Optical Behaviour of Carbon Dot:Spectral, Dynamical and Structural Aspects

Photoluminescent carbon dots (CDs) represent an exciting new class of oxygen containing carbonaceous emitters. Serendipitous discovery of carbon dot as a by-product of single walled carbon nanotube purification has triggered extensive studies to explore fluorescence properties of CDs. Since then various synthetic methods have been developed to produce highly luminescent CDs. Previously reported CDs were of low photoluminescence quantum yield (PLQY) and the composition of these CDs was not known. Most of the CDs exhibit either excitation dependent emission and /or multiexponential decay behaviour. Thus, the first challenge was to understand the molecular origin of photoluminescence of CD. There is ambiguity regarding the location of fluorescing moiety in CD. Some researchers have surmised that the fluorescing unit is at the surface, whereas, some researchers have mentioned that the fluorescing unit is inside CD. However, there is no direct evidence regarding the location of fluorescing unit. Hence, it is necessary to investigate the exact location of fluorescing unit in CD. In order to utilise the CD for bioimaging it was necessary to increase the PLQY of CD in aqueous medium. Most of the carbon based nanomaterials have been shown to act as fluorescence quencher. Thus, to increase applicability of the CD it was necessary to show that CD does not act as a fluorescence quencher. It would be better if it is possible to show that CD can act as fluorescence donor especially in resonance energy transfer (RET) based experiment. For many nanomaterials it has been shown that Forster model of RET cannot explain the experimental observables of RET. It was thus necessary to check whether the vastly applied Forster model can be useful to understand the RET phenomenon involving CD. Previously reported CDs have been shown to exhibit multiexponential PL decay, however, fluorescence dynamical analyses becomes much easier if the PL decay is single exponential. Moreover, multiexponential PL decay rises concern about the origin of different decay channels. Those problems can be alleviated if the PL decay is single exponential.