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.     

Molecular-scale investigation of tolane disulfide self-assembled monolayers on Au(111) using scanning tunneling microscopy

Molecular-scale surface structures of self-assembled monolayers (SAMs) formed by the spontaneous adsorption of tolane disulfides (TDS) on Au(111) in a 1 mM mixed solution of ethanol/N, N'-dimethylformamide (9:1) were examined using scanning tunneling microscopy (STM). The STM study revealed that TDS SAMs formed after a 24 h immersion at room temperature were composed of two-dimensional (2D) ordered phases with inhomogeneous surface morphologies and no clear domain boundaries. However, after 2 h immersion at 50 degrees C, uniform 2D ordered domains with clear domain boundaries were observed, which could be described as c(3 x square root of 3) structures with centered rectangular unit cell. Interestingly, a unique intermediate ordered phase with a low surface coverage was also observed. After a longer immersion for 24 h at 50 degrees C, only the uniform c(3 x square root of 3) domains were observed with a corrugation that may have reflected surface reconstruction of the Au(111) surfaces. From this study, we found that 2D ordered TDS SAMs with large and uniform domains on Au(111) can be obtained by controlling the solution temperature and immersion time.     

Polymer films studied by scanning tunneling microscopy

Polymer films with thicknesses up to 300 nm were investigated by scanning tunneling microscopy. It was demonstrated that the films contain areas whose images change depending on the scan parameters, which can be explained by the emission processes.     

NaCl multi-layer islands grown on Au(111)-([Formula: see text]) probed by scanning tunneling microscopy

The growth of multi-layer NaCl islands on Au(111)-([Formula: see text]) surfaces was investigated using scanning tunneling microscopy (STM). We observed that the aspect of the NaCl islands drastically differs depending on the tunneling conditions. It is therefore possible to observe the layers forming an NaCl island or to image the gold reconstruction below the first NaCl layer. Atomically resolved STM images obtained on the first NaCl layer demonstrate that NaCl grows as an epitaxial crystalline film on Au(111)-([Formula: see text]). STM images also suggest that some NaCl layers can be non-crystalline.     

Initial epitaxial growth of (111) Au/(111) Cu and (111) Cu/(111) Au

The room temperature modes of growth of Au/(111) Cu and Cu/(111) Au are described. For the former growth mode initial deposits (2.4 Å) of gold on copper form smooth flat islands delineated by coincidence lattice misfit dislocations. For 6.0 Å of gold deposit, both thick and thin gold areas were observed with almost complete substrate coverage. For a 10 Å deposit, surface coverage was complete. Strain measurements and dislocation densities obtained on the (111) Au/(111) Cu films suggest the presence of two separate misfit dislocation networks at the interface. The coincidence lattice networks were large enough for transmission electron microscopy observation but contributed little to total overlayer strain. The (van der Merwe) natural lattice misfit dislocations were too closely spaced for direct observation but their presence was inferred because of the strain measurements. The initial epitaxy of Cu/(111) Au was similar to the Stranski-Krastanov model: the initial monolayer of copper (also delineated by coincidence misfit dislocations) grew smoothly on the gold; additional copper formed essentially stress-free “nuclei” on top of the initial copper layer.     

Boron surface phase on Si(111): atomic structure and Si overgrowth studied by scanning tunneling microscopy and work function measurement

Surface reconstructions of boron (B) on silicon (Si) have been well known for several years. One reconstruction of special interest to us is the so called R30° boron surface phase (BSP) on Si(111). This reconstruction can occur in two different forms, one with B located on T₄ lattice sites (B-T₄), the second one with B residing in S₅ sites (B-S₅) directly underneath a Si adatom in a T₄ site. The two forms of the R30° reconstruction are expected to exhibit completely different properties due to their different chemical binding. In this paper we present work function measurements of these surface phases which clearly show their distinctively different behaviour and allow the determination of the temperature at which the boron atoms migrate from the T₄ sites to the S₅ sites. Furthermore, STM results concerning the overgrowth of BSPs with Si films of variable thicknesses and its effect on the BSP itself are shown.     

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.     

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.     

GaAs/AlGaAs nanowire heterostructures studied by scanning tunneling microscopy

We directly image the interior of GaAs/AlGaAs axial and radial nanowire heterostructures with atomic-scale resolution using scanning tunneling microscopy. We show that formation of monolayer sharp and smooth axial interfaces are possible even by vapor-phase epitaxy. However, we also find that instability of the ternary alloys formed in the Au seed fundamentally limits axial heterostructure control, inducing large segment asymmetries. We study radial core-shell nanowires, imaging even ultrathin submonolayer shells. We demonstrate how large twinning-induced morphological defects at the wire surfaces can be removed, ensuring the formation of wires with atomically flat sides.     

