Reaction of chlorinated benzenes with Si(1 0 0)2 × 1: a theoretical study

Theoretical (HF + DFT) investigations of the adsorption of chlorobenzene (ClPh), 1,2- and 1,4-dichlorobenzene (1,2-diClPh and 1,4-diClPh) on a silicon (1 0 0) surface are reported for the first time, and are compared with one another and with benzene. Binding energies for various structures with the molecules attached on-top and in-between the surface dimer rows are correlated with the STM experimental data. Novel structures with the molecules linking two dimer rows, stabilised by detachment of Cl (or H)-atoms forming Cl-Si (or H-Si) bonds, are described. For 1,4 and 1,2 binding, these linking structures are predicted to attach the phenyl ring parallel or perpendicular to the Si surface, respectively, while preserving its aromaticity. The potential-energy barriers between several different structures are evaluated, and compared with available experimental evidence. For 1,4-diClPh it is shown that the potential-energy barrier for the second Cl transfer is significantly lower than for the first one in contrast to the gas-phase, and comparable to the barrier for lifting the Bz-ring into a vertical position and forming a singly bonded ‘displaced’ structure. The predicted barrier-heights are consistent with the experimentally observed relative occurrence of the on-top, linking, and displaced structures.     

Site selective atomic chlorine adsorption on the Si(1 1 1)7 × 7 surface

The spontaneous dissociation of trichloroethylene molecules on the Si(1 1 1)7 × 7 surface was investigated using STM. Chlorine atoms were identified by using voltage dependent imaging and by observing voltage dependent tip-induced diffusion. At low coverage, we identify one chlorine that dissociates and binds to an adatom, leaving a nearby chlorovinyl group as the other product bound to the surface. Chlorine atoms show strong site selectivity for corner adatoms and some preference for the faulted half of the unit cell. This result differs significantly from previous studies of chlorine on this surface and a site-selective mobile precursor model is used to explain this discrepancy. The observed site-selectivity is consistent with the high electronegativity value for chlorine.     

The chemisorption of pentacene on Si(0 0 1)-2 × 1

Adsorption structures of the pentacene (C₂₂H₁₄) molecule on the clean Si(0 0 1)-2 × 1 surface were investigated by scanning tunneling microscopy (STM) in conjunction with density functional theory calculations and STM image simulations. The pentacene molecules were found to adsorb on four major sites and four minor sites. The adsorption structures of the pentacene molecules at the four major sites were determined by comparison between the experimental and the simulated STM images. Three out of the four theoretically identified adsorption structures are different from the previously proposed adsorption structures. They involve six to eight Si-C covalent chemical bonds. The adsorption energies of the major four structures are calculated to be in the range 67-128 kcal/mol. It was also found that the pentacene molecule hardly hopped on the surface when applying pulse bias voltages on the molecule, but was mostly decomposed.     

An STM study of the localized atomic reaction of 1,2- and 1,4-dibromobenzene at Si(1 1 1)-7 × 7

A comparative study is reported of the thermal reaction of 1,2- and 1,4-dibromobenzene (1,2- and 1,4-diBrPh) on Si(1 1 1)-7 × 7, investigated by STM. Some results are given for the intermediate case of 1,3-diBrPh. The STM images gave evidence of a different pattern of reaction to yield pairs of Br-Si for 1,2-, 1,3- and 1,4-diBrPh. The ratio of pairs of Br-Si to single bromination events was 1:2 for 1,2-diBrPh and 1:3 for 1,4-diBrPh. In many cases organic residue from the bromination reaction, R-Si, was evident in the STM image. The products R-Si and Br-Si were found to be bound to adjacent Si, for both 1,2- and 1,4-diBrPh. The mean Br?Br pair separation at the surface depended on the parent molecule, being 7.6 Å for 1,2-diBrPh, 10.3 Å for 1,3-diBrPh, and 11.3 Å for 1,4-diBrPh. These separations are, in each case, about 4 Å greater than the separation of the Br-atoms in the intact parent molecule, which increases systematically down the series. There was a marked decrease in the percentage of R-Si accompanying the Br-Si in going down the series, decreasing from 70% for 1,2- to 20% for 1,4-diBrPh; this was interpreted as being due to a decrease in the percentage of `benzene-mediated' reaction dynamics, in which the benzene ring was bound to the surface. At moderately increased surface temperature (45 °C) the reaction of 1,2- and also 1,4-diBrPh no longer resulted in R-Si formation, suggesting that the dynamics had altered from benzene-mediated to `bromine-mediated'.     

