From Solid to Fluid: Novel Approaches in Neuromorphic Engineering.

Neuromorphic engineering is rapidly developing as an approach to mimicking processes in brains using artificial memristors, devices that change conductivity in response to the electrical field (resistive switching effect). Memristor-based neuromorphic systems can overcome the existing problems of slow and energy-inefficient computing that conventional processors face. In the Introduction, the basic principles of memristor operation and its applications are given. The history of switching in sandwich structures and granular metals is reviewed in the Historical Overview. Particular attention is paid to the fundamental articles from the pre-memristor era (the 1960s-70s), which demonstrated the first evidence of resistive switching and predicted the filamentary mechanism of switching. Multi-dimensionality in neuromorphic systems: Despite the powerful computational abilities of traditional memristor arrays, they cannot repeat many organizational characteristics of biological neural networks, i.e., their multi-dimensionality. This part reviews the unconventional nanowire- and nanoparticle-based neuromorphic systems that demonstrate incredible potential for use in reservoir computing due to the unique spiking change in conductance similar to firing in neurons. Liquid-based neuromorphic devices: The transition of neuromorphic systems from solid to liquid state broadens the possibilities for mimicking biological processes. In this section, ionic current memristors are reviewed and, the working principles of which bring us closer to the mechanisms of information transmittance in real synapses. Nanofluids: A novel direction in neuromorphic engineering linked to the application of nanofluids for the formation of reconfigurable nanoparticle networks with memristive properties is given in this section. The Conclusion t summarizes the bullet points of the Review and provides an outlook on the future of liquid-state neuromorphic systems.

One-step synthesis of photoluminescent nanofluids by direct loading of reactively sputtered cubic ZrN nanoparticles into organic liquids.

ZrN nanofluids may exhibit unique optoelectronic properties because of the matching of the solar spectrum with interband transitions and localized surface plasmon resonance (LSPR). Nevertheless, these nanofluids have scarcely been investigated, mainly because of the complexity of the current synthetic routes that involve aggressive chemicals and high temperatures. This work aims to validate reactive dc magnetron sputtering of zirconium in Ar/N2 as an environmentally benign, annealing-free method to produce 22 nm-sized, highly crystalline, stoichiometric, electrically conductive, and plasmonic ZrN nanoparticles (NPs) of cubic shape and to load them into vacuum-compatible liquids of different chemical compositions (polyethylene glycol (PEG), paraffin, and pentaphenyl trimethyl trisiloxane (PTT)) in one step. The nanofluids demonstrate LSPR in the red/near-IR range that gives them a bluish color in transmittance. The nanofluids also demonstrate complex photoluminescence behavior such that ZrN NPs enhance the photoluminescence (PL) intensity of paraffin and PEG, whereas the PL of PTT remains almost invariable. Based on DFT calculations, different energetic barriers to charge transfer between ZrN and the organic molecules are suggested as the main factors that influence the observed optoelectronic response. Overall, our study provides a novel approach to the synthesis of transition metal nitride nanofluids in an environmentally friendly manner, deepens the understanding of the interactions between ZrN and organic molecules, and unveils new optoelectronic phenomena in such systems.

Plasmonic Ag/Cu/PEG nanofluids prepared when solids meet liquids in the gas phase.

Since the time of Faraday's experiments, the optical response of plasmonic nanofluids has been tailored by the shape, size, concentration, and material of nanoparticles (NPs), or by mixing different types of NPs. To date, water-based liquids have been the most extensively investigated host media, while polymers, such as poly(ethylene glycol) (PEG), have frequently been added to introduce repulsive steric interactions and protect NPs from agglomeration. Here, we introduce an inverse system of non-aqueous nanofluids, in which Ag and Cu NPs are dispersed in PEG (400 g mol-1), with no solvents or chemicals involved. Our single-step approach comprises the synthesis of metal NPs in the gas phase using sputtering-based gas aggregation cluster sources, gas flow transport of NPs, and their deposition (optionally simultaneous) on the PEG surface. Using computational fluid dynamics simulations, we show that NPs diffuse into PEG at an average velocity of the diffusion front of the order of µm s-1, which is sufficient for efficient loading of the entire polymer bulk. We synthesize yellow Ag/PEG, green Cu/PEG, and blue Ag/Cu/PEG nanofluids, in which the color is given by the position of the plasmon resonance. NPs are prone to partial agglomeration and sedimentation, with a slower kinetics for Cu. Density functional theory calculations combined with UV-vis data and zeta-potential measurements prove that the surface oxidation to Cu2O and stronger electrostatic repulsion are responsible for the higher stability of Cu NPs. Adopting the De Gennes formalism, we estimate that PEG molecules adsorb on the NP surface in mushroom coordination, with the thickness of the adsorbed layer L < 1.4 nm, grafting density σ < 0.20, and the average distance between the grafted chains D > 0.8 nm. Such values provide sufficient steric barriers to retard, but not completely prevent, agglomeration. Overall, our approach offers an excellent platform for fundamental research on non-aqueous nanofluids, with metal-polymer and metal-metal interactions unperturbed by the presence of solvents or chemical residues.

