CHAPTER 3 EFFECT OF COBALT DOPING ON THE OPTICAL AND MAGNETIC PROPERTIES OF CADMIUM SULFIDE NANOCRYSTALS

Transcription

1 49 CHAPTER 3 EFFECT OF COBALT DOPING ON THE OPTICAL AND MAGNETIC PROPERTIES OF CADMIUM SULFIDE NANOCRYSTALS 3.1 INTRODUCTION Diluted Magnetic Semiconductor Diluted magnetic semiconductors (DMSs) (Twardowski 1996) are important because of the strong exchange interaction between the hybridized sp 3 d orbitals of the magnetic ions and charge carriers. This manifests itself most clearly in the giant Zeeman splitting observed in both the conduction and valence bands when the material is placed in an external magnetic field. At low magnetic ion concentrations, these materials generally exhibit 'frustrated' paramagnetism, with the number of spin singlet states being reduced by nearest-neighbour anti-ferromagnetic spin-pairing (Fatah et al 1994). At low magnetic fields (< 8 T) these spins remain locked and cannot contribute to the paramagnetism of the material; however experiments carried out under very high magnetic fields have been able to break these spindoublets. The magnetic ion itself sits substitutionally on a cation site and can generally be incorporated to high concentrations. The sp-d exchange interactions in bulk Co 2+ -doped II-VI semiconductors have been shown to be significantly larger than those in the analogous Mn 2+ -doped DMS materials (Zielinski et al 1996 and Seong et al 2001). The possibility that novel magneto-optical phenomena may arise from

2 50 quantum confinement in these DMS materials has motivated to synthesis and characterization of Co 2+ -doped II-VI DMS QDs. Previous studies on Co 2+ - doped II-VI nanocrystals have used only 113 Cd NMR (Ladizhansky and Vega 2000) and semiconductor band gap emission (Yang et al 2001) to probe doping. 113 Cd NMR spectra of Co 2+ :CdS QDs prepared by aqueous co-precipitation showed transferred hyperfine interactions between Co 2+ and Cd 2+ ions indicating that at least some of the Co 2+ ions were incorporated substitutionally in the particles (Ladizhansky and Vega 2000) Cadmium Sulfide Based DMS Cadmium sulfide (CdS) with its direct band gap of 2.42 ev has been investigated in terms of preparation and characterisation of its properties based on its potential application in the fabrication of solar cells, photoelectric devices and photocatalysts (Acharya et al 2006). The photoelectric properties of undoped CdS are not very interesting from the technical viewpoint (Franc et al 2006). Dopants create new energy levels, which are responsible for the optical absorption at longer wavelengths (Bube 1960). The photoelectric efficiency and sensitivity in the region nm considerably increase by the introduction of donors and acceptors. The donors for CdS are metals of group III (Al, Ga and In) and elements of group VII (halogens). Not only the dopants, but also crystal imperfections (sulphur vacancies) can act as donors. The acceptors are metals of group Ib (Cu, Ag and Au), elements of group V and oxygen, cadmium vacancies and halogens forming complexes with vacancies left by cadmium (Franc et al 2006). The presence of magnetic ions leads to a number of unusual electronic and optical properties, which are due to the interaction of the magnetic ion with band electrons as well as to the exchange interaction between the magnetic ions themselves (Brandt and Moshchalkov 1984, Furdyna 1982, Furdyna and Samarth 1987). NMR studies on Co 2+ doped CdS QDs with strong sp-d mixing is achieved, which is

