Charge decay in the spatial afterglow of plasmas and its impact on diffusion regimes.

The spatial afterglow is a region at the boundary of a non-equilibrium plasma where charged species relax into ambient equilibrium. In many applications, the spatial afterglow is the part of the plasma that interacts with surfaces, such as suspended particles or a material substrate. However, compared to the bulk plasma, there has been little effort devoted to studying the properties of the spatial afterglow, and a fundamental analysis has not yet been developed. Here, we apply double Langmuir probe measurements and develop an advection-diffusion-recombination model to provide a detailed description of charged species in the spatial afterglow over a wide range of pressures, temperatures, plasma dimensions, and flow rates. We find that the density of charged species in the spatial afterglow decays by orders of magnitude, which leads to a transition from ambipolar to free diffusion. These insights can be used to explain or predict experimental observations of phenomena, such as the charging of dust grains and the dose of charged species to a biomaterial.

Plasma Electrochemistry for Carbon-Carbon Bond Formation via Pinacol Coupling.

The formation of carbon-carbon bonds by pinacol coupling of aldehydes and ketones requires a large negative reduction potential, often realized with a stoichiometric reducing reagent. Here, we use solvated electrons generated via a plasma-liquid process. Parametric studies with methyl-4-formylbenzoate reveal that selectivity over the competing reduction to the alcohol requires careful control over mass transport. The generality is demonstrated with benzaldehydes, benzyl ketones, and furfural. A reaction-diffusion model explains the observed kinetics, and ab initio calculations provide insight into the mechanism. This study opens the possibility of a metal-free, electrically-powered, sustainable method for reductive organic reactions.

Rate, Efficiency, and Mechanisms of Electrochemical Perfluorooctanoic Acid Degradation with Boron-Doped Diamond and Plasma Electrodes.

The removal of per- or polyfluorinated alkyl substances (PFAS) has received increasing attention because of their extreme stability, our increasing awareness of their toxicity at even low levels, and scientific challenges for traditional treatment methods such as separation by activated carbon or destruction by advanced oxidation processes. Here, we performed a direct and systematic comparison of two electrified approaches that have recently shown promise for effective degradation of PFAS: plasma and conventional electrochemical degradation. We tailored a reactor configuration where one of the electrodes could be a plasma or a boron-doped diamond (BDD) electrode and operated both electrodes galvanostatically by continuous direct current. We show that while both methods achieved near-complete degradation of PFAS, the plasma was only effective as the cathode, whereas the BDD was only effective as the anode. Compared to the BDD, plasma required more than an order of magnitude higher voltage but lower current to achieve similar degradation efficiency with more rapid degradation kinetics. All these factors considered, it was noted that plasma or BDD degradation resulted in similar energy efficiencies. The BDD electrode exhibited zero-order kinetics, and thus, PFAS degradation using the conventional electrochemical method was kinetically controlled. On the contrary, analysis using a film model indicated that the plasma degradation kinetics of PFAS using plasma were mass-transfer-controlled because of the fast reaction kinetics. With the help of a simple quantitative model that incorporates mass transport, interfacial reaction, and surface accumulation, we propose that the degradation reaction kinetically follows an Eley-Rideal-type mechanism for the plasma electrode, and an intrinsic rate constant of 2.89 × 108 m4 mol-1 s-1 was obtained accordingly. The investigation shows that to realize the true kinetic potential of plasma degradation for water treatment, mass transfer to the interface must be enhanced.

Microscale Bipolar Charge Distributions on Surfaces after Liquid Wetting and Evaporation in an Electric Field.

Studies have shown that when insulator surfaces become electrostatically charged, complex spatial distributions of charge are produced, which are made up of micrometer-scale regions of both charge polarities. The origin of these charge patterns, often called "charge mosaics", is not understood. Here, we carried out controlled Kelvin force microscopy experiments on microfabricated interdigitated electrode systems to show that the process of wetting a surface by a liquid followed by evaporation of the liquid in an electric field can lead to neighboring micrometer-scale regions of positive and negative charge, which remain stable long after the electric field is removed. We thus suggest that local electric fields, perhaps due to the existing charge on the surface, can act in concert with liquid evaporation to contribute to the creation of charge mosaics.

