Vanadium nanoclusters on Si(1 1 1) 7 × 7 surface studied by scanning tunneling microscopy

The size distribution and shape transition of self-assembled vanadium silicide clusters on Si(1 1 1) 7 × 7 have been investigated by scanning tunneling microscopy. Nanoclusters were formed by submonolayer vanadium deposition at room temperature followed by subsequent annealing (solid phase epitaxy - SPE). At room temperature, initially V-nanoclusters are formed which occupy sites avoiding the corner hole parts of the unit cells in the Si(1 1 1) 7 × 7 surface. Upon annealing, strong metal-silicon reaction occur leading to the formation of vanadium silicide nanoclusters. As a function of temperature, both, flat (2D) and three dimensional (3D) clusters have been obtained. After annealing at temperatures around 900 K many faceted clusters are created, whereas at higher annealing temperature, around 1300 K, predominantly 3D clusters are formed. The size distribution of SPE grown clusters could be well controlled in the range of 3-10 nm. The cluster size depends on the annealing temperature as well as on the initial vanadium coverage. Based on high resolution STM images a structure model for one kind of vanadium disilicide clusters exposing atomically flat surfaces was proposed.     

STM characterization of size-selected V1, V2, VO,and VO2 clusters on a TiO2(110)-(1 × 1) surface at room temperature

We use ultra-high vacuum scanning tunneling microscopy (UHV–STM) to probe, at the atomic level, the structure of mass-selected isolated V₁, V₂, VO and VO₂ clusters deposited on rutile TiO₂(110) by ion soft landing. All four species interact differently with the TiO₂ surface and the ultimate binding site and configuration strikes a balance between the gas-phase structure and the ligation of this cluster by the TiO₂ surface. Our results show that vanadium atoms prefer to bind in the upper threefold hollow sites on the surface and have a slight tendency to pair along the [1–10] direction, while vanadium dimers bind to the surface oriented along the [001] direction exclusively. VO clusters bind with the vanadium atom in the upper threefold hollow site and with the oxygen atom bound to an adjacent fivefold coordinated Ti atom (5c-Ti). The VO₂ cluster also binds with the vanadium atom in the upper threefold hollow site and with both oxygen atoms bound to adjacent 5c-Ti atoms or with only one oxygen bound to the surface and the other directed out of the plane of the surface.     

Ultra thin V2O3 films grown on oxidized Si(1 1 1)

The growth of V₂O₃(0 0 0 1) has been investigated by scanning tunnelling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). Direct evaporation of vanadium onto the Si(1 1 1)-7 × 7 substrate gives rise to massive surface intermixing and consequent silicide formation. In order to obtain the vanadium oxide with good quality, the 7 × 7 surface was initially partially oxidized which leads to a smooth oxygen–silicon surface layer which in turn prevents a complete vanadium–silicon alloy formation. Finally a vanadium oxide film of V₂O₃ stoichiometry was created. The grown film exposes single crystalline areas of stepped surfaces which appear azimuthally randomly-oriented.     

Structural properties of size-selected FeCo nanoparticles deposited on W(110)

Size-selected iron and iron–cobalt alloy clusters have been studied with high resolution transmission electron microscopy (HRTEM) and scanning tunneling microscopy (STM). The clusters were produced by a continuously working arc cluster ion source and subsequently size-selected by an electrostatic quadrupole deflector. The crystalline structure of pure clusters has been investigated with HRTEM to ensure a reliable determination of the lattice parameter for the alloy clusters. The composition of the alloy clusters was checked with energy dispersive X-ray spectroscopy (EDX). The height of the deposited FeCo clusters on the (110) surface of tungsten was determined via STM. These results were compared with the lateral size distribution being investigated by TEM and allow a conclusion on the shape of the deposited alloy clusters. Furthermore, the behavior of the alloy clusters on the W(110) surface at elevated temperatures has been examined, at which the clusters show anisotropic spreading.     

