Search

A study on the nanoscale electrocrystallization of pure Sb and the compound semiconductor AlSb on Au(111) is conducted by in situ scanning probe techniques (STM and STS). Conductance spectra of these clusters display the main features of the electronic structure of the bulk semiconductor AlSb, with a band gap of 2.0 plusmn 0.2 eV.

We have investigated the thickness and surface structure of surface freezing films in Ga–Bi and Ga–Pb alloys over a wide temperature range between room temperature and the respective surface freezing transitions by x-ray photoelectron spectroscopy (XPS) and scanning tunnelling microscopy (STM). For the example of a Ga–Bi alloy dilute in Bi, XPS measurements show that the surface freezing film has a nearly constant value of ∼25 Å between the surface freezing temperature of 130 °C and room temperature if the sample is cooled slowly (5 K/h). On heating to 130 °C the film thickness exhibits a clear hysteresis on melting. On quenching the alloy sample (>100 K/h) the film thickness increases by almost a factor of 10. These observations indicate that the surface freezing films are metastable. The surface structure of the surface freezing films of various Ga-rich Ga–Bi and Ga–Pb alloys has been probed for the first time by STM at different temperatures below and above the bulk eutectic point. Atomically resolved STM images show the surface structures of pure Bi (0001) and Pb (111), respectively, at room temperature. On heating above the eutectic temperature the surface structure of the films does not change significantly as judged from the size and thickness of Pb or Bi terraces. These observations together with the film thickness variation with temperature indicate that the surface freezing films behave like a metastable independent surface phase. These results together with the wetting characteristics of these alloys suggest that surface freezing in these systems is a first order surface phase transition between wetting and metastable surface freezing films. The energy barrier for nucleation is strongly reduced due to a lowering of the interfacial energy if the nucleus is completely immersed in the respective wetting layer. [ABSTRACT FROM AUTHOR]

Phase formation of surface alloying by spinodal decomposition has been studied at an electrified interface. For this aim Zn was electrodeposited on Au(111) from the ionic liquid AlCl3-MBIC (58:42) containing 1 mM Zn(II) at different potentials in the underpotential range corresponding to submonolayer up to monolayer coverage. Structure evolution was observed by in situ electrochemical scanning tunneling microscopy (STM) at different times after starting the deposition via potential jumps and at temperatures of 298 and 323 K. Spinodal or labyrinth two-dimensional structures predominate at middle coverage, both in deposition and in dissolution experiments. They are characterized by a length scale of typically 5 nm which has been determined from the power spectral density of STM images. Structure formation and surface alloying are governed by slow kinetics with a rate constant k with activation energy of 120 meV and preexponential factor of 0.17 s-1. The evolution of the structural features is described by a continuum model and is found to be in good agreement with the STM observations. From the experimental and model calculation results we conclude that the two-dimensional phase formation in the Zn on Au(111) system is dominated by surface alloying. The phase separation of a Zn-rich and a Zn-Au alloy phase is governed by two-dimensional spinodal decomposition. [ABSTRACT FROM AUTHOR]

We have studied the interfacial characteristics of Ga-rich liquid Ga-Bi alloys at various compositions and temperatures up to 300 °C by different electron spectroscopies under ultrahigh vacuum conditions. In particular, the thickness and thickness profiles of Bi-rich wetting and surface freezing films have been measured. It is found that at the surface freezing transition the Bi film thickness jumps from ∼5 Å to macroscopic values of over 100 Å. This distinct behavior is discussed in comparison with the results in Ga-Pb. In addition, we report first scanning tunneling microscopy observations of the surface structure of a solidified Ga0.989Bi0.011 alloy after passing a surface freezing transition. The topology at room temperature is characterized by extended, atomically flat Bi terraces covered by nanosized triangular two-dimensional Bi islands. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present the first experimental investigation and pertinent theoretical modeling of an interfacial oscillatory instability in a binary fluid alloy, the Ga—Pb system. It is characterized by spinodal decomposition at elevated temperatures and by a complete wetting transition at liquid—liquid coexistence. For the alloy Ga0.95Pb0.05 the fluid interface has been probed by second harmonic generation SHG under UHV conditions at temperatures between 740 and 550 K. At conditions inside the miscibility gap clear oscillations of the SHG-intensity with a period of ∼30 min are found for different cooling cycles and also at constant temperatures. These interfacial oscillatory instabilities simultaneously induce temperature oscillations in the bulk fluid with the same period. This phenomenon can be explained by a periodic variation of the fluid interfacial emissivity. A model has been developed which describes the wetting—dewetting dynamics by hydrodynamic equations within the Reynolds approximation. It is found that the interfacial oscillatory instability is determined by capillary-gravitation instability. The model quantitatively describes the time evolution of the interfacial and temperature oscillations and gives the correct value of the oscillation period. A detailed comparison of the experimental and model results is given. [ABSTRACT FROM AUTHOR]

The two-dimensional (2D) phase formation and growth of Ag on Au(111) are studied under pulsed electrodeposition conditions in comparison with conventional underpotential range (UPD). The evolution of the 2D nanostructures as a function of time and potential is followed by in situ scanning tunneling microscopy (STM).

