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Effect of Mn co-doping on the structural, optical and magnetic properties of ZnS:Cr nanoparticles
Journal of Alloys and Compounds 537 (2012) 208–215
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Effect of Mn co-doping on the structural, optical and magnetic properties of ZnS:Cr nanoparticles D. Amaranatha Reddy a, S. Sambasivam b, G. Murali a, B. Poornaprakash a, R.P. Vijayalakshmi a,⇑, Y. Aparna c, B.K. Reddy a, J.L. Rao a a b c
Department of Physics, Sri Venkateswara University, Tirupati 517 502, India Department of Physics, Pukyong National University, Busan 608-737, South Korea Department of Physic, JNTU College of Engineering, Hyderabad, India
a r t i c l e
i n f o
Article history: Received 3 February 2012 Received in revised form 18 April 2012 Accepted 28 April 2012 Available online 18 May 2012 Keywords: Nanostructured materials Chemical synthesis PL EPR Magentisation
a b s t r a c t ZnS and Zn0.97Mn0.03S, Zn0.97Cr0.03S and Zn0.94Mn0.03Cr0.03S nanoparticles were synthesized by chemical co-precipitation method using ethylene diamine tetra acetic acid (EDTA) as stabilizer. energy-dispersive spectroscopy (EDS) confirmed the presence of Mn and Cr in the samples in near stoichiometric ratio. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) studies showed that Mn and Cr dopants entered the ZnS cubic lattice as substituents. The band-gap was found to be in the range of 3.81–4.09 eV from diffusion reflectance spectral (DRS) studies. In PhotoLuminescence (PL) spectra a sulfur–vacancy related PL band around 430 nm, a PL band associated with the 4T1 ? 6A1 transition of Mn2+ and PL peaks associated with Cr were observed. Electron paramagnetic resonance (EPR) spectra exhibited resonance signals characteristic of Mn2+ and Cr3+. Vibrating sample magnetometer (VSM) studies revealed that ZnS:Cr nanoparticles exhibited room temperature ferromagnetism and in ZnS:Cr nanoparticles co-doped with Mn a suppression of room temperature ferromagnetism was noticed. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Search for magnetic semiconductors or diluted magnetic semiconductors (DMS) which exhibit room temperature ferromagnetism (RTFM) is in full swing realizing their promising applications in spintronic devices. DMS are a class of magnetic semiconductors that are obtained by adding a fraction of transition or rare earth metal ions such as Mn, Fe, Co, Cr, Ni, Cu, Sm, Er, Dy, Gd to II–VI, IV–VI and III–V compounds. The novel spintronic applications of DMS are due to the spin and charge degrees of freedom of the charge carriers. The exchange interaction between the spin of the dopant atoms and the carriers in the semiconductor host is expected to bring global ferromagnetic order in the entire lattice [1]. Spintronics becomes an active area because spin based multifunctional electronic devices have several advantages such as lower power consumption, nonvolatility, higher integration densities and data processing speed over the conventional charge-based devices [2]. In order to make these applications practically viable, it is important to find magnetic semiconductors which are ferromagnetic at or above room temperature. In this pursuit, considerable work has been carried out on transition metal doped Zn based chalcogenides namely ZnS, ZnSe and ZnTe [3–10]. Among the most studied host materials ⇑ Corresponding author. Tel.: +91 877 226 0211 E-mail address: [email protected] (R.P. Vijayalakshmi). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.04.115
for DMS, ZnS is one with great potential for device applications and it has been identified as an excellent host semiconductor for supporting RTFM when doped with a variety of 3d transition metal ions such as Mn [11,12], Co [13], Cu [14] Fe [15] and Cr [16]. New semiconductor materials with RTFM are required for the development of new generation spintronic devices. The type, mobility and concentration of the carriers have a significant impact on the magnetic properties, which provide an opportunity to manipulate the ferromagnetism of DMS. Growth techniques and conditions also influence strongly the magnetic properties of the DMS. Another important technique for changing the magnetic properties of semiconductor materials is co-doping. Co-doping is a fairly simple and effective method to alter the number of vacancies and interstitial host metal atoms, which is useful to explore the mechanism of RTFM. On the other hand, co-doping under appropriate conditions can modulate the intrinsic point defects of the TM-doped DMS and influence the properties of the carriers to regulate the magnetism. There are extensive reports [17–21] on the influence of TM codoping on the magnetic properties of nanocrystalline ZnO materials. Surprisingly as per our knowledge, there are no experimental reports on RTFM in co-doped ZnS nanoparticles, although the above mentioned reports indicate the possibility of RTFM in a similar system ZnO with TM co-dopants. The present authors have earlier reported [22] enhanced RTFM in Cu co-doped ZnS:Cr nanoparticles. With a view of looking for more enhanced RTFM in ZnS:Cr
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nanoparticles in the present study an attempt has been made to incorporate Mn as a co-dopant in room temperature ferromagnetic ZnS:Cr nanostructured lattice. Mn is chosen as co-doping element because unlike many other transition metals (i) it can be incorporated into a ZnS host in large proportions without altering the crystal structure. (ii) It has a relatively large magnetic moment and also creates stable polarized state due to its half filled 3d bands. (iii) It is electrically neutral in ZnS host, thus avoiding the formation of any acceptor or donor impurities in the crystal (iv) Mn is antiferromagnetic, which makes this system more clean in terms of metal precipitate induced ferromagnetism. (v) The chemistry of the Mn atom is governed by its 4s2 electrons, while its magnetic properties are determined by the 3d5 shell. No other transition metal shows this dichotomy of behavior. This paper reports for the first time the synthesis of (Mn,Cr) co-doped ZnS nanoparticles by chemical co-precipitation method using EDTA as capping agent and the effect of co-doping on structural, optical and magnetic properties of ZnS nanoparticles. 2. Experimental All chemicals used in the present work were of analytical grade and were used without further purification. Ultra-pure de-ionized water was used in all synthesis steps. In this study ZnS, Zn0.97Mn0.03S, Zn0.97Cr0.03S and Zn0.94Mn0.03Cr0.03S nanocrystals were synthesized using Zn(CH3COO)2 and Na2S as precursors and C6H9MnO62(H2O) and CrCl36(H2O) as source materials for Mn and Cr, respectively. EDTA was used as the capping agent. The source materials were weighed according to the stoichiometry as per the target compositions and were dissolved in distilled water to make a 0.2 M solution. Sodium sulfide and Mn and Cr solutions were added simultaneously drop wise to the zinc acetate solution under continuous stirring for 8 h at room temperature till a fine precipitate was formed. The precipitate was filtered out separately and washed with de-ionized water and methanol to remove unnecessary impurities formed during the preparation process. The obtained product was placed in oven for 8 h at 60 °C. Chemical analysis was carried out using scanning electron microscopy (SEM) with EDS attachment (CARL-ZEISS EVO MA 15). The X-ray diffraction (XRD) patterns were recorded to characterize the phase and structure of the nanoparticles using Seifert 3003 TT X-ray Diffractometer. Transmission electron microscopy (TEM) were recorded in (JEOL–TEM 2010) with an accelerating voltage of 200 kV. Diffuse reflectance measurements of dry powders were performed using Jasco V-670 double-beam spectrophotometer for energy gap determination. Photo-luminescence (PL) studies were carried out using JOBIN YVON Fluorolog-3 Spectrophotometer with a 450 W Xenon arc lamp as an excitation source. FTIR studies were carried out using Thermo Nicolet FTIR-200 thermo Electron Corporation. Electron paramagnetic resonance spectra (EPR) were recorded at room temperature using a JEOL-FE1X EPR spectrometer. To know the magnetic state of the samples prepared, room temperature magnetization is studied as a function of applied magnetic field in the range of 10,000 to +10,000 G using a Lakeshore Vibrating sample magnetometer, VSM-7410.
