In-situ study on thermal decomposition of 1,3-disilabutane to silicon carbide on Si(1 0 0) surface

The intermediates of thermal decomposition of 1,3-disilabutane (SiH₃CH₂SiH₂CH₃, DSB) to form SiC on Si(1 0 0) surface were in situ investigated by reactive ion scattering (RIS), temperature programmed reactive ion scattering (TPRIS), temperature programmed desorption (TPD), and auger electron spectroscopy (AES). DSB as a single molecular precursor was exposed on Si(1 0 0) surface at a low temperature less than 100 K, and then the substrate was heated up to 1000 K. RIS, TPD, and AES investigations showed that DSB adsorbed molecularly and decomposed to SiC via some intermediates on Si(1 0 0) surface as substrate temperature increasing. Between 117 and 150 K molecularly adsorbed DSB desorbed partially and decomposed to CH₄Si₂, which is the first observation on Si(1 0 0) surface, and further decomposed to CH₄Si between 150 and 900 K. CH₄Si lost hydrogen and formed SiC over 900 K.     

High resolution X-ray photoelectron spectroscopy study on initial oxidation of 4H-SiC(0 0 0 1)-(√3 × √3)R30° surface

An initial oxidation dynamics of 4H-SiC(0 0 0 1)-(√3 × √3)R30° surface has been studied using high resolution X-ray photoelectron spectroscopy and supersonic molecular beams. Clean 4H-SiC(0 0 0 1)-(√3 × √3)R30° surface was exposed to oxygen molecules with translational energy of 0.5 eV at 300 K. In the first step of initial oxidation, oxygen molecules are immediately dissociated and atomic oxygens are inserted into Si-Si back bonds to form stable oxide species. At this stage, drastic increase in growth rate of stable oxide species by heating molecular beam source to 1400 K was found. We concluded that this increase in growth rate of stable oxide is mainly caused by molecular vibrational excitation. It suggests that the dissociation barrier is located in the exit channel on potential energy hypersurface. A metastable molecular oxygen species was found to be adsorbed on a Si-adatom that has two oxygen atoms inserted into the back bonds. The adsorption of the metastable species is neither enhanced nor suppressed by molecular vibrational excitation.     

Electron surface states in short-period superlattices: (GaAs)2/(AlAs)2(1 0 0)-c(4 × 4)

The electronic structure of (GaAs)₂/(AlAs)₂(1 0 0)-c(4 × 4) superlattice surfaces was studied by means of angular-resolved photoelectron spectroscopy (ARUPS) in the photon energy range 20-38 eV. Four samples with different surface termination layers were grown and As-capped by molecular beam epitaxy (MBE). ARUPS measurements were performed on decapped samples with perfect c(4 × 4) reconstructed surfaces. An intensive surface state was, for the first time, observed below the top of the valence band. This surface state was found to shift with superlattices’ different surface termination in agreement with theoretical predictions.     

Initial stages of the autocatalytic oxidation of the InAs(0 0 1)-(4 × 2)/c(8 × 2) surface by molecular oxygen

The initial stages of oxidation of the In-rich InAs(0 0 1)-(4 × 2)/c(8 × 2) surface by molecular oxygen (O₂) were studied using scanning tunneling microscopy (STM) and density functional theory (DFT). It was shown that the O₂ dissociatively chemisorbs along the rows in the [1 1 0] direction on the InAs surface either by displacing the row-edge As atoms or by inserting between In atoms on the rows. The dissociative chemisorption is consistent with being autocatalytic: there is a high tendency to form oxygen chemisorption sites which grow in length along the rows in the [1 1 0] direction at preexisting oxygen chemisorption sites. The most common site size is about 21-24 Å in length at ∼25% ML coverage, representing 2-3 unit cell lengths in the [1 1 0] direction (the length of ∼5-6 In atoms on the row). The autocatalysis was confirmed by modeling the site distribution as non-Poisson. The autocatalysis and the low sticking probability (∼10⁻⁴) of O₂ on the InAs(0 0 1)-(4 × 2)/c(8 × 2) are consistent with activated dissociative chemisorption. The results show that is it critical to protect the InAs surface from oxygen during subsequent atomic layer deposition (ALD) or molecular beam epitaxy (MBE) oxide growth since oxygen will displace As atoms.     

