  Metal nanoparticle (NP) surface coatings are known to significantly enhance
the ultra-violet luminescence intensity of ZnO. Although there is general
agreement that resonantly excited Localized Surface Plasmons (LSPs) in metal
NPs can directly couple to excitons in the semiconductor increasing their
spontaneous emission rate, the exact mechanisms involved in this phenomenon are
currently not fully understood. In this work, LSP-exciton coupling in a ZnO
single crystal and ZnO nanorods coated with a 2 nm Al layer has been
investigated using correlative photoluminescence and depth-resolved
cathodoluminescence and time-resolved photoluminescence spectroscopy.
Temperature-resolved cathodoluminescence and photoluminescence measurements
from 10 K to 250 K show enhancement factors up to 12 times of the free exciton
(FX) emission at 80 K. The FX couple more efficiently to the LSPs in Al
compared to the localized donor-bound excitons. Furthermore, a strong
polarization dependence of the LSPs with respect to the FX was observed with
higher enhanced FX transitions polarized in the same direction as the electric
field of the incident excitation. These results indicate that selective
enhancement of the ultra-violet excitonic PL in ZnO can be achieved by careful
alignment of the crystallographic axes of the ZnO relative to the electric
vector of the excitation source.