Low reactivity of molecular oxygen with Si(1 1 1)-c(2 × 8)

The adsorption of molecular oxygen on the c(2 × 8) reconstruction of quenched Si(1 1 1) surfaces has been studied at the atomic scale using scanning tunneling microscopy (STM) at room temperature (RT). It has been found that clean well reconstructed c(2 × 8) adatoms do not react with O₂ molecules but that a limited oxidation can start where adatom sites arranged in reconstructed structures are present. Comparison between O₂ adsorption on Si(1 1 1)-c(2 × 8) and Si(1 1 1)-7 × 7 reconstructions coexisting on the same quenched silicon surface has been carried out in detail. For each atomic site present on the surface the variation of reacted sites with exposure has been measured. For low O₂ exposures, bright and dark oxygen induced sites appear on the Si(1 1 1)-7 × 7, while Si(1 1 1)-c(2 × 8) does not oxidized at all. At high O₂ exposures, large oxidation areas have spread on the 7 × 7 reconstruction, preferentially on the faulted halves of the unit cell, and smaller oxidation areas induced by topological defects have grown all around clean un-reacted c(2 × 8) regions.     

Vanadium nanoclusters on Si(1 1 1) 7 × 7 surface studied by scanning tunneling microscopy

The size distribution and shape transition of self-assembled vanadium silicide clusters on Si(1 1 1) 7 × 7 have been investigated by scanning tunneling microscopy. Nanoclusters were formed by submonolayer vanadium deposition at room temperature followed by subsequent annealing (solid phase epitaxy - SPE). At room temperature, initially V-nanoclusters are formed which occupy sites avoiding the corner hole parts of the unit cells in the Si(1 1 1) 7 × 7 surface. Upon annealing, strong metal-silicon reaction occur leading to the formation of vanadium silicide nanoclusters. As a function of temperature, both, flat (2D) and three dimensional (3D) clusters have been obtained. After annealing at temperatures around 900 K many faceted clusters are created, whereas at higher annealing temperature, around 1300 K, predominantly 3D clusters are formed. The size distribution of SPE grown clusters could be well controlled in the range of 3-10 nm. The cluster size depends on the annealing temperature as well as on the initial vanadium coverage. Based on high resolution STM images a structure model for one kind of vanadium disilicide clusters exposing atomically flat surfaces was proposed.     

An STM study of the Si(0 0 1) (2 × 7)-Gd, Dy surface

Both Gd and Dy induce two different reconstructions of the Si(0 0 1) surface with 2 × 4 and 2 × 7 unit cells. Detailed examination by scanning tunneling microscopy shows that the structure of both phases is essentially the same for both metals. Furthermore, the 2 × 7 unit cell contains structural subunits that are the same as the 2 × 4 structure. The similarities and differences between the two superstructures induced by the two metals are discussed.     

STM study of Si(1 1 3) 3 × 2-Ga surface

The Ga-adsorbed structure on Si(1 1 3) surface at low coverage has been studied by scanning tunneling microscopy (STM). The bright protrusion corresponding to the position of the dimer without the interstitial Si atom of the clean surface disappeared in the filled-state STM image after Ga adsorption, although the protrusion due to the Si adatom still remained. On the basis of the adatom-dimer-interstitial (ADI) model, this result indicates that the Ga atom is adsorbed interstitially at the center of another pentamer that does not have the interstitial Si atom. An ab initio calculation was performed and STM images were simulated.     

Si nucleation on Si(1 1 1)-7 × 7: From cluster pairs to 2D islands

Nucleation of 2D islands in Si/Si(1 1 1)-7 × 7 molecular beam epitaxy is studied using scanning tunneling microscopy (STM). A detailed analysis of the population of small amorphous clusters coexisting on the surface with epitaxial 2D islands has been performed. It is shown that small clusters tend to form pairs. The pairs serve as precursors for 2D islands as confirmed by direct STM observations of the smallest 2D islands covering two adjacent half-unit cells of the 7 × 7 reconstruction. It is proved with scaling arguments that the critical nucleus for 2D island formation consists not only of the pair itself, but also includes additional adatoms not belonging to the stable clusters.     

