A novel method for elimination of the gold-islands formed in the self-assembled monolayers of benzeneselenol on Au(1 1 1) surface

Molecular ordering of benzeneselenol (BSe) self-assembled monolayers (SAMs) on Au(1 1 1) substrates have been investigated by scanning tunnelling microscopy (STM) [1]. After short immersion time (10 min), elevated islands with height of 2.4 Å were found to cover the entire gold surface. On and among the islands, the STM results exhibited the formation of a highly ordered phase (α-phase) by BSe species. In the present study, a novel method is presented to completely eradicate the elevated gold-islands. The method depends on a repetitive STM scanning over the same part of the SAM at restricted tunnelling conditions. After almost 6 h of successive scanning, the surface becomes clean and free of the elevated islands. Moreover, this method was found to induce phase transformation into β-phase. The size of the ordered domains of the β-phase was found to exceed five times that of α-phase. Such a long-range ordering of the β-phase at room temperature has not been previously observed for any system on Au(1 1 1). After detailed analyses, the β-phase was found to have a 33.5% of lower packing density than that of α-phase.     

Alloy formation in the Au{1 1 1}/Ni system - An investigation with scanning tunnelling microscopy and medium energy ion scattering

Using the techniques of scanning tunnelling microscopy (STM) and medium energy ion scattering (MEIS), we examine the growth and annealing behaviour of ultrathin Ni films on Au{1 1 1} at 300 K. As has been shown previously, submonolayer growth of Ni on Au{1 1 1} is strongly influenced by the presence of the herringbone reconstruction with two-dimensional clusters nucleating at herringbone elbows. Second layer growth commences prior to the completion of the monolayer. After multiple layers have been deposited, the surface morphology retains a similar cluster-like appearance. Annealing produces surfaces exhibiting long range Moiré structures and, at higher temperature, triangular misfit dislocations. We use MEIS to examine the composition and structure of these surface alloy phases and conclude that in each case, they consist of an essentially pure Au surface layer on a bimetallic second layer.     

Adsorption of PTCDI on Au(1 1 1): Photoemission and scanning tunnelling microscopy

The adsorption of perylene-3,4,9,10-tetracarboxylic-3,4,9,10-diimide (PTCDI) on Au(1 1 1) has been studied using synchrotron-based X-ray photoelectron spectroscopy and in situ scanning tunnelling microscopy. Direct topographic and surface coverage information provided by the scanning probe measurements have enabled us to correlate peaks in the relatively complex carbon core-level photoemission to interactions of the surface with different parts of the PTCDI molecule. A strong interaction between the imide ends of the molecule with the underlying gold substrate is evidenced by a large chemical shift in the imide carbon peaks, which is observed only for the first adsorbed layer.     

Enhanced surface evolution induced by the molecular desorption in dodecanethiol self-assembled monolayer on Au(1 1 1)

The evolution of the surface structure in dodecanethiol self-assembled monolayer on Au(1 1 1) substrate has been studied with ultra high vacuum scanning tunneling microscopy at several temperatures. The structure of substrate Au(1 1 1) surface changed suddenly at a temperature of 110 °C. The enhanced mobility of the substrate gold atoms at this temperature is attributed to the desorption of the dodecanethiol molecules.     

Faceting of nanoscale fingers on the (1 1 1) surface of gold

Gold fingers, one atomic layer (0.25 nm) high, 4-5 nm wide, and several hundred nm long are formed on the (1 1 1) surface of gold at room temperature by a combination of atomic manipulation and surface self-organisation. Each finger has two parallel edges (type A and type B, respectively) running along its length. The type A step is found to have higher step energy and become nanofaceted when disturbed by either thermal energy or the electric field under the STM tip, leading to the transformation of fingers to “nano-knives”. Our findings reveal the important role of step energy in the process of nanostructure fabrication on surfaces. The gold fingers also provide an ideal system for the investigation of meta-stable nanostructures.     

