Johannes Berndt

Spectroscopic characterization of micro- and nanoparticle suspensions with size dynamics in plasmas

Mitic et al (2011 Opt. Lett. 36 3699) proposed a spectroscopic method of the in situ measurement of the size and optical properties of spherical micro- and nanoparticles with monotonically variable size. The method requires three optical channels: one for the illumination of a particle suspension by white light and two for the measurements of the spectra of scattered light. This allows one to determine the optical properties of the particles in a wide spectral range. In this work an extended description of the experimental setup and data analysis technique are given. Performance of the method is illustrated on particle suspensions in plasmas, exhibiting increase and decrease of particle size.

Optical diagnostics of dusty plasmas during nanoparticle growth

Carbon-based thin films deposited on surfaces exposed to a typical capacitively-coupled RF plasma are sources of molecular precursors at the origin of nanoparticle growth. This growth leads to drastic changes of the plasma characteristics. Thus, a precise understanding of the dusty plasma structure and dynamics is required to control the plasma evolution and the nanoparticle growth. Optical diagnostics can reveal some particular features occurring in these kinds of plasmas. High-speed imaging of the plasma glow shows that instabilities induced by nanoparticle growth can be constituted of small brighter plasma regions (plasmoids) that rotate around the electrodes. A single bigger region of enhanced emission is also of particular interest: the void, a main central dust-free region, has very distinct plasma properties than the surrounding dusty region. This particularity is emphasized using optical emission spectroscopy with spatiotemporal resolution. Emission profiles are obtained for the buffer gas and the carbonaceous molecules giving insights on the changes of the electron energy distribution function during dust particle growth. Dense clouds of nanoparticles are shown to be easily formed from two different thin films, one constituted of polymer and the other one created by the plasma decomposition of ethanol.

Growth process of nanosized aluminum thin films by pulsed laser deposition for fluorescence enhancement

Pulsed laser deposition was used to deposit aluminum thin films of various thicknesses (tAl) ranging from 5 to 40 nm and to investigate their growth process when they are deposited onto SiO2 and Y2O3. Atomic force microscopy and x-ray reflectivity measurements show that the structure of the Al films are related to the wettability properties of the underlaying layer. Onto SiO2, ultra-smooth layers of aluminum are obtained, due to a perfect wetting of SiO2 by Al. In contrast when deposited onto Y2O3, percolated Al layers are observed with apparent pore size decreasing from 200 to 82 nm as t(Al) is increased from 5 to 40 nm, respectively. This particular morphology is related to partial dewetting of Al on Y2O3. These two different growth mechanisms of aluminum depend therefore on the surface properties of SiO2 and Y2O3. The plasmon resonance of such Al nanostructures in the UV region was then analyzed by studying the coupling between Eu(3+) rare earth emitters and Al.

Development of voids in pulsed and CW- driven reactive plasmas with large nanoparticle density

This contribution deals with the nanoparticle distribution inside of plasma with large nanoparticle density. In particular, the formation of voids i.e. dust-free regions in pulsed and CW discharge regime is analyzed. The discharge was ignited in a mixture of argon and acetylene in the conditions favorable for nanoparticles formation and growth. The temporal evolution of the spatial distribution of nanoparticles during their growth is studied experimentally by means of a laser light scattering technique. The experimental results show that the void expands much faster in the case of pulsed plasma than in the CW plasma operated for the same gas mixture and at the same (average) input power. In the CW plasma, we observed a rapid expansion of the void after some tens of minutes, whereas the corresponding process in the pulsed plasma occurred after some tens of seconds.

A review on Light Extinction Spectrometry as a diagnostic for dust particle characterization in dusty plasmas

In this article, a detailed description of the light extinction spectrometry diagnostics is given. It allows the direct in-situ measurement of the particle size distribution and absolute concentration of a dust cloud levitating in plasmas. Using a relatively simple and compact experimental setup , the dust cloud parameters can be recovered with a good accuracy making minimum assumptions on their physical properties. Special emphasises are given to the inversion of light extinction spectra and all the required particle shape, refractive index and the light extinction models. The parameter range and the limitations of the diagnostic are discussed. In addition, two examples of measurements in low-pressure gas discharges are presented: in a DC glow discharge in which nanopar-ticles are growing from the sputtering of a tungsten cathode, and in an Argon-Silane radio-frequency discharge.

