Electron correlation effects on non-linear optical properties of diamond-shaped graphene quantum dots
Tista Basak1 and Tushima Basak2
1Mukesh Patel School of Technology Management & Engineering, NMIMS University, Mumbai-56, India.
2Department of Physics, Mithibai College, Mumbai 400056, India.
Abstract
Extensive experimental [1] and theoretical [2] studies on carbon-based systems have demonstrated that electron correlations play a significant role in determining the one-photon allowed optical states. However, the one-photon and two-photon allowed optical states are distinct in centrosymmetric systems. The electron correlation effects are more pronounced in two-photon optical states which involve excitations of two electrons and two holes as compared to one-photon states which are mainly due to contribution of one electron and one hole. According to existing literature [3] on quasi-1D π-conjugated polymers such as trans- polyacetylene and polyenes, incorporation of electron correlation effects leads to "reversed" excited state ordering in which the lowest energy two-photon 2Ag state occurs below the first one-photon 1Bu state. This is in disagreement with the predictions of one-electron Hückel and mean-field Hartree-Fock theories. However, as the dimension of the system increases from 1D to 2D and thereafter 3D, "reversed" excited state ordering is not expected due to decreasing influence of electron correlations.
In this work, we have demonstrated the existence of "reversed" excited state ordering in fairly 2D π-conjugated systems, namely, diamond-shaped graphene quantum dots (DQDs) as their size increases. The systems considered are DQD-16, DQD-30 and DQD-48, consisting of 16, 30 and 48 carbon atoms, respectively. It is observed that at the PPP-CI level, the 2Ag state (3.68 eV) is slightly above the 1Bu state (3.60 eV) for DQD-16 while they are almost degenerate for DQD-30. However, for DQD-48, the 2Ag state (1.71 eV) occurs significantly below the 1Bustate (1.96 eV). This proves that the strength of electron correlation becomes dominant with increasing size of DQDs. This is in contradiction to the Hückel model results which always predicts the occurrence of 2Ag state above 1Bu state, for DQD-16, DQD-30 and DQD-48. Our computed photo-induced absorption (PA) as well as two-photon absorption (TPA) spectra at the configuration interaction (CI) level also reveal that these structures exhibit reversed excited state ordering with increasing size, in sharp contrast to independent-particle results. In addition, our results indicate that in contrast to strictly 1D system like trans-polyacetylene, several even parity states give rise to strong absorptions in the TPA and PA spectra of these 2D structures. Hence, DQDs exhibit more intricate optical properties as compared to 1D systems. We hope that our results will play a significant role in fabricating graphene-based nonlinear optical devices.
The authors gratefully acknowledge the financial support received from DST-SERB (Grant No. ECR/2016/000793) for conducting this work.
References:
1. F. Wang, G. Dukovic, L. E. Brus, and T. F. Heinz, Science 308, 838 (2005)
2. T. Basak, H. Chakraborty, and A. Shukla, Phys. Rev. B 92, 205404 (2015)
3. S. N. Dixit, D. Guo, and S. Mazumdar, Phys. Rev. B 43, 6781 (1991)