The complete bilayer is commonly
considered as the termination
of the (0001) surface of hexagonal ice. Experiments on thin crystalline
ice structures grown on Cu(111) demonstrated a termination by admolecule
structures on top of the bilayer. Modeling of complex admolecule terminations
including admolecule clusters and decorated hexagon adrows within
density functional theory and high-resolution STM imaging are combined
for the structural analysis and to reveal possible causes for the
apparent distinction. A dominant admolecule structure that appears
during a short anneal at 130 K is identified as an arrangement of
water dimer and trimers. By the combined approach, detailed models
for decorated hexagon adrows are derived. Such structures possess
low energy; however, the proton-ordered bilayer is more favorable
at a small margin. Yet, energetically unfavorable bonding of water,
for example, in thin ice films may drive the formation of admolecule
terminations, for which kinetic effects still are an important factor.
The results also shine light on the edge termination of bilayer islands.
We have investigated the response of the work function, W, of low-index aluminum surfaces to tangential strain by using first-principles calculations based on density functional theory. This response parameter is a central quantity in electrocapillary coupling of metal electrodes relating to the performance of porous metal actuators and surface stress based sensing devices. We find that Al surfaces exhibit a positive response for all orientations considered. By contrast, previous studies reported negative-valued response parameters for clean surfaces of several transition metals. We discuss separately the response of W to different types of strain and the impact of the strain on the Fermi energy and the surface dipole. We argue that the reason for the abnormal positive sign of the Al response parameter lies in its high valence electron density.
The electromechanical coupling at the silicon (100) and (111) surfaces was studied via density functional theory by calculating the response of the ionization potential and the electron affinity to different types of strain. We find a branched strain response of those two quantities with different coupling coefficients for negative and positive strain values. This can be attributed to the reduced crystal symmetry due to anisotropic strain, which partially lifts the degeneracy of the valence and conduction bands. Only the Si(111) electron affinity exhibits a monotonously linear strain response, as the conduction band valleys remain degenerate under strain. The strain response of the surface dipole is linear and seems to be dominated by volume changes. Our results may help to understand the mechanisms behind electromechanical coupling at an atomic level in greater detail and for different electronic and atomic structures. V
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