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Superparamagnetic Magnetite Nanoparticle Superstructures for Optical Modulation/ Chopping Serkan Zorba,*,† Randolph Thomas Maxwell,† Constantine Farah,† Le He,‡ Miaomiao Ye,‡,§ and Yadong Yin‡ Department of Physics and Astronomy, Whittier College, California 90608, Department of Chemistry, UniVersity of California, RiVerside, California 92521, and State Key Laboratory of Urban Water Resource and EnVironment, Harbin Institute of Technology, Harbin 150090, P.R. China ReceiVed: June 16, 2010; ReVised Manuscript ReceiVed: August 9, 2010
We experimentally demonstrate proof of concept operation of superparamagnetic magnetite nanoparticles and magnetite-TiO2 peapod superstructures for laser intensity modulation and chopping. The frequency of the modulation is shown to be twice that of the driving signal and a function of the size of the particles. Specifically, optical modulation with round nanoparticles of sizes 86, 140, and 190 nm is compared with optical modulation with magnetite-TiO2 peapod superstructures of lengths of around 1 µm. The former gave rise to modulations of up to 2 kHz in frequency, a number comparable to that of commercial optical choppers, the latter up to 100 Hz. We also show that particle shape asymmetry and anisotropy enhance optical modulation and that modulation frequency is inversely proportional to the inertia of the particles used. Introduction Magnetic nanoparticles suspended in a carier fluid (ferrofluid) are increasingly studied for their potential use in magneto-optical applications such as magnetochromatics,1 optical switches,2 photonic crystals,3 and light transmission modulation.4 However, an important problem with ferrofluids is particle aggregation due to magnetic attraction and settling due to gravity.4-7 To circumvent both of these problems, researchers are fabricating superparamagnetic nanoparticles (SPNs) with surfactant/polymer coating and/or high surface charges (due to ionic groups) around them.3,8 SPNs’ biocompatibility, high magnetic susceptibility, and lack of strong magnetic interactions under zero magnetic field (no superparamagnetic-to-ferromagnetic transition) make them ideal for biomedical and photonic crystal applications. The idea in these applications is to use external stimulus, such as a magnetic field, to coax and assemble magnetic nanostructures into ordered and aligned systems in a reversible way, which results in enhanced optical, electrical, and magnetic properties.4,6,9-11 Similar investigations have been conducted with electric fields on dielectric nanoparticles and nanowires as well.12,13 For example, Edwards et al. demonstrated synchronous electrorotation of Au nanowires in water by an alternating current electric field.12 This has potential applications in reconfigurable polarization filters, optoelectronic light valves, microfluidic valves, and localized fluid stirring devices. The controlled alignment of rod- or disk-like particles that are suspended in a transparent liquid has become increasingly attractive for particle light valve (PLV) applications.2,4 In fact, there are already commercial applications of PLVs for smart glass applications based on electro-orientation.6 With the progess made in nanofabrication of highly monodisperse and uniformly clustered SPNs and complex superstructures, the research on magnetic nanostructures is accelerating.14,15 * To whom correspondence should be addressed. Phone: (562) 907-4200. E-mail:
[email protected]. † Whittier College. ‡ University of California, Riverside. § Harbin Institute of Technology.
For example, Bao et al. synthesized a bifunctional gold-magnetite composite nanoparticle system that enabled them to separate proteins simply with the help of a magnet.16 Yuan et al. assembled magnetite nanoparticles onto Te nanorods rendering them magnetic, which in turn enabled alignment of the nanorods by applying a magnetic field.17 After the alignment, they removed the magnetite nanoparticles by an acid etch. In another study, iron oxide nanoparticles and quantum dots were incorporated into silica microspheres with a capability to respond to an external magnetic field and be able to luminesce simultanouesly.18 Recently, we fabricated, for the first time, magnetically responsive colloidal photonic crystals by self-assembling highly charged superparamagnetic magnetite (Fe3O4) colloidal nanocrystal clusters (CNCs).3,14 Each cluster consisted of many single-magnetite crystallites of uniform size as subunits. We also recently synthesized a novel composite system (a superstructure made of different constituents) composed of magnetite nanoparticles encapsulated in a polystyrene structure.19 Here, we report the use of CNCs and their composites such as magnetite-TiO2 peapod-like structures for magnetically controlled light intensity modulation and chopping. Although magnetic field modulation of light transmission through ferrofluid films has been reported in the literature, these studies were mostly limited to the application of dc magnetic fields.1,2,4,5,7 The reported magneto-optical studies that use ac magnetic fields as an external stimulus either employ very-low-frequency fields (less than 1 Hz)4,20 or do not focus on the light modulation but rather on the transmission coefficient as a function of frequency.21 Currently, light chopping needs are mostly met by mechanical spoked-wheel-based optical choppers. Our work demonstrates proof of concept operation of an optical modulator and chopper based on superparamagnetic magnetite CNCs and magnetiteTiO2 peapod superstructures in a carrier fluid. As no mechanical moving parts are involved, the proposed system may find applications in optical circuits with advantages including high reliability, long lifetime, and great flexibility and convenience for a high degree of device integration.
