Oxidant aggregate-induced porosity in vapour-deposited polymer films and correlated impact on electrochemical properties

ABSTRACT

Persistently doped conjugated polymers are integral for energy storage, flexible electronics, and biosensors due to their unique ability to interact with both ionic and electronic currents. To maximise the performance of devices across these fields, research has focused on controlling material properties to optimise conductivities of both types of charge carriers. The challenge lies in improving ionic transport, which is typically the rate-limiting step in redox processes, without sacrificing electronic conductivity or desirable mechanical properties. Here we report on control of nanostructure in vapour deposited conducting polymer films and correlate changes in film structure with resulting electrochemical properties. Structural control is enabled by exploiting the growth of oxidant nanoaggregates during the reactive vapour deposition process. Relative to dense films, porous films exhibit faster response times in electrochemical testing. Scan rate analysis confirms a transition away from diffusion-limited charging kinetics and demonstrates the important role that porosity can play in ion transport through electroactive polymers. Advantageously, continuous polymer networks remain evident in nanostructured films, ensuring that high electronic conductivities are maintained along with high porosity. We find that such enhanced properties are retained even as polymer thickness increases ten-fold. The films reported herein may serve as robust electrodes in flexible electrochemical devices.     

Electrical and magnetic properties of thin films containing metal clusters

A large variety of granular polymer thin films can be synthetized in a capacitively coupled coplanar diode radiofrequency (r.f.) discharge system in which an argon-monomer mixture is injected at low pressure (20 mTorr). This approach offers several advantages over the other techniques, e.g. easy control of the metal content in the film from a few % up to 100%. The d.c. electrical properties of gold containing plasma polymerized tetrafluoroethylene (PPTFE) and the magnetic behavior of cobalt containing plasma polymerized propane (PPP) are reported in this paper.     

Electrical and mechanical properties of thin metal films: Size effects

A general expression for the function of electron scattering in thin films of a grain structure is derived which takes account of both the external and internal size effects. Limiting and particular values of this function are given for polycrystalline and single-crystal films. Theoretical results are compared to the experimental results obtained for aluminium and tin. The relationship between size effects and resistivity, temperature coefficient of resistance, longitudinal and transverse strain coefficient of resistance and thermoelectric power is studied. The dependence of the orientation of monocrystalline copper and aluminium films on thermal stress, the direction of the energy density of elastic strains and the anistropy of elastoresistance coefficients in these films are examined. An expression for the gauge factor in single-crystal metal films of given orientations is derived.     

Control of optical properties of gel-derived oxide coating films containing fine metal particles

Control of the optical properties of gel-derived oxide films containing fine metal particles is described. The duration of the aging of Si(OC₂H₅)₄-derived sols and the amount of water for hydrolyzing Si(OC₂H₅)₄ were found to greatly affect the size and the shape of Au particles formed in the silica matrix, and accordingly the optical absorption of the Au/SiO₂ composite films. Employing dielectric media with high refractive indices like TiO₂ was shown to shift the absorption peak of Au particles to longer wavelengths. Pd/TiO₂ and Pt/TiO₂ composite films showed absorption in the visible region.     

Magnetic properties of transition metal films and islands on W(110)

In this contribution we report on magnetic phenomena of iron and cobalt islands on a W(110). In our experiment we employed the technique of magnetic circular dichroism in photoemission from the Fe and Co 3p semi core levels. Using highly circularly polarized synchrotron radiation in the soft X-ray region our photoemission data clearly show a remanent ferromagnetic effect for cobalt island on tungsten. The phase transition from the epitaxial iron and cobalt film to the island structure was additionally studied with LEED and atomic force microscopy.     

