Properties of Bilayer Graphene Quantum Dots for Integrated Optics: An Ab Initio Study

Abstract

Due to their bandgap engineering capabilities for optoelectronics applications, the study of nano-graphene has been a topic of interest to researchers in recent years. Using a first-principles study based on density functional theory (DFT) and thermal DFT, we investigated the electronic structures and optical properties of bilayer graphene quantum dots (GQDs). The dielectric tensors, absorption spectra, and the refractive indexes of the bilayer GQDs were obtained for both in-plane and out-of-plane polarization. In addition, we calculated the absorption spectra via time-dependent DFT (TD-DFT) in the linear response regime. The TDDFT results show that a blue shift occurs in the absorption spectrum, which is consistent with the experimental results. In this investigation, we consider triangular and hexagonal GQDs of various sizes with zigzag and armchair edges. Our simulations show that unlike monolayer GQDs, for which light absorption for out-of-plane polarization occurs in the ultraviolet wavelength range of 85–250 nm, the out-of-plane polarization light absorption peaks in the bilayer GQDs appear in the near-infrared range of 500–1600 nm, similar to those in bilayer graphene sheets. The out-of-plane polarization light absorption peaks in the near-infrared range make bilayer GQDs suitable for integrated optics and optical communication applications.

Tuning the properties of graphene quantum dots by passivation

“…This results in reducing the bandgap of all the three GQDs when their surface is passivated with electron-withdrawing substituents 33 which has also been confirmed experimentally. 57 Passivation with an electronegative group element lowers the LUMO energy 28,65 and hence the observed band-gap reduction resulting in narrowing of the bandgap which is evidenced from the energy band diagram as shown in Fig. 3.…”

“…The band gaps of monolayer and bilayer GQDs were found to lie in the range 1.2 to 2.8 eV and 0.9 to 3.0 eV, respectively. 28 The optical absorption spectrum of monolayer GQDs for the parallel polarization of the incident light has been reported to shift from visible to IR as the atoms in GQDs are increased. However, when their size was increased for perpendicular polarisation, the absorption spectra migrated from UV to a deep UV region of 85-250 nm.…”

Finite Temperature Ultraviolet-Visible Dielectric Functions of Tantalum Pentoxide: A Combined Spectroscopic Ellipsometry and First-Principles Study

“…Prior to the dielectric properties calculations, the structural optimization was performed, where the energy cutoff was set as 520 eV and the maximum forces acting on each atom were smaller than 10 −6 eV/Å. With the relaxed lattice structure, the ground-state electronic energy state and transition dipole moment were calculated to predict the dielectric function [37][38][39][40]. With the conventional DFT, the "frozen" atom was adopted and all the properties were predicted at the absolute 0 K. To explore the temperature effect, we applied the first-principles molecular dynamics simulations to introduce lattice vibration effect and to collect the thermally equilibrated configurations at finite temperatures.…”

Engineering functionalization and properties of graphene quantum dots (GQDs) with controllable synthesis for energy and display applications

“…57 Theoretically, single-layer GQDs exhibit band gaps in the range of 1.2 to 2.8 eV, while double-layer GQDs display band gaps between 0.9 and 3.0 eV. 58 The band gap of GQDs is influenced by several factors, which can be summarized as follows. First, as the lateral size of GQDs decreases, the band gap increases.…”