Design and fabrication of an electrostatic precipitator for infrared spectroscopy

Infrared (IR) spectroscopy is a direct measurement technique for chemical characterization of aerosols that can be applied without solvent extraction thermal treatment a priori. This technique has been used for chemical speciation, source apportionment, and detailed characterization of the complex organic fraction of atmospheric particles. Currently, most IR analyses are performed by transmission through porous membranes on which the particles are collected via filtration. The membrane materials interfere with the IR spectra through scattering and absorption that not only make extracting the chemical information on aerosol harder but also limit the lower extent of detection. An alternative IR measurement method that does not inherit such limitations is to collect the particles on an IR transparent material. We present an electrostatic precipitator (ESP) design that enables such measurements by collection on a zinc selenide (ZnSe) crystal. Through numerical simulations and rapid prototyping with 3D printing, we design and fabricate a device which is tested with polydispersed ammonium sulfate particles to evaluate the quantitative chemical composition estimates against particle count reference. Furthermore, with an image analysis procedure and using variable aperture of the IR spectrometer, we analyze the radial mass distribution. The collector has high collection efficiency (82 +/- 8 %) and linear response to mass loading (R-2 > 0.94) with a semi-uniform deposition. The method of design and fabrication is transferable to other applications, and the current ESP collector can provide directions for further design improvements.

Nanoscale Magnetometry with Single Fluorescent Nanodiamonds Manipulated in an Anti-Brownian Electrokinetic Trap

Metin Kayci

Studies on single-molecule spectroscopy and nanoscale detection have been remarkably driven by an interest to reveal quantum and conformational states of single particles, the intra-molecular dynamics and their response to physical observables hidden by ensemble level measurements. A straightforward practice used in enhancing the signal from single particles is either to immobilize them on an engineered substrate or to embed them in a solid matrix. Given that the biophysical properties of the host environment introduce new perturbations and the particles will not behave as in their native environment, such approaches are inefficient to reflect the real dynamics. Therefore, recent advances in the field of single-molecule have led to a renewed interest in novel trapping methods, increased efforts into the development of promising tools for extended investigation, and the manipulation of solution-phase bio-molecules in real time. Despite the variety of successful passive trapping techniques, precise manipulation through non-perturbative forces is a big challenge for nano-sized particles. Such techniques either exert high power to the sample or compel special operating conditions disturbing the native environment. Therefore, an active trapping scheme guiding non-perturbative forces can break the trade-off between the particle size and the excreted power. This dissertation presents the development of an active trapping set-up using non-perturbative electrokinetic feedback and demonstrates its performance on nano-sized single particles for aims in biophysics. The essential theme is the engineering aspect of the technique, including the feedback configurations for various fluidic devices, the corresponding particle tracking schemes and the integration of the trapping platform to an integrated circuit pattern for advanced manipulation aims. The second theme is on specialized single fluorescence nanodiamonds (FNDs) as scanning magnetometer in fluidics. The implemented active trapping tool is employed for the manipulation of a rotationally free single FND to detect the localized magnetic field through an optically detected magnetic resonance (ODMR) spectrum. While the laser beam used in particle tracking can serve in optical excitation, an external radio frequency (RF) source is not sufficient to achieve microwave manipulation. Therefore, an RF antenna is designed to transmit the microwave signal to the proximity of the trapping chamber for electron spin resonance (ESR) spectroscopy. A nanostage positioning controller introduces scanning ability to the sample plane, in relative position of the trapped particle, in order to map the distribution of the detected fields over a fluidic volume. As FNDs are also sensitive to many other physical quantities, nanoscale single particle trapping and diamond photonics linkages are realized in this work, which provide an outstanding alternative for detection and imaging in complex fluidic environments that are closed to AFM-like physically supported probes.

EPFL2016

Etude de l'origine et de la croissance de particules submicrométriques dans les plasmas radiofréquence réactifs

