ARTICLE pubs.acs.org/Langmuir
Intermetallic Magnetic Nanoparticle Precipitation by Femtosecond Laser Fragmentation in Liquid Takashi Yamamoto,† Yasuhiko Shimotsuma,*,† Masaaki Sakakura,‡ Masayuki Nishi,† Kiyotaka Miura,† and Kazuyuki Hirao† † ‡
Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 615-8510, Japan Office of Society-Academia Collaboration for Innovation, Kyoto University, Kyoto 615-8510, Japan ABSTRACT: Intermetallic Nd2Fe14B nanoparticles with an average diameter of 30 nm, which are smaller than a theoretical single magnetic domain size of 220 nm, were successfully prepared by the femtosecond laser fragmentation in liquid. The self-passivating amorphous carbon layer resulting from the decomposition of the surrounding solvent prevents the Nd2Fe14B nanoparticle from aggregation and oxidation. The coercivity of Nd2Fe14B nanoparticle increases with increase of the laser irradiation time, despite the reduction of crystallinity.
1. INTRODUCTION The size and shape of nanoscale materials provide excellent control over many of the physical and chemical properties, including electrical and thermal conductivity, magnetic properties, luminescence, and catalytic activity.1 In particular, the synthesis and morphological control of nanosized metal particles, which exhibit surprising and novel phenomena based on the unique property called the quantum size effect, are attractive to chemists and physicists. In recent years, metal nanoparticles are widely used in many applications ranging from biosensing,2,3 plasmonic devices,4,5 and multifunctional catalysts.6,7 There are a wide variety of techniques that are capable of creating nanoparticles with various morphology and production yield. These nanoparticle formation approaches are typically grouped into two categories: “top-down” and “bottom-up”. Colloidal chemists have gained excellent controlled nanosized particles for several spherical metal and semiconductor compositions, which has led to the discovery of quantum size effect in colloidal nanocrystals.8 However, these processes require high temperature, low pressure, complex procedures, extended growth period, or the use of catalysts that could be embedded on the nanostructure tip, which are unfavorable conditions. On the other hand, top-down approaches have been developed for producing metal and semiconductor nanowires,9 nanobelts,10 and nanoprisms.11 In particular, the laserinduced ablation method has become an increasingly popular approach for making nanoparticles due to the applicability to various target materials in an ambient atmosphere.1216 Recently, various shape-controlled nanoparticles, such as nanowires,17 nanotubes,18 and composite nanostructures,19 have been fabricated by this technique. More recently, nanosecond2022 and femtosecond23,24 pulsed laser ablation in liquid has become profoundly intrigued for preparing nanoparticles from the viewpoint of the concise procedure and the ease of handling. In a previous study, we have successfully prepared one-dimensional copper nanoparticles from r 2011 American Chemical Society
microflakes using femtosecond laser pulse irradiation in alcohol solution at room temperature.25 It should be interesting to investigate the preparation and evolution of copper nanowires under intense ultrashort light fields, which reveals that the fragmented anisotropically shaped copper nanoclusters resulting from the interaction between the incident light field and the surface plasmon wave act as nuclei, and then these uniaxially grow to nanowires.26,27 Here we report the formation of intermetallic Nd2Fe14B magnetic nanoparticles smaller than a theoretical critical singledomain size under intense ultrashort light fields in liquid. The prepared Nd2Fe14B nanoparticles had an about twice higher coercivity than that of initial particles. Beyond the basic understanding, such metal nanoparticles have possible applications in the areas of plasmonic devices,28 surface-enhanced Raman scattering (SERS),29 and medicinal imaging.30
2. EXPERIMENTAL SECTION Using the combination technique of hydrogen decrepitation and jet milling, the ingots of Nd2Fe14B alloy were crushed into small particles with an average size of about 1 μm. A small amount of Nd2Fe14B particles was mixed with 4.0 mL of 99.5% cyclohexane in a rectangular quartz vessel of 12.5 12.5 45 mm3. The wall thickness of this vessel is 1.25 mm. In order to prevent Nd2Fe14B particles from oxidizing, the suspension with a particle concentration of 0.1 wt % was degassed for 10 min by bubbling with N2 gas. The schematic of the experimental setup is shown in Figure 1a. The laser radiation in Gaussian mode produced by a regenerative amplified mode-locked Ti:sapphire laser (Coherent Inc., 100 fs pulse duration, 1 kHz repetition rate) operating at a wavelength of 800 nm was focused via a microscope objective (Nikon; LU Plan Fluor, 20 0.40 N.A.) into the suspension for 60 min. The laser focus was Received: November 24, 2010 Revised: May 27, 2011 Published: June 08, 2011 8359
dx.doi.org/10.1021/la201211e | Langmuir 2011, 27, 8359–8364
Langmuir
ARTICLE
Figure 1. (a) Schematic of experimental setup. (b) Photograph around the focal region during femtosecond laser irradiation.