Assembling of redox proteins on Au(111) surfaces: A scanning probe microscopy investigation for application in bio-nanodevices

The morphology and conductive properties of azurin molecules, chemically attached to sulfhydryl terminated alkanethiol monolayer assembled on Au(111) surface, are mapped at single molecule level and compared with those observed for the same molecule immobilised on bare Au(111). High-resolution Tapping Mode Atomic Force Microscopy shows that the protein molecules immobilised on modified gold, better reproduces the crystallographic height of the protein, than that immobilised on bare gold. Such a height recovering is also found in the Scanning Tunnelling Microscopy images. Consistently, a good tunnelling conduction of azurins on the modified gold electrode is demonstrated by Tunnelling Spectroscopy. Cyclic voltammetry measurements show, in addition, that the redox activity of azurin molecules covalently immobilised on sulfhydryl functionalised Au(111) surface is retained. These results are discussed in connection with possible use of this linker in the assembling of nano-hybrid systems.     

Electrochemical scanning tunneling microscope imaging of self-assembled monolayer of double-stranded DNA on Au(111)

Self-assembled monolayer of a double-stranded DNA, which was formed by a 20-nt 5'-thiol-modified oligonucleotide at the interface of Au(111)/solution, has been imaged and studied with electrochemical scanning tunneling microscopy (ECSTM). The results showed that the preferred directions of the adsorbed DNA were <110> and <112>. The measured width of the DNA stripes was 0.95 +/- 0.02 nm, which was consistent with the width of dsDNA base pairs. The contour of DNA in the STM image consisted of a series of blobs, which were due to the anchoring of the bases to the substrate along the dsDNA chain. The imaging of a large-scale array of DNA lying flat on the substrate offers a convenient method for the further application of high-resolution STM to the study of the interaction between DNA and a variety of molecules.     

Bis(phthalocyaninato)yttrium grown on Au(111): Electronic structure of a single molecule and the stability of two-dimensional films investigated by scanning tunneling microscopy/spectroscopy at 4.8 K

Scanning tunneling microscopy/spectroscopy (STM/STS) at 4.8 K has been used to examine the growth of a double-decker bis(phthalocyaninato)yttrium (YPc₂) molecule on a reconstructed Au(111) substrate. Local differential conductance spectra (dI/dV) of a single YPc₂ molecule allow the characteristics of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) to be identified. Furthermore, lateral distributions of the local density of states (LDOS) have also been obtained by dI/dV mapping and confirmed by first principles simulations. These electronic feature mappings and theoretical calculations provide a basis for understanding the unique STM morphology of YPc₂, which is usually imaged as an eight-lobed structure. In addition, we demonstrate that bias-dependent STM morphologies and simultaneous dI/dV maps can provide a way of understanding the stability of two-dimensional YPc₂ films.      

Topography and transport properties of oligo(phenylene ethynylene) molecular wires studied by scanning tunneling microscopy

Conjugated phenylene(ethynylene) molecular wires are of interest as potential candidates for molecular electronic devices. Scanning tunneling microscopic study of the topography and current-voltage (I-V) characteristics of self-assembled monolayers of two types of molecular wires are presented here. The study shows that the topography and I-Vs, for small scan voltages, of the two wires are quite similar and that the electronic and structural changes introduced by the substitution of an electronegative N atom in the central phenyl ring of these wires does not significantly alter the self-assembly or the transport properties.     

Nanoassembly of meso-tetraphenylporphines on surfaces of carbon materials: initial steps as studied by molecular mechanics and scanning tunneling microscopy

We employed MM+ molecular mechanical modeling and scanning tunneling microscopy (STM) in order to analyze the initial steps of nanoassembly of meso-tetraphenylporphine H2TPP and its cobalt(II) complex CoTPP on the surface of highly-oriented pyrolytic graphite (HOPG) and single-walled carbon nanotubes (SWNTs). According to the MM+ results, monolayer H2TPP adsorption is more favorable energetically than the formation of porphyrin stacks on both graphite and nanotube sidewall; the formation of parallel interacting chains of H2TPP on graphite is more preferable than the growth of long single chains; and the assembly into a long-period helix is favored versus the formation of a short-period helix on SWNT sidewall. STM observations of CoTPP complex deposited onto bare HOPG and onto the graphite with deposited SWNTs are consistent with theoretical results. At the same time, both CoTPP single chains and ribbons were observed on HOPG. The formation of short-period helices on the nanotube sidewalls was concluded to be more likely than the long-period helical nanoassembly.     

