Article pubs.acs.org/JPCC
Synthesis of Crystalline Pyramidal ε‑FeSi and Morphology- and SizeDependent Ferromagnetism Xiang Wang, Zhiqiang He, Shijie Xiong, and Xinglong Wu* Key Laboratory of Modern Acoustics, MOE, Institute of Acoustics, Department of Physics, Nanjing University, Nanjing 210093, People’s Republic of China S Supporting Information *
ABSTRACT: Crystalline pyramidal ε-FeSi particles smaller than 1 μm in size with {111} lateral facets are synthesized by a spontaneous chemical vapor deposition method. The nanocrystals initially nucleate from the amorphous film via self-clustering forming a rectangular ε-FeSi (001) terrace as a result of the cubic crystalline structure and subsequent anisotropic accumulation on the terrace produces the pyramidal morphology. Roomtemperature ferromagnetism is observed from ε-FeSi particles larger than 250 nm and having the {111} facets. A model is postulated to explain the morphology- and size-dependent ferromagnetism based on the nonuniform Fe atomic arrangement that forms atomic-scale islands on the surface and dipole interaction among these islands in the large enough particles. The morphology- and size-dependent ferromagnetism allows control of the magnetic moments of mesostructures and is important to spintronics and other applications.
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carriers with localized dangling bond spins.19 This and other discoveries have aroused tremendous interest in making nonmagnetic semiconductors magnetic.20−23 One way is to introduce magnetic dopants such as Co into FeSi and the resulting Fe1−xCoxSi structures display different magnetic behavior including ferromagnetism.24−26 Furthermore, by taking advantage of defects as native “dopants” ferromagnetism can be induced.27 However, some ferromagnetic materials have been observed to lose magnetism when the size decreases to several nanometers.28 The morphology that determines the surface atomic arrangement and coordination is another important parameter to modulate the materials characteristics.29,30 Polyhedron particles with many faces, angles, and edges have many special physical and chemical properties and despite potential applications as catalysts, gas sensors and optical devices,31−34 the relationship between the morphology and magnetic properties is not well understood. In this paper, we report direct growth of pyramidal crystalline ε-FeSi particles and observe room-temperature ferromagnetism from the crystals with the {111} lateral facets. However, ε-FeSi particles that are less than 250 nm in size exhibit no magnetism. Our theoretical calculation discloses that the atomic-scale islands on the {111} surface caused by nonuniform Fe atomic arrangement and dipole interaction among these islands in the relatively large particles are responsible for the observed morphology- and size-dependent room-temperature ferromagnetism.
INTRODUCTION Spintronics utilizes the spin degree of freedom of electrons in addition to charge degree of freedom to enhance the performance of solid-state electronic and photonic devices.1−3 It may also be able to inject, manipulate, and detect electron spins in semiconductor devices thereby allowing quantum operations demanded by quantum computing.4 In particular, silicon-based ferromagnetic materials compatible with microelectronics processing are potentially important building blocks for future spintronic devices.5 However, despite a long spin coherence lifetime6,7 and mature processing technology siliconbased spintronic devices are less developed than those based on the more popular Ga1‑xMnxAs magnetic semiconductor8 because of the slow technological advances so far combined with insufficient understanding of silicon-based magnetic semiconductor materials.5 ε-FeSi, a narrow bandgap semiconductor with a cubic structure (space group P213), has been classified as a hybridization-bandgap semiconductor or Kondo insulator.9,10 It has attracted interest from scientists mainly because of the unusual magnetic properties.11−15 Single-crystal bulk ε-FeSi exhibits paramagnetism and very weak ferromagnetism has been observed at 4 K overlapping the dominant paramagnetism contribution.16 Different from the bulk materials, ferromagnetism observed from ε-FeSi films stems from another mechanism involving the expanded lattice constant and difference in stoichiometry.17 Kim et al. have recently synthesized epitaxially grown freestanding FeSi nanowires using a catalyst-free chemical vapor transport method.18 Directional growth of nanowires was correlated with initially epitaxially formed FeSi seed crystals. Single-crystal ε-FeSi nanowires with certain orientations have been reported to exhibit room-temperature ferromagnetism because of the interaction between charge © 2014 American Chemical Society