10.1021/jp1055599 2010 American Chemical Society Published on Web 09/24/2010
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Experimental Section Chemicals. Diethylene glycol (DEG, reagent grade), ethanol (denatured), and sodium hydroxide (NaOH, 98.8%) were purchased from Fisher Scientific. Anhydrous iron(III) chloride (FeCl3, 98%) was purchased from Riedel-de Hae¨n. Poly(acrylic acid) (PAA, Mw ) 1800) was obtained from Sigma-Aldrich. All chemicals were directly used as received without further treatment. Synthesis of Superparamagnetic Fe3O4 CNCs. Magnetite CNCs were prepared in DEG solution at high temperature. In a typical synthesis, a mixture of 4 mmol of PAA, 0.4 mmol of FeCl3, and 16 mL of DEG was heated to 220 °C in a nitrogen atmosphere for 60 min with vigorous stirring to form a transparent, light yellow solution. A 1.73 mL amount of NaOH/ DEG stock solution (2.5 mol/L) was then injected into the above solution, which slowly turned black after about 2 min. The resulting mixture was further heated for 1 h to yield ∼86 nm Fe3O4 CNCs. CNCs with size of 140 and 190 nm were synthesized by injecting 1.80 and 1.87 mL of NaOH/DEG, respectively. These colloids were first washed with a mixture of deionized (DI) water and ethanol and then with pure water several times and finally dispersed in 3 mL of DI water. Synthesis of Fe3O4/SiO2 Core/Shell Structures. The interlayer of SiO2 was prepared through a modified Sto¨ber method.27 In a typical process, 3 mL of Fe3O4 aqueous solution was mixed with 20 mL of ethanol and 1 mL of ammonium hydroxide under vigorous magnetic stirring. Tetraethylorthosilicate (TEOS, 0.1 mL) was injected into the solution every 20 min until the total amount of TEOS reached 0.3 mL. After washing with ethanol three times, the products were redispersed in 3 mL of absolute ethyl alcohol. Synthesis of Fe3O4/SiO2/TiO2 Peapod-Like Structures. Typically, 1.5 mL of Fe3O4/SiO2 aqueous solution was mixed with 0.12 mL of distilled water, 50 mg of hydroxypropyl cellulose (HPC), and 25 mL of absolute ethyl alcohol under vigorous magnetic stirring. A 0.4 mL amount of tetrabutyl titanate (TBOT) previously dissolved in 5 mL of ethanol was introduced drop by drop under magnetic stirring at a rate of 1150 rpm, followed by reflux at 85 °C for 90 min. The final products were washed with ethanol several times. Characterization of Fe3O4 CNCs and Fe3O4/SiO2/TiO2 Peapod-Like Structures. The morphology and size distribution of the Fe3O4 CNCs and Fe3O4/SiO2/TiO2 peapod-like structures were characterized under a Tecnai T12 transmission electron microscope (TEM) and an XE-70 AFM by Park Systems. Colloids dispersed in water at an appropriate concentration were cast onto a carbon-coated copper grid, followed by evaporation under vacuum at room temperature. We performed magnetic property measurements on peapodlike structures. The magnetite clusters, composed of small primary nanocrystals, behaved as superparamagnetic at room temperature with no remanence or coercivity (see Supporting Information, Figure S1). Results and Discussion Synthesis and Characterization of Magnetite-CNCs and Magnetite/SiO2/TiO2 Peapod Superstructures. Fe3O4 CNCs were synthesized through a one-pot high-temperature hydrolysis reaction.22 Briefly, It involved the hydrolysis of FeCl3 by NaOH at around 220 °C in a diethylene glycol solution with poly(acrylic acid) (PAA) as a surfactant. The particle size can be tuned simply by changing the amount of NaOH. As shown in the transmission electron microscopy (TEM) image in Figure 1, these magnetite CNCs have a unique cluster-like structure.
