Appl Phys A (2011) 102: 393–399 DOI 10.1007/s00339-010-6047-8
Optical, structural and magnetic properties of Zn0.9 Cd0.1 S:yCo nanoparticles Amit Kumar Chawla · Sonal Singhal · Hari Om Gupta · Ramesh Chandra
Received: 1 July 2010 / Accepted: 2 September 2010 / Published online: 17 September 2010 © Springer-Verlag 2010
Abstract Zn0.9 Cd0.1 S:yCo nanoparticles were prepared by a co-precipitation method at low temperature. The obtained products were identified to be of cubic structure without any impurity phase. Cobalt incorporation leads to an increase in the local strain value and a decrease in the lattice constants as measured from XRD. Magnetic measurements showed that cobalt was incorporated in the Zn0.9 Cd0.1 S lattice as Co2+ and substituted for the Zn site as there was no evidence of the presence of metallic cobalt. Transmission electron microscopy suggests the crystalline nature of nanoparticles, with average particle size of ∼3.5 nm. UV-Vis measurements showed a red shift with respect to undoped nanoparticles in energy band gap with increasing cobalt concentration. Photoluminescence spectra reveal the defect-related emissions. The decay time constant is found to be in the nanosecond regime and is attributed to the spatial confinement of photo generated electron–hole pairs.
1 Introduction Transition metal (TM) doped II–VI and III–V semiconductors have been investigated extensively due to their wide range applications in electroluminescence devices such as light-emitting displays, optical sensors, etc. [1–3]. Tuning of physical and chemical properties by changing the particle A.K. Chawla · S. Singhal · R. Chandra () Nanoscience Laboratory, Institute Instrumentation Center, Indian Institute of Technology Roorkee, Roorkee 247667, India e-mail:
[email protected] S. Singhal · H.O. Gupta Department of Electrical Engineering, Indian Institute of Technology Roorkee, Roorkee 247667, India
size could cause problems in many applications, in particular, if unstable small particles (less than 2 nm) are used [4, 5]. Recent advances have led to the exploration of tunable optical properties by changing their constituent stoichiometries in mixed ternary nanoparticles [6, 7]. Undoped ternary nanoparticles, whose bandgap is tuned as a function of their composition, were reported including Zn1 − x Cdx Se [5, 7, 8] and CdSe1 − x Tex [9]. II–VI semiconductor material, Zn1 − x Cdx S is also considered to be a promising host material [10]. The introduction of transition metal (TM) into nonmagnetic semiconductors provides another possible way for generation of diluted magnetic semiconductors (DMS) [11, 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 [13]. With the hope of combining the unique advantages of quantum confinement with those of DMS, a huge interest has developed in recent times to study the DMS nanoparticles. Successful doping of magnetic TM in semiconductor nanoparticles yields a new paradigm as a zero dimensional counterpart of the well developed DMS and photoluminescent bulk and thin-film systems, which could potentially lead to the fabrication of unusual magneto-optical and magneto-electronic devices [3, 14, 15]. Continuous attempts are being made to synthesize sulfide nanomaterials with controlled sizes, shapes, and phase purity by various chemical routes [16–20]. 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. [21] reported the one-dimensional nanocomposites of CdS/ZnS. Mehta et al. [22] synthesized the ZnS nanoparticles via facile CTAB aqueous micellar solution route.
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Recent studies of Mn doped ZnS nanoparticles revealed significant increase of luminescence intensity. The increase of luminescence intensity is attributed to the strong interaction of Mn2+ d electrons with s–p electrons of the host nanocrystalline ZnS [23]. The sp–d exchange interactions in Co2+ doped II–VI semiconductors have been found to be much larger than those in the Mn2+ doped counterparts [18, 19]. In this study, Zn0.9 Cd0.1 S:yCo nanoparticles with different cobalt concentrations were prepared by the coprecipitation method [24]. With the help of magnetic measurements and structural analysis, we demonstrated that the dopants are embedded within the nanoparticles. The relationship of optical properties of Zn1 − x Cdx S:yCo nanoparticles to the doping amount is explored systematically.
