Thermal Vibrations Of Surface Atoms Research Articles

Ionic Behavior in Highly Concentrated Aqueous Solutions Nanoconfined between Discretely Charged Silicon Surfaces.

Through molecular dynamics simulations considering thermal vibration of surface atoms, ionic behaviors in concentrated NaCl solutions confined between discretely charged silicon surfaces have been investigated. The electric double layer structure was found to be sensitive to the density and distribution of surface charges. Due to the discreteness of the surface charge, a slight charge inversion appeared which depended on the surface charge density, bulk concentration, and confinement. In the nanoconfined NaCl solutions concentrated from 0.2 to 4.0 M, the locations of accumulation layers for Na(+) and Cl(-) ions remained stable, but their peak values increased. The higher the concentration was, the more obvious the charge inversion appeared. In 4.0 M NaCl solution, Na(+) and Cl(-) ions show obvious alternating layered distributions which may correspond to the solidification found in experiments. By changing surface separation, the confinement had a large effect on the ionic distribution. As both surfaces approached each other, many ions and water molecules were squeezed out of the confined space. Two adjacent layers in ion or water distribution profiles can be forced closer to each other and merge together. From ionic hydration analysis, the coordination number of Na(+) ions in highly confined space was much lower than that in the bulk.

Surface segregation at the binary alloy CuAu (100) studied by low-energy ion scattering

We present results from an experimental study of segregation at the CuAu (100) surface. It is shown that Au enrichment in the top surface layer persists up to temperatures far beyond the bulk order–disorder transition temperature. From the gradual desegregation at higher temperatures a segregation energy of −0.30eV was determined. Our results are in quantitative agreement with calculations by Tersoff predicting oscillatory concentration depth profiles with decreasing amplitudes at higher temperatures. For the layer selective surface analysis we used low-energy He+ and Na+ scattering. Data interpretation and quantification were supported by numerical simulations with the MARLOWE code to which we had added the special features of trajectory resolved analysis and anisotropic thermal vibrations of surface atoms.

Focusing of particles scattered by a surface

It has been shown by computer simulation that the coefficient of reflection of argon atoms scattered by crystalline aluminum and germanium targets at glancing angles of less than 4° is close to unity and the beam of scattered particles exhibits focusing (the angular distributions of particles are strongly compressed). Whereas beam focusing with respect to the azimuth is well known and has already been studied, sharp focusing in the surface-normal direction at small glancing angles has not been studied so far. This effect is confirmed by the experimental results. It is associated with multiple scattering of incident particles by the atomic chain. The simulation results allowed finding quite accurately the amplitude of thermal vibrations of surface atoms ((0.123 ± 0.007) A for aluminum), which agrees well with the experiment.

Size-dependent of compression yield strength and deformation mechanism in titanium single-crystal nanopillars orientated [0001] and [112̄0

Different size effects and deformation mechanisms are revealed in α-titanium (Ti) single crystal nanopillars orientated for [112̄0] and [0001] based on molecular dynamics simulations. The strength–size relationship changes from “smaller is stronger” to “smaller is much weaker” when the width of nanopillars reduces from 19nm to 3nm. The “smaller is much weaker” is attributed to that the surface effect caused by a thermal vibration of surface atoms leads to the initiation and growth of surface dislocations.

Structural phase transition and thermal vibration of surface atoms studied by reflection high-energy electron diffraction

We have developed a new system for reflection high-energy electron diffraction (RHEED) with two pairs of magnetic coils to measure rocking curves in a short time. Since the measurement time of the system is short and the intensity measurement is very accurate compared with mechanical angle adjustment systems currently in use, this system is the most suitable tool to determine structural phase transitions at high temperatures. For the Si(111) surface, we have discovered an additional phase just below the melting temperature, in which the thermal vibration of surface atoms is strongly enhanced. For the Si(001) surface, the asymmetric dimer structure, which is believed to be the most stable at low temperature, transforms into the symmetric dimer structure, as confirmed by comparison with a rocking curve from a hydrogen-terminated Si(001) 2 × 1 surface.