In situ scanning tunneling microscopy studies of bimetallic cluster growth: Pt-Rh on TiO2(1 1 0)

The growth of Pt on clusters on TiO₂(1 1 0) in the presence and absence of Rh was investigated by scanning tunneling microscopy (STM) for Pt deposited on top of 0.3 ML Rh clusters (Rh + Pt). In situ STM studies of Pt growth at room temperature show that bimetallic clusters are produced when Pt is directly incorporated into existing Rh clusters or when newly nucleated clusters of pure Pt coalesce with existing Rh clusters. Low energy ion scattering experiments demonstrate that Rh is still present at the surface of the clusters even after deposition of 2 ML of Pt, indicating that Rh atoms can diffuse to the cluster surface at room temperature. Rh clusters were found to seed the growth of Pt clusters at room temperature as well as 100 K and 450 K. Furthermore, clusters as large as 100 atoms were observed to be mobile on the surface at room temperature and 450 K, but not at 100 K. Pt deposition at 100 K exhibited more two-dimensional cluster growth and higher cluster densities compared to room temperature experiments due to the lower diffusion rate. Increased diffusion rates at 450 K resulted in more three-dimensional cluster growth and lower densities for pure Pt growth, but cluster densities for Pt + Rh growth were the same as at room temperature.     

Periodic nanostructures observed by STM on vanadium surface preilluminated with a cw Yag-laser

Metallic vanadium samples were oxidized in air by a cw Nd:YAG laser. The structure of developed V₂O₅ crystals was investigated by optical and scanning tunneling microscopy (STM). Ripple morphologies with submicron spatial period have been observed. The grating-like structures have been attributed to the interference of surface scattered electromagnetic waves with incident laser light. The dispersion of the surface capillary waves has been taken into account since the oxide has developed partly in a liquid phase.     

Elastic strain at the interface in Ge clusters on Si substrate—a Raman spectroscopic measurement

Ge clusters are grown on Si substrate at room temperature (Ge-RT) and also at liquid nitrogen temperature (Ge-LNT) by cluster evaporation technique. These clusters show blue luminescence. Raman measurement demonstrates the increase in strain with annealing in diffused disordered Si at the interface between Ge-LNT clusters and Si substrate. This manifests in strain-relaxation in the clusters as observed by Photoluminescence (PL) measurements. The decrease in PL intensity for Ge-RT with annealing has been attributed to reduction in surface oxide species, which is supported by Raman spectroscopic measurements. The objective of the paper is to understand the effect of thermal annealing on both interfacial strain and interdiffusion of elemental Si at the interface, together with luminescence characteristics of the clusters.     

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.     

Growth of PTCDA crystals on H:Si(1 1 1) surfaces

The room temperature deposition of PTCDA on hydrogen passivated Si(1 1 1), as a function of evaporation temperature and dosing time, has been studied by STM. At low evaporation temperature, 200 °C, clusters with an average size of 3.5 nm are formed on the surface. The mobility of the small clusters is so high, even at room temperature, that most of the clusters are trapped at surface defects. By increasing the evaporation temperature to 230 °C, larger clusters are formed which have lower mobility. The growth process is identified as a Volmer-Weber mechanism. On increasing the evaporation temperature further to 250 °C, crystals with dendritic shape are formed with an average size of 150 nm. The terraces of the crystal are formed with the (1 0 2) basal plane of the α-phase. Molecular resolution on the terrace also allows us to identify the molecular mechanism involved in the growth of the dendritic crystals.     

用扫描隧道显微镜操纵Cu亚表面自间隙原子

在超高真空环境下使用扫描隧道显微镜研究了吸附有双甘氨肽分子的Cu(001)表面.在一定的 偏压条件下，针尖在该表面扫描后会形成纳米尺度的Cu团簇，这些团簇可以根据意愿排列成 字母或图形.团簇的高度同偏压、隧道电流以及时间等条件有密切关系.在室温下可以稳定存 在的团簇为制造纳米器件提供了技术上的可能性.实验结果表明，形成团簇的Cu原子不是来 自Cu衬底表面或是针尖.化学吸附在Cu表面的双甘氨肽分子，受到隧道电场的作用会在Cu表 面形成张应变场，Cu亚表面自间隙原子在张应变场作用下迁移到表面是形成团簇的原因. 关键词： 扫描隧道显微镜 纳米尺度Cu团簇 自间隙原子     

