Ordered silicon nanowire arrays for surface enhanced Raman scattering

Abstract

Surface enhanced Raman scattering (SERS) has attracted intensive research interest since its discovery because of its ultrahigh sensitivity up to single molecule detection. Silver nanoparticles (AgNPs) coated on silicon nanowires (SiNWs) as SERS substrates achieved a detection limit of probed molecule concentration of about 10-12 M. However, the high enhancement of AgNP/SiNW substrate was usually accompanied by low signal reproducibility and stability that hindered its practical applications. In this thesis, I firstly analyzed the enhancing units of metal-nanoparticles-based SERS systems and summarized the reasons leading to SERS irreproducibility, based on which I proposed ordered array of enhancing monomers as a solution. Then I described the fabrication of ordered SiNW arrays with tunable sizes by Nanosphere Lithography and Metal-assisted Chemical Etching. Enhanced Raman scattering of the array in relation to wire diameters and lengths were revealed and interpreted with a surface electrical wave model based on the Finite Difference Time Domain (FDTD) simulation. Following this, based on the resonant surface wave of SiNW arrays, I coated silver (Ag) on SiNW (Ag/SiNW) arrays as a monomer-based SERS substrate, and conducted multiplex SERS mapping to characterize and prove their reproducibility. Finally, I used optical waveguide theory and FDTD simulation to study the optical modes of the Ag/SiNW arrays and control their energies for future sensing and photovoltaic applications. The thesis is composed of five chapters. In the first chapter, I reviewed the SERS systems based on metal nanoparticles (MNPs) and summarized their common features. After analyzing the reasons for the low SERS reproducibility, I further introduced two monomer-based SERS systems that exhibited improved reproducibility, and finally proposed ordered arrays of enhancing monomers as a solution. Fabrication methods of Nanoshpere Lithography and Metal-assisted Chemical Etching were described to make ordered hexagonal-packed vertical SiNW arrays, and characterization tools of morphology, optical properties and near-field electric intensities were introduced. In the second chapter, Raman enhancement per unit volume (REV) were observed upon ordered vertical SiNW arrays with varying diameters of 450-900 nm and varying lengths of 0.54-7.3 um. The REV increased with decreasing wire diameters and oscillated with wire length, and significantly the Raman signals of seven 450-nm-diameter 3-um-long SiNWs was about 160 times larger than that of a Si wafer. The REV variations were attributed to the helical resonant surface wave resulting from the finite-length cylinder structures of the SiNW array. This assumption was supported by FDTD simulation that could visualize the near-field electrical field intensities of SiNW arrays. In the third chapter, a description was given to characterize continuous Ag film being coated on SiNWs of 110 nm in diameter and 700 nm in length to make a 300 nm-apart Ag/SiNW arrays as a monomers-based SERS system. The Ag/SiNW monomer demonstrated high SERS reproducibility with a small relative standard deviation of <10% in the mono-analyte detection and ~15% in the tri-analyte detection from 4624 Raman spectra mapping covering an area of 200 × 200 um2. The enhancement factor of the substrate could be increased from 1.12 × 105 to 1.02 × 106 by increasing the wire length from 500 nm to 700 nm. The high spot-to-spot reproducibility was due to the hexagonal array of enhancing monomers in the form of Ag/SiNWs that exhibited large open wire surface with broad electric fields to allow uniform molecule distribution and excitation. In the fourth chapter, I explained how I used optical waveguide theory to calculate the guided modes and cutoffs of single SiNW under excitation of 400 – 800 nm to guide the location of leaky modes. Using FDTD simulation, I found that SiNW arrays began to exhibit TM leaky modes at 564 nm and became very leaky at 609 nm. Isolated and coupled surface plasmon resonances (SPR) of the Ag nanotube arrays were also studied and demonstrated by FDTD models. Finally, the combination of SiNWs leaky modes and SPR of Ag nanotubes led to a model of hybrid surface modes of an Ag/SiNW arrays. An Ag/SiNW arrays with 20-nm Ag thickness, 700 nm in length and 150 nm in diameter exhibited hybrid guided mode at 504 nm and hybrid leaky mode at 541 nm, as well as isolated SPR at 444 nm and far-field coupled SPR at 645 nm. The model was agreed with by the stronger SERS of 4-ABT adsorbed on the Ag/SiNW arrays excited by a 633-nm laser than by a 514-nm laser. Tuning of the hybrid surface modes of the Ag/SiNW arrays were demonstrated by changing wire diameters, lengths and periodicity. Finally, in concluding the presented experiments and theories of optical property of the Ag/SiNW arrays, I discussed the challenges of the fabrication methods to make the Ag/SiNW arrays as a reproducible SERS substrate. Comparison between the light trapping and Raman enhancement of SiNWs was made and an AgNP/SiNWs plasmonic solar water splitting system was suggested as future work.

Date of Award	2 Oct 2013
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Wenjun ZHANG (Supervisor) & Shuit Tong LEE (Supervisor)