Roles of Impurity and Sample Heterogeneity in Intriguing Photoluminescence Properties of Zero-Dimensional (0D) Carbonaceous Materials
Krishna Mishra, Manas Barai, Subhadip Ghosh
Abstract
Zero-dimensional (0D) carbonaceous materials, such as graphene quantum dots (GQDs) and fluorescent carbon dots (FCDs), are in the limelight for their intriguing optical and material properties. Excellent results of initial photophysical studies with these emergent nanodots garnered intense interest among the researchers that suggested an immediate opportunity of replacing cytotoxic fluorescent dyes with carbonaceous dots (CDs) in bioimaging and related applications. Several exciting properties of CDs, including the excitation-dependent multicolor emission, slow carrier cooling, subpicosecond (subps) interfacial charge transfer, and quantum emission, are of paramount importance for various real applications. Despite the initial success, their large-scale implementations in daily life remained challenging, mainly because of several serious issues, including elusive structures (especially for FCDs), the controversial origin of their emissions, and a lack of proper knowledge while correlating an unusual photophysical property to an intrinsic phenomenon. Such limitations not only present hurdles to their practical utilizations but also call into question their real potential. Recent reports have suggested that many of these exciting properties stemmed from external factors like the presence of a molecular impurity, sample heterogeneity (i.e., a mixture of CD subsets with different spectral identities), and the adopted synthesis methodology (i.e., top-down vs bottom-up). Hence, only a rigorous sample purification cannot help in realizing the actual merit of these emergent materials unless a sample-specific synthesis procedure is followed. A combination of both renders the maximum integrity to the CD samples. This review highlights several exciting properties of these coveted materials and how they are disturbed by the parameters discussed above. We further emphasize the actual promises of these materials, as shown by the recent fluorescence-based temporal and spectral studies. The results of these studies are even more fascinating compared to that of inadequately purified samples. Finally, we propose a standard sample purification methodology applicable to all types of carbonaceous materials.