Ultrathin DLC and SiOx layer deposition on poly(ethylene terephthalate) and restriction of surface dynamics

The hydrophilicity of oxygen plasma‐reated polymer surfaces decays with storing time in air environments. Because they are dense, highly crosslinked, and chemically stable, diamond‐like carbon (DLC) films and silicon oxide films (SiO_x) were deposited on poly(ethylene terephthalate) by plasma‐enhanced chemical vapor deposition to restrict polymer surface dynamics. In this study, the effects of ultrathin films on surface dynamics of these polymers were investigated. The layers were deposited on substrates with thickness below 100 Å. The thickness of films was measured with a scanning analyzer ellipsometer, while ATR‐IR spectroscopy and Raman spectroscopy were performed to observe the chemical structure of the films. Films below 50 Å were also shown to be effective in stabilizing the plasma treated polymer surfaces. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 1158–1164, 2000     

Electrochemical investigation of the influence of thin SiOx films deposited on gold on charge transfer characteristics

The paper reports on the investigation of the electrochemical behavior of a thin gold film electrode coated with silicon dioxide (SiO_x) layers of increasing thickness. Stable thin films of amorphous silica (SiO_x) were deposited on glass slides coated with a 5 nm adhesion layer of titanium and 50 nm of gold, using plasma-enhanced chemical vapor deposition (PECVD) technique. Scanning electrochemical microscopy (SECM) and electrochemical impedance spectroscopy (EIS) were used to investigate the electrochemical behavior of the interfaces. In the case of SECM, the influence of the SiO_x thicknesses on the electron transfer kinetics of three redox mediators was investigated. Normalized current-distance curves (approach curves) were fitted to the theoretical model in order to find the effective heterogeneous first order rate constant (k_eff) at the sample. EIS was in addition used to confirm the diffusion barrier character of the SiO_x interlayer.     

Morphological,compositional, structural,and optical properties of Si-nc embedded in SiOx films

Structural, compositional, morphological, and optical properties of silicon nanocrystal (Si-nc) embedded in a matrix of non-stoichiometric silicon oxide (SiO_x) films were studied. SiO_x films were prepared by hot filament chemical vapor deposition technique in the 900 to 1,400°C range. Different microscopic and spectroscopic characterization techniques were used. The film composition changes with the growth temperature as Fourier transform infrared spectroscopy, energy dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy reveal. High-resolution transmission electron microscopy supports the existence of Si-ncs with a diameter from 1 to 6.5 nm in the matrix of SiO_x films. The films emit in a wide photoluminescent spectrum, and the maximum peak emission shows a blueshift as the growth temperature decreases. On the other hand, transmittance spectra showed a wavelength shift of the absorption border, indicating an increase in the energy optical bandgap, when the growth temperature decreases. A relationship between composition, Si-nc size, energy bandgap, PL, and surface morphology was obtained. According to these results, we have analyzed the dependence of PL on the composition, structure, and morphology of the Si-ncs embedded in a matrix of non-stoichiometric SiO_x films.     

Compositional and optical properties of SiO x films and (SiO x /SiO y ) junctions deposited by HFCVD

In this work, non-stoichiometric silicon oxide (SiO _x ) films and (SiO _x /SiO _y ) junctions, as-grown and after further annealing, are characterized by different techniques. The SiO _x films and (SiO _x /SiO _y ) junctions are obtained by hot filament chemical vapor deposition technique in the range of temperatures from 900°C to 1,150°C. Transmittance spectra of the SiO _x films showed a wavelength shift of the absorption edge thus indicating an increase in the optical energy band gap, when the growth temperature decreases; a similar behavior is observed in the (SiO _x /SiO _y ) structures, which in turn indicates a decrease in the Si excess, as Fourier transform infrared spectroscopy (FTIR) reveals, so that, the film and junction composition changes with the growth temperature. The analysis of the photoluminescence (PL) results using the quantum confinement model suggests the presence of silicon nanocrystal (Si-nc) embedded in a SiO _x matrix. For the case of the as-grown SiO _x films, the absorption and emission properties are correlated with quantum effects in Si-nc and defects. For the case of the as-grown (SiO _x /SiO _y ) junctions, only the emission mechanism related to some kinds of defects was considered, but silicon nanocrystal embedded in a SiO _x matrix is present. After thermal annealing, a phase separation into Si and SiO₂ occurs, as the FTIR spectra illustrates, which has repercussions in the absorption and emission properties of the films and junctions, as shown by the change in the A and B band positions on the PL spectra. These results lead to good possibilities for proposed novel applications in optoelectronic devices.

