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Understanding Nature’s glue

Mussel-inspired adhesion has much to teach about the
chemistry and processing of polymers and provides a glimpse of some remarkable
physical properties exhibited by this complex fluid. . Mussel adhesive processing
has been mimicked by Waite, Tirrell and Israelachvili mixing a
recombinant mussel adhesive protein (MAP) with hyaluronic acid (HA) to form a
fluid coacervate. Significantly, these unique materials were shown to be shear-thinning,
while also having high friction coefficients (>1.2), and extremely low
interfacial energies (2).

The computational studies were performed using cutting-edge first-principles techniques based on density functional theory and hybrid functionals.  Details of the calculations are included in J. L. Lyons, A. Janotti, and C. G. Van de Walle, Appl. Phys. Lett. 95, 252105 (2009).  The figure at the top shows a ball-and-stick model of the ZnO crystal with a nitrogen acceptor on an oxygen site, and isosurfaces of the spin density.  This spin density can be used to compare with experimental measurements of hyperfine parameters using electron spin resonance.
As to the reason why previous experiments seemed to show that nitrogen acted as a shallow acceptor: In optical studies, the near-band-gap photoluminescence line most commonly associated with nitrogen is now known to be caused by stacking faults.  Optical absorption and emission associated with the nitrogen deep acceptor in fact occurs at much lower energies, at wavelengths that have been all but ignored in prior studies (see lower figure).  When it comes to electrical measurements of acceptor-doped ZnO, there are many potential pitfalls, as addressed in an IRG-2 paper published last year [O. Bierwagen, T. Ive, C. G. Van de Walle, and J. S. Speck, Appl. Phys. Lett. 93, 242108 (2008)], casting doubt on most of the p-type conductivity reports published to date.
Results on interstitial doping using fluorine were published in A. Janotti, E. Snow, and C. G. Van de Walle, Appl. Phys. Lett. 95, 172109 (2009).