Film Interior Research Articles

Electrical currents resulting from reduction/oxidation processes of tested particles on electrodes modified with metal-containing polymer films

Metal-containing films are considered as systems including a multitude of microelectrodes, the role of which is played by metal clusters distributed in both the film interior and film interfaces. In the case of electrode reactions occurring exclusively on the surface of such metal inclusions and the film thickness significantly exceeding clusters’ radii, it is shown that the concentration of species participating in such processes (tested particles, further on) satisfies a diffusion equation complicated with an accompanying electrochemical reaction. By assuming reversibility of the electron transfer processes between polymer fragments, one can arrive at solutions of the derived equation and calculate the corresponding currents resulting from tested particles’ reactions. The role of such factors of inclusion of metal clusters into conducting polymer films, as its density and inhomogeneity is discussed in the context of their influence on the measured currents of reduction/oxidation processes of tested particles. Non-stationary effects that might be observed in cycling voltammetry and potential clamp measurements with electrodes modified by metal-containing films are also analyzed. The performed analysis shows that electrochemical properties of the modified electrodes in question practically coincide with those of the electrodes made of the corresponding metals and immersed into the same solutions.

Direct observation of two-dimensional nematic and smectic ordering in freely suspended films of a bolaamphiphilic liquid crystal

The T-shaped bolaamphiphile CT2, a liquid crystal with a hydroxyl-terminated, rod-shaped core and semifluorinated side chains, forms the Lam-Iso, Lam-Nem, and Lam-Sm lamellar phases, which are reported to exhibit respectively smectic layering of the side chains with additional in-layer isotropic (Iso), nematic (Nem) and smectic-like (Sm) ordering of the rigid molecular cores. Optical observation of 2–16 layer thick freely suspended films enables visualization and quantitative study of these transitions in two dimensions (2D). The 2D-Iso to 2D-Nem transition is found to be first order, occurring first in the film surface layers and then in the film interior on further cooling. The second order 2D-Nem to 2D-Sm transition is marked by the continuous evolution of the orientational texture from 2D nematic-like to 2D focal conic. The film geometry stabilizes the lamellar ordering, significantly increasing the birefringence associated with the in-plane ordering in the films.

Nanofilm Biomaterials: Localized Cross-Linking To Optimize Mechanical Rigidity and Bioactivity

Nanofilm biomaterials, formed by the layer-by-layer assembly of charged macromolecules, are important systems for a variety of cell-contacting biomedical and biotechnological applications. Mechanical rigidity and bioactivity are two key film properties influencing the behavior of contacting cells. Increased rigidity tends to improve cells attachment, and films may be rendered bioactive through the incorporation of proteins, peptides, or drugs. A key challenge is to realize films that are simultaneously rigid and bioactive. Chemical cross-linking of the polymer framework--the standard means of increasing a film's rigidity--can diminish bioactivity through deactivation or isolation of embedded biomolecules or inhibition of film biodegradation. We present here a strategy to decouple mechanical rigidity and bioactivity, potentially enabling nanofilm biomaterials that are both mechanically rigid and bioactive. Our idea is to selectively cross-link the outer region of the film, resulting in a rigid outer skin to promote cell attachment, while leaving the film interior (with any embedded bioactive species) unaffected. We propose an approach whereby an N-hydroxysulfosuccinimide (sulfo-NHS) activated poly(L-glutamic acid) is added as the terminal layer of a multilayer film and forms (covalent) amide bonds with amino groups of poly(L-lysine) placed previously within the film. We characterize film assembly and cross-linking extent via quartz crystal microbalance with dissipation monitoring (QCMD), Fourier transform infrared spectroscopy in attenuated total reflection mode (FTIR-ATR), and laser scanning confocal microscopy (LSCM) and measure the attachment and metabolic activity of preosteoblastic MC3T3-E1 cells. We show cross-linking to occur primarily at the film surface and the subsequent cell attachment and metabolic activity to be enhanced compared to native films. Our method appears promising as a means to realize films that are simultaneously mechanically rigid and bioactive.