Atomic structure of Bi-dimer row selectively adsorbed on Si(5 5 12)-2 × 1 surface

In order to understand the atomic structure of nanostructures self-assembled on the template with one-dimensional symmetry, Bi/Si(5 5 12) system has been chosen and Bi-adsorption steps have been studied by STM. With Bi adsorption, the clean Si(5 5 12) is transformed to (3 3 7) terraces with disordered boundary due to mismatched periodicities between (3 3 7) and (5 5 12), and Bi-dimer rows are formed inside the (3 3 7) unit as follows: Initially, when Bi atoms are deposited at the adsorption temperature of about 450 °C, they selectively replace Si-dimers and Si-adatoms and form adsorbed Bi-dimers and Bi-adatoms, respectively. If additional Bi is supplied, the Bi-dimers adsorb on the Bi-dimers and Bi-adatoms in the first layer. These adsorbed dimers in the second layer are facing each other to form a Bi-dimer pair with relatively stable p³bonding. Finally, a single Bi-dimer adsorbs above the Bi-dimer pair in the second layer, at which point the Bi layer thickness saturates. It has been concluded that the Bi-dimer pair with stable p³ bonding is the composing element in the second layer and such site-selective adsorption is possible due to the substrate-strain relaxation through inserting Bi-buffer layer limited to specific sites of the substrate.     

Mechanism of GeH4 dissociation on Si(1 1 1)-(7 × 7)

Results of an STM study of dissociative GeH₄ adsorption on Si(1 1 1)-(7 × 7) at 300 K show that GeH₄ adsorbs under scission of two Ge-H bonds according to GeH₄(g) + 4db → GeH₂(ad) + 2H(ad). GeH₂ binds to two adatom dangling bonds in a bridged configuration, while the two released hydrogen atoms saturate two additional dangling bonds. The GeH₄ sticking coefficient under these conditions is 1.2 × 10⁻⁶, one order of magnitude smaller than for SiH₄.     

(6 × 2) Reconstruction of the Ag/Si(1 1 1) surface at 77 K

The ground state of the Ag/Si(1 1 1)-(3 × 1) has been investigated by low temperature scanning tunneling microscopy (STM) and density-functional theory. The Fourier transform of the STM image reveals a (6 × 2) reconstruction, which is theoretically found to yield a reconstruction with lower energy than the (3 × 1). The most stable (6 × 2) structural model leads to excellent correspondence between experimental and simulated STM images, and reveals a dimerization of the silver atoms in the channels formed by neighbouring honeycomb Si chains.     

Formation of carbon-induced dimer vacancy defects on Si(0 0 1)-2 × 1 by thermal decomposition of organic molecules-lack of dependence on the molecules’ structure

Two large and complex adsorbed organic molecules, coronene (C₂₄H₁₂) and C₆₀, have been used to produce Si dimer vacancy defects on Si(0 0 1) by thermal decomposition. Studies by STM show that the aligned structural arrangement of the dimer vacancy defects produced is independent of the chemical structure of the organic molecules. This indicates that the chemistry of the thermally decomposed carbon species produced by decomposition of the organic molecule controls the organization of the Si dimer vacancy defects. It is found that ∼1 C atom is responsible for each dimer vacancy defect for both molecules in accordance with earlier studies of C₂H₂ decomposition on Si(0 0 1).     

Investigation of 1,1-dichloroethene interacting with the Si(1 1 1)-7 × 7 surface studied by scanning tunneling microscopy

Adsorption of 1,1-dichloroethene (1,1-DCE) at the Si(1 1 1)-7 × 7 surface has been investigated using scanning tunneling microscopy. 1,1-DCE dissociates upon adsorption by breaking one or both CCl bonds. The appearance of reacted adatoms in the 7 × 7 reconstruction is found to vary for both positive and negative sample bias voltages in the range of 0.8 V to 2.5 V. Dissociated Cl atoms bond to adatom sites and appear bright for bias voltages higher than ±1.4 V. The other dissociated species appear dark for bias voltages below ±1.85 V with a preference of 2:1 for bonding to center relative to corner adatom sites. The faulted half unit cell is preferred. It is demonstrated that rest atoms are active in the dissociation of two-thirds of the 1,1-DCE molecules.     

Scanning tunneling microscopy study on initial stage of atomic hydrogen adsorption on the Si(1 1 1) 7 × 7 surface

Initial hydrogen adsorption on the Si(1 1 1) 7 × 7 surface was studied by scanning tunneling microscopy (STM) in an ultrahigh vacuum. Room temperature adsorbed hydrogen on the adatom in the 7 × 7 reconstruction led to depression of adatoms in the STM images. The hydrogen uptake curve at the adatom site as a function of hydrogen exposure time was well represented by Langmuir adsorption. No preferential adsorption was seen among four inequivalent adatoms in the 7 × 7 reconstruction. Adsorption of the adjacent center and corner adatoms respectively showed ∼10% higher adsorption. Even though the number of reacted adatoms in the half unit of the 7 × 7 reconstruction was statistically random, the number of reacted adatoms in the nearest neighbor half unit was enhanced as the number of reacted sites increased in the half unit.     

