Corner Adatom Sites Research Articles

Bidentate Surface Structures of Glycylglycine on Si(111)7×7 by High-Resolution Scanning Tunneling Microscopy: Site-Specific Adsorption via N–H and O–H or Double N–H Dissociation

The early adsorption stage of glycylglycine on Si(111)7×7 surface has been studied by scanning tunneling microscopy (STM). Filled-state imaging shows that glycylglycine adsorbs dissociatively in a bidentate fashion on two adjacent Si adatoms across a dimer wall or an adatom-restatom pair, with the dissociated H atoms on neighboring restatoms. The present STM result validates our hypothesis that both bidentate configurations involving N-H and O-H dissociation and double N-H dissociation are equally probable. Our STM results further show that the relative surface concentrations of the five bidentate configurations follow a specific ordering. This suggests that N-H dissociation at a center adatom site would likely be followed by N-H dissociation at an adjacent restatom, while N-H dissociation at a corner adatom site would be succeeded by O-H dissociation at an adatom across the dimer wall. Evidently, the strong bidentate interactions also inhibit surface diffusion of the adsorbed glycylglycine fragment, and the adsorption apparently follows random sequential adsorption statistics. The random nature of adsorption is also supported by the similar relative occupancies of the center adatom and corner adatom sites, indicating that the relative reactivities of these adatom sites do not play a significant role. Our DFT computational study shows that all three bidentate (Si-)NHCH(2)CONHCH(2)COO(-Si) adatom-adatom configurations (center-center, corner-corner, center-corner) have similar adsorption energies for a double adatom-adatom pair across the dimer wall, while the (Si-)NHCH(2)CON(-Si)CH(2)COOH bidentate adatom-restatom configuration is energetically favorable. The free -CONH- and -COOH groups remaining on the respective bidentate adstructures could facilitate adsorption of the second adlayer through the formation of hydrogen bonding.

Low-flux elucidation of initial growth of Ge clusters deposited onSi(111)−7×7observed by scanning tunneling microscopy

Deposition of Ge on $\text{Si}(111)\text{\ensuremath{-}}7\ifmmode\times\else\texttimes\fi{}7$ under very low Ge flux is examined using ultrahigh vacuum scanning tunneling microscopy; Ge atoms are deposited at $150\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$ under a flux of $\ensuremath{\sim}0.005$ or 0.05 ML/min. Initially Ge atoms are substituted for Si atoms on corner adatom sites of faulted half unit cells. At a Ge coverage of 0.08 ML under the lower flux, hollow-centered hexagonal Ge clusters with six protrusions are formed preferentially on faulted half unit cells, which are uniform and separated from other clusters. At the higher flux various types of clusters grow, frequently neighboring with others. This indicates that the low flux is needed to elucidate the stable type of Ge clusters grown on $\text{Si}(111)\text{\ensuremath{-}}7\ifmmode\times\else\texttimes\fi{}7$.

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(111)–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 0.8V to 2.5V. Dissociated Cl atoms bond to adatom sites and appear bright for bias voltages higher than ±1.4V. The other dissociated species appear dark for bias voltages below ±1.85V 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.

Negative differential resistance of TEMPO molecules on Si(1 1 1)

Negative differential resistance (NDR) has been observed for individual 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) molecules on Si(1 1 1) in ultra high vacuum (UHV) scanning tunneling microscopy (STM) and spectroscopy (STS) measurements at room temperature. NDR effects were observed exclusively at negative bias voltage using an n-type Si(1 1 1) sample. At 77 K no NDR effects were observed, but the I( V) curves were similar in shape to those recorded on bare Si(1 1 1) sites. TEMPO was observed to adsorb preferentially at corner adatom sites of the Si(1 1 1)-7 × 7 structure. Although the Si(1 1 1)-7 × 7 reconstruction was conserved, local defects were frequently observed in the vicinity of the TEMPO adsorbates.