Local and Average Glass Transitions in Polymer Thin Films

In the companion paper (DOI 10.1021/ma101098d), we presented a quantitative theory for the suppression of the glass transition in a thin polymer film. Our delayed glassification (DG) model follows a proposal by de Gennes that free volume can be transmitted from surface to film interior via kinks transported along molecular strands or loops. In this paper, we use the DG model to predict the effects of molecular weight and film thickness on the film-averaged glass transition for a polystyrene sample. Our predictions for both freestanding and supported films of polystyrene illustrate that the DG model is able to account for some, but not all, of the experimental trends. This leads us to confront a number of issues, including how to average local glass transitions to yield a sample value as well as how to rationalize the nature of the molecular weight dependence for transitions in the thinnest freestanding films.

Measurement of the interior structure of thin polymer films using grazing incidence diffuse x-ray scattering

A method is developed for calculating the small-angle x-ray scattering originating from within the interior of a thin film under grazing incidence illumination. This offers the possibility of using x-ray scattering to probe how the structure of polymers is modified by confinement. When the diffuse scattering from a thin film is measured over a range of incident angles, it is possible to separate the contributions to scattering from the interfaces and the contribution from the film interior. Using the distorted-wave Born approximation the structure factor, S(q), of the film interior can then be obtained. We apply this method to analyze density fluctuations from within the interior of a silicon supported molten polystyrene (PS) film. Measurements were made as a function of film thickness ranging from one to ten times the polymer radius of gyration (Rg). The compressibility, calculated by extrapolating the measured S(q) to q=0, agrees well with that of bulk PS for thick films, but thinner films exhibit a peak in S(q) near q=0. This peak, which grows with decreasing thickness, is attributed to a decreased interpenetration of chains and a consequent enhanced compressibility.

Thermally Reversible Surface Morphology Transition in Thin Diblock Copolymer Films

Many phase transitions exhibit ordering transitions at the boundary of the material that are distinct from its interior where intermolecular interactions can be significantly different. The present work considers the existence of a surface thermodynamic order-order transition between two distinct morphologies in thin block copolymer (BCP) films that are of interest in nanomanufacturing applications. Specifically, we find a thermally reversible interfacial transition between sphere-like structures and cylinders in flow-coated films of poly(styrene-block-methyl methacrylate) (PS-b-PMMA), where the BCP forms a cylinder microphase in the bulk. We present direct evidence from atomic force microscopy (AFM) of ion-etched films and grazing-incidence small-angle X-ray scattering (GISAXS) on films without etching, which shows that the order-order transition is restricted to the outer layer of the film, while the film interior remains in the cylinder state. Moreover, we find this order-order transition to be insensitive to film thickness over the range investigated (40-170 nm). This morphological transition is of importance in characterizing the thermodynamics and dynamics of thin BCP films used as templates in nanomanufacturing applications.

Mn Valency at La0.7Sr0.3MnO3/SrTiO3 (0 0 1) Thin Film Interfaces

This article presents a (scanning) transmission electron microscopy (TEM) study of Mn valency and its structural origin at La 0.7 Sr 0.3 MnO 3/SrTiO3(0 0 1) thin film interfaces. Mn valency deviations can lead to a breakdown of ferromagnetic order and thus lower the tunneling magnetoresistance of tunnel junctions. Here, at the interface, a Mn valency reduction of 0.16 +/- 0.10 compared to the film interior and an additional feature at the low energy-loss flank of the Mn-L3 line have been observed. The latter may be attributed to an elongation of the (0 0 1) plane spacing at the interface detected by geometrical phase analysis of high-resolution images. Regarding the interface geometry, high-resolution high-angle annular dark-field scanning TEM images reveal an atomically sharp interface in some regions whereas the transition appears broadened in others. This can be explained by the presence of steps. The performed measurements indicate that, among the various structure-related influences on the valency, the atomic layer termination and the local oxygen content are most important.