Electron hopping conductivity and vapor sensing properties of flexible network polymer films of metal nanoparticles

Films of monolayer protected Au clusters (MPCs) with mixed alkanethiolate and omega-carboxylate alkanethiolate monolayers, linked together in a network polymer by carboxylate-Cu2+-carboxylate bridges, exhibit electronic conductivities (sigma(EL)) that vary with both the numbers of methylene segments in the ligands and the bathing medium (N2, liquid or vapor). A chainlength-dependent swelling/contraction of the film's internal structure is shown to account for changes in sigma(EL). The linker chains appear to have sufficient flexibility to collapse and fold with varied degrees of film swelling or dryness. Conductivity is most influenced (exponentially dependent) by the chainlength of the nonlinker (alkanethiolate) ligands, a result consistent with electron tunneling through the alkanethiolate chains and nonbonded contacts between those chains on individual, adjacent MPCs. The sigma(EL) results concur with the behavior of UV-vis surface plasmon adsorption bands, which are enhanced for short nonlinker ligands and when the films are dry. The film conductivities respond to exposure to organic vapors, decreasing in electronic conductivity and increasing in mass (quartz crystal microgravimetry, QCM). In the presence of organic vapor, the flexible network of linked nanoparticles allows for a swelling-induced alteration in either length or chemical nature of electron tunneling pathways or both.     

Growth modes of ultrathin metal films on dissimilar metal substrates

This paper outlines some of the outstanding problems associated with the experimental determination of the growth modes of thin metal films on dissimilar metal substrates, and suggests approaches for overcoming them. Specifically, it discusses ways to differentiate between Stranski-Krastanov and alloy growth, the quantitative fitting of “Auger signaltime” plots, and the determination of the stoichiometry of surface alloys. A comparison of the observed growth modes with those predicted by the Macroscopic Atom Method shows that the results predicted by this method must be treated with extreme caution.     

Infrared-optical double resonance spectroscopic investigation of trifluoromethylphenols and their water complexes

The hydrogen bonding behavior of trifluoromethylphenols and their water complexes were investigated using IR-UV double resonance spectroscopy. Both ortho- and meta-trifluoromethylphenols exist in the syn conformer, which is the global minimum in both the cases. The IR spectrum in the O-H stretching region reveals the absence of an intramolecular O-H···F hydrogen bond in the syn-o-trifluoromethylphenol, which is in contrast to the results reported in the literature. The water complexes of both o-trifluoromethylphenol and m-trifluoromethylphenol are characterized by formation of O-H···O hydrogen bonds between the donor phenolic OH group and the acceptor water molecule. In addition, the o-trifluoromethylphenol-(water)(2) complex was also observed.     

Dispersion of functional tetraphenylporphyrin-ligated metal into ultra-thin flexible acrylate films. 2. Oxygen-uptake properties

Cobalt(II) meso-tetraphenylporphyrin (CoTPP)/acrylate hybrid thin films were prepared by CoTPP sublimation and reactive monomer evaporation onto the glass substrate in vacuum conditions. Deposited CoTPP/acrylate thin films were in situ photopolymerized. The oxygen-uptake behaviors of CoTPP/acrylate films were investigated by means of sorption measurements, monitored by gravimetric means, and analyzed using dual mode sorption model. The loading percent of CoTPP in the film was adjusted up to 60% by controlling the CoTPP sublimation rate. The thickness of the CoTPP/acrylate hybrid film was about 200 nm and oxygen-uptake data obtained from the sorption measurements indicated that CoTPP molecules in the CoTPP/acrylate hybrid films were able to bind oxygen molecules reversibly.     

Thin metal films on quasicrystalline surfaces

Thin metal films with a thickness of one or over one monolayer formed on quasicrystalline surfaces were studied using reflection high-energy electron diffraction, X-ray photoelectron spectroscopy, X-ray photoelectron diffraction and scanning tunneling microscopy. The substrates were the 10f surface of d-Al–Ni–Co and the 5f surface of i-Al–Pd–Mn. The metals deposited were Au, Pt, Ag and In. None of these metals forms any ordered layer by deposition onto clean quasicrystalline surfaces. However, if a submonolayer of In is present atop the 10f surface, an epitaxial layer of multiply-twinned AuAl₂ crystals is formed by Au deposition and subsequent annealing. This is also the case for Pt deposition, but not for Ag deposition. Although the surfactant effect of In is also observed in the case of Au deposition on the 5f surface of i-Al–Pd–Mn, the ordered layer formed is a film of Au–Al alloy with icosahedral symmetry. No ordered films are formed by Pt or Ag deposition onto the 5f surface, regardless of the presence of an In-precovered layer. A Sn film monolayer induced by surface diffusion was also studied.     

