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Synthesis and influence of ultrasonic treatment on luminescence of Mn incorporated ZnS nanoparticles
Optical Materials 72 (2017) 533e539
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Synthesis and influence of ultrasonic treatment on luminescence of Mn incorporated ZnS nanoparticles A.-I. Cadis a, L.E. Muresan a, I. Perhaita a, V. Munteanu a, Y. Karabulut b, J. Garcia Guinea c, A. Canimoglu d, M. Ayvacikli b, N. Can b, e, * a
Babes-Bolyai University, Raluca Ripan Institute for Research in Chemistry, Fantanele 30, 400294, Cluj-Napoca, Romania Manisa Celal Bayar University, Faculty of Arts and Sciences, Department of Physics, Muradiye-Manisa, Turkey Museo Nacional Ciencias Naturales, Jose Gutierrez Abascal 2, Madrid, 28006, Spain d Omer Halisdemir University, Faculty of Arts and Sciences, Physics Department, Nigde, Turkey e Physics Department, Jazan University, P.O. Box 114, 45142, Jazan, Saudi Arabia b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 February 2017 Received in revised form 17 June 2017 Accepted 27 June 2017
Manganese (Mn) doping of ZnS phosphors was achieved by precipitation method using different ultrasound (US) maturation times. The structural and luminescence properties of the samples were carried out by means of X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), photoluminescence (PL), and cathodoluminescence (CL). The real amount of manganese incorporated in ZnS lattice was calculated based on ICP-OES results. According with XRD patterns, the phase structure of ZnS:Mn samples is cubic. EDS spectra reveal deviations of the Mn dopant concentration from the target composition. Both 300 K PL and CL emission spectra of the Mn doped ZnS phosphors display intense orange emission at 590 and 600 nm, respectively, which is characteristic emission of Mn ion corresponding to a 4T1/6A1 transition. Both PL and CL spectra confirmed manganese is substitutionally incorporated into the ZnS host as Mn2þ. However, it is suggested that the origin of broad blue emission around 400 nm appeared in CL is due to the radiative recombination at deep level defect states in the ZnS. The ultrasound treatment at first enhances the luminescent intensity by ~3 times in comparison with samples prepared by classical way. This study gives rise to an optimization guideline, which is extremely demanded for the development of new luminescent materials. © 2017 Elsevier B.V. All rights reserved.
Keywords: ZnS Mn XRD SEM Luminescence
1. Introduction As is well known, zinc sulphide (ZnS) is one of the first wideband gap (3.6e3.9 eV) II-VI compound semiconductors discovered to display remarkable properties. The pioneering work on Mndoped ZnS semiconductor nanocrystals at room temperature using a chemical process was initiated Bhargava et al. [1]. The interesting, unique physical properties and strong application potential of some wide bandgap II-VI compound semiconductors have received considerable attention among researchers. ZnS has a high absorption coefficient, high refractive index, and high transmittance in the visible light range of the optical spectrum because of its wide band gap. Particularly, this makes it convenient for use as a host lattice
* Corresponding author. Manisa Celal Bayar University, Faculty of Arts and Sciences, Department of Physics, Muradiye-Manisa, Turkey. E-mail address: [email protected] (N. Can). http://dx.doi.org/10.1016/j.optmat.2017.06.056 0925-3467/© 2017 Elsevier B.V. All rights reserved.
for a large variety of dopants. Therefore, during the last several decades, there are many studies on II-VI semiconductor nanoparticles doped with different metal ions. Examples include CdS:Mn [2e4], CdS:Eu [5], ZnO:Co,Ni [6], ZnO:Al,Ni [7], ZnSe:Mn [8,9], ZnS:Cu [10,11], ZnS:Pb,Cu [12], ZnS:Mn [13,14], which is the focus of this article. ZnS can also be used in light emitting diodes (LEDs), optoelectronic devices and electro luminescent devices [15,16]. Its band gap value is ideal for wavelength optoelectronic applications [17]. Warm white light and WLEDs can be achieved using ZnS based phosphors like ZnS:Ag [18], ZnCds:Ag,Cl [19]. Energy bands of orange-yellow emitting Mn doped ZnS phosphors [20] and luminescence centers can be changed via simple adjusting relative doping concentration [21]. Several methods have been popularly adopted to synthesize ZnS nanoparticles doped with different Mn2þconcentrations.ZnS phosphors activated with Mn at the different concentrations synthesised by hydrothermal method exhibit a 495 nm defect related blue emission and orange emission at 587 nm assigned to the 4T1/6A1 transition within the 3d shell of
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the Mn2þ on Zn2þ sites [22]. Konisthi et al. [23] prepared ZnS:Mn nanocrsystals by hybridizing with polymerized acrylic acid and investigated the PL properties. It was observed that PL enhancement monitored at 580 nm due to d-d transition of Mn2þ ions is larger for ZnS:Mn nanocrystals modified by acrylic acid. Peng et al. [24] studied the structural and optical investigation of Mn doped ZnS nanocrsytals synthesised using a solution based chemical method. PL spectrum in visible region measured at room temperature was recorded. It was seen that the Mn2þ ions doped into ZnS results in the improvement of both structural and luminescence properties. The emission color of Mn doped ZnS phosphor could be useful when the phosphor is stacked on a blue or green emitting phosphor because a white emission could be achieved by mixing the emission colors of two phosphors. As seen above, there is a good correlation between luminescence properties of the synthesised materials and synthesis conditions. With this in mind, we aim to prepare Mn doped ZnS synthesised using precipitation reaction as ZnS is insoluble in water and present results of PL and CL measurements. The novelty of our work brings together the advantages of both ultrasound assisted techniques and simultaneous addition of reagents (SimAdd) with a controlled flow during the precipitation in order to bring improvements on the structural and luminescent characteristics of ZnS:Mn powders. Our previous work showed that the SimAdd technique can be successfully used for the synthesis of ZnS:Mn [25]. This work is focused to investigate the role of sonication time during the precipitation of ZnS:Mn. We found that under US treatment combined with a controlled delivery of the reagents, the luminescent characteristics of phosphor samples were improved. 2. Introduction 2.1. Materials preparation The preparation of manganese doped zinc sulphide phosphors with general formula Zn1-xMnxS (x ¼ 0.08) was performed by precipitation using zinc acetate dihydrate (98e101%, Alfa Aesar), manganese sulfate monohydrate (99%, Alfa Aesar) and sodium sulphide nanohydrate (98%, Alfa Aesar) as starting reagents. The synthesis method consists in the simultaneous addition with a controlled flow (10 ml/min) of equal volumes of reagent solutions of Zn-Mn (1 M) and Na2S (1 M) into a diluted bottom solution of ZnMn (0.01 M) under continuous stirring. The precipitation of ZnS:Mn was carried out at 40 C, under different sonication times (35 kHz ultrasonic waves at 360 kW output power). The resulting ZnS:Mn powders were carefully washed with deionised water and isopropyl alcohol, centrifuged, and finally dried at 80 C.
