In this article, we report the synthesis of single-crystalline nickel silicide nanowires (NWs) via chemical vapor deposition method using NiCl2·6H2O as a single-source precursor. Various morphologies of δ-Ni2Si NWs were successfully acquired by controlling the growth conditions. The growth mechanism of the δ-Ni2Si NWs was thoroughly discussed and identified with microscopy studies. Field emission measurements show a low turn-on field (4.12 V/μm), and magnetic property measurements show a classic ferromagnetic characteristic, which demonstrates promising potential applications for field emitters, magnetic storage, and biological cell separation.
With the miniaturization of electronic devices, one-dimensional (1-D) nanostructures have attracted much attention due to their distinct physical properties compared with thin film and bulk materials. One-dimensional materials, such as nanorods, nanotubes, nanowires (NWs), and nanobelts, are promising to be utilized in spintronics, thermoelectric and electronic devices, etc. [1–5]. Metal silicides have been widely synthesized and utilized in the contemporary metal-oxide-semiconductor field-effect transistor as source/drain contact materials, interconnection , and Schottky barrier contacts. One-dimensional metal silicides have shown excellent field emission [7, 8] and magnetic properties [9–11]. Hence, recently, the synthesis and study of 1-D metal silicide nanostructures and silicide/silicon or silicide/siliconoxide nanoheterostructures have been extensively investigated [9, 12–18]. Among various silicides, Ni silicide NWs with low resistivity, low contact resistance, and excellent field emission properties [19, 20] are considered as a promising material in the critical utilization for the future nanotechnology. Thus, plenty of methods have been reported to synthesize Ni silicide NWs. Wu et al. have formed NiSi NWs by the chemical reaction between coated Ni metal layers and pre-fabricated Si NWs . In addition, metal-induced growth, chemical vapor deposition (CVD), and chemical vapor transport method have been successfully applied to synthesize NiSi [21, 22], Ni31Si12, Ni3Si , and Ni2Si  NWs, and their physical properties have been investigated. For simplification of the whole processing, metal chloride compounds such as Fe(SiCl3)2(CO)4, CoCl2[11, 25], or NiCl2 are commonly used as single-source precursors (SSPs) in synthesizing metal-silicide NWs. In this work, δ-Ni2Si NWs were synthesized via CVD method with SSP of NiCl2. The morphology and yield of δ-Ni2Si NWs can be mastered through parameter control. The δ-Ni2Si NWs were structurally characterized via high-resolution transmission electronic microscopy (HRTEM). The growth mechanisms of δ-Ni2Si NWs and NiSi phases were identified through structural analysis by X-ray diffraction (XRD) and TEM. Electrical measurements showed an outstanding field emission property, and magnetic property measurements demonstrated a classic ferromagnetic behavior of the δ-Ni2Si NWs.
The synthesis of the silicide NWs was carried out in the three-zone furnace via a chemical vapor deposition process. Commercial single-crystalline Si substrates were firstly cleaned in acetone for 10 min by ultrasonication. In order to remove the native oxide layer, substrates were dipped in dilute HF solutions for 30 s and then dried by nitrogen gas flow. The nickel chloride (NiCl2) precursor was placed in an aluminum boat at the upstream and flown by carrier gas Ar at 30 sccm, while Si substrates were put at the downstream. The temperatures of the precursor and substrates were controlled at 600°C and 400°C, respectively, and held for 15 to 30 min with a 10°C/min ramping rate. The vacuum pressure was controlled in the range of 6 to 15 Torr. The morphologies were investigated by field emission scanning electron microscopy. XRD and TEM were utilized in structural characterization. The noise of the atomic images was filtered by fast Fourier transform (FFT). The field emission property was measured using a Keithley power supply (Keithly Instruments Inc., Cleveland, OH, USA) with an anode probe of 180 μm in diameter. A superconductive quantum interference device (SQUID; MPMS XL, SQUID Technology, Heddington, Wiltshire, UK) was utilized for magnetic property measurements.
Results and discussion
Figure 1a,b,c,d shows the SEM images of samples grown at different pressures (6, 9, 12, 15 Torr, respectively), indicating that the geometry on the surface of substrates varied with the ambient condition. With lower partial pressure of the precursor, as shown in Figure 1a, Ni silicide NWs were not formed due to insufficient supply of the Ni source; however, small nanowhiskers can be observed on the surface. As the ambient pressure was raised to the range of 9 to 12 Torr (Figure 1b,c), NWs with high aspect ratios were obtained for proper concentrations of precursors and growth conditions. The diameter of the NWs slightly increased with the increase of the ambient pressure (from 30 to 50 nm to 40 to 70 nm). This may be attributed to the fact that higher precursor concentration is more suitable for the formation of δ-Ni2Si system. Furthermore, when the pressure was higher than 15 Torr, the concentration of the Ni source was oversaturated and the morphology of the product turned into islands instead of NWs. Those islands may result from the condition change to decrease the surface energy of the system by transforming into bulk-like structures, as shown in Figure 1d. Thus, the diameter of the NWs can be controlled under specific pressure range and the ambient pressure plays an important role in maintaining the morphology of the NWs.
