Aggregation quenching in thin films of MEH-PPV studied by near-field scanning optical microscopy and spectroscopy

Aggregates in thin films of conjugated polymers form excimer states and significantly reduce the photo- and electroluminescence efficiency in devices produced from these materials. We have studied the aggregate formation in thin films of poly(2-methoxy,5-(2′-ethyl-hexyloxy)-p-phenylene-vinylene) (MEH-PPV) by near-field scanning optical microscopy and spectroscopy. Local photoluminescence spectroscopy and photo-bleaching experiments have been used to show that thin films of MEH-PPV are homogeneously aggregated and do not form aggregated domains.     

Conductive Poly(3-methylthiophene) Thin Films

The work described herein represents an efficient method in the deposition of poly(3-methylthiophene),P3MeTh,thin films utilizing a microwave plasma system in combination with a simultaneous doping with iodine.It was envisaged that,an alternative poly(3-methylthiophene),P(3MeTh),with an electron donating methyl substituent,would reduce the degree of ring opening which reportedly occur to a certain extent during the plasma polymerization process of its parent compound polythiophene.An alkyl substituent would also increase the solubility of the materials.P(3MeTh)thin film deposition has been performed utilizing microwave-induced plasma polymerization in order to directly grow films on glass substrates.Moreover,simultaneous doping of the so-formed polymer with iodine has been carried out as opposed to the post-synthesis doping method.This is aimed to prolong electrical conducting lifetime of the materials.The synthesized films were characterized by attenuated total reflection fourier transform infrared(ATR FT-IR)spectroscopy and energy dispersive X-ray spectroscopy(EDS)to confirm the incorporation of iodine dopant into the films.Scanning electron microscopy showed uniformly deposited films.It has been observed that the electrical conductivity of the doped film is 2 orders of magnitude higher than the undoped counterpart.The doped fabricated films exhibited UV-vis spectra indicative of increased π-conjugation(536 nm).Furthermore,electrical conductivity of the in situ doped P(3MeTh)is more highly sustained over a longer period of time.     

Dielectric and Energy Storage Properties of the Heterogeneous P(VDF-HFP)/PC Composite Films

Polymer-based materials with a high discharge energy and low energy loss have attracted considerable attention for energy storage applications. A new class of polymer-based composite films composed of amorphous polycarbonate (PC) and poly(vinylidene fluoride-hexafluoropropylene) [P(VDF-HFP)] has been fabricated by simply solution blending followed by thermal treatment under vacuum. The results show that the diameter of the spherical phase for PC and the melting temperature of P(VDF-HFP) increase, and the crystallinity and crystallization temperature of P(VDF-HFP) decrease with increasing PC content. The phase transition from the polar β phase to weak polarity γ phase is induced by PC addition. Moreover, the Curie temperature of the P(VDF-HFP)/PC composite films shifts to a lower temperature. With the addition of PC, the permittivity, polarization and discharge energy of the P(VDF-HFP)/PC composite films slightly decrease. However, the energy loss is significantly reduced.     

Onset of thermal degradation in poly(p-phenylene vinylene) films deposited by chemical vapor deposition

We report on the thermal degradation characteristics of heat treated poly(p-phenylene vinylene) (PPV) films deposited using chemical vapor deposition. It was found that no decrease in the thickness of the films (110 nm) occurred after isothermal heat treatment in vacuum at 450 °C for up to 1 h, while films treated at 500 °C for 1 h decreased to 70% of their original thickness. In situ mass spectrometry during heat treatment of the film at 500 °C showed the release of significant amounts of material including toluene and xylene ion fragments. However, Fourier transform infrared spectroscopy shows no significant change in bond structure, indicating that the decrease in film thickness after heat treatment is due to release of material and densification, not crosslinking or bond breaking.     

An examination of the optical absorption spectrum of poly(p-phenylene sulphide) films when exposed to arsenic pentafluoride: comparison with poly(p-phenylene)

The oxidative doping of poly(p-phenylene sulphide) (PPS) films with arsenic pentafluoride has been followed by transmission absorption spectroscopy in the range 0.5 to 6.0 eV. A uniform dopant distribution resulted from the doping of very thin melt-formed films, allowing resolution of the bands assigned to both the polaron and bipolaron defects. In contrast to the predictions of simple theory, the polaron levels are not symmetrically displaced about the band-gap centre, and this may be attributed to the dopant-ion ‘impurity’ potentials. Published spectra for doped poly(p-phenylene) can be similarly interpreted; in particular, the polaron levels are asymmetrically displaced about mid-gap and both materials show the transition between the two polaron levels in the gap to be unexpectedly weak. Prolonged exposure of the PPS films to AsF₅ resulted in reaction towards a poly(benzothiophene) structure. The rate and extent of this reaction was much greater for crystalline, solution-formed films than for less crystalline melt-formed films. The fully-transformed polymer had a band edge at 2.5 eV and π − π^* gap of 2.9 eV. Redoping of this material gave below-gap features consistent with the formation of bipolaron and asymmetrically displaced polaron states.     

