Surface Defect Passivation of Graphene Quantum Dots by Amino Functionalization and Photoluminescence Emission Enhancement

Abstract

In this work, graphene oxide sheets are cut into graphene quantum dots (GQDs) by acidic oxidation, then GQDs are hydrothermally treated with ammonia (NH3) at 100°C to form amino-functionalized graphene quantum dots (N-GQDs). Atomic force microscopy (AFM) shows smaller dots in ammonia treated GQDs, and holey graphene structure is directly observed. Fourier transform infrared (FTIR) spectra confirm that NH3 can effectively react with epoxy and carboxyl groups to form hydroxylamine and amide groups, respectively. The absorption and photoluminescence (PL) properties of the samples are determined by ultraviolet-visible-near infrared (UV-Vis-NIR) spectra and steady-state fluorescence spectra. Three PL excitation peaks occurring at around 250, 290, and 350 nm are attributed to C＝C related π-π* transition, C-O-C and C＝O related nπ* transitions, respectively. After amino functionalization, the C-O-C related n-π* transition is suppressed, and the PL emission spectrum of N-GQDs is less excitation wavelength. The fluorescence quantum yield of the N-GQDs is 9.6%, which is enhanced by 32 times compared with that of the unmodified GQDs (~0.3%). Timeresolved PL spectra are also used to investigate the N-GQDs. The PL lifetimes depend on the emission wavelength and coincide with the PL spectrum, and are different from most fluorescent species. This result reveals the synergy and competition between defect derived photoluminescence and amino passivation of the N-GQDs. Compared with oxygen-related defects, nitrogen-related localized electronic states are expected to have a longer lifetime and enhanced radiative decay rates.

Structural defects in graphene quantum dots: A review

“…Wang et al [89] reported on surface defect passivation, where GQDs were composed of epoxy and carboxyl groups. Ammonia reacted with them to form hydroxyl amine and amide groups [90].…”

“…In general, edge defects occur in a material, when a row of atoms get missing in any particular region [89]. The influence of edge defects on a structure similar to that of GQDs has been studied.…”

One-Pot Synthesis of Bright Blue Luminescent N-Doped GQDs: Optical Properties and Cell Imaging

“…The cells displayed enhanced blue (405 nm laser excitation) or green (488 nm laser excitation) fluorescence around their nucleus (Figure 7), indicating that the N-GQDs were able to label the cell membrane and the cytoplasm. Studies have shown that N-GQDs are likely to enter the cytoplasm, which can be attributed to the smaller amount of carboxyl on the surface of N-GQDs [68,[73][74][75][76]. The abundant surface functional groups in N-GQDs (carboxyl, carbonyl, hydroxyl, and amino) ensure that they adhere easily to the negatively charged cell membrane [77][78][79], thus achieving effective uptake by cells.…”