Effect of Er doping on optical band gap energy of TiO2 thin films prepared by spin coating

In order to evaluate the effect of Er doping in the range of 0–1.0 mol% on optical indirect band gap energy (E_g) of the film, the Er-doped TiO₂ (Er-TiO₂) thin films were spin-coated onto fluorine-doped SnO₂ coated (FTO) glasses. Glancing angle X-ray diffraction (GAXRD) results indicated that the films whose thickness was 550 nm consisted of pure anatase and FTO substrate. The anatase (101) TiO₂ peaks became broader and weaker with the rise in Er content. The apparent crystallite size decreased from 12 nm to 10 nm with increasing the amount of Er from 0 mol% to 1.0 mol%. UV–vis spectrophotometry showed that the values of E_g decreased from 3.25 eV to 2.81 eV with the increase of Er doping from 0 to 0.7 mol%, but changed to 2.89 eV when Er content was 1.0 mol%. The reduction in E_g might be attributed to electron and/or hole trapping at the donor and acceptor levels in the TiO₂ band structure.     

Dewetting of thin polymer films

We study the dewetting of thin polymer films deposited on slippery substrate. Recent experiments on these systems have revealed many unexpected features. We develop here a model that takes into account the rheological properties of polymer melts, focussing on two dewetting geometries (the receding of a straight edge, and the opening of a hole). We show that the friction law associated with the slippage between the film and the substrate has a direct influence on the dewetting dynamic. In addition, we demonstrate that residual stresses, which can be stored in the films due to their viscoelasticity, are a source of destabilization for polymer films, and accelerate the dewetting process.     

Dynamics in thin polymer films by dielectric spectroscopy

After a brief review on the recent developments of the dielectric studies in thin polymer films, our recent results on dynamics in thin films of poly(methyl methacrylate) (PMMA) and polyisoprene (PIP) are shown. For PMMA, the tacticity effect on dynamics in thin films has been investigated and the disappearance of this effect was found below a critical thickness. For PIP, the motion of entire polymer chains, namely, the normal mode, has been investigated. The dielectric-loss spectrum of the normal mode is much more sensitive to the decrease in film thickness than that due to the -process. The broadening of dielectric-loss spectra of the normal mode is observed for film thicknesses below about 150 nm, while the position of the loss peak does not change in the thickness range down to about 50 nm. Anomalous increase in dielectric loss between the -process and the normal mode was observed, which is consistent with the recent report on the existence of an additional relaxation process.Received: 1 January 2003, Published online: 14 October 2003PACS: 64.70.Pf Glass transitions - 68.60.-p Physical properties of thin films, nonelectronic     

The spin Hall effect in single-crystalline gold thin films

The spin Hall effect has been investigated in 10-nm-thick epitaxial Au(001) single crystal films via H-pattern devices,whose minimum characteristic dimension is about 40 nm. By improving the film quality and optimizing the in-plane geometry parameters of the devices, we explicitly extract the spin Hall effect contribution from the ballistic and bypass contribution which were previously reported to be dominating the non-local voltage. Furthermore, we calculate a lower limit of the spin Hall angle of 0.08 at room temperature. Our results indicate that the giant spin Hall effect in Au thin films is dominated not by the interior defects scattering, but by the surface scattering. Besides, our results also provide an additional experimental method to determine the magnitude of spin Hall angle unambiguously.     

Electrical breakdown of thin polymer films

Data are presented on the dielectric strength of thin polymer films. The conclusion is drawn that the electron avalanche concept is inapplicable to the breakdown of thin films. It is proposed to consider electrical breakdown as a consequence of an abrupt local field enhancement induced by evolution of the space charge injected into the polymer from electrodes. It is shown that the lifetime of polymer films depends exponentially on electric field strength.     

Structure distortion,optical and electrical properties of ZnO thin films co-doped with Al and Sb by sol-gel spin coating

ZnO thin films co-doped with Al and Sb with different concentrations and a fixed molar ratio of AlCl 3 to SbCl 3 at 1:2, are prepared by a sol-gel spin-coating method on glass annealed at 550°C for 2 h in air. The x-ray diffraction results confirm that the ZnO thin films co-doped with Al and Sb are of wurtzite hexagonal ZnO with a very small distortion, and the biaxial stresses are 1.03×10 8 , 3.26×10 8 , 5.23×10 8 , and 6.97×10 8 Pa, corresponding to those of the ZnO thin films co-doped with Al and Sb in concentrations of 1.5, 3.0, 4.5, 6.0 at% respectively. The optical properties reveal that the ZnO thin films co-doped with Al and Sb have obviously enhanced transmittance in the visible region. The electrical properties show that ZnO thin film co-doped with Al and Sb in a concentration of 1.5 at% has a lowest resistivity of 2.5Ω·cm.     

