Monoatomic Steps Research Articles

Acoustical Phonons Transport through a Stepped Quantum Wire

This work presents a theoretical approach for the study of phonon dynamics and scattering properties of an infinite linear atomic chain perturbed by a mono atomic step. The coherent transmittance scattering cross-sections for incident phonons on the atomic waveguide structure are calculated using the Landauer-Buttiker electron scattering description and the matching method formalism with the nearest and next nearest neighbour interactions. Numerical results for different configurations yield an understanding of the chain dynamical properties and the effects on phonon transmittance due to incoming phonons. The reflectance and transmittance coefficients show spectral characteristic features depending on the cut-off frequencies for the propagating phonons. They illustrate the occurrence of Fano resonances in the scattering spectra that result from degeneracy of step localized modes and propagating continuum modes due to the breakdown of the translation symmetry in the propagating direction. Furthermore, the interferences between diffused and reflected waves in the step regions generate Fabry-Perot oscillations whose number is determined by the distance between steps and the number of terraces.

New insight on the behavior of the irreversible adsorption and underpotential deposition of thallium on platinum (111) and vicinal surfaces in acid electrolytes

We report, for the first time, the electrochemical behavior of thallium irreversibly adsorbed on Pt (111) and platinum stepped surfaces composed of (111) terraces and monoatomic steps. Similar to the case of thallium UPD, the voltammograms obtained after thallium irreversible adsorption present three characteristic features. After a careful analysis of the effect of the thallium concentration, the concentration and nature of the anion of the supporting electrolyte and the pH of the solution on these voltammetric features, we have been able to ascribe these processes to Tl/Tl+ oxidation and anion adsorption on the Tl-modified surface. In addition, the results obtained with stepped surfaces, indicate that some of the features are clearly associated to the presence of (111) surface domains, and thus they could be used for the quantification of these sites.

Anisotropic dispersion and partial localization of acoustic surface plasmons on an atomically stepped surface: Au(788).

Understanding acoustic surface plasmons (ASPs) in the presence of nanosized gratings is necessary for the development of future devices that couple light with ASPs. We show here by experiment and theory that two ASPs exist on Au(788), a vicinal surface with an ordered array of monoatomic steps. The ASPs propagate across the steps as long as their wavelength exceeds the terrace width, thereafter becoming localized. Our investigation identifies, for the first time, ASPs coupled with intersubband transitions involving multiple surface-state subbands.

Self-propelled motion of Au–Si droplets on Si(111) mediated by monoatomic step dissolution

By Low Energy Electron Microscopy, we show that the spontaneous motion of gold droplets on silicon (111) is chemically driven: the droplets tend to dissolve silicon monoatomic steps to reach the temperature-dependent Au–Si equilibrium stoichiometry. According to the droplet size, the motion details are different. In the first stages of Au deposition small droplets nucleate at steps and move continuously on single terraces. The droplets temporarily pin at each step they meet during their motion. During pinning, the growing droplets become supersaturated in Au. They depin from the steps when a notch nucleate on the upper step. Then the droplets climb up and locally dissolve the Si steps, leaving behind them deep tracks formed by notched steps. Measurements of the dissolution rate and the displacement lengths enable us to describe quantitatively the motion mechanism, also in terms of anisotropy of Si dissolution kinetics. Scaling laws for the droplet position as a function of time are proposed: x ∝ tn with 1/3<n<2/3.

Surface morphological studies of Nafion®/Pt(100) interface

The structure and molecular processes occurring at the Nafion® membrane/electrode interface remain poorly understood. The present study aims at elucidating the nano-scale properties of the Pt(100)/Nafion® interface using scanning probe techniques, namely STM and AFM. In STM, the surface morphology of the thin Nafion® film appears smooth on the atomic scale; monoatomic steps of the substrate are well visible. The thickness of the Nafion® membrane is determined using AFM tip penetration experiments and also by scratching of the Nafion® layer from the electrode surface. Force deflection (“approach”) curves demonstrate that the tip penetrates the Nafion® layer at a normal force greater than 5 nN.

