Etching characteristics of GaN by plasma chemical vaporization machining

A high‐quality bulk gallium nitride (GaN) substrate, which is suitable for high‐quality homoepitaxial growth, is indispensable for realizing high‐performance GaN devices. With improvement in the quality of the bulk GaN substrate, the removal of subsurface damage induced during surface polishing has become increasingly necessary. To remove the subsurface damage from the bulk GaN substrate, a chemical finishing method that does not produce further damage is required. We applied plasma chemical vaporization machining (CVM) to remove the subsurface damage from the bulk GaN substrate. In this study, we investigated the etching characteristics of GaN by plasma CVM applying atmospheric pressure Cl₂/He plasma. The maximum removal rate in the depth direction by plasma CVM was 9100 nm/min, which is seven times greater than that of reactive ion etching (RIE). The activation energy in plasma CVM was estimated to be 2.1 kcal/mol, which is 1.75 times greater than that in RIE. It is supposed that some of the energy required for the removal reaction in RIE is supplied by ion bombardment, but plasma CVM depends on only a chemical reaction without high‐energy ion collision. This result suggests that plasma CVM is a finishing method that causes less subsurface damage than RIE. Copyright © 2008 John Wiley & Sons, Ltd.     

Charge transport properties of MDMO PPV thin films cast in different solvents

Charge transport properties in thin films of Poly(2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐1,4‐phenylenevinylene) (MDMO PPV) cast using either chloroform (CF), toluene (TOL), or chlorobenzene (CB) as solvent were investigated. Hole mobility (μ) in these thin films measured using time‐of‐flight transient photoconductivity showed an increasing trend with respect to the solvent used in the same order, that is, μ_CF (2.4 × 10^?7 cm²/Vs) < μ_TOL (6.9 × 10^?7 cm²/Vs) < μ_CB (2.3 × 10^?6 cm²/Vs). Observed variations in mobilities were attributed to different morphologies of MDMO PPV chains in thin films cast using the aforesaid solvents. Nature of the interchain interactions and aggregate formation were obtained using photoluminescence (PL), Raman spectroscopy, and AFM studies. Ratio of PL peak intensities of 0–0 and 0–1 transitions, which is a direct measure of interchain interaction, was the highest in CB and lowest in CF. Variation in the relative intensities of out‐of‐plane wagging of vinylene group (～963 cm^?1 mode) in Raman spectra suggested different extent of coiling of polymer chains in these thin films. From these observations, it was elicited that aggregate size and interchain interactions are highest in CB and least in CF. AFM‐based topographic images of thin films further supported these variations in the size of aggregates. Variation in the aggregate sizes and interchain interactions explained the corresponding variation in the mobility. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1431–1439     

Improvement in carrier mobility of poly(3,4‐ethylenedioxythiophene) nanowires synthesized in porous alumina templates

Polymeric nanowires of poly(3,4‐ethylenedioxythiophene) (PEDOT) are electrochemically synthesized using porous anodic alumina oxide (AAO) membranes as templates. Four‐point resistivity measurements on more than 100 PEDOT nanowires with different diameters (50–250 nm) reveal a statistically significant size‐dependent phenomenon in which the nanowires with a smaller diameter exhibit higher conductivity. Structural characterization with Raman spectroscopy and doping level estimation with energy‐dispersive X‐ray spectrometry and X‐ray photoelectron spectroscopy indicate that the observed conductivity enhancement can be attributed to improved carrier mobility in PEDOT nanowires having an elongated conjugation structure because of the effect of the AAO template. From the estimated doping levels (～5%) and conductivity data (～100 S/cm), it is found that the carrier mobility reach 2.0 cm²/V s for the nanowire with the smallest diameter, as compared with 4.0 × 10^?4 cm²/V s for a bulk PEDOT film. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Damage effects of 120‐keV electron radiation on AG‐80 resin

AG‐80 epoxy resin, namely tetraglycidyl diaminodiphenyl methane (TGDDM), was irradiated with electrons of 120 keV. The results show that the irradiation leads to four major damage effects on the surface layer of the AG‐80 resin, including the discharging, mass loss, degradation, and carbon enrichment. With increasing fluence to 1.05 × 10¹⁶ cm^?2, the mass loss ratio increases gradually, and then tends to level off. The mass loss behavior can be attributed to a combined effect of the formation of gaseous radiolytic products and a degraded layer, the surface ablation due to discharging, and the skin carbon enrichment. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 177–184, 2006     

