Probing the Effect of Molecular Orientation on the Intensity of Chemical Enhancement Using Graphene‐Enhanced Raman Spectroscopy

Abstract

A rational approach to investigate the effect of molecular orientation on the intensity of chemical enhancement using graphene-enhanced Raman spectroscopy (GERS) is developed. A planar molecule, copper phthalocyanine (CuPc), is used as probe molecule. Annealing allows the CuPc molecule in a Langmuir-Blodgett film to change orientation from upstanding to lying down. The UV-visible absorption spectra prove the change of the molecular orientation, as well as the variation of the interaction between the CuPc molecule and graphene. The Raman spectra of the molecule in the two different orientations are compared and analyzed. The results show that chemical enhancement is highly sensitive to the molecular orientation. The different molecular orientations induce different magnitudes of the interaction between the molecule and graphene. The stronger the interaction, the more the Raman signal is enhanced. Furthermore, the sensitivity of GERS to molecular orientation is promising to determine the orientation of the molecule on graphene. Based on this molecular orientation sensitive Raman enhancement, quantitative calculation of the magnitude of the chemical enhancement is implemented for a series of Pc derivatives. It shows that the magnitude of the chemical enhancement can be used to evaluate the degree of interaction between the molecules and graphene. Moreover, an understanding of the chemical enhancement in GERS is promoted and the effect of molecular orientation on the intensity of chemical enhancement is explained.

Direct measurement of the Raman enhancement factor of rhodamine 6G on graphene under resonant excitation

“…2014, 7(9): 1271-1279 molecule orientation [3], electronic energy levels [4][5], graphene thickness [6], and the incident conditions [7], were then implemented, and the enhancement mechanism was attributed solely to a chemical mechanism (CM). However, one of the most commonly used Raman probes, rhodamine 6G (R6G), yielded confusing results about the Raman enhancement effect on grapheme [1,8].…”

Tuning surface-enhanced Raman scattering from graphene substrates using the electric field effect and chemical doping

“…[11][12][13] Various experimental studies have been carried out to unravel the SERS enhancement mechanism of graphene: Ling et al has shown that the enhancement depends on the orientation of the molecules in its "first-layer" vicinity; 14,15 Xu et al has further shown that SERS enhancement of graphene can be modulated by tuning its Fermi level with a graphene field-effect transistor (GFET) device. 16,17 These studies suggest that SERS enhancement of graphene is a CE effect.…”

“…5 In SERS, it is widely believed that there are two contributions to its enhancement: an electromagnetic mechanism (EM) through intense enhancement of the localized electromagnetic fields around metallic nanostructures [6][7][8][9][10] and a chemical mechanism (CE) through a combination of metal-molecule chemical interactions. [11][12][13] Various experimental studies have been carried out to unravel the SERS enhancement mechanism of graphene: Ling et al has shown that the enhancement depends on the orientation of the molecules in its "first-layer" vicinity; 14,15 Xu et al has further shown that SERS enhancement of graphene can be modulated by tuning its Fermi level with a graphene field-effect transistor (GFET) device. 16,17 These studies suggest that SERS enhancement of graphene is a CE effect.…”

A Review on Applications of Two-Dimensional Materials in Surface-Enhanced Raman Spectroscopy

“…For example, CuPc molecule will be in lying-down position after annealing, which results in a molecular orientation that isoindole group is in parallel with graphene surface. 1530 cm −1 mode, which represents symmetric stretching of isoindole groups of CuPc molecules, has highest enhancement than other vibrational modes of CuPc after annealing [39].…”

“…The graphene electrons involvement in the Raman scattered process can enhance the electron-phonon coupling and thus induce the enhancement of the Raman signals. When a vibrational mode involves the lone pair or electrons, which has stronger coupling with graphene [20,39], the vibrational mode with particular molecular orientations could have highest Raman enhancement, following the SERS selection rules. For example, CuPc molecule will be in lying-down position after annealing, which results in a molecular orientation that isoindole group is in parallel with graphene surface.…”