Polyvinyl Alcohol Thin Film Research Articles

Linear and nonlinear optical properties of Co_3O_4 nanoparticle-doped polyvinyl-alcohol thin films

Thin films made from a composite of the polymer polyvinyl-alcohol and cobalt oxide (Co3O4) nanoparticles were fabricated by spin coating. Linear and nonlinear optical properties of thin films with thicknesses of hundreds of nanometers were investigated. The refractive index and absorption coefficient were measured and two direct band gaps (Eg = 1.38 eV and 2.0 eV) were determined from the absorption spectrum. Reversed saturable absorption and saturable absorption were observed when the films were illuminated with the different fluences. Optical nonlinearities corresponding to reverse saturable absorption were measured by the z-scan technique. A nonlinear refractive index (n2) of ~10−10 cm2/W and nonlinear absorption (β) of ~103 cm/GW have been measured from 425 nm to 675 nm. The experimental results show that the Co3O4 nanoparticle/PVA composite is a promising material for nonlinear optical devices in the visible, since it takes advantages of the high optical nonlinearities of transition metal oxides and the superior mechanical properties and convenient fabrication properties of polymers.

Mechanical characterizations of cast Poly(3,4-ethylenedioxythiophene):Poly(styrenesulfonate)/Polyvinyl Alcohol thin films

The polymer Poly(3,4-ethylenedioxythiophene):Poly(styrenesulfonate), hereafter referred to as PEDOT:PSS, has electrical properties superior to those of most conducting polymers, but it is too brittle to be employed in many applications. Blending PEDOT:PSS with other polymers is a promising route to reach a good trade-off between electrical and mechanical properties. This paper describes the mechanical characterization of PEDOT:PSS/PVA (Polyvinyl Alcohol) blends. The PEDOT:PSS/PVA films used in this study are produced by casting, and uniaxial tensile tests are performed to characterize the Young's modulus, fracture strain, tensile strength, and plastic deformation behavior of the blends as a function of the weight fraction of the components. For pure PVA, the Young's modulus, fracture strain and tensile strength are found to be, respectively, 41.3 MPa, 111% and 41.3 MPa. The strength exhibits a nearly perfect bimodal behavior, suddenly increasing by a factor 2 at a PEDOT:PSS content of 30%. Importantly, the ductility remains extremely high (∼94%, only 20% lower than pure PVA) up to PEDOT:PSS fractions of ∼50%. The Young's modulus monotonically increases with PEDOT:PSS content, reaching 1.63 GPa at 50%. SEM imaging and XRD analysis allows correlation of these evolutions to substantial morphological changes in the PEDOT:PSS/PVA microstructure. When combined with a previously published electrical characterization study, the current work suggests that a PEDOT:PSS/PVA polymer blend with 30–40 wt% of PEDOT:PSS provides the best trade-off of conductivity and ductility. For non free-standing films, higher PEDOT:PSS fractions (70%) might be preferable.

Fabrication of three-dimensional polymer quadratic nonlinear grating structures by layer-by-layer direct laser writing technique

We demonstrate the fabrication of a three-dimensional (3D) polymer quadratic nonlinear (χ(2)) grating structure. By performing layer-by-layer direct laser writing (DLW) and spin-coating approaches, desired photobleached grating patterns were embedded in the guest-host dispersed-red-1/poly(methylmethacrylate) (DR1/PMMA) active layers of an active-passive alternative multilayer structure through photobleaching of DR1 molecules. Polyvinyl-alcohol and SU8 thin films were deposited between DR1/PMMA layers serving as a passive layer to separate DR1/PMMA active layers. After applying the corona electric field poling to the multilayer structure, nonbleached DR1 molecules in the active layers formed polar distribution, and a 3D χ(2) grating structure was obtained. The χ(2) grating structures at different DR1/PMMA nonlinear layers were mapped by laser scanning second harmonic (SH) microscopy, and no cross talk was observed between SH images obtained from neighboring nonlinear layers. The layer-by-layer DLW technique is favorable to fabricating hierarchical 3D polymer nonlinear structures for optoelectronic applications with flexible structural design.

