Solvent Effects on Light-Emitting Properties of Si Nanocrystals

It is presently accepted that the band gap widens as a result of quantum confinement, which allows Si crystallites sized below 5 nm to exhibit visible photoluminescence (PL). However, many research groups have reported that when the crystallite size decreases to a few nm, the PL peak energy does not increase much beyond 2.1 eV even when the crystallite size drops well below 3 nm in porous silicon (PS).It has become increasingly clear that the analysis is partially hampered by diffusion drift of excitons prior to recombination from small crystallites to larger ones and by averages over large distributions. Thus, dispersion of the material into ultrasmall particles allows the isolation of single particle effects and opens the way for direct theoretical examination. In previous investigations, Si nanocrystals crumbled
from PS are usually mixed with a polar solvent to achieve uniform distribution. The solvent effect on the light-emitting properties of the composite system has not been investigated extensively and is relatively unknown. Hence, it is necessary to design some experiments to fathom the phenomenon. Using ultrasonic vibrations of as-made porous silicon in water, benzene, chloroform, and toluene, four groups of suspensions of Si nanocrystals are prepared in order to explore the influence of solvents on
the light-emitting properties of Si nanocrystals. Photoluminescence (PL) examinations show that the strongest PL spectrum appears from the toluene suspension with the smallest HOMO/LUMO gap.This result can be explained on the basis of the frontier orbital theory and toluene chemisorption model. In addition, shifts of the PL peaks are observed in different suspensions. This behavior is believed to be due to the influence of the solvent polarity.