Friction force microscopy at a regularly stepped Au(665) electrode: Anisotropy effects

Abstract Using friction force microscopy, friction was determined for the AFM-tip scanning parallel and vertically to the monoatomic steps of Au(665) electrode for different coverages of Cu in sulfuric acid. When the tip was scanning parallel to the steps, the results were similar to those obtained before for a Au(111) surface: a higher coverage of Cu leads to an increased friction. However, differently from Au(111), no transitions in the friction coefficient were observed with increasing load. Atomic stick slip was observed both for the Au surface and the 3 × 3 honeycomb Cu adlayer with a Cu coverage of 2/3. When the tip was scanning perpendicular to the steps, friction did not depend much on coverage; astonishingly, atomic stick slip was also observed.

Desorption of water from distinct step types on a curved silver crystal.

We have investigated the adsorption of H2O onto the A and B type steps on an Ag single crystal by temperature programmed desorption. For this study, we have used a curved crystal exposing a continuous range of surface structures ranging from [5(111) × (100)] via (111) to [5(111) × (110)]. LEED and STM studies verify that the curvature of our sample results predominantly from monoatomic steps. The sample thus provides a continuous array of step densities for both step types. Desorption probed by spatially-resolved TPD of multilayers of H2O shows no dependence on the exact substrate structure and thus confirms the absence of thermal gradients during temperature ramps. In the submonolayer regime, we observe a small and linear dependence of the desorption temperature on the A and B step density. We argue that such small differences are only observable by means of a single curved crystal, which thus establishes new experimental benchmarks for theoretical calculation of chemically accurate binding energies. We propose an origin of the observed behavior based on a “two state” desorption model.

Remarkable effects of disorder on superconductivity of single atomic layers of lead on silicon

In bulk materials superconductivity is remarkably robust with respect to non-magnetic disorder. In the two-dimensional limit however, the quantum condensate suffers from the effects produced by disorder and electron correlations which both tend to destroy superconductivity . The recent discovery of superconductivity in single atomic layers of Pb, the striped incommensurate (SIC) and $\sqrt{7}\times\sqrt{3}$ Pb/Si(111), opened an unique opportunity to probe the influence of well-identified structural disorder on two-dimensional superconductivity at the atomic and mesoscopic scale. In these two ultimate condensates we reveal how the superconducting spectra loose their conventional character, by mapping the local tunneling density of states. We report variations of the spectral properties even at scales significantly shorter than the coherence length. Furthermore, fine structural differences between the two monolayers, such as their atomic density, lead to very different superconducting behaviour. The denser SIC remains globally robust to disorder, as are thicker Pb films, whereas in the slighly more diluted $\sqrt{7}\times\sqrt{3}$ system superconductivity is strongly fragilized. A consequence of this weakness is revealed at monoatomic steps of $\sqrt{7}\times\sqrt{3}$, which disrupt superconductivity at the atomic scale. This effect witnesses that each individual step edge is a Josephson barrier. At a mesoscopic scale the weakly linked superconducting atomic terraces of $\sqrt{7}\times\sqrt{3}$ form a native network of Josephson junctions. We anticipate the Pb/Si(111) system to offer the unique opportunity to tune the superconducting coupling between adjacent terraces, paving a new way of designing atomic scale quantum devices compatible with silicon technology.