Precursor to surface melting of Si(111) at high temperature

The surface structures of Si(111) in the temperature range between 1220 and 1540 K. in which an additional phase is discovered [Phys. Rev. Lett. 85, 5150 (2000)], have been investigated with rocking curves of reflection high-energy electron diffraction (RHEED). From the analysis using the dynamical RHEED calculation, we found that the thermal vibration of surface atoms shows an anisotropic feature, where the mean-square amplitude of the thermal vibration parallel to the surface is about twice as large as the normal component, and the amplitude is extremely enhanced around 1540 K, just below the surface melting temperature, with an enhancement of the anisotoropy. The anharmonic increase in the thermal vibration of the surface atoms plays an important role in the appearance of the additional phase around 1540 K. This result suggests that this structural phase with a large vibrational amplitude acts as a precursor to the surface melting.

Ne neutral atoms 180° backscattered from a Pt(111) surface in the three-atom scattering model

The intensity of 180° backscattered neutral atoms is investigated using numerical calculations based on the three-atom scattering model (THASM). The calculations are performed for 2 keV Ne + ions incident along the [1̄1̄2] direction of a Pt(111) surface. The THASM calculations well reproduce the maximum intensity in the polar-angle scan, which has been observed experimentally, and the peak position is strongly dependent on the interatomic potential. Also, the peak width (FWHM) is sensitive to thermal vibrations of surface atoms, especially to their anisotropies. The THASM results show that the Molière approximation of the Thomas-Fermi potential with a reduced Firsov screening length, multiplied by a factor of 0.85, is suitable for the Ne-Pt interatomic potential, and that the vertical component of the surface Debye temperature is 143 K.

Anomalous dynamical behavior of the Cu(110) surface.

The thermal vibrations of surface atoms on Cu(110) have been studied as a function of temperature with ion-scattering spectroscopy in the impact-collision mode. By comparing the experimental data to Monte Carlo simulations of the scattering process, mean-square displacements of surface atoms are obtained that agree well with the results of helium diffraction and inverse photoemission experiments. Below 550 K the atom vibrations can be described quite well by a Debye temperature of 224 K obtained from phonon slab calculations. Above 550 K, strong anharmonic vibrations are observed. In this temperature region we find thermally activated adatoms to be produced with an activation energy of 0.27\ifmmode\pm\else\textpm\fi{}0.07 eV. Both effects lead above 1000 K to structural changes of the surface that can be described in terms of a rough surface or a quasiliquid layer on the surface.

Thermal vibrations at copper surfaces studied by low energy ion scattering

The thermal vibrations of surface atoms on Cu(110) have been studied as a function of temperature with impact collision ion scattering spectroscopy. Monte Carlo simulations of the scattering along chains of surface atoms show that for large vibration amplitudes more than two atoms have to be included to explain the experimental data. The mean square displacements obtained from the comparison between experiment and calculations agree well with the results of atom scattering and inverse photoemission experiments and the results from surface phonon calculations. The influence of correlations and anisotropy of the surface vibrations on impact collision ion scattering spectra is discussed.

Energy distribution of Ni + ions recoiled from a Ni(100) surface

A Ni(100) single-crystal surface was bombarded by 5 keV Ne +, Ar + and kr + ions along [100] and [110] azimuths at an incident angle of 15° (measured from the surface). The various features observed in the energy distribution caused by direct and deflected recoils, i.e. knock-on and blocking processes, are identified by computer simulation of binary collisions involving different collision sequences, inelastic energy loss and thermal vibrations of surface atoms. Good agreement is obtained between experiment and computer simulation.

Mechanisms of recoil ejection of Ni+ ions from a Ni(100) surface.

A Ni(100) single-crystal surface was bombarded by 5-keV ${\mathrm{Ne}}^{+}$ ions along [100] and [110] azimuths at an incident angle of 15\ifmmode^\circ\else\textdegree\fi{}. The energy distribution of fast recoil ${\mathrm{Ni}}^{+}$ ions was measured as a function of various exit angles. The important features in the energy spectra caused by direct knock-on and blocking processes are identified by computer simulation of binary collisions involving different collision sequences, inelastic energy loss, thermal vibrations of surface atoms, initial charge state of recoil, and subsequent neutralization during exit from surface. Good agreement is obtained between experiment and computer simulation.

