L. Hellner

Evolution and properties of nanodiamond films deposited by direct current glow discharge

Nanocrystalline carbon films possessing a prevailing diamond character are deposited by a direct current glow discharge chemical vapor deposition method using a 9:91 vol % methane to hydrogen gas mixture. In the present work the evolution and properties of nanodiamond films deposited by this method onto silicon substrates as a function of time were studied by various complementary techniques. Our analysis showed that prior to formation and growth of continuous films of a predominantly nanodiamond character, a graphitic phase is formed. After the nanodiamond phase is stabilized, near edge x-ray adsorption fine structure measurements proved the predominant diamond character of the film to be about 80%. By electron energy loss spectroscopy analysis the sp2-like character of the nanodiamond grain boundaries has been determined. The nanodiamond films were found to be thermally stable up to temperatures of ∼950 °C as established by vacuum heating. By scanning electron microscopy and atomic force microscopy the ...

Sensitivity of near-edge x-ray absorption fine structure spectroscopy to ion beam damage in diamond films

In the present work, we study the sensitivity of the near-edge x-ray absorption fine structure (NEXAFS) spectroscopy to ion induced defects in polycrystalline diamond films. The ion bombardment of hydrogenated films is performed using 30 keV Xe+ ions at room temperature for doses ranging from 2×1013 ions/cm2, producing local point defects, to 2×1015 ions/cm2, which results in almost complete amorphization of the diamond surface. Auger electron spectroscopy measurements are not sensitive to the modifications induced by the lowest implantation dose. Whereas partial electron yield (PEY) NEXAFS measurements, applied in surface and bulk-sensitive modes, using 35, 15, and 8 eV secondary electrons, respectively, reveals the formation of a defective structure and gradual deterioration of diamond in the near-surface region. From PEY NEXAFS spectra measured using 15 eV secondary electrons, the position of C(1s) binding energy is measured. The x-ray photoelectron spectra of the samples were measured using an inciden...

A low symmetry adsorption state of benzene on Si(111)7 × 7 studied by photoemission and photodesorption

Benzene adsorption on Si(111)7 × 7 has been studied by valuence band and Si2p photoemission and photodesorption. We hypothesize a molecular adsorption of benzene on Si(111)7 × 7, involving simultaneously an adatom and a rest atom in the adsorption process. The assignment of the benzene adsorption features has been made by comparison with gas-phase benzene and with ARUPS spectra of benzene on Si(100)2 × 1. We find that the degenerate orbitals of the e type lose their degeneracy with decrease of symmetry. We suppose that two opposite carbon atoms are involved in the adsorption process, leading to a partial rehybridization of the two carbon atoms from sp2 to sp3 that makes the adsorbate state intermediate between benzene and 1,4 cyclohexadiene. We also observed a high desorption rate of benzene from the silicon surface that shows a weak benzene-silicon bond. D+ is the only ion produced in the Si2p energy range, due to the enhanced production of secondary electrons in correspondence of the ionization threshold.

Nanosize diamond formation promoted by direct current glow discharge process: Synchrotron radiation and high resolution electron microscopy studies

Diamond nucleation on Si(100) surfaces can be promoted by a dc-glow discharge process, using a CH4/H2 gas mixture. However, the phase composition and structure of the carbon film deposited during the dc-glow discharge pretreatment are still unclear. In the present work, we report on a combined study of near edge x-ray absorption fine structure (NEXAFS), and high resolution scanning electron microscopy (HR-SEM) of this film as a function of substrate temperature. NEXAFS measurements of the films deposited by the dc-glow discharge process render unambiguous evidence of diamond phase formation in the 880–900 °C substrate temperature range. It is determined from HR-SEM measurements that in this temperature range, nanosize diamond particles are formed. At lower and higher substrate temperatures the NEXAFS results indicate the predominant formation of graphitic carbon. The changes in the film composition as a function of substrate temperature during the dc-glow discharge process is expressed in terms of relativ...

Study of the interaction of SO2 with TiO2 (110) surface

Abstract The adsorption of SO2 on TiO2(110) surfaces at 100 K has been studied by means of the X-ray photoemission and ultra-violet photoemission spectroscopy, and photon stimulated desorption techniques using Synchrotron radiation. For a saturation dose at 100 K, the SO2 is molecularly adsorbed and a chemisorbed interface is formed by SO42− and SO32− species on the surface. Annealing of the adsorbed layer to room temperature leads to the formation of sulfur derived species.

