1s Components Research Articles

Competing Pathways in N-Allylurea Adsorption on Si(111)-(7 × 7)

Functionalization of silicon surfaces with N-allylurea (CH2═CH–CNH–CO–NH2) represents a valuable strategy to obtain covalently bonded Si–C interfaces with amino and/or carbonyl termination. In this work, we studied N-allylurea adsorption on the Si(111)-(7 × 7) surface by combining X-ray and ultraviolet photoemission spectroscopy (XPS and UPS) with high resolution energy loss spectroscopy (HREELS) measurements. XPS core level analysis provides information on the molecular attachment process. Si–C covalent bonding is evidenced by the presence of a C 1s component at 284.8 eV, while interaction through N–Si bonding is proved by the presence of a N 1s component at 397.8 eV. Three different adsorption mechanisms are envisaged: (I) [2 + 2]-like cycloaddition occurring at the rest atom–adatom dimer through cleavage of the vinyl group, (II) Si–N bonding at adatom sites upon cleavage of NH2 and rearrangement of the ureic group to form an imidol species (−N═C–OH), with release of a H atom, and (III) hydrosilylation ...

Effects of Substrate Temperature on Structural, Electrical and Optical Properties of Amorphous In-Ga-Zn-O Thin Films

We studied the effects of substrate temperature (Ts) on the surface roughness, resistivity (ρ), mobility (μ), charge carrier concentrations (n), transmission and optical bandgap (Eg) of amorphous In-Ga-Zn-O thin films (∼250 nm thickness) deposited by radio-frequency magnetron sputtering. As Ts increased from RT to 300°C, n increased (2.6 × 1019 → 5.0 × 1019 /cm3), ρ decreased (7.0 × 10−3 → 4.7 × 10−3 Ω−cm), and Eg (3.9 → 3.7 eV) along with the average transmission (89 → 82%) in the visible region decreased. Investigation of the O 1s core level and the Ga 3d, In 4d, and Zn 3d shallow-core levels spectra obtained by high-resolution X-ray photoelectron spectroscopy revealed that as Ts increased, an O 1s component representing the oxygen vacancies increased in amount and that the intensity ratio of In/Ga increased but that of Zn/Ga decreased. The analysis suggests that the increase of oxygen vacancies could explain the increase of charge carrier concentration and that the compositional change could explain the change of Eg.

Bond cutting in K-doped tris(8-hydroxyquinoline) aluminium

A series of Al 2p, K 2p, O 1s and N 1s core-level spectra have been used to characterize the interaction between potassium (K) and tris(8-hydroxyquinoline) aluminium (Alq(3)) molecules in the K-doped Alq(3) layer. All core-level spectra were tuned to be very surface sensitive in selecting various photon energies provided by the wide-range beamline at the National Synchrotron Radiation Research Center, Taiwan. A critical K concentration (x = 2.4) exists in the K-doped Alq(3) layer, below which the K-doped atoms generate a strained environment near the O and N atoms within 8-quinolinoline ligands. This creates new O 1s and N 1s components on the lower binding-energy side. Above the critical K coverage, the K-doped atoms attach the O atoms in the Al-O-C bonds next to the phenoxide ring and replace Al-O-C bonds by forming K-O-C bonds. An Alq(3) molecule is disassembled into Alq(2) and Kq by bond cutting and bond formation. The Alq(2) molecule can be further dissociated into Alq, or even Al, through subsequent formations of Kq.

Surface Composition and Electrical and Electrochemical Properties of Freshly Deposited and Acid-Etched Indium Tin Oxide Electrodes