Evidence of temperature dependence of initial adsorption sites of Ge atoms on Si(111)-7×7

Small amounts of Ge atoms are deposited on Si(111)-7×7 surfaces at room temperature (RT) and at 100°C to clarify the initial adsorption sites using scanning tunneling microscopy. At RT Ge atoms are adsorbed at high coordination B2 sites around the rest atoms, as predicted by Cho and Kaxiras [Surf. Sci. 396, L261 (1998)]. On one hand, at 100°C Ge atoms are adsorbed on corner adatom sites. With increasing Ge coverage the corner sites are gradually occupied, followed by Ge adsorption at center adatom sites, resulting in Ge cluster growth with a size of the half unit cell.

Bonding Configurations and Collective Patterns of Ge Atoms Adsorbed onSi(111)−(7×7)

We report scanning tunneling microscopy observations of Ge deposited on the Si(111)-(7 x 7) surface for a sequence of submonolayer coverages. We demonstrate that Ge atoms replace so-called Si adatoms. Initially, the replacements are random, but distinct patterns emerge and evolve with increasing coverage, until small islands begin to form. Corner adatom sites in the faulted half unit cells are preferred. First-principles density functional calculations find that adatom substitution competes energetically with a high-coordination bridge site, but atoms occupying the latter sites are highly mobile. Thus, the observed structures are indeed more thermodynamically stable.

Characteristic adsorption ofXeon aSi(111)−(7×7)surface at low temperature

Site-dependent adsorption structures of Xe atoms on a $\mathrm{Si}(111)\ensuremath{-}7\ifmmode\times\else\texttimes\fi{}7$ surface were studied by scanning tunneling microscopy at 8 K. Xe atoms initially adsorb as dimers on the site between a rest atom and its two neighboring center adatoms, and then near the corner adatom site. Similar structures were formed at both the faulted half (FH) and the unfaulted half (UH) of the $7\ifmmode\times\else\texttimes\fi{}7$ unit. The observed structure is in excellent agreement with that estimated by the rigid ball model, however, site-dependent bias dependence and different stabilities were observed for them, indicating the existence of different interactions between Xe and Si atoms in the $7\ifmmode\times\else\texttimes\fi{}7$ unit despite the stable electronic structure of Xe. Based on the results with thermal desorption spectroscopy, the adsorption energies are estimated to be 200 and 220 meV for FH and UH units, respectively.

Spectroscopy of Si vacancies formed with a scanning tunneling microscope

The electronic structure of adatom vacancies on a Si (111)-7×7 surface is described based on their observation by scanning tunneling spectroscopy. Differential conductance (dI/dV) images show that electrons with energies 0.4 and 1.3 eV below the Fermi energy (EF) completely fill the vacancies. Spectra of (dI/dV)/(I/V ) versus bias voltage and the differential conductance images show characteristic bands at 0.4 and 1.3 eV below EF, which are associated with the dangling bonds and back bonds of vacancies, respectively. The electron densities associated with these bonds are lower at vacancies of center adatom sites than at those of corner adatom sites. This provides a possible explanation for the site-selective extraction of adatoms from a Si surface.

Study of aniline on a Si(111)7×7 surface by scanning tunneling microscopy

The adsorption of aniline on a Si(111)7×7 surface was examined by scanning tunneling microscopy (STM). In the STM images at negative sample bias, aniline molecules were observed as bright ring-like features. We have found that aniline molecules adsorb on the center adatom site, with its amino group oriented to the rest atom site. The differences in reactivity of the various sites on the Si(111)7×7 surface to aniline were observed. The center adatom site is more reactive than the corner adatom site. With increasing aniline exposure, an aniline chain was observed. From the STM images obtained at various sample biases, this aniline chain is seen to have different electronic structure from an aniline molecule singly adsorbed on the Si(111)7×7 surface. This suggests that aniline molecules are associated with each other on the Si(111)7×7 surface.