Molecular precursor-mediated methanol dissociation on Si(1 1 1)7 × 7: ab initio study

We present an ab initio study of methanol interaction with the Si(1 1 1)7 × 7 surface using a Si(1 1 1)4 × 2 model. The study of the methanol dissociation on Si(1 1 1)4 × 2 shows that pair dissociation on adatom-restatom dangling bonds is largely favoured, in agreement with the experimental observations. The “center” type adatom is slightly more reactive than the “corner” type one, although the difference is weak. Similar behaviour is observed in both adatom types. Our results for a direct CH₃OH dissociation favouring a basic cleavage (adsorption of OH and CH₃ fragments) rather than an acidic one (adsorption of H and OCH₃ fragments), we are finally led to take a kinetic effect into consideration to reconcile theory with experiment. We show that the presence of molecular precursor states is possible. Different orientations with respect to the silicon dangling bonds of these molecular precursors are investigated. However, the corresponding energies are very close and, considering their relative energies, it is finally difficult to discriminate between acidic and basic cleavages.     

RT growth of acetonitrile and acrylonitrile on Si(0 0 1)-2 × 1 studied by XPS and LEED

In this work we show the adsorption of acetonitrile (CH₃CN) and acrylonitrile (CH₂CHCN) on Si(0 0 1)-2 × 1 at room temperature by increasing the molecular doses. Especially, by means of XPS and LEED data, we stress the action of these molecules on the silicon surface locating the dangling-bonds quasi-saturation within 10 L. The shortage of nitrogen XPS signal and some anomalies in carbon spectra point to an invading action from a traditional X-ray source (Al-K_α line) against chemisorbed molecules. In particular, we think that a long exposure to this radiation could break carbon-silicon bonds changing some adsorption geometries and making desorb molecular fragments.     

Growth and structure of vanadium oxide on anatase (1 0 1) terraces

The interaction of vanadium oxide with epitaxial anatase films exposing (1 0 1) terraces was characterized. The TiO₂ films were grown on vicinal LaAlO₃ (1 1 0) substrates by oxygen plasma-assisted molecular beam epitaxy (OPA-MBE); reflection high energy and low energy electron diffraction (RHEED and LEED) indicated that the films exposed (1 0 1) terraces of the anatase TiO₂ polymorph. When a vanadium oxide monolayer was deposited onto the anatase surface by OPA-MBE at 725 K, only (1 × 1) RHEED and LEED patterns were observed. The V X-ray photoelectron spectroscopy (XPS) peak intensities indicated that the monolayer wetted the anatase surface and so the diffraction patterns were attributed to an epitaxial vanadia layer. Analysis of the vanadium oxide monolayer by X-ray and ultraviolet photoelectron spectroscopies revealed that the V was predominantly 5+. When the vanadia coverage was increased at 725 K, Auger electron spectra showed only very slow attenuation of the anatase Ti peaks while spots began to develop in RHEED patterns recorded along the LaAlO₃ direction; both indicative of 3-D cluster formation. In the orthogonal direction, the RHEED patterns showed unusual diagonal streaks. Meanwhile, the (1 × 1) LEED pattern persisted even after 30 nm of vanadia was deposited. This was attributed to gaps between the 3-D clusters exposing the epitaxial monolayer. Core level XPS spectra of the 3-D clusters revealed a broad V 2p_3/2 peak that was centered at the position expected for V⁴⁺ but could be deconvoluted into three peaks corresponding to V³⁺, V⁴⁺, and V⁵⁺. It is shown that crystallographic shear that accommodates such variations in the oxygen content of V oxides can lead to the diagonal streaks in RHEED patterns recorded along the LaAlO₃ [0 0 1] direction even as the pattern in the orthogonal direction shows sharp transmission spots. The results show that vanadia growth on anatase (1 0 1) proceeds through the Stranski-Krastanov mode with a strong vanadia-titania interaction stabilizing a dispersed vanadia monolayer. The results are compared with previous data for vanadia growth on anatase (0 0 1) where smooth, epitaxial VO₂ films grow ad infinitum.     