3 51 assignable to Co exchange interactions in the core (Ladizhansky and Vega 2000). Previous report (Radovanovic and Gamelin 2001) has shown that normal soft chemical processing does not render cobalt substitution in the CdS matrix and that the dopants are diverted to the surface and observed that no magnetic properties are examined. Attention was focused on Co-containing systems (Jonker et al 1988), as Co (II) (3d7) has an orbital singlet ground state in a tetrahedral coordination environment and it is anticipated to be easily introduced into II VI compounds. The substitution of zinc and cadmium by cobalt causes a significant decrease in the unit cell parameters. The optical properties of Co (II) in II VI compounds have been studied extensively (Niu et al 1990). CdS-based dilute magnetic semiconductors are good candidates for applications such as magneto optical devices (magnetic field sensors, spintronics, isolators, magnetic recording and magneto-optical switches), field emission displays, solar cells and gas sensors. However, very limited information is available on the film quality, optical and electronic properties of the cobalt-doped CdS-based DMS materials (Liu et al 2007 and Wolf et al 2001). Co 2+ -doped II VI systems exhibit stronger coupling due to the increased Co d-orbital mixing with the valence band and conduction band of the II VI host. Mn 2+ -doped CdS nanocrystals (NCs) have shown evidence for size dependent magnetism (Feltin et al 1999). Recently, several researchers have reported the use of direct chemical synthetic methods to prepare a variety of colloidal DMS NCs and a great deal for the syntheses of such materials and research has been addressed on dopant carrier exchange interactions in such colloidal DMS NCs (Schwartz et al 2003, Bryan and Gamelin 2005). In the present investigation, Cd 1-x Co x S (x = 0.02, 0.04, 0.06) NCs were synthesized by simple chemical co-precipitation method. Significant changes in the optical and magnetic properties of CdS due to Co

4 52 doping have been observed. The possibility of its conversion to a diluted magnetic semiconductor for spintronics applications has also been studied. 3.2 EXPERIMENTAL PROCEDURE Sample Preparation Cobalt-doped CdS NCs have been synthesized by the aqueous co-precipitation method. 0.1 M of cadmium acetate dihydrate (Cd(Ac) 2.2H 2 O) and cobalt acetate tetrahydrate (Co(Ac) 2.4H 2 O) were prepared with DI water. Simultaneously 0.1 M of sodium sulfide (Na 2 S) solution was also prepared with DI water with the precursor ratio of 1:1. Aqueous solution of Na 2 S was added drop wise into the mixed solution, which resulted in an orange yellowish solution of Co:CdS NCs. It was then refluxed with constant stirring at 80 C for 90 min to attain saturation. After attaining the room temperature small quantity of acetone added with stirring to precipitate the NCs. The precipitate was ultrasonicated for 60 min. It was further washed with distilled water and ethanol for several times to remove the organic residues present in the NCs and then dried in a hot air oven for 24 h to get cobalt-doped CdS NCs. 0.1 M aqueous solution of Cd(Ac) 2.2H 2 O 0.1 M aqueous solution of Co(Ac) 2.4H 2 O Stirring at room temperature for 1 h Aqueous solution of 0.1 M sodium sulphide refluxed at 80 C & stirring for 90 min Addition of Acetone to precipitate Co:CdS NCs Ultrasonication for 60 min washed with DI water and ethanol Dried in a hot air oven for 24 h

5 53 Cd 1-x Co x S samples for three different concentrations (x = 0.02, 0.04, 0.06) were prepared by the procedure schematically illustrated above. The samples were then subjected to powder X-ray diffraction (XRD) analysis for structural characterisation using PANalytical X-ray diffractometer with Cu K radiation ( = Å) in the range of (2 ) at a scanning rate of 0.05 /min. Optical absorption was studied using CARY 5E UV VIS NIR diffuse reflectance mode spectrophotometer and luminescence properties were studied by fluorescence spectroscopy using JOBIN-VYON FLUROLOG-3-11 spectroflurometer with Xenon lamp source of 450 W. Crystallinity and average particle size of the synthesized samples were studied by high resolution transmission electron microscope (HRTEM) and SAED analysis using 200 kev JEOL JEM 3010 TEM with LaB 6 filament. XPS measurements were carried out using PHI Quantera SXM (ULVAC- PHI) with monochromatic Al Kα as a X-ray source. Surface area of the NCs was calculated using BET surface area analysis with micromeritics with liquid nitrogen as adsorptive. Room temperature magnetisation was measured using a vibrating sample magnetometer (VSM-EG&G PARC, Model 4500). Raman measurement were analysed with Horiba Jobin-Yvon T64000 photon design micro Raman spectrophotometer using Ar ion laser with the excitation of nm. 3.3 RESULTS AND DISCUSSION X-ray Diffraction Analysis The XRD pattern of 2, 4 and 6 at. % of Co-doped CdS NCs shown in Figure 3.1 were assigned to the -phase cubic zinc blende (hawleyite) structure. The diffraction pattern for (111) peak position shows a clear shift towards higher angle (lower d value) with increasing cobalt concentration. This clearly implies the lattice compression, consistent with the smaller ionic

