Influence of Cobalt Doping on the Physical Properties of Zn0.9Cd0.1S Nanoparticles
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Zn0.9Cd0.1S nanoparticles doped with 0.005–0.24 M cobalt have been prepared by co-precipitation technique in ice bath at 280 K. For the cobalt concentration >0.18 M, XRD pattern shows unidentified phases along with Zn0.9Cd0.1S sphalerite phase. For low cobalt concentration (≤0.05 M) particle size, dXRDis ~3.5 nm, while for high cobalt concentration (>0.05 M) particle size decreases abruptly (~2 nm) as detected by XRD. However, TEM analysis shows the similar particle size (~3.5 nm) irrespective of the cobalt concentration. Local strain in the alloyed nanoparticles with cobalt concentration of 0.18 M increases ~46% in comparison to that of 0.05 M. Direct to indirect energy band-gap transition is obtained when cobalt concentration goes beyond 0.05 M. A red shift in energy band gap is also observed for both the cases. Nanoparticles with low cobalt concentrations were found to have paramagnetic nature with no antiferromagnetic coupling. A negative Curie–Weiss temperature of −75 K with antiferromagnetic coupling was obtained for the high cobalt concentration.
KeywordsCobalt doping Paramagnetism Quantum confinement
Semiconductor nanoparticles have generated great fundamental and technical interests due to novel size-tunable properties and, consequently, in potential applications as optoelectronic devices and biomedical tags [1, 2, 3, 4, 5]. In the last two decades, the main efforts have been focused on the preparation of different colour-emitting binary or core–shell nanoparticles with different particle sizes [6, 7, 8, 9]. However, the tuning of physical and chemical properties by changing the particle size could cause problems in many applications, in particular, if unstable small particles (less than 2 nm) are used . Recent advances have led to the exploration of tunable optical properties by changing their constituent stoichiometries in mixed ternary nanoparticles . The introduction of transition metal (TM) into non-magnetic semiconductors provide another possible way for generation of diluted magnetic semiconductors (DMS) [3, 12]. DMS can play a vital role in the field of spintronics because of its ability to accommodate electron charge and its spin degrees of freedom into single matter and their interplay can explore new functionality . There are contradictory reports on magnetic behaviour of these materials such as many people have reported presence of ferromagnetism in DMS systems, whereas some reported its absence [12, 13, 14, 15]. Continuous attempts are being made to synthesize sulphide nanomaterials with controlled sizes, shapes, and phase purity by various chemical routes [16, 17, 18]. The advantages of chemical routes over other synthesis methods are: (a) easier control of the oxidation states, (b) ability to make nanostructures of different sizes and shapes, (c) relatively cheap. Wang et al.  reported the one-dimensional nanocomposites of CdS/ZnS. Mehta et al.  synthesized the ZnS nanoparticles via facile CTAB aqueous micellar solution rout. It has been found that nanocrystals with dopants inside their crystal lattice can exhibit different properties from those with ones on their surface . However, experimental data is still lacking on the fundamental question of whether different dopant positions inside nanocrystals can affect physical properties of doped nanocrystals. Homogeneously substitutional doping is one of the most important goals for achieving novel physical properties in TM-doped nanosized semiconductors [20, 21].
In nanoparticles the systematic tuning of their band gap can be controlled by alloy formation as well as by size variation. Sung et al. and Yang et al. demonstrated that for undoped ternary nanoparticles, energy band gap can be tuned as a function of their composition including Zn1− xCdxSe [22, 23, 24] and CdSe1−xTex. As an II–VI semiconductor, Zn1−xCdxS is considered to be a promising host material. Zhong et al.  and Bhargava et al.  studies reveal that Mn-doped ZnS nanoparticles show significant increase in luminescence intensity and is due to the strong interaction of d electrons of Mn2+ with s–p electrons of the host nanocrystalline Zn. Zielinski et al.  and Seong et al.  reported that the sp–d exchange interactions in Co2+-doped II–VI semiconductors are much larger than those in the Mn2+-doped counterparts. In this study, cobalt-doped Zn0.9Cd0.1S alloyed (Zn0.9Cd0.1S: y Co) nanoparticles with different cobalt doping concentrations were prepared by the co-precipitation method. With the aid of structural, magnetic and quantitative analyses, we demonstrated that the dopants are embedded within the nanoparticles. The relationship of physical properties of Zn0.9Cd0.1S: y Co nanoparticles to the doping amount is explored systematically.
Cobalt-doped Zn0.9Cd0.1S alloyed nanoparticles were synthesized using the co-precipitation method without capping ligand or surfactant. Requisite amounts of 0.5 M zinc nitrate, 0.05 M cadmium nitrate and appropriate molar amount of cobalt nitrate aqueous solution were mixed thoroughly. 0.5 M sodium sulphide aqueous solution was added into the above mixture drop by drop along with continuous stirring at 280 K in ice bath. The particles were then centrifuged, rinsed with distilled water and dried in a hot air oven at 320 K. A series of Zn0.9Cd0.1S alloyed nanoparticles doped with cobalt concentrations of 0.0, 0.005, 0.01, 0.015, 0.025, 0.05, 0.12, 0.18 and 0.24 M were prepared. Doping concentrations of cobalt were determined by Electron Probe Micro Analyzer (Cameca SX 100). The particle size, shape and orientations of the nanoparticles were determined by transmission electron microscope (FEI TECNAI-G2). X-ray analysis was performed using a Bruker D8 Advance diffractometer with Cu Kα target (λ = 1.54056 Å) radiation. Optical absorption was measured in the 200–800 nm wavelength range using UV–Vis–NIR spectrophotometer (Varian Cary 5000). Magnetic measurements were taken with superconducting quantum interference device (SQUID) magnetometer (QD MPMS-XL).