Electrochemical formation and properties of two-component films of transition metal complexes and C60 or C70

The electrochemically active polymers have been formed during electro-reduction carried out in solution containing fullerenes, C₆₀ or C₇₀, and transition metal complexes of Pd(II), Pt(II), Rh(III), and Ir(I). In these films, fullerene moieties are covalently bounded to transition metal atoms (Pd and Pt) or their complexes (Rh and Ir) to form a polymeric network. All films exhibit electrochemical activity at negative potentials due to the fullerene cages reduction process. For all studied metal complexes, yields of formation of films containing C₇₀ are higher than yields of electrodeposition of their C₆₀ analogs. C₇₀ /M films also exhibit higher porosity in comparison to C₆₀/M layers. The differences in film morphology and efficiency of polymer formation are responsible for differences in electrochemical responses of these films in acetonitrile containing supporting electrolyte only. C₇₀/M films shows more reversible voltammeric behavior in negative potential range. They also show higher potential range of electrochemical stability. Processes of film formation and electrochemical properties of polymers depend on the transition metal ions or atoms bonding fullerene cages into polymeric network. The highest efficiency of polymerization was observed for fullerene/Pd and fullerene/Rh films. In the case of fullerene/Pd films, the charge transfer processes related to the fullerene moieties reduction in negative potential range exhibit the best reversibility among all of the studied systems. Capacitance performances of C₆₀/Pd and C₇₀/Pd films deposited on the porous Au/quartz electrode were also compared. Capacitance properties of both films are significantly affected by the conditions of electropolymerization. Only a fraction of the film having a direct contact with solution contributes to pseudocapacitance. Capacitance properties of these films also depend on the size of cations of supporting electrolyte. The C₇₀/Pd film exhibits much better capacitance performance comparison to C₆₀/Pd polymer.     

Two applications of metal cyanide square grid monolayers: studies of evolving magnetic properties in layered films and templating Prussian blue family thin films

Two applications of previously described square grid network monolayers prepared at the air/water interface are explored. The monolayer networks are single layers of Prussian blue like mixed-metal cyanide networks that are formed via the interface-directed condensation of amphiphilic pentacyanometallate complex and subphase metal ions. In the first application, the monolayers are deposited onto solid supports and the magnetic properties of the networks are evaluated, as the transferred films evolve from a monolayer to a bilayer to multilayers. In the second application, the network monolayers are used to derivatize a surface, providing a seed layer for the subsequent deposition of solid-state metal cyanide molecule-based magnets. Improved surface wetting results in continuous, transparent magnetic films.     

Physico-chemical properties of Chitosan films

Chitosan films obtained by dry phase inversion were prepared from an aqueous solution of chitosan in acetic acid. The films, of thickness less than 20 μm, were transparent, very flexible and had smooth surfaces. Increasing the film thickness induced an increase of the internal tensions and the consequent formation of a rough surface. Structural investigations by X-ray diffraction and Fourier transform IR analysis, showed that the chitosan films, as prepared, are amorphous. Further annealing to evaporate acetic acid and water traces, changed the amorphous phase into a more ordered phase, characterized by diffraction peaks at 2θ values of 9, 17, 20 and 23 degrees. Thermal investigations by TG, DTG, and DTA revealed that the decomposition of the chitosan films as prepared proceeds in two stages, starting from 180°C and 540°C.     

Antibacterial properties of polyaniline-silver films

In situ polymerised thin polyaniline (PANI) films produced on polystyrene dishes were tested for their antibacterial activity with respect to Escherichia coli and Staphylococcus aureus, representing both gram-positive and gram-negative bacteria. PANI films were subsequently used for the reduction of silver ions to metallic Ag. PANI salt and base in original forms and after the deposition of Ag were studied. PANI salt showed a significant antibacterial effect against both bacteria strains while the efficacy of neat PANI base was only marginal. After the Ag deposition, the PANI base exhibited different levels of antibacterial effect depending on the type of the bacterial strain; the growth of gram-positive Staphylococcus aureus was inhibited depending on the Ag concentration on the film, while Escherichia coli remained uninfluenced. Efficacy of the PANI salt with deposited Ag against both bacteria strains was comparable with that of PANI alone and was not affected by the Ag concentration. The results show that Ag deposition can be a suitable method for the preparation of PANI base films with improved antibacterial properties.     

