Highly resolved non-contact atomic force microscopy images of the Sn/Si(111)-([Formula: see text]) surface

The Sn/Si(111)-([Formula: see text]) surface is observed by using non-contact atomic force microscopy (NC-AFM) at room temperature. The images at relatively far tip-surface distances show four protrusions in each ([Formula: see text]) unit cell, which are similar to previously reported scanning tunnelling microscopy (STM) images. On the other hand, it is found that, at closer tip-surface distances, eight protrusions are clearly resolved, which indicates that the spatial resolution of NC-AFM is higher than that of STM as far as imaging this surface is concerned. Our high-resolution NC-AFM images are in good agreement with a recently proposed model based on 13 Sn atoms per unit cell.     

Assembly behavior and monolayer characteristics of OH-terminated alkanethiol on Au(111): in situ scanning tunneling microscopy and electrochemical studies

Self-assembled monolayers (SAMs) of 6-mercapto-1-hexanol (MHO) on an Au(111) electrode were prepared in an electrochemical system. The adsorption behavior of MHO and the time-dependent organization of the SAM were investigated by in situ scanning tunneling microscopy (STM) and cyclic voltammetry (CV). The results show that a potential higher than 0.28?V (relative to RHE) is required to induce the adsorption of MHO. At 0.28?V, the MHO molecules adsorb in a flat-lying orientation, forming an ordered striped phase with a molecular arrangement of ([Formula: see text]). However, the adlayer is not stable at this potential. The adsorbed striped phase may recover to the herringbone feature of the gold substrate due to the desorption of adsorbed MHO. At a higher potential (0.35?V), the adlayer becomes stable and can undergo a phase evolution from the striped phase to a condensed structure, identified as c([Formula: see text]). This structure can also be described as a c(4 × 2) superlattice of a [Formula: see text] hexagonal adlattice. The surface coverage of the MHO SAM is identical to the saturated structure of an 11-mercapto-1-undecanol (MUO) SAM reported in a previous work, [Formula: see text]. However, the STM image of MHO adlayer shows a modulation in intensity, reflecting the presence of various conformations of adsorbed molecules. This result is attributed to the shorter chain length of MHO, which gives a weaker?van der Waals interaction between adsorbed molecules. This effect also results in a higher charge permeability across the adlayer and a lower striping potential to an MHO SAM.     

Adsorption, manipulation and self-assembling of TBrPP-Co molecules on a Ag/Si(111) surface by scanning tunnelling microscopy

Individual adsorption and two-dimensional assembling of 5,10,15,20-tetrakis-(4-bromophenyl)-porphyrin-Co (TBrPP-Co) molecules on a Si(111)-[Formula: see text] Ag reconstructed surface have been studied using low-temperature scanning tunnelling microscopy (STM). All the isolated molecules are observed in a planar shape with slight distortion. The isolated molecules can be controllably rotated with an STM tip to the orientation along the trigonal lattice ([Formula: see text] direction) of the substrate. With an increased coverage (0.07?ML) and appropriate annealing, the molecules assemble to form three types of ordered phase. The long-range ordered structures, however, disappear at higher coverage (0.75?ML).     

Self-assembly of rubrene on Cu(111)

We performed an ultra-high vacuum scanning tunneling microscopy (STM) investigation of the self-assembly of rubrene at room temperature on Cu(111), a metal surface with threefold symmetry. Rubrene self-assembles into two different structures called row and trimer. Both are different than the structures already observed on Cu(110) and Cu(100). Row and trimer structures have comparable molecular packing densities and are equally distributed across the surface. In the row structure the molecules are oriented with their backbone along the same high symmetry directions of the surface: [[Formula: see text]], [[Formula: see text]] or [[Formula: see text]]. The trimer structure is composed of units of three rubrene molecules, oriented along the high symmetry surface directions. These units are chiral, as revealed by height profile measurements by STM, and self-assemble in domains containing only one type of enantiomer.     

Variable-temperature scanning tunneling microscopy and scanning tunneling spectroscopy study on copper phthalocyanine ultrathin films on a Au(111) surface

Variable-temperature high-resolution scanning tunneling microscopy (STM) images reveal that well-ordered copper phthalocyanine (CuPc) strips can be self-assembled by depositing CuPc molecules on a Au(111) surface. The self-assembled strips are supposed to result from the balance of the intermolecular interaction and the interaction between the molecules and substrate during annealing. The energy band (approximately 1.9-2.1 eV) of CuPc, measured by scanning tunneling spectroscopy (STS), is comparable to the optical band gap (approximately 1.7 eV). Spectroscopic measurements confirm that a dipole layer and/or an effect of image force exist at the CuPc/Au(111) interface.     

