Mixed Composition Films Research Articles

Atmospherically induced defects in (FASnI3)0.6(MAPbI3−3xCl3x)0.4 perovskites

The effects of brief atmospheric exposure are assessed for mixed composition perovskites comprised of a combination of formamidinium tin iodide (FASnI3) and methylammonium lead iodide + chloride (MAPbI3−3xCl3x) as well as MA0.7FA0.3PbI3 perovskite for use as absorber layers in high-efficiency thin film solar cells. Photothermal deflection spectroscopy (PDS) is employed to probe the relative defect density of all films by way of extracting the Urbach energy characterizing the width of the sub-bandgap absorption tail. Exposure to atmosphere for ~3 min prior to PDS measurement is found to degrade mixed composition films such that exposed films had an average increase of 36% in Urbach energies compared to those that are fully kept out of atmosphere through immersion in inert C6F14 liquid. By contrast, the same exposure to atmosphere induced no change in Urbach energy for MA0.7FA0.3PbI3. These results demonstrate the influence of ambient exposure on perovskites, particularly those containing Sn, and highlight the fact that handling procedure itself throughout all fabrication and subsequent characterization steps influences the measured results.

Optical properties and laser damage threshold of HfO2–SiO2 mixed composite thin films

HfO(2)-SiO(2) mixed composite thin films have been deposited on fused silica substrate by co-evaporation of HfO(2) and SiO(2) through the reactive electron-beam evaporation technique. The composition-dependent refractive index and the absorption coefficient have been analyzed using different effective medium approximation (EMA) models in order to evaluate the suitability of these models for such mixed composite thin films. The discrepancies between experimentally determined and EMA-computed values are explained through microstructural and morphological evolutions observed in these mixed composite films. Finally, the dependence of the laser damage threshold as a function of silica content has been investigated, and the improved laser-induced damage threshold for films having more than 80% silica content has been explained through the defect-assisted multiphoton ionization process.

Modulation of the Deswelling Temperature of Thermoresponsive Microgel Films

We demonstrate fine-tuning of the deswelling temperatures of thermoresponsive microgels within a biologically relevant range (30-40 °C). This was achieved by copolymerizing N-isopropylacrylamide and N-isopropylmethacrylamide (NIPAm and NIPMAm, respectively) in varying ratios; the parent homopolymers are well-known thermoresponsive polymers. Polyelectrolyte layer-by-layer (LbL) assemblies of these microgels retain the temperature response properties as demonstrated by temperature-dependent light scattering. Furthermore, films composed of more than one type of microgel building block were shown to have multiple temperature responses similar to those observed for the individual building blocks, permitting further tailoring of the temperature responsive interface. Additional experiments with mixed composition films, investigating multiple assembly processes, show that the location of the microgels within the film does not interfere with the temperature response. This suggests that microgels within the polyelectrolyte assembly behave independently of neighboring microgels with respect to their thermally induced deswelling.

Growth and Characterization of Films Containing Fullerenes and Water Soluble Porphyrins for Solar Energy Conversion Applications

Thin films consisting of two fulleropyrrolidine derivatives 1 or 2 and a water-soluble porphyrin, TPPS4, were prepared by the Langmuir-Schäfer (LS, horizontal lifting) method. In particular, a solution of the fulleropyrrolidine in chloroform and dimethyl sulfoxide was spread on the water surface, while the porphyrin (bearing peripheral anionic sulfonic groups) was dissolved into the aqueous subphase. To the best of our knowledge, such a versatile method for film fabrication of fullerene/porphyrin mixed composite films has never been used by other researchers. Evidence of the effective interactions between the two components at the air-water interface was obtained from the analysis of the floating layers by means of surface pressure vs area per molecule Langmuir curves, Brewster angle microscopy, and UV-visible reflection spectroscopy. The characterization of the LS films by UV-visible spectroscopy reveals that in each case the two constituents behave as strongly interacting pi systems. The use of polarized light suggests the existence of a preferential direction of the TPPS4 macrocyclic rings with an edge-on arrangement with respect to the substrate surface, regardless which fulleropyrrolidine derivative is in the composite film. Atomic force microscopy investigations give evidence of morphologically flat layers even for LS transfer at low surface pressures. Photoaction spectra were recorded from films deposited by only one horizontal lifting onto indium-tin-oxide (ITO) electrodes, and the observed photocurrent increased notably with increasing transfer surface pressure for both 1/TPPS4 and 2/TPPS4 composite films. IPCE values are larger for 2/TPPS4 systems in comparison with 1/TPPS4 composite layers. Finally, a nonconventional approach to photoinduced phenomena is proposed by differential spectroscopy in the FT-IR attenuated total reflectance (ATR) mode.