Figure 3. Typical TEM images of the Nd2Fe14B particles before (a) and after the femtosecond laser pulse irradiation for 60 min (b). Inset shows the electron diffraction pattern of point A or B in each figure. (c) High magnified TEM image of the dotted region in figure (b). Points of C, D, and E in figure (c) show the analysis points of the EDX. (d) The corresponding EDX spectra for the points marked A to E.
3. RESULTS AND DISCUSSION
Figure 2. Secondary electron images of the Nd2Fe14B particles before (a) and after (b) the femtosecond laser pulse irradiation for 60 min in cyclohexane. (c) Backscattering electron image on the same surface of (b). (d) Histogram for the particle size distribution and log-normal fit curve (red line). (e) Absorption spectrum of the nanoparticles produced by the femtosecond laser irradiation for 60 min and the fitted extinction spectrum of an iron nanosphere (R = 35 nm) coated with an amorphous carbon layer (t = 2 nm) based on Mie theory. Inset shows the calculation model, where R and t are the radius of the iron nanosphere and the thickness of the amorphous carbon layer, respectively. (f) Particle size distribution of the prepared nanoparticles obtained from dynamic light scattering. positioned roughly in the center of a quartz vessel (Figure 1b). During the experiments, no apparent damage was observed on the surface of the quartz vessel because the laser fluence did not reach the laser damage threshold. A transmittance at 800 nm of 0.1 wt % Nd2Fe14B suspension was 50%. To keep the uniform dispersion status of magnetic nanoparticles in cyclohexane during laser irradiation, we continuously agitated the suspension by the intense ultrasonic wave. The beam was focused into the suspension with a beam waist diameter and laser energy fluence estimated at ∼4 μm and 2.4 103 J/cm2, respectively. All of the experiments were carried out at ambient temperature and pressure. After femtosecond laser irradiation, absorption spectra of the suspension were measured by a spectrophotometer (Jasco, V-570). The size distribution of the prepared nanoparticles was measured by the dynamic lightscattering technique (NanoSight, LM20). After liquid evaporation at room temperature in a N2 purged glovebox, the processed Nd2Fe14B particles were characterized by a field-emission scanning electron microscope (JEOL, JSM-6700F) and a transmission electron microscope (Hitachi, HF-2000) equipped with an energy dispersive X-ray spectrometer (EDX). The magnetic properties were measured by using a vibrating sample magnetometer (Toyo Corp., Model 7400) at room temperature.
Characterization of Nd2Fe14B Nanoparticles. Figure 2a,b shows the typical secondary electron images (SEIs) of the Nd2Fe14B particles before and after the femtosecond laser pulse irradiation for 60 min in cyclohexane. The backscattering electron image (BEI) corresponding to the SEI is also shown in Figure 2c. The SEI and BEI on the same surface were compared. It is well-known that the SEI reveals the surface morphology of a sample, while the BEI is sensitive to the atomic weight of the elements or the density of material constituting the observation surface. The BEI reveals that the prepared spherical nanoparticles with a Feret’s diameter of 30 nm (Figure 2c,d), which are covered with the low-molecular-weight layer, resulting in the lack of clarity and sharpness of SEI (Figure 2b). Figure 2d indicates the size distribution obtained by measuring the diameters of more than 100 particles in sight on a given SEIs. The average size of the produced nanoparticles after the femtosecond laser pulse irradiation for 60 min in cyclohexane was obtained to be 30.4 ( 2.6 nm. We have also observed there is little change in the average size and the standard deviation with increasing the laser irradiation time up to 240 min. Figure 2e shows that the absorption spectrum of the nanoparticles prepared by the femtosecond laser irradiation for 60 min and the fitted extinction spectrum of an iron nanosphere coated with an amorphous carbon layer based on Mie theory.31 The fitted wavelength range was 250900 nm. In this calculation for simplicity, the radius of the iron nanosphere (R) and the thickness of the amorphous carbon layer (t) were used as fitting parameters. A comparison of the experimental and simulation results indicates that the simulation was good agreement with the experiment when R and t were 35 and 2 nm, respectively (Figure 2e). The differences between Feret’s diameter and the radius estimated from the optical absorption may be due to the aggregation of the nanoparticles during absorption spectra acquisition. Figure 2f also shows the particle size distribution obtained from the dynamic light scattering. Although a broad size distribution peaking at about 198 and 265 nm suggesting a partial aggregation of the nanoparticles was observed, the sharp peaks at 39 and 54 nm indicate the average diameter of the nanoparticles were produced by the femtosecond 8360
