Carbon dots (CDs) have been fascinating researchers since their discovery in 2004 due to their extraordinary properties. Despite widespread studies of CDs, the origin of their photoluminescence (PL) emission remains mysterious. Here, we explain the origin of PL emission from carbon dots with the help of solvent dependent PL emission. Carbon dots are synthesized using a solvothermal technique, and their optical and structural properties are studied. The average size of more than 350 particles, determined from transmission electron microscope (TEM) images, is 19.5 nm. X-ray photoelectron spectroscopy (XPS) shows that the CDs are mainly composed of carbon, oxygen and nitrogen. Fourier-transform infrared spectroscopy and XPS analysis show that the CDs contain hydroxyl, amine and carboxyl groups on their surface. The optical properties of the CDs show strong dependence on the solvent they are dispersed in. In water, they show a very weak blue PL emission. However, when they are dispersed in organic water-miscible solvents (such as alcohols, acetone, etc. in a 9:1 ratio with water), they show a strong green PL emission, in addition to the blue emission. The PL quantum yield (QY) of the CDs increases with decreasing polarity of the solvent. For instance, the PL QY in methanol is 1.9 %, and in tetrahydrofuran its value is 7.7 %. Time-resolved PL measurements show that the PL emission has two components: one short lived (~2 ns) and the other relatively long (~6 ns - 8 ns).<br/>The CDs in nonpolar water-immiscible solvents (such as dichloromethane, benzene, etc.), show much stronger PL emission. PL QY in these solvents is significantly greater than that in water-miscible polar solvents. In benzene, the PL QY is 42 %, which is almost 22-fold greater than that recorded in methanol. PL emission decays with a single exponential with lifetime of 4.8 nm. Furthermore, strong vibronic features are also observed in the PL emission and excitation spectra when measured in water-immiscible solvents. These properties are like those of molecular fluorophores, which indicates that the PL of the CDs originates form molecular structures. TEM imaging confirms that the CD particles do not retain their structures in nonpolar solvents, instead they decompose into small molecules giving rise to molecule-like optical properties. From the results, it is concluded that the CDs are composed of small molecular fluorophores that becomes luminescent only when they are not in aggregated form.

Elucidation of the structure and origin of photoluminescence from carbon dots

ACS Spring Meeting 2021

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