2 Experimental Undoped and cobalt doped Zn0.9 Cd0.1 S nanoparticles were synthesized by employing co-precipitation method. Synthesis procedure was carried out without addition of any capping ligand or surfactant. Requisite amounts of zinc nitrate (0.5 M), cadmium nitrate (0.05 M) and appropriate molar amount of cobalt nitrate aqueous solution were thoroughly mixed and kept at 280 K using ice bath. 0.5 M sodium sulfide aqueous solution was added into the above solution with a dripping speed of 10 drops/minute. Uniform magnetic stirring was provided for the better atomic diffusion during the length of the reaction. The resulting precipitates were filtered off and washed several times in distilled water and acetone. The precipitates were dried in hot air oven at 323 K. Final products were then obtained by crushing dried precipitate using pestle mortar. A series of Zn0.9 Cd0.1 S nanoparticles doped with cobalt concentration of 0.005, 0.01, 0.015, 0.025, and 0.05 M were prepared. Doping concentrations of cobalt were determined by energy dispersive spectrometer attached with FE-SEM (FEI Quanta 200 F). The particle size, morphology, structure and electron diffraction were determined by transmission electron microscopy (FEI TECNAI-G2 ). X-ray analysis was performed by using a Bruker D8 Advance Diffrac-
tometer with CuKα target (λ = 1.54056 Å) radiation. Optical absorption was measured in the 200–800 nm wavelength range using UV-Vis-NIR spectrophotometer (Cary 5000 Varian). Photoluminescence emission spectra were recorded at room temperature with a photoluminescence spectrometer (Perkins Elmer LS 55). Fluorescence life times were recorded using Horiba Jobin Yvon Fluorescence Lifetime system using LEDs and LDs as excitation sources. Magnetic measurements were taken with superconducting quantum interference device (SQUID) magnetometer (QD MPMS-XL).
3 Results and discussions Determination of composition, structure and particle size is very important for the discussion on the physical properties. EDS (Energy dispersive spectroscopy) analysis shows that the cobalt concentration between different bunches of particles, is very consistent and suggest a homogeneous distribution of cobalt in our samples. Molar cobalt values (y) as obtained by EDS are shown in Table 1. Figure 1 shows the XRD patterns of the Zn0.9 Cd0.1 S:yCo nanoparticles along with the undoped sample. Broad diffraction peaks in all patterns are in agreement with the characteristics of nanosized materials. It can be seen that all samples exhibited a cubic structure, and is in consistence with the results reporting that ZnS exist in cubic structure at low temperature [20, 23]. However the diffraction peak (111) shifted to a lower angle from 28.6 to 28.45◦ from the standard cubic structure of ZnS [20, 23]. This shift toward lower angle is believed to result from the incorporation of Cd ions into the ZnS lattice, and the larger ionic radius of Cd2+ as compared to that of Zn2+ (Cd2+ : 0.97 Å, Zn2+ : 0.88 Å) [25, 26]. Average crystallite size of 3.5 nm is estimated from the full width at half maximum of the major XRD peak using the Scherrer equation [27]. Strain analysis has been carried out to find out the extent of strain that have induced in the host lattice due to cobalt incorporation. Local strain is calculated by making use of Scherrer’s formula of k vs k (the scattering vector k =
Table 1 Co2+ molar concentrations in starting solution, y, analyzed from EDS, average particle sizes, local strain, lattice constant as obtained by XRD, energy band gap as determined by UV-Vis measurements, and decay time constant obtained from life time measurements Molar Co
y of
d XRD
in starting solution
Zn1 − x Cdx S:yCo
(nm)
0.005
0.0045
3.8
0.010
0.0077
3.8
0.015
0.0091
0.025 0.050
Local strain
Lattice constant
Band gap
Decay constant
(Å)
(eV)
(ns)
0.0425
5.391
3.81
2.58
0.0428
5.387
3.76
3.05
3.6
0.0432
5.384
3.71
2.39
0.0122
3.5
0.0524
5.376
3.66
3.39
0.0202
3.01
0.0876
5.364
3.60
3.48
Optical, structural and magnetic properties of Zn0.9 Cd0.1 S:yCo nanoparticles
Fig. 1 X-ray diffraction patterns of Zn0.9 Cd0.1 S:yCo (y = 0.0, 0.005, 0.01, 0.015, 0.025, and 0.05 M) nanoparticles
Fig. 2 Variation in local strain and lattice constant with cobalt concentration Zn0.9 Cd0.1 S:yCo (0.005, 0.01, 0.015, 0.025, and 0.05 M)
