Atomic and electronic properties of furan on the Si(0 0 1)-(2 × 2) surface

The atomic and electronic properties of the adsorption of furan (C₄H₄O) molecule on the Si(1 0 0)-(2 × 2) surface have been studied using ab initio calculations based on pseudopotential and density functional theory. We have considered two possible chemisorption mechanisms: (i) [4 + 2] and (ii) [2 + 2] cycloaddition reactions. We have found that the [4 + 2] interaction mechanism was energetically more favorable than the [2 + 2] mechanism, by about 0.2 eV/molecule. The average angle between the CC double bond and Si(1 0 0) surface normal was found to be 22°, which is somewhat smaller than the experimental value of 28°, but somewhat bigger than other theoretical value of 19°. The electronic band structure, chemical bonds, and theoretical scanning tunneling microscopy images have also been calculated. We have determined a total of six surface states (one unoccupied and five occupied) in the fundamental band gap. Our results are seen to be in good agreement with the recent near edge X-ray absorption fine structure and high resolution photoemission spectroscopy data.     

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.     

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-).     

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.     

A theoretical study of c-C5H8 adsorption on Ge (0 0 1)-2 × 1 and on dimer vacancies on the surface: Electronic structure and bonding

In this work we analyzed the geometry and the chemical interactions for c-C₅H₈ adsorption on Ge (0 0 1), using density functional theory calculations (DFT). We examined the changes in the atomic interactions using a slab model. We considered two cases, the cyclopentene adsorption on Ge(0 0 1) and on dimer vacancies on the surface. We found an average distance H-Ge, -C-Ge and C-Ge of 1.50, 1.70 and 1.65 Å, respectively, on dimer vacancies; and an average C-Ge distance of 2.05 Å on Ge-Ge dimer. We also computed the density of states (DOS) and the DOS weighted overlap populations (OPDOS) corresponding to C-C, C-Ge, C-H, and Ge-Ge bonds. During adsorption the main contribution are the CC double bond in both cases, and the next C and the H's belonging to this bonds in the case of adsorption on dimer vacancies. The orbital contribution includes participation of the 2p_y and 2p_z orbitals corresponding to unsaturated C atoms, 2p_z corresponding to side saturated C, and the 4p orbitals of Ge for the adsorption on dimer vacancies; 2s and 2p_z orbitals corresponding to double bond C atoms, 4s and 4p_z orbitals of Ge for the adsorption on Ge(0 0 1).     

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).     

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.     

Site-blocking effects of preadsorbed H on Pt(1 1 1) probed by 1,3-butadiene adsorption and reaction

The influence of hydrogen coadsorption on hydrocarbon chemistry on transition metal surfaces is a key aspect to an improved understanding of catalytic selective hydrogenation. We have investigated the effects of H preadsorption on adsorption and reaction of 1,3-butadiene (H₂CCHCHCH₂, C₄H₆) on Pt(1 1 1) surfaces by using temperature-programmed desorption (TPD) and Auger electron spectroscopy (AES). Preadsorbed hydrogen adatoms decrease the amount of 1,3-butadiene chemisorbed on the surface and chemisorption is completely blocked by the hydrogen monolayer (saturation) coverage (θ_H = 0.92 ML). No hydrogenation products of reactions between coadsorbed H adatoms and 1,3-butadiene were observed to desorb in TPD experiments over the range of θ_H investigated (θ_H = 0.6-0.9 ML). This is in strong contrast to the copious evolution of ethane (CH₃CH₃, C₂H₆) from coadsorbed hydrogen and ethylene (CH₂CH₂, C₂H₄) on Pt(1 1 1). Hydrogen adatoms effectively (in a 1:1 stoichiometry) remove sites from interaction with chemisorbed 1,3-butadiene, but do not affect adjacent sites. The adsorption energy of coadsorbed 1,3-butadiene is not affected by the presence of hydrogen on Pt(1 1 1). The chemisorbed 1,3-butadiene on hydrogen preadsorbed Pt(1 1 1) completely dehydrogenates to H₂ and surface carbon upon heating without any molecular desorption detected, which is identical to that observed on clean Pt(1 1 1). In addition to revealing aspects of site blocking that should have broad implications for hydrogen coadsorption with hydrocarbon molecules on transition metal surfaces in general, these results also provide additional basic information on the surface science of selective catalytic hydrogenation of butadiene in butadiene-butene mixtures.     