dx.doi.org/10.1021/la201211e |Langmuir 2011, 27, 8359–8364
Langmuir
ARTICLE
Table 1. Ion Fragments and Structural Assignment for Positive and Negative Ion Spectra in Figure 4 ion fragments
positive ion
ion fragments
positive ion
[m/z]
structure
[m/z]
structure
27 41
C2H3þ C3H5þ C5H3þ C6H5þ C7H7þ
13 16
CH O
49
C4H
73
C6H
63 77 91
Figure 4. (a) TOF-SIMS spectra of positive fragment ions from the Nd2Fe14B nanoparticles (upper row) and the reference (down row). (b) TOF-SIMS spectra of negative fragment ions from the Nd2Fe14B nanoparticles (upper row) and the reference (down row). Numbers in figure refer to the typical ion fragments.
laser irradiation. Indeed, there were few large particles (>100 nm) in SEM observation (Figure 2c). These results clearly indicate that the size of the prepared Nd2Fe14B nanoparticles is smaller than that of the critical single domain diameter at room temperature (Ds ∼ 218 nm), which can be estimated by the following equation:32 Ds ¼ 18γw =ðμ0 Ms 2 Þ
ð1Þ
where γw (= 4(AK)1/2) is the domain wall energy per surface area, μ0 is vacuum permeability, Ms is the saturation magnetization, A is the exchange constant, and K is the anisotropy constant. In this calculation, the bulk values of the saturation magnetization (Ms = 16.1 kOe)33 and the domain wall energy density (γw = 24 erg/cm2)34 were used. Detailed characterization of the prepared nanoparticles was carried out by means of TEM (Figure 3). Figures 3a and 3b show the typical TEM images of the Nd2Fe14B particles before and after the femtosecond laser pulse irradiation for 60 min, respectively. The electron diffraction patterns of point A and B are also shown in the inset of each figure. Since no apparent diffraction patterns can be observed after laser irradiation, suggesting that the tetragonal Nd2Fe14B crystal structure was changed to amorphous. Additionally, a very small amount of nanoparticles with sizes of a few nanometers which are comparable to superparamagnetic limit (∼4 nm)35 was observed (Figure 3b). Furthermore, detailed TEM observation and EDX analysis (Figure 3c,d) indicate that the composition of the nanoparticles after laser irradiation changed from that of the initial particle. The peak intensity ratios of INdLR/IFeKR X-ray fluorescence spectra obtained from the points A, B, C, and D (Figure 3d) were estimated to be 0.09, 0.05, 0.04, and 2.05, respectively. Especially the prepared nanoparticles were partially oxidized and Nd-rich phase was formed from the surface to the depth of about 5 nm due to the surface segregation of Nd. Moreover, the Nd2Fe14B nanoparticles were covered with an amorphous carbon layer with a thickness of about 2 nm (Figure 3c,d). In order to reveal characteristics of the amorphous carbon layer, time-of-flight secondary ion
mass spectrometry (TOF-SIMS) measurements were performed on the Nd2Fe14B nanoparticles produced by the femtosecond laser irradiation. TOF-SIMS spectra were acquired by using a PHI TRIFT V nanoTOF system equipped with a Bi liquid metal primary ion source operating at 30 kV beam voltage. We also analyzed a silicon wafer as a reference, which was prepared by immersing in pure cyclohexane solution without laser irradiation. Figure 4 shows TOFSIMS spectra of positive and negative fragment ions from the Nd2Fe14B nanoparticles and the reference. Ion fragments and structural assignment for positive and negative ion spectra in Figure 4 are listed in Table 1. Although slight differences were observed in the low mass fragments, the general fragmentation pattern of the Nd2Fe14B nanoparticles was similar to that of reference, indicating that the amorphous carbon layer was originated from the decomposition of the surrounding cyclohexane molecules. During the focused femtosecond laser radiation in cyclohexane, white-light emission caused by nonlinear optical effects of high intensity laser field36 was observed around the focal point. Additionally, very fine bubbles, which could be produced by the decomposition of cyclohexane molecules to elementary carbon through laser-induced breakdown,37 were generated around the focal point. To discuss the energy transfer to the laser radiation to the light-matter interaction, we observed white light emission generated in cyclohexane with and without the Nd2Fe14B particles (Figure 5). Furthermore, we have also measured the laser power transmitted through the focus inside vessel filled with cyclohexane without the Nd2Fe14B particles (see inset of Figure 5). The broad white light continuum ranging from visible to near-infrared was observed with or without the Nd2Fe14B particles. The attenuated laser power was estimated by the following equation: A ¼ Iin TOL Tcell LD Iout
