The influence of edge-plane defects and oxygen-containing surface groups on the voltammetry of acid-treated, annealed and “super-annealed” multiwalled carbon nanotubes

The role of edge-plane-like defects at the open ends of multiwalled carbon nanotubes (MWCNTs) and at hole defects in the tube walls is explored using cyclic voltammetry with two charged redox probes, namely potassium ferrocyanide and hexaamineruthenium(III) chloride in unbuffered aqueous solutions, and one neutral redox probe, norepinephrine, in pH 5.7 buffer. Further, the presence of oxygen-containing functional groups (such as phenol, quinonyl and carboxyl groups), which decorate the edge-plane defect sites on the voltammetric response of the MWCNTs, is also explored. To this end, three different pre-treatments were performed on the pristine MWCNTs made using the arc-discharge method (arc-MWCNTs). These were (a) arc-MWCNTs were subjected to acid oxidation to form acid-MWCNTs—open-ended MWCNTs also possessing numerous hole defects revealing a large number of edge-plane-like sites heavily decorated with surface functional groups; (b) acid-MWCNTs, which were subsequently vacuum-annealed at 900 °C to remove the functional groups but leaving the many undecorated edge-plane-like sites exposed (ann-MWCNTs); (c) ann-MWCNTs, which were subjected to a further vacuum “super-annealing” stage at 1,750 °C (sup-MWCNTs), which caused the hole defects to close and also closed the tube ends, thereby, restoring the original, pristine, almost edge-plane defect-free MWCNTs structure. The results of the voltammetric characterisation of the acid-, ann- and sup-MWCNTs provide further evidence that edge-plane-like sites are the electroactive sites on MWCNTs. The presence of oxygen-containing surface groups is found to inhibit the rate of electron transfer at these sites under the conditions used herein. Finally, the two charged, “standard” redox probes used were found to undergo strong interactions with the oxygen-containing surface groups present. Thus, we advise caution when using these redox probes to attempt to voltammetrically characterise MWCNTs, and by extension, graphitic carbon surfaces.     

Epilogue to the Gerald Maggiora Festschrift: a tribute to an exemplary mentor,colleague, collaborator,and innovator

Journal of Computer-Aided Molecular Design - In May 2022, JCAMD published a Special Issue in honor of Gerald (Gerry) Maggiora, whose scientific leadership over many decades advanced the fields of...     

Specific binding of large aggregates of amphiphilic molecules to the respective antibodies

The Binding of nonylphenol to respective antibodies immobilized on solid substrates was studied with the methods of total internal reflection ellipsometry (TIRE) and QCM (quartz crystal microbalance) impedance spectroscopy. The binding reaction was proved to be highly specific having an association constant of KA=1.6x10(6) mol(-1) L and resulted in an increase in both the adsorbed layer thickness of 23 nm and the added mass of 18.3 microg/cm2 at saturation. The obtained responses of both TIRE and QCM methods are substantially higher than anticipated for the immune binding of single molecules of nonylphenol. The mechanism of binding of large aggregates of nonylphenol was suggested instead. Modeling of the micelle of amphiphilic nonylphenol molecules in aqueous solutions yielded a micelle size of about 38 nm. The mechanism of binding of large molecular aggregates to respective antibodies can be extended to other hydrophobic low-molecular-weight toxins such as T-2 mycotoxin. The formation of large molecular aggregates of nonylphenol and T-2 mycotoxin molecules on the surface was proved by the AFM study.     

Is the isolated pentagon rule merely a suggestion for endohedral fullerenes? The structure of a second egg-shaped endohedral fullerene--Gd3N@C(s)(39663)-C82

The structure of Gd3N@Cs(39663)-C82 has been determined through single crystal X-ray diffraction on Gd3N@Cs(39663)-C82.NiII(OEP).2(C6H6) The carbon cage has a distinct egg shape because of the presence of a single pair of fused pentagons at one apex of the molecule. Although 9 IPR structures are available to the C82 cage, one of the 39709 isomeric structures that do not conform to the IPR was found in Gd3N@Cs(39663)-C82. The egg-shaped structure of Gd3N@Cs(39663)-C82 is similar to that observed previously for M3N@Cs(51365)-C84 (M = Gd, Tm, Tb). As noted for other non-IPR endohedral fullerenes, one Gd atom in Gd3N@Cs(39663)-C82 is nestled within the fold of the fused pentagons.     

Metal-selective coordination and enhanced fluorescence of a self-assembling ligand scaffold

A fluorescent hydroxyiminonicotinonitile (HINT) coordinating group can be incorporated into self-assembling ligand scaffolds, and shows metal-selective assembly and enhanced fluorescence. The assembly process is dependent on ligand coordination angle, and coordination only occurs for oxophilic first row transition metal ions, with affinity enhanced by the supramolecular assembly process. The ligands are weakly emissive, and show strong enhancement in fluorescence upon Zn²⁺ coordination, whereas Co²⁺ or Fe²⁺ cause complete quenching.     

A neoteric interferometer for use in holographic photoelasticity

A shortcoming of the conventional holographic interferometer used in photoelasticity is that, for the double-exposure cases, the resulting fringe patterns are a complex combination of conventional isochromatic and isopachic fringes. This paper describes a holographic interferometer that may be used to obtain separate but simultaneous isochromatic- and isopachic-fringe patterns for photoelastic models in states of plane stress. The method requires a model with a partially reflecting front surface. Isopachics, which are proportional to the thickness change, are recorded using holographic interferometry from the transmitted light. The isochromatics are obtained from the transmitted light by conventional means. General equations relating the surface displacement of the specimen to the observed fringe patterns are developed, and examples of static and dynamic loadings are shown.     