Dual-Mode Solution Plasma Processing for the Production of Chitosan/Ag Composites with the Antibacterial Effect.

The development of novel biocompatible and biodegradable materials for medical applications has been drawing significant interest in the scientific community for years. Particularly, chitosan loaded with silver nanoparticles (Ag NPs) has a strong antimicrobial potential and could be applied, for example, as wound dressing material. In this work, chitosan/Ag NP composites were produced utilizing a single-step plasma-solution process, which is simple and environmentally friendly. An acetic solution of chitosan containing AgNO3 was treated by the direct current (DC) atmospheric pressure glow discharge, with the liquid serving as either cathode or anode. The plasma-solution system with liquid anode is more useful for the production of Ag NPs. Nevertheless, the NP size is comparable for both cases. The plasma treatment with both polarities led to chitosan degradation. The cleavage of glucosidic chains mostly occurred in the system with the liquid cathode, whereas the side oxidation reactions took place when the solution served as the anode. The oxidation processes were possibly induced by the hydrogen peroxide H2O2 efficiently formed in the last case. The composite materials produced with both polarities of liquid electrode demonstrated the bactericidal action against Gram-negative Escherichia coli, Gram-positive Staphylococcus aureus, and Gram-positive Bacillus subtilis.

Plasma Polymerization of Acrylic Acid for the Tunable Synthesis of Glassy and Carboxylated Nanoparticles.

Polymer nanoparticles (NPs) can be highly attractive in numerous applications, including biomedicine, where the use of inorganic matter may be detrimental for living tissues. In conventional wet chemistry, polymerization and functionalization of NPs with specific chemical groups involves complex and often numerous reactions. Here, we report on a solvent-free, single-step, low-temperature plasma-based synthesis of carboxylated NPs produced by the polymerization of acrylic acid under the conditions of a glow discharge. In a monomer-deficient regime, the strong fragmentation of monomer molecules by electron impact results in the formation of 15 nm-sized NPs with <1% retention of the carboxyl groups. In an energy-deficient regime, larger 90 nm-sized NPs are formed with better retention of carboxyl groups that reaches 16%. All types of NPs exhibit a glass transition above room temperature, which makes them highly stable in an aqueous environment with no dissolution or swelling. The NPs are also found to degrade thermally when heated above 150 °C, with a decrease in the mean NP size but with retention of the chemical composition. Thus, plasma polymerization proves to be a versatile approach for the production of polymer NPs with a tunable size distribution, chemical composition, and physical properties.

Infrared Absorption Spectroscopy of Albumin Binding with Amine-Containing Plasma Polymer Coatings on Nanoporous Diamond Surfaces.

Nanocrystalline diamond (NCD) layers functionalized with amine-containing functional groups have generated considerable interest as biocompatible substrates for attachment of biomolecules and cells with a view to biosensor and tissue engineering applications. Here we prepare nanoporous diamond layers with the surfaces modified by hydrogen plasma, oxygen plasma, and conformal 7 nm amine-containing plasma polymer (PP). Immobilization of bovine serum albumin (BSA) molecules is characterized on such surfaces. Grazing angle reflectance infrared spectroscopy as well as X-ray photoelectron spectroscopy show that concentration of amine-containing bonds after BSA exposure depends on the type of NCD surface modification. AFM measurements reveal that BSA proteins are physisorbed on H- and O-terminated diamond surfaces in different thicknesses and morphology. When the diamond layers are coated with the amine-containing PP, BSA molecules assume similar thickness and morphology, and their adhesion is significantly increased on both types of the diamond surfaces.

