The adsorption of C6H5Cl on Si(111)7 × 7 studied by STM

The adsorption of chlorobenzene on Si(111)7 × 7 at room temperature was studied by scanning tunneling microscopy (STM). Selective chemisorption was observed at different adatom sites. It was found that the center adatoms were more reactive than the corner adatoms, and the faulted half of the unit cell was more reactive than the unfaulted. The mechanism is discussed in terms of the electronic and atomic structures in Si(111)7 × 7. Both preferences indicate that chlorobenzene was present initially in a mobile precursor state.     

Atomic motion induced by a scanning tunneling microscope tip on the Si(111) surface

Using scanning tunneling microscopy (STM), we have observed the STM-tip-induced atomic motion of the Si adatoms in the Si(111)−(7 × 7) surface structure near out-of-phase boundaries in the structure. In order to excite the motion of Si adatoms, the tip was fixed at a certain height at every pixel in the lateral scan and the sample bias was changed stepwise. Although the motion of Si adatoms occurs in a complicated manner, analysis shows that its elemental process is quite simple. Namely, the Si adatom jumps from an occupied T₄ site to an unoccupied T₄ site.     

Hydrogen adsorption on Mo1−xRex(110) (x=0–0.25) surfaces

The effects of adsorbed H on the Mo_1−xRe_x(110), x=0, 0.05, 0.15, and 0.25, surfaces have been investigated using low-energy electron diffraction (LEED) and high-resolution electron energy loss spectroscopy (HREELS). For the x=0.15 alloy only, a c(2×2) LEED pattern is observed at a coverage Θ0.25 ML. A (2×2) pattern is observed for H coverages around Θ0.5 ML from surfaces with x=0, 0.05, and 0.15. Both c(2×2) and (2×2) patterns are attributed to reconstruction of the substrate. At higher coverages, a (1×1) pattern is observed. For the alloy surface with x=0.25, only a (1×1) pattern is obtained for all H coverages. Two H vibrations are observed in HREELS spectra for all Re concentrations, which shift to higher energies at intermediate coverages. Both peaks exhibit an isotopic shift, confirming their assignment to hydrogen. For Re concentrations of x=0.15 and higher, a third HREELS peak appears at 50 meV as H (D) coverage approaches saturation. This peak does not shift in energy with isotopic substitution, yet cannot be explained by contamination. The intrinsic width of the loss peaks depends on the Re concentration in the surface region and becomes broader with increasing x. This broadening can be attributed to surface inhomogeneity, but may also reflect increased delocalization of the adsorbed hydrogen atom.     

Dissociative adsorption and site specificity in the initial stages of tetraethoxysilane (TEOS) interaction with Si(111)-(7×7)

We report results of a scanning tunneling microscopy and X-ray photoelectron spectroscopy study on the interaction of tetraethoxysilane (TEOS), a precursor for chemical vapor deposition of silicon dioxide, with Si(111)-(7×7) at room temperature. Under these conditions the interaction of TEOS with the surface is predominantly dissociative. The main adsorption products are ethyl- and triethoxysiloxane groups, which probably evolve in a four-center reaction of TEOS with two neighboring surface dangling bonds. Adsorption of the dissociation products is highly site-selective: triethoxysiloxane groups adsorb on the Si adatoms while ethyl groups react with the rest-atom dangling bonds. Center adatoms are over three times more reactive towards this reaction than corner adatoms. This two-fold selectivity is explained within the concept of local electron donor/acceptor properties.     

Mechanism and energetics of dimerization of SiH2 radicals on H-terminated Si(0 0 1)-(2×1) surfaces

The mechanism and energetics are presented of the dimerization of two adsorbed surface SiH₂ groups on the H-terminated Si(0 0 1)-(2 × 1) surface to form Si₂H₄ species during the initial stages of growth in plasma deposition of hydrogenated amorphous silicon (a-Si:H) films. The reactions are observed during classical molecular-dynamics (MD) simulations of a-Si:H film deposition from SiH₂ radical precursors impinging on an initially H-terminated Si(0 0 1)-(2 × 1) surface and substrate temperature, T, over the range 500T700 K. The Si₂H₄ species resulting from the surface SiH₂ dimerization reactions undergo surface conformational changes resulting in either a non-rotated (NRD) or a rotated dimer (RD) configuration. The RD configuration is found to be the energetically favorable one. The MD simulation results for the structure of the NRD and RD surface Si₂H₄ configurations corroborate with ab initio calculations of optimized adsorption configurations of SiH₂ radicals on crystalline Si surfaces, as well as results of STM imaging of the thermal decomposition of disilane on Si(0 0 1).     