A comparative study of coordination between host polymers and Li ions in UV-cured gel polymer electrolytes

Crosslinked gel polymer electrolytes are prepared via free radical photo-polymerization of 1,6-hexanediol diacrylate (HDDA) or tri(ethylene glycol) diacrylate (TEGDA) with 1 M LiClO₄ dissolved in a solvent mixture of ethylene carbonate (EC) and propylene carbonate (PC). TEGDA-based gel polymer electrolytes containing a polar moiety of ethylene oxide exhibit relatively high ionic conductivities over a temperature range from − 15 to 65 °C in comparison to those based on HDDA. The coordination structure between polar moieties of a polymer backbone and Li⁺ ions is examined using a Fourier transform infrared (FT-IR) spectroscopy. The results of FT-IR analyses manifest that the CO and COC groups of TEGDA-based polymer matrix form the complex with Li⁺ ions.     

Misfit driven azimuthal orientation of NaCl domains on Ag(1 0 0)

We investigated the growth of thin NaCl films on Ag(1 0 0) by spot-profile-analysis low energy electron diffraction (SPA-LEED), varying extensively the growth temperature (200–500 K) and the film thickness (0.5–14 ML). The incommensurate growth of NaCl on Ag(1 0 0) yields (1 0 0)-terminated epitaxial NaCl domains, which are preferentially oriented with their [0 1 0] axis parallel to that of the substrate. At 300 K, the NaCl domains exhibit an azimuthal mosaicity by 14° around this orientation and the NaCl unit cell is laterally contracted in the first layers by 0.9% with respect to the bulk. At higher growth temperatures, the azimuthal mosaic distribution sharpens and additional distinct orientations appear, presumably due to a higher-order commensurability. The evolution of the azimuthal mosaic distribution with increasing temperature can be ascribed to both the NaCl thermal expansion and higher diffusion rates of NaCl on Ag(1 0 0). The best epitaxy, i.e. that with the highest selectivity of a specific azimuthal domain orientation, is achieved by growing NaCl films at low deposition rate (0.1 ML min⁻¹) on the Ag(1 0 0) substrate at constant high temperature (450–500 K). The observations made here can probably be applied more generally to other heterogeneous interfaces and, in particular, be used to improve the quality of thin insulating films.     

Interaction of 2,2,6,6-tetramethyl-3,5-heptanedione with the Si(1 0 0)-2 × 1 surface: Scanning tunneling microscopy and density functional theory study

Room temperature adsorption and reaction of 2,2,6,6-tetramethyl-3,5-heptanedione (dpmH) on the Si(1 0 0)-2 × 1 surface has been studied with ultra-high vacuum scanning tunneling microscopy (UHV-STM) and temperature programmed desorption (TPD). The molecule is found to chemisorb as a mixture of at least five distinct species. Density functional theory (DFT) was used to calculate the structures and adsorption energies of 12 possible addition products. Unique bonding assignments for each experimental feature are proposed by consideration of a common intermediate reaction network, and a comparison of possible reaction pathways leading to the final products. These assignments are: OH inter-dimer dissociation, OH intra-dimer dissociation, 1,5 intra-dimer addition, 1,5 inter-dimer addition, and intra-dimer [2 + 2]CO addition with OH dissociation on an adjacent dimer. TPD and STM results show that the molecule dissociates completely upon annealing to 700 °C with formation of the c(4 × 4) phase at low exposures, and SiC islands for exposures exceeding 0.15 L.     

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.     

Adsorption of phenylalanine on single crystal rutile TiO2(1 1 0) surface

The adsorption of the aromatic amino acid, phenylalanine on a TiO₂ rutile (1 1 0) single crystal surface has been investigated with photoemission and NEXAFS (near edge X-ray absorption fine structure) spectroscopy. The results indicate initial adsorption via the carboxylate group in a bidentate configuration with the phenyl ring oriented at approximately 25° to the surface normal. The amino group remains as NH₂. Subsequent layers of phenylalanine appear to adsorb as neutral molecules with H-bonding between NH₂ and CO groups.     

