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Ferromagnetic Iron Boride (Fe3B) Nanowires Yan Li, Emma Tevaarwerk, and R. P. H. Chang* Department of Materials Science and Engineering and, Materials Research Institute, Northwestern UniVersity, 2220 Campus DriVe, EVanston, Illinois 60208-3108 ReceiVed January 10, 2006. ReVised Manuscript ReceiVed March 21, 2006
Single-crystal iron boride (Fe3B) nanowires were synthesized on Pt and Pd (Pt/Pd) coated sapphire substrates by a chemical vapor deposition method at 800 °C using boron triiodide (BI3) and iron iodide (FeI2) as precursors. Morphology of the Fe3B nanowires can be controlled by manipulating the Pt/Pd film thickness and the growth time. Transmission electron microscopy and selected area electron diffraction were used to analyze the crystal structures of these novel materials. Electron energy-loss spectroscopy and X-ray energy-dispersive spectroscopy studies on these nanowires confirm that they are composed of boron and iron. Scanning electron microscopy was employed to observe the morphology of these nanomaterials. The typical size of the iron boride nanowires is about 5-50 nm in width and 2-30 µm in length. The vapor-liquid-solid (VLS) growth process is shown to be the growth mechanism of the Fe3B nanowires. Room temperature magnetic force microscopy investigations on the iron boride nanowires suggest that they are ferromagnetic nanowires with a single-domain configuration.
One-dimensional (1D) ferromagnetic nanostructures are expected to have interesting magnetic properties, as the geometrical dimensions of the material become comparable to key magnetic length scales, such as the exchange length or the domain wall width.1,2 Due to the strong shape anisotropy, 1D ferromagnetic nanostructures usually exhibit hysteresis loops with high squareness, ideal for data storage media.3,4 Ferromagnetic nanowires with single domains have potential for use as magnetic force microscope (MFM) tips. This is because single-domain nanowires create a well-known stray field, which simplifies MFM image interpretation. The high aspect ratio of nanowires also allows for better resolution.2 Due to their anisotropic magnetoresistance5 and giant magnetoresistance,6,7 ferromagnetic nanowires could be building blocks in magnetic sensors, magnetic recording heads, and magnetic memories. Ferromagnetic nanowires can also have applications in magnetic composites. Iron borides are promising structural and soft magnetic metals, which show a unique combination of excellent mechanical properties and good soft magnetic properties.8-14 * To whom correspondence should be addressed. Phone: 1-847-491-3598. Fax: 1-847-491-4181. E-mail:
[email protected].
(1) Dennis, C. L.; Borges, R. P.; Buda, L. D.; Ebels, U.; Gregg, J. F.; Hehn, M.; Jouguelet, E.; Ounadjela, K.; Petej, I.; Prejbeanu, I. L.; Thorntorn, M. J. J. J. Phys.: Condens. Matter 2002, 14, R1175. (2) Henry, Y.; Ounadjela, K.; Piraux, L.; Dubois, S.; George, J.-M.; Duvail, J.-L. Eur. Phys. J. B 2001, 20, 35. (3) Garcia, J. M.; Thiaville, A.; Miltat, J. J. Magn. Magn. Mater. 2002, 249, 163. (4) Spaldin, N. A. Magnetic materials: fundamentals and deVice applications; Cambridge Universty Press: New York, 2003. (5) Wegrowe, J.-E.; Kelly, D.; Franck, A.; Gilbert, S. E.; Ansermet, J.-P. Phys. ReV. Lett. 1999, 82, 3681. (6) Dubois, S.; Marchal, C.; Beuken, J. M.; Piraux, L.; Duvail, J. L.; Fert, A.; George, J. M.; Maurice, J. L. Appl. Phys. Lett. 1997, 70, 396. (7) Piraux, L.; George, J. M.; Despres, J. F.; Leroy, C.; Ferain, E.; Legras, R.; Ounadjela, K.; Fert, A. Appl. Phys. Lett. 1994, 65, 2484. (8) Dehlinger, A. S.; Pierson, J. F.; Roman, A.; Bauer, P. Surface & Coatings Technology 2003, 174, 331. (9) Sen, S.; Ozbek, I.; Sen, U.; Bindal, C. Surf. Coat. Technol. 2001, 135, 173.