Flattening Technique of (551) Silicon Surface Using Xe/H2 Plasma

The interface microroughness between the insulator film and the silicon substrate is one of the critical issues to the gate insulator film reliability and the performance of MOSFETs [1,2]. The atomically flat surface of Si(100) has been obtained by annealing wafers at the temperature of 800 ºC or above [3-7]. The MOSFETs having atomically flat interface between the silicon substrate and the radical gate SiO2 film show higher performances than the MOSFETs having conventional interfaces [3,4,7]. Furthermore, the speed performance of LSI is enhanced by introducing the MOS transistor fabricated on (551) surface silicon substrate [8]. In order to utilize (551) surface silicon substrate, flattening process of (551) silicon surface is required. In this paper, flattening technique of (551) silicon surface are developed by using Xe/H2Plasma.Boron–doped Cz Si(551) wafers (8-12 Ω·cm) were used for the flattening experiment. Before flattening these wafers, the native oxides were removed by 0.5% dilute HF (DHF) and the surface microroughness (Ra) were about 0.12 nm.(I) We applied the flattening technique by the thermal annealing in the ultraclean Ar ambience at the atmospheric pressure. Figure 1 shows 3×3 μm2AFM images of silicon surface after 850 °C annealing for 3 hours. Ra on each terrace of Si(100) surface is 0.03~0.04 nm, which is AFM noise level, and step height has only mono atomic step of 0.135 nm in Fig.1(a). On the other hand, Ra of Si(551) surface is 0.640 nm and step height has a few atomic step in Fig.1(b). Therefore Si(551) surface is drastically roughened by the ultraclean Ar annealing which can flatten Si(100) surface. Figure 2 shows the annealing temperature dependence of Ra of Si(551) surface. With decreasing the annealing temperature from 850 °C to 600 °C, Ra of Si(551) surface is decreased. It is considered that Si atoms on Si(551) surface easily migrate at lower temperature than Si(100). However, atomically flat silicon surface cannot be observed on Si(551) yet.(II) We performed Xe/H2 plasma process at low temperature less than 600 °C in order to flatten Si(551) surface. Figure 3 shows 3×3 μm2 AFM images before and after flattening Si(551) surface by using Xe/H2 plasma. Plasma process was carried out in a microwave-excited high-density Xe (90%)/H2 (10%) mixture plasma at a pressure of 266 Pa at 400 °C for 3 min. Ra of Si(551) surface was decreased by Xe/H2 plasma flattening. Here, Ra of Si(551) surface did not change by annealing in 100% H2 ambient at a pressure of 266 Pa at 400 °C for 10 min without plasma excitation. Therefore it is considered that the surface migration of Si atoms on Si(551) surface increasingly progresses because Si(551) surface became activated by Xe/H2plasma.This work was supported by JSPS KAKENHI Grant Number 22000010. Reference [1] T. Ohmi, et al., Electron. Device Lett., 12, P. 652,1991.[2] T. Ohmi, et al., T-ED 39, P. 537, 1992.[3] R. Kuroda, et al., T-ED, 56, P. 291, 2009.[4] R. Kuroda, et al., Jpn. J. Appl. Phys., 48, P. 04C048, 2009.[5] X. Li, T et al., ECS Trans. 28 (2010) 299.[6] X. Li, et al., Microelec. Eng., 88, P. 3133, 2011.[7] X. Li, et al., Jpn. J. Appl. Phys., 50, P. 10PB05, 2011.[8] T. Ohmi, et al., T-ED, 54. P. 1471, 2007.

Density functional studies of surface potentials for hydrogen and oxygen atoms on diamond (111) surfaces

The adsorption of hydrogen and oxygen atoms on diamond (111)-(1 × 1) surfaces is investigated by a molecular orbital method based on the density functional theory. The potential energy for hydrogen and oxygen adatoms on a flat surface and a surface with monoatomic steps are studied. The oxygen adatom is found to have a much lower energy barrier for migration than the hydrogen adatom. On the basis of the variations of potential energy, surface diffusion coefficients of adatoms are calculated. The potential energy for the oxygen adatom is much lowered near the step edge in comparison with those on flat regions. It is suggested that the oxygen atoms adsorbed on the diamond (111) surface are preferentially trapped near atomic steps after migration.