3. Results and discussions 3.1. Elemental analysis In order to analyze the amount of Cr and Mn contents and their distribution in doped ZnS, the doped nanoparticles were examined by EDS. Fig. 1(a) and (b) show typical EDS spectra of ZnS and Zn0.94Cr0.03Mn0.03S nanoparticles, respectively. The above figures confirm the presence of Zn, S, Mn and Cr. No traces of other elements were noticed in the spectra indicating the purity of the samples. The estimated atomic percentages of Zn, S, Mn and Cr were close to the nominal (target) values. The observed deviations in the estimated compositions were small and were within ±3% by weight. 3.2. Morphological studies SEM is a powerful tool to study the surface morphology especially by observing the top and the cross-sectional views. The surface morphologies of the ZnS, Zn0.97Mn0.03S, Zn0.97Cr0.03S and Zn0.94Cr0.03Mn0.03S nanoparticles were shown in Fig. 2. The images
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were obtained at 20,000 magnifications. As seen in these images, that the agglomeration is decreasing with doping. It is obvious from the images that the morphologies of Cr, Mn and (Cr, Mn) co-doped samples are different from that of the undoped samples. In undoped samples only clouds of agglomerations are seen where as in doped samples the agglomerated particles are distinctly visualized. It is also noticed that in doped samples, the agglomerated particles appear to be nearly spherical with the distribution becoming nearly homogeneous. These features may be attributed to the differences in the ionic fractions of the bond which may be correlated to the bond’s electro negativity [23]. 3.3. Structural studies To check the crystallinity and crystal structures of the synthesized nanoparticles, XRD studies were carried out and the results are shown in Fig. 3. The XRD scan was carried out for a 2-theta angle range of 20–80° with a scan step size of 0.01°. The XRD patterns of the doped and undoped nanoparticles show a single-phase with cubic zincblende structure with three diffraction peaks corresponding to (1 1 1), (2 2 0) and (3 1 1) diffraction planes of cubic zincblende structure as per the standard card (JCPDS No. 80-0020). No secondary phase was detected indicating that Cr and Mn are incorporated into the ZnS host lattice as substituent atoms. Broad XRD peaks indicate the nanocrystalline nature of the particles. The average crystallite size was obtained from the most prominent XRD peak, (1 1 1) using Scherrer formula D = 0.89k/bCosh, where D is the average particle size, k is wavelength of Cu-Ka irradiation, b is the full width at half maximum intensity of the diffraction peak and h is the diffraction angle. The calculated diameters of the particles were 10, 8.5 6.9 and 6 nm for ZnS, ZnS:Mn, ZnS:Cr, ZnS:(Mn, Cr) respectively. Because of the surface scattering of nanoparticles, the co-doped ZnS nanoparticles are of smaller size compared to undoped nanoparticles. The lattice parameters are calculated by the formula: 1/d2 = 1/a2(h2 + k2 + l2) where ‘a’ is lattice parameter, dhkl is the interplanar separation corresponding to Miller indices h, k, and l. The calculated lattice constants are 5.390, 5.387, 5.380 and 5.374 Å for ZnS, ZnS:Mn, ZnS:Cr, ZnS:(Mn, Cr) samples, respectively. It is evident from Fig. 3, that the XRD peak positions have slightly shifted towards higher 2h values on co-doping, indicating a small decrease in lattice parameters. This may be due to the substitution of Zn2+ ions (Zn2+ = 0.78 Å) with atoms of different ionic radius (Mn2+ = 0.80 Å) and Cr3+ ions (Cr3+ = 0.63 Å), which results in a small decrease in lattice constant. Partly this could also be due a decrease in grain size in doped nanoparticles. Ummartyotin et al. [24] also reported similar decrease of crystallite size in Mn and Cu co-doped ZnS nanoparticles. Fig. 4 shows typical TEM images of Zn0.97Mn0.03S and Zn0.94Cr0.03Mn0.03S nanoparticles. The calculated grain sizes from TEM images are in the range of 6–15 nm. 3.4. Optical absorption studies UV–VIS absorption spectra of doped and co-doped samples along with the undoped samples are shown in Fig. 5. ZnS nanoparticles exhibit optical absorption edge around 326 nm. The corresponding band gap value is 3.81 eV. This is blue shifted compared to the bulk absorption edge (338 nm). This blue shift is attributed to the quantum size effect due to the small size of the synthesized nanoparticles. The absorption maxima for samples doped with Mn, Cr and co-doped with Mn and Cr lie at 313, 308 and 303 nm respectively. The band gap values corresponding to these maxima are 3.97, 4.03 and 4.09 eV. Both Cr and Mn dopants in ZnS have caused a decrease in particle size and also a blue shift in the absorption edge indicating the substitution of the dopant metal ions in the ZnS host lattice. In the (Mn, Cr) co-doped samples
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Fig. 1. EDS spectra of (a) ZnS and (b) Zn0.94Cr0.03Mn0.03S nanoparticles.
similar blue shift compared to the undoped samples, may be expected due to the localized energy levels of excitation states of the composite center of two metal ions. This blue shift in turn can vary with the nature of dopant ion, the absorption spectra characteristics. There are no reports of band-gap studies on (Cr, Mn) co-doped ZnS nanoparticles for comparison. However, similar blue shift in band gap has been observed in nanoparticles of ZnS:Mn co-doped with Cu [24], Cd [25] and Ni [26]. 3.5. Photoluminescence (PL) studies Room-temperature PL spectra of ZnS, Zn0.94Cr0.03S, Zn0.97 Mn0.03S, and Zn0.94Cr0.03Mn0.03S nanoparticles are shown in Fig. 6. PL spectra recorded for the undoped ZnS nanoparticles showed broad emission band at 435 nm, which is higher than what would be expected from a band gap emission process. This blue emission can be ascribed to a self-activated center presumably formed between a Zn vacancy and a shallow donor associated with a sulfur vacancy [27]. Sulfur vacancies at the surface are expected to give rise to Zn dangling bonds that form shallow donor levels. Thus, the recombination is mainly between these shallow donor levels and the valence band arising from a very fast energy transfer from the electron hole pair excited across the band gap of the nanocrystal. The emission spectra of Cr doped and undoped samples appear similar. An appreciable luminescence quenching is observed in Cr doped samples compared to the undoped samples, with the PL peaking around the same wavelength namely 435 nm. The
luminescence quenching as a result of Cr doping is attributed to repeated excitations within the Cr sites and thermal escape of charge carriers from confined states to other states [28]. It is seen that Mn2+-doped ZnS samples exhibit three emission bands at 414, 445 and 592 nm. The peak at 414 nm is assigned to interstitial sulfur, where as the peak at 445 nm is assigned to zinc interstitial since sulfur ion has larger ionic radius (1.7 Å) than that of zinc ion (0.74 Å), interstitial sulfur produces more strain in the ZnS lattice and thus the electron levels due to this site will have smaller binding energy. Therefore, interstitial sulfur energy levels must be closer to valence band than the interstitial zinc energy levels to the conduction band. Similarly sulfur vacancy states are closer to conduction band edge than zinc vacancies states to the valence band edge. The PL peak at 592 nm is assigned to Mn2+ ion which arises due to 4T1 ? 6A1 transition within the 3d shell of Mn2+ [29]. When Mn2+ ions substitute Zn2+ cation sites in ZnS lattice, the mixing of s–p electrons of host ZnS into the 3d electrons of Mn2+ causes strong hybridization and makes the forbidden transition of 4T1 ? 6A1 partially allowed, this yields orange emission at 592 nm. For samples co-doped with (Mn, Cr), the blue emission wavelength is extended to lower energy. The PL spectra exhibit two emission peaks at 445 and 598 nm. It is noticed that in the co-doped samples the PL characteristics of individual dopants namely Cr and Mn are retained with the intensity of the blue emission in the intermediate range. However, slight variations in the emission wavelength and intensity of orange emission are observed in co-doped and Mn doped ZnS nanoparticles.
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Fig. 2. SEM images of (a) ZnS (b) ZnS:Mn (c) ZnS:Cr and (d) ZnS:(Cr, Mn) nanoparticles.
3.6. FTIR analysis FTIR spectra of ZnS and ZnS:(Cr,Mn) nanoparticles recorded at room temperature in the wavelength range of 4000–400 cm 1 are shown in Fig. 7. The templates like EDTA-type compounds belong to the class called complexones, a group of polyaminocarboxylic acids or their salts, which are derivatives of iminodiacetic acid. A characteristic arrangement of these compounds is a nitrogen atom connected with two carboxymethyl groups N(CH2COOH)2. From
ZnS ZnS:Mn ZnS:Cr ZnS:(Cr,Mn)
Intensity (a.u)
(111)
(220) (311)
the point of view of EDTA-type molecular structure, it may be treated as a tertiary amine or acetic acid derivative. The spectral band at 2352 cm 1 in Fig. 7 is associated with the N–H3+ stretching vibration of the amino acid zwitterions. The broad band around 3393 cm 1 is assigned to the O–H stretching vibration because all FTIR spectra are recorded by mixing samples with KBr. Hence there may be some adsorbed water vapor, as KBr is hygroscopic. The spectral transitions at 1583 and 1400 cm 1 are, respectively, related to the antisymmetric and symmetric C@O stretching vibrations of the carboxylate anion associated with EDTA [30]. On the other hand, EDTA shows no vibrational band around 2100 cm 1, suggesting that EDTA in a ZnS matrix does not form a double betaine structure. However, the carboxylate ion groups are still active in the entrapped EDTA in ZnS as evidenced by the observed spectral bands at 1637 and 1401 cm 1 in Fig. 7. The peaks appearing at 473 and 663 cm 1 are assigned to the Zn–S stretching vibration. This obviously excludes the possibility that intact EDTA simply coexists with the ZnS nanoparticles. Since the surface capping of sulfide particles by carboxylate ion groups is attributed to coordination of the deprotonated EDTA groups to the sulfide surfaces, these IR spectra strongly confirm the surface capping of the ZnS nanoparticles through direct bonding of the EDTA, presumably to the ZnS site at the surface. The close similarity of the FTIR spectra of doped and undoped samples indicates that Cr and Mn have entered the ZnS lattice substitutionally. 3.7. EPR studies
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Fig. 3. X-ray diffraction patterns of ZnS, ZnS:Mn, ZnS:Cr and ZnS:(Cr, Mn) nanoparticles.
