Detection of nanoparticle agglomerates trapped in a low pressure RF (Radio-Frequency) plasma discharge using LIBS (Laser-Induced Breakdown Spectroscopy)
Résumé
Nanotechnology is often said to be the industry of the 21st century. With the advent of nanostructured materials has emerged a new kind of nanostructured particles with different sizes, shapes and chemical compositions referred to as NOAA (Nano Object, their Aggregates and their Agglomerates). Either when being synthesized or accidentally released in the course of the manufacturing process, adapted metrology tools allowing their detection / characterization in situ and in real time (if possible) are to be developed for applications such as process control and workplace surveillance. LIBS was deemed as a promising technique to deal with such issue [1-3]. With the final aim in view to enhance detection of these objects, preliminary experiments were carried out coupling a LIBS analyzer with a low pressure RF (Radio Frequency) plasma discharge (instead of the usual flow cell) acting as a particle trap [4]. Agglomerates of composite nanoparticles of SiC and Al2O3 were injected into the RF plasma discharge where they were trapped and remained in levitation. Nanosecond laser pulses were then focused on the trapped particles, thereby producing the LIBS signal. LIBS detection was therefore achieved with a satisfying signal to noise ratio at a reduced pressure of 0.25 mBar instead of the atmospheric pressure. Thus, this coupling presents several advantages. All injected particles are trapped and can potentially be analyzed, thereby improving sampling. Organic particle analysis is made possible without interferences with the C, H, N, O, elements already present in the air when using an inert gas such as argon or helium for RF plasma generation. In addition, the signal-to-noise ratio may be higher at reduced pressure than at atmospheric pressure. The obtained results, though qualitative so far, have demonstrated the potentialities of such coupling for elemental identification of the elements composite nanoparticle agglomerates are made of. Future experiments are intended to optimize the LIBS detection, to assess the detection limits and to obtain quantitative results when tackling issues such as process control or ambient air monitoring. The use of other laser diagnostics could be envisaged as well for size and morphology measurements.