Porosity Analyzer. Adsorption-desorption isotherms were measured close to the boiling point of nitrogen (77 K). Gas pressure was gradually increased to the saturation value when condensation of the adsorbate (N2) occurs into the powder pores. The chemical analysis of ZnS:Mn powders was carried out from hydrochloric acid by inductively coupled plasma optical emission spectroscopy (ICP-OES) using a PerkinElmer, OPTIMA 2100DVspectrometer. The manganese and zinc detection was made at 257.61 nm (detection limit 1 mg/L) and 206.20 nm (detection limit 0.41 mg/L). Photoluminescence emission (PL) and photoluminescence excitation (PLE) spectra were registered with JASCO FP- 6500 spectrofluorimeter Wavell (PMT R928 photomultiplier; glass filter WG 320- Reichmann Feinoptik). The cathodoluminescence (CL) spectra were recorded using a MONOCL3 Gatan probe with a blue sensitive photomultiplier tube (PMT) equipped with ESEM XL30 microscope. The microscope has a chemical EDS probe. PMT used for UV and visible detection covers the entire wavelength range of 250e850 nm and it is most sensitive in the blue parts of the spectrum. The electron beam with a few nanometers diameter was accelerated using an energy of 25 keV for the CL measurements.
3. Results and discussion 3.1. Crystal structure As described in the experimental section, all samples were prepared by the precipitation method at 313 K. Wide-angle X-ray diffraction (XRD) spectra of all the ZnS:Mn samples synthesised at different ultrasound time were illustrated in Fig. 1. It could be seen that the XRD lines of Mn doped ZnS samples were branded with three main broad diffraction peaks located at ~28.33 , ~47.98 and~57.07 corresponding to (111), (220), and (311) Bragg's reflections of ZnS. All the synthesised ZnS:Mn displays the peak characteristics of cubic phase ZnS (JCPD No. 79-43) that are consistent with literature [14,26]. There were no diffraction peaks observed for the hexagonal ZnS phase, at least within the resolution limit of the diffractometer. The XRD data's were used to determine the particle size of the synthesised ZnS:Mn samples. For this, the Scherrer's formula was used to calculate the grain size of the particles.
2.2. Materials characterisation The X-ray diffraction (XRD) patterns of the prepared Mn doped ZnS samples at various sonication times were evaluated by a Philips PW-1710/00 diffractometer using CuKa (1.5418 Å) radiation. The XRD patterns were recorded by step scanning from 2 to 80 2q and then compared with the XRD PDF2 card files of the Joint Committee on Powder Diffraction Standards using X-powder diffraction software. Size, morphology and composition of the samples were revealed by scanning electron microscopy (SEM) using a Hitachi 8230 SEM electron microscope (JAPAN) and energy dispersive X-ray spectroscopy (EDX) using an Oxford Instruments (UK). Specific surface area and porosity measurements were made on decontaminated samples of any traces of moisture or other adsorbents, using a Micromeritics, TriStar II 3020 - Surface Area and
Fig. 1. XRD patterns of Mn activated ZnS samples at different ultrasonic times.
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D¼
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3.3. Energy dispersive X-ray spectroscopy
kl bcosq
where, D is the grain size (nm), k is correction factor, l is the X-ray wavelength (Cu Ka; 1.5405 Å), b is the full width at half maximum (FWHM in radians, q is the Bragg's angle in degrees. The calculated crystallite sizes of ZnS nanopowders found to be 2.5 nm (no US); 3.4 nm (20 min); 2.4 nm (40 min); 2.7 nm (60 min) and 2.6 nm (80 min) respectively. It is worth noting that the Debye-Scherrer equation for calculating particle size makes sense to use in a few special cases. The value of the constant in the above equation accounts for the shape of the particle. It does not take into consideration the presence of a size distribution or the existence of intrinsic point defects formed in the lattice. Hence, diameter of the grains from FWHM of the peak can overestimate the real value as the larger grains contribute more to the diffraction pattern than smaller grain in terms of intensity.
Energy dispersive X-ray spectroscopy diagram (EDX) was also obtained to calculate the elemental compositions of the ZnS samples. As an example, Fig. 3 depicts the EDX spectrum of ZnS:Mn nanoparticles after 20 min ultrasonic treatment. It is clear the presence of Mn atom along with the very strong peaks for zinc and sulfur suggesting the incorporation of Mn into the ZnS host lattice. Based on EDX results, the atomic composition of no US sample (% Zn ¼ 50.97, %S ¼ 47.86, % Mn ¼ 1.16) and 20 min US sample (% Zn ¼ 51.70, %S ¼ 47.07 and % Mn ¼ 1.23) reveals that the incorporation of manganese in the ZnS lattice takes place in smaller amounts than the theoretical ones (% Zn ¼ 46.0, %S ¼ 50.00 and % Mn ¼ 4.00). This is expected because the precipitation takes place in a multi-metal containing medium and competition between precipitation of Zn-Mn ions occurs.
3.4. ICP-OES analyses 3.2. Morphological study The size and morphology of samples were investigated by SEM and TEM and are shown in Fig. 2. SEM images reveal that very small ZnS particles are present in the as-prepared powders with rounded particles held together by a porous irregular network and some plates have been mostly formed by the aggregation. The TEM micrographs for no US and 60 min US samples (Fig. 2 c,d) are very similar. It was found that ZnS nanoparticles are spherical with diameters up to 30 nm that tends to agglomerate due to the high surface making difficult to identify of a single particle.