Christian Deschenaux

The formation of sub-micron sized particulates has been studied in a low pressure (0.1 mbar) radio-frequency (13.56 MHz) capacitively coupled plasma discharge. Particles are studied in situ by infrared absorption spectroscopy, laser illumination and Cavity Ring Down Absorption Spectroscopy. Transmission Electron Microscopy was applied ex situ for particulates collected during the plasma. We propose that two principal mechanisms may be involved in the formation of these particulates: formation and trapping of negative ions in the gaseous phase, and the recycling of material deposited on the electrodes during the discharge, by film erosion or delamination. The relative importance of these two mechanisms was studied in silane, methane, ethylene and acetylene plasmas. The growth of the particulates in silane plasmas, monitored with Infrared and Cavity Ring Down Absorption Spectroscopy, allowed us to distinguish an increasing volume growth phase followed by an agglomeration phase at constant volume, in agreement with the literature. A slight powder and film recycling effect is detected as the experiments are repeated, as observed by an increasing particle size. The hydrogen content of powder formed in silane plasmas was estimated in situ by infrared absorption spectroscopy and changes in the infrared absorption spectrum could be observed for silane diluted with hydrogen and argon. In particular, the porous structure of the powder was observed from the high content of surface related vibration bonds relative to bulk related bonds. Photoluminescence was detected for the particles at 2.15eV that could possibly be attributed to the crystalline structure of the particles. Infrared Absorption Spectroscopy and mass spectrometry applied in situ showed that acetylenic compounds are preferentially produced in methane, ethylene and acetylene plasmas and that the hydrogen content of the other species produced in these plasmas depends on the initial gas hydrogen content. Negative ions were evidenced in these plasmas and a polymerization reaction is proposed between acetylene and C2nHx- negative ions to explain the formation of particle's precursors. The formation of aromatic structures is suggested for n>3. The different relative abundance of C2Hx- ions is proposed to explain the different powder production rates with different initial gases or different fluxes. Hydrogen may inhibit the formation of high-mass negative ions and particles. Fast growth of spherical amorphous carbon particles (∼100s) was observed for a low pressure (0.1 mbar) acetylene plasma. The sp3 structure was confirmed in situ by infrared absorption spectroscopy. Slow growth (∼1000s) of irregularly shaped particles is observed for methane plasmas at the same pressure and power. Particle heterogeneity was studied with Transmission Electron Microscopy and showed graphitic onions, multiwalled nanotubes, flakes and agglomerates attributed to the erosion or the delamination of the deposited carbon layer. The influence of the recycling of the deposited layer on the powder formation was observed in situ from enhanced particle formation as the experiments were repeated. A similar behavior was observed for discharges containing mixtures of hexamethyldisiloxane, oxygen and helium. Keeping in mind that the control of powder homogeneity implies a control over the reactor contamination and its history, we conclude that an important part of the particulates formed in the discharges studied depends on the gas phase reactions, determined by the plasma chemistry.

EPFL2002

Design and fabrication of an aerosol collector for infrared spectroscopy analysis

Nikunj Dudani

Particulate matter (PM) pollution causes adverse health effects and millions of deaths each year. PM or aerosol, is difficult to characterize because of its wide range of particle sizes, constituents (various organic and inorganic compounds), concentration, morphology, state (liquid or solid), and time-dependent modification. Infrared (IR) spectroscopy is a non-destructive method which provides useful chemical information about the constituents of PM. Current methods for collecting samples use filters made of materials which interfere with the IR spectra and thus have lower detection capabilities. In order to improve the chemical resolution, collection on an IR-transparent substrate is desirable. However, as most current collector types modify or preferentially sample certain size ranges, chemical composition, morphology or state, a new collector design is required. Electrostatic precipitation (ESP), impaction and filtration are common methods for aerosol collection. Impactors preferentially sample certain size range and liquids and suffer from particle bounce-off effects. Filtration onto filters uses a high pressure-drop that can modify the aerosol chemical composition and has inherent size-dependence. ESP is a versatile method of aerosol collection and does not suffer from high pressure-drop, or from bounce-off effects and is highly tunable, allowing its use in various applications. However, most ESP designs in the public domain have been designed for different purposes and face limitations in direct application in quantitative chemical composition measurement using IR spectroscopy. In order to rapidly develop prototypes and test the feasibility of such a device we used numerical simulations along with 3D-printing. A device was fabricated and tested for mass loading response and surface deposition profile with image analysis and variable aperture IR-spectroscopy. After observing near-quantitative response of the collector, we developed an analytical model to evaluate the particle collection response over a wider range of geometries and operating variables, with a goal to achieve specific objectives with the collector. Low size dependence, low chemical interference, and high collection efficiency are required to obtain an aerosol sample identical to the aerosol in air. High spatial uniformity in the deposition pattern is important for reducing the optical artefacts or the spectrometer dependence, and a high collection mass flux reduces the sample collection time needed for making a confident claim. The analytical model embodies the physics of particle migration trajectories due to fluid dynamics and electrostatics in a two-stage ESP device. We evaluated it against numerical simulations, which can only be run for a limited number of configurations. The scalable model allowed translating the objectives into quantifiable variables and to relate the ESP design to the collection performance, and evaluate the trade-offs to arrive at a design that is optimized across multiple length scales. The proposed collector should provide high chemical- and time-resolution information of PM in laboratory or ambient sampling settings. It has applications in laboratory studies into volatile and labile aerosol species that have low life-time, some of which are linked to harmful oxidative-stress in the human body.

EPFL2021