Crystal growth activation barrier

The growth rate of silicon crystals by either MBE or CVD is relatively slow, and so there is ample time for adsorbed atoms and molecules to diffuse to energetically more favorable sites. Experimental rates of diffusion of silicon on silicon and the activation barriers, however, are not known well. Experimental estimates of the activation barrier for silicon atoms diffusing on the Si(lll) surface have ranged from 58 kcal/mole when the pyrolysis of silane is used to produce surface silicon atoms , to 4.6 kcal/mole for the direct deposition of silicon atoms under ultrahigh vacuum Furthermore, a comparison of these... 

First we consider the transformation mechanism of anatase to rutile in order to determine the reason for the dependence of the transformation rate on particle size. Penn and Banfield (1998 1999) showed that the oriented assembly of nanoparticles to form larger crystals (see below for details) is accompanied by formation of twins that introduce new atomic arrangements at particle-particle interfaces. In the case of anatase, a 112 twin represents a slab of brookite and thus, a structural state intermediate between anatase and rutile. Penn and Banfield (1999) proposed that the activation barrier for rutile nucleation is lowered by the presence of these twins. Simultaneously, it was noted that the transformation of anatase to rutile in air (Gribb and Banfield 1997) and under hydrothermal conditions (Penn and Banfield 1999) rarely generates partially reacted crystals, suggesting a high activation barrier for rutile nucleation but rapid rutile growth. 

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