Results and Discussions
Cobalt molar concentrations in starting solution, y analysed from EPMA, average particle size, local strain, lattice constant as obtained by XRD, and energy band gap as determined by UV–Vis measurements
Molar Cobalt in starting solution
y of Zn1−xCdxS:y Co
Lattice constant (Å)
Band gap (eV)
Unknown phase appeared
TEM observation reveals the particle size of ~3.5 nm for both the samples considered. XRD measurements reveal the particle size of ~3.5 nm for the low cobalt concentration (≤0.05 M) and are in agreement with the result obtained from the TEM. For high cobalt concentration (>0.05 M) there exist a large discrepancy in particle sizes obtained via XRD and TEM. Difference in the particle sizes calculated from XRD and observed from TEM can be attributed to the distorted lattice structure, where both anion (S2−) and cation (Zn2+) deviate from standard tetrahedral coordination. Ren et al.  have also reported the similar discrepancy in the particle sizes on increasing the cobalt doping in ZnS nanoparticles.
The usual method of determining band gap is to plot a graph between αh ν and h ν and look for that value of n which gives best linear graph in the band edge region . We plotted (αh ν)1/n versus h ν for Zn0.9Cd0.1S: y Co nanoparticles for each of the cobalt concentration, and the best fit were obtained for n = 1/2 for the samples for low cobalt concentration (≤0.05 M) indicating a direct transition. For the high cobalt concentration (>0.05 M) the best fit was obtained for n = 2, giving an evidence of an indirect transition. The appearance of change in gradients in the absorbance spectra at higher cobalt concentration might be caused by the deviation of lattice structure from undoped sample.
Bouloudein et al.  have considered in their study that the ferromagnetism in DMS is originated from the exchange interaction between free delocalized carriers and the localized d spins of the cobalt ions. Presence of free carriers is therefore necessary for the appearance of ferromagnetism. Free carriers can be induced either by doping or by defects or by cobalt ions in another oxidation state like Co3+. Above explanation suggests that our samples have limited number of impurities or defects, which may explain the absence of free carriers and consequently the ferromagnetism.
The most direct and immediate evidence for the alloying process for undoped Zn0.9Cd0.1S nanoparticles can be probed from the XRD peak position and the energy band gap obtained from UV–Vis measurement, found in consistent with the Vegard’s law, indicating the homogeneous distribution of ZnS and CdS in the alloyed nanocrystals. We also believe that there is no signature of CoS or other impurity phases in our samples. XRD does not show any detectable signal of Co or CoS, which means that the content of CoS or Co in the samples is at most less than 5% (5% is the detection limit of XRD). TEM diffraction pattern also supports our argument as we did not find any other diffraction rings in our TEM diffraction pattern that cannot be indexed by sphalerite structure. Also, if there is even a trace amount of ferromagnetic Co in the precipitates, the sample will exhibit ferromagnetism. Nanoparticles with low cobalt concentration are paramagnetic at 5 K, while the nanoparticles with high cobalt concentrations give rise to antiferromagnetism coupling, but the ferromagnetism did not appear at all. This eliminates the possibility of ferromagnetic cobalt existing in the samples. In this study, we found that the physical properties of Zn0.9Cd0.1S: y Co nanoparticles produced at different cobalt concentration are obviously different. Low cobalt concentration samples (≤0.05 M) have less distortion of the tetrahedral coordination of Co2+ ions and direct band gap absorption, while high cobalt concentration samples (>0.05 M) have more distortion of the tetrahedral coordination of Co2+ and indirect band gap absorption.
In summary, we have presented the synthesis of Zn0.9Cd0.1S: y Co alloyed nanoparticles from a solution-based synthetic route. Structural, optical and magnetic characterizations confirm that the cobalt doping is substitutional for zinc cations in the host lattice. The doping concentration in the alloyed nanoparticles can be divided into two distinct regions, low (≤0.05 M) and high (>0.05 M) cobalt concentration corresponding to the Co2+ molar percentage in the starting solution. Accordingly the structural, optical and magnetic properties were found distinctively different. A red shift in the energy band gap is found with increasing cobalt concentration. Cobalt-doped Zn0.9Cd0.1S nanoparticles are found to have paramagnetic nature for the nanoparticles with cobalt concentration of 0.05 M (low cobalt concentration). Nanoparticles with cobalt concentration of 0.18 M (high cobalt concentration) found to have antiferromagnetic coupling with negative Curie–Weiss temperature of −75 K. These findings have important implications for the optical and the magnetic materials, where the physical properties get significantly affected on increasing the doping.
One of the authors, Amit Kumar Chawla, thanks CSIR, New Delhi for the award of research associate. The financial support by DST [Grant No. SR/S5NM-32/2005] New Delhi is gratefully acknowledged.
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