A study of orange photoluminescence in cubic nanocrystals ZnS: Mn2+

JOURNAL OF SCIENCE OF HNUE Mathematical and Physical Sci., 2012, Vol. 57, No. 7, pp. 51-59 This paper is available online at A STUDY OF ORANGE PHOTOLUMINESCENCE IN CUBIC NANOCRYSTALS ZnS:Mn2+ Nguyen Minh Thuy, Nguyen Hong Quan, Nguyen Manh Nghia Nguyen Thi Van Anh, Tran Minh Thi and Le Thi Hong Hai1 Faculty of Physics, Hanoi National University of Education 1Faculty of Chemistry, Hanoi National University of Education Abstract. Pure ZnS and Mn2+-doped ZnS nanocrystals were synthesized using the co-precipitation method. ZnS:Mn2+ samples were produced by changing the V/Ti ratio from 0.0 at.% to 4.5 at.%. The ZnS and low doped ZnS:Mn2+ samples are single phase with sphalerite (cubic) crystal structure. The average particle size is about 3 - 5 nm. The optical absorption spectra of ZnS sample are blue shifted compared to that from the bulk material, reflecting a nano-size effect. The absorption edge of the ZnS:Mn2+ samples shifts to long wavelength with increasing Mn2+ content. The photoluminescence intensity of ZnS:Mn2+ samples at 588 nm reached a maximum of 1.0 at.% Mn2+. The high Mn doping samples show a red shift in PL peak position and the PL band becomes more broad. The calculated density of states (DOS) by density functional theory (DFT) shows that the Mn2+ doping can make Mn-3d states in the band gap which are close to the top of the valence band. The calculated result of optical absorption also indicated this trend, the absorption edge shifted to lower energy, which is in good agreement with experimental results. Keywords: Semiconductors, photoluminescence, absorption, nanocrystals, emission. 1. Introduction Zinc sulphide (ZnS) is an important II-VI semiconductor material with a wide, direct band gap of 3.65 eV in bulk. It has potential applications in optoelectronic Received February 2, 2011. Accepted Aipril 26, 2012. Physics Subject Classification: 62 44 07 01. Contact Nguyen Minh Thuy, e-mail address: thuynm@hnue.edu.vn 51 N.M. Thuy, N.H. Quan, N.M. Nghia, N.T.V. Anh, T.M. Thi and L.T.H. Hai devices such as blue emitting diodes, electroluminescent devices and photovoltaic cells. Semiconductor nanoparticles have quantum size effects and non-linear optical characteristics as well as high quantum efficiency, making them next-generation materials that can be applied in various optoelectronic devices [1-4]. Manganese-doped materials represent a class of phosphors that have already found their way into applications. The 4T1 → 6A1 transition within the 3d5 configuration of the divalent Mn2+ ion has been studied extensively and its orange-yellow luminescence in ZnS is well documented. This luminescence was also observed in nanocrystalline ZnS:Mn2+ and applications have been suggested [2, 4]. The quantum efficiency and luminescence of nanocrystalline ZnS:Mn2+ are dependent on Mn2+ concentration and its maximum was reached at 0.12 at.% Mn2+ relative to Zn2+ [5] or 1.0 at.% [1] and 2.0 at.% Mn2+ [6]. Those variations in the quantum efficiency of nanoparticle luminescence mentioned above have been shown to be related to different surface areas and different roles of surface defects in the luminescence process. Previous studies have shown that luminescence quantum efficiency is extremely sensitive to the synthesis condition which largely affects the particle size in the sample. With the growth of an additional ZnS shell on ZnS:Mn2+ nanocrystals or when highly monodisperse ZnS nanocrystals were obtained by adding a polymer, surface defects will be greatly reduced [7]. In our study, we have prepared nano-sized ZnS and ZnS:Mn2+ powders using the co-precipitation method. We investigated the effect of the Mn2+ doping concentration; and we are focusing on attempting to obtain a strong orange emission of this material. It has been observed that ZnS nanopowder of 1.0 at.% Mn2+ has maximal PL emission at 590 nm. 2. Content 2.1. Experiments The pure ZnS and Mn2+-doped ZnS nanopowders are prepared by the co- precipitation method from very pure initial chemical solutions as follows (more detail in [9]). (CH3COO)2Zn and (CH3COO)2Mn were added to an aqueous solution. For the undoped sample, only (CH3COO)2Zn was used. After stirring for about 10 minutes, the Na2S solution was injected into the solution. Immediately after the injection of the Na2S solution, a turbid white fluid was obtained. Then this fluid was filtered by a filtering system to obtain the nano particles. These particles were rinsed with distilled water and ethanol, and dried in a vacuum at 500C. The ZnS:Mn2+ samples were produced with Mn2+ calculated at initial nominal concentration in the range of 0.0 to 20 at.