chemical doping. We also found that furan spontaneously form rings with one of three different bonding configurations and the electronic properties of the ring formed by furan on graphene can be described using by AFM, NEXAFS and CLPES, respectively.

Chemical Doping of Graphene by Altretamine(2,4,6-Tris [dimethylamino]-1,3,5-Triazine)

The electronic properties of altretamine(2,4,6-tris [dimethylamino]-1,3,5-triazine) adsorbed on epitaxial graphene (EG) were investigated by core-level photoemission spectroscopy (CLPES) in conjunction with low energy electron diffraction (LEED). We found that altretamine molecule adsorbed onto interface layer (S1) of graphene as we confirm decrement of S1 peak using CLPES and haziness of LEED pattern. Moreover, the measured work function changes verified that increased adsorption of the altretamine on graphene layer showed n-type doping characteristics due to charge transfer from altretamine to graphene through the nitrogens. Two distinct nitrogen bonding feature associated with the N 1s peak was clearly observed in the core-level spectra indicating two different chemical environments.

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.

Adsorption Configuration of Serine on Ge(100): Competition between the Hydroxymethyl and Carboxyl Groups of Serine During the Adsorption Reaction

We investigated the adsorption structures of serine on a Ge(100) surface by core-level photoemission spectroscopy (CLPES) in conjunction with density functional theory (DFT) calculations. The adsor...

Adsorption structure and doping effect of azidotrimethyltin on graphene

The adsorption structure and the electronic property of azidotrimethyltin (ATMT) on monolayer graphene were investigated using scanning tunneling microscopy and core-level photoemission spectroscopy. We also confirmed the n-type doping effect by scanning tunneling spectroscopy and work function measurements. We will systematically demonstrate the variation of characteristic of graphene induced by the chemical functionalized molecule as we confirmed the results using scanning tunneling microscopy in conjunction with core-level photoemission spectroscopy.

Nondestructive estimation of depletion layer profile in Nb-doped SrTiO3/(La,Ba)MnO3 heterojunction diode structure by hard x-ray photoemission spectroscopy

We report the hard x-ray (hv=7.94 keV) core level photoemission spectroscopy to determine the depletion layer profile in Nb-doped SrTiO3/(La,Ba)MnO3 n-p+ diode structures. The Sr 2p3/2 spectra were detected penetrating even through (La,Ba)MnO3 over layer with thickness of 20 nm. The spectrum for a 0.01 wt % Nb doped SrTiO3 diode exhibited large shift to lower binding energy, whereas the spectrum for a 0.5 wt % Nb doped SrTiO3 diode exhibited small shift with broader width. The depletion layer lengths were experimentally estimated as 182 nm for 0.01 wt % one and 15 nm for 0.5 wt % one, respectively, in good agreement with semiconductor theory.

Electronic structure of elemental curium studied by photoemission

The electronic structure of curium metal has been investigated by valence-band and $4f$ core-level photoemission spectroscopy using a high-purity $^{248}\mathrm{Cm}$ sample. The experimental spectra are dominated by poorly screened features and indicate full localization of the $5f$ electrons, in strong contrast to the lighter actinides, including Pu and Am. The results obtained are in good agreement with the Cm photoemission obtained by atomic multielectronic calculations for a $5{f}^{7}$ configuration and with the total $5f$ density of states calculated ab initio within a correlated band theory model based on the self-consistent, local-density-matrix approximation to the dynamical mean-field theory.

Multiple phosphorus chemical sites in heavily phosphorus-doped diamond

We have performed high-resolution core level photoemission spectroscopy on a heavily phosphorus (P)-doped diamond film in order to elucidate the chemical sites of doped-phosphorus atoms in diamond. P 2p core level study shows two bulk components, providing spectroscopic evidence for multiple chemical sites of doped-phosphorus atoms. This indicates that only a part of doped-phosphorus atoms contribute to the formation of carriers. From a comparison with band calculations, possible origins for the chemical sites are discussed.

Si 2<i>p</i> Core Level Shifts of the Epitaxial SiON Layer on a SiC(0001), Studied by Photoemissin Spectroscopy

The epitaxial silicon oxynitride (SiON) layer grown on a 6H-SiC(0001) surface is studied with core level photoemission spectroscopy. Si 2p spectra show three spectral components other than the bulk one. Chemical shifts and emission angle dependence of these components are well explained within a framework of a determined structure model of the SiON layer.

Enhanced Chemical Reactivity of Under-Coordinated Atoms at Pt−Rh Bimetallic Surfaces: A Spectroscopic Characterization

The electronic structure and chemical reactivity changes of highly under-coordinated Rh atoms on a Pt(111) surface versus coordination number were studied by a combination of synchrotron radiation core level photoemission spectroscopy and density functional theory. The properties of Rh adatoms are strongly modified by the Pt substrate, when compared with the equivalent atomic configuration in the homometallic environment. A remarkable linear relationship is found between core level shifted spectral components, originating from different atomic geometrical structures, and the corresponding induced d-band center shifts. This finding strongly underscores the relevance of core level shifts as reliable experimental descriptors of chemical reactivity, also for under-coordinated atoms in bimetallic transition metal systems. The availability of an experimental tool capable of detecting local surface alloy configurations with enhanced chemical reactivity opens up the possibility to investigate more complex systems...

Soft X-Ray Spectroscopic Study of Dense Strontium-Doped Lanthanum Manganite Cathodes for Solid Oxide Fuel Cell Applications

The evolution of the Mn charge state, chemical composition, and electronic structure of (LSMO) cathodes during the catalytic activation of solid oxide fuel cell (SOFC) has been studies using X-ray spectroscopy of as-processed, exposed, and activated dense thin LSMO films. Comparison of O -edge and Mn -edge X-ray absorption spectra from the different stages of LSMO cathodes revealed that the largest change after the activation occurred in the Mn charge state with little change in the oxygen environment. Core-level X-ray photoemission spectroscopy and Mn resonant photoemission spectroscopy studies of exposed and as-processed LSMO determined that the SOFC environment ( ambient pressure of ) alone results in La deficiency (severest near the surface with Sr doping ) and a stronger contribution, leading to the increased insulating character of the cathode prior to activation. Meanwhile, O -edge X-ray absorption measurements support enrichment nearer the surface, along with the formation of mixed and/or passive and SrO species.

Soft x-ray absorption and photoemission spectroscopy study of superoxideKO2

The electronic structure of superoxide ${\text{KO}}_{2}$ was investigated by employing soft x-ray absorption spectroscopy (XAS) and core-level photoemission spectroscopy (PES). The clean and single component was observed in both the $\text{O}\text{ }1s$ and $\text{K}\text{ }2p$ core-level PES spectra of ${\text{KO}}_{2}$, which confirms the good quality of the ${\text{KO}}_{2}$ sample employed in this work. The measured $\text{O}\text{ }1s$ XAS spectrum was compared with the calculated density of states (DOSs), obtained from the first-principles electronic-structure calculations that include both the spin-orbit (SO) effect and the on-site Coulomb interaction $(U)$ between $\text{O}\text{ }2p$ electrons. In the $\text{O}\text{ }1s$ XAS, both the unoccupied $\text{O}\text{ }2p$ antibonding ${\ensuremath{\pi}}_{g}$ and ${\ensuremath{\sigma}}_{u}$ states are identified. It is found that the unoccupied states, observed in the $\text{O}\text{ }1s$ XAS, agree reasonably well with the calculated DOSs, which suggests the importance of the SO and $U$ effects for ${\text{KO}}_{2}$ having the high-symmetry tetragonal structure.