Computational study of linear optical properties in diamond-shaped graphene quantum dots
Tushima Basak1 and Tista Basak2
1Department of Physics, Mithibai College, Mumbai-56, India
2Mukesh Patel School of Technology Management & Engineering, NMIMS University, Mumbai-56, India
Abstract
The unique electronic, mechanical and thermal properties of two-dimensional graphene have significant application potential in the field of nanoelectronics [1]. However, the zero band-gap of pure graphene confines its utilization in the field of optoelectronics. The creation of a finite band-gap by reducing the dimensionality of graphene to one-dimensional graphene nanoribbons (GNRs) and zero-dimensional graphene quantum dots (GQDs) overcomes this limitation. Recent experimental and theoretical studies [2-4] have demonstrated that the optical properties of graphene nanostructures can be tuned by altering its size, shape and edge structure. This has motivated us to explore the electronic structure and linear optical absorption of diamond-shaped graphene quantum dots (DQDs) of varying sizes, consisting of 16 carbon atoms (DQD-16), 30 carbon atoms (DQD-30) and 48 carbon atoms (DQD-48), respectively.
In this work, we employ the π-electron Pariser-Parr-Pople (PPP) model Hamiltonian along with configuration interaction (CI) methodology to determine the influence of electron correlation effects on the ground and excited states of DQDs. From the results, it is evident that the atoms occupying the projected corners of zigzag edge contribute more to the charge density as compared to the other atoms in these systems. Hence, the optical properties of these systems can be suitably tuned by attaching appropriate functional groups at these sites. Our computed linear absorption spectrum at the PPP-CI level for DQD-16 and DQD-30 [5] are in excellent agreement with the experimental data [6-8] of the hydrogen saturated counterparts of DQD-16 and DQD-30 i.e., pyrene and dibenzo[bc,kl] coronene, respectively, in terms of peak position and symmetry assignments. Our computations have indicated that the first peak of the optical spectra representing the optical gap is not the most intense peak, in accordance with the experimental data. However, this result is in striking contrast to the predictions from tight- binding model, signifying the substantial importance of electron correlation effects in determining the optical properties of DQDs. In addition, it is observed that the absorption spectrum is red-shifted with increasing size of the quantum dots. The authors gratefully acknowledge the financial support received from DST-SERB (Grant No. ECR/2016/000793) for conducting this work.
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