Chemical synthesis of highly stable PVA/PANI films for supercapacitor application

Polyvinyl alcohol (PVA)/polyaniline (PANI) thin films were chemically synthesized by adopting two step process: initially a thin layer (200 nm) of PVA was spin coated by using an aqueous PVA solution onto fluorine doped tin oxide (FTO) coated glass substrate, afterwards PANI was chemically polymerized from aniline monomer and dip coated onto the precoated substrate. The thickness of PANI layer was varied from 293 nm to 2367 nm by varying deposition cycles onto the precoated PVA thin film. The resultant PVA/PANI films were characterized for their optical, morphological and electrochemical properties. The FT-IR and Raman spectra revealed characteristic features of the PANI phase. The SEM study showed porous spongy structure. Electrochemical properties were studied by electrochemical impedance measurement and cyclic voltammetry. The electrochemical performance of PVA/PANI thin films was investigated in 1 M H 2SO 4 aqueous electrolyte. The highest specific capacitance of 571 Fg −1 was observed for the optimized thickness of 880 nm. The film was found to be stable for more than 20,000 cycles. The samples degraded slightly (25% decrement in specific capacitance) for the first 10,000 cycles. The degradation becomes much slower (10.8% decrement in specific capacitance) beyond 10,000 cycles. This dramatic improvement in the electrochemical stability of the PANI samples, without sacrificing specific capacitance was attributed to the optimized PVA layer.

Whitening of polyvinyl alcohol used as restoration material for Shohekiga

Polyvinyl alcohol (PVA) has been used to prevent flaking of paint pigments on traditional Japanese Shohekiga painted screens and panels. However, over time these PVA coatings whiten. To date, the mechanism of this whitening has not been studied from the point of view of polymer degradation. In this study, a piece of whitened PVA was obtained from a painted wooden panel. The PVA whitening mechanism was studied in this piece using scanning electron microscopy (SEM) and functional group analysis by Fourier transform-infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS). SEM showed that the surface of the whitened PVA thin film was irregular and rugged, and many micrometer-sized cracks were observed. This suggests that efficient scattering of all visible light from the cracks causes whitening. FT-IR and XPS measurements revealed that inter- and intra-chain dehydration and chemical crosslinking (C–O ether linkage) occur on the surface of the whitened PVA. Polyvinyl alcohol (PVA), which has been used to prevent flaking of paint pigments on traditional Japanese Shohekiga, became white and we investigated its phenomenon. The surface of whitened PVA was irregular and rugged, and many micrometer size cracks were observed. This suggests that efficient scattering of all visible region light from the cracks causes whitening. Fourier transform-infrared spectroscopy and X-ray photoelectron spectroscopy measurements revealed that interchain dehydration and chemical crosslinking (C–O ether linkage) occurred on the surface of whitened PVA.

Self-quenching of uranin: Instrument response function for color sensitive photo-detectors.

Concentration is a key determining factor in the fluorescence properties of organic fluorophores. We studied self-quenching of disodium fluorescein (uranin) fluorescence in polyvinyl alcohol (PVA) thin films. The concentration dependent changes in brightness and anisotropy were followed by a lifetime decrease. We found that at a concentration of 0.54 M, the lifetime decreases to 7 ps. At a concentration of 0.18 M the lifetime was 10 ps with the relatively high quantum yield of 0.002. In these conditions the fluorescence intensity decay was homogeneous (well approximated by a single lifetime). We realized that such a sample was an ideal fluorescence lifetime standard for spectroscopy and microscopy, and therefore characterized instrument response functions for a time-domain technique. We show that self-quenched uranin enables measurements free of the color effect, making it a superior choice for a lifetime reference over scattered light.

