Abstract
In this work, the electronic and optical properties of hybrid boron-nitrogen-carbon structures (h-BNCs) with embedded graphene nanodisks are investigated. Their molecular affinity is explored using pyridine as model system and comparing the results with the corresponding isolated graphene nanodisks. Time-dependent density functional theory (TDDFT) analysis of the electronic excited states was performed in the complexes in order to characterize possible surface and charge transfer resonances in the UV region. Static and dynamic (hyper)polarizabilities were calculated with coupled-perturbed Kohn-Sham theory (CPKS) and the linear and nonlinear optical responses of the complexes were analyzed in detail using laser excitation wavelengths available for (Hyper)Raman experiments and near-to-resonance excitation wavelengths. Enhancement factors around 103 and 108 were found for the polarizability and first order hyperpolarizability, respectively. The quantum chemical simulations performed in this work point out that nanographenes embedded within hybrid h-BNC structures may serve as good platforms for enhancing the (Hyper)Raman activity of organic molecules immobilized on their surfaces and for being employed as substrates in surface enhanced (Hyper)Raman scattering (SERS and SEHRS). Besides the better selectivity and improved signal-to-noise ratio of pristine graphene with respect to metallic surfaces, the confinement of the optical response in these hybrid h-BNC systems leads to strong localized surface resonances in the UV region. Matching these resonances with laser excitation wavelengths would solve the problem of the small enhancement factors reported in Raman experiments using pristine graphene. This may be achieved by tuning the size/shape of the embedded nanographene structure.
Highlights
Even though a chemical mechanism based on charge transfer excitations could yield larger enhancement factors (EF), it is less likely that these factors might reach the Raman enhancements obtained with metallic nanoparticles, which were reported to be larger than ten orders of magnitude [15]
Among the electronic features relevant for this work, we are mainly interested in the electron delocalization patterns and electric polarizabilities
The results obtained for the six-center delocalization indices of the nonequivalent benzenoid rings in the hybrid boron-nitrogen-carbon structures (h-BNCs) studied are incorporated to Figure 1
Summary
Since the discovery of the Surface Enhanced Raman Scattering (SERS) activity of extended graphene sheets [1], known as Graphene Enhanced Raman Scattering (GERS) phenomena [2,3,4], experimental and theoretical efforts have been devoted to elucidate the possible enhancement mechanisms [5,6,7,8,9,10,11,12,13].It is well-known that the closeness of the energy gap in extended graphene structures makes the optical activity and plasmon emission to be located within the THz region [14], ruling out the possibility of electromagnetic enhancement using UV laser sources. Since the discovery of the Surface Enhanced Raman Scattering (SERS) activity of extended graphene sheets [1], known as Graphene Enhanced Raman Scattering (GERS) phenomena [2,3,4], experimental and theoretical efforts have been devoted to elucidate the possible enhancement mechanisms [5,6,7,8,9,10,11,12,13]. The experiments confirmed the small enhancement factor [2,3,4], reaching two orders of magnitude at most, and its origin in charge transfer (CT) transitions between the surface and the adsorbed molecule [1,6,10,12]. Even though a chemical mechanism based on charge transfer excitations could yield larger enhancement factors (EF), it is less likely that these factors might reach the Raman enhancements obtained with metallic nanoparticles, which were reported to be larger than ten orders of magnitude [15].
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