Adsorption and thermal reaction of short-chain alkanethiols on GaAs(1 0 0)

By means of temperature-programmed desorption (TPD) and X-ray photoemission spectroscopy (XPS) with synchrotron radiation, we investigated the adsorption and thermal decomposition of alkanethiols (RSH, R = CH₃, C₂H₅, and C₄H₉) on a GaAs(1 0 0) surface. All chemisorbed alkanethiols can deprotonate to form thiolates below 300 K via dissociation of the sulfhydryl hydrogen (-SH). Two types of thiolates species are observed on GaAs(1 0 0), according to adsorption on surface Ga and As sites. The thiolates adsorbed on a Ga site preferentially recombine with surface hydrogen to desorb as a molecular thiol at 350-385 K. The thiolate on the As site exhibits greater thermal stability and undergoes mainly dissociation of the C-S bond at ∼520 K, independent of the alkyl chain length. The decomposition of CH₃S either directly desorbs CH₃ or transfers the CH₃ moiety onto the surface. The surface CH₃ further evolves directly from the surface at 665 K. The dissociations of C₂H₅S and C₄H₉S yield surface C₂H₅ and C₄H₉, which further decompose to desorb C₂H₄ and C₄H₈, respectively, via β-hydride elimination. The complete decomposition of alkanethiol leads to the formation of surface S without deposition of carbon. Adsorption of CH₃SSCH₃ results in the formation of surface CH₃S at initial exposures via scission of the S−S bond. Compared with the adsorption of CH₃SH, the CH₃S on the Ga site exhibits greater thermal stability because surface hydrogen is absent. At a high exposure, CH₃SSCH₃ can absorb molecularly on the surface and decompose to desorb CH₃SCH₃ via formation of a CH₃SS intermediate.     

Surface reactions of 2-iodopropane on GaAs(1 0 0)

The surface reactions of 2-iodopropane ((CH₃)₂CHI) on gallium-rich GaAs(1 0 0)-(4 × 1), was studied by temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). CH₃CHICH₃ adsorbs molecularly at 120 K but dissociates below room temperature to form chemisorbed 2-propyl ((CH₃)₂CH) and iodide (I) species. Thermal activation causes desorption of the molecular species at 240 K, and this occurs in competition with the further reactions of the (CH₃)₂CH and I chemisorbed species. Self-coupling of the (CH₃)₂CH results in the formation of 2,3-dimethylbutane ((CH₃)₂CH-CH(CH₃)₂) at 290 K. β-Hydride elimination in (CH₃)₂CH yields gaseous propene (CH₃CHCH₂) at 550 K while reductive elimination reactions of (CH₃)₂CH with surface hydrogen yields propane (CH₃CH₂CH₃) at 560 K. Recombinative desorption of the adsorbed hydrogen as H₂ also occurs at 560 K. We observe that the activation barrier to carbon-carbon bond formation with 2-propyls on GaAs(1 0 0) is much lower than that in our previous investigations involving ethyl and 1,1,1-trifluoroethyl species where the β-elimination process was more facile. The difference in the surface chemistry in the case of 2-propyl species is attributable to its rigid structure resulting from the bonding to the surface via the second carbon atom, which causes the methyl groups to be further away from the surface than in the case of linear ethyl and 1,1,1-trifluoroethyl species. The β-hydride and reductive elimination processes in the adsorbed 2-propyl species thus occurs at higher temperatures, and a consequence of this is that GaI desorption, which is expected to occur in the temperature range 550-560 K becomes suppressed, and the chemisorbed iodine leaves the surface as atomic iodine.     

