Recovery of surfaces from impurity poisoning during crystal growth

Growth and dissolution of crystal surfaces are central to processes as diverse as pharmaceutical manufacturing(1,2), corrosion(3), single-crystal production(4) and mineralization in geochemical and biological environments(5,6). Impurities are either unavoidable features of these processes or intentionally introduced to modify the products. Those that act as inhibiting agents induce a so-called 'dead zone', a regime of low supersaturation where growth ceases. Models based on the classic theory of Cabrera and Vermilyea(7) explain behaviour near the dead zone in terms of the pinning of elementary step motion by impurities(8,9). Despite general acceptance of this theory, a number of commonly investigated systems exhibit behaviour not predicted by such models(10). Moreover, no clear microscopic picture of impurity-step interactions currently exists, Here we use atomic force microscopy to investigate the potassium dihydrogen phosphate {100} surface as it emerges from the dead zone, We show that traditional models are not able to account for the behaviour of this system because they consider only elementary steps, whereas it is the propagation of macrosteps (bunches of monolayer steps) that leads to resurrection of growth out of the dead zone. We present a simple physical model of this process that includes macrosteps and relates characteristics of growth near the dead zone to the timescale for impurity adsorption.