Scanning tunneling microscopy investigation of leucine and asparagine adsorbed on Cu(1 1 1)

Adsorption of leucine and asparagine on Cu(1 1 1) surface has been studied by means of scanning tunneling microscopy (STM) in ultra-high vacuum (UHV) conditions. It has been found that leucine can form (3 × 2) superstructure on Cu(1 1 1) and copper steps facet to 〈1 1 0〉 directions. On the basis of our STM images, model has been proposed for the Cu(1 1 1)(3 × 2)-leucine superstructure. The model explains quite naturally the motivation behind the step faceting process. On the other hand, asparagine cannot form any ordered superstructure on Cu(1 1 1) and no copper step faceting can be observed either. The role of amino group in the side chain played in asparagine adsorption geometry is discussed in this paper.     

Structure of the SiC (0 0 0 1) 3 × 3 reconstruction studied by surface X-ray diffraction

The surface structure of the 3 × 3 reconstruction of the 4H-SiC (0 0 0 1) surface was investigated with surface X-ray diffraction (SXRD).Of the studied models, the twist model proposed by Starke et al. [U. Starke, J. Schardt, J. Bernhardt, M. Franke, K. Reuter, H. Wedler, K. Heinz, J. Furthmuller, P. Kackell, F. Bechstedt, Phys. Rev. Lett. 80 (1998) 758] gave the best fit to the experimental data. The structural parameters were determined accurately.     

Surface structure and phase transition of Ge(1 1 1)-3 × 3-Pb studied by reflection high-energy positron diffraction

We studied the structures and the phase transition of Pb/Ge(1 1 1) surface by using the reflection high-energy positron diffraction. The surface structures at 60 K and 293 K have the 3 × 3 and √3 × √3 periodicities, respectively. The rocking curves measured at both temperatures are nearly the same. This indicates that the equilibrium positions of the surface atoms do not change according to the phase transition. From the analysis of the rocking curve based on the dynamical diffraction theory, we found that at both temperatures the surface structures are composed of the so-called one-up and two-down model. The 3 × 3-√3 × √3 phase transition for the Pb/Ge(1 1 1) surface is interpreted in terms of order-disorder transition.     

Atomic structure of Cs layer grown on Si(0 0 1)(2 × 1) surface at room temperature

The atomic structure of Cs atoms adsorbed on the Si(0 0 1)(2 × 1) surface has been investigated by coaxial impact collision ion scattering spectroscopy. When 0.5 ML of Cs atoms are adsorbed on Si(0 0 1) at room temperature, it is found that Cs atoms occupy a single absorption site on T3 with a height of 3.18 ± 0.05 Å from the second layer of Si(0 0 1)(2 × 1) surface, and the bond length between Cs and the nearest Si atoms is 3.71 ± 0.05 Å.     

Ruthenium adsorption and diffusion on the GaN(0 0 0 1) surface

We report first principles calculations to analyze the ruthenium adsorption and diffusion on GaN(0 0 0 1) surface in a 2×2geometry. The calculations were performed using the generalized gradient approximation (GGA) with ultrasoft pseudopotential within the density functional theory (DFT). The surface is modeled using the repeated slabs approach. To study the most favorable ruthenium adsorption model we considered T₁, T₄ and H₃ special sites. We find that the most energetically favorable structure corresponds to the Ru- T₄ model or the ruthenium adatom located at the T₄ site, while the ruthenium adsorption on top of a gallium atom (T₁ position) is totally unfavorable. The ruthenium diffusion on surface shows an energy barrier of 0.612 eV. The resultant reconstruction of the ruthenium adsorption on GaN(0 0 0 1)- 2×2 surface presents a lateral relaxation of some hundredth of Å in the most stable site. The comparison of the density of states and band structure of the GaN(0 0 0 1) surface without ruthenium adatom and with ruthenium adatom is analyzed in detail.     

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.     

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.     

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.     

STM studies of the reconstructed Au(1 1 1) thin-film at elevated temperatures

High temperature scanning tunneling microscope (HT-STM) was used to investigate a reconstructed Au(1 1 1) film evaporated on mica. The experiment was carried out at elevated temperatures in the range of 300-500 K. A herringbone reconstruction was observed at a wide range of temperatures. However, at the highest temperatures studied a break down of the reconstruction long range order was noticed. Finally, the presence of a triangular-like reconstruction was reported. Changes in the reconstruction were explained in terms of the change in surface stress arising as a result of the tension at the gold-mica interface.     

The gold and oxygen (3 × 1) structures on W(1 1 2)

The adsorption of two very different adsorbates, gold and oxygen, induce the formation of a (3 × 1) surface structure on both W(1 1 2) and Mo(1 1 2). In spite of similar adsorbate unit cells, the surface electronic structure, derived from photoemission, exhibits pronounced differences for the two adsorbates. Indeed, both experiment and simulations indicate substantial changes in electronic structures of (1 × 1) and (3 × 1) gold overlayers supported by highly anisotropic (1 1 2) plane. We speculate that (3 × 1) is a favored periodicity in the atomic rearrangement of the (1 1 2) surfaces of molybdenum and tungsten due in part as a result of the initial state band structure of these surfaces.     

