Surface Structural Transition Research Articles

The kinetics of the (5 × 20) → (1 × 1) structural transition for Au(100)

The kinetics of the anion-induced (5 × 20) → (1 × 1) surface structural transition of reconstructed Au(100) electrodes was studied in sulfate-containing solutions by current transients. It is shown that lifting of the reconstruction follows a nucleation-and-growth type behavior which can be described by the Avrami equation. Moreover, for high positive potentials, i.e., high anion coverages, the current-transient analysis reveals instantaneous nucleation, whereas for low transition overpotentials and long transition times a mechanism with constant transition rate prevails. Apparent activation energies, which depend strongly on the electrode potential, are derived from the temperature dependence of the Avrami plots.

Surface structural transition of adsorption of oxygen on Ag(100)

High resolution electron energy loss spectroscopy (HREELS) and low energy electron diffraction (LEED) were used to study the oxygen adsorption on Ag(100). An ordered c(2 × 2) superstructure occurs after low temperature adsorption, in which the stretching mode at 37 meV was observed. The energy loss at 30 meV is attributed to the ordered p(1 × 1) structure after the adsorption at room temperature. The structure transition from the c(2 × 2) to the p(1 × 1) has been observed when heating the adsorbed surface from low temperature to room temperature. Cooling of the adsorbed surface at room temperature down to 180 K results in the surface transition from the p(1 × 1) structure to the coexistence of the p(1 × 1) and c(2 × 2) structures.

Critical dynamics near a surface structural phase transition.

Using the Mori projection-operator formalism, we develop a continued-fraction expansion for the dynamical structure factor of a system near a structural phase transition. The theory is applied to a W(100) surface and compared with recent He scattering data. Good agreement is obtained between theory and experiment.

Kinetics of the irreversible transition Pt(110)1×1→1×2 as observed by LEED

The kinetics of the surface structure transition of Pt(110)1×1→1×2 is observed by LEED. The integral intensity of a superstructure spot is recorded as a function of time at various temperatures as the transition develops from the metastable 1×1 phase towards reconstruction. An initial fast increase is followed by a very slow growth which leads to a quasi-saturation level. Both the time constant and the magnitude of the level vary with temperature according to an activation energy of about 0.26 eV. Thermal equilibrium, i.e. full reconstruction is only approached at temperatures beyond 800 K. An Avrami type growth law fits the experiments only approximately but fails for long times and increasing temperatures.

Second-harmonic and sum-frequency generation in reflection: probing of GaAs surface structure and laser-induced phase transitions

The dynamics of pulsed-laser annealing of an ion-implanted GaAs surface is studied by second-harmonic generation in reflection. The results are explained in terms of surface melting. Sum-frequency and second-harmonic generation in reflection are shown to be sensitive, convenient techniques having high temporal and spatial resolution for studying the structure of GaAs surfaces that have been laser annealed or randomized by ion implantation and of laser-induced phase transitions. The tensor nature of the second-order nonlinearity provides an opportunity to make polarizational measurements that are sensitive to small structural changes.

Two-dimensional lattice hamiltonian for surface reconstruction

We demonstrate that the strong coupling surface structural phase transition as observed on the (001) face of bcc group transition metals can be studied with an effective two-dimensional lattice Hamiltonian. This Hamiltonian provides an accurate description of the zero temperature properties and is convenient for a detailed study of the finite temperature phase diagram and critical fluctuation properties of the structural phase transition.

Summary Abstract: Effective two dimensional lattice dynamic Hamiltonian for surface structural phase transition

Summary Abstract: Effective two dimensional lattice dynamic Hamiltonian for surface structural phase transition

Optical study of the transition (2×1) → (2×8) of germanium versus the temperature

The irreversible surface structure transition Ge(111) (2×1) → Ge(111)(2×8) is studied by ellipsometry as a function of temperature. The optical constants of these surface layers are determined. The temperature dependence of the optical properties of Ge(111)(2 × 8) measured at λ = 6328 Å is closely connected with the dispersion measurements of Aspnes, measured at room temperature.

LEED fine structure and the W(001) surface structural transition

Fine structure dispersion relations are determined from accurate, very low energy, 0.5 eV resolution LEED measurements and compared with those obtained from electron reflection data for the clean W(001) surface at the structural transition. The two experimental methods are found to give consistent results. From the fine structure dispersions, surface-weighted effective potentials may be determined which give some insight into the possible change in surface structure at the W(001) thermally induced transition.

