Effects of AlN Layer and Impurities on Optical Properties of GaN

IntroductionIn recent years Ga N and the related - group semiconductors have shown suitability forhigh- temperature,high- power electronic devicesand blue- ultraviolet light emitters due to theirstrong binding energies and large direct energyband gaps[1,2 ] .A considerable effortis being devot-ed to the improvement of the quality of epitaxiallayers,material characterization methods and tech-niques for device processing.New substrate mate-rials and various buffer layers have been utilized to…     

Effects of ZnO Buffer Layer Thickness on Properties of MgxZn1-xO Thin Films Deposited by MOCVD

High-quality MgxZn1-xO thin films were grown on sapphire（0001 ） substrates with a ZnO buffer layer of different thicknesses by means of metal-organic chemical vapor deposition. Diethyl zinc, bis-cyclopentadienyl-Mg and oxygen were used as the precursor materials. The crystalline quality, surface morphologies and optical properties of the Mg, Zn1-xO films were investigated by X-ray diffraction, atomic force microscopy and photoluminescence spectrometry. It was shown that the quality of the MgxZn1-xO thin films depends on the thickness of the ZnO buffer layer and an Mg, Zn1-xO thin film with a ZnO buffer layer whose thickness was 20 nm exhibited the best crystal-quality, optical properties and a flat and dense surface.     

Studies on the oxygen sensitivity and microstructure of sol–gel based organic–inorganic hybrid coatings doped with platinum porphyrin dye

Hybrid organic–inorganic nanocomposite coatings were prepared by copolymerizing tetraethylorthosilicate with ethyltriethoxysilane with an acid catalysis process. Oxygen sensor coatings were fabricated by doping the hybrid sol with platinum meso-tetra(pentfluorophenyl) porphyrin. Photophysical properties and oxygen sensitivity of the sensor coatings were studied. The microstructure of the coatings was examined using optical microscopy and scanning electron microscopy. The effect of sol–gel process conditions like precursor silane molar ratio, acid concentration and stirring time of the sol on the oxygen sensitivity and surface microstructure of the sensor coating was studied. Oxygen sensitivity and surface morphology of the coatings were dependent on the sol–gel process parameters.     

Nanometer-scale wetting of the silicon surface by its equilibrium oxide

Despite the extremely broad technical applications of the Si/SiO2 structure, the equilibrium wetting properties of silicon oxide on silicon are poorly understood. Here, we produce new results in which a solid-state buffer method is used to systematically titrate oxygen activity about the Si/SiO2 coexistence value. The equilibrium morphology at the Si(001) surface over >8 decades of PO2 about coexistence is revealed to be a uniform sub-stoichiometric SiOx film of sub-nanometer thickness, coexisting with secondary island structures which coarsen with annealing time. A new thermodynamic method using chemical potential to stabilize and control surficial oxides in nanoscale devices is suggested.     

Effect of oxygen plasma treatment on the surface properties of tin-doped indium oxide substrates for polymer LEDs

The effect of oxygen plasma treatment on the surface properties of tin-doped indium oxide (ITO) substrates and the changes in surface properties of treated ITO substrates with ageing time were investigated by X-ray photoelectron spectroscopy (XPS), contact angle and surface free energy measurements. Experimental results show that oxygen plasma treatment increases the oxygen concentration, decreases the carbon concentration, and enhances the surface free energy and polarity, and thereby improves the surface properties of ITO substrates. However, the improved ITO surface properties tended to decay and the surface free energy decreased, with ageing time. In addition, the ageing effect of treated ITO substrates on the performance of polymer light-emitting diodes (LEDs) was studied with respect to the driving voltage, electroluminescent luminance and efficiency. We observe that the ITO substrates aged for various times result in significant differences in optical and electrical characteristics which become worse as the ageing time increases. The optical and electrical performance of polymer LEDs is closely related to the surface properties of ITO substrate and the interface characteristics of ITO/polymer.     

