Gerko Oskam

Phase-pure TiO2 nanoparticles: anatase, brookite and rutile

We report on the synthesis of phase-pure TiO(2) nanoparticles in anatase, rutile and brookite structures, using amorphous titania as a common starting material. Phase formation was achieved by hydrothermal treatment at elevated temperatures with the appropriate reactants. Anatase nanoparticles were obtained using acetic acid, while phase-pure rutile and brookite nanoparticles were obtained with hydrochloric acid at a different concentration. The nanomaterials were characterized using x-ray diffraction, UV-visible reflectance spectroscopy, dynamic light scattering, and transmission electron microscopy. We propose that anatase formation is dominated by surface energy effects, and that rutile and brookite formation follows a dissolution-precipitation mechanism, where chains of sixfold-coordinated titanium complexes arrange into different crystal structures depending on the reactant chemistry. The particle growth kinetics under hydrothermal conditions are determined by coarsening and aggregation-recrystallization processes, allowing control over the average nanoparticle size.

Electrochemical deposition of metals onto silicon

The general concepts governing the electrochemical deposition of metal films onto semiconductors are discussed. Deposition onto semiconductor surfaces is complicated due to the band structure of the semiconductor, which affects both the thermodynamics and the kinetics of metal deposition processes. The influence of the potential distribution at the semiconductor/solution interface on the charge transfer mechanisms involved in deposition of metals is discussed. Models for electrochemical nucleation and growth are described and the influence of the unique physical properties of semiconductors is analysed. Finally, we present recent results for electrochemical deposition of gold, copper and platinum onto n-type silicon.

Influence of solvent on the growth of ZnO nanoparticles

We have synthesized ZnO nanoparticles by precipitation from zinc acetate in a series of n-alkanols from ethanol to 1-hexanol as a function of temperature. In this system, nucleation and growth are relatively fast and, at longer times, the average particle size continues to increase due to diffusion-limited coarsening. During coarsening, the particle volume increases linearly with time, in agreement with the Lifshitz-Slyozov-Wagner (LSW) model. The coarsening rate increases with increasing temperature for all solvents and increases with alkanol chain length. We show that the rate constant for coarsening is determined by the solvent viscosity, surface energy, and the bulk solubility of ZnO in the solvent.

Electrochemical Deposition of Copper on n‐Si/TiN

In this paper we report on the electrochemical deposition of copper onto n‐type silicon with a 30 nm TiN barrier film. We show that the mechanism of nucleation and growth is dependent on the applied potential. At potentials more negative than −0.35 V, instantaneous nucleation of hemispherical clusters is followed by diffusion‐limited growth. In this potential regime, the nucleus density is essentially constant at about . At potentials more positive than −0.35 V, the nucleation and growth kinetics are more complex, and clusters consisting of several nuclei are formed. The cluster density decreases to about at −0.05 V.

Island growth in electrodeposition

Electrochemical deposition of metals onto foreign substrates usually occurs through Volmer–Weber island growth. The mechanism of island nucleation and growth dictates the shape, orientation and number density of islands, and ultimately, the structure and properties of thin films. With increasing emphasis on deposition of ultrathin films and nanostructures, it is critically important to understand the kinetics of nucleation and growth. Here we provide a comprehensive review of island growth in electrodeposition and summarize methods for mechanistic analysis in both the kinetic and diffusion limited regimes.

Electrochemical nucleation and growth of gold on silicon

Abstract In this paper we report on the nucleation and growth of gold on n-type silicon from solution by electrochemical deposition. We show that deposition occurs by progressive nucleation and diffusion limited growth of 3D hemispherical islands. Gold films were prepared by nucleation at a potential where the maximum nucleus density is obtained, followed by growth under kinetic control. Transmission electron microscopy confirmed that the films were continuous and polycrystalline with a 〈111〉 texture. The electrical properties of the Si/Au junctions were comparable to junctions prepared by evaporation or sputtering.

Electrochemistry of Gold Deposition on n‐Si(100)

The electrochemical deposition of gold on n-type silicon from KAu(CN) 2 solutions was investigated by performing a detailed study of the nucleation and growth kinetics. Deposition occurs by progressive nucleation and diffusion-limited growth of three-dimensional hemispherical islands over a wide range of potentials and KAu(CN) 2 concentrations. It is shown that for a silicon/gold electrode, the applied potential is dropped over the silicon/gold interface at potentials more positive than 0 V, while at potentials more negative than 0 V, the applied potential is dropped over the Helmholtz layer at the gold/solution interface. The influence of these observations on the applicability of nucleation models derived for metal-on-metal deposition to metal deposition onto semiconductors is discussed.

A reappraisal of the frequency dependence of the impedance of semiconductor electrodes

Abstract The frequency dependence of the impedance of polarizable semiconductor/metal and semiconductor/electrolyte solution interfaces is reconsidered. No frequency dispersion of the polarization capacitance is found with n-GaAs/Au contacts whereas the capacitance of n-GaAs/electrolyte solution interfaces show considerable dispersion. The frequency dispersion depends on the microroughness of the electrode surface and on the specific conductivity of the electrolyte solution. As for polarizable metal electrodes, the relationship between the capacitance and the frequency is closely related to the relationship between the capacitance and the specific conductivity of the electrolyte solution. It is concluded that the main origin of frequency dispersion should not be sought in the solid but is related to the development of the electric double layer at the electrolyte side of the interface. A model is presented to account for these results.

The formation of porous GaAs in HF solutions

Abstract The electrochemical etching of n-type GaAs in HF solutions in the dark results in the formation of a porous layer. The pore density, the pore dimensions and the structure of the porous layer depend on the doping density and the crystallographic orientation of the surface. The pore morphology of porous GaAs is essentially independent of the applied current. The pore front velocity is linearly proportional to the current and the porous layer can be grown to any thickness. The primary pores in GaAs grow in the 〈111〉 a direction which is in contrast with silicon where the pores grow in the 〈100〉 direction. The pore diameters increase from 80 nm for highly doped GaAs to 400 nm for undoped GaAs. The combination of electrochemical methods and structural analysis techniques, including transmission electron microscopy and small angle neutron scattering, leads to a better understanding of anisotropic etching of semiconductors.

Electrochemical nucleation and growth of copper on Si(111)

In this paper we report on the deposition of copper on n-Si(1 1 1) from 1 mM CuSO4+0.1 M H2SO4 (pH=1) solution. Voltammograms revealed a deposition peak characteristic of diffusion limited growth. Atomic force microscopy (AFM) images after deposition of a few monolayers showed that deposition occurs by Volmer–Weber island growth. From analysis of AFM images obtained as a function of deposition time, we show that the nucleus density increases linearly with time, consistent with progressive nucleation. Deposition transients follow the rate law for progressive nucleation and 3D diffusion limited growth over a wide range of potentials. Ex situ AFM imaging of copper deposition on annealed miscut surfaces revealed that the copper cluster density is higher in regions of high step density and that nucleation occurs preferentially at step edges.