Measuring the implantation depth of silver clusters in graphite

Scanning tunnelling microscopy (STM) and molecular dynamics (MD) simulations have been used to investigate the implantation of Ag₇ ^- clusters into the graphite surface. An experimental measure of the implantation depth of individual clusters is gained via thermal oxidation of the bombarded graphite surfaces. This process results in etching of the cluster-induced defects to form etch pits which grow laterally whilst retaining the depth of the implanted cluster. STM imaging of the etch pits reveals the distribution of implantation depths for deposition energies of 2 keV and 5 keV. Molecular dynamics simulations for clusters of 5 keV energy show that the implantation depth for Ag₇ ^- is largely independent of the impact site on the graphite surface and the cluster orientation. The implantation depth found by MD lies at the upper edge of the experimental depth distribution. Received 30 November 2000     

STM analysis of triosmium carbonyl cluster adsorption at HOPG

The study of metallic carbonyl clusters as precursors in tailoring the heterogeneous metal catalysts has been of great importance. The catalytic nature of the adsorbed clusters in thin film form depends on the chemical properties of the substrate used. The metal-support interaction will determine various properties such as the surface morphology, adsorption features and the structural orientations. We report a scanning tunneling microscopy (STM) study of an osmium carbonyl cluster (Os₃(CO)₁₁(NCCH₃)) adsorbed on highly oriented pyrolytic graphite (HOPG). STM measurements showed that the osmium carbonyl cluster interacts with HOPG in such a way that it adsorbs on the basal plane showing regular lattice structure, whereas the axial planes of the HOPG surface shows no ordered structure. The regular cluster lattice structure of the carbonyl cluster on the basal plane of the graphite has lattice parameters of a=1.4 nm and b=1.5 nm. We believe that the regular orientation of the cluster indicates a monolayer adsorption of the cluster on the graphite basal planes. Scanning tunneling spectroscopy (STS) measurements also indicated an insulating behavior for the cluster molecules on HOPG, with a significant energy gap value of ca. 300 mV. The cluster interaction at the active sites, i.e. axial planes of the graphite, was also observed by in situ STM measurements.     

Growth behavior and field-induced diffusion of Ni clusters/particles on SrTiO3 (001) observed by UHV-TEM/STM

The fundamental and technological importance of metal clusters and particles on oxide surfaces is growing. Here, room temperature deposited Ni clusters and particles on clean SrTiO₃ (001) surfaces were analyzed with a UHV-TEM/STM combined system to investigate reaction, growth, morphology, and crystal structure consistently. STM observation revealed their growth process from isolated clusters almost of the size of the nuclei to bigger particles. From TEM observation, it was found that small clusters have a semi-commensurate epitaxial orientation relationship, but that bigger ones grow into an incommensurate cube-on-cube epitaxial orientation relationship. STS measurement on Ni particles caused field-induced diffusion of Ni atoms, in which piling up of Ni was recognized at the positions of the STM tips. This is assumed to be related with interfacial reaction.     

Processes involved in the formation of silver clusters on silicon surface

We analyze scanning electron microscopy measurements for structures formed in the deposition of solid silver clusters onto a silicon(100) substrate and consider theoretical models of cluster evolution onto a surface as a result of diffusion and formation of aggregates of merged clusters. Scanning electron microscopy (SEM) data are presented in addition to energy dispersive X-ray spectrometry (EDX) measurements of the these films. Solid silver clusters are produced by a DC magnetron sputtering source with a quadrupole filter for selection of cluster sizes (4.1 and 5.6 nm or 1900 and 5000 atoms per cluster in this experiment); the energy of cluster deposition is 0.7 eV/atom. Rapid thermal annealing of the grown films allows analysis of their behavior at high temperatures. The results exhibit formation of cluster aggregates via the diffusion of deposited solid clusters along the surface; an aggregate consists of up to hundreds of individual clusters. This process is essentially described by the diffusion-limited aggregation (DLA) model, and thus a grown porous film consists of cluster aggregates joined by bridges. Subsequent annealing of this film leads to its melting at temperatures lower than to the melting point of bulk silver. Analysis of evaporation of this film at higher temperatures gives a binding energy in bulk silver of ɛ₀= (2.74 ± 0.03) eV/atom. The text was submitted by the authors in English.     

