Self-Assembled Magnetic Nanohead-FeSi Nanowire Epitaxial

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Self-Assembled Magnetic Nanohead-FeSi Nanowire Epitaxial Heterojunctions by Chemical Vapor Deposition S. Liang,*,† X. Fang,† Tian-Long Xia,‡ Yujun Qing,† and Zhi-Xin Guo† Department of Chemistry, Renmin UniVersity of China, Beijing 100872, China, and Department of Physics, Renmin UniVersity of China, Beijing 100872, China ReceiVed: April 21, 2010; ReVised Manuscript ReceiVed: August 11, 2010

Self-assembled magnetic nanohead-FeSi nanowire (NW) epitaxial heterojunctions by chemical vapor deposition (CVD) are reported. Free-standing FeSi NWs, ∼30 nm in diameter and several micrometers long, were grown by reduction of Fe2O3 in a H2 atmosphere on a heated silicon substrate at 750 °C. These NWs shows very uniform dimensions, which is thought to be due to a higher diffusion activation energy for the nanostructure. A magnetic Fe3O4 head with a diameter of ∼30 nm was observed on each FeSi NW. The epitaxial relationship of these junctions is Si[001]//Fe3O4[2j01], Si(001)//Fe3O4(102). Ferromagnetic behavior of the nanoheads was identified, and the coercivity was characterized as ∼220 Oe at 10 K. This one-step CVD approach for self-assembling epitaxially magnetic heads could enable device integration for spintronics and sensors. Particularly, such FeSi nanowires with highly localized magnetic nanoheads are ideal for high resolution magnetic force microscopy (MFM). Nanowires and nanostuctures have emerged as promising building blocks for fundamental studies and possible technological applications.1,2 Incorporating nanoparticles and nanoheterostructures into a nanosystem is important both for theoretical exploring their properties and for creating practical new functionalities. Controlled assembly of nanowire heterostructures is of particular interest because the incorporation of functional elements, such as quantum wells,3 and p-n junctions,4 etc., supports the development of advanced device architectures. Silicide NWs with various properties are attracting expanding efforts due to their following unique properties:5 excellent crystal quality, generally single crystal structure, and ease of fabrication and self-assembly. In addition, metallic, magnetic, and semiconducting properties of silicide NWs are readily available. Transition-metal silicides are widely used as contacts and interconnects in current microelectronics6 and potential optoelectronic devices.7 Iron silicides are particularly interesting since they form a rich variety of iron-silicon binary compounds with a wide range of magnetic, electric, and optic properties. The semiconducting phase, β-FeSi2, with a band gap of 0.87 eV,8,9 is favorable for optical fiber communication systems. β-FeSi2 was reported as a light emitter10,11 and is an important thermoelectric material with a thermoelectric figure of merit (ZT) value of 0.4 (850-1200 K).12 Ferromagnetic properties are observed in Fe3Si and Fe5Si3.13 ε-FeSi is a narrow-bandgap semiconductor with a cubic structure (space group P213) that has been considered as a hybridization-bandgap semiconductor or Kondo insulator.14,15 Growth of various silicide NWs has been observed on surfaces16 and as free-standing.17,18 CVD is the widely used method for silicide NW growth.13,19,20 Heterostructures within NWs have been fabricated during growth4,21 and by postsynthesis processing.22-25 Template-assisted growth, through periodically electrodepositing different materials into pores in an anodized aluminum oxide template, offers good * Corresponding author. E-mail: [email protected], [email protected]. † Department of Chemistry. ‡ Department of Physics.