To get information about the oxidation state and site occupancy of the transition metal ion in the host ZnS lattice, and also to understand the interplay between the carrier concentration and magnetic exchange coupling at the microscopic level, EPR experi-
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Wavelength (nm) Fig. 6. PL spectra of ZnS, ZnS:Mn, ZnS:Cr and ZnS:(Cr, Mn) nanoparticles.
ments were conducted on these samples. No EPR signal was detected in the spectra of ZnS indicating that the host materials used in the present study are free from paramagnetic impurities.
Absorbance (a.u)
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Fig. 4. TEM images of (a) ZnS:Mn and (b) ZnS:(Cr, Mn) nanoparticles.
When manganese ions were doped into the ZnS matrix the EPR spectra of all the investigated samples exhibited resonance signals. Fig. 8 (a) shows the room temperature EPR spectra for Zn0.97Mn0.03S and Zn0.94Mn0.03Cr0.03S nanoparticles. To make the comparison clear, the EPR spectrum of ZnS:Cr (3 at.%) are also shown in Fig. 8(b). From Fig. 8 (a) it is clear that Mn doped ZnS nanoparticles exhibit well-resolved resonance signal at g = 2.08, with a six line hyperfine structure. The hyperfine structure originates from the interaction between the Mn2+ electron cloud and 55 Mn nucleus with the spin I = 5/2. This indicates the presence of the paramagnetic Mn2+ ions in the samples. This unambiguously confirms the successful doping of paramagnetic Mn2+ ions in the ZnS crystal lattice. The electron configuration of Mn2+ ion is 3d5 (S state). In the case of d5 metal ions, it is known that the axial distortion of octahedral symmetry gives rise to three Kramer’s doublets |±5/2), |±3/2) and |±1/2). An application of Zeeman field will split the spin degeneracy of the Kramer’s doublets. As the crystal field splitting is normally much greater than the Zeeman field, the resonances observed are due to transitions within the Kramer’s
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Wavelength (nm) Fig. 5. Absorption spectra of ZnS, ZnS:Mn, ZnS:Cr and ZnS:(Cr, Mn) nanoparticles.
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Wavenumber (cm ) Fig. 7. FTIR spectra of ZnS and ZnS:(Cr, Mn) nanoparticles.
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Fig. 8. EPR spectra of (a) ZnS:Mn and ZnS:(Cr, Mn) (b) ZnS:Cr nanoparticles.
doublets split by the Zeeman field [31]. The resonance at g 2.08 is due to Mn2+ ions in an environment close to an octahedral symmetry and is known to arise from the transition between the energy levels of the lower doublet |±1/2). Another resonance signal at g = 4.03, arising from the transition between the Zeeman split energy levels of middle Kramer’s doublet of |±3/2), is also observed. Usually the intensity of the resonance signal at g = 4.03 is much lower than that at g = 2.08. ZnS:Cr samples exhibit single resonance signal at g = 1.989. Generally EPR spectrum of chromium powder comprises three lines, but in our studies we obtained single broad EPR signal because of exchange interaction. The present authors have earlier reported [16] spectral studies with different Cr concentrations in ZnS nanoparticles. Zn0.94Cr0.03Mn0.03S nanoparticles also exhibit well-resolved resonance signal at g = 2.08, with a six line hyperfine structure. But compared to Zn0.97Mn0.03S samples the intensity of the EPR signal drops significantly. The spins of manganese and chromium ions in Zn0.94Cr0.03Mn0.03S are coupled by the exchange interaction that narrows the EPR signal and its intensity in the paramagnetic phase, and g factors of the manga-
nese and chromium ions are nearly 2.0. Hence, we assume that the resonance signal with g = 2.08 in the EPR spectrum is related to the manganese and chromium ions in the paramagnetic state. The strong decrease in EPR signal intensity indicates the probability of ferromagnetism in Zn0.96Mn0.03Cr0.03S samples. The peak-to-peak linewidth (DH) values of the hyperfine lines obtained from the EPR signals are 560 G for Zn0.97Mn0.03S and 436 G for Zn0.94Mn0.03Cr0.03S and 367 G for ZnS:Cr nanoparticles. Generally the change in linewidth mainly depends on the variation of temperature and concentration of the dopants. In the present case it is almost independent of temperature because all EPR studies are carried out at room temperature only. So the decreased linewidth in Zn0.96Mn0.03Cr0.03S might be an indication of considerable changes in the magnitude of the exchange interaction on addition of chromium and manganese in the host lattice of ZnS. The number of spins (Ns) participating in the resonance is calculated by using the formula: Ns = 0.285 I (DH)2 where ‘I’ is the peak-to-peak height and DH is the linewidth (in G) and are found to be 1.2 106, 7.4 105 and 1.7 105 for ZnS:Mn, ZnS:Cr and ZnS:(Cr, Mn),
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respectively. The values of hyperfine splitting constant (A) are determined from the average value of hyperfine splitting of successive allowed hyperfine lines of the central sextet and are found to be 90 G for Zn0.97Mn0.03S and 79 G for Zn0.94Mn0.03Cr0.03S. The strength of the hyperfine splitting (A) depends on the matrix into which the ion is dissolved and is mainly determined by the electronegativity of the neighbors. This means ‘A’ is a direct qualitative measure of the degree of the covalency of the bonding in the matrix. The magnitude of the hyperfine splitting constant (A) in the present samples indicates the bonding between Mn2+ and the surrounding ligands is moderately covalent. Further, by the addition of Cr in ZnS:Mn, the hyperfine splitting constant decreases indicating that there is decrease in the strength of ionic bonding on addition of Cr to ZnS:Mn.
3.8. VSM studies Fig. 9 shows the field-dependent magnetization (M–H) curves of ZnS, Zn0.97Mn0.03S, Zn0.97Cr0.03S and Zn0.94Cr0.03Mn0.03S nanoparticles at room temperature in the field range of 0 to ±10 000 Oe. A typical diamagnetic behavior has been observed in the host ZnS and is attributed to the absence of unpaired electrons of its‘d’ orbital. From the figure it is obvious that Mn doped ZnS nanoparticles exhibit straight lines indicating that the samples are paramagnetic at room temperature. It may be noted that in the present case, the absence of ferromagnetism in Mn-doped ZnS nanoparticles can be explained with theoretical ab initio calculations [32]. Mn 3d states in the ZnS lattice, with a cubic crystal-field, split into an e doublet and a t2 triplet. e Levels have a lower energy than t2 levels. t2 Levels are fully occupied leading to the absence of a hole for the coupling. It can, however, be ferromagnetic if a double exchange via the localized holes is predominant. In EPR a well resolved EPR signal is observed as mentioned in the previous section, which also confirmed the paramagnetic nature of ZnS:Mn nanoparticles. ZnS:Cr nanoparticles exhibit a strong ferromagnetic ordering at room temperature. The present authors have previously reported ferromagnetism and EPR studies on ZnS:Cr nanoparticles [16]. In brief, the observed ferromagnetism in ZnS:Cr nanoparticles was attributed to the substitution of Cr for Zn and the room temperature EPR signal showed the presence of Cr3+ ions in the samples, which provide the necessary unpaired spins for ferromagnetism rather
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4. Conclusions
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than the Cr clusters. Thus, the observed ferromagnetism in the Cr doped ZnS samples was attributed to the exchange interaction between localized ‘d’ spins on the Cr ions and the free delocalized carriers. The present study reports the effect of Mn co-doping in Cr (3 at.%) doped ZnS nanoparticles. Zn0.94Cr0.03Mn0.03S nanoparticles also exhibit room temperature ferromagnetism. The roomtemperature ferromagnetic phase could not be due to Mn-related oxides, such as MnO, MnO2, Mn2O3 and Mn3O4, because none of them shows ferromagnetism above room temperature. Similarly this is not due to the Cr related metal clusters like phases of magnetic (CrO2) or antiferromagnetic (Cr2O3 and Cr3O4) either since different phases were not noticed in XRD and TEM studies. Hence the observed ferromagnetism in ZnS: (Cr, Mn) nanoparticles is due to substitution of Cr and Mn at Zn sites. This means the observed ferromagnetism may be attributed solely to the doping of (Cr, Mn) in ZnS lattice and not due to the presence of any secondary phases. Here Mn and Cr ions, provide the necessary unpaired spins for ferromagnetism rather than the Cr, Mn clusters. Thus, the ferromagnetism in the Zn0.94Cr0.03Mn0.03S samples could be considered as a result of exchange interaction between localized ‘d’ spins on the Cr, Mn ions and the free delocalized carriers. In this study small amount of Mn co-doping, however, brought drastic changes in magnetic properties as illustrated in Fig. 9. ZnS doped with both Mn and Cr showed a suppression in magnetization. Initially, as the electron density increases with Mn doping, the magnetization decreases. This is consistent with the bound magnetic polaron model, it provides a mechanism whereby holes that are localized at or near the Mn2+ ions are responsible for mediating ferromagnetism. However, only a fraction of the Mn ions are expected to order ferromagnetically due to competing superexchange antiferromagnetic interactions between neighboring Mn ions. The remaining Mn ions may form antiferromagnetic Mn clusters as a result of decreased Mn–Mn separation leading to the suppression of ferromagnetism in (Mn, Cr) co-doped samples. The values of coercive fields (Hc) are 152.30, 363.5 G. The values of magnetization (Ms) are 27.964 10 3, 20.186 10 3 emu/g and the values of retentivity (Mr) are 3.3979 10 3, 5.8944 10 3 emu/g for Zn0.97Cr0.03S and Zn0.94Cr0.03Mn0.03S, respectively. In co-doped samples coercive field (Hc) and retentivity increase but magnetization decreases. In co-doped samples the increased coercivity is attributed to the strong antiferromagnetic coupled interaction between Mn ions. There are no reports on magnetic studies of Cr, Mn co-doped ZnS nanoparticles for comparison. More detailed investigations on the dopant induced ferromagnetism are needed to understand the mechanism of magnetism in these materials. Such investigations are under progress in our laboratory. Obviously, the co-doping of Mn does indeed play a key role in tuning the ferromagnetism of the ZnS:Cr nanoparticles.