Studies on Growth of N-Polar InN Films by Pulsed Metal-organic Vapor Phase Epitaxy

We reported the growth of N-polar InN films on N-polar GaN/sapphire substrates by pulsed metal-organic vapor phase epitaxy. The crystalline quality, surface morphology, optical and electrical properties of N-polar InN films were investigated in details by varying the breaking time and trimethylindium(TMIn) duration of pulse cycle. It has been found that when the breaking time and the TMIn duration in each cycle remain at 30 and 60 s, respectively, the N-polar InN film obtained exhibits a better crystalline quality and greater optical properties. Meanwhile, the surface morphology and electrical properties of the N-polar InN films also greatly depend on the given growth conditions.     

Effect of reactive oxygen species generated with ultraviolet lamp and plasma on polyimide surface modification

Understanding the effect of reactive oxygen species (ROS), such as singlet oxygen molecule and atomic oxygen, on polyimide (PI) film properties, such as wettability, morphology, and chemical bonding state, is essential for further development of PI‐based surfaces. We investigated the effect of different ROS generated during ultraviolet (UV) and plasma treatment in oxygen gas on surface modification of Kapton PI. Different surface modification techniques, UV and plasma treatment, are known to generate different ROS. In this work, we demonstrate the effect of different ROS on PI surface modification. From the diagnostics of ROS by means of electron spin resonance and optical emission spectroscopy, we confirmed that during UV treatment, excited singlet oxygen molecules are the main ROS, while plasma treatment mainly generated atomic oxygen. The wettability of PI surface treated by UV and plasma resulted in hydrophilic PI surfaces. XPS results show that the wettability of PI samples is mainly determined by their surface O/C ratio. However, chemical bonding states were different: while UV treatment tended to generate C=O bonds, while plasma treatment tended to generate both C―O and C=O bonds. Singlet oxygen molecules are concluded to be the main oxidant during UV treatment, and their main reaction with PI was concluded to be of the addition type, leading to an increase of C=O groups on the surface of PI film. Meanwhile, atomic oxygen species were the main oxidant during plasma treatment, reacting with the PI surface through both etching and addition reaction, resulting in a wider variety of bonds, including both C―O and C=O groups.     

Amorphous GdN buffer-layer-enhanced switching properties of Gd switchable mirrors

Thin gadolinium nitride (GdN) films deposited on a glass surface may serve as excellent substrates for uniform growth of gadolinium metal layers. We have deposited GdN films on glass substrates at room temperature by Ar/N₂ mixed gas-plasma-radio frequency sputtering method. An improvement of the optical properties and the lifetime of switchable mirrors is obtained by placing a thin GdN buffer layer between the glass substrate and the optically active, rare earth layer. The GdN buffer layer affects the structural properties of the Gd films, and enhances the switching properties of Gd switchable mirrors.     

Electrochemical polymerization of imidazolum‐ionic liquids bearing a pyrrole moiety

A series of imidazolium‐based ionic liquid monomers bearing a terminal pyrrole moiety were synthesized and electrochemically polymerized. It is found that the polymerizability of the synthesized ionic liquids is strongly dependent on the type of the counteranions. Although bromide monomer is not polymerizable, well‐defined polymeric films can be formed on various substrates in the cases of flour‐containing anions (BF₄^?, PF₆^?). The performed characterizations show that all resulting polypyrrole films are electroactive, and the imidazolium‐based ionic liquid moieties are correctly incorporated in polymer films during the electropolymerization process. This work not only provides a facile new method to immobilize ionic liquids on solid surface. Interestingly, without use of any template unique “knit” morphology and nanostructure, even hierarchical structures could also be produced by the electropolymerization of these new functionalized pyrrole monomers. We found that the properties of the pendant ionic liquid units on the surface of the formed polymer films preserved, and by simple anion exchange their surface energy and tension could be easily tuned without loss of the electrical, optical properties, and morphology of the polypyrrole films. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 4151–4161, 2008     