Adsorption of hydrazine on the perfect and defective copper (111) surface: A dispersion-corrected DFT study

We have investigated the adsorption of hydrazine (N2H4) on perfect and defect-containing copper (111) surfaces by first-principles calculations. The long-range interactions are included in the geometry optimization through the application of the generalised gradient approximation with dispersion correction, DFT-D2 in the method of Grimme. We have studied three types of defects at the surfaces: monoatomic steps, Cu-adatoms and vacancies, where our calculations show that the adsorption energy increases as the coordination of the adsorption sites decreases. The ideal (111) is the most stable surface with the weakest adsorption of hydrazine, whilst the stepped (111) surface is the least stable and hence more reactive surface with the highest adsorption energy, where the hydrazine bridges across the step edge. We found that inclusion of the dispersion correction results in significant enhancement of molecule–substrate binding, thereby increasing the adsorption energy. This strong adsorption results in a bridging adsorption geometry for hydrazine, with a rotation around the NN bond where the torsional angle changes from a gauche towards an eclipsed conformer to help the molecule to bridge through both nitrogen atoms, in agreement with experimental evidence. The core-level binding shifts for the N(1s) states upon N2H4 adsorption have been calculated at DFT level to provide further insight into the N2H4 adsorption process on the copper surfaces.

Generation of surface steps on Pt(9 7 7) induced by the catalytic oxidation of CO

The catalytic oxidation of CO has been investigated on the stepped Pt(977) surface at near ambient pressure and elevated temperature. It has been found that the reactivity is maximum at approximately the stoichiometric proportion of reactants and that surface monoatomic steps are generated by the reaction itself, being most abundant when the reaction proceeds with maximum efficiency.

Surface stresses on symmetric (2 × 1) reconstructed Si(001) calculated from surface energy variations

On Si(001) vicinals, the orientation of the dimers on the (2×1) reconstructed terrace is mutually perpendicular on either side of a monoatomic step. Understanding the surface stresses on (2×1) reconstructed Si(001) is crucial for the discussion of forces representing steps and possibly stability issues at vicinal surfaces. The surface stresses are calculated from the surface energy variations in function of the deformation parallel to the surface. For the deformation parallel to the dimer rows (x), the first monolayer stress is high as for unreconstructed surfaces. For the other direction (y) parallel to the dimer axis, the first monolayer is weakly stressed while the second monolayer stress is high due to the bond between dimer atoms. In the direction diagonal between x and y, the first monolayer stress is decreased at the expense of the other surface monolayers. The surface stresses are calculated by using the MEAM potential. This potential considers the second nearest neighbouring (2NN) bond between dimer atoms while the 2NN bonds are screened in bulk. This paper validates clearly the Shuttleworth formula.

Adsorption of PTCDA on NaCl(100) and KCl(100)

The adsorption of PTCDA on KCl and NaCl(100) surfaces has been investigated by means of first principles calculations. Besides a variety of adsorbate structures for single molecules and a monolayer of flat lying molecules on plain terraces, the influence of monoatomic steps and the different defect site at step edges has been studied in detail.

Atomic structure of (110) anti-phase boundaries in GaP on Si(001)

High quality III/V-layers grown on Si enable a variety of optoelectronic devices. The performance of such devices is limited by anti-phase domains forming at monoatomic steps on the Si-surface. To date the atomic structure of anti-phase boundaries, which affects the charge distribution at polar interfaces, is unknown. Here, we use CS-corrected scanning transmission electron microscopy to reveal the atomic structure of the anti-phase boundaries in III/V-semiconductors, choosing GaP as a model system. We observe boundaries on (110) lattice planes which are atomically abrupt and also facetted ones, which introduces locally charged regions influencing device performance.

High-precision nanoscale length measurement

Modern lithographical methods used to create linear measures for nanometer-range dimensions and the main factors which limit the applications of such gages have been analyzed in the paper. Prospects for developing high-precision measures based on an atomically structured crystalline surface (containing monoatomic steps) whose parameters are bound to the crystallographic parameters of the crystal (traceable to the length measure) are shown. A method which can be used to create such measures based on controlling the surface morphology of monocrystalline silicon at an atomic level due to the effects of self-organization arising at the atomically clean surface as a result of annealing in ultrahigh vacuum is proposed. A description of the set of high-precision gages of vertical dimensions STEPP-IFP-1 in a size range of 0.31–31 nm with an error in the whole interval of gages of less than 0.05 nm is presented. The set of high-precision gages after carrying out state testing is included into the state registry of measuring means as measuring type no. 48115-11 (Federal Agency on Technical Regulating and Metrology order no. 6290 of October 31, 2011).

