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Aluminum nanoparticles (Al NPs) are interesting for energetic and plasmonic applications due to their enhanced size-dependent properties. Passivating the surface of these particles is necessary to avoid forming a native oxide layer, which can degrade energetic and optical characteristics. This work utilized a radiofrequency (RF)-driven capacitively coupled argon/hydrogen plasma to form surface-modified Al NPs from aluminum trichloride (AlCl3) vapor and 5% silane in argon (dilute SiH4). Varying the power and dilute SiH4 flow rate in the afterglow of the plasma led to the formation of varying nanoparticle morphologies: Al–SiO2 core–shell, Si–Al2O3 core–shell, and Al–Si Janus particles. Scanning transmission electron microscopy with a high-angle annular dark-field detector (STEM-HAADF) and energy-dispersive X-ray spectroscopy (EDS) were employed for characterization. The surfaces of the nanoparticles and sample composition were characterized and found to be sensitive to changes in RF power input and dilute SiH4 flow rate. This work demonstrates a tunable range of Al–SiO2 core–shell nanoparticles where the Al-to-Si ratio could be varied by changing the plasma parameters. Thermal analysis measurements performed on plasma-synthesized Al, crystalline Si, and Al–SiO2 samples are compared to those from a commercially available 80 nm Al nanopowder. Core–shell particles exhibit an increase in oxidation temperature from 535 °C for Al to 585 °C for Al–SiO2. This all-gas-phase synthesis approach offers a simple preparation method to produce high-purity heterostructured Al NPs.