The dehydrogenation of CH4 on Rh(1 1 1), Rh(1 1 0) and Rh(1 0 0) surfaces: A density functional theory study

CH₄ dehydrogenation on Rh(1 1 1), Rh(1 1 0) and Rh(1 0 0) surfaces has been investigated by using density functional theory (DFT) slab calculations. On the basis of energy analysis, the preferred adsorption sites of CH_x (x = 0-4) and H species on Rh(1 1 1), Rh(1 1 0) and Rh(1 0 0) surfaces are located, respectively. Then, the stable co-adsorption configurations of CH_x (x = 0-3) and H are obtained. Further, the kinetic results of CH₄ dehydrogenation show that on Rh(1 1 1) and Rh(1 0 0) surfaces, CH is the most abundant species for CH₄ dissociation; on Rh(1 1 0) surface, CH₂ is the most abundant species, our results suggest that Rh catalyst can resist the carbon deposition in the CH₄ dehydrogenation. Finally, results of thermodynamic and kinetic show that CH₄ dehydrogenation on Rh(1 0 0) surface is the most preferable reaction pathway in comparison with that on Rh(1 1 1) and Rh(1 1 0) surfaces.     

Hydrogen adsorption on GaAs(0 0 1)-c(4 × 4)

Exposure of the As-terminated GaAs(0 0 1)-c(4 × 4) reconstructed surface to atomic hydrogen (H) at different substrate temperatures (50-480 °C) has been studied by reflection high-energy electron diffraction (RHEED) and scanning tunnelling microscopy (STM). Hydrogen exposure at low temperatures (∼50 °C) produces a disordered (1 × 1) surface covered with AsH_x clusters. At higher temperatures (150-400 °C) exposure to hydrogen leads to the formation of mixed c(2 × 2) and c(4 × 2) surface domains with H adsorbed on surface Ga atoms that are exposed due to the H induced loss of As from the surface. At the highest temperature (480 °C) a disordered (2 × 4) reconstruction is formed due to thermal desorption of As from the surface. The results are consistent with the loss of As from the surface, either through direct thermal desorption or as a result of the desorption of volatile compounds which form after reaction with H.     

A density functional theory study of CH4 dehydrogenation on Co(1 1 1)

The dehydrogenation of CH₄ on the Co(1 1 1) surface is studied using density functional theory calculation (DFT). It is found that CH₄ is favored to dissociate to CH₃ and then transforms to CH₂ and CH by sequential dehydrogenation, and CH₄ activation into CH₃ and H is the rate-determining step on the Co(1 1 1) surface. CH₂ is quite unstable on Co(1 1 1) surface. CH dehydrogenation into C and H is difficult. CH₃ and H prefer to adsorb on 3-fold hollow hcp and fcc sites, and CH₂, CH and C prefer to adsorb on hcp sites.     

Hydrogen desorption kinetics and band bending for 6H-SiC(0 0 0 1) surfaces

The desorption kinetics of hydrogen from polished 6H-SiC(0 0 0 1) surfaces exposed to various sources of hydrogen have been determined using temperature programmed desorption (TPD). For (3 × 3) 6H-SiC(0 0 0 1) surfaces prepared via annealing and cooling in SiH₄, desorption of 0.2 ± 0.05 monolayer of molecular hydrogen was observed to occur at ≈590 °C. This β₁ H₂ desorption peak exhibited second order kinetics with an activation energy of 2.4 ± 0.2 eV. For (3 × 3) 6H-SiC surfaces exposed to atomic hydrogen generated via either a hot rhenium filament or remote hydrogen plasma, low energy electron diffraction patterns showed an eventual conversion back to (1 × 1) symmetry. Spectra acquired using Auger electron and X-ray photoelectron spectroscopies revealed that the atomic hydrogen exposure removed the excess Si. Photoelectron spectroscopy results also showed a 0.5 eV increase in binding energy for the Si2p and C1s core levels after removal of the Si-Si bilayer that is indicative of a decrease in band bending at the SiC surface. TPD from the (3 × 3) 6H-SiC(0 0 0 1) surfaces exposed to atomic hydrogen showed substantially more molecular hydrogen desorption (1-2 ML) through the appearance of a new desorption peak (β_2,3) that started at ≈200 °C. The β_2,3 peak exhibited second order desorption kinetics and a much lower activation energy of 0.6 ± 0.2 eV. A third smaller hydrogen desorption state was also detected in the 650-850 °C range. This last feature could be resolved into two separate desorption peaks (α₁ and α₂) both of which exhibited second order kinetics with activation energies of 4.15 ± 0.15 and 4.3 ± 0.15 eV, respectively. Based on comparisons to hydrogen desorption from Si and diamond surfaces, the β and α desorption peaks were assigned to hydrogen desorption from Si and C sites, respectively.     