Iron monosulfide (FeS) oxidation by dissolved oxygen: characteristics of the product layer

Samples of FeS were oxidized by oxygen‐bearing acidic solutions at 25 °C and different initial pH values (2.75 ≤ pH ≤ 3.45). The reacted FeS samples were investigated by scanning electron microscopy (SEM), nitrogen adsorption/desorption isotherms measurements, and FT‐Raman spectroscopy. A sulfur rich phase (layer) is inferred to be form on reacting FeS surface based on sequential extraction with acidic chromium(II) chloride solutions. The sulfur‐rich layer (SRL) formed on oxidized FeS samples is traversed by various mesopores with average pore size < 450Å. It is reasonable to assume that the mesopores facilitate the migration of iron and sulfur from FeS structure into solution across SRL. The FT‐Raman spectra of oxidized FeS samples show an intense peak at 162 cm^?1, which can be attributed to SRL developed on FeS surface. A model for SRL development on FeS during its oxidation by dissolved oxygen has been proposed from the present results and previous studies. Copyright © 2009 John Wiley & Sons, Ltd.     

Hydrophobically modified polyelectrolyte for improved oxygen barrier in nanobrick wall multilayer thin films

The influence of attaching hydrophobic side groups to a polyelectrolyte, used for deposition of a multilayer oxygen gas barrier thin film, was investigated. Polyethyleneimine (PEI) was labeled with pyrene and deposited in “quadlayers” of PEI, poly(acrylic acid), PEI, and sodium montmorillonite clay using layer‐by‐layer assembly. Thin films made of three repeating quadlayers using unmodified PEI had much lower density (1.24 g/cm³) than pyrene‐labeled PEI‐based films (1.45 g/cm³), which is believed to be the result of greater chain coiling from the increased hydrophobicity of pendant pyrene groups. This increased density in pyrene‐labeled PEI layers allowed three quadlayers to match the oxygen transmission rate of a four quadlayer film made with unmodified PEI. This discovery provides an additional tool for tailoring the barrier behavior of clay‐based multilayer thin films that could prove useful for a variety of packaging applications. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 1153–1156     

Effect of biaxial orientation on dielectric and breakdown properties of poly(ethylene terephthalate)/poly(vinylidene fluoride‐co‐tetrafluoroethylene) multilayer films

Polymer films with enhanced dielectric and breakdown properties are essential for the production of high energy density polymer film capacitors. By capitalizing on the synergistic effects of forced assembly nanolayer coextrusion and biaxial orientation, polymer multilayer films using poly(ethylene terephthalate) (PET) and a poly(vinylidene fluoride‐co‐tetrafluoroethylene) [P(VDF‐TFE)] copolymer were produced. These films exhibited breakdown fields, under a divergent field using needle/plane electrodes, as high as 1000 kV mm^?1. The energy densities of these same materials, under a uniform electric field measured using plane/plane electrodes, were as high as 16 J cm^?3. The confined morphologies of both PET and P(VDF‐TFE) were correlated to the observed breakdown properties and damage zones. On‐edge P(VDF‐TFE) crystals induced from solid‐state biaxial stretching enhanced the effective P(VDF‐TFE) layer dielectric constant and therefore increased the dielectric contrast between the PET and P(VDF‐TFE) layers. This resulted in additional charge buildup at the layer interface producing larger tree diameters and branches and ultimately increasing the breakdown and energy storage properties. In addition to energy storage and breakdown properties, the hysteresis behavior of these materials was also evaluated. By varying the morphology of the P(VDF‐TFE) layer, the low‐field dielectric loss (or ion migration behavior) could be manipulated, which in turn also changed the observed hysteresis behavior. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 882–896     

Enhanced Performance of Solution‐Processed TESPE‐ADT Thin‐Film Transistors

A solution‐processed anthradithiophene derivative, 5,11‐bis(4‐triethylsilylphenylethynyl)anthradithiophene (TESPE‐ADT), is studied for use as the semiconducting material in thin‐film transistors (TFTs). To enhance the electrical performance of the devices, two different kinds of solution processing (spin‐coating and drop‐casting) on various gate dielectrics as well as additional post‐treatment are employed on thin films of TESPE‐ADT, and p‐channel OTFT transport with hole mobilities as high as ～0.12 cm² V^?1 s^?1 are achieved. The film morphologies and formed microstructures of the semiconductor films are characterized in terms of film processing conditions and are correlated with variations in device performance.     