We compare the near-surface composition and electroactivity of commercial indium tin oxide (ITO) thin films, activated by plasma cleaning or etching with strong haloacids, with ITO films that have been freshly deposited in high vacuum, before and after exposure to the atmosphere or water vapor. Conductive-tip AFM, X-ray photoelectron spectroscopy (XPS), and the electrochemistry of probe molecules in solution were used to compare the relative degrees of electroactivity and the near-surface composition of these materials. Brief etching of commercial ITO samples with concentrated HCl or HI significantly enhances the electrical activity of these oxides as revealed by C-AFM. XPS was used to compare the composition of these activated surfaces, focusing on the intrinsically asymmetric O 1s line shape. Energy-loss processes associated with photoemission from the tin-doped, oxygen-deficient oxides complicate the interpretation of the O 1s spectra. O 1s spectra from the stoichiometric indium oxide lattice are accompanied by higher-binding-energy peaks associated with hydroxylated forms of the oxide (and in some cases carbonaceous impurities) and overlapping photoemission associated with energy-loss processes. Characterization of freshly sputter-deposited indium oxide (IO) and ITO films, transferred under high vacuum to the surface analysis environment, allowed us to differentiate the contributions of tin doping and oxygen-vacancy doping to the O 1s line shape, relative to higher-binding-energy O 1s components associated with hydroxyl species and carbonaceous impurities. Using these approaches, we determined that acid activation and O2 plasma etching create an ITO surface that is still covered with an average of one to two monolayers of hydroxide. Both of these activation treatments lead to significantly higher rates of electron transfer to solution probe molecules, such as dimethyferrocene in acetonitrile. Solution electron-transfer events appear to occur at no more than 4x10(7) electroactive sites per cm2 (each with diameters of ca. 50-200 nm) (i.e., a small fraction of the geometric area of the electrode). Electron-transfer rates correlate with the near-surface tin dopant concentration, suggesting that these electroactive sites arise from near-surface tin enrichment.

Kinetics of oxynitridation of 6H–SiC( [formula omitted] and the interface structure analyzed by ion scattering and photoelectron spectroscopy

Clean and pre-oxidized 6H–SiC( 1 1 2 ¯ 0 ) surfaces were annealed in NO at temperatures ranging from 800 to 1000 °C under a pressure of 1 × 10 −3 Torr. The growing surface and interface structures were analyzed in situ by high-resolution medium energy ion scattering (MEIS) and photoelectron spectroscopy using synchrotron-radiation-light. The present result reveals growth of double-layered structure of SiO 2/SiO x N y on SiC for the samples annealed at 1000 °C in NO with and without pre-oxidation in O 2. Oxynitridation takes place only at SiO 2/SiC interfaces. The thickness of growing layers is saturated at ∼0.2 nm of SiO 2 and 0.3–0.4 nm of SiO x N y layers with the elemental compositions unchanged. For the samples pre-oxidized in 18O 2 followed by annealing in N 16O, the exchange reaction between 18O and 16O occurs at the surface and interface. No nitrogen removal was observed by annealing the oxy-nitrided sample in O 2 at 1000 °C and 1 × 10 −3 Torr. We also observed the C 1s, N 1s and Si 2p spectra and identified the N 1s and Si 2p components originating from Si-oxynitride layers.

Interface of atomic layer deposited Al2O3 on H‐terminated silicon

AbstractAl2O3 films 1 to 20 nm thick were deposited as alternative high‐κ gate dielectric on hydrogen‐terminated silicon by Atomic Layer Deposition (ALD) and characterized by Synchrotron X‐ray Photoelectron Spec‐troscopy (SXPS), Fourier Transform Infrared (FTIR) absorption spectroscopy and admittance measure‐ments. The SXPS results indicate that about 60% of the original Si–H surface bonds are preserved at the Al2O3/Si interface and this is confirmed by monitoring the Si–H stretching modes by FTIR spectroscopy in the Attenuated Total Reflection (ATR) mode both before and after ALD of Al2O3. The remaining 40% of Si–H bonds are replaced by Si–O bonds as verified by SXPS. In addition, a fraction of a monolayer of SiO2 forms on top of the Al2O3 dielectric during deposition. The presence of OH‐groups at a level of 3% of the total oxygen content was detected throughout the Al2O3 layer through a chemically shifted O 1s component in SXPS. Admittance measurements give a dielectric constant of 9.12, but a relatively high density of interface traps between 1011 and 1012 cm–2 eV–1. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

In situ XPS study of Pd(1 1 1) oxidation. Part 1: 2D oxide formation in 10 −3 mbar O 2