Synthesis and luminescent characteristics of one-dimensional europium doped Gd2O3 phosphors

One-dimensional (1D) Gd₂O₃:Eu³⁺ nano-rods and micro-rods were prepared using a facile sol-gel precipitation method, without a template and with a post-growth heat treatment in air. Based on scanning electron microscopy (SEM) and X-ray diffraction (XRD) data, hexagonal Gd(OH)₃:Eu³⁺ micro-rods, 60∼90 nm in diameter and 700 nm to 1 μm in length, were synthesized and then transformed by calcining (800°C, 2 hr) to cubic Gd₂O₃:Eu³⁺ with the same morphology and dimensions. Nano-rods of Eu³⁺ doped Gd(OH)₃ and calcined Gd₂O₃, 60∼90 nm diameter and 150∼300 nm length, were prepared by adding polyethylene glycol (PEG) as a capping agent during the sol-gel synthesis. Photoluminescence (PL) spectra exhibited the ⁵D₀–⁷F₂ transitions of Eu³⁺ at 612 and 627 nm from excitation at 280 nm. Photoluminescence excitation (PLE) data showed that a small fraction of PL from Eu³⁺ resulted from direct excitation, but most PL resulted from the oxygen to europium charge-transfer band (CTB) between 250 and 280 nm.     

Synthesis and properties of colloidal ternary ZnGa2O4:Eu nanocrystals

A nonhydrolytic hot solution synthesis technique was used to grow monodisperse ternary oxide nanocrystals of ZnGa₂O₄:Eu³⁺. The shape of ZnGa₂O₄:Eu³⁺ nanocrystals was a function of the type of precursor, and their size was controlled by changing the concentration ratio of Zn precursor to surfactant. The crystal structure of synthesized ZnGa₂O₄ nanocrystals was a cubic spinel with no detectable secondary phases. Photoluminescence of red-emitting ZnGa₂O₄:Eu³⁺ nanocrystals resulted in a high (⁵D₀-⁷F₂)/(⁵D₀-⁷F₁) intensity ratio, suggesting that the Eu³⁺ ions occupy tetrahedral Zn²⁺ sites or distorted octahedral Ga³⁺ sites with no inversion symmetry in ZnGa₂O₄ nanocrystals.     

High-Power GaN Electronic Devices

Gallium Nitride (GaN) and related materials (especially AlGaN) recently have attracted a lot of interest for applications in high-power electronics capable of operation at elevated temperatures and high frequencies. The AlGaInN system offers numerous advantages. These include wide bandgaps, good transport properties, the availability of heterostructures (particularly AlGaN/GaN), the experience base gained by the commercialization of GaN-based laser and light-emitting diodes and the existence of a high growth rate epitaxial method (hydride vapor phase epitaxy, HVPE) for producing very thick layers or even quasisubstrates. These attributes have led to rapid progress in the realization of a broad range of GaN electronic devices. Al_xGa_1-xN (x=0 ～.25) Schottky rectifiers were fabricated in a lateral geometry employing p⁺-implanted guard rings and rectifying contact overlap onto an SiO₂ passivation layer. The reverse breakdown voltage (V_B) increased with the spacing between Schottky and ohmic metal contacts, reaching 9700 V for Al_0.25Ga_0.75N and 6350 V for GaN, respectively, for 100-µm gap spacing. Assuming lateral depletion, these values correspond to breakdown field strengths of <9.67×10⁵ Vcm^?2, which is roughly a factor of 5 lower than the theoretical maximum in bulk GaN. The figure of merit (V_B)²/R_ON, where RON is the on-state resistance, was in the range 94 to 268 MWcm^?2 for all the devices. Edge-terminated Schottky rectifiers were also fabricated on quasibulk GaN substrates grown by HVPE. For small-diameter (75?µm) Schottky contacts, Vs measured in the vertical geometry was ～700?V, with an on-state resistance (RON) of 3?mΩcm², producing a figure-of-merit V_B ²/R_ON of 162.8?MW-cm^?2. GaN p-i-n diodes were also fabricated. A direct comparison of GaN p-i-n and Schottky rectifiers fabricated on the same GaN wafer showed higher reverse breakdown voltage for the former (490?V vs. 347?V for the Schottky diodes), but lower forward turn-on voltages for the latter (～3.5?V vs. ～5?V for the p-i-n diodes). The forward I-V characteristics of the p-i-n rectifiers show behavior consistent with a multiple recombination center model. The reverse current in both types of rectifiers was dominated by surface perimeter leakage at moderate bias. Finally, all of the devices we fabricated showed negative temperature coefficients for reverse breakdown voltage, which is a clear disadvantage for elevated temperature operation. Bipolar devices are particularly interesting for high current applications such as microwave power amplifiers for radar, satellite, and communication in the l～5?GHz range, powers >l00?W, and operating temperatures >425°C. pnp Bipolar Junction Transistors and pnp Heterojunction Bipolar Transistors were demonstrated for the first time. For power microwave applications, small area self-aligned npn GaN/AlGaN HBTs were attempted. The devices showed very promising direct current characteristics.     

A technique for reducing the fringe frequency in large-displacement holography

The analytical method of superposition is combined with the experimental technique of multiple-exposure holography to decrease the sensitivity of holographic measurement by at least an order of magnitude. Moiré fringes of a lower frequency are produced which simultaneously extend the range of measurement to larger displacements. The method is demonstrated for the case of a clamped circular plate subjected to a concentrated load centrally applied.