Surface-enhanced Raman Scattering on the warped nanopatterns fabricated by multiple exposure interference lithography

Abstract

Laser interference lithography (LIL) is a large-area patterning strategy with submicron features. LIL utilizes the interference pattern between two coherent beams to realize selective irradiation. In contrast to conventional photolithography, LIL is a cost-effective process which can produce reproducible structures without a photomask. The process also possesses high-resolution capability, which is highly attractive as a surface-enhanced Raman scattering (SERS) fabrication platform. SERS represents substantial enhancement of Raman scattered electromagnetic field, thereby characterizing the ultra-trace chemical contents of analyte. SERS strongly depends on near field enhancement governed by localized surface plasmon resonance (LSPR), where the wavelength and coupling efficiency of LSPR are determined by the geometry of metallic nanostructures. Therefore, appropriate design and precise fabrication of metallic structures are important to realize the high performance, reproducible sensors. Herein, we fabricate warped nanopatterns using triple-exposure of LIL and functionalize the patterned photoresist structure as a completed SERS sensor by e-beam evaporation of gold or electroplating of silver. Although the resulting patterns appear more disordered than the typical hole or dot patterns, we show that the complex patterns exhibit sufficient periodicity and signal enhancement. We evaluate the performance of our SERS sensor by calculating the Raman enhancement factor (EF) of Rhodamine 6G. EF results 0.387*105for gold evaporated SERS and 1.451*105 for silver evaporated SERS. The uniformity and periodicity of patterns are confirmed by scanning electron microscopy and atomic force microscopy imaging with our mathematical model. Due to the periodic nature of the complex patterns, Raman measurement with spatial variance shows small relative standard deviation = 5.96%. Moreover, in virtue of increased complexity, we find that our complex pattern has no apparent peak in the simulated absorption spectrum on account of multi-shape feature. Such optical characteristics may highly attractive as a multi-frequency operation of SERS. Additionally, we explore the curvature induced effect on near-field enhancement of nanoparticles placed on the sinusoidally warped metallic surface.