Modified Capillary Cell for Foam Film Studies Allowing Exchange of the Film-Forming Liquid

Many of the macroscopic properties of foams and emulsions are controlled by the mesoscopic properties of the thin films separating the bubbles or droplets. The properties of these films depend on contributions (1) from the adsorbed surface layers and (2) from the liquid that separates these adsorbed layers. To separate in the experimental studies the effects of these two contributions, we developed a new modified version of the capillary cell for foam film studies (originally developed by Scheludko and Exerowa (Scheludko, A.; Exerowa, D. Kolloid Z. 1959, 165, 148-151), which allows exchange of the film-forming liquid between the air-water surfaces. This modified cell allows one to distinguish between the role of the adsorbed species (e.g., proteins, particles, or long-chain synthetic polymers) and the species present in the film interior (e.g., particles, electrolytes, or surfactants). The film properties that can be studied in this way include film stability, rate of film thinning, and surface forces stabilizing the film. These properties are of significant interest in understanding and controlling the stability of dispersed systems. The experimental procedure and the capabilities of the modified cell are demonstrated in several examples.

Loading of Myoglobin into Multilayer Films Assembled by ZrO<SUB>2</SUB> Nanoparticles and Phytic Acid: Electrochemistry and Electrocatalysis

Small-molecular phytic acid (PA) with its unique structure was successfully assembled with ZrO2 nanoparticles into {PA/ZrO2}n layer-by-layer films on solid surfaces, which was confirmed by quartz crystal microbalance (QCM) and cyclic voltammetry (CV) with K3Fe(CN)6 as the electroactive probe. Myoglobin (Mb) could be gradually "absorbed" or loaded into the films when the films were immersed into Mb solutions. The Mb-loaded films at pyrolytic graphite (PG) electrodes, designated as {PA/ZrO2}n-Mb, demonstrated well-defined and quasi-reversible CV responses for Mb Fe(III)/Fe(II) redox couple and good electrocatalytic properties toward oxygen and H2O2. The driving force of the film assembly and the interaction between Mb and {PA/ZrO2}n films were explored and discussed in detail. The coordination interaction between PA and ZrO2 is believed to be the main driving force for the assembly of {PA/ZrO2}n multilayer films, and the electrostatic attraction between oppositely charged Mb and the film components is the main interaction for Mb loading into the films. The loading behavior of the {PA/ZrO2}n films toward different proteins with different size indicates that while the porosity of the films is necessary for the protein loading, only those proteins with the size smaller than the average pore size of the films can be incorporated in the interior of films.

PHONON SCATTERING VIA AN ATOMIC WELL IN FREE STANDING THIN SOLID FILMS

A theoretical model is presented for the study of the scattering of phonons at an extended inhomogeneous boundary separating thin solid monatomic films. The model system consists of two solid films with otherwise stress-free surfaces on either side of an atomic well boundary. The coherent reflection and transmission scattering cross sections for phonons incident from the interior of the thin films on the inhomogeneous atomic well boundary are calculated in accordance with the Landauer–Büttiker coherent scattering description, using the matching method with nearest and next-nearest neighbor elastic force constants. This is done specifically for two different cases of elastic interactions on the boundary to investigate the consequences of their softening and hardening for the coherently scattered spectra. The numerical results yield an understanding for the effects on coherent phonon conductance due to phonon incidence and to the elastic nature of the boundary. The coherent reflection and transmission scattering cross sections show characteristic spectral features that are invariant with the change of the boundary force constants, depending solely on the cutoff frequencies for the propagating phonons and on incidence angle. They also show the Fano resonances that result from the interactions of propagating phonons with the localized vibrational Rayleigh-like modes on the boundary, depending on the boundary elastic force constants. The evolution of the system average conductance per phonon mode with incidence illustrates an interesting effect. An experimental observable this average conductance remains constant at half a phonon in intensity over significant frequency intervals. The effect is remarkable inasmuch as it permits in principle the possibility of a constant intensity phonon source.

