Adsorption of cobalt phthalocyanine on a Si(1 0 0) surface with Bi-line structures as evaluated by scanning tunneling microscopy

We report the reaction dynamics of cobalt phthalocyanine (CoPc) molecules with Bi-line structures (BLSs) on a Si(1 0 0) surface, investigated using scanning tunneling microscopy (STM). When CoPc molecules were deposited on a Si(1 0 0) surface with BLSs at room temperature, single-spot protrusions were observed in the STM image instead of four-spot images corresponding to CoPcs flat molecular structure. Moreover, domains with a c(4 × 4) periodicity appeared on the terraces of the Si(1 0 0) surface. This indicates that CoPc molecules may have decomposed on the surface by catalytic reaction with Bi atoms.     

The calculation of transition probabilities for atomic oxygen

In this contribution we present scanning tunnelling microscopy (STM) and spectroscopy (STS) investigations on isolated cobalt clusters in contact with Ge(001). Mass-filtered nanoparticles with diameters ranging from 3 to 11 nm are generated using an arc cluster ion source (ACIS) and deposited under soft landing conditions (E_kin/atom < 0.5 eV). Since the tip radius is of the same order as the nanoparticle diameters the recorded STM images are significantly affected by tip folding. By means of the “blind reconstruction method" it is possible to approximate the tip shape. After a respective deconvolution of the image structural features of the particle facets become observable. According to the equilibrium shape of the clusters being a truncated octahedron in the size range under investigation, hexagonal and rectangular features appear in the images. STS is sensitive to occupied and unoccupied states near the Fermi level and reveals the existence of distinct states in the tunnelling conductivity of the substrate as well as on the clusters. The richly structured density of states of the germanium surface serves here as tip condition test. First measurements of the tunnelling conductivity of the Co_N/Ge(001) are presented and discussed.     

MeV Au ion induced modifications at Co/Si interface

We report on room temperature MeV Au ion induced modifications at the Co/Si interfaces. Nanometers size thin film of Co and Si were grown by ultra high vacuum (UHV) electron beam evaporation technique on Si(1 1 1) surface and were irradiated by 1.5 MeV Au²⁺ ions at a fluence of 5 × 10¹⁴ ions cm⁻². High-resolution transmission electron microscopy (HRTEM) along with energy filter imaging technique has been employed to study the formation of Co-Si alloy at the interface. Formation of such surface alloy has been discussed in the light of ion-matter interaction in nanometer scale regime.     

Growth of Ag thin films on ZnO(0 0 0 −1) investigated by AES and STM

The growth of Ag films on ZnO(0 0 0 −1) has been investigated by Auger electron spectroscopy (AES) and scanning tunneling microscopy (STM). A high density of islands is nucleated at the earliest stages of the growth. An upstepping mechanism causes these islands to coalesce while the uncovered fraction of the ZnO surface remains constant (30%).     

Structure and properties of TiO<Subscript>2</Subscript> surfaces: a brief review

Titanium oxides are used in a wide variety of technological applications where surface properties play a role. TiO₂ surfaces, especially the (110) face of rutile, have become prototypical model systems in the surface science of metal oxides. Reduced TiO₂ single crystals are easy to work with experimentally, and their surfaces have been characterized with virtually all surface-science techniques. Recently, TiO₂ has also been used to refine computational ab initio approaches and to calculate properties of adsorption systems. Scanning tunneling microscopy (STM) studies have shown that the surface structure of TiO₂(110) is more complex than originally anticipated. The reduction state of the sample, i.e. the number and type of bulk defects, as well as the surface treatment (annealing in vacuum vs. annealing in oxygen), can give rise to different structures, such as two different (1×2) reconstructions, a ‘rosette’ overlayer, and crystallographic shear planes. Single point defects can be identified with STM and influence the surface chemistry in a variety of ways; the adsorption of water is discussed as one example. The growth of a large number of different metal overlayers has been studied on TiO₂(110). Some of these studies have been instrumental in furthering the understanding of the ‘strong metal support interaction’ between group-VIII metals and TiO₂, as well as low-temperature oxidation reactions on TiO₂-supported nanoscopic gold clusters. The growth morphology, interfacial oxidation/reduction reaction, thermal stability, and geometric structure of ultra-thin metal overlayers follow general trends where the most critical parameter is the reactivity of the overlayer metal towards oxygen. It has been shown recently that the technologically more relevant TiO₂ anatase phase can also be made accessible to surface investigations. Received: 4 March 2002 / Accepted: 20 October 2002 / Published online: 5 February 2003 RID="*" ID="*"Corresponding author. Fax: +1-504/862-8279, E-mail: diebold@tulane.edu     