Diffusion by bond hopping of hydrogen atoms on the Si(111)-7×7 surface

Site-specific adsorption and diffusion of atomic hydrogen on the Si(111)-7\ifmmode\times\else\texttimes\fi{}7 surface at elevated temperatures are studied by scanning tunneling microscopy. Hydrogen atoms are found to adsorb preferentially on rest-atom sites rather than adatom sites with a binding-energy difference of \ensuremath{\sim}0.2 eV. The adsorption causes a reverse charge transfer from rest atoms back to adatoms. Above \ensuremath{\sim}280 \ifmmode^\circ\else\textdegree\fi{}C, atomic hopping between two rest-atom sites within a half-cell can occur which is mediated by an adatom site. Above \ensuremath{\sim}330 \ifmmode^\circ\else\textdegree\fi{}C, H atoms start to hop across the cell boundary via two adatom sites, or they can diffuse across the surface. The activation barrier for hopping from a rest-atom site to a corner adatom site is \ensuremath{\sim}50 meV lower than that to an edge adatom site. Thus, in cross boundary jumps, they hop preferentially via two corner adatom sites. From Arrhenius plots, the hopping barriers within the cell and across the cell boundary are determined. The hopping paths, relative binding energies, and site selectivity of hydrogen atoms on the Si(111)-7\ifmmode\times\else\texttimes\fi{}7 surface agree in general with theoretical results, but our result are both site and path specific. The dynamic behavior of two to three H atoms inside a half-cell is also investigated.

Site-sensitive yield of atomic emission induced by laser irradiation on Si(111)−7×7 surface

The atomic structure of the Si(111)−7×7 surface after irradiation of 2.48 eV laser pulses has been studied using scanning tunneling microscopy. In the range of fluence at which no indication of melting is observed, the examination of more than 300 unit cells on irradiated surface has revealed that the number of vacancies formed at the center-adatom site is much larger than that at the corner-adatom site, showing the strong site-dependent yield of laser-induced atomic emission.

Highly site-selective adsorption of trimethylphosphine on a Si(111)-(7 × 7) surface studied by a scanning tunneling microscope (STM)

Adsorption of trimethylphosphine (TMP) on a Si(111)-(7 × 7) surface has been studied using a scanning tunneling microscope (STM). We find that most of the TMP molecules are adsorbed preferably on center adatom sites at the surface. It is observed that the TMP molecule on a corner adatom is moved to a center adatom site at RT. TMP is more stable on the center adatom sites than on the corner adatom sites.

Scanning tunneling microscopy study of the reaction of AlCl3 with the Si(111) surface

The initial stage of the reaction of aluminum trichloride (AlCl3) with the Si(111)-(7×7) surface and annealing effects of the adsorbed surface are investigated with a scanning tunneling microscope. Reacted and unreacted sites manifest in the contrast of adatom sites on the AlCl3-exposed surface. An AlCl3 molecule dissociatively adsorbs onto the Si(111)-(7×7) surface at room temperature, which yields chlorine atoms. Preferential adsorption site is found to be the center adatom site rather than the corner adatom site, and the chlorine atom adsorbs onto the top site of the center Si adatom. The migration of adsorbed molecules around the rest atom in the 7×7 unit cell is observed at room temperature. The Si–Clx species desorb from the surface by thermal annealing at 600 °C, leaving vacancies behind, after annealing at 1200 °C. Aluminum deposition occurs in a limited area and the surface shows the √7×√7 reconstruction.

Surface properties of Si(111)7 x 7 upon H and NH2 adsorption: A local-density-functional study.

The structural and electronic properties of H and ${\mathrm{NH}}_{2}$ adsorbed onto the Si(111)7\ifmmode\times\else\texttimes\fi{}7 surface are studied using first-principles local-density total-energy and atomic force calculations carried out for cluster models. The results show that different adsorption sites, i.e., the center adatom site, the corner adatom site, and the first-layer ``rest-atom'' site, exhibit different chemisorption properties. The results obtained from energy minimization for H and ${\mathrm{NH}}_{2}$ on these possible sites deduced from the Auger electron spectroscopy and photoemission yield spectroscopy studies confirm the model of a two-step adsorption process. The electronic structures of the adsorption systems were determined and a spectrum of orbital overlap population ${\mathit{P}}_{\mathit{O}}$(\ensuremath{\varepsilon}) versus energy \ensuremath{\varepsilon} was calculated that is found to bear close relationship with images obtained from scanning tunneling microscope (STM) studies. We conclude from the \ensuremath{\varepsilon} properties of ${\mathit{P}}_{\mathit{O}}$(\ensuremath{\varepsilon}) that H and ${\mathrm{NH}}_{2}$ adsorbed on the rest-atom site may not be detected by the STM but are still visible when absorbed on the adatom sites.

