Single-crystalline δ-Ni2Si nanowires with excellent physical properties
© Chiu et al.; licensee Springer. 2013
Received: 10 May 2013
Accepted: 7 June 2013
Published: 19 June 2013
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.
KeywordsCVD Ni2Si nanowires Field emission Ferromagnetic characteristic
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
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.
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].
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.
Chemical vapor deposition
Fast Fourier transform
High-resolution transmission electronic microscopy
Superconductive quantum interference device
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.
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