Excitation Wavelength Independence: Toward Low-Threshold Amplified Spontaneous Emission from Carbon Nanodots.

Carbon nanodots (CDs) are known to be a superior type of lasing material due to their low cost, low toxicity, high photostability, and photobleaching resistance. Significant attention has been paid to synthesizing CDs with high fluorescence quantum yields (FLQYs) to achieve higher optical gains. In this report, we reveal that excitation wavelength-independent (λex-independent) photoluminescence (PL) characteristics, rather than high FLQYs, should be given priority to realize CD-based light amplification. CDs with excitation wavelength-dependent (λex-dependent) PL characteristics and FLQYs as high as 99% and 96% were found not to exhibit amplified spontaneous emission (ASE), while those with λex-independent PL characteristics and FLQYs of only 38% and 82% realized ASE with low thresholds. The difficulty of achieving ASE using CDs with λex-dependent PL characteristics is likely attributable to their high contents of C-O-H or C-O-C groups. These groups can induce numerous localized electronic states within the n-π* gap, which could decentralize the excited electrons, thus increasing the difficulty of population inversion. In addition, the radiative transition rates and stimulated emission cross sections of CDs with λex-independent PL characteristics were found to be significantly higher than those of CDs with λex-dependent PL characteristics. ASE in a planar waveguide structure, which is a practical structure for solid-state lasing devices, was also demonstrated for the first time using CDs with λex-independent PL characteristics. These results provide simple and effective guidelines for synthesizing and selecting CDs for low-threshold lasing devices.