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Introduction
There is significant interest in using semiconductor nanoparticles for diverse applications such as non-toxic biological markers, luminescent devices, and single particle transistors. Because many proposed applications rely on particle size control, being able to accurately manipulate, monitor, and preserve individual particle sizes is of critical importance. Additionally, many applications require specifically functionalized particle surfaces to elicit desired characteristics or responses. In the case of a single nanoparticle transistor, a thin coating of a high dielectric material may be desirable.
This document outlines some initial results regarding silicon-based nanostructures functionalized in an atmospheric pressure aerosol chemical vapor deposition (CVD) apparatus. As an initial step towards single nanoparticle transistor production, growth kinetics of aerosolized silicon nanoparticles coated with various oxides (SiO 2 , HfO 2 , and ZrO 2 ) will also be examined.
Methods
Silicon nanoparticles are produced by decomposing silane (SiH 4 ) in a small RF plasma reactor (Mangolini et al., 2005). Particles are extracted from the plasma, aerosolized, and immediately diluted in an inert carrier gas to inhibit growth by coagulation. The extracted aerosol is directed through a pre-conditioning furnace set at ~500°C followed by a unipolar charger. (For SiO 2 experiments, the preconditioning furnace was not used.) A monodisperse size band of particles is then selected from the charged aerosol stream using a nano Differential Mobility Analyzer (nDMA). (Equivalent mobility diameters studied in this work range from 6-15 nm.) That narrow size band of particles is then subjected to various oxygen-rich or metal oxide precursor vapor-rich environments over wide temperature ranges. Particle size changes as a result of oxide layer growth are quantified with a second nDMA in conjunction with an ultrafine condensation particle counter.
Preliminary Results
Thermal pre-conditioning causes an apparent initial reduction in silicon particle diameter at temperatures greater than ~300°C. As the preconditioning furnace temperature is increased from 300 to 400°C, particle diameters decrease further until a maximum decrease of approximately 0.3 nm is exhibited. At temperatures greater than 400°C, particles also exhibit the apparent size decrease, but do not appreciably shrink more than 0.3 nm. This initial size reduction is similar over the range of particle sizes studied.
Reaction activation energies and growth rates of SiO 2 for 6 and 10 nm particles were on the order of 100 kJ/mol and 0.02 m/s. Metal oxide thin film growth has also been studied. For precursor flow rates and initial temperatures investigated to date, a maximum ZrO 2 film thickness of ~0.9 nm was obtained.
Discussion
The onset of oxidative growth appears to be delayed until particle temperatures have reached 300-400°C, the temperature range consistent with hydrogen evolution characteristics found in similar materials (Beyer and Wagner, 1983). Since the silicon particles are formed in a hydrogen-rich environment, the likelihood of hydrogen terminated particle surfaces is high. Because detected size decreases are on the order of the bond length between H and Si, and the temperatures at which the size decreases are detectable are well below those required for thermal restructuring, attributing the delayed growth to surface hydrogen desorption is not unrealistic.
Although reaction kinetics parameters are significantly different when comparing growth rates on bulk silicon to the nanoparticles studied here (Liao et al., 2005), they do not appreciably change when comparing 6 nm particles to 10 nm particles. Initial growth rate estimates of ZrO 2 films were comparable to CVD of ZrO 2 on bulk Si wafers.
References
Beyer, W. and Wagner, H. (1983). The role of hydrogen in a-Si:H - results of evolution and annealing studies, J. Non-Cryst. Sol ., 59-60, 161.
Liao, Y.-C., Nienow, A. , Roberts, J. T. (submitted 2005) Surface chemistry of aerosolized nanoparticles: thermal oxidation of silicon. J. Phys. Chem. B.
Mangolini, L., Thimsen, E., and Kortshagen, U. (2005). High-yield plasma synthesis of luminescent silicon nanocrystals, Nanoletters , 5(4), 655. |
