Design and fabrication of vertically aligned single-crystalline Si nanotube arrays and their enhanced broadband absorption properties

We propose and demonstrate a new and room-temperature approach for the fabrication of well-ordered arrays of vertically aligned, diameter-, length-, and interspacing-controllable Si nanotubes on (0 0 1)Si substrates. In this approach, a unique, hexagonally-ordered Au disk/hole dual nanostructure pattern with uniform size and spacing was first produced on the surface of (0 0 1)Si substrate and was then used as the catalyst to etch vertically downward into the Si substrate by Au-catalyzed chemical etching. All the produced vertical Si nanotubes were identified to be single crystalline with the same axial orientation of [0 0 1] and their lengths could be readily tuned by adjusting the etching time. The produced long vertical Si nanotubes with catalytic Au disk/hole dual nanostructures show greatly enhanced hydrophobicity (water contact angle: ~146°) and broadband absorption properties (average visible-light absorptance: ~96%; average near-IR absorptance: >70%) compared to the corresponding bare (0 0 1)Si substrate and Si nanorod samples. The obtained results present the exciting prospects that the new approach proposed here promises to open opportunities for the design and construction of various 1-D hollow semiconductor nanodevices with multi-function.