The electric resistivities of crystalline iron borides are around 36-80 µΩ cm.15 The hardness and the reduced Young’s modulus of amorphous iron boride films deposited on glass substrates are measured to be 20 GPa and 200 Gpa, respectively.8 Fe3B can be prepared by crystallization of amorphous alloys of the approximate composition Fe75B2511 or Fe75Si15B10.16,17 It has two crystal structures: the tetragonal phase and the orthorhombic phase.11-13 Both structures are metastable at room temperature.11-13 The orthorhombic Fe3B phase is believed to be less stable than the tetragonal phase. It is suggested that at high temperature (>1160 °C), tetragonal Fe3B could be an equilibrium phase.11 Magnetic property studies of tetragonal Fe3B flakes made by isothermal annealing of amorphous Fe76B24 flakes show that tetragonal Fe3B is a soft magnetic metal with a saturation magnetization of 1.6 T, a coercive field µ0Hc around 0.3 T, a curie temperature of 786 K, and a domain wall thickness of 20 nm.11,12 The magnetocrystalline anisotropy of Fe3B in tetragonal modification is proposed to be uniaxial along 〈001〉.18 However magnetic domain observations on Fe3B flakes by means of Lorentz transmission electron microscopy (TEM) indicate that the magnetocrystalline anisotropy of tetragonal Fe3B is in-plane with a preferential orientation along 〈110〉.12 (10) Dorofeev, V. Y.; Selevtsova, I. V. Powder Metall. Met. Ceram. 2001, 40, 452. (11) Coehoorn, R.; Demooij, D. B.; Dewaard, C. J. Magn. Magn. Mater. 1989, 80, 101. (12) Coene, W.; Hakkens, F.; Coehoorn, R.; Demooij, D. B.; Dewaard, C.; Fidler, J.; Grossinger, R. J. Magn. Magn. Mater. 1991, 96, 189. (13) Kong, Y.; Li, F. S. Phys. ReV. B 1997, 56, 3153. (14) Chien, C. L.; Musser, D.; Gyorgy, E. M.; Sherwood, R. C.; Chen, H. S.; Luborsky, F. E.; Walter, J. L. Phys. ReV. B 1979, 20, 283. (15) Johnson, O.; Joyner, D. J.; Hercules, D. M. J. Phys. Chem. 1980, 84, 542. (16) Illekova, E.; Kuhnast, F. A.; Matko, I.; Naguet, C. Mater. Sci. Eng., A 1996, 215, 150. (17) Illekova, E.; Matko, I.; Duhaj, P.; Kuhnast, F. A. J. Mater. Sci. 1997, 32, 4645. (18) Livington, L. D. J. Appl. Phys. 1981, 52, 2506.
10.1021/cm060068z CCC: $33.50 © 2006 American Chemical Society Published on Web 04/21/2006
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Figure 1. Schematic of the CVD apparatus.