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Applied Field (G) Fig. 9. VSM spectra of ZnS, ZnS:Mn, ZnS:Cr and ZnS:(Cr, Mn) nanoparticles.
ZnS and ZnS:Mn, ZnS:Cr and ZnS: (Mn, Cr) nanoparticles were synthesized successfully through chemical co-precipitation using EDTA as the capping agent. EDS spectral studies confirmed the presence of Mn and Cr in the samples. XRD, studies showed that the particles crystallized in cubic Zincblende structure with the dopant metal atoms entering the lattice as substituents. FTIR spectra confirmed that EDTA played the role of template and simply coexisted in the lattice. Band gap values from DRS spectra were in the range of 3.81–4.09 eV which are higher than the bulk ZnS values due to size quantization effect. In PL spectra a sulfur–vacancy-related PL band at around 430 nm, another PL orange emission band associated with the 4T1 ? 6A1 transition of Mn2+ were observed. Magnetization studies indicated that ZnS nanoparticles exhibited diamagnetic behavior, where as ZnS:Mn were paramagnetic and ZnS:Cr exhibited
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strong room temperature ferromagnetism. Suppression in room temperature ferromagnetism was observed in Mn co-doped ZnS:Cr nanoparticles due to Mn–Mn antiferromagnetic interactions. Acknowledgments The authors are grateful to the University Grants Commission (UGC), New Delhi, Government of India, for financial support. One of the authors (D. Amaranatha Reddy) is thankful to the UGC, for the award of the UGC-BSR Fellowship in Sciences for Meritorious Students. The authors are thankful to Mr. N. Sivaramakrishnan and Mr. Muttukumar, SAIF, IIT Madras, Chennai for extending the VSM. References [1] K. Sato, H. Katayama-Yoshida, Hyperfine interactions and magnetism of 3d transition-metal-impurities in II–VI and III–V compound-based diluted magnetic semiconductors, Hyperfine Interactions 136 (137) (2001) 737–742. [2] S.A. Wolf, D.D. Awschalom, R.A. Buhrman, J.M. Daughton, S. von Molnar, M.L. Roukes, A.Y. Chtchelkanova, D.M. Treger, Spintronics: a spin-based electronics vision for the future, Science 294 (2001) 1488–1495. [3] Lijuan. Zhao, Bei. Zhang, Qi. Pang, Shihe. Yang, Xixiang. Zhang, Weikun. Ge, Jiannong. Wang, Chemical synthesis and magnetic properties of dilute magnetic ZnTe:Cr crystals, Appl. Phys. Lett. 89 (2006) 092111–092113. [4] H. Saito, V. Zayets, S. Yamagata, K. Ando, Room-temperature ferromagnetism in highly Cr-doped II–VI diluted magnetic semiconductor Zn1-xCrxTe, J. Appl. Phys. 93 (2003) 6796–6798. [5] G. Krishnaiah, N. Madhusudhana Rao, B.K. Reddy, D. Raja Reddy, T. Mohan Babu, S. Sambasivam, P. Sreedhara Reddy, EPR and magnetic properties of vapour phase grown Zn1 xCrxTe crystals, Phys. Lett. A 372 (2008) 6429–6433. [6] Taejoon Kang, Joonho Sung, Wooyoung Shim, Heesung Moon, Jaehun Cho, Younghun Jo, Wooyoung Lee, Bongsoo Kim, Synthesis and magnetic properties of single-crystalline Mn/Fe-doped and Co-doped ZnS nanowires and nanobelts, J. Phys. Chem. C 113 (2009) 5352–5357. [7] D.V. Martyshkin, V.V. Fedorov, C. Kim, I.S. Moskalev, S.B. Mirov, Mid-IR random lasing of Cr-doped ZnS nanocrystals, J. Opt. 12 (2010) 024005–024009. [8] Kunio Ichino, Yasuharu Morimoto, Hiroshi Kobayashi, Molecular beam epitaxy and structural properties of ZnCrS, Phys. Status Solidi C 3 (2006) 776–779. [9] Ming Luo, N.Y. Garces, N.C. Giles, Utpal N. Roy, Yunlong Cui, Arnold Burger, Optical and electron paramagnetic resonance spectroscopies of diffusiondoped Co2+:ZnSe, J. Appl. Phys. 99 (2006) 073115–073709. [10] Amit D. Lad, Ch. Rajesh, Mahmud Khan, Naushad Ali, I.K. Gopalakrishnan, S.K. Kulshreshtha, Shailaja Mahamuni, Magnetic behavior of manganese-doped ZnSe quantum dots, J. Appl. Phys. 101 (2007) 103906–103910. [11] P. Vinotha Boorana Lakshmi, K. Sakthi Raj, K. Ramachandran, Synthesis and characterization of nano ZnS doped with Mn, Cryst. Res. Technol. 44 (2009) 153–158. [12] M.M. Rashad, D.A. Rayan, K. El-Barawy, Hydrothermal synthesis and magnetic properties of Mn doped ZnS nanoparticles, J. Phys.: Conference Series 200 (2010) 072077–072081. [13] S. Sambasivam, D. Paul Joseph, J.G. Lin, C. Venkateswaran, Doping induced magnetism in Co–ZnS nanoparticles, J. Solid State Chem. 182 (2009) 2598– 2601.
