pubs.acs.org/NanoLett
Diffusion-Driven Crystal Structure Transformation: Synthesis of Heusler Alloy Fe3Si Nanowires Kwanyong Seo,† Nitin Bagkar,† Si-in Kim,‡ Juneho In,† Hana Yoon,† Younghun Jo,§ and Bongsoo Kim*,† †
Department of Chemistry and ‡ Graduate School of Nanoscience and Technology, KAIST, Daejeon 305-701, Korea and § Nano Materials Research Team, KBSI, Daejeon 305-333, Korea ABSTRACT We report fabrication of Heusler alloy Fe3Si nanowires by a diffusion-driven crystal structure transformation method from paramagnetic FeSi nanowires. Magnetic measurements of the Fe3Si nanowire ensemble show high-temperature ferromagnetic properties with Tc . 370 K. This methodology is also successfully applied to Co2Si nanowires in order to obtain metal-rich nanowires (Co) as another evidence of the structural transformation process. Our newly developed nanowire crystal transformation method would be valuable as a general method to fabricate metal-rich silicide nanowires that are otherwise difficult to synthesize. KEYWORDS Metal silicide, nanowire, crystal transformation, magnetic materials, Heusler alloy
M
The key concept underlying this work is metal-enrichment of metal silicide NWs by thermal diffusion. This conversion method could prove highly valuable, since novel metal-rich silicide NWs that are difficult to synthesize but possess interesting physical properties can be fabricated from other metal silicide NWs. In this study, FeSi NWs grown on a Si substrate was placed in the center of a 1 in. diameter horizontal quartz tube furnace. FeSi NWs were synthesized via a vapor transport-based method.6a NW crystal transformation was carried out at 900 °C under a flow rate of 250 sccm of 20% O2/Ar at atmospheric pressure for 30 min. A schematic briefly illustrates the crystal transformation process from FeSi to Fe3Si (Figure 1a). Transmission electron microscopy (TEM) images (Figure 1b-d) show distinct change of the NW morphology and diameter. A FeSi NW
etal silicide nanowires (NWs) can have diverse metal-silicon compositions, leading to a broad spectrum of physical properties.1,2 Ferromagnetic metal silicide NWs, in particular, being compatible with Sibased electronics, are viewed as potentially important building blocks for future spintronics. Chemical vapor transport (CVT)-based method has been highly effective for the syntheses of metal silicide NWs with a metal to silicon ratio e1.5,3-9 as seen in the recent syntheses of CoSi, FeSi, MnSi, Fe5Si3, and Fe1-xCoxSi NWs, which show interesting electromagnetic properties.3-7 However, changing the composition of metal silicide NWs in a wider range, especially achieving a composition of a metal to silicon ratio g2, has been quite difficult.9 Accordingly, developing efficient and reliable synthetic methods to vary the elemental compositions in metal silicide NWs more flexibly would be valuable for the fabrication of practical spintronic and nanoelectronic devices. Fe3Si is a Heusler alloy with a high Curie temperature of 840 K.10-13 The Heusler alloys have been attracting interests because of possible applications in spintronics.13 Herein, we report a diffusion-driven crystal structure transformation method to fabricate ferromagnetic Fe3Si NWs from paramagnetic FeSi NWs. To the best of our knowledge, this is the first report of the synthesis of Fe3Si NWs. Furthermore, this methodology can be extended to the synthesis of other metal-rich silicide NWs. We also present transformation of cobalt silicide NWs to Co NWs to show broad generality of our new method.
FIGURE 1. (a) Schematic illustration of NW crystal transformation process. FeSi is converted to Fe3Si by high-temperature thermal annealing in diluted O2 condition and subsequent wet etching by 5% HF. Low-resolution TEM images of FeSi (b), Fe3Si@SiO2 core-shell (c), and Fe3Si NW after shell-etching (d). Scale bars are 20 nm.
* To whom correspondence should be addressed. E-mail:
[email protected]. FAX: +82-42-350-2810. Received for review: 6/14/2010 Published on Web: 08/02/2010 © 2010 American Chemical Society
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FIGURE 2. EDS analysis of the Fe3Si@SiO2 core-shell NW. (a) TEM image of the core-shell NW. (b) EDS line profiles for the core-shell NW from S to E in panel a. (c) EDS spectrum from the NW core, area 1 in panel a. (d) EDS spectrum from the NW shell, area 2 in panel a.