Atomic steps on the MgO(100) surface

Defects on insulating surfaces have been studied much less compared to conductors due to experimental difficulties. Here we report quantitative structure analysis of atomic steps on the MgO(100) surface combining aberration-corrected high resolution transmission electron microscopy and density function theory calculations. While the broad faces show little relaxation or rumpling, the atoms at the step sites have significant displacements, depending on the atomic coordination. The general trend is to smooth the steps and lower their formation energy. The angles at the upper and lower corners of the mono-atomic step increase from unrelaxed 90\ifmmode^\circ\else\textdegree\fi{} to 94\ifmmode^\circ\else\textdegree\fi{} and 101\ifmmode^\circ\else\textdegree\fi{}, respectively. The experimentally measured positions of the atoms at the step sites match the density function theory results within several picometers.

LEED analysis of a nickel cylindrical single crystal

Using a low energy electron diffraction analysis, we have investigated the surface structure of a large cylindrical nickel single crystal. We show that a single cleaning procedure results for the low and high step density regions in a clean surface that contains only monoatomic steps. In between the low and high step density regions, some streaking is observed in the diffraction patterns. The streaking is attributed to increased disorder and partial reconstruction of the surface. For the (211) vicinal surfaces, streaking is related to the presence of an adsorbate, most likely oxygen, on the steps. We conclude that large sections of the circumference of the cylindrical crystal can be used for studies relating local surface structure to chemical reactivity for stepped surfaces in the 110 zone.

Structural variations of CO adlayers on a Pt(100) electrode in 0.1 M HClO4 solution: an in situ STM study

In the present study, we have investigated structures of a CO adlayer on a well-defined Pt(100) electrode surface in 0.1 M HClO4 aqueous solutions saturated with N2, 1% CO/He and 100% CO by using in situ STM. The in situ STM images with molecular resolution demonstrated that highly ordered structures of the CO adlayer, denoted (2 × n) - 2(n - 1)CO with CO coverages of (n - 1)/n, dynamically varied with the electrode potential and the CO partial pressure in solution. As the CO partial pressure increased, more compressed structures of the CO adlayer formed on the electrode surface. In each solution, a phase transition of the CO adlayer on the terrace site was observed to be triggered by increasing the electrode potential, accompanied by a partial desorption of surface CO without charge transfer. A series of in situ STM images revealed transient local structures during the phase transition of the CO adlayer. Specifically, unique structures were found to appear in the vicinity of monoatomic steps in N2- and 1% CO/He-saturated solution, but not in 100% CO-saturated solution.

Nitrate Reduction on Platinum (111) Surfaces Modifiedl with Bi: Single Crystalsl and Nanoparticles

Abstract Nitrate reduction on well-oriented platinum surfaces modified with Bi adatoms has been studied. The quantification of the electrocatalytic enhancement of the reaction rate due to the presence of Bi at different coverages was made on Pt(111) and the vicinal surfaces Pt(554) and Pt(332). These contain 9 and 5 atoms-width (111) terraces, respectively, separated by (110) monoatomic steps. The study was then extended to preferentially {111}Pt oriented nanoparticles. In all cases, Bi catalyzes nitrate reduction at high potentials, but the catalytic current suddenly drops when Bi is reduced. The analysis of the variation of catalytic activity with Bi coverage reveals the participation of a third body effect, meaning that Bi impedes the NO formation on the surface that acts as a poison for the nitrate reduction. The poisoning effect was also quantified by measuring the stripping of adsorbed NO spontaneously formed by contacting, with nitrate solutions, electrodes with different Bi coverage. The results of both single crystals and preferentially oriented nanoparticles agree with the supposed third body effect. The comparison of Pt nanoparticles with the stepped surfaces, Pt(554) and Pt(332), suggests that the main differences are related to the size of the terraces and not to the existence of defects/steps that do not seem to play any significant contribution to the catalysis.