Depth profiling in secondary ion mass spectrometry for ultra‐thin layer with nanometer order thickness by mesa‐structure fabrication

We attempted to make an accurate depth profiling in secondary ion mass spectrometry (SIMS) including backside SIMS for ultra‐thin nanometer order layer. The depth profiles for HfO₂ layers that were 3 and 5 nm thick in a‐Si/HfO₂/Si were measured using quadrupole and magnetic sector type SIMS instruments. The depth profiling for an ultra‐thin layer with a high depth resolution strongly depends on how the crater‐edge and knock‐on effects can be properly reduced. Therefore, it is important to control the analyzing conditions, such as the primary ion energy, the beam focusing size, the incidence angle, the rastered area, and detected area to reduce these effects. The crater‐edge effect was significantly reduced by fabricating the sample into a mesa‐shaped structure using a photolithography technique. The knock‐on effect will be serious when the depth of the layer of interest from the surface is located within the depth of the ion mixing region due to the penetration of the primary ions. Finally, we were able to separately assign the origin of the distortion to the crater‐edge effect and knock‐on effect. Copyright © 2012 John Wiley & Sons, Ltd.     

Bimetallic Metal‐Organic Framework‐Derived Nanosheet‐Assembled Nanoflower Electrocatalysts for Efficient Oxygen Evolution Reaction

Non‐noble metal‐based metal–organic framework (MOF)‐derived electrocatalysts have recently attracted great interest in the oxygen evolution reaction (OER). Here we report a facile synthesis of nickel‐based bimetallic electrocatalysts derived from 2D nanosheet‐assembled nanoflower‐like MOFs. The optimized morphologies and large Brunauer–Emmett–Teller (BET) surface area endow FeNi@CNF with efficient OER performance, where the aligned nanosheets can expose abundant active sites and benefit electron transfer. The complex nanoflower morphologies together with the synergistic effects between two metals attributed to the OER activity of the Ni‐based bimetallic catalysts. The optimized FeNi@CNF afforded an overpotential of 356 mV at a current density of 10 mA cm^?2 with a Tafel slope of 62.6 mV dec^?1, and also exhibited superior durability with only slightly degradation after 24 hours of continuous operation. The results may inspire the use of complex nanosheet‐assembled nanostructures to explore highly active catalysts for various applications.     

Evaluation of piezoelectric PVDF polymers for use in space environments. II. Effects of atomic oxygen and vacuum UV exposure

The effects of atomic oxygen (AO) and vacuum UV radiation simulating low Earth orbit conditions on two commercially available piezoelectric polymer films, poly(vinylidene fluoride) (PVDF) and poly(vinylidene fluoride‐trifluoroethylene) P(VDF‐TrFE), have been studied. Surface erosion and pattern development are significant for both polymers. Erosion yields were determined as 2.8 × 10^?24 cm³/atom for PVDF and 2.5 × 10^?24 cm³/atom for P(VDF‐TrFE). The piezoelectric properties of the residual material of both polymers were largely unchanged after exposure, although a slight shift in the Curie transition of the P(VDF‐TrFE) was observed. A lightly cross‐linked network was formed in the copolymer presumably because of penetrating vacuum ultraviolet (VUV) radiation, while the homopolymer remained uncross‐linked. These differences were attributed to varying degrees of crystallinity and potentially greater absorption, and hence damage, of VUV radiation in P(VDF‐TrFE) compared with PVDF. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2503–2513, 2005     

Optimization of Electrochemical Total Creatine Kinase Biosensors Based on Three‐Au‐Electrode

For point‐of‐care examination, total CK (creatine kinase: adenosine‐5‐triphosphate‐creatine phosphotransferase, EC 2.7.3.2) biosensors were developed and optimized. The biosensors were fabricated with three‐Au‐electrode system modified with polyvinylpyridine‐osmium‐wired horseradish peroxidase (PVP‐Os‐HRP) redox polymer film. The reagents were separately immobilized on the single layer biosensor and double layer biosensor which contained lens paper layer and the surface layer of the working electrode. The mediator, the working potential, the structure of working electrode and the stabilizer agent were studied. The biosensor with double reagent layer showed good stability at room temperature (≥2 months) and the biosensor with single reagent layer had excellent response signal (a sensitivity of 11 nA L U^?1 cm^?2).     

