Study of initial dust formation in an Ar-SiH4 discharge by laser induced particle explosive evaporation

Abstract

The initial step of particulate growth in a dust forming low pressure radio-frequency discharge has been studied in situ by laser induced particle explosive evaporation (LIPEE). With respect to the conventional light scattering, this method has been found much more efficient to observe small nanometer size particles, especially in the case of UV excimer laser radiation. Experimental results interpreted by a simple model of laser-particle interaction show that the intensity of LIPEE continuum emission depends on the particle radius roughly as r4. This interaction is essentially different from Rayleigh scattering, as the latter varies as r6. A study of time evolution of powder formation by LIPEE emission reveals the initial formation of nanometer size crystallites and the coalescence process leading to larger scale particles. It could be demonstrated that the critical step of dust formation is the initial clustering process leading to nanometer scale crystallites.

Production of gas phase zinc oxide nanoclusters by pulsed laser ablation

“…In the case of non-radiative recombination of the free carriers, all the photon energy is directly converted to heat. We estimate the increase in temperature of a ZnO cluster with size of about 1.8 nm after absorption of one photon of 6.4 eV energy to be about 20 K. The number of photons absorbed by a particle irradiated at the fluence of 30 mJ/cm 2 used in our experiments is estimated to be about 15 taking into account the nanocluster absorption efficiency [15]. This number gives a relatively good approximation of the cluster temperature of 550 − 600 K estimated from the PL spectra.…”

“…At the observation point situated far from the focal plane, the fluence of the focused ArF laser beam was about 200 mJ/cm 2 . This fluence is fairly high to induce the evaporation of atomic species from the nano-aggregates and their fragmentation [15,16]. The corresponding luminescence spectra are shown in Fig.…”

“…These results provide evidence for the growth and/or coalescence of the nanoclusters. It is known that the efficiency of light absorption by nanoparticles increases with the particle size (see [15] and references therein). Thus, when the clusters grow up they absorb more photons and more electron-hole pairs are created leading to the increase in the PL intensity.…”

From molecules to particles in silane plasmas

“…In low power, low pressure silane plasmas, the appearance of particles after plasma ignition can take many seconds to several minutes [7,8]. Negative ions are retained by the p!asma sheaths, which suggests that plasma polymerisation might proceed via an anionic pathway [3,6,9-121.…”

“…The precursors themselves may be neutral or ionic (positive or negative), and the growth reactions may also involve neutrals and/or ions. Different plasma conditions for a given process may favor different polymerisation pathways [a.In low power, low pressure silane plasmas, the appearance of particles after plasma ignition can take many seconds to several minutes [7,8]. Negative ions are retained by the p!asma sheaths, which suggests that plasma polymerisation might proceed via an anionic pathway [3,6,9-121.…”