The
growth and electronic levels of molecular monolayer structures
of helical polyalanine-based peptides (PA) on Au(111) surfaces were
investigated by scanning tunneling microscopy (STM), spectroscopy
(STS), and optical ellipsometry under ambient conditions. The self-assembled
monolayer (SAM) films revealed a high degree of lateral and rotational
order. Moreover, because of the formation of Au–S bonds, resulting
from the termination of the helix by cysteine, the PA molecules are
oriented, and their intrinsic dipole moment is tilted by around 50°
with respect to the surface normal. This charge ordering within the
SAM facilitates internal electric fields of up to 1 V/nm, which may
renormalize the molecular orbital energies along the helix, thus enabling
a high conductance through these peptides. The characteristic bonding
scheme and ordering, found in this study for chiral and polar PA molecules,
will be important to understand the accompanied spin polarization
of propagating electrons as well as the spin exchange mechanism at
the hybrid interface.
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