Received: November 3, 2013 Revised: January 14, 2014 Published: January 14, 2014 2222
dx.doi.org/10.1021/jp410813z | J. Phys. Chem. C 2014, 118, 2222−2228
The Journal of Physical Chemistry C
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Article
MATERIALS AND METHODS
Materials. Crystalline pyramidal ε-FeSi particles were synthesized in a horizontal two-zone pipe furnace by chemical vapor deposition (CVD). The quartz tube was evacuated to a pressure of 1 × 10−3 Pa by mechanical and turbomolecular pumps and then purged with Ar (99.99%) afterward. During synthesis, Ar was bled in at a flow rate of 100 sccm (cubic centimeter per minute) and the (100) Si substrates were cleaned with 1 wt %-buffered HF prior to synthesis. A 0.5 g sample of anhydrous FeCl3 powders (98% from Alfa Aesar) was used as the precursor and the silicon substrates were placed in the upstream and downstream zones, respectively. The upstream zone was heated from room temperature to 300 °C and the downstream zone was heated to 650 °C in 20 min. When the furnace reached the set temperature, the FeCl3 precursors that produced vapor-phase FeCl3 or Fe2Cl6 were introduced together with the carrier gas flow downstream. The reaction proceeded for 2 h, and afterward the furnace was cooled to room temperature slowly. Characterization. The morphologies of iron silicide particles were examined with a field-emission scanning electron microscopy (FE-SEM, Hitachi S4800). The microstructure and chemical compositions were characterized using a 300 kV transmission electron microscope (TEM, Tecnai G2 F30 STwin) equipped with an energy dispersive X-ray (EDS) spectrometer. The samples were put into ethanol and then ultrasonically treated using a frequency and power of 40 kHz and 150 W, respectively. Several drops of suspensions after ultrasonic treatment were put on a copper mesh initially coated with a thin carbon. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were conducted on a X’TRA X-ray diffractometer with a Cu Kα source and PHI5000 VersaProbe, respectively. The magnetic properties were determined on a vibrating sample magnetometer (VSM). An atomic force microscope (AFM, Digital Instruments, Nanoscope IIIa) in the magnetic force microscopic (MFM) mode was utilized to measure the surface topography and magnetic domains. In the MFM experiments, the sample was the initial state without any magnetic treatment. The AFM tips were provided by Nano World Ltd. Co. (Type MESP). The cantilever oscillating frequency was 60−100 Hz and the module of elasticity was 1−5 N/m. The lift mode was adopted to obtain the magnetic micrographs and the height was 50−80 nm.
Figure 1. (a) SEM image of the pyramidal crystalline ε-FeSi particle film. (b,c) Magnified images of two particles. (d) Schematic diagram of a pyramidal particle. The bottom surface and four lateral sides are (001) and {111}, respectively.
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RESULTS AND DISCUSSION Charaterization of Pyramid-Shaped ε-FeSi Particles. As shown in Figure 1a, there are many particles with smooth surfaces on the silicon substrate with sizes less than 1 μm. The particles are embedded in the film without a specific morphology. To determine the shape some particles are magnified and five surfaces consisting of one square and four equilateral triangles are observed (Figure 1b,c). They are consistent with the pyramid shape shown in Figure 1d. Several irregular particles are also found but they are the minority. Although particles with a partial pyramid shape may grow imperfectly, this should not alter the crystal facets of the five surfaces. Here, we would like to mention that in all the SEM images, no octahedral ε-FeSi paricles with all facets of {111} are observed. Figure 2a depicts the low-magnification TEM image of an individual particle. The particle resembles a rhombus with an edge length of about 650 nm and an internal angle of about
Figure 2. (a) TEM image of a pyramidal crystalline ε-FeSi particle taken from the an ethanol suspension. (b) SEM image and (c) Simulated image of a pyramidal particle from the same view. (d) HRTEM image of the particle [110] edge. (e) HR-TEM image and (f) SAED pattern near the [110] edge of the particle.