Influence of oxygen plasma treatment on poly(ether sulphone) films

Poly(ether sulphone) (PES) is one of the most widely used materials in the micro-electronics industry and a good candidate for the substrates of flexible optoelectronic devices. In this work, the influences of oxygen plasma treatment on the surface chemical composition, surface morphology and optical transparency of PES films were investigated by means of X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM) and UV-visible spectrophotometry. The possible relations between the optical transparency of the substrate and the surface roughness and chemical composition were also studied. The oxygen plasma treatment seriously changed the surface chemical composition and made the surface more rough. Considerable amounts of sulphate species were found on the plasma-treated surface and the surface roughness values (R_a) increased monotonically with the increase of the treatment time. The PES films treated by 5 min, 15 min, 30 min and 45 min oxygen plasma demonstrated transmission of approximately 98, 94, 68 and 46%, respectively, in the wavelength range of 400-780 nm. The oxygen plasma induced decline of optical transparency of PES films might be attributed to both the increase of surface roughness and the changes of chemical composition of the film surface.     

Optical properties of passivated silicon nanoclusters: the role of synthesis

The effect of preparation conditions on the structural and optical properties of silicon nanoparticles is investigated. Nanoscale reconstructions, unique to curved nanosurfaces, are presented for silicon nanocrystals and shown to have lower energy and larger optical gaps than bulk-derived structures. We find that high-temperature synthesis processes can produce metastable noncrystalline nanostructures with different core structures than bulk-derived crystalline clusters. The type of core structure that forms from a given synthesis process may depend on the passivation mechanism and time scale. The effect of oxygen on the optical of different types of silicon structures is calculated. In contrast to the behavior of bulklike nanostructures, for noncrystalline and reconstructed crystalline structures surface oxygen atoms do not decrease the gap. In some cases, the presence of oxygen atoms at the nanocluster surface can significantly increase the optical absorption gap, due to decreased angular distortion of the silicon bonds. The relationship between strain and the optical gap in silicon nanoclusters is discussed.     

Preparation of ZnO nanoparticles in a reverse micellar system and their photoluminescence properties

ZnO nanoparticles with spherical morphology and narrow size distribution were obtained by calcination of Zn(OH)2 nanoparticles, which were prepared in a polyethylene glycol mono-4-nonylphenyl ether (NP-5)/cyclohexane reverse micellar system and incorporated into polyurea (PUA) via an in situ polymerization of hexamethylene diisocyanate (HDI). The resulting ZnO nanoparticles demonstrated a near-UV emission and a green emission, the intensity ratio of which depended on calcination conditions. For the nanoparticles studied, the calcination atmosphere influenced remarkably the photoluminescence properties such as intensity ratio of the near-UV emission to green emission, rather than the size, morphology, and crystallinity of the ZnO nanoparticles. The green emission decreased by calcination in O2 flow but increased by calcination in N2 flow, as compared with the case calcined in air flow. This finding suggests that the green emission is enhanced with the increase of the number of oxygen vacancies of the ZnO nanoparticles and thus the photoluminescence properties of the nanoparticles were successfully controlled by the calcination condition, without changing the size and morphology.     

Morphologies and surface plasmon resonance properties of monodisperse bumpy gold nanoparticles

The influence of the morphology of gold nanoparticles on the surface plasmon resonance was investigated experimentally and theoretically. Highly monodisperse bumpy gold nanoparticles of increasing size were synthesized, and the surface plasmon resonance wavelength shifted to longer wavelengths more rapidly with increasing particle size for bumpy particles than for spherical gold nanoparticles. The detailed surface morphology of bumpy gold nanoparticles was characterized by AFM, TEM, and SEM, and the optical properties were investigated on a single particle level. The comparison of the plasmon resonant properties between bumpy and spherical gold nanoparticles was also examined with a theoretical model.     

Tuning, via counter anions, the morphology and catalytic activity of CeO2 prepared under mild conditions

Advanced synthetic methods under mild and controlled conditions for the synthesis of nanocrystals with specific shapes and exposed surfaces are very important for understanding the surface related properties and to explore their structure-property relationship for various potential applications. Here, we report the synthesis of highly uniform CeO(2) nanorods and nanoflowers in large scale using non-hydrothermal homogeneous precipitation method with urea as a precipitating agent and CTAB as a shape directing agent. Uniform microstructures of CeO(2) samples were selectively synthesized using chloride and nitrate as the counter anions. The samples were characterized by thermal analysis, X-ray diffraction, N(2) adsorption-desorption isotherms, SEM, TEM, UV-Vis-DRS, and Raman spectroscopy, and temperature programmed reduction as well as desorption methods. The results show that the physicochemical and optical properties of CeO(2) samples significantly differ with their surface microstructure and morphology. They also strongly influence the redox property, oxygen storage capacity, and surface acidity of the CeO(2) samples. The CeO(2) samples with different morphologies were tested for their soot oxidation activity. The CeO(2) sample with nanorod morphology was found to be more active due to larger CeO(2)/soot interface than the CeO(2) sample with nanoflower morphology.     

