Regular step formation on concave-shaped surfaces on 6H–SiC(0 0 0 1)

A concave-shaped surface has been prepared in a 6H–SiC(0 0 0 1) substrate by mechanical grinding. As a consequence, the different crystallographic planes building up the 6H–SiC polytype are cut under continuously changing polar angles in all azimuthal directions. Through hydrogen etching, this curved surface breaks up into a whole set of surfaces vicinal to the initial 6H(0 0 0 1) orientation. The local structural reorganisation after hydrogen etching has been studied by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Two types of local bond environments are present at the step edges leading to a strong anisotropy in the surface etching with hydrogen. As a result, the distribution of the terrace width and the step heights varies with the azimuthal angle and reflects the sixfold symmetry of the bulk crystal. For most azimuthal directions, an alternation of large and small terraces, separated by steps of 0.75 nm heights (height of half the 6H polytype, three bilayers) is observed and only for well defined azimuthal directions, equally spaced terraces separated by steps of 1.5 nm height (one unit cell of 6H–SiC, six bilayers) are found. In addition, the polar variations have been studied by taking various line-scans along the concave-shaped surface with AFM. It seems that for polar angles above 3°, step bunching of several SiC steps occurs whereas below 3° the bimodal terrace width distribution is observed.     

Thin film quantum size effects: I. The effect of defect structure at the vacuum/film interface

The quantum size oscillations observed in the transmitted current for very low energy electrons normally incident on thin epitaxial (111) films of Cu and Ag on W(110) are sensitive to the structure of both the vacuum/film and the film/substrate interface. We report here the contribution of the vacuum/film interface as a function of oxygen adsorption, ion bombardment damage and annealing temperature. We find that the quantum size oscillations are always reduced in amplitude with increasing disorder on an atomic scale at the film surface. Random oxygen adsorption generally suppresses the quantum size effect (QSE) structure. The QSE amplitude is also significantly reduced by defect structure in the form of a random, irregular array of monatomic steps at the film surface. Using the LEED (00) beam width as an index of surface roughness, we find that the amplitude of the quantum size oscillations decreases linearly with surface step atom density. The QSE amplitude is reduced by a factor of two for step atom densities as low as 6.5%, and nearly extinguished for a step atom density of 12%. The QSE structure provides a relative indication of surface roughness at least as sensitive as LEED beam broadening and work function change measurements. We conclude that a relatively well-ordered and uniform film surface is a necessary (but not sufficient) condition for the observation of quantum size structure.     

Confining barriers for surface state electrons tailored by monatomic Fe rows on vicinal Au111 surfaces

We fabricated monatomic Fe wires on vicinal Au(111) surfaces and found that decoration of step edges with Fe adatoms has a significant influence on the behavior of surface state electrons confined between regularly arranged steps. On a surface with Fe monatomic rows, angle-resolved photoemission spectra measured in the direction perpendicular to the steps shows parabolic dispersion, in contrast to one-dimensional quantum-well levels observed on a clean surface. Simple analysis using a one-dimensional Kronig-Penney model reveals potential barrier reduction from 20 to 4.6 eV A, suggesting an attractive nature of the Fe adatoms as scatterers.     

单畴的单原子In纳米线阵列的制备与研究

利用Si（001）向［110］方向偏4°角的斜切表面作为衬底，成功地制备了分布均匀的单畴的单原子In链阵列.扫描隧道显微镜分析表明，沉积的In原子优先吸附在台面上沿着台阶内边缘的位置，并在两个Si的二聚体链之间形成稳定的In二聚体.In二聚体组成直的单原子链，其生长机理与Car提出的“表面聚合反应”相一致.另外，衬底具有非常窄的台面和双原子层台阶边的特殊结构是形成单畴的单原子链的关键. 关键词： 铟单原子链 硅邻近面 扫描隧道显微镜     

The influence of sulfur adsorption on the step structure of vicinal Mo(100): a LEED and STM study

The effect of sulfur adsorption on the step structure of vicinal Mo(100) surfaces has been studied with scanning tunneling microscopy (STM) and low energy electron diffraction (LEED). STM was used for low densities of steps on a nominally flat (100) surface. LEED was used to study the more highly and regularly stepped (910), (911), and (28,4,1) oriented surfaces. Steps and the sulfur adsorbate were found to have a strong interaction. The presence and orientation of steps on the surface governed the formation of ordered domains of sulfur, and sulfur modified the structure of the steps on the surface. Both techniques show that monatomic steps predominate on the clean surfaces. When sulfur was adsorbed on the surfaces, however, steps coalesced to form groups of steps, double atomic height steps, and multiple height steps, depending on sulfur coverage and initial step density. The results are discussed in the framework of the theory of equilibrium crystal shape and provide information on how the Mo step-step interactions are affected by sulfur adsorption.     

