Pulsed Cathodoluminescence of ZnWO 4 :Eu Single Crystals and Nanocrystals

Pulsed Cathodoluminescence of ZnWO4:Eu Single Crystals and Nanocrystals

Viktor Lisitsyn1a, Damir Valiev1b*, Irina Tupitsyna2c, Elena Polisadova1d, Ludmila Lisitsyna3e, Lubov Andryuschenko2f, Aleksander Dubovik2g,

Anna Yakubovskaya2h, Oleg Vovk4i 1 National Research Tomsk Polytechnic University, Tomsk 634050, Russia

2 Institute for Scintillation Materials, Kharkov 61001, Ukraine

3 Tomsk State University of Architecture and Building, Tomsk 634003, Russia

4 Institute for Single Crystals, Kharkov 61001, Ukraine

[email protected], [email protected], [email protected], [email protected], [email protected], [email protected], [email protected],

[email protected], [email protected]

*Corresponding author

Keywords: time-resolved luminescence, scintillation material; ZnWO4:Eu3+, decay kinetics.

Abstract: Pulsed cathodoluminescence of zinc tungstate samples doped with europium in the form

of single crystals and nanocrystals in the transparent organosilicic matrix is studied. The effect of

the size of the crystal on spectral and kinetic parameters of the oxygen luminescence centers is

interpreted within the model of self-organization of activator nanodefects during crystal growing.

Introduction

Zinc tungstates are among the most promising scintillation materials. They are inferior if

compared to the materials based on cadmium tungstate, however the low level of their intrinsic

radioactivity is low and they are non-toxic [1]. Nanocomposite materials (zinc tungstate

nanocrystals in a polymer matrix) which can be used to obtain qualitatively new devices with

desired functional characteristics are of interest [2, 3, and 4]. The production technology for

nanocrystals and nanocrystal-based nanocomposite materials is quite well developed. However, the

information required to optimize the properties of these materials is still insufficient. This paper

presents the results of the study of pulsed cathodoluminescence (PCL) of zinc tungstate samples

doped with europium in the form of single crystals and nanocrystals in a transparent organosilicic

matrix to find the dependence of luminescence characteristics on the crystal size.

Experimental details

We investigated and compared spectral-kinetic characteristics of luminescence activator centers

in two types of single crystals and two types of nanocrystals. ZnWO4 and ZnWO4:Eu3+

single

crystals were grown by Czochralski method in air followed by heat treatment to relieve stresses

during their growing. The crystals were grown from the charge obtained by solid-phase synthesis

from ZnO (99, 995%) and WO3 (99,995%) oxides. The europium was added in the form of an

oxide.

ZnWO4:Eu3+

nano-sized crystals were obtained by microwave-hydrothermal synthesis. We used

the following starting materials: Na2WO4·2H2O and Zn(NO3)2·6H2O and NH3⋅H2O of analytical

grade purity and HNO3 (ρ = 1,40) (Merk, GR for analysis). The nanocrystals were particles of two

types: grains ~25 nm in length, and rods ~100–200 nm in length and ~5–10 nm in diameter. The

images of nanocrystals in the form of grains and rods obtained by electron microscopy are shown in

Fig. 1.

Advanced Materials Research Vol. 872 (2014) pp 128-133© (2014) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.872.128

All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,www.ttp.net. (ID: 194.44.181.106-13/12/13,15:51:05)

http://www.ttp.net

The samples to study the luminescence characteristics were polymer films of low-molecular

heat-resistant organosilicic rubber film with incorporated ZnWO4:Eu nanocrystals.

Fig. 1 Images of ZnWO4:Eu

3+ samples with a rod (a) and grain (b) structure obtained by

transmission electron microscopy

The tested pure single crystals and doped crystals were transparent in the range from 1100 nm to

420 nm; in the range from 420 nm to 330 nm we could observe a monotonic increase of the

absorption up to about 10 cm–1

; starting from 320 nm absorbance sharply increases. In the

nanocomposite samples with ZnWO4 nanocrystals incorporated into the polymer matrix with Eu in

the form of rods and grains the absorption spectra were similar.