Contributions to the Characterization of Chlorinated Polyisoprene Surfaces

Summary: The present work is focused on the characterization of the surface properties and the mechanical properties of chlorinated polyisoprene films. Cross-linked polyisoprene films were treated with acidified hypochlorite solution and the influence of the chlorination time on the surface properties was determined by spectroscopic techniques including FT-IR spectroscopy and X-ray photoelectron spectroscopy (XPS). The surface morphology was investigated by scanning electron microscopy (SEM), optical microscopy and contact angle measurements. In addition, the effect of the chlorination time on the tensile strength and ageing stability of natural rubber latex gloves was investigated.     

Optical and Electrical Properties of the PLT Films with Various Compositions Deposited by Sol-Gel Process

In order to study optical and electrical properties of (Pb_1-xLa_x)TiO₃ (PLT) films with varying La concentration, the PLT films were deposited by sol-gel process. X-ray diffraction revealed that a pseudocubic phase of the PLT film became dominant with increasing La concentration due to decrease of lattice constant of c-axis. Three-dimensional atomic force microscopy images showed that the grain size and root mean square surface roughness decreased by addition of La. The optical band gap of the PLT films became wider when Pb was substituted with La. The addition of La increased the transparency of the PbTiO₃ film and shifted the threshold for initiation of absorption to shorter wavelength. Hysteresis loops of the PLT films showed that remanent polarization and coercive field decreased with increasing La concentration. In addition, we modified the surface of the PLT film with La concentration of 5% using a keV oxygen ion beam at different doses. The optical band gap of the PLT film was changed by the oxygen ion beam irradiation although the XRD patterns and the transmittance values were not significantly changed. In measuring AFM images of the surface of modified PLT film, significant changes of the grain shape and size were not found. Moreover, polarization and dielectric constant were not changed after oxygen ion irradiation. These results suggested that addition of La could affect the optical and electrical properties of the PbTiO₃ and PLT films and that surface modification by oxygen ion beam modification with 1 keV energy can change the surface property but not bulk property.     

Effect of cold plasma process on the surface wettability of NBR and the kerosene resistance of NBR/PTFE composites

In this study, first the acrylonitrile‐butadiene rubber (NBR5080) was modified by argon (Ar), air, and oxygen plasma at low temperature, and the effect of plasma process (power, time, and pressure) on the surface properties of NBR5080, the interfacial properties, physical properties, and the mechanical properties of NBR5080/polytetrafluoroethylene (PTFE) composites were investigated. The state contact angle and the surface free energy were applied to characterize the surface wettability of NBR5080. The scanning electron microscope and the atomic force microscope were used to observe the surface morphology of the NBR5080. The chemical changes on the NBR5080 surface were verified by X‐ray photoelectron spectroscopy. The average water contact angle the NBR5080 declined obviously when NBR5080 was treated by Ar (100 W/600 s/30 Pa). The active oxygen groups were introduced onto the surface of NBR5080 by cold plasma treatment and more active group containing oxygen were observed on the samples treated by Ar plasma. The peel strength between the NBR5080 and the PTFE was increased obviously, which increased from 0 to 44.2 N?m^?1 for Ar plasma treatment. The mass and the dimension of NBR5080 increase sharply after immersing in kerosene, whereas the NBR5080/PTFE composites changed a little. The mechanical properties of NBR5080 and NBR5080/PTFE composites decreased as the immersion time in kerosene increased, but the decreased degree of NBR5080 is higher than NBR5080/PTFE composites.     