Underpotential deposition (UPD) of Ag on Au(111) is studied with two different electrolytes such as aqueous 0.1 M H(sub 2)SO(sub 4), solution in comparison with the ionic liquid 1-butyl-3-methylimidazolium chloride BMICI plus AlCl(sub 3). Cyclic and linear sweep voltammetry in combination with situ electrochemical scanning tunneling microscopy (STM) is used to investigate the distinct behavior of 2D phase formation at both interfaces.

Studies the infrared spectra of the carbon dioxide anions isolated in solid argon. Effect of the laser ablation of transition metal targets; Density-functional theory (DFT) calculations using the GAUSSIAN 94 program; Metal independent infrared absorptions.

We have measured the electron spin resonance (ESR) spectra of saltrich KxKCl1-x melts at temperatures near 800 °C and over a broad composition range, 10-4≤x≤10-1, approaching the metal–nonmetal (M–NM) transition in these solutions. Emphasis has been given to a precise in situ variation of composition which has been achieved for the first time in a high temperature ESR experiment by Coulometric titration. The spectra are characterized by a motional linewidth narrowing with a Lorentzian line shape. The ESR characteristics as determined from the Lorentz fits of the spectra exhibit the following features as a function of x: In the nonmetallic regime the g factor (g=1.9938±0.0002) and the peak-to-peak halfwidth (ΔBPP=0.2 mT) stay constant up to x∼0.05. Above this composition a clear increase of ΔBPP is observed indicating the NM–M transition. The spin susceptibility χs has been determined from the imaginary part of the Lorentz fits and has been calibrated against a sapphire signal measured simultaneously with the KxKCl1-x liquid samples. The spin susceptibility strongly deviates from simple Curie behavior even at low x which gives evidence for spin paired states in the nonmetallic solutions. Above x∼0.05 χs rises steeply with x. The overall variation of χs with x is interpreted by two limiting models: a thermodynamic defect model focusing on the particle character of localized paramagnetic and diamagnetic states for x≤0.05, and a simple band model describing the strong increase of χs approaching the NM–M transition. Both approaches yield a satisfactory description of the observed variation of χs. Further support of this interpretation is found in the spin dynamics which are qualitatively discussed. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report a detailed optical investigation of the wetting phenomena in fluid metalrich KxKCl1-x solutions for temperatures up to 820 °C and x≥0.86. To this end the fluid sample–sapphire interface has been probed by second harmonic generation (SHG) with continuous variation of temperature T at various x. At coexistence a wetting transition near Tw≊500 °C is signaled by a clear change in the temperature dependence of the SH intensity. This observation is in agreement with previous ellipsometric measurements. The precise location of the transition in these systems is strongly influenced by the presence of oxygen impurities segregating at the interface. This is manifested in a strong enhancement of the SH signal in the vicinity of 450 °C. In the homogeneous metal-rich phase the SH intensity shows a pronounced maximum as a function of temperature for T>=Tw. The occurrence of these maxima is explained consistently by assuming a salt-rich microscopic wetting film in between the metallic bulk fluid and the sapphire window whose thickness changes from thick to thin with increasing temperature. The line connecting these maxima positions in the x–T plane exhibits all the characteristics of a prewetting line. In particular, it is curved away from the coexistence curve as has been predicted theoretically for systems with screened Coulombic interactions, but is found here for the first time experimentally. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Optical absorption spectra are reported for liquid KxKCl1-x (10-5≤x≤5×10-2) and NaxNaBr1-x (10-3≤x≤3×10-2) solutions at 800 °C in the energy range 0.5≤hν≤5 eV. The composition has been varied in situ by Coulometric titration of the metal. With this method of simultaneous measurement of optical spectra and metal activity relative changes in the spectra are detected with high precision. In this way it has been possible, for the first time, to separate distinct components in the optical spectra of metal solutions. A contribution which is quantitatively described by the Drude model is attributed to weakly localized electrons in conduction band tail states with low mobility. The determined electronic mobilities are of the order of 0.1 cm2 V-1 s-1. The concentration dependence of the corresponding electronic conductivity quantitatively agrees with independent measurements of the dc-electronic conductivity. The main component in the absorption spectra is due to excitations of localized electrons which are different in character in the KxKCl1-x and NaxNaBr1-x melts. This is concluded from the distinct concentration dependence and is discussed in the light of quantum molecular dynamics calculations and further results of the electronic properties. [ABSTRACT FROM AUTHOR]

Glasses with electronic properties intermediate between the ionic and metallic state are obtained. This is achieved by melts of Bi-BiCl(sub 3) and Bi-BiCl(sub 3)-KCl mixtures being rapidly quenched into the glassy state at cooling rates of ~10(super 8) K/s.