Nanoparticles (NPs), which typically range in size from a few to 100 nm diameter, are readily found in nature or can be fabricated from a broad range of materials (metallic, organic, inorganic). The combination of their material composition, nanoscale size (large surface area to volume ratio), and shape produce unique properties that are significantly different from that of their bulk counterpart. As such, NPs have been heavily investigated in the past few decades for use in various applications, including color pigmentation, catalysis, filtration, and medicine [1, 2].
Nanoparticles are commonly synthesized from inorganic-organic precursors or in combination with organic functional molecules to provide colloidal stability. As a result, post-synthesis cleaning of the NPs is typically required to remove the organic layer. In addition, as their properties are largely dictated by size and shape, the ability to finely tune particle size and morphology for different applications is critical. With relatively low applied powers, our plasma cleaners are well-suited for treating NPs and limiting surface alteration to the nanoscale level. Here, we explore one use of plasma to clean metallic NPs and alter their size and shape following deposition.
COARSENING METALLIC NANOPARTICLES
Duan et al. studied the effect of oxygen (O2) plasma on silver (Ag) NPs deposited on smooth silicon (Si) substrates and water-predosed zinc oxide (ZnO) nanopowder substrates, which provide a rough substrate surface.
Their research showed that the process of particle coarsening and agglomeration (Ostwald ripening) depends on the initial Ag NP density and plasma exposure time. Plasma treatment resulted in Ag oxidation and agglomeration of NPs into larger particles for NPs deposited on both smooth and rough substrates. However, for the ZnO nanopowder substrate, smaller particles were formed (5-10 nm) with narrower size distribution.
Higher initial Ag particle density resulted in larger particles after plasma exposure. In addition, O2 plasma was effective in removing carbon contamination and residual fluorine from the Ag precursor used during NP deposition. After plasma, the nanoparticle morphology became more disordered, similar to porous Ag. With continuous Ag film on Si substrate (high initial Ag density), higher power and longer plasma exposure resulted in Ag NPs becoming larger, eventually forming nanorods.
Winkler et al. studied the impact of deposition and plasma cleaning on the morphology of gold (Au) nanoparticles on Si substrates. Initially, as-deposited Au NPs were deformed (flattened) with low contact angles. After O2 plasma treatment, significant nanoparticle coarsening (Ostwald ripening) was observed for both low- and high-density Au deposition.
For low-density coating, plasma-treated NPs reverted to a more spherical shape with increased radius of ~7 nm (variation in radius from 4 to 30 nm). For high-density coatings, plasma exposure resulted in the formation of larger, albeit irregular and elongated shapes of up 70 nm length. For both densities, a roughly 2 fold decrease in surface coverage was observed following plasma treatment. Despite the plasma-induced shape change, the Au NPs still retained their face-centered cubic (FCC) crystalline structure.
Relevant Articles from Harrick Plasma Users
- Duan Y, Rani S, Newberg JT and Teplyakov AV. “Investigation of the influence of oxygen plasma on supported silver nanoparticles”. J. Vac. Sci. Technol. A (2018) 36(1): 01B101.
- Winkler K, Wojciechowski T, Liszewska M, Gorecka E and Fialkowski M. “Morphological changes of gold nanoparticles due to adsorption onto silicon substrate and oxygen plasma treatment”. RSC Adv. (2014) 4: 12729.
Supplemental References (Do not report using Harrick Plasma instruments)
 Heiligtag FJ and Niederberger M. “The fascinating world of nanoparticle research.” Mat. Today (2013) 16(7/8): 262-271.
 Salata, OV. “Applications of nanoparticles in biology and medicine.” J. Nanobiotechnol. (2004) 2: 1-6.