Phototropic and skototropic growth in inorganic films

We report herein phototropic growth toward, and skototropic growth away from, an incoherent, unstructured, low-intensity light stimulus in the deposition of nonliving, inorganic material. Fundamental light-matter interactions yield growth of inorganic deposits with ordered mesostructures that are a function of the characteristics of the light stimulus. Illumination at normal incidence, as well as at substantial inclination, resulted in phototropic growth toward the direction of incidence. However, illumination incident at shallow inclinations resulted in growth away from the stimulus, a skototropic response. We show that the skototropic behavior of these materials is an emergent phenomenon produced primarily by nearest-neighbor optical scattering interactions during growth. We moreover observe similar scaling relations between the morphologies of the inorganic deposits and the growth habits of trees and forest stands, despite the differences in growth mechanisms and differences of >10⁸ in scale length.