(4π/λ)Sin θ ) [28]. The (111), (220), and (311) peaks are fitted linearly to obtain the local strain values. Calculated values of local strain are shown in Table 1. On the left axis of Fig. 2 local strain values are shown with variation in molar cobalt concentration. It can be seen from Fig. 2 that there is no observable change in the strain values when the cobalt concentration is ≤0.015 M. However with further increase in cobalt concentration the local strain in the nanoparticles increases due to the presence of large amount of Co2+ ions in the host lattice. Semiconductor alloys (solid solutions) have been proposed to obey Vegard’s law [29], revealing the linear relationship between the lattice constant and composition as follows: 0 0 0 aA (x) = (1 − x)aAC + xaBC , 1−x Bx C
(1)
0 where aA is the natural constant of the ternary form 1−x Bx C 0 and a 0 are the natural constants of the A1 − x Bx C and aAC BC
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binaries AC and BC respectively, and x is the mole fraction of binary BC. In the case of cubic Zn0.9 Cd0.1 S nanoparticles, the ideal lattice constant with Cd concentration of 0.05 0 = 5.345 Å (PDF # M is 5.392 Å, where we have cubic aZnS 0 80-0020) and aCdS = 5.820 Å (PDF # 01-0647). Lattice constants have also been calculated by XRD patterns. Lattice constant for undoped sample is 5.390 Å, which is comparable to the lattice constant obtained by Vegard’s law. Also, from Fig. 1, it can be observed that there is a slight shift in the XRD peak position to higher angles with increase in cobalt concentration. On the right axis of Fig. 2 we showed variation in lattice constants of Zn0.9 Cd0.1 S:yCo nanoparticles for different cobalt concentrations. From Fig. 2 it is also observed that the lattice constant of doped nanoparticles does not vary significantly. Similar dependence of the lattice constant on cobalt concentration is usually observed in the reported results [30]. Moreover, this also reflects that Co2+ ions were substituted without changing the cubic structure. This is quite expected as the ionic radii of Co2+ (0.745 Å) in the tetrahedral coordination are nearly the same as that of Zn2+ site (0.88 Å) [26, 31]. As a result the unit cell parameters do not vary significantly with increase in doping concentration. Field dependent magnetization (M–H ) curves of the sample Zn0.9 Cd0.1 S:0.0202Co with highest cobalt concentration at 5, 50, 100 and 300 K temperatures are shown in Fig. 3(a). No hysteresis is observed in our samples. It is found that the magnetic moment M increases with increasing external field H for the temperatures considered (5, 50, 100 and 300 K), indicating paramagnetic behavior. For the sake of comparison M–H measurement of undoped sample is also taken at 5 & 300 K and shows diamagnetic behavior. Corresponding graphs are shown in Fig. 3(b). No difference is observed between the magnetizations measured at 5 and 300 K for undoped sample. Fig. 3(c) shows the temperature dependence of magnetization of the selected sample Zn0.9 Cd0.1 S:0.0202Co in a field of 200 Oe. From 300 to 75 K the sample shows a very small value of magnetic moment, while below 75 K it increases sharply with decrease in temperature. In this temperature range the measured magnetic moment can be described by the Curie–Weiss Law M ∝ 1/T [32]. Inverse susceptibility as a function of temperature is shown in the inset of Fig. 3(c). The Curie–Weiss temperature for the selected sample is found close to zero by extrapolating the high temperature linear part. This indicates that the antiferromagnetic exchange between cobalt magnetic moments is very weak. The results are different from those of bulk cobalt doped ZnS where Chen et al. [33] have reported a large negative Curie–Weiss temperature as well as strong antiferromagnetic interactions. In addition, from the slope in the plot of χ −1 versus T shown in the inset of Fig. 3(c), the effective magnetic moment can be calculated [32], and is found to be 2.89 μB /Co, where μB is
