Thermal stability of surface freezing films in Ga-based alloys: an x-ray photoelectron spectroscopy and scanning tunneling microscopy study.

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 approximately 25 A between the surface freezing temperature of 130 degrees C and room temperature if the sample is cooled slowly (5 Kh). On heating to 130 degrees C the film thickness exhibits a clear hysteresis on melting. On quenching the alloy sample (>100 Kh) 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.

Microscopic and electronic structure of semimetallic Sb and Semiconducting AlSb fabricated by nanoscale electrodeposition: An in situ scanning probe investigation.

The nanoscale electrocrystallization of pure Sb and the compound semiconductor AlSb on Au(111) has been studied by in situ scanning probe techniques (STM and STS) employing an ionic liquid electrolyte, {AlCl3-[C4mim]+Cl-} (1:1) containing SbCl3. The characteristic changes of the electronic structures with varying potentials have been probed for the first time by normalized differential conductance spectra, (dI/dU)/(I/U). In the underpotential deposition range of Sb the formation of two layers is observed. For the first monolayer a (square root 3 x square root 3)R30 degrees structure is determined from atomically resolved STM images. During the deposition and dissolution of the Sb monolayers characteristic wormlike or spinodal structures appear indicating surface alloying of antimony with the gold substrate. Under overpotential conditions two different Sb structures have been observed. If the deposition potential is continuously stepped to -0.1 V, Sb nanostripes form. On the other hand, randomly dispersed small clusters occur if the potential is jumped from 0.0 to -0.3 V vs Al/Al(III). Both modifications exhibit typical semimetallic behavior as shown by the STS spectra. At -1.1 V the cyclic voltammogram shows a clear reduction wave that is assigned to AlSb compound formation. Deposits in this potential range are characterized by a homogeneous distribution of clusters with diameters of approximately 20 nm. Conductance spectra of these clusters exhibit the main features of the electronic structure of the bulk semiconductor AlSb, with a band gap of 2.0 +/- 0.2 eV. Electrodeposition experiments on both sides of the compound deposition potential show a strong doping effect that is manifest in the corresponding conductance spectra.

Pulsed electrodeposition of two-dimensional Ag nanostructures on Au(111).

One-step pulsed potential electrodeposition of Ag on Au(111) in the underpotential deposition (UPD) region has been studied in 0.5 mM Ag2SO4 + 0.1 M H2SO4 aqueous electrolyte at various pulse durations from 0.2 to 500 ms. Evolution of the deposited Ag nanostructures was followed by in situ scanning tunneling microscopy (STM) and by measurement of the respective current transients. At short pulse durations a relatively high number density (4 x 10(11) cm(-2)) of two-dimensional Ag clusters with a narrow size and distance distribution is observed. They exhibit a remarkably high stability characterized by a dissolution potential which lies about 200 mV more anodically than the typical potential of Ag-(1 x 1) monolayer dissolution. To elucidate the underlying nucleation and growth mechanism, two models have been considered: two-dimensional lattice incorporation and a newly developed coupled diffusion-adsorption model. The first one yields a qualitative description of the current transients, whereas the second one is in nearly quantitative agreement with the experimental data. In this model the transformation of a Ag-(3 x 3) into a Ag-(1 x 1) structure indicated in the cyclic voltammogram (peaks at 520 vs 20 mV) is taken into account.

Nanoscale electrocrystallisation of Sb and the compound semiconductor AlSb from an ionic liquid.

Aluminium antimonide nanoclusters with an apparent band gap energy of 0.92 +/- 0.2 eV have been electrodeposited from the neutral ionic melt AlCl3-1-butyl-3-methylimidazolium chloride {AlCl3-[C4mim]+Cl-} at room temperature and have been characterized in-situ by electrochemical scanning tunneling microscopy (STM) and spectroscopy (STS).

Two-dimensional connective nanostructures of electrodeposited Zn on Au (111) induced by spinodal decomposition.

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 AlCl(3)-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.

Comparative investigation of underpotential deposition of Ag from aqueous and ionic electrolytes: An electrochemical and in situ STM study.

Underpotential deposition (UPD) of Ag on Au(111) has been studied with two different electrolytes: aqueous 0.1 M H2SO4 solution in comparison with the ionic liquid 1-butyl-3-methylimidazolium chloride BMICl + AlCl3. Of particular interest is the distinct behavior of 2D phase formation at both interfaces, which has been investigated by cyclic and linear sweep voltammetry in combination with in situ electrochemical scanning tunneling microscopy (STM). It is found that one monolayer (ML) of Ag is formed in the UPD region in both electrolytes. In aqueous solution, atomically resolved STM images at 500 mV versus Ag/Ag+ show a (3 x 3) adlayer of Ag, whereas after sweeping the potential just before the commencement of the bulk Ag deposition, a transition from expanded (3 x 3) to pseudomorphic ML of Ag on Au(111) occurs. In BMICl-AlCl3, the first UPD process of Ag exhibits two peaks at 410 and 230 mV indicating that two distinct processes on the surface take place. For the first time, STM images with atomic resolution reveal a transition from an inhomogeneous to an ordered phase with a (square root of 3 x square root of 3)R30 degrees structure and an adsorption of AlCl4- anions having a superlattice of (1.65 x square root of 3)R30 degrees preceding the deposition of Ag.