Here we report the synthesis and characterization of singlecrystal tetragonal Fe3B nanowires by a chemical vapor deposition (CVD) method, with iron iodide (FeI2) and boron triiodide (BI3) as precursors and Pt/Pd as catalysts. MFM studies on the Fe3B nanowires suggest that they are ferromagnetic nanowires with a single-domain configuration. The Fe3B nanowires could have superior mechanical properties as well as excellent magnetic properties. They may have potential applications in magnetic composite, magnetic sensors, MFM tips, and magnetic data storage media. Sapphire (C plane, one side polished, Coating and Crystal Technology Inc.) substrates were sputter coated with 0.10.5 nm Pt/Pd film on the polished sides using a Cressington 208 HR sputter coater with a Pt/Pd foil (80% Pt, 20% Pd, Refining Systems Inc., Las Vegas) as the target. A schematic of the CVD apparatus is shown in Figure 1. The whole reactor is made of quartz or glass to avoid metal contamination. A Pt/Pd coated sapphire substrate was placed on a quartz boat in the center of the tube furnace 2. The tube furnace 2 was then heated to 800 °C under Ar flow. At the same time, the tube furnace 1, which controls the temperature of the FeI2 sublimator, was heated to 445 °C. Once the desired temperatures of the two tube furnaces were reached, the FeI2 container was pushed to the center of the tube furnace 1. The FeI2 container has an opening on one side, which is connected to a feed through quartz tubing, and a small hole on the other side. BI3 vapor was introduced from the BI3 sublimator, the temperature of which was controllable and usually kept at 35 °C. Ar gas was introduced as a carrier gas for BI3 vapor, a carrier gas for FeI2 vapor, and as a diluting gas. The typical flow rates were 10, 20, and 200 sccm, respectively. The BI319 (98+%, Aldrich) and FeI2 (99.99+%, Aldrich) powders were transferred into their sublimators inside a glovebox to prevent them from reacting with H2O vapor and O2 in air. The reaction time was varied from 15 min to 2 h. After reaction, the valves of the BI3 sublimator were closed, the FeI2 container was pulled back to the inlet of the reactor, and the two tube furnaces were cooled to room temperature under Ar gas flow. Scanning electron microscopy (SEM, Hitachi S-4500 FESEM) was used to observe the morphology of the nanowires formed on the substrates. Transmission electron microscopy (TEM, Hitachi H8100 and Hitachi HF2000) and selected area electron diffraction (SAD, Hitachi H8100) were employed to characterize the crystalline nature of the nanowires. TEM specimens were made by dragging holey carbon grids (400 (19) Li, Y.; Fan, Z. Y.; Lu, J. G.; Chang, R. P. H. Chem. Mater. 2004, 16, 2512.
Figure 2. SEM images from the sample made on a 0.1 nm thick Pt/Pd film coated sapphire substrate at 800 °C with (a) 20 sccm Ar gas carried FeI2 vapor (FeI2 sublimator temperature: 445 °C), (b) 10 sccm Ar gas carried BI3 vapor (BI3 sublimator temperature: 35 °C), and (c) 200 sccm diluting Ar gas flowing for 2 h.
mesh Cu, SPI supplies) along the surface of the samples. Chemical composition of the nanowires was studied by electron energy-loss spectrometry (EELS, Hitachi HF2000 with a Gatan imaging filter (GIF) system, JEOL JEM-2100F with a GIF system) and X-ray energy-dispersive spectrometry (XEDS, Hitachi HF2000 with a UTW X-ray detector, JEOL JEM-2100F with a UTW X-ray detector). Magnetic force microscopy (Digital Instruments Nanoscope IIIa microscope with Quadrex Extender under ambient conditions and Co/ Cr coated high-moment MESP tips) was used to observe the magnetic domain structure of the nanowires. The nanowires were dispersed in isopropyl alcohol by sonication and then deposited onto a SiO2 on Si substrate for MFM studies. Before imaging, the tip was magnetized along the tip axis by exposing it to the magnetic field of a small permanent magnet. The sample was imaged as grown, with no external magnetic field applied to the sample prior to imaging. SEM images from the samples made on Pt/Pd film coated sapphire substrates at 800 °C indicate that lots of nanowires are produced all over the surface. No nanowire forms if a bare sapphire is used as the substrate. Parts a-c of Figure 2 present the SEM images from the sample made on a 0.1 nm thick Pt/Pd film coated sapphire substrate at 800 °C with 20 sccm Ar gas carried FeI2 vapor (FeI2 sublimator tempera-
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Figure 3. SEM images of the samples made on (a) 0.1, (b) 0.3, and (c) 0.5 nm thick Pt/Pd film coated sapphire substrates. The other growth conditions are the same as those of the sample with SEM images presented in Figure 2.