Figure 2a,b shows a series of SEM images of NWs with different growth times at a constant gas flow rate (30 sccm) and ambient pressure (9 Torr). The yield and density increased prominently when the growth time was raised from 15 to 30 min. The XRD analysis of different reaction time is shown in Figure 2c. The characteristic peaks were examined and identified to be orthorhombic δ-Ni2Si and NiSi according to the JCPDF data base. From Figures 1 and 2, SEM images indicate that there were two types of microstructures (NWs and islands) in the products. In order to identify each phase of the microstructures of the as-grown products, structural analysis of the NWs has been performed. Figure 3a is the low-magnification TEM image of the NW with 30 nm in diameter. HRTEM image (Figure 3b) shows the NW of  growth direction with 2-nm-thick native oxide. FFT diffraction pattern of the lattice-resolved image is shown in the inset of Figure 3b, which represents the reciprocal lattice planes with  zone axis. The phase of the NW has been identified to be δ-Ni2Si, constructed with the orthorhombic structure by lattice parameters of a = 0.706 nm, b = 0.5 nm, and c =0.373 nm. Therefore, the as-deposited layer would be ascribed to NiSi.
The schematic illustration of the growth mechanism is in Figure 4. In the Ni-Si binary alloy system, it has been investigated that Ni atoms are the dominant diffusion species during the growth of orthorhombic δ-Ni2Si and NiSi . The reaction and phase transformation between δ-Ni2Si and NiSi have also been reported . Based on these previous studies, the reaction of the as-deposited Ni metal film occurred to form δ-Ni2Si with a diffusion-controlled kinetics at 300°C to 400°C [27, 28]. Then, partial transformation from δ-Ni2Si into NiSi thin-film structures could happen if the thickness of the Ni is below 40 nm because NiSi would form on Si substrates with a low Si/NiSi interface energy [26, 29]. Then, the continuous supply of Ni atoms may induce further growth of δ-Ni2Si phase NWs via surface diffusion kinetics  on the remnant δ-Ni2Si phase grains or NiSi bulks. There are two plausible and reversible formation paths of δ-Ni2Si, which can be described in the following equations [11, 24, 31]:
The two equations correspond well with the experiment results: higher ambient pressure will enhance the reaction to form Ni2Si according to LeChatelier's principle, contributing to the formation and agglomeration of larger amount of δ-Ni2Si NWs and islands at the surface.
Due to the metallic property and special 1-D geometry, investigation of field emission properties has been conducted. Figure 5 shows the plot of the current density (J) as a function of the applied field (E) and the inset is the ln(J/E2)−1/E plot. The sample of δ-Ni2Si NWs was measured at 10−6 Torr with a separation of 250 μm. According to the Folwer-Nordheim relationship, the field emission behavior can be described by the following equation:
The turn-on field was defined as the applied field attained to a current density of 10 μA/cm2 and was found to be 4.12 V/μm for our Ni2Si NWs. The field enhancement factor was calculated to be about 1,132 from the slope of the ln(J/E2)−1/E plot with the work function of 4.8 eV  for Ni2Si NWs. Based on the measurements, Ni2Si NWs exhibited remarkable potential applications as a field emitter like other silicide NWs [20, 25, 33].
The saturated magnetization (MS) and coercivity (HC) of δ-Ni2Si NWs were measured using SQUID at 2 and 300 K, respectively. Figure 6 shows the hysteresis loop of the as-grown NWs of 30 nm in diameter with the applied magnetic field perpendicular to the substrates. The inset highlighted the hysteresis loop, which demonstrates a classic ferromagnetic characteristic. The HC was measured to be 490 and 240 Oe at 2 and 300 K, respectively, and MS was about 0.64 and 0.46 memu, correspondingly. For the magnetization per unit volume (emu/cm3), normalization has been introduced through cross-sectional and plane-view SEM images (not shown here) to estimate the density of NWs and the average volume of δ-Ni2Si NWs. The estimated values are 2.28 emu/cm3 for 2 K and 1.211 emu/cm3 for 300 K, respectively. With the normalized value, we may build up a database of the magnetic property of Ni2Si NWs, which may be utilized in applications such as cell separation in biology .