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[14] Huiyu Yan, Yuqi Li, Yanrui Guo, Qinggong Song, Yifei Chen, Ferromagnetic properties of Cu-doped ZnS: a density functional theory study, Physica B 406 (2011) 545–547. [15] S. Sambasivam, D. Paul Joseph, D. Raja Reddy, B.K. Reddy, C.K. Jayasankar, Synthesis and characterization of thiophenol passivated Fe-doped ZnS nanoparticles, Mater. Sci. Eng. B 150 (2008) 125–129. [16] D. Amaranatha Reddy, G. Murali, R.P. Vijayalakshmi, B.K. Reddy, Roomtemperature ferromagnetism in EDTA capped Cr-doped ZnS nanoparticles, Appl. Phys. A 105 (2011) 119–124. [17] Jinghai Yang, Lianhua Feia, Huilian Liua, Yang Liua, Ming Gao, Yongjun Zhang, Lili Yang, A study of structural, optical and magnetic properties of Zn0.97 xCuxCr0.03O diluted magnetic semiconductors, J. Alloys Compd. 509 (2011) 3672–3676. [18] L.B. Duan, G.H. Rao, Y.C. Wang, J. Yu, T. Wang, Magnetization and Raman scattering studies of (Co, Mn) codoped ZnO nanoparticles, J. Appl. Phys. 104 (2008) 013909–013913. [19] Y. He, Parmanand Sharma, Krishanu Biswas, E.Z. Liu, Naofumi Ohtsu, A. Inoue, Y. Inada, M. Nomura, J.S. Tse, S. Yin, J.Z. Jiang, Origin of ferromagnetism in ZnO codoped with Ga and Co: experiment and theory, Phys. Rev. B 78 (2008) 155202–155208. [20] X.C. Liu, E.W. Shi, Z.Z. Chen, H.W. Zhang, B. Xiao, L.X. Song, High-temperature ferromagnetism in (Co, Al)-co doped ZnO powders, Appl. Phys. Lett. 88 (2006) 252503–252505. [21] O.D. Jayakumar, I.K. Gopalakrishnan, K. Shashikala, S.K. Kulshreshtha, C. Sudakar, Magnetic properties of hydrogenated Li and Co doped ZnO nanoparticles, Appl. Phys. Lett. 89 (2006) 202507–202509. [22] D. Amaranatha Reddy, G. Murali, B. Poornaprakash, R.P. Vijayalakshmi, B.K. Reddy, Structural, optical and magnetic properties of Zn0.97-xCuxCr0.03S nanoparticles, Appl. Surf. Sci. 258 (2012) 5206–5211. [23] Shubra Singh, Shubra Singh, E. Senthil Kumar, M.S. Ramachandra Rao, Microstructural, optical and electrical properties of Cr-doped ZnO, Scripta Materialia 58 (2008) 866–869. [24] S. Ummartyotin, N. Bunnak, J. Juntaro, M. Sain, H. Manuspiya, Synthesis and luminescence properties of ZnS and metal (Mn, Cu)-doped-ZnS ceramic powder, Solid State Commun. 14 (2012) 299–304. [25] J.Z. Liu, P.X. Yan, G.H. Yue, J.B. Chang, D.M. Qu, R.F. Zhuo, Red light photoluminescence emission from Mn and Cd co-doped ZnS onedimensional nanostructures, J. Phys. D: Appl. Phys. 39 (2006) 2352–2356. [26] Ping. Yang, Lu. Mengkai, Xu. Dong, Duorong. Yuan, Chunfeng Song, Suwen Liu, Xiufeng Cheng, Luminescence characteristics of ZnS nanoparticles co-doped with Ni2+ and Mn2+, Opt. Mater. 24 (2003) 497–502. [27] D. Denzler, M. Olschewski, K. Sattler, Luminescence studies of localized gap states in colloidal ZnS nanocrystals, J. Appl. Phys. 84 (1998) 2841–2845. [28] D. Amaranatha Reddy, G. Murali, R.P. Vijayalakshmi, B.K. Reddy, B. Sreedhar, Effect of Cr doping on the structural and optical properties of ZnS nanoparticles, Cryst. Res. Technol. 46 (2011) 731–736. [29] Sameer Sapra, Ankita Prakash, Ajit Ghangrekar, N. Periasamy, D.D. Sarma, Emission properties of manganese-doped ZnS nanocrystals, J. Phys. Chem. B 109 (2005) 1663–1668. [30] J. Ryczkowski, IR studies of EDTA alkaline salts interaction with the surface of inorganic oxides, Appl. Surf. Sci. 252 (2005) 813–822. [31] A. Pedro, Gonzalez Beermann, Bruce R. McGarvey, Subra Muralidharan, Raymond C.W. Sung, EPR Spectra of Mn2+ Doped ZnS Quantum Dots, Chem. Mater. 16 (2004) 915–918. [32] Amit D. Lad, Ch. Rajesh, Mahmud Khan, Naushad Ali, I.K. Gopalakrishnan, S.K. Kulshreshtha, Shailaja Mahamuni, Magnetic behavior of manganese-doped ZnSe quantum dots, J. Appl. Phys. 101 (2007) 103906–103910.
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Effect of Mn co-doping on the structural, optical and magnetic properties of ZnS:Cr nanoparticles D. Amaranatha Reddy a, S. Sambasivam b, G. Murali a, B. Poornaprakash a, R.P. Vijayalakshmi a,⇑, Y. Aparna c, B.K. Reddy a, J.L. Rao a a b c
Department of Physics, Sri Venkateswara University, Tirupati 517 502, India Department of Physics, Pukyong National University, Busan 608-737, South Korea Department of Physic, JNTU College of Engineering, Hyderabad, India
a r t i c l e
i n f o
Article history: Received 3 February 2012 Received in revised form 18 April 2012 Accepted 28 April 2012 Available online 18 May 2012 Keywords: Nanostructured materials Chemical synthesis PL EPR Magentisation
a b s t r a c t ZnS and Zn0.97Mn0.03S, Zn0.97Cr0.03S and Zn0.94Mn0.03Cr0.03S nanoparticles were synthesized by chemical co-precipitation method using ethylene diamine tetra acetic acid (EDTA) as stabilizer. energy-dispersive spectroscopy (EDS) confirmed the presence of Mn and Cr in the samples in near stoichiometric ratio. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) studies showed that Mn and Cr dopants entered the ZnS cubic lattice as substituents. The band-gap was found to be in the range of 3.81–4.09 eV from diffusion reflectance spectral (DRS) studies. In PhotoLuminescence (PL) spectra a sulfur–vacancy related PL band around 430 nm, a PL band associated with the 4T1 ? 6A1 transition of Mn2+ and PL peaks associated with Cr were observed. Electron paramagnetic resonance (EPR) spectra exhibited resonance signals characteristic of Mn2+ and Cr3+. Vibrating sample magnetometer (VSM) studies revealed that ZnS:Cr nanoparticles exhibited room temperature ferromagnetism and in ZnS:Cr nanoparticles co-doped with Mn a suppression of room temperature ferromagnetism was noticed. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Search for magnetic semiconductors or diluted magnetic semiconductors (DMS) which exhibit room temperature ferromagnetism (RTFM) is in full swing realizing their promising applications in spintronic devices. DMS are a class of magnetic semiconductors that are obtained by adding a fraction of transition or rare earth metal ions such as Mn, Fe, Co, Cr, Ni, Cu, Sm, Er, Dy, Gd to II–VI, IV–VI and III–V compounds. The novel spintronic applications of DMS are due to the spin and charge degrees of freedom of the charge carriers. The exchange interaction between the spin of the dopant atoms and the carriers in the semiconductor host is expected to bring global ferromagnetic order in the entire lattice [1]. Spintronics becomes an active area because spin based multifunctional electronic devices have several advantages such as lower power consumption, nonvolatility, higher integration densities and data processing speed over the conventional charge-based devices [2]. In order to make these applications practically viable, it is important to find magnetic semiconductors which are ferromagnetic at or above room temperature. In this pursuit, considerable work has been carried out on transition metal doped Zn based chalcogenides namely ZnS, ZnSe and ZnTe [3–10]. Among the most studied host materials ⇑ Corresponding author. Tel.: +91 877 226 0211 E-mail address: [email protected] (R.P. Vijayalakshmi). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.04.115
for DMS, ZnS is one with great potential for device applications and it has been identified as an excellent host semiconductor for supporting RTFM when doped with a variety of 3d transition metal ions such as Mn [11,12], Co [13], Cu [14] Fe [15] and Cr [16]. New semiconductor materials with RTFM are required for the development of new generation spintronic devices. The type, mobility and concentration of the carriers have a significant impact on the magnetic properties, which provide an opportunity to manipulate the ferromagnetism of DMS. Growth techniques and conditions also influence strongly the magnetic properties of the DMS. Another important technique for changing the magnetic properties of semiconductor materials is co-doping. Co-doping is a fairly simple and effective method to alter the number of vacancies and interstitial host metal atoms, which is useful to explore the mechanism of RTFM. On the other hand, co-doping under appropriate conditions can modulate the intrinsic point defects of the TM-doped DMS and influence the properties of the carriers to regulate the magnetism. There are extensive reports [17–21] on the influence of TM codoping on the magnetic properties of nanocrystalline ZnO materials. Surprisingly as per our knowledge, there are no experimental reports on RTFM in co-doped ZnS nanoparticles, although the above mentioned reports indicate the possibility of RTFM in a similar system ZnO with TM co-dopants. The present authors have earlier reported [22] enhanced RTFM in Cu co-doped ZnS:Cr nanoparticles. With a view of looking for more enhanced RTFM in ZnS:Cr
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nanoparticles in the present study an attempt has been made to incorporate Mn as a co-dopant in room temperature ferromagnetic ZnS:Cr nanostructured lattice. Mn is chosen as co-doping element because unlike many other transition metals (i) it can be incorporated into a ZnS host in large proportions without altering the crystal structure. (ii) It has a relatively large magnetic moment and also creates stable polarized state due to its half filled 3d bands. (iii) It is electrically neutral in ZnS host, thus avoiding the formation of any acceptor or donor impurities in the crystal (iv) Mn is antiferromagnetic, which makes this system more clean in terms of metal precipitate induced ferromagnetism. (v) The chemistry of the Mn atom is governed by its 4s2 electrons, while its magnetic properties are determined by the 3d5 shell. No other transition metal shows this dichotomy of behavior. This paper reports for the first time the synthesis of (Mn,Cr) co-doped ZnS nanoparticles by chemical co-precipitation method using EDTA as capping agent and the effect of co-doping on structural, optical and magnetic properties of ZnS nanoparticles. 2. Experimental All chemicals used in the present work were of analytical grade and were used without further purification. Ultra-pure de-ionized water was used in all synthesis steps. In this study ZnS, Zn0.97Mn0.03S, Zn0.97Cr0.03S and Zn0.94Mn0.03Cr0.03S nanocrystals were synthesized using Zn(CH3COO)2 and Na2S as precursors and C6H9MnO62(H2O) and CrCl36(H2O) as source materials for Mn and Cr, respectively. EDTA was used as the capping agent. The source materials were weighed according to the stoichiometry as per the target compositions and were dissolved in distilled water to make a 0.2 M solution. Sodium sulfide and Mn and Cr solutions were added simultaneously drop wise to the zinc acetate solution under continuous stirring for 8 h at room temperature till a fine precipitate was formed. The precipitate was filtered out separately and washed with de-ionized water and methanol to remove unnecessary impurities formed during the preparation process. The obtained product was placed in oven for 8 h at 60 °C. Chemical analysis was carried out using scanning electron microscopy (SEM) with EDS attachment (CARL-ZEISS EVO MA 15). The X-ray diffraction (XRD) patterns were recorded to characterize the phase and structure of the nanoparticles using Seifert 3003 TT X-ray Diffractometer. Transmission electron microscopy (TEM) were recorded in (JEOL–TEM 2010) with an accelerating voltage of 200 kV. Diffuse reflectance measurements of dry powders were performed using Jasco V-670 double-beam spectrophotometer for energy gap determination. Photo-luminescence (PL) studies were carried out using JOBIN YVON Fluorolog-3 Spectrophotometer with a 450 W Xenon arc lamp as an excitation source. FTIR studies were carried out using Thermo Nicolet FTIR-200 thermo Electron Corporation. Electron paramagnetic resonance spectra (EPR) were recorded at room temperature using a JEOL-FE1X EPR spectrometer. To know the magnetic state of the samples prepared, room temperature magnetization is studied as a function of applied magnetic field in the range of 10,000 to +10,000 G using a Lakeshore Vibrating sample magnetometer, VSM-7410.