ture: 445 °C), 10 sccm Ar gas carried BI3 vapor (BI3 sublimator temperature: 35 °C), and 200 sccm diluting Ar gas flowing for 2 h, respectively. Figure 2a-c reveals that nanowires with diameters of 5-20 nm and lengths of 6-15 µm are synthesized all over the surface. There is a particle attached to one end of each nanowire. The morphology of the nanowires can be manipulated by changing the thickness of the Pt/Pd films and the growth time. Parts a-c of Figure 3 show SEM images of the samples made on 0.1, 0.3, and 0.5 nm thick Pt/Pd film coated sapphire substrates, respectively. The other growth conditions are the same as those of the sample with SEM images presented in Figure 2. As shown in Figure 3, the diameters of the nanowires synthesized on 0.1, 0.3, and 0.5 nm Pt/Pd film coated sapphire substrates are in the range of 5-20 nm, 1050 nm, and 20-100 nm, respectively. Figure 3 suggests that increasing the Pt/Pd film thickness while keeping the other growth conditions the same can enlarge the diameters of the nanowires. Parts a-c of Figure 4 present SEM images of the samples with 15 min, 0.5 h, and 1 h growth time, respectively. The other growth conditions are the same as those of the sample with an SEM image shown in Figure 3b. Figure 4a reveals that nanoparticles with diameters of 10-50 nm are produced on the surface if the growth time is 15 min. This also indicates that there is an incubation time for the growth of the nanowires. When the growth time extends to 0.5 h, a few short nanowires grow out of the nanoparticles, as displayed in Figure 4b. Lots of long nanowires are synthesized as the growth time is increased to 1 h, as shown in Figure 4c. The nanowires grow longer when the growth time goes to 2 h (presented in Figure 3b). Figure 4a-c and Figure 3b suggest that if the other growth conditions are kept unchanged, longer nanowires can be produced with longer growth time. The nanowires synthesized on Pt/Pd film coated sapphire substrates have the same crystal structure and chemical compositions. Figure 5a shows a typical low-magnification TEM image of the nanowires. A high-resolution TEM (HRTEM) image taken from part of a nanowire and the corresponding SAD pattern are shown in Figure 5b. The SAD pattern can be indexed as a tetragonal Fe3B phase (JCPDS 39-1316) with lattice constants of a ) 8.6736 Å and c ) 4.3128 Å recorded along the [1h10] zone axis. The HR-TEM image clearly displays the 0.216 nm d spacing of the (002) planes. The axis of the nanowires is usually along the 〈110〉 direction. Both the SAD pattern and the HR-TEM image suggest that the nanowires are single crystals. As indicated in Figure 5b, there is a 2-5 nm thick amorphous layer on
Figure 4. SEM images of the samples with (a) 15 min, (b) 0.5 h, and (c) 1 h growth time. The other growth conditions are the same as those of the sample whose SEM image is shown in Figure 3b.
Figure 5. (a) A typical low-magnification TEM image of the nanowires. (b) The HR-TEM image and the corresponding SAD pattern (inset) taken from part of a nanowire.
the surface of each nanowire. EELS and XEDS in scanning transmission electron microscopy (STEM, 1 nm probe size) mode were performed to study the element distribution in the nanowires. Parts a and b of Figure 6 show typical EELS spectra taken from the center and the edge of a nanowire, respectively. They both exhibit the characteristic boron K
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Figure 6. (a) An EELS (in STEM mode) spectrum taken from the center of a nanowire. (b) An EELS (in STEM mode) spectrum taken from the edge of the same nanowire.
Figure 8. SEM images of (a) a 0.3 nm Pt/Pd film covered sapphire substrate and (b) the same sample after heated to 800 °C.
Figure 7. (a) An XEDS (in STEM mode) spectrum taken from the center of a nanowire. (b) An XEDS (in STEM mode) spectrum taken from the edge of the same nanowire.