Pd adsorption on Si(1 1 3) surface: STM and XPS study

Pd-induced surface structures on Si(1 1 3) have been studied by scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). In the initial process of the Pd adsorption below 0.10 ML, Pd silicide (Pd₂Si) clusters are observed to form randomly on the surface. By increasing the Pd coverage to 0.10 ML, the clusters cover the entire surface, and an amorphous layer is formed. After annealing the Si(1 1 3)-Pd surface at 600 °C, various types of islands and chain protrusions appears. The agglomeration, coalescence and crystallization of these islands are observed by using high temperature (HT-) STM. It is also found by XPS that the islands correspond to Pd₂Si structure. On the basis of these results, evolution of Pd-induced structures at high temperatures is in detail discussed.     

NaCl islands decorated with 2D or 3D 3,4,9,10-perylene-tetracarboxylic-dianhydride nanostructures

The formation of PTCDA (3,4,9,10-perylene-tetracarboxylic-dianhydride) nanostrutures on Au(1 1 1)-() covered with NaCl islands has been studied using scanning tunneling microscopy (STM). Atomically resolved STM images show that NaCl grows as (1 0 0)-terminated layers on Au(1 1 1)-(). Local atomic hexagonal packing has also been observed in the NaCl(1 0 0) layer. At submonolayer NaCl coverage, PTCDA forms two-dimensional islands on the Au(1 1 1) surface and nucleate preferentially at the NaCl island step edges. When the Au surface is fully covered with NaCl layers, PTCDA molecules form three-dimensional molecular clusters decorating the step edges of NaCl layers.     

Growth and characterization of nanostructured anatase phase TiO<Subscript>2</Subscript> thin films prepared by DC reactive unbalanced magnetron sputtering

Titanium dioxide thin films were deposited on three different unheated substrates by unbalanced magnetron sputtering. The effects of the sputtering current and deposition time on the crystallization of TiO₂ thin films were studied. The TiO₂ thin films were deposited at three sputtering current values of 0.50, 0.75, and 1.00 A with different deposition times of 25, 35, and 45 min, respectively. The surface morphology of the films was investigated by atomic force microscopy (AFM). The structure was characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The film thickness was determined by field emission scanning electron microscopy (FE-SEM), and the optical property was evaluated with spectroscopic ellipsometry. The results show that polycrystalline anatase films were obtained at a low sputtering current value. The crystallinity of the anatase phase increases as the sputtering current increases. Furthermore, nanostructured anatase phase TiO₂ thin films were obtained for all deposition conditions. The grain size of TiO₂ thin films was in the range 10–30 nm. In addition, the grain size increases as the sputtering current and deposition time increase.     

Quasi-one-dimensional structures on the Si(1 1 1) surface induced by Ba adsorption

Ba-induced quasi-one-dimensional reconstructions of the Si(1 1 1) surface have been investigated by low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). While the 3 × ‘2’ surface shows double-periodicity along the stripes in STM images consistent with half-order streaks observed in LEED patterns, no sign of the double-periodicity along the chain direction was detected for the 5 × 1 surface. The 5× stripes in STM images show internal structures with multiple rows. The two rows comprising the boundaries of a 5× stripe in the filled-state STM image are found to have 3a × √3/2 spacing across the stripe. The observation of the successive 3× and 2× spacings between the boundary rows supports a structural model proposed for the Ba-induced 5 × 1 Si reconstruction composed of honeycomb chains and Seiwatz chains. The highest coverage 2 × 8 surface does not reveal a quasi-1D row structure in STM images.     

Investigation of the 4H-SiC surface

The silicon carbide (SiC) surface is more complex than that of silicon and can be carbon-terminated or silicon-terminated, and can exist as several reconstructions. Investigations of the surface structure as a function of temperature, under ultrahigh vacuum (UHV) conditions using scanning tunneling microscopy (STM) and low energy electron diffraction (LEED), are presented. The 4H-SiC surface can be passivated using a silicon deposition/evaporation technique to reconstruct the surface. This has a significant effect on the electrical behaviour of metal contacts to the silicon carbide surface, critical in any electronic device. Atomic resolution STM studies of the 4H-SiC surface have revealed step features and micropipe defects in unprecedented detail. STM has also been used to image clusters of metal deposited on the 4H-SiC surface. The effect of annealing on the behaviour of these nickel clusters is also presented. The surface of the silicon carbide is extremely important in the fabrication of silicon carbide electronic devices and this paper presents a discussion of the SiC surface with particular reference to its impact on SiC device applications in power electronics.     

