Reaction of acetonitrile with the silicon(0 0 1) surface: A combined XPS and FTIR study

The adsorption of acetonitrile on the Si(0 0 1) surface has been investigated using X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared spectroscopy (FTIR). XPS and FTIR spectra indicate that adsorbed acetonitrile forms two correlated binding configurations, a CN species with a strong FTIR absorption at 1540 cm⁻¹ and a CCN (ketenimine) species that has a very strong FTIR absorption at 1952 cm⁻¹. The CCN FTIR peak at 1952 cm⁻¹ shows a striking polarization dependence, with the infrared transition dipole almost entirely in the plane of the sample and parallel to the SiSi dimer axis. Our data suggests that the primary CCN structure results from cleavage of two C-H bonds, forming a structure in which the N and terminal C atom are both linked to the surface. Temperature-dependent experiments help to elucidate the complicated reaction mechanism for acetonitrile adsorbing onto the Si(0 0 1) surface. Dosing at higher temperature increases the amount of CCN relative to CN species while heating leads to direct transformation of the CN to the CCN species. Our results indicate that previous studies, which considered only products formed by cleavage of a single C-H bond, have misidentified the primary ketenimine product. A reinterpretation of the earlier results, combined with data presented here, sheds new light onto the products and mechanism of interaction of acetonitrile with Si(0 0 1).     

Chemisorption of 1,1-dichloroethene on the Si(1 1 1)-7 × 7 surface

Chemisorption of 1,1-dichloroethene (Cl₂CCH₂) to a Si(1 1 1)-7 × 7 surface was studied by means of X-ray photoelectron spectroscopy using synchrotron radiation, recording chlorine 2p and carbon 1s spectra. For carbon 1s, spectral assignment of the chemisorbed species is based on quantum chemical calculations of chemical shifts in model compounds.The results confirm the identity of covalently bonded 1-chlorovinyl (-CClCH₂) and vinylidene (CCH₂) adspecies. Upon chemisorption at room temperature it was found that about one-third of the molecules break one C-Cl bond while about two-thirds of the adsorbates break two C-Cl bonds. We do not, however, find evidence for isomerization of CCH₂ to di-bonded vinylene (-CHCH-).     

Influence of thermal treatment of low dielectric constant SiOC(H) films using MTES/O2 deposited by PECVD

Low dielectric constant SiOC(H) films are deposited on p-type Si(100) substrates by plasma enhanced chemical vapor deposition (PECVD) using methyltriethoxysilane (MTES, C₇H₁₈O₃Si) and oxygen gas as precursors. The SiOC(H) films are deposited at room temperature, 100, 200, 300 and 400 °C and then annealed at 100, 200, 300 and 400 °C temperatures for 30 min in vacuum. The influence of deposition temperature and annealing on SiOC(H) films are investigated. Film thickness and refractive index are measured by field emission scanning electron microscopy and ellipsometry, respectively. Chemical bonding characteristics of as-deposited and annealed films are investigated by Fourier transform infrared (FTIR) spectroscopy in the absorbance mode. As more carbon atoms are incorporated into the SiOC(H) films, both film density and refractive index are decreased due to nano pore structure of the film. In the SiOC(H) film, CH₃ group as an end group is introduced into OSiO network, thereby reducing the density to decrease the dielectric constant thereof. The dielectric constant of SiOC(H) film is evaluated by C-V measurements using metal-insulator-semiconductor (MIS), Al/SiOC(H)/p-Si structure and it is found to be as low as 2.2 for annealed samples deposited at 400 °C.     

Alkali adsorption on Ni(1 1 1) and their coadsorption with CO and O

The adsorption of alkalis (Na, K) on Ni(1 1 1) and their coadsorption with CO and O were studied by high-resolution electron energy loss spectroscopy. Loss measurements of clean alkali adlayers provided the expected behaviour of the alkali-substrate vibration energy as a function of the alkali coverage. This result was achieved by eliminating any trace of CO contamination from the alkali adlayer. As a matter of fact, a significant softening of the alkali-Ni vibration energy was revealed in the alkali + CO coadsorbed phase. Moreover, alkali coadsorption with oxygen caused a weakening of the ONi bond and a strengthening of the alkaliNi bond.     

