Edge Modification and Site-Selective Functionalization of Graphene Quantum Dots: A Versatile Technique for Designing Tunable Optoelectronic and Sensing Devices.

Abstract

This work successfully traces the imprints of zigzag/armchair-edge modification and site-selective functionalization by different chemical species in dictating the structural, electronic, and optical properties of low-symmetry structural isomers of graphene quantum dots (GQDs). Our time-dependent density functional theory-based computations reveal that the electronic band gap reduction is greater for zigzag-edge functionalization than for armchair-edge modification by chlorine atoms. The computed optical absorption profile of functionalized GQDs exhibits an overall red shift with respect to their pristine counterpart, with the shift being more pronounced at higher energies. Markedly, the optical gap energy is regulated more substantially by zigzag-edge chlorine passivation while the armchair-edge chlorine functionalization is more effective in altering the position of the most intense (MI) absorption peak. The MI peak energy is exclusively decided by the significant perturbation in the electron-hole distribution produced by the structural warping of the planar carbon backbone realized by edge-functionalization while the interplay between frontier orbital hybridization and structural distortion governs the energies of the optical gap. In particular, the enhanced tunability range of the MI peak in comparison to the optical gap variation signals that the structural warping plays a more decisive role in modulating the MI peak characteristics. The energy of the optical gap and the MI peak along with the charge-transfer character of the excited states is critically dependent on the electron-withdrawing capability and the site of the functional group. This comprehensive study is extremely crucial for promoting the application of functionalized GQDs in designing highly efficient tunable optoelectronic devices.