ROOM TEMPERATURE FERROELECTRIC PROPERTIES OF SrBi2Ta2O9- AND (Bi, La)4Ti3O12-INCORPORATED BiFeO3 THIN FILMS

ABSTRACT SrBi2Ta2O9 (5%)- and (Bi, La)4Ti3O12 (5%)-incorporated BiFeO3 thin films were fabricated by chemical solution deposition on Pt/Ti/SiO2/Si (100) structures. The mixed composition films showed lower leakage current density at high electric fields than pure BFO films. Because of the low leakage current density in the mixed films, polarization hysteresis loops with the remanent polarization of 80 μC/cm2 and 40 μC/cm2 were obtained at 1 kHz frequency in SBT- and BLT-incorporated films, respectively.

Langmuir–Shäfer Transfer of Fullerenes and Porphyrins: Formation, Deposition, and Application of Versatile Films

Thin films consisting of a fulleropyrrolidine derivative 1 and a novel water-soluble porphyrin 2 were prepared by the Langmuir-Shäfer (LS, horizontal lifting) method. In particular, a solution of 1 in chloroform and dimethyl sulfoxide was spread on the water surface, while porphyrin 2 (bearing peripheral anionic groups) was dissolved into the aqueous subphase. To the best of our knowledge, such a versatile method of film fabrication for fullerene/porphyrin mixed composite films has never been used before. Evidence of the effective interactions between the two moieties at the air-water interface was obtained from the analysis of the floating layers by means of surface pressure versus area per molecule Langmuir curves, Brewster angle microscopy and UV-visible reflection spectroscopy. The characterisation of the LS films by UV-visible spectroscopy reveals that the two constituents behave as discrete and weakly interacting pi systems. The use of polarised light suggests the existence of a preferential direction of the macrocyclic rings with an edge-on arrangement with respect to the substrate surface. Finally, photoaction spectra were recorded from films deposited by only one horizontal lifting onto indium-tin-oxide (ITO) electrodes and the observed photocurrent increased notably with increasing transfer surface pressure.

Scanning Tunneling Microscopy Study of the Structure and Orbital-Mediated Tunneling Spectra of Cobalt(II) Phthalocyanine and Cobalt(II) Tetraphenylporphyrin on Au(111): Mixed Composition Films

Binary thin films of cobalt(II) phthalocyanine (CoPc) and cobalt(II) tetraphenylporphyrin (CoTPP) were prepared at submonolayer coverage on Au(111)/mica substrates byvapor deposition. All sample preparation and analysis were done under an ultrahigh vacuum. Scanning tunneling microscopy (STM) constant-current images of CoPc/CoTPP mixtures showed two close-packed surface structures, with different compositional percentages and some disorder. CoPc was also observed exclusively in one-dimensional chains and as single, isolated molecules below 220 K. Occupied and unoccupied orbital energy levels were identified by STM and tunnel-diode-based orbital-mediated tunneling spectroscopy. Occupied energy levels were also confirmed by ultraviolet photoelectron spectroscopy. The transient oxidation of the Co d(z2) orbital is identified in STM dI/dV(V) curves just negative of the 0 V sample bias for both molecules. Nearly identical constant-current contours are observed over the central Co2+ ions of CoTPP and CoPc, indicating that the attenuation of the d(z)2 orbital-mediated tunneling current induced by the structure of TPP relative to Pc is at most a factor of about 10. The orbital-mediated tunneling spectra of CoTPP and CoPc are distinctly different and allow these structurally similar species to be differentially identified.

Mixed Composition Films of Spans and Tween 80 at the Air−Water Interface

Sorbitan fatty acid esters (Spans) and the corresponding polyoxyethylene (POE) adducts (Tweens) have a wide range of applications in the pharmaceutical and other industries. In some formulations, mixtures of Span surfactants and Tween surfactants are required, but some of the physical properties of these mixtures are not well defined. To assess the mixing behavior of Spans and Tweens, selected mixtures of these surfactants were studied using Langmuir compression isotherms. Our data indicate that mixtures of Tween 80 (polyoxyethylene sorbitan monoleate) with Span 80 (sorbitan monooleate), Span 83 (sorbitan sesquioleate), or Span 85 (sorbitan trioleate) form expanded mixed monolayer films at the planar air−water interface. Mixing of Tween 80 with these three Spans exhibits significant repulsive nonideality except at low proportions of Tween 80 in Span 85. A model to explain the observed phase behavior is proposed.

