  Nanostructured metals subject to local optical interrogation can generate
open-circuit photovoltages potentially useful for energy conversion and
photodetection. We report a study of the photovoltage as a function of
illumination position in single metal Au nanowires and nanowires with nanogaps
formed by electromigration. We use a laser scanning microscope to locally heat
the metal nanostructures via excitation of a local plasmon resonance and direct
absorption. In nanowires without nanogaps, where charge transport is diffusive,
we observe voltage distributions consistent with thermoelectricity, with the
local Seebeck coefficient depending on the width of the nanowire. In the
nanowires with nanogaps, where charge transport is by tunneling, we observe
large photovoltages up to tens of mV, with magnitude, polarization dependence,
and spatial localization that follow the plasmon resonance in the nanogap. This
is consistent with a model of photocurrent across the nanogap carried by the
nonequilibrium, "hot" carriers generated upon the plasmon excitation.