As a confirmation for the incorporation degree of manganese in ZnS powders, ICP-OES measurements were performed. It was found that the real concentration of the manganese is lower than the theoretical values and varies from 0.65 mol % up to 1.8 mol % leading to ZnS:Mn phosphors with 1.6 mol% (no US), 1.8 mol % ( 20 min US), 1.7 mol % (40 min US), 0.65 mol % ( 60 min US) and 1.4 mol % (80 min US). This behaviour can be explained due to selective precipitation of metallic ions (Zn2þ, Mn2þ). Lewis [27] presents a comprehensive overview regarding the mechanism of metal sulphides precipitation. It seems that the precipitation rate of the two metallic sulfides is different as a consequence of the large
Fig. 2. SEM and TEM images of ZnS obtained without (a, c) and with (b, d) US treatment for 60 min.
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Fig. 3. EDS spectra of ZnS:Mn after US treatment for 20 min.
difference between their solubility products namely 2.5 1022 for cubic ZnS form and 2.5 1010 for MnS amorphous form [28]. Zinc ions precipitate first, followed by the manganese ions which precipitate onto the zinc sulfide particles to produce ZnS:Mn. Moreover, increase of the acidity during washing the samples may lead to redissolving of a certain amount of Zn-Mn sulfide.
3.5. Physical characterisation Surface area and porosity measurements were performed to demonstrate the changes of the ZnS powders surface states during the synthesis process. Adsorption/desorption isotherms for samples prepared without and with US treatment are shown in Fig. 4a. The isotherms are all similar in shape to each other and narrow hysteresis indicating that samples present small pore volumes. Isotherm shape of phosphors is similar to type III that are usually meet in the materials with low porosity and weak adsorbateeadsorbent interactions [28,29]. The hysteresis loops are identified to be type H3 which indicate aggregates of plate-like particles giving rise to slit-shaped pores [29]. The quantity of adsorbed nitrogen for sample prepared without US treatment is slightly higher compared with sonicated phosphors, confirmed by pore volume measurements (0.57 cm3/g no US and 0.51 cm3/g for US-60min). Moreover, the sonication causes the decrease of the specific surface area of phosphors from 104.3 m2/g (no US) to 87.4 m2/g (60 min US). BJH method described by Baret, Joyner, and Halenda was employed to determine surface area and volume of the meso-and macropores using adsorption and desorption techniques. Fig. 4b exhibits the pore volume distributions of samples. Sample prepared in the absence of US shows a distribution curve which covers abroad range that includes both meso-and macropores with various maxima situated at 4 nm, 22 nm, 32 nm, 54 nm and 78 nm, confirming the heterogeneity of the powder. The aspect of the pore volume curve for ZnS powder prepared with US is smoother and becomes bimodal with pore volume concentrated at 4 nm and 67 nm respectively. In other words, the sonication treatment improves the homogeneity of the powders. The pore size strongly depends on the sonication time as can be seen in Fig. 4c. Powder with largest pore size are obtained when precipitation take place under US for 20 min.
3.6. Luminescence properties There are different ways of exciting the luminescence of a solid and each approach has its own advantages and limitations. The most common ones consist of irradiating this solid with a light of convenient wavelength (Photoluminescence, PL) which can wavelength selective in the choice of site that is stimulated (e.g. nanoparticles) or varying the electron energy (Cathodoluminescence, CL). For surfaces, there is partial control of excitation depth by PL with strongly absorbed light or varying the electron energy in CL. Here the PL properties are presented both to understand the fundamental properties of the particles and for comparison to the CL data. It is expected that dopant materials play a significant role for the host in specific applications, which can effectively impose a material's microstructure, properties and also function. Room temperature PL emission spectra of Mn doped ZnS powders prepared without and with US treatment at different ultrasonic time under the UV excitation are shown in Fig. 5a. The transition metallic ions create excellent luminescence centers in phosphors lattice which give rise to characteristic emissions due to different d-d transitions. In this study, Mn2þ derived from MnSO4$H2O (from starting chemical reagent) can be substituted into ZnS lattice. As can be seen from Fig. 5a the broad emission band extending in visible region from 500 nm to 700 nm with maxima located at 595 nm (intense orange emission) at room temperature does not shift with increasing ultrasound time. Under the ultrasonic treatment Mn2þ ions quickly diffuse into the ZnS lattice. As seen in Fig. 5a, there is no steady increase in the peak intensity of the ZnS:Mn with increase in ultrasonic time. Intensity of this peak reaches at maximum intensity at ultrasonic time of 60 min. On the other hand, there is a slight decrease in the emission intensity over the entire spectral range at ultrasonic time of 10 min compared with no ultrasonic process. Ultrasonic treatment also gives rise to dislocation motion and leads them to emerge on the surface. In other words, a longer ultrasonic treatment time may induce more defects, causing the increase of trapping or nonradiative recombination centers. In this case, emission intensity should be decreased with increase US time but the pattern in Fig. 5a does not follow this. Possible explanation for such a pattern should take in consideration several aspects namely: the real amount of manganese incorporated in the ZnS lattice; quenching
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Fig. 4. Adsorptionedesorption isotherms (a); pore volume distribution curves (b) and effect of US maturation time on pore size (c) of ZnS samples prepared with or without US.
