Single-crystalline delta-Ni2Si nanowires with excellent physical properties

N A N O E X P R E S S

Open Access

Single-crystalline

δ-Ni

₂

Si nanowires with excellent

physical properties

Wen-Li Chiu

, Chung-Hua Chiu

, Jui-Yuan Chen

, Chun-Wei Huang

, Yu-Ting Huang

, Kuo-Chang Lu

,

Cheng-Lun Hsin

, Ping-Hung Yeh

and Wen-Wei Wu

Abstract

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 ofδ-Ni2Si NWs were

successfully acquired by controlling the growth conditions. The growth mechanism of theδ-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. Keywords: CVD, Ni2Si nanowires, Field emission, Ferromagnetic characteristic

Background

With the miniaturization of electronic devices, one-dimensional (1-D) nanostructures have attracted much at-tention due to their distinct physical properties compared with thin film and bulk materials. One-dimensional mate-rials, such as nanorods, nanotubes, nanowires (NWs), and nanobelts, are promising to be utilized in spintronics, thermoelectric and electronic devices, etc. [1-5]. Metal sili-cides have been widely synthesized and utilized in the con-temporary metal-oxide-semiconductor field-effect transistor as source/drain contact materials, interconnection [6], and Schottky barrier contacts. One-dimensional metal silicides have shown excellent field emission [7,8] and magnetic prop-erties [9-11]. Hence, recently, the synthesis and study of 1-D metal silicide nanostructures and silicide/silicon or silicide/ siliconoxide nanoheterostructures have been extensively in-vestigated [9,12-18]. Among various silicides, Ni silicide NWs with low resistivity, low contact resistance, and excel-lent 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 [13]. In addition, metal-induced growth, chemical vapor deposition (CVD), and chemical vapor transport method have been successfully applied to synthesize NiSi [21,22], Ni31Si12[20], Ni3Si [23],

and Ni2Si [24] NWs, and their physical properties have been

investigated. For simplification of the whole processing, metal chloride compounds such as Fe(SiCl3)2(CO)4 [9],

CoCl2[11,25], or NiCl2[19] 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 ofδ-Ni2Si NWs and NiSi phases

were identified through structural analysis by X-ray diffrac-tion (XRD) and TEM. Electrical measurements showed an outstanding field emission property, and magnetic property measurements demonstrated a classic ferromagnetic behav-ior of theδ-Ni2Si NWs.

Methods

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

* Correspondence:[email protected];[email protected]

3

Department of Electrical Engineering, National Central University, Tao Yuan 320, Taiwan

1

Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu 300, Taiwan

Full list of author information is available at the end of the article

© 2013 Chiu et al.; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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), in-dicating 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 sili-cide NWs were not formed due to insufficient supply of the Ni source; however, small nanowhiskers can be ob-served 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 concen-trations of precursors and growth conditions. The diam-eter of the NWs slightly increased with the increase of

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 dif-ferent reaction time is shown in Figure 2c. The charac-teristic 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, struc-tural analysis of the NWs has been performed. Figure 3a

Figure 1 SEM images of as-synthesized NWs at vacuum pressures of (a) 6, (b) 9, (c) 12, and (d) 15 Torr. The temperature was fixed at 400°C, reaction time was 30 min, and carrier gas flow rate was held at 30 sccm.

Figure 2δ-Ni2Si NWs grown at (a) 15 and (b) 30 min, and

(c) corresponding XRD analysis of products. The temperature was fixed at 400°C, ambient pressure was 9 Torr, and the carrier gas flow rate was 30 sccm.

is the low-magnification TEM image of the NW with 30 nm in diameter. HRTEM image (Figure 3b) shows the NW of [010] 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 repre-sents the reciprocal lattice planes with [1] 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 [26]. The reaction and phase transformation be-tween δ-Ni2Si and NiSi have also been reported [25].

