Structural, optical and magnetic properties of Co doped CdS nanoparticles

Structural, optical and magnetic properties of Co doped CdS nanoparticles

Accepted Manuscript Structural, optical and magnetic properties of Co doped CdS nanoparticles G. Giribabu, G. Murali, D. Amaranatha Reddy, Chunli Liu,...

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Accepted Manuscript Structural, optical and magnetic properties of Co doped CdS nanoparticles G. Giribabu, G. Murali, D. Amaranatha Reddy, Chunli Liu, R. P. Vijayalakshmi PII: DOI: Reference:

S0925-8388(13)01707-6 http://dx.doi.org/10.1016/j.jallcom.2013.07.082 JALCOM 29006

To appear in: Received Date: Revised Date: Accepted Date:

12 April 2013 17 June 2013 13 July 2013

Please cite this article as: G. Giribabu, G. Murali, D. Amaranatha Reddy, C. Liu, R. P. Vijayalakshmi, Structural, optical and magnetic properties of Co doped CdS nanoparticles, (2013), doi: http://dx.doi.org/10.1016/j.jallcom. 2013.07.082

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Structural, optical and magnetic properties of Co doped CdS nanoparticles G. Giribabu1, G. Murali1, D. Amaranatha Reddy2, Chunli Liu2 and R. P. Vijayalakshmi*,1 1 2

Department of Physics, Sri Venkateswara University, Tirupati-517502, India. Department of Physics, Hankuk University of Foreign Studies YongIn, Gyeonggi Do 449-791, South Korea.

 

Abstract Pure and cobalt doped CdS nanoparticles were successfully synthesized by surfactant assisted simple chemical co-precipitation method. Size of the particles around 3nm and cubic zincblende structure were revealed by the X- ray diffraction pattern. From TEM images particle size was found be around 4 to 5 nm. Redshift of absorption edge and bandgap narrowing (from DRS spectra) can be attributed to increase in the carrier concentration by the inclusion of cobalt ions and creation of defect levels in the bandgap. three characteristic reflectance minima only in cobalt doped samples can be related to d-d transitions 4 A2(F)→3/2U1, 4A2(F) →E11 and 4A2(F) →5/2 U1 .Strong green emission was exhibited by all the samples through photoluminescence studies can be attributed to the transitions of trapped electrons from donor levels to the valance band or mid gap surface states. Intensity of the luminescence peak was found to be maximum for 2% and quenching was observed beyond 2%. Well defined room temperature ferromagnetic hysteresis loop was observed for 4% Co doped CdS nanoparticles. Saturation of magnetization MS and retentivity MR mounts up with cobalt content up to 4% and then decreases. Key words: chemical synthesis; nanostructured materials; luminescence; magnetic measurements.

*

Corresponding author: [email protected], Phone: +91 877 226 0211

                                                            

1. Introduction When a fraction of cations of semiconducting material is substitutionally replaced by transition metal ions that have partially filled d states ( Sc, Ti, Cr, Mn, Fe, Co, Ni and Cu) or rare earth ions that have partially filled f states (Eu, Gd and Er), dilute magnetic semiconductors (DMSs) are formed [1]. As incorporation of transition metal ions in II-VI semiconductor is more effective than other systems, they are particularly interesting in the DMS field [2]. Dilute magnetic semiconductors with room temperature ferromagnetism (RTFM) are potential candidates for spintronic applications [3,4] based on the function of charge and spin of the electron. CdS is one of the wide bandgap II-VI semiconductors with wide range of applications in the fields like X-ray detectors, window material for hetero junction solar cells, light emitting diodes, photocatalysis, biological sensors, address decoders and gas detectors [5-12]. A large number of reports are available on the CdS based DMSs with transition metals other than cobalt. Tambidurai and his co workers studied structural, optical and electrical properties of CdS:Co nanoparticles [13, 14] but they have not studied the magnetic properties of their products. A few reports are available on Co doped thin films [15-17]. Kashinath et al. noticed un expected weak ferromagnetism in CdS:Co synthesized by high energy electron irradiation method [18]. Saravanan et al. reported RTFM in CdS:Co nanoparticles synthesized by chemical co precipitation method [19]. Strong RTFM in Co doped CdS nanocrystals synthesized by gas liquid phase mechanism was reported by Tingting Hu [20]. Most of the earlier reports are on the wurtuzite structure of CdS:Co DMSs. In this paper we are reporting a systematic study on structural, optical and room temperature ferromagnetic properties of cubic zincblende CdS:Co DMSs.