centers; disorders in crystalline arrangement of ZnS. Generally, the emission intensity of manganese increases with the increase of doping level in the lattice [31]. Fig. 5b reveals the variation of PL intensity for 590 nm band with Mn concentration of samples after 60 min of US treatment. The real amounts of manganese (from ICP-OESresults) are also given for comparison with theoretical values. When the manganese concentration exceeds the optimal
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values (experimental 0.4÷0.65 mol% or theoretical 4÷8 mol %) the luminescence decreases due to the so called “concentration quenching” (Fig. 5b). The process occurs due the emergence of mutual interactions between too many Mn emission centers that will cause the decrease of radiant transitions. This is added the formation of black MnS phase that will absorb a part from the emitted light. In order to have a better view of PL evolution, we have prepared additional samples for every 10 min US treatment. Moreover, we repeated the synthesis for several times. We found the same evolution of increasing of emission up to 60 min followed by a decrease (see Fig. 5c). Fig. 5c presents the effect of US maturation time on emission intensity at 595 nm of ZnS: Mn samples. As shown in Fig. 5c, the real Mn concentration decrease from 1.57% (10min US) to 0.33% (60 min US) so that Mn-Mn interactions decrease in favour of the radiant transitions enhancing the luminescence. The lowest manganese concentration gives the highest luminescence, due to the optimal ratio between luminescent centers, quenching centers and structural ZnS disorder. A prolonged US treatment (>60 min), amplifies the structural disorders, disturbing the above-mentioned ratio and leading to decrease in luminescence. The PL emission peaked at 590 nm is the characteristic emission of Mn2þ ion corresponding to a 4T1/6A1 transition. This result is fully consistent with the results of previous reports [27e29]. Zn2þ ions (0.74 nm) match better with the Mn2þ ions (0.83 nm). Therefore Mn2þ ions are prior to Zn2þ ions sites, which will reduce the Zn2þ vacancies. The presence of Mn2þ emission at 590 nm indicated that the Mn2þ ions occupy the Zn2þ host lattice, nicely matching those reported previously in the literature [14]. It is worth viewing PL signals in terms of various excitation wavelengths. This is shown in Fig. 6 as an isometric plot of signal against wavelength and excitation wavelength. Intensity of emission at 590 nm is remarkably changed with change in excitation wavelengths. The orange emission located at 590 nm is clearly observed with increase in excitation wavelength up to 360 nm. It is conceivable that the emission colors observed under excitation with light of different wavelengths are different due to the different emission characteristics. Looking at CL data, Fig. 7 depicts the CL emission spectra from the ZnS:Mn powder. The primary electrons get scattered throughout the host crystal and eventually the electrons will transfer only a fraction of its energy to the Mn2þ. We observed a broad emission band located 600 nm and a low intensity peak at about 400 nm as shown in Fig. 7. Ultrasonic treatment at 20 min does not affect CL spectrum but there is a significant increase in CL intensity at 40 min compared to no US treatment and again a sharp decrease at 80 min. Large red shift and linewidth broadening have been observed when compared CL spectrum to the PL spectrum of the same sample. Full width of maximum (FWHM) of the peak is 94 nm, whereas PL one is 65 nm. The broadness of the emission peak is assigned to emission from more than one energy level. The redshift of the CL decreases as the electron voltage was increased (not shown here). We speculate that charging of the nanoparticles during the electron beam excitation responsible for the redshift and broadening. It is well known that point defects (i.e. vacancies, interstitials, and antisites) create new energy levels in the forbidden band gap of semiconductor materials. The CL spectra exhibit a broad blue emission band located at about400 nm, which is attributed to radiative recombination involving defect sites at the surface of ZnS host material [30] while the tail of CL spectrum extends all the way up to 500 nm, due to the presence of sulfur vacancies in the lattice [31]. As clearly seen in PL or CL, Mn doped ZnS exhibits dominating orange emission related to Mn2þ d-d transition. Normally, when
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Fig. 5. (a) PL emission spectra of the ZnS: Mn under different US time; (b) effect of manganese concentration on emission intensity of ZnS: Mn samples sonicated for 60min; (c) effect of US maturation time on emission intensity appeared at 590 nm of ZnS: Mn samples. Each point is an average value of three experiments and the error bar shows the standard deviation.
Fig. 6. Three-dimensional PL spectra depending on the excitation wavelength from 260 nm to 440 nm for ZnS:Mn2þ.
the electrons of the ground state taking part in the luminescence process are excited under different excitation sources they transfer to the higher excited state. After excitation, electrons may return to the ground state via two main relaxation mechanisms. One is the relaxation process through surface defects such as donors and acceptors. This relaxation will result in a non-radiative recombination. The other emission is due to an intra-configurational 3d5
Fig. 7. Comparison of CL emission spectra of the ZnS nanoparticles with Mn2þ without and with ultrasonic (US) treatment. The inset shows magnified view of spectral range of 350 nme450 nm.
transition on the Mn2þ ion located within the band gap of ZnS host material. This 4T1/6A1 energy transition will give rise to an orange luminescence. The based on the results, the energy level scheme of different electronic transitions taking place in Mn doped ZnS is illustrated in Fig. 8.
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Fig. 8. Schematic energy level diagram for ZnS:Mn2þ.
4. Conclusions Mn doped ZnS cubic phase nanoparticles were prepared by precipitation method under different ultrasound maturation time. ZnS:Mn powders consist of rounded particles of about 20 nm kept together in agglomerates. Presence of Mn in ZnS host lattice is confirmed by EDX and ICP-OES, but the incorporated amount is around 4 time smaller than theoretical ones due to the differences in the solubility products of metallic ions. Photoluminescence (PL) and Cathodoluminescence(CL) spectra confirm that our samples are doped with Mn2þ due to characteristic orange emission. The PL intensity increases with Mn amount up to optimal concentration (0.4e0.65 mol%) followed by a decrease due to quenching processes. Ultrasonic treatment exhibits strong influences on PL and CL properties. In general, PL performance increases with a longer ultrasonic time (i.e 60 min) but decrease sharply at ultrasonic time of 80 min. CL performance also follows a pattern similar to PL at ultrasonic time of 80 min. The PL and CL emissions located at 590 nm and 600 nm, respectively, are characteristic emissions of Mn2þ ion which can be assigned to a 4T1/6A1 transition. Blue emission appeared about 400 nm in the CL data is very small and broad and originates from the recombination involving defect states in the ZnS host lattice. On the basis of the promising findings illustrated here, further research to investigate luminescence properties of the samples based on parameters such as laser power, electron beam energy is still continuing and will be presented in our future papers. Acknowledgments This work was supported by a grant of the Romanian National Authority for Scientific Research, CNCS e UEFISCDI, project number PN-II-RU-TE-2014-4-1391. References [1] R.N. Bhargava, D. Gallagher, T. Welker, Doped nanocrystals of semiconductors - a new class of luminescent materials, J. Lumin. 60 (1994) 275e280. [2] Z.K. Heiba, M.B. Mohamed, N.G. Imam, Biphasic quantum dots of cubic and hexagonal Mn doped CdS; necessity of Rietveld analysis, J. Alloys Compd. 618 (2015) 280e286. [3] M.J. Tanaka, Photoluminescence properties of Mn2þ-doped IIeVI