Based on these previous studies, the reaction of the as-deposited Ni metal film occurred to formδ-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 sub-strates with a low Si/NiSi interface energy [26,29]. Then, the continuous supply of Ni atoms may induce further growth ofδ-Ni2Si phase NWs via surface diffusion

kinet-ics [30] on the remnant δ-Ni2Si phase grains or NiSi

bulks. There are two plausible and reversible formation

Figure 3 Low-magnification (a) and high-resolution TEM images (b) of_δ-Ni2Si NWs grown at 400°C, 9 Torr, and 30-sccm

Ar flow. The image shows that there exists an oxide layer with 2 nm in thickness on the NW. The inset in (b) shows the

corresponding FFT diffraction pattern with a [1] zone axis and [010] growth direction.

Figure 4 The schematic illustration of the growth mechanism.

Figure 5 The field emission plot of_δ-Ni2Si NWs. The inset shows

the corresponding ln(J/E2_{)−1/E plot.}

Figure 6 M-H curve ofδ-Ni2Si NWs measured at different

reaction to form Ni2Si according to LeChatelier's

principle, contributing to the formation and agglomer-ation of larger amount of δ-Ni2Si NWs and islands at

the surface.

Due to the metallic property and special 1-D geom-etry, investigation of field emission properties has been conducted. Figure 5 shows the plot of the current dens-ity (J) as a function of the applied field (E) and the inset is the ln(J/E2_{)−1/E plot. The sample of δ-Ni}

2Si 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:

J ¼ Aβ2_E2_=ψ
exp_−Bψ3=2_=βE_{: ð3Þ}

The turn-on field was defined as the applied field attained to a current density of 10μA/cm2and was found to be 4.12 V/μm for our Ni2Si NWs. The field

enhance-ment 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 [32] 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 ap-plied magnetic field perpendicular to the substrates. The inset highlighted the hysteresis loop, which demonstrates a classic ferromagnetic characteristic. The HC was

mea-sured to be 490 and 240 Oe at 2 and 300 K, respectively, andMSwas 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 ofδ-Ni2Si NWs. The estimated values are 2.28 emu/cm3

for 2 K and 1.211 emu/cm3for 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 [34].

magnetization per unit volume. This work has demon-strated future applications of Ni2Si NWs on biologic cell

separation, field emitters, and magnetic storage.

Abbreviations

CVD:Chemical vapor deposition; FFT: Fast Fourier transform; HC: Coercivity;

HRTEM: High-resolution transmission electronic microscopy; MS: Saturated

magnetization; NWs: Nanowires; Oe: Oersted; SQUID: Superconductive quantum interference device; SSPs: Single-source precursors.

Competing interests

The authors declare that they have no competing interests.

Authors’ contributions

WLC synthesized the Ni2Si nanowires. WLC and YTH performed the field

emission and magnetization experiments. JYC and CWH analyzed the diffraction data and atomic structure via TEM. CHC analyzed the structure through XRD spectra and demonstrated the illustration of growth mechanism. WLC and WWW conceived the study and designed the research. PHY supported the field emission experiments. WLC, KCL, CLH, and WWW wrote the paper. All authors read and approved the final manuscript.

Acknowledgments

WWW, CLH, and KCL acknowledge the support by National Science Council through grants 2628-E-009-023-MY3, 101-2218-E-008-014-MY2, and 100-2628-E-006-025-MY2.

Author details

1_{Department of Materials Science and Engineering, National Chiao Tung} University, Hsinchu 300, Taiwan.2Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan. 3

Department of Electrical Engineering, National Central University, Tao Yuan 320, Taiwan.4_{Department of Physics, Tamkang University, New Taipei City} 25137, Taiwan.

Received: 10 May 2013 Accepted: 7 June 2013 Published: 19 June 2013

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doi:10.1186/1556-276X-8-290

Cite this article as: Chiu et al.: Single-crystallineδ-Ni2Si nanowires with

excellent physical properties. Nanoscale Research Letters 2013 8:290.

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