Magnetron Sputtering of Polymeric Targets: From Thin Films to Heterogeneous Metal/Plasma Polymer Nanoparticles.

Magnetron sputtering is a well-known technique that is commonly used for the deposition of thin compact films. However, as was shown in the 1990s, when sputtering is performed at pressures high enough to trigger volume nucleation/condensation of the supersaturated vapor generated by the magnetron, various kinds of nanoparticles may also be produced. This finding gave rise to the rapid development of magnetron-based gas aggregation sources. Such systems were successfully used for the production of single material nanoparticles from metals, metal oxides, and plasma polymers. In addition, the growing interest in multi-component heterogeneous nanoparticles has led to the design of novel systems for the gas-phase synthesis of such nanomaterials, including metal/plasma polymer nanoparticles. In this featured article, we briefly summarized the principles of the basis of gas-phase nanoparticles production and highlighted recent progress made in the field of the fabrication of multi-component nanoparticles. We then introduced a gas aggregation source of plasma polymer nanoparticles that utilized radio frequency magnetron sputtering of a polymeric target with an emphasis on the key features of this kind of source. Finally, we presented and discussed three strategies suitable for the generation of metal/plasma polymer multi-core@shell or core-satellite nanoparticles: the use of composite targets, a multi-magnetron approach, and in-flight coating of plasma polymer nanoparticles by metal.

Cu nanoparticles constrain segmental dynamics of cross-linked polyethers: a trade-off between non-fouling and antibacterial properties.

Copper has a strong bactericidal effect against multi-drug resistant pathogens and polyethers are known for their resistance to biofilm formation. Herein, we combined Cu nanoparticles (NPs) and a polyether plasma polymer in the form of nanocomposite thin films and studied whether both effects can be coupled. Cu NPs were produced by magnetron sputtering via the aggregation in a cool buffer gas whereas polyether layers were synthesized by Plasma-Assisted Vapor Phase Deposition with poly(ethylene oxide) (PEO) used as a precursor. In situ specific heat spectroscopy and XPS analysis revealed the formation of a modified polymer layer around the NPs which propagates on the scale of a few nanometers from the Cu NP/polymer interface and then transforms into a bulk polymer phase. The chemical composition of the modified layer is found to be ether-deficient due to the catalytic influence of copper whereas the bulk polymer phase exhibits the chemical composition close to the original PEO. Two cooperative glass transition phenomena are revealed that belong to the modified polymer layer and the bulk phase. The former is characterized by constrained mobility of polymer segments which manifests itself via a 30 K increase of dynamic glass transition temperature. Furthermore, the modified layer is characterized by the heterogeneous structure which results in higher fragility of this layer as compared to the bulk phase. The Cu NPs/polyether thin films exhibit reduced protein adsorption; however, the constrained segmental dynamics leads to the deterioration of the non-fouling properties for ultra-thin polyether coatings. The films are found to have a bactericidal effect against multi-drug resistant Gram-positive Methicillin-Resistant Staphylococcus aureus and Gram-negative Pseudomonas aeruginosa.

Magnetron-sputtered copper nanoparticles: lost in gas aggregation and found by in situ X-ray scattering.

Magnetron discharge in a cold buffer gas represents a liquid-free approach to the synthesis of metal nanoparticles (NPs) with tailored structure, chemical composition and size. Despite a large number of metal NPs that were successfully produced by this method, the knowledge of the mechanisms of their nucleation and growth in the discharge is still limited, mainly because of the lack of in situ experimental data. In this work, we present the results of in situ Small Angle X-ray Scattering measurements performed in the vicinity of a Cu magnetron target with Ar used as a buffer gas. Condensation of atomic metal vapours is found to occur mainly at several mm distance from the target plane. The NPs are found to be captured preferentially within a region circumscribed by the magnetron plasma ring. In this capture zone, the NPs grow to the size of 90 nm whereas smaller ones sized 10-20 nm may escape and constitute a NP beam. Time-resolved measurements of the discharge indicate that the electrostatic force acting on the charged NPs may be largely responsible for their capturing nearby the magnetron.

