DISPERSION OF ZnSe QUANTUM DOTS IN KBr MATRIX. * (Received 26 February 2008) Abstract. Introduction

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1 DISPERSION OF ZnSe QUANTUM DOTS IN KBr MATRIX A. Aksas 1, A. Chelouche 1, D. Djouadi 1*, B. Boudine 2, and O. Halimi 2 1 Laboratory of Genius Environment, University A. Mira of Bejaia, Algeria 2 Laboratory of Crystallography, University Mentouri of Constantine, Algeria * djameldjouadi@yahoo.fr (Received 26 February 28) Abstract In this work we present some results concerning the growth and the structural and optical characterizations of ZnSe nanocrystals dispersed in KBr matrix. The growth of the samples was carried out by the Czochralski method and the incorporation of the ZnSe semiconductor nanocrystallites was realised in the liquid phase of the matrix. The introduction of ZnSe crystallites into the matrix was revealed by the X-ray diffraction. The UV-visible optical absorption measurement showed a displacement of the absorption threshold of the sample towards high energies (short wavelengths) as compared to the ZnSe bulk crystal one. This shift is due to the quantum confinement effect induced by size reduction. Introduction The study of the nanometric size semiconductor properties has become an important research field for a few years. It was possible to control material absorption by concentration and size of introduced semiconductor crystallites in various matrices [1-4]. Semiconductor nanocrystals are a new material class, which, according to their size, are located between the molecule and the bulk crystal. They have interesting properties, being controlled only by the size, the form, and the medium surrounding the nanoparticle [5]. Containing hundreds to thousands of atoms, 2-2 Å in diameter, nanocrystals maintain a crystalline core with the periodicity of the bulk semiconductor. However, the structure of the electronic levels and the resultant optical and electrical properties are greatly modified. Upon reducing the semiconductor size to the nanocrystal regime, a characteristic blue shift of the band gap appears, and a discrete level structure develops, as a result of the quantum size effect. Bulk ZnSe semiconductor has a large gap and, at the nanoscale, it presents very interesting optical properties, which make it a privileged material for optical applications in UVvisible region [6]. Several works were carried out on study of ZnSe nanocrystals using various methods, like Physical Vapor Transport Technique [7] and sol-gel [8-1], and dispersed in various matrices, like polymers [11], glasses [12], and colloids [13, 14]. However, a few works are concerned with dispersion of ZnSe semiconductor in alkali halides. This medium is favourable for study of the optical properties in this UV-visible region. In this work, we have dispersed ZnSe nanocrystals in KBr single crystal of very good optical quality obtained by the Czochralski method. Obtained samples were characterized by optical absorption and X-ray diffraction. Experimental procedure During pulling of the KBr monocrystals by the Czochralski method, very small quantities of a very fine powder of ZnSe are added by sowing systematically and during equal time

2 A. Aksas, A. Chelouche et al. intervals. The grains of nanometer size are absorbed by the matrix and the others are deposited on the bottom of the crucible. We obtain KBr single crystal, where ZnSe crystallites are dispersed. The single crystal, thus obtained, has a cylindrical form (5 cm high and 2 cm in diameter). Then, we cut thin wafers of a few mm thick. The wafers with faces parallel to (1) planes undergo polishing with a silk fabric and distilled water, then the rigorously plane faces are cleaned with ethanol. The optical density was measured at room temperature by the double beam UV-visible 31 Shimadzu spectrophotometer, and the X-ray diffraction (XRD) of ZnSe embedded in KBr matrix was performed by using the K α of copper λ =1.54 Å of the D8 Advanced Siemens Diffractometer. Results and discussion X-ray diffraction of the KBr matrix (Fig. 1a) reveals the presence of (2), (4), and (6) peaks situated at the positions 2 θ = 27.5, 55.65, and 88.96, respectively. These peaks correspond to the KBr cubic structure. In addition, these peaks are harmonics testifying that the obtained wafers are of the single-crystal character ,5 (2) KBr (2) 27,4 ZnSe (cubic) (111) 27,3 Intensity (c/s) ,65 (4) 88,96 (6) θ ( ) a Intensity (c/s) , 25,5 26, 26,5 27, 27,5 28, 2θ ( ) b Figure 1. X-ray diffraction of pure KBr single-crystal (a) and ZnSe crystallites dispersed in KBr single-crystal matrix (b). The dispersion of ZnSe nanocrystals in KBr single-crystal matrix is confirmed by X-ray diffraction characterization (Fig. 1b). A peak at 2θ = 27.4, which corresponds to the (2) plane of KBr cubic structure, and another peak of relatively low intensity with 2θ = 27.3, which corresponds to the (111) plane of ZnSe cubic structure, are observed. According to the data base (ASTM), it can be seen that this peak is shifted by.75 as compared to that of the ZnSe bulk crystal (2θ = ). This shift towards great angles of diffraction is probably due to the contribution of the surface. In nanocrystals the number of atoms at the surface is much greater than the number of atoms in the volume. Indeed, surface relaxation, which results in the reduction in distances between the diffracting planes, leads to diffraction peak shift towards great angles. Moreover, the contribu- 141