ð2Þ
where Iin and Iout are the input and output laser power, TOL and Tcell are the transmission of objective lens (=0.8) and quartz vessel (=0.87), and LD is the detection loss which is negligibly small. The consumed laser power through the focus inside vessel filled with cyclohexane increases in proportion to the input laser power, indicating the attenuation was maintained constant of about 54%. Such laser energy consumption would be derived from the lightmatter interaction including the decomposition of the cyclohexane molecules and the white-light continuum generation. It is well-known that the magnetic properties strongly depend on the size and shape of the particles. Figure 6ad shows the hysteresis loops of the Nd2Fe14B particles before and after the femtosecond laser irradiation for 20, 40, and 60 min, respectively. The coercivity (Hc) and the normalized remanent magnetization (Mr/Ms) of the Nd2Fe14B nanoparticles as a function of the laser irradiation time are also shown in Figure 6e. These results indicate that the coercivity and the normalized remanent magnetization of 8361
dx.doi.org/10.1021/la201211e |Langmuir 2011, 27, 8359–8364
Langmuir
ARTICLE
Figure 5. White-light emission spectra generated in cyclohexane with and without the Nd2Fe14B particles. Inset shows attenuation of laser power transmitted through the focus inside vessel filled with cyclohexane without the Nd2Fe14B particles. Dotted line indicates a guide for the eye.
the Nd2Fe14B nanoparticles increased with increasing in laser irradiation time and then saturated. Finally, the coercivity of the Nd2Fe14B nanoparticles smaller than the critical single domain size (Ds) indicates twice as high as that of the initial particles. In our experiments, since the sizes of the prepared nanoparticles after femtosecond laser irradiation for 20, 40, and 60 min remained almost unchanged and were smaller than the Ds, the Nd2Fe14B particle size reduction to enhance magnetic properties was achieved within at least 20 min (Figure 6e). In spite of the degradation of magnetic anisotropy due to the sphere shape and the decrease in crystallinity, the coercivity enhancement was observed after laser irradiation, indicating the particle size dependence of coercivity.38 It should be noted that the composition variation within the prepared nanosphere also influences the magnetic properties. In our experiments, the Nd-rich layer covering the surface of Nd2Fe14B nanoparticles was partially oxidized. Furthermore, the slight change in the core composition resulting from the Nd segregation to the surface of nanoparticles was also observed. The oxidized Nd-rich layer reduces the overall magnetization, indicating the decrease of Ms (increase of Mr/Ms). On the other hand, this layer induces pinning of the magnetic moments in the core, indicating the increase of Hc because a higher field is required to change their magnetic moment orientation. Furthermore, the change of Hc and Mr/Ms could also be attributed to the change in the core composition. Indeed, it is well-known that Nd2Fe14B magnets including such Nd-rich phase exhibits a high coercivity but a low saturation magnetization.39,40 Another possible explanation of changing Hc and Mr/Ms, which needs further investigation, is the boron segregation. If boron also segregates to the surface, the core will be slightly less amorphous, signifying that the core will have a higher anisotropy (higher Hc). Mechanisms of Nd2Fe14B Nanosphere Formation. Assuming femtosecond-laser irradiation into a continuous agitation of the suspension, the initial particle with an average size of ∼1 μm can move only 30 nm while the fragmented Nd2Fe14B nanoparticles with a radius of about 20 nm (Figure 2) can travel at least 149 nm within an interpulse time (τint) of 1 ms, which was estimated by the Brownian motion of suspended nanoparticles described by the StokesEinstein equation: Æx2 æ ¼ kB Tτint =ð3πηdÞ
ð3Þ
Figure 6. Hysteresis loops of the Nd2Fe14B particles before (a) and after the femtosecond laser irradiation for 20 (b), 40 (c), and 60 min (d). (e) Coercivity and the normalized remanent magnetization of the Nd2Fe14B nanoparticles as a function of the laser irradiation time.