PACS

61.05.-a; 68.37.Og; 61.05.cp; 78.55.-m; 68.37.Ps; 81.15.Gh     

A facile approach to the fabrication of ultrathin polymer films and application to optical lenses

A facile approach to the fabrication of ultrathin polymer films on a flat or curved substrate is presented. Polymers with unsaturated pendant groups were spin-coated on a photoinitiator tethered surface, which was then photoirradiated and washed with a solvent. The obtained films were uniform, smooth (R_a < 0.2 nm) and exhibited robustness toward solvents. The thickness of the films was determined by the molecular weight of the coated polymer and was not dependent on the initial spin-coated thickness. A mechanism for the formation of the ultrathin film and application to optical lenses is presented.     

Possibility of graphene growth by close space sublimation

Carbon films on the Si/SiO₂ substrate are fabricated using modified method of close space sublimation at atmospheric pressure. The film properties have been characterized by micro-Raman and X-ray photoelectron spectroscopy and monochromatic ellipsometry methods. Ellipsometrical measurements demonstrated an increase of the silicon oxide film thickness in the course of manufacturing process. The XPS survey spectra of the as-prepared samples indicate that the main elements in the near-surface region are carbon, silicon, and oxygen. The narrow-scan spectra of C1s, Si2p, O1s regions indicate that silicon and oxygen are mainly in the SiO _x (x ≈ 2) oxide form, whereas the main component of C1s spectrum at 284.4 eV comes from the sp²-hybridized carbon phase. Micro-Raman spectra confirmed the formation of graphene films with the number of layers that depended on the distance between the graphite source and substrate.     

Preparation of surface initiated polystyrenesulfonate films and PEDOT doped by the films

Various substrates such as glass slides and silicon wafers were modified by styrylethyltrimethoxysilane to attach double bonds to those surfaces. The double bond layer was initiated and capped by benzoyl peroxide (BPO) and 2,2,6,6-tetramethylpiperidirooxy (TEMPO), respectively, to form ‘living’ free radical layer from which polystyrene brushes were grown. The density of double bonds on the surface controlled the orientation of polystyrene brush or film. The polystyrene films were then sulfonated by fuming sulfuric acid (H₂SO₄·xSO₃) to obtain polystyrenesulfonic acid (PSS) films with controlled polymer chain alignment. The lower double bond density led to a lower degree of polymer chain alignment. 3,4-Ethylenedioxythiophene monomer was diffused into PSS film and then polymerized. A conductive polyethylenedioxythiophene (PEDOT)/PSS film was obtained. The films were characterized by four-point probe, AFM and UV-VIS. The conductivity of PSS/PEDOT film measured along the direction which is normal to polymer chain alignment, is lower than that from commercial PSS/PEDOT.     

Impact of thickness on CO2 concentration profiles within polymer films swollen near the critical pressure

The isothermal swelling of polymer thin films by a supercritical fluid does not increase monotonically with increasing chemical potential (pressure), but rather a maximum in swelling is generally observed near the critical pressure. A reactive templating approach utilizing the condensation of silica within hydrophilic domains of a swollen amphiphilic polymer film enables visualization of the qualitative concentration profile of CO₂ by the changes in the size of hydrophobic domains (pores) with cross sectional TEM microscopy; specifically, isothermal swelling of poly(ethylene oxide-propylene oxide-ethylene oxide) films by CO₂ at 60 °C is examined. Films that contain thickness gradients are used to avoid any uncertainties in the impact of thickness due to variations in the temperature or pressure during the silica modification. A uniform pore size (local swelling) is observed for all film thicknesses when the pressure is outside of the anomalous maximum in the film swelling, except for a small increase at the buried interface due to preferential adsorption of CO₂ to the native silicon oxide surface of the substrate. However at this swelling maximum, a gradient in the pore size is observed at both interfaces. These swelling gradients at interfaces appear to be responsible for the anomalous maximum in thin films. As the film thickness increases beyond 350 nm, there is a decrease in the maximum swelling at the free interface.     