Selective surface chemistry of allyl alcohol and allyl aldehyde on Si(1 0 0)2×1: Competition of [2 + 2] CC cycloaddition with O-H dissociation and with [2 + 2] CO cycloaddition in bifunctional molecules

Competition between the CC functional group with the OH group in allyl alcohol and with the CO group in allyl aldehyde in the adsorption and thermal chemistry on Si(1 0 0)2×1 has been studied by X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD), as well as density-functional theory (DFT) calculations. The similarities found in the C 1s and O 1s spectra for both molecules indicate that the O-H dissociation product for allyl alcohol and [2 + 2] CO cycloaddition product for allyl aldehyde are preferred over the corresponding [2 + 2] CC cycloaddition products. Temperature-dependent XPS and TPD studies further show that thermal evolution of these molecules gives rise to the formation of ethylene, acetylene, and propene on Si(1 0 0)2×1, with additional CO evolution only from allyl alcohol. The formation of these desorption products also supports that the [2 + 2] CC cycloaddition reaction does not occur. In addition, the formation of SiC at 1090 K is observed for both allyl alcohol and allyl aldehyde. We propose plausible surface-mediated reaction pathways for the formation of these thermal evolution products. The present work illustrates the crucial role of the Si(1 0 0)2×1 surface in selective reactions of the Si dimers with the O−H group in allyl alcohol and with the CO group in allyl aldehyde over the CC functional group common to both molecules.     

Resonant Auger spectroscopy study of charge transfer phenomena in N 1s core-excited acetonitrile adsorbates on Si(0 0 1)-2 × 1

Acetonitrile (CH₃CN) adsorbs on Si(0 0 1)-2 × 1 at room temperature under two forms, a cycloaddition-like adduct (Si-CN-Si) and a pendent cyano (Si-CH₂-CN) resulting from the decomposition of the molecule. Resonant Auger spectroscopy has been used to study the excited-state-dependent electron transfer from the N 1s core-excited molecular adsorbate to the silicon substrate, using the core-hole lifetime (∼6 fs) as an internal clock. It is shown that the π_CN^∗ NEXAFS state lies within the silicon bandgap because of a core-excitonic effect. Therefore no charge transfer of the excited electron to the substrate is observed. On the other hand the π_CN^∗ NEXAFS state is placed within the silicon conduction band. Excitation to this orbital leads to valence/Auger spectra in which both resonant and normal Auger contributions are observed. Therefore there is evidence for a charge transfer from the pendent CN to the silicon surface, on a timescale estimated to tens of femtoseconds.     

The surface modification of nanoporous SiOx thin films with a monofunctional organosilane

The surfaces of nanostructured, porous SiO_x/Si (air-oxidized Si) and SiO_x thin films, deposited by excimer laser ablation in He and He + O₂ gas ambients, respectively, have been modified by the deposition of a monofunctional organosilane. They were characterized using photoacoustic Fourier-transform infrared (FTIR) X-ray photoelectron (XPS) spectroscopies, and field-emission scanning electron microscopy (FESEM). Photoacoustic FTIR analysis indicates that the organosilane has hydrolyzed to form a silanol, which has chemically reacted with SiO_x through its surface silanol (SiOH) group, to form siloxane (SiOSi) structures. An enhanced IR spectral signal is found, due to the expansion and contraction of both the pores of the solid and the gas within them.     

DFT calculations of XPS/NEXAFS and IR spectra to elucidate the reaction products of acetonitrile with Si(0 0 1)-2 × 1

NEXAFS data [S. Rangan et al., Phys. Rev. B 71 (2005) 165319] and FTIR data [M.P. Schwartz, R.J. Hamers, Surf. Sci. 601 (2007) 945] apparently do not converge in the identification of the reaction products of acetonitrile (CH₃CN) with Si(0 0 1)-2 × 1 at room temperature. Using DFT calculations of core-excited/core-ionized spectra and of IR vibrational frequencies and intensities, we show the consistency of the body of experimental data. Three species are present on the surface in equivalent amounts, a CN moiety, a pendent CN and a CCN ketenimine submitted to a strong twist imposed by the Si bond directionality. More generally, the paper shows the usefulness of spectroscopic data simulations in the elucidation of complex surface chemistry problems.     

