Journal of Magnetism and Magnetic Materials 322 (2010) 3926–3931
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Cobalt nanoparticles doped emaraldine salt of polyaniline: A promising room temperature magnetic semiconductor Shadie Hatamie a, M.V. Kulkarni b, S.D. Kulkarni c, R.S. Ningthoujam d, R.K. Vatsa d, S.N. Kale a,n a
Department of Electronic Science, Fergusson College, Pune 411004, India Center for Materials for Electronics Technology, Pashan, Pune 411008, India c Center for Materials Characterization, National Chemical Laboratory, Pune 411008, India d Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India b
a r t i c l e in f o
a b s t r a c t
Article history: Received 4 February 2010 Received in revised form 1 August 2010 Available online 13 August 2010
Incorporation of magnetic nanoparticles in polymers with organic functional groups working as semiconducting substrate is of immense interest in the field of dilute magnetic semiconductors (DMS) and spintronics. In this article we report on synthesis and evaluation of dilutely doped (0–10 wt%) cobalt nanoparticles in emaraldine salt (ES) of polyaniline in the presence of dodecyl benzene sulfonic acid (DBSA) and p-toluene sulfonic acid (p-TSA) using a sonochemical-assisted-reduction approach as a possible DMS candidate. The X-ray diffraction pattern and high resolution transmission electron microscopy (HRTEM) image show the ES to be polycrystalline, in which 10 nm sized Co nanoparticles get embedded in its FCC structural form. From Fourier transform infrared (FT-IR) and UV–visible (UV–vis) spectroscopy studies, it is predicted that cobalt particles get electrostatically bound to the specific SO 3 ion sites of ES, thereby modifying torsional degrees of freedom of the system. The applied field dependent magnetization study shows that the sample exhibits hysteresis loop with a minimal doping of 3 wt% of Co nanoparticles and increases with the amount of Co nanoparticles in ES due to dipolar interaction. The electron transport data show that with increase in Co wt% there is a gradual shift from ohmic to non-ohmic response to the sample bias, accompanied by opening of electrical hysteresis and an increased resistance. The non-linear response of higher doped systems has been attributed to the combination of direct and Fowler–Nordheim tunneling phenomena in these systems. Persistence of optical and transport properties of the polymer, with an introduction of magnetic moment in the system, envisages the system to be a fine magnetic semiconductor. & 2010 Elsevier B.V. All rights reserved.
Keywords: Magnetic semiconductor Polyaniline Cobalt nanoparticles
1. Introduction Semiconducting host matrices with magnetic ion doping are of considerable interest and intense scientific studies have been carried out over past several years in this area [1–3]. Such materials, popularly termed as diluted-magnetic semiconductors (DMS), if realized in a perfect form without extra phases, have considerable impact on the field of spintronics as well as magnetooptics. Several haunting issues have been around these interesting systems, which include non-uniformity in dopant distribution, dopant cluster formation, defect states and secondary phases. These have been the case with most inorganic semiconducting host matrices, typically ZnO, SnO2 and TiO2 [4–6] with magnetic dopants being mainly, Mn, Fe and Co. In such scenario, most of the
n Correspondence to: Department of Applied Physics, Defence Institute of Advanced Technology (DIAT), Girinagar, Pune 411025, India. Tel.: + 91 20 2430 4091. E-mail addresses:
[email protected] (R.K. Vatsa),
[email protected] (S.N. Kale).