并联导热结构的金刚石/铜基复合材料的制备

为提高金刚石/铜基复合材料的导热性能,在芯材表面预先化学气相沉积(CVD)高质量金刚石膜,获得柱状金刚石棒,再将其垂直排列,填充铜粉后真空热压烧结,制备并联结构的金刚石/铜基复合材料。分别采用激光拉曼光谱(Raman)与扫描电子显微镜(SEM)对CVD金刚石膜的生长进行分析,并通过数值分析讨论复合材料的热性能。结果表明:金刚石/铜基复合材料结构致密,密度为9.51g/cm3;CVD金刚石膜构成连续的导热通道,产生并联式导热,复合材料的热导率为392.78 W/(m·K)。     

磁控溅射氧化钒薄膜的相成分及电阻-温度特性

采用磁控溅射法在p-Si(100)衬底上制备了VOx薄膜。利用X射线衍射(XRD)分析、原子力显微镜(AFM)及四探针测试方法，研究了制备工艺条件对薄膜的相成分和电阻．温度特性的影响，并测试分析了薄膜的电学热稳定性。结果表明：通过这种方法制备的氧化钒薄膜具有较高的电阻温度系数及良好的电学热稳定性，可作为微测辐射热计的热敏材料。     

Structural, morphological, luminescent and electronic properties of sprayed aluminium incorporated iron oxide thin films

Thin films of aluminium incorporated Fe₂O₃, synthesized by simple chemical spray pyrolysis on to glass substrates using aqueous solutions of analytical reagent grade ferric trichloride and aluminium nitrate as precursors. The influence of aluminium doping on to morphological properties, contact angle, X-ray photoelectron spectroscopy, photoluminescence and thermal conductivity properties have been investigated. The preparative parameters have been optimized to obtain good quality thin films which are uniform and well adherent to the substrate. The FE-SEM and AFM micrographs depict the films are compact and homogeneous (spindle-shaped hematite nanostructures) with varying grain sizes (average grain size ~ 20-60 nm). Contact angle measurement show the films are hydrophobic in nature. The chemical composition and valence states of constituent elements in Fe₂O₃ are analyzed by X-ray photoelectron spectroscopy. The excitonic strong violet emission has been observed in photoluminescence. The specific heat and thermal conductivity study shows the phonon conduction behavior is dominant in these polycrystalline films. We studied interparticle interactions like grains, grain boundary effects using complex impedance spectroscopy.     

Synthesis and optical properties of oligo- and poly(2,5-dialkoxy-1,4-para-phenylenevinylene)s

Model compounds based on 1,4-distyrylbenzene and its 2,5-dialkoxy derivatives were synthesized by the Wittig and Wittig-Horner methods. Corresponding poly(2,5-dialkoxy-1,4-para-phenylenevinylene)s (DAPPVs) were obtained from their 2,5-bis (chloromethyl)benzene monomers. The photophysical behavior of the model compounds and DAPPVs was investigated through absorbance, excitation and fluorescence spectroscopy. The absorbance and fluorescence spectra of DAPPVs are red shifted in comparison to the corresponding three- and five-ring model compounds by 4600 and 7000 cm⁻¹, respectively. We also report the fluorescence excitation and emission spectra of the model compounds deposited as solid films on quartz and as solid solutions in poly(methyl methacrylate) (PMMA). The photophysical properties of these materials are interpreted from their capacity to form excimer-like molecular dimers. The fluorescence excitation spectra of DAPPV solutions are concentration dependent. We show evidence for the existence of fluorescent aggregates in the solid state.     

Electrochemical synthesis of free-standing poly(para-phenylene) films in composite electrolytes of boron trifluoride diethyl etherate and sulfuric acid

Free-standing poly(para-phenylene) (PPP) films with tensile strength of 30-50 MPa and conductivity in the order of 10⁻³ S cm⁻¹ have been electrosynthesized by direct oxidation of benzene at a stainless steel electrode in a composite solvent of boron trifluoride diethyl etherate (BFEE) and concentrated sulfuric acid. The films were very shiny and flexible. The structure and morphology of the films were also studied by FT-infrared spectroscopy and scanning electron microscopy.     