where kB, T, η, and d are the Boltzmann constant, the temperature, the viscosity of cyclohexane, and the diameter of the Nd2Fe14B nanoparticle, respectively. In order to understand the origin of the Nd2Fe14B nanosphere formation by femtosecond laser fragmentation in cyclohexane, the following explanation of the heating mechanism is proposed. Since the light intensity in the focus of the beam is of 1 1013 W/cm2, the plasma is produced by multiphoton ionization in the focal volume. Once a high free electron density is produced by multiphoton ionization, the material has the properties of plasma and will absorb the laser energy via absorption mechanism of inverse Bremstrahlung heating.41 The maximum electron temperature can be roughly estimated by a simple formula:42 Temax ¼ 8RF=ð3cε0 nls ne τp Þ 1:5 105 K
ð4Þ
where R (= 4πls/λ) is the absorption coefficient, F is the laser energy fluence (= 2.4 103 J/cm2), c is the speed of light, ε0 is the vacuum permittivity, ls (= c/(2ωk) ∼ 17.3 nm) is the skin depth, ne is the electron plasma density, and τp is the pulse width. For simplicity, we consider the complex refractive index of iron (n þ ik = 3.0 þ 3.7i) at the laser wavelength (λ) of 800 nm in this estimation. Under such high electron temperature, not only the fragmentation of the Nd2Fe14B surface but also the decomposition of the cyclohexane molecules can simultaneously occur within the focal volume during the femtosecond laser irradiation, resulting in the formation of the Nd2Fe14B nanoparticles covered with an amorphous carbon layer. Such very high electron temperature decreases with increase of the lattice temperature, then it reaches to the same temperature as lattice temperature with a time scale of several picoseconds. Because of the low particle concentration of the initial suspension and the freely suspended state of the fragmented Nd2Fe14B nanoparticles, we consider the local temperature elevation in cyclohexane. Assuming that the initial temperature of focal volume reaches ΔT0 = 2000 K, which is much higher than the melting point of Nd2Fe14B, after the 8362
dx.doi.org/10.1021/la201211e |Langmuir 2011, 27, 8359–8364
Langmuir
ARTICLE
interaction in liquid will open new opportunities in nanomaterial fabrication, material processing, optical trapping, and manipulation.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
Figure 7. Contour plot of the calculated temperature distribution after the femtosecond single pulse irradiation.
electronphonon coupling, the thermal diffusivity can be calculated by the following equation:43 0 13 w0 =2 B C ΔTðr, tÞ ¼ ΔT0 @qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiA 2 ðw0 =2Þ þ 4Dth t ! r2 exp ðw0 =2Þ2 þ 4Dth t
ð5Þ
where w0 (= 1.22λ/NA ∼ 2.4 μm) is the laser beam waist, t is the time after the irradiation, r is the distance from the focus, and Dth is the thermal diffusion constant. Because the thermal diffusion in Nd2Fe14B is the slowest compared with that in cyclohexane (∼8.3 102 m2/s), we used the thermal diffusion coefficient of bulk Nd2Fe14B (Dth = 3.1 106 m2/s).44 Figure 7 indicates the contour plot of the calculated temperature distribution after the femtosecond single pulse irradiation. Because of the repetition rate of 1 kHz, i.e., the interpulse time of 1 ms in the experiments, this calculation apparently indicates that the heat induced by the first pulse can diffuse away from the focal region before the arrival of the successive pulse. Indeed, no apparent temperature rise of liquid occurred after the femtosecond laser irradiation for 60 min. The temperature within the focal volume rapidly dropped to ambient condition during about 1 μs, indicating that the cooling rate can be estimated as ∼109 K/s. Under such situation, the ablated and molten Nd2Fe14B nanoparticles are instantaneously quenched, and then the spherical and amorphous Nd2Fe14B nanoparticles are formed without composition variation. Simultaneously, the fragments derived from cyclohexane interact with the surface of Nd2Fe14B nanosphere, resulting in the formation of the self-passivating layer consisting of an amorphous carbon.
