EPITAXY VERSUS ORIENTED HETEROGENEOUS NUCLEATION OF ORGANIC-CRYSTALS ON IONIC SUBSTRATES

It is plausible to assume that epitaxy is a special case of heterogene ous nucleation in which a restrictive crystallographic relationship ex ists between substrate and deposit orientations. This would mean that epitaxial substrates should always induce a perceptible reduction in t he critical supercooling for nucleation of the deposit. To test this h ypothesis, the critical supercoolings of six organic compounds were me asured on glass and 11 single-crystal cleaved substrates including (00 01) graphite, (001) mica, (111) BaF2, SrF2, and CaF2, and (100) KCl, K Br, KI, NaCl, NaF, and LIF. Reductions in supercooling (with reference to glass substrates) were checked many limes for repeatability and re producibility and shown in almost all cases to have a standard deviati on of 1 degrees C or less. Acetanilide, benzoic acid, and p-bromochlor obenzene showed a wide range of supercooling reductions and were orien ted on all crystalline substrates. Naphthalene and p-dibromobenzene sh owed only slight supercooling reductions but were oriented on all subs trates, including glass. Benzil showed strong supercooling reductions only for mica and KI but was oriented not only in these cases but also with KI, BaF2, CaF2, and graphite. There was little correlation betwe en degree of lattice match and either supercooling reduction or degree of preferred orientation. These results suggest that, for the systems and geometry studied, forces such as molecular dipole binding and gro wth anisotropy had a stronger effect than lattice match.