Center 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.

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.

Study of adsorption of Al atom on Si(1 1 1)7 × 7 surface

We investigated adsorption of an Al atom on the Si(1 1 1)7 × 7 surface by using a scanning tunneling microscope. The evaporated Al atom displaced a Si center adatom of the surface; the expelled Si atom diffused inside a half unit cell of the Si(1 1 1)7 × 7 surface. The adsorbed single Al atom, which shows bias voltage dependency, hopped among three center adatom sites inside the half unit cell. Ab initio molecular orbital method was used to investigate the adsorption of Al atom on the surface.

Hopping Al atom on Si(111) 7 × 7 surface studied by scanning tunneling microscopy

AbstractAdsorption of Al atoms on the Si(111) 7 × 7 surface was investigated by STM at room temperature. The adsorbed Al atom displaced a Si center adatom on the surface, and the expelled Si atom diffused inside a half unit cell. The adsorbed Al atom hopped among center adatom sites with a frequency of about 0.01s−1. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Molecular and dissociative bonding of amines with the Si [formula omitted]-(7×7) surface

The interaction of trimethylamine (TMA) and dimethylamine (DMA) with the Si(1 1 1)-(7×7) surface has been studied by scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and ultraviolet photoemission spectroscopy (UPS). STM data for TMA at low coverage show molecular features exhibiting a strong preference for bonding at the center adatom sites. XPS data show that at low coverage the majority of molecules form a highly ionic dative-bonded molecular adduct in which the N atom donates electron density to the surface, leading to a very high N(1s) binding energy of 402.4 eV. UPS data show that the interaction of TMA with the Si(1 1 1)-(7×7) surface also involves the restatom, suggesting that formation of dative bonds may also alter the restatom state. At very high exposures, a new, dissociative pathway becomes important, leading to dissociation and the appearance of new fragments with lower N(1s) binding energies of 399.1 eV. Corresponding studies for DMA only show dissociative bonding on Si(1 1 1), forming H atoms and N(CH 3) 2 species. While the N(CH 3) 2 species bonds primarily to the adatoms, the H atoms can bond to either adatoms or restatoms. Possible reaction mechanism and the reactivity of the different types of surface silicon atoms are discussed.

Selective formation of cumulative double bonds (C[double bond]C[double bond]N) in the attachment of multifunctional molecules on Si(111)-7 x 7.

The cumulative double bond (C[double bond]C[double bond]N), an important intermediate in synthetic organic chemistry, was successfully prepared via the selective attachment of acrylonitrile to Si(111)-7 x 7. The covalent binding of acrylonitrile on Si(111)-7 x 7 was studied using high-resolution electron energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), scanning tunneling microscopy (STM) and DFT calculations. The observation of the characteristic vibrational modes and electronic structures of the C[double bond]C[double bond]N group in the surface species demonstrates the [4 + 2]-like cycloaddition occurring between the terminal C and N atoms of acrylonitrile and the neighboring adatom-rest atom pair, consistent with the prediction of DFT calculations. STM studies further show the preferential binding of acrylonitrile on the center adatom sites of faulted halves of Si(111)-7 x 7 unit cells.

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.

Site preferences of oxygen and boron atoms during dissociative reaction of HBO2 molecules onto the Si(111)-7×7 surface

Using scanning tunneling microscopy (STM), we succeeded in observing the initial process of the HBO molecule on the Si(111)-7×7 surface for the first time. Since HBO2 molecules are commonly used as the B source in the molecular beam epitaxy system, and the dominant flux from the HBO2 source in the ordinary condition is the form of HBO, analysis of the HBO reaction process is highly important. When the substrate temperature was kept at room temperature during the molecular irradiation, the adsorbed HBO molecules remained on the surface without dissociation. With the substrate temperature being increased, the dissociation process was activated, and the oxygen- and boron-related adatoms were produced at the corner and center adatom sites in the 7×7 units, respectively. The site dependence was well determined by the bias voltage dependent STM. The selective reaction site of oxygen atoms was comprehensively explained by their electronegativity and charge distribution in the Si(111)-7×7 surface adatoms. On the other hand, regarding the selectivity for the boron atoms, introduction of the surface strain effect was found to be essential.

Adsorption of LiF on Si(111)-7×7 surface studied by scanning tunneling microscopy and low energy electron diffraction

Modification of the Si(111)-7×7 surface by LiF adsorbates and its annealing behavior are investigated using Scanning Tunneling Microscopy and Low Energy Electron Diffraction. The preferred adsorption sites are found to be the center adatom sites. Whereas, at the coverage higher than 0.4 monolayer, the 7×7 structure disappears. One monolayer (ML) is referred to as the site density of the unreconstructed surface. For the LiF-covered surface (0.4 ML), the 7×7 structure begins to recover at an annealing temperature of 325°C, and at 800°C it reconstructs into the 7×7 structure. In the temperature range of 300 ∼ 800°C, the Si(111) surface and LiF adsorbates are suggested to undergo the processes including dissociation of LiF adsobates, fluorination of Si atoms, desorption of the fluorides, etching of the surface and reconstruction of the etched surface.