Low‐energy excitations in stressed poly(methyl methacrylate)

Raman study of flexure stressed poly(methyl methacrylate) (PMMA) plates at the frequency range of libration mode (80 cm⁻¹), boson peak (14 cm⁻¹), and quasi‐elastic light scattering (QLS) (below 20 cm⁻¹) is presented. The reversible changes in the vicinity of the Rayleigh line were attributed to partial disturbance of the midrange order due to stress‐induced redistribution of random fluctuations of orientation. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1133–1136, 2000     

Impact of average free‐volume element size on transport in stereoisomers of polynorbornene. I. Properties at 35 °C

This article demonstrates that transport of gases through glassy polymers is significantly influenced not only by the absolute amount but also by the distribution of free volume. Two stereoisomers of polynorbornene with nearly equivalent total free volume, but markedly different average free‐volume sizes, were evaluated. The free‐volume element size was probed with positron annihilation lifetime spectroscopy, wide‐angle X‐ray scattering, gas sorption, and molecular modeling. The permeation, sorption, and diffusion of light gases were measured in each stereoisomer at 35 °C. All analytical techniques indicated that one isomer (labeled as Architecture II) had a larger average free‐volume element size but fewer elements. This isomer also had a very slightly higher bulk density (1.000 vs 0.992 g/cm³ for the other stereoisomer). Architecture II also had gas sorption and diffusion coefficients that were two to three times those of the less dense counterpart. These differences have been attributed to differences in the free‐volume element size available within the polymer matrix. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2185–2199, 2003     

Theoretical and experimental infrared spectra of hydrated and dehydrated Nafion

The time‐dependent IR spectra during dehydration of fully hydrated Nafion show the reversible disappearance of the 1061 cm^?1 and 969 cm^?1 concurrent with the emergence of peaks at ～928 cm^?1 and ～1408 cm^?1. The first pair of group modes is associated with a dissociated exchange group (sulfonate) with a local C_3V symmetry. The C_3V group modes shift with state‐of‐hydration: The 969 cm^?1 peak completely vanishes and the 1061 cm^?1 is reduced to a small shoulder at 1070 cm^?1 at end of dehydration. The C_3V group modes are replaced by the pair of group modes of an associated exchange group (sulfonic acid) with C₁ local symmetry. The density functional theory normal mode analysis confirms that the sulfonic acid/sulfonate site plays a dominant role in the C₁ and C_3V group modes, respectively. This work clarifies the importance of assigning fluoropolymers peaks as group modes rather than traditional single functional group assignments as is often the case with the ～1061 cm^?1 and ～969 cm^?1 C_3V group modes. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013 , 51, 1329–1334     

Tetrabrominated Lead Naphthalocyanine for Optical Power Limiting

The complex 2,(3)‐tetrabromo‐3,(2)‐tetra[(3,5‐di‐tert‐butyl)phenyloxy]‐naphthalocyaninato lead [Br₄(tBu₂C₆H₃O)₄NcPb, 1 ] has been prepared and its optical limiting properties for ns light pulses have been measured. Complex 1 behaves as a reverse saturable absorber within the spectral range 440–720 nm with a limiting threshold of 0.1 J cm^?2 at 532 nm. The lifetime of the absorbing triplet excited state has been evaluated as 3.8×10^?7 s and the quantum yield of triplet formation has been measured as 0.07 in toluene. The nonlinear optical transmission properties of complex 1 have also been determined in Plexiglas [naphthalocyanine content: 5.0×10^?4 M (0.1 % by weight)]. A reversible nonlinear absorption was again observed for a fluence above 0.4 J cm^?2, but through different excited‐state dynamics. This may be rationalized in terms of aggregation of the molecule in the polymer matrix.     

