Expansion-Limited Nanocluster Growth in a Plume Formed by MHz-Pulse-Rate Laser Ablation
A.V. Rode, N.R. Madsen, E.G. Gamaly and B. Luther-Davies
The Australian national University, AU
laser ablation, nanocluster formation
Material parameters of a solid experience a dramatic change when cluster size becomes less than a certain critical value. Hence, one can control material properties through control over cluster size. Previously we have produced and characterized a unique nanostructured material, pure carbon nanofoam, which has strong paramagnetic properties. Now we aim to control the properties of carbon nanofoam by changing the size of carbon nanoclusters, which are the “building blocks” for this material. Conventionally, using low-repetition rate ns laser pulses, clusters are formed through the interaction of a laser-ablated plume with a noble gas filled chamber through diffusion-limited aggregation. The gas fill serves as confinement for the ablated atomic plume reducing its diffusion velocity and therefore retaining atoms at a temperature and density appropriate for atom-to-atom sticky collisions, and cluster formation.
We present the results on carbon cluster formation in a laser plume formed by high repetition rate, in the range 0.15 MHz – 28 MHz, 12-ps laser ablation of graphite and glassy carbon targets. We demonstrate experimentally and describe theoretically that the carbon nanoclusters are created by single picosecond laser pulses even in the conditions when the time gap between the pulses is in the sub-microsecond time domain. We demonstrate that the time for the plume’s adiabatic expansion in vacuum appears to be sufficient for the many successive collisions to occur that result in expansion-limited aggregation of nanoclusters. This time, which exclusively depends on the laser and solid parameters, is the major factor determining the cluster size. Hence, we show that the average size of a nanocluster is determined exclusively by single laser pulse parameters and it is independent from the nature of the filling gas (He, Ar, Kr, Xe) and its pressure in a range from 20mTorr to 200 Torr. Simple kinetic theory allows estimates of the cluster size, which are in qualitative agreement with the experimental data. In these experiments we conclude that the role of the buffer gas at pressures greater than approximately 1 Torr is to induce formation of the fractal-like nanofoam, while in vacuum a solid film containing clusters is formed.
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Nanotech 2006 Conference Program Abstract