control over pitch length in NW superlattice and heterostructure fabrication.26,27 However, assembling magnetic heads on semiconducting NWs has received little attention. Such heterojunction holds interest for device integration for spintronics and sensors. Particularly, these NWs with highly localized magnetic heads are ideal for high-resolution magnetic force microscopy (MFM). There are a large number of reports on growth of magnetic NWs either through template or by CVD,28-31 but few reports exist on magnetic heterojunctions and localized one-dimensional magnetic probes.32 A previous report on magnetic nanoparticles assembled on NWs showed magnetic heads with big diameter (>60 nm, our magnetic heads on NWs can be fabricated as small as 30 nm in diameter) and no epitaxial relationship between the magnetic heads and NWs. In addition, they require much more complicated steps than the approach reported here for such structure fabrication.32 In the present work, we report on self-assembled magnetic nanohead-FeSi NW epitaxial heterojunctions created by onestep chemical vapor deposition. Free-standing FeSi NWs, ∼30 nm in diameter and several micrometers long, were grown by reduction of Fe2O3 in a H2 atmosphere on a heated silicon substrate at 750 °C. This simple CVD approach for self-assembling epitaxially magnetic heads could enable device integration for spintronics and sensors.33 In addition, this new one-step CVD approach for self-assembly of a magnetic head-NW heterostructure could be easily extended to other material systems with different properties. The synthesis for the magnetic head-NW heterostructure was performed using a CVD method: all silicon(100) substrates were cleaned by acetone; some substrates were treated additionally with 2% buffered HF. After cleaning, all substrates were coated with a 5-nm-thick gold thin film as catalyst by sputter-coating. They were placed in a horizontal 1-in.-diameter tube furnace between the center and the downstream end of the quartz tube. Analytic purity Fe2O3 powder (99.8%) was placed in an alumina boat at the center of the furnace, upstream from the substrates. After the furnace was pumped down to 0.1 Torr, an inert atmosphere was maintained with a flow rate of 30 sccm Ar and

10.1021/jp1035822  2010 American Chemical Society Published on Web 09/07/2010

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Figure 1. SEM micrograph of FeSi NWs grown at 750 °C on Si (100) showing magnetic heads.

20 sccm H2. The temperature at the center of the furnace was set to ramp up to 1100 °C at a rate of 30 °C/min. When the center of the furnace reached 1100 °C, the iron source was heated to produce vapor-phase Fe2O3 and possibly iron ion inside the quartz tube atmosphere. The precursor vapors were carried by the Ar flow to the downstream silicon substrate where they reacted with the silicon to form silicide. The reaction was held under these conditions for 1-2 h, and then the furnace was allowed to cool to room temperature at a rate of 30 °C/min. After being taken out of the CVD system, the silicon substrates were covered by a “fluffy” white powder composed of aggregates of FeSi NWs. The morphology of the FeSi nanowires was analyzed with a JEOL 7401F field-emission scanning electron microscope (SEM) and a JEOL 2010 transmission electron microscope (TEM) with EDS X-ray-mapping capabilities. Ensemble nanowire magnetization was measured with a Quantum Design physical property measurement system (QD PPMS-14T) with VSM option. Spatially resolved magnetic properties were measured by a Veeco D3100 magnetic force microscope. The morphology of the magnetic nanohead-FeSi NW heterojunctions was examined using SEM and TEM. Figure 1 shows representative SEM images. Free-standing FeSi NWs, ∼30 nm in diameter and several micrometers long, are estimated. To make NWs applicable in nanodevices, it is desirable to accurately control their morphology and positioning. Figure 2 shows FeSi NW dimensions for different growth durations: (a) 1 and (b) 2 h. In Figure 2a, the inset shows

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Figure 3. FeSi NWs growth on HF-etched Si(100) substrate.

nanowires with a diameter around 15-40 nm; whereas in part b, the diameters of the nanowires are around 20-50 nm, not a significant increase for growth duration from 1 to 2 h. The average diameter of FeSi NWs changed from ∼30 to ∼40 nm. The FeSi NWs, even between different runs, show a much narrower dimension distribution compared to other reported FeSi NWs, such as the reported diameter ranging from 5-80 nm,19,20 and lengths from submicrometer to tens of micrometers.19 NW dimensions could be fine-tuned through rational control of growth parameters, such as precursor evaporation temperature, substrate temperature, pressure of the growth atmosphere, substrate treatment, etc. A plausible explanation for the stable and narrow dimension distribution of the FeSi NWs is that the growth of these FeSi NWs is silicon-diffusion-limited. To verify this hypothesis, we designed a control experiment: FeSi NWs were grown on a 2% HF-dipped silicon substrate (native oxide removed). Compared to the NWs grown on as-received silicon(100) substrate (shown in Figure 1 and Figure 2), where a native oxide film, 1-2 nm, exists, the FeSi NWs on the HF-treated substrate are significantly longer (4-5 µm instead of 1-2 µm; see Figure 3). This is thought to be caused by the removal of a diffusion barrier between the iron and silicon. It was shown that the β-FeSi2 bulk growth follows parabolic growth kinetics.34 In addition, the activation energy for thin film (1.5 eV34) and NWs (1.8 eV35) are highter than bulk (0.83 eV34), which indicates a stronger degree of temperature dependence for nanostructure growth. Silicon diffusion in FeSi follows a behavior similar to that in β-FeSi2;34 therefore, NW dimension control by temperature

