Electron correlation effects and two-photon absorption in diamond-shaped graphene quantum dots

Abstract

In quasi-1D π-conjugated polymers such as trans-polyacetylene and polyenes, electron correlation effects determine the "reversed" excited state ordering in which the lowest two-photon 2A g state lies below the lowest one-photon 1B u state. In this work, we present conclusive theoretical evidence of reversed excited state ordering in fairly 2D π-conjugated systems, namely, diamond-shaped graphene quantum dots (DQDs). Our electron correlated calculations show that DQDs begin to exhibit reversed excited ordering with increasing size, in disagreement with independent-particle picture. This signals the onset of strong correlation effects which renders them nonluminescent.Further, we calculate and analyze the two-photon absorption (TPA) spectra as well as photoinduced absorption (PA) spectra of these systems and find excellent agreement with the available experimental results. Our investigations demonstrate that unlike a strictly 1D system like transpolyacetylene, the non-linear and excited state absorptions in DQDs are highly intricate, with several even parity states responsible for strong absorptions. Our results could play an important role in the design of graphene-based non-linear optical devices.

Graphene quantum dots with Stone-Wales defect as a topologically tunable platform for visible-light harvesting

“…1, with the parameters U and V ij representing the on-site and long-range Coulomb interactions, respectively. The one-electron hopping matrix elements t ij are restricted between nearest-neighbor carbon atoms i and j, with the value t 0 = -2.4 eV corresponding to uniform carbon-carbon bond-length r 0 = 1.4 Å, in accordance with our earlier calculations on conjugated polymers, polyaromatic hydrocarbons, and graphene quantum dots [18][19][20][21][22][23][24] . For the non-uniform bond-lengths, the values of corresponding t ij are determined from the exponential formula t ij = t 0 e (r 0 −r ij )/δ , extensively used by us earlier 25 , in which r ij is the distance between ith and jth carbon atoms (in Å), t 0 = -2.4 eV r 0 = 1.4 Å, and δ = 0.73 Å is a parameter depicting electron-phonon coupling.…”

“…where κ i,j is the dielectric constant of the system representing the screening effects, and R i,j is the distance between ith and jth carbon atoms (in Å). In the present set of computations, we have adopted the "screened parameters" 27 with U = 8.0 eV, κ i,j = 2.0 (i = j), and κ i,i = 1.0, consistent with our earlier works on π-conjugated systems and graphene quantum dots [18][19][20][21][22][23][24] .…”

Correlated Electronic States of a Few Polycyclic Aromatic Hydrocarbons: A Computational Study

“…30 Even the lowest two-photon gap compares well with their results from the standard parameters. 31 Triphenylene molecule having C 3 symmetry shows two two-photon levels below…”

“…They find that the effect of electron-electron correlation gets enhanced with the increase in the size of graphene quantum dots. 31 Motivated by this large number of experimental and theoretical data on PAHs, the current study focuses on the effect of strong electron-electron correlations on one-photon and two-photon absorptions in PAHs. So far, a thorough study of this class of molecules inclusive of strong long range electron-electron correlations has not been reported to the best of our knowledge.…”

“…They also studied the two-photon absorption and photoinduced absorption of these molecules. They found that the effect of electron–electron correlation increases with the increase in the size of graphene quantum dots . Motivated by this large number of experimental and theoretical data on PAHs, the current study focuses on the effect of strong electron–electron correlations on one-photon and two-photon absorptions in PAHs.…”