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Bifunctional Magneto-Optical FePt-CdS Hybrid Nanoparticles Shuli He,†,§ Hongwang Zhang,‡ Savas Delikanli,† Yueling Qin,† Mark T. Swihart,‡ and Hao Zeng*,† Department of Physics, and Department of Chemical and Biological Engineering, UniVersity at Buffalo, SUNY, Buffalo, New York 14260, and Department of Physics, The Capital Normal UniVersity, Beijing 100037, Peoples Republic of China ReceiVed: July 15, 2008; ReVised Manuscript ReceiVed: October 11, 2008
This paper describes preparation of binary FePt-CdS hybrid nanoparticles by spontaneous heteroepitaxial nucleation and growth of the CdS component onto FePt-seed nanoparticles in high-temperature organic solution. FePt nanoparticles with average sizes of 4 and 9 nm were used as seeds. Lattice mismatch makes complete coating of the seed particles unfavorable, resulting in peanut-like and flower-like FePt-CdS nanoparticles. The CdS photoluminescence is quenched in 9 nm FePt-CdS hybrid nanoparticles. The band edge emission is observed in 4 nm FePt-CdS nanoparticles, albeit with substantially lower intensity than in pure CdS nanoparticles. We suggest that the strong FePt-size dependent photoluminescence intensity results from the interplay between interface charge transfer and quantum confinement effects. Introduction In the past two decades, considerable progress has been made in the field of nanomaterials synthesis.1-8 These materials may have important technological applications that take advantage of novel size-dependent properties arising from confined dimensions.9,10 Multicomponent hybrid nanoparticles are being explored as a means to achieve increased complexity and functionality in nanomaterials.11,12 They are composed of discrete domains of different materials, and thus can exhibit the properties of different components in the same structure. Such hybrid structures open up new possibilities for investigating the interactions between different nanoscale components. Shared interfaces in hybrid nanoparticles facilitate charge, energy, and spin transfer between different components.13 Such transfer noticeably modulates the properties of each component in the hybrid nanoparticles by the conjugating ones. Even entirely novel properties may arise because of the coupling between different domains. Such examples include the enhancement of photoluminescence and stability,14,15 shifts of metal surface plasmon absorption,16,17 enhanced magnetic properties,18 and increased catalytic activity.19,20 These hybrid nanostructures may have a broad range of applications not available in homogeneous nanoparticles, such as in heterogeneous catalysis,20,21 multifunctional biolabels,22-24 and as building blocks for optoelectronic devices.25 In general, hybrid nanoparticles can be synthesized by either sequential growth of a second component on presynthesized nanoparticles,14,26 or by growth of two separate particle domains in a one-pot synthesis.27-29 By rationally tuning the synthetic parameters, hybrid nanoparticles with different morphologies have been prepared, which can be classified as core/shell nanostructures,14,18,24,29-32 hetero-dimers/-trimers,27,33-36 and rodlike heterostructures.37-41 Here, we report high temperature organic synthesis of binary FePt-CdS nanoparticles, based on * Corresponding author. E-mail:
[email protected]. † Department of Physics, University at Buffalo. ‡ Department of Chemical and Biological Engineering, University at Buffalo. § The Capital Normal University.
spontaneous heteroepitaxial nucleation and growth of CdS onto preformed FePt seeds. Different sized FePt seeds were used to investigate the effects of the size of the semiconductor domain relative to the magnetic domain on the optical and magnetic properties of the binary FePt-CdS nanoparticles. While the magnetic properties of FePt are retained, the photoluminescence (PL) intensity of CdS can be tuned by the size of FePt, a result of the interplay between interface charge transfer and quantum confinement. Experimental Section Platinum acetylacetonate (Pt(acac)2, Pt 49.8%), lead oxide (99.999%), sulfur (powder 99.99%), and cadmium oxide (99.999%) were purchased from Alfa Aesar; phenyl ether (99%), benzyl ether (99%), iron pentacarbonyl (Fe(CO)5, 90%), tri-noctylamine (98%), oleic acid (90%), and oleylamine (70%) were purchased from Aldrich. All chemicals were used as received. FePt nanoparticles were synthesized by decomposition of Fe(CO)5, and reduction of platinum acetylacetonate (Pt(acac)2) by 1,2-hexadecanediol in dioctyl ether at high temperature, following published procedures.4 After separation and purification, FePt nanoparticles were dispersed in hexane and used as seeds for further growth of the semiconductor component. The size of the FePt nanoparticles can be tuned by tuning the heating rate and amount of surfactant. Spherical/cubic-shaped FePt can be obtained by tuning the ratio of oleic acid to oleylamine and the amount of surfactants. For the synthesis of binary FePt-CdS hybrid nanoparticles, 0.5 mmol CdO, 3 mmol oleic acid, and 10 mL of trioctylamine were loaded into a 125 mL four-necked flask set in a heating mantle. Under argon gas flow, the mixture was heated to 180 °C until a Cd-oleate complex was formed. Then, 0.05 mmol FePt nanoparticle seeds in hexane were injected, and the hexane was distilled out, followed by the injection of a 2.5 mL portion of 0.1 M sulfur solution (sulfur dissolved in a mixture of oleylamine and phenyl ether with 1:4 volume ratio of oleylamine to phenyl ether). The mixture was further heated to 280 °C for 30 min. After cooling to room temperature, the mixture was
10.1021/jp806247f CCC: $40.75 2009 American Chemical Society Published on Web 12/11/2008
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He et al.