Low temperature catalytic performance of nanosized TiNiO for oxidative dehydrogenation of propane to propene

Nanosized TiNiO catalysts prepared by a modified sol-gel method have been investigated in the oxidative dehydrogenation of propane (ODP) to propene. At 300 °C the yield to propene of 12.1% was obtained on 9.1 wt.% TiNiO catalyst with the selectivity of 43%. The continued variety of lattice parameter and variation of chemical value of nickel and titanium ion on the surface indicates that there are strong interactions of TiO₂ and NiO. The decreased low temperature oxygen desorption and the weaker reducibility seems to be responsible for the decreased activity and enhanced selectivity of propane oxidative dehydrogenation over TiNiO catalysts.     

Ab initio studies of electronic spectra of the linear aluminum-bearing carbon chains AlC2nH (n = 1-5)

Geometries and stabilities of the linear aluminum-bearing carbon chains AlC_2nH (n = 1-5) in their ground states have been explored by the DFT-B3LYP and RCCSD(T) methods. Structures of the X¹Σ⁺ and 1¹Π electronic states have also been optimized by the CASSCF approach. The studies indicate that these species have single-triple bond alternate pattern, AlCCCC?CCH, and the electronic excitation from X¹Σ⁺ to 1¹Π leads to the shortening of the AlC bonds. The vertical excitation energies of the 1¹Π ← X¹Σ⁺ and 2¹Π ← X¹Σ⁺ transitions for AlC_2nH (n = 1-5) have been investigated by the CASPT2, EOM-CCSD, and TD-B3LYP levels of theory with the cc-pVTZ basis set, respectively. CASPT2-predicted 1¹Π ← X¹Σ⁺ transition energies are 3.57, 3.44, 3.33, 3.26, and 3.21 eV, respectively. For AlC₂H, our estimate agrees very well with the experimental value of 3.57 eV. In addition, the AlC bond dissociation energies and the exponential-decay curves for these vertical excitation energies are also discussed.     

Pre-treatments applied to oxidized aluminum surfaces to modify the interfacial bonding with bis-1,2-(triethoxysilyl)ethane (BTSE): Part I. High-purity Al with native oxide

A remote microwave-generated H₂ plasma and heating to 250 °C were separately used to modify high-purity oxidized aluminum surfaces and to assess whether these treatments can help enhance adhesion with bis-1,2-(triethoxysilyl)ethane (BTSE) coatings. Different initial oxide surfaces were considered, corresponding to the native oxide and to surfaces formed by the Forest Products Laboratory (FPL) treatment applied for either 15 or 60 min. BTSE is applied from solution at pH 4, and competing processes of etching, protonation (to form OH groups) and coupling (to form AlOSi interfacial bonds) occur at the solid-liquid interface. Scanning electron microscopy (SEM) was used to determine how the topographies of the modified Al surfaces changed with the different pre-treatments and with exposure to a buffer solution of pH 4. Secondary-ion mass spectrometry (SIMS) was used to determine the direct amount of AlOSi interfacial bonds by measuring the ratio of peak intensities 71-70 amu, while X-ray photoelectron spectroscopy (XPS) was used to determine the overall strength of the silane coating adhesion by measuring the Si 2p signals before and after application of an ultrasonic rinse to the coated sample. Measured Al 2p and O 1s spectra helped assess how the different pre-treatments modified the various Al oxidized surfaces prior to BTSE coating. Pre-treated samples that showed increased AlOSi bonding after BTSE coating corresponded to surfaces, which did not show evidence of significant etching after exposure to a pH 4 environment. This suggests that such surfaces are more receptive to the coupling reaction during exposure to the BTSE coating solution. These surfaces include all H₂ plasma-treated samples, the heated native oxide and the sample that only received the 15 min FPL treatment. In contrast, other surfaces that show evidence of etching in pH 4 environments are samples that received lower amounts of AlOSi interfacial bonding. Overall, heating improved the BTSE adhesive bonding for the native Al oxide, while H₂ plasma treatment improved the BTSE bonding for surfaces that had initially been FPL-treated for 15 and 60 min.     