Translational transitions at domain boundaries in octanethiol monolayers on Au(1 1 1)

Systematic studies of domain boundaries of self-assembled monolayers of octanethiols on (1 1 1) oriented gold surfaces have been performed by ultrahigh vacuum scanning tunnelling microscopy (STM). The monolayers consist of domains that exhibit the c(4×2) superstructure of the hexagonal structure of alkanethiols on gold. By high-resolution images domain boundaries are displayed with molecular resolution. These are used to deduce the exact relation between the adsorption sites of the molecules belonging to different domains on the gold substrate. A translational transition is shown here in detail and it is demonstrated that hexagonal close-packed (hcp), and face-centred cubic (fcc) triple hollow sites are occupied similarly by octanethiols.     

Vibrationally assisted diffusion of H2O and D2O on Pd(1 1 1)

The adsorption and diffusion of isolated H₂O and D₂O molecules on Pd(1 1 1) were studied using scanning tunneling microscopy at low temperature (∼40 K). At low tunneling voltage the diffusion is thermally activated. The diffusion rate is enhanced by several orders of magnitude when the tunneling electron has enough energy to excite the vibrational “scissor” mode of the molecule. An isotope effect was observed in the threshold voltage for electron-assisted diffusion of H₂O and D₂O.     

Scanning tunnelling microscopy and spectroscopy of the reduced TiO2(1 0 0) surface

Scanning tunnelling microscopy and current imaging tunnelling spectroscopy were used to study the topographic and electronic structure of a reduced TiO₂(1 0 0) surface. The STM results showed that the TiO₂(1 0 0) surface is capable to form (1 × 7) reconstruction which can transform to (1 × 3) reconstruction due to reoxidation of the surface. The CITS results showed that the (1 × 7) reconstruction is much more metallic in compared to the (1 × 3) reconstruction showing pronounced surface states at energy 1.3 eV and 0.8 eV below the Fermi level and at energy 1.0-1.2 eV above the Fermi level.     

Room and high-temperature scanning tunnelling microscopy and spectroscopy (HT-STM/STS) investigations of surface nanomodifications created on the TiO2(1 1 0) surface

High-temperature scanning tunnelling microscopy, scanning tunnelling spectroscopy and current imaging tunnelling spectroscopy (HT-STM/STS/CITS) were used to study the topographic and electronic structures changes due to surface modifications of the TiO₂(1 1 0) surface caused by the STM tip. In situ high-temperature STM results showed that the created modifications were stable even at elevated temperatures. The STS/CITS results showed the presence of energy gap below the Fermi level on the untreated regions. The disappearance of energy gap below the Fermi level on the modifications created by the tip was observed. It is assumed that the presence of the tip can change the chemical stoichiometry of the surface from TiO_2−x towards Ti₂O₃.     

Phosphorus and hydrogen atoms on the (0 0 1) surface of silicon: A comparative scanning tunnelling microscopy study of surface species with a single dangling bond

We present a comparative scanning tunnelling microscopy (STM) study of two features on the Si(0 0 1) surface with a single dangling bond. One feature is the Si-P heterodimer—a single surface phosphorus atom substituted for one Si atom of a Si-Si dimer. The other feature is the Si-Si-H hemihydride—a single hydrogen atom adsorbed to one Si atom of a Si-Si dimer. Previous STM studies of both surface species have reported a nearly identical appearance in STM which has hampered an experimental distinction between them to date. Using voltage-dependent STM we are able to distinguish and identify both heterodimer and hemihydride on the Si(0 0 1) surface. This work is particularly relevant for the fabrication of atomic-scale Si:P devices by STM lithography on the hydrogen terminated Si(0 0 1):H surface, where it is important to monitor the distribution of single P dopants in the surface. Based on the experimental identification, we study the lateral P diffusion out of nanoscale reservoirs prepared by STM lithography.     

