T. Rindzevicius, M.S. Schmidt, K. Wu, A.H. Thilsted, R.K. Lauridsen, T.S. Alstrøm, M. Palla, J. Yang, A. Boisen
Technical University of Denmark,
Keywords: SERS, plasmonic nanopillars, Si RIE, SERS detection, SERS substrate
Summary:Nanosized noble metal structures support localized surface plasmon (LSPR) resonances that depend on their dimensions, shapes and compositions. Particle LSPR’s can be used to spatially confine the incident light and produce enormous electromagnetic field enhancement spots that are often referred to as “hot spots” . This property has been widely utilized in surface-enhanced Raman scattering (SERS) spectroscopy for chemical and biological sensing enabling molecular detection down to a single molecule limit . A desirable SERS substrate should display (i) high (>107) and reproducible enhancement factors (EF), and (ii) nanofabrication process should be simple, fast and cost-effective. Leaning Au or Ag coated Si nanopillar (NP) structures are good candidates to fulfill the aforementioned requirements . First, the nanofabrication process is simple, fast and can be employed to fabricate SERS substrate on 4” or 6” Si substrates. Second, it does not involve lithography steps which reduces substantially fabrication costs. The process is essentially based on two nanofabrication steps: (i) Si mask-less reactive ion etching (RIE) and (ii) metal deposition. The Si NP density and pillar height can be tuned via Si RIE process. The optical properties of Ag-coated Si NP structures were analyzed using FEM simulations and dark-field (DF) scattering microscopy . The DF scattering results show that the SERS substrate exhibits a broad LSPR line shape centered around 800 nm. Theoretical simulations show that single Ag-coated NPs supports two LSPR modes, i.e. the particle mode and the Ag cap cavity mode. The most prominent cavity mode is located in the near-infrared spectral region. The LSPR peak position can be adjusted by tuning the diameter of the Si NP. The electric field distribution calculations show that the highest E-field enhancement is located at the bottom of the Ag cap which is important for practical SERS-based sensing. We illustrate that Ag or Au coated Si NP SERS substrates can be used for molecular detection at ultra-low concentrations. Reproducible and repeatable SERS signal intensities can be obtained across large surface areas (>cm2). SERS signals from low concentration (10nm-100pM) of trans-1,2-bis(4-pyridyl) ethylene (BPE) can be obtained using Ag coated Si NP structures. Other prominent examples are detection of TAMRA-labeled vasopressin and HCN which are important markers that signal hemorrhagic shock and cystic fibrosis, respectively. Finally, we demonstrate that the fabrication approach can be extended to optically transparent materials such as fused silica. By tuning the plasma etching parameters it is possible to obtain nanograss and nanocylinder structures that can be utilized for both LSPR and SERS-based molecular sensing. References:  Xu, H.; Aizpurua, J.; Käll, M.; Apell, P. Phys. Rev. E 2000, 62, 4318−4324.  Le Ru, E. C.; Etchegoin, P. G. Annu. Rev. Phys. Chem. 2012, 63, 65−87.  Wu, K.; Rindzevicius, T.; Schmidt, M. S.; Mogensen, K. B.; Hakonen, A.; Boisen, A. J. Phys. Chem. C 2015, 119, 2053-2062.  Wu, K.; Rindzevicius, T.; Schmidt, M. S.; Mogensen, K. B.; Xiao, S.; Boisen, A. OPTICS EXPRESS, 2015, 23(10), 12965-12978.