Synchrotron-based High-resolution Photoemission Research Articles

Synchrotron-based high-resolution photoemission spectroscopy study of ZIRLO cladding with H2O adsorption: Coverage and temperature dependence

The coverage and temperature dependence of ZIRLO cladding with H2O adsorption are studied using synchrotron-based high-resolution photoemission spectroscopy (HRPES). Based on the analytical results of the Zr 3d, O 1 s, C 1 s, and Sn 3d HRPES profiles prior to H2O adsorption, we determine the surface compositions of O2−, hydroxyl OH−, chemisorbed H2O, zirconium carbide, adventitious carbon, Sn metal, and SnO2 in ZIRLO. When ZIRLO is exposed to H2O molecules, the relative proportion of zirconium metal decreases, whereas that of the total zirconium oxides increases, suggesting the reaction between H2O and the zirconium metal in ZIRLO. On annealing a sample with 1000 L H2O on ZIRLO at 300 °C, Zr2O3 and ZrO2 decompose, and oxygen diffuses into the bulk, thereby reducing the oxidation states of zirconium on the surface. Moreover, at this temperature, the excess H2O molecules on ZIRLO are thoroughly desorbed and tin element is diffused into the bulk in ZIRLO.

Enhanced surface transfer doping of diamond by V2O5 with improved thermal stability

Surface transfer doping of hydrogen-terminated diamond has been achieved utilising V2O5 as a surface electron accepting material. Contact between the oxide and diamond surface promotes the transfer of electrons from the diamond into the V2O5 as revealed by the synchrotron-based high resolution photoemission spectroscopy. Electrical characterization by Hall measurement performed before and after V2O5 deposition shows an increase in hole carrier concentration in the diamond from 3.0 × 1012 to 1.8 × 1013 cm−2 at room temperature. High temperature Hall measurements performed up to 300 °C in atmosphere reveal greatly enhanced thermal stability of the hole channel produced using V2O5 in comparison with an air-induced surface conduction channel. Transfer doping of hydrogen-terminated diamond using high electron affinity oxides such as V2O5 is a promising approach for achieving thermally stable, high performance diamond based devices in comparison with air-induced surface transfer doping.

Surface transfer doping of diamond by MoO3: A combined spectroscopic and Hall measurement study

Surface transfer doping of diamond has been demonstrated using MoO3 as a surface electron acceptor material. Synchrotron-based high resolution photoemission spectroscopy reveals that electrons are transferred from the diamond surface to MoO3, leading to the formation of a sub-surface quasi 2-dimensional hole gas within the diamond. Ex-situ electrical characterization demonstrated an increase in hole carrier concentration from 1.00 × 1013/cm2 for the air-exposed hydrogen-terminated diamond surface to 2.16 × 1013/cm2 following MoO3 deposition. This demonstrates the potential to improve the stability and performance of hydrogen-terminated diamond electronic devices through the incorporation of high electron affinity transition metal oxides.

Structural and Electronic Properties of PTCDA Thin Films on Epitaxial Graphene

In situ low-temperature scanning tunneling microscopy is used to study the growth of 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) on epitaxial graphene (EG) on 6H-SiC(0001), as well as on HOPG for comparison. PTCDA adopts a layer-by-layer growth mode, with its molecular plane lying flat on both surfaces. The PTCDA films grow continuously over the EG step edges, but not on HOPG. STS performed on single-layer PTCDA on monolayer EG shows a wide band gap larger than 3.3 eV, consistent with pristine PTCDA films. Synchrotron-based high-resolution photoemission spectroscopy reveals weak charge transfer between PTCDA and EG. This suggests weak electronic coupling between PTCDA and the underlying EG layer.

