Morphology-Graded Silicon Nanowire Arrays via Chemical Etching: Engineering Optical Properties at the Nanoscale and Macroscale

Abstract

We report on a quick, simple, and cost-effective solution-phase approach to prepare centimeter-sized morphology-graded vertically aligned Si nanowire arrays. Gradients in the nanowire diameter and shape are encoded through the macroscale substrate via a "dip-etching" approach, where the substrate is removed from a KOH etching solution at a constant rate, while morphological control at the nanowire level is achieved via sequential metalassisted chemical etching and KOH etching steps. This combined approach provides control over light absorption and reflection within the nanowire arrays at both the macroscale and nanoscale, as shown by UV−vis spectroscopy and numerical three-dimensional finite-difference time-domain simulations. Macroscale morphology gradients yield arrays with gradually changing optical properties. Nanoscale morphology control is demonstrated by synthesizing arrays of bisegmented nanowires, where the nanowires are composed of two distinct segments with independently controlled lengths and diameters. Such nanowires are important to tailor light−matter interactions in functional devices, especially by maximizing light absorption at specific wavelengths and locations within the nanowires.

Recent Advances in Vertically Aligned Nanowires for Photonics Applications

“…Furthermore, Wendisch et al recently reported a solution-phase shaping method by the combination of MACE and KOH etching for producing the morphology-graded vertical Si nanowires [ 74 ]. MACE and KOH etching steps were sequentially conducted to yield bi-segmented Si nanowires with controlled lengths and diameters ( Figure 3 f), which provides another way to maximize absorbability of vertical Si nanowire arrays in specific wavelengths of light.…”

“…( a ) reprinted by permission from Springer Nature: Springer Scientific Reports [ 61 ], Copyright 2019; ( b ) reprinted with permission from [ 21 ]. Copyright 2012 American Chemical Society; ( c , d ) reprinted from [ 70 ], with the permission of AIP Publishing; ( e ) reprinted by permission from Springer Nature: Springer Nature Nanotechnology [ 73 ], Copyright 2008; ( f ) reprinted with permission from [ 74 ]. Copyright 2020 American Chemical Society.…”

Silicon Nanowires Synthesis by Metal-Assisted Chemical Etching: A Review

“…The most used one remains the gold due to the best quality of fabricated Si NWs, for the stability in the etching solution compared to the silver, and for the fast and high tunable etching rate compared to other metals. Ti and other material such as AZO were also used as an adhesion layer for thick Au films, permitting to improve the film stability [ 126 , 127 , 138 ]. Kim et al report the application of an Au/Ag bilayer mesh to avoid the issue on the Ag stability and anodic dissolution during the etching [ 113 ].…”

Spatial-Tunable Au Nanoparticle Functionalized Si Nanorods Arrays for Surface Enhanced Raman Spectroscopy

“…Other than the design of narrow gap between metal nanostructures, the manipulation of incident light by three-dimensional nanostructures has also been considered as a potential strategy [ 24 ]. For example, hexagonal-packed silver-coated silicon nanowire arrays could be made as a nanogap-free SERS system with long-range electric fields [ 25 ].…”