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Chemical Decomposition of CdTe and CdBr2 Dopants in KBr 1
A. Bensouici, 1M. Ayadi, 2M. Iosin, 3G. Damian, 4J. L. Plaza, 2S. Astilean, 1M. Sebais Department of Physics, Mentouri-Constantine University, Constantine 25000, Algeria
[email protected] 2 Molecular Spectroscopy Dept. Faculty of Physics & Nanobiophotonics Center, Babes-Bolyai University, Cluj-Napoca, Romania 3 Departement of Biomedical Physics, Faculty of Physics, Babes-Bolyai University, Cluj-Napoca, Romania 4 Crystal Growth Laboratory (CGL), Departamento de Física de Materiales, Universidad Autónoma de Madrid, Madrid, Spain 1
ABSTRACT Chemical decomposition of CdTe and CdBr2 dopants in KBr single crystals was confirmed by optical absorption, photoluminescence and electron paramagnetic resonance (EPR). An intense UV-emission band observed at 393 nm is assigned to optical response of Cadmium impurities; this intense UV emission band can be useful in light-emitting devices and optoelectronic applications. Keywords : Cadmium, KBr, Photoluminescence, EPR. 1. INTRODUCTION Recently, fabrication of composite consisting of semiconductor nanocrystals have stimulated intense investigation. Semiconductor nanocrystals exhibit modified electronic and optical properties due to quantum confinement and the study of their physical properties and their influence on host matrix seems to be an attractive research profile [1-4]. However, fabrication of nanocrystals in crystalline matrices was less attracted, the reason was limitations encountered during materials handling and methods used for embedding. In such matrices, low concentration of penetration may be confronted due to symmetry and crystalline order which can limit incorporation of solid inclusions comparing to growth in glass matrices. In addition, chemical decomposition may be one of encountered obstacles during crystal growth at high temperature. In our previous works [5,6] where we have elaborate KBr:CdTe and KBr:CdBr2 using Czochralski technique we have encountered chemical decomposition of dopants: CdTe and CdBr2. XRD and optical absorption results (Fig. 1) show cadmium response. The obtained optical absorption spectra exhibit an absorption band near 250 nm that we attribute to cadmium impurities response. In this case, emission study near this absorption band and EPR measurements seems necessary to confirm our results. In this paper we present photoluminescence and EPR measurements results at room temperature. Confirmatory evidence for chemical decomposition of CdTe and CdBr2 during crystal growth at high temperature is obtained by correlating luminescence and electron paramagnetic resonance. 5
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Figure 1. Optical absorption of KBr:CdTe/CdBr2. 2. EXPERIMENTAL Our samples were prepared in Crystal Growth Laboratory at Autonoma de Madrid University by using Czochralski (Cz) technique. CdTe/CdBr2 powder added to KBr (0.5% mol) has been obtained by mechanical grinding. The Cz technique in this work is carried out by a standard Czochralski chamber, more experimental details was mentioned previously [5,6]. Room temperature emission spectra were recorded by using a Jasco LP6500 luminescence spectrometer. The EPR experiments were carried out using Bruker-Biospin EMX spectrometer operating in the X-band (9 – 10 GHz) with a field modulation of 100 kHz (quartz capillary: Wilmad Labglass).
978-1-4799-0683-3/13/$31.00 ©2013 IEEE
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Figure 2. KBr:CdTe/CdBr2 obtained crystals. 3. RESULTS AND DISCUSSION Emission spectra of KBr:CdTe and KBr:CdBr2 at room temperature excited at 280 nm are represented in Fig. 3. The emission band has a maximum at 393 nm and half width 30 nm, we attribute this emission band to optical response of cadmium adsorbed in KBr. We note the total absence of any emission band of pure KBr near 393 nm. Strong excitation band at 265 nm for the 393 nm emission band was observed in KBr:CdTe/CdBr2 (Fig. 4). This result is in agreement with absorption results which confirm that emission at 393 nm is due to cadmium impurities, incorporated in KBr which can be occur if there is a chemical decomposition of the dopants at high temperature during crystal growth. The aim of our previous work [4,5] was the isolation of CdTe nanocrystals in KBr host to study their optical properties in visible region and compare these one with CdBr2 optical properties. However, our aim was truncated due to decomposition of dopants, nevertheless the intense emission of cadmium in UV region may be very interesting for light-emitting devices and photonic applications. 393 nm
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Figure 3. Emission spectra of: (a) KBr pure, (b) KBr:CdTe and (c) KBr:CdBr2. The emission band has a maximum at 393 nm and half-width 30 nm. 80
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Figure 4. Excitation spectrum of emission at 393nm for (a) KBr pure, (b) KBr:CdTe and (c) KBr:CdBr2.
