Uniformity investigation of MOCVD‐grown LED layers

Doped or undoped gallium nitride compounds (GaN/InGaN), usually grown by metal‐organic chemical vapor deposition (MOCVD) method, are at the heart of blue and green light emitting diodes (LEDs). Growth uniformities, such as the excited wavelength, luminous intensity and film thickness, critically influence their application in LED devices. In this paper, growth of GaN compounds in a MOCVD reactor, capable of a one‐time production of 36 × 2” wafers of nitrides, has been investigated. To examine growth uniformity across the wafer and from wafer to wafer, the reactor is divided into Zone A, Zone B and Zone C according to distance to the center of the graphite susceptor. Comparative analysis of each zone offers a straightforward view of the mean excitation wavelength, luminous intensity, film thickness and their standard deviations. Conformity of the growth uniformity in each zone is further checked comprehensively through averaging across‐wafer and wafer‐to‐wafer variables and their standard deviations. Zone B is found to retain excellent wavelength uniformity, since it is located at the middle of the susceptor with weaker effects of the susceptor edge and of the inlet gas flow. Zone A, at the center of the reactor, has the best mean intensity and thickness uniformities due to a well control of the infrared temperature measurement during the growth. And Zone C is worst in all uniformities and should be the main focus when optimizing the reactor. The above experimental analysis reveals the principles common to the MOCVD technique, and provides a basic for further optimization of the process window to improve the cycles with considerable reduction of the costs.     

Vibrational spectroscopy,electrical characterization,nonlinear optical properties and DFT calculation of (NEt4)(H2AsO4)(H3AsO4)2

An organic-inorganic compound of tetraethylammonium dihydrogenarsenate bis(arsenic acid) salts of formula (NEt₄)(H₂AsO₄)(H₃AsO₄)₂, a potential new nonlinear optical material, was prepared by a slow evaporation technique and characterized by IR and Raman spectroscopy accomplished with DFT calculation and electrical-dielectrical measurements. The structure has been solved using direct method and refined by least-squares analysis. In this case, the structure consists of infinite parallel two-dimensional planes built of mutually H₂AsO₄^?, H₃AsO₄ tetrahedra connected by strong O–H?O hydrogen bonding giving birth to trimers. The geometry, first hyperpolarizability and harmonic vibrational wavenumbers were calculated by means of density functional theory (DFT) with the B3LYP/6-31G(d) level of theory. Good consistency was found between the calculated and the experimental structure, IR, and Raman results. The first hyperpolarizability β_tot of the title compound is about 1.75 times more than that of the reference crystal KDP. The complex impedance has been investigated in relation to the temperature and frequency ranges 297 and 373 K and 1 to 100 KHz, respectively. The conductivity temperature variation shows a typical Arrhenius-type behavior with a linear dependence on logarithm of conductivity. Transport properties in this material appear to be due to proton hopping mechanism.     

Crystal structure,optical properties,and theory study of a 1-D bromoplumbate stabilized by in situ generated N-alkylated DABCO cation

We report the exploration of the stabilization effect of the in situ generated N-alkylated DABCO (DABCO = 1,4-diazabicyclo[2.2.2]octane) cation in the family of bromoplumbates and a 1-D bromoplumbate, (Et₂DABCO)_2n(Pb₃Br₁₀)_n (1), has been prepared by solvothermal conditions. Optical diffuse reflectance determination shows the band gap of 1 is 3.69 eV, which manifests that 1 is a wide band gap semiconductor. Compared with the band gap of bulk PbBr₂ (3.84 eV), 1 exhibits 0.15 eV red shift of absorption edge. While for the reported iodo analogs of this compound, (MPDA)_2n(Pb₃I₁₀)_n and (Et₂DABCO)_2n(Pb₃I₁₀)_n, they exhibit 0.53 and 0.47 eV blue shift of the energy gaps compared with the measured value of 2.30 eV for bulk PbI₂, respectively. The photoluminescent study of 1 shows that it exhibits a broad emission band centered at 697 nm upon photoexcitation by 345 nm (amount to 3.59 eV). The calculated density of states manifests the theoretical value of the band gap of 1 is 3.422 eV and the origination of photoluminescence can be ascribed to the transition of bonding electrons of Br^– anion to the empty orbits of Pb(II) ion.     