δ-Ni2Si phase NWs have been successfully synthesized through CVD using a single precursor, NiCl2·6H2O. The influence of the chamber pressure on the product morphology has been discussed. SEM, TEM, and XRD studies were conducted to analyze the growth mechanism and reaction paths. Electrical measurements show that the field emission property of the δ-Ni2Si NWs makes them attractive choices for emitting materials. Magnetic measurements via SQUID at different temperatures show the ferromagnetic property of the δ-Ni2Si NWs, and normalization has been applied to calculate the value of magnetization per unit volume. This work has demonstrated future applications of Ni2Si NWs on biologic cell separation, field emitters, and magnetic storage.
WWW, CLH, and KCL acknowledge the support by National Science Council through grants 100-2628-E-009-023-MY3, 101-2218-E-008-014-MY2, and 100-2628-E-006-025-MY2.
Department of Materials Science and Engineering, National Chiao Tung University
Department of Materials Science and Engineering, National Cheng Kung University
Department of Electrical Engineering, National Central University
Department of Physics, Tamkang University
Wu XC, Song WH, Huang WD, Pu MH, Zhao B, Sun YP, Du JJ: Simultaneous growth of alpha-Si3N4 and beta-SiC nanorods. Mater Res Bull 2001, 36: 847–852. 10.1016/S0025-5408(01)00571-2View Article
Morales AM, Lieber CM: A laser ablation method for the synthesis of crystalline semiconductor nanowires. Science 1998, 279: 208–211. 10.1126/science.279.5348.208View Article
Sun Y, Ndifor-Angwafor NG, Riley DJ, Ashfold MNR: Synthesis and photoluminescence of ultra-thin ZnO nanowire/nanotube arrays formed by hydrothermal growth. Chem Phys Lett 2006, 431: 352–357. 10.1016/j.cplett.2006.09.100View Article
Dai ZR, Pan ZW, Wang ZL: Novel nanostructures of functional oxides synthesized by thermal evaporation. Adv Funct Mater 2003, 13: 9–24. 10.1002/adfm.200390013View Article
Zhang HL, Li F, Liu C, Cheng HM: The facile synthesis of nickel silicide nanobelts and nanosheets and their application in electrochemical energy storage. Nanotechnology 2008, 19: 165606. 10.1088/0957-4484/19/16/165606View Article
Maszara WP: Fully silicided metal gates for high-performance CMOS technology: a review. J Electrochem Soc 2005, 152: G550-G555. 10.1149/1.1924307View Article
Xiang B, Wang QX, Wang Z, Zhang XZ, Liu LQ, Xu J, Yu DP: Synthesis and field emission properties of TiSi2 nanowires. Appl Phys Lett 2005, 86: 243103. 10.1063/1.1948515View Article
Lin HK, Tzeng YF, Wang CH, Tai NH, Lin IN, Lee CY, Chiu HT: Ti5Si3 nanowire and its field emission property. Chem Mater 2008, 20: 2429–2431. 10.1021/cm800079cView Article
Schmitt AL, Bierman MJ, Schmeisser D, Himpsel FJ, Jin S: Synthesis and properties of single-crystal FeSi nanowires. Nano Lett 2006, 6: 1617–1621. 10.1021/nl060550gView Article
Schmitt AL, Higgins JM, Jin S: Chemical synthesis and magnetotransport of magnetic semiconducting Fe1-xCoxSi alloy nanowires. Nano Lett 2008, 8: 810–815. 10.1021/nl072729cView Article
Seo K, Varadwaj KSK, Mohanty P, Lee S, Jo Y, Jung MH, Kim J, Kim B: Magnetic properties of single-crystalline CoSi nanowires. Nano Lett 2007, 7: 1240–1245. 10.1021/nl070113hView Article
Ham MH, Lee JW, Moon KJ, Choi JH, Myoung JM: Single-crystalline ferromagnetic Mn4Si7 nanowires. J Phys Chem C 2009, 113: 8143–8146.View Article
Wu Y, Xiang J, Yang C, Lu W, Lieber CM: Single-crystal metallic nanowires and metal/semiconductor nanowire heterostructures. Nature 2004, 430: 61–65. 10.1038/nature02674View Article
Weber WM, Geelhaar L, Graham AP, Unger E, Duesberg GS, Liebau M, Pamler W, Cheze C, Riechert H, Lugli P, Kreupl F: Silicon-nanowire transistors with intruded nickel-silicide contacts. Nano Lett 2006, 6: 2660–2666. 10.1021/nl0613858View Article