Scanning tunneling microscopy studies of self-assembled monolayers of alkanethiols on gold

Scanning tunneling microscopy was used to probe the molecular scale structures of pure and mixed composition self-assembled monolayers of CH3(CH2)15SH and CH3O2C(CH2)15SH on gold. A predominant defect structure is found for both films, and is assigned as a void defect in the thiolate overlayer. The mixed composition films are observed to phase segregate into domains of the pure component thiolates. Time-lapse series of scanning tunneling microscope images reveal kink-driven motion at step edges and exchange of thiolate molecules on terraces.

Crystallization and diffusion in composite TiO2-SiO2 thin films

The crystallization behavior of evaporated TiO2-SiO2 mixed composition films and its dependence on composition, temperature, time and type of mixing (codeposited or alternating layers) were studied. All codeposited films annealed between 600 and 900 °C with 15%–90% molar TiO2 exhibited crystallization in the anatase phase. Crystallite size increased with Ti content of the film and with temperature. TiO2 in alternating layered films, which had layer thicknesses in the 65–1000 Å range, crystallized in the anatase phase in the 400–600 °C range, with thin layered films requiring higher temperatures for crystallization. For temperatures of 900–1100 °C, codeposited films were transformed into rutile films, whereas alternating layered films remained as anatase. Diffusivity of Ti in the mixed composition film was calculated to be 3×10−14 and 3×10−13 cm2/s from the study of precipitation kinetics at 950 and 1050 °C, respectively. Morphology of intermediate composition analog films (25%–65% atomic TiO2) remained virtually unaltered from its as-deposited state after annealing and crystallization.

Intrinsic stress and structural properties of mixed composition thin films

Intrinsic stress and microstructure of mixed composition films were investigated for several binary systems of IR optical materials. These properties were measured for the entire range of compositions and for mixing obtained by codeposition and by layering of alternate pure components. The variation of stress with composition was observed to be significantly different, depending on the method of mixing. Microstructural analysis revealed a corresponding difference in the grain structure of the films. Low compressive stress films were obtained by coevaporation of high tensile stress materials. These can be used to produce thick and mechanically stable gradient-index optical coatings.

Intrinsic Stress and Structure in Mixed Composition thin films

ABSTRACTIntrinsic stress and microstructure of mixed composition films were investigated for several material systems useful in the IR wavelength range. Stress in coevaporated mixed films (analog) deviated from a linear dependence on composition, while in discrete alternating films (digital) it varied linearly with composition. For MgF2-Ge and CeF3-Ge, analog film stress was small and compressive over a large range of intermediate compositions despite the fact that the constituent materials had high tensile stress. The stress behavior correlated with changes in the microstructure. Low stress compositions were observed to correspond to enhanced grain size in the mixed film. These results have significant implications for the design of gradient-index thin-film structures.

Dielectric Films for Ge Planar Devices

A series of passivating and masking films was developed and evaluated for use in a Ge planar transistor technology. In the search for satisfactory films, silicon dioxide, aluminum oxide, silicon nitride and multilayer combinations of these films, as well as some doped and mixed-composition films, were studied. The films, formed by pyrolytic deposition or by sputtering, were evaluated and compared with respect to the following properties: etch rate; dopant masking; mechanical stress; oxygen, hydrogen and water permeability; stability with respect to elevated temperature electrical-bias stressing; and Ge-insulator interface electrical condition. The most important results of our experiments are the following: Silicon nitride appears to be the only satisfactory mask for Ga diffusions, although silicon dioxide is adequate for As, P and Sb diffusion masking. The dielectric properties of pyrolytically deposited SiO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> can be made to approach those of thermal SiO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> by high temperature densification. The surface electrical properties of the Ge-SiO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> interface are more stable to thermal annealing than is any other Ge-insulator combination. However, aluminum oxide has been shown to be much less permeable to oxygen, hydrogen and water vapor than is SiO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> . Since the Ge-insulator interface electrical properties are sensitive to these ambient gases, a satisfactory passivating film structure for Ge consists of an underlayer of SiO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> and an overlayer of Al <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> , silicon nitride or other relatively impermeable film.