Synthesis of Silicon Nanoparticles in a Novel CO2 Laser Pyrolysis Reactor With an Elongated Reaction Zone

We demonstrated silicon nanoparticle synthesis using a novel CO 2 laser pyrolysis reactor. The reactor was designed to have an elongated reaction zone more than 10 times longer than conventional laser pyrolysis systems. Such elongation was achieved by aligning the laser beam and precursor gas stream. SiH 4 gas was used to synthesize the silicon nanoparticles. The yield of the nanoparticles was 40.9%, as calculated by comparing the masses of the synthesized nanoparticles and precursor gas used. Silicon nanoparticles synthesized by using a typical reactor with identical gas ow rate conditions and without a focusing lens had a nanoparticle yield of 1.7%, which was far smaller than for the new reactor. The average diameter of as-synthesized silicon nanoparticles was 26.7 nm. Considering that high power CO 2 lasers are often used for large scale nanoparticle production by laser pyrolysis, our proposed reactor serves as a proof of concept that demonstrates its potential for large scale nanoparticle synthesis.


Introduction
Laser pyrolysis techniques have been widely employed to synthesize nanoparticles ever since Haggerty et al reported the synthesis of silicon and silicon nitride nanoparticles. 1 This technique employs a CO 2 laser beam as a heat source and gaseous precursors as feedstock. The precursor molecules dissociate when the coupled energy provided by the laser beam is su cient, which subsequently initiates the nucleation and growth of nanoparticles. These processes occur on timescales on the order of 10 − 3 seconds. The inherent characteristics of a laser beam make it an advantageous heat source for nanoparticle synthesis, speci cally enabling contact-free, continuous, and high-yield processing of nanoparticles. In addition, process parameters such as the laser beam, precursor gases, and the pressure inside the reaction chamber can be independently controlled, thus enabling the judicious design and modi cation of processes to yield tailor-made nanoparticles. To date, the range of nanoparticles that can be produced by laser pyrolysis has steadily expanded. For instance, reports have exploited laser pyrolysis to generate silicon, 2-5 germanium, 6 silicon germanium alloy, 7,8 boron, 9 titanium oxide, 10 fullerene, 11 and iron oxide 12 nanoparticles. The versatility of the processing parameters permits various strategies for laser pyrolysis that can be employed to synthesize alloyed, 6,13 core-shell 14,15 and doped nanoparticles, 16,17 particularly by manipulating processing parameters or the con guration of the set-up.
Laser pyrolysis for the nanoparticle synthesis requires overlap of the cross-sectional areas of the incident laser beam and precursor gases. A typical set-up is con gured to allow the laser to intersect the precursor gases orthogonally, with the cross-sectional area de ning the reaction zone. The laser beam is focused by using an optical lens to increase the laser intensity in the reaction zone, unless the power of the unfocused laser beam is su cient, e.g. greater than ~ 10 3 Watt. The reaction time and the reaction zone are limited to small values that hinder the use of various gases as precursors. Additional photosensitizer gases can be used, in particular when there is insu cient absorption cross-section between precursor gases and the laser beam. But including photosensitizer molecules potentially introduces contaminants.
From a practical viewpoint, intensifying the laser beam promotes the nanoparticle synthesis by improving nanoparticle yield, but causes thermal lensing effects 18 or thermal damage to the optical components 19 that are problematic and occur more frequently at increased laser intensities.
In this paper, we proposed a novel reactor that was designed with an elongated reaction zone that is more than 10 times longer than conventional laser pyrolysis systems. Such elongation was achieved by aligning the laser beam and precursor gas stream as shown in Fig. 1(a). By comparison, typical laser pyrolysis reactors have a much smaller reaction zone that is located at the intersection of the focused laser beam and precursor gas stream line, as shown in Fig. 1(b). The length of the reaction zone is approximately 180 mm in the new reactor which is much longer than that of typical reactor set-up with a few mm.