0304-8853/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2010.08.022
recent literature reviews and scientific studies [6,7] speculate that meticulous cluster engineering in the host matrix, rather than the substitutional site elemental doping, may not be a bad idea for realizing a good DMS system. Further, organic host matrices have also been explored to introduce DMS effects with magnetic ion doping [8,9]. Polyaniline is one such interesting polymer, which shows a wide range of electronic structure change depending upon the doping. 50% oxidized and base-treated polyaniline (called emeraldine base) shows poor conductivity as compared to protonated emaraldine salt (ES). It results in modification of the structure and thereby semiconductor to metallic nature can be brought out. This has been attributed to the electron–hole transport mechanism in ES [10–12]. The intrinsically conductive nature of the ES arises from a unique bonding structure along the polymer backbone, consisting of alternating double (s and p) and single (s) bonds forming a benzoid and quinoid structure, respectively. If an electron is added to the conjugated polymer backbone (via reduction, n-type doping) or removed from it (via oxidation, p-type doping) during the chemical or electrochemical doping process, then the charge can freely travel down these
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conjugation paths when an electrical potential is applied. The electrical resistivity covers a wide range from insulators ( 107 O cm) to semiconductors (105–101 O cm) and metals (10 2–10 5 O cm), the range depending on the extent of doping. The conductivity achieved depends strongly on the type of dopant, the polymer characteristics (such as specific repeat unit and molecular mass, chain defects such as branching and chemical heterogeneity) and on how the polymer was processed [13]. There have also been observations that the ES exhibit granular morphologies with a pattern of fully protonated conducting clusters surrounded with non-conducting regions. These properties are of tremendous interest from DMS perspective, mainly because of the speculated tunneling transport between their conducting islands and the opportunity to manipulate the transport and optical properties and induce magnetism via appropriate dopants. In this context, we report on the synthesis of Co nanoparticles doped ES (Co 1–10 wt%). These are characterized by ultraviolet– visible (UV–vis) absorption, Fourier transform infrared (FT-IR) transmittance and X-ray diffraction (XRD) techniques. Morphology study from the high resolution transmission electron microscopy (HRTEM) suggests the homogenous distribution of Co nanoparticles of 10 nm size in ES matrix. Optical, magnetic and electrical transport studies at room temperature establish the magnetic spin-induced electrical conductivity, which is a signature for dilute magnetic semiconductor.
2. Experimental details 2.1. Synthesis of ES and Co nanoparticles All samples were of analytical grade with 99.95% purity. The cobalt nanoparticles of average size of 10 nm, with a fine citrate coat, were produced using a direct reduction method, as has been described elsewhere [14]. Polyaniline in the form of emaraldine salt (ES) was synthesized in the presence of dodecyl benzene sulfonic acid (DBSA) and p-toluene sulfonic acid (p-TSA). The solutions were prepared in doubly distilled water. The polymerization of the monomer, aniline (0.1 M), was initiated by the drop wise addition of the oxidizing agent, (NH4)2S2O8 (0.1 M) under constant stirring at low temperature between 0 and 5 1C in an acidified solution containing p-TSA and DBSA. The monomer to oxidizing agent ratio was kept as 1:1. After complete addition of oxidizing agent, the reaction mixture was kept under constant stirring for 24 h. The dark green colored ES phase of the polymer was further utilized for the synthesis of the doped samples (Co:ES) using toluene as a solvent.
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powder samples were characteristically green in color, which were subjected to conventional characterizations. The samples were characterized using VSM (EG&G, PAR4500 model), UV–vis (1700 Shimadzu), FT-IR (Shimadzu IRA Affinity-1), HRTEM (JEOL 2010 UHR TEM microscope) and XRD (Philips PW 1710 Diffractometer). For transport measurements, the samples were given silver four point contacts and copper wires were used for connectivity to Keithley (current source Model 6221) and nanovoltmeter (Keithley 2182AE). IEEE-488 interface was used for computer interface.
