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Anisotropic electrical transport of flexible tungsten carbide nanostructures: towards nanoscale interconnects and electron emitters
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Download details: IP Address: 132.174.250.220 This content was downloaded on 30/08/2017 at 15:04 Manuscript version: Accepted Manuscript sun et al To cite this article before publication: sun et al, 2017, Nanotechnology, at press: https://doi.org/10.1088/1361-6528/aa87c3 This Accepted Manuscript is: © 2017 IOP Publishing Ltd During the embargo period (the 12 month period from the publication of the Version of Record of this article), the Accepted Manuscript is fully protected by copyright and cannot be reused or reposted elsewhere. As the Version of Record of this article is going to be / has been published on a subscription basis, this Accepted Manuscript is available for reuse under a CC BY-NC-ND 3.0 licence after the 12 month embargo period. After the embargo period, everyone is permitted to copy and redistribute this article for non-commercial purposes only, provided that they adhere to all the terms of the licence https://creativecommons.org/licences/by-nc-nd/3.0 Although reasonable endeavours have been taken to obtain all necessary permissions from third parties to include their copyrighted content within this article, their full citation and copyright line may not be present in this Accepted Manuscript version. Before using any content from this article, please refer to the Version of Record on IOPscience once published for full citation and copyright details, as permission will likely be required. All third party content is fully copyright protected, unless specifically stated otherwise in the figure caption in the Version of Record. When available, you can view the Version of Record for this article at: http://iopscience.iop.org/article/10.1088/1361-6528/aa87c3
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Anisotropic electrical transport of flexible tungsten carbide nanostructures: towards nanoscale interconnects and electron emitters
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Anisotropic electrical transport of flexible tungsten carbide nanostructures: towards nanoscale interconnects and electronemitters
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Bo Sun, Yong Sun* and Chengxin Wang*
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State Key Laboratory of Optoelectronic Materials and Technologies, School of Materials and Engineering, The Key Laboratory of Low-Carbon Chemistry & Energy Conservation of Guangdong Province, Sun Yat-Sen (Zhongshan)University, Guangzhou 510275, China
*Correspondence and requests for materials should be addressed to C. X. Wang and Y. Sun.
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Tel & Fax: +86-20-84113901; E-mail: [email protected]; [email protected]
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Abstract: Due to the coexistence of metal- and ionic-bonds in hexagonal tungsten carbide (WC)
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plane-expanded
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lattice, the disparate electrons behaviors were found in basal plane and along c-axial direction, which may create interesting anisotropic mechanical and electrical performance. To demonstrate this topic, the low-dimensional nanostructures like nanowires and nanosheets are suitable for investigation, because they usually grow in single crystals with special orientations. Herein, we report the experimental research about the anisotropic conductivity of [0001] grown WC nanowires and basal
7.86×103–1∙m–1 and 7.68×104–1∙m–1 respectively. This conforms to the fact that the highly localized W d state align along the c direction, while there is little intraplanar directional bonding in the W planes. With which
resulted
in
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nanosheets,
conductivity
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advanced micro-manipulation technology, the conductivity of a nanowire was tested approximately constant even under considerable bending state. Moreover, the field electron emission of WC was evaluated based on large area emission and single nanowire (nanosheet) emission. A single nanowire exhibits stable electrons emission performance, which can output emission currents > 3uA before fusing. These results provide useful references to assess low-dimensional WC nanostructures as
ce
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electronic materials in flexible devices, such as nanoscale interconnects and electron emitters.
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AUTHOR SUBMITTED MANUSCRIPT - NANO-114048.R2
AUTHOR SUBMITTED MANUSCRIPT - NANO-114048.R2 Anisotropic electrical transport of flexible tungsten carbide nanostructures: towards nanoscale interconnects and electron emitters
1
Introduction Tungsten carbide (WC) alloys see a wide range of use in different fields as special
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ceramics, hard alloys, metallic carbides, and other settings that utilize their favorable
4
mechanical properties [1-2]. The hard and brittle characteristics of WC lead to its frequent use
5
in cutting and drilling tools [3-4], and WC is also useful as a protective coating as it is an acid
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and alkali resistant and antioxidant material [5]. As discovered recently, it also has good
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performance as a catalyst in electrocatalysis field, owing to the Pt-like electron behavior
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arising from the contraction of the d-band electrons [6], making it a possible substitute for Pt
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in such processes. Metallic bonding in the basal plane also endows the material with excellent
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conductivity.
In terms of crystallography, h-WC can be considered as stacked regular hexahedrons,
12
with three adjacent W atoms composing the base plane and C atoms occupying the two vertex
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position along the [0001] direction, or as sandwich stacking layers of -W-C-W- [7]. Therein,
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anisotropic atomic bonds are dominant, i.e., metallic W-W bonds in the base plane and ionic
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W-C bonds perpendicular to those. As such, the electric properties of WC are unique
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compared to most other carbides [8]. However, these properties have rarely been investigated
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in WC materials, especially for low-dimensional nanostructures.