Surface spin waves in ferromagnetic thin films

Surface spin-wave modes are calculated for (100) surface of simple cubic structure. An isotropic part as well as an anisotropic part are introduced in the spin Hamiltonian. The anisotropic factor is allowed to vary on the first two surface layers. Dispersion curves are derived by a Green function formalism.The authors are indebted to Dr A. P. Legrand for his help in numerical calculations and to Dr J. Korringa for his continuous encouragement and suggestions.     

Structure distortion, optical and electrical properties of ZnO thin films co-doped with Al and Sb by so--gel spin coating

ZnO thin films co-doped with Al and Sb with different concentrations and a fixed molar ratio of AlCl₃ to SbCl₃ at 1:2, are prepared by a sol--gel spin-coating method on glass annealed at 550 ℃ for 2 h in air. The x-ray diffraction results confirm that the ZnO thin films co-doped with Al and Sb are of wurtzite hexagonal ZnO with a very small distortion, and the biaxial stresses are 1.03×10⁸, 3.26×10⁸, 5.23×10⁸, and 6.97×10⁸ Pa, corresponding to those of the ZnO thin films co-doped with Al and Sb in concentrations of 1.5, 3.0, 4.5, 6.0 at% respectively. The optical properties reveal that the ZnO thin films co-doped with Al and Sb have obviously enhanced transmittance in the visible region. The electrical properties show that ZnO thin film co-doped with Al and Sb in a concentration of 1.5 at% has a lowest resistivity of 2.5Ω·cm.     

Molecular mobility in polymer thin films

Fluorescence recovery after photobleaching was used to measure in-plane dye-probe diffusion coefficients, D, in thin films of monodisperse polystyrene supported on fused quartz substrates. The substrates were prepared with a high density of surface hydroxyl groups which interact favorably with repeat units of the polymer. The effects of temperature and film thickness were investigated, at temperatures above the bulk glass transition of the polymer, T(g), and in the range of film thicknesses from 1-10(2) times the radius of gyration (R(g)) of individual polymer molecules. As the film thickness decreases towards R(g) the value of D increases above the bulk values, with significant effects first appearing in films approximately 20R(g). In the thinnest films studied, about 4R(g), the values of D lie as much as two orders over bulk values. At the same time, the temperature dependence of D becomes much weaker than in bulk. Analysis by free volume theory indicates that apparent values of both T(g) and the thermal expansion coefficient for liquid state, alpha(L), decrease as the film thickness decreases. The possible effects of surface segregation of the dye probe are discussed.     

Reduced viscosity in thin polymer films

The dewetting of thin polystyrene films (20-500 nm) on a liquid substrate is studied at time scales that are long compared to the reptation time. It is shown that the kinetics correspond to those of purely viscous flow and that the viscosity measured by this technique is, for the thickest films, consistent with bulk measurements. Films on the order of the coil size are then studied. The effective viscosity of these films displays a large decrease when the film thickness h is below several radius of gyration, R(g). This viscosity reduction is found to depend only on the ratio h/R(g).     

Glass transitions in thin polymer films

Freely standing polystyrene films show an anomalous drop of the glass temperature when the molecular weight is high and the thickness smaller than the coil size R₀. We present here a tentative explanation for these features, where two types of motions compete: a) standard motions, controlled by the free volume, and independent of chain length, b) collective motions along the chain, which require a weaker free volume (except for the end groups). For bulk systems, the standard motion always wins because of the end group hindrance. But for films thinner than the coil size, the dominant process may be the collective motion of a “loop” which does not involve the chain ends. What matters then is not the overall polymerisation index (N), but the length g of a typical loop starting from the surface (which is a more fluid region) and reaching deep into the film. With these ingredients, some surprising aspects of may possibly be understood. Received 7 December 1999     

Acoustic instabilities in thin polymer films

The spatial confinement of a fluctuation spectrum leads to forces at the confining boundaries. While electromagnetic (EM) fluctuations lead to the well-known dispersion forces, the acoustic analogy has widely been neglected. We show that the strength of the forces resulting from confined acoustic modes may be of the same order of magnitude as van der Waals forces. Additionally, the predicted scaling behavior is identical to the non-retarded case of the EM fluctuations. Our results suggest that dewetting experiments using polymer films are strongly influenced by the acoustic dispersion forces. Received 5 March 2002 and Received in final form 21 May 2002     