Isomeric effects on room-temperature chemisorption and thermal evolution of iso-, cis- and trans-dichloroethylene on Si(1 1 1)7 × 7

The room-temperature adsorption and thermal evolution of iso-, cis- and trans-dichloroethylene (DCE) on Si(1 1 1)7 × 7 have been studied by vibrational electron energy loss spectroscopy and thermal desorption spectrometry (TDS). The presence of the Si-Cl stretch at 510 cm⁻¹ suggests that, upon adsorption, all three isomers dissociate via C-Cl bond breakage on the 7 × 7 surface to form mono-σ bonded chlorovinyl , which could, in the case of iso-DCE, further dechlorinate to vinylidene (:CCH₂) upon insertion into the back-bond. The higher saturation exposure for the Si-Cl stretch at 510 cm⁻¹ observed for cis- and trans-DCE than iso-DCE suggests that Cl dissociation via the CHCl group in the cis and trans isomers is less readily than the CCl₂ group in iso-DCE. Our TDS data show remarkable similarities in both molecular desorption near 360 K and thermal evolution of the respective adstructures for all three isomers on Si(1 1 1)7 × 7. In particular, upon annealing to 450 K, the mono-σ bonded chlorovinyl adspecies is found to further dechlorinate to either vinylene di-σ bonded to the Si surface or acetylene to be released from the surface. Above 580 K, vinylene could also become gaseous acetylene or undergo H abstraction to produce hydrocarbon or SiC fragments. All three DCE isomers also exhibit TDS features attributable to an etching product SiCl₂ at 800-950 K and recombinative desorption products HCl at 700-900 K and H₂ at 650-820 K. The stronger Cl-derived TDS signals and Si-Cl stretch at 510 cm⁻¹ over 450-820 K for trans-DCE than those for cis-DCE indicate stronger dechlorination for trans-DCE than cis-DCE, which could be due to less steric hindrance resulting from the formation of the chlorovinyl adspecies for trans-DCE during the initial adsorption/dechlorination process. Finally, our density functional calculations qualitatively support the thermodynamic feasibility and relative stabilities of the proposed adstructures involving chlorovinyl, vinylidene, and vinylene adspecies.     

Structure and reaction pathways of methyl lactate on Pd(1 1 1)

The adsorption and reaction of methyl lactate (CH₃CH(OH)COOCH₃) is studied in ultrahigh vacuum on a Pd(1 1 1) surface using temperature-programmed desorption (TPD) and reflection–absorption infrared spectroscopy (RAIRS). Methyl lactate reacts at relatively low temperatures (220 K) by O–H bond scission. This intermediate can either react with hydrogen to reform methyl lactate at 280–300 K or undergo β-hydride elimination to form flat-lying methyl pyruvate. This decomposes to form acetyl and methoxy carbonyl species as found previously following methyl pyruvate adsorption on Pd(1 1 1). These species predominantly react to form carbon monoxide, methane and hydrogen.     

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

The surface chemistry of proline is explored on Pd(1 1 1) using a combination of temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy. Proline adsorbs on Pd(1 1 1) at temperatures of 250 K and below into second and subsequent layers prior to the saturation of the first layer, where approximately 70% of the adsorbed proline is present in its zwitterionic form. Molecular proline desorbs between ∼315 K and ∼333 K depending on coverage. When adsorbed at ∼300 K, only the first monolayer is formed, and the proline is present as zwitterions, oriented such that all of the carbons are detected equally by XPS. Proline decomposes by scission of the C-COO bond, where the carboxylate moiety desorbs as carbon monoxide and carbon dioxide, while the nitrogen-containing moiety desorbs as to HCN, and evolves pyrrole at ∼390 K, pyrrolidine at ∼410 K, and final species that desorbs at ∼450 K that cannot be unequivocally assigned but may be 2-butenenitrile (CH₃-CHCH-CN), 3-butenenitrile (CH₂CH-CH₂-CN), 2-methyl-2-propenenitrile (CH₂C(CH₃)-CN) or cyclopropanecarbonitrile.     

