(551e) Electronic Relaxation Dynamics at the ZnO (10-10) Surface

ZnO is a remarkably versatile material with
applications in catalysis, optoelectronics, and photovoltaics. ZnO nanowires
and nanoparticle films have shown promise as nanoscale light sources,
photodetectors, optical switches, and anodes in solar cells. Because of the
intrinsic anisotropy of the wurtzite unit cell, a large fraction of the surface
area in these nanostructured devices belongs to the low-energy {10-10} family
of planes. Consequently, an understanding of excited state electron dynamics at
these surfaces is essential for realization of such exciting technologies.

In this talk, I will present a detailed study of
electron relaxation dynamics at the ZnO (10-10) surface using femtosecond
time-resolved two-photon photoelectron spectroscopy (TR-2PPE). Efficient
emission of longitudinal optical phonons results in electron lifetimes shorter
than 30 fs within the Γ valley of the bulk conduction band, in agreement
with simple perturbation theory calculations. At or below the band minimum, dynamics
are accurately described by electronic relaxation within a quasi-continuum of
defect-derived surface states whose density decreases exponentially into the
band gap. Existence of these states is consistent with observed upward (n-type)
band-bending and Fermi level pinning at the (10-10) surface.