Carbon dots prepared by hydrothermal synthesis: challenges and perspectives

As a relatively new and rapidly developing field, carbon nanoparticle research has garnered immense interest over the last decade. This is, in large part, due to abundant and inexpensive precursors and relatively simple synthesis methods (such as hydrothermal carbonisation) that can be used for preparation of carbon nanoparticles. However, the reaction products are ferequently heterogeneous ensembles of particles and other byproducts hence there is significant confusion in the field about the atomic structure and origins of the electronic and optical properties of these systems. In this review, we look specifically into carbon dots prepared by the hydrothermal carbonisation and by continuous hydrothermal flow synthesis. We show that these carbon dots represent a new class of carbon nanoparticles, hypothesise their potential atomic structure and suggest a standardised framework for their characterisation and analysis that would enable a direct comparison of the results between various research groups. We also provide an outlook for future research and suggest a number of advanced methods that will take the characterisation of optical properties beyond the ensemble average by providing information on dispersion of various physical properties of carbon dots.