%. The real Mn2+ contents in the samples were determined from atomic absorption spectroscopy measurements obtained using a Perkins Elmer 3300 spectometer, which are in the range of 52 A study of orange photoluminescence in cubic nanocrystals ZnS:Mn2+ 0.0 to 4.5 at.%. In the next part we talk about only the real Mn2+ contents in the samples. The structure and crystallinity were characterized by X-ray diffraction (XRD), using a SIEMENS D5005 X-ray diffractometer, and selected area electron diffraction (SAED). The photoluminescence (PL emission and excitation) spectra were recorded by a fluorescence spectrophotometer HP340-LP370 using a 325 nm laser beam at room temperature. The photoluminescence spectral range was 360 - 880 nm. The photoluminescence excitation (PLE) measurements were obtained by exciting the doped nanocrystals in the wavelength region 320 - 550 nm while monitoring the Mn2+ emission or the ZnS emission. The average particle size was measured using a scanning electron microscope (SEM), a transmission electron microscope (TEM) and HRTEM. Absorption measurements were obtained using a Jasco V-670 spectrometer. 2.2. Results and discussion 2.2.1. Structure of ZnS and ZnS:Mn2+ nanoparticles The XRD patterns of ZnS and low doping ZnS:Mn2+ powders (Figure 1) show that the nanoparticles present the zinc blend cubic (sphalerite) phase. The XRD patterns from nanocrystal samples exhibit three broad peaks (111, 220 and 311 of cubic phase).The spectral locations, widths and intensities of the XRD peaks were found to be nearly the same for all samples. Analyzing the peaks (111) of all XRD one can see that the peak position shifts to the 2-theta larger with an increase in Mn2+ concentration. This is evidence that Mn2+ is incorporated into the ZnS host lattice. The average crystallite size calculated using the Scherer’s formula gave a value of about 4 - 5 nm. This result is agreed with the TEM images (Figure 3, right). Figure 1. XRD patterns of the ZnS:Mn2+ samples with Mn2+ concentration 0.0% (a), 0.25% (b), 0.75%(c), 1.0% (d) and 1.5% (e) 53 N.M. Thuy, N.H. Quan, N.M. Nghia, N.T.V. Anh, T.M. Thi and L.T.H. Hai The absorption edges of ZnS and ZnS:Mn2+ are presented in Figure 2. The absorption spectra of ZnS nanocrystals show the blue shift compared to that of the bulk material (3.65eV). The absorption edge of the ZnS:Mn2+ samples shift to long wavelength with increasing Mn2+ content due to the effect of the dopant Mn2+ [8]. Figure 2. Absorption spectra of the ZnS:Mn2+ samples with Mn2+ concentration 0.0% (a), 0.25% (b), 0.75%(c) and 1.5% (d) Figure 3. The TEM (a), HRTEM (b) and SEAD (c) images of ZnS with 1.0%Mn (d) and (e) show the interplanar lattice distances in detail Figure 3 presents the TEM (a), high-resolution HRTEM (b) and SAED (c) images of the ZnS:1.0%Mn2+ powders. The TEM image (Figure 3.a) shows that the diameters of the grains are mainly distributed in the 4 - 6 nm range. The HRTEM image (Figure 3.b) of ZnS:Mn2+ demonstrates the high crystallinity of cubic ZnS nanocrystals and the distances (3.15 A˚) between the adjacent lattices and the interplanar distances of the ZnS (111) plane. Figure 3.d gives a more detailed image of the grainy area that’s in the middle 54 A study of orange photoluminescence in cubic nanocrystals ZnS:Mn2+ of Figure 3.b. Figure 3.e is the analyzed result of the interplanar distances from Figure 3.d using the Gatan software packet, which shows that the distance between the adjacent lattices is 3.15 Angstrom. The selected area electron diffraction (SEAD) pattern (Figure 3.c) shows the three diffraction rings, which are perfectly indexed to the same position as those from ZnS. 2.2.2. Photoluminescence properties of Mn2+-doped ZnS The photoluminescence (PL) measurements were carried out at room temperature using an excitation wavelength of 325 nm. Figure 4 presents the PL spectrum of Mn2+ doped samples. It was found that the PL spectra were generally fitted by the superposition of two Gaussian curves peaked at 465 nm and at 588 nm. The emission peak at ∼ 465 nm is related to the self-activated emission caused by Zn vacancies in the lattice of the ZnS. It is a native PL emission of the host ZnS compound [8]. The luminescence at 465 nm in the Mn2+-doped ZnS might be related to a de-excitation of Mn2+ ion in the ZnS matrix via the 4A1 + 4E1 → 6A1(S) transition, as illustrated in the middle diagram in Figure 5. Figure 4. PL spectra of the ZnS:Mn2+ samples with Mn2+ concentration 0.0% (a), 0.1% (b), 1.0%(c), 1.5%(d) and 4.5% (e) The PL emission at 588 nm