3 Moldavian Journal of the Physical Sciences, Vol.7, N2, 28 tion of the crystallite defects (existing even for nanocrystals) consists in plane continuity breaking; thus, X-rays do not diffract any more in the same direction, contributing to the peak position shift on the diagram. To highlight the increase of the crystallite gap and to estimate their size, the samples were characterized by optical absorption in the UV-visible region. As shown in Fig. 2a, a pure KBr matrix practically does not present any absorption in 6-25 nm spectral region. However, it should be noted that the sample presents a light absorption, which is probably explained by the presence of structure defects created by mechanical vibrations of the system during growth step. For the wave length below 21 nm a very important absorption is observed. The optical absorption spectrum of the ZnSe crystallites dispersed in KBr single-crystal wafer is represented in Fig. 2b. The presence of two peaks in the absorption spectrum is observed: the first one, with a relatively weak intensity, centred at 3 nm, and the second one, more intense, centred around nm. These bands correspond to band-to-band transitions between LUMO-HUMO levels in ZnSe crystallites. These peaks enable us to determine the optical gap of the ZnSe crystallites embedded in KBr single-crystal matrix. 3,5 3, Absorption (c/s) 2,5 2, 1,5 1, w ave length (nm ) a 5, 2 Absorption (c/s) 4,5 4, 3,5 3, 2, nm Seconde Derivative (u.a) 3. nm -2 3, 3,2 3,4 3,6 3,8 4, Energy (ev) 3.45 ev 2, 1, wave length (nm) b Figure 2. Absorption spectrum of a pure KBr single-crystal wafer (a) and ZnSe crystallites dispersed in KBr single-crystal matrix (b). 142

4 A. Aksas, A. Chelouche et al. The transition corresponding to the peak located at 3 nm corresponds to direct transition of the Γ type from the highest valence band to the lowest conduction band at v c the 8 Γ 6 Γ point, and the band centred around nm corresponds to at energy E + Δ (Fig. 3 [15]). g Γ v c 7 Γ 6 transition, occurring Figure 3. Discrete energy level structure of small ZnSe cluster [15]. It is necessary to note a shift of the absorption edge towards the great wavelengths compared to the absorption spectrum of the pure KBr wafer. The energy difference between the two positions of these peaks, estimated at.37 ev, is equal to the spin-orbit splitting between light-hole and heavy-hole level positions. This value is of the same order of magnitude as that obtained by Pejova et al. [15]. The presence of the second peak is a result of the discretization of the electronic states observed in very low size crystallites. In this case, the oscillator strength is concentrated on transitions between discrete electronic levels. The two peaks are relatively large, because ZnSe crystallites have different sizes. There is a superposition of peaks corresponding to various sizes. The peak shape informs us about the size distribution [16]. The presence of the absorption peaks and the absorption edge blue shift of our sample can inform us about the presence of ZnSe crystallites in the KBr single-crystal matrix. ZnSe crystallite optical gap determined by the second derivative method of optical absorption spectrum is about 3.45 ev (Fig. 2b, insertion); it is shifted towards high energies as compared to the bulk ZnSe gap (E g = 2.7 ev [11]). This result confirms the incorporation of ZnSe crystallites in KBr single-crystal matrix. The increase of the optical gap value (ΔE g =.75 ev) is a result of the quantum confinement effect observed in crystallites. According to the parabolic band approximations and supposing that ZnSe crystallites are spherical, the ZnSe crystallite average size is estimated to be 4 nm [17]. This result indicates that ZnSe crystallites are in regime of strong confinement (the Bohr radius of ZnSe bulk crystal is about 3.8 nm [18]). Conclusions ZnSe nanocrystals of 4-nm average size were dispersed in KBr single-crystal matrix by the Czochralski method. The obtained samples are of high crystalline quality. X-ray diffraction characterization shows the introduction of ZnSe crystallites into the KBr single-crystal matrix. Optical absorption characterization confirms the incorporation of the ZnSe crystallites into the KBr matrix. 143

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