Carboxyl-Functionalized Nanoparticles Produced by Pulsed Plasma Polymerization of Acrylic Acid.

Carboxyl-enriched and size-selected polymer nanoparticles (NPs) may prove to be very useful in biomedical applications for linker-free binding of biomolecules and their transport to cells. In this study, we report about the synthesis of such NPs by low-pressure low-temperature pulsed plasma polymerization of acrylic acid. Gas aggregation cluster source was adapted to operate plasma with a constant pulse period of 50 µs and with varying duty cycle. The NPs were produced with the size ranging from 31 ± 5 to 93 ± 14 nm and with retention of the carboxyl groups ranging from 4.0 to 12.0 atom %. Two regimes of the NP formation were identified. In the large duty cycle regime, the NP growth was interfered with by positive ion bombardment which resulted in the ion-driven detachment of the carboxyl species and in the formation of carboxyl-deficient NPs. In the small duty cycle regime, the NP growth was accompanied by the radical-driven chain propagation with the attachment of intact monomer molecules. Improved efficacy of the monomer retention resulted in increased concentration of the carboxyl groups.

In situ electrochemical AFM monitoring of the potential-dependent deterioration of platinum catalyst during potentiodynamic cycling.

A platinum catalyst undergoes complex deterioration process during its operation as a cathode in a proton exchange membrane fuel cell. By using in situ electrochemical atomic force microscopy (EC-AFM) with super-sharp probes, we quantitatively describe the roughening of platinum thin films during electrochemical cycling to different upper potentials, which simulate critical operation regimes of the proton exchange membrane fuel cell. The comprehensive quantitative analysis of morphology changes obtained using common roughness descriptors such as the root mean square roughness, the correlation length and the roughness exponent is correlated with cyclic voltammetry performed simultaneously.

Advances and challenges in the field of plasma polymer nanoparticles.

This contribution reviews plasma polymer nanoparticles produced by gas aggregation cluster sources either via plasma polymerization of volatile monomers or via radio frequency (RF) magnetron sputtering of conventional polymers. The formation of hydrocarbon, fluorocarbon, silicon- and nitrogen-containing plasma polymer nanoparticles as well as core@shell nanoparticles based on plasma polymers is discussed with a focus on the development of novel nanostructured surfaces.

Single-step generation of metal-plasma polymer multicore@shell nanoparticles from the gas phase.

Nanoparticles composed of multiple silver cores and a plasma polymer shell (multicore@shell) were prepared in a single step with a gas aggregation cluster source operating with Ar/hexamethyldisiloxane mixtures and optionally oxygen. The size distribution of the metal inclusions as well as the chemical composition and the thickness of the shells were found to be controlled by the composition of the working gas mixture. Shell matrices ranging from organosilicon plasma polymer to nearly stoichiometric SiO2 were obtained. The method allows facile fabrication of multicore@shell nanoparticles with tailored functional properties, as demonstrated here with the optical response.

In Situ Nanocalorimetric Investigations of Plasma Assisted Deposited Poly(ethylene oxide)-like Films by Specific Heat Spectroscopy.

In recent years, highly cross-linked plasma polymers have started to unveil their potential in numerous biomedical applications in thin-film form. However, conventional diagnostic methods often fail due to their diverse molecular dynamics conformations. Here, glassy dynamics and the melting transition of thin PEO-like plasma assisted deposited (ppPEO) films (thickness 100 nm) were in situ studied by a combination of specific heat spectroscopy, utilizing a pJ/K sensitive ac-calorimeter chip, and composition analytical techniques. Different cross-linking densities were obtained by different plasma powers during the deposition of the films. Glassy dynamics were observed for all values of the plasma power. It was found that the glassy dynamics slows down with increasing the plasma power. Moreover, the underlying relaxation time spectra broaden indicating that the molecular motions become more heterogeneous with increasing plasma power. In a second set of the experiment, the melting behavior of the ppPEO films was studied. The melting temperature of ppPEO was found to decrease with increasing plasma power. This was explained by a decrease of the order in the crystals due to formation of chemical defects during the plasma process.

Microphase-Separated PE/PEO Thin Films Prepared by Plasma-Assisted Vapor Phase Deposition.