for Mn2+-doped samples may be attributed to the 4T1−6A1 (in Td symmetry) or A2 − A1 (in C3v symmetry) transition of the Mn2+ ion. The orange emission of bulk material ZnS:Mn is 583 nm [8], the red-shift of orange emission (at 588 nm) in the samples may come from the quantum confinement effect in nanoparticles which leads to the charge of the crystal field surrounding theMn2+ ions. It is well known that the association of Mn2+ ion and S2− vacancy in ZnS can also lead to C3v symmetry instead of Td symmetry alone [10]. As a result of the mixed contribution, the 55 N.M. Thuy, N.H. Quan, N.M. Nghia, N.T.V. Anh, T.M. Thi and L.T.H. Hai peak of this emission is expected to be relatively broadened as shown in Figure 4e. This 588 nm-peak becomes more pronounced from the samples with middle doping (curves b, c in Figure 4). Figure 5. Electronic levels of d5 free ions and d5 ions in cubic and hexagonal crystal fields We investigated the Mn doping concentration in order to ascertain the maximum orange at 588 nm emission of this material. It is observed that the orange-PL relative intensity considerably increases with increasing Mn2+ concentration and reaches a maximum in a sample of 1.0% Mn2+ (see Figure 4c). By our preparation technique the orange emission reaches a maximum in the doped sample ZnS with 1.0% Mn2+ (corresponding to 10% of the initial nominal concentration of Mn2+). This result agrees with other research [1]. A reversed trend in emission intensity variation existed at higher doping levels (Figure 4d and 4e). The higher intensity at the shorter wavelength (465 nm) and broadened 588 nm PL peak imply an excessive presence of Mn2+ in the sample. When the Mn2+ concentration is more than optimal, some Mn2+ ions are distributed on the surface of the nanoparticles so that the orange emission is quenched [8]. The optimal value for Mn2+ concentration to obtain maximum orange emission varies, depending on the preparation method, from between 0.3 and 2.0 at.% Mn2+ concentrations [1, 2, 7, 8]. We found that by our preparation method the orange emission reaches a maximum in the doped sample with 1.0%Mn2+. 56 A study of orange photoluminescence in cubic nanocrystals ZnS:Mn2+ 2.3. Density of states calculation We used Density functional theory (DFT) [11] to calculate the band structure and density of states (DOS) of ZnS and ZnS:Mn2+. Local density approximations (LDA) and generalized gradient approximation (GGA) [12] were used to describe the exchange-correlation effects. The Monkhorst-Pack scheme k-points grid sampling was used to irreducible Brillouin zone with a kinetic energy cutoff of 380.0 eV. The primitive unit cell of ZnS in the cubic structure and the 2×2×1 supercell model was considered in this work. In the calculations, a 32-atom cubic supercell (16 Zn and 16 S atoms) was used in constructing the Mn2-doped structures. To investigate the influent of the Mn dopant effect we substituted an Mn atom with one Zn atom giving a dopant concentration of 6.25%. To study native impurities we make the S vacancies of 6.25%. The energy bands of ZnS cubic are presented in Figure 6. The DOS of ZnS:6.25%Mn is plotted in Figure 6-right with Fermi energy being 0 eV on the energy axis. Figure 6 shows that the calculated band gap value (2.65eV) of ZnS is smaller than that obtained from experimental data (3.7eV- see Figure 2) due to the well known limitation of DFT, which is similar to that reported in [11]. The DOS of Mn2+-doped ZnS in Figure 7-left show that a new state (of 3d Mn2+) appears in the gap at the top of the valence band tail (VBT) and several states mix with the conduction band edge of cubic ZnS. The results are in good agreement with the absorption calculation (Figure 7 - right) and measurements (see Figure 2), and its similar to that in [6]. Figure 6. Calculated band structure (left) and density of states (right) of cubic ZnS structure The top of valence band is taken as the energy zero point 57 N.M. Thuy, N.H. Quan, N.M. Nghia, N.T.V. Anh, T.M. Thi and L.T.H. Hai Figure 7. Calculated density of states of cubic ZnS:Mn2+ structure (left) and optical absorption band of ZnS and ZnS:6.25%Mn2+ (right) 3. Conclusion We have prepared ZnS and Mn2+-doped ZnS nanocrystals using the co-precipitation method. The structure studies show that the nanocrystals are of sphalerite phase and have a diameter in the range of 3 - 5 nm. The absorption spectra of ZnS nanocrystals are blue shifted compared to that of the bulk material. The absorption spectra of the ZnS:Mn2+ samples show a red shift effect with increasing Mn2+ content. 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