Kinetic Monte Carlo Predictions of the Conditions Necessary to Grow Helical (Chiral) and Vertical Columnar Structures

Abstract

Rare Event Dynamics Monte Carlo simulations have been used to investigate the possibility of growing porous crystalline material possessing vertical columnar morphology. This is achieved using a deposition process in which the incoming beam of atoms is held at a grazing angle and, importantly, is rotated azimuthally. For incident beams 80° from normal incidence, rotational speeds of around 1 revolution per second to 1 per 10 seconds were found to produce columnar thin film growth with essentially vertical walls. Slower rotational speeds, around 1 revolution per 30 seconds, produced a helical, but still vertical, columnar structure. Such so-called chiral structures are postulated to be potentially important optical materials. The effect of raising the temperature of the substrate is to decrease the density of the columns, but to broaden their size. This suggests that a temperature-rotational speed set of parameters could be found to produce designer pore sizes with relatively little variation in inter-column spacing and height and either with or without helical structure. Data for one such representation are presented. A movie of the evolution of the nanostructural features of a vertical columnar material is included.