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Synthesis and influence of ultrasonic treatment on luminescence of Mn incorporated ZnS nanoparticles A.-I. Cadis a, L.E. Muresan a, I. Perhaita a, V. Munteanu a, Y. Karabulut b, J. Garcia Guinea c, A. Canimoglu d, M. Ayvacikli b, N. Can b, e, * a
Babes-Bolyai University, Raluca Ripan Institute for Research in Chemistry, Fantanele 30, 400294, Cluj-Napoca, Romania Manisa Celal Bayar University, Faculty of Arts and Sciences, Department of Physics, Muradiye-Manisa, Turkey Museo Nacional Ciencias Naturales, Jose Gutierrez Abascal 2, Madrid, 28006, Spain d Omer Halisdemir University, Faculty of Arts and Sciences, Physics Department, Nigde, Turkey e Physics Department, Jazan University, P.O. Box 114, 45142, Jazan, Saudi Arabia b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 February 2017 Received in revised form 17 June 2017 Accepted 27 June 2017
Manganese (Mn) doping of ZnS phosphors was achieved by precipitation method using different ultrasound (US) maturation times. The structural and luminescence properties of the samples were carried out by means of X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), photoluminescence (PL), and cathodoluminescence (CL). The real amount of manganese incorporated in ZnS lattice was calculated based on ICP-OES results. According with XRD patterns, the phase structure of ZnS:Mn samples is cubic. EDS spectra reveal deviations of the Mn dopant concentration from the target composition. Both 300 K PL and CL emission spectra of the Mn doped ZnS phosphors display intense orange emission at 590 and 600 nm, respectively, which is characteristic emission of Mn ion corresponding to a 4T1/6A1 transition. Both PL and CL spectra confirmed manganese is substitutionally incorporated into the ZnS host as Mn2þ. However, it is suggested that the origin of broad blue emission around 400 nm appeared in CL is due to the radiative recombination at deep level defect states in the ZnS. The ultrasound treatment at first enhances the luminescent intensity by ~3 times in comparison with samples prepared by classical way. This study gives rise to an optimization guideline, which is extremely demanded for the development of new luminescent materials. © 2017 Elsevier B.V. All rights reserved.
Keywords: ZnS Mn XRD SEM Luminescence
1. Introduction As is well known, zinc sulphide (ZnS) is one of the first wideband gap (3.6e3.9 eV) II-VI compound semiconductors discovered to display remarkable properties. The pioneering work on Mndoped ZnS semiconductor nanocrystals at room temperature using a chemical process was initiated Bhargava et al. [1]. The interesting, unique physical properties and strong application potential of some wide bandgap II-VI compound semiconductors have received considerable attention among researchers. ZnS has a high absorption coefficient, high refractive index, and high transmittance in the visible light range of the optical spectrum because of its wide band gap. Particularly, this makes it convenient for use as a host lattice
* Corresponding author. Manisa Celal Bayar University, Faculty of Arts and Sciences, Department of Physics, Muradiye-Manisa, Turkey. E-mail address: [email protected] (N. Can). http://dx.doi.org/10.1016/j.optmat.2017.06.056 0925-3467/© 2017 Elsevier B.V. All rights reserved.
for a large variety of dopants. Therefore, during the last several decades, there are many studies on II-VI semiconductor nanoparticles doped with different metal ions. Examples include CdS:Mn [2e4], CdS:Eu [5], ZnO:Co,Ni [6], ZnO:Al,Ni [7], ZnSe:Mn [8,9], ZnS:Cu [10,11], ZnS:Pb,Cu [12], ZnS:Mn [13,14], which is the focus of this article. ZnS can also be used in light emitting diodes (LEDs), optoelectronic devices and electro luminescent devices [15,16]. Its band gap value is ideal for wavelength optoelectronic applications [17]. Warm white light and WLEDs can be achieved using ZnS based phosphors like ZnS:Ag [18], ZnCds:Ag,Cl [19]. Energy bands of orange-yellow emitting Mn doped ZnS phosphors [20] and luminescence centers can be changed via simple adjusting relative doping concentration [21]. Several methods have been popularly adopted to synthesize ZnS nanoparticles doped with different Mn2þconcentrations.ZnS phosphors activated with Mn at the different concentrations synthesised by hydrothermal method exhibit a 495 nm defect related blue emission and orange emission at 587 nm assigned to the 4T1/6A1 transition within the 3d shell of
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the Mn2þ on Zn2þ sites [22]. Konisthi et al. [23] prepared ZnS:Mn nanocrsystals by hybridizing with polymerized acrylic acid and investigated the PL properties. It was observed that PL enhancement monitored at 580 nm due to d-d transition of Mn2þ ions is larger for ZnS:Mn nanocrystals modified by acrylic acid. Peng et al. [24] studied the structural and optical investigation of Mn doped ZnS nanocrsytals synthesised using a solution based chemical method. PL spectrum in visible region measured at room temperature was recorded. It was seen that the Mn2þ ions doped into ZnS results in the improvement of both structural and luminescence properties. The emission color of Mn doped ZnS phosphor could be useful when the phosphor is stacked on a blue or green emitting phosphor because a white emission could be achieved by mixing the emission colors of two phosphors. As seen above, there is a good correlation between luminescence properties of the synthesised materials and synthesis conditions. With this in mind, we aim to prepare Mn doped ZnS synthesised using precipitation reaction as ZnS is insoluble in water and present results of PL and CL measurements. The novelty of our work brings together the advantages of both ultrasound assisted techniques and simultaneous addition of reagents (SimAdd) with a controlled flow during the precipitation in order to bring improvements on the structural and luminescent characteristics of ZnS:Mn powders. Our previous work showed that the SimAdd technique can be successfully used for the synthesis of ZnS:Mn [25]. This work is focused to investigate the role of sonication time during the precipitation of ZnS:Mn. We found that under US treatment combined with a controlled delivery of the reagents, the luminescent characteristics of phosphor samples were improved. 2. Introduction 2.1. Materials preparation The preparation of manganese doped zinc sulphide phosphors with general formula Zn1-xMnxS (x ¼ 0.08) was performed by precipitation using zinc acetate dihydrate (98e101%, Alfa Aesar), manganese sulfate monohydrate (99%, Alfa Aesar) and sodium sulphide nanohydrate (98%, Alfa Aesar) as starting reagents. The synthesis method consists in the simultaneous addition with a controlled flow (10 ml/min) of equal volumes of reagent solutions of Zn-Mn (1 M) and Na2S (1 M) into a diluted bottom solution of ZnMn (0.01 M) under continuous stirring. The precipitation of ZnS:Mn was carried out at 40 C, under different sonication times (35 kHz ultrasonic waves at 360 kW output power). The resulting ZnS:Mn powders were carefully washed with deionised water and isopropyl alcohol, centrifuged, and finally dried at 80 C.
