Search

We explore the effect of crystallographic anisotropy on the optical properties of high-quality β-Ga2O3 thin films by conducting experimental measurements and theoretical simulations. High resolution x-ray diffraction measurements confirm the high-quality and the single crystalline quality, ruling out the effect of defects for all films. Raman spectra reveal the presence of anisotropy, as evident from a stronger B g (2) mode related to GaIO4 chain libration in (100) orientation. Conversely, a stronger A g (10) mode corresponding to tetrahedral bonds is evident in the (010) orientation, while it is suppressed in the (100) orientation. Low-temperature photoluminescence spectra indicate the presence of intrinsic impurity-related emissions in the ultraviolet and blue regions for all films. An anisotropic bandgap is observed, wherein the lowest bandgap value is related to the (010) oriented sample. Spectroscopic ellipsometry measurements confirm different refractive indices and extinction coefficient values depending on crystallographic orientation, which are in good agreement with the theoretical values obtained by density functional theory, confirming the anisotropic characteristics. The theoretical calculations of charge density show that the strength of covalent bonding depends on the β-Ga2O3 orientation, while experimental findings demonstrate that as the covalent bonding character increases, the film bandgap and the refractive index decrease. The anisotropy of β-Ga2O3 with respect to the crystal orientation leads to variations in the extinction coefficient, refractive index, and bandgap energy. [ABSTRACT FROM AUTHOR]

The authors present a second-generation broadband 4 × 4 Mueller-matrix (MM) ellipsometer for ultrasensitive infrared-spectroscopic (8000 − 800 cm − 1 ) studies of complex nanometer-thin films. In a modular design, the instrument employs retractable achromatic retarders and various sets of tandem polarizers. Using high-transmittance free-standing wire-grid polarizers, the device reaches an unparalleled precision of up to 5 ⋅ 10 − 5 in the important fingerprint region, even for block-offdiagonal MM elements. Broadband and signal-to-noise optimized access to the full 4 × 4 MM provides in-depth information on the sample's polarimetric properties and opens the door for detailed explorations of depolarizing and anisotropic materials. The authors discuss examples of highly depolarizing nonuniform polyimide membranes, uniaxial-to-biaxial anisotropy changes in ultrathin polymer films, and azimuthal off-axis effects in 2D-structured silica arrays. Diverse optical modeling approaches based upon anisotropic layer stacks and rigorous coupled-wave analysis are used to quantify the optical, structural, and chemical properties of the sample. [ABSTRACT FROM AUTHOR]

Cellulose is widely regarded as an environmentally friendly, natural and low cost material which can significantly contribute to the sustainable economic growth. In this study, cellulose composite membranes were prepared via regeneration of trimethylsilyl cellulose (TMSC), an easily synthesized cellulose derivative. The amorphous hydrophilic feature of the regenerated cellulose enabled fast permeation of water vapour. The pore-free cellulose layer thickness was adjustable by the initial TMSC concentration and acted as an efficient gas barrier. As a result, a 5000 GPU water vapour transmission rate (WVTR) at the highest ideal selectivity of 1.1 × 106 was achieved by the membranes spin coated from a 7% (w/w) TMSC solution. The membranes maintained a 4000 GPU WVTR with a selectivity of 1.1 × 104 in mixed-gas experiments, surpassing the performances of previously reported composite membranes. This study provides a simple way to not only produce high performance membranes but also advance cellulose as a low-cost and sustainable membrane material for dehumidification applications. [ABSTRACT FROM AUTHOR]

Hydrophobic polymers of intrinsic microporosity (PIM‐1) are successfully demonstrated as proton conducting separators in the all‐vanadium redox flow battery with the unprecedented infinite proton/vanadium selectivity. A battery fabricated with a nanocomposite comprising a microporous support membrane and nanometer‐thick selective PIM layer shows an energy efficiency of up to nearly 99%, while it is also stable during 100 cycles in cyclic performance. [ABSTRACT FROM AUTHOR]