Laser-induced incandescence applied to dusty plasmas

This paper reports on the laser heating of nanoparticles (diameters ≤1 μm) confined in a reactive plasma by short (150 ps) and intense (~63 mJ) UV (355u2009nm) laser pulses (laser-induced incandescence, LII). Important parameters such as the particle temperature and radius follow from analysis of the emission spectrum of the heated nanoparticles. The nanoparticles are not ideal black bodies, which is taken into account by calculating their emissivity using a light-scattering theory relevant to our conditions (Mie theory). Three sets of refractive index data from the literature serve as model input. The obtained radii range between 100 and 165u2009nm, depending on the choice of refractive index data set. By fitting the temperature decay of the particles to a heat exchange model, the product of their mass density and specific heat is determined as (1.3±0.5) J K−1 cm−3, which is considerably smaller than the value for bulk graphite at the temperature our particles attain (3000 K): 4.8 J K−1 cm−3. The particle sizes obtained in situ with LII are compared with ex situ scanning electron microscopy analysis of collected particles. Quantitative assessment of the LII measurements is hampered by transport of particles in the plasma volume and the fact that LII probes locally, whereas the samples with collected particles have a more global character.

Nanoparticle formation in a low pressure argon/aniline RF plasma

The formation of nanoparticles in low temperature plasmas is of high importance for different fields: from astrophysics to microelectronics. The plasma based synthesis of nanoparticles is a complex multi-scale process that involves a great variety of different species and comprises timescales ranging from milliseconds to several minutes. This contribution focuses on the synthesis of nanoparticles in a low temperature, low pressure capacitively coupled plasma containing mixtures of argon and aniline. Aniline is commonly used for the production of polyaniline, a material that belongs to the family of conductive polymers, which has attracted increasing interest in the last few years due to the large number of potential applications. The nanoparticles which are formed in the plasma volume and levitate there due to the collection of negative charges are investigated in this contribution by means of in-situ FTIR spectroscopy. In addition, the plasma is analyzed by means of plasma (ion) mass spectroscopy. The experiments reveal the possibility to synthesize nanoparticles both in continuous wave and in pulsed discharges. The formation of particles in the plasma volume can be suppressed by pulsing the plasma in a specific frequency range. The in-situ FTIR analysis also reveals the influence of the argon plasma on the characteristics of the nanoparticles.The formation of nanoparticles in low temperature plasmas is of high importance for different fields: from astrophysics to microelectronics. The plasma based synthesis of nanoparticles is a complex multi-scale process that involves a great variety of different species and comprises timescales ranging from milliseconds to several minutes. This contribution focuses on the synthesis of nanoparticles in a low temperature, low pressure capacitively coupled plasma containing mixtures of argon and aniline. Aniline is commonly used for the production of polyaniline, a material that belongs to the family of conductive polymers, which has attracted increasing interest in the last few years due to the large number of potential applications. The nanoparticles which are formed in the plasma volume and levitate there due to the collection of negative charges are investigated in this contribution by means of in-situ FTIR spectroscopy. In addition, the plasma is analyzed by means of plasma (ion) mass spectroscopy. The ex...

Interaction of nanosecond ultraviolet laser pulses with reactive dusty plasma

Even though UV laser pulses that irradiate a gas discharge are small compared to the plasma volume (≲3%) and plasma-on time (≲6u2009×u200910−6%), they are found to dramatically change the discharge characteristics on a global scale. The reactive argon–acetylene plasma allows the growth of nanoparticles with diameters up to 1u2009μm, which are formed inside the discharge volume due to spontaneous polymerization reactions. It is found that the laser pulses predominantly accelerate and enhance the coagulation phase and are able to suppress the formation of a dust void.

Plasma response to nanoparticle growth

The influence of nanoparticle growth on the plasma characteristics is studied by analyzing the spatiotemporal evolution of several argon lines. It appears that some lines are promoted in dusty areas while other ones have an enhanced emission in dust-free regions like the void. This effect is related to the spatial dependence of the electron energy distribution: the particularly small electron energy in dust-free regions of a dusty plasma results in enhanced excitation of certain argon energy levels from metastable atoms.

Nanoparticle Growth in Ethanol Based Plasmas

Nanoparticles are grown in a capacitively-coupled radio-frequency discharge (ccrf) in argon from the sputtering of a carbonaceous film deposited on the electrodes. This brown film was previously formed from the ethanol decomposition obtained in argon/ethanol plasmas. During the nanoparticle growth, optical emission spectroscopy reveals the evolution of some typical carbonaceous molecules. The nanoparticle formation also disturbs the plasma equilibrium and induces several plasma instabilities consisting in some cases in regular plasma rotation at very low frequencies. Once nanoparticles are large enough to be observed, they constitute a dense cloud trapped in between the electrode with one central or two symmetrical voids. Ex-situ analysis by scanning electron microscopy evidences that grown nanoparticles can have original surface stuctures.