Nanoplasmonics and surface enhanced spectroscopy

Investigation of surface-enhanced Raman scattering on gold nanoislands substrates with different geometrical parameters and analytes
P. Pál1, A. Bonyár2, M. Veres3, L. Juhász4, I. Csarnovics1 
1) Department of Experimental Physics, Institute of Physics, Faculty of Science and Technology, University of Debrecen, Debrecen, Hungary
E-mail: csarnovics.istvan@science.unideb.hu
2) Department of Electronics Technology, Budapest University of Technology and Economics, Budapest, Hungary
3) Institute for Solid State Physics and Optics, Wigner Research Centre for Physics, Budapest, Hungary
4) Department of Solid State Physics, Institute of Physics, Faculty of Science and Technology, University of Debrecen, Debrecen, Hungary

In this work, the process of surface-enhanced Raman scattering (SERS) was studied on gold and silver nanoisland arrangements. Systematic investigations were carried out on different gold and silver nanoisland samples to determine how SERS enhancement depends on the morphology (particle size, gap distance) and optical parameters (plasmon wavelength) of the nanostructures. 
For this purpose, the metallic nanostructures were created by annealing thin film layers of gold and silver deposited by vacuum thermal evaporation onto glass substrates. The heat treatment (solid-state dewetting) took place at different temperatures (400 °C, 450 °C, 500 °C, and 550 °C) for different periods (15, 30, 60 and 120 minutes), resulting in a variety of geometrical parameters (e.g. particle diameter, interparticle distance). The parameters of the created nanoislands, such as equivalent particle diameter, gap distance, and plasmon wavelength on the SERS enhancement efficiency were studied. The SERS capabilities of the samples were evaluated by measuring an analyte solution of benzophenone, riboflavin, and rhodamine with three different excitation laser wavelengths (473, 532, and 633 nm). Scanning electron microscopy was used to measure the average particle size and average interparticle distance to correlate them with the obtained SERS enhancements. The influence of the geometric parameters of metallic nanoparticles and the laser wavelength on the surface-enhanced Raman process was evaluated. Furthermore, we have examined the effect of the analyte on the process of Surface-Enhanced Raman Scattering (SERS). The enhancement factor was obtained for the analytes, and the influence of the analytes was studied as well. SERS EF showed a positive linear correlation with nanoparticle size and a negative exponential relationship with interparticle distance for both types of nanomaterials. This is due to the higher scattering efficiency (larger particle size) and the higher density of near-field hot spots on tightly packed particles. Besides it, the results have shown the result of analytes on their plasmon peak position and so on the SERS EF as well. 

Acknowledgments
This work was financially supported by the grant GINOP-2.3.2-15-2016-00041. The projects are co-financed by the European Union and the European Regional Development Fund. Istvan Csarnovics is grateful for the support of the János Bolyai Research Scholarship of the Hungarian Academy of Sciences (BO/348/20) and the support through the New National Excellence Program of the Ministry of Human Capacities (ÚNKP-20-5-DE-107). The research reported in this paper and partially carried out at the Budapest University of Technology and Economics has been supported by the National Research Development and Innovation Fund (TKP2020) based on the charter of bolster issued by the National Research Development and Innovation Office under the auspices of the Ministry for Innovation and Technology.