Lu KC, Wu WW, Wu HW, Tanner CM, Chang JP, Chen LJ, Tu KN: In situ control of atomic-scale Si layer with huge strain in the nanoheterostructure NiSi/Si/NiSi through point contact reaction. Nano Lett 2007, 7: 2389–2394. 10.1021/nl071046uView Article
Wu WW, Lu KC, Wang CW, Hsieh HY, Chen SY, Chou YC, Yu SY, Chen LJ, Tu KN: Growth of multiple metal/semiconductor nanoheterostructures through point and line contact reactions. Nano Lett 2010, 10: 3984–3989. 10.1021/nl101842wView Article
Chiu CH, Huang CW, Chen JY, Huang YT, Hu JC, Chen LT, Hsin CL, Wu WW: Copper silicide/silicon nanowire heterostructures: in situ TEM observation of growth behaviors and electron transport properties. Nanoscale 2013, 5: 5086–5092. 10.1039/c3nr33302gView Article
Hsin CL, Yu SY, Wu WW: Cobalt silicide nanocables grown on Co films: synthesis and physical properties. Nanotechnology 2010, 21: 485602. 10.1088/0957-4484/21/48/485602View Article
Lee CY, Lu MP, Liao KF, Lee WF, Huang CT, Chen SY, Chen LJ: Free-standing single-crystal NiSi2 nanowires with excellent electrical transport and field emission properties. J Phys Chem C 2009, 113: 2286–2289. 10.1021/jp809029qView Article
Lee CY, Lu MP, Liao KF, Wu WW, Chen LJ: Vertically well-aligned epitaxial Ni31Si12 nanowire arrays with excellent field emission properties. Appl Phys Lett 2008, 93: 113109. 10.1063/1.2981703View Article
Decker CA, Solanki R, Freeouf JL, Carruthers JR, Evans DR: Directed growth of nickel silicide nanowires. Appl Phys Lett 2004, 84: 1389–1391. 10.1063/1.1650877View Article
Dong LF, Bush J, Chirayos V, Solanki R, Jiao J, Ono Y, Conley JF, Ulrich BD: Dielectrophoretically controlled fabrication of single-crystal nickel silicide nanowire interconnects. Nano Lett 2005, 5: 2112–2115. 10.1021/nl051650+View Article
Song YP, Jin S: Synthesis and properties of single-crystal β3-Ni3Si nanowires. Appl Phys Lett 2007, 90: 173122. 10.1063/1.2732828View Article
Song YP, Schmitt AL, Jin S: Ultralong single-crystal metallic Ni2Si nanowires with low resistivity. Nano Lett 2007, 7: 965–969. 10.1021/nl0630687View Article
Tsai CI, Yeh PH, Wang CY, Wu HW, Chen US, Lu MY, Wu WW, Chen LJ, Wang ZL: Cobalt silicide nanostructures: synthesis, electron transport, and field emission properties. Cryst Growth Des 2009, 9: 4514–4518. 10.1021/cg900531xView Article
Foll H, Ho PS, Tu KN: Transmission electron microscopy of the formation of Nickel silicides. Philos Mag A 1982, 45: 31–47.View Article
Dheurle F, Petersson CS, Baglin JEE, Laplaca SJ, Wong CY: Formation of thin-films of NiSi - metastable structure, diffusion mechanisms in intermetallic compounds. J Appl Phys 1984, 55: 4208–4218. 10.1063/1.333021View Article
Guliants EA, Anderson WA, Guo LP, Guliants VV: Transmission electron microscopy study of Ni silicides formed during metal-induced silicon growth. Thin Solid Films 2001, 385: 74–80. 10.1016/S0040-6090(00)01916-7View Article
Toman K: The structure of NiSi. Acta Cryst 1951, 4: 462–464. 10.1107/S0365110X51001458View Article
Maex K: Properties of metal silicides. London: IEE; 1995.
Lian OY, Thrall ES, Deshmukh MM, Park H: Vapor-phase synthesis and characterization of epsilon-FeSi nanowires. Adv Mater 2006, 18: 1437–1440. 10.1002/adma.200502721View Article
Kittl JA, Pawlak MA, Lauwers A, Demeurisse C, Opsomer K, Anil KG, Vrancken C, van Dal MJH, Veloso A, Kubicek S, Absil P, Maex K, Biesemans S: Work function of Ni silicide phases on HfSiON and SiO2: NiSi, Ni2Si, Ni31Si12, and Ni3Si fully silicided gates. Ieee Electr Device L 2006, 27: 34–36.View Article
Kim DJ, Seol JK, Lee MR, Hyung JH, Kim GS, Ohgai T, Lee SK: Ferromagnetic nickel silicide nanowires for isolating primary CD4+ T lymphocytes. Appl Phys Lett 2012, 100: 163703. 10.1063/1.4704924View Article
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