4. CONCLUSIONS Nanoparticles of intermetallic Nd2Fe14B with a diameter below the critical single-domain size were successfully prepared by the femtosecond laser ablation in cyclohexane. Nd2Fe14B nanoparticle was coated with the self-passivating layer consisting of an amorphous carbon originated from the decomposition of solvent molecules. The prepared Nd2Fe14B nanospheres exhibit twice higher coercivity than the initial particles in spite of the decrease in magnetic anisotropy and crystallinity. Beyond the basic understanding, we anticipate that such light-matter
’ ACKNOWLEDGMENT This work is partially supported by the New Energy and Industrial Technology Development Organization (NEDO), Grant-in-Aid for Scientific Research (B), MURATA Science Foundation and Engineering. We also thank Dr. Masato Sagawa (Intermetallics Co., Ltd.) for preparation of the Nd2Fe14B alloy. ’ REFERENCES (1) Lieber, C. M. Solid State Commun. 1998, 107, 607–616. (2) Elghanian, R.; Storhoff, J. J.; Mucic, R. C.; Letsinger, R. L.; Mirkin, C. A. Science 1997, 277, 1078–1081. (3) Anker, J. N.; Hall, W. P.; Lyandres, O.; Shah, N. C.; Zhao, J.; Van Duyne, R. P. Nature Mater. 2008, 7, 442–453. (4) Maier, S. A.; Kik, P. G.; Atwater, H. A.; Meltzer, S.; Harel, E.; Koel, B. E.; Requicha, A. A. G. Nature Mater. 2003, 2, 229–232. (5) Ferry, V. E.; Sweatlock, L. A.; Pacifici, D.; Atwater, H. A. Nano Lett. 2008, 8, 4391–4397. (6) Lu, A.-H.; Schmidt, W.; Matoussevitch, N.; Bonnemann, H.; Spliethoff, B.; Sch€uth, F. Angew. Chem., Int. Ed. 2004, 43, 4303–4306. (7) Lin, Y.-S.; Wu, S.-H.; Hung, Y.; Chou, Y.-H.; Chang, C.; Lin, M.-L.; Tsai, C.-P.; Mou, C.-Y. Chem. Mater. 2006, 18, 5170–5172. (8) Alivisatos, A. P. Science 1996, 271, 933–937. (9) Hu, J. T.; Odom, T. W.; Lieber, C. M. Acc. Chem. Res. 1999, 32, 435–445. (10) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947– 1949. (11) Jin, R.; Cao, Y. W.; Mirkin, C. A.; Kelly, K. L.; Schatz, G. C.; Zheng, J. G. Science 2001, 294, 1901–1903. (12) Link, S.; Burda, C.; Mohamed, M. B.; Nikoobakht, B.; El-Sayed, M. A. J. Phys. Chem. A 1999, 103, 1165–1170. (13) Tamaki, Y.; Asahi, T.; Masuhara, H. J. Phys. Chem. A 2002, 106, 2135–2139. (14) Sylvestre, J. -P.; Kabashin, A. V.; Sacher, E.; Meunier, M.; Luong, J. H. T. J. Am. Chem. Soc. 2004, 126, 7176–7177. (15) Jia, T. Q.; Zhao, F. L.; Huang, M.; Chen, H. X.; Qiu, J. R.; Li, R. X.; Xu, Z. Z.; Kuroda, H. Appl. Phys. Lett. 2006, 88, 111117–13. (16) Tull, B. R.; Carey, J. E.; Sheehy, M. A.; Friend, C.; Mazur, E. Appl. Phys. A: Mater. Sci. Process. 