2. Experimental Cd1-XCoXS (x = 0, 0.02, 0.04, 0.06, 0.08) nanoparticles capped with 2mercaptoethanol were synthesized by simple co precipitation route. All chemicals (Cd(CH3COO)2.2H2O, Co(CH3COO)2.4H2O and Na2S used in the present study are of AR grade and used without further purification. Aqueous solutions of precursors (0.2M) were separately prepared as per the stoichiometric ratio and stirred for 30 minutes. Anionic precursor solution was added drop wise to the mixture of cationic precursor solutions and 0.5 ml of surfactant (2-Mercaptoethanol) and was stirred for 7 hours. Precipitates were washed several times with de-ionized water, dried at 600 C for 8 hours and then made in to fine powders. Crystal structure of the synthesized nanoparticles was studied using Serifert 3003 TT X-ray Diffractometer with Cu-Kα radiation with a wavelength of 1.540Å. Chemical composition of the target samples were analysed using Energy Dispersive Analysis of X-rays (EDAX) attachment (CARL-ZESIS EVO MA 15). Average particle size of the synthesized samples was estimated by Transmission Electron Microscope (Tecnai-12, FEI, Netherlands). Diffuse reflectance measurements of dry powders were performed using Jasco V-670 doublebeam spectrophotometer for energy gap determination. Photo-luminescence (PL) studies were carried out using JOBIN-YVON Fluorolog-3 Spectrophotometer with a 450W Xenon arc lamp as an excitation source. Fourier transformation infrared spectroscopic studies were recorded using Thermo Nicolet IR200 spectrometer with a resolution of 4cm-1. The FTIR

spectra were obtained in the wave number range 500-4000cm-1. The magnetic measurements (M-H and M-T) were performed in a vibrating sample magnetometer (VSM) Lakeshore 7410 model. The XPS measurement was performed using Thermo Fisher Scientific (U.K) Theta Probe AR-XPS system. 3. Results and Discussion 3.1. Elemental studies Fig. 1 shows the energy dispersive X-ray analysis (EDAX) of Cd1-XCoXS (x = 0 and 0.04) nanoparticles. Elements Cd, S and Co were found to be in a near stoichiometric ratio. EDAX analysis rules out the presence of impurities in synthesized nanoparticles. 3.2. Structural studies X-ray diffraction pattern of Cd1-XCoXS (x = 0, 0.02, 0.04, 0.06 and 0.08) nanoparticles is shown in Fig. 2. Two broad peaks observed in all the samples can be indexed to (111) and (220) planes of the cubic zincblende structure. As dopant concentration increases peak positions in the X-ray diffraction pattern were shifted to higher 2θ values which can be attributed to the smaller ionic radii of Co2+ (0.74Å) when compared to Cd2+(0.96Å). This result supports the substitution of Cd2+ ion by the Co2+ ion in the as synthesized Co doped CdS nanoparticles. The average grain size of the nanoparticles was calculated using Scherrer formula [21] D = 0.89λ/β cosθ where D is the average particle size, λ is the wavelength of Cu-Kα radiation, β is the full width at half maximum intensity of the diffraction peak and θ is the diffraction angle. From the XRD studies, calculated diameters of the particles are found to be around 2 to 3 nm. Transmission electron microscope (TEM) images of Cd1-XCoXS (x = 0, 0.06 and 0.08) and selected area energy diffraction (SEAD) pattern of Cd1-XCoXS (x = 0.04) are shown in Fig. 3. Size of the nanoparticles was found to be around 4 to 5 nm. Concentric circles of SEAD pattern corresponds to the (111), (220) and (311) planes of cubic phase reveal the small size of the synthesized nanoparticles.