6 54 radii of Co 2+ (0.82 Å) than that of Cd 2+ (0.97 Å), confirming the dopant incorporation in the synthesized CdS NCs, similar results were obtained by Hanif et al (2002) for Cd 1-x Co x Se QDs. The width of the diffraction peaks is broadened indicating that the cobalt-doped CdS NCs has nanoscale dimensions. The substitution of Cd 2+ by Co 2+ decreased the XRD peak intensities of the Co 2+ - occupied lattice planes due to the lower atomic scattering factor of Co 2+ with respect to Cd 2+. (The variation of d-spacing with corresponding (hkl) planes for different cobalt concentration are (111) Å, (220) Å, (311) Å and a = Å, V = Å 3 for 2% Co:CdS, (111) Å, (220) Å, (311) Å and a = Å, V = Å 3 for 4% Co:CdS and (111) Å, (220) Å, (311) Å and a = Å, V = Å 3 for 6% Co:CdS.) Average particle size was calculated using Scherrer formula and the size distribution was found to be 6 nm. Upon doping, no additional reflections were observed indicating that the cubic phase of CdS structure is not disturbed by the cobalt substitution and there are no impurities present in the sample. Figure 3.1 XRD pattern of the cobalt doped cadmium sulfide nanocrystals with three different cobalt concentrations

7 Structural and Elemental Analysis Elemental analysis was done using X-ray fluorescence spectrometry and for 4% Co:CdS, at. % of cadmium, at. % of cobalt and at. % of sulphur and for 6% Co:CdS, at. % of cadmium, at. % of cobalt and at. % of sulphur were estimated, which indicates the presence of dopants in the samples. It also represents that the inclusion of cobalt replaces cadmium ion and the sulphur content remains approximately same for all the samples. EDX spectrum and the elemental mapping of 4% cobalt doped cadmium sulfide nanocrystals shown in Figure 3.2 (a) and (b), also confirm the presence of dopant ions in the sample. Figure 3.2 (a) EDX pattern of the 4 at. % of cobalt doped cadmium sulfide nanocrystals

8 56 Co Cd S Figure 3.2 (b) Elemental mapping represents the elements present in cobalt doped CdS nanocrystals XPS Studies X-ray photoelectron spectroscopy (XPS) analysis of cobalt doped CdS nanocrystals is shown in Figure 3.3 (a-d) for 6 at. % Co concentration. The survey spectrum (Figure 3.3a) reveals the presence of characteristic peaks with binding energies as follows: C 1s (285.0 ev), O 1s (531.8 ev), Cd 3d 5/2 (405.3 ev), Cd 3d 3/2 ( ev) and S 2p (major peak at ev and a low intensity peak at ev). In this case, the Co 2p 3/2 binding energy is observed at ev. The binding energy observed for Co 2p 3/2 in the synthesised Co:CdS nanocrystals shows cobalt in the Co 2+ state, and not in the metallic form (Wagner et al 1978). The binding energy of S 2p at ev also agrees well with binding energy with blue shift compared to the observed S 2p in pure CdS nanoparticles (Winkler et al 1999). The appearance of low intensity peak at ev is due to oxidation of sulphur in nanoparticles (Wagner et al 1978).

9 57 Figure 3.3 XPS spectra for (a) wide energy (b) Cd 3d 5/2 and Cd 3d 3/2 in the Co:CdS nanocrystals

10 58 Figure 3.3 XPS spectra for (c) S 2p 3/2 and (d) Co 2p 3/2 and Co 2p 1/2 in the Co:CdS nanocrystals

