Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Effects of reabsorption and spatial trap distributions on the radiative
  quantum efficiencies of ZnO   Ultrafast time-resolved photoluminescence spectroscopy following one- and
two-photon excitation of ZnO powder is used to gain unprecedented insight into
the surprisingly high external quantum efficiency of its "green" defect
emission band. The role of exciton diffusion, the effects of reabsorption, and
the spatial distributions of radiative and nonradiative traps are comparatively
elucidated for the ultraviolet excitonic and "green" defect emission bands in
both unannealed, nanometer-sized ZnO powders and annealed, micrometer-sized
ZnO:Zn powders. We find that the primary mechanism limiting quantum efficiency
is surface recombination because of the high density of nonradiative surface
traps in these powders. It is found that unannealed ZnO has a high density of
bulk nonradiative traps as well, but the annealing process reduces the density
of these bulk traps while simultaneously creating a high density of
green-emitting defects near the particle surface. The data are discussed in the
context of a simple rate equation model that accounts for the quantum
efficiencies of both emission bands. The results indicate how defect
engineering could improve the efficiency of ultraviolet-excited ZnO:Zn-based
white light phosphors.

30
The categories of this paper are: q-alg
13
The categories of this paper are: quant-ph
12
The categories of this paper are: nucl-th
23
The categories of this paper are: solv-int
8
The categories of this paper are: hep-ex
6
The categories of this paper are: astro-ph
1
The categories of this paper are: cs
5
The categories of this paper are: gr-qc
2
The categories of this paper are: math
29
The categories of this paper are: funct-an
26
The categories of this paper are: bayes-an
17
The categories of this paper are: q-fin
14
The categories of this paper are: math-ph
20
The categories of this paper are: acc-phys
10
The categories of this paper are: q-bio
15
The categories of this paper are: nucl-ex