3. Results and discussions 3.1. Elemental analysis In order to analyze the amount of Cr and Mn contents and their distribution in doped ZnS, the doped nanoparticles were examined by EDS. Fig. 1(a) and (b) show typical EDS spectra of ZnS and Zn0.94Cr0.03Mn0.03S nanoparticles, respectively. The above figures confirm the presence of Zn, S, Mn and Cr. No traces of other elements were noticed in the spectra indicating the purity of the samples. The estimated atomic percentages of Zn, S, Mn and Cr were close to the nominal (target) values. The observed deviations in the estimated compositions were small and were within ±3% by weight. 3.2. Morphological studies SEM is a powerful tool to study the surface morphology especially by observing the top and the cross-sectional views. The surface morphologies of the ZnS, Zn0.97Mn0.03S, Zn0.97Cr0.03S and Zn0.94Cr0.03Mn0.03S nanoparticles were shown in Fig. 2. The images
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were obtained at 20,000 magnifications. As seen in these images, that the agglomeration is decreasing with doping. It is obvious from the images that the morphologies of Cr, Mn and (Cr, Mn) co-doped samples are different from that of the undoped samples. In undoped samples only clouds of agglomerations are seen where as in doped samples the agglomerated particles are distinctly visualized. It is also noticed that in doped samples, the agglomerated particles appear to be nearly spherical with the distribution becoming nearly homogeneous. These features may be attributed to the differences in the ionic fractions of the bond which may be correlated to the bond’s electro negativity [23]. 3.3. Structural studies To check the crystallinity and crystal structures of the synthesized nanoparticles, XRD studies were carried out and the results are shown in Fig. 3. The XRD scan was carried out for a 2-theta angle range of 20–80° with a scan step size of 0.01°. The XRD patterns of the doped and undoped nanoparticles show a single-phase with cubic zincblende structure with three diffraction peaks corresponding to (1 1 1), (2 2 0) and (3 1 1) diffraction planes of cubic zincblende structure as per the standard card (JCPDS No. 80-0020). No secondary phase was detected indicating that Cr and Mn are incorporated into the ZnS host lattice as substituent atoms. Broad XRD peaks indicate the nanocrystalline nature of the particles. The average crystallite size was obtained from the most prominent XRD peak, (1 1 1) using Scherrer formula D = 0.89k/bCosh, where D is the average particle size, k is wavelength of Cu-Ka irradiation, b is the full width at half maximum intensity of the diffraction peak and h is the diffraction angle. The calculated diameters of the particles were 10, 8.5 6.9 and 6 nm for ZnS, ZnS:Mn, ZnS:Cr, ZnS:(Mn, Cr) respectively. Because of the surface scattering of nanoparticles, the co-doped ZnS nanoparticles are of smaller size compared to undoped nanoparticles. The lattice parameters are calculated by the formula: 1/d2 = 1/a2(h2 + k2 + l2) where ‘a’ is lattice parameter, dhkl is the interplanar separation corresponding to Miller indices h, k, and l. The calculated lattice constants are 5.390, 5.387, 5.380 and 5.374 Å for ZnS, ZnS:Mn, ZnS:Cr, ZnS:(Mn, Cr) samples, respectively. It is evident from Fig. 3, that the XRD peak positions have slightly shifted towards higher 2h values on co-doping, indicating a small decrease in lattice parameters. This may be due to the substitution of Zn2+ ions (Zn2+ = 0.78 Å) with atoms of different ionic radius (Mn2+ = 0.80 Å) and Cr3+ ions (Cr3+ = 0.63 Å), which results in a small decrease in lattice constant. Partly this could also be due a decrease in grain size in doped nanoparticles. Ummartyotin et al. [24] also reported similar decrease of crystallite size in Mn and Cu co-doped ZnS nanoparticles. Fig. 4 shows typical TEM images of Zn0.97Mn0.03S and Zn0.94Cr0.03Mn0.03S nanoparticles. The calculated grain sizes from TEM images are in the range of 6–15 nm. 3.4. Optical absorption studies UV–VIS absorption spectra of doped and co-doped samples along with the undoped samples are shown in Fig. 5. ZnS nanoparticles exhibit optical absorption edge around 326 nm. The corresponding band gap value is 3.81 eV. This is blue shifted compared to the bulk absorption edge (338 nm). This blue shift is attributed to the quantum size effect due to the small size of the synthesized nanoparticles. The absorption maxima for samples doped with Mn, Cr and co-doped with Mn and Cr lie at 313, 308 and 303 nm respectively. The band gap values corresponding to these maxima are 3.97, 4.03 and 4.09 eV. Both Cr and Mn dopants in ZnS have caused a decrease in particle size and also a blue shift in the absorption edge indicating the substitution of the dopant metal ions in the ZnS host lattice. In the (Mn, Cr) co-doped samples
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Fig. 1. EDS spectra of (a) ZnS and (b) Zn0.94Cr0.03Mn0.03S nanoparticles.
similar blue shift compared to the undoped samples, may be expected due to the localized energy levels of excitation states of the composite center of two metal ions. This blue shift in turn can vary with the nature of dopant ion, the absorption spectra characteristics. There are no reports of band-gap studies on (Cr, Mn) co-doped ZnS nanoparticles for comparison. However, similar blue shift in band gap has been observed in nanoparticles of ZnS:Mn co-doped with Cu [24], Cd [25] and Ni [26]. 3.5. Photoluminescence (PL) studies Room-temperature PL spectra of ZnS, Zn0.94Cr0.03S, Zn0.97 Mn0.03S, and Zn0.94Cr0.03Mn0.03S nanoparticles are shown in Fig. 6. PL spectra recorded for the undoped ZnS nanoparticles showed broad emission band at 435 nm, which is higher than what would be expected from a band gap emission process. This blue emission can be ascribed to a self-activated center presumably formed between a Zn vacancy and a shallow donor associated with a sulfur vacancy [27]. Sulfur vacancies at the surface are expected to give rise to Zn dangling bonds that form shallow donor levels. Thus, the recombination is mainly between these shallow donor levels and the valence band arising from a very fast energy transfer from the electron hole pair excited across the band gap of the nanocrystal. The emission spectra of Cr doped and undoped samples appear similar. An appreciable luminescence quenching is observed in Cr doped samples compared to the undoped samples, with the PL peaking around the same wavelength namely 435 nm. The
luminescence quenching as a result of Cr doping is attributed to repeated excitations within the Cr sites and thermal escape of charge carriers from confined states to other states [28]. It is seen that Mn2+-doped ZnS samples exhibit three emission bands at 414, 445 and 592 nm. The peak at 414 nm is assigned to interstitial sulfur, where as the peak at 445 nm is assigned to zinc interstitial since sulfur ion has larger ionic radius (1.7 Å) than that of zinc ion (0.74 Å), interstitial sulfur produces more strain in the ZnS lattice and thus the electron levels due to this site will have smaller binding energy. Therefore, interstitial sulfur energy levels must be closer to valence band than the interstitial zinc energy levels to the conduction band. Similarly sulfur vacancy states are closer to conduction band edge than zinc vacancies states to the valence band edge. The PL peak at 592 nm is assigned to Mn2+ ion which arises due to 4T1 ? 6A1 transition within the 3d shell of Mn2+ [29]. When Mn2+ ions substitute Zn2+ cation sites in ZnS lattice, the mixing of s–p electrons of host ZnS into the 3d electrons of Mn2+ causes strong hybridization and makes the forbidden transition of 4T1 ? 6A1 partially allowed, this yields orange emission at 592 nm. For samples co-doped with (Mn, Cr), the blue emission wavelength is extended to lower energy. The PL spectra exhibit two emission peaks at 445 and 598 nm. It is noticed that in the co-doped samples the PL characteristics of individual dopants namely Cr and Mn are retained with the intensity of the blue emission in the intermediate range. However, slight variations in the emission wavelength and intensity of orange emission are observed in co-doped and Mn doped ZnS nanoparticles.
