An atomic force microscopic investigation of surface degradation of potassium hydrogen phthalate (KAP) crystals caused by removal from solution

Atomic force microscopy (AFM) has been used to examine the nature and extent of surface degradation of the {0 1 0} faces of potassium hydrogen phthalate (KAP) crystals removed from aqueous solution. Crystals separated From their growth solutions by passage through a layer of n-hexane or using a jet of n-hexane exhibited clearly preserved growth spirals. The step terraces, however, were found to be extensively pitted, typically to a depth of approximately 1.4 nm which corresponds to one lattice spacing in the b-direction. In addition to such nanoscale pitting, larger pits several microns in lateral dimensions were also observed. These were frequently hexagonal in shape usually with a shallow mound within or adjacent to the pit. More rapid removal of solution from crystal surfaces was effected using a pulse of compressed air or argon. Under optimised conditions this approach yielded much better surface protection with step terraces in this case being essentially smooth. Step heights on the spirals shown were found to be 1.4 +/- 0.2 nm and thus of single unit cell height. The occurrence of pits on the terraces is attributed to deposition of KAP by two dimensional nucleation on the terraces, producing partial coverage and thus the apparently pitted surface.