Tailoring Optical Properties of Silicon Nanowires by Au Nanostructure Decorations: Enhanced Raman Scattering and Photodetection

Metallic nanoparticles (NPs) decorated semiconductor nanowires (NWs) heterostructures show significant promise in enhanced optical and opto-electrical properties due to the coupling of surface plasmon to nanowires. Here, we demonstrate a galvanic displacement based strategy to achieve in situ nucleation of Au nanoparticles and then postgrowth into higher order Au nanostructures such as dimers, nanorods, and nanoprisms along the same Si nanowires (SiNWs). The presence of Au nanostructures significantly enhances the optical properties of nanowires. Particularly, a 24 times enhancement of Si Raman scattering signal was achieved with a Au dimer decoration. A Au nanorod aligned in parallel along nanowire strongly enhances the anisotropy of Si Raman scattering, with more than 28 times stronger signal under parallel polarization than that under perpendicular polarization, demonstrating for the first time the surface plasmon enhanced antenna effect. In addition, we demonstrate that surface plasmon enhances photocurrent of SiNW by almost 100%, which is higher than previous reports. Our studies show that SiNWs decorated with metallic nanostructures by in situ galvanic displacement exhibit significant promise toward high efficiency photodetection and light harvesting applications. ■ INTRODUCTION Recently, increasing interests have been drawn on metallic nanoparticles decorated semiconductor nanomaterials, especially SiNWs, due to the expanded functionality and considerable promise in a wide range of applications. For instance, selectively deposited metallic nanoparticles can be used for a secondary growth of nanowires, which enables the unconventional synthesis of semiconductor heterostructures with encoded novel properties for logic gates and addressable transistors. Metallic nanoparticles−SiNW heterostructures also exhibit enhanced interaction between photons and nanowires, leading to excellent photocatalytic properties. For example, Pt NPs-decorated SiNW arrays have been shown to enhance photoconversion efficiency in photoelectrochemical solar cells. What makes these heterostructures more unique is the interaction between surface plasmon resonance of noble metal (e.g., Pt, Ag, or Au) and the SiNWs. These metal nanocrystals can not only serve as hot spots supported by the large area of the nanowire surface but also improve the optical and opto-electrical properties of the semiconductor nanowires themselves. Thus, extended research and applications have also been carried out on surface-enhanced Raman scattering (SERS) and biosensing. However, in this system, optimal interface with close interconnection is crucial for applications based on plasmon enhancement, since the localized electromagnetic field on the metal surface exponentially decreases with a characteristic decay length scale of ∼2 nm. Galvanic displacement is an effective way to decorate SiNWs with metallic nanoparticles, such as Au or Ag, with clean and heteroepitaxial interface. In this process, silicon nanowire surface serves as both the template for particles decoration and the source of electrons that reduce the metal ions in solution. Fresh SiNW surface can provide the necessary reduction potential to facilitate the formation of uniform Ag or Au nanoparticles. Upon the supply of hydrofluoric (HF) acid, continuous metallic ion reduction can be sustained and Received: October 24, 2011 Revised: January 19, 2012 Published: January 24, 2012 Article