The PCL spectra were measured by two techniques:

1. The PCL of the materials was studied using time-resolved spectrometry with nanosecond time

resolution. The method enabled to record the PCL spectra of the samples with the time

resolution of ~10 ns via the monochromator MDR–23 (inverse linear dispersion of 1.3 nm),

PMT-84, PMT-97 and TDS 2014 digital oscilloscope. The PCL was excited by pulsed EB with

the pulse duration of 10 ns, average electron energy of 250 keV and the excitation energy

density of 15 mJ/cm2. The PCL of the crystals was investigated in the spectral range of 1.6–4.1

eV at 300 K in the time interval of 1·10–8

– 1·10–2

s after the end of EB action. To record the

luminescence we used PMT-106, PMT-84, a digital oscilloscope Gwinstek GDS-2204 (200

MHz bandwidth) and MDR-3.

2. The spectrum was measured after exposure of a single pulse EB (“a spectrum per pulse”). After

excitation by pulsed EB the integrated PCL spectra were recorded in the spectral range of 1.5–

4.0 eV by high-precision fiber optic spectrometer AvaSpec-2048 without time resolution. The

measurement module AvaSpec-2048 of the highly sensitive spectrophotometer is based on

AvaBench-75 platform with the symmetrical Czerny-Turner optical bench and 2048 element

CCD detector array.

Result and discussion

Fig. 2 shows the results of the study of luminescence spectral-kinetic characteristics for ZnWO4

single crystal without intentionally introduced impurities. The sample was excited by pulsed EB of

nanosecond pulse duration. In the spectrum measured by the “spectrum per pulse” technique we

observe one non-elementary luminescence band with the maximum at 480 nm and a “shoulder” in

the region of 570 nm that is characteristic of luminescence in these crystals [5–8]. The

luminescence spectrum is an integral spectrum of all possible time and spectral components that are

excited by pulsed EB. The luminescence spectra measured after 25 ns, 5 µs and 10 µs after the

excitation pulse are of similar shape. Over time after the end of the excitation pulse, the shape of the

spectrum does not change significantly. The integral spectrum is mainly determined by this spectral

component that can be seen from their comparison.

Advanced Materials Research Vol. 872 129

Fig.2. PCL spectra of zinc tungstate crystal measured by “spectrum per pulse” technique (a);

with the time-resolution (b): after 25 ns (1), 5 µs (2) and 10 µs (3).

We studied the luminescence spectra of ZnWO4:Eu3+

single crystals with europium of 3 mol%,

6 mol% and 9 mol% in the mixture. In comparison to the “pure” single crystal, in the integrated

luminescence spectra of the europium-doped crystals we observe a set of narrow bands in the region

of 610–720 nm. The intensity of the bands increases as the concentration of europium in the crystal

grows; the maximum is a characteristic narrow band at 614 nm (Fig. 3, a). As the concentration of

europium rises, the band responsible for the luminescence of europium ions grows, and the ratio of

bands at 480 nm and 614 nm changes in its favor.

Fig. 3 (b, c) shows the luminescence spectra of ZnWO4 single crystals doped with 3 mol% Eu

measured with different time delay after the end of pulsed EB. In the spectra measured by

”spectrum per pulse” technique the band at 480 nm is dominant, and it decays with no distinct

change in the shape of the spectrum. After 100 µs, the intensity of this band becomes low, and the

band at 614 nm line becomes distinct, which then eventually becomes dominant in the spectrum and

decays within a millisecond range.

Fig. 3 PCL spectra for ZnWO4-3 mol% Eu crystal measured by ”spectrum per pulse” technique

(a) with the time resolution of 25 ns after 5 µs and 10 ms (b), 250 µs, 500 µs and 1 ms (c)

Pulsed EB excitation of composite samples in the form of nanocrystal grains about 25 nm in size

in the polymer matrix enhances luminescence with the characteristics shown in Fig. 4. The

luminescence spectra measured by “spectrum per pulse” technique are substantially identical to

those in single crystals. The spectrum contains a broad band with the maximum at 480 nm and

narrow bands with the maximum band at 614 nm. In the spectra measured with different time delay,

up to 50 ns the dominant is the broad band at 400 nm; over time, this band disappears, and after 100

ns the band at ~480 nm dominates in the spectrum. After 100 µs the band at ~480 nm disappears,

and a narrow band at 614 nm becomes dominant.