Improvement of photoluminescence properties of porous silicon by silica passivation

Porous silicon (PS) was passivated by silica film using a sol-gel method; the photoluminescence (PL) properties were significantly improved; namely, PL intensity and stability increased and PL peak shifted to shorter wavelength. Scanning electron microscope (SEM) and Fourier transformed infrared spectroscope (FTIR) results indicated that silica passivation produced a compact film on the PS surface and modified the surface state of PS. The number of stable surface bonds (HSiO₃, HSiSiO₂ and H₂SiO₂) increased due to the oxidation of SiH back-bonds during the gelation process, and thus the PL intensity and stability were improved. Moreover, the blue-shift of PL peak was determined due to the increase in the ratio of SiO/SiH.     

Effect of defects on the reflectivity of Cr/C multilayer soft X-ray mirror at 4.48 nm

Cr/C is a promising material combination for multilayer mirror in the “near water window region” (4.4-6.7 nm). In the present paper, the effect of defects on the reflectivity of Cr/C soft X-ray multilayer mirror deposited by magnetron sputtering was studied. Formation of thin interlayer due to the interdiffusion, rough interface due to the non-sharp layer and contamination of O happened during the deposition process were found by a method combined by XPS, soft X-ray reflectivity at 4.48 nm and grazing incidence hard X-ray reflectivity at 0.154 nm. The XPS results show that both interlayers (Cr-on-C and C-on-Cr) are mixture composed of C sp2, C sp3, CO, CO, CrCr and CrO bondings. No chromium carbide was found at the interlayer probably due to the blocking of oxides’ formation. Through the analysis of X-ray reflectivity, we obtained the multilayer structure parameters (thickness and roughness) and optical constants of each layer at 4.48 nm. Based on those results, a further calculation was carried out. The result shows that the formation of the thin interlayer contributes little to the decrease of the reflectivity, the rough interface decreases the reflectivity most and the contaminant (O) not only decreases the reflectivity but also shifts the position of the peak.     

Carbon monoxide adsorption on Pd-deposited Cu(1 1 0) surface: Infrared reflection absorption and temperature programmed desorption studies

We investigated carbon monoxide (CO) adsorption and desorption behaviors on 0.1-nm-, 0.15-nm-, and 0.3-nm-thick-Pd-deposited Cu(1 1 0) surfaces using infrared reflection absorption (IRRAS) and temperature-programmed desorption (TPD) spectroscopic methods. CO was exposed to the 0.1-nm-thick-Pd/Cu(1 1 0) surface at the substrate temperature of 90 K. The IR band attributable to CO bonded to Cu atoms emerged at 2092 cm⁻¹: the band was located at 2100 cm⁻¹ at saturation coverage, with a shoulder at 2110 cm⁻¹. In addition to these bands, weak absorptions attributable to the PdCO bonds appeared at 2050 and 1960 cm⁻¹. With increasing Pd thickness, the Pd related-bands became increasingly prominent. Particularly at the early stage of exposure, the band at 2115 cm⁻¹ became visible. The band at 2117 cm⁻¹ dominated the spectra all through the exposures for the 0.3-nm-thick-Pd surface. The TPD spectra of the surfaces showed two remarkable features at around 220-250 and 320-390 K, ascribable ,respectively, to CuCO and PdCO. The desorption peaks shifted to higher temperatures with increasing Pd thickness. Based on the TPD and IRRAS results, we discuss the adsorption-desorption behaviors of CO on the Pd/Cu(1 1 0) surfaces.     

Benzene and NO on a Ru(0 0 1) surface: Electronic structure and bonding

We have theoretically studied the co-adsorption of benzene and NO on a Ru surface. The calculations were performed using the atom superposition and electron delocalisation-tight-binding (ASED-TB) method. We have modelled the Ru(0 0 1)-p(3 × 3)-4C₆D₆ + 2NO co-adsorbed layer from experimental data.We have confirmed that the more stable sites are hcp on the Ru for both benzene and NO co-adsorbates.The NO states are more stabilized in the co-adsorbed system. There is more bonding between RuN than RuC. We have described an important interaction of on H (from benzene) and the O (from a close NO). That results explain previous experimental reports and confirm suggested direct interaction.     