Phenol adsorption on Si(111)7×7 studied by synchrotron radiation photoemission and photodesorption

Phenol adsorption on Si(111)7 x 7 has been studied by synchrotron radiation photoemission and photodesorption in the valence band Si 2p energy ranges both at room temperature and at approximate to 50 K. The assignment of the phenol adsorption features has been made by comparison of the gas-phase spectrum. At low exposures the surface rest atoms are selectively quenched, while the adatoms are involved in the adsorption process only for higher exposures. The comparison of the spectra at different temperatures allows us to infer that the adsorption on the rest atoms is molecular. However, the relative intensity of the spectral features changes when different sites are involved in the adsorption process. In particular, the feature attributed to the sigma(O-H) orbital does not increase in intensity. although the other features do, suggesting that the adsorption is dissociative. The photon-stimulated desorption pattern shows a D+ yield in the Si 2p energy range, due to the enhanced production of secondary electrons in correspondence with the ionization threshold, similarly to the system benzene-Si(111)7 x 7

Synchrotron radiation photoemission and photostimulated desorption of deuterated methanol on Si(111)7 × 7 and Si(100)2 × 1

Abstract Deuterated methanol adsorption on Si(111)7 × 7 and Si(100)2 × 1 surfaces has been investigated, at room temperature, by synchrotron radiation photoemission and photostimulated desorption. Photoemission experiments as a function of methanol coverage show that deuterated methanol adsorption is dissociative on both surfaces and occurs via б O-D bond breakage and б SiO bond formation. Site selectivity is displayed by the methoxy species on Si(111)7 × 7 surface, since t S2 surface state related to the rest atoms is quickly quenched upon low coverage adsorption. The S1 surface state is quenched only subsequently at much higher coverages. The photostimulated desorption pattern is similar for the two surfaces and strongly depends on the investigated energy range. D+ is always the most abundant ion produced, but higher mass fragments are present too in the energy range including the C 1s threshold, where multiple desorption processes are possibly taking place.

Methanol adsorption on Si(111)-(7 × 7), investigated by core-line photoemission and mass spectrometry of photodesorbed ions

Deuterated methanol adsorption on Si(111)-(7 × 7) has been studied by C 1s and O 1s core-line synchrotron radiation photoemission and mass spectrometry of photodesorbed ions. Experiments were performed as a function of coverage and temperature. Two different C 1s photoemission peaks hint at two different adsorbed species, the first one related to the methoxy fragment produced after dissociative adsorption, the second one is attributed to a CDx species. Both peaks disappear after heating at 350°C, though the methoxy species is more reactive, and a third peak appears. The nature of the species producing the third peak was deduced by means of the mass spectra after heating the sample at 400°C and attributed to C dispersed on the silicon surface.

Photon-stimulated desorption of methanol on Si(111)7 × 7 and Si(100)2 × 1 at the C 1s and O 1s thresholds

Abstract Photostimulated desorption experiments have been performed on deuterated methanol adsorbed on Si(111)7 × 7 and Si(100)2 × 1 at the C 1s and O 1s thresholds. D + and the masses of the series CD + x are produced in the photofragmentation process in both energy ranges. A comparison has been made with the photofragmentation spectra of methanol in the gas phase and two different desorption mechanisms have been hypothesized for the desorption of D + and higher masses from the silicon surfaces at the C 1s threshold.

Adsorption of ethylene on Si(111)7×7 by synchrotron radiation photoemission

Abstract Ethylene adsorption on Si(111)7×7 was studied by valence band and Si2p core line synchrotron radiation photoemission. Experiments were performed at room temperature, as a function of coverage. The simultaneous quenching of the S 1 and S 2 silicon surface states upon adsorption shows that ethylene adsorbs in a molecular form, on a bridging site between a silicon adatom and rest atom. An electron donation from silicon to ethylene, which indicates a rather strong SiC bond, induces an upward shift of the shallower molecular orbitals. The CC double bond is stretched and becomes intermediate between single and double bond.