Inhomogeneous Photoluminescence Characteristic in Carbon Nanodots and Electrophotoluminescence Measurements

Photoluminescence (PL) spectra, time-resolved PL spectra, and PL decay profiles have been observed for carbon nanodots (CDs) with different excitation wavelengths in an embedded solid film and in solution. PL excitation spectra have been also observed with different monitoring wavelengths. Then, it is found in both solid film and solution that not only the location of the PL spectra but also the peak of the excitation spectra show a significant red shift, as the excitation and monitoring wavelengths become longer, respectively, indicating that the emitting states of the excitation-dependent PL are the real state to which direct absorption occurs from the ground state, not the transient trapped states produced by photoexcitation. It is shown that not only the excitation-dependent PL but also the excitation-independent PL with a peak at ∼375 nm exist. The lifetimes of both PL emissions are very sensitive to the surroundings. Multiple emitting states that give excitation-dependent PL are ascribed to the inhomogeneous properties in prepared carbon nanodots, which is supported by the fluorescence lifetime image measurements. Electrophotoluminescence spectrum, that is, the electric-field-induced change in PL spectrum, has also been observed for the excitation-independent PL of CDs embedded in a poly(methyl methacrylate) film, and the magnitude of the change in electric dipole moment and molecular polarizability following emission has been determined.