Surface-enhanced Raman spectroscopy (SERS) using Ag nanoparticle films produced by pulsed laser deposition

Abstract

Thin silver nanoparticle films, of thickness 7 nm, were deposited onto glass microslides using pulsed laser deposition (PLD). The films were then characterised using UV–vis spectroscopy and scanning transmission electron microscopy before Rhodamine 6G was deposited onto them for investigation using surface-enhanced Raman spectroscopy (SERS). The sensitivity obtained using SERS was compared to that obtained using a colloidal silver suspension and also to a commercial SERS substrate. The reproducibility of the films is also examined using statistical analysis.

Introduction

Since the discovery of surface-enhanced Raman spectroscopy (SERS) in the latter half of the 1970s [1], [2] there has been much focus on the phenomenon, both in terms of explaining its origins and of maximising its applicability to chemical analysis. Possessing an ability to enhance Raman scattering by orders of 10⁶ and higher, it is a technique of huge potential, though its progress has often been hampered with issues regarding reproducibility. The enhancement of the Raman signal is thought to be due to plasmonic enhancement of the incident radiation intensity in the vicinity of nanoscale features on a metallic surface, and also due to chemical enhancements arising from adsorption of the target material on the nanostructured metal substrate [3].

Nanoparticle metal films (nanoparticles deposited on solid substrates) with feature sizes in the range of 1–100 nm have attracted much interest due their novel magnetic, catalytic and optical properties [4]. Several physical deposition techniques such as sputtering, chemical vapour deposition, pulsed laser deposition (PLD) and thermal evaporation have been used for the fabrication of metal nanoparticle films. In particular, PLD is a relatively simple and effective nano-fabrication technique [5]. In PLD, a high power pulsed laser is focused on the target surface. For a sufficiently high laser fluence (for metals ∼1 J/cm²), each laser pulse vaporises, or ablates, a small amount of material which expands rapidly from the target surface in vacuum. This ablated material provides the deposition flux for thin film growth, and the morphology and size of nanoparticles can be controlled by the number of laser pulses. Nanostructured Ag substrates prepared using PLD in a background gas have proved to be a promising candidate for SERS [6], and have previously performed well in comparison with commercial substrates [7].

Raman enhancement can also be obtained by using colloidal particles in solution, where their nanoscale dimensions leads to the excitation of localised plasmon oscillations. In this paper we use Rhodamine 6G, a dye commonly used in SERS characterisation studies, and benzotriazole to investigate the relative SERS sensitivity of an Ag nanoparticle film prepared by ns-PLD, a solution of silver colloids and a commercial substrate. Statistical analysis was used to quantify the SERS performance of each of the three approaches.