Porosity Analyzer. Adsorption-desorption isotherms were measured close to the boiling point of nitrogen (77 K). Gas pressure was gradually increased to the saturation value when condensation of the adsorbate (N2) occurs into the powder pores. The chemical analysis of ZnS:Mn powders was carried out from hydrochloric acid by inductively coupled plasma optical emission spectroscopy (ICP-OES) using a PerkinElmer, OPTIMA 2100DVspectrometer. The manganese and zinc detection was made at 257.61 nm (detection limit 1 mg/L) and 206.20 nm (detection limit 0.41 mg/L). Photoluminescence emission (PL) and photoluminescence excitation (PLE) spectra were registered with JASCO FP- 6500 spectrofluorimeter Wavell (PMT R928 photomultiplier; glass filter WG 320- Reichmann Feinoptik). The cathodoluminescence (CL) spectra were recorded using a MONOCL3 Gatan probe with a blue sensitive photomultiplier tube (PMT) equipped with ESEM XL30 microscope. The microscope has a chemical EDS probe. PMT used for UV and visible detection covers the entire wavelength range of 250e850 nm and it is most sensitive in the blue parts of the spectrum. The electron beam with a few nanometers diameter was accelerated using an energy of 25 keV for the CL measurements.
3. Results and discussion 3.1. Crystal structure As described in the experimental section, all samples were prepared by the precipitation method at 313 K. Wide-angle X-ray diffraction (XRD) spectra of all the ZnS:Mn samples synthesised at different ultrasound time were illustrated in Fig. 1. It could be seen that the XRD lines of Mn doped ZnS samples were branded with three main broad diffraction peaks located at ~28.33 , ~47.98 and~57.07 corresponding to (111), (220), and (311) Bragg's reflections of ZnS. All the synthesised ZnS:Mn displays the peak characteristics of cubic phase ZnS (JCPD No. 79-43) that are consistent with literature [14,26]. There were no diffraction peaks observed for the hexagonal ZnS phase, at least within the resolution limit of the diffractometer. The XRD data's were used to determine the particle size of the synthesised ZnS:Mn samples. For this, the Scherrer's formula was used to calculate the grain size of the particles.
2.2. Materials characterisation The X-ray diffraction (XRD) patterns of the prepared Mn doped ZnS samples at various sonication times were evaluated by a Philips PW-1710/00 diffractometer using CuKa (1.5418 Å) radiation. The XRD patterns were recorded by step scanning from 2 to 80 2q and then compared with the XRD PDF2 card files of the Joint Committee on Powder Diffraction Standards using X-powder diffraction software. Size, morphology and composition of the samples were revealed by scanning electron microscopy (SEM) using a Hitachi 8230 SEM electron microscope (JAPAN) and energy dispersive X-ray spectroscopy (EDX) using an Oxford Instruments (UK). Specific surface area and porosity measurements were made on decontaminated samples of any traces of moisture or other adsorbents, using a Micromeritics, TriStar II 3020 - Surface Area and
Fig. 1. XRD patterns of Mn activated ZnS samples at different ultrasonic times.
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D¼
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3.3. Energy dispersive X-ray spectroscopy
kl bcosq
where, D is the grain size (nm), k is correction factor, l is the X-ray wavelength (Cu Ka; 1.5405 Å), b is the full width at half maximum (FWHM in radians, q is the Bragg's angle in degrees. The calculated crystallite sizes of ZnS nanopowders found to be 2.5 nm (no US); 3.4 nm (20 min); 2.4 nm (40 min); 2.7 nm (60 min) and 2.6 nm (80 min) respectively. It is worth noting that the Debye-Scherrer equation for calculating particle size makes sense to use in a few special cases. The value of the constant in the above equation accounts for the shape of the particle. It does not take into consideration the presence of a size distribution or the existence of intrinsic point defects formed in the lattice. Hence, diameter of the grains from FWHM of the peak can overestimate the real value as the larger grains contribute more to the diffraction pattern than smaller grain in terms of intensity.
Energy dispersive X-ray spectroscopy diagram (EDX) was also obtained to calculate the elemental compositions of the ZnS samples. As an example, Fig. 3 depicts the EDX spectrum of ZnS:Mn nanoparticles after 20 min ultrasonic treatment. It is clear the presence of Mn atom along with the very strong peaks for zinc and sulfur suggesting the incorporation of Mn into the ZnS host lattice. Based on EDX results, the atomic composition of no US sample (% Zn ¼ 50.97, %S ¼ 47.86, % Mn ¼ 1.16) and 20 min US sample (% Zn ¼ 51.70, %S ¼ 47.07 and % Mn ¼ 1.23) reveals that the incorporation of manganese in the ZnS lattice takes place in smaller amounts than the theoretical ones (% Zn ¼ 46.0, %S ¼ 50.00 and % Mn ¼ 4.00). This is expected because the precipitation takes place in a multi-metal containing medium and competition between precipitation of Zn-Mn ions occurs.
3.4. ICP-OES analyses 3.2. Morphological study The size and morphology of samples were investigated by SEM and TEM and are shown in Fig. 2. SEM images reveal that very small ZnS particles are present in the as-prepared powders with rounded particles held together by a porous irregular network and some plates have been mostly formed by the aggregation. The TEM micrographs for no US and 60 min US samples (Fig. 2 c,d) are very similar. It was found that ZnS nanoparticles are spherical with diameters up to 30 nm that tends to agglomerate due to the high surface making difficult to identify of a single particle.
