Influence of Substrate Properties on the Dewetting Dynamics of Viscoelastic Polymer Films

We studied the dewetting process of thin polystyrene (PS) films on silicon substrates, coated with a thin, irreversibly adsorbed polydimethylsiloxane (PDMS) layer, by optical microscopy and atomic force microscopy. Besides demonstrating the exceptional potential of dewetting for a sensitive characterization of rheological properties of PS thin films, characterized by a stress-relaxation time, τ₁, we focused on the influence of the frictional behaviour (energy dissipation mechanism) at the interface between the PDMS-coated silicon wafer and the PS film on the dewetting process. Our results show that the initial stages of dewetting depend sensitively on the thickness and the way the PDMS layer was adsorbed. The maximum width of the dewetting rim at τ₁ increased with increasing PDMS layer thickness, which can be interpreted as an increase of the effective, velocity-dependent slippage length. Interestingly, τ₁ was found to be almost independent of the substrate properties. Our results demonstrate that dewetting is a really powerful approach for rheological and frictional studies of thin polymer films.     

Spectroscopic ellipsometry characterization of TiO2 thin films prepared by the sol–gel method

TiO₂ thin films were prepared on SiO₂/Si(100) substrates by the sol–gel process. XRD results indicate that the major phase of TiO₂ thin films is anatase. The surface morphology and cross-section are observed by FE-SEM. The surface of thin films is dense, free of cracks and flat. The average grain size is about 60–100 nm in diameter. The thickness of single layer TiO₂ thin films is about 60 nm, which increases with the concentration of solution. Ellipsometric angles ψ, Δ are investigated by spectroscopic ellipsometry. The optical constant and the thickness of TiO₂ thin films are fitted according to Cauchy dispersion model. The results reveal that the refractive index and the extinction coefficient of TiO₂ thin films in wavelength above 800 nm are about 2.09–2.20 and 0.026, respectively. The influences of processing conditions on the optical constants and thicknesses of TiO₂ thin films are also discussed.     

Unique molecular aggregation of novel naphthalocyanine (near IR absorbing dye)

A series of organic-soluble naphthalocyanine derivatives (Y_mMNcX₄) have been synthesized. Their spectroscopic properties in organic solutions and in thin films were studied. MNcX₄ such as MNc(t-Bu)₄ (5a-5c) and MNc(On-Bu)₄ (5j) formed H-aggregates even in dilute solutions. MNc(CO₂R')₄ (5d-5i) had much stronger H-aggregation properties compared with those ofMNc(t-Bu)₄ and MNc(On-Bu)₄. Especially, MNc(CO₂R')₇ complexes (M=Cu, Pd and Ni) existed exclusively without monomers in the aggregated state even in highly dilute solutions. MNcX₄, which has strong H-aggregation properties, showed merely H- aggregate absorption maxima in thin films. In contrast, Y₂MNcX₄ (4), which has sterically hindered Y groups, showed monomerically pure characteristics in solution. However, thin films of Y₂MNcX₄ have a J-type molecular arrangement, exhibiting a red shift of Q-band absorption. The monomeric properties of Y₂MNcX₄ in solutions and J-type molecular arrangement in thin films arise from steric hindrance of two Y groups, such as R₃SiO-in Y₂MNcX₄, which prevents strong H-aggregation of naphthalocyanine.     

ELECTROLESS PLATING OF GLASS AND SILICON SUBSTRATES THROUGH SURFACE PRETREATMENTS INVOLVING PLASMA-POLYMERIZATION AND GRAFTING PROCESSES

Electroless plating of nickel (or copper) was carried out on glass (or silicon) substrates that were previously surface modified by using plasma-polymerization and grafting processes, and then activated by immersion in a simple acidic PdCl₂ solution. Three pretreatments based on the deposition of plasma-polymerized thin films (PACVD process) on O₂ plasma-cleaned substrates were investigated. They include film deposition of (1) amorphous hydrogenated carbon (a-C:H) grown from CH₄, whose surface is subsequently plasma-functionalized in NH₃ or N₂; (2) amorphous hydrogenated carbon nitride (a-CN_x:H) grown from CH₄/NH₃ or CH₄/N₂ mixtures; and (3) amorphous hydrogenated carbon nitride grown from volatile organic precursors (allylamine, acetonitrile).