Immobilization of methylene blue on self-assembled iodine monolayers on gold

The immobilization of methylene blue (MB) on iodine-covered Au(111) is studied by electrochemical techniques, scanning tunneling microscopy (STM), Auger Electron Spectroscopy (AES), and Raman spectroscopy. Results show that MB species are efficiently adsorbed on the square root of 3 x square root of 3 R30 degrees I lattice on Au(111). The electrochemical behavior of the adsorbed MB molecules is reversible, indicating a relatively fast electron transfer from the Au(111) surface to the immobilized MB species through the iodine layer. STM images with molecular resolution are consistent with adsorption of MB dimers on a square root of 3 x square root of 3 R30 degrees I lattice placed atop of the Au(111) substrate. Results are compared to those obtained for MB immobilized on Au(111) covered by S(n) (n = 3-8) surface structures.     

Facet and in-plane crystallographic orientations of GaN nanowires grown on Si(111)

We have determined the in-plane orientation of GaN nanowires relative to the Si (111) substrate on which they were grown. We used x-ray diffraction pole figure measurements to evidence two types of crystallographic orientation, all the nanowires having [Formula: see text] lateral facets. The proportion of these two orientations was determined and shown to be influenced by the pre-deposition of Al(Ga)N intermediate layers. In the main orientation, the GaN basal [Formula: see text] directions are aligned with the [Formula: see text] directions. This orientation corresponds to an in-plane coincidence of GaN and Si lattices.     

Metal-free phthalocyanine (H2Pc) molecule adsorbed on the Au(111) surface: formation of a wide domain along a single lattice direction

Using low-temperature scanning tunneling microscopy (STM), we observed the bonding configuration of the metal-free phthalocyanine (H₂Pc) molecule adsorbed on the Au(111) surface. A local lattice formation started from a quasi-square lattice aligned to the close-packed directions of the Au(111) surface. Although we expected the lattice alignment to be equally distributed along the three crystallographically equivalent directions, the domain aligned normal to the ridge of the herringbone structure was missing in the STM images. We attribute this effect to the uniaxial contraction of the reconstructed Au(111) surface that can account for the formation of a large lattice domain along a single crystallographical direction.     

Growth of oriented Ag nanocrystals on air-oxidized Si surfaces: An in-situ reflection high energy electron diffraction study

Growth of Ag nanoislands on air-oxidized Si(001), (111) and (110) surfaces has been investigated by reflection high energy electron diffraction (RHEED), scanning tunneling microscopy (STM) and cross-sectional transmission electron microscopy. We have shown that the oriented nanocrystalline Ag, similar to the epitaxial growth of Ag on clean Si surfaces, can be grown on oxide-covered Si surfaces. A thin oxide layer (~ 2-3 nm thick) is formed on ultra-high vacuum (UHV)-cleaned Si surfaces via exposure of the clean reconstructed surface to air. Deposition of Ag was carried out under UHV at different substrate temperatures and monitored by RHEED. RHEED results reveal that Ag deposition at room temperature leads to the growth of randomly oriented Ag islands while, in spite of the presence of the oxide layer between Ag islands and Si, preferred orientations with an epitaxial relationship with the substrate evolve when Ag is deposited at higher substrate temperatures. STM images of the oxidized surfaces, prior to Ag deposition, apparently do not show any order. However, Fourier transforms of STM images show the presence of a short range order on the oxidized surface following the unit cells of the underlying reconstructed Si surface. It is intriguing that Ag nanoislands follow an epitaxial orientational relationship with the substrate in spite of the presence of a 2-3 nm thick oxide layer between Ag and Si. Apparently, the short range order existing on the oxide surface influences the orientation of the Ag nanoislands.     

Resonant electron tunneling through azurin in air and liquid by scanning tunneling microscopy

Electron transfer through the redox metalloprotein azurin immobilized on Au (111) by its disulphide bridge is studied by scanning tunneling microscopy (STM) in buffer solution and (for the first time) in air. STM analysis gives evidences of a stable and robust binding of the molecules in both cases. Bright spots, associated with azurin molecules, are clearly visible in STM images. The image contrast of adsorbed azurin is highly affected by the bias potential with proteins visible only for some well-defined voltage values. This experimental demonstration of the possibility to induce tunneling through azurin in air could disclose very interesting perspectives for the development of protein-based hybrid nanodevices operating in nonliquid environments.     