Wetting properties of polyacetylene films

The wetting characteristics of free-standing polyacetylene films were determined by using a standard series of wetting liquids. As-prepared films of cis-polyacetylene were found to contain a significant polar contribution to the surface free energy, which became nearly entirely dispersive upon thermal isomerization to the trans form. Both isomeric forms are characterized by a critical surface tension of wetting γ_c ≈ 51 mN/m, which is considerably higher than that normally obtained from organic polymers. These results have been interpreted with respect to surface oxidation and tested by examining an oxidized film.     

Epitaxy and thermal behaviour of metastable metal films

We demonstrate the complexity of metal-on-metal epitaxy. Low-energy ion scattering and medium-energy electron diffraction were used to study the growth, structure and thermal stability of iron deposited on a Cu(001) surface. The system exhibits as a function of film thickness a rich variety of morphological and structural phases. At smallest coverages (< 2 ML) iron does not grow layer-by-layer at room temperature. Iron is even partially incorporated into the copper substrate. Near 2 ML the substrate is covered for the most part (90%) with Fe and at even higher coverages layer-by-layer growth occurs, leading to well-ordered fcc iron films. Above 10 ML a structural phase transition into the bcc equilibrium modification is observed. All of the deposited films exhibit additional thermal metastability. Heating the samples causes enrichment of the surface with copper, resulting in a Cu/Fe/Cu sandwich morphology with a Cu overlayer of initially monoatomic height on top of the iron layers, which remain essentially intact. The onset temperature of Cu diffusion depends on the film thickness, but is always far below the values for regular bulk diffusion.     

Transport properties of chitosan films

Effect of the history of chitosan films on their transport characteristics in relation to elimination of drugs from the body is studied.     

Effect of metal cations on polydiacetylene Langmuir films

Polydiacetylene (PDA) Langmuir films (LFs) are a unique class of materials that couple a highly aligned conjugated backbone with tailorable pendant side groups and terminal functionalities. The films exhibit chromatic transitions from monomer to blue polymer and finally to a red phase that can be activated optically, thermally, chemically, and mechanically. The properties of PDA LFs are strongly affected by the presence of metal cations in the aqueous subphase of the film due to their interaction with the carboxylic head groups of the polymer. In the present study the influence of divalent cadmium, barium, copper, and lead cations on the structural, morphological, and optical properties of PDA LFs was investigated by means of surface pressure-molecular area (π-A) isotherms, atomic force microscopy, optical absorbance, and Raman spectroscopy. The threshold concentrations for the influence of metal cations on the film structure, stability, and phase transformation were determined by π-A analyses. It was found that each of the investigated cations has a unique influence on the properties of PDA LFs. Cadmium cations induce moderate phase transition kinetics with reduced domain size and fragmented morphology. Barium cations contribute to stabilization of the PDA blue phase and enhanced linear strand morphology. On the other hand, copper cations enhance rapid formation of the PDA red phase and cause fragmented morphology of the film, while the presence of lead cations results in severe perturbation of the film with only a small area of the film able to be effectively polymerized. The influence of the metal cations is correlated with the solubility product (K(sp)), association strength, and ionic-covalent bond nature between the metal cations and the PDA carboxylic head groups.     

Mechanical properties of metal hydrides

The role of metal hydrides in the hydrogen embrittlement of a number of metal-hydrogen systems is reviewed. The stress-induced formation and cleavage of hydrides at crack tips and the thermodynamics of the hydrides in the stress field of the crack tip are discussed.The mechanical response of the hydrides to applied stresses is described. Many of the hydrides exhibit some plastic deformation under favorable stress systems with the amount of this deformation increasing as the temperature of testing is increased. Phonon properties and the elastic moduli of the hydrides are briefly discussed and it is suggested that these parameters do not provide a basis for understanding the mechanical properties of the hydrides. The deformation response of the hydrides is consistent with the expected behavior of dislocations in the ordered hydride structures.     

Corrosion of metal films in ice: the dynamics of the conductivity of films

A decrease in the conductivity of copper, aluminum, iron, and silver films brought in contact with ice as ice underwent thermally stimulated structural relaxation and of a copper film in contact with water undergoing crystallization was studied. Conductivity decrease was found to be irreversible and caused by metal corrosion.