The oxidation of the Pd(1 1 1) surface was studied by in situ XPS during heating and cooling in 3 × 10 −3 mbar O 2. A number of adsorbed/dissolved oxygen species were identified by in situ XPS, such as the two dimensional surface oxide (Pd 5O 4), the supersaturated O ads layer, dissolved oxygen and the ( 67 × 67 ) R 12.2° surface structure. Exposure of the Pd(1 1 1) single crystal to 3 × 10 −3 mbar O 2 at 425 K led to formation of the 2D oxide phase, which was in equilibrium with a supersaturated O ads layer. The supersaturated O ads layer was characterized by the O 1s core level peak at 530.37 eV. The 2D oxide, Pd 5O 4, was characterized by two O 1s components at 528.92 eV and 529.52 eV and by two oxygen-induced Pd 3d 5/2 components at 335.5 eV and 336.24 eV. During heating in 3 × 10 −3 mbar O 2 the supersaturated O ads layer disappeared whereas the fraction of the surface covered with the 2D oxide grew. The surface was completely covered with the 2D oxide between 600 K and 655 K. Depth profiling by photon energy variation confirmed the surface nature of the 2D oxide. The 2D oxide decomposed completely above 717 K. Diffusion of oxygen in the palladium bulk occurred at these temperatures. A substantial oxygen signal assigned to the dissolved species was detected even at 923 K. The dissolved oxygen was characterised by the O 1s core level peak at 528.98 eV. The “bulk” nature of the dissolved oxygen species was verified by depth profiling. During cooling in 3 × 10 −3 mbar O 2, the oxidised Pd 2+ species appeared at 788 K whereas the 2D oxide decomposed at 717 K during heating. The surface oxidised states exhibited an inverse hysteresis. The oxidised palladium state observed during cooling was assigned to a new oxide phase, probably the ( 67 × 67 ) R 12.2° structure.

Atomic structure of the 6H–SiC(0 0 0 1) nanomesh

The atomic structure of the carbon nanomesh template (the so-called 6 3 × 6 3 R 30 ° reconstruction) on the 6H–SiC(0 0 0 1) surface was investigated in detail by scanning tunneling microscopy (STM), low energy electron diffraction (LEED), synchrotron photoemission spectroscopy (PES) and density-functional theory (DFT) calculations. We propose that the origin of the carbon nanomesh template arises from the self-organization of excess carbon atoms forming a novel honeycomb arrangement atop the 6H–SiC(0 0 0 1) surface after annealing at 1100 °C. Two carbon nanomesh-related C 1s components are observed at binding energies of 285.1 eV ( S 1) and 283.8 eV ( S 2) by angle-resolved synchrotron PES experiments. We assign the S 2 component to carbon atoms that bond to one underlying Si atom, and the S 1 component to carbon atoms bonded to other carbon atoms without Si–C bond formation. Further annealing results in the formation of crystalline graphitic layers on top of the surface.

Surface and Interface Studies of Si-Rich 4H-SiC and SiO<sub>2</sub>

A photoemission study of Si-rich polar and nonpolar 4H-SiC surfaces before andafter initial oxidation are presented. Core level and valence band data recorded usingsynchrotron radiation are utilized to explore the properties of these surfaces and of theinterfaces.The Si-rich surfaces showed three prominent surface Si 2p components. These componentsare strongly attenuated upon oxygen exposures and can not be detected after an exposure of3000 L. After exposures of ≥200 L the number of oxidation states, i.e. Si+1, Si+2 and Si+4oxidation states, and also their shifts are found to be the same as after initial oxidation of thesame surfaces prepared by in situ heating.Only one sharp C 1s core level is observed on the Si-rich surfaces. This is quite differentcompared to earlier findings on the same surfaces prepared by in situ heating whereprominent surface shifted C 1s components are found. After oxygen exposure of ~ 200 L theC 1s peak is broadened and an asymmetry is observed on the high binding energy side. Thistogether with the Si 2p results show that oxygen exposures of ~200 L affect not only the Sioverlayers. Layers in the SiC substrate are also affected since the C 1s peak is broadened andthe Si 2p spectrum show the same oxidation states as after large exposures and after oxidationof these surfaces prepared without Si overlayers.

XPS with fast-frozen samples: A renewed approach to study the real mineral/solution interface

The procedure for XPS study of quick-frozen solutions developed at the end of 1970s [K. Burger, E. Fluck, Inorg. Nucl. Chem. Lett. 10 (1974) 171] was modified and applied to investigate the mineral/solution interface formed at the surface of silica, gibbsite, goethite and manganite in aqueous sodium chloride solution at variable pH and ionic strength. The Na/Cl atomic ratio (frozen samples) allows determination of the pH range where the mineral surface charge is close to zero. This ratio can be used also to prove an ion pair formation at the surface. From the intensity of the O 1s component corresponding to water, an interface thickness of 0.5 nm was estimated. A “built-in” potential at the interface, evincing as matrix elements photoelectron lines shifts after water loss, was found. This potential arises due to the spatial separation of the charged mineral surface and counter-ions in the electrical double layer (EDL). Its value does not exceed 250 mV and the sign depends on the charge of the mineral surface.