2006, 83, 341–346. (17) Morales, A. M.; Lieber, C. M. Science 1998, 279, 208–211. (18) Rao, A. M.; Eklund, P. C.; Bandow, S.; Thess, A.; Smalley, R. E. Nature 1997, 388, 255–259. (19) Zhang, Y.; Suenaga, K.; Colliex, C.; Iijima, S. Science 1998, 281, 973–975. (20) Mafune, F.; Kohno, J. Y.; Takeda, Y.; Kondow, T.; Sawabe, H. J. Phys. Chem. B 2000, 104, 9111–9117. (21) Kawasaki, M.; Masuda, K. J. Phys. Chem. B 2005, 109, 9379– 9388. (22) Kazakevich, P. V.; Simakin, A. V.; Voronov, V. V.; Shafeev, G. A. Appl. Surf. Sci. 2006, 252, 4373–4380. (23) Kabashin, A. V.; Meunier, M.; Kingston, C.; Luong, J. H. T. J. Phys.Chem. B 2003, 107, 4527–4531. (24) Barcikowski, S.; Hahn, A.; Kabashin, A. V.; Chichkov, B. N. Appl. Phys. A: Mater. Sci. Process. 2007, 87, 47–55. (25) Shimotsuma, Y.; Yuasa, T.; Homma, H.; Sakakura, M.; Nakao, A.; Miura, K.; Hirao, K.; Kawasaki, M.; Qiu, J.; Kazansky, P. G. Chem. Mater. 2007, 19, 1206–1208. 8363
dx.doi.org/10.1021/la201211e |Langmuir 2011, 27, 8359–8364
Langmuir
ARTICLE
(26) Chang, G.; Shimotsuma, Y.; Sakakura, M.; Yuasa, T.; Homma, H.; Oyama, M.; Miura, K.; Qiu, J.; Kazansky, P. G.; Hirao, K. Appl. Surf. Sci. 2008, 254, 4992–4998. (27) Nakao, A.; Shimotsuma, Y.; Nishi, M.; Miura, K.; Hirao, K. J. Ceram. Process. Res. 2008, 9, 425–429. (28) Liu, S.-D.; Cheng, M.-T.; Yang, Z.-J.; Wang, Q.-Q. Opt. Lett. 2008, 33, 851–853. (29) Sauer, G.; Brehm, G.; Schneider, S.; Nielsch, K.; Choi, J.; G€oring, P.; G€osele, U.; Miclea, P.; Wehrspohn, R. B. Appl. Phys. Lett. 2006, 88, 023106. (30) Desvaux, C.; Amiens, C.; Fejes, P.; Renaud, P.; Respaud, M.; Lecante, P.; Snoeck, E.; Chaudret, B. Nature Mater. 2005, 4, 750–753. (31) Bohren, C. F.; Huffman, D. R. Absorption and Scattering of Light by Small Particles; Wiley: New York, 1983. (32) Kittel, C. Rev. Mod. Phys. 1949, 21, 541–583. (33) Sagawa, M.; Fujimura, S.; Yamamoto, H.; Matsuura, Y.; Hirosawa, S. J. Appl. Phys. 1985, 57, 4094–4096. (34) Livingston, J. D. J. Appl. Phys. 1985, 57, 4137–4139. (35) Weller, D.; Moser, A.; Folks, L.; Best, M. E.; Lee, W.; Toney, M. F.; Schwickert, M.; Thiele, J.-U.; Doerner, M. F. IEEE Trans. Magn. 2000, 36, 10–15. (36) Brodeur, A.; Chin, S. L. Phys. Rev. Lett. 1998, 80, 4406–4409. (37) Kuzmin, P. G.; Shafeev, G. A.; Bukin, V. V.; Garnov, S. V.; Farcau, C.; Carles, R.; Warot-Fontrose, B.; Guieu, V.; Viau, G. J. Phys. Chem. C 2010, 114, 15266–15273. (38) Kneller, E. F.; Luborsky, F. E. J. Appl. Phys. 1963, 34, 656–658. (39) Wecker, J.; Schnitzke, K.; Cerva1, H.; Grogger, W. Appl. Phys. Lett. 1995, 67, 563–565. (40) Ram, S. J. Mater. Sci. 1997, 32, 4133–4148. (41) Shimotsuma, Y.; Kazansky, P. G.; Qiu, J.; Hirao, K. Phys. Rev. Lett. 2003, 91, 247405. (42) Gamaly, E. G.; Rode, A. V.; Tikhonchuk, V. T.; Luther-Davies, B. Appl. Surf. Sci. 2002, 197198, 699–704. (43) Sakakura, M.; Terazima, M.; Shimotsuma, Y.; Miura, K.; Hirao, K. Opt. Express 2007, 15, 16800–16807. (44) Bradley, J. R.; Perry, T. A.; Schroeder, T. J. Magn. Magn. Mater. 1993, 124, 143–150.
8364
dx.doi.org/10.1021/la201211e |Langmuir 2011, 27, 8359–8364