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.     

The growth and structure of titanium dioxide films on a Re(1 0 −1 0) surface: Rutile(0 1 1)-(2 × 1)

Titanium dioxide films were grown on Re(1 0 −1 0) by Ti vapor deposition in oxygen at T = 830 K and studied by means of low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), low-energy ion scattering (LEIS) and X-ray diffraction (XRD). The Ti oxide stoichiometry was determined by XPS as Ti:O = 1:2, with the Ti oxidation state (4+). The TiO₂ growth was monitored by means of LEED as a function of film thickness. Extending the coverage from the submonolayer into the multilayer regime gives rise to a p(2 × 2) pattern, a (poorly ordered) (1 × 1), and, finally, a stable (2 × 2) structure, the latter being associated with a homogeneous TiO₂ phase. For normal electron incidence, the (2 × 2) LEED pattern exhibits systematically extinguished beams at (n ± 1/2, 0) positions, indicating a glide mirror plane. The pg(2 × 2) structure could be explained by both a rutile(0 1 1)-(2 × 1) reconstructed surface and a bulk truncated brookite(0 0 1) surface. Faceting phenomena, i.e. running LEED spots, observed with thin TiO₂ films point to the formation of a rutile(0 1 1)-(2 × 1) surface with two domains and {0 1 1}-(2 × 1) facets and rule out the brookite alternative. Confirmation of this assignment was obtained by an XRD analysis performed at the Berlin synchrotron facility BESSY.     

Atomic and molecular adsorption on Pt(1 1 1)

The adsorption of several atomic (H, O, N, S, and C) and molecular (N₂, HCN, CO, NO, and NH₃) species and molecular fragments (CN, CNH₂, NH_2, NH, CH₃, CH₂, CH, HNO, NOH, and OH) on the (1 1 1) facet of platinum, an important industrial and fuel cell catalyst, was studied using self-consistent periodic density functional theory (DFT-GGA) calculations at a coverage of 1/4 ML. The best binding site, energy, and position, as well as an estimated diffusion barrier, of each species were determined. The binding strength for all the species can be ordered as follows: N₂ < NH₃ < HCN < NO < CO < CH₃ < OH < NH₂ < H < CN < NH < O < HNO < CH₂ < NOH < CNH₂ < N < S < CH < C. Although the atomic species generally preferred fcc sites, there was no clear trend in site preference by the molecular species or molecular fragments. The vibrational frequencies of all the stable adsorbates in their best and second best adsorption sites were calculated and found to be in good agreement with experimental values reported in the literature. Finally, the decomposition thermochemistry of NOH, HNO, NO, NH₃, N₂, CO, and CH₃ was analyzed.     

A theoretical study of CH4 dissociation on Pt(1 0 0) surface

Density functional theory calculations have been performed on the adsorption of H and CH₃, and the dissociation of CH₄ on Pt(1 0 0) surface. It was found that H was adsorbed on the top and bridge sites, while CH₃ was adsorbed only on the top site. The coadsorption of methyl and hydrogen which has also been investigated shows that the interaction between the two adsorbates is stabilising. In addition, two distinct pathways were explored, differing by the initial adsorbed state of CH₄. They converge readily to the same transition state corresponding to an activation energy value of 0.53 eV. These results compare favourably with existing data in the literature for Pt(1 1 1) and Pt(1 1 0).     