Destruction of SO2on Au and Cu Nanoparticles Dispersed on MgO(100) and CeO2(111)

When going from periodic surfaces to isolated clusters or nanoparticles, there is a big increase in the reactivity of Au and Cu toward SO(2). Density functional calculations indicate that the enhancement in the SO(2) adsorption energy is due to the presence of corner sites (i.e., metal atoms with a low coordination number) and the fluxionality of the nanoparticles. Therefore, small Au particles bind SO(2) stronger than a periodic Au(100) surface. However, the S <--> Au and O <--> Au interactions are not strong enough to induce the rupture of the S-O bonds. In contrast, the dissociation of SO(2) on Cu particles is a very exothermic process, even more exothermic than on a periodic Cu(100) surface. Experiments of synchrotron-based high-resolution photoemission and X-ray absorption spectroscopy show big differences in the DeSOx activity of Au and Cu nanoparticles dispersed on MgO(100) and CeO(2)(111). The heat of adsorption of the SO(2) on Au nanoparticles supported on MgO(100) or CeO(2)(111) was 0.2 to 0.4 eV larger than on Au(100) with negligible dissociation of the molecule. The full decomposition of SO(2) was observed only after O vacancies were introduced in the ceria support. The O vacancies in ceria either played a direct role in the dissociation of SO(2) (cracking of the molecule at the oxide-metal interface) or enhanced the chemical activity of the supported Au nanoparticles. The addition of Cu particles to MgO(100) or CeO(2)(111) generates systems that are extremely active for the destruction of SO(2). At 100-150 K, the SO(2) adsorbs molecularly on the supported Cu particles. Heating to temperatures above 200 K leads to massive dissociation of the SO(2). A comparison of the behavior of SO(2) on Cu/MgO(100) and Cu/CeO(2-x)(111) shows how important the reducibility of the oxide support in DeSOx operations can be.

Ultrathin Films of Diindenoperylene on Graphite and SiO2

In situ low-temperature scanning tunneling microscopy (LT-STM), synchrotron-based high-resolution photoemission spectroscopy, and near-edge X-ray absorption fine structure measurements were used to study the supramolecular packing, molecular orientation, and electronic structures of ultrathin films of diindenoperylene (DIP) on graphite and SiO2. LT-STM measurements reveal that monolayer DIP on highly ordered pyrolytic graphite (HOPG) adopts a long-range ordered “brick-wall” arrangement with their molecular π-planes orientated parallel to the graphite surface, arising from the DIP−graphite interfacial π−π interaction. In contrast, DIP molecules stand upright on inert SiO2 due to the weak interfacial interaction. The ionization potential (IP) of DIP films largely depends on their molecular orientation, i.e., the IP for the standing-up DIP film on SiO2 is 0.40 ± 0.05 eV lower than that of the lying-down film on graphite. This is attributed to the intrinsic surface dipoles that originate from the intramolecular dipolar C−H bonds exposed at the surfaces of the standing-up thin film.

Molecular Orientation-Dependent Ionization Potential of Organic Thin Films

Orientation-dependent ionization potentials (IPs) of organic thin films have been investigated with the combination of synchrotron-based high-resolution photoemission spectroscopy and near-edge X-ray absorption fine structure measurements. Organic thin films of copper(II) phthalocyanine (CuPc) and its fully fluorinated counterpart of copper hexadecafluorophthalocyanine (F16CuPc) with well-controlled orientation are used as model systems. Both molecules lie flat on the graphite surface and stand upright on Au(111) terminated by self-assembled monolayer of octane-1-thiol. The IP for the standing-up CuPc thin film (IP = 4.75 eV) is 0.40 eV lower than that of the lying-down film (IP = 5.15 eV). In contrast, the IP of the standing up F16CuPc (IP = 6.50 eV) is 0.85 eV higher than that of the lying-down film (IP = 5.65 eV). This reversed orientation dependence in IP is explained by the opposite surface dipoles in the standing-up CuPc and F16CuPc molecular layers originating from the different intramolecular dipo...