A variable energy positron annihilation lifetime spectroscopy study of physical aging in thin glassy polymer films

The influence of physical aging on the profile of free volume characteristics in thin polysulfone (PSF) films was investigated using variable energy positron annihilation lifetime spectroscopy. The PSF films exhibited decreasing o-Ps lifetime during physical aging, while o-Ps intensity remained constant. The o-Ps lifetime was reduced at lower implantation energies, indicating smaller free volume elements near the film surface (i.e., in the top ∼50 nm). These near-surface regions of the films age dramatically faster than bulk PSF. The accelerated aging is consistent with the notion of enhanced mobility near the film surface, which allows polymer near the surface to reach a lower free volume state more quickly than the bulk. No influence of the silicon wafer support on aging behavior was detected. Additionally, the impact of CO₂ conditioning on physical aging was briefly examined. The results from these studies were compared to aging behavior of ultrathin PSF films tracked by gas permeability measurements, and favorable agreement was found.     

Chemical anchoring of silica nanoparticles onto polyaniline chains via electro-co-polymerization of aniline and N-substituted aniline grafted on surfaces of SiO2

Chemical anchoring of silica nanoparticles onto polyaniline (PANI) chains was conducted through electro-co-polymerization of aniline and N-substituted aniline grafted on surfaces of silica nanoparticles. The grafting of N-substituted aniline on surfaces of silica nanoparticles were realized through hydrolysis of triethoxysilylmethyl N-substituted aniline (ND42) and the following condensation reaction with silanol groups on surfaces of SiO₂. Organic-inorganic interactions between PANI and SiO₂ involved in electro-co-polymerization process pushed the polymer chains apart and so facilitated the 1D growth of the polymer. Hence, the obtained hybrid film PANI/ND42-SiO₂ displayed nano-fibrous morphologies (ca. 50 nm in diameter). Consequently, PANI/ND42-SiO₂ exhibited an average specific capacitance of 380 F g⁻¹, ca. 40% higher than that of PANI/SiO₂ (276 F g⁻¹). The hybrid film also showed improved cyclic stability.     

Characteristics of waterborne polyurethane/poly(N‐vinylpyrrolidone) composite films for wound‐healing dressings

For ideal wound‐healing dressings, a series of waterborne polyurethane (WBPU)/poly(N‐vinylpyrrolidone) (PVP) composite films (transparent film dressings) were prepared by in situ polymerization in an aqueous medium. Stable WBPU/PVP composites, which had a high remaining weight greater than 98.4%, were obtained. The maximum content of PVP for stable WBPU/PVP dispersions was found to be about 15 wt %. The water absorption (%) and equilibrium water content (%) of the WBPU/PVP composite films remarkably increased in proportion to the PVP content and the time of water immersion. The maximum water absorption and equilibrium water content of the WBPU/PVP composite films were in the range of 21–158 and 22–56%, respectively. The water vapor transmission rate of the WBPU/PVP composite films was in the range of 1816–2728 g/m²/day. These results suggest that WBPU/PVP composite films may have high potential as new wound‐dressing materials that provide and maintain the moist environment needed to prevent scab formation and dehydration of the wound bed. By a wound‐healing evaluation using a full‐thickness rat model experiment, it was found that a wound covered with a typical WBPU/PVP composite film (15 wt % PVP) was completely filled with new epithelium without any significant adverse reactions. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

In situ monitoring of thermal transitions in thin polymeric films via optical interferometry