3.3. Optical studies Diffuse reflectance spectra of pure and Co doped CdS nanoparticles are shown in Fig. 4. In all the samples absorption edge is blueshifted compared to bulk CdS (512 nm) and which is the direct evidence of quantum confinement associated with nanoscale regime of the present particles. Redshift of absorption edge with increasing cobalt concentration indicates bandgap narrowing. This may be due to increase in the carrier concentration by the inclusion of cobalt ions and creation of defect levels in the bandgap [19]. From the DRS spectra it can be seen that the three characteristic minima peaks are present in all Co doped samples at 686, 729 and 750 nm and not in pure CdS. These characteristic peaks confirm the substitution of Co in the doped samples. They can be attributed to the d-d transitions 4A2(F)→3/2U1, 4A2(F) →E11and 4A2(F) →5/2 U1 of tetrahedrally co-ordinated Co2+ ions [16,22]. Presence of these reflectance minima peaks and their sharpness may be due to the high spin state (S=3/2) in a

tetrahedral crystal symmetry [23]. Similar kind of reflectance minima peaks related to d-d transitions due to cobalt substitution was also reported in Co doped ZnS films [24, 25]. For analysis purposes the diffuse-reflectance (R) of the samples can be related to the Kubelka–Munk function F(R) by the relation F(R) = (1 − R) 2/2R (Fig. 5) [26]. Direct bandgap of the pure and Co doped CdS nanoparticles was estimated by plotting [F(R) x hν]2 versus hν by extrapolating the linear part of the curve to zero as shown in the figure. Bandgap of pure and Co doped samples lie in the range 3.02 to 2.62 eV. Narrowing of bandgap with the incorporation of Co can be attributed to the sp-d exchange interactions between the band electrons in CdS and the localized d electrons of the Co2+ [16, 22]. The room temperature photoluminescence spectra of Cd1-XCoXS (x = 0, 0.02, 0.04, 0.06, 0.08) nanoparticles with an excitation wavelength of 390 nm are shown in Fig. 6. From the figure it is observed that all the samples are exhibiting strong green luminescence peak around 530 nm and very weak red emission shoulder around 670 nm. The green emission is due to the transition of electrons from defect donor levels to the valance band/ midgap states [19, 27] and the red emission can be ascribed to sulpher vacancies [28]. In addition, the luminescence intensity of 2% Co doped sample is found to increase drastically when compared to the undoped sample. Here Co acts as the sensitizing agent and responsible for the enhancement of radiative recombination process. It is interesting to note that the luminescence intensity is maximum for 2% cobalt doping. Hence the favourable doping level of cobalt in CdS for luminescent properties seems to be 2%. Beyond 2%, intensity of the luminescence peak is gradually decreasing up to 6% (still greater than pure CdS) and suddenly quenched in 8%. Dopant induced quenching of photoluminescence intensity is a natural phenomena in semiconductor nanoparticles. This dopant effect/ formation of deep centers can inhibit more electrons (holes) to be excited and lead to the enhancement of non radiative recombination process and may be responsible for quenching of luminescence intensity beyond 2% Co [29,30]. Similar type of observations was reported in Co2+ doped ZnS nanocrystallines [29]. Shift in the emission peak can be attributed to the variation of position of the defect levels within the band gap.

FTIR spectra of Cd1-XCoXS (x = 0, 0.02 and 0.06) are shown in Fig. 7. FTIR spectra were carried out by the samples with KBr. As KBr is hygroscopic, there may be some adsorbed water vapour which leads to a broad intermolecular hydrogen bonded O-H stretch around 3398cm-1. As there are no other functional groups from aliphatic chains falls in the region from 2000 to 1300cm-1, peaks at 1407 and 1554cm-1 can also be attributed to O-H group [31,32]. Stretches found at 1040 and 1005cm-1 corresponds to C-C bonding. Peaks at 2923 and 655cm-1 can be associated with the presence of -CH2 group and C-S group respectively. This analysis could provide evidence that 2-mercaptoethanol act as a ligand to control the size of the nanoparticles. 3.4. Magnetic studies The field dependent magnetization (M-H) curves at room temperature for pure and Co doped CdS nanoparticles with applied magnetic field ranging from -15KG to 15KG are shown in Fig.8. From the figure it is evident that pure CdS nanoparticles exhibit diamagnetic behaviour where as all other Co doped samples exhibit ferromagnetic nature. The origin of RTFM in Co doped CdS could arise from a number of possibilities such as native defects,