Studies of NO on 4H-SiC(0001) using synchrotron radiation

Detailed studies of the effects induced on the R 30° 4H-SiC(0001) surface after different NO exposures, at a substrate temperature of800 °C, have been made. Photoemission experiments using synchrotron radiation were performed in orderto study the properties of the interface formed after gas exposures. Recorded Si 2p spectra showthree shifted components, besides the bulk SiC peak. These are assigned as originating fromSi3N4 orSi1+ sub-oxide, N–Si–Oand SiO2. It wasconcluded that SiO2 does grow on top of N–Si–O and that Si3N4/Si1+ is located at the interface. Two N 1s components are observed after NO exposures.The main one, located at around 398.05 eV, is assigned as originating fromSi3N4 and the weaker one is suggested to correspond to N–Si–O bonding. Theassignments are made with the aid of Si 2p and N 1s spectra collected afterNH3 and O2 exposures under similar conditions. No graphite-like carbon or carbonby-product at the interface can be detected after large NO orO2 exposures.

Ab initiocalculation of graphite surface-bulk core-level splitting

The origin of two carbon 1s components observed recently in photoelectron energy spectra of graphite have been analyzed quantitatively by means of electronic structure and total-energy calculations for supercell structures. The all-electron pseudopotential technique within the local-density approximation was applied. Two different contributions to the core-level shift were studied: the initial-state shift and the final-state shift from a core-hole relaxation. The results of calculations show that final-state relaxations plays a crucial role and that the doublet cannot be understood within an initial-state picture.

A Fast XPS study of the surface chemistry of ethanol over Pt{1 1 1}

The adsorption and reaction of ethanol over Pt{1 1 1} has been investigated by Fast XPS and TPD. Ethanol adsorbs molecularly at 100 K, with a saturation coverage of 0.44 ML giving rise to C 1s components with binding energies of 283.7 eV (CH 3–) and 284.8 eV (–H 2COH). Ethanol multilayers desorb above 150 K, while ∼60% of the monolayer desorbs intact above 200 K in competition with decomposition pathways. Reaction initially proceeds via progressive dehydrogenation to form a metastable acetyl intermediate with components at 283.5 eV (CH 3–) and 285.2 eV (CO), which in turn undergoes decarbonylation above 250 K to chemisorbed CO and methyl groups. A significant fraction of the latter are hydrogenated above 270 K, desorbing as CH 4, with the remainder further decomposing to liberate H 2 and surface CH x moeities.

Effects of nitrogenmonoxide on 4H–SiC(0 0 0 1)

Effects induced by nitrogenmonoxide (NO) exposures on the √3×√3 R30° reconstructed 4H–SiC(0 0 0 1) surface are reported. NO exposures from 0.3 to 1 × 10 6 L at a substrate temperature of 800 °C are investigated. Recorded Si 2p spectra show three shifted components, besides the bulk SiC peak. These are assigned to originate from SiO 2, N–Si–O and Si 3N 4/Si +1 (since we cannot distinguish between Si 3N 4 and an Si +1 oxidation state). It can be concluded that SiO 2 does grow on top of N–Si–O and that Si 3N 4/Si +1 is located at the interface. Two N 1s components are observed after NO exposures. A main one, located at around 398.05 eV, assigned to originate from Si 3N 4 and a weaker one suggested to correspond to N–Si–O bonding. The assignments are made using Si 2p and N 1s spectra collected after NH 3 and O 2 exposures under similar conditions. No graphite like carbon or carbon by-product at the interface can be detected after large NO or O 2 exposures.

Chemical configuration of nitrogen in ultrathin Si oxynitride on Si(100)

Chemical bonding structures of nitrogen within very thin oxynitride films on Si(100) have been investigated by high-resolution photoemission spectroscopy using synchrotron radiation. The oxynitride films nitrided by the rapid thermal process with NO and N 2 O are systematically compared. Two distinct N 1s components are resolved for both films with a binding-energy difference of ∼0.61 eV, which are assigned to the N atoms at the interfaces and those in the SiO 2 matrix. Both components are unambiguously attributed to represent a N-Si 3 like chemical configuration with three nearest-neighbor Si atoms. The energy shift of 0.61 eV between these components is thought be due to the second-nearest-neighbor effect combined with core-hole screening. The difference between N 2 O- and NO-nitrided films and the interface suboxide species identified by Si 2p core levels are discussed.