Multiple modulator-induced syntheses,crystal structures,and luminescent properties on hippuric acid and bovine serum albumin of Ln(III) complexes with 2,2′-oxybis(benzoic acid)

Six new coordination complexes, Ln₂(2,2′-oba)₂(phen)₂(ox)(H₂O)₂ (Ln = Eu 1, Tb 2), Ln₄(2,2′-oba)₆(phen)₂ (Ln = Eu 3, Tb 4), Eu₄(2,2′-oba)₆(phen)₂(H₂O) (5), and K[Eu(2,2′-oba)₂(phen)₂] (6) [2,2′-H₂oba = 2,2′-oxybis(benzoic acid), phen = 1,10-phenanthroline, H₂ox = oxalic acid] were synthesized by hydrothermal reactions with the same compound molar ratios but different modulatory reagents (MRs). Complexes 1–5 have different 1-D chain structures and 6 shows a mononuclear structure. These complexes form diverse 3-D supramolecular networks through hydrogen bonds. The interaction between these complexes and hippuric acid (HA) or bovine serum albumin (BSA) was investigated by fluorescence spectral analysis. Interestingly, the hippuric acid could quench the luminescence of these complexes while the fluorescence of BSA could be quenched by these complexes. Results suggested that the complexes may be potential luminescent testing reagents for HA or BSA by significant fluorescence quenching of Ln³⁺ or BSA, respectively, through a static and dynamic quenching process.     

Supercritical Fenton Oxidation: A New Method for Total Organic Carbon Measurement

A new method for total organic carbon (TOC) measurement was established based on supercritical Fenton oxidation. The organic pollutants in wastewater were oxidized to carbon dioxide in supercritical water by Fenton reagents that was detected using a nondispersive infrared detector. The influence of temperature from 380 to 480°C, oxidant coefficient from 1 to 20, pH from 2.2 to 5.2, and Fe²⁺ concentration from 0.2 to 0.8?mg?L^?1 was characterized; the optimal conditions were at 420°C, an oxidant coefficient n?≥?5, a pH of 4.4, and Fe²⁺ concentration of 0.8?mg?L^?1. Using these parameters, the recovery of potassium hydrogen phthalate exceeded 98.2%. The introduction of Fenton oxidation based on supercritical water lowered the temperature and reduced the oxidant coefficient required for TOC determination.     

Determination of Volatile Organic Compounds by a Novel Polymer Spin-Coated Thin Film and Surface Plasmon Resonance

Here, the synthesis, characterization, and volatile organic compound (VOCs) sensing of a 1,3-dimethyl polyphenylene vinylene polymer is reported. The synthesis was performed by a Witting condensation through the reaction of 1,4-terphthaldehyde with the phosphonium chloride of meta-xylene. The material was characterized by infrared spectroscopy, elemental analysis, and thermogravimetric analyses. Thin films of the polymer were prepared by spin coating at speeds from 1000 to 5000?rpm. Ultraviolet–visible spectroscopy and surface plasmon resonance were used to characterize the spin-coated films. The thicknesses of the films were estimated by fitting the curves and were between 4.5 and 24.5?nm depending on the speed. The refractive index of the new polymer was 1.72. The polymer spin-coated films were exposed to volatile organic vapors to characterize their sensing properties by surface plasmon resonance as a function of time. The results showed that the new material responded rapidly, sensitively, and reversibly to VOCs.     