Characterization of polymer thin films obtained by pulsed laser deposition

The development of laser techniques for the deposition of polymer and biomaterial thin films on solid surfaces in a controlled manner has attracted great attention during the last few years. Here we report the deposition of thin polymer films, namely Polyepichlorhydrin by pulsed laser deposition. Polyepichlorhydrin polymer was deposited on flat substrate (i.e. silicon) using an NdYAG laser (266 nm, 5 ns pulse duration and 10 Hz repetition rate).The obtained thin films have been characterized by atomic force microscopy, scanning electron microscopy, Fourier transform infrared spectroscopy and spectroscopic ellipsometry.It was found that for laser fluences up to 1.5 J/cm² the chemical structure of the deposited polyepichlorhydrin polymer thin layers resembles to the native polymer, whilst by increasing the laser fluence above 1.5 J/cm² the polyepichlorohydrin films present deviations from the bulk polymer.Morphological investigations (atomic force microscopy and scanning electron microscopy) reveal continuous polyepichlorhydrin thin films for a relatively narrow range of fluences (1-1.5 J/cm²).The wavelength dependence of the refractive index and extinction coefficient was determined by ellipsometry studies which lead to new insights about the material.The obtained results indicate that pulsed laser deposition method is potentially useful for the fabrication of polymer thin films to be used in applications including electronics, microsensor or bioengineering industries.     

Photothermal deflection spectroscopy of polymer thin films

Photothermal Deflection Spectroscopy (PDS) is known to be one of the most sensitive techniques for measuring the absorption of weakly absorbing materials. We have applied PDS for measuring the optical absorption of a few polymer thin-film samples over the wavelength region from 0.4–2.0 m. The results are useful for optical evaluation of these polymers.     

Spinodal-like dewetting of thermodynamically-stable thin polymer films

Energetic considerations indicate that long-range Van der Waals forces stabilize thin polystyrene (PS) films against height fluctuations on silicon substrates. Nevertheless, we report here on the amplification of capillary waves of specific wavelengths for 15 nm thick PS films on silicon, ultimately leading to dewetting in a spinodal-like process. However, the temporal dependence of the wavelength of the growing instability does not agree with the classical spinodal dewetting mechanism. Therefore, this phenomenon is ascribed to the existence of structural forces resulting either from the restructuring of the films or from density variations within the films during annealing, in accordance with recent theoretical treatments. The process is shown not to be limited to polystyrene films, which indicates the generality of our findings.Received: 1 August 2003PACS: 68.15. + e Liquid thin films - 47.20.-k Hydrodynamic stability - 47.20.Ma Interfacial instability - 68.08.-p Liquid-solid interfaces     

Supramolecular structure of electroactive polymer thin films

This paper presents the results of an experimental investigation of the supramolecular structure of polydiphenylenephthalide thin films that exhibit effects of resistive switching. The supramolecular structure of the polymer has been investigated using small-angle neutron scattering in conjunction with atomic force microscopy. It has been found that the internal structure of polymer films consists of structural elements in the form of spheroids. The sizes of the structural elements, which were obtained from the neutron scattering data and analysis of the atomic force microscopy images, correlate well with each other. A model of the formation of polymer layers has been proposed. The observed structural elements in polymer films are formed due to the association of macromolecules in the initial polymer solution.     

Study of terahertz radiation generated by optical rectification on thin gold films

Emission of terahertz (THz) radiation as a result of optical rectification of intense femtosecond laser pulses on thin gold films has been studied by time-domain THz spectroscopy. The THz amplitude was measured as a function of film thickness and incidence angle. The experiments reveal that the emitted THz field is suppressed for a thickness below 100 nm, which gives evidence of the nonlocal character of the response. The variation of incidence angle allows us to estimate the components of susceptibility tensor chi2ijk. For thicker films and near grazing incidence, the emitted THz field attains a peak value of 4 kV/cm.     

Ultra-smooth metal surfaces generated by pressure-induced surface deformation of thin metal films

We present a mechanical pressing technique for generating ultra-smooth surfaces on thin metal films by flattening the bumps, asperities, rough grains and spikes of a freshly vacuum deposited metal film. The method was implemented by varying the applied pressure from 100 MPa to 600 MPa on an e-beam evaporated silver film of thickness 1000 Å deposited on double-polished (100)-oriented silicon surfaces, resulting in a varying degree of film smoothness. The surface morphology of the thin film was studied using atomic force microscopy. Notably, at a pressure of ～600 MPa an initial silver surface with 13-nm RMS roughness was plastically deformed and transformed to an ultra-flat plane with better than 0.1 nm RMS. Our demonstration with the e-beam evaporated silver thin film exhibits the potential for applications in decreasing the scattering-induced losses in optical metamaterials, plasmonic nanodevices and electrical shorts in molecular-scale electronic devices.     

Phase-sensitive acoustic microscopy of polymer thin films

The three-dimensional images obtained by scanning acoustic microscopy with vector contrast (PSAM), contain significant qualitative and quantitative information that is not easily obtainable by other methods. We employ this technique to examine homopolymer and polymer blend thin films. The complex V(z) functions derived from the images, and the results obtained by image processing and meticulous analysis are employed to render the morphology, composition and micro-mechanical properties of the polymer films. In addition, ways by which the information inherent in the phase images can be extracted are examined. This is highly desirable, as the phase images contain very useful additional information.