A novel, low-cost, rapid, accurate, non-invasive and high throughput method based on the principles of Optical Interferometry (OPTI method) has been developed and applied for the in situ monitoring in one simple run of first (melting) and second (glass transition) order transitions as well as of the thermally induced decomposition of various thin polymeric films spin coated on flat reflective substrates (untreated silicon wafers). The new method has been applied successfully for measuring the glass transition, melting and decomposition temperatures of six commercially available polymers [poly(methyl methacrylate) (PMMA), poly(2-hydroxyethyl methacrylate), (PHEMA), poly(vinyl acetate-co-crotonic acid), (PVACA), poly(vinyl pyrrolidone) (PVP), poly(vinyl chloride-co-vinyl acetate) (PVCVA) and crystalline poly(vinylidene fluoride-co-hexafluoropropylene) (PVFHP)] of known T_gs or T_ms. The recorded interferometric signals were identified and characteristic signal patterns were qualitatively correlated to specific transitions. The monitoring of first and second order transitions in thin polymeric films is based on detectable differentiations of the total energy of a fixed wavelength laser beam incident almost vertically (angle of incidence <5°) onto a thin polymeric film spin coated on a flat reflective substrate. These differentiations are caused by film thickness and/or refractive index changes of the polymeric film both resulted from the significant change of the polymer's free volume taking place on the transitions. For film thicknesses over approx. 200-250 nm, the T_g or T_m of the polymeric films measured with the OPTI method were in excellent agreement with the corresponding values of the polymer, measured by DSC. An investigation on the trends of the T_g of PHEMA and PMMA films in a wide thickness range (30-1735 nm) was also carried out. Ultra-thin (∼30 nm) films of PMMA and PHEMA showed significant increase in their T_g values by approx. 30 °C upon comparing to their corresponding bulk T_gs. This behavior was attributed to an enhanced polymer-surface interaction through hydrogen bonding and/or to changes in the tacticity of the polymer.     

Deposition of carbon nitride films from single-source s-triazine precursors

Trisubsituted derivatives of s-triazine (1,3,5-triazine) with N(i-Pr)₂, N(i-Bu)₂, NH(t-Bu), pyridyl, and NHNHMe ligands were used as single-source precursors to produce carbon nitride (CN_x) thin films via hot wall CVD. The precursors are either commercially available or were synthesized in straightforward, one-pot procedures, and the synthesis and characterization of tris-2,4,6-methlyhydrazino-1,3,5-triazine (TMHT) is reported for the first time. All of the precursors studied are thermally stable and volatilize below 250 °C. They thermally decompose between 500 and 1000 °C, resulting in CN_x films with x ranging from 0.95 to 0.03. The film deposition temperature and nitrogen content depend upon the structure and stability of the precursor. The film properties vary from disordered insulating structures with high nitrogen content (CN_0.95) to low nitrogen content turbostratic carbon films. The films on Si and SiO₂ substrates were characterized by Auger surface analysis, FT-IR and Raman spectroscopy, X-ray diffraction, and scanning electron microscopy.     

Preparation of highly stable superhydrophobic TiO2 surfaces with completely suppressed photocatalytic activity

TiO₂ nanoparticles with a mean size of 20–30 nm were covered by ultrathin polydimethylsiloxane (PDMS) film, which shows hydrophobic properties. Surfaces consisting of the PDMS-coated TiO₂ particles showed water contact angles close to 170°. In contrast to the hydrophobic films consisting of organic molecules, which can be photocatalytically decomposed on TiO₂ in the presence of UV light, PDMS-coating on TiO₂ was highly stable. The PDMS-coating completely suppressed the photocatalytic activity of TiO₂. The unique properties of PDMS-coating can be exploited for UV protection layer and self-cleaning surfaces.     

Study on polystyrene thin film on glass substrate by scanning acoustic microscope

Acoustic micrograph and V(z) curves of polystyrene thin films on hydrophobic modified and unmodified alumino silicate glass substrates were studied in the frequency range from 170 to 450 MHz by a scanning acoustic microscope. The bright and dark parts in the acoustic micrograph of the unmodified glass samples appeared owing to permeation of water into the film. The blister was observed after about 20 s from dropping water. Sizes of the blister depended on the time and the thickness of thin films. On the other hand, the acoustic micrograph of the hydrophobic modified samples was a uniform image and the peeling of the thin film was not observed. V(z) curves of polystyrene thin film on the modified glass substrates had two oscillation periods in a certain frequency range that depended on the thickness of thin films. The short cycle and the long cycle components were assigned to a leaky surface acoustic wave (LSAW) and a leaky pseudo Sezawa wave, respectively. Velocities of the LSAW decreased linearly with an increase in film thickness.     

