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 CO2 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 sufficient, 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, specifically 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 modification 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 oxide12 nanoparticles. The versatility of the processing parameters permits various strategies for laser pyrolysis that can be employed to synthesize alloyed,6,13 core-shell14,15 and doped nanoparticles,16,17 particularly by manipulating processing parameters or the configuration 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 configured to allow the laser to intersect the precursor gases orthogonally, with the cross-sectional area defining 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 sufficient, e.g. greater than ~ 103 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 insufficient 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 effects18 or thermal damage to the optical components19 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.