3. Results and discussion 3.1. FT-IR study Fig. 1 shows the FT-IR spectra of ES (a), Co1:ES (b), Co3:ES (c), Co5:ES (d), Co7:ES (e) and Co10:ES (f). As seen in Fig. 1(a), clear signatures of ES are observed at 1031, 1215, 1409, 3167, 2967 and 3731 cm 1. The bands at 1032 and 1257 cm 1 belong to the symmetric and asymmetric stretching frequencies of SO 3, respectively [15,16]. These SO ions exist due to the acid doping. 3 The bands around 3180–3480 cm 1 can be assigned to the N–H stretching vibration of aromatic amine, and bands near 1400 cm 1 are due to C¼ C stretching vibrations. The bands centered near 1300 cm 1 are assigned to C–N stretching band of ES. Bands near 1215 and 850 cm 1 are designated to be aromatic CH out-of-plane bending. It was observed that as the Co nanoparticles’ doping increases, there is a shift in the maximum band absorption near 1200–1122 cm 1 (indicated by black arrows in the figure) along with the new bands less than 1200 cm 1 and thus, there is a kind of chemical bond between surfaces of Co nanoparticles with SO 3 groups of ES. Additionally, there is also a shift in the NH vibrational frequency signatures to 3351 cm 1 (indicated by blue arrows in the figure). This implies that Co nanoparticles could contain a positively charged surface, electrostatically interacting with SO 3 of ES, thereby changing the torsional angle of the ES molecule, which would also change the vibrational frequencies of NH groups in accordance with the experimental observations. This argument is supported by the results of Yue et al. [17] on
2.2. Syntheses of Co:ES samples For syntheses of doped samples, 0.1 mg of ES was suspended in toluene and ultrasonicated for about 5 min. Co nanoparticles were weighed (1–10%, by weight) and dispersed in toluene using ultrasonication for about 20 min. This suspension was then mixed with ES solution under constant ultrasonic action at room temperature for 10 min to get cobalt nanoparticles-impregnated-ES (in order to differentiate we refer the samples with the doping percentage as the number: ES, Co1:ES, Co3:ES, Co5:ES, Co7:ES and Co10:ES). 2.3. Characterization For powder characterizations, the samples were dried in vacuum (10 3 torr) at room temperature. For transport measurements, the samples were spin coated (1000 rpm, 2 min) on glass substrates and then dried in vacuum. The film and
Fig. 1. FT-IR spectra of ES (a), Co1:ES (b), Co3:ES (c), Co5:ES (d), Co7:ES (e) and Co10:ES (f).
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Fig. 2. Schematic representation of the molecular structure Co nanoparticles are represented by black spherical symbol.
of
Co:ES. Fig. 3. UV–vis spectra of ES (a), Co1:ES (b), Co3:ES (c), Co5:ES (d), Co7:ES (e) and Co10:ES (f).
polyaniline backbone, which states that, as polyaniline is converted into its conducting structure by sulphonation procedure + using sulphuric acid, there are charged groups of SO 3 and NH in the molecule, which offer the torsional degrees of freedom to the structure, thereby generating a strained structure, which greatly affect their transport properties. In our case, there is an additional Co doping, which furthers this process. It is important to mention here that the SO 3 groups of the protonating dopant (acid) is not bound to the ES chain. It merely remains in the vicinity of the molecule due to the hole developed on the quinoid group of ES. Hence, the main features of the ES molecule do not alter much, which is the beauty of this system, imparting magnetic property to the material without hampering their original property range. As we further confirm in the following paragraphs, this unique feature of ES imparts it a probable DMS character, which is uniquely different from doped inorganic-metal-oxide. The molecular structure so formed is shown in Fig. 2. It is to be noted that the figure is just a schematic and torsional modifications are not shown. In order to confirm the above mentioned morphological changes, the samples were further analyzed. 3.2. UV–vis absorption study Fig. 3 shows UV–vis absorption spectra of ES (a), Co1:ES (b), Co3:ES (c), Co5:ES(d), Co7:ES (e) and Co10:ES (f). As seen in Fig. 2(a), clear absorption band signatures of ES are observed at 322, 442 and 744 nm, which correspond to a p–p* transition and forms polaronic and bipolaronic bands [18]. It is observed that there is a gradual polaronic band shift significantly from 744 to 900 nm with incorporation of Co nanoparticles (shown by black arrows). It is suggested that the change in delocalization of electrons/polaron in ES structure (i.e. variation in energy gap between HOMO and LUMO) has taken place with incorporation of Co nanoparticles in ES. This will reflect in the electrical transport study. Such modifications in the ES electronic structure with doping of different organic/metal/ metal oxides have been well reported in recent papers [19–21]. 3.3. Magnetization study Fig. 4 shows the magnetization vs. applied magnetic field (M– H) data for Co1:ES (a), Co3:ES (b), Co5:ES (c), Co7:ES (d) and
Fig. 4. VSM data for Co1:ES (a), Co3:ES (b), Co5:ES (c), Co7:ES (d) and Co10:ES (e). The insets show M vs. T and room temperature hysteresis loops for Co nanoparticles.
