Revealing the trap emission in graphene-based nanostructures

Enhancing the photoconversion efficiency of graphene-based nanomaterials, especially graphene quantum dots (GQDs)and graphene nanoribbons (GNRs), has been a long pursued interest in diverse optical applications. Though chemical modification has shown its effectiveness to enhance the photoluminescence efficiencies of these materials, an accurate, comprehensive fundamental understanding of this enhancement is demanded for further development. Herein, density functional theory (DFT)and time-dependent DFT (TD-DFT)calculations reveal that chemical species on the graphene nanostructure surface perturb the intrinsic electronic structures of the graphene moiety, with the perturbation manner and extent crucially depending on the binding positions of the species. In several configurations studied, the surface species lead to significantly enlarged oscillator strengths for the lowest optical transition (S₁→S₀)and localized excitons (bounded electron and hole)compared to those of the pristine nanographene structures, indicative of their role in enhancing the photoluminescence as emission centers. By contrast, the surface species in the other configurations show trivial influence on the emission efficiency, as indicated by the only slightly altered electronic/excitonic structures and oscillator strengths. These results provide a fundamental understanding of the trap emission mechanism and also the tuning/engineering of photoluminescence efficiencies in relevant materials.