Figure 1. TEM images of 86, 140, and 190 nm Fe3O4 CNCs.
As we reported previously, each cluster is composed of many interconnected primary nanocrystals with a size of ∼10 nm.22 The unique and complex structure allows Fe3O4 CNCs to retain the superparamagnetic behavior at room temperature even though their overall size exceeds the critical size distinguishing ferromagnetic and superparamagnetic magnetite (30 nm). CNCs were diluted in distilled water to a concentration of 0.1 mg/mL (∼2 × 1010 particles/ml). The average distance between CNCs was calculated to be around 4 µm. Fe3O4/SiO2/TiO2 peapod-like structures were obtained by encapsulating magnetite/SiO2 core/shell structures in a tubular TiO2 shell. The detailed synthesis approach and structural analysis of the peapod structures could be found in our previous work,28,29 in which the SEM and TEM images show clearly the detailed morphologies of the peapod structures. The synthesis process involves chaining of the Fe3O4/SiO2 cores during magnetic stirring and subsequent fixing of the interparticle connection during TiO2 coating. The thickness of SiO2 and TiO2 can be controlled by tuning the amount of silica and titanium precursor, respectively. The thickness of the SiO2 layer increases with the increasing amount of silica precursor.27 In the meantime, the amount of titanium precursor and magnetic stirring rate are the two important factors that determine the final size and morphology of the Fe3O4/SiO2/TiO2 nanostructures, as well as the numbers of nanoparticles in each peapod. The composite structures can be seen in the TEM image shown in Figure 2. The average size of the peapods was about 0.2 µm in diameter and about 2 µm in length. The peapod-like particles were diluted in distilled water to a concentration of 0.27 mg/ ml (∼6 × 107 particles/ml). The average distance between them was calculated to be around 10 µm. Both of our solutions have stayed stable for about 10 months.
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Figure 2. TEM image of Fe3O4/SiO2/TiO2 peapod-like superstructure.
Figure 3. Schematic diagram of the experiemental setup used for modulation of laser intensity.
Optical Modulation with Magnetite CNCs and MagnetiteTiO2 Peapods. Figure 3 shows a schematic diagram of the experimental setup used for optical modulation. An aqueous solution containing nanoparticles is placed in a cylindrical glass cell. The length and diameter of the glass cell are 1 cm. The glass cell is positioned inside a solenoid (15 cm in length) which is driven by an audio amplifier which in turn is driven by a function generator. A compensating capacitor is used to counteract the inductive reactance due to the solenoid for a given frequency. Alternating (ac), linearly polarized magnetic fields of a few mT magnitude were facilely obtained at the center of the solenoid with tunable frequency capability. A constant intensity polarized laser beam is directed at the sample, goes through the sample, and finally is detected by a photodetector which in turn is monitored by an oscilloscope. Throughout this paper, the top and bottom signals in the oscilloscope screenshots are presented at different scales to render visibility. The loss
Zorba et al. generated by our ferrofluid was large. We could not obtain any modulation amplitude better than 10% (highest) of the initial light intensity. However, this is largely due to the long length of the path light has to traverse, which is 1 cm (work is underway to investigate ferrofluid films of a few micrometers). A typical result of the optical intensity modulation with 140 nm magnetite CNCs is shown in Figure 4 in an oscilloscope screenshot. The top signal drives the solenoid magnetic field. Unless stated otherwise, the rms value of the magnetic field used was 4.5 mT. The bottom signal is what is observed at a photodetector due to the modulated laser light. Notice the doubling of the frequency: the laser beam comes out of the CNC ferrofluid with its intensity modulated at twice the frequency of the driving magnetic field signal. We used CNCs of 86, 140, and 190 nm. The modulation was obtained with these particles for both sinusoidal and square wave driving signals of up to 1 kHz, resulting in up to 2 kHz modulation frequency. Note that the latter is comparable with the frequency of commercial optical choppers (Supporting Information, Figure S3). Sinusoidal Driving Signal. Figure 4 shows the modulation, obtained using a sinusoidal driving signal, with 140 nm CNCs at 210 Hz and that with 190 nm CNCs at 905 Hz. The modulated signals are also sinusoidal in form. We believe this is due to the small size of the particles (small inertia) and low viscosity