Structure and electronic properties of gold adsorbed on Ti(0 0 0 1)

The Au/Ti(0 0 0 1) adsorption system was studied by low energy electron diffraction (LEED) and photoemission spectroscopy with synchrotron radiation after step-wise Au evaporation onto the Ti(0 0 0 1) surface. For adsorption of Au at 300 K, no additional superstructures were observed and the (1 × 1) pattern of the clean surface simply became diffuse. Annealing of gold layers more than 1 ML thick resulted in the formation of an ordered Au-Ti surface alloy. Depending on the temperature and annealing time, three surface reconstructions were observed by LEED: (√3 × √3) R30°, (2 × 2) and a one-dimensional incommensurate (√3 × √3) rectangular pattern. The Au 4f core level and valence band photoemission spectra provided evidence of a strong chemical interaction between gold and titanium. The data indicated formation of an intermetallic interface and associated valence orbital hybridization, together with diffusion of gold into the bulk. Au core-level shifts were found to be dependent on the surface alloy stoichiometry.     

Stable reconstruction and adsorbates of InAs(1 1 1)A surface

We investigated the surface properties of InAs(1 1 1)A by low-temperature scanning tunneling microscopy (LT-STM) with atomic resolution and first-principles calculation. Very clear atom image was observed, showing that the surface reconstruction is an In-vacancy structure. We also observed two kinds of adsorbates on the surface. The first-principles calculations indicate that the In-vacancy structure is the most stable surface reconstruction under any experimental conditions, which is consistent with the LT-STM observation. Investigations of adsorption properties of an In atom, an As atom, and an As₂ molecule by the first-principles calculations imply that the observed adsorbates are an In atom and an As₂ molecule.     

New reconstruction-stoichiometry correlation for GaAs(0 0 1) surface treated by atomic hydrogen

The structure, stoichiometry and electronic properties of the GaAs(0 0 1)-(2 × 4)/c(2 × 8) surface treated by cycles of atomic hydrogen (AH) exposure and subsequent annealing in UHV were studied with the aim of preparing the Ga-rich surface at low temperatures. Low energy electron diffraction showed reproducible structural transformations in each cycle: AH adsorption at the (2 × 4)/c(2 × 8) surface led to the (1 × 4) structure at low AH exposure and to a (1 × 1) surface at higher AH exposure with subsequent restoration of the (2 × 4)/c(2 × 8) structure under annealing at 450 °C. The cycles of AH treatment preserved the atomic flatness of the GaAs(1 0 0) surface, keeping the mean roughness on to about 0.15 nm. The AH treatment cycles led to the oscillatory behavior of 3dAs/3dGa ratio with a gradual decrease to the value characteristic for the Ga-rich surface. Similar oscillatory variations were observed in the work function. The results are consistent with the loss of As from the surface as a result of the desorption of volatile compounds which are formed after reaction with H. The prepared Ga-rich GaAs(0 0 1) surface showed the stability of the (2 × 4)/c(2 × 8) structure up to the annealing temperature of 580 °C.     

Charge transfer in the atomic structure of Ge (1 0 5)

The atomic structure and charge transfer on the Ge (1 0 5) surface formed on Si substrates are studied using scanning tunneling microscopy and spectroscopy (STM and STS). The bias-dependent STM images of the whole Ge (1 0 5) facets formed on a Ge “hut” structure on Si (0 0 1) are observed, which are well explained by the recently confirmed structure model. The local surface density of states on the Ge (1 0 5) surface is measured by STS. The localization of the electronic states expected from charge transfer mechanism is observed in the dI/dV spectra. The surface band gap is estimated as 0.8-0.9 eV, which is even wider than the bulk bandgap of Ge, indicating the strong charge transfer effect to make the dangling bonds stable. The shape of normalized tunnel conductance agrees with the theoretical band structure published recently by Hashimoto et al.     

Synergetic effect between ion energy and sample temperature in the formation of distinct dot pattern on Si(1 1 0) by ion-sputter erosion

We observed a synergetic effect between ion energy and sample temperature in the formation of distinct dot pattern on Si(1 1 0) by Ar⁺ ion sputtering. The ion flux was 20 μA/cm², a value smaller than those used in preceding reports by one or two orders of magnitude. In experiments, the ion energy was from 1 to 5 keV, and the temperature from room temperature to 800 °C. A phase diagram indicating the ranges of ion energy and temperature within which distinct dot patterns can be achieved has been obtained. Data analyses and simulation results reveal that the synergetic effect is consistent with the effect of the Ehrlich-Schwoebel step-edge barrier, rather than the Bradley-Harper model.     