Ag nanostructures on Au(7 8 8): A self-assembled superlattice of metallic quantum resonators

We have studied by scanning tunneling microscopy (STM) the effect of the reconstruction of a stepped Au(1 1 1) surface on the growth of silver sub-monolayer deposition. For narrow terraces, the reconstruction is disturbed and its pattern changes, Ag growth is therefore influenced. Thus growth of Ag on Au(7 8 8) vicinal surface can be controlled and leads to the formation of a highly ordered superlattice of nanostructures. Moreover, we show by tunneling conductance images that Ag islands exhibit electronic confinement effects of the Shockley surface state. Due to the homogeneity of their shapes and sizes, all the nanostructures of the self-assembled superlattice should exhibit similar electronic properties.     

Characterization and protonation behavior of bipyridine thiol self-assembled monolayer on Au(1 1 1) studied using X-ray photoelectron spectroscopy and scanning tunneling microscopy

The adsorption process, molecular arrangement and protonation behavior of self-assembled monolayers (SAMs) of bipyridine thiol on Au(1 1 1) were investigated using X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM), with a view towards constructing a molecular ion sensor. When the bipyridine SAMs were immersed in a strong CF₃SO₃H solution for protonation of the bipyridine group, additional N(1s) XPS peaks were generated at higher binding peak positions where the origin of the peak was considered to be the N-H species. We further investigated the relationship between the immersion time for the SAMs and the probability of protonation. We observed a decrease in the probability of protonation with a longer immersion time for the SAMs. We consider that both the bipyridine molecular arrangements and the molecular density on the Au surface are crucial for controlling the probability of protonation based on the STM and XPS data.     

Growth of Au thin film on Cu-modified Si(1 1 1) surface

Miniaturizing of electronic devices requires that conductive elements maintain advanced electrical characteristics upon reducing their geometrical sizes. For gold, which is valued for its high electrical conductivity and stability against ambient conditions, creation of extra-thin films on silicon is hampered by formation of the quite complex Au/Si interface. In the present work, by forming a Si(1 1 1)5.55 × 5.55-Cu surface reconstruction prior to Au deposition we formed Au films with smoother surface morphology and higher surface conductivity compared to Au film grown on a pristine Si(1 1 1)7 × 7 surface. Scanning tunnelling microscopy and four-point probe measurements were used to characterize the growth mode of the Au film on a Si(1 1 1)5.55 × 5.55-Cu reconstruction at room temperature.     

Au island growth on a Si(1 1 1) vicinal surface

Au island nucleation and growth on a Si(1 1 1) 7 × 7 vicinal surface was studied by means of scanning tunneling microscopy. The surface was prepared to have a regular array of step bunches. Growth temperature and Au coverage were varied in the 255-430 °C substrate temperature range and from 1 to 7 monolayers, respectively. Two kinds of islands are observed on the surface: Au-Si reconstructed islands on the terraces and three-dimensional (3D) islands along the step bunches. Focusing on the latter, the dependence of island density, size and position on substrate temperature and on Au coverage is investigated. At 340 °C and above, hemispherical 3D islands nucleate systematically on the step edges.     

Sulfur-induced mobilization of Au surface atoms on Au(1 1 1) studied by real-time STM

The interaction of sulfur with gold surfaces has attracted considerable interest due to numerous technological applications such as the formation of self-assembled monolayers and as a chemical sensor. Here, we report on the interaction of sulfur with Au(1 1 1) at two different temperatures (300 K and 420 K) studied by real-time scanning tunnelling microscopy, low energy electron diffraction and Auger electron spectroscopy. In the low coverage regime (<0.1 ML), S adsorption lifts the herringbone reconstruction of the clean Au(1 1 1) surface indicating a lateral expansion of the surface layer. An ordered (√3 × √3)R30° sulfur adlayer develops as the coverage reaches ∼0.3 ML. At higher S coverages (>0.3 ML) gold surface atoms are removed from regular terrace sites and incorporated into a growing gold sulfide phase. At 300 K this process leads to the formation of a rough pit and mound surface morphology. This gold sulfide exhibits short-range order and an incommensurate, long-range ordered AuS phase develops upon annealing at 450-525 K. In contrast, formation of an ordered AuS phase via rapid step-retraction rather than etch pit formation is observed during S-interaction with Au(1 1 1) surfaces at 420 K. Our results shed new light on the S-Au(1 1 1) interaction.     