a Si NW through oxidation of surface Si atoms.16 Apparently, Fe atoms diffuse inward during thermal annealing, resulting in recrystallization as well as diameter reduction of the core part. Figure 3a shows a high-resolution TEM (HRTEM) image of a 30 nm diameter Fe3Si NW, which is transformed from a FeSi NW, showing clear lattice fringes and confirming the single-crystalline nature. The selected-area-electron diffraction (SAED) pattern obtained from the NW (Figure 3b) shows a regular spot pattern, which can be fully indexed to the fcc Fe3Si crystal structure (JCPDS card no. 45-1207). This pattern agrees well with an artificially generated standard SAED pattern of a fcc Fe3Si crystal structure along the [011] zone axis (Figure 3d). The EDS spectrum in Figure 3c from a single NW shows that Fe and Si are the only elements in the NW (the peaks for Cu and C are from the TEM grid), occurring in approximately a 3:1 (Fe/Si) atomic ratio. We performed additional XRD measurements on the NW samples for phase identification. XRD pattern of Fe3Si NWs is shown in the Figure 4 (red spectrum), which shows the peaks due to Fe3Si and FeSi phases. We did not observe peaks due to any other iron or iron oxide phases. Hence the magnetic properties are solely originated from Fe3Si NWs. The peaks due to FeSi phase in XRD spectrum can be attributed to the presence of thick FeSi film layer deposited on the Si substrate prior to FeSi NW growth. Careful TEM measurements of about 60 NWs indicated that more than 80% of the FeSi NWs were transformed into Fe3Si NWs in the annealing condition.
with a diameter of about 100 nm was transformed into a core-shell structure, producing a thinner metal-rich silicide NW after eliminating a SiO2 layer by application of a dilute HF (5%) solution. The core part of the NW remained inert. TEM studies indicate that the shell part of an intermediate core-shell NW (Figure 1c) is composed of SiO2. Interestingly, iron oxide was not observed in the shell. Figure 2 shows composition line profiles along the diameter of a NW and energy-dispersive X-ray spectrometry (EDS) spectra of a single Fe3Si@SiO2 core-shell NW. One EDS spectrum is taken at the center of the NW (Figure 2c) and the other is taken at the amorphous layer of the NW (Figure 2d). The EDS spectrum in Figure 2c shows that Fe and Si are the major elements in the NW (lines due to Cu are from the TEM grid). Figure 2d shows that the amorphous layer is composed of Si and O with an atomic ratio close to 1:1. The composition of Fe and Si in the NW core is estimated to be approximately 3:1 taking into account the Si concentration due to the oxide layer. The cross-sectional line profile in Figure 2b also shows a uniform composition of Fe and Si along the NW diameter. The formation of metal-free SiO2 layers by thermal oxidation of silicide films on Si substrates has been well investigated. The mechanism of this process can be attributed to the diffusion of metal and Si in the opposite directions, where Si diffuses out from the silicide to form a SiO2 shell.14 Note that the oxidation of Si would occur prior to that of Fe in a FeSi structure, because six-order higher magnitude of oxygen pressure is required for Fe oxidation than for Si oxidation.15 Chen et al. also reported diameter reduction of © 2010 American Chemical Society
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more, free energy calculation for a Fe-rich silicide phase at 700 °C suggests that stoichiometric Fe3Si is the most stable composition and would form preferentially.17 Although formation of other Fe-rich phases such as Fe5Si3 cannot be ruled out, the limited phase stability and relatively higher free energy of Fe5Si3 could be ascribed to the predominant formation of a Fe3Si phase. During annealing, the reaction kinetics is mostly governed by diffusion of Fe and Si. In this study, the length of R′ and r in Figure 1a was determined by the annealing time and temperature (Supporting Information), suggesting again that the NW crystal transformation is diffusion controlled. If we control the oxidation and diffusion processes more precisely, it would be possible to systematically synthesize FexSi (x g 1) NWs. Magnetic properties of the Fe3Si NW ensemble on a Si substrate have been studied using a superconducting quantum interference device (SQUID) magnetometer. M(H) curves in Figure 5a show hysteresis loops measured at 5 and 300 K, respectively, and provide a ferromagnetic signature. Figure 5b displays temperature-dependent magnetization M(T) curves measured under a magnetic field of 500 Oe after zero-field cooling (ZFC) and field cooling (FC), respectively. The curves show steady behavior in the measured temperature range of 0 to 370 K, suggesting a Curie temperature much higher than 370 K. This agrees with the reported Curie temperature of bulk Fe3Si (840 K).10-13 Note that the crystal transformation from FeSi NWs to Fe3Si NWs changes their magnetic properties from paramagnetic to ferromagnetic. The plot of M is also given as a function of T at an applied field of 1000 Oe from the FeSi NW ensemble in Figure 5b, which confirms that FeSi NW is nonmagnetic. The bulk FeSi crystallizes in a simple cubic B20 structure, where each Fe atom is surrounded by seven Si atoms. During the transformation from simple cubic FeSi into fcc Fe3Si, Fe atoms replace Si atoms and occupy their sites,18 building up a completely new crystal structure that is ferromagnetic at high temperatures. Further investigation into the high degree of spin polarization of Fe3Si NWs is underway. We have also successfully applied this methodology to Co2Si NWs to obtain metal rich NWs (Co) as further evidence of the structural transformation process. Co2Si NWs were synthesized via a vapor transport-based method.9 NW crystal transformation was carried out at 900 °C under a flow rate of 250 sccm of 20% O2/Ar at atmospheric pressure for 10 min. TEM images (Figure 6a,b) show distinct change of the NW morphology and diameter. Figure 6c shows an HRTEM image of a NW with clear lattice fringes, which reflects the single-crystalline nature of the NW. The SAED pattern of the NW shows a regular spot pattern, confirming again the single-crystalline nature of the NWs. The diffraction pattern is fully indexed to the fcc Co structure. In summary, we have shown that Fe3Si NWs can be fabricated by a simple diffusion-driven annealing method from FeSi NWs. Thus-synthesized Fe3Si NWs are ferromagnetic at high temperatures as well as single-crystalline.
