We describe a new and widely applicable method for functionalizing and passivating the surfaces of aerosolized silicon nanoparticles. In one set of experiments, organic monolayers derived from amines, alkenes, and alkynes were deposited by spontaneous adsorption from the gas phase. In another set of experiments, multilayer films of group IV metal oxides, including zirconium dioxide, were deposited via chemical vapor deposition. The results are important because they describe a new approach for manipulating and controlling interfacial properties in nanoparticles that have materials applications. More generally, the results are among the first kinetic and mechanistic studies of aerosolized silicon nanoparticle reactivity.

In these experiments, free-flowing streams of crystalline silicon nanoparticles were extracted from a low-temperature, low-pressure plasma synthesis chamber into an atmospheric pressure flow tube reactor. Using nitrogen as the carrier gas, the particle streams were sent through a furnace for thermal preactivation, through a bipolar diffusion charger to establish a known charge distribution on the particles, and then through a differential mobility analyzer (DMA-1). DMA-1 was used to create streams of monodisperse particles; selected mobility diameters were in the 10-20 nm range. The particle streams were swept into a reaction zone, which was a heated copper tube with a valve for the precursor introduction. The reaction zone was designed for maximal flexibility, with variable temperature (25-200 ºC), particle residence time (1-10 s), and gas-phase composition. Particles that exited the reaction zone were analyzed in three ways: (1) for size changes, with a second DMA capable of measuring diameter changes as small as 1%, (2) for nano-structural and nano-morphological changes that are induced by deposition, using transmission electron microscopy (TEM), for functional groups, using infrared spectroscopy.

Results will be presented on the adsorption of a homologous series of organic amines, on the adsorption of terminal alkenes, on the adsorption of alkynes, and on the deposition of zirconium oxide from zirconium nitrate. Organic layers were deposited between 100 and 200 ºC. In all cases, adsorption led to an increase in particle mobility diameter, with adsorption saturating at a particle diameter that was consistent with monolayer uptake. In the case of a homologous series of primary amines (C_nH_2n+1NH₂, n = 4-6), the maximum diameter change increased with carbon chain length, at a rate of roughly 0.4 nm per additional carbon atom. Similar results were obtained for alkene and alkyne adsorption. In the case of zirconium oxide deposition, films were deposited from the anhydrous metal nitrate Zn(NO₃)₄. Layers were deposited onto silicon particles of initial mobility diameter between 10 and 20 nm. Layers as thick as 1 nm could be deposited under relatively conditions. Growth rates, expressed as the change in mobility diameter with respect to time, uniformly increased with deposition temperature and reactor residence time The growth rates were studied as functions of thermal pre-activation temperature, precursor flow rate, and deposition temperature. Growth rates also depend on the thermal pre-activation temperature. Film growth is most rapid for particles are pre-treated at temperatures of 500 ºC or more, which is close to the temperature at which hydrogen desorbs from silicon wafers.