GaN based high electron mobility transistors (HEMTs) have demonstrated extraordinary features in the applications of high power and high frequency devices. In this paper, we review recent progress in AlGaN/GaN HEMTs, including the following sections. First, challenges in device fabrication and optimizations will be discussed. Then, the latest progress in device fabrication technologies will be presented. Finally, some promising device structures from simulation studies will be discussed.
Abstract:The application of bioprinting allows precision deposition of biological materials for bioengineering applications. Here we propose a 2 stage methodology for bioprinting using a back pressure-driven, automated robotic dispensing system. This apparatus can prepare topographic guidance features for cell orientation and then bioprint cells directly onto them. Topographic guidance features generate cues that influence adhered cell morphology and phenotype. The robotic dispensing system was modified to include a sharpened stylus that etched on a polystyrene surface. The same computer-aided design (CAD) software was used for both precision control of etching and bioink deposition. Various etched groove patterns such as linear, concentric circles, and sinusoidal wave patterns were possible. Fibroblasts and mesenchymal stem cells (MSC) were able to sense the grooves, as shown by their elongation and orientation in the direction of the features. The orientated MSCs displayed indications of lineage commitment as detected by fluorescence-activated cell sorting (FACS) analysis. A 2% gelatin bioink was then used to dispense cells onto the etched features using identical, programmed co-ordinates. The bioink allows the cells to contact sense the pattern while containing their deposition within the printed pattern.
The complete set of 2333 isomers of C(26) fullerene composed of square, pentagonal, hexagonal, and heptagonal faces together with some noncage structures is investigated at the Hartree-Fock and density functional theory (DFT) levels. For the singlet states, a nonclassical isomer C(26)-10-01 with a square embedded is predicted by the DFT method as the lowest energy isomer, followed by the sole classical isomer C(26)-00-01. Further explorations reveal that the electronic ground state of C(26)-10-01 is triplet state in C(s) symmetry, while that of C(26)-00-01 corresponds to its quintet in D(3h) symmetry. Both the total energies and nucleus independent chemical shift values at DFT level favor the classical isomer. It is found that both C(26)-00-01 and C(26)-10-01 possess high vertical electron affinity. The addition of electron(s) to C(26)-10-01 increases its aromatic character and encapsulation of Li atom into this cage is highly exothermic, indicating that it may be captured in the form of derivatives. To clarify the relative stabilities at elevated temperatures, the entropy contributions are taken into account based on the Gibbs free energy at the B3LYP/6-311+G( *) level. C(26)-10-01 behaves thermodynamically more stable than the classical isomer over a wide range of temperatures related to fullerene formation. The IR spectra of these two lowest energy isomers are simulated to facilitate their experimental identification.
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