Phototropic growth control of nanoscale pattern formation in photoelectrodeposited Se-Te films

Abstract

Photoresponsive materials that adapt their growth rates dynamically to the local incident electromagnetic field would provide a remarkable route to the synthesis of complex three-dimensional mesoscale structures via feedback between illumination and the morphol. that develops in response to the optical excitation. We report the spontaneous development of ordered, complex nanoscale lamellar patterns in electrodeposited seleniumtellurium (Se-Te) alloy films that are grown under uniform illumination on unpatterned substrates in an isotropic electrolyte soln. These inorg. nanostructures exhibited phototropic growth in which lamellar stripes grew towards the incident light source, adopted an orientation parallel to the light polarization direction, and showed an increased growth rate with increasing light intensity. The illumination wavelength controlled the lamellar period, which varied from 130 nm for UV light to 412 nm for near-IR light. The height modulation of the lamellar morphol. was detd. by the ratio of the photocurrent-mediated growth rate to the growth rate mediated by the dark current at a given applied potential. Highly anisotropic features, exhibiting av. lamellar heights of 938 nm and widths of 164 nm, were formed under incident light intensities of 18.6 mW/cm2 at a potential of -0.40 V referenced to a std. calomel electrode. Furthermore, the patterns responded dynamically to changes during growth in the polarization, wavelength, and angle of the incident light, enabling the templatefree and pattern-free synthesis of woodpile, spiral, or branched structures. Full-wave electromagnetic simulations were used to model the light-matter interactions in Se-Te films. In combination with Monte Carlo growth simulations, this approach produced a model for the morphol. evolution of the lamellar structures under phototropic growth conditions. The simulations and expts. are consistent with a phototropic growth mechanism in which the optical near-field intensity profile selects and reinforces the dominant morphol. mode in the developing nanoscale patterns.