Exploring the Emissive States of Heteroatom-Doped Graphene Quantum Dots

Abstract

The photoluminescence (PL) emission states of heteroatom-doped graphene quantum dots (GQDs) remain unknown, particularly the assignment of the low-energy excitation band (more than 330 nm). To address these issues, this work synthesized three different types of GQDs: undoped GQDs (UGQDs), nitrogen-doped GQDs (NGQDs), and boron-doped GQDs (BGQDs), with similar sizes, chemical compositions (types and compositions of surface functional groups), and defects using a constant potential electrolysis method. The PL emissive states in these GQDs and the effects of the dopant heteroatom on the PL were revealed based on the combination of spectroscopic methods and theoretical calculations. The results indicated that the GQDs exhibit multiemissive centers for the PL emission mechanism. An excitation-independent PL emission band (band I) results from a highenergy transition originating from the quantum confinement of the carbon core (carbon π−π* transitions in sp 2 domain), and an excitation-dependent PL emission band (band II) originates from a low-energy edge band transition, which is attributed to radiative recombination associated with both the n−π* transition of N/O/B-containing groups and the π−π* charge transfer between the carbon core and the edge of the GQDs. Moreover, the PL emission maxima (both bands I and II) for NGQDs and BGQDs show a blue shift and a red shift, respectively, relative to UGQDs because of the doping that led to the alteration in the electronic structure and the distribution of molecular orbitals in the GQDs. These results clarify previous inconsistencies regarding the PL emission mechanism and the electronic properties of GQDs and can thus provide a foundation for the application of doped GQDs in electronics, photonics, and biology.

Gamma irradiation of graphene quantum dots with ethylenediamine: Antioxidant for ion sensing

“…Although a great scientific effort was invested in finding a reason for this GQDs property, there is still a lot of discussion in order to better elucidate the mechanism behind this phenomenon [50]. Different models explaining the PL behavior of GQDs coexist in the literature: size-dependent quantum confinement effects [53], surface trap states determined by functional groups [54], armchair vs. zigzag configuration of edges [55], the electronegativity of heteroatoms [56,57]. Each model explains multicolor, excitation-depended PL emission at some level.…”

Colloidal N-Doped Graphene Quantum Dots with Tailored Luminescent Downshifting and Detection of UVA Radiation with Enhanced Responsivity

“…Moreover, the enhancement in UV absorption intensity of the N-GQD samples with high dopant concentration (Figure 3a, inset) gives a notion to their use as an LDS material in photovoltaic applications. 27 However, compared to the N-GQD3 sample, N-GQD4 exhibits quenching in absorption intensity, which is a consequence of reduction in the amount of sp 2 content within the N-GQD4 sample 54 and is due to excessive insertion of graphitic N inside the core.…”

The Role of Carbon Quantum Dots in Organic Photovoltaics: A Short Overview

“…Another strategy to tune the PL properties of CDs is the doping with etheroatoms. To this end, the work of Saavedra et al [77] is interesting, where the effect of electron-deficient (boron atom) and electron-rich (nitrogen-atom) dopants on the band gaps and PL emission of GQDs were studied. They synthesized three different types of GQDs with similar sizes, chemical compositions and defects: undoped GQDs (UGQDs), boron-doped GQDs (BGQDs), and nitrogen-doped GQDs (NGQDs).…”

“…Experimentally measured HOMO and LUMO energy levels for nitrogen-doped GQDs (NGQDs), undoped GQDs (UGQDs), and boron-doped GQDs (BGQDs) and their emission wavelengths. Reprinted with the permission of Reference[77]. Copyright 2018 American Chemical Society.…”