In the three cases, X-ray photoelectron spectroscopy (XPS) results show that chemisorption of the catalyst occurs on the nitrogen-containing functionalities created by plasma polymerization and grafting and thus that the electroless deposition is possible. Differences were observed depending on the nature and thickness of the plasma-polymerized thin films, as well as on the nature and concentration of the nitrogen-containing functionalities present or grafted at the surface. Practical adhesion of Ni films was investigated using a Scotch® tape test. Ni films up to 3 or 4 μm in thickness were shown to pass this test successfully, i.e., without causing any metal detachment.     

Morphology of poly(styrene‐block‐dimethylsiloxane) copolymer films

The morphologies of poly(styrene‐block‐di‐methylsiloxane) (PS‐b‐PDMS) copolymer thin films were analyzed via atomic force microscopy and transition electron microscopy (TEM). The asymmetric copolymer thin films spin‐cast from toluene onto mica presented meshlike structures, which were different from the spherical structures from TEM measurements. The annealing temperature affected the surface morphology of the PS‐b‐PDMS copolymer thin films; the polydimethylsiloxane (PDMS) phases at the surface were increased when the annealing temperature was higher than the PDMS glass‐transition temperature. The morphologies of the PS‐b‐PDMS copolymer thin films were different from solvent to solvent; for thin films spin‐cast from toluene, the polystyrene (PS) phase appeared as pits in the PDMS matrix, whereas the thin films spin‐cast from cyclohexane solutions exhibited an islandlike structure and small, separated PS phases as protrusions over the macroscopically flat surface. The microphase structure of the PS‐b‐PDMS copolymer thin films was also strongly influenced by the different substrates; for an asymmetric block copolymer thin film, the PDMS and PS phases on a silicon substrate presented a lamellar structure parallel to the surface at intervals. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 1010–1018, 2007     

Sol-gel processed barium titanate thin films

Ferroelectric barium titanate (BaTiO₃) thin films have been prepared by sol-gel technique from barium acetate [Ba(CH₃COO)₂] and titanium (IV) isopropoxied [Ti(CH₃)₂CHO)₄] precursors. The as-grown films were found to be amorphous which crystallized to tetragonal phase after annealing at 700°C for one hour in air. The room temperautre dielectric constant (ε) and loss tangent (tanδ) of the films were found to be 370 and 0.02 respectively. The values of the spontaneous polarization (P_s), remanent polarization (P_r) and coercive field (E_c) of the films determined from the polarization-field (P-E) hysteresis were found to be 14.0, 3.2μC-cm^-2 and 53KV-cm^-1 respectively. The coercive field of the film determined from the capacitance-voltage (C-V) characteristics is slightly lower than that determined from the P-E hysteresis loop.     

Preparation, structural and optical characterization of BaWO4 and PbWO4 thin films prepared by a chemical route

Polycrystalline BaWO₄ and PbWO₄ thin films having a tetragonal scheelite structure were prepared at different temperatures. Soluble precursors such as barium carbonate, lead acetate trihydrate and tungstic acid, as starting materials, were mixed in aqueous solution. The thin films were deposited on silicon, platinum-coated silicon and quartz substrates by means of the spinning technique. The surface morphology and crystal structure of the thin films were investigated using scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction, and specular reflectance infrared Fourier transform spectroscopy, respectively. Nucleation stages and surface morphology evolution of thin films on silicon substrates have been studied by atomic force microscopy. XRD characterization of these films showed that BaWO₄ and PbWO₄ phase crystallize at 500 °C from an inorganic amorphous phase. FTIR spectra revealed the complete decomposition of the organic ligands at 500 °C and the appearance of two sharp and intense bands between 1000 and 600 cm⁻¹ assigned to vibrations of the antisymmetric stretches resulting from the high crystallinity of both thin films. The optical properties were also studied. It was found that BaWO₄ and PbWO₄ thin films have Eg=5.78 eV and 4.20 eV, respectively, of a direct transition nature. The excellent microstructural quality and chemical homogeneity results confirmed that soft solution processing provides an inexpensive and environmentally friendly route for the preparation of BaWO₄ and PbWO₄ thin films.     

