Bioinsipired Inorganic Phototropic Growth via Photoelectrochemical Deposition

Photosynthetic plants exhibit a phenomenon known as phototropic growth wherein the physical extension of the biological system proceeds preferentially towards the time-averaged position of the sun. By capitalizing on anisotropies intrinsic to nanoscale light-material interactions, natural phototropism can be mimicked via photoelectrochemical deposition to effect the template-free generation of ordered semiconductor mesostructures with nanoscale features over macroscale areas. This process mirrors the phenotypic plasticity exhibited by plants, wherein despite a fixed genotype a wide range of different morphologies can be displayed depending on the environmental conditions: the precise structure produced is determined by the characteristics of the growth illumination and wide array of morphologies can be generated despite a consistent material composition and phase. By controlling the incident direction of the light, the orientation and growth direction of the structures with respect to the substrate can be defined without relying on epitaxial growth but rather mirroring the way palm trees develop an observable tilt toward the average position of the solar azimuth. Changing the incident direction during growth was observed to drive reorientation of the newly added material and zig-zag structures could be produced by oscillating the incidence about the surface normal. However, the interfacial morphology generated after an input direction shift was observed to be function of not only the new input direction but also the preexisting structure and thus the prior input direction. Thus, different structures could exhibit discrete adaptations to the same illumination input. Modeling of the growth using a combination of electromagnetic simulations of light absorption coupled with Monte Carlo simulations of mass addition successfully reproduced the experimentally observed morphologies indicating structure generation was a result of optical processes. The growth process was observed to be a highly emergent phenomenon involving optical communication between neighboring features including cooperative scattering and synergistic absorption and thus is like the way neighboring plants exchange information and avoid competition for light resources.