Microplasmas for Advanced Materials and Devices.

Microplasmas are low-temperature plasmas that feature microscale dimensions and a unique high-energy-density and a nonequilibrium reactive environment, which makes them promising for the fabrication of advanced nanomaterials and devices for diverse applications. Here, recent microplasma applications are examined, spanning from high-throughput, printing-technology-compatible synthesis of nanocrystalline particles of common materials types, to water purification and optoelectronic devices. Microplasmas combined with gaseous and/or liquid media at low temperatures and atmospheric pressure open new ways to form advanced functional materials and devices. Specific examples include gas-phase, substrate-free, plasma-liquid, and surface-supported synthesis of metallic, semiconducting, metal oxide, and carbon-based nanomaterials. Representative applications of microplasmas of particular importance to materials science and technology include light sources for multipurpose, efficient VUV/UV light sources for photochemical materials processing and spectroscopic materials analysis, surface disinfection, water purification, active electromagnetic devices based on artificial microplasma optical materials, and other devices and systems including the plasma transistor. The current limitations and future opportunities for microplasma applications in materials related fields are highlighted.

Catalyst-free, highly selective synthesis of ammonia from nitrogen and water by a plasma electrolytic system.

There is a growing need for scalable ammonia synthesis at ambient conditions that relies on renewable sources of energy and feedstocks to replace the Haber-Bosch process. Electrically driven approaches are an ideal strategy for the reduction of nitrogen to ammonia but, to date, have suffered from low selectivity associated with the catalyst. Here, we present a hybrid electrolytic system characterized by a gaseous plasma electrode that facilitates the study of ammonia formation in the absence of any material surface. We find record-high faradaic efficiency (up to 100%) for ammonia from nitrogen and water at atmospheric pressure and temperature with this system. Ammonia measurements under varying reaction conditions in combination with scavengers reveal that the unprecedented selectivity is achieved by solvated electrons produced at the plasma-water interface, which react favorably with protons to produce the key hydrogen radical intermediate. Our results demonstrate that limitations in selectivity can be circumvented by using catalyst-free solvated electron chemistry. In the absence of adsorption steps, the importance of controlling proton concentration and transport is also revealed.

Nanoparticle based simple electrochemical biosensor platform for profiling of protein-nucleic acid interactions.

The analysis of protein-nucleic acid interactions is essential for biophysics related research. However, simple, rapid, and accurate methods for quantitative analysis of biomolecular interactions are lacking. We herein establish an electrochemical biosensor approach for protein-nucleic acid binding analysis. Nanoparticle based sensors are fabricated by highly-controlled inkjet printing followed by plasma conversion. A novel bioconjugation method is demonstrated as a simple and rapid approach for protein-based biosensor fabrication. As a proof of concept, we analyzed the binding interaction between unwinding protein 1 (UP1) and SL3ESS3 RNA, confirming the accuracy of this nanoparticle based electrochemical biosensor approach with traditional biophysical methods. We further accurately profiled and differentiated a unique binding interaction pattern of multiple G-tract nucleic acid sequences with heterogeneous nuclear ribonucleoprotein H1. Our study provides insights into a potentially universal platform for in vitro biomolecule interaction analysis using a nanoparticle based electrochemical biosensor approach.

Electrically Conductive, Reduced Graphene Oxide Structures Fabricated by Inkjet Printing and Low Temperature Plasma Reduction.

Here, an environmentally-friendly and scalable process is reported to synthesize reduced graphene oxide (RGO) thin films for printed electronics applications. The films are produced by inkjet printing GO flakes dispersed binder-free in aqueous solutions followed by treatment with a nonthermal, radio-frequency (RF) plasma containing only argon (Ar) gas. The plasma process is found to heat the substrate to temperatures no greater than 138 °C, enabling RGO to be printed directly on a wide range of temperature-sensitive substrate materials including photo paper. Unlike other low-temperature methods such as electrochemical reduction, plasma reduction is friendly to moisture absorbent materials. Moreover, the plasma treatment can be performed on nonconducting substrates, eliminating the need for film transfer. From an applications perspective, the printed, plasma-reduced RGO exhibits excellent electrical, mechanical, and electrochemical properties. As a technology demonstrator, the working electrodes of hydrogen peroxide (H2O2) sensors fabricated from plasma-reduced GO show a sensitivity of 277 ± 80 µA mm-1 cm-2, which is comparable to RGO working electrodes made by electrochemical reduction.

