An Investigation of Enhanced Raman Scattering on 2D Material Substrates
Open Access
- Author:
- Anil, Vivek
- Area of Honors:
- Interdisciplinary in Engineering Science and Physics
- Degree:
- Bachelor of Science
- Document Type:
- Thesis
- Thesis Supervisors:
- Mauricio Terrones, Thesis Supervisor
Reginald Felix Hamilton, Thesis Honors Advisor
Richard Wallace Robinett, Thesis Honors Advisor - Keywords:
- graphene
WS2
Raman spectroscopy
Graphene enhanced Raman spectroscopy (GERS)
Surface enhanced Raman spectroscopy (SERS)
fluorescent dyes - Abstract:
- Non-technical: Graphene has demonstrated the ability to enhance Raman signals from low concentrations of adsorbed molecules; this phenomenon is known as graphene enhanced Raman scattering (GERS). More recently, other 2D materials, such as MoS2, have also demonstrated this Raman-enhancing ability. Compared to other similar platforms, such as surface enhanced Raman spectroscopy (SERS) which makes use of metallic nanoparticles, 2D materials are more biocompatible, and so the Raman-enhancing effect on 2D materials offers promise for biosensing applications. While this phenomenon has been demonstrated on 2D materials, questions remain about the mechanism of enhanced Raman scattering. This study seeks to provide experimental data that could be used to assess a proposed mechanism for enhanced Raman scattering on 2D materials based on a charge transfer between the 2D substrate and the adsorbed molecule. Further, this charge transfer mechanism is strengthened by aligning an energy level in the substrate with an energy level in the molecule and using a laser with energy approximately equal to the energy gap in the molecules. We synthesize 2D material substrates, graphene and WS2, and prepare fluorescent dyes and collect Raman spectra of these dyes adsorbed on the substrates with different lasers. We compare the ability of each substrate to probe vibrational modes of each of the dyes. Our results give support for the proposed mechanism not only in graphene, but also in WS2. Technical: Previous research asserts that the observation of enhanced Raman signals from molecules adsorbed on graphene and doped graphene substrates are due to charge transfer between graphene and the molecules. It is further proposed that this charge transfer is strengthened by the alignment of the molecule’s LUMO with graphene’s Fermi energy and an excitation nearly resonant with the HOMO-LUMO gap of the molecule. More recently, other 2D materials have similarly shown the ability to enhance Raman signals from adsorbed analytes. This research seeks to test the aforementioned proposed charge transfer mechanism by collecting Raman spectra of fluorescent dye solutions, 10-5 M crystal violet (CV), 10-5 M rhodamine 6G (R6G), and 10-5 M rhodamine B (RhB), on graphene and WS2 substrates at different laser excitations. Based on the charge transfer mechanism, we develop a hypothesis for which combinations of dyes, substrates, and lasers would allow us to observe the best Raman enhancement of the dyes’ vibrational modes. For CV, we observed a few vibrational modes on graphene and none on WS2, for R6G many modes on WS2 and some on graphene, and for RhB some modes on WS2 but fewer on graphene. In addition, the ability of the substrates to quench the fluorescence of the dyes was influenced by energy-level alignment. Our results were mostly in agreement with our hypothesis and therefore give support to the proposed mechanism in graphene as well as its extension to WS2.