Investigating the surface state of graphene quantum dots

Abstract

A universal route to GQDs is developed based on "solution phase-based scissor" methods. The PL centers of the GQDs are systematically studied and are proved to be the surface state. This is related to the hybridization structure of the edge groups and the connected partial graphene core. Through experiment and analysis, we have preliminarily proved that the efficient edge groups for green emission are mainly carboxyl, carbonyl and amide. This is indicated by the following three factors: firstly, the PL of GQDs is enhanced by UV exposure, during which partial -OH groups are converted into carboxyl groups; secondly, the PL properties of GQDs can be further improved by one-step solvothermal treatment, in which partial carboxyl groups are converted to amide groups and the surface state of the GQDs is enhanced; thirdly, reduced m-GQDs possess more -OH groups compared with reduced GQDs, resulting in more blue PL centers (the carboxyl, carbonyl and amide-based green centers are converted to -OH-based blue centers). The present work highlights a very important direction for the understanding of the PL mechanism of GQDs and other related carbon-based materials.

Green synthesis of fluorescent carbon dots from carrot juice for in vitro cellular imaging

“…The excitation dependent PL phenomenon might be caused by emissive traps and surface states, which resulted from abundant self-passivated oxygen and nitrogen-containing functional groups (Fig. 3) on the surface of CDs [6,7], agreeing well with other reported CDs prepared from natural sources [4,14,15,27,28].…”

“…Furthermore, on the basis of their excitation-dependent photoluminescence (PL) properties, they are excellent optical nanoprobes candidates for bioimaging applications. Although the mechanism of the excitation wavelength-dependent PL properties of CDs has not yet been explained, it is generally believed that these properties are related to their unique surface energy trap structure containing a graphitized sp 2 in the core and abundant functional groups in the shell [3,6,7].…”

A fluorescence-electrochemical study of carbon nanodots (CNDs) in bio- and photoelectronic applications and energy gap investigation

“…2–4 For the mechanism for light emission in CNDs, 7, 8 some workers have proposed that the bandgap transitions responsible for fluorescence arise from conjugated π-domains consisting of sp 2 hybridized islands rich in π-electrons, bond disorder induced energy gaps, 9, 10 or giant red-edge effects that give rise to strong excitation wavelength dependent fluorescence. 11, 12 These mechanisms are similar to those used to understand the emissive properties of single-layer graphene and graphene oxides. 13, 14 Other workers ascribe the light emission characteristics to quantum confinement effects, 15 size-dependent optical properties, 16 surface-related defect sites, 17 and radiative recombination of excited surface states.…”

Graphene and carbon nanodots in mesoporous materials: an interactive platform for functional applications

“…Graphene quantum dots (GQDs) and carbon quantum (CQDs) dots are two specific subclasses of the broader family of carbon nanodots [72], even if the two groups show small differences, they both have some peculiar properties which allows a clear distinction [73,74]. The GQDs have a graphene lattice which is clearly recognizable independently of the dot size.…”