MOLECULAR ORIENTATION AND ENERGY LEVEL ALIGNMENT AT THE CuPc/SAMs INTERFACE

This article highlights recent progress in the use of functionalized self-assembled monolayers for organic electronics, with particular emphasis on the investigation of the CuPc-SAMs interface properties, particularly the energy level alignment and molecular orientation. Synchrotron-based high-resolution photoemission spectroscopy (PES) and near-edge X-ray absorption fine structure measurements (NEXAFS) are used to address these issues. It is found that the energy level alignment at the CuPc-SAMs interface depends on the chain length of SAMs. Fermi level pinning occurs at the interface of CuPc with short chain SAMs of 4-trifluoromethyl-benzenethiol (CF3-SAM) and 4-methyl-benzenethiol (CH3-SAM), whereas the vacuum level aligns at the interface of CuPc with long chain SAMs including 1-(p-thiophenyl)-4-phenylbenzene (BBB), 4-(p-thiophenyl)-2, 2', 5, 5'-tetramethoxy-biphenyl (BOO), 1-(p-thiophenyl)-4-(2', 5'-dimethoxyphenyl)-tetrafluorobenzene (BFO) and 4-pentafluorophenyl-1-(p-thiophenyl)-2, 5-dimethoxybenzene (BOF). A significant reduction of the hole injection barrier (Δh) by up to 0.75 eV was observed after deposition of 5 nm CuPc on BOF/Au(111) as compared to the CuPc/Au(111) (Δh = 0.9 eV). Angular-dependent NEXAFS measurements reveal that CuPc molecules adopt a standing up configuration on all SAMs. This suggests that the interface charge transfer has negligible effect on the molecular orientation of CuPc on various SAMs.

Facet Stability in Oxygen-Induced Nanofaceting of Re(123̅1)

The stability of the various facets in oxygen-induced faceting of Re(1231) has been studied by low-energy electron diffraction, scanning tunneling microcopy, and synchrotron-based high-resolution X-ray photoemission spectroscopy. When Re(1231) is annealed at 800-1200 K in oxygen (10(-7) Torr), the surface becomes completely covered with nanometer-scale facets, and its morphology depends on the substrate temperature and oxygen exposure. Especially, the (1121) facet competes with the (1011) facet in determining the surface morphology, and the stability of each facet relies on oxygen coverage. Using density functional theory, the O-Re binding energies on the facets for various oxygen concentrations are calculated to explain how the oxygen coverage affects the anisotropy of surface free energy, which in turn determines the morphology of the faceted surface.

Surface Transfer p-Type Doping of Epitaxial Graphene

Epitaxial graphene thermally grown on 6H-SiC(0001) can be p-type doped via a novel surface transfer doping scheme by modifying the surface with the electron acceptor, tetrafluoro-tetracyanoquinodimethane (F4-TCNQ). Synchrotron-based high-resolution photoemission spectroscopy reveals that electron transfer from graphene to adsorbed F4-TCNQ is responsible for the p-type doping of graphene. This novel surface transfer doping scheme by surface modification with appropriate molecular acceptors represents a simple and effective method to nondestructively dope epitaxial graphene for future nanoelectronics applications.

Probing the interaction at the C 60–SiC nanomesh interface

Synchrotron-based high-resolution photoemission spectroscopy (PES) and in situ scanning tunneling microscopy (STM) are used to investigate the interaction at the C 60–SiC nanomesh interface during the sequential deposition and subsequent desorption of C 60 molecules. A weak charge transfer occurs at the C 60–nanomesh interface, involving electrons transferring from nanomesh to C 60 overlayer. The interface interaction originated from the weak charge transfer at the C 60–nanomesh interface is stronger than C 60 intermolecular interaction (e.g., van-der-Waals force), facilitating the layer-by-layer growth for the first two layers of C 60 on SiC nanomesh. The highly corrugated nanomesh surface results in an anisotropic diffusion and high diffusion barrier of C 60 on top, and thereby leads to the formation of irregularly shaped C 60 islands under submonolayer condition. In contrast, C 60 diffusion on HOPG and Ag(1 1 1) surfaces is rather isotropic, resulting in the formation of hexagonally shaped C 60 islands with smooth domain boundaries. STM results show the partial desorption of C 60 molecules from the SiC nanomesh surface after annealing the 1 ML C 60 sample (complete wetting layer of C 60 on SiC nanomesh) at around 150 °C for 20 min. Thorough desorption of C 60 molecules and full recovery of the clean SiC nanomesh are observed after annealing at around 200 °C for 20 min. In situ PES and STM experiments clearly demonstrate that C 60 adsorption and desorption processes do not affect the underlying SiC nanomesh structure, revealing its thermal stability and chemical inertness to C 60 molecules.

