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Nuclear Instruments and Methods in Physics Research B 310 (2013) 23–26

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Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb

Comparative study of the luminescence properties of macro- and nanocrystalline MgO using synchrotron radiation A.I. Popov a,b,⇑, L. Shirmane a,⇑, V. Pankratov a,c, A. Lushchik d, A. Kotlov f, V.E. Serga e, L.D. Kulikova e, G. Chikvaidze a, J. Zimmermann g a

Institute of Solid State Physics, University of Latvia, Kengaraga 8, Riga LV-1063, Latvia Institut Laue-Langevin, 6 rue Jules Horowitz, 38042 Grenoble, France Department of Physics, University of Oulu, P.O. Box 3000, FIN-90014 University of Oulu, Finland d Institute of Physics, University of Tartu, Riia 142, Tartu 51014, Estonia e Institute of Inorganic Chemistry, Riga Technical University, Miera 34, Salaspils LV-2169, Latvia f HASYLAB, DESY, Notkestrasse 85, Hamburg D-22761, Germany g Electronic Materials Division, Institute of Materials Science, Darmstadt University of Technology, Darmstadt D-64287, Germany b c

a r t i c l e

i n f o

a b s t r a c t

Article history: Received 22 March 2013 Received in revised form 14 May 2013 Available online 1 June 2013

MgO nano-powder with an average crystallite size of nanoparticles ranging 10–15 nm was synthesized using the extractive-pyrolytic method and was studied by room temperature VUV spectroscopy under synchrotron radiation excitation. Comparative analysis of their luminescent properties with that of macrocrystalline powder analogues and an MgO single crystal, grown by the arc-fusion method, has been performed under excitation by pulsed VUV synchrotron radiation. Special attention was paid to VUV spectral range, which is not reachable with commonly used lamp and laser sources. A considerable blue shift of about 0.3 eV in the excitation spectra of 2.95 eV emission band, was revealed in nanocrystalline MgO samples. Ó 2013 Elsevier B.V. All rights reserved.

Keywords: MgO Nano particles Excitation spectroscopy Luminescence Synchrotron radiation

1. Introduction Wide-gap magnesium oxide (Eg = 7.8 eV) continues to attract attention due to its fundamental interest and applications. The electronic, optical and radiation properties in the bulk have been investigated in details [1–13]. It has been also well recognized that MgO has several types of bulk intrinsic defects, including oxygen and magnesium vacancies, interstitials, their agglomerates, etc. [3,14–20]. Among them, neutral (F centers) and positively charged (F+ centers) oxygen vacancies have been extensively studied during the past decades [3,21–28]. The F and F+ centers exhibit broad photoluminescence (PL) bands peaking at 2.3 eV (500 nm) and 3.2 eV (400 nm), respectively, whereas the optical absorption of both types of centers is peaked essentially at the same energy of 5 eV (250 nm). There have been many studies of the luminescence of nominally pure and impurity-doped MgO crystals after exposure to a variety of ionizing radiation, including UV-light, X-ray, gamma, electron or ⇑ Corresponding authors at: Institute of Solid State Physics, University of Latvia, Kengaraga 8, Riga LV-1063, Latvia. E-mail addresses: [email protected] (L. Shirmane).

(A.I.

Popov),

[email protected]

0168-583X/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2013.05.017

neutron irradiation. Spectral analysis of this emission shows broad features in the blue, orange and red with the relative emission intensity in each regions being strongly dependent on temperature and radiation conditions. Synchrotron radiation, which is the concern of this report, was used for the optical spectroscopy studies of MgO single crystals more than once [29–33] and references there in. To our knowledge, nevertheless there is only report on VUV synchrotron radiation spectroscopy of MgO nano-powder which was synthesized using the combustion method [33]. The goal of the present study was to compare the luminescent properties of nanocrystalline MgO which was prepared by the extractive-pyrolytic method with macrocrystalline powder analogues and a single crystal. Special attention was paid to VUV spectral range, which is not reachable with commonly used lamp and laser sources.

2. Experimental The nanopowder of magnesium oxide (MgO) was prepared by the extractive-pyrolytic method at the Institute of Inorganic Chemistry, Salaspils. The X-ray diffraction measurements have been performed in order to investigate the crystalline structure and to provide an average crystallite size of nanoparticles (10–15 nm).

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Single crystals of MgO were grown by the arc-fusion method at the Institute of Physics, Tartu. The SUPERLUMI facility of HASYLAB was used for the measurements of emission and excitation spectra [34]. This experimental set-up is unique tool for investigations of wide band gap materials as well as nanocrystalline semiconductors [30,31,33–42]. It is necessary to note that highly pure MgO belong to the class of radiation-resistant oxide materials in which radiation damage occurs via particles elastic collisions only that is not the case for VUV (3.7–40 eV) photons [15,43].