Metal clusters on rare gas layers – growth and spectroscopy

Gold and lead clusters were grown by the evaporation of metal atoms on rare gas layers on Au(111) and Pb(111). The growth was investigated with scanning tunneling microscopy (STM) and ultraviolet photoelectron spectroscopy (UPS). We studied the electronic coupling to the surface and charging effects in the photoemission process. For a rare gas film thickness of 60 ML we observed an electronic decoupling: the cluster spectra are no longer referenced to the Fermi edge, but to the vacuum energy of the substrate. For Pb clusters on a Au(111) substrate this leads to the remarkable result of a cluster-photoelectron signal above the Fermi energy defined by the ground level of the sample holder. PACS 33.60.Cv; 36.40.Mr; 68.37.Ef; 61.46.+w; 73.22.-f     

Local surface cleaning and cluster assembly using contact mode atomic force microscopy

Conventional contact mode atomic force microscopy (AFM) has been used for local surface cleaning and cluster alignment. By using the AFM tip to sweep and push in contact mode, we have demonstrated that Cu clusters, prepared by vacuum evaporation onto Dow Cyclotene 3022 polymer and subsequent exposure to atmosphere, can easily be moved by the AFM tip, and assembled at the outer edge of the scanned region to form a line of clusters. We have found that the force applied by the tip plays an important role in the ease of cluster motion. Cyclotene surface treatment that enhances cluster adhesion hinders this ability, and may be used as a method of nanofabrication.     

钒氧化物团簇正离子与乙烯在流动管反应器中的反应

利用一套自建的配有团簇产生和化学反应源的飞行时间质谱装置，研究了钒氧化物团簇正离子(VmOn+)与乙烯(C2H4)在气相条件下的反应．激光溅射产生的钒等离子体与氧气分子在超声喷射下反应，产生m=1?10和n=1?26 (具体数字依赖于m) 的VmOn+，然后与流动管反应器中的乙烯反应． 该实验条件下， 氧饱和团簇(2n?5m)往往含有氢原子(VmOnH2x+)．与乙烯反应时，相对于VmOnH2x+，VmOn+表现出比较高的反应活性，而且，小团簇比大团簇具有较高的反应活性．观测到的反应产物大都为碰撞结合体V     

Diffusion of tungsten clusters on tungsten (110) surface

Using molecular dynamics simulation and modified analytic embedded-atom method, we have investigated the self-diffusion of clusters on a tungsten (110) surface. As compared to the linear-chain configuration, the close-packed islands for tungsten clusters containing more than nine adatoms have been predicted to be more stable with the relatively lower binding energies. The migration energies show an interesting and oscillating behavior with increasing cluster size. The tetramer, hexamer and octamer have obviously higher migration energies than the others. The different atomic configurations and diffusion mechanisms have been determined during the diffusion processes. It is clear that the dimer-shearing mechanism occurs inside the hexamer, while it occurs at the periphery of heptamer. The successive hopping mechanism of individual atom is of critical importance in the migration of the clusters containing five or fewer adatoms. In addition, the diffusion of a cluster with nine adatoms is achieved through the changes of the cluster shape.     

Doping of magic nanoclusters in the submonolayer In/Si(100) system

Si(100)4 x 3-In reconstruction is essentially a superlattice of magic (identical-size) Si7In6 nanoclusters. Using scanning tunneling microscopy (STM) observations, we have found that under appropriate growth conditions up to 35% of these clusters can be modified; namely, two Si atoms in the cluster can be replaced by two In atoms, thus forming a Si5In8 cluster. This modification can be considered as a doping of the magic cluster, as it changes the electronic properties of the cluster from semiconducting towards metallic. The doped cluster is less rigid than the ordinary one and swings in the electrical field of the STM tip. The atomic structure and stability of the doped magic cluster have been examined using first-principles total-energy calculations.     

Pinning of size-selected Co clusters on highly ordered pyrolytic graphite

Deposition and implantation of size-selected Co⁺ _50±5 cluster ions on/in highly ordered pyrolytic graphite (HOPG) have been performed. Cobalt clusters were produced by laser ablation using the second harmonic (532 nm) of a Nd:YAG laser. They were deposited/implanted with energies of 250–4850 eV/cluster and examined using scanning tunneling microscopy (STM). For the highest energies the clusters created craters and wells with residual clusters at the bottom of the wells. Decrease of the impact energy led to formation of bumps whichconsist of damaged graphite areas mixed with fragmented cobalt clusters. Further decrease of the impact energy to 250–450 eV/cluster probably corresponds to the so-called pinning regime, when the impacting cluster creates defects in the surface layer and becomes bound to them. The transition from implantation to pinning with a decrease of impact energy was confirmed by etching experiments showing the depth of the damage introduced by the cluster collisions with HOPG.