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.     

Synthesis,Structure, and Properties of Nonlinear Optical Crystal Na2SiF6

Nonlinear optical crystals of fluosilicate Na₂SiF₆ are synthesized via hydrothermal method and its structure is determined by single‐crystal X‐ray diffraction (XRD). The space group of Na₂SiF₆ is P321 with cell parameters a = 8.8715(3) Å, c = 5.0484(5) Å, Z = 3, V = 344.09(4) ų. The properties of the crystal are measured by powder XRD, infrared (IR) spectroscopy, ultraviolet/visible (UV/Vis) near‐infrared (NIR) diffuse reflectance spectroscopy, thermogravimetric (TG), and differential scanning calorimetry (DSC) analysis. The bandgap calculated using CASTEP is 7.41 eV, indicating that the cut‐off edge of the Na₂SiF₆ crystal can be down to deep‐UV energy region. The first‐principles studies were performed to elucidate the structure/property relationship of Na₂SiF₆.     

A Practical High-Energy Cathode for Sodium-Ion Batteries Based on Uniform P2-Na0.7CoO2 Microspheres

Layered metal oxides have attracted increasing attention as cathode materials for sodium-ion batteries (SIBs). However, the application of such cathode materials is still hindered by their poor rate capability and cycling stability. Here, a facile self-templated strategy is developed to synthesize uniform P2-Na_0.7CoO₂ microspheres. Due to the unique microsphere structure, the contact area of the active material with electrolyte is minimized. As expected, the P2-Na_0.7CoO₂ microspheres exhibit enhanced electrochemical performance for sodium storage in terms of high reversible capacity (125 mAh g⁻¹ at 5 mA g⁻¹), superior rate capability and long cycle life (86 % capacity retention over 300 cycles). Importantly, the synthesis method can be easily extended to synthesize other layered metal oxide (P2-Na_0.7MnO₂ and O3-NaFeO₂) microspheres.     

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.     

Carbon dots doped with heteroatoms for fluorescent bioimaging: a review

Carbon dots (CDs) possess superior fluorescent properties in that they do not blink, are biocompatible, chemically inert, have small size and well tunable photoluminescence (PL), can be easily functionalized with biomolecules, and can be multi-photon excited to give up-converted PL. This review (with 141 refs.) summarizes recent progress in the field of imaging using carbon dots doped with heteroatoms (X-CDs). Following an introduction, we discuss top-down and bottom-up strategies for synthesis and methods for surface modification. We also compare the differences in synthesis for undoped CDs and X-CDs. Specifically, CDs doped with heteroelemets nitrogen, phosphorus, sulfur, selenium, boron and silicium are treated. We then discuss method for determination of the properties (particle size, ZP), how doping affects fluorescence (spectra, quantum yields, decay times), and how dopants affect upconversion (UC, anti-Stokes luminescence). We finally review the progress made in fluorescent imaging of cells tissue, and other biomatter. This review also gives new hints on how to use synthetic methods for tuning the structure of X-CDs, how doping affects properties, and how to achieve new bioimaging applications.

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Graphical abstract Carbon dots doped with heteroatoms (X-CDs) are a kind of fluorescent nanomaterials that display bright fluorescence, high quantum yield, photostability, biocompatibility and low toxicity. Hence, they possess large potential for both in-vitro and in-vivo bioimaging.

     

The chemical bonding and spectral assignments of rhodium(III)‐catalyzed closo‐dodecaborate complexes: Ab initio study

The chemical bonding and spectral assignments of rhodium(III)‐catalyzed closo‐dodecaborate (RhCD) complex are systematically studied using the density functional theory calculations. It is found that the calculated main bond lengths of framework are in good agreement with experimental X‐ray observation, and the pronounced hybridization of B‐2p and Rh‐4d states is responsible for the structural stability, reflected by the large dissociation energy and HOMO–LUMO energy gap. The AdNDP chemical bonding analysis indicates that the RhCD complex can be stabilized by two H‐bridged 3c‐2e σ‐bonds (B‐H‐Rh triangles). Additionally, the theoretical calculations reproduce well the main experimental IR spectrum, and the characteristic peaks are properly assigned. These results will be helpful for further insight into the unique electronic structure of the species, and provide valuable references for potential applications in novel materials.     

Ion association in lithium metaborate solution: a Raman and ab initio insight

Ion association and hydration clusters in aqueous lithium borate solution are extremely important to understand some extraordinary properties of lithium borates. In the present work, polyborate distribution in aqueous LiBO₂ solution was investigated through Raman and thermodynamics equilibrium analysis. Geometry and stability of hydrated clusters LiB(OH)₄(H₂O)_n up to n = 8 were calculated at the B3LYP/aug-cc-pVDZ level. Three different types of ion association, namely, contact ion pairs (CIP), solvent-shared ion pairs (SIP) and solvent separated ion pairs (SSIP) were obtained; characteristics of all of these stable configurations were determined, and the most stable hydrated clusters were chosen. Then the mechanisms of ion aggregation and crystal nuclei formation in the LiB(OH)₄ solution were proposed. The tight four-hydrated sphere of Li⁺ makes it difficult for the dehydrated form of its first hydration sphere to from a CIP, which is the passible reason that lithium borate always has a large super-saturation degree.     

Preparations and photophysical properties of thermally activated delayed fluorescence materials based on N-phenyl-phenothiazine-S,S-dioxide

Four compounds (TM-1～4) based on N-(4-trifluoromethylphenyl)phenothiazine-S,S-dioxide and N-[3,5-bis(trifluoromethyl)phenyl]phenothiazine-S,S-dioxide are synthesized by Buchwald-Hartwig cross coupling reaction. The effect of different substituents in acceptors on the electronic, photophysical and electrochemical properties are studied by UV–Vis, fluorescence spectroscopy, cyclic voltammetry curves and theoretical calculations. The energy gaps (ΔE_ST) between singlet excited states (S₁) and triplet excited states (T₁) of these compounds decrease by addition of one para-substituted CF₃ or two meta-substituted CF₃ on N-phenyl in order to enhance the electron-withdrawing capability of the acceptors. Small ΔE_ST of these compounds range from 0.09 eV to 0.18 eV. TM-2～4 exhibit thermally activated delayed fluorescence (TADF) characteristics with short delayed fluorescence lifetimes (τ_d) within 0.69-1.16 μs. TM-3 emits blue light with an emission wavelength of 475 nm.     

Iodine-catalyzed C3-formylation of indoles using hexamethylenetetramine and air

An efficient iodine-catalyzed chemoselective 3-formylation of free (N–H) and N-substituted indoles was achieved by using hexamethylenetetramine (HMTA) in the presence of activated carbon under air atmosphere. This new method could provide 3-formylindoles in moderate to excellent yields with fairly short reaction times. Moreover, this catalytic formylation of indoles procedure can be applied to gram-scale synthesis.