Immiscible polymer blends tend to undergo phase separation with the formation of nanoscale architecture which can be used in a variety of applications. Different wet-chemistry techniques already exist to fix the resultant polymeric structure in predictable manner. In this work, an all-dry and plasma-based strategy is proposed to fabricate thin films of microphase-separated polyolefin/polyether blends. This is achieved by directing (-CH2-)100 and (-CH2-CH2-O-)25 oligomer fluxes produced by vacuum thermal decomposition of poly(ethylene) and poly(ethylene oxide) onto silicon substrates through the zone of the glow discharge. The strategy enables mixing of thermodynamically incompatible macromolecules at the molecular level, whereas electron-impact-initiated radicals serve as cross-linkers to arrest the subsequent phase separation at the nanoscale. The mechanism of the phase separation as well as the morphology of the films is found to depend on the ratio between the oligomeric fluxes. For polyolefin-rich mixtures, polyether molecules self-organize by nucleation and growth into spherical domains with average height of 22 nm and average diameter of 170 nm. For equinumerous fluxes and for mixtures with the prevalence of polyethers, spinodal decomposition is detected that results in the formation of bicontinuous structures with the characteristic domain size and spacing ranging between 5 × 10(1) -7 × 10(1) nm and 3 × 10(2)-4 × 10(2) nm, respectively. The method is shown to produce films with tunable wettability and biologically nonfouling properties.

Vacuum thermal degradation of poly(ethylene oxide).

Thermal degradation of poly(ethylene oxide) (PEO) was studied under vacuum conditions. PEO macromolecules degrade predominantly by random chain scission of a backbone with elimination of oligomer fragments. The reactions include the mechanism of radical termination by disproportionation. The eliminated fragments form thin film deposits which have chemical composition close to the original PEO. Activation of the evaporated flux with a glow discharge leads to further fragmentation and recombination of the released species and can be used to tune the properties of the resulting thin films.

Nanocomposite Ti/hydrocarbon plasma polymer films from reactive magnetron sputtering as growth support for osteoblast-like and endothelial cells.

Nanocomposite Ti/hydrocarbon plasma polymer (Ti/ppCH) films were deposited by DC magnetron sputtering of titanium target in n-hexane, argon, or a mixture of these two gases. The resultant films were heterogeneous, with inorganic regions of nanometer scale distributed within a plasma polymer matrix. The titanium content was controlled by adjusting the argon/n-hexane ratio in the working gas. In the pure n-hexane atmosphere, the Ti concentration was found to be below 1 at %, whereas in pure argon it reached 20 at %, as measured by Rutherford backscattering spectroscopy and elastic recoil detection analysis (RBS/ERDA). A high level of titanium oxidation is detected with TiO(2), substoichiometric titania, and titanium carbide, composing an inorganic phase of the composite films. In addition, high hydrogen content is detected in films rich with titanium. Ti-deficient and Ti-rich films proved equally good substrates for adhesion and growth of cultured human osteoblast-like MG 63 cells. In these cells, the population densities on days 1, 3, and 7 after seeding, spreading area on day 1, formation of talin-containing focal adhesion plaques as well as concentrations of talin and osteocalcin (per mg of protein) were comparable to the values obtained in cells on the reference cell culture materials, represented by microscopic glass coverslips or a polystyrene dish. An interesting finding was made when the Ti/ppCH films were seeded with calf pulmonary artery endothelial cells of the line CPAE. The cell population densities, the spreading area and also the concentration of von Willebrand factor, a marker of endothelial cell maturation, were significantly higher on Ti-rich than on Ti-deficient films. On Ti-rich films, these parameters were also higher or similar in comparison with the reference cell culture materials. Thus, both types of films could be used for coating bone implants, of which the Ti-rich film remains effective in enhancing the endothelialization of blood contacting artificial materials.

Mechanistic studies of plasma polymerization of allylamine.

Plasma polymerization of allylamine is performed both in continuous wave and pulsed mode. Chemical derivatization is applied to determine primary and secondary amine concentration. Primary amines are efficiently formed, but secondary amines are more abundant. A polymerization mechanism is proposed to account for the difference in amine content obtained from comparison between continuous wave and pulsed mode plasma polymerization. The AFM measurements performed on ultrathin (1-10 nm) plasma polymers confirm the continuity of films and that the film growth on silicon occurs via a layer-by-layer mechanism because no islandlike structures were detected.