Figure 2. FeSi NW growth for different times: (a) 1 and (b) 2 h. Insets are high magnification SEM images, respectively.

Magnetic Nanohead-FeSi NW Heterojunctions

Figure 4. SAED pattern for FeSi NWs, which is indexed down the [111j] zone axis.

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Figure 6. Magnetization measurements of the nano Fe3O4-headed FeSi NWs at 10 and 300 K. Inset shows MFM scan over magnetic nanoheaded NWs.

Figure 7. Temperature-dependent magnetization of the nano Fe3O4headed FeSi NWs. Figure 5. Representative HRTEM of magnetic Fe3O4-FeSi NW heterojunctions. Inset is the two-dimensional FFT of the FeSi NW.

is effective. Surprisingly, the nanomagnetic heads were maintained at a small size, which is comparable to the NW diameters. This phenomenon may relate to the constraint by the epitaxial relationship between the nano-Fe3O4-head and FeSi NW. Figure 4 shows the selected area diffraction micrograph of the NWs, which is indexed as ε-FeSi (simple cubic lattice constant, a ) 4.488 Å, PDF00-038-1397). NW growth is along the [110] direction, consistent with a previous report,20 but along the [111] direction was also reported.19 It was demonstrated that the catalyst composition, NW diameter, growth rate, etc. control the growth axis.36,37 For example, silicon NWs grow in the [111] direction for large wires (diameter > 20 nm) but along [110] for small wires (diameter < 10 nm).36 The chemical reaction in these FeSi NWs formation seems straightforward: Fe2O3 is reduced to Fe and then reacts with silicon to form silicide. However, our silicide NW growth appears to have a mechanism that is different from the widely accepted VLS growth.38,39 No catalyst metal, gold, was detected in our EDS spectrum, and the magnetic caps are identified to be Fe3O4, as shown in Figure 5. We tried to grow on silicon substrate without Au catalyst, and there grew FeSi NWs, but with sparse density. Figure 5, the HRTEM of the heterojunctions, shows the epitaxial relationship between FeSi NW and magnetic Fe3O4 head: Si[001]//Fe3O4[2j01], Si(001)//Fe3O4(102). There are stacking faults with Burger vectors perpendicular to the NWs growth direction. Silicon NWs with the most commonly observed polytypes, diamond hexagonal (2H) or rhombohedral (9R), were reported. The planes with stacking faults are parallel to the NW growth axis. When compared to ZnS NWs with a polytype

modulated superlattice which was formed via intermittent laser ablation-catalytic growth,40 our FeSi NWs were formed by a simple one-step CVD, and the 2H polytype was identified. This polytype modulated 1D structure could possibly lead to distinct properties, such as optic and thermoelectric properties, due to modification40 of the thermal properties, which justifies further research in this direction. To examine the magnetic properties of the grown Fe3O4 heads on FeSi NWs, the magnetic properties were measured using the Quantum Design physical property measurement system (QD PPMS) with VSM option. The magnetization curves are shown in Figures 6 and 7. We see slight hysteresis at 300 K while more enhanced remanence at 10 K, which is consistent with the temperature-dependent magnetization curve shown in Figure 6. The coercivity is about 60 Oe at 300 K and about 220 Oe at 10 K. The signal is mostly from Fe3O4 instead of FeSi NWs, which have a much weaker magnetism compared to Fe3O4. Basically, the temperature dependence of magnetization curve is quite different for the two materials.19,41 Figure 7 shows the temperature dependent magnetization measured at zero field cooling (ZFC) and field cooling (FC) conditions with applied field H ) 1000 Oe, and the measurements are done with the temperature increasing. The blocking temperature is at about 60 K (as indicated in the inset of Figure 7), below which the nanoparticles show magnetization increasing with temperature and above which the magnetization decreases with temperature. The result is consistent with previous results on Fe3O4 nanoparticles,42 except for the lower transition temperature, which could be possibly caused by the size difference and the existence of a broadened distribution of energy barriers.

