Phosphorescence is a type of photoluminescence related to fluorescence. They exhibited size-dependent properties especially in the optical absorption and the photoluminescence (PL). Carbon nanotubes have useful absorption, photoluminescence (fluorescence), and Raman spectroscopy properties. Such materials exhibit room-temperature photoluminescence in the monolayer limit. In contrast to optical absorption and photoluminescence, THz absorption may involve all exciton states. Immediate photon emission is called fluorescence, a type of photoluminescence. These graphene quantum dots were functionalized with organic ligands that experience photoluminescence from UV-Vis absorption. In phosphor thermometry, the temperature dependence of the photoluminescence process is exploited to measure temperature. Photoluminescence properties were also observed to change with ligand moiety. This light emission (photoluminescence) is illustrated in the figure on the right. Photoluminescence is light resulting from absorption of photons. Similar to single interstitials, divacancies do not produce photoluminescence.