Peculiarity of the liquid/vapour interface of an ionic liquid: study of surface tension and viscoelasticity of liquid BMImPF6 at various temperatures.

We have measured the surface tension and the capillary wave spectra at the liquid/vapour interface of the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate at various temperatures up to 400 K. From the weak temperature dependence of the surface tension a low value of the surface excess entropy of approximately 3.5 x 10(5) J K(-1) m(-2) results which is consistent with a strongly aligned surface layer of imidazolium cations previously predicted by MD-calculations. The capillary wave spectra recorded at different wave numbers in the range 170 cm(-1) < or = q < or =500 cm(-1) exhibit strong deviations from the behaviour expected for the free surface of simple liquids. With an extended dispersion relation including the contributions of surface dipole moment density gamma and shear surface excess viscosity mu the spectra have been analyzed. It is found that mu is negligibly small, whereas gamma substantially influences the capillary wave spectra. The electrostatic potential across the interface, which corresponds to the measured dipole moment densities, qualitatively agrees with simulation calculations. The distinct temperature dependence of gamma suggests that with increasing temperature an order-disorder transformation occurs in the surface layer.

Effect of vertical temperature variation on the oscillatory wetting instability in a fluid Ga-Pb alloy.

Oscillatory wetting instabilities driven by capillary-gravitation forces have been explored very recently in the binary fluid Ga-Pb alloy [A. Turchanin, R. Tsekov and W. Freyland, J. Chem. Phys., 2004, 120, 11 171]. This system is characterized by a complete wetting transition at liquid-liquid coexistence. Due to its metallic nature the bulk and interfacial instabilities are strongly coupled via variation of the respective emissivities. In our previous work we have investigated these phenomena at different cooling cycles and at constant temperature inside the miscibility gap. In this study we present for the first time the observations of the oscillatory wetting instabilities also in heating cycles. The interfacial properties of a Ga0.95Pb0.05 alloy at conditions inside the miscibility gap have been investigated by following the second harmonic generation (SHG) intensity changes. Corresponding model calculations of the Pb-rich wetting film instabilities have been performed taking into account the effect of a temperature variation vertical to the bulk sample. The influence of this temperature variation on the occurrence of the oscillations is discussed.

Electrodeposition of monodispersed Fe nanocrystals from an ionic liquid.

Monodispersed Fe nanocrystals up to approximately 2 nm thick, approximately 50 nm wide and approximately 120 nm long have been electrodeposited from the ionic melt AlCl(3)-1-methyl-3-butylimidazolium chloride [AlCl3-[MBIm](+)Cl(-)] at room temperature on Au(111) and have been characterized in-situ by electrochemical scanning tunneling microscopy.

Oscillatory wetting instability induced by liquid-liquid decomposition in a Ga--Pb alloy.

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 Ga(0.95)Pb(0.05) 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 approximately 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.

Electron spectroscopy and scanning tunneling microscopy study of quasi-two-dimensional freezing at the liquid/vapor interface of Ga-Bi alloys.

We have studied the interfacial characteristics of Ga-rich liquid Ga-Bi alloys at various compositions and temperatures up to 300 degrees 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 approximately 5 A to macroscopic values of over 100 A. 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 Ga(0.989)Bi(0.011) 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.

SAMs of shape-persistent macrocycles: structure and binding on HOPG and Au(111).

Self-assembled monolayers of shape-persistent macrocycles have been adsorbed on highly oriented pyrolytic graphite and Au(111) substrates. Their structure and binding characteristics have been investigated by scanning tunneling microscopy, X-ray photoelectron spectroscopy, and ultraviolet photoelectron spectroscopy experiments. Special functionalization of the macrocycles for the first time has enabled their self-assembling and binding on a metal substrate and opened a new route for new functional nanostructures.

Spectroscopic evidence of a wetting and prewetting transition in liquid K-KCl mixtures

Spectroscopic ellipsometry and simultaneous reflectivity measurements of liquid KxKCl1-x solutions clearly exhibit a first order wetting transition in metal-rich melts. At the sample/substrate interface, salt-rich wetting films of mesoscopic thickness ( approximately 100 nm) are observed at and off of coexistence. They are uniquely characterized by the liquid F-center absorption band. However, crossing the prewetting line towards metal-rich concentrations, the F bands disappear. From the observation of the liquid F-center band, it is concluded that a strong undercooling of the wetting films of about 200 K may occur in binary metallic fluids, which is demonstrated here for the first time.