The extremely intense and highly confined electromagnetic fields near plasmonic nanostructures enhance spectral signatures for plasmoinc biosensing, including surface-enhanced Raman scattering (SERS), metal-enhanced fluorescence (MEF), and plasmon resonance energy transfer (PRET). The phenomenon results from the interactions between the incident light and surface electrons of metal nanostructures. This interaction induces localized surface plasmon resonance (LSPR) that strongly depends on the composition, size, interstitial gap spacing, and dielectric environment of metal and its surrounding medium. Tailoring the LSPR of varying size, shape and material has been a major research interest. Since the main contribution of plasmon-enhanced optical signal arises from the excitation of the LSPR of plasmonic nanostructures. Although considerable progress has been made recently in the rational design and fabrication of plasmonic nanostructures to vary with the size and shape, and material, unavoidable limitations associated with conventional nanofabrication techniques still remain.
Solid-state dewetting of thin metal film recomes of much interest in implementing large area plasmonic nanostructures, which allow tailoring the strong optical fields near metal nanostructures. In particular, surface-enhanced Raman scattering (SERS) takes the full benefits of plasmonic nanostructures for providing the most highly sensitive biochemical sensing techniques even at single molecule level without fluorescent labeling. The main contribution of SERS signal arises from the excitation of LSPR of plasmonic nanostructures. Therefore, constructing strong local electromagnetic fields called hot spots and optimizing the location of LSPR and excitation wavelengths is required to achieve maximum SERS performance in order to take full benefits of electromagnetic enhancement of SERS signals. This works demonstrate a systematic investigation for improving plasmonic enhancement of SERS in terms of hotspot engineering and LSPR tailoring of plasmonic nanostructure by using solid-state dewetting.
First, the strong correlation between LSPR and SERS signals experimentally reveals by tuning LSPR with novel deformable nanoplasmonic membrane. Modified solid-state dewetting, i.e., solid immersion metal film dewetting (SIMD), enables the successful fabrication of silver nanoislands with size distribution on a thin circular elastomeric membrane. This unique step facilitates not only thermal dewetting of thin silver film at low temperature but also the stable fixation of silver nanoislands on the PDMS, which allows easy transfer of silver nanoislands onto a membrane. A thin circular elastomeric membrane provides continuous, monotonic, uniform, and large tuning of LSPR with 1 nm in tuning resolution. The correlation visibly indicates the individual SERS peaks have the maximum gains at the maximum products of extinction values at an excitation and the corresponding Raman scattering wavelengths. In addition, relative SERS peak heights substantially vary depending on LSPR. This observation provides a guideline of plasmonic enhancement in not only additionally increasing SERS signals but also constructing SERS substrates.
Second, engineering of hotspot for SERS was systematically investigated by using repeated solid-state dewetting of thin gold film. Conventional solid-state dewetting is very simple and facile but unmanageable for simultaneously controlling both the size and gap of nanoislands for the precise control of LSPR. Very distinct from a conventional dewetting process, repeated solid-state dewetting can easily control both the size and gap of gold nanoislands. The results clearly exhibit that the nanoisland size increases by more than twice but the gap spacing relatively remains constant after the repeated dewetting with double film thickness for different film thicknesses. In addition, based on the comparison between a single-step and repeated solid-state dewetting, the effective diameter increases but the gap spacing remain constant as the total Au film thickness increases and therefore it concludes that the repeated solid-state dewetting enables the fabrication of small gap spacing Au nanoislands. Consequently, enlarged gold nanoislands with small gap spacing increase the number of electromagnetic hotspots and thus enhance the extinction intensity as well as the tunability for LSPR. The nanoislands from repeated dewetting substantially increase SERS enhancement factor over one order-of-magnitude higher than those from a single-step dewetting process and they allows ultrasensitive SERS detection of a neurotransmitter with extremely low Raman activity.
Lastly, wide-range tailoring plasmon resonance with straightforward approach was demonstrated with AuAg alloyed nanoislands. Successive thin films evaporation and thermal annealing enable fabrication of AuAg alloyed nanoislands with well-defined sizes and shapes. The complete miscibility of Au and Ag leads to programmable tuning of the plasmon resonance over the entire visible wavelength region by varying corresponding film thickness. Such wide-range tuning capability of AuAg alloyed nanoislands is expected to be excellent candidates for SERS-based sensing and imaging applications.
In conclusion, this works demonstrate a systematic investigation for improving plasmonic enhancement of SERS in terms of hotspot engineering and LSPR tailoring of plasmonic nanostructure by using solid-state dewetting. Continuous deformation of nanoplasmonic membrane from solid immersion metal film dewetting provides precise tuning of the LSPR wavelength, which enables revealing the correlation between LSPR and SERS signals. New approach by using repeated solid-state dewetting enables tuning of the LSPR wavelength as well as engineering of hotspots. In addition, gold-silver alloyed nanoislands was fabricated as means of the LSPR tailoring. The combination of gold and silver enables tuning of the LSPR wavelength over the entire visible wavelength region. Based on the observation for relationship between the LSPR wavelength and SERS, hotspot engineering and LSPR tailoring will provide a clear guideline for taking full benefits of plasmonic enhancement in not only additionally increasing SERS signals but also constructing novel plasmonic nanoparticles or nanoplasmonic substrates for highly efficient SERS based biosensing and bioimaging at a single molecule level.