Ultrafast optical spectroscopy of surface-modified silicon quantum dots: unraveling the underlying mechanism of the ultrabright and color-tunable photoluminescence

Abstract

In this work, the fundamental mechanism of ultrabright fluorescence from surface-modified colloidal silicon quantum dots is investigated in depth using ultrafast spectroscopy. The underlying energy band structure corresponding to such highly efficient direct bandgap-like emissions in our surface-modified silicon quantum dots is unraveled by analyzing the transient optical spectrum, which demonstrates the significant effect of surface molecular engineering. It is observed that special surface modification, which creates novel surface states, is responsible for the different emission wavelengths and the significant improvement in the photoluminescence quantum yields. Following this essential understanding, surface-modified silicon quantum dots with deep blue to orange emission are successfully prepared without changing their sizes. Ultrafast spectroscopy shows that novel surface states give rise to ultrabright photoluminescence from surface-modified silicon quantum dots. Specifically, researchers from Jilin University and Fudan University in China used femtosecond transient absorption spectroscopy in combination with time-resolved fluorescence spectroscopy to investigate the underlying mechanism for the ultrabright, colour-tunable photoluminescence, which they had previously observed from nitrogen-containing, colloidal silicon quantum dots passivated by organic ligands. They discovered that the mechanism was related to exciton wave functions being modulated by surface molecular engineering. Based on this insight, they produced surface-modified silicon quantum dots that had identical sizes and yet whose emissions ranged in colour from deep blue to orange. This novel approach involves tailoring the surfaces of nanoparticles in contrast to the conventional approach of varying the nanoparticle size.