Collision-induced diffusion and vacancy migration in alkanethiol monolayers on Au(1 1 1)

Scanning tunneling microscopy is used to characterize the collision-induced migration of molecules within well-ordered octanethiol and nonanethiol self-assembled monolayers. A seeded molecular beam is used to create xenon atoms with a kinetic energy of 1.3 eV, and collisions with these atoms cause measurable changes in alkanethiol monolayer surface structure. Migration rates are calculated and compared for molecules in close-packed domains, at domain-boundary defects, and along the perimeter of vacancy-island defects. The number of nearest-neighbor molecules (within the 5 Å lattice distance) is strongly predictive of molecular stability with respect to xenon bombardment, and the overall dependence of stability on nearest neighbors is well fit by a simple exponential curve. The incident direction of the molecular beam is not observed to influence the direction of molecular motion; however, in some cases, migration correlates strongly to surface lattice directions. Finally, there is no evidence that substrate restructuring or gold-atom diffusion accompanies alkanethiol migration under these non-equilibrium conditions.     

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.     

Room temperature observation of nitric oxide on Ir(1 1 1) by scanning tunneling microscopy

Room temperature (RT) adsorption of nitric oxide (NO) on Ir(1 1 1) was studied by scanning tunneling microscopy (STM). At low exposures, NO molecules can not be imaged by STM, because at RT the diffusion of NO is much faster than the STM scanning speed. At high exposures near the saturation coverage, however, a well-ordered 2 × 2 structure is observed. The coverage of the major 2 × 2 species is 0.25 and they can be assigned to the NO molecules adsorbed on the Ir ontop sites. A small number of less bright spots are assigned to nitrogen atoms produced by dissociation. Their number increases by annealing the NO-saturated surface at 380 K. A small number of another dissociation product, oxygen, are observed as black lines, indicating that the diffusion of oxygen atoms is fast. Scratch-like noise features were also detected by the STM, which suggests that a mobile precursor state exists, which was clearly shown by the effects of electron irradiation from the STM tip. These results are consistent with the previous molecular beam studies. Hopping of the 2 × 2 ordered NO species was frequently observed at the anti-phase domain boundaries and edges of the 2 × 2 islands.     

Nanogroove formation during homoepitaxial Au electrodeposition on reconstructed Au( 1 1 1)

In situ scanning tunneling microscopy (STM) studies of homoepitaxial electrodeposition on Au(1 1 1) from hydrochloric acid solution reveal an unusual deposit morphology in the potential regime of the Au surface reconstruction, where the deposited Au islands are separated by nanoscale grooves with preferred widths of 6 and 12 nm. The formation of these structures is attributed to a hindered coalescence of the islands, caused by elastic energy contributions of the reconstructed bottom of the grooves.     

Degradation of octanethiol self-assembled monolayers from hydrogen-atom exposure: A molecular-scale study using scanning tunneling microscopy

Octanethiol self-assembled monolayers were exposed to gas-phase hydrogen atoms, and the resulting changes in the order and chemical structure of the surface were monitored using scanning tunneling microscopy (STM). Extensive damage to the monolayer was observed in the form of both dark and bright features in STM images. These changes began along domain boundaries and moved into close-packed regions of the monolayer as hydrogen-atom exposure time increased. Increasing exposure also results in an accelerated rate of observed surface changes, indicating that the reactivity of the surface increases as a result of initial gas-surface reactions. Complex restructuring of the alkanethiol monolayer is observed, including defect formation and the disordering of the alkanethiol monolayer. However, in some cases the monolayer demonstrates the capability of self-healing, with local annealing and reordering of close-packed domains. This annealing and reordering likely results from increased mobility of surface-bound alkanethiolates in the vicinity of monolayer defects, or from diffusion and readsorption of transiently formed alkanethiol molecules.