Synthesis of metal nanoparticles by electrolytical STM and electrochemical discharges

As conventional fabrication techniques reach their limits novel techniques are needed for fabrication of structures in the nanometer range. This thesis investigates new methods to create metal nanostructures or nanoparticles with an STM tip or a sharp metal tool working inside an electrolyte. Electrochemical synthesis has the advantage that since deposition/etching are controlled by electrode potentials, it can be relatively simple to control and in addition STM allows in situ observation. Mathematical models based on finite difference have been developed to estimate electric field and ion distributions in the tip-sample gap region and quantitatively describe the dominant structuring methods – (a) double-layer mixing, (b) ultra-short voltage pulses and (c) diffusion governed deposition. Further, Monte Carlo analysis is used to look at cluster formation for the particular case of 2d growth. The mathematical models provide important insights and can be used for optimization. Experimental work with STM has been carried out on two systems – (a) deposition of Pt on graphite (HOPG) and (b) deposition of platinum on gold (Au(111)), from solutions containing chloroplatinic ions. In the case of HOPG, microsecond voltage pulses of large amplitudes (∼5 V) are shown to produce local structures in the ∼10-100 nm range. It is shown that deposition occurs from ions adsorbed on the tip surface. In the case of Au(111), by applying a train of nanosecond voltage pulses between tip and surface, Pt clusters of ∼10 nm and 1-2 monolayers high can be created in a zone of ∼50 nm. Deposition occurs from platinic ions adsorbed on Au surface. The role of the pulse parameters, tip material and geometry and solution concentration are discussed. General rules are established for local deposition by train of ultra-short voltage pulses. The last part of the thesis presents a method to produce metal nano particles by using electrochemical discharges. Electrochemical discharges occur under extreme current densities in an aqueous electrolyte and is characterized by a sudden breakdown of the conductivity accompanied by the formation of an insulating gas layer around the electrode. Electrical discharges occur across the gas layer. In this work, it is shown that this method is applicable to a large number of metals and is suitable for alloy nanoparticles. A simple method to control the size of the particles is developed. Metal nanoparticles ranging from a few nm to ∼200 nm can be synthesized.

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