The Transformation from Intraband Transition to Localized Surface Plasmon Resonance with Crystal Phase Change in Self-Doped Ag₂Se Nanocrystals

Recently Localized Surface Plasmon Resonance(LSPR) in a semiconductor has attracted a lot of attention due to its unique property that combines LSPR and the quantized energy, which are the characteristics of metal and semiconductor, respectively. Here, we present the appearance of LSPR in self-doped Ag₂Se nanocrystals mixed with the intraband excitonic character. By fitting the experimental data to the k.p model, it was observed that absorption peaks deviated as the size of nanocrystals increased while they agreed well with the model including the surface plasmon resonance term, which indicates that the LSPR property gradually mixed with the excitonic transition. Additionally, the intensity of photoluminescence attenuated with increasing the size, demonstrating the weakening of excitonic transition and emergence of LSPR property. Interestingly, the peak was observed to split as the LSPR character increases, which simultaneously occurs with the crystal phase transformation from cubic to tetragonal. By applying the electrochemical potential to the nanocrystals, it was revealed that both split peaks share the same state, 1S_e, which indicated that the peak splitting results from the nondegenerate 1P_e state. The observed transformation of optical property in self-doped Ag₂Se nanocrystals paves the way for the new colloidal quantum to plasmon hybrid optoelectronics and provides an opportunity to investigate the correlation between the crystal phase and carrier density.