The Fundamental Photophysics of Fluorescent Carbon Nanodots

Abstract

Carbon Nanodots (CDs) are a new protagonist of carbon-based nanoscience by which the paradigm of carbon as a black material unable to emit light has been completely revolutionised. They have been emerging as a new frontier in Nanoscience at the beginning of 2000s, and their potential is evident from the explosion of the number of studies, now ranging in the thousands per year. CDs are nanoparticles composed by carbon, oxygen and hydrogen with a size smaller than 10 nm. Their most important hallmark is their strong luminescence, which is combined with many additional benefits as the low cost and ease of synthesis, the high water solubility, the biocompatibility and non-toxicity, the great sensitivity to the external environment, and a marked electron donating and accepting capabilities. The combination of all of these characteristics guarantees the possibility to use CDs in a very broad range of applications which encompasses many different fields as optoelectronics or sensing. Actually, the term CDs includes nanomaterials which display a wide range of possible structures and variable optical properties. Indeed, in the literature, it is common to find different sub-types of CDs: they can be graphitic, amorphous, disks of graphene or with a C3N4 core; they can be hydrophilic or hydrophobic; they can emit blue, green, or red light; their emission can be independent of the excitation wavelength, or more commonly tunable (peak of the emission depends on the excitation wavelength); their fluorescence intensity can be sensitive to one particular ion in solution or they respond to a variety of interactions with other systems, such as carbon nanotubes. Besides, CDs can be synthesized by many different procedures which yield subtypes of CDs capable of emitting fluorescence at different wavelengths. In all the synthesis approaches, their surface is passivated by external agents to get bright emission. All of this leads to the existence in the literature of diverse sub-types of CDs with different core and surface structure, and different specific optical characteristics. Despite of this, there are common characteristics which are recurrent almost in every type of CDs as the small size and the core+corona structure which are found to be crucial to obtain CDs with a visible photoluminescence with a high emission efficiency. Carbon dots research is still in a developing phase despite thousands of studies have already been published on the subject, and several scientific open questions exist about their optical behaviour, the fundamental nature of the electronic states, the key factors determining their bright fluorescence, and the relation between structure and emission. As a consequence, a large effort is in progress to find the most effective ways to tailor them for specific applications. In this Thesis, we carried out an investigation of the fundamental physics of different families of CDs with the aim to achieve an exhaustive understanding of their entire photocycle from femtosecond to the steady state. To do this, it was necessary to relate the optical properties to the structural ones, and to study the influence of various possible interactions with external agents. Thus, electronic transitions of CDs having different structures or exposed to different environments have been studied by the combined use of several experimental techniques. In particular, as an important novelty in the field, a variety of new methods which involve ultrafast spectroscopic techniques have been used. Ultrafast spectroscopy is a powerful tool to investigate in real time electronic and vibrational processes with picosecond and femtosecond time resolution. With the use of these methods it was possible to map the photocycle of CDs revealing several dynamics which occur on short time scale as solvation, charge transfer, and fluorescence quenching.