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Fig. 2. SEM images of (a) ZnS (b) ZnS:Mn (c) ZnS:Cr and (d) ZnS:(Cr, Mn) nanoparticles.
3.6. FTIR analysis FTIR spectra of ZnS and ZnS:(Cr,Mn) nanoparticles recorded at room temperature in the wavelength range of 4000–400 cm 1 are shown in Fig. 7. The templates like EDTA-type compounds belong to the class called complexones, a group of polyaminocarboxylic acids or their salts, which are derivatives of iminodiacetic acid. A characteristic arrangement of these compounds is a nitrogen atom connected with two carboxymethyl groups N(CH2COOH)2. From
ZnS ZnS:Mn ZnS:Cr ZnS:(Cr,Mn)
Intensity (a.u)
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(220) (311)
the point of view of EDTA-type molecular structure, it may be treated as a tertiary amine or acetic acid derivative. The spectral band at 2352 cm 1 in Fig. 7 is associated with the N–H3+ stretching vibration of the amino acid zwitterions. The broad band around 3393 cm 1 is assigned to the O–H stretching vibration because all FTIR spectra are recorded by mixing samples with KBr. Hence there may be some adsorbed water vapor, as KBr is hygroscopic. The spectral transitions at 1583 and 1400 cm 1 are, respectively, related to the antisymmetric and symmetric C@O stretching vibrations of the carboxylate anion associated with EDTA [30]. On the other hand, EDTA shows no vibrational band around 2100 cm 1, suggesting that EDTA in a ZnS matrix does not form a double betaine structure. However, the carboxylate ion groups are still active in the entrapped EDTA in ZnS as evidenced by the observed spectral bands at 1637 and 1401 cm 1 in Fig. 7. The peaks appearing at 473 and 663 cm 1 are assigned to the Zn–S stretching vibration. This obviously excludes the possibility that intact EDTA simply coexists with the ZnS nanoparticles. Since the surface capping of sulfide particles by carboxylate ion groups is attributed to coordination of the deprotonated EDTA groups to the sulfide surfaces, these IR spectra strongly confirm the surface capping of the ZnS nanoparticles through direct bonding of the EDTA, presumably to the ZnS site at the surface. The close similarity of the FTIR spectra of doped and undoped samples indicates that Cr and Mn have entered the ZnS lattice substitutionally. 3.7. EPR studies
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Fig. 3. X-ray diffraction patterns of ZnS, ZnS:Mn, ZnS:Cr and ZnS:(Cr, Mn) nanoparticles.
To get information about the oxidation state and site occupancy of the transition metal ion in the host ZnS lattice, and also to understand the interplay between the carrier concentration and magnetic exchange coupling at the microscopic level, EPR experi-
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ments were conducted on these samples. No EPR signal was detected in the spectra of ZnS indicating that the host materials used in the present study are free from paramagnetic impurities.
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Fig. 4. TEM images of (a) ZnS:Mn and (b) ZnS:(Cr, Mn) nanoparticles.
When manganese ions were doped into the ZnS matrix the EPR spectra of all the investigated samples exhibited resonance signals. Fig. 8 (a) shows the room temperature EPR spectra for Zn0.97Mn0.03S and Zn0.94Mn0.03Cr0.03S nanoparticles. To make the comparison clear, the EPR spectrum of ZnS:Cr (3 at.%) are also shown in Fig. 8(b). From Fig. 8 (a) it is clear that Mn doped ZnS nanoparticles exhibit well-resolved resonance signal at g = 2.08, with a six line hyperfine structure. The hyperfine structure originates from the interaction between the Mn2+ electron cloud and 55 Mn nucleus with the spin I = 5/2. This indicates the presence of the paramagnetic Mn2+ ions in the samples. This unambiguously confirms the successful doping of paramagnetic Mn2+ ions in the ZnS crystal lattice. The electron configuration of Mn2+ ion is 3d5 (S state). In the case of d5 metal ions, it is known that the axial distortion of octahedral symmetry gives rise to three Kramer’s doublets |±5/2), |±3/2) and |±1/2). An application of Zeeman field will split the spin degeneracy of the Kramer’s doublets. As the crystal field splitting is normally much greater than the Zeeman field, the resonances observed are due to transitions within the Kramer’s
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Wavelength (nm) Fig. 5. Absorption spectra of ZnS, ZnS:Mn, ZnS:Cr and ZnS:(Cr, Mn) nanoparticles.
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Fig. 8. EPR spectra of (a) ZnS:Mn and ZnS:(Cr, Mn) (b) ZnS:Cr nanoparticles.
doublets split by the Zeeman field [31]. The resonance at g 2.08 is due to Mn2+ ions in an environment close to an octahedral symmetry and is known to arise from the transition between the energy levels of the lower doublet |±1/2). Another resonance signal at g = 4.03, arising from the transition between the Zeeman split energy levels of middle Kramer’s doublet of |±3/2), is also observed. Usually the intensity of the resonance signal at g = 4.03 is much lower than that at g = 2.08. ZnS:Cr samples exhibit single resonance signal at g = 1.989. Generally EPR spectrum of chromium powder comprises three lines, but in our studies we obtained single broad EPR signal because of exchange interaction. The present authors have earlier reported [16] spectral studies with different Cr concentrations in ZnS nanoparticles. Zn0.94Cr0.03Mn0.03S nanoparticles also exhibit well-resolved resonance signal at g = 2.08, with a six line hyperfine structure. But compared to Zn0.97Mn0.03S samples the intensity of the EPR signal drops significantly. The spins of manganese and chromium ions in Zn0.94Cr0.03Mn0.03S are coupled by the exchange interaction that narrows the EPR signal and its intensity in the paramagnetic phase, and g factors of the manga-
nese and chromium ions are nearly 2.0. Hence, we assume that the resonance signal with g = 2.08 in the EPR spectrum is related to the manganese and chromium ions in the paramagnetic state. The strong decrease in EPR signal intensity indicates the probability of ferromagnetism in Zn0.96Mn0.03Cr0.03S samples. The peak-to-peak linewidth (DH) values of the hyperfine lines obtained from the EPR signals are 560 G for Zn0.97Mn0.03S and 436 G for Zn0.94Mn0.03Cr0.03S and 367 G for ZnS:Cr nanoparticles. Generally the change in linewidth mainly depends on the variation of temperature and concentration of the dopants. In the present case it is almost independent of temperature because all EPR studies are carried out at room temperature only. So the decreased linewidth in Zn0.96Mn0.03Cr0.03S might be an indication of considerable changes in the magnitude of the exchange interaction on addition of chromium and manganese in the host lattice of ZnS. The number of spins (Ns) participating in the resonance is calculated by using the formula: Ns = 0.285 I (DH)2 where ‘I’ is the peak-to-peak height and DH is the linewidth (in G) and are found to be 1.2 106, 7.4 105 and 1.7 105 for ZnS:Mn, ZnS:Cr and ZnS:(Cr, Mn),
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respectively. The values of hyperfine splitting constant (A) are determined from the average value of hyperfine splitting of successive allowed hyperfine lines of the central sextet and are found to be 90 G for Zn0.97Mn0.03S and 79 G for Zn0.94Mn0.03Cr0.03S. The strength of the hyperfine splitting (A) depends on the matrix into which the ion is dissolved and is mainly determined by the electronegativity of the neighbors. This means ‘A’ is a direct qualitative measure of the degree of the covalency of the bonding in the matrix. The magnitude of the hyperfine splitting constant (A) in the present samples indicates the bonding between Mn2+ and the surrounding ligands is moderately covalent. Further, by the addition of Cr in ZnS:Mn, the hyperfine splitting constant decreases indicating that there is decrease in the strength of ionic bonding on addition of Cr to ZnS:Mn.