Optical properties and orientation in polyethylene blown films

We report structural factors affecting the optical properties of blown polyethylene films. Two types of blown polyethylene films of similar degrees of crystallinity were made from (1) single‐site‐catalyst high‐density polyethylene (HDPE; STAR α) and (2) Ziegler–Natta‐catalyst HDPE (ZN) resins. The STAR α film exhibited high clarity and gloss, whereas the ZN film was turbid. Small‐angle X‐ray scattering (SAXS), small‐angle light scattering (SALS), and optical microscopy gave quantitative and qualitative information regarding structure and orientation in the films. A new approach is described for determining the three‐dimensional lamellar normal orientation from SAXS. Both the clear STAR α and turbid ZN films had similar lamellar crystalline structures and long periods but displayed different degrees of orientation. It is demonstrated that optical haze is related to surface features that seem to be linked to the bulk morphology. The relationship between haze and structural orientation is described. The lamellar orientation is linked to rodlike structures seen in optical microscopy and SALS through a stacked lamellar or cylindrite morphology on a nanometer scale and through a fiberlike morphology on a micrometer scale. The micrometer‐scale, rodlike structures seem directly related to surface roughness in a comparison of index‐matched immersion and surface micrographs. The higher haze and lower gloss of the ZN film was caused by extensive surface roughness not observed in the STAR α film. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2923–2936, 2001     

Formation of Oxygen Vacancies on the {010} Facets of BiOCl and Visible Light Activity for Degradation of Ciprofloxacin

BiOCl nanosheets with oxygen vacancies on the exposed {010} facets were assistant-synthesized by triethanolamine(TEOA) via hydrothermal method. We explored the surface properties, crystal structure, morphology and optical absorption ability of the prepared samples via various characterization technologies. The results indicate that the morphologies and microstructures of the obtained samples depend on the amount of TEOA in the synthesis. The addition of TEOA induces the production of oxygen vacancy on the surface of the samples. Therefore, the synthesized samples with TEOA-assistance hold higher photoactivity for the degradation of colorless antibiotic agent ciprofloxacin(CIP) under visible light(λ ≥ 420 nm). The obtained sample upon the addition of 20 mL of TEOA exhibits the highest photocatalytic performance, which is nearly 14 times as high as that of the sample prepared without TEOA and twice as high as that of the prepared samples with NaOH or NH₃·H₂O. The possible degradation mechanism was discussed on the basis of the experiment results.     

Effect of protons on the optical properties of oxide nanostructures

Site-specific functionalization of oxide nanostructures gives rise to novel optical and chemical surface properties. In addition, it can provide deeper insights into the electronic surface structure of the associated materials. We applied chemisorption of molecular hydrogen, induced by ultraviolet (UV) light, followed by vacuum annealing to MgO nanocubes to selectively decorate three-coordinated oxygen ions (oxygen corner sites, for simplicity) with protons. Fully dehydroxylated nanocubes exhibit 3.2 +/- 0.1 eV photoluminescence induced by 4.6 eV light, where both emission and absorption are associated with three-coordinated oxygen sites. We find that partially hydroxylated nanocubes show an additional photoluminescence feature at 2.9 +/- 0.1 eV. Interestingly, the excitation spectra of the 2.9 and 3.2 eV emission bands, associated with protonated and nonprotonated oxygen corner sites, respectively, nearly coincide and show well-pronounced maxima at 4.6 eV in spite of a significant difference in their local atomic and electronic structures. These observations are explained with the help of ab initio calculations, which reveal that (i) the absorption band at 4.6 eV involves four-coordinated O and Mg ions in the immediate vicinity of the corner sites and (ii) protonation of the three-coordinated oxygen ions eliminates the optical transitions associated with them and strongly red-shifts other optical transitions associated with neighboring atoms. These results demonstrate that the optical absorption bands assigned to topological surface defects are not simply determined by the ions of lowest coordination number but involve contributions due to the neighboring atoms of higher coordination. Thus, we suggest that the absorption band at 4.6 eV should not be regarded as merely a signature of the three-coordinated O2- ions but ought to be assigned to corners as multiatomic topological features. Our results also suggest that optical absorption signatures of protonated and nonprotonated sites of oxide surfaces can be remarkably similar.