Metal-free phthalocyanine (H2Pc) molecule adsorbed on the Au(111) surface: formation of a wide domain along a single lattice direction

Abstract

Using low-temperature scanning tunneling microscopy (STM), we observed the bonding configuration of the metal-free phthalocyanine (H₂Pc) molecule adsorbed on the Au(111) surface. A local lattice formation started from a quasi-square lattice aligned to the close-packed directions of the Au(111) surface. Although we expected the lattice alignment to be equally distributed along the three crystallographically equivalent directions, the domain aligned normal to the ridge of the herringbone structure was missing in the STM images. We attribute this effect to the uniaxial contraction of the reconstructed Au(111) surface that can account for the formation of a large lattice domain along a single crystallographical direction.     

Zinc porphyrin-driven assembly of gold nanofingers

Nanofingers of gold covered by porphyrins are prepared by a combination of atomic manipulation and surface self-organization. A submonolayer of zinc(II) 5,10,15,20-tetrakis(4-tert-butylphenyl)-porphyrin (ZnTBPP) axially ligated to a self-assembled monolayer of 4-aminothiophenol (4-ATP) on Au(111) is prepared and studied using a combination of ultrahigh vacuum techniques. Under the electric field produced by the STM tip, the relatively weakly bound Au surface atoms along the discommensuration lines become mobile due to the strong bond to 4-ATP, while the tendency of the porphyrins towards self-assembly result in a collective motion of gold clusters. The clusters diffuse onto the surface following well-defined pathways along the [112] direction and then reach the step edges where they assembled, thus forming nanofingers. First-principles density functional theory calculations demonstrate the reduction of the binding energies between the surface gold clusters and the substrate induced by adsorption of thiols. Scanning tunneling microscopy images show assemblies across three adjacent discommensuration lines of the Au(111)-(22 x square root 3) reconstruction, which collectively diffuse along these lines to form islands nucleated at step edges.     

STM study of Au adsorption on Si(111)-7 x 7 surface: voltage and temperature dependence

We presented the scanning tunneling microscopy (STM) results on the study of Au adsorption at Si(111)-7 x 7 reconstructed surface. The voltage-dependent STM measurements indicated that there are at least three kinds of Au clusters and two types of single Au atoms adsorption on Si(111)-7 x 7 surface with Au coverage of about 0.2 monolayer. After the Au adsorbed Si surface was annealed at about 250 degrees C, the adsorbed Au clusters would diffuse into the Si substrate, and consequently create surface defects on the Si substrate. Sequentially annealed the sample at about 500 degrees C, the diffused Au would emerge out again and form larger 3-dimension Au clusters at the Si surface.     

MgB2 Superconducting Tips for Scanning Tunneling Microscopy Study

We report a simple method for the fabrication of reproducible, clean, and stable MgB₂ superconducting tips. The quality of these tips has been verified by imaging the surface of a thin Au(111) film sample, using a low temperature scanning tunneling microscopy (STM). Using the MgB₂ superconducting tip, high-quality semiatomically resolved STM surface images of the thin Au(111) film sample have been observed, which unambiguously indicates that the fabrication of relatively superconducting MgB₂, suitable for use as STM tips, is feasible.     

MgB2 Superconducting Tips for Scanning Tunneling Microscopy Study

We report a simple method for the fabrication of reproducible, clean, and stable MgB₂ superconducting tips. The quality of these tips has been verified by imaging the surface of a thin Au(111) film sample, using a low temperature scanning tunneling microscopy (STM). Using the MgB₂ superconducting tip, high-quality semiatomically resolved STM surface images of the thin Au(111) film sample have been observed, which unambiguously indicates that the fabrication of relatively superconducting MgB₂, suitable for use as STM tips, is feasible.     