The clean thermally induced W {001} (1 × 1) → (√2 × √2) R 45° surface structure transition and its crystallography

A (√2 × √2)R45° surface structure on W {001} produced only by cooling below ~370 K, first reported by Yonehara and Schmidt, has been investigated by LEED, AES, work function change, characteristic loss and low energy Auger fine structure measurements. No significant changes at any energy up to 520 eV occur in the standard Auger spectrum upon cooling to 220 K for as long as 30 min after a flash to >2 500 K. The work function of the (√2 × √2) R45° at 210 K is 20 ± 10 mV below that of the (1 × 1) surface, and a sensitive feature in the fine structure of the N 7VV AES transition shows approximately 60% attenuation. Unlike for H 2 adsorption, the “surface plasmon” loss peak exhibits little if any measurable attenuation and no measurable shift in energy as the crystal cools to form the (√2 × √2)R45°. The rate of intensity buildup in the 1 2 -order LEED beams is strictly temperature dependent, and significant differences exist between the 1 2 -order LEED spectra produced by cooling and those produced by H 2 adsorption. Only 2-fold symmetry was observed in the LEED beam intensities at exactly normal incidence, rather than 4-fold as expected for statistically equal numbers of rotationally equivalent domains. The LEED I- V spectra for 24 fractional order beams and 12 integral order beams, taken over large energy ranges at normal incidence, clearly establish that the beam intensities display 2 mm point group symmetry, and hence a preference of one domain orientation over the other. No beam broadening or splitting effects were apparent, implying only incoherent scattering from the various domains. The half-order beam spectra (± h/2, ± h/2) are identical in relative intensity to the (± h/2, ± h/2) spectra but different in absolute intensity by a constant factor, which can be explained only by domains with p2mg space group symmetry rather than just p2mm. Adsorption of H 2 onto the cooled (√2 × √2)R45° structure restores the 4-fold symmetry in the LEED beam intensities at normal incidence, giving a c(2 × 2) hydrogen structure, the same as when adsorbing H 2 onto the above room temperature (1 × 1) crystal. This strongly supports the observed p2mg symmetry as being a true property of the cooled (√2 × √2)R45° surface structure. These results show that the (1 × 1) → (√2 × √2) R45° transition produced by cooling is a transition involving displacement of surface W atoms, and that it apparently can be characterized as an order-order, second degree, homogeneous nucleation process, which is strongly prohibited by the presence of impurities or defects.

Atomic geometry of semiconductor surfaces

Abstract While considerations of the space‐group symmetries of semiconductor surfaces as inferred from elastic low energy electron diffraction (ELEED) spot patterns have been given for decades,1 only during the past 2 years have quantitative analyses of ELEED intensities been performed to determine the atomic geometry of semiconductor surfaces. Specific systems which have been studied using ELEED intensity analyses include the {100} faces of rocksalt structure the low index faces of the wurtzite structure semiconductor Zn06–9 the cleavage faces of several zincblende structure semiconductors,9–14 the {100}15–17 and {11 l}18.19 faces of silicon, and the {OOOl} faces of MoSZ and NbSel.zo.zL The important finding emerging from these analyses is the common occurrence of substantial deviations of the actual surface structures from those characteristic of a truncated but otherwise “bulk” solid: a phenomenon which is referred as “reconstruction” (or “rearrangement”) of the surface. Moreover, these reconstructions both are pervasive (i.e., occurring for all but possibly the most ionic materials)3–5 and involve large (Δa∼0.5 Å) measurements of atomic species in the surface layer and possibly in the layers beneath as well. These results, which are in marked contrast to those obtained for metals,22 are manifestations of the differences between metallic, covalent, ionic and Van der Wads' bonding. Indeed, it seems possible22 that a covalent‐ionic surface structural phase transition occurs within the tetrahedrally coordinated compound semiconductors, all of which exhibit covalent bonding from the perspective of their bulk structure.23

Thermodynamics of surface phase systems: V. Contribution to the thermodynamics of the solid-gas interface

A thermodynamic fundamental equation is derived for the surface phase of a solid exposed to an adsorbing gas. This equation is more general than the corresponding equation given by Gibbs in the sense that it is applicable with less restriction as to the state of strain of the solid surface. An equation relating the surface tension to the reversible cleavage work which is similar to the Shuttleworth equation is shown to be implied in the fundamental equation deduced. Some new thermodynamic expressions are obtained for the effect of temperature and gas pressure changes on the surface tension and on the reversible cleavage work. Measurements of adsorbent dimensional changes, heats of adsorption and surface structure transitions are discussed on the basis of the formalism developed.

Influence of the leed beam on the structure of the vanadium pentoxide (010) surface

A surface structure transition of the V 2O 5 (010) surface into V 12O 26 (010), induced by the electron beam, is observed during LEED experiments. This letter concerns the different phenomena connected with the low energy electron bombardment and which are liable to play a part in the transition mechanism. Briefly, these phenomena are space charge build up, residual gas ionisation, adsorption, desorption and chemical reactions at the surface.