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To measure spatially resolved magnetic properties of these NWs, we carried out MFM measurement. Dynamic operational mode (phase-lock-loop excitation of the cantilever at its resonance frequency, ∼67 kHz.) was used to monitor the cantilever frequency as a function of the lateral position. The inset in Figure 6 shows the MFM scan over Fe3O4 nanoparticle headed NWs using an AppNano MAGT cantilever. The sign of the signal (positive frequency shift, ∼3.2 Hz) is consistent with the repulsive force expected for the probe and sample being magnetized in the opposite direction. The magnetic properties of nanoparticles depend on their geometry, size, crystal orientation, etc.43 Magnetic nanoparticles have a single-domain structure when their sizes are smaller than certain critical values (∼100 nm). Through controlling the growth conditions, MFM probes with a single NW tip and single-domain magnetic nanoparticle could be produced. The new MFM tip structure and, consequently, calculation of the tip-sample interaction can be significantly simplified, compared with commercial tips coated with a magnetic film of multiple domains.44 The tip-sample perturbation and convolution are also reduced due to the small volume of the single domain magnetic nanoparticle. Therefore, these magnetic nanohead-FeSi nanowire epitaxial heterojunctions are very promising candidates for highly sensitive MFM probes. Previous studies on magnetic nanoparticles assembled on NWs by electrodeposition show magnetic heads with a big diameter (>60 nm, our magnetic head on a NW is only ∼30 nm in diameter) and no epitaxial relationship between the magnetic heads and the NWs. In addition, they require several complicated steps for the fabrication of such structures.32 Magnetic heads by electrodeposition have a gold core, which makes the magnetic signal weaker. In summary, self-assembled magnetic nanohead-FeSi nanowire (NW) epitaxial heterojunctions by CVD are reported. Free standing FeSi NWs, ∼30 nm in diameter and several micrometers long, were grown by reduction of Fe2O3 in a H2 atmosphere on a heated silicon substrate at 750 °C. A magnetic Fe3O4 head, with a diameter of ∼30 nm was observed on each FeSi NW. The epitaxial relationship of these junctions is: Si[001]// Fe3O4[2j01], Si(001)//Fe3O4(102). Ferromagnetic behavior of the heterojunctions was identified, and the coercivity is characterized to be ∼60 Oe at 300 K and ∼220 Oe at 10 K. This simple CVD approach for self-assembling epitaxially magnetic heads could enable device integration for spintronics and sensors. Particularly, such FeSi nanowires with highly localized magnetic heads are ideal for high-resolution magnetic force microscopy. In addition, this one-step CVD approach for self-assembling of magnetic nanohead-NW heterostructure could be easily extended to other material (such as nickel, manganese, chromium, etc.) systems with different properties. We are performing MFM measurements using probes equipped with FeSi nanowires and will report results elsewhere. Acknowledgment. This work was supported by the Starting Foundation of Renmin University of China. References and Notes (1) Dai, Z. R.; Pan, Z. W.; Wang, Z. L. AdV. Funct. Mater. 2003, 13, 9–24. (2) Law, M.; Goldberger, J.; Yang, P. D. Ann. ReV. Mater. Res. 2004, 34, 83–122. (3) Thelander, C.; Martensson, T.; Bjork, M. T.; Ohlsson, B. J.; Larsson, M. W.; Wallenberg, L. R.; Samuelson, L. Appl. Phys. Lett. 2003, 83, 2052– 2054. (4) Gudiksen, M. S.; Lauhon, L. J.; Wang, J.; Smith, D. C.; Lieber, C. M. Nature 2002, 415, 617–620.

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