Tuning the Hole Injection Barrier at the Organic/Metal Interface with Self-Assembled Functionalized Aromatic Thiols

Self-assembled functionalized aromatic thiols (oligophenylenes composed of building blocks of dimethoxy-substituted phenylenes, perfluoro-substituted phenylenes, and a terminal thiol group) were used to tune the hole injection barrier (Delta(h)) of copper(II) phthalocyanine (CuPc) on Au(111). Synchrotron-based high-resolution photoemission spectroscopy study reveals a significant reduction of Delta(h) by as much as 0.75 eV from Delta(h) = 0.9 eV for CuPc/Au(111) to Delta(h) = 0.15 eV for CuPc/BOF/Au(111), where BOF represents 4-pentafluorophenyl-1-(p-thiophenyl)-2,5-dimethoxybenzene. The delocalized pi orbitals of these functionalized aromatic thiols greatly facilitate effective charge transfer (hole or electron) across the SAM interface as compared to alkanethiols, hence making this novel interface modification scheme a simple and effective way to tune the hole injection barrier. This method has potential applications in molecular electronics, organic light-emitting diodes (OLED), organic field-effect transistors (OFETs), and organic solar cells.

The chemisorption of SO 2 on the Cu/Au(1 1 1) surface: Interplay between ensemble and electronic effects

Ion-scattering spectroscopy (ISS), synchrotron-based high-resolution photoemission spectroscopy (PES) and first-principles density-functional (DF) calculations were used to study the adsorption of SO 2 on Cu/Au(1 1 1). ISS experiments for Cu/Au(1 1 1) clearly demonstrate that Cu intermixes with the Au(1 1 1) substrate at 300 K or higher temperature, but a substantial fraction of the Cu atoms stays on the Au(1 1 1) surface when the deposition is done at 100 K. When 1 ML of Cu was pre-deposited onto Au(1 1 1) at 300 K, the Cu/Au(1 1 1) surface exhibited a negligible reactivity towards SO 2 very similar to that of clean Au(1 1 1). However, when Cu was pre-deposited at 100 K, the Cu/Au(1 1 1) system bonded SO 2 well and the adsorbate remained on the surface up to temperatures above 300 K. DFT calculations for Cu/Au(1 1 1) show electronic perturbations in the Cu overlayer that should enhance their chemical reactivity, but the Cu atoms were more stable when penetrating into the substrate rather than sitting on the surface. Calculations for SO 2 adsorption on Cu/Au(1 1 1) surfaces with different θ Cu showed that the value of θ Cu determines the binding energy of SO 2. In general, the more Cu atoms in the surface, the stronger the bonding energy towards SO 2. Cu/Au(1 1 1) behaves differently from pure Cu. A temperature increase does not lead to S–O bond cleavage on the bimetallic system. Instead, the Cu atoms in the surface migrate into Au, accompanied by SO 2 desorption. Ensembles of the active sites necessary for the dissociation of SO 2 are not available. For Cu/Au(1 1 1), ensemble effects clearly overcome electronic effects. The Cu/Au(1 1 1) interface illustrates how effective ensemble effects can be for the prevention of the corrosion of alloys by SO 2.