3. Results and discussion In the Fig. 1 the room temperature luminescence spectra of a MgO crystal, commercial macropowder and nanoparticle samples under excitation at 200 nm (6.2 eV) are presented. The emission band in the 2.9–3.0 eV region has almost Gaussian shape with the maximum at 2.93; 2.93 and 3.01 eV and half-width 0.78; 0.86 and 0.99 eV for single crystal, macropowder and nanoparticle samples, respectively. In addition, in the case of MgO single crystal, strong well-known Cr3+ band at about 700 nm was also observed [5] and references therein. Fig. 2 displays the excitation luminescence spectra for all these three samples measured at 2.95 eV (420 nm) in spectral range 4.5– 10.0 eV. Two definite conclusions can be drawn here, namely: (a) The excitation spectra for crystal and commercial macropowder samples have maximum at almost the same energy (at about 5.75 eV), while the appropriate peak for nanoparticle samples reveals blue shift at about 0.3 eV, which could be connected with the nano-size particles of the sample. (b) The excitation spectra for both macro- and nano-particle samples show the clear shoulder at about 5.0 eV, where the both F and F+ centers have their optical absorption with peak essentially at the same energy of 5.0 eV, while such feature is absent at all in the case of single crystals. It is known for a long time that several intrinsic and extrinsic emission bands in MgO, are contributing to the luminescence in 3.0 eV spectral region. The emission of F+ centers has maximum at 3.16–3.20 eV [3,21] whereas the emission bands of deformation-induced centers – vacancy complexes [44] and Sn2+ centers [45], are both situated at 2.9 eV. The emission of Ge2+ centers at

Fig. 1. Luminescence spectra of the MgO crystal, commercial macropowder and nanoparticle samples under excitation 200 nm (6.2 eV).

Fig. 2. Excitation spectra of the emission kem = 420 nm.

3.18 eV excited at 4s2 ? 4s4p electron transitions in Ge2+ ions (4.8–6.4 eV) was also reported [18]. Concerning the plastic deformation, it was reported more than once that it produces an optical absorption band at about 220 nm (5.7 eV) [2,4,44,46]. Excitation in this band in deformed samples produces a luminescence band centering at about 2.9 eV (430 nm) and the emission localized mainly at the slip planes [44]. Furthermore, an emission band at 3.0 eV (417 nm) was also found using the cathodoluminesce mode of the scanning electron microscope, where the slip planes can be seen with higher resolution [47,48]. It was suggested that 5.7 eV absorption band is due to point defects produced by deformation and the emission at 2.9 eV is due to photoexcitation of these defects [2,44]. It is important to note here that one of three absorption bands (5.7, 4.8 and 4.3 eV) induced in MgO crystals by heat treatment in oxygen also has a peak at this energy [49]. Possible connection of this band with Fe3+ was discussed in [49,50]. In [50] it was also pointed out that there is no direct correlation between the cathodoluminescence at 2.9 eV and the absorption band at 5.7 eV. Furthermore, studies of the temperature behavior shows that the absorption band at 5.7 eV annealed out at about 850 K, while the luminescence disappeared at around 1100 K [4]. In addition, in [51] it was investigated the strain dependence of the absorption and photoluminescence bands and found that the latter reached a maximum saturation before the former. Further, Fig. 3(a–c) represents the results of the Gaussian deconvolution of the excitation spectra of 420 nm luminescence band for nanoparticle (a), single crystal (b) and macropowder samples (c). In all cases, the deconvolution analysis gives three Gaussian and their maxima are summarized in Table 1. From this Table, we can conclude that: (a) In both macropowder and single crystal sample, above-discussed the well-known 5.7 eV excitation band caused by deformation-induced defects/vacancy complexes is revealed. (b) The excitation bands at 4.95 eV in the case of macropowder and 5.16 eV in the case of nanoparticle sample can be connected with F+ center, while there is no any manifestation of these defects in the case of single crystals. (c) The 6.06 eV excitation band observed in nanoparticle samples is similar to that at 5.7 eV found in both macropowder and single crystal sample. This blue shift of about 0.35 eV, could be connected with the nano-size particles of the sample. Probably, this situation is similar to that recently

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Fig. 3. Deconvolution analysis of the excitation spectra of the 420 nm luminescence in the MgO nanocrystals (a), single crystal (b) and macropowder (c).

Table 1 Positions of the excitation band peaks for nano-, macro and single crystaline MgO. Peaks (eV) Nanopowder Macropowder Crystal

4. Conclusion

5.16 4.95

vacancy complexes. In this case, the situation looks similar to that well known for D-bands observed in alkali halides doped with Tl+-ions [53].

6.06 5.75 5.74

6.77 6.60 6.32

7.11

described in [33], where they have observed 0.6 eV blue shift of the intrinsic low-temperature UV emission from nanocrystalline MgO ceramics. It is quite interesting to compare similar effects in semiconductor materials, for example CuCl with lower band gap energy 3.39 eV, but quite similar exciton radius 10 Å [52]. As it is known exciton radius in MgO is 8.0 Å [1]. Note also, that recombination processes and exciton relaxations processes in CuCl are rather different with comparison with MgO. From Fig. 2 we can conclude that in the case of MgO single crystal, the exciton peak is quite well resolved, while in the case of macro and nano powder materials, its exact position is not easy to determine. (d) Others excitation bands (6.32; 6.60; 6.77 and 7.11 eV) can be thus connected with charge-transfer excitations which are more far away from the deformation-induced defects/

The excitation spectra of 2.95 eV emission band observed in nanocrystalline MgO with an average crystallite size (10–15 nm), macrocrystalline samples as well as MgO single crystal have been studied under synchrotron radiation at room temperature. A considerable blue shift of about 0.3 eV in the excitation spectra, was revealed in nanocrystalline samples.

Acknowledgements A.I. Popov was supported by ERAF Project 2010/0272/2DP/ 2.1.1.1.0/10/APIA/VIAA/088 and DAAD fellowship. The work of L. Shirmane has been supported by the European Social Fund within the project « Support for Doctoral Studies at University of Latvia ». V. Pankratov and G. Chikvaidze thank ERAF project Nr.2010/0245/ 2DP/2.1.1.1.0/10/APIA/VIAA/114 for support. A. Lushchik thanks Estonian Research Council – Institutional Research Funding IUT02-26. The experiments at DESY leading to these results have received funding from the European Community’s Seventh

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