Scanning Tunneling Microscopy Observation of the Reaction of AlCl3 on Si(111)-7×7 Surface

AbstractThe initial stage of the reaction of aluminum trichloride (AlCl3) with the Si(111)-(7×7) surface and the annealing effects of the adsorbed surface are investigated with a scanning tunneling microscope (STM). Reacted and unreacted sites manifest in the contrast of adatom sites on the AlCl3-exposed surface. An AlCl3 molecule dissociatively adsorbs onto the Si( 111)-(7×7) surface at room temperature, which yields C1 atoms. Preferential adsorption site is found to be the center adatom site rather than the corner adatom site, and Cl atom adsorbs onto the top site of the center Si adatom. The migration of adsorbed molecules (AlClx) in the (7×7) unit cell is observed at room temperature. The SiCl,, species desorb from the surface by thermal annealing at 600 'C, leaving vacancies behind. After annealing at 1200 0C, Al deposition occurs in a limited area and the surface shows the V/7×V7 reconstruction, which is a characteristic Al-induced surface structure.

Atom-resolved surface chemistry: The early steps of Si(111)-7×7 oxidation

The early stages of Si(111)-7×7 oxidation using scanning tunneling microscopy and scanning tunneling and photoemission spectroscopies have been studied. It has been found that there are at least two different oxygen-containing sites being formed. Their different surface site distributions and behavior as a function of O2- exposure show them to be two distinct early products. By correlating the spectroscopic results and the results of theoretical calculations one is able to identify one of these products as involving an O atom tying up an adatom dangling bond with a second O atom inserted in one of the adatom back bonds, while the other involves a single O atom inserted in an adatom back bond. The preference of these products for the faulted half of the 7×7 unit cell and for corner-adatom sites is explained in terms of a site-dependent sticking coefficient involving a process analogous to the gas-phase ‘‘harpooning’’ processes. It is shown that the majority of the resulting molecular precursors involve O2 interacting with a single dangling-bond site. Finally, mechanisms are proposed for the early steps of the oxidation process.

Atom-resolved surface chemistry studied by scanning tunneling microscopy and spectroscopy.

We have used scanning tunneling microscopy and spectroscopy to study the reaction of Si(111)- (7×7) with NH3. We have found that by use of topographs obtained at different energies, as well as atom-resolved spectra, reacted and unreacted surface sites can be imaged selectively. Thus we have been able to probe the spatial distribution of the surface reaction on an atom-by-atom basis. We find that there are significant differences in reactivity between the various dangling-bond sites on the Si(11l)-(7×7) surface. Specifically, rest-atom sites are more reactive than adatom sites and, moreover, center-adatom sites are more reactive than corner-adatom sites. We ascribe the reduced reactivity at adatom sites to the delocalized nature of their dangling-bond state. We suggest that a bonding interaction between adatoms and the Si atoms directly below them is responsible for this behavior—a suggestion supported by electronic-structure calculations. Thus, while reaction at a rest-atom site can be considered a dangling-bond saturation process, reaction at an adatom site involves the formation of a hypervalent (fivefold-coordinated) adatom. We tentatively ascribe the reactivity differences between center and corner adatoms to differences in the strain they induce upon reaction on the dimer bonds. Atom-resolved spectroscopy allows us to probe interactions and charge transfer between surface sites, and for the first time, we can directly observe how chemisorption affects the substrate electronic structure at neighboring unreacted sites.