STM study of low pressure adsorption of silane on Si(111)7 × 7

We report a study of silane adsorption on the Si(111)7 × 7 surface. We have been interested in the first stages of chemisorption at room temperature. Reactive sites of the unit cell have been clearly identified on Scanning Tunneling Microscopy (STM) images: the reaction involves the rest atom and the adjacent adatom of the DAS structure with preferential adsorption on the center adatom. We propose an original chemisorption mechanism which leads to the formation of two SiH₂ species by chemisorption and involves the breaking of Si---Si backbonds of the adatom.     

Surface reactions of monoethylgermane on Si(100)-(2×1)

The adsorption and decomposition of monoethylgermane (GeH₃Et) on the Si(100)-(2×1) surface was investigated with the intent of elucidating the surface processes leading to the deposition of germanium. The low-temperature adsorption of the molecule was explored, as well as its thermal decomposition. H₂ and C₂H₄ are observed as the desorption products in temperature-programmed desorption experiments. The ethylene is produced by a hydride elimination reaction within the adsorbed ethyl groups. The amount of Ge which can be deposited in a reaction cycle is correlated with the number of sites occupied by the ethyl groups upon the dissociation of GeH₃Et.     

Potassium adsorption on graphite(0001)

Potassium adsorption on graphite has been studied with emphasis on the two-dimensional K adlayer below one monolayer. Data are presented for the work function versus coverage, high-resolution electron energy loss spectroscopy (HREELS) vibrational spectra of K-adlayers, low energy electron diffraction and ultraviolet photoemission spectroscopy (UPS) spectra at different coverages. The data provide information regarding the vibrational properties of the K-adlayer, the metallization of the adlayer at submonolayer coverages, and the charge transfer from the K adatoms to the graphite substrate. Analysis of the work function, HREELS, and UPS data provides a qualitatively consistent picture of the charge state of the K adatoms, where at low coverages, below a critical coverage θ_c (θ_c=0.2–0.3), the K adatoms are dispersed and (partially) ionized, whereas at θ>θ_c islands of a metallic 2×2 K phase develops that coexist with the dispersed a K adatoms up to θ=1. We show that it is possible to understand the variation of the work function data based on a two-phase model without invoking a depolarization mechanism of adjacent dipoles, as is normally done for alkali-metal adsorption on metal surfaces. Similarly, the intensity variation as a function of coverage of the energy loss peak at 17 meV observed in HREELS, and the photoemission peak at E_b=0.5 eV seen in UPS can be understood from a two-phase model. A tentative explanation is presented that connects apparent discrepancies in the literature concerning the electronic structure of the K adlayer. In particular, a new assignment of the K-induced states near the Fermi level is proposed.     

TPD, HREELS and UPS study of the adsorption and reaction of methyl nitrite (CH3ONO) on Pt(111)

The adsorption and reaction of methyl nitrite (CH₃ONO, CD₃ONO) on Pt(111) was studied using HREELS, UPS, TPD, AES, and LEED. Adsorption of methyl nitrite on Pt(111) at 105 K forms a chemisorbed monolayer with a coverage of 0.25 ML, a physisorbed second layer with the same coverage that desorbs at 134 K, and a condensed multilayer that desorbs at 117 K. The Pt(111) surface is very reactive towards chemisorbed methyl nitrite; adsorption in the monolayer is completely irreversible. CH₃ONO dissociates to form NO and an intermediate which subsequently decomposes to yield CO and H₂ at low coverages and methanol for CH₃ONO coverages above one-half monolayer. We propose that a methoxy intermediate is formed. At least some C–O bond breaking occurs during decomposition to leave carbon on the surface after TPD. UPS and HREELS show that some methyl nitrite decomposition occurs below 110 K and all of the methyl nitrite in the monolayer is decomposed by 165 K. Intermediates from methyl nitrite decomposition are also relatively unstable on the Pt(111) surface since coadsorbed NO, CO and H are formed below 225 K.     

The role of repulsive interactions in molecular bromine adsorption and patterning of Si(100)-2×1

Scanning tunneling microscopy (STM) was used to investigate the role of repulsive interactions in the adsorption and patterning of molecular bromine on the Si(100) surface. At room temperature and low coverage, chemisorption of bromine occurs dissociatively on the same side of adjacent dimers of the same row. Using the STM tip as a probe, we demonstrate the existence of repulsive interactions at adjacent sites on the Si(100)-2×1 surface. These repulsive interactions also contribute to the arrangement of adatoms on the surface. In particular, we report the presence of a stable c(4×2) surface phase that results after exposing the Si(100) surface to bromine under certain conditions. This phase involves adsorption on non-neighboring dimers and is stabilized by repulsive interactions that force bromine adatoms to occupy alternating dimers within rows with an out-of-phase occupancy between adjacent rows.     