Electron spectroscopic studies of nanocomposite PVD TiAlBN coatings

A range of TiAlBN coatings have been deposited by electron beam evaporation and the optimised composition exhibits a 150% improvement over single phase TiAlN coatings in drilling tests. The coatings have been analysed by core level XPS to investigate the phase composition and bonding. Grain sizes have been determined by TEM. Al substitutes into the TiN phase and the coatings show a phase composition in agreement with that predicted by the (Ti,Al)BN equilibrium phase diagram. The optimum microstructure for wet drilling applications consists of 26 nm (Ti,Al)N grains separated by 3 nm regions of amorphous BN. As the grain size increases, a new B 1s component develops, attributable to B either incorporated into the (Ti,Al)N phase or located at grain boundaries.

ANGLE-SCANNED PHOTOELECTRON DIFFRACTION: FROM CLEAN SURFACES TO COMPLEX ADSORPTION SYSTEMS

We exploit the capabilities of photoelectron diffraction (PED) to provide quantitative information on the local structure of the first layers of clean and adsorbate-covered surface systems. Selected studies of low-energy PED are presented, highlighting the advantages of the angle-scanning approach. In the first experiment, we evaluate the clean Rh(110) surface layer relaxation by employing the PED of the Rh 3d5/2 surface component. The resulting relaxation is in good accord with previous low energy electron diffraction data. In the second experiment, a system lacking long-range order is examined, namely the saturation layer formed by nitrogen monoxide on Rh(100) at 123 K. Preliminary results confirm the bridge adsorption geometry model. The last example is a chemical shift PED study of the c(4 × 2) phase of carbon monoxide on Pt(111). In this system, CO molecules are coadsorbed at two different adsorption sites, the energy separation of the respective C 1s components being 0.7 eV. Structural determination has been achieved by an independent analysis of the diffraction yield originated by the two chemically shifted C 1s components. The structure of Pt (111) + c(4 × 2)-2CO has been refined with an automated search of the best parameters using a modified version of MSCD (multiple scattering calculation of diffraction) code.

XPS of chemically modified low‐density polyethylene surfaces: observations on curve‐fitting the C 1s spectrum

AbstractThe curve‐fitting of the complex C 1s envelopes (obtained under high‐energy‐resolution XPS conditions) resulting from the surface chemical modification of low‐density polyethylene has been investigated. The main problem is modelling of the dominant, asymmetric component due to unmodified hydrocarbon. Two approaches have been compared, namely the use of a full (four‐component) vibrational progression or a single peak with a lineshape recently introduced by Gelius. As well as goodness of fit criteria, the agreement between the measured O 1s intensity and that predicted from the intensities of the high‐binding‐energy C 1s components (from oxygen‐containing functional groups) was used in the assessment. Under the relatively low‐level, but practically relevant, modification conditions studied, the two approaches are equally effective. However, they can give rise to significant differences in the relative intensities of high‐binding‐energy components. Broadening of the various components as a function of treatment also has been studied. Copyright © 2002 John Wiley & Sons, Ltd.

Electronic states of hydrogen in zirconium oxide

First-principles molecular orbital calculations for the electronic states of hydrogen in ZrO 2 have been carried out using the discrete variational-Xα cluster method. Both the band gap energy and the valence band structure of ZrO 2 are in good agreement with experimental results. By the introduction of a hydrogen atom, a new impurity level appears below the conduction band and the band gap is greatly reduced. Calculated partial density of states of the H 1s component in the band gap reproduces the experimental results obtained by means of Mott–Schottky analysis. The dominant chemical bond in ZrO 2 is found to be the strong ionic bond between Zr and O atoms, and the bonding character is hardly modified by the hydrogen content.

Vibrationally resolved C 1s photoemission from CO absorbed on Rh(1 1 1): the investigation of a new chemically shifted C 1s component

High-resolution core-level photoemission at an energy resolution better than the intrinsic width of the C 1s level has been used to study the CO/Rh(1 1 1) overlayer system. C 1s spectra have been measured in a large CO coverage range; from low coverages, where the CO molecules only adsorb in on-top positions, to saturation coverage, where CO molecules occupy both on-top and threefold hollow sites. Fine structure components due to vibrational excitation of the C–O stretch mode are clearly resolved in the emission peak from each site. The vibrational splittings and intensity ratios are found to be different for the different adsorption sites. A third C 1s component, which has not been resolved earlier, is found at intermediate CO coverages. Some possible explanations for the origin of this extra component are discussed. In the light of its binding energy and vibrational energy in photoemission, together with supporting evidence from high-resolution electron energy loss spectroscopy data and photoelectron diffraction, this is assigned to bridging CO species at intermediate coverages.