Results And Discussion
The CO 2 laser beam had a diameter of 4 ± 1 mm and operated at a wavelength of 10.6 µm, under continuous wave conditions, and without a focusing lens. A laser power of 57W was used, which was 95% of the maximum laser power. The nozzle system of the new reactor had three ports. Inert gas (N 2 ) is owed through the bottom and the upper nozzles. The N 2 gas from the bottom nozzle prevents the deposition of nanoparticles and chemical reactions involving the precursor gas on the ZnSe window. The N 2 gas from the upper nozzle guides and con nes the precursor gases. The N 2 gas ow rates were 150 and 5000 standard cubic centimeters per minute (sccm) at the bottom and upper nozzle, respectively.
Precursor gas, which was SiH 4 here, was introduced into the reactor from the middle nozzle at a ow rate of 50 sccm. The ow rates of all gases were controlled by mass ow controllers. The body of the reactor could be designed in a cylindrical shape as shown in Fig. 1(a) because the gas mixtures and the laser beam were aligned in the same way. Therefore, the middle part of the reactor was built with a quartz tube in order to observe and monitor the reaction zone. A rotary vacuum pump was connected to the exhaust of the reactor. The gas mixture was eventually puri ed through a dry gas scrubber. The pressure inside the reactor was 400 Torr. Synthesized nanoparticles were captured in a membrane lter in the collector, which was installed between the reactor and a rotary vacuum pump. After synthesis, the collector was separated from the reactor system and transferred to an N 2 -lled glove box to avoid exposure to the oxidizing effects of air.
As soon as the synthesis process initiated, it was able to observe red straight light through the quartz tube which was from the laser diode pointer (635 nm wavelength) scattered by the synthesized nanoparticles. However, strong reaction ame was not observed. Typically, bright reaction ame is observable at the con ned reaction zone in the conventional reactor due to the thermal radiation from the heated solids or liquids. 4 Fig. 2 shows images of as-synthesized silicon nanoparticles prepared by using the new reactor. Images of silicon nanoparticles that were captured on the membrane lter are shown in Fig. 2(a). Silicon nanoparticles were densi ed and deposited on to the membrane. These nanoparticles were brown in color and were easily separated from the membrane as shown in Fig. 2(b). The yield of the nanoparticles was 40.9%, as calculated by comparing the masses of the synthesized nanoparticles and precursor gas used. To compare the production yield, we synthesized silicon nanoparticles by using a conventional reactor which con gured as in Fig. 1(b). Identical experimental conditions including gas ow rates and the laser beam were applied to the synthesis. The production yield of silicon nanoparticles was 1.7%, which was far smaller than for the new reactor. In conventional laser pyrolysis reactors, increases in either the laser beam intensity or density of precursor molecules lead to higher nanoparticle yields. Thus, collectively, these results demonstrate that the elongated reaction zone of the new reactor dramatically enhances the yield of silicon nanoparticles, especially at low laser intensities.
Transmission electron microscopy (TEM) images of the as-synthesized silicon nanoparticles are shown in Fig. 3. The silicon nanoparticles have quasi-spherical shapes and are aggregated (Fig. 3(a)). Lattice fringes were clearly visible in the magni ed TEM image provided in Fig. 3(b). The internal region of the nanoparticle appears to be polycrystalline, which we believe originates from individual nanoparticles merging while traveling through the elongated reaction zone. Most of nanoparticles were polycrystalline. It is assumed that the temperature of the reaction zone is insu cient to recrystallize the nanoparticles. Xray diffraction (XRD) patterns of as-synthesized silicon nanoparticles ( Fig. 4(a)) exhibit peak patterns that correspond to crystalline silicon. Calculations by using the Scherrer equation reveals that the size of the crystallites was 19 nm. Nanoparticle size was also established by measuring the sizes of 89 individual nanoparticles from several TEM images, yielding an average size of 26.7 nm. Such a large discrepancy in the estimates of average size from XRD and TEM is reasonable because small domains within crystallites contribute to broad XRD peaks that would favor smaller size estimates. Figure 4(b) shows a histogram of the sizes of 89 silicon nanoparticles measured from TEM images. Large aggregates of nanoparticles of various shapes with characteristic lengths greater than 50 nm were frequently observed in TEM images. Such aggregated nanoparticles are composed of a collection of individual nanoparticles, as opposed to a single homogeneous particle, due to insu ciently high temperatures in the reaction zone. The elongated reaction zone increases production yield dramatically, however, is assumed to promote the growth of inhomogeneous nanoparticles.

Conclusions
We have demonstrated silicon nanoparticle synthesis using a new reactor and it was con rmed that our proposed reactor extends the reaction zone and thus reaction time, which improves production yield.
Considering that high power CO 2 lasers are often used for large scale nanoparticle production by laser pyrolysis, our proposed reactor serves as a proof of concept that demonstrates its potential for large scale nanoparticle synthesis. The low laser powers required in the new reactor also provide advantages with respect to maintaining the delity of optical components, which may be able to tolerate even higher laser powers that would be bene cial for large scale nanoparticle production. Moreover, the elongated reaction zone could permit su cient reaction times for precursors that have low absorption coe cients when using CO 2 laser beams. This research presents and demonstrates a novel laser pyrolysis reactor that we believe will meaningfully impact progress in the eld of nanoparticle synthesis.  Figure 1