Growth of copper phthalocyanine on hydrogen passivated vicinal silicon(1 1 1) surfaces

Using ultra-high vacuum scanning tunneling microscopy (UHV-STM), we show that copper-phthalocyanine (CuPc) grows in a well ordered manner on hydrogen passivated vicinal silicon surfaces. CuPc grows one-dimensionally parallel to the monatomic steps on the vicinal silicon surface. Surprisingly, elongated clusters of the CuPc parallel to the step directions are formed even on the middle of the terraces well away from the step edges. The one-dimensional growth mode continues even after the full monolayer coverage on the substrate which results in strongly oriented growth mode of a thin film of CuPc on the vicinal silicon surfaces.     

Carbide induced reconstruction of monatomic steps on Ni(1 1 1) - A density functional study

We present density functional calculations for carbon adsorption at the two types of monatomic steps on a Ni(1 1 1) surface. We show that it is thermodynamically favourable to make a carbon induced clock-type reconstruction at the close-packed step with a [1 1 1] step geometry, which creates fourfold sites at the step-edge. It is furthermore possible to extend the carbide with the clock reconstructed geometry onto the upper terrace with a net energy gain compared to adsorption of carbon on unreconstructed close-packed steps or terrace sites on Ni(1 1 1). Our findings explain the fact that carbide islands start to grow preferentially on the close-packed steps as has been observed using scanning tunneling microscopy.     

Asymmetric peak broadening in photoemission of surface states as a consequence of lateral electron confinement

In addition to the inverse lifetime, the photoemission peak width of a surface state depends on the quality of the surface. For a stepped surface, in particular, there is an energy shift of the surface state owing to the confinement of electrons. A random distribution of steps causes an asymmetric peak broadening, which is examined as a function of the shift parameter, the inverse lifetime and the mean terrace width. The model is discussed for the Cu(111) sp-surface state for which an influence of steps on the peak shape is obtained particularly for mean terrace widths less than 100 Å.     

Instability of the distribution of atomic steps on Si(111) upon submonolayer gold adsorption at high temperatures

The gold adsorption effect on the distribution of monatomic steps on the (111) silicon surface is studied in situ by ultrahigh vacuum reflection electron microscopy at temperatures of 850–1260°C. A new effect of the instability of silicon surface morphology has been detected. This effect leads to the redistribution of regular steps (RSs) to step bunches (STs) and vice versa on a surface covered with a gold submonolayer. For the crystal heated by directly passing an electric current, the behavior of the RS ? SB morphological transitions on the silicon surface is investigated as a function of the gold coverage and the direction of the heating current. Thus, isothermal annealing at 900°C is accompanied by the following transitions on the silicon surface with predeposited 0.75 monolayer gold coverage: RS (0.72) ? SB (0.42) ? RS (0.24) ? SB (0.07) RS ? (0). The numbers given in parentheses are estimated values of the critical gold coverage measured in the monolayers at which the morphological transitions are observed. A change in the direction of the electric current used to heat the crystal leads to the reversible changes RS ? SB and SB ? RS at the same values of the critical gold coverage.     

Extended one-dimensional supramolecular assembly on a stepped surface

2,6-naphthalene-dicarboxylic acid was adsorbed on a Ag110 surface with an average terrace width of only some tens of a nm. Scanning tunneling microscopy shows that the adsorbates self-assemble into one-dimensional mesoscale length chains. These extend over several hundred nanometers and thus the structure exhibits an unprecedented tolerance to monatomic surface steps. Density functional theory and x-ray photoelectron spectroscopy explain the behavior by a strong intermolecular hydrogen bond plus a distinct template-mediated directionality and a high degree of molecular backbone flexibility.     

Structure and thermal stability of the Au(334) surface and Au(111) thin films in air: A scanning tunneling microscopy study

A Au(334) surface cleaned and annealed under ultra-high vacuum conditions was examined in air using a scanning tunneling microscope. The STM images reveal a smooth surface with a plateau structure. On top of the plateaus parallel rows of monatomic height steps can be observed. Annealing this surface in air to around 600 ° C results in segregation of impurities that form surface oxides. STM examination of this air annealed surface shows a structure where the plateaus have developed into large (111) facets. These facets are atomically smooth except for some steps near the edges. Au(111) films on mica, examined in air, were found to be atomically smooth over thousands of ångströms, with the surface having occasional monatomic height steps. Several transient effects encountered in STM imaging in air are discussed.     

Growth of single-domain monatomic In chain arrays on the vicinal Si(0 0 1) surface

Using the annealed vicinal Si(0 0 1) surface with 4° miscut toward the [1 1 0] direction as a substrate, single-domain monatomic In chain arrays have been fabricated. High-resolution STM images reveal that deposited In atoms preferentially form In dimers between two neighboring Si dimer rows along the step edges on the lower terrace. Formation of In dimers removes the surface dangling bonds and saturates the In valency. With increasing coverage, the In dimers develop into straight monatomic In chains along the step running direction. It is found that the ordered narrow terrace and rebonded double-layer (D_B) step edge are the keys for the formation of monatomic In chains.     