11 High Resolution Transmission Electron Microscopy HRTEM images of Cd 1-x Co x S (x = 0.02, 0.04) nanocrystals and the corresponding lattice fringe pattern are shown in Figure 3.4. The image of NCs establishes the reasonable uniformity of the spherical particle with the average size of ~10 nm, which agrees with the XRD data. The high resolution TEM image exhibits well defined lattice fringes of Co:CdS NCs (Figure 3.4 c, d). The inset in Figure 3.4 (c) shows the SAED pattern of observable rings with the indexing of diffraction planes, which confirms the cubic phase formation of the NCs. An overview image shows the individual nanocrystal with size of about 8 nm (indicated by arrows). High resolution TEM images in Figure 3.5 a-f for 6% Co:CdS NCs also demonstrate the synthesized samples contain monodisperse spherical particles. Figure 3.4 (a), (b) HRTEM image of 2 and 4% of Co:CdS nanocrystals and (c), (d) lattice fringe pattern of 4% Co:CdS, inset in (c) shows SAED pattern

12 60 (a) (b) (c) (d) (e) (f) Figure 3.5 (a-f) HRTEM images of 6% of Co:CdS nanocrystals UV-reflectance Studies UV-reflectance spectra of cobalt-doped CdS NCs are shown in Figure 3.6. The size dependent optical properties of nano sized particles provide a very convenient and useful way to monitor their formation. The peaks at higher energy than the fundamental absorption edge of bulk CdS indicate quantum confinement effects in nanoparticles. The absorption spectra clearly show the variation in optical band gap of 2.70 ev (458 nm), 2.64 ev (468 nm) and 2.60 ev (476 nm) for 2, 4 and 6% of Co:CdS systems,

13 61 respectively, which were estimated with the extrapolation of line in the spectra. It is clearly seen from the spectra that the absorption edge shifts towards higher wavelength (i.e. decrease in the band gap) with increasing cobalt concentration. This may be due to the increase in the carrier concentration by the inclusion of cobalt ions and creation of defect levels in the band gap. This can be attributed to the increase in the doping concentration, which may induce the lattice disorder in the NCs. The observed blue shift of the absorption edge compared with bulk CdS indicates that the synthesized material is in the nanoscale regime. Figure 3.6 Reflectance UV spectra of cobalt doped cadmium sulfide nanocrystals with different concentrations of cobalt Fluorescence Studies The room temperature fluorescence emission spectra for Co:CdS samples excited at 450 nm are shown in Figure % cobalt-doped CdS NCs exhibits a sharp emission (low frequency) peak with broad feature with maximum intensity centered at 560 nm, which is assigned to the

14 62 recombination of charge carriers (electron hole pair) in deep traps of surface localized states (Xu et al 2002). This indicates that a fraction of Co 2+ ions reside on CdS nanocrystal surfaces when Co 2+ :CdS NCs are prepared by the co-precipitation method (Radovanovic and Gamelin 2001). It is to be noticed that this emission is strong, suggesting that the synthesized nanoparticles are like Cd 2+ rich, as suggested by the observation that excess Cd 2+ as opposed to excess S 2- can result in higher fluorescence quantum yields (Harruff and Bunker 2003). This peak disappears when the cobalt concentration increases. The broadening of the peak is due to small size of the particles, which is in agreement with HRTEM results. The emission peak centered at 515 nm appears for all the concentrations and shifts towards blue end of the visible spectrum with respect to bulk CdS. These high frequency peaks are attributed to the transitions of trapped electrons from donor levels to the valence band. This shift in the emission spectra confirms the cobalt ion can replace the cadmium in the lattice of CdS NCs. Because of the dopant incorporation evenly in the CdS NCs and increase in the concentration, the emission intensity decreases for 6% Co:CdS. Figure 3.7 Fluorescence spectra of cobalt doped cadmium sulfide nanocrystals with different concentrations of cobalt.

