Pulsed Laser Synthesis of Carbon Nanostructures from Organic Molecular Liquids: Structure, Kinetics and Photophysical Properties.

The striking one-step bottom-up synthesis of carbon nanoparticles (CNPs) from visible and near infrared (NIR) light transparent air-equilibrated liquid aromatic compounds (benzene, toluene, chlorobenzene, aniline, pyrrole, and thiophene) under unfocused pulsed nanosecond laser irradiation (532/1064 nm) is reported. The formation of CNPs follows zero-order kinetics with an induction period dependent on the reactivity of the organic precursor. Experimental evidence suggests a surface-catalyzed photochemical process involving a partial change in C-atom hybridization from sp² to sp³, with oxygen-based functional groups passivating the surface of the nanostructures. The presence of additional heteroatoms depends on the structure and composition of the precursor. Contingent on the starting compound, the prepared CNPs can be classified as amorphous carbon nanodots (CNDs) or layered curved-graphene quantum dots (c-GQDs), resembling nano-onion fragments. CNDs are obtained from substituted benzenes or heterocyclic compounds, whereas c-GQDs can be synthesized from benzene. Photophysical characterization of the CNPs shows both wavelength-dependent excitation and emission bands, with constant emission quantum yields in the 1-10% range, and wavelength-dependent emission decays displaying several lifetime components in the range 1-20 ns. Triplet exciton lifetimes longer than 5 µs and wavelength-dependent singlet oxygen production quantum yields in the 10-40% range have been measured.