Self-Organization of Ordered Nanopore Arrays Stimulated by Isotropic Optical Inputs

Optically stimulated electrodeposition of semiconducting Se-Te alloys produced spontaneous self-organization of pore arrays. The arrays were characterized by subwavelength scale dimensions and were generated uniformly over the macroscopic substrate area. In contrast, deposition in the absence of photoexcitation resulted in material without substantial extended order. The growth substrate was free of lithographic patterning and templating. There was no epitaxial relation between the deposit and substrate, and the same morphologies were generated regardless of the substrate identity. The deposition electrolyte contained no chemical structure directing agents and no optically active solutes. The optical input was fully isotropic. Photoexcitation was supplied by an incoherent, unpolarized, low intensity (mW cm-2) light-emitting diode (LED) source (laser excitation not necessary). No photomask was utilized, and the light-field had spatially uniform intensity. Nevertheless, the deposited material exhibited discrete order with six-fold symmetric domains of pores. The pore size and pitch could be tuned by adjusting wavelength of the stimulating light with larger dimensions observed for the use longer wavelengths. Electron microscopy in tandem with two-dimensional Fourier transform analysis indicated that interfacial order of the deposits increased with increasing deposition charge as the pore structure developed with increased film thickness. Electrodeposition was hypothesized to be preferentially accelerated in a spatially anisotropic manner by photovoltage generated by light absorption in the deposited material. Electrodeposition was simulated by a two-step iterative model in which the local optical absorption profile was calculated using electromagnetic simulations and then deposition was simulated with a Monte Carlo simulation in which the local probability of material addition was a function of the local absorption magnitude. This modeling reproduced the experimental structures indicating the self-organization was principally controlled by light absorption. Additional optical simulations using idealized structures indicated self-organization was an emergent consequence of coordinated light scattering at the deposit interface.