Direct, Transfer-Free Growth of Large-Area Hexagonal Boron Nitride Films by Plasma-Enhanced Chemical Film Conversion (PECFC) of Printable, Solution-Processed Ammonia Borane.

Synthesis of large-area hexagonal boron nitride (h-BN) films for two-dimensional (2D) electronic applications typically requires high temperatures (∼1000 °C) and catalytic metal substrates which necessitate transfer. Here, analogous to plasma-enhanced chemical vapor deposition, a nonthermal plasma is employed to create energetic and chemically reactive states such as atomic hydrogen and convert a molecular precursor film to h-BN at temperatures as low as 500 °C directly on metal-free substrates-a process we term plasma-enhanced chemical film conversion (PECFC). Films containing ammonia borane as a precursor are prepared by a variety of solution processing methods including spray deposition, spin coating, and inkjet printing and reacted in a cold-wall reactor with a planar dielectric barrier discharge operated at atmospheric pressure in a background of argon or a mixture of argon and hydrogen. Systematic characterization of the converted h-BN films by micro-Raman spectroscopy shows that the minimum temperature for nucleation on silicon-based substrates can be decreased from 800 to 500 °C by the addition of a plasma. Furthermore, the crystalline domain size, as reflected by the full width at half-maximum, increased by more than 3 times. To demonstrate the potential of the h-BN films as a gate dielectric in 2D electronic devices, molybdenum disulfide field effect transistors were fabricated, and the field effect mobility was found to be improved by up to 4 times over silicon dioxide. Overall, PECFC allows h-BN films to be grown at lower temperatures and with improved crystallinity than CVD, directly on substrates suitable for electronic device fabrication.

Tuning Optical Signatures of Single- and Few-Layer MoS2 by Blown-Bubble Bulge Straining up to Fracture.

Emerging atomic layer semiconducting crystals such as molybdenum disulfide (MoS2) are promising candidates for flexible electronics and strain-tunable devices due to their ultrahigh strain limits (up to ∼20-30%) and strain-tunable bandgaps. However, high strain levels, controllable isotropic and anisotropic biaxial strains in single- and few-layer MoS2 on device-oriented flexible substrates permitting convenient and fast strain tuning, remain unexplored. Here, we demonstrate a "blown-bubble" bulge technique for efficiently applying large strains to atomic layer MoS2 devices on a flexible substrate. As the strain increases via bulging, we achieve continuous tuning of Raman and photoluminescence (PL) signatures in single- and few-layer MoS2, including splitting of Raman peaks. With proper clamping of the MoS2 crystals, we apply up to ∼9.4% strain in the flexible substrate, which causes a doubly clamped single-layer MoS2 to fracture at 2.2-2.6% strain measured by PL and 2.9-3.5% strain measured by Raman spectroscopy. This study opens new pathways for exploiting 2D semiconductors on stretchable substrates for flexible electronics, mechanical transducers, tunable optoelectronics, and biomedical transducers on curved and bulging surfaces.

Microplasma-Induced in Situ Formation of Patterned, Stretchable Electrical Conductors.

We report a microplasma-based process to fabricate stretchable, electrically conductive metal patterns from metal-cation containing polymers. The technique is compatible with prestraining strategies, allowing films to remain conductive with almost no drop in resistance up to 35% strain. We show that the stretchability of the films is related to uniform strain delocalization which is made possible by how the metallized layer is formed in situ, growing from within the polymer matrix rather than by deposition, to create a quasi-monolithic structure without a well-defined metal-polymer interfacial boundary.

Nonthermal Plasma Synthesis of Nanocrystals: Fundamental Principles, Materials, and Applications.