Investigation of the corrosion protection of SiOx-like oxide films deposited by plasma-enhanced chemical vapor deposition onto carbon steel

The paper reports on the corrosion behavior of carbon steel coated with thin SiO_x-like oxide films. The SiO_x-like coatings were deposited by plasma-enhanced chemical vapor deposition (PECVD) and their thickness was varied between 20 and 200 nm. The coated carbon steel interfaces were investigated for their corrosion protection efficiency when immersed in an aqueous saline solution of 3% NaCl. FTIR measurements and electrochemical impedance spectroscopy (EIS) experiments revealed that thin SiO_x-like coating layers (20 nm thick) do not prevent the carbon steel from corrosion, while thicker silica layers (d ≥ 100 nm) protect efficiently carbon steel interfaces in highly saline media with a protection efficiency of about 96% for a 200 nm thick coating.     

Surface and mechanical properties of hydrophobic silica contained hybrid films of waterborne polyurethane and fluorinated polymethacrylate

The surface and mechanical properties of hybrid films of waterborne polyurethane (WPU) and fluorinated polymethacrylate (FPMA), and high-hydrophobic silica (SiO₂) contained hybrid films of FPMA/WPU were investigated. X-ray photoelectron spectroscopy confirmed that the surfaces of hybrid films exhibited notable fluorine enrichment. Scanning electron microscopy observation demonstrated that micro-scale rough structures consisted of sub-micro papillae and micro-scale wrinkles formed on the surfaces of FPMA/WPU. This was attributed to the enhanced phase separation of WPU and the incompatibility of low-surface-energy FPMA and WPU. Colloidal SiO₂ was modified by polydimethylsiloxane and the modified SiO₂ was reactive and high-hydrophobic. After the addition of reactive SiO₂, the rough structures became micro-scale striped wrinkles studded with nano- and sub-micro papillae formed by the high-hydrophobic SiO₂. The combination of the fluorine enrichment and the rough structures accounted for the high hydrophobic FPMA/WPU film and superhydrophobic SiO₂/FPMA/WPU film.     

Surface relief terraces and self-assembled nanostructures in thin block copolymer films with solvent annealing

We have investigated the temporal evolution of relief terraces and microphase-separated nanodomains in poly(styrene-block-ethylene oxide) P(S-b-EO) on silicon substrates (SiO_x/Si) annealed under toluene/butanol vapors. For thin films, solvent annealing causes the formation of relief terraces. With solvent annealing for 12 h, hexagonal arrays of nanospheres of three sizes are dominant over the surface morphology of relief-terraced microstructures with epitaxial packing. The coexistence of nanospheres of three sizes is ascribed to a morphology triggered by the constraint of the film thickness. Upon exposure to toluene/butanol vapors for 16 and 20 h, these nanospheres become metastable and grow further into nanocylinders lying on the surface. During dewetting, the undulations of the local thickness and the nanodomain transitions are inseparable, consequently forming relief terraces comprising multiple layers of lying nanocylinders. The mechanisms and causes of nanodomain transitions of varied routes in solvent-annealed P(S-b-EO) films are analyzed and discussed.     

Morphology-dependent spin Seebeck effect in yttrium iron garnet thin films prepared by metal-organic decomposition

In this study, we examined the dependence of surface morphology and spin Seebeck effect (SSE) voltages on the poly[vinylpyrrolidone] (PVP) concentration in polycrystalline Y₃Fe₅O₁₂ (YIG) ultrathin films on a silicon substrate synthesized by metal-organic decomposition followed by a crystallization process. During fabrication, PVP concentrations of 0.5–2 g were used while all other conditions remained fixed. Atomic force microscopy and grazing incidence X-ray diffraction (XRD) measurements revealed a strong dependence of crystallinity and sample morphology on PVP concentration. The 1-g PVP sample had the smoothest surface, with a root mean square roughness of 0.2 nm, as well as superior bulk uniformity with respect to the shape and intensity of XRD reflection peaks. This was confirmed by scanning electron microscopy measurements of a cross-section of the sample that revealed a uniform film without pores. SSE measurements were performed to obtain the output SSE voltages (V_SSE) of all samples, to which a platinum layer was added as a spin-detection layer. Repeatedly, the 1-g PVP sample had the best performance, demonstrating the importance of film crystallinity and morphology in the spin-to-charge conversion efficiency of YIG films.