Field-desorption microscopy study of the deformation of a tungsten tip subjected to thermal treatment in an electric field

Deformation of a tungsten tip 500 to 1000 nm in radius subjected to heating in an electric field is studied using field-emission microscopy and continuous-mode field-desorption microscopy. Measurements are performed immediately after the thermal field treatment without any smoothing of the tip by either heating or field evaporation. The edges of the tip (which is shaped into a polyhedron) are found to consist of monatomic steps about 1 nm wide and about 100 nm long. Microscopic protrusions about 10 nm in size are shaped like pyramids or wedges with single-atom apexes or monatomic edges and facets that are a continuation of the facets of the reconstructed tip or an outgrowth on which the protrusions are situated. The outgrowths are shaped like stepped truncated pyramids with monatomic edges. The observed phenomena are explained in terms of competing processes of surface diffusion, crystal growth in an electric field, and field evaporation.     

Bistability in the shape transition of strained islands

The equilibrium shape of a monatomic strained island on a substrate depends on the step free energies and the difference in surface stress between the island and the substrate. For small island sizes the step free energies dominate, resulting in compact islands. Beyond a critical island size, however, the strain energy becomes dominant and the island maximizes its perimeter, resulting in elongated islands. Here we show that for strained islands with force monopoles pointing in opposing directions at neighboring steps, a regime exists near the critical island size where both compact and elongated shapes can coexist.     

Elastic interaction of surface steps: effect of atomic-scale roughness

Elastic interactions of atomic steps can greatly impact surface morphology. Recent atomistic calculations and experimental observations find the standard dipole model of steps is valid only for very large step separations. In this Letter, a new model is presented that displays remarkable agreement with atomistic predictions for step separations larger than just a few step heights. It is shown that the interaction energy of steps exhibits a novel intermediate-ranged behavior and that, for particular systems, step interactions switch from repulsive to attractive as separation distance decreases.     

Topology-induced spin frustrations at the Cr(001) surface studied by spin-polarized scanning tunneling spectroscopy

The magnetic structure of the Cr(001) surface was investigated by spin-polarized scanning tunneling spectroscopy by making use of the spin-polarized surface state located close to the Fermi level. Periodic alternations of the intensity of the surface state peak in local tunneling spectra measured above different ferromagnetic terraces separated by monatomic steps confirm the topological antiferromagnetic order of the Cr(001) surface. Screw dislocations cause topology-induced spin frustration, leading to the formation of domain walls with a width of about 120 nm.     

Time dependence of vacuum arc parameters

Time-resolved investigations of the expanded plasma of vacuum arc cathode spots are described, including the study of the ion charge state distribution, the random cathode spot motion, and the crater formation. It was found that the ion charge state distribution changes over a timescale on the order of hundreds of microseconds. For the random spot motion two timescales were observed: a very short spot residence time of tens of nanoseconds which gives, combined with the step width, the diffusion parameter of the random motion, and a longer timescale on the order of 100 μs during which the diffusion parameter changes. Crater formation studies by scanning electron microscopy indicate the occurrence of larger craters at the end of crater chains. The existence of a timescale much longer than the elementary times for crater formation and spot residence can be explained by local heat accumulation     

Low energy electron diffraction studies of high index crystal surfaces of platinum

Several high Miller Index crystal surfaces of platinum have been examined by low energy electron diffraction (LEED) and have been shown to consist of low index (111) or (100) terraces of constant width, linked by steps of monatomic height. The ordered stepped surfaces were found to be stable in ultra-high vacuum up to 1500 K, and the reasons for this remarkable stability are discussed. The presence of ordered atomic steps may be considered as a general structural property of high index surfaces regardless of the chemical bonding in the crystal. A nomenclature is suggested to identify the surface structures of stepped surfaces.     

Electromagnetic coupling at an atomic scale

We report the first spectrally resolved measurements of scanning tunnelling microscope induced fluorescence from monatomic steps. As a function of lateral distance from the step edge, on a sub nanometre scale, increasingly large red (blue) shifts of the emission are observed as the tip approaches the lower (upper) step edge.     

A statistical model for the scattering of atoms from randomly stepped surfaces

The scattering of an atomic beam from a randomly stepped surface has been calculated using the hard corrugated wall model. Under the basic assumption that scattering from the step edges may be neglected the scattering equation can be solved without any further approximation. The solution displays the usual diffraction peaks each of them is broaden by a term characterizing the step configuration. If the step distribution is ergodic and stationnary in space this term is the characteristic function of the difference of level between two point of the surface considered as a random variable. Statistical models for the step repartition at the surface are proposed and the scattering intensity is derived in a closed form. The main result is that the broadening of the peaks varies from no broadening to a maximum according as the interferences from waves reemitted by the various terraces are constructive or destructive. A comparison is made with previous experimental data from which an estimation of the average step separation can be drawn. The sensitivity of the atomic beam scattering to steps is found to be more than one step every one hundred crystal atoms.