Silicon nanoparticle formation depending on the discharge conditions of an atmospheric radio-frequency driven microplasma with argon/silane/hydrogen gases

Atmospheric radio-frequency driven non-equilibrium microplasma jets in an argon/silane/ hydrogen gas mixture are characterised and analysed with respect to the reaction pathway, which leads to the formation of silicon nanoparticles. Optical emission spectroscopy is used to obtain initial information about possible plasma chemistry processes and high-timeresolution images uncover the mode of operation of the discharge. It is demonstrated that the effect of the way the electric field is applied (parallel or perpendicular to the gas flow), the gas flow magnitude, and varying the gas mixture can result in three different operation modesfilamentary plasma with a stationary filament, diffuse-like plasma with the filament changing its position, and a diffuse non-filamentary plasma-being formed in the one millimetre inner diameter tube with ring electrodes, which apply an electric field parallel to the gas flow (a parallel-field plasma). An electric field applied perpendicular to the gas flow (a cross-field plasma) results only in a homogeneous diffuse discharge with low plasma density. The nanoparticles synthesised in the microplasma jet are studied by scanning electron microscopy and dynamic light scattering. The experimental results reveal that the silane precursor can very probably be fully dissociated in the parallel-field plasma and particles with sizes almost independent of silane concentration are generated. In contrast, silane is only weakly fragmented in the cross-field plasma and negative ions are formed. Particle size reacts very sensitively to silane concentration in this case and is a result of a condensation of radicals or ions on the particle surface.