Atmospheric Pressure Microplasmas for the Synthesis of Nm-Size Nanocrystals from Pure Metal Wires

Abstract

Nanoparticles of metallic elements are interesting for a wide range of scientific fields. Particles with diameters ranging from 10 nm up to hundreds of nanometres exhibit attractive properties for various applications (e.g.: catalysis, chemical sensors etc.). Some of the most studied characteristics are for instance related to the possibility of tuning localized surface plasmon resonance, which depends on the nanoparticle size, shape and surrounding environment. Meanwhile, particles with diameters below 10 nm down to and below 1 nm approach a new regime with properties of interest to electronics and semiconductor science. Model calculations for such quantum dots of metallic elements predict, depending on the composition, the transition to a semiconductive state, i.e.: opening of an energy band gap and the consequent appearance of visible absorption and photoluminescence effects. However, most of the synthesis methods that yield narrow size distributions employ high vacuum methods or use surfactants to sterically control particle size during their synthesis.Atmospheric pressure microplasmas recently revealed their value for nanomaterial synthesis 1. Microplasmas simultaneously offer a cheap alternative to low pressure plasmas and some unique peculiarities. These plasmas at atmospheric pressure typically show also high electron densities and enhanced rate of ion-neutral interactions, which are mainly responsible for charging and selective heating of particles inside a plasma. The former preventing agglomeration of particles and the latter allowing to achieve higher particle temperatures compared to the gas background. A clear advantage of microplasmas for nanomaterial synthesis is the small residence time of nucleating particles inside the active plasma region, which can be reached with a flow-through type reactor, limiting the in-flight growth of particles.In the current work, nm-size nanocrystals of metallic elements, or quantum dots of different elements such as bismuth, antimony, tin 2, copper 3, zinc and nickel have been produced with an atmospheric pressure microplasma of inert gases using the corresponding solid bulk form as precursor; we discuss the experimental observations concerning the synthesis process for different precursor materials and reactor geometries and the model calculations about the plasma physics phenomena involved in order to reach further understanding of the underlying mechanism of formation.References Mariotti, D. & Sankaran, R. M. Microplasmas for nanomaterials synthesis. J. Phys. D. Appl. Phys. 43, (2010).Haq, A. U. et al. Size-dependent stability of ultra-small α-/β-phase tin nanocrystals synthesized by microplasma. Nat. Commun. 1–8 doi:10.1038/s41467-019-08661-9.Brunet, P. et al. Surfactant-free synthesis of copper nanoparticles and gas phase integration in CNT-composite materials. Nanoscale Adv. 3, (2021).