15 BET Surface Area Analysis Nanocrystals synthesized at higher temperatures result in more hard aggregates and with the lower reaction temperature (<150 C) the NCs possess highest surface area. A sufficient fraction of primary particles remain either unaggregated or loosely aggregated, so that reasonable high surface areas of , and m 2 /g for 2, 4 and 6% Co:CdS were observed. When the concentration of cobalt increases the surface area slightly decreases, which supports the results observed in the absorbance and emission studies Raman Studies The Raman spectra for all the samples of 2%, 4% and 6% cobalt doped CdS nanocrystals are shown in Figure 3.8. It is well known that in a crystalline semiconductor, the observed Raman shifts are usually associated with the longitudinal optical phonons (LO), while in general, other modes, such as the transverse optical phonons (TO) and the surface phonons (SP), are not as observable because of symmetry restrictions and weaknesses in the observed intensities, respectively (Wang et al 2008). The 1LO and 2LO phonon modes of CdS, located around 295 and 595 cm 1 respectively, are clearly identified in the Raman spectra of all the samples, illustrating characteristic Raman shifts analogous to those of pure crystalline CdS (Suh and Lee 1997). The observed phonon peaks are shifted toward lower frequency, as expected from bulk, likely due to effects of small size and high surface area. The Raman spectrum is sensitive to the size of the nanostructures, in particular the size could be inferred from the 2LO/1LO intensity ratio (Scamarcio et al 1996). The full width at half maximum (FWHM) of the 1LO mode is determined by different contributions, e.g. the nanoparticle size and disorder. A FWHM value of 1LO found to be 22 cm 1. However, the FWHM

16 64 decreases for decreasing nanoparticle size, as deduced form the 2LO/1LO intensity ratio. Figure 3.8 Raman spectra of the cobalt doped cadmium sulfide nanocrystals Magnetic Studies The room temperature field dependent magnetisation curves using VSM studies with the field range of 7000 Oe, for 4 and 6% of Co:CdS are shown in Figure 3.9 (a, b) respectively. In II VI semiconductor nanomaterials, the nature of magnetic properties depends on the magnitude of the transition metal ion exchange coupling with electronic levels. The cubic zinc blende and hexagonal wurtzite structure of CdS differ significantly with the bond structures between a centre ion and its Next-Nearest-Neighbour (NNN) cations. The 42 NNN atoms in the cubic zinc blende lattice can be classified according to four non-equivalent bond structures. They can be characterised by trans trans (tt), gauche trans (gt), trans gauche (tg) and

17 65 gauche gauche (gg) conformations (Gavish et al 1993). The ferromagnetic behaviour with weak magnetic moment was observed for 4% cobalt-doped CdS NCs. The spontaneous moment observed in Co:CdS NCs indicates that only a fraction of Co 2+ ions are magnetically ordered with respect to applied magnetic field. Here, the diamagnetic nature of the CdS reduces the magnetic moment of cobalt ions for low concentration, which follows the previous reports for 1 and 3% cobalt-doped CdS NCs (Bogle et al 2008). A complete ferromagnetic behaviour is observed for 6% cobaltdoped CdS, which reflect the contribution of high magnetic moment for obtaining the ferromagnetic hysteresis. The room temperature ferromagnetism arises from the creation of a spin-split impurity band at the Fermi level, below the conduction band due to the hybridization between the charge carriers of cobalt and cadmium. Here, we assume that the resulting ferromagnetic behaviour is derived from the substitution of Co 2+ ions for Cd 2+ ions without changing its zinc blende structure. (a) Figure 3.9 (a) Room temperature magnetisation spectrum for 4% of Co:CdS nanocrystals

18 66 (b) Figure 3.9 (b) Room temperature magnetisation spectrum for 6% of Co:CdS nanocrystals 3.4 CONCLUSION Nanocrystals of Cd 1-x Co x S have been synthesized by chemical co-precipitation method in aqueous medium with three different cobalt concentrations. XRD pattern confirms the cubic zinc blende structure of CdS, with a small shift in the peaks at higher angle due to the incorporation of cobalt in the lattice. HRTEM image reveals that the sample is in the nanoscale regime and shows the quantum confinement effect. SAED pattern confirms the cubic phase formation of Co:CdS. The variation in optical and luminescence properties supports the Co 2+ dopant incorporation in CdS NCs. The strong emission peak at ~560 nm in 2% Co:CdS clearly shows that the low concentration of cobalt does not affect the emission of CdS NCs. Room temperature magnetisation measurements indicate ferromagnetic behaviour for 6% Co:CdS with well defined ferromagnetic hysteresis. This study reveals that the presence of cobalt ion may create some defects or vacancy sites which play a key role in varying the optical and magnetic properties of these cobalt-doped CdS NCs for spintronics applications.