Reaction of SO2 with Au∕CeO2(111): Importance of O vacancies in the activation of gold

Synchrotron-based high-resolution photoemission was used to study the adsorption and chemistry of SO(2) on AuCeO(2)(111) and AuO(x)CeO(2) surfaces. The heat of adsorption of the molecule on Au nanoparticles supported on stoichiometric CeO(2)(111) was 4-7 kcalmol larger than on Au(111). However, there was negligible dissociation of SO(2) on the AuCeO(2)(111) surfaces. The full decomposition of SO(2) was observed only after introducing O vacancies in the ceria support. AuO(x)CeO(2) surfaces were found to be much less chemically active than AuCeO(2)(111) or AuCeO(2-x)(111) surfaces. The active sites in {Au + AuO(x)}ceria catalysts should involve pure gold nanoparticles in contact with O vacancies.

The decomposition and chemistry of Ru 3(CO) 12 on TiO 2(1 1 0) studied with X-ray photoelectron spectroscopy and temperature programmed desorption

The decomposition process of triruthenium dodecacarbonyl (Ru 3(CO) 12) on single crystal TiO 2(1 1 0) has been investigated using synchrotron-based high resolution photoemission spectroscopy and temperature programmed desorption (TPD). Ruthenium carbonyl adsorbs molecularly on TiO 2(1 1 0) at 100 K. When dosing at room temperature, the molecule dissociatively adsorbs forming a Ru 3(CO) m ( m ⩽ 10) species on the surface. TPD spectra for ruthenium carbonyl dosed at 100 K and 300 K are quite different in peak numbers and shape, indicating that the ruthenium carbonyl decomposition is sensitive to the preparation procedure. Both TPD and photoemission demonstrate that around 700 K, the decarbonylation process is completed yielding almost pure Ru particles. A small amount of CO adsorbs on the obtained Ru particles after large CO exposures (60–100 L) at 300 K. The Ru particles can be oxidized to form RuO 2 at 600 K with an O 2 pressure of ∼1 × 10 −6 Torr (>400 L exposure). In addition, the chemical activity of the Ru metal particles dispersed on TiO 2(1 1 0) has also been examined with SO 2 and compared to that of a Ru(0 0 1) single crystal. SO 2 is easily decomposed by the Ru particles at room temperature, producing S and O adatoms. SO 4 is the main product of the reaction of SO 2 with the titania support, and the presence of the Ru particles favors a SO 4,ads → S ads + nO ads + mO 2,gas transformation at elevated temperature.

Core-level photoemission and near-edge x-ray absorption fine-structure studies of GaN surface under low-energy ion bombardment

We have investigated compositional changes on GaN surfaces under low-energy Ar ion bombardment using synchrotron-based high-resolution core-level photoemission measurements and near-edge x-ray absorption fine-structure (NEXAFS) spectroscopy. The low-energy ion bombardment of GaN produces a Ga-rich surface layer which transforms into a metallic Ga layer at higher bombarding energies. At the same time, the photoemission spectra around the N 1s core level reveal the presence of both uncoordinated nitrogen and nitrogen interstitials, which we have analyzed in more detail by x-ray absorption measurements at the N K-edge. We have proposed a mechanism for the relocation and loss of nitrogen during ion bombardment, in agreement with some recent experimental and theoretical studies of defect formation in GaN. We have also demonstrated that photoemission spectroscopy and NEXAFS provide a powerful combination for studying compositional changes and the creation of point defects at GaN surface.

Compositional changes on GaN surfaces under low-energy ion bombardment studied by synchrotron-based spectroscopies

We have investigated compositional changes on GaN surfaces under Ar-ion bombardment using synchrotron-based high-resolution x-ray photoemission (PES) and near-edge x-ray absorption fine structure (NEXAFS) spectroscopy. The low-energy ion bombardment of GaN produces a Ga-rich surface layer which transforms into a metallic Ga layer at higher bombarding energies. At the same time, the photoemission spectra around N 1s core levels reveal the presence of both uncoordinated nitrogen and nitrogen interstitials, which we have analyzed in more details by x-ray absorption measurements at N K edge. We have demonstrated that PES and NEXAFS provide a powerful combination for studying the compositional changes on GaN surfaces. A mechanism for the relocation and loss of nitrogen during ion bombardment in agreement with some recent experimental and theoretical studies of defect formation in GaN has been proposed.