Chemistry of Alanine on Pd(1 1 1): Temperature-programmed desorption and X-ray photoelectron spectroscopic study

The adsorption of alanine is studied on a Pd(1 1 1) surface using X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD). It is found that alanine adsorbs into the second and subsequent layers prior to completion of the first monolayer for adsorption at ∼250 K, while at ∼300 K, alanine adsorbs almost exclusively into the first monolayer with almost no second-layer adsorption. Alanine adsorbs onto the Pd(1 1 1) surface in its zwitterionic form, while the multilayer contains about 30-35% neutral alanine, depending on coverage. Alanine is thermally stable on the Pd(1 1 1) surface to slightly above room temperature, and decomposes almost exclusively by scission of the CCOO bond to desorb CO₂ and CO from the COO moiety, and the remaining fragment yields ethylamine and HCN.     

Conformational isomers of stilbene on Si(1 0 0)

Stilbene (1,2-diphenylethylene) has shown an intriguing isomerisation behavior and may serve as a model system for “molecular switches” incorporating a CC double bond. To evaluate the possible use of such molecules as molecular switches on semiconductor surfaces, the adsorption of cis- and trans-stilbene on Si(1 0 0) has been investigated. Identification of both isomers is achieved by differences in adsorption geometry as revealed by NEXAFS, and differences in electronic structure in the occupied and unoccupied molecular orbitals. For both isomers, bonding takes place via the CC double bond to the Si dimer atoms allowing for free movement of the aromatic rings, a necessary prerequisite for photoinduced isomerisation on the surface. Our experimental results agree well with theoretical calculations.     

Preparation of visible-light responsive PF-codoped TiO2 nanotubes

Fabrication of PF-codoped TiO₂ nanotubes was carried out using a one-step electrochemical anodization process by tailoring the composition of the electrolyte with the aim of PF-codoping to extend the optical absorption of TiO₂ to the visible-light region. The as-prepared PF-codoped TiO₂ nanotubes were characterized by SEM, XPS, and UV-vis diffuse reflectance absorption spectra (DRS). The results showed that the tube diameter of the nanotubes was approximately 100 nm and the tube length was approximately 510 nm. The phosphorus and fluorine were successfully doped into TiO₂ nanotubes, as evidenced by XPS. Moreover, the PF-codoped samples displayed remarkably strong visible-light response.     

Adsorption structure of phenylphosphonic acid on an alumina surface

The adsorption structure of phenylphosphonic acid (PPOA) on an alumina surface was investigated as a function of exposure and temperature using infrared reflected absorption spectroscopy (IRAS) and a Kelvin probe. The alumina surface was held at room temperature during deposition. At monolayer adsorption, PO and Phenyl-P bands are observed, which indicate the creation of POAl bonds. The aromatic ring plane is positioned perpendicular to the surface, i.e. deprotonated PPOA stands vertically on the surface. At multilayer adsorption, PO and POH bands appear in the spectra. The multilayer PPOA film starts to desorb at 400 K. From 400 to 700 K, the IRAS spectra are similar to the monolayer spectrum, indicating that the multilayer structure reverses to the monolayer-like PPOA adlayer by heating. The acid molecules start to desorb at 700 K.     

Effect of CCC bond bending vibration on the photodissociation of cyclobutane

The effect of CCC bond bending on the photodissociation of cyclobutane to form two ethylene molecules was investigated by performing semiclassical electron-radiation-ion dynamics simulations and also by examining the potential energy surfaces of the electronic ground state and lowest excited states. These potential energy surfaces, calculated at the CASSCF/MRPT2 level with 6-31G* basis sets along a reaction path determined by the semiclassical dynamics simulations, show well-defined energy minima and maxima in the intermediate state region. It is found that in addition to rotation of the molecule around the central CC bond, CCC bond bending plays an important role in determining the features of the potential energy surfaces for the intermediate species.