Nonthermal plasmas have emerged as a viable synthesis technique for nanocrystal materials. Inherently solvent and ligand-free, nonthermal plasmas offer the ability to synthesize high purity nanocrystals of materials that require high synthesis temperatures. The nonequilibrium environment in nonthermal plasmas has a number of attractive attributes: energetic surface reactions selectively heat the nanoparticles to temperatures that can strongly exceed the gas temperature; charging of nanoparticles through plasma electrons reduces or eliminates nanoparticle agglomeration; and the large difference between the chemical potentials of the gaseous growth species and the species bound to the nanoparticle surfaces facilitates nanocrystal doping. This paper reviews the state of the art in nonthermal plasma synthesis of nanocrystals. It discusses the fundamentals of nanocrystal formation in plasmas, reviews practical implementations of plasma reactors, surveys the materials that have been produced with nonthermal plasmas and surface chemistries that have been developed, and provides an overview of applications of plasma-synthesized nanocrystals.

The effect of shear flow on nanoparticle agglomeration and deposition in in vitro dynamic flow models.

Traditional in vitro toxicity experiments typically involve exposure of a mono- or co-culture of cells to nanoparticles (NPs) in static conditions with the assumption of 100% deposition (i.e. dose) of well-dispersed particles. However, cellular dose can be affected by agglomeration and the unique transport kinetics of NPs in biological media. We hypothesize that shear flow can address these issues and achieve more predictable dosage. Here, we compare the behavior of gold NPs with diameters of 5, 10 and 30 nm in static and dynamic in vitro models. We also utilize transport modeling to approximate the shear rate experienced by the cells in dynamic conditions to evaluate physiological relevance. The transport kinetics show that NP behavior is governed by both gravity and diffusion forces in static conditions and only diffusion in dynamic conditions. Our results reveal that dynamic systems are capable of producing a more predictable dose compared to static systems, which has strong implications for improving repeatability in nanotoxicity assessments.

The solvation of electrons by an atmospheric-pressure plasma.

Solvated electrons are typically generated by radiolysis or photoionization of solutes. While plasmas containing free electrons have been brought into contact with liquids in studies dating back centuries, there has been little evidence that electrons are solvated by this approach. Here we report direct measurements of solvated electrons generated by an atmospheric-pressure plasma in contact with the surface of an aqueous solution. The electrons are measured by their optical absorbance using a total internal reflection geometry. The measured absorption spectrum is unexpectedly blue shifted, which is potentially due to the intense electric field in the interfacial Debye layer. We estimate an average penetration depth of 2.5 ± 1.0 nm, indicating that the electrons fully solvate before reacting through second-order recombination. Reactions with various electron scavengers including H(+), NO2(-), NO3(-) and H2O2 show that the kinetics are similar, but not identical, to those for solvated electrons formed in bulk water by radiolysis.

Solid state transformations in consequence of electrospraying--a novel polymorphic form of piroxicam.

The aim of the research was to verify that electrospraying of piroxicam yielded a new polymorphic form of this drug. In the experiments, piroxicam was dissolved in chloroform and the solution was atomised electrostatically. Subsequently, the charged droplets were neutralised and dried. The solid drug particles were collected and analysed by scanning electron microscopy, X-ray diffraction, differential scanning calorimetry, high performance liquid chromatography, and infrared and Raman spectroscopy. The X-ray diffractogram measured for the electrosprayed piroxicam particles did not match with any of the known piroxicam crystal structures (Cambridge Crystallographic Data Centre). The variable temperature X-ray diffraction showed that the structure recrystallised completely into piroxicam polymorphic formI during heating. No degradation products or solvate removal was detected by high performance liquid chromatography and thermal analysis. The infrared and Raman spectra of the electrosprayed piroxicam were compared to those of formI, and some notable differences in the peak positions, shapes and intensities were detected. The results indicate that electrospraying leads to piroxicam crystallisation in a currently unknown polymorphic form.

Particle size effects in particle-particle triboelectric charging studied with an integrated fluidized bed and electrostatic separator system.