Figure 1. TEM images of peanut-like FePt-CdS hybrid nanocrystals synthesized at a reaction temperature of 280 °C using (a) spherical 4-5 nm FePt seeds and using (b) 9 nm cubic-shaped FePt nanoparticles as seeds.
treated with ethanol under air followed by centrifugation, then redispersed in hexane. The UV absorption spectra were collected using a Shimadzu model 3101PC UV-vis-NIR scanning spectrophotometer over a wavelength range from 300 to 800 nm. The samples were measured against hexane as a reference. The photoluminescence spectra were obtained using a Fluorolog-3 spectrofluorometer. The nanoparticle size and morphology were characterized by transmission electron microscopy (TEM) using a JEOL model JEM-100CX microscope and by high resolution transmission electron microscopy (HRTEM) using a JEOL model 4000EX microscope. X-ray diffraction patterns were recorded using a Bruker AXS D8-Advanced diffractometer with Cu KR radiation (λ ) 1.5418 Å). The magnetic hysteresis loops were measured using a Quantum Design Physical Property Measurement System model 6000. Results and Discussion High temperature reaction at 280 °C leads to the nucleation and growth of CdS lobes onto FePt to form binary FePt-CdS nanoparticles, as shown in Figure 1a,b, respectively. In the TEM images, the darker region is FePt while the lighter area is CdS. The contrast between the darker and the lighter areas is due to the different electron penetration efficiencies through the metallic FePt domain and the semiconductor CdS domain. By using 4 nm FePt as seeds, the morphology of the binary nanoparticles is predominantly peanut-like, with a single CdS component in each particle. However, for 9 nm FePt seeds, the binary particles are flower-like, with multiple CdS lobes surrounding the FePt. In the X-ray diffraction pattern (Figure 2) of FePt-CdS hybrid nanoparticles prepared using 9 nm cubic-shaped FePt nanoparticles as seeds, the position and relative intensity of all diffraction peaks match well with the standard face-centered-cubic (fcc) FePt and zinc blende CdS powder diffraction patterns. HRTEM images (Figure 3a,b) show that CdS domains in these binary particles are single-crystalline, with a lattice spacing of 0.334 and 0.337 nm, for 4 and 9 nm FePt nanoparticle seeds, respectively. The lattice spacing can be indexed to the (111) plane of the cubic zinc blende CdS structure and matches well with the XRD pattern of CdS. It is difficult to resolve lattice images of both FePt and CdS in a single hybrid, because of the rare occurrence for both lower index zone axes to be parallel to the electron beam. From the few images we obtain, it is possible to derive the heteroepitaxial relationship between CdS and FePt. As can be seen from Figure 3c, lattices of both FePt and CdS can be resolved. They are parallel to each other, suggesting the coherent interfaces between two components. The lattices are indexed to be CdS(220) with a spacing of 2.05 Å
Figure 2. X-ray diffraction pattern of FePt-CdS hybrid nanoparticles using 9 nm cubic-shaped FePt nanoparticles as seeds indicates that FePt and CdS domains have the expected fcc and zinc blende structures, respectively.