Metal plasmon enhanced europium complex luminescence

The plasmon enhanced luminescence of a rare-earth complex Tris(6, 6, 7, 7, 8, 8, 8-heptafluoro-2, 2-dimethyl-3, 5-octanedionato) europium (Eu(fod) 3) was investigated. A polyvinyl alcohol (PVA) thin film was successfully adopted as a spacer to separate the Eu complex from the silver island film (SIF), and five-fold enhancement of the radiative decay rate of the Eu complex on SIF was demonstrated based on the luminescence intensity and lifetime measurement. Investigation of the distance dependent luminescence indicates that 7 nm is an optimal distance for SIF enhanced Eu luminescence. Plasmon enhanced rare-earth luminescence based on an organic film spacer would find potential applications in plasmon enhanced organic light emitting diode (OLED) devices.

Enhanced control of diamond nanoparticle seeding using a polymer matrix

We have improved the diamond nanoparticle seeding approach for chemical vapor deposition diamond growth in a novel process that consists of embedding the nanoparticles into a polymer matrix. We used a thin film of polyvinyl alcohol (PVA) doped with nanoparticles, which burns away during the initial stages of growth, leaving a stable distribution of nanoparticles on the substrate to initiate growth. The study shows that by varying the initial concentration of nanoparticles in the polymer preparation, it is possible to control the density of nanoparticles on the surface, over a wide range of densities. In some experimental conditions, the high densities of diamond seeding values obtained compare well with the highest values reported by the state-of-the-art. Moreover, the technique also opens up the route to very large area seeding, and this onto most types of substrates. In situ x-ray photoelectron spectroscopy (XPS) analyses showed that after pyrolysis of the polymer under H2 plasma, no significant residual carbon from the polymer was observed. Also, in the case of growth on silicon substrates, no silicon carbide was observed at the surface, showing that no reaction takes place between the polymer and the silicon surface itself. Finally, XPS also demonstrated that the polymer has not modified significantly the surface of the diamond nanoparticles after its pyrolysis. This approach improves the reproducibility of diamond nanoparticle seeding on flat surfaces and is more versatile as it may be applied to complex three-dimensional structures or cavities.

Selective mode excitation in dye-doped DNA polyvinyl alcohol thin film

A solid-state laser based on a dye-doped deoxyribonucleic acid (DNA) matrix is described. A thin solid film of DNA has been fabricated by treating with polyvinyl alcohol (PVA) and used as a host for the laser dye Rhodamine 6G. The edge emitted spectrum clearly indicated the existence of laser modes and amplified spontaneous emission. Lasing was obtained by pumping with a frequency-doubled Nd:YAG laser at 532 nm. For a pump energy of 10 mJ/pulse, an intense line with FWHM approximately 0.2 nm was observed at 566 nm due to selective mode excitation.

Hybrid gas sensing utilizing localized surface plasmons of gold colloid and a quartz crystal microbalance

Gas sensing properties were investigated using localized surface plasmons (LSPs) and a quartz crystal microbalance (QCM). A thin film of polyvinylalcohol (PVA) and gold colloid (20 nm in diameter) was deposited on the QCM substrate, and the gas responses of the optical absorption of gold colloid LSP and the QCM frequency were simultaneously observed. Vapors of water, ethanol and methylamine in N 2 were detected. The optical absorption spectra changed with the gas adsorption and desorption, and the gas response could be repeatedly observed. Furthermore, the relationship between the QCM frequency and the optical properties depended on the vapor species. Hybrid sensing can be used for detection and discrimination of various vapor adsorptions.

Photophysical properties of commercial red dyes in polymer films

A fluorescence study was conducted on commercial dyes (squaraines; triphenylmethane dyes; porphyrins and chlorins) in thin films of polyvinyl alcohol (PVA), poly (methyl methacrylate) (PMMA) and Hival 5108, a commercial polystyrene (PS) from Ashland. Fluorescence quantum yields were measured for dye–polymer system and compared to liquid- and solid-phase media values. All dyes were found to have fluorescent yields and stabilities that were dependent on the polymer film material. Squaraine dyes, SQ3 and OH-SQ, exhibited the highest quantum yields (Φf) when miscible in polymer films, reaching values as high as 0.89 in PS and 0.79 in PMMA. Acceptable Φf at wavelengths higher than 650nm were observed for SQ3, OH-SQ, TPP and PP-IX-DME. Moreover, fluorescence decay was measured after 240s of CW irradiation for all the dye–polymer films studied.

