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August 20th, 2008
What happens when you fire a silicon nanoparticle at a silicon surface at 900 m/s? What about at 2,000 m/s? As it turns out, two completely different things occur; bouncing or sticking, and the transition between the two responses occurs at a speed between 1,250 and 1,550 m/s.
Why would anyone care about this? As scientists and engineers seek to create new materials by exploiting nanoscale features and the different physical and chemical effects that dominate at these small scales, control of the entire process becomes key. Control over nanoparticle formation in the gas phase, and deposition onto a surface are key for "manufacturing novel nanostructured surfaces and thin films," according to, M. Suri and T. Dumitrica, a pair of mechanical engineers at the University of Minnesota, and the authors of an upcoming paper in Physical Review B.
The authors look at the example of coating a surface with silicon spheres, which has been problematic. The difficulty is due to the different physics that play a dominant role at such short length and time scales. On the macroscale, inelastic impacts have their energy released due to the formation of dislocations within the material, but according to the authors there is not enough time or space for this mechanism to occur within a nanoparticle. Secondly, silicon nanospheres have been found to be extremely hard, over four times the bulk yield stress was required to generate yield in these tiny balls. Finally the system is complicated by the chemistry of the surface of the particle and surface material. If they are both bare silicon, then the nanoparticle will easily become chemically bound by a substrate. If the surfaces are passivated with hydrogen, then bonds do not form as easily.
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