Synthesis and Characterisation of Stable Fluorescent Silicon Nanoclusters Produced Using Liquid Jet Method

Fluorescent silicon nanoclusters were produced via a novel liquid jet method (using water or alcohols). The jet initially passes through a plume of atomized silicon in vacuum. The silicon atoms are captured in the jet, which then agglomerate to nanoclusters, and are ultimately deposited on a cold trap. In this method, several millilitres of sample can be produced in only a few minutes. AFM measurements show the nanoclusters produced have a size of ~1 nm. Samples can be produced in different solvents such as water, ethanol, and isopropanol. Fluorescence emission spectra showed two different fluorescence peaks at 310 nm and 365 - 440 nm; the former is constant, but the latter’s wavelength is highly dependent on the solvent used. This strong solvent sensitivity showed that fluorescence originated from an electronic state localized on the cluster surface. Measurements over long time periods prove these fluorescent particles are chemically and optically stable in solution over several years without further [chemical] stabilization. Samples of silicon deposited in water jets showed a fluorescence quantum yield of 8 - 10% three years after production. Solvent exchange shows the wavelength of fluorescent peaks not only depends on the chemical reactions of silicon particles on/in the liquid jet during nanocluster growth, but also in the solvents in which the nanoclusters were solvated. Solvent transfer experiments show fluorescence peaks shift depending on solvent in a reversible manner. A sample transferred to a specific solvent is equivalent to a sample directly deposited in that solvent. Nanocluster fluorescence lifetime measurements show decay times of a few nanoseconds (between 3.7 to 5.6 ns) that depend on the solvent; Si-water samples have shorter lifetimes with respect to alcoholic samples. Film samples produced by evaporating the solvent show the first and second fluorescence peaks at ~300 - 310 nm and at 420 - 440 nm, respectively. Film samples present longer fluorescence lifetimes and higher fluorescence intensities than liquid samples. Changing the temperature shows the particles have a shorter fluorescence lifetime at higher temperatures and fluorescence intensity decreases with increasing temperature. Chemical analysis of nanoparticles using XPS and ATR revealed that practically all the silicon was oxidized, and clusters produced in water are in the highest oxidation states. Also, the number of silicon particles which interact with the solvent is greater in water than in alcohols. Infrared absorption bands were attributed to SiOH, SiH, SiO, SiO2 and SiOx, (x > 2) species. The solvent exchange experiments suggest that several stable forms of silicon nanoclusters in different oxidation states exist in solution. These can be interchanged by reversible reduction and oxidation depending on the solvent. Our observations suggest that an intrinsically stable form of silicon nanocluster in water exists, and that the deep-blue fluorescence we observed emerges from oxygen-rich states.