Linear and Nonlinear Optical Properties of Graphene Quantum Dots: A Computational Study

Abstract

Because of the advantages of tunability via size, shape, doping, and relatively low level of loss and high extent of spatial confinement, graphene quantum dots (GQDs) are emerging as an effective way to control light by molecular engineering. The collective excitation in GQDs shows high energy plasmon frequency along with frequencies in the terahertz (THz) region, making these systems powerful materials for photonic technologies. Here, we report a systematic study of the linear and nonlinear optical properties of large varieties of GQDs (∼400 systems) in size and topology utilizing the strengths of both semiempirical and firstprinciples methods. Our detailed study shows how the spectral shift and trends in the optical nonlinearity of GQDs depend on their structure, size, and shape. Among the circular, triangular, stripe, and random shaped GQDs, we find that GQDs with inequivalent sublattice atoms always possess lower HOMO−LUMO gap, broadband absorption, and high nonlinear optical coefficients. Also, we find that a majority of the GQDs with interesting linear and nonlinear optical properties have zigzag edges, although the reverse is not always true. We strongly believe that our findings can act as guidelines to design GQDs in optical parametric oscillators, higher harmonic generators, and optical modulators.

Optical properties of graphene nanoflakes: Shape matters

“…In Figure 6 (a) the correlation between the gap energy and the inverse square root of the number of atoms is depicted. The general observation is a decrease in the gap energy with increasing the size due to quantum confinement 71 , also found in previous calculations 22,42,72 . This explains the red-shift in the absorption spectra.…”

“…The electronic structure of the system was described using the self-consistent charge density functional tight binding (SCC-DFTB) method, which is based on a second-order expansion of the Kohn-Sham energy around a reference density of the neutral atomic species 54 . This method has been extensively used for the study of graphene and graphene-based structures 26,31,38,42,53,55 and has been benchmarked with respect to DFT for several graphene based systems with defects, with bond length discrepancies around 2% and formation energies matching DFT values within 1.5% 56 . We have used the DFTB+ implementation of DFTB 57 , with the mio-1-1 set of parameters 58 , to obtain the hamiltonian, overlap matrix and the initial ground state (GS) single-electron density matrix.…”

“…The quantum confinement caused by reduced dimensionality of these structures opens a tunable bandgap 7,17 . Several studies have characterized the electronic properties of GNF 5,18-39 and some have shed light on their optical features [20][21][22][23][24]27,33,34,[40][41][42][43][44] . Yamijala et al 42 performed a systematic study of the linear and nonlinear optical properties for ∼400 graphene quantum dots at the ZINDO/S semiempirical level.…”

Tunable optical properties of OH-functionalised graphene quantum dots

“…18,[23][24][25][26][27][28][29][30][31] Recently, Yamijala et al studied the nonlinear optical properties of G-QDs and found that QDs with zigzag edges show interesting optical properties. 32 The tunability of the emission of QDs has been recently analysed considering the effect of different functional groups. 18 Herein, we evaluate the effect of the OH functionalisation on the ionisation potentials, absorption and emission properties of GO-QD zigzag edge models.…”

Tuning the Optically Bright and Dark States of Doped Graphene Quantum Dots

“…In our earlier work, we studied in details the edge state transition and how ground state spin multiplicity plays a crucial roles. 34 Main point is that for all shaped GQDs, with different edges the strength of the coherent peak increases with the system size, with lowering of transition energy (also see S.I.). Fig.1(b) shows the difference between fundamental nature of the coherent peak and the peak generated from the edge states transition as the oscillator strengths of the edge states remain more or less unchanged with increase of the size of the system, while the oscillator strengths of the coherent peaks increase steadily with the system size due to more and more positive in-phase combination of the transition dipoles of the de-localized states (also see S.I.…”