AN IN-SITU AFM INVESTIGATION OF CANAVALIN CRYSTALLIZATION KINETICS

We present the results of an in-situ atomic force microscopy investiga tion of the kinetics of canavalin crystal growth. The results show tha t, depending on the supersaturation, growth occurs on steps of one gro wth unit in height generated either by simple and complex screw disloc ation sources, 2D nucleating islands, or macroclusters which sediment onto the surface before spreading laterally as step bunches. The step velocity of canavalin al three different pHs (pH = 7.0, 7.7 and 8.0 va ries linearly with concentration and gives a kinetic coefficient beta which depends strongly on pH, with beta approximate to 2.6 x 10(-3) cm s(-1) at pH 7.3 to beta approximate to 5.8 x 10(-4) cm s(-1) at pH 8. 0. Analysis of the velocity of single steps versus that of step bunche s created by macroclusters, as well as the occurrence of 2D nucleation on broad terraces, constrains the length scale for diffusion to be of the order of I mm. A simple diffusion analysis is presented which ind icates that surface diffusion rather than bulk diffusion is the contro lling mechanism iii solute transport to the steps. A demonstration of step homogenization with an exponential time dependence fur step pair decay is presented, and is found to be in qualitative agreement with p redictions of the models of Schwoebel and Shipsey and Ehrlich and Hudd a [R.L. Schwoebel, E.J. Shipsey, J. Appl. Phys. 37 (1966) 3682: G. Ehr lich, F.G. Hudda, J. Chem. Phys. li (1966) 1039]: showing that tile be havior of the system is consistent with a model of surface-diffusion c ontrolled growth coupled with an up-step diffusion bias. The relations hip between step speed and terrace width during step homogenization wa s investigated quantitatively using the model of Gilmer a al. [G.H. Gi lmer, R. Ghez, N. Cabrera, J. Crystal Growth 8 (1971) 79]. The best fi t to the data is obtained with a surface diffusion length of 0.4-0.9 m u m, and leads to estimates for values of the activation energy for ad sorption to the terrace E-ad, and for incorporation at the step E-inc, of 0.27 and < 0.1 eV, respectively. The results of this analysis are compared to those obtained from interferometric measurements on NH4H2P O4 (ADP), a common inorganic crystal, for which the kinetic coefficien t is three orders of magnitude larger. The comparison indicates that t he main reason for this difference is the slow adsorption rate to the surface for canavalin as compared to ADP. (C) 1997 Elsevier Science B. V.