The polarization and surface reactivity
of the ZnO nanorods (NRs)
grown on the (0001̅) N-polar GaN substrate and the (0001) Ga-polar
GaN substrate by hydrothermal process were studied. We found that
the polarization direction of the ZnO NRs grown on the GaN substrates
depends on the surface polarity of the GaN growth substrates. The
surface planes of ZnO NRs on the Ga-polar GaN substrate and the N-polar
GaN substrate are the Zn-polar (positive-charged surface) and the
O-polar (negative-charged surface) ZnO surface plane, respectively.
The O-polar ZnO plane is more chemically stable than the Zn-polar
ZnO plane. After the water dissolution process, the Zn-polar ZnO NRs
transformed to the tube-like nanostructure and the O-polar ZnO NRs
formed hexagonal facets on the top of ZnO NRs. X-ray absorption spectroscopy
(XAS) was used to analyze the electronic structures of the growth
surface of the ZnO NRs on the two GaN growth substrates. The absorption
peaks in the XAS spectral reveal the density of the dangling bonds
on the terminal surface of ZnO NRs and the occupancy probability of
the hybridized orbitals of the terminal surface of ZnO NRs. With the
information on the electronic structure of the surface planes of the
ZnO NRs, the surface plane of the ZnO NRs on the different GaN substrates
can be verified.
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