Co10:ES (e). The samples show clear hysteresis loop at room temperature, even for an extremely small Co doping (Co3:ES), indicating ferromagnetic behavior. Pure ES sample shows a paramagnetic behavior, which has been documented [22] and has been also checked by us (results not shown). As expected, the samples show an increased magnetization from 0.11 to 1.34 emu/g with the increase of Co nanoparticles from 1 to 10 wt%. The increase in magnetization is related to an increase in dipolar interaction among Co particles [23] because particles proximity increases with increase of Co particles in ES. It is to be noted that this magnetization value is much less than the bulk value of Co (161 emu/g) [24] because, in this study, magnetization value includes weight of ES matrix and thereby reduction in magnetization occurs. Introduction of magnetization in organic system due to magnetic ions is not new [25]; however, most of these references are in context
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Fig. 5. HRTEM image of Co5:ES (a), SAED pattern for Co nanoparticles (b), the SAED pattern for ES matrix (c) and the XRD pattern for ES matrix (d).
of composites and not doped systems, wherein the properties of organic molecules change drastically, which is an adverse modification for DMS systems. As observed, the coercivity of all the samples (Fig. 4(a)–(e)) is rather low and can be of concern to evaluate the magnetic semiconductor basic issue. Hence the pure Co nanoparticles were subjected to M vs. T measurements as well as room temperature hysteresis measurements, which are shown in the insets of Fig. 4. A clear onset of transition is observed at temperature 4780 K ( 1121 K) with the coercivity of hysteresis 85 Oe. Small coercivity observed in our Co samples could be due to the following reasons: (1) small particle size (10 nm); usually, small particles do show low coercivity and magnetization as compared to bulk and (2) cobalt is basically in its FCC geometry, which has low coercivity as compared to hexagonal Co, as was also observed in our earlier studies [14]. These FCC Co particles are crystalline, which is supported by XRD pattern and TEM as well, as discussed below. Unfortunately, since the samples have a polymeric host matrix, we were unable to do high temperature M–T measurements, which could have thrown more light on the origin of magnetism (clustering (or otherwise)). One more point to be mentioned here is that Co nanoparticles are protected from surface oxidation since SO2 3 group of ES acts as stabilizer. Bare Co nanoparticles are prone to oxidation of surface of Co particle. Thus, the synthesis route will be ideal for formation of the polymer based magnetic semiconductors.
˚ is observed. The particle size is found to be (d(1 1 1) ¼2.06 A) 10 nm and distribution of Co particles in ES matrix is homogenous. Its SAED pattern evidences the formation of crystalline Co particles. The SAED pattern for pure ES matrix (Fig. 5(c)) shows polycrystalline nature, but exact determination of crystal system is not known to us and this was supported by the XRD pattern (Fig. 5(d)), where there are peaks in 2y ¼15–301. Such polycrystalline nature is related to the orderly arrangement of polymer units in 2D or 3D structure, but their arrangement is confined to a local domain region [26]. Using Bragg’s relation, l ¼2dsiny (l is wavelength of X-ray and y is angle found in XRD pattern), d-spacing between layers of crystalline polymer can be calculated. There have been reports on SAED patterns of ES as amorphous [27–29] as well as polycrystalline [30]. Homogeneous distribution of ferromagnetic Co particles can be seen from Fig. 5(a). As magnetic characterizations show neither any anomaly in the emerged hysteresis loops nor a shift in coercivity, one can establish the ferromagnetic doping to be uniform in the ES matrix. Though few advanced tools are definitely needed to evaluate if the samples have uniform Co doping, or it is a clustered system, our preliminary findings do suggest that ES matrix comprises of orderly polymeric arrangements, with Co nanoparticles being uniformly distributed in it.