As a confirmation for the incorporation degree of manganese in ZnS powders, ICP-OES measurements were performed. It was found that the real concentration of the manganese is lower than the theoretical values and varies from 0.65 mol % up to 1.8 mol % leading to ZnS:Mn phosphors with 1.6 mol% (no US), 1.8 mol % ( 20 min US), 1.7 mol % (40 min US), 0.65 mol % ( 60 min US) and 1.4 mol % (80 min US). This behaviour can be explained due to selective precipitation of metallic ions (Zn2þ, Mn2þ). Lewis [27] presents a comprehensive overview regarding the mechanism of metal sulphides precipitation. It seems that the precipitation rate of the two metallic sulfides is different as a consequence of the large
Fig. 2. SEM and TEM images of ZnS obtained without (a, c) and with (b, d) US treatment for 60 min.
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Fig. 3. EDS spectra of ZnS:Mn after US treatment for 20 min.
difference between their solubility products namely 2.5 1022 for cubic ZnS form and 2.5 1010 for MnS amorphous form [28]. Zinc ions precipitate first, followed by the manganese ions which precipitate onto the zinc sulfide particles to produce ZnS:Mn. Moreover, increase of the acidity during washing the samples may lead to redissolving of a certain amount of Zn-Mn sulfide.
3.5. Physical characterisation Surface area and porosity measurements were performed to demonstrate the changes of the ZnS powders surface states during the synthesis process. Adsorption/desorption isotherms for samples prepared without and with US treatment are shown in Fig. 4a. The isotherms are all similar in shape to each other and narrow hysteresis indicating that samples present small pore volumes. Isotherm shape of phosphors is similar to type III that are usually meet in the materials with low porosity and weak adsorbateeadsorbent interactions [28,29]. The hysteresis loops are identified to be type H3 which indicate aggregates of plate-like particles giving rise to slit-shaped pores [29]. The quantity of adsorbed nitrogen for sample prepared without US treatment is slightly higher compared with sonicated phosphors, confirmed by pore volume measurements (0.57 cm3/g no US and 0.51 cm3/g for US-60min). Moreover, the sonication causes the decrease of the specific surface area of phosphors from 104.3 m2/g (no US) to 87.4 m2/g (60 min US). BJH method described by Baret, Joyner, and Halenda was employed to determine surface area and volume of the meso-and macropores using adsorption and desorption techniques. Fig. 4b exhibits the pore volume distributions of samples. Sample prepared in the absence of US shows a distribution curve which covers abroad range that includes both meso-and macropores with various maxima situated at 4 nm, 22 nm, 32 nm, 54 nm and 78 nm, confirming the heterogeneity of the powder. The aspect of the pore volume curve for ZnS powder prepared with US is smoother and becomes bimodal with pore volume concentrated at 4 nm and 67 nm respectively. In other words, the sonication treatment improves the homogeneity of the powders. The pore size strongly depends on the sonication time as can be seen in Fig. 4c. Powder with largest pore size are obtained when precipitation take place under US for 20 min.
3.6. Luminescence properties There are different ways of exciting the luminescence of a solid and each approach has its own advantages and limitations. The most common ones consist of irradiating this solid with a light of convenient wavelength (Photoluminescence, PL) which can wavelength selective in the choice of site that is stimulated (e.g. nanoparticles) or varying the electron energy (Cathodoluminescence, CL). For surfaces, there is partial control of excitation depth by PL with strongly absorbed light or varying the electron energy in CL. Here the PL properties are presented both to understand the fundamental properties of the particles and for comparison to the CL data. It is expected that dopant materials play a significant role for the host in specific applications, which can effectively impose a material's microstructure, properties and also function. Room temperature PL emission spectra of Mn doped ZnS powders prepared without and with US treatment at different ultrasonic time under the UV excitation are shown in Fig. 5a. The transition metallic ions create excellent luminescence centers in phosphors lattice which give rise to characteristic emissions due to different d-d transitions. In this study, Mn2þ derived from MnSO4$H2O (from starting chemical reagent) can be substituted into ZnS lattice. As can be seen from Fig. 5a the broad emission band extending in visible region from 500 nm to 700 nm with maxima located at 595 nm (intense orange emission) at room temperature does not shift with increasing ultrasound time. Under the ultrasonic treatment Mn2þ ions quickly diffuse into the ZnS lattice. As seen in Fig. 5a, there is no steady increase in the peak intensity of the ZnS:Mn with increase in ultrasonic time. Intensity of this peak reaches at maximum intensity at ultrasonic time of 60 min. On the other hand, there is a slight decrease in the emission intensity over the entire spectral range at ultrasonic time of 10 min compared with no ultrasonic process. Ultrasonic treatment also gives rise to dislocation motion and leads them to emerge on the surface. In other words, a longer ultrasonic treatment time may induce more defects, causing the increase of trapping or nonradiative recombination centers. In this case, emission intensity should be decreased with increase US time but the pattern in Fig. 5a does not follow this. Possible explanation for such a pattern should take in consideration several aspects namely: the real amount of manganese incorporated in the ZnS lattice; quenching
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Fig. 4. Adsorptionedesorption isotherms (a); pore volume distribution curves (b) and effect of US maturation time on pore size (c) of ZnS samples prepared with or without US.
