On the mechanism responsible of Raman enhancement on carbon allotropes surfaces: the role of molecule‐surface vibrational coupling in SERS

Abstract

The ability of different carbon allotropes surfaces as potential substrates for enhanced Raman spectroscopy is analysed theoretically and the factors responsible of the Raman-enhancing mechanism deeply scrutinised. Our analysis is based on the partition of the Raman tensor into molecule and surface terms, which leads to three different contributions to the Raman activity ('molecule', 'surface' and 'intermolecular'). Both static and pre-resonance conditions are considered in our analysis of the Raman spectra of pyridine adsorbed on model planar and curved surfaces and the three contributions to the Raman activity obtained separately. At static conditions, there is a general decrease in the Raman activity of vibrational modes associated to the molecule, proportional to the strength of the molecule-surface interaction. This stems from a reduction of the polarizability of pyridine upon its adsorption on the carbon surface. Under pre-resonance conditions, the surface contributes significantly to the Raman activity of the pyridine vibrational modes, even if the electronic transition involves exclusively energy levels from the surface. This is because of small vibrational couplings between molecular and surface modes which are negligible in metallic surface-like silver. It suggests also the possibility of finding similar effects in metallic surface built from lighter atoms like silicon or aluminium.

Potential Application of h-BNC Structures in SERS and SEHRS Spectroscopies: A Theoretical Perspective

“…These EFs may certainly lead to Raman and Hyper-Raman enhancements on the line of electromagnetic enhancements produced by metallic nanoparticles. In fact, this result was theoretically verified, only for Raman scattering, in isolated carbon nanodisks [26,27]. Calculations of the Raman spectra reflected EFs around 10 8 using on-resonance conditions.…”

“…Thus, contrary to extended graphene sheets, these resonances do not correspond to charge transfer excitations but electronic excitations involving surface electrons. In the case of isolated graphene nanodisks and upon on-resonance conditions, surface electronic excitations were found to lead to strong Raman enhancements of adsorbed molecules such as pyridine, porphyrin or phthalocyanine [26,27]. However, the ground-to-excited state electron deformations indicate the strong resonances are not localized within the carbon nanodisk but within the outer carbon structure in C24BN and C54BN.…”

“…Recent theoretical works demonstrated that certain carbon nanostructures may display electronic transitions with plasmonic features in the UV range [24,25], suggesting the possibility of using large- and medium-size polycyclic aromatic hydrocarbons to enhance the Raman activity of smaller molecules attached to their surfaces. Such possibility was explored recently by some of us on different carbon allotropes, including graphene nanodisks, fullerenes and nanotubes, with time-dependent density functional theory (TDDFT) [26,27]. The following important conclusions were extracted from these works regarding the Raman enhancement mechanism by carbon surfaces: when approaching resonance, CT may give rise to enhancement factors comparable to resonance Raman mechanisms; CT occurs for large molecule-surface contacts, such as in stacking interactions; surface resonance effects in the carbon structure may lead to huge enhancement of the Raman activity of the adsorbed molecule favored by small molecule-surface vibrational couplings.…”

Recent advances in linear and non‐linear Raman spectroscopy. Part X

“…Under pre‐resonance conditions, the surface contributes significantly to the Raman activity of the pyridine vibrational modes, even if the electronic transition involves exclusively energy levels from the surface. This is because of small vibrational couplings between molecule and surface modes which are negligible in metallic surface‐like silver . Sloufova et al .…”

“…This is because of small vibrational couplings between molecule and surface modes which are negligible in metallic surface-like silver. [25] Sloufova et al have identified two Ag-2,2′:6′,2″-terpyridine (tpy) surface species on Ag nanoparticle surfaces by excitation wavelength dependence of SERS spectra and factor analysis which provides evidence for the chemical mechanism contribution to SERS of Ag(0)-tpy. [26] Applications of SERS Caglayan and co-workers have described two SERS-based assays for the sensitive detection of modafinil in urine matrix.…”