Adsorption of nitrogen-containing aromatic molecules on Si(1 1 1)-7 × 7

The adsorption configurations of pyrimidine and triazine on Si(1 1 1)-7 × 7 were investigated using high-resolution electron energy loss spectroscopy (HREELS) X-ray photoelectron spectroscopy and density functional theory calculations. The HREELS spectra of chemisorbed monolayer show the coexistence of the C(sp²)-H and C (sp³)-H stretching modes together with the observation of the unconjugated CN(C) vibrational feature suggesting that the carbon atom and its para-nitrogen atom of pyrimidine and triazine directly participate in binding with the surface to form Si-C and Si-N σ-linkages. The core levels of the C-atom and its opposite nitrogen atom directly binding with Si-atoms experience a down-shifting by 1.8-1.9 and 1.4-1.6 eV, respectively. These experimental findings are consistent with the density functional theory calculations indicating that the carbon atom and its para-nitrogen atom favorably link with the adjacent adatom and rest atom pair to form C-Si and N-Si linkages.     

Adsorption of organic molecules by photochemical reaction on Cl:Si(1 1 1) and H:Si(1 1 1) evaluated by HREELS

The covalent attachment of alkyl groups to silicon surfaces, via carbon-silicon bond formation, has been attempted using gas-surface reactions starting from Cl-terminated Si(1 1 1) or H:Si(1 1 1) under ultraviolet light irradiation. The formation of Cl-terminated Si(1 1 1) and its resulting stability were examined prior to deposition of organic molecules. High-resolution electron energy loss spectroscopy (HREELS) was utilized for detecting surface-bound adsorbates. The detection of photo-deposited organic species on Cl:Si(1 1 1) from gas-phase CH₄ or CH₂CH₂ was not significant. On H:Si(1 1 1), it was evident that after the photoreaction with gas-phase C₂H₅Cl, C₂H₅ groups were chemically bonded to the surface Si atoms through single covalent bonds. The C₂H₅ groups were thermally stable at temperatures below 600 K. Alkyl monolayers prepared on silicon surfaces by dry process will lead to a new prospective technology of nanoscale fabrication and biochemical applications.     

Formamide reactions on rutile TiO2(0 1 1) surface

The reaction of formamide over the (0 1 1) faceted TiO₂(0 0 1) surface has been studied by Temperature Programmed Desorption (TPD) and X-ray Photoelectron Spectroscopy (XPS). Two main reactions were observed: dehydration to HCN and H₂O and decomposition to NH₃ and CO. The dehydration reaction was found to be three to four times larger than the decomposition at all coverages. Each of these reactions is found to occur in two temperature domains which are dependent upon surface coverage. The low temperature pathway (at about 400 K) is largely insensitive to surface coverage while the high temperature pathway (at about 500 K) shifts to lower temperatures with increasing surface coverage. These two temperature pathways may indicate two adsorption modes of formamide: molecular (via an η¹(O) mode of adsorption) and dissociative (via an η²(O,N) mode of adsorption). C1s and N1s XPS scans indicated the presence of multiple species after formamide absorption at 300 K. These occurred at ca. 288.5 eV (-CONH-) and 285 eV (sp³/sp² C) for the C1s and 400 eV-(NH₂), 398 eV (-NH) and 396 eV (N) for the N1s and result from further reaction of formamide with the surface.     

Degradation behavior and products of malathion and chlorpyrifos spiked in apple juice by ultrasonic treatment

Apple juice (13 °Brix) spiked with malathion and chlorpyrifos (2–3 mg l⁻¹ of each compound) was treated under different ultrasonic irradiations. Results showed that ultrasonic treatment was effective for the degradation of malathion and chlorpyrifos in apple juice, and the output power and treatment time significantly influenced the degradation of both pesticides (p < 0.05). The maximum degradations were achieved for malathion (41.7%) and chlorpyrifos (82.0%) after the ultrasonic treatment at 500 W for 120 min. The degradation kinetics of both pesticides were fitted to the first-order kinetics model well (R²  0.90). The kinetics parameters indicated that chlorpyrifos was much more labile to ultrasonic treatment than malathion. Furthermore, malaoxon and chlorpyrifos oxon were identified as the degradation products of malathion and chlorpyrifos by gas chromatography–mass spectrometry (GC–MS), respectively. The oxidation pathway through the hydroxyl radical attack on the PS bond of pesticide molecules was proposed.