High-resolution photoemission study of hydrogen interaction with polar and nonpolar GaAs surfaces

We have used synchrotron-based high-resolution core-level photoemission and valence-band emission measurements to characterize hydrogen chemisorption on nonpolar GaAs(110) and polar GaAs(100) and GaAs(001) surfaces. Chemisorbed atomic hydrogen forms both Ga-H and As-H bonds on all three surfaces, causing chemical shifts of core-level binding energies and changing the valence-band emission. For low hydrogen exposures arsenic desorbs from all surfaces. However, at higher exposures, the (110) surface transforms into a Ga-rich structure with traces of metallic Ga, while the (100) surface transforms into an As-rich structure. We have also observed some additional changes in the binding energy of bulk components of Ga and As core levels as a function of hydrogen exposure, which may be explained by hydrogen-induced changes in band bending.

Interaction of thiophene with stoichiometric and reduced rutile TiO2(1 1 0) surfaces: role of Ti3+ sites in desulfurization activity

The adsorption of thiophene (C4H4S) on almost stoichiometric TiO2(1 1 0) and defective TiO2−x(1 1 0) surfaces has been studied with a combination of synchrotron-based high-resolution photoemission spectroscopy, thermal desorption mass spectrometry (TDS), and first-principles density-functional (DF) slab calculations. The bonding nature between C4H4S and Ti or O sites of TiO2(1 1 0), and point defects (oxygen vacancies) of TiO2−x(1 1 0) was investigated. Over an almost stoichiometric TiO2(1 1 0) surface, the adsorption and desorption of C4H4S is completely reversible. In the submonolayer regime, four adsorption states in the temperature range of 150–450 K were identified in TDS. No thiophene decomposition was observed up to 800 K under ultra-high vacuum (UHV) conditions. The results of DF calculations indicate that at small coverage the molecule should be bonded with its ring nearly parallel to the surface. At high coverages of thiophene, bonding through the S atom becomes more stable because it reduces adsorbate ↔ adsorbate repulsion. On a defective TiO2−x(1 1 0) surface with ∼45% of Ti3+ and Ti2+, a small fraction of the adsorbed thiophene molecules (<0.05 ML) decomposed. Our experimental and theoretical studies indicate that the vacancy states of TiO2−x(1 1 0) interact poorly with the LUMO of thiophene. This is a behavior opposite to that found for the adsorption of S2, CH3S and SO2. The role of Ti3+ sites in the desulfurization activity of MoSx/TiO2 catalysts is analyzed in light of these results.

Coverage effects and the nature of the metal-sulfur bond in S/Au(111): high-resolution photoemission and density-functional studies.

The bonding of sulfur to surfaces of gold is an important subject in several areas of chemistry, physics, and materials science. Synchrotron-based high-resolution photoemission and first-principles density-functional (DF) slab calculations were used to study the interaction of sulfur with a well-defined Au(111) surface and polycrystalline gold. Our experimental and theoretical results show a complex behavior for the sulfur/Au(111) interface as a function of coverage and temperature. At small sulfur coverages, the adsorption of S on fcc hollow sites of the gold substrate is energetically more favorable than adsorption on bridge or a-top sites. Under these conditions, S behaves as a weak electron acceptor but substantially reduces the density-of-states that gold exhibits near the Fermi edge. As the sulfur coverage increases, there is a weakening of the Au-S bonds (with a simultaneous reduction in the Au --> S charge transfer and a modification in the S sp hybridization) that facilitates changes in adsorption site and eventually leads to S-S bonding. At sulfur coverages above 0.4 ML, S(2) and not atomic S is the more stable species on the gold surface. Formation of S(n)(n > 2) species occurs at sulfur coverages higher than a monolayer. Very similar trends were observed for the adsorption of sulfur on polycrystalline surfaces of gold. The S atoms bonded to Au(111) display a unique mobility/reactivity not seen on surfaces of early or late transition metals.