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We have taken the electron diffraction pattern at different regions on the copper grid for each sample. Figure 4(a) shows the TEM image for the particular sample Zn0.9 Cd0.1 S:0.0202Co. All samples across the different concentrations of cobalt show TEM images with nearly spherical particles and having an average diameter of 3– 4 nm. Figure 4(b) shows the selected area electron diffraction pattern characteristic of a cubic phase. In Fig. 4(b) the first ring indicates a periodical structure with length of 3.1 Å, which is coincident with the standard cubic Zn0.9 Cd0.1 S interplanar distance of 3.126 Åin (111) direction, which is essentially the same as that obtained by Vegard’s law [29] within experimental uncertainties, establishing the internal consistency between independent measurement. We deduce that there is no CoS or Co in our samples for the following reasons: firstly, the structure and XRD pattern of CoS and Co is totally different from that of Zn1−x Cdx S. There is no XRD peak corresponding to that of Co or CoS, if it is present at all, it is beyond the detection limit (5%) of XRD. Secondly, if there is even a trace amount of ferromagnetic Co in the precipitates, the sample should exhibit ferromagnetism, while the highest cobalt doped sample show paramagnetic behavior as depicted in Fig. 3 (SQUID measurements). Also, we did not find any other diffraction rings in our TEM diffraction pattern that cannot be indexed by the cubic structure. This eliminates the possibility of ferromagnetic Co in our samples. Absorption edges of semiconductors correspond to the threshold for charge transition between the highest nearly filled band and the lowest nearly empty band. According to interband absorption theory, the optical band of the nanoparticles can be calculated using the following relation [35] αhν = A(hν − Eg )n ,
Fig. 3 (a) Magnetization versus applied magnetic field measured at 5, 50, 100, and 300 K of Zn0.9 Cd0.1 S:0.0202Co nanoparticles. (b) Magnetization versus applied magnetic field measured at 5 and 300 K of undoped sample. (c) Temperature dependent χ (magnetic mass susceptibility) measured under a magnetic field of 200 Oe for Zn0.9 Cd0.1 S:0.0202Co nanoparticles. Inset: χ −1 (Inverse susceptibility) as a function of temperature
the Bohr magneton. A similar value of effective magnetic moment for cobalt doped ZnO is reported in the literature [32, 34].
where α is the absorption coefficient, A is the probability parameter for the transition, h is Planck’s constant, ν is the photon frequency, Eg is the optical bandgap and n is 1/2 for direct band gap semiconductor. In the case of alloyed nanoparticles, the bandgap energies are determined by their size and composition i.e. quantum confinement effect and alloying effect. In bulk CdS–ZnS alloyed crystals their composition (x) dependent bandgap energies Eg (x), can be expressed by the relation [29] Eg (x) = Eg (ZnS) + Eg (CdS) − Eg (ZnS) − b x + bx 2 , (2) where Eg (ZnS) and Eg (CdS) are the band gap energies for bulk ZnS and CdS, respectively, and b is the bowing parameter and has the value 0.61 [29, 36]. For Zn1 − x Cdx S nanoparticles with average particle size of 3.5 nm as determined by XRD, Cd concentration of 0.05 M, the bandgap
Optical, structural and magnetic properties of Zn0.9 Cd0.1 S:yCo nanoparticles
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Fig. 4 (a) TEM image of Zn0.9 Cd0.1 S:0.0202Co nanoparticles. (b) Corresponding SAED pattern showing cubic structure
Fig. 5 (αhν)2 versus hν plots of Zn0.9 Cd0.1 S:yCo (y = 0.0, 0.005, 0.01, 0.015, 0.025 and 0.05 M) nanoparticles
energy for the host system i.e. Zn0.9 Cd0.1 S can be calculated. Shifts of 0.40 and 0.52 eV in the band gap values for ZnS and CdS are found in the quantum confinement regime using the Brus equation [37]. Therefore instead of using 3.6 eV for ZnS and 2.38 eV for CdS [38], 4.005 eV for ZnS nanoparticles and 2.907 eV for CdS nanoparticles are plugged into (2) and the resulting composition (x) dependent bandgap energy of 3.84 eV is found for Zn0.9 Cd0.1 S nanoparticles in the quantum confinement regime. Figure 5 shows the plot of (αhν)2 against the photon energy (hν) of the Zn1−x Cdx S:yCo nanoparticles with increasing cobalt concentration. The direct band gap of these nanoparticles is determined by taking an extrapolation of the linear region of a plot of (αhν)2 . The band gaps for an undoped sample (Eg = 3.87 eV) obtained from UV-Vis measurements are in agreement with the composition dependent quantum confined energy band gap (Eg = 3.84 eV). It can be observed that there is a decrease in band gap values with increase in cobalt concentration. The red shift of the energy band gap with increasing cobalt concentration is interpreted as being