130 Nanomaterials for Structural, Functional and Biomedical Applications

Fig. 4. PCL spectra for ZnWO4:Eu nanocrystals (grains) measured by “spectrum per pulse”

technique (a) with the time resolution of 25 ns, 1 µs and 10 µs (b), 250 µs, 500 µs and 1 ms (c) after

the end of excitation.

Similar results were obtained for the nanocomposite materials in the form nanocrystal rodes

10 nm in diameter and 200 nm in length added into the polymer matrix.

The kinetic curves of the luminescence decay for the tested samples are shown in Fig. 5. The

decay kinetics of the band at ~ 480 nm for single crystals and nanocrystals is significantly different

(Fig. 5, a, c). The main share of luminescence in this spectral region, which determines the light

yield of the scintillator, occurs within a microsecond range. In single crystals, by 12 µs, the band at

~480 nm reduces 2.5 times as much, while in the nanocrystal it reduces twentyfold. The

characteristic decay time of the luminescence in the region of 480 nm in the single crystal is 15 µs,

and in a nanocrystal it is 3 µs. Decrease in the characteristic luminescence decay period caused by

europium occurs in the region of 614 nm as well. In the single crystals and nanocrystals it is 0.5 µs

and 0.3 µs, respectively. The results obtained show that change in the band at 614 nm in single

crystals and nanocrystals with time is much smaller than that in the band at 480 nm. Note that the

kinetic characteristics of the samples with nanocrystals in the form of rods and grains are identical.

Fig. 5. Luminescence decay kinetics of the bands at 480 nm and 614 nm in excitation by electron

beam for single crystals (a, b) and nanocrystals (c, d) doped with 3 mol% Eu.


Conclusion

Doping single crystals and nanocrystals with europium gives rise to a set of narrow bands in the

luminescence spectrum due to intracenter transitions characteristic of the optical transitions 5D0→

7Fn

of Eu3+

ion. The line with the maximum at 614 nm which corresponds to intracenter electron

transitions 5D0−

7F2 is of maximal intensity. As the concentration of the impurity grows the

luminescence intensity of these bands increases. The duration of the impurity luminescence is

larger, and the kinetics over time after pulsed EB excitation is well described by an exponential

function with the characteristic time of 0.5 ms and 0.3 ms for single crystals and nanocrystals,

respectively. The luminescence center, a europium ion, is excited probably by successive capture of

an electron and hole which are formed in the center when exposed to pulsed EB or in transitions

with charge transfer.

In addition to the impurity luminescence, a broad band with the maximum at 480 nm

characteristic of all tested “pure” and doped single crystals can be observed in the luminescence

spectrum. The values of the luminescence intensity in the maxima of the bands at 614 nm and ~ 480

nm are comparable; their relation depends on the concentration of the impurity. The spectral

characteristics of the luminescence in the band at 480 nm are similar in all the crystals, and this

indicates the identical nature of the luminescence centers responsible for this band. However, the

kinetic characteristics of the luminescence in the band at ~480 nm in single crystals and

nanocrystals are significantly different. The luminescence kinetics with time after pulsed EB is well

described by an exponential function with the characteristic time of 15 µs and 3 µs for single

crystals and nanocrystals, respectively. It is known that the luminescence in this region is due to the

centers with an oxygen ion as a structural element [9–11].

The oxygen center responsible for luminescence is a component of a nanodefect, complex

nanodefect with the structure different from that of a regular crystal which contains a set of intrinsic

point defects in addition to the luminescence center [12]. This defect is formed in the crystal in the

process of its synthesis since stoichiometry is difficult to maintain. The dimensions of the

nanodefect are no less than 2 ... 3 nm.

