Electrical characterization and cathodoluminescence microanalysis of AlN/GaN heterostructures

Access full text - https://doi.org/10.1016/S0921-5107(01)01045-5Low-pressure MOCVD is used to grow AlN/GaN MIS-type heterostructures with AlN thickness between 3 and 35 nm. The two-dimensional electron gas (2DEG) Hall mobility was found to decrease with AlN thickness. The measured room temperature and 20 K mobilities for a sample with 15 nm thick AlN were 465 cm2V−1s−1 (ns=1.72×1013 cm−2) and 877 cm2 V−1 s−1 (ns=1.57×1013 cm−2), respectively. Cathodoluminescence (CL) spectra consist of two GaN-related bands with the maxima at 3.4 and 1.9–2.3 eV. Under surface excitation the intensity of the red–yellow CL relative to the intensity of the UV emission was found to increase with AlN film thickness. This increase was found to correlate with the decrease in 2DEG Hall mobility

Controlled Bending of Microscale Au-Polyelectrolyte Brush Bilayers

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Fröhlich modes in GaN columnar nanostructures

Access full text - https://doi.org/10.1103/PhysRevB.64.233317GaN columnar nanostructures fabricated by electrochemical dissolution of bulk material have been studied by micro-Raman spectroscopy. The anodization induces an increase in the intensity of Raman scattering accompanied by a breakdown of the polarization selection rules and by the appearance of a new mode at 716cm−1, i.e., in the frequency gap between the transverse optical and longitudinal optical bulk phonons. We present a Raman line-shape analysis based on the effective dielectric function of a composite that brings to light the Fröhlich character of this mode

On the Cis to Trans Isomerization of Prolyl–Peptide Bonds under Tension

The cis peptide bond is a characteristic feature of turns in protein structures and can play the role of a hinge in protein folding. Such cis conformations are most commonly found at peptide bonds immediately preceding proline residues, as the cis and trans states for such bonds are close in energy. However, isomerization over the high rotational barrier is slow. In this study, we investigate how mechanical force accelerates the cis to trans isomerization of the prolyl–peptide bond in a stretched backbone. We employ hybrid quantum mechanical/molecular mechanical force-clamp molecular dynamics simulations in order to describe the electronic effects involved. Under tension, the bond order of the prolyl–peptide bond decreases from a partially double toward a single bond, involving a reduction in the electronic conjugation around the peptide bond. The conformational change from cis to extended trans takes place within a few femtoseconds through a nonplanar state of the nitrogen of the peptide moiety in the transition state region, whereupon the partial double-bond character and planarity of the peptide bond in the final trans state is restored. Our findings give insight into how prolyl–peptide bonds might act as force-modulated mechanical timers or switches in the refolding of proteins

Structural specializations of A2, a force-sensing domain in the ultralarge vascular protein von Willebrand factor

The lengths of von Willebrand factor (VWF) concatamers correlate with hemostatic potency. After secretion in plasma, length is regulated by hydrodynamic shear force-dependent unfolding of the A2 domain, which is then cleaved by a specific protease. The 1.9-Å crystal structure of the A2 domain demonstrates evolutionary adaptations to this shear sensor function. Unique among VWF A (VWA) domains, A2 contains a loop in place of the α4 helix, and a cis-proline. The central β4-strand is poorly packed, with multiple side-chain rotamers. The Tyr-Met cleavage site is buried in the β4-strand in the central hydrophobic core, and the Tyr structurally links to the C-terminal α6-helix. The α6-helix ends in 2 Cys residues that are linked by an unusual vicinal disulfide bond that is buried in a hydrophobic pocket. These features may narrow the force range over which unfolding occurs and may also slow refolding. Von Willebrand disease mutations, which presumably lower the force at which A2 unfolds, are illuminated by the structure