Fig. 6 Photoluminescence spectra of Zn0.9 Cd0.1 S:yCo nanoparticles with cobalt concentration of 0.005, 0.01, 0.015, 0.025, and 0.05 M
mainly due to the sp–d exchange interactions between the band electrons and the localized d electrons of the Co2+ ions substituting host ions and is consistent with the reported results [39]. Photoluminescence (PL) spectra can be used to study the defect-related emissions. Figure 6 depicts the respective PL spectra of the doped nanoparticles. When excited with ultraviolet light of wavelength 275 nm, nanoparticles exhibit a broad defect band. The peak at ∼400 nm (corresponding to label A in Fig. 6) may be ascribed to the hole traps originating from unsaturated sp3 orbitals of surface S atoms [40]. The emission band centering at ∼431 (corresponding to label B in Fig. 6) nm is ascribed to S vacancies [41]. Emission band ∼464 (corresponding to label C in Fig. 6) nm has been attributed to a self-activated center presumably formed between a Zn vacancy and a shallow donor associated with a sulfur vacancy [42]. Cd doping in ZnS can also result in a green emission band. The Zn1−x Cdx S nanoparticles had been reported to have a broad PL emission around 510–
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with cobalt concentration of 0.025 M show the maximum photoluminescence intensity. Furthermore, only a straightforward precipitation process in aqueous solution using simple inorganic precursors (metal nitrates and sodium sulfides) is used. No organic solvents or toxic gases (e.g. H2 S) are involved. Therefore the optical properties in the current work are obtained via a “green pathway”, which should be considered as an important factor for practical applications. Acknowledgements We are grateful to Dr. A.C. Pandey, Director, Nanophospher Application Centre, Allahabad for providing the Photo Luminescence facility. The financial support by DST [Grant No. SR/S5/NM-32/2005], New Delhi is gratefully acknowledged. Fig. 7 Fluorescence decay curves of Zn0.9 Cd0.1 S:0.0202Co nanoparticles with time calibration scale of 0.111541 nanosecond/Channels
550 nm with the Cd content ranging between 0.5 and 1.5% [43]. A broad peak centering at ∼504 (corresponding to label D in Fig. 6) nm was found in the PL spectrum and is attributed to the Cd content. Figure 7 shows the luminescence decay curve for Zn0.9 Cd0.1 S:0.0202Co. We have estimated the characteristics times of luminescence decay τ as the time of the signal decrease to a 1/e level. The decay curve exhibits the typical exponential behavior. The decay time constant is found 2.58 ns for the sample under investigation. Values of τ for all samples fall in a nanosecond range and are in agreement with the previously reported values of the cobalt doped system [44, 45]. Decay time constants for all samples are shown in Table 1. This ultrafast recombination is a result of the high degree of localization that occurs in these confined structures. Bhargava et al. [23] have reported that the trapping and recombination times are dramatically faster than the same transitions in bulk material. These trapping times in doped nanoparticles represent ultrafast minority carrier lifetimes and are expected to play a critical role in ultrafast devices.
4 Conclusions A solution-based low temperature synthetic route is adopted to synthesize the Zn0.9 Cd0.1 S:yCo nanoparticles. Both XRD and TEM show that the nanoparticles are of cubic structure and have an average particle size of ∼3.5 nm. We found a systematic variation in the lattice constants with variation in cobalt concentration. Local strain in the nanoparticles is found to increase with the increased cobalt concentration. A red shift in the energy band gap is found with increasing cobalt concentration. Cobalt doped Zn0.9 Cd0.1 S nanoparticles are found to have paramagnetic nature and confirms the substitutional doping of cobalt. PL spectrum shows the broad spectrum covering the visible range. Nanoparticles
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