Quasi-one-dimensional In atomic chains on Si(1 1 1) at low temperature studied by reflection high-energy positron diffraction and scanning tunneling microscopy

We have investigated a quasi-one-dimensional structure of Si(1 1 1)-8 × 2-In surface using reflection high-energy positron diffraction (RHEPD) and scanning tunneling microscopy (STM). From the RHEPD rocking curve analyses, we confirmed the formation of the In hexagon structure proposed by González et al. [C. González, F. Flores, J. Ortega, Phys. Rev. Lett. 96 (2006) 136101]. Furthermore, we found that the empty-state STM image at 44 K is consistent with that calculated with the optimum hexagon structure by the RHEPD analyses.     

Morphology of pentacene films deposited on Cu(1 1 9) vicinal surface

We investigate the morphology of a pentacene (C₂₂H₁₄) film adsorbed on the Cu(1 1 9) vicinal surface by scanning tunnelling microscopy (STM). Thermal treatment of a thick film of molecules generates a long-range ordered structure. Series of molecular rows are alternated with areas where the molecules assume two equivalent orientations. STM data analysis suggests that the ordered structure can be described by a rippled morphology. The behaviour of the film at different annealing temperatures suggests a possible explanation of the film structure as due to an adsorbate-induced modification of the substrate.     

Electronic and structural studies of immobilized thiol-derivatized cobalt porphyrins on gold surfaces

The immobilisation of thiol-derivatized cobalt porphyrins on gold surfaces has been studied in detail by means of combined scanning tunnelling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). S-thioacetyl has been used as a protective group for the thiol. Different routes for deprotection of the acetyl groups were performed in acidic and in basic conditions. The results show the formation of monolayer films for the different preparation schemes. The immobilisation of the molecules on the gold surface takes place through the thiol-linkers by the formation of multiple thiolate bonds. In the case of layers formed with protected porphyrins approximately 60% of the linkers are bonded to the gold surface whereas for deprotected layers the amount of bonded linkers is increased up to about 80%. STM measurements revealed that the molecules arrange in a disordered overlayer and do not exhibit mobility on the gold surface. Annealing experiments have been performed in order to test the stability of the porphyrin layers. Disordered patterns have been observed in the STM images after annealing at T = 400 °C. XPS revealed that the sulphur content disappeared completely after annealing at T = 180 °C and that the molecules did undergo significant modifications.     

Hydrogen-induced metallization on Ge(1 1 1) c(2 × 8)

We have studied hydrogen adsorption on the Ge(1 1 1) c(2 × 8) surface using scanning tunneling microscopy (STM) and angle-resolved photoelectron spectroscopy (ARPES). We find that atomic hydrogen preferentially adsorbs on rest atom sites. The neighbouring adatoms appear higher in STM images, which clearly indicates a charge transfer from the rest atom states to the adatom states. The surface states near the Fermi-level have been followed by ARPES as function of H exposure. Initially, there is strong emission from the rest atom states but no emission at the Fermi-level which confirms the semiconducting character of the c(2 × 8) surface. With increasing H exposure a structure develops in the close vicinity of the Fermi-level. The energy position clearly indicates a metallic character of the H-adsorbed surface. Since the only change in the STM images is the increased brightness of the adatoms neighbouring a H-terminated rest atom, we identify the emission at the Fermi-level with these adatom states.     

Influences of the interfacial state between Co and a Si substrate on the magnetic properties of Co/Si(1 1 1) films

Morphology and magnetic properties of Co/Si(1 1 1) interfaces have been investigated using scanning tunneling microscope and surface magneto-optic Kerr effect techniques. As deposited at room temperature for Co/Si(1 1 1), defects have been observed with shapes of dark patches and bright islands on the surface with different Co coverage. The defect formation causes a rough interface. For subsequently deposited Co layers, the interfacial state between Co and the Si substrate results in the appearance of both the longitudinal and polar Kerr loops. After annealing treatments, interdiffusion of Co atoms and Si(1 1 1) substrate occurs as revealed by Auger electron spectroscopy. Scanning tunneling microscope images show the formation of Si clusters with average diameter of 10 nm at high temperatures. The disappearance of ferromagnetism of the films occurs due to the structural and compositional changes.     

