Exploration of organic acid chain length on water-soluble silicon quantum dot surfaces

Surface functionalization of silicon quantum dots influences oxidation of the silicon core while affording control of physical properties and maintaining optical stability. An effective method for surface modification is photochemical hydrosilylation in which the hydride-terminated Si surface is reacted with an unsaturated C-C bond resulting in a covalent Si-C bond at the surface. The physical properties (e.g., reactivity and solvent compatibility) of the nanocrystals are thus dictated by those of the pendant functional group. Water-soluble nanoparticles can be produced by extending polar functional groups, such as carboxylic acids, from the surface. Previous literature reports have shown acrylic acid to be an attractive starting material for creating water-soluble Si nanocrystals. To date, a detailed study of the effects of differing surface groups (i.e., carboxylic acids of varying carbon chain lengths) has not been offered. Here, we investigate the effects of carboxylic acid surface moieties with increasing carbon chain length on various silicon nanocrystal properties. Oxidative and optical stability was improved by increasing the length of the carbon spacer between the silicon surface and the polar carboxylic acid group. As well, increased chain length was found to enhance nanocrystal dispersibility in polar solvents. Of important note, however, the use of acrylic acid as a precursor led to poly(acrylic acid) formation under the reaction conditions studied, leading to anomalous behavior compared to precursors with longer carbon chains.