Morphological change in FePt nanogranular thin films induced by swift heavy ion irradiation

We have investigated morphology change of FePt nanogranular films (FePt) 47(Al 2O 3) 53 under irradiation with 210 MeV Xe ions. Here, electron tomography technique was extensively employed to clarify three-dimensional (3D) structure in irradiated specimens, in addition to conventional transmission electron microscopy (TEM) techniques such as bright-field observation and scanning TEM energy dispersive X-ray spectroscopy (STEM-EDX) analysis. The ion irradiation induces the coarsening of FePt nanoparticles with elongation along the beam direction. Electron tomography 3D reconstructed images clearly demonstrated that when the fluence achieves 5.0 × 10 14 ions/cm 2, well-coarsened FePt balls have been formed on the irradiated surface, and the particles in the film interior have been deformed into rods along the ion trajectory. The alloy particles become inhomogeneous in composition after prolonged irradiation up to 1.0 × 10 15 Xe ions/cm 2. The particle center is enriched with Pt, while Fe is slightly redistributed to the periphery.

Nonthermal Water Splitting on Rutile TiO2: Electron-Stimulated Production of H2 and O2 in Amorphous Solid Water Films on TiO2(110)

The electron-stimulated reactions leading to H2 and O2 and the electron-stimulated desorption (ESD) of H2O from 0 − 60 ML films of amorphous solid water (ASW) adsorbed on TiO2(110) are investigated as a function of film thickness and isotopic composition at 100 K. For 100 eV incident electrons, both the H2 and O2 ESD yields have maxima when the ASW coverage is ∼20 monolayers (ML), while the H2O ESD yield increases monotonically with water coverage. All the products reach a coverage-independent yield above 40−50 ML. Experiments using layered films of H2O and D2O demonstrate that the molecular hydrogen is produced in reactions that occur preferentially at or near both the ASW/TiO2 interface and the ASW/vacuum interface. However, electronic excitations or ionic defects created within the interior of the ASW films by the energetic electrons can subsequently migrate to the interfaces where they initiate reactions. Electron irradiation of ASW films results in the formation of bridge-bonded hydroxyls on TiO2(110...

Formation of Solid Splats During Thermal Spray Deposition

This paper reviews the findings of recent research on the formation of solid splats by the impact of thermal spray particles on solid substrates. It discusses methods of describing the substrate, by characterizing both chemical (oxide layers) and physical (surface topography, adsorbed and condensed contaminants) aspects. Recent experiments done to observe impact of thermal spray particle are surveyed and techniques used to photograph particle impact and measure cooling rates described. The use of numerical modeling to simulate impact and deformation of impacting particles is appraised. Two different break-up mechanisms are identified: solidification around the edges of splats; and perforations in the interior of thin liquid films created by droplet spreading without solidification. These two modes can be reproduced in numerical models by varying the value of thermal contact resistance between the splat and substrate. A simple criterion to predict the final splat shape is presented.

Asymmetric lipid bilayer sandwiched in polyelectrolyte multilayer films through layer-by-layer assembly

Lipid bilayer of 1,2-dimyristoyl-sn-glycero-3-phosphoethanolamine (DMPE) was formed on the surface of a linearly growing polyanion/polycation polyelectrolyte multilayer (PEM) film prepared by the layer-by-layer (LbL) technique. The PEM films were prepared with two strong polyelectrolytes—poly(sodium 4-styrenesulfonate) (PSS) as the polyanion and poly-(diallyldimethylammonium chloride) (PDADMAC) as the polycation. The DMPE was always deposited onto the PEM cushion terminated with positively charged PDADMAC. The surface charge of the DMPE-coated film depends on the pH of the buffer solution. Formation of PEM could be continued on the top of the lipid bilayer by tuning the headgroup charge of the DMPE. An asymmetric structure of polycation/lipid/polyanion “sandwich” was created in the case where anionic PSS was used as the first layer of the subsequent PEM. In this system, the DMPE lipid membrane was constructed as a charge asymmetric barrier sandwiched in the interior of the PEM films. The formation of PEM architectures incorporated with an asymmetric DMPE bilayer was confirmed by ζ potential, UV-vis absorption, quartz crystal microbalance (QCM) and neutron reflectometry experiments.