STRUCTURE, MECHANICAL PROPERTIES AND THERMAL STABILITY OF DIAMOND-LIKE CARBON FILMS PREPARED BY ARC ION PLATING

Diamond-like Carbon (DLC) films have been prepared on Si(lO0) substrates by arc ion plating in conjunction with pulse bias voltage under He atmosphere. The deposited films have been characterized by scanning electron microscopy and atomic force microscopy. The results show that the surface of the film is smooth and dense without any cracks, and the surface roughness is low. The bonding characteristic of the films has been studied by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. It shows the sp^3 bond content of the film deposited at -200V is 26. 7%. The hardness and elastic modulus of the film determined by nanoindentation technique are 30.8 and 250.1GPa, respectively. The tribological characteristic of the films reveals that they have low friction coefficient and good wear-resistance. After deposition, the films have been annealed in the range of 350-700℃ for 1h in vacuum to investigate the thermal stability. Raman spectra indicate that the ID/IG ratio and G peak position have few detectable changes below 500℃. Further increasing the annealing temperature, the hydrogen can be released, the structure rearranges, and the phase transition of sp^3 configured carbon to sp^2 configured carbon appears.     

Investigation of annealing effects on microstructure of hybrid nanocrystal–polymer solar cells by impedance spectroscopy

We have fabricated hybrid bulk heterojunction solar cells with blends of MEH-PPV (poly[2-methoxy,5-(2-ethylhexoxy)-1,4-phenylene vinylene]) and nano-CdS as electron donor and electron acceptor, respectively. The effect of thermal annealing was investigated with impedance spectroscopy (IS) and capacitance–voltage (C–V) characteristic measurements on devices. The IS and C–V characteristic manifested very different before and after thermal annealing. We performed equivalent circuit to explain the effect of thermal annealing. The thermal treatment of nanocrystal–polymer films is seen to aid in the formation of a continuous network for electron transport between nanorods, and hence improves devices performance. The method based on the IS is available to probe the microstructure of hybrid bulk heterojunction solar cells before and after thermal annealing, and therefore detect the mechanism for the annealing improvements.     

LaNiO3/石墨烯复合薄膜的制备及光催化性能

采用改进的Hummer法制备出薄层石墨烯材料,采用溶胶-凝胶法结合旋转涂膜技术以玻璃基底制备出石墨烯基LaNiO3复合薄膜。采用X射线衍射(XRD)仪,透射电镜(TEM),扫描电镜(SEM),拉曼光谱,X射线衍射光电子能谱(XPS)、紫外-可见(UV-Vis)吸收光谱和荧光光谱(PL)等测试手段对复合薄膜进行表征。结果表明,晶粒粒径约为20 nm的LaNiO3较均匀地生长在了石墨烯片上。光催化性能研究表明,LaNiO3/石墨烯复合薄膜的光催化活性显著优于LaNiO3薄膜,当石墨烯的含量约为4%时,复合薄膜4 h内对酸性红的降解效率比LaNiO3薄膜提高了1倍。     

EFFECT OF ANNEALING TREATMENT ON THE STRUCTURE OF CdS FILMS

The effect of annealing treatment on the structure of CdS films was investigated. The cadmium sulfide thin films were prepared by chemical bath deposition, and were annealed at nitrogen atmosphere at different temperatures. The films were characterized by SEM and XPS (X-ray photoelectron spectroscopy). X-ray photoelectron spectroscopy was used to examine the chemical states on the CdS films surface. It was found that thermal annealing could produce large grains of CdS thin films, remove the air contamination and reduce the oxygen content on the CdS films surface. Therefore, the CdS films changed more uniform and smoother surface after thermal annealing.     

沉积温度和退火处理对BCN薄膜结构的影响

采用脉冲激光沉积技术,在Si(100)基片上制备了BCN薄膜,研究了沉积温度和退火处理对BCN薄膜组分和结构的影响。利用傅里叶变换红外光谱(FTIR)和X射线光电子能谱(XPS)对制备的BCN薄膜进行了表征。结果表明:沉积温度升高时,BCN薄膜的组分无明显改变。所制备的BCN薄膜包含B—N,C—B和C—N化学键,是由杂化的B—C—N键构成的化合物。真空退火温度为700℃时,BCN薄膜结构稳定;大气退火温度达到600℃时,BCN薄膜表面发生氧化分解,同时有C≡N键形成,表明C≡N键具有较好的高温热稳定性。     

Corrosion behaviour in physiological fluids of surface films formed on titanium alloys