It should be noted that changes in the kinetic characteristics of the luminescence decay in

europium-doped single crystals and nanocrystals at ~480 nm and 614 nm are significantly different.

The kinetics of luminescence relaxation in the two luminescence centers, a europium ion center and

an oxygen-containing center, depends on the crystal size. However, this change is much more

significant for the oxygen-center than for the europium ion center. We suggest the following

hypothesis. A europium ion is less than a nanometer in size even in pair with the donor. A

nanodefect is several nanometers in size which is comparable to the size of a nanocrystal. These

may be the factors which account for the effect of the size and surface area of the crystal with the

dimensions of 10 ... 25 nm on the processes occurring in nanodefects.

Acknowledgments

The research was supported by the Ministry of Education and Science of the Russian Federation,

«Science», No 2.3302.2011.

References

[1] L.L. Nagornaya, B.V. Grinyov, A.M. Dubovik et al., Large volume ZnWO4 crystal scintillators

with excellent energy resolution and low background, IEEE Trans. Nucl. Sci. 56 (2009) 994–997.

[2] N.V. Klassen, V.N. Kurlov, S.N. Rossolenko et al., Scintillation fibers and nanoscintillators for

improve of spatial, spectrometric and temporal resolution of radiation detectors, Izvestiya. A series

of physical. 73 (2009) 1451–1456.

[3] N.V. Klassen, V.V. Kedrov, V.N. Kurlov et al. IEEE Transact. Nucl. Sci. 55 (2008) 1536.

[4] J.P. Reithmaier, L. Keldysh, V. Kulakovskii et al. Nature. 432 (2004) 197.

132 Nanomaterials for Structural, Functional and Biomedical Applications

[5] A.E. Ovechkin, V.D. Ryzhikov, G. Tamulaitis, аnd A. Zukauskas., Luminescence of ZnWО4

and CdWО4 crystals, Phys. stat. sol. (a) 103 (1987) 285–2908.

[6] A.E. Ovechkin, L.V. Viktorov, and L.L. Nagornaya, Pulsed Cathodoluminescence of CdWО4

and ZnWО4 crystals, J. of App. Spectroscopy. 48 (1988) 396–401.

[7] V. Nagirnyi, L. Jonsson, M. Kirm, A. Kotlov, A. Lushchik, I. Martinson, A. Watterich, B. I.

Zadneprovski, Luminescence study of pure and Fe- or Mo-doped ZnWO4 crystals, Rad. Measur. 38

(2004) 519–522.

[8] L. Grigorjeva, D. Millers, S. Chernov, V. Pankratov, A.Watterich, Luminescence and transient

absorption in ZnWO4 and ZnWO4–Fe crystals, Radiat. Meas. 33 (2001) 645.

[9] L.A. Lisitsyna, V.I. Oleshko, S.N. Putintseva, V.M. Lisitsyn, Pulsed cathodoluminescence of

irradiated LiF–O and LiF(U)–O crystals, Opt. and Spectroscopy 105 (2008) 531–537.

[10] L.A. Lisitsyna, V.I. Oleshko, V.M. Lisitsyn, S.N. Putintseva, The Luminescence of uranium in

LiF crystals, Proceedings of Higher Education 51 (2009) 49–55.

[11] L.A. Lisitsyna, V.I. Korepanov, V.M. Lisitsyn, A.E. Eliseev, N.N. Timoshenko, A.K

Dauletbekova, Impurity cathodoluminescence of oxygen-containing LiF crystals, Opt. and

Spectroscopy 110 (2011) 529–533.

[12] L.A. Lisitsyna, V.M. Lisitsyn, Composition nanodefects in doped lithium fluoride crystals,

Solid State Phys. 55 (2013) 2183–2190.


Pulsed Cathodoluminescence of ZnWO 4 :Eu Single Crystals and Nanocrystals

Documents

Transcript of Pulsed Cathodoluminescence of ZnWO 4 :Eu Single Crystals and Nanocrystals