Angular distributions of reflected MeV light ions from a single crystal surface at glancing incidence

The angular distributions of ions reflected from a SnTe(001) surface are measured for a glancing angle incidence of MeV protons and He + ions, with the glancing angle less than 7 mrad. The observed angular distributions are compared with the calculated distributions derived from the stochastic theory taking account of both the thermal vibration of surface atoms and the scattering by electrons near the surface. From comparison of the experimental and calculated results the surface Debye temperature is determined to be 72 ±6 K.

An X-ray diffraction study of soft surface vibrations of FCC fine metal particles

The lattice parameter and the characteristic temperature theta p of the fine crystalline particles of copper, silver, gold and lead in a range from 60 to 2000 AA in diameter were determined by the X-ray diffraction method. The lattice parameters did not change appreciably from the bulk value in contrast to the observations by electron diffraction by Vermaak and Kuhlmann-Wilsdorf (1968) and by Solliard and Buffat (1977). The characteristic temperature theta p of copper, silver and gold decreases with decreasing particle size. This is due to the softening of the thermal vibration of surface atoms in fine metal particles. The surface characteristic temperature theta s was estimated from the particle size dependence of theta p on the assumption that the effective thickness of the surface is of the order of one atomic layer. The results were in good agreement with LEED experiments. However, theta p of lead particles was not found to change with the decrease of particle size.

Influence of thermal vibrations on ion surface scattering near 180° scattering angle in the two-atom scattering model

Based on the two-atom scattering model, the ion surface scattering near 180° scattering angle is investigated for the analysis of a new ion beam technique, i.e., impact collision ion scattering spectroscopy (ICISS). The shadowing effect of elastic scattering and the ion trajectory focusing effect play important roles for large-angle backscattering at the solid surface. Numerical results show that these two effects are very sensitive to the thermal vibrations of surface atoms, especially to their anisotropy. The angle of incidence at the maximum intensity and the peak width (fwhm) of the backscattering intensity is an increasing function of u z / u x , where u x and u z are the rms displacements parallel to and perpendicular to the surface, respectively. In the case of a TiC crystal, it was found that the ratio of u z / u x is nearly 1.6.

Analysis of the outermost atomic layer of a surface by low-energy ion scattering

Abstract Noble gas ion scattering is used to study the surface of solid targets. It is shown that this technique can be used to obtain a mass spectrum of the first atomic layer of the surface. Since the outermost atoms will largely determine the chemical reactivity and physical properties of the surface, this is an important property for an analytic tool to have. In addition to reflected ions, high-energy recoil ions are sometimes observed.Both the reflected and the recoil ions provide information about the atomic structure of the surface. The various possibilities of ion scattering are demonstrated for bromine, oxygen, sodium and potassium adsorbed on a Si(111) surface, for halogens adsorbed on Ni(100) and for GaP (110), (111) ( 111 ) surfaces. The influence of thermal vibrations of surface atoms and of electronic excitation on the spectra is discussed.

Small angle multiple reflection of low energy (6keV) noble gas ions from single crystal surfaces as a means to study surface texture and contamination

The elemental composition of a surface can be derived from the energy spectrum of singly reflected noble gas ions. It is shown that it is also possible to obtain information about certain structural features on the surface from the energy spectra of multiply reflected ions. By carefully studying the multiple reflection energy spectra when the angle of incidence ψ is varied at a given scattering angle θ, many different multiple reflection peaks can be resolved and identified. The ideal conditions for measurements on atomically clean and undisturbed surfaces have been closely approached by working with a low residual gas pressure (10 −10 torr) in the collision chamber and small primary ion current densities ( < 10 −8 A cm 2 ). For Ar + and Kr + on (100)-Cu, peaks have been found resulting from an ideal, flat surface and peaks resulting from surface ridges (steps) and surface vacancies. This technique opens possibilities to study nucleation and early stages of growth of thin films, surface defect formation as a result of ion bombardment, the annealing behaviour of surface defects, thermal vibrations of surface atoms and neutralization processes in multiple collision sequences.