The properties of the surface films formed on Ti and its Ti–5Al–4V and Ti–6Al–3.5Fe alloys by chemical (immersion in 10 M NaOH) and thermal treatment (500 °C) were studied in this paper. Their corrosion behaviour in Ringer solution was investigated. After treatments, the sample microhardness was measured. Also, the sample surfaces were observed by an optical microscope before and after immersion in Ringer solutions. The electrochemical behaviour of the surface films obtained by the chemical and thermal treatments was studied using the methods of the electrochemical impedance spectroscopy (EIS) and monitoring of open circuit potentials at different immersion periods (up to 60 days) in Ringer solution. Microhardness increased with the increasing loads for the studied materials, showing the existence of a compact protective layer. Microscopic observations exhibited more compact coatings after 60 days of exposure in Ringer solution, due to the formation of an apatite layer. EIS spectra revealed surface films with two layers: an inner, barrier protective layer and an outer, porous apatite layer. The impedance increases in time denoting that the films have grown by apatite nucleation. Also, EIS spectra showed that the complex treatment by chemical + heat method is the most efficiently. For the samples treated by the two processes (chemical + heat) the values of the open circuit potentials are nobler than of the chemical treated samples, denoting better protective, bioactive films.     

Thermal stability studies of atmospheric plasma deposited siloxane films deposited on Vycor™ glass

The thermal stability of siloxane films deposited on Vycor glass and silicon substrates using atmospheric pressure plasma was studied. Siloxane films were deposited from a liquid tetraethyl orthosilicate (TEOS) precursor which was nebulised into a helium/oxygen plasma. The thickness of the siloxane films was in the range 1-12 nm as measured using both ellipsometry and X-ray reflectometry. Film composition and chemical properties were examined using Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy analysis. The thermal stability of the films was evaluated under argon atmosphere at temperatures up to 700 °C. The films were found to exhibit good adhesion without cracks or delamination after the thermal treatment. A decrease in coating thickness and a reduction in surface roughness was however observed indicating, a change in coating chemistry. This was confirmed by the reduction in carbon concentration observed by XPS. Particulates generated by excess gas phase reaction of TEOS were observed on the surface of the coating. A larger reduction in particulates size was observed after thermal treatment compared with the reduction in thickness of the bulk coating. This indicates that the particulates may have a different composition to that of the coating.     

Influence of thermal and plasma treatments on the electrical properties of poly(para-phenylene vinylene)-based diodes

We have studied the influence of plasma and thermal treatments on the electrical properties of poly(para-phenylene vinylene) (PPV) thin films in metal-polymer-metal structures. Thermal heating of the samples modifies the diode characteristic by reducing the voltage threshold and by favouring the formation of the rectifying contact at the bottom side. Plasma treatments with oxygen or nitrogen drastically change the conductivity of the polymer with disappearance of the rectifying behaviour and increase the current by many orders of magnitude. Attempts to explain the modifications of the electrical property of the treated samples are given, using previous results from X-ray photoelectron spectroscopy (XPS) which highlight the role of the interfacial layer formed by chemical reaction between the polymer and metallic electrode.     

Cyclic oxidation resistance of Ni–Al alloy coatings deposited on steel by a cathodic arc plasma process

The cyclic oxidation resistance of nickel-aluminide coatings deposited on steel using a cathodic arc plasma (CAP) process has been investigated. Our results show that nickel-aluminide films can be successfully deposited on carbon steel and stainless steel substrates by this process; NiAl₃ is the major phase in the deposited films. The thermal cycling behaviour suggests that such coatings can resist oxidation through physical blocking of oxygen, either by the coating itself or by the aluminium oxide scale subsequently formed in-service. Aluminium diffusion inwards to the substrate may also be beneficial to the thermal oxidation resistance. The coating protects stainless steel substrate materials at 500°C by transforming the NiAl₃ phase into NiAl, producing aluminium oxide on the open substrate surface. At 800°C, oxide flaking is suppressed by the trace amounts of nickel or aluminium which have partially diffused into the substrate.     

Photo-oxidation of polymers used in electroluminescent devices

Two electroluminescent polymers, poly(2,5-bis(cholestanoxy)-1,4-phenylene vinylene) (BCHA-PPV) and poly(3-octylthiophene) (P3OT), have been characterized on Au and Al substrates in terms of photo-oxidative stability after exposure to UV and visible irradiation from a Hg lamp. Thin polymer films were analyzed in situ using IR reflection absorption spectroscopy (IRRAS), and ex situ by X-ray photoelectron spectroscopy (XPS) and attenuated total reflection (ATR) IR spectroscopy. The oxidation of BCHA-PPV is quite extensive, resulting in the formation of an ester and volatile aldehyde species. Singlet oxygen, formed by energy transfer from the BCHA-PPV triplet state, appears responsible for at least one photo-oxidation pathway. BCHA-PPV triplet exciton states will also form during electroluminescent device operation (the singlet exciton state is responsible for luminescence in the film) resulting in the formation of reactive singlet oxygen. Thus, it appears that BCHA-PPV will slowly self-degrade in the presence of oxygen when used as the active layer in electroluminescent devices. P3OT shows no detectable oxidation during similar exposure.