Fundamental studies of triboelectric charging of granular materials via particle-particle contact are challenging to control and interpret because of foreign material surfaces that are difficult to avoid during contacting and measurement. The measurement of particle charge itself can also induce charging, altering results. Here, we introduce a completely integrated fluidized bed and electrostatic separator system that charges particles solely by interparticle interactions and characterizes their charge on line. Particles are contacted in a free-surface fluidized bed (no reactor walls) with a well-controlled fountain-like flow to regulate particle-particle contact. The charged particles in the fountain are transferred by a pulsed jet of air to the top of a vertically-oriented electrostatic separator consisting of two electrodes at oppositely biased high voltage. The free-falling particles migrate towards the electrodes of opposite charge and are collected by an array of cups where their charge and size can be determined. We carried out experiments on a bidisperse size mixture of soda lime glass particles with systematically varying ratios of concentration. Results show that larger particles fall close to the negative electrode and smaller particles fall close to the positive electrode, consistent with theory and prior experiments that larger particles charge positively and smaller particles charge negatively. The segregation of particles by charge for one of the size components is strongest when its collisions are mostly with particles of the other size component; thus, small particles segregate most strongly to the negative sample when their concentration in the mixture is small (and analogous results occur for the large particles). Furthermore, we find additional size segregation due to granular flow, whereby the fountain becomes enriched in larger particles as the smaller particles are preferentially expelled from the fountain.

Ligand-free Ni nanocluster formation at atmospheric pressure via rapid quenching in a microplasma process.

The production of metal nanoclusters composed of less than 10(3) atoms is important for applications in energy conversion and medicine, and for fundamental studies of nanomaterial nucleation and growth. Unfortunately, existing synthesis methods do not enable adequate control of cluster formation, particularly at atmospheric pressure wherein formation typically occurs on sub-millisecond timescales. Here, we demonstrate that ligand-free, unagglomerated nickel nanoclusters can be continuously synthesized at atmospheric pressure via the decomposition of bis(cyclopentadienyl)nickel(II) (nickelocene) in a spatially-confined microplasma process that rapidly quenches particle growth and agglomeration. The clusters were measured on line by ion mobility spectrometry (IMS) and further analyzed by atomic force microscopy (AFM). Our results reveal that stable clusters with spherical equivalent mean diameters below 10 Åare produced, and by controlling the nickelocene concentration, the mean diameter can be tuned up to ∼50 Å. Although diameter is often the sole metric used in nanocluster and nanoparticle characterization, to infer the number of atoms in AFM and IMS detected clusters, we compare measured AFM heights and IMS inferred collision cross sections to theoretical predictions based on both bulk matter approximations and density functional theory and Hartree-Fock calculated Ni nanocluster structures (composed of 2-15 atoms for the latter). The calculations suggest that Ni nanoclusters composed of less than 10(2) atoms can be produced repeatably with simple microplasma reactors.

Fabrication of electrically conductive metal patterns at the surface of polymer films by microplasma-based direct writing.

We describe a direct-write process for producing electrically conductive metal patterns at the surface of polymers. Thin films of poly(acrylic acid) (PAA) loaded with Ag ions are reduced by a scanning, atmospheric-pressure microplasma to form crystalline Ag features with a line width of 300 µm. Materials analysis reveals that the metallization occurs in a thin layer of ∼5 µm near the film surface, suggesting that the Ag ions diffuse to the surface. Sheet resistances of 1-10 Ω/sq are obtained independent of film thickness and Ag volume concentration, which is desirable for producing surface conductivity on polymers while minimizing metal loading.

Decoupling interfacial reactions between plasmas and liquids: charge transfer vs plasma neutral reactions.

Plasmas (gas discharges) formed at the surface of liquids can promote a complex mixture of reactions in solution. Here, we decouple two classes of reactions, those initiated by electrons (electrolysis) and those initiated by gaseous neutral species, by examining an atmospheric-pressure microplasma formed in different ambients at the surface of aqueous saline (NaCl) solutions. Electrolytic reactions between plasma electrons and aqueous ions yield an excess of hydroxide ions (OH(-)), making the solution more basic, while reactions between reactive neutral species formed in the plasma phase and the solution lead to nitrous acid (HNO2), nitric acid (HNO3), and hydrogen peroxide (H2O2), making the solution more acidic. The relative importance of either reaction path is quantified by pH measurements, and we find that it depends directly on the composition of the ambient background gas. With a background gas of oxygen or argon, electron transfer reactions yielding excess OH(-) dominate, while HNO2 and HNO3 formed in the plasma and by the dissolution of nitrogen oxide (NOx) species dominate in the case of air and nitrogen. For pure nitrogen (N2) gas, we observe a unique coupling between both reactions, where oxygen (O2) gas formed via water electrolysis reacts in the bulk of the plasma to form NOx, HNO2, and HNO3.