Band-gap modulation in single-crystalline Si1-xGex nanowires

We report the energy band-gap modulation of single-crystalline Si1-xGex (0 <= x <= 1) nanowires ranging from near-infrared (NIR) to visible regions by optical band-edge absorption. Single-crystalline Si1-xGex nanowires were grown by an Au catalyst-assisted chemical vapor synthesis using SiH4 and GeH4 precursors, and the relative composition of Si and Ge was reproducibly directed in the whole range of 0 <= x <= 1 by controlling the kinetics of catalytic decomposition of precursors near the eutectic temperature with Au. We show that, by the appropriate alloying of Si and Ge to form random solid solutions at the nanometer scale, the energy band-gap of Si1-xGex is tuned from 0.68 to 2.25 eV. Specifically, we demonstrate that with respect to the fundamental energy band-gap of bulk Si, the optical-absorption band edge shifts to a lower energy with increasing Ge content, and also that the band edge shifts to a higher energy with decreasing diameter of the nanowires below certain sizes. Our finding demonstrates that the energy band-gap of Si1-xGex nanowires can be modulated in a wider energy range and suggests implications for group-IV semiconductor nanowire photonics.