Excitation Energy Migration of Acridine Orange Intercalated into Deoxyribonucleic Acid Thin Films

The excitation energy migration of acridine orange (AO) immobilized in deoxyribonucleic acid (DNA) thin film was investigated by time-resolved fluorescence and fluorescence anisotropy decay measurements. Polyvinylalcohol (PVA) was used as a reference matrix. The absorption and fluorescence spectra indicated that the molecular aggregation was depressed in DNA thin films rather than in PVA thin films. Time-resolved fluorescence spectra of AO in DNA films suggested effcient energy transfer from monomer to trap sites, dimers, or aggregates. Fluorescence lifetime of AO in DNA films was decreased with increasing fractions of AO, which suggested efficient energy migration between intercalated AOs. The fluorescence anisotropy measurements directly supported the efficient energy migration between intercalated AOs. These findings indicate that the double-helix DNA contributes to the efficient energy migration between intercalated dyes.

Effect of donor-acceptor interaction strength on excitation energy migration and diffusion at high donor concentrations

The migration and diffusion modulated excitation energy transfer has been studied in a new dye pair 7-diethylamino-4-methylcoumarin (donor) to 3,3'-dimethyloxacarbocyanine iodide (acceptor) by steady-state and picosecond time-resolved spectroscopy. To reduce the artifact of self-absorption, at high donor concentrations, the time-resolved studies have been carried out in thin films of polyvinyl alcohol (solid matrix) and in methanol (liquid phase) at front-face geometry of excitation. The Forster-type (nonradiative) energy transfer [Discuss. Faraday Soc. 27, 7 (1959)] takes place directly from donor to acceptor in case of solid matrix, while Yokota-Tanimoto model [J. Phys. Soc. Jpn. 22, 779 (1967)] for diffusion has been found to be operating in the liquid phase. It has been found here that the high interaction strength between donor and acceptor molecules as compared to that among donors masks the effect of energy migration and diffusion at high donor concentrations. The rate and efficiency of energy transfer increase with increasing the acceptor concentration. This has been confirmed by the study of acceptor kinetics.

Effect of particle size and inter-electrode distance on the field-emission properties of nanocrystalline CdS thin films grown in a polymer matrix by chemical bath deposition

The field-emission property of transparent nanocrystalline CdS thin films, grown by chemical bath deposition (CBD) within the pores of polyvinyl alcohol (PVA) on Si and glass substrates, has been studied. X-ray diffraction and transmission electron microscopy studies confirmed the nanocrystalline nature of the CdS particles with sizes lying in the range 6–12 nm. The Fowler–Nordhiem plots of the emission current from the nano-CdS/PVA thin films are almost straight line. The turn-on fields of the grown nano-CdS/PVA thin films are found to be in the range of 3.6–6.6 V/μm. The turn-on field decreases with the decrease of particle size. Only PVA thin film and bulk CdS/PVA composite thin film grown on Si substrates have not showed field-emission property under the same conditions.

Preparation, characterization and application of bilayer surfactant-stabilized ferrofluids

Fe 3O 4 magnetic nanoparticles (MNPs) were prepared by the coprecipitation of Fe 2+ and Fe 3+ using ammonium hydroxide (NH 4OH) as precipitating agent. Transmission electronic microscopy (TEM) showed that the particle-size is around 10 nm. X-ray powder diffraction (XRD) indicated the sole existence of inverse cubic spinel phase of Fe 3O 4. The surface of MNPs was coated with oleate sodium as the primary layer and polyethylene glycol 4000 (PEG-4000) as the secondary layer to improve the stability of water-based ferrofluids (FFs). The dosages of oleate sodium and PEG-4000 were found to have an important effect on increasing the solid content. Gouy magnetic balance showed that the saturation magnetization could be as high as 1.44 × 10 5 A/m. Laser particle-size analyzer determined the aggregate size in FFs. The Fe 3O 4 MNPs did not change through the preparation of FFs. Differential scanning calorimetry–thermogravimetry (DSC–TG) and Fourier transform infrared spectroscopy (FT-IR) analysis showed existence of two distinct surfactants on the particle surface. The concentrated and diluted FFs were characterized by UV-vis spectrophotometer and excellent stability was found. The rheological measurements indicated that viscosity increased with the increase of solid content and applied magnetic field, but decreased with the increase of temperature. The FFs showed the non-Newtonian behavior of shear-thinning when the solid content was high. The mechanical properties of polyvinylalcohol (PVA) thin film can be greatly improved by adding FFs.