Hydrolysis of Sodium Atoms on Water−Ice Films. Characterization of Reaction Products and Interfacial Distribution of Sodium and Hydroxide Ions

We have examined the hydrolysis reaction of Na atoms at the surface of thin water−ice films by using the techniques of Cs+ reactive ion scattering and low energy sputtering. Na atoms are adsorbed onto amorphous D2O−ice films, and the reaction products formed at the ice film surfaces are examined for ionic (both positive and negative) species as well as neutral molecules for the temperature range of 95−135 K and Na coverage below the multilayer regime. The hydrolysis reaction produces isolated sodium and hydroxide ions as well as a sodium hydroxide molecule. Efficient solvation of these products by water molecules at high temperature alters their distribution near the ice film surface. Hydroxide ions tend to reside at the film surface, whereas sodium ions migrate to the film interior. NaOD molecules are efficiently solvated by water molecules at high temperature, but a substantial portion of the molecules remains undissociated.

Ferromagnetic resonance in ferromagnetic/ferroelectric Fe∕BaTiO3∕SrTiO3(001)

Single Fe(001) films (30nm thick) have been epitaxially grown on ferroelectric BaTiO3∕SrTiO3(001) substrates at different growth temperatures to study the mutual interaction between the multiferroic components. This paper reports on the as-grown magnetic properties of the structures as a precursor to a full investigation of the multiferroic interactions. Ferromagnetic resonance measurements were carried out at 36GHz cavity and variable frequency microstrip resonators. Dual resonance modes are observed in the film, which are attributed to relaxed Fe in the film interior and strained Fe at the interface. Fourfold anisotropy is present for both modes with energy density consistent with that of bulk Fe. The interface mode is characterized by a large out-of-plane anisotropy comparable and opposite in sign to the shape anisotropy. This strained interfacial Fe should serve to couple the multiferroic components in this system. Dispersion curves show both optic and acoustic branches along the hard axis [110], with the optic branch resulting from resonance below saturation, indicating high quality Fe in these samples. Growth temperature has minimal influence on the observed anisotropy energies.

Processing and thin film formation of TiO2–Pt nanocomposites

AbstractThin films of TiO2‐Pt nanocomposites containing 4 at% Pt have been processed via spin‐coating. Film characterization involved XRD, Raman as well as XPS and scanning surface potential microscopy (SSPM). After annealing at 500 °C the thin films consisted of nanocrystalline anatase and a few nm Pt nanoclusters. Annealing at 600 °C resulted in the formation of a high volume fraction of rutile, ∼70%, and a coarsening of the microstructure, including Pt nanoparticles which attained a mean particle size of up to 11 nm. These results contrasted with those of pure TiO2 films obtained at 600 °C which showed only a limited amount of rutile formation, namely 9%. Raman spectra of Pt‐containing samples exhibited a fluorescence emission, as background to the Raman features, which was attributed to photoinduced luminescence from Pt nanoparticles supported by their surface plasmon resonance. Emission intensity being much higher in 600 °C film indicated a difference between the two films in terms of the (Pt) particle size and crystallinity, in agreement with the XRD results. XPS investigations revealed different oxidation states of Pt at the surface and in the film interior. The spectra suggested a slight oxidation of Pt at the surface while mainly metallic Pt was revealed in the film interior. The morphology and distribution of the Pt nanoparticles in the films annealed at 600 °C were investigated using SSPM. Discrete Pt nanoparticles, mainly distributed in the vicinity of TiO2 grain boundaries were revealed. Nanocomposite film formation, Pt distribution and morphology are explained in terms of the limited solubility of Pt in the TiO2 lattice and its higher surface energy in comparison to that of TiO2. Both effects are believed to lead to the formation of Pt nanoparticles at the (anatase or rutile) grain boundaries. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Xerogel Optical Sensor Films for Quantitative Detection of Nitroxyl

