Low-temperature photoluminescence of C₆₀ single crystals intercaled with nitrogen molecules in the wide range of temperatures

The optical properties of C₆₀ single crystals, intercalated with nitrogen molecules, were studied by the spectral-luminescence method at a temperature T = 30 K and excitation by the He–Ne laser (E_exc = 1.96 eV). Intercalation was carried out at a pressure of 30 atm in a temperature range of 200–550 °C. It was found that at sorption temperatures up to 400 °C, the bands of the low-temperature luminescence spectrum of the C₆₀–N₂ solutions are broadened without significant energy shift. As a rule, this situation is typical for the case of an increased contribution of the luminescence of "deep X-traps". The concentration of such exciton emission centers is determined by the degree of occupation of the octahedral cavities of the fullerite fcc lattice by intercalated molecules. This indicates the formation of an equilibrium C₆₀–N₂ interstitial solution as a result of physisorption. At sorption temperatures above 400 °C, a significant shift of the luminescence spectrum towards low energies with a considerable inhomogeneous broadening of spectral bands was found for the first time. The shift and significant nonuniform broadening of the luminescence bands are explained by the emergence of a chemical interaction (chemisorption) of nitrogen with fullerene molecules, with the formation of a new nitrogen-containing substance in this case. (author)