Humidity sensors made on polyvinyl-alcohol film coated SAW devices

Highly sensitive surface acoustic wave (SAW) humidity sensors were fabricated by spin-coating polyvinyl-alcohol (PVA) thin film on the SAW resonators. The transmission properties of the SAW sensor, including centre frequency and insertion loss, were investigated at a relative humidity ranging from 30 to 90%. The results justify the suitability for the PVA film coated SAW resonators in the application of sensing humidity at a higher relative humidity.

Polarization properties of polymer films with a birefringence gradient

This paper briefly describes a method of fabricating polymeric optical elements with a new property: an axial birefringence gradient (BRG), and presents the results of an experimental study of their optical polarization properties. The BRG elements were fabricated from thin films of polyvinyl alcohol (PVA, d=100−150 µm) and polyethylene terephthalate (PET, d=190 µm). It is shown that the birefringence gradient Delta n in the middle of the BRG element increases exponentially along the central z axis, with Delta nmaxapproximately0.02. Increasing the forming temperature of the BRG element from 80 to 90 °C improves the linearity of the Delta n=f(z) dependence, even though it is degraded and becomes irregular in the peripheral zone.

Excitation energy transfer efficiency of dipole–dipole interaction in a dye pair in polymer medium

Excitation energy transfer efficiency (η) of dipole–dipole interaction has been studied in the dye pair 3,3′-dimethyloxacarbocyanine iodide (DMOCI) (donor) to o-(6-diethylamino-3diethylimino-3H-xanthen-9-yl) benzoic acid (Rhodamine B, RB) (acceptor) in polyvinyl alcohol (PVA) thin films by steady-state and ps time-resolved fluorescence spectroscopy. In the presence of the acceptor the fluorescence intensity of the donor decreases, while that of the acceptor increases as a function of the added acceptor concentration. Time-resolved study of the donor at various acceptor concentrations suggest that the non-radiative energy transfer mechanism as proposed by Forster is responsible for the observed behaviour along with some modifications at very low acceptor concentrations. Modified η values have been simulated and compared with those obtained experimentally. It is found that the value of η increases with acceptor concentration.

Giant electrooptic response of excitons in porphyrin J-aggregates

J-aggregates of tetraphenyl porphyrin tetrasulfonic acid (TPPS) doped in a polyviny-lalcohol (PVA) thin film were studied by highly sensitive electroabsorption spectroscopy using a multichannel lock-in amplifier. A change in the static polarizability Δ α of the aggregate upon electronic excitation was determined as 20 000–39 000 Å 3, from which the size of coherent delocalization of excitons was evaluated to be 60–80 molecules. This is ten times larger than that previously reported. At least one of the reasons of the difference is found to be the content of water molecules in the polymer matrix.

Giant electrooptic response of excitons in porphyrin J-aggregates

J-aggregates of tetraphenyl porphyrin tetrasulfonic acid (TPPS) doped in a polyvinylalcohol (PVA) thin film were studied by highly sensitive electroabsorption spectroscopy using a multichannel lock-in amplifier. A change in the static polarizability Δ α of the aggregate upon electronic excitation was determined as 20 000–39 000 Å 3, from which the size of coherent delocalization of excitons was evaluated to be 60–80 molecules. This is 10 times larger than that previously reported. At least one of the reasons of the difference is found to be the content of water molecules in the polymer matrix.