3.4. XRD and HRTEM studies
3.5. Transport study
In order to understand microstructure of Co nanoparticles doped ES, HRTEM image of one of the doped systems, namely Co5:ES, is shown in Fig. 5(a) and the corresponding SAED (selected area electron diffraction) pattern is shown in Fig. 5(b). From HRTEM image, Co nanoparticles with inter-planar distance
In order to understand relationship between electron transport and magnetic spins in Co nanoparticles doping in ES, the current vs. applied voltage (I–V) characteristics for ES (a), Co1:ES (b), Co3:ES (c), Co7:ES (d) and Co10:ES (e) are shown in Fig. 6. Here, the current is noted by changing direct and reversed
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Fig. 6. I–V characteristics of the samples, typically for ES (a), Co1:ES (b), Co3:ES (c), Co7:ES (d) and Co10:ES (e).
voltages. The up and down arrows show the data during ramp-up and ramp-down, respectively. Basic observation from the current values of all graphs shows that with increase in Co doping level the sample resistance increases. Increase in resistance with Co doping up to 10 wt% suggests that mean free path of electron decreases because of introduction of extra scattering by small Co nanoparticles in conducting polymer. The ES sample shows repeatable, linear behavior during the bias ramp-up and rampdown with no hysteresis effect, which is also true for the lower Co percentages. This was a much expected result, since, as seen from the TEM images, the distribution of Co in ES is rather uniform and the ES matrix is also in an orderly fashion. However, for the higher Co doped samples (Co3:ES and above), the samples show nonlinear behavior with transport hysteresis loop between the rampup and ramp-down scans. Area of hysteresis loop increases with increase in percentage of Co nanoparticles doping (Fig. 6(c)–(e)). Such non-linearity in the I–V characteristics for doped/composite polyaniline has been reported before [31,32,18]. Ouyang et al. [31] have fitted the non-linear behavior of transport using a temperature independent tunneling equation for current in their gold-doped polyaniline films as a combination of direct tunneling (tunneling through a square barrier) and Fowler–Nordheim tunneling (tunneling through a triangular barrier). According to them, for lower voltages, direct tunneling is dominant and at high voltage Fowler–Nordheim tunneling becomes the dominant conduction mechanism, which we envisage to be also the case in our higher doped samples. It is interesting and important to mention here that, in our case, the Co nanoparticles are magnetic in nature, which have been bound at specific sites of ES matrix. If it is assumed that the ES matrix is not coupled to the magnetic nanoparticles, the behavior of all samples from Co3:ES to Co10:ES should be similar to ES, except for increased resistance. However, owing to the observations that the samples show a clear hysteresis and nonohmic response, which increases with Co percentage, it can be envisaged that the ES matrix gets coupled to the magnetic dopants. With applied voltage, the spins of Co nanoparticles get oriented in the applied field, thus making the whole matrix develop a spin-polarized transport. As the voltage polarity is reversed, the current changes its direction. As the spins (m) take
time to now re-orient along the direction of conduction electrons, the sample experiences more resistance at initial stage. When directions of current and spins (k) are the same in reversed voltage, the resistance drops. Thus, resistances for both direct and reversed voltages are different because of the presence of magnetic spins. This develops the transport hysteresis loop. As amount of the magnetic doping increases, the area under hysteresis curve also increases. Interestingly, as mentioned above, since the Co nanoparticles are associated with the SO 3 groups of ES chains and are only indirectly linked to the ES main polymeric chains, though the conductivity and magnetism are dominated by magnetic dopants, the main ES matrix maintains its optical and structural properties intact, to much appreciable level, as are seen from the UV–vis and FT-IR signatures. This, in some sense, can be a good signature of the so-formed DMS system, for further applications in spintronics.
4. Conclusions In conclusion, synthesis of dilutely doped (0—10 wt%) cobalt nanoparticles in emaraldine salt (ES) of polyaniline using a sonochemical-assisted-reduction approach has been reported. The TEM images and SAED pattern confirm the phase formation and particle size of Co10 nm. Images also show uniform distribution of Co nanoparticles in the ES matrix. The FT-IR analysis illustrates that cobalt gets electrostatically bound to the SO 3 ion sites in ES matrix, thereby introducing torsional modifications in the basic ES structure but keeping the polymeric backbone intact. This is supported by persistence but systematic shifting in the UV–visible spectroscopy signatures. The magnetic moment gets introduced in the sample with a minimal doping of 3 wt%, which increases as doping increases. The transport data show ohmic response for no/low Co doped systems, which becomes non-linear with increased hysteresis as the doping percentage increases. The non-linear response in such high doped systems has been understood using tunneling transport mechanisms and hysteresis effect has been attributed to the spinpolarized transport mechanism. The results project promising applications of such systems as dilute magnetic semiconductors and in spintronics applications.
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