Laser vaporization cluster growth

Figure. 1. Schematic of essential components of the Exxon group cluster laser vaporization source and fast flow tube chemical reactor. On the far left is a 1 mm diameter pulsed nozzle that emits an -200 ysec long pulse of helium which achieves an average pressure of approximately one atmosphere above the sample rod. Immediately before the sample rod position the tube is expanded to 2 mm diameter. The length of this extender section can be varied form 6 mm to 50 mm depending upon the desired integration time for cluster growth. The reactor flow tube is 10 mm in diameter and typically 50 mm long. The reactants diluted in helium are added and mixed with the flow stream via the second pulsed valve.

It was later shown in two independent studies that the relative importance of NbmC met-cars and nanocrystals in the mass spectrum is critically dependent on the experimental conditions. It has been proved that the concentration of hydrocarbon in the carrier gas, the nature of the laser used as the vaporization source, the laser power selected, and, finally, the direet detection of cluster ions or the photoionization of neutral species drastically influence the relative proportion of met-cars to nanocrystals. This selectivity has been attributed to distinct mechanisms of cluster growth for met-cars and nanocrystals (Section 5.9.2.3). Laser-induced photodissociation of 3 X 3 X 3 nanocrystals M14C13+ (M = Ti, V) and larger clusters assumed to have a fee crystal structure has been reported by Pilgrim and Duncan. The titanium carbide cluster corresponding to the 1044-amu peak in the mass spectrum has been assigned to the 3 x 3 x 4 fee fragment in which one tita-... 

A method combining laser ablation cluster formation and vapor-liquid-solid (VLS) growth was recently developed for the synthesis of single crystal semiconductor siiicon and germanium nanowhiskers [74], Specificaily, iaser abiation was used to prepare dusters of moiten metal catalyst particles with a nanometer diameter. The droplet diameter defines the diameter of the resulting nanowhiskers. Buik quantities of uniform silicon and germanium nanowhiskers with diameters from 6 to 20 and from 3 to 9 nanometers, respectiveiy, and lengths from 10 to 300 nanometers were obtained. 

The chapter consists of three major sections. The first is a brief review of the processes of nucleation and growth in supersaturated vapors for the formation of clusters and nanoparticles. The second section deals with the application of laser vaporization for the synthesis of nanoparticles in a diffusion cloud chamber. In the third section, we present some examples of nanoparticles synthesized using this approach and discuss some selected properties. 

The mechanism of nanotube formation in chemical vapor deposition features characteristics rather distinct from those found for the synthesis by arc discharge or laser ablation. Contrary to the latter, a solution of small carbon clusters in and subsequent diffusion through catalyst particles play a minor role in the deposition from the gas phase. The employed hydrocarbons decompose directly on the surface of the catalytic particle. The carbon, therefore, becomes immediately available for nanotube growth. 

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