centers; disorders in crystalline arrangement of ZnS. Generally, the emission intensity of manganese increases with the increase of doping level in the lattice [31]. Fig. 5b reveals the variation of PL intensity for 590 nm band with Mn concentration of samples after 60 min of US treatment. The real amounts of manganese (from ICP-OESresults) are also given for comparison with theoretical values. When the manganese concentration exceeds the optimal
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values (experimental 0.4÷0.65 mol% or theoretical 4÷8 mol %) the luminescence decreases due to the so called “concentration quenching” (Fig. 5b). The process occurs due the emergence of mutual interactions between too many Mn emission centers that will cause the decrease of radiant transitions. This is added the formation of black MnS phase that will absorb a part from the emitted light. In order to have a better view of PL evolution, we have prepared additional samples for every 10 min US treatment. Moreover, we repeated the synthesis for several times. We found the same evolution of increasing of emission up to 60 min followed by a decrease (see Fig. 5c). Fig. 5c presents the effect of US maturation time on emission intensity at 595 nm of ZnS: Mn samples. As shown in Fig. 5c, the real Mn concentration decrease from 1.57% (10min US) to 0.33% (60 min US) so that Mn-Mn interactions decrease in favour of the radiant transitions enhancing the luminescence. The lowest manganese concentration gives the highest luminescence, due to the optimal ratio between luminescent centers, quenching centers and structural ZnS disorder. A prolonged US treatment (>60 min), amplifies the structural disorders, disturbing the above-mentioned ratio and leading to decrease in luminescence. The PL emission peaked at 590 nm is the characteristic emission of Mn2þ ion corresponding to a 4T1/6A1 transition. This result is fully consistent with the results of previous reports [27e29]. Zn2þ ions (0.74 nm) match better with the Mn2þ ions (0.83 nm). Therefore Mn2þ ions are prior to Zn2þ ions sites, which will reduce the Zn2þ vacancies. The presence of Mn2þ emission at 590 nm indicated that the Mn2þ ions occupy the Zn2þ host lattice, nicely matching those reported previously in the literature [14]. It is worth viewing PL signals in terms of various excitation wavelengths. This is shown in Fig. 6 as an isometric plot of signal against wavelength and excitation wavelength. Intensity of emission at 590 nm is remarkably changed with change in excitation wavelengths. The orange emission located at 590 nm is clearly observed with increase in excitation wavelength up to 360 nm. It is conceivable that the emission colors observed under excitation with light of different wavelengths are different due to the different emission characteristics. Looking at CL data, Fig. 7 depicts the CL emission spectra from the ZnS:Mn powder. The primary electrons get scattered throughout the host crystal and eventually the electrons will transfer only a fraction of its energy to the Mn2þ. We observed a broad emission band located 600 nm and a low intensity peak at about 400 nm as shown in Fig. 7. Ultrasonic treatment at 20 min does not affect CL spectrum but there is a significant increase in CL intensity at 40 min compared to no US treatment and again a sharp decrease at 80 min. Large red shift and linewidth broadening have been observed when compared CL spectrum to the PL spectrum of the same sample. Full width of maximum (FWHM) of the peak is 94 nm, whereas PL one is 65 nm. The broadness of the emission peak is assigned to emission from more than one energy level. The redshift of the CL decreases as the electron voltage was increased (not shown here). We speculate that charging of the nanoparticles during the electron beam excitation responsible for the redshift and broadening. It is well known that point defects (i.e. vacancies, interstitials, and antisites) create new energy levels in the forbidden band gap of semiconductor materials. The CL spectra exhibit a broad blue emission band located at about400 nm, which is attributed to radiative recombination involving defect sites at the surface of ZnS host material [30] while the tail of CL spectrum extends all the way up to 500 nm, due to the presence of sulfur vacancies in the lattice [31]. As clearly seen in PL or CL, Mn doped ZnS exhibits dominating orange emission related to Mn2þ d-d transition. Normally, when
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Fig. 5. (a) PL emission spectra of the ZnS: Mn under different US time; (b) effect of manganese concentration on emission intensity of ZnS: Mn samples sonicated for 60min; (c) effect of US maturation time on emission intensity appeared at 590 nm of ZnS: Mn samples. Each point is an average value of three experiments and the error bar shows the standard deviation.
Fig. 6. Three-dimensional PL spectra depending on the excitation wavelength from 260 nm to 440 nm for ZnS:Mn2þ.
the electrons of the ground state taking part in the luminescence process are excited under different excitation sources they transfer to the higher excited state. After excitation, electrons may return to the ground state via two main relaxation mechanisms. One is the relaxation process through surface defects such as donors and acceptors. This relaxation will result in a non-radiative recombination. The other emission is due to an intra-configurational 3d5
Fig. 7. Comparison of CL emission spectra of the ZnS nanoparticles with Mn2þ without and with ultrasonic (US) treatment. The inset shows magnified view of spectral range of 350 nme450 nm.
transition on the Mn2þ ion located within the band gap of ZnS host material. This 4T1/6A1 energy transition will give rise to an orange luminescence. The based on the results, the energy level scheme of different electronic transitions taking place in Mn doped ZnS is illustrated in Fig. 8.
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Fig. 8. Schematic energy level diagram for ZnS:Mn2þ.
4. Conclusions Mn doped ZnS cubic phase nanoparticles were prepared by precipitation method under different ultrasound maturation time. ZnS:Mn powders consist of rounded particles of about 20 nm kept together in agglomerates. Presence of Mn in ZnS host lattice is confirmed by EDX and ICP-OES, but the incorporated amount is around 4 time smaller than theoretical ones due to the differences in the solubility products of metallic ions. Photoluminescence (PL) and Cathodoluminescence(CL) spectra confirm that our samples are doped with Mn2þ due to characteristic orange emission. The PL intensity increases with Mn amount up to optimal concentration (0.4e0.65 mol%) followed by a decrease due to quenching processes. Ultrasonic treatment exhibits strong influences on PL and CL properties. In general, PL performance increases with a longer ultrasonic time (i.e 60 min) but decrease sharply at ultrasonic time of 80 min. CL performance also follows a pattern similar to PL at ultrasonic time of 80 min. The PL and CL emissions located at 590 nm and 600 nm, respectively, are characteristic emissions of Mn2þ ion which can be assigned to a 4T1/6A1 transition. Blue emission appeared about 400 nm in the CL data is very small and broad and originates from the recombination involving defect states in the ZnS host lattice. On the basis of the promising findings illustrated here, further research to investigate luminescence properties of the samples based on parameters such as laser power, electron beam energy is still continuing and will be presented in our future papers. Acknowledgments This work was supported by a grant of the Romanian National Authority for Scientific Research, CNCS e UEFISCDI, project number PN-II-RU-TE-2014-4-1391. References [1] R.N. Bhargava, D. Gallagher, T. Welker, Doped nanocrystals of semiconductors - a new class of luminescent materials, J. Lumin. 60 (1994) 275e280. [2] Z.K. Heiba, M.B. Mohamed, N.G. Imam, Biphasic quantum dots of cubic and hexagonal Mn doped CdS; necessity of Rietveld analysis, J. Alloys Compd. 618 (2015) 280e286. [3] M.J. Tanaka, Photoluminescence properties of Mn2þ-doped IIeVI
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