and FePt(200) with a lattice spacing of 1.94 Å. The mismatch is about 5.7%. Careful observation can even identify the stacking faults at the interface. The hybrid nanoparticles form peanutand flower-like structures instead of core/shell structures to reduce the strain due to this mismatch. When grown at a relatively low temperature of 160 °C, the FePt seeds are partially covered by a thin layer of material faintly visible in Figure 4a. Figure 4b shows the nanoparticles synthesized at 160 °C using 9 nm cubic-shaped FePt nanoparticles as seeds. These FePt seeds are covered by a shell material and are somewhat agglomerated. This is similar to a previous report by Xu et al. on FePt@CdS core-shell nanoparticles synthesized at relatively low reaction temperatures (100 °C).33 Optical measurements of these hybrid nanoparticles show neither an absorption peak nor photoluminescence. No electron diffraction patterns have been observed from the shell, suggesting that it may be a sulfur-rich amorphous phase.33 The formation mechanism of the binary nanoparticles can be intuitively understood as follows: at low temperatures, sulfur is adsorbed at the surface of FePt because of the strong Pt-S bond, followed by the formation of a continuous CdS shell. Upon heating, CdS dewets the FePt surface and forms small droplets. The atoms at the surface of nanoparticles are rather mobile, and smaller droplets come together to form larger CdS, much like small water droplets aggregate to form a larger one. The high temperature appears critical for the formation of the peanutlike and flower-like structures. In a recent publication,42 when the reaction temperature was kept well-below 300 °C, only core-shell FePt-CdS nanoparticles are formed. Heterodimers are only formed after refluxing in benzyl ether at 300 °C. In this work, TOA was used as the solvent and a refluxing temperature of 280 °C is needed to form the heterodimers. We therefore conclude that temperature is the key factor in the formation of heterodimers. For 4 nm FePt seeds, all CdS can aggregate into a single component to form a peanut-like structure, while for large FePt seeds, because of limited diffusion length, the CdS can form a few droplets, thus forming the flower-like morphologies. The PL spectra for the pure CdS nanoparticles and FePt-CdS hybrid nanoparticles are shown in Figure 5. The free CdS nanoparticles have a strong emission peak at 454 nm and a broad low-energy peak, corresponding to the trap-state emission arising from surface defect sites. For the FePt-CdS hybrid nanoparticles using 9 nm FePt seeds, the PL of CdS is completely quenched. When 4 nm FePt particles were used as seeds, the resulting
Magneto-Optical FePt-CdS Hybrid Nanoparticles
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Figure 3. HRTEM image of peanut-like FePt-CdS hybrid nanocrystals synthesized at a reaction temperature of 280 °C using (a) spherical 4-5 nm FePt seeds. The lattice spacing of the CdS domain is 0.334 nm, corresponding to the (111) plane of zinc blende CdS. (b) HRTEM image using 9 nm cubic-shaped FePt nanoparticles as seeds. The lattice spacing of CdS domain is 0.337 nm, corresponding to the (111) plane of zinc blende CdS. (c) A typical peanut-like FePt-CdS hybrid nanocrystal showing the lattices of both the FePt and CdS components with coherent interfaces suggesting heteroepitaxial growth.
Figure 4. TEM images of FePt-CdS hybrid nanoparticles synthesized at a reaction temperature of 160 °C using (a) spherical 4-5 nm FePt seeds and using (b) 9 nm cubic-shaped FePt nanoparticles as seeds.
Figure 5. Photoluminescence spectra from pure CdS nanoparticles and FePt-CdS hybrid nanoparticles produced using 4 nm FePt seeds.
