  We observe a systematic red shift of the band-edge of passivated
GaAs/Al0.35Ga0.65As core-shell nanowires with increasing shell thickness up to
100 nm. The shift is detected both in emission and absorption experiments,
reaching values up to 14 meV for the thickest shell nanowires. Part of this red
shift is accounted for by the small tensile strain imposed to the GaAs core by
the AlGaAs shell, in line with theoretical calculations. An additional
contribution to this red shift arises from axial piezoelectric fields which
develop inside the nanowire core due to Al fluctuations in the shell.
