Adsorption and switching of single azobenzene molecules at silver nanohemisphere facets studied by Surface-enhanced Raman Scattering
Abstract
The promise of molecular machines to control mechanical and electronic components at atomic level has brought molecular switches to the spotlight in the last decade. Among many switching molecules, azobenzene (AB) is one of the most extensively studied owing to its chemical simplicity, and its efficient and reversible photoswitching between a trans (t-AB) and a cis (c-AB) form. Immobilization of AB on surfaces is needed for the precise design and control of molecular devices. However, preserving the switching of surface adsorbed AB has been a challenge. Hence, understanding the governing factors of the conformational dynamics of surface adsorbed AB has been a major interest. With the objective of understanding how AB interacts with metal surfaces, the present work studies the adsorption and switching of AB on a metal surface (i.e., Ag facets) at single molecule (SM) level by Surface-enhanced Raman Scattering (SERS). In the present work, the SM-SERS measurements reveal three distinct species of both t-AB and c-AB on the surface of plasmonic Ag nanohemispheres (NHSs). With the help of DFT calculations, the observed SM-SERS jumps are assigned to surface adsorbates of t-AB and c-AB on three facets of different crystal orientation (i.e., (111), (100) and (110)) available on Ag NHSs. The experimental and computational results indicate that adsorption mechanism and the resulting adsorbate geometry of AB varies significantly on surfaces of different crystal orientation. The reversible photoswitching of AB molecules is also observed at single molecule level. The SM-SERS spectra exhibiting switching of AB indicate that surface decoupling of AB is required for switching of AB. The Ag-on-Ge SERS substrates which provide SM sensitivity in the present study are also investigated. As experimentally demonstrated and computationally elucidated, NHSs exhibit remarkably different plasmonic characteristics as compared to nanospheres. The by asymmetric particle shape leads to dipolar modes parallel (in-plane) and normal to the base (out-of-plane), which are significantly different in terms of energy, excitation dependence on polarization, and electromagnetic coupling. The unique characteristics discovered in the present study point to potential applications of Ag-on-Ge substrates in various plasmonic applications, including photovoltaics, photocatalysis, UV plasmonics, and chemical detection (i.e., LSPR and SERS).
Collections
- OSU Dissertations [11222]