Preconcentration and Determination of a Phospholipid at a Surface Modified by Layer‐by‐Layer Assembly

Electrochemical oxidation of a phospholipid, phosphatidylcholine (PC), was accomplished at a 4‐aminothiophenol (ATP)‐modified gold electrode coated with a layer‐by‐layer assembly of an electrochemical catalyst (dirhodium phosphomolybdic acid), a trapping agent for PC (a cyclophane, CP, derivative, 1,4‐xylylene‐1,4‐phenylene‐diacetate), and a spacer (generation‐4 polyamidoamine dendrimer, PAMAM). The layer‐by‐layer assembly process and the trapping of PC was verified by quartz crystal microbalance measurements; Au|ATP|CP|PAMAM|CP trapped (1.5±0.4)×10^?9 mol cm^?2 of PC. The electrocatalytic oxidation of PC yielded a current that varied linearly with concentration over the range 1–50 μM; the R² value was 0.996.     

A new near‐infrared switchable electrochromic polymer and its device application

A new near‐infrared switchable electrochromic polymer containing carbazole pendant (poly‐SNSC), synthesized by electrochemical polymerization of 2,5‐bis‐dithienyl‐1H‐pyrrole (SNS) main chain, has been prepared. The electrochemical and optical properties of SNSC monomer and its polymer have been investigated. Because of having two different electro‐donor moieties; that is, carbazole and SNS, SNSC gave two separate electrochemical oxidation and also light brown color of the film in the neutral state turn into gray on oxidation. An electrochromic device, contructed in the sandwich configuration [indium tin oxide (ITO)‐coated glass/anodically coloring polymer (poly‐SNSC)//gel electrolyte//cathodically coloring polymer (PEDOT)/ITO‐coated glass] and exhibited a high coloration efficiency (1216 cm² C^–1), a very short response time (about 0.3 s), low driving voltage, and a high redox stability. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010     

Infrared spectroscopic characterization of oriented polyamide 66: Band assignment and crystallinity measurement

Having found much ambiguity in the infrared band assignments for polyamide 66 (PA66), we revisited some of these assignments before using infrared spectroscopy to assess microstructure changes resulting from multiple thermal treatments. We discovered that earlier assignments of the 1144 and 1180 cm⁻¹ bands to the amorphous (noncrystalline) phase were incorrect, whereas the bands at 924 and 1136 cm⁻¹ can be attributed unambiguously to the noncrystalline phase. We also confirmed that PA66 bands at 936 and 1200 cm⁻¹ are crystalline bands. The normalized absorbance of the 1224‐cm⁻¹ fold band increases in proportion to crystallinity, indicating that chain folding is the predominant mechanism of thermal crystallization in PA66. We demonstrated that infrared spectroscopy can be used to estimate the degree of crystallinity of PA66, and two methods were explored. One is a calibration method in which the band ratio of 1200 and 1630 cm⁻¹ is plotted against crystallinity measured by density. The other is an independent infrared method based on the assumption that PA66 satisfies a two‐phase structure model. The crystallinity determined by the independent infrared method showed good agreement with the crystallinity obtained from density measurements. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 516–524, 2000     

Regioregular poly(3‐alkanoylthiophene): Synthesis and electrochemical,photophysical, charge transport,and photovoltaic properties

Head‐to‐tail regioregular poly(3‐heptanoylthiophene) (PHOT) was synthesized by Ni‐catalyzed polycondensation of the 2,2‐dimethyl‐1,3‐propanediol‐protected Grignard monomer followed by deprotection. Cyclic voltammetric (CV) study demonstrates that the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of PHOT are 0.5 eV lower in energy than those of the head‐to‐tail poly(3‐hexylthiophene) (HT‐P3HT). Their optical band gaps are essentially the same. Incomplete photoluminescence (PL) quenching was observed in thin films of the 1:1 blend of PHOT and HT‐P3HT. PHOT displayed a glass transition at ～269 °C and decomposed at ～300 °C according to differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Wide‐angle X‐ray diffraction (WAXD) study showed that PHOT exists in a not highly ordered state in solid films especially in the π‐stacking direction. Only p‐channel activity was observed in field‐effect transistors (FETs) for PHOT. The hole mobility was on the order of 10^?4 cm² V^?1 s^?1. Photovoltaic devices with an active layer of 1:1 blend of PHOT and PC₇₁BM had a power conversion efficiency (PCE) of ～0.5%. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010