Ab initio studies of pristine and oxidized Cu3Au(100) and (111) surfaces

We review the previous literature and our recent work on first-principles studies of Cu₃Au(100) and (111) surfaces, with focus on the segregation of atomic species to the surface at pristine conditions and in the presence of oxygen. In particular, the combined use of experimental and theoretical tools to achieve chemical identification at an atomic level of the surface species is emphasized and discussed.     

Properties of Co on Cu(111) revealed by AES and DEPES

Auger electron (AES) and directional elastic peak electron (DEPES) spectroscopies were used to investigate the Co/Cu(111) interface. The change in the Auger Cu (M_2,3VV transition at 66 eV) peak intensity (h_Cu) recorded at room temperature shows that the Co growth mechanism is not a layer by layer type. This proves that Co does not wet the Cu substrate. The recorded DEPES profiles reveal the coexistence of fcc and hcp Co structures already at an early stage of growth (0.8 ML). The Co coverage increase leads to the reduction of intensities associated with the fcc structure and significant increase of the signal from the hcp structure. The comparison between experimental and theoretical data at large coverages (43 ML) shows a major contribution of the hcp structure within the Co layers in the recorded DEPES profiles.     

Superconductivity and scanning tunneling microscopy (STM) / spectroscopy (STS) of transition metal oxide artificial lattices

High T_c superconducting artificial lattices and superlattices have been constructed by a laser molecular beam epitaxy. Layer-by-layer growth method controlled by RHEED and STM is a promising method for the construction of these artificial lattices. Formation and properties of superconducting artificial lattices and super-conducting / ferromagnetic superlattices are presented as the typical examples. In order to investigate the electronic state of carrier doped strongly correlated electronic system, STS measurements have been performed on La_2–xSr_xCuO₄, La_2–xSr_xNiO₄, and La_1-xSr_xMnO₃ thin films. Based on the similarities and the differences between the STS spectra of the three types of the films, electronic states of the strongly correlated system are discussed.     

Dynamic probe of ZnTe(110) surface by scanning tunneling microscopy

The reconstructed surface structure of the II–VI semiconductor ZnTe (110), which is a promising material in the research field of semiconductor spintronics, was studied by scanning tunneling microscopy/spectroscopy (STM/STS). First, the surface states formed by reconstruction by the charge transfer of dangling bond electrons from cationic Zn to anionic Te atoms, which are similar to those of IV and III–V semiconductors, were confirmed in real space. Secondly, oscillation in tunneling current between binary states, which is considered to reflect a conformational change in the topmost Zn–Te structure between the reconstructed and bulk-like ideal structures, was directly observed by STM. Third, using the technique of charge injection, a surface atomic structure was successfully fabricated, suggesting the possibility of atomic-scale manipulation of this widely applicable surface of ZnTe.     

Dynamic probe of ZnTe(110) surface by scanning tunneling microscopy

Abstract

The reconstructed surface structure of the II–VI semiconductor ZnTe (110), which is a promising material in the research field of semiconductor spintronics, was studied by scanning tunneling microscopy/spectroscopy (STM/STS). First, the surface states formed by reconstruction by the charge transfer of dangling bond electrons from cationic Zn to anionic Te atoms, which are similar to those of IV and III–V semiconductors, were confirmed in real space. Secondly, oscillation in tunneling current between binary states, which is considered to reflect a conformational change in the topmost Zn–Te structure between the reconstructed and bulk-like ideal structures, was directly observed by STM. Third, using the technique of charge injection, a surface atomic structure was successfully fabricated, suggesting the possibility of atomic-scale manipulation of this widely applicable surface of ZnTe.     

Sensitizers in inelastic electron tunneling spectroscopy: a first-principles study of functional aromatics on Cu(111)

Low sensitivity is a key problem in inelastic electron tunneling spectroscopy (IETS) with the scanning tunneling microscope. Using first-principles simulations, we predict different means to tune the IETS sensitivity of symmetrical functional aromatics on a Cu(111) surface. We show how the IET-spectra of phenyl-NO? compounds can be greatly enhanced as compared to pristine phenyl. More precisely, the NO? substituent qualifies as a sensitizer of low-frequency wagging modes, but also as a quencher of high-frequency stretching modes. At variance, the CO? substituent is found to suppress the whole IET-activity. The head-up (non-anchoring) and head-down (anchoring) configurations of the functional group lead to minor changes in the signals, nevertheless allowing access to discriminate configurational features. It is shown how to disentangle the electronic and steric effects of the substituent in the STM junction.