Nano-Phase Deposition Methodology

8.1.2.2. Nano-Phase Deposition Methodology

1. The Growth in Nano Beaker

In this approach, our knowledge on ‘electro-chemistry’ is applied to produce materials in the nanometer range. This approach consists in growing nano-scale materials by the use of nano-beakers, which involves using the pores in nano-porous membrane as ‘templates’ in order to produce nano-sized particles of the desired material. The membranes, i.e. the anodized aluminium or track-etch polymers, have cylindrical pores of uniform diameter.

When a polymer, metal, semiconductor, or carbon is synthesized by the electro-chemical means, within one of these pores, a nano-cylinder of the desired material is obtained, which ultimately de- pends on the type of material and the chemistry of pores wall that decide whether this cylinder may be hollow, i.e. tubular, or solid, i.e. fibril. The metal nano-tube membrane can also serve as ion-selective membrane. The nano-tube diameter can be as small as 0.8 nm, and the length of the nano-tube can span the complete thickness of the membrane. A metal nano-tube can act as “cation or anion selective”, which actually depends on the applied potential.

2. The Scanning Probe Nano Lithography

For a surface with nanometer scale defects, STM is used to modify so as to induce the nucleation of the deposited material at these sites of the defects. The silver pillars 10 nm-30 nm in diameter and 4 nm-10 nm high on a STM-modified highly ordered pyrolytic graphite surface have already been re- ported in the USA, and thus a nanometer scale galvanic cell composed of copper and silver nano-pillars can also be fabricated.

3. The Epitaxial Growth of Quantum Dots

A short discussion on ‘quantum dots’ will be given later. Here, the epitaxial growth is mentioned. At Weizmann Institute, the Scientists have produced epitaxially oriented CdSe ‘quantum dots’ with diameters of about 5 nm with a controllable spatial distribution and narrow size distribution by electro- deposition of the nano-crystals on the evaporated gold substrates. An interesting feature of this work is that the size of these ‘quantum dots’ is believed to be controlled by the strain that is induced by the mismatch between the CdSe and the Au lattice.

4. In-Situ Studies of Epitaxial Growth

When one of the precursors is in low concentration in the solution and the layer is grown at high over potential, the composition is graded throughout the layer with a (time) 1/2 dependence. The superlattices grown at lower potential in which both the reactants are deposited under activation con- trol have square profiles, e.g. for three Pb-Ti-O superlattices that were grown by pulsing between 70 MV and 150, 230, or 260 MV versus SCE.

NANO MATERIALS

The composition of the 150 MV layer is relatively constant at 64% Pb, whereas the Pb content of the 260 MV layer varies from 39 to 76% through the layer. The graded composition profile may be desirable for applications, e.g. by grading the composition, the lattice may inhibit ‘misfit dislocation formation’ in strained-layer superlattices. In semiconductor devices for optical or electronic applica- tions, it is desirable to have square composition profiles. The electro-chemical method is ideal for both measuring and tailoring the interface symmetry, and the composition profile is in real time on a nano meter scale [2].