FePt-CdS nanoparticles showed clear band edge emission, although with an intensity reduced by a factor of about 10 compared with pure CdS nanoparticles. Furthermore, the defect emission at longer wavelength was completely quenched. Previous studies have demonstrated that the optical properties of the hybrid nanocrystals are more than a simple linear combination of the properties of the individual components.11,34,43 Banin et al. observed PL quenching in CdSe nanorods upon increasing the size of Au tips grown on the ends of the rods.41 A similar result was also observed for the CdS-Au hybrid nanorods by the nucleation and growth of Au nanocrystals onto CdS rods.43 In contrast, some other work reported that the semiconductor component in a magnetic-semiconductor binary hybrid can maintain its optical properties. Kim et al. reported a Co/CdSe core/shell structure,24 and Xu et al. reported a binary FePt/CdS structure, both of which exhibited PL.33 Previous work suggested that the emission quenching is a result of charge (electron) transfer at the interface for Au-CdS nanostructures.41,43
However, the Au-size dependence of PL quenching was not understood. We suggest that the PL reduction in FePt-CdS originates from similar mechanisms: when FePt and CdS make contact, because the initial Fermi level of CdS is lower than that of FePt, electrons will transfer from FePt to CdS so that their Fermi levels eventually match. This leads to the downward band bending of CdS, causing electron accumulation at the interface, that is, the formation of a Schottky barrier. The electrons generated by photoexcitation can only transfer from CdS to FePt but not vice versa because of the presence of this barrier. This leads to charge separation at the interface that prevented charge recombination within the semiconductor, leading to quenched emission. For very small FePt nanoparticles, however, the charging energy is very high (on the order of a few hundred millielectronvolts for a 4 nm particle). The Coulomb repulsion will prevent multiple electrons from occupying the same FePt nanoparticle. When the rate of electron-hole pair generation exceeds the rate of discharging from FePt to the surrounding media, interface charge transfer is partially blocked and the PL intensity is only reduced but not completely quenched. The amount of reduction is ultimately determined by the rate of photoelectron generation and the rate of discharging from FePt into the surrounding media (the rate of interface charge transfer is likely fast enough that it does not play a significant role). One would expect surface trap-state emission to be more susceptible to quenching than band gap emission because of its longer lifetime, and thus the defect state emission is preferentially removed. This means, by tuning the size of the FePt, it is possible to tune the emission properties of the semiconductor component. The highly FePt-size dependent PL reduction is therefore interpreted as the interplay between interface charge transfer and quantum confinement. Comparing the PL spectra of CdS with that of FePt-CdS, we further notice a significant redshift for the hybrid particles. While a slight difference in size can contribute to some of the redshift, the amount of the shift cannot be attributed solely to size difference. For the hybrids, there will be some interface states that may locate in the forbidden band gap. This may reduce the effective band gap and cause a redshift. The magnetic hysteresis loops of 4 nm FePt-CdS and 9 nm FePt-CdS nanoparticles measured at 10 K are shown in Figure 6a,b respectively. As indicated by the M-H curves, the FePt parts showed ferromagnetic properties and gave a coercivity of Hc ) 20 mT for 4 nm FePt and 40 mT for 9 nm FePt. The hybrid nanoparticles show reduced remanence ratio compared to FePt seeds, indicating the weak exchange coupling between hybrid nanoparticles in the presence of the semiconductor part. No difference of coercivity between FePt (not shown) and
90 J. Phys. Chem. C, Vol. 113, No. 1, 2009
Figure 6. Magnetic hysteresis loops of peanut-like FePt-CdS hybrid nanocrystals measured at a temperature of 10 K (a) using spherical 4-5 nm FePt seeds and (b) using 9 nm cubic-shaped FePt nanoparticles as seeds.
FePt-CdS is observed. This is similar to the behavior observed for Co-CdSe core-shell nanoparticles reported by Kim et al.24 The coercivity of binary FePt-CdS nanoparticles is determined mainly by the magnetocrystalline anisotropy, which is insensitive to surface modifications. The magnetic and optical measurements confirm that the properties of FePt and CdS are both preserved, which enable the hybrid nanoparticles to combine those magnetic and optical properties. Conclusions This report demonstrates that binary FePt-CdS nanoparticles can be synthesized by the sequential growth of the semiconductor component on the magnetic seeds. This binary hybrid retains the magnetic property of FePt with the luminescence properties tuned by the size of FePt. Reduced band edge emission intensity was observed in 4 nm FePt-CdS, while the PL of 9 nm FePt-CdS was completely quenched. These magnetic-optical hybrid nanostructures demonstrate bifunctionality with potential biological applications. The magnetic domain can be used for magnetic detection or navigation, while the fluorescent domain can be optically detected. In addition, our results demonstrated that the relative size of the magnetic and semiconductor domains could influence the optical properties of the hybrid nanostructure. By carefully controlling the synthesis conditions, the bifunctionality of these hybrids can be maintained. Acknowledgment. The work was supported by the NSF DMR-0547036, NSF-CBET 0652042, UB Integrated Nanostructured Systems Instrument Facilities, and the National Science Foundation of China. References and Notes (1) Peng, X. G.; Manna, L.; Yang, W. D.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature 2000, 404, 59.
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