Scanning tunneling microscopy, high-resolution electron energy loss spectroscopy, and theoretical studies of trimethylphosphine (TMP) on a Si(111)-(7×7) surface

Adsorption of trimethylphosphine (TMP) on a Si(111)-(7×7) surface was studied using scanning tunneling microscopy (STM), high-resolution electron energy loss spectroscopy (HREELS), and theoretical calculations. It was found from STM and HREELS that most of the TMP molecules are adsorbed molecularly on center adatom sites at room temperature. A semi-empirical calculation (AM1 method) verified that this is energetically favorable. It is also found that one of the protrusions (unoccupied state images) on the center adatom site moves to the rest atom site and three protrusions (occupied state images) appear on the corner and center adatom sites in the second scan (interval of a few minutes). The results of a theoretical calculation indicate that adsorption of the three CH 3 groups and the P atom on the adatom and rest atom sites, respectively, is energetically favorable. This explains the STM images well. The HREELS result suggests that the CH 3 group is split off and dissociated into CH 2 (or CH) and H upon heating the sample. The result of the theoretical calculation shows that the dissociation of CH 3 into CH 2 and H is energetically preferable when CH 2 is bridged between an Si adatom and an Si atom in the second layer and H is on the rest atom site.

Site selectivity in the initial oxidation of the Si(111)-7×7 surface

The site selectivity of oxygen in the initial oxidation of the Si(111)-7×7 surface was investigated using a scanning tunnelling microscope. At room temperature, two different oxygen-induced features, bright and dark sites, were observed indicating two different oxygen chemisorption configurations. In particular, the major dark features were observed preferentially at the center adatom sites of the faulted half of the dimer–adatom–stacking-fault (DAS) structure over corner ones, which is different from previous results. We explain this in terms of the difference in the potential energy curves of two sites combined with the effect of the slow oxygen exposure (1×10 −9 Torr) that we used. At low oxygen pressure, oxygen atoms have sufficient diffusion time to be adsorbed on the most stable sites. This proposal is supported by the pressure dependence of the site selectivity that we measured. Furthermore, when the sample was exposed to oxygen at high temperatures, the dark features still preserved the strong site preference on the center adatom sites supporting both our hypothesis and the previously reported idea that center adatom sites are thermally more stable than the corner ones.

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.

Initial stages of the nitridation of the Si(111) surface observed by scanning tunneling microscopy

The initial stages of the nitridation processes on Si(111) have been investigated by scanning tunneling microscopy. Two nitridation processes are tried: one is an exposure to a clean Si(111)7×7 surface at room temperature followed by annealing, and another is an exposure on the hot substrate. In either case, the initial reaction occurs at the center adatom sites and they tend to pair with each other across the adjacent dimer row. At high coverage, very flat monolayer films with an 8×8 superstructure are clearly observed, the surface of which seems to contain misfit dislocation presumably due to relaxation of the surface stress.

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.

Local atomic structures near the domain boundary between the Al-√3×√3 and the 7×7 phases on Si(111): Substitutional defects

Both Al and Si substitutional defects have been observed in the 7×7 domain and the √3×√3 domain, respectively, on the specimen prepared by higher temperature annealing up to 900 °C by scanning tunneling microscopy. At positive sample bias (+1 V) the protrusions in the √3×√3 domain appear ∼1 Å higher on the average than those in the 7×7 domain, though slight modulation in brightness can be observed in each domain. The image taken at negative sample bias (−1 V) reveals clearly the substitutional defects in both domains. The Si adatoms substituting Al in the √3×√3 domain appear in almost the same contrast as Si adatoms in the 7×7 domain. The Al adatoms substituting Si in the 7×7 domain appear ∼1.5 Å higher than Al atoms in the √3×√3 domain. These results suggest a distinct difference in the electronic structure between the Al (or Si) adatoms in both domains derived from the surrounding atomic geometry. In addition, Al preferentially substitutes the center adatom site in the 7×7 unit cell.

Adsorption and Desorption of AlCl3 on Si(111)7×7 Observed by Scanning Tunneling Microscopy and Atomic Force Microscopy

The initial stage of the reaction of aluminum chloride (AlCl3) with the Si(111)-(7×7) surface is investigated using a scanning tunneling microscope (STM) and an atomic force microscope (AFM). Reacted and unreacted sites are manifested in the contrast of adatom sites on the AlCl3-exposed surface. AlCl3 molecules dissociatively adsorb onto the Si(111)-(7×7) surface at room temperature. The preferential adsorption site is found to be the center-adatom site. Thermal annealing at 1200°C results not only in anisotropic etching of the Si surface but also in the deposition of Al.

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.

Site-specific measurement of adatom binding energy differences by atom extraction with the STM.

Using a scanning tunneling microscope, single adatoms can be extracted from a Si(111)7\ifmmode\times\else\texttimes\fi{}7 surface by field evaporation, when the sample voltage is pulsed at 4 V or more in either polarity. Statistically, adatoms at the center of the 7\ifmmode\times\else\texttimes\fi{}7 unit cell are more frequently removed than those near the corner holes, by a ratio of 1.6:1. This difference can be explained by assuming that the binding energy of center adatoms is approximately 0.01 eV less than for corner adatoms. The relationship of this result to previous observations of greater chemical reactivity at center adatom sites is discussed.