FIGURE 3. Detailed structural analysis of a Fe3Si NW. (a) HRTEM image of a Fe3Si NW. Inset in panel a shows the FFT. (b) SAED pattern of a Fe3Si NW down the [011] zone axis. (c) TEM-EDS spectrum corresponding to panel a. (d) Standard SAED pattern of a fcc Fe3Si crystal structure down the [011] zone axis.
FIGURE 4. XRD pattern of NWs on the Si substrate before and after thermal annealing. Black and red spectra indicate before and after annealing of NWs on the Si substrate, respectively. Inset shows the magnified Fe3Si (220) peak for a clear guide to eye of the peak shift.
We propose that the NW crystal transformation is mainly controlled by free energy change and thermally driven reaction kinetics. In the present case, the transformation mechanism of Fe3Si NWs can be explained by thermodynamic stability and diffusion-controlled growth of a Fe3Si phase. The Fe-Si phase diagram shows that the D03 type Fe3Si phase is stable over a wide composition range (Si concentration of 11-25 at. %) at 600-700 °C.17 Further© 2010 American Chemical Society
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FIGURE 5. Magnetic properties of the Fe3Si NW ensemble. (a) Plot of M as a function of H at 5 and 300 K, respectively. The inset shows the loops on an enlarged scale. (b) Plot of M as a function of T at an applied field of 500 Oe. Red triangles and black circles represent the FC and ZFC data, respectively. Plot of M as a function of T at an applied field of 1000 Oe from the FeSi NW ensemble is also presented.
30 min. In the transformation of cobalt silicide NW, annealing time is changed to 10 min. X-ray diffraction (XRD) patterns of the as-grown NW ensembles were recorded on a Rigaku D/max-rc (12 kW) diffractometer operated at 40 kV and 100 mA with the filtered 0.15405 nm CuKR radiation. Transmission electron microscope (TEM) images, high-resolution TEM (HRTEM) images, and selected area electron diffraction (SAED) patterns were taken on JEOL JEM-2100F TEM operated at 200 kV. Chemical compositions of the NWs were studied by X-ray energy-dispersive spectrometry (EDS) attached to the TEM. The samples for TEM analysis were prepared either by dispersing the NWs in solvent followed by placing a drop of the solution on a carbon coated copper grid or by dragging the grids along the surface of the sample. The temperature and field dependence of the magnetization were measured by a commercial SQUID magnetometer (Quantum Design, MPMS7). Acknowledgment. This research was supported by NRF through the NRL (20090083138), SRC (20100001484), and CNMT (2010K000350) under “21st Century Frontier R&D Programs” of the MEST, Korea. Y.J. was supported by the KBSI Grant (T30513). TEM analysis were performed at the KBSI in Daejeon.
FIGURE 6. Transformation of cobalt silicide NWs using Co2Si NWs as a starting material. (a) TEM image of an orthorhombic Co2Si NW. (b) TEM image of the core-shell NW after annealing at 900 °C for 10 min. (c) HRTEM image of the core part in panel b shows clear lattice fringes and confirms the single-crystalline nature of the core part. (d) SAED pattern from the core part in panel b. A regular spot pattern can be fully indexed to the fcc Co crystal structure (JCPDS card no. 15-0806).
Supporting Information Available. Detailed descriptions of materials and methods and supplementary figures. This material is available free of charge via the Internet at http:// pubs.acs.org.
Furthermore, we also showed transformation of cobalt silicide NWs to Co NWs to show the generality of this method. It is anticipated that our newly developed method, which enriches the metal component in the metal silicide NWs by thermal diffusion, could prove highly valuable as an efficient alternative method to fabricate novel metal-rich silicide NWs, which are otherwise difficult to synthesize. Methods. As-grown FeSi NWs on a Si substrate were transformed to Fe3Si NWs by high-temperature thermal annealing in O2 condition and subsequent wet etching by 5% hydrofluoric acid (HF). The NW crystal transformation (from FeSi to Fe3Si) was carried out at 900 °C under flow rate of 250 sccm of 20% O2/Ar at atmospheric pressure for © 2010 American Chemical Society
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DOI: 10.1021/nl102093e | Nano Lett. 2010, 10, 3643-–3647
