2008

Excitonic luminescence in ZnO nanopowders and ceramics

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L. Grigorjeva , D. Millers , K. Smits , V. Pankratov , W. Łojkowski, J. Fidelus , T. Chudoba , K. Bienkowski , C. Monty 

Fast photoluminescence spectra in the spectral region of 3.1–3.45 eV in ZnO and ZnO:Al ceramics were
studied at 14 and 300 K. Ceramics with grains smaller than 100 nm were sintered from nanopowders
by high pressure (8 GPa) and low temperature (350 C). Ceramics with grain sizes 1–5 lm were sintered
at 1400 C. It is shown that excitonic luminescence spectra depend on the ceramics grain size, post preparing
annealing and doping. The excitonic luminescence decay time was faster than 2 ns and the afterglow
at 30 ns was 0.05%.

Optical Materials 2009

doi:10.1016/j.optmat.2008.10.052

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Time-resolved absorption and luminescence following electron-hole pair creation in ZnO

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R. Andrew Wall, Kyle C. Lipke, K. B. Ucer, R. T. Williams, D. Millers, K. Smits, and L. Grigorjeva

We report transient absorption induced by electron-hole excitation
in undoped ZnO. A laser pump/continuum probe
method covers 2–300 ps, and an electron pulse with lamp
transmission covers 8–300 ns. The broad absorption spectrum
increases monotonically with wavelength from 900 to 1600
nm. Following a reasonable hypothesis that the free-carrierlike
induced infrared absorption is proportional to the total
number of free carriers, excitons, and shallow-trapped carriers
in the sample, these data allow setting an upper limit on
the quantum efficiency of a specified lifetime component of
luminescence. For the undoped commercial ZnO studied in
this report, the quantum efficiency of room temperature excitonic
luminescence is less than 5%. This means that there is
significant room for improvement in applications aiming to
use room-temperature excitonic blue luminescence of ZnO
for fast scintillators and light sources. Direct observation that
a large majority of excitations are tied up for more than 100
nanoseconds in shallow traps confirms the premise for studies
undertaken at Oak Ridge National Laboratory to improve the
excitonic luminescence yield and decay rate of ZnO by donor
doping. The preliminary results presented here on undoped
ZnO suggest that induced absorption measurements should be
a useful diagnostic of quantum efficiency while studying such
dopant effects.

Physica Status Solidi (C) Current Topics in Solid State Physics

DOI 10.1002/pssc.200879896

phys. stat. sol. (c) 6, No. 1, 323–326 (2009)

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Radiative Decay of Electronic Excitations in ZrO Nanocrystals and Macroscopic Single Crystals

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Krisjanis Smits, Larisa Grigorjeva, Donats Millers, Janusz D. Fidelus, and Witold Lojkowski

Abstract—The time-resolved luminescence was studied for
ZrO:Y single crystal and nanocrystals. The similar recombination
centres were found in both—single crystal and nanocrystals.
Luminescence decay is within 200 ns in nanocrystals, whereas it
extends to the microseconds in single crystal. It was shown that
the defects responsible for transient absorption were not involved
directly in radiative recombination process.

IEEE Transactions on Nuclear Science, VOL. 55, NO. 3, JUNE 2008

Digital Object Identifier 10.1109/TNS.2008.924077

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Luminescence Properties of ZnO Nanocrystals and Ceramics

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Larisa Grigorjeva, Donats Millers, Janis Grabis, Claude Monty, Aleksandr Kalinko, Krishjanis Smits,
Vladimir Pankratov, and Witold Łojkowski

Abstract—The luminescence excitation spectra, luminescence
spectra and the nanosecond-scale decay kinetics were studied.
The ZnO and ZnO:Al nanopowders were prepared by vaporization-
condensation in a solar furnace using different raw powders:
commercial, hydrothermal and those obtained by plasma synthesis.
Exciton-phonon as well as exciton-exciton interaction
processes in nanopowders, a bulk crystal and ZnO ceramics were
studied and compared. The fast decay and low afterglow intensity
of ZnO nanopowders and ceramics support these materials for
scintillators.

IEEE Transactions on Nuclear Science, VOL. 55, NO. 3, JUNE 2008

Digital Object Identifier 10.1109/TNS.2008.921931

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Transient and stable color centers in neutron irradiated MgO

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Vera Skvortsova, Nina Mironova-Ulmane, Larisa Grigorjeva,
Donats Millers, Krisjanis Smits

Abstract
The transient absorption and luminescence induced by the pulsed electron beam have been investigated in the MgO single crystal
containing transition metal ion (Cr, Mn, Fe) impurities and preliminary irradiated by the fast neutrons. It is supposed that the different
behavior of the absorption spectra of the MgO samples preliminary irradiated by the different fast neutron fluence is connected with the
destruction of the hole centers and with the creation of interstitial protons and the formation of the microphase Mg(OH)2. We assume
that the luminescence band at 3.2 eV is connected with F+ color centers.

Nuclear Instruments and Methods in Physics Research B 266 (2008) 2941–2944

doi:10.1016/j.nimb.2008.03.143

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