An alkyl spacer effect on the structure of 4-fluorobenzenethiol and 4-fluorobenzenemethanethiol self-assembled monolayers on Au(111)

Self-assembled monolayers (SAMs) formed by the adsorption of 4-fluorobenzenethiol (4-FBT) and 4-fluorobenzenemethanethiol (4-FBMT) on Au(111) were examined by scanning tunneling microscopy (STM) to understand the effect of a flexible methylene spacer between the sulfur head-group and phenyl group and the effect of solution temperature on the formation and structure of the SAMs. Although the adsorption of 4-FBT on Au(111) at room temperature for 24 h generated only disordered phase SAMs containing gold adatom islands, 4-FBT at 75 degrees C for 2 h formed mixed SAMs: disordered phases and ordered (2 x 12√(2))R10 degrees superlattice with a rectangular unit cell containing six adsorbed molecules. On the other hand, SAMs formed from 4-FBMT, with a methylene spacer, at room temperature for 24 h on Au(111) had irregularly ordered phases containing uniformly distributed VIs with lateral dimensions of 2-5 nm; SAMs formed from 4-FBMT at 75 degrees C for 2 h were composed of slightly improved ordered phases and larger VIs with lateral dimensions of 5-12 nm as a result of Ostwald ripening. From this study, we found that the methylene spacer plays an important role in controlling the structure of SAMs formed from p-substituted fluorinated aromatic thiols.     

Investigation of the composition and electrical properties of gold–H-terminated silicon (111) interface

Experimental studies of the interface between hydrogen-terminated Si(111) and evaporated Au are reported. Auger Electron Spectroscopy (AES) and Scanning Tunneling Microscopy (STM), in ambient conditions, were used to study the composition and electrical (I–V) properties of the Au/Si junction, as a function of Au film thickness. Au films of 5 nm present a rectifying behavior with the tunneling current for negative tip bias. This result is explained by the fact that Si is known to migrate through the Au layer. The I–V measurements point to the fact that enough Si segregates on top of the Au layer to form a reverse diode. It is also shown that the first monolayers of gold chemically bind to the silicon. Such a surface presents electrical metallic properties at this initial stage of gold growth on silicon.     

Evaluation of molecular orientation and alignment of poly(3-hexylthiophene) on Au (111) and on poly(4-vinylphenol) surfaces

The orientation and alignment of regioregular poly(3-hexylthiophene) (P3HT) molecules on Au (111) surface and on poly(4-vinylphenol) (PVP) thin film were investigated. The P3HT molecules on the smooth Au (111) are oriented with both the backbones and the side chains parallel to the substrate (plane-on orientation) as revealed by the scanning tunneling microscope (STM) images. However, the P3HT molecules on the PVP thin films are preferably oriented with side chains perpendicular to the surface (edge-on orientation). Surface modification of the PVP by hexamethyldisilazane (HMDS) can increase the crystalline size in the P3HT semicrystalline films. The performance of an all-polymer organic field-effect transistor (OFET) with the drop-cast P3HT semiconductor layer and the crosslinked PVP gate insulator on poly(ethylene naphthalate) (PEN) substrate was evaluated.     

PTCDA on Cu(111) partially covered with NaCl

The organic molecule 3,4,9,10-perylene-tetracarboxylic dianhydride (PTCDA) was studied by means of scanning tunneling microscopy (STM) on thin insulating NaCl films grown on a Cu(111) single crystal. The deposition of approximately two monolayers (ML) of sodium chloride onto a Cu(111) substrate at a sample temperature of about 350 K causes a rather rough growth of (100)-oriented NaCl islands up to a local height of 4 ML. For submonolayer coverages (0.1 and 0.4 ML) of PTCDA on a Cu(111) surface partly covered with NaCl, two different rod structures of PTCDA were found on the copper surface, which are in contrast to previously published data for PTCDA on Cu(111) showing a herringbone-like arrangement. These findings can be explained by the formation of a Na(x)-PTCDA complex. On NaCl covered areas, single PTCDA molecules adsorb at vacancies of [010] and [001] oriented steps of the NaCl(100) islands. In this case, the electrostatic forces between the polar step edges and the PTCDA molecules are dominant. The terraces of the alkali halide surface are free of PTCDA molecules.     

Cerium oxide layers on Pt(111): a scanning tunneling microscopy study

Ultrathin epitaxial layers of cerium oxide were prepared by oxidation of layers of an ordered Pt–Ce surface alloy on top of a Pt(111) single crystal. They consist of low-dimensional CeO₂ islands. Atomically resolved scanning tunnel microscopy (STM) images indicate a surface structure of the fluorite-type CeO₂(111) 1×1 phase and the presence of surface defects.