Structures of sulfur on TiO2(1 1 0) determined by scanning tunneling microscopy, X-ray photoelectron spectroscopy and low-energy electron diffraction

The temperature dependent adsorption of sulfur on TiO₂(1 1 0) has been studied with X-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM), and low-energy electron diffraction (LEED). Sulfur adsorbs dissociatively at room temperature and binds to fivefold coordinated Ti atoms. Upon heating to 120°C, 80% of the sulfur desorbs and the S 2p peak position changes from 164.3±0.1 to 162.5±0.1 eV. This peak shift corresponds to a change of the adsorption site to the position of the bridging oxygen atoms of TiO₂(1 1 0). Further heating causes little change in S coverage and XPS binding energies, up to a temperature of 430°C where most of the S desorbs and the S 2p peak shifts back to higher binding energy. Sulfur adsorption at 150°C, 200°C, and 300°C leads to a rich variety of structures and adsorption sites as observed with LEED and STM. At low coverages, sulfur occupies the position of the bridging oxygen atoms. At 200°C these S atoms arrange in a (3×1) superstructure. For adsorption between 300°C and 400°C a (3×3) and (4×1) LEED pattern is observed for intermediate and saturation coverage, respectively. Adsorption at elevated temperature reduces the substrate as indicated by a strong Ti³⁺ shoulder in the XPS Ti 2p_3/2 peak, with up to 15.6% of the total peak area for the (4×1) structure. STM of different coverages adsorbed at 400°C indicates structural features consisting of two single S atoms placed next to each other along the [0 0 1] direction at the position of the in-plane oxygen atoms. The (3×3) and the (4×1) structure are formed by different arrangements of these S pairs.     

Ordering of Si atoms on the ten-fold surface of the decagonal Al72Ni11Co17 quasicrystal

The deposition of Si at room temperature on the ten-fold quasicrystalline surface of d-Al-Ni-Co has been investigated by scanning tunnelling microscopy. At a coverage of 0.30 ML, Si pentagons in two orientations related by inversion symmetry are observed on the same substrate terrace. The side-length of the pentagons is 4.2 ± 0.2 Å. At this coverage, the Si adlayer displays quasiperiodic order. Depressions related to pentagonal features observed in STM images of the clean d-Al-Ni-Co substrate are proposed as plausible adsorption sites for the Si adatoms. As the Si coverage is increased, the well-defined structures observed are no longer distinguishable. At coverages above the monolayer, the Si overlayer follows a rough three-dimensional growth mode.     

A LEED, TPD and HREELS investigation of NO adsorption on Sn/Pt(111) surface alloys

The adsorption of NO on Pt(111), and the (2 × 2)Sn/Pt(111) and (√3 × √3)R30°Sn/Pt(111) surface alloys has been studied using LEED, TPD and HREELS. NO adsorption produces a (2 × 2) LEED pattern on Pt(111) and a (2√3 × 2√3)R30° LEED pattern on the (2 × 2)Sn/Pt(111) surface. The initial sticking coefficient of NO on the (2 × 2)Sn/Pt(111) surface alloy at 100 K is the same as that on Pt(111), S₀ = 0.9, while the initial sticking coefficient of NO on the (√3 × √3)R30°Sn/Pt(111) surface decreases to 0.6. The presence of Sn in the surface layer of Pt(111) strongly reduces the binding energy of NO in contrast to the minor effect it has on CO. The binding energy of β-state NO is reduced by 8–10 kcal/mol on the Sn/Pt(111) surface alloys compared to Pt(111). HREELS data for saturation NO coverage on both surface alloys show two vibrational frequencies at 285 and 478 cm⁻¹ in the low frequency range and only one N-O stretching frequency at 1698 cm⁻¹. We assign this NO species as atop, bent-bonded NO. At small NO coverage, a species with a loss at 1455 cm⁻¹ was also observed on the (2 × 2)Sn/ Pt(111) surface alloy, similar to that observed on the Pt(111) surface. However, the atop, bent-bonded NO is the only species observed on the (√3 × √3)R30°Sn/Pt(111) surface alloy at any NO coverage studied.     