formation of Co related secondary phases, metallic Co and carrier mediated intrinsic property. As our pure CdS sample exhibits diamagnetic behaviour under similar conditions using VSM, the native defects alone could not originate RTFM in Co doped CdS nanoparticles [33, 34]. Secondary phases of CoO or CoS could not be a reason for the origin of observed RTFM in the synthesized samples because neither XRD nor XPS could detect any such secondary phases of Co. Moreover possibilities for the formation of oxides or sulphides of Co in our samples is remote as their formation requires elevated temperatures [35] where as our samples were synthesized at room temperature. X-ray photo electron spectrum (XPS) for the 4% Co doped CdS sample is shown in Fig. 9. As per the obtained XPS results, the Co 2p3/2 binding energy is 781.7eV , the Co 2p1/2 binding energy is 797.6 eV and in turn shows 16.1eV of energy difference between those two states. This result supports that cobalt in our sample is in Co2+ state. If Co exist in the metallic form Co(0) , binding energies of Co 2p3/2 and Co 2p1/2 should be around 778 and 793 eV with a difference of binding energies between those two states 15.05eV[36,37].Hence through XPS analysis we can rule out the origin of RTFM in our products due to metallic Co clusters. Bandgap modification due to quantum confinement, reflectance minima observed in DRS spectra and the absence of secondary phases of Co in the doped CdS nanoparticles clearly indicates the substitution of Cd2+ ions by Co2+ ions in to CdS lattice. Hence finally we can attribute the origin of RTFM in CdS:Co to dopant related intrinsic property. As the Co concentration increases, saturation magnetic moment in our samples increase and attains a maximum value for 4%. Beyond 4%, MS value decreases with increasing Co. Bound magnetic Polaron (BMP) is the basic unit for the mechanism of ferromagnetic behaviour in DMSs with localized charge carriers. There are two independent competing mechanisms present in DMSs arising from two different processes. The long range carrier mediated component is ferromagnetic where as the short range direct exchange interaction is anti ferromagnetic. At lower concentration of magnetic ion impurities, anti ferromagnetic component can be neglected but at a certain concentration of this magnetic ion, anti ferromagnetic interactions play an important role and reduces the FM behaviour of the product [1,38]. Hence, the variation of saturation magnetic moment with Co concentration in the present study can be attributed to the coupling between ferromagnetic and anti ferromagnetic components. Thus in our investigation we observed well defined room temperature ferromagnetism in CdS:Co (4%) where as earlier workers reported at 2% [20], 3% [18] and 6% [19] with different synthesis routes. Savas Delikanli et.al reported the Curie temperature of Mn doped CdS rods synthesized by high temperature organic solution phase technique around 700K [39]. We made an attempt to determine Curie temperature ( TC ) of 4% Co doped CdS nanoparticles and found it to be around 519 K [Fig. 10]. This result also supports that the RTFM is not due to Co metal (TC > 1300K) clusters. No previous reports on TC in Co doped CdS nanoparticles are available for comparison. Magnetic parameters such as saturation magnetization MS, retentivity MR and coercivity HC of CoXCd1-XS nanoparticles were obtained from the hysteresis loops and are presented in Table 1. From the table it is evident that MS and MR values follow a bowing trend where as HC values decrease with increasing x. 4. Conclusions

In summery we successfully synthesized very small DMS nanoparticles with cubic structure via simple chemical co precipitation route. Substitution of Cd2+ ion by the Co2+ ion was evidenced by XRD and DRS studies. Three characteristic reflectance minima peaks present in Co doped samples can be related to d-d transitions. For luminescent properties, favourable doping level of cobalt in to CdS was found to be 2%. Well defined room temperature ferromagnetic nature was observed in 4% Co doped CdS nanoparticles which may be useful for devices in spintronic applications. Acknowledgements One of the authors G.Giribabu is thankful for the financial support from University Grants Commission, Govt. of India for providing Teacher Fellowship under Faculty Development Programme. Chunli Liu is thankful to Hankuk University of Foreign Studies, South Korea for providing financial support to carryout XPS studies.