An electrostatic force microscope study of Si nanostructures on Si(1 0 0) as a function of post-annealing temperature and time

The evolution of Si nanostructures induced by Ar⁺ ion sputtering on Si(1 0 0) was studied with electrostatic force microscopy (EFM) as a function of post-annealing temperature (T = room temperature-800 °C) and time (t = 0-160 min). The post-annealing of the nanostructure was conducted in vacuum. It was found that with T increasing, the EFM contrast degraded steadily and became nearly undetectable at T = 800 °C; with t increasing at T = 800 °C, the EFM contrast fell down steadily as well. However, the surface morphology and roughness were much less affected after annealing. The results suggest that the as-formed Si nanostructures may not be epitaxially grown on Si(1 0 0) substrate as claimed before. A plane capacitance model supported this conclusion.     

Interaction of atomic hydrogen with the Si(100)2×1 surface

The interaction of atomic hydrogen with the Si(100)2×1 surface has been investigated in detail by a field ion-scanning tunneling microscope (FI-STM). At low exposure, hydrogen atoms reside singly on top of the dimerised Si atoms, and are imaged brightly. The hydrogen chemisorption induces the buckling of dimers, indicating the strong bonding between Si and hydrogen atoms. The adsorption geometry changed from the (2×1) monohydride phase to the (1×1) dihydride phase with increasing exposure of hydrogen. The former is imaged dark compared with the unreacted Si dimers due to the reduction of the density of electronic states near the Fermi level. Surface etching was also observed during the formation of the dihydride phase. The behavior of hydrogen desorption from the H-saturated Si(100) surface was investigated as a function of annealing temperatures. Our STM results suggest that the desorbing H₂ molecules are formed by two hydrogen atoms on the same dihydride species.     

Optimization of VI/II pressure ratio in ZnTe growth on GaAs(0 0 1) by molecular beam epitaxy

ZnTe epilayers were grown on GaAs(0 0 1) substrates by molecular beam epitaxy (MBE) at different VI/II beam equivalent pressure (BEP) ratios (R_VI/II) in a wide range of 0.96-11 with constant Zn flux. Based on in situ reflection high-energy electron diffraction (RHEED) observation, two-dimensional (2D) growth mode can be formed by increasing the R_VI/II to 2.8. The Te/Zn pressure ratios lower than 4.0 correspond to Zn-rich growth state, while the ratios over 6.4 correspond to Te-rich one. The Zn sticking coefficient at various VI/II ratios are derived by the growth rate measurement. The ZnTe epilayer grown at a R_VI/II of 6.4 displays the narrowest full-width at half-maximum (FWHM) of double-crystal X-ray rocking curve (DCXRC) for (0 0 4) reflection. Atomic force microscopy (AFM) characterization shows that the grain size enlarges drastically with the R_VI/II. The surface root-mean-square (RMS) roughness decreases firstly, attains a minimum of 1.14 nm at a R_VI/II of 4.0 and then increases at higher ratios. It is suggested that the most suitable R_VI/II be controlled between 4.0 and 6.4 in order to grow high-quality ZnTe epitaxial thin films.     

Structure and composition of chemically prepared and vacuum annealed InSb(0 0 1) surfaces

The InSb(0 0 1) surfaces chemically treated in HCl-isopropanol solution and annealed in vacuum were studied by means of X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED) and electron energy-loss spectroscopy (EELS). The HCl-isopropanol treatment removes indium and antimony oxides and leaves on the surface about 3 ML of physisorbed overlayer, containing indium chlorides and small amounts of antimony, which can be thermally desorbed at 230 °C. The residual carbon contaminations were around 0.2-0.4 ML and consisted of the hydrocarbon molecules. These hydrocarbon contaminations were removed from the surface together with the indium chlorides and antimony overlayer. With increased annealing temperature, a sequence of reconstructions were identified by LEED: (1 × 1), (1 × 3), (4 × 3), and (4 × 1)/c(8 × 2), in the order of decreasing Sb/In ratio. The structural properties of chemically prepared InSb(0 0 1) surface were found to be similar to those obtained by decapping of Sb-capped epitaxial layers.     

The adsorption of In on the surface of (0 0 1) CdTe

To understand CdTe doping with In, first-principle calculations are performed to obtain the various kinds of surface-structure for In on CdTe (0 0 1) surface. Of all the structures examined, the structure of CdTe (0 0 1) as caused by In adsorption atoms at the fourfold hollow sites with 0.25 monolayer coverage is the most energetically favorable. In atoms are adsorbed on the Cd-terminated surface, whereas below the Te-terminated surface. For the Cd-terminated surface, cadmium vacancy can form spontaneously and is energetically favorable. In atoms are likely to be adsorbed/incorporated at an interstitial site on Te-terminated CdTe (0 0 1) surfaces for most of the range of the chemical potential.