Magnetism of Fe clusters formed by buffer-layer assisted growth on Pt(997)

The growth and magnetism of nanometer size Fe clusters on stepped Pt surfaces is investigated by scanning tunneling microscopy (STM) and magneto-optical Kerr effect measurements (MOKE). The clusters are formed on xenon buffer layers of varying thickness and then brought into contact with the substrate by thermal desorption of the Xe. The cluster size is controlled by the thickness of the Xe layer. It is found that clusters of diameter smaller than the Pt terrace width of 2 nm are aligned along the step edges of the Pt(997), thus forming linear cluster chains. In this arrangement, the clusters are ferromagnetic with an easy axis in the direction along the surface normal. If the cluster diameter is larger than the terrace width then the alignment along the step edges is not observed and rather large agglomerates are found which are randomly distributed over the surface. Despite their increased volume, such agglomerates are superparamagnetic with in-plane easy magnetization axis. The enhanced magnetic anisotropy energy in the smallest clusters is originating from hybridization effects at the Fe-Pt interface.     

Atomic structure of the carbon induced Si(0 0 1)-c(4 × 4) surface

The atomic and electronic structures of the Si(0 0 1)-c(4 × 4) surface have been studied by scanning tunneling microscopy (STM) and density functional theory (DFT). To explain the experimental bias dependent STM observations, a modified mixed ad-dimer reconstruction model is introduced. The model involves three tilted Si dimers and a carbon atom incorporated into the third subsurface layer per c(4 × 4) unit cell. The calculated STM images show a close resemblance to the experimental ones.     

Observation of p(1×1)-type rows on TiO2(110)-p(1×2) surface by scanning tunneling microscope (STM)

The TiO₂(110) surfaces were observed by a Scanning Tunneling Microscope (STM). We found two types of bright p(1×1)-type rows on the p(1×2) surface. One p(1×1)-type formed independently and corresponds to the bridging oxygen rows. The second p(1×1)-type appeared in a bright grouping, forming narrow rows, and corresponds to the five-fold titanium rows. The above results suggest the following two conclusions. First, the density of state (DOS) on the bridging oxygen rows becomes higher than that on the five-fold titanium atom rows when a bridging oxygen row exists independently on the p(1×2) surface. Second, the bright rows on a TiO₂(110)-p(1×1) surface correspond to the five-fold titanium atom rows. The results further show the validity of DOS calculations on the TiO₂(110)-p(1×1) surface by Diebold et al. [Phys. Rev. Lett. 77 (1996) 1322]. The difference of width for Ti₂O₃ unit rows on the p(1×2) and p(1×3) surfaces in STM images are also discussed.     

Effects of thermal treatment on the morphology of Alq3 thin layers on Si(1 1 1) substrate

Alq₃ thin layers were vapor deposited onto a single crystal of Si(1 1 1) and the morphology of the surface was investigated by the scanning tunneling microscope under ultrahigh vacuum conditions. The STM imaging showed considerable influence of the thermal processing onto the topography of the sample. Slowly raising the sample temperature to ∼160 °C caused a complete desorption of Alq₃ molecules and uncovering the clean surface of Si(1 1 1). A fast rise of the temperature (flashing) to ∼600 °C led to decomposition of the Alq₃ and resulted in remnants of a carbon-rich surface species. Then heating or flashing this surface to a temperature in excess of 1000 °C brought about the occurrence of regular shape object on the Si(1 1 1) surface.     

On the growth of conversion chromate coatings on 2024-Al alloy

The initial growth of chromate conversion coatings on aluminium 2024-T3 alloy has been investigated by scanning Auger microscopy, scanning electron microscopy and X-ray photoelectron spectroscopy. The coating initiation is shown to be influenced by the alloy microstructure. In agreement with previously proposed growth models, Cr(VI) to Cr(III) reduction begins on the Al-Cu-Fe-Mn intermetallic second-phase particles, which act as cathodic sites, and then over the entire Al matrix surface. The less noble Al-Cu-Mg second-phase particles demonstrate dual behaviour during the initial stage of coating; some dealloy, with formation of a Cu-rich sponge-like structure, while others show no evidence for etching during the first few seconds and coating deposits on them similar to the situation for the Al-Cu-Fe-Mn particles. XPS measurements show more Cr(III) at the very initial stage of nucleation and growth, whereas the amount of Cr(VI) in the coating increases with the length of the chromating treatment. This is discussed in relation to Raman spectroscopy measurements made in a separate study.