Function of subsurface boron on Si(0 0 1)-2 × 1: water adsorption

Using density functional theory calculations we investigate the function of subsurface boron in determining surface properties of Si(0 0 1). To demonstrate its effect on surface reactivity we compare the behaviors of water adsorption on the clean and B-modified surfaces. We find that subsurface boron brings about a significant change in surface chemical properties by altering charge polarization of Si(0 0 1) locally. As a consequence, water adsorption on the B-modified surface shows a distinctively different feature from that on the clean surface.     

Imprinting Br-atoms at Si(1 1 1) from a SAM of CH3Br(ad), with pattern retention

The formation of molecular-scale patterns at surfaces as a self-assembled monolayer (SAM) demands adsorbate mobility, whereas devices require stability. We describe a method of photo- or electron-imprinting a SAM of CH₃Br(ad) as covalently-bound Br-Si(s) at Si(1 1 1)7 × 7, with pattern-retention. This imprinting process, it is proposed, involves charge-transfer to the adsorbate, followed by downward recoil of Br⁻ to give chemical attachment of Br at the reactive Si-atom beneath the parent physisorbed methyl bromide. The electron may subsequently be returned to the substrate along with the excess energy, accounting for the localised Br-imprint.     

Adsorption, ordering, and chemistry of nitrobenzene on Si(1 0 0)-2 × 1

Surface chemistry of nitrobenzene on Si(1 0 0)-2 × 1 has been investigated using multiple internal reflection Fourier-transform infrared spectroscopy (MIR-FTIR), Auger electron spectroscopy (AES) and thermal desorption mass spectrometry. Molecular adsorption of nitrobenzene at submonolayer coverages is dominating at cryogenic temperatures (100 K). As the surface temperature is increased to 160 K, chemical reaction involving nitro group occurs, while the phenyl entity remains intact. Thus, a barrier of approximately 40.8 kJ/mol is established for the interaction of the nitro group of nitrobenzene with the Si(1 0 0)-2 × 1 surface. Further annealing of the silicon surface leads to the decomposition of nitrobenzene. The concentration of nitrogen and oxygen remains constant on a surface within the temperature interval studied here. AES studies also suggest that the majority of carbon-containing products remain bound to the surface at temperatures as high as 1000 K. The only chemical reaction leading to the release of the gaseous products is benzene formation around 670 K. The amount of benzene accounts only for a few percent of the surface species, while the rest of the phenyl groups connected to the silicon surface via a nitrogen linker remain stable even at elevated temperatures, opening an opportunity for stable surface coatings.     

Silicon carbide nanocrystals growth on Si(1 0 0) and Si(1 1 1) from a chemisorbed methanol layer

SiC nanocrystals are grown at high temperature on Si(1 0 0) and Si(1 1 1) surfaces starting from a chemisorbed layer of methanol. The decomposition of this layer allows to have a well defined amount of carbon to feed SiC growth. Nanocrystals ranging from 10 nm to 50 nm with density from 100 μm⁻² to 1500 μm⁻² are obtained, and the total volume of produced SiC corresponds to carbon provided by the chemisorbed organic layer. Large differences in nanocrystal size and density, as well as in surface roughness, are observed depending on substrate orientation. The internal structure, crystallinity and epitaxy of nanocrystals grown on Si(1 0 0) are studied using cross-sectional transmission electron microscopy (XTEM), methanol adsorption and surface evolution using scanning tunnelling microscopy (STM). The joint application of XTEM and STM techniques allows a complete characterization of the geometry and chemical composition of these nanostructures.     

Growth of Si nanostructures on Ag(0 0 1)

The first stages of the growth of silicon on Ag(0 0 1) at moderate temperatures start by the formation of a p(3 × 3) superstructure, which continuously evolves with increasing coverage toward a more complex superstructure. In this paper, the atomic arrangement of the p(3 × 3) and of the “complex” superstructure has been investigated using scanning tunnelling microscopy, surface X-ray diffraction and low energy electron diffraction. The atomic model retained for the p(3 × 3) reconstruction consists in four silicon atoms (tetramers) adsorbed near hollow and bridge sites of the top most Ag(0 0 1) surface layer. For higher coverages, i.e., when the “complex” superstructure starts to develop, the silicon overlayer forms periodic stripes, most probably bi-layers, with a graphitic like structure.