ABSTRACT The sorption of compressed carbon dioxide and methane in a series of all-aromatic poly(etherimide) (PEI) thin films is presented. The polymer films are derived from the reactions between an arylether diamine (P1) and four different dianhydrides [3,3′,4,4′-oxydiphthalic dianhydride (ODPA), 3,3′,4,4′ biphenyltetra-carboxylic dianhydride (BPDA), 3,3′,4,4′-benzo-phenonetetracarboxylic dianhydride (BTDA), and pyromellitic dianhydride (PMDA)] that have been selected to systematically change the flexibility of the polymer backbone, the segmental mobility, and the nonequilibrium excess free volume (EFV) of the polymer. The EFV, gas sorption capacities, and sorption- and temperature-induced dynamic changes in film thickness and refractive index have been investigated by spectroscopic ellipsometry. The sorption capacity depends to a great extent on the PEI backbone composition. PMDA-P1 shows the highest carbon dioxide sorption, combined with the lowest sorption selectivity because of the predominant sorption of methane in the EFV. For ODPA-P1, the highest sorption selectivity is obtained, while it shows little long-term relaxations at carbon dioxide pressures up to 25 bar. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 986-993 [ABSTRACT FROM AUTHOR]

Thin film and bulk, sulfonated poly(ether ether ketone) (SPEEK) have been subjected to a thermal treatment at 160-250 °C for up to 15 h. Exposing the films to 160 °C already causes partial desulfonation, and heating to temperatures exceeding 200 °C results in increased conjugation in the material, most likely via a slight cross-linking by H-substitution. It is well-known that the sulfonate proton plays a major role in the desulfonation reactions, and exchanging the protons with other cations can inhibit both protodesulfonation as well as electrophilic cross-linking reactions of the sulfonate group with other chains. Yet, the implications of such ion-exchange for the thermal processing of sulfonated polymer films has not been recognized. Our study demonstrates that the ion exchange stabilizes thin films and bulk SPEEK up to temperatures exceeding 200 °C, opening up ways for the thermal processing of SPEEK in the temperature range of 160-220 °C without adverse effects. [ABSTRACT FROM AUTHOR]

We discuss the formation of TiO2 thin films via DC reactive magnetron sputtering. The oxygen concentration during sputtering proved to be a crucial parameter with respect to the final film structure and properties. The initial deposition provided amorphous films that crystallise upon annealing to anatase or rutile, depending on the initial sputtering conditions. Substoichiometric films (TiOx < 2), obtained by sputtering at relatively low oxygen concentration, formed rutile upon annealing in air, whereas stoichiometric films formed anatase. This route therefore presents a formation route for rutile films via lower (< 500 °C) temperature pathways. The dynamics of the annealing process were followed by in situ ellipsometry, showing the optical properties transformation. The final crystal structures were identified by XRD. The anatase film obtained by this deposition method displayed high carriers mobility as measured by time-resolved microwave conductance. This also confirms the high photocatalytic activity of the anatase films. [ABSTRACT FROM AUTHOR]

In situ time-resolved spectroscopic ellipsometry is used to study the dynamics of n-hexane diffusion into, and the corresponding induced swelling of, ultra-thin polystyrene films. The experimental conditions are carefully selected to facilitate the observation of anomalous Case II diffusion in the system, thereby allowing the probing of the chain-relaxation dynamics of a sharp moving penetrant front within the films. It has been found that the two different approaches to the obtained data are in quantitative agreement and suggest the existence of a finite thickness region of 14 ± 3 nm in the outer film interface that is instantly swollen after contact with the penetrant. The thickness of this fast swelling layer is found to be independent of swelling temperature and physical aging time. After the interface is swollen, the diffusion front velocity shows no significant spatial variations in the direction perpendicular to the substrate, but is strongly dependent on temperature and sample aging history. [ABSTRACT FROM AUTHOR]

Ultra thin ZIF-8 nanofilms are prepared by facile step-by-step dip coating. A critical withdrawal speed allows for films with a very uniform minimum thickness. The high refractive index of the films denotes the absence of mesopores. The dynamic response of the films to CO2 exposure resembles behaviour observed for nonequilibrium organic polymers. [ABSTRACT FROM AUTHOR]

Front Cover: The cover illustrates proton (H)‐ and sodium (Na)‐ form of sulfonated poly(ether ether ketone) membrane films. In the image, the film H‐membrane undergoes degradation and cross‐linking upon heating to 190 °C, resulting in a red color of the material, while the Namembrane remains unchanged. The significantly enhanced thermal stability of Na‐membranes allows for processing at elevated temperatures without adverse effects. Further details can be found in the article by B. T. Koziara, E. J. Kappert, W. Ogieglo, K. Nijmeijer, M. A. Hempenius and N. E. Benes on page 71. [ABSTRACT FROM AUTHOR]