Dynamics of Silicon Nanoparticle Synthesis by Pulsed Laser Ablation
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Dynamics of Silicon Nanoparticle Synthesis by Pulsed Laser Ablation P.T. Murray1,2 and L. Grazulis1 1- Research Institute 2- Graduate Materials Engineering University of Dayton Dayton, OH 45469-0162 USA ABSTRACT Si nanoparticles have been synthesized by ablating a Si target in Ar with 355 nm laser radiation. The nanoparticle size distribution has been determined in real time by laser-induced time of flight mass spectrometry. Under these conditions, nanoparticles that are formed in 1.0 and 2.0 Torr of background Ar gas exhibit log-normal size distributions with most probable diameters of 2.6 and 3.0 nm, respectively. The speed distribution of the nanoparticles has been determined by varying the time delay between the ablation and photoionization lasers. The results indicate that the most probable speed of the nanoparticles, after formation and a 25 mm drift in background Ar, is 100 m/s. Finally, there is a deviation of the size distribution from the log-normal distribution at small nanoparticle sizes. This is attributed to multiple ionization of the nanoparticles. Confirming evidence for multiple ionization is provided by the atomic and mass spectra which show peak broadening due to Coulomb explosion. INTRODUCTION Thin films of Si and SiOx nanoparticles can be formed by pulsed laser ablation, and such structures have potential applications in optoelectronic devices such as electroluminescent lightemitting diodes. Several reports have been published that deal with Si nanoparticle formation dynamics, and the majority of these have involved the use of spectroscopic techniques [1-9] such as laser-induced fluorescence and Rayleigh scattering to explore the process. More recently, Geohegan and coworkers [4,5] have reported elegant time-resolved imaging of SiOx nanoparticle synthesis. Their results showed a dramatic difference in propagation dynamics when Si was ablated into a He or an Ar atmosphere. In the former case, He was found to slow the Si plume through a series of small angle scattering events, resulting in the formation of a turbulent smoke ring that propagated at 10 m/s. In the latter case, Ar was found to backscatter some of the Si flux which resulted in a stationary, uniformly distributed nanoparticle cloud. The SiOx nanoparticles were analyzed ex situ by transmission electron microscopy. Typical sizes for the SiOx nanoparticles were 4 to 5 nm. The purpose of the work described here was to determine the feasibility of determining Si nanoparticle size distributions in real time by using time of flight mass spectrometry (TOFMS) and to obtain more fundamental understanding of the process. EXPERIMENTAL The experiments were carried out in an apparatus that was comprised of an ablation chamber and four steps of differential pumping that enabled the ablation chamber to be maintained at a pressure of 1 Torr, while the final stage of differential pumping (containing the TOFMS) was at ~ 1x10-8 Torr. The base pressure for the entire system was 2x10-9 Torr. Samples of single crystal Si were ablated with the frequ
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