THE EFFECT OF PRESSURE ON AZEOTROPES

Recently determined electrical conductivities of anodic AgCl, A1C1₃, TiBr₄, and MgCl₂ films are compared to well established conductivities of anodic Ta₂O₅; and A1₂O₃. All of these films exhibit high-field barrier layer condition. The exponential constant, β. is about the same value for the salts as for oxides but the preexponential constants and conductivities are orders of magnitude larger for the salts. The salts, AlCl₃, TiBr₄, and MgCl₂, in addition, have a series ohmic resistance which is attributed to an outer, continuously-forming and dissolving, hydrated salt layer.     

Composition fluctuations before dewetting in polystyrene/poly(vinyl methyl ether) blend thin films

We report structural development in blend thin films of deuterated polystyrene (dPS) and poly(vinyl methyl ether) (PVME) below 200 nm in two phase region during the incubation period before dewetting using neutron reflectivity (NR) and atomic force microscopy (AFM). As was predicted by the former optical microscope (OM) and small-angle light scattering (LS) measurements on blend thin films of protonated PS and PVME [Ogawa H, Kanaya T, Nishida K, Matsuba G. Polymer 2008;40:254–62.], the NR results clearly showed that the tri-layer structure consisting of the surface PVME layer, the middle blend layer and the bottom PVME layer was formed in the one phase region. After the temperature jump into the two phase region, it was found that the phase separation of the middle blend layer proceeded in the depth direction during the incubation period before dewetting, suggesting that the dewetting was induced by the composition fluctuations during the incubation period.     

Voltammetric study of halide ion adsorption on platinum in perchloric acid solutions

Periodic current/potential curves were taken potentiostatically at 30 mV/sec on smooth platinum in 1 N HClO₄ with different additions of HCl, HBr, and HI, in the potential range between hydrogen evolution and deposition of oxygen or of the respective halide ion. The effect of increasing halide ion concentration, starting at 10⁻⁶ M, on hydrogen adsorption and oxygen adsorption was studied systematically. Impedance measurements were made at 1000 c/s under the same conditions as the measurements of the current/potential curves. It is concluded from the dependence of the double layer capacitance upon the bulk concentration of halide ions in the double layer region that nearly a monolayer of adsorbed ions is present there for ₀c_Cl⁻ ≈ 10⁻² M, ₀c_Br ≈ 10⁻⁴ M, and ₀c_HI ≈ 10⁻⁵ M. The measurements at larger concentrations, especially for I⁻, show additional effects which are due to the deposition of halides and to the reverse reaction.     

Chemical Solution Deposition of Columnar-Grained Metallic Lanthanum Nitrate Thin Films

Columnar and (100)-oriented LaNiO₃ thin films were prepared on silicon substrates by a chemical solution deposition (CSD) process using a 0.05 M solution. By reducing the individual layer thickness to 10 nm, columnar LaNiO₃ films with a lateral grain size of ∼120 nm were obtained. The success of this approach required restricting the individual layer thickness to a value below the grain size observed for equiaxed films. This change in microstructure resulted in an improvement in conductivity. The columnar LaNiO₃ film with a thickness of 300 nm showed a resistivity of 4.5 × 10⁻⁵Ω·cm, which is lower by one order of magnitude than that of fine-grain equiaxed films that typically result from CSD methods.     

Fabrication of La2Ti2O7 thin films by a sol-gel technique

Sol-gel thin films of La₂Ti₂O₁₇ were deposited on fused silica and Si( 100) substrates by a spin-coating process. The La₂Ti₂O₁₇ precursor solution for the spin-coating was prepared from lanthanum acetylacetonate and titanium iso-propoxide dissolved in 2-methoxyethanol. Crystalline and crack-free films of ∼ 0.3 μm thickness were deposited on the above sulastrates using a single coating and followed by annealing at a temperature of 800 "C. Microstructural studies revealed that these films contained extremely fine grains of ∼ 0.1 μm. Thin film X-ray diffraction patterns indicated the formation of grain oriented films along [100] direction on these substrates.     

