Fabrication of SERS Active Substrates by Nanoimprint Lithography
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Fabrication of SERS Active Substrates by Nanoimprint Lithography Kebin Li, Bo Cui, Liviu Clime, and Teodor Veres Industrial Materials Institute, National Research Council, Canada, 75, de Mortagne, Boucherville, J4B 6Y4, Canada ABSTRACT A method for low-cost and high throughput fabrication of SERS substrates based on nanoimprint lithography (NIL) has been developed. The SERS effect for detection of rhodamine 6G (R6G) molecules is demonstrated on two model nanostructures comprising either Au nanocrescents or Ag nano-wells fabricated. Numerical simulations based on discrete dipole approximation (DDA) show that the observed enhancement of the SERS signal for the given geometries originates from hot-spots localized at the tips of the nanocrescent. For the nanowell, the hotspots are mainly localized inside the cavity, on the side of the nanodonut and at the edge of the bottom nanodisc when it is excited by a laser at the wavelength of 785 nm. INTRODUCTION Surface enhanced Raman scattering (SERS) has been proven to be a sensitive technique allowing for single molecule detection. Because of its chemical specificity and label-free nature, it has attracted increasing attention in analytical, biomedical, environmental, as well as global and homeland security domains. The enhancement of Raman signal in SERS is attributed to two well-known mechanisms, namely, the electromagnetic mechanism and charge-transfer mechanism [1-5]. The enhancements of the local electromagnetic (EM) field close to roughened metal surfaces is due to the excitation of intrinsic surface plasmon resonance (SPR) of individual nano-structure as well as plasmon coupling effects between adjacent nano-structures. SERS enhancement factor of 1014 has been demonstrated in aggregated colloids of metallic (silver or gold) nanoparticles. [6, 7] However, the limited stability and reproducibility of metal colloids often limit their practical application. Alternatively, various types of nano-structures have been suggested for enhancing the local EM field, such as sharp metal tips [8-10], sharp metallic grooves and nano-wedges [11-14], nanoburgers [15], triangular nano-prisms [16], nano-pins [7], nano-cubes [17], as well as the recently reported nano-crescents [18-20]. In addition to these isolated nanostructure, SERS has also been demonstrated in some nanostructures with nanovoids such as nanoporous gold[21-23], silver cavity array[24] and circular holes[25, 26]. Therefore, it is desirable to fabricate the SERS active substrates in a reproducible and controllable way with high throughput in order to meet the requirement in practical application. In this work, we report on the fabrication of arrays of noble metallic nanocrescents and nanowells over large area by using the NIL method. The period of the array is 200 nm and the feature size of the fabricated nanostructures can be as small as 30 nm. The fabricated nanostructures are SERS active in the detection of R6G molecules. The fabrication process is reliable, repeatable and affordable. Numerical simulation
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