of distilled water, which was used as the suspending fluid of the particles. The modulated signal frequency is twice that of the driving signal. Furthermore, there is a constant phase relationship between the driving and the modulated signals. To ascertain the validity of the small inertia argument and to investigate the effects of shape anisotropy, we looked at rodlike magnetite-TiO2 peapods, which are much larger nanoparticles than 190 nm. Figure 5 shows light modulation achieved with magnetite-TiO2 peapod superstructures of sizes of around 1-3 µm (see the Supporting Information, Figure S2). The frequency doubling and constant phase relation are still in effect. However, the modulated signal is not a sinusoidal signal anymore (Figure 5a). Rather, it is a sawtooth waveform, which reflects the large intertia of the particles. The optical modulation obtained for these large peapod particles was only achieved at low frequency values (50 Hz). The modulation slowly disappeared as the driving signal frequency was increased. For example, for a driving frequency of 1000 Hz, almost no modulation was observed (Figure 5b). Essentially, the laser intensity modulation is due to a change of the transmission cofficient of the fluid used due to the alignment of the scatters present in the fluid, which are our engineered nanoparticles in this case. The modulation by the
Figure 4. Oscilloscope screenshot of light modulation with magnetite CNCs of size (a) 140 nm at 210 Hz and (b) 190 nm at 905 Hz. The top signal is the signal driving the solenoid magnetic field. The bottom signal is the modulated laser intensity as recorded on a photodetector. Note the frequency doubling in the modulated light signal.
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Figure 5. Oscilloscope screenshot of light modulation with magnetite-TiO2 peapod superstructures of sizes 1-3 µm at (a) 50 and (b) 1000 Hz. The top signal is the signal driving the solenoid magnetic field. The bottom signal is the modulated laser intensity as recorded on a photodetector. Note the frequency doubling in the modulated light signal. The field strength is 4.5 mT.
point to the rotation of the particles themselves, indicate that the Brownian mechanism is the dominant mechanism of relaxation in our experiment. The rotation is resisted by the viscous torque due to the carrier fluid and characterized by the Brownian time (also called decay time) of rotational diffusion Figure 6. Depiction of how light modulation is caused during a full cycle of the magnetic field oscillation. A cross-sectional view with respect to laser beam’s direction is shown: Peapods are aligned (a) parallel, (b) perpendicular, and (c) antiparallel to the initial orientation of the magnetic field, B. Thus, peapods orient themselves parallel to the field twice within one full cycle, hence twice allowing maximum light passage, causing frequency doubling.
peapod nanostructures is readily understood by their large anisotropic structure as depicted in Figure 6, where we show a cartoon of how peapod particles would behave under the application of an ac field. The intensity modulation obtained from CNCs, which are almost spherical and hence symmetrical in shape, is not so facily understood. However, we do know from previos work that spherical magnetite particles in a fluid under the application of a magnetic field do form reversible short linear chains5,7 (not unlike the magnetite CNC particles aligned inside our peapod nanostructures) due to the induced magnetic dipolar forces.1,4,23-25 For this reason, we refer the reader to the same cartoon in Figure 6 to explain the light modulation obtained with the CNCs. The results agree with what this model suggests in terms of light transmission variation. As shown in Figure 6, for a full period of the periodic signal driving the magnetic field, the peapods align themselves twice, once for the +B direction and the other for the -B direction, allowing maximum light passage each time. This explains the frequency doubling. This is similar to full-wave rectification of an ac electric current in rectifier circuits. The high fidelity in the waveform of the modulated signal and the constant-phase relation between the driving signal and the modulated signal are due to the small inertia of the particles and the low viscosity of water. It is worth noting here that when the magnetic field direction is flipped, the particle magnetization attempts to regain equilibrium and can do so by one of two mechanisms.26 The first one is the Neel mechanism, whereby the magnetic moment rotates relative to the crystal axis but not so the particle itself. The Neel relaxation is dominant in small particles (