75°. It is consistent with the SEM image of the pyramidal particle illustrated in Figure 2b and the simulated pyramid in Figure 2c. The EDS spectrum shows that it is composed of Fe and Si with an atomic ratio of 51.4:48.6, which is close to 1:1 2223
dx.doi.org/10.1021/jp410813z | J. Phys. Chem. C 2014, 118, 2222−2228
The Journal of Physical Chemistry C
Article
for ε-FeSi (Figure SI-1 of Supporting Information). Figure 2d displays the high-resolution TEM (HR-TEM) image of the selected area in Figure 2a clearly illustrating the single-crystal structure of the particle. The d-spacing of 0.200 nm (Figure 2e) corresponds to the ε-FeSi (210) plane and the selected area electron diffraction (SAED) pattern acquired from the same location also shows the (210) plane (Figure 2f). In addition, the (2̅10) plane is present. No amorphous disfusive diffraction spot is observed suggesting that the particle is a single crystal. The angle between the (210) plane and right edge is 18.4° (Figure 2d), which agrees well with the angle of 18.4° formed by the (210) and (110) planes. Hence, it can be inferred that the right edge of the particle is [110]. The pyramidal shape suggests that the square undersurface is (001) and the four equilateral triangular sides are {111}. The growth process and formation mechanism of the pyramidal ε-FeSi particles are shown in Figure SI-3 of Supporting Information). ε-FeSi has a cubic structure.19 Initially, ε-FeSi nanocrystals nucleate from the amorphous film and then the irregularly distributed nanocrystals self-cluster into square FeSi (001) terraces. In the process of terrace formation, the terrace surface accumulates many nanoparticles. The surface energy of the {001} planes is higher than that of the {111} planes in ε-FeSi. This causes growth along the ⟨001⟩ direction to be faster than that along ⟨111⟩ via crystallization of the surface nanoparticles. Hence, the {111} faces are preserved. As the reaction proceeds, the missing parts are filled and pyramidal ε-FeSi particles are eventually formed. Because of the existence of initial terrace, the other half of the pyramid is suppressed and thus the octahedral-shape particles cannot be observed. Magnetic Property of Pyramid-Shaped ε-FeSi Particles. To characterize the magnetic characteristics of the FeSi particles, magnetic measurements are performed by VSM at ambient temperature with the substrate perpendicular to the magnetic fields (Figure 3). Curve 1 is the magnetic character-
has been reported that single-crystalline bulk FeSi is paramagnetic at room temperature and amorphous FeSi is similar to the bulk materials.16,35 In the first growth period, some ε-FeSi nanocrystals nucleate from the amorphous film that contains small ε-FeSi nanocrystals (Figure SI-4 of Supporting Information). The size of the newly formed ε-FeSi crystals is several nanometers. Crystallization continues and the crystals become bigger as time elapses. However, a random growth orientation is observed from these nanocrystals (Figure SI-4 of Supporting Information). Some crystals show the (210) plane but some others show the {111} plane. The TEM results agree well with the XRD pattern of sample 2 (Figure SI-5a of Supporting Information) in which an amorphous peak and four other peaks corresponding to ε-FeSi (110), (111), (210), and (211) exist (JCPDS card no. 86-0798). Besides these peaks, the (200) peak appears from sample 1 (Figure SI-5b of Supporting Information) because some rectangular ε-FeSi (001) terraces are formed in sample 1. The ε-FeSi nanocrystals without a specific orientation appear to possess superparamagnetic properties at room temperature. This suggests that the room-temperature ferromagnetic properties of sample 1 may be attibuted to the pyramidal ε-FeSi particles. To elucidate the origin of the ferromagnetism, MFM measurements were performed on sample 1. Figure 4a shows a wide-range scanned map. Four pyramidal particles grow on the film and some irregular particles are also found. This is consistent with the SEM picture in Figure 1a. According to the corresponding magnetic image, the magnetic domain can only be found from the pyramidal particles (marked by an elliptic dashed box). In contrast, the particle with an irregular shape at the lower right corner (marked by a rectangular dashed box) shows no magnetism at all. Many small pyramidal particles (