3.8. VSM studies Fig. 9 shows the field-dependent magnetization (M–H) curves of ZnS, Zn0.97Mn0.03S, Zn0.97Cr0.03S and Zn0.94Cr0.03Mn0.03S nanoparticles at room temperature in the field range of 0 to ±10 000 Oe. A typical diamagnetic behavior has been observed in the host ZnS and is attributed to the absence of unpaired electrons of its‘d’ orbital. From the figure it is obvious that Mn doped ZnS nanoparticles exhibit straight lines indicating that the samples are paramagnetic at room temperature. It may be noted that in the present case, the absence of ferromagnetism in Mn-doped ZnS nanoparticles can be explained with theoretical ab initio calculations [32]. Mn 3d states in the ZnS lattice, with a cubic crystal-field, split into an e doublet and a t2 triplet. e Levels have a lower energy than t2 levels. t2 Levels are fully occupied leading to the absence of a hole for the coupling. It can, however, be ferromagnetic if a double exchange via the localized holes is predominant. In EPR a well resolved EPR signal is observed as mentioned in the previous section, which also confirmed the paramagnetic nature of ZnS:Mn nanoparticles. ZnS:Cr nanoparticles exhibit a strong ferromagnetic ordering at room temperature. The present authors have previously reported ferromagnetism and EPR studies on ZnS:Cr nanoparticles [16]. In brief, the observed ferromagnetism in ZnS:Cr nanoparticles was attributed to the substitution of Cr for Zn and the room temperature EPR signal showed the presence of Cr3+ ions in the samples, which provide the necessary unpaired spins for ferromagnetism rather
0.03
M (emu/g)
0.02
ZnS ZnS:Mn ZnS:Cr ZnS:Mn&Cr
4. Conclusions
0.01
0.00
-0.01
-0.02
-0.03 -10000
than the Cr clusters. Thus, the observed ferromagnetism in the Cr doped ZnS samples was attributed to the exchange interaction between localized ‘d’ spins on the Cr ions and the free delocalized carriers. The present study reports the effect of Mn co-doping in Cr (3 at.%) doped ZnS nanoparticles. Zn0.94Cr0.03Mn0.03S nanoparticles also exhibit room temperature ferromagnetism. The roomtemperature ferromagnetic phase could not be due to Mn-related oxides, such as MnO, MnO2, Mn2O3 and Mn3O4, because none of them shows ferromagnetism above room temperature. Similarly this is not due to the Cr related metal clusters like phases of magnetic (CrO2) or antiferromagnetic (Cr2O3 and Cr3O4) either since different phases were not noticed in XRD and TEM studies. Hence the observed ferromagnetism in ZnS: (Cr, Mn) nanoparticles is due to substitution of Cr and Mn at Zn sites. This means the observed ferromagnetism may be attributed solely to the doping of (Cr, Mn) in ZnS lattice and not due to the presence of any secondary phases. Here Mn and Cr ions, provide the necessary unpaired spins for ferromagnetism rather than the Cr, Mn clusters. Thus, the ferromagnetism in the Zn0.94Cr0.03Mn0.03S samples could be considered as a result of exchange interaction between localized ‘d’ spins on the Cr, Mn ions and the free delocalized carriers. In this study small amount of Mn co-doping, however, brought drastic changes in magnetic properties as illustrated in Fig. 9. ZnS doped with both Mn and Cr showed a suppression in magnetization. Initially, as the electron density increases with Mn doping, the magnetization decreases. This is consistent with the bound magnetic polaron model, it provides a mechanism whereby holes that are localized at or near the Mn2+ ions are responsible for mediating ferromagnetism. However, only a fraction of the Mn ions are expected to order ferromagnetically due to competing superexchange antiferromagnetic interactions between neighboring Mn ions. The remaining Mn ions may form antiferromagnetic Mn clusters as a result of decreased Mn–Mn separation leading to the suppression of ferromagnetism in (Mn, Cr) co-doped samples. The values of coercive fields (Hc) are 152.30, 363.5 G. The values of magnetization (Ms) are 27.964 10 3, 20.186 10 3 emu/g and the values of retentivity (Mr) are 3.3979 10 3, 5.8944 10 3 emu/g for Zn0.97Cr0.03S and Zn0.94Cr0.03Mn0.03S, respectively. In co-doped samples coercive field (Hc) and retentivity increase but magnetization decreases. In co-doped samples the increased coercivity is attributed to the strong antiferromagnetic coupled interaction between Mn ions. There are no reports on magnetic studies of Cr, Mn co-doped ZnS nanoparticles for comparison. More detailed investigations on the dopant induced ferromagnetism are needed to understand the mechanism of magnetism in these materials. Such investigations are under progress in our laboratory. Obviously, the co-doping of Mn does indeed play a key role in tuning the ferromagnetism of the ZnS:Cr nanoparticles.
-5000
0
5000
10000
Applied Field (G) Fig. 9. VSM spectra of ZnS, ZnS:Mn, ZnS:Cr and ZnS:(Cr, Mn) nanoparticles.
ZnS and ZnS:Mn, ZnS:Cr and ZnS: (Mn, Cr) nanoparticles were synthesized successfully through chemical co-precipitation using EDTA as the capping agent. EDS spectral studies confirmed the presence of Mn and Cr in the samples. XRD, studies showed that the particles crystallized in cubic Zincblende structure with the dopant metal atoms entering the lattice as substituents. FTIR spectra confirmed that EDTA played the role of template and simply coexisted in the lattice. Band gap values from DRS spectra were in the range of 3.81–4.09 eV which are higher than the bulk ZnS values due to size quantization effect. In PL spectra a sulfur–vacancy-related PL band at around 430 nm, another PL orange emission band associated with the 4T1 ? 6A1 transition of Mn2+ were observed. Magnetization studies indicated that ZnS nanoparticles exhibited diamagnetic behavior, where as ZnS:Mn were paramagnetic and ZnS:Cr exhibited
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strong room temperature ferromagnetism. Suppression in room temperature ferromagnetism was observed in Mn co-doped ZnS:Cr nanoparticles due to Mn–Mn antiferromagnetic interactions. Acknowledgments The authors are grateful to the University Grants Commission (UGC), New Delhi, Government of India, for financial support. One of the authors (D. Amaranatha Reddy) is thankful to the UGC, for the award of the UGC-BSR Fellowship in Sciences for Meritorious Students. The authors are thankful to Mr. N. Sivaramakrishnan and Mr. Muttukumar, SAIF, IIT Madras, Chennai for extending the VSM. References [1] K. Sato, H. Katayama-Yoshida, Hyperfine interactions and magnetism of 3d transition-metal-impurities in II–VI and III–V compound-based diluted magnetic semiconductors, Hyperfine Interactions 136 (137) (2001) 737–742. [2] S.A. Wolf, D.D. Awschalom, R.A. Buhrman, J.M. Daughton, S. von Molnar, M.L. Roukes, A.Y. Chtchelkanova, D.M. Treger, Spintronics: a spin-based electronics vision for the future, Science 294 (2001) 1488–1495. [3] Lijuan. Zhao, Bei. Zhang, Qi. Pang, Shihe. Yang, Xixiang. Zhang, Weikun. Ge, Jiannong. Wang, Chemical synthesis and magnetic properties of dilute magnetic ZnTe:Cr crystals, Appl. Phys. Lett. 89 (2006) 092111–092113. [4] H. Saito, V. Zayets, S. Yamagata, K. Ando, Room-temperature ferromagnetism in highly Cr-doped II–VI diluted magnetic semiconductor Zn1-xCrxTe, J. Appl. Phys. 93 (2003) 6796–6798. [5] G. Krishnaiah, N. Madhusudhana Rao, B.K. Reddy, D. Raja Reddy, T. Mohan Babu, S. Sambasivam, P. Sreedhara Reddy, EPR and magnetic properties of vapour phase grown Zn1 xCrxTe crystals, Phys. Lett. A 372 (2008) 6429–6433. [6] Taejoon Kang, Joonho Sung, Wooyoung Shim, Heesung Moon, Jaehun Cho, Younghun Jo, Wooyoung Lee, Bongsoo Kim, Synthesis and magnetic properties of single-crystalline Mn/Fe-doped and Co-doped ZnS nanowires and nanobelts, J. Phys. Chem. C 113 (2009) 5352–5357. [7] D.V. Martyshkin, V.V. Fedorov, C. Kim, I.S. Moskalev, S.B. Mirov, Mid-IR random lasing of Cr-doped ZnS nanocrystals, J. Opt. 12 (2010) 024005–024009. [8] Kunio Ichino, Yasuharu Morimoto, Hiroshi Kobayashi, Molecular beam epitaxy and structural properties of ZnCrS, Phys. Status Solidi C 3 (2006) 776–779. [9] Ming Luo, N.Y. Garces, N.C. Giles, Utpal N. Roy, Yunlong Cui, Arnold Burger, Optical and electron paramagnetic resonance spectroscopies of diffusiondoped Co2+:ZnSe, J. Appl. Phys. 99 (2006) 073115–073709. [10] Amit D. Lad, Ch. Rajesh, Mahmud Khan, Naushad Ali, I.K. Gopalakrishnan, S.K. Kulshreshtha, Shailaja Mahamuni, Magnetic behavior of manganese-doped ZnSe quantum dots, J. Appl. Phys. 101 (2007) 103906–103910. [11] P. Vinotha Boorana Lakshmi, K. Sakthi Raj, K. Ramachandran, Synthesis and characterization of nano ZnS doped with Mn, Cryst. Res. Technol. 44 (2009) 153–158. [12] M.M. Rashad, D.A. Rayan, K. El-Barawy, Hydrothermal synthesis and magnetic properties of Mn doped ZnS nanoparticles, J. Phys.: Conference Series 200 (2010) 072077–072081. [13] S. Sambasivam, D. Paul Joseph, J.G. Lin, C. Venkateswaran, Doping induced magnetism in Co–ZnS nanoparticles, J. Solid State Chem. 182 (2009) 2598– 2601.
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