Langmuir probe characterisation of an Ar–H2 non-thermal plasma loaded with carbon nanoparticles

Abstract

In this contribution, we describe the development of a test-bed for the characterisation of non-thermal dusty plasmas via Langmuir probe. This technique, while allowing the precise determination of plasma parameters and electron energy distribution function (EEDF), is notoriously difficult to apply in dust-forming chemistries. We overcome this limitation by utilising a two-plasmas system in which the particle precursor, in this case acetylene, is fully consumed and converted into nanoparticles in a first plasma reactor, followed by the injection of the dust into a second plasma reactor where the Langmuir probe measurement is performed. This approach allows studying the influence of the variation of process parameters on the dusty plasma properties, all while leaving the nucleation and growth phase of the particles unaffected and fully decoupled from the discharge in which the measurement takes place. We have applied this approach to the case of graphitic carbon nanoparticles dispersed in an argon–hydrogen mixture. We have monitored the quality of the Langmuir probe measurement, and found that it is minimally affected by the presence of the graphitic particles even after several EEDF measurements. Our measurements confirm the unipolar charging of nanoparticles in non-thermal plasmas, consistent with previous observations and theoretical predictions. We also observe an unexpected trend with plasma input power: the charge carried by the particles does not monotonically increase with increased power, instead starts decreasing at sufficiently high input power levels.