edge, oxygen K edge, and iron L edges. However the oxygen K edge in Figure 6b is more obvious than that in Figure 6a, which suggests that the amorphous layer on the surface of each nanowire is an oxide layer. To compare the chemical composition of the edge and the center of a nanowire, the EELS spectra were quantified. The atomic ratios of B/O/Fe are 0.99/0.06/1 and 0.82/0.12/1 for the center and the edge, respectively. Compared to the center of a nanowire, the edge of a nanowire has more iron and oxygen. Typical XEDS spectra taken from the center and the edge of a nanowire are displayed in parts a and b of Figure 7, respectively. Figure 7b indicates that the amorphous oxide layer may contain a small amount of Si. From the HR-TEM, SAD, EELS, and XEDS studies presented above, the as-grown nanowires are single-crystal tetragonal Fe3B coated with thin amorphous iron borate layers. To explore the growth process of the Fe3B nanowires synthesized on Pt/Pd film coated substrates at 800 °C, a fresh sapphire substrate deposited with 0.3 nm Pt/Pd film was studied by SEM. After the SEM study, the sample was put in the CVD system and was heated under 200 sccm Ar gas flow to 800 °C. Once the desired temperature was reached, the sample was cooled under Ar gas flow to room temperature and was taken out for a SEM observation. Parts a and b of Figure 8 show SEM images of a 0.3 nm Pt/Pd film covered sapphire substrate and the same sample after it was heated to 800 °C, respectively. Figure 8a indicates that the surface of the 0.3 nm Pt/Pd film coated sapphire substrate is quite smooth. At 800 °C, nanoparticles with diameters of
Figure 9. (a) TEM image and (b) the corresponding SAD pattern of a bundle of the nanoparticles produced by heating a 0.3 nm coated sapphire substrate to 800 °C under Ar gas flow. (c) A typical XEDS spectrum from one of the nanoparticles. The diffraction rings in the SAD pattern (from center to edge) can be indexed as the (111), (200), (220), (311), and (421) peaks of a cubic phase with a lattice constant of 3.88 Å.
10-20 nm form on the surface (see Figure 8b). The nanoparticles were transferred to a TEM grid and studied using TEM, SAD, and XEDS. Parts a and b of Figure 9 show a TEM image and the corresponding SAD pattern of a bundle of the nanoparticles, respectively. A typical XEDS spectrum from an individual nanoparticle presented in Figure 9c indicates that the nanoparticles are mainly composed of Pt and Pd. The XEDS spectrum was quantified. The atomic ratio of Pt to Pd is about 65/35. The SAD pattern can be indexed as a cubic phase with a lattice constant of 3.88 Å (JCPDS 87-0647). From the Pd-Pt phase diagram,20 Pt and Pd form a solid-solution (Pt, Pd) over the whole composition range under 1555 °C. The nanoparticles can be considered as solid-solutions of Pt and Pd.19 Figure 4a-c and Figure 3b show the time evolution of Fe3B nanowires produced on (20) Okamoto, H. Desk Handbook: Phase Diagrams for Binary Alloys; ASM International: Materials Park, OH, 2000.
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Figure 10. (a) TEM image of a nanoparticle synthesized at 15 min, (b) the corresponding SAD pattern, and (c) a typical XEDS spectrum from a single nanoparticle. The SEM image of the sample is shown in Figure 4a.
0.3 nm Pt/Pd film coated sapphire substrates at 800 °C from 15 min to 2 h. At 15 min, there are only nanoparticles on the surface. Parts a-c of Figure 10 exhibit the TEM, SAD, and XEDS results, respectively, from these nanoparticles. EELS spectra of the nanoparticles do not present boron K edge, which indicates that the nanoparticles do not contain boron. The XEDS spectrum shown in Figure 10c suggests that the nanoparticles are composed of Pt, Pd, and Fe. The Cu signal in the XEDS spectrum is from the TEM grid. Quantification of the XEDS spectrum indicates that the atomic ratio of Fe/Pt/Pd is about 40/38/22. As we discussed above, Pt and Pd are miscible over the whole composition range. The phase diagram of Fe-Pt20 indicates that intermediate solid-solution Pt3Fe phase forms at 800 °C within 21-41% atomic percent of Fe. The SAD pattern displayed in Figure 10b can be indexed as a cubic Pt3Fe phase with a lattice constant of 3.87 Å (JCPDS 00-029-0716) recorded along the [001] zone axis. The nanoparticles synthesized for a growth time of 15 min could be considered as a solidsolution (Pt, Pd)3Fe phase. As the growth time extends to half an hour, nanowires begin to grow out of the nanoparticles, as shown in Figure 4b. More nanowires are synthesized for a growth time of 1 h, and the nanowires grow longer with longer growth time, indicated by Figure 4c and Figure 3b, respectively. Each nanowire usually has a particle attached to one of its ends, as shown by the SEM images of the nanowires and the TEM image in Figure 5a. XEDS and EELS studies (see Figure 11a,b) on the nanoparticles attached to the nanowires suggest that they are composed of Pt, Pd, Si, and a small amount of B and Fe. XEDS quantification shows that the atomic ratio of Si/Pt/Pd is about 54/36/10. The SAD pattern (the inset in Figure 11a) taken from the nanoparticle attached to a nanowire can be indexed as an orthorhombic PtSi phase (JCPDS 71-0523, a ) 5.916 Å, b ) 5.577 Å, c ) 3.587 Å) recorded close to the [203h] zone axis. The nanoparticle attached to each nanowire may be considered as a solid-solution of (Pt, Pd)Si, Fe, and B. The existence of a nanoparticle at one end of each nanowire suggests that the growth mechanism could be the vapor-liquid-solid (VLS)21 growth process. As shown in
Li et al.
