Study of Chemical Enhancement Mechanism in Non-plasmonic Surface Enhanced Raman Spectroscopy (SERS)

Abstract

Surface enhanced Raman spectroscopy (SERS) has been intensively investigated during the past decades for its enormous electromagnetic field enhancement near the nanoscale metallic surfaces. Chemical enhancement of SERS, however, remains rather elusive despite intensive research efforts, mainly due to the relatively complex enhancing factors and inconsistent experimental results. To study details of chemical enhancement mechanism, we prepared various low dimensional semiconductor substrates such as ZnO and GaN that were fabricated via metal organic chemical vapor deposition process. We used three kinds of molecules (4-MPY, 4-MBA, 4-ATP) as analytes to measure SERS spectra under non-plasmonic conditions to understand charge transfer mechanisms between a substrate and analyte molecules leading to chemical enhancement. We observed that there is a preferential route for charge transfer responsible for chemical enhancement, that is, there exists a dominant enhancement process in non-plasmonic SERS. To further confirm our idea of charge transfer mechanism, we used a combination of 2-dimensional transition metal dichalcogenide substrates and analyte molecules. We also observed significant enhancement of Raman signal from molecules adsorbed on 2-dimensional transition metal dichalcogenide surface that is completely consistent with our previous results. We also discuss crucial factors for increasing enhancement factors for chemical enhancement without involving plasmonic resonance.

Multilayer Gold-Silver Bimetallic Nanostructures to Enhance SERS Detection of Drugs

“…Despite Au@AgNRs and, particularly, AuNRs presenting a higher SPR intensity at 633 nm (wavelength used as Raman source), their SERS enhancement is lower, suggesting that the local field enhancement cannot account for the determined AEF values. The establishment of chemical interactions between Au@Ag@AuNRs and portions of Rh6G molecule might have a charge transfer nature, able to affect the bond polarizability; as previously reported, nanoparticle-adsorbate charge transfer interactions boost SERS enhancement [37]. The higher performances detected for Au@Ag@AuNRs can also be due to the higher electron density on these samples' surface.…”

Vertically-Oriented WS2 Nanosheets with a Few Layers and Its Raman Enhancements

“…In our case, with visible (2.33 eV) excitation, charge transfer can only occur from the valance band (VB) of WS 2 to the lowest unoccupied molecular orbital (LUMO) level of R6G. The largest Raman enhancement thus occurs when the energy difference between VB and LUMO matches the excitation energy of the incident laser [ 31 , 32 ]. The samples grown at different temperatures have different thicknesses and different impurity contents, leading to different positions of each energy level.…”

Review of Integrated Optical Biosensors for Point-of-Care Applications

“…The chemical mechanism depends not only on the substrate but also analyte molecules. It involves several different transitions/processes, which charge transfer between the energy levels of analyte molecules, with the fermi level of the substrate metal surface playing a critical role [ 41 ].…”