MODIFIED CALCITE DEPOSITION DUE TO ULTRATHIN ORGANIC FILMS ON SILICONSUBSTRATES

In order to study the effect of organic surface chemistry on calcite n ucleation, attachment, and growth, calcium carbonate was precipitated in the presence of various ultrathin-film organosilane-modified silico n wafers. The chemistry of the aminosilane surfaces was systematically changed by the coupling of various acidic molecules, without creating a geometric lattice of acidic functional groups. Optical microscopy, scanning electron microscopy with image analysis, and X-ray scattering were employed to characterize crystallite density and orientation nor mal to the surface. Calcite grown on amino-modified surfaces was produ ced with the equilibrium rhombohedral habit and had the [104] orientat ion. Surfaces of the silicon oxide, carboxylate, iminodiacetate, or ph osphoramidate tended to favor the orientation of surface crystals alon g [001] or near the [001] axes of the crystal. Primarily this is a res ult of the affinity of the surface for cations, but functional-group-m ediated ion ordering and/or stereochemical matching is also suggested by the much greater amount of crystal nucleation on the long-chain car boxylates when compared to short-chain carboxylates. Coupling of nitri lotriacetic acid (NTA) favored appearance of [110], [113], and [116] o riented crystals when compared to the other acid surfaces. Growth of c alcite with relatively larger {110} faces was observed when the microc rystals were synthesized in the presence of freely soluble NTA. Appear ance of these faces is a result of a relatively suppressed growth rate due to face-specific adsorption on the growing crystallites. Similarl y, the enhancement of specific crystal surface binding by the substrat e bound NTA is probably the mechanism influencing orientation of surfa ce microcrystals. Two common structural features of the {110}, {113}, and {116} faces are the tilt of the carbonate plane at large angles fr om the face and the same angle of rotation of the carbonates about the ir 3-fold symmetry axes. That angle may enhance the ability of two NTA carboxylates to simultaneously occupy carbonate sites of these calcit e faces. The fact that crystallite density and orientation are influen ced by submonolayers of functional groups attests to the importance of electrostatic and stereochemical recognition of certain crystal faces even without matching of the geometric lattice.