Design of donor–acceptor–donor (D–A–D) type small molecule donor materials with efficient photovoltaic parameters

Four Donor–Acceptor–Donor (D–A–D) type of donor molecules (M1‐M4) with triphenylamine (TPA) as donor moiety, thiophene as bridge, and thiazolothiazole as acceptor unit were designed and its photovoltaic parameters were equated with reference molecule “R.” DFT functional CAM‐B3LYP/6‐31G (d,p) was found best for geometry optimization and TD‐CAM‐B3LYP/6‐31G (d,p) was found suitable for excited state calculations. Among designed donor molecules, M4 manifests suitable lowest band gap of 4.73 eV, frontier molecular orbital energy levels as well as distinctive broad absorption of 455.3 nm due to the stronger electron withdrawing group. The electron‐withdrawing substituents contribute to red shifts of absorption spectra and better stabilities for designed molecules. The theoretically determined reorganization energies of designed donor molecules suggested excellent charge mobility property. The lower λ_e values in comparison with λ_h illustrated that these four donor materials would be ideal for electron transfer and M4 would be best amongst the investigated molecules with lowest λ_e of 0.0177. Furthermore, the calculated Voc of M4 is 2.04 V with respect to PC₆₀BM (phenyl‐C61‐butyric acid methyl ester). This study revealed that the designed donor materials are suitable and recommended for high performance organic solar cell devices.     

Effect of SIMS ionization probability on depth resolution for organic/inorganic interfaces

Secondary ion mass spectrometry (SIMS) relies on the fact that surface particles ejected from a solid surface are ionized under ion bombardment. By comparing the signal of molecular secondary ions desorbed from an organic film with that of the corresponding sputtered neutral precursor molecules, we investigate the variation of the molecular ionization probability when depth profiling through the film to the substrate interface. As a result, we find notable variations of the ionization probability both at the original surface and in the interface region, leading to a strong distortion of the measured SIMS depth profile. The experiments show that the effect can act in two ways, leading either to an apparent broadening or to an artificial sharpening of the observed film‐substrate transition. As a consequence, we conclude that care must be taken when assessing interface location, width, or depth resolution from a molecular SIMS depth profile.     

Interrelationship between total volatile organic compounds emissions,structure and properties of natural rubber/polycaprolactone bio-blends cross-linked with peroxides

Natural rubber/polycaprolactone (NR/PCL) bio-based blends with different organic peroxides were prepared using an internal batch mixer and subsequently cross-linked at 170 °C. Two types of commonly used organic peroxides, dicumyl peroxide and di(tert-butylperoxyisopropyl)benzene peroxide, were applied as free-radical initiator. Cross-linking efficiency of NR/PCL blends were investigated using oscillating disc rheometer measurements, followed by infrared spectroscopy, thermogravimetric analysis, differential scanning calorimetry, dynamic mechanical analysis and tensile testing. Total volatile organic compounds (TVOCs) emissions were determined using headspace analysis integrated with gas chromatography with flame ionization detector. Determined TVOCs emissions varying in range 21.6–52.1 μg/g and generally value of this parameter decreased with increasing content of PCL phase in studied blends or with application of more efficient di(tert-butylperoxyisopropyl)benzene peroxide as cross-linking agent. It was found that increasing of TVOCs parameter indicated deterioration of mechanical properties of NR/PCL blends, which corresponded with the changes in chemical structure and thermal properties of cross-linked NR/PCL. This confirms that evaluation of TVOCs parameter is interesting alternative for “conventional methods” to characterization of the studied bio-based blends.     

Effect of organic acid chain length on foam performance in a porous medium

We have systematically studied the effect of organic acid chain length on surface dilatational properties and foam flow performance in a porous medium. Surface dilatational properties were studied by oscillating drop module (ODM). ODM results in deionized water show that sufficient long chain length of organic acid is an essential requirement for high surface dilatational modulus. While, to various salinities, surfactant to acid ratio of achieving high surface dilatational modulus varies. Foam flow tests show that surface dilatational modulus has decisive effect on produced foam size, which partially determines foam flow pressure drop. Both surface dilatational modulus and surface tension determine foam flow pressure drop. Besides, surface loss modulus also contributes to pressure drop. Bulk foam tests show that addition of organic acids with proper chain length can enhance foam tolerance to oil significantly. Compared with alkane chain length, acid with longer chain has good ability in stabilizing foam. At last, foam flooding tests show that surface dilatational modulus and foam tolerance to oil play important roles in foam enhanced oil recovery.