Simon Chouteau

Gold nanoparticles (AuNPs) are attractive substrates for surface-enhanced Raman scattering (SERS) because of their strong visible plasmon resonance, chemical stability, and biocompatibility. However, the enhancement they provide depends on how accessible the gold surface is to the analyte. Tannic acid (TA) is an effective green reductant and stabilizer for AuNPs. Conventional TA-based syntheses, however, require large doses and boiling, leaving a dense organic shell on the particles. Herein, an argon cold atmospheric-pressure plasma jet combined with a low dose of TA reduced HAuCl₄ to AuNPs at room temperature without additional chemical reductants. Well-dispersed colloids of quasi-spherical, crystalline particles about 10 nm in size formed with 25-fold less TA than was required without plasma. In situ UV–Vis spectroscopy showed that low-dose TA largely removed the induction period of plasma reduction and that particle growth continued after the discharge was switched off. This result suggests that plasma supplies most of the reducing power, whereas TA stabilizes the nascent nuclei. XPS confirmed metallic gold, with a small positive Au 4f shift consistent with surface-bound TA. The lightly capped AuNPs gave stronger methylene blue SERS signals than both TA-free plasma AuNPs and AuNPs prepared with 25-fold more TA. This room-temperature route therefore provides clean colloidal AuNPs as promising SERS substrates for chemical sensing.

We investigate a low-power (<100 W), low-flow (0.3 slm) argon planar gliding discharge (GD) open to ambient air using spatiotemporally resolved optical emission spectroscopy coupled with a collisional-radiative model to characterize electron and neutral gas temperatures under two discharge current conditions. At low current (≈0.05 A), the electron temperature reaches 1.7 ± 0.2 eV, while the neutral gas temperature remains at 810 ± 20 K, indicating a strongly non-equilibrium state. At higher current (≈0.5 A), the neutral gas temperature increases to 1280 ± 10 K, whereas the electron temperature remains nearly unchanged, revealing distinct energy coupling pathways for electrons and heavy species. Temporal scans of the electron temperature at fixed spatial positions along the discharge axis show little variation, while measurements near the cathode exhibit substantial variations (1.5–4 eV), indicating varying conditions within the near-cathode region. These results demonstrate that the present GD operates in a purely non-equilibrium regime and stress the importance of diagnostic platforms enabling localized, spatiotemporal characterizations.

The effect of a nitrogen admixture to argon gas, on the densities of the Ar 1s 5 and 1s 3 metastable atoms and the intensities of Ar 2p → 1s spectral lines (in Paschen notation) radiated by a capacitively coupled plasma is investigated experimentally and by a computational model comprising particle simulation of the discharge and a fluid (diffusion-reaction-radiation) description for the excited Ar atoms. The nitrogen admixture has a twofold effect on the plasma and its radiation processes: N 2 molecules (i) efficiently quench excited Ar atoms thereby influencing their densities and thus also the ionization and excitation balance, and (ii) due to the lower threshold energy for inelastic excitation channels, the high-energy part of the electron energy probability function is strongly depleted. Both of these effects result in significant changes of the Ar-I emission spectrum due to the roles of the excited (Ar 1s) atoms in the stepwise excitation, and of the high-energy electrons in the direct excitation of Ar 2p levels from which the investigated transitions originate. The simulations make it possible to separate these two contributions and to reveal the excitation and depopulation pathways of the excited Ar-I levels. Experimentally, the densities of the Ar 1s 5 and 1s 3 metastable atoms are measured with tunable diode laser absorption spectroscopy while the Ar 2p → 1s spectral line intensities are acquired with conventional optical emission spectroscopy (with relative calibration). The computational results are found to be in fair agreement with these measured quantities over the whole range of parameters considered ( p = 2.5–40 Pa, [ N 2 ] = 0%–5%) thus confirming the conclusions from the simulations.