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Figure Captions Fig. 1 EDAX spectra of Cd1-XCoXS (x = 0, and 0.04) nanoparticles. Fig. 2 X-ray diffraction pattern of Cd1-XCoXS (x = 0, 0.02, 0.04, 0.06 and 0.08) nanoparticles. Fig. 3 (a, b and c) TEM images of Cd1-XCoXS (x = 0, 0.06 and 0.08) nanoparticles. (d) SEAD pattern of Cd1-XCoXS (x= 0.04) nanoparticles. Fig. 4 Diffuse reflectance spectra of Cd1-XCoXS (x = 0, 0.02, 0.04, 0.06 and 0.08) nanoparticles. Fig. 5 Kubelka- Munk plots of Cd1-XCoXS (x = 0, 0.02, 0.04, 0.06 and 0.08) nanoparticles. Fig. 6 Room temperature photoluminescence spectra of Cd1-XCoXS (x = 0, 0.02, 0.04, 0.06 and 0.08) nanoparticles. Fig. 7 FTIR spectra of Cd1-XCoXS (x = 0, 0.02 and 0.06) nanoparticles. Fig. 8 Room temperature M-H plots of Cd1-XCoXS (x = 0, 0.02, 0.04, 0.06 and 0.08) nanoparticles. Fig. 9 (a) XPS survey spectrum for 4% CdS:Co nanoparticles and (b) XPS spectrum for Co 2p states in 4% CdS:Co nanoparticles. Fig. 10 M-T behaviour of 4% CdS:Co nanoparticles with constant applied field 5KGauss.

Table.1 Magnetic properties of CoXCd1-XS (x=0.02, 0.04, 0.06 and 0.08) nanoparticles. Property↓ Sample→

X=0.02

X=0.04 -4

X=0.06 -4

X=0.08 -4

MS (emu/g)

7.88x10

87.69x10

27.40x10

MR (emu/g)

3.65x10-4

21.98x10-4

11.29x10-4

HC (Gauss)

470

454

422

1.27x10-4 350

 

Fig. 1 EDAX spectra of Cd1-XCoXS (x = 0, and 0.04) nanoparticles.        

 

Fig. 2 X-ray diffraction pattern of Cd1-XCoXS (x = 0, 0.02, 0.04, 0.06 and 0.08) nanoparticles. 

 

Fig. 3 (a, b and c) TEM images of Cd1-XCoXS (x = 0, 0.06 and 0.08) nanoparticles. (d) SEAD pattern of Cd1-XCoXS (x= 0.04) nanoparticles.  

 

Fig. 4 Diffuse reflectance spectra of Cd1-XCoXS (x = 0, 0.02, 0.04, 0.06 and 0.08) nanoparticles.      

 

 

Fig. 5 Kubelka- Munk plots of Cd1-XCoXS (x = 0, 0.02, 0.04, 0.06 and 0.08) nanoparticles.    

 

Fig. 6 Room temperature photoluminescence spectra of Cd1-XCoXS (x = 0, 0.02, 0.04, 0.06 and

0.08) nanoparticles.    

 

Fig. 7 FTIR spectra of Cd1-XCoXS (x = 0, 0.04 and 0.06) nanoparticles.    

 

Fig. 8 Room temperature M-H plots of Cd1-XCoXS (x = 0, 0.02, 0.04, 0.06 and 0.08) nanoparticles.  

 

Fig. 9 (a) XPS survey spectrum for 4% CdS:Co nanoparticles and (b) XPS spectrum for Co 2p states in 4% CdS:Co nanoparticles.

Fig. 10 M-T behaviour of 4% CdS:Co nanoparticles with constant applied field 5KGauss.

Table.1 Magnetic properties of CoXCd1-XS (x=0.02, 0.04, 0.06 and 0.08) nanoparticles. Property↓ Sample→

MS (emu/g) MR (emu/g) HC (Gauss)

X=0.02 7.88x10-4 3.65x10-4 470

X=0.04 87.69x10-4 21.98x10-4 454

X=0.06 27.40x10-4 11.29x10-4 422

X=0.08 1.27x10-4 350

¾ Co doped CdS nanoparticles were successfully synthesized by co - precipitation method. ¾ PL emission intensity versus doping level in CdS: Co nanoparticles were studied for the first time in detail. ¾ Well defined room temperature ferromagnetic behaviour was observed. ¾ We are the first to determine TC for CdS: Co nanoparticles.