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Figure 5 show the EPR response of KBr:CdTe and KBr:CdBr2 recorded at room temperature, proving the existence of cadmium, exhibiting a signal consisting of a central line at 3261.55 Gauss and 3249.11 Gauss with g factor, respectively, equal to 2.188 and 2.193 is detected, our results are in agreement with S. C. Jain et al. [7] results. According to S. V. Nistorl et al. [8] the impurities enter alkali halides lattice accompanied by an equal number of charge compensating cation vacancies, usually at lattice sites near the IIB impurity (cadmium). The elements of the IIB-group are expected to enter the crystal lattice in their +2 valence state. In the case of the alkali halides, several effects are to be expected: – The segregation coefficient during the growth of such doped crystals is larger than in the case of the doping with monovalent impurities. Consequently a smaller concentration of IIB-group impurities is found in the resulting crystals. – The impurity-charge compensating vacancy pairs have the tendency to aggregate, even at room temperature. More detailed EPR and photoluminescence study of KBr:CdTe/CdBr2 after gamma irradiation is carried out in the near future.
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(b) Figure 5. EPR spectra of: (a) KBr:CdTe, (b)KBr:CdBr2.
4. CONCLUSION In conclusion, the analysis of various studies, optical absorption, EPR and photoluminescence of KBr:CdTe/CdBr2 crystals confirm the chemical decomposition of the CdTe and CdBr2 during crystal growth at high temperature. The intense fluorescence response of cadmium in UV region can be useful in light-emitting devices and optoelectronic applications. ACKNOWLEDGMENTS This work has been supported by the Laboratory of Crystallography of Mentouri-Constantine University (Algeria) under a CNEPRU project: “D00920070075” from the Ministry of High Education and Scientifique research. One of the authors (J.L. Plaza) acknowledges the support under a Ramon y Cajal Project from the Ministry of Education and Science. This work was also partially supported by the following Projects: ESP200609935, Spanish “Ministerio de Educación y Ciencia”; S-0505/MAT-0279, Spanish “Comunidad de Madrid”; FP7-SEC-2007-01, European Commission, and Contract number 14240/00/NL/SH, European Space Agency. REFERENCES [1] M.H. Yükselici, Ç. Allahverdi: Size-dependent photo-induced shift of the first exciton band in CdTe quantum dots in glass prepared by a two-stage heat-treatment process, Journal of Luminescence 128 (2008) 537-545.
[2] M.H. Yükselici, Ç. Allahverdia, H. Athalin: Zinc incorporation into CdTe quantum dots in glass, Materials Chemistry and Physics 119 (2010) 218-221.
[3] Andrey L. Rogach, Nicholas A. Kotov, D.S. Koktysh, A.S. Susha, F. Caruso: II–VI semiconductor nanocrystals in thin films and colloidal Crystals, Colloids and Surfaces A: Physicochemical and Engineering Aspects 202 (2002) 135-144.
[4] Synthesis and optical properties of CdTe nanocrystals with improved optical properties, Indian Journal of Engineering & Materials Sciences 16 (2009) 188-192.
[5] A. Bensouici, J. L. Plaza, E. Diéguez , O. Halimi, B. Boudine, S. Addala, L. Guerbous, M. Sebais: CdTe aggregates in KBr crystalline matrix, Journal of Luminescence 129 (2009) 948-951.
[6] A. Bensouici, J. L. Plaza, E. Dieguez, O. Halimi, L. Guerbous, M. Sebais: Optical and structural characterization of KBr crystals doped cadmium bromide, Journal of Luminescence 130 (2010) 688-691 .
[7] S. C. Jain, S. Radhakrishna: Optical, electrical, and EPR studies of KBr crystals doped with cadmium, Physical Review 172-3 (1968) 972-982.
[8] S. V. Nistorl, D. Schoemaker, Review of the EPR data on ns1 centers in crystals, 16-3/4 (1994) 193-223.
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