Fine-tuning bandgap, lifetime, and phonon characteristics in CdSe nanocrystals for enhanced optoelectronic device applications

CdSe nanocrystals (NCs) hold significant promise for advanced optoelectronic devices due to their facile synthesis and remarkable properties. The performance of these devices is strongly influenced by the size and characteristics of the CdSe NCs. Raman spectroscopy has revealed that NCs smaller than 5 nm are prone to surface defects, while those larger than 5 nm display well-crystalline structures with substantially fewer defects. This assessment is based on the analysis of optical phonon peaks, including surface optical (SO) and longitudinal optical (LO) modes. Our findings indicate that NCs larger than 5 nm possess narrower optical bandgaps (approximately 1.86 eV) and longer radiative lifetimes (approximately 2.5 ns), whereas smaller NCs (below 5 nm) demonstrate wider bandgaps (approximately 1.92 eV) and shorter radiative lifetimes (around 1.0 ns). The observed level spacing, significantly exceeding the LO phonon energies, suggests weak multi-phonon scattering processes in these NCs. Through comprehensive optical absorption and time-resolved photoluminescence measurements, our research highlights the critical role of energy level transitions in understanding the complex dynamics of CdSe NCs.