| This thesis aims to characterize the fluorescence brightness and size of silicon nanocrystals in solution by measuring equilibrium fluctuations through the techniques of Fluorescence Correlation Spectroscopy (FCS) as well as Photon Counting Histogram (PCH). It was found that Si nanocrystals are comparably bright to fluorescein, a standard organic fluorophore, as well as comparably small (∼1.1nm in diameter). Imaging results on single Si nanocrystals show that individual nanocrystals are photostable for over 150s of continuous illumination, orders of magnitude longer than possible with traditional organic fluorophores under similar conditions. Due to the poorly controlled sonication step in the preparation of the Si nanocrystal colloid from the porous Si precursor, it was desirable to quantify the heterogeneity of the Si nanocrystal colloid. This was achieved by extending the techniques of FCS and PCH by scanning the excitation energy. In this way each fraction excited at a given wavelength could be counted, and a spectrum of number density versus excitation wavelength could be built up. By directly measuring the molecular heterogeneity in this way it was found that there exists significant heterogeneity in the Si nanocrystal preparations (i.e. the number density changes as a function of excitation wavelength). This important new observable (number density spectrum) can now be used as a control variable in refining the production of Si nanocrystal colloids in the effort to produce homogenous samples, which would be a necessary condition for applications. Traditional ensemble techniques (fluorescence emission/excitation spectra, fluorescence lifetime) are also performed, corroborating the conclusion of heterogeneity, though such techniques are not able to quantify it at the molecular level. |