Dewetting and Adsorption in Homopolymer Films Containing Triblock Copolymers: Role of Chain Architecture and Anchoring Block Molar Fraction

Triblock copolymer additives are found to stabilize thin-film dewetting of B-type homopolymers with degree of polymerization (DOP) P deposited on silicon oxide. The triblock copolymers' architectures are ABA and BAB, where A and B represent anchoring and nonadsorbing blocks with DOP's N_A and N_B, respectively. Upon adding 1 vol.% of the ABA additive, dewetting is only observed for anchoring block molar fractions, f_A, below 4%. Dewetting is arrested in films containing 1 vol.% ABA, BAB, or AB that have similar values of f_A ∼ 8%, showing that chain architecture is not the only indicator of a successful additive. Compared with films containing diblock copolymers, the interfacial excess, z*, of triblock copolymers at the melt/substrate interface is relatively small as measured by low-energy forward-recoil spectrometry. Because adsorbed copolymers can reduce the capillary driving force for dewetting and participate in entanglements with matrix chains, the higher coverage and grafting density observed for diblock copolymers suggests that diblocks are more effective than triblocks in improving thin-film stability.     

Galvanostatic desorption of hydrogen from palladium layers—I. Theory

The theory of the galvanostatic desorption of hydrogen held in a finite palladium layer (thickness, l 10⁻³−10⁻² cm), in a thin palladium layer (thickness, λ 10⁻⁶−10⁻⁵ cm) and in a composite finite layer of a non-noble metal protected with a thin palladium layer is developed. The diffusion equations are solved by the Laplace transform and their solutions in term of the concentration of dissolved N_i and weakly adsorbed N_w hydrogen are given. Further the Iτ-functions are estimated for small values of the current, which allows the calculation of the characteristic parameters both of the metallic layer and of the partial steps involved, ie the diffusion and the transfer. The transient overpotentials occurring during the electrochemical desorption for a reversible and an irreversible oxidation step are also calculated.     

Reptation in polymer blends

Forward recoil spectrometry (FRES) was used to measure the tracer diffusion coefficients D*_PS and D*_PXE of deuterated polystyrene (d-PS) and deuterated poly(xylenyl ether) (d-PXE) chains in high molecular weight protonated blends of these polymers. The D*s were shown to be independent of matrix molecular weights and to decrease as M⁻², where M is the tracer molecular weight, suggesting that the tracer diffusion of both species occurs by reptation. These D*s were used to determine the monomeric friction coefficients ζ_0,PS and ζ_0,PXE of the individual PS and PXE macromolecules as a function of ф, the volume fraction of PS in the PS:PXE blend. Since ζ_0,PSζ_0,PXE at each ф, the rate at which a PS molecule reptates is much greater than that of a PXE molecule, even though both chains are diffusing in identical surroundings. Part of this difference may be due to the difficulty of backbone bond rotation of the PXE molecule. However, even when measured at a constant temperature increment above the glass transition temperature, ζ_0,PS and ζ_0,PXE were observed to be markedly composition dependent. In addition the ratio ζ_0,PS/ζ_0,PXE varied from a maximum of 4 × 10⁻² near ф=0.85 to a minimum of 5 × 10⁻⁵ for ф=0.0. These results show that intramolecular barriers do not solely determine the ζ₀s of the components in this blend. Clearly, the interactions between the diffusing chains and the matrix chains also influence ζ₀.     

Structural and Optical Properties of Barium Strontium Titanate (Ba0.5Sr0.5TiO3) Thin Films

Ferroelectric Barium Strontium Titanate (Ba_0.5Sr_0.5TiO₃) or BST thin films on quartz substrates have been prepared by using a modified sol gel processing technique. The starting materials are Barium 2-ethylhexanoate Ba[CH₃(CH₂)₃CH(C₂H₅)CO₂]_2, Strontium 2-ethylhexanoate Sr[CH₃(CH₂)₃CH(C₂H₅)CO₂]₂ and Titanium(IV) isopropoxide [TiOCH(CH₃)₂]₄. The precursors except [TiOCH(CH₃)₂]₄ were synthesized in the laboratory. Transparent and crack-free films were fabricated on quartz substrates by spin coating. The as-fired films were found to be amorphous, which crystallized to cubic phase after annealing at 550°C in air for 1 hr. In this paper we report the structural and optical properties of BST films prepared by the modified sol-gel process.