Figure 11. (a) An XEDS spectrum and (b) an EELS spectrum from the nanoparticle attached to a nanowire. The inset in part a shows a SAD pattern taken from the nanoparticle.
Figure 8b, the Pt/Pd film turns into Pt/Pd nanoparticles at a reaction temperature of 800 °C. It has been reported that FeI2 decomposes at 498 °C (FeI2 f Fe + I2),22 while the decomposition of BI3 happens at 800-1000 °C (2BI3 f 2B + 3I2).23 FeI2 and BI3 may also react to form FeB3 (2FeI2 + 6BI3 f 2FeB3 +11I2). Once the precursor vapor is introduced, FeI2 may decompose before it reaches the center of the tube furnace 2, which is often kept at 800 °C. At the beginning of the reaction, Fe molecules get adsorbed to the surface of the (Pt, Pd) nanoparticles and diffuse into them. As the composition of Fe in the (Pt, Pd) nanoparticles arrives 21% atomic percent, (Pt, Pd)3Fe intermediate phase forms. BI3 vapor can only decompose at the center of the tube furnace 2 with a temperature of 800 °C. It takes some time (∼15 min) for the BI3 vapor to reach the high-temperature zone and decompose into boron and iodine. Boron molecules are then adsorbed on the surface of the (Pt, Pd)3Fe nanoparticles and diffuse into them. Pt and B can form an eutectic alloy at 825 °C. Because of the high surface-to-volume ratio, the melting temperature of nanoparticles is size and shape dependent and is much lower than that of their corresponding bulk materials.24-26 For a nanoparticle of 5 nm in diameter, the melting temperature is estimated to be 60% of the melting point for its corresponding bulk material.24 Thus the melting point of the eutectic Pt-B nanoscale alloy could be much lower than 825 °C. At a reaction temperature of 800 °C, liquid droplets of B, Pt, Pd, Fe, and Si can form. Si may come from the quartz tube (the reaction chamber is made of quartz). It has been reported that FeI2 reacts with SiO2 (2FeI2 + SiO2 f SiI4 + 2FeO;22 SiI4 f Si + 2I223), which provides Si vapor. Although detailed ternary phase diagram of B-SiPt could not be found in the literature,27 it is possible that the existence of Si may also lower the melting temperature of B-Pt eutectic alloy. For amorphous boron nanowire (21) Levitt, A. P. Whisker Technology; Wiley-Interscience: New York, 1970. (22) Zaugg, W. E.; Gregory, N. W. J. Phys. Chem. 1966, 70, 486. (23) Holleman, A. F.; Wiberg, E. Inorganic Chemistry; Academic Press: New York, 2001. (24) Qi, W. H.; Wang, M. P. Mater. Chem. Phys. 2004, 88, 280. (25) Jiang, Q.; Zhang, S.; Zhao, M. Mater. Chem. Phys. 2003, 82, 225. (26) Wautelet, M. Phys. Lett. A 1998, 246, 341. (27) Villars, P.; Prince, A.; Okamoto, H. Handbook of ternary alloy phase diagrams; ASM International: Materials Park, 1995.