A scanning tunneling microscopy study of water on Si(111)-(7 × 7): adsorption and oxide desorption

Scanning tunneling microscopy and temperature-programmed desorption have been used to investigate the chemistry of water on Si(111)-(7 × 7) substrates which were misoriented 2° toward the [ 10] direction. Upon room temperature exposure to water, the adatoms of the (7 × 7) unit cell are still evident even after high exposures, implying that major modifications of the substrate do not occur. At high coverages, the distribution of reacted adatoms shifts from one controlled by dissociative adsorption across the adatom-rest atom pair to a statistical distribution based on the availability of dangling bonds. Desorption of the oxide layer which remains after water adsorption and the desorption of hydrogen have also been characterized. The oxide desorption occurs along well-defined wavefronts which originate at step edges and advance in directions consistent with the underlying substrate symmetry, primarily the [ 2] direction (i.e. the wave vector points in the [ 2] direction). In regions of the surface where the oxide has desorbed, the (7 × 7) unit cell can be seen clearly. Vacancies resulting from the loss of surface silicon atoms (via the etching) coalesce into islands in the clean regions of the terraces, but unlike desorption of oxide layers from Si(100), the desorption does not occur from the boundaries of these vacancy islands.     

Scanning tunneling microscopy and molecular orbital calculation of pentacene molecules adsorbed on the Si(100)2×1 surface

Adsorption of the organic molecule pentacene on Si(100)2×1 surfaces was imaged using scanning tunneling microscopy (STM). The molecular images exhibit distinct shapes corresponding to the expected shapes for adsorption configurations. Semi-empirical molecular orbital (MO) calculations reveal a local surface density of states for the adsorbed pentacene on the Si surface. In the cases where the pentacene molecule is adsorbed on an Si dimer row, the calculated MOs are in good agreement with the molecular images observed in STM. In the case of pentacene adsorbed on two or three Si-dimer rows, however, the MOs of the pentacene do not correlate directly with the observed STM images. It is thus considered that the STM images are produced by a combination of Si dimer states and MO.     

Chlorine/silicon surface reactions under heating

Reactions on Cl-adsorbed Si(111) and Si(100) surfaces—(Cl/Si(111) and Cl/Si(100))—under heating in ultrahigh vacuum (UHV) and in a Cl₂ atmosphere were studied. Auger electron spectroscopy (AES) and low-energy electron energy loss spectroscopy (LEELS) were used for examination of surface changes. Heating in UHV at 820°C for 30 s successfully removed almost all Cl atoms, both on Cl/Si(111) and Cl/Si(100). Variance in LEELS spectra shows that decomposition of SiCl_x (x > 1), a small amount of which was present on Cl/Si(111), occurs under heating on Si(111) both in UHV and in Cl₂ and desorbs reaction products, leaving the Si---Cl bonds on the surfaces. Such Si---Cl bonds specific to those on Cl/Si(111) are formed also on Cl/Si(100) heated in C1₂ at 820°C. On Cl/Si(100) heated in C1₂, there are various surface changes: relaxation of the 2 × 1 structure remaining on the Cl/Si(100), desorption of reaction products, and formation of Si---Cl bonds specific to those on Cl/Si(111). The Si---Cl bonds, both on Cl/Si(111) and Cl/Si(100), decomposed under longer heating and under heating at higher temperatures in UHV.     

Structure of the Si(011)-(16 × 2) surface and hydrogen desorption kinetics investigated using temperature-programmed desorption

D₂ temperature-programmed desorption (TPD) was used to probe the structure of the Si(011)-(16 × 2) surface. Deuterium was adsorbed at 200°C to coverages θ_D ranging up to complete saturation (approximately 1.1 ML) and the sample heated at 5°C s⁻¹. TPD spectra exhibited three second-order desorption peaks labelled β₂, β*₁ and β₁ centered at 430, 520 and 550°C. Of the proposed models for the Si(011)-(16 × 2) reconstruction, the present TPD results as a function of θ_D provide support for the adatom/dimer model with the β₂ peak assigned to D₂ desorption from the dihydride phase, while the β*₁ and β₁ peaks arise from adatom and surface-atom monohydride phases.     

Ge在Si(111)7×7表面的选择性吸附

利用超高真空扫描隧道显微镜研究了室温条件下Ge在Si(111)7×7表面上初期吸附过程.在Ge所形成团簇中存在一个临界核.这些Ge团簇的吸附中心总是在三个增原子所围成的区域中.它们的电子结构具有类似半导体的性质,即其局域态密度在远离费米面的能级处很大,而在费米面附近的能级处非常小. 关键词： 扫描隧道显微镜 Si(111)7×7表面 Ge团簇     

Adsorption of potassium on a Si(111) surface

The adsorption of potassium on several sites (T₁, T₄, H₃) of a Si(111) surface is studied using the extended Hückel method and cluster models. The results show that the chemisorption energy on the T₄ site is almost equal to that on the H₃ site for an ideal surface. However, the analysis of chemical bonding of adatoms on a Si surface suggests that the chemisorption of adatoms might induce a kind of reconstruction on the Si(111) surface. Rough calculations for the adsorption on this reconstructed surface are carried out. In this paper, charge transfer, density of states and characterization of the bonds between the adatom and substrate Si atoms are also investigated in detail.