Communicated by Prof. E. C. Subbarao     

Particle nucleation for dewetting initiation of thin polymer films

A quantitative comparison between spontaneous dewetting and particle nucleation for thin (thickness = 17nm) polystyrene (PS) films on nonwettable silicon (Si) surfaces is presented both experimentally and theoretically. Performing experiments in a class 100 clean room, we found that ～ 23% of the observed dry patches formed because of dust particles, while the majority of the holes formed via the well known spontaneous dewetting process. The result was verified qualitatively by diffusion theory, which, however, predicted a diminished role for the airborne particles, leading to the conclusion that pre‐existing particles on the Si surfaces and/or the polymer solutions contribute substantially to the dewetting process. The driving force of particle motion into the polymer film is examined by placing aluminum oxide (Al₂O₃) particles on PS films. Finally, the effect of particle geometry is studied by placing gold (Au) disks on the free surface of PS films. An optically continuous PS film is found to be present around the periphery of the disk particles, even after the completion of the dewetting process in the rest of the sample. An attempt to explain dewetting inhibition at the vicinity of the micro‐disks, on the basis of molecular interactions developed in the system Au/PS/Si, is finally presented. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 138–145, 2005     

The preparation and properties of PZT thin film by sputering method

The ferroelectric PZT thin films were prepared on Pt/Ti/SiO₂/Si substrate by RF sputtering method followed by the rapid thermal annealing. The preparation of the Pt and Ti thin films as bottom electrode, and their influences on the PZT thin films were studied in details. The substrate temperature during sputtering was room temperature; the rapid thermal annealing temperature was 500°C-750°C and the annealing time was 30-70s. The influences of different preparation parameters on the structure and electric properties were studied with X-ray diffraction technique and RT66A Standardized Ferroelectric Test System. The electric properties of the prepared PZT thin film was: P_a=39μc/cm₂, P_r = 9.3 μc/cm², E_c=28KV/mm, ε=300, p=10⁹ω⋅cm.     

Effect of ammonium polyacrylate on rheology of anatase TiO2 nanoparticles dispersed in silicon alkoxide sols

Ammonium polyacrylate (NH₄PA) was introduced into powdered mixtures consisting of anatase-structured TiO₂ nanoparticles and silicon alkoxide precursors at the sol level, and the rheological behavior of the mixtures was examined under various solid loadings (φ=0.05–0.13 in volumetric ratios), shear rates (  s⁻¹) and NH₄PA concentrations. The alkoxide precursors were mixtures of tetraethyl orthosilicate (TEOS, Si(OC₂H₅)₄), ethyl alcohol (C₂H₅OH), H₂O and HCl in a constant [H₂O]/[TEOS] ratio of 11. The nanoparticle–sol mixtures generally exhibited a pseudoplastic flow behavior over the shear-rate regime examined. The NH₄PA appeared to serve as an effective surfactant which facilitates the suspension flow by reducing the flow resistance at low NH₄PA concentrations. At φ=0.10, a viscosity reduction ca. 85% was found at  s⁻¹ when the NH₄PA concentration was held at 2.5 wt.% of the solids. As the NH₄PA exceeded a critical level, e.g., [NH₄PA]≥3.0 wt.%, the NH₄PA acted as a catalyst which quickly turned the TiO₂–silica sol mixtures (φ=0.10) into a gelled structure, resulted in a pronounced increase of mixture viscosity. The maximum solids concentration (φ_m) of the mixtures was experimentally determined from a derivative of relative viscosity, i.e., (1−η_r^−1/2)–φ dependence. The estimated φ_m increased from 0.127 to 0.165 when NH₄PA of 0.5 wt.% was introduced into the TiO₂–silica sol mixtures.