Ferromagnetic Iron Boride (Fe3B) Nanowires
Figure 12. (a) Tapping mode AFM image of the nanowires. (b and c) The corresponding MFM phase images with opposite tip magnetization. The reversal of contrast with opposite tip magnetization confirms the magnetic origin of the signal.
synthesis, Si was added to lower the melting temperature of Au-B eutectic alloy.28 Si was also found in the catalyst particle attached to each CaB6 nanowire produced by a CVD method.29 The liquid droplets have a larger accommodation coefficient and are preferential sites for the adsorption of the chemical vapor. The continuous supply of adsorbent Fe, B, and Fe3B clusters causes the liquid droplets to be (28) Wu, Y. Y.; Messer, B.; Yang, P. D. AdV. Mater. 2001, 13, 1487. (29) Xu, T. T.; Zheng, J. G.; Nicholls, A. W.; Stankovich, S.; Piner, R. D.; Ruoff, R. S. Nano Lett. 2004, 4, 2051.
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supersaturated. Single-crystal Fe3B is precipitated out of the liquid droplets and grows into Fe3B nanowires. The surface of each Fe3B nanowire is oxidized by the residual oxygen in the chamber. A thin iron borate layer then forms on the periphery of every Fe3B nanowire. As the tube furnaces are cooled to room temperature, the liquid droplets turn into (Pt, Pd)Si nanoparticles with a small amount of B and Fe. Parts a and b of Figure 12 show a tapping mode atomic force microscopy (AFM) and the corresponding MFM image of the Fe3B nanowires, respectively. Figure 12c presents the MFM image taken from the same nanowires after reversing the tip magnetization. For MFM scanning, lift mode was employed with a lift height of 40 nm. There are two nanowires shown in Figure 12. The diameter of each nanowire was estimated by measuring its height. Nanowire (i) has a diameter of 8 nm, nanowire (ii) a diameter of 6 nm. Both have bright/dark phase contrast at their two ends which flips with opposite tip magnetization (see Figure 12b,c), proving the magnetic origin of this part of the signal. The comparatively weaker bright contrast along the length of the nanowires does not flip upon reversal of the tip magnetization and is therefore attributed to topographic interactions between the tip and sample during the MFM scan. The bright/dark contrast at the nanowire ends proves that they are ferromagnetic at room temperature. Such MFM patterns have been observed in cobalt nanowires and were shown to be consistent with a single-domain axially magnetized bar magnet model.2 Because of the strong shape anisotropy, 1D ferromagnetic nanowires are usually magnetized along their long axis. As discussed above, the axis of the Fe3B nanowires is the 〈110〉 direction, which is also the preferential orientation of its magnetocrystalline anisotropy.12 Single-crystal Fe3B nanowires were synthesized on Pt/Pd coated sapphire substrates by a CVD method. BI3 and FeI2 were used as precursors. Ar was employed as the carrier gas and the diluting gas. MFM studies show that the synthesized Fe3B nanowires are ferromagnetic at room temperature. The morphology of the Fe3B nanowires can be manipulated by changing the Pt/Pd film thickness and the growth time. Their growth process was shown to be the VLS growth mechanism. The results also demonstrate that metal boride nanostructures may be synthesized by introducing boron triiodide and metal iodide vapor to catalyst coated substrates at elevated temperature. Acknowledgment. This work was supported by the B NIRT NSF grant (29212S/WU-HT-02-33/NSF-EEC-0210120). We appreciate helpful discussions with Prof. Venkat Chandrasekhar, Dr. Yongho Seo, Dr. Gajendra S. Shekhawat, Dinna Geraldine Ramlan (on the MFM experiment), and Dr. D. Bruce Buchholz (on the CVD system). We acknowledge the use of MRSEC facilities: EPIC, NIFTI, and J.B. Cohen XRD Facilities at Northwestern University. CM060068Z
