Using Low-Temperature Plasmas to Engineer Nano-Energetic Materials

Low-temperature plasmas are a versatile and effective tool for nanoparticle production. The presence of high-energy electrons in these systems is well-known to lead to the nucleation and growth of particles. While this is a major source of contamination in the semiconductor industry, the plasma community has now learnt to optimize low-temperature plasma processes for the deliberate formation of particles with desirable properties. For instance, low-temperature plasmas can produce particles sufficiently small to show quantum confinement effects [1] and localized surface plasmon resonance.[2] Due to their capability of producing particles with very small sizes (&lt;10 nm), these systems are of obvious interest for application in energetic materials. In this talk, we will discuss two examples of how low-temperature plasmas can be used to produce magnesium-based and silicon-based energetic nanomaterials.<br/>In a first example, magnesium nanoparticles are first produced via thermal evaporation of a bulk magnesium source. Following homogeneous nucleation, the particles are injected in a RF driven low-temperature plasma to which hydrogen is added. Optical emission spectroscopy is used to measure the atomic hydrogen density, which is found to be quite high (~10¹⁴ cm^-3). As a consequence, the magnesium particles are rapidly converted into magnesium hydride as they are aerodynamically dragged through the plasma, within a residence time of a few tens of milliseconds. XRD analysis confirms that the MgH₂ molar fraction can be as high as 25% after in-flight plasma processing. Characterization of the combustion properties confirms that the hydrogenated magnesium particles have a significantly lower ignition temperature compared to Mg particles of the same size, as confirmed by time-of-flight mass spectrometry. Modelling of the energy exchange between the plasma and the nanoparticles suggests that while in the plasma, the particles are sufficiently hot to desorb hydrogen, limiting the degree of hydrogenation to roughly 25% for our current reactor design. This insight provides guidelines towards the development of magnesium-based particles with a significant degree of hydrogenation.<br/>In a second example, we will discuss the case of ultra-fine (&lt;10 nm) silicon nanoparticles which can be easily grown by RF excitation of a silane-containing plasma. Due to their large surface area, it is necessary to carefully control the surface chemistry of these particles. One way to do that, is to utilize a two-steps plasma reactor in which the particles are grown in a first plasma and then passed onto a second flow-through reactor to which a fluorocarbon source is added. TEM analysis suggests that this approach leads to the growth of a highly conformal CF_x shell onto the silicon core. We have found that this approach is very effective at preventing the formation of a native oxide. The core-shell particles form a nanothermite structure, whose combustion leads to the formation of SiF_x gaseous species, as confirmed by time-of-flight mass spectrometry measurements.<br/><br/>1. Wang, K., R.P. Cline, J. Schwan, J.M. Strain, S.T. Roberts, L. Mangolini, J.D. Eaves, and M.L. Tang, <i>Efficient photon upconversion enabled by strong coupling between silicon quantum dots and anthracene.</i> Nature Chemistry, 2023.<br/>2. Alvarez Barragan, A., N.V. Ilawe, L. Zhong, B.M. Wong, and L. Mangolini, <i>A Non-Thermal Plasma Route to Plasmonic TiN Nanoparticles.</i> The Journal of Physical Chemistry C, 2017. <b>121</b>(4): p. 2316-2322.