Ce travail porte sur la croissance de couches minces nanocomposites dans un procédé à plasma « brumeux » à basse pression, couplant dépôt chimique en phase vapeur assisté par plasma et injection de solution colloïdale. Cinq études sont réalisées. Dans un premier temps, le mécanisme de croissance des couches est étudié par ellipsométrie spectroscopique in situ : l’apport de matière est alternativement riche en nanoparticules, puis en matériau matrice. La deuxième étude identifie l’ébullition « flash » comme étant le mécanisme de formation des sprays dans le milieu plasma à basse pression. Cette connaissance a permis d’acquérir un contrôle fin sur la taille des gouttelettes constituant l’aérosol, et donc sur l’uniformité des couches minces nanocomposites synthétisées. Ensuite, un modèle collisionnel-radiatif a permis l’analyse poussée de spectres d’émission optique résolus en temps, afin d’étudier l’interaction aérosol-plasma. Ainsi, les évolutions temporelles de la densité d’argon métastable, de la température électronique, de la densité électronique, et de la fréquence de désexcitation collisionnelle ont pu être déterminées par inférence bayésienne au cours de l’injection de liquide dans le plasma. De plus, l’étude de l’interaction aérosol-surface a permis de mettre en évidence l’apparition l’évaporation « stick-slip » puis de l’effet Leidenfrost sur les surfaces chaudes. Enfin, les connaissances tirées de ces cinq études ont été mises en application lors de la synthèse de couches minces nanocomposites contenant des nanomatériaux de natures diverses.

Optical emission spectroscopy (OES) is widely used for non-invasive plasma characterisation. OES data is often coupled with a collisional-radiative model (CRM) to get insight on the fundamental plasma properties. A CRM comes with many adjustable parameters, whose value must provide the best agreement between the measured and simulated spectra. In this work, a CRM is used to analyse time-resolved OES measurements of low-pressure ‘misty’ plasmas, in this case Ar plasmas in which liquid droplets are injected as aerosols in a pulsed fashion. It is shown that series of liquid pulses can lead to significant quenching of Ar(1 s ), even though a single injection does not. The search for the best-matching spectrum is framed in terms of probability distributions, taking full advantage of the principles of Bayesian inference. Some of the tools provided by Bayesian analysis are illustrated in the specific case of OES-CRM analysis of Ar 2 p -1 s transitions. Both experimental and modelling uncertainties are considered in a rigorous mathematical framework, and results are expressed as probability distributions instead of single values. The question of model selection is addressed through the estimation of the Bayes factor, applied as a quantitative filter for superfluous adjustable parameters.

Misty plasmas have recently emerged as a promising tool for nanocomposite thin films deposition. However, aerosol-plasma interactions remain poorly documented, especially at low working pressure. In this work, optical emission spectroscopy is used to probe the temporal evolution of three fundamental plasma parameters during pulsed liquid injection in an inductively coupled argon plasma at low-pressure. Time-resolved values of metastable argon density, electron temperature, and electron density are determined from radiation trapping analysis and particle balance equations of selected argon 1s and 2p levels. Pulsed liquid injection is found to induce a sudden drop in metastable density and electron temperature, and an increase in electron density. These results are attributed to the lower ionization thresholds of the injected molecular species compared to the one of argon. In addition, upstream liquid temperature is found to affect the transitory kinetics for non-volatile solvents more than volatile ones, in accordance with a previously reported flash boiling atomization mechanism.

Misty plasma processes based on colloidal solutions sprayed into low-pressure plasmas have recently shown great potential for multifunctional thin film deposition. In such processes, nanoparticle accumulation in ring-shaped structures remains the main obstacle to the synthesis of high-quality coatings containing abundant, small-scale, and evenly dispersed nanoparticles. These local buildups appear after a colloidal droplet evaporates from a substrate. Accordingly, controlling the droplets' size in the spray is of key importance to ensure a uniform nanoparticle content in the plasma-deposited nanocomposite film. In this work, it is shown that the use of more volatile solvents produces finer droplets on the substrate, thereby improving nanoparticle dispersion in the matrix. A one-dimensional evaporation model is further developed and used to show that, contrary to what one might expect, this result cannot be attributed to faster evaporation during droplet transport in the low-pressure plasma. Instead, a so-called "flash" boiling atomization mechanism is discussed to support the experimental findings.

Mist-assisted methods have recently attracted much attention for plasma deposition in high-quality (multi)functional thin films. However, very little is known on plasma interactions with misted colloidal solutions and their role in plasma process kinetics. Time-resolved optical diagnostics have been carried out to study the deposition of TiO₂–SiO₂ nanocomposite thin films in low-pressure oxygen-argon plasmas with organosilicon precursors and TiO₂ suspensions. Each pulsed injection of the dispersion was followed by a pressure rise due to solvent evaporation. This caused a significant reduction in the electron temperature and density, which mitigated matrix precursor fragmentation and SiO₂ deposition as TiO₂ nanoparticles were supplied to the film. Comparing injections with and without nanoparticles, misty plasma effects were dominated by plasma droplets rather than plasma-nanoparticle interactions. Successive matrix-rich and nanoparticle-rich deposition steps were confirmed by in situ spectroscopic ellipsometry.