An in-situ AFM investigation of canavalin crystallization kinetics

We present the results of an in-situ atomic force microscopy investigation of the kinetics of canavalin crystal growth. The results show that, depending on the supersaturation, growth occurs on steps of one growth unit in height generated either by simple and complex screw dislocation 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 varies 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 bunches 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 indicates that surface diffusion rather than bulk diffusion is the controlling 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 predictions of the models of Schwoebel and Shipsey and Ehrlich and Hudda [R.L. Schwoebel, E.J. Shipsey, J. Appl. Phys. 37 (1966) 3682: G. Ehrlich, F.G. Hudda, J. Chem. Phys. li (1966) 1039]: showing that the behavior of the system is consistent with a model of surface-diffusion controlled growth coupled with an up-step diffusion bias. The relationship between step speed and terrace width during step homogenization was investigated quantitatively using the model of Gilmer a al. [G.H. Gilmer, R. Ghez, N. Cabrera, J. Crystal Growth 8 (1971) 79]. The best fit to the data is obtained with a surface diffusion length of 0.4-0.9 mu m, and leads to estimates for values of the activation energy for adsorption 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 NH4H2PO4 (ADP), a common inorganic crystal, for which the kinetic coefficient is three orders of magnitude larger. The comparison indicates that the 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.