Xerogel sensing films were synthesized via sol-gel chemistry were used to fabricate optical nitroxyl (HNO) sensors [corrected] Selective detection of HNO in solution was achieved by monitoring the rates of manganese(III) meso-tetrakis(4-sulfonatophenyl) porphyrinate (MnIIITPPS) reductive nitrosylation in the anaerobic interior of aminoalkoxysilane-derived xerogel films. Nitroxyl permeability in sensor films deposited in round-bottom 96-well plates was enhanced via incorporation of trimethoxysilyl-terminated poly(amidoamine-organosilicon) dendrimers in the xerogel network. The selectivity of MnIIITPPS for HNO, the overall sensitivity, and the working dynamic range of the resulting sensors were characterized. The HNO-sensing microtiter plates were used to quantify pH-dependent HNO generation by the recently described HNO-donor sodium 1-(isopropylamino)diazene-1-ium-1,2-diolate (IPA/NO), and compare HNO production efficiency between IPA/NO and Angeli's salt, a traditional HNO-donor.

Preparation and photoelectric performance of ITO/TiO 2/CdS composite thin films

ITO/TiO 2/CdS composite thin films were successfully prepared by electrodeposition of CdS on ITO/TiO 2 substrate. The prepared ITO/TiO 2/CdS thin films were characterized in detail by X-ray diffraction, surface photovoltage, and scanning electronic microscopy techniques. The results indicated that the deposited CdS layer with about 1:1 stoichiometry of Cd and S distributes not only on the surface with cauliflower-like structure but also the TiO 2 film interior and even in the immediate proximity to the surface of ITO substrate. The UV–vis absorption spectrum results suggested that the absorption peak of the ITO/TiO 2/CdS film shifts from the ultraviolet region to the visible region in comparison to that of the ITO/TiO 2 film. Furthermore, by the photocurrent–potential behavior and incident monochromatic photon to current conversion efficiency measurements, it can be also noted that the deposited ITO/TiO 2/CdS thin films possess higher photoelectric conversion efficiency than the ITO/TiO 2 films. The sensibilization of TiO 2 with CdS can effectively improve steady photocurrent. Therefore, the modification of CdS for ITO/TiO 2 can strongly ameliorate the photoelectric capability of ITO/TiO 2 thin film, and the prepared ITO/TiO 2/CdS thin films were expected to possess more excellent photoelectrochemical performance than the ITO/TiO 2 films.

Use of Ultrasonic Force Microscopy to Image the Interior Nanoparticles in ${\rm YBa}_{2}{\rm Cu}_{3}{\rm O}_{7-{\rm x}}$ Films

Nanoparticles present in the interior of YBa2Cu3O7-x (YBCO) films were successfully imaged for the first time by using an ultrasonic force microscope (UFM), which can also operate as a conventional atomic force microscope (AFM). Nanoparticles of Y2BaCuO5 and BaSnO3 were introduced into YBCO films using pulsed laser ablation to improve critical current density via enhanced flux pinning. The scanning speed and ultrasonic frequencies in the range of 300-500 kHz were optimized for each sample such that the nanometer sized particles on the surface as well as from the film interior can be imaged with good contrast and resolution. UFM and AFM scans taken of the same locations were compared to show the advantages of using UFM over AFM. We demonstrate that UFM can be used nondestructively to both characterize the interior nanoparticles introduced in YBCO films and provide high resolution images of the screw dislocation induced terraces present in the films.