The mechanism of amine formation on Si(1 0 0) activated with chlorine atoms

The dissociation of NH₃ on a Si(1 0 0) surface activated with Cl atoms was investigated using X-ray photoelectron spectroscopy. Gas phase UV-Cl₂ (0.1-10 Torr Cl₂ for 10-600 s under 1000 W Xe lamp illumination) completely replaced the H-termination on aqueous-cleaned Si(1 0 0) with 0.82 ± 0.06 ML of Cl at 298 K. A single spin-orbit split Cl 2p doublet indicated that the Cl atoms were bound to Si dimer atoms, forming silicon monochloride (Cl-Si-Si-Cl). Exposing the Cl-terminated surface at 348 K to NH₃ (1-1000 Torr for 5-60 min) replaced one Cl atom with one N atom up to a coverage of 0.33 ± 0.02 ML. Cl atoms lowered the activation energy barrier for reaction to form a primary amine (Si-NH₂). Oxygen was coadsorbed due to competition by H₂O contamination. The presence of Cl on the surface even after high NH₃ exposures is attributed to site blocking and electrostatic interactions among neighboring Cl-Si-Si-NH₂ moieties. The results demonstrate a low temperature reaction pathway for depositing N-bearing molecules on Si surfaces.     

Methanethiolate structural phases on Cu{1 1 1} observed using a novel fibre-optic low-energy electron diffraction instrument

We have used a novel fibre-optic low-energy electron diffraction (FO-LEED) instrument, capable of low flux measurements that minimise electron beam damage to surface overlayers, to study methanethiolate (CH₃-S-) structural phases formed on Cu{1 1 1} at temperatures between 110 and 300 K. Three structural phases were seen: a (√3 × √3)R30° phase that forms at 110-140 K; a (4 × 4) phase which was observed transiently at 110 K; and a pseudo-{1 0 0} reconstructed phase which forms at room temperature. We discuss these in the context of previous studies of this system, and demonstrate the ability of the FO-LEED instrument to record high-quality LEED patterns and intensity data from a strongly beam-sensitive surface.     

DFT study on dissociative adsorption of SiH4 and GeH4 on SiGe(1 0 0)-2 × 1 surface

SiH₄ and GeH₄ dissociative adsorptions on a buckled SiGe(1 0 0)-2 × 1 surface have been analyzed using density functional theory (DFT) at the B3LYP level. The Ge alloying in the Si(1 0 0)-2 × 1 surface affects the dimer buckling and its surface reactivity. Systematic Ge influences on the reaction energetics are found in SiH₄ and GeH₄ reactions with four dimers of Si^∗-Si, Ge^∗-Si, Ge^∗-Ge, and Si^∗-Ge (∗ denotes the protruded atom). On a half H-covered surface, the energy barriers for silane and germane adsorption are higher than those on the pristine surface. The energy barrier for silane adsorption is higher than the corresponding barrier for germane adsorption. Rate constants are also calculated using the transition-state theory. We conclude that the SiGe surface reactivity in adsorption reaction depends on the Ge presence in dimer form. If the surface Ge is present in form of Ge^∗-Ge, the surface reactivity decreases as the Ge^∗-Ge content increases. If the surface Ge prefers to be in form of Ge^∗-Si at low Ge contents, the surface reactivity increases first, then decreases at high Ge surface contents when Ge^∗-Ge prevails. The calculated rate constant ratio of GeH₄ adsorption on Si^∗-Si over Ge^∗-Ge at 650 °C is 2.1, which agrees with the experimental ratio of GeH₄ adsorption probability on Si(1 0 0) over Si(1 0 0) covered by one monolayer Ge. The experimental ratio is 1.7 measured through supersonic molecular beam techniques. This consistency between calculation and experimental results supports that one monolayer of Ge on Si(1 0 0) exists in form of Ge^∗-Ge dimer.