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an
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Here, we focus on the electronic properties of low-dimensional WC nanostructures for
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potential use in nanoscale electronics [9-10]. Besides their high conductivity (5.21×106Ω-1m-1),
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WC materials have other advantages such as outstanding physical and chemical stability and
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robust mechanical properties, implying that WC nanostructures can be used in complex and
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hard environments as interconnectors of nanoscale devices. On the other hand, flexibility is
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also present in their low-dimensional nanostructures, as hard and brittle materials can show
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novel mechanical performance including robust elasticity and plasticity. In this case, WC
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nanowires and nanobelts could be utilized for their high stability, excellent hardness, high
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Young’s modulus (450-650GPa) [11-12], considerable flexibility and high conductivity. Therefore, for flexible devicesthat undergo mechanical deformation (bending, stretching), the reversible elastic behavior of WC would be dominantover a wider strain amplitude
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(maximally tested ~20% of nanowires) [11] compared to pure metals such as Au, Ag, and Cu,
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owing to its large Young’s modulus and yield strength (~6GPa) [13-14]; irreversible plastic
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Anisotropic electrical transport of flexible tungsten carbide nanostructures: towards nanoscale interconnects and electron emitters
deformation and lattice destruction [15-16] that would degrade device performance can be
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avoided in this way. In this case, low-dimensional WC nanostructures are more stable under
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mechanical deformation in flexible devices, and both W and C are more widely available
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compared to the noble metals that often serve similar functions. Noticeably, the mechanical
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properties and antioxidant ability of WC is more prominent than that of tungsten [17], which
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itself is widely used in industrial electronemitters [18]. From this perspective, WC is also
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promising in vacuum micro-nano electronics as a cold cathode electron emitter [19].
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Herein, we investigated the electric properties of single WC nanostructures using advanced
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micro-nano manipulation technology. Conductivity, flexibility performance and field emission
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(FE) properties of single nanowires and nanosheets were examined. Remarkable anisotropic
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conductivity was found on WC nanostructures grown in different directions. Considerable
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large-current FE characteristics (~80mA/cm2) were also demonstrated. Moreover, both the
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nanosheets and nanowires exhibit outstanding bendability and fracture resistance, deviating
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from our common understanding of hard and brittle bulk WC. These findings can be used to
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assess the use of WC in electronics, especially for nanoscale flexible conductors and large
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current electronemitters.
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1. Experimental section
The synthesis of WC low-dimensional nanostructures: WC nanostructures were grown on
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tungsten substrate at 1310℃ with CH4 and H2 at flow of 5 sccm and 100 sccm respectively.
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The equipment used is home-made high temperature and vacuum horizontal furnace. The
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detailed introduction can be found in our previous reports [7]. Simply, Al source was used to
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alloy with tungsten at low temperature (the melting points of Al, ~660℃). After carbon being
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incorporated in at higher temperature (above 1000℃), W-Al-C ternary liquid alloy would
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form. The WC nanostructures grew via novel eutectic precipitation. The structural analysis
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was employed in HRTEM (FEI, F30 300 kV).
The in situ manipulation and test of nanowires: SEM, EBID (electron beam induced
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AUTHOR SUBMITTED MANUSCRIPT - NANO-114048.R2
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deposition), micro-manipulation and electrical measurement were carried out in dual-beam
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SEM/FIB working platform (Carl Zeiss, Auriga) integrated with GIS (gas injection system)
AUTHOR SUBMITTED MANUSCRIPT - NANO-114048.R2 Anisotropic electrical transport of flexible tungsten carbide nanostructures: towards nanoscale interconnects and electron emitters
and 3D moving W-probes. During the experiments, SE2 imaging mode was applied for SEM
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observation, including the content in Figure 2, 3, and 4. The vacuum degree of the chamber
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reached the level of ~10-7 torr. For Pt/C deposition, electron beam was accelerated by a 5kV
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voltage in high-current mode. As soon as the organo platinum gas released, the vacuum raised
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to 2×10-5 torr. The manipulation of a nanowire and nanosheet was processed using one or two
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W-probes linked to micro electric actuator, which can move along three dimensions with
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minimum step-size less than 100nm. The electrical test of two-electrode device was employed
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using two W-probes connected to a dual-channel system source meter (Keithley, 2634B).
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2. Results and discussion
Figure 1 shows high-resolution transmission electron microscopy (HRTEM) images of a
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nanowire and a nanosheet. Figure 1(a) shows the simulative lattice configuration as described
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previously, while a typical TEM image of a nanosheet is shown in Figure 1(b). Figure 1(c)
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and (d) correspond to the HRTEM image and selected area electron diffraction (SAED)
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pattern of a select nanosheet region, respectively. As evidenced in Figure 1(c), the nanosheet
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expanded in the basal plane according to the growth characteristics of hexagonal crystals,
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where the hexagonal lattice can be distinguished as marked using red hexagons. The grey and
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blue spheres represent C and W sites respectively, as shown in the inset of Figure 1(c). The
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typical indices of the crystal plane are marked in (d), with zone axis of [0001]. Noticeably, all
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of the 2D WC nanostructures possess the same growth orientation although their shapes are
20
distinctly different. Figure 1(e-g) show the lattice analysis of a nanowire grew along the
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c-axis direction with zone axis of [1010]. Figure 1(f) is the corresponding HRTEM image,
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where only one type of …ABAB… stacking layers could be distinguished. Figure 1(g) is the
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SAED pattern of the nanowire, indicating very high quality.
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A double-electrode configuration based on a nanobelt was used to test the electrical
ce
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properties of these materials, as illustrated in Figure 2(a). Ion-beam induced deposition (IBID) was used to deposit a layer of Pt/C at both ends (Figure 2(b)). Figure 2(c) shows the I-V curve obtained from in situ measurements with two W-tips as electrodes. There is an abrupt
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increase of conductivity at a voltage of 0.8V, which was attributed to removal of the oxide
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layer due to heating, as found in our previous research [7]. The inset shows the I-V curve
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Anisotropic electrical transport of flexible tungsten carbide nanostructures: towards nanoscale interconnects and electron emitters
measured in the low-voltage range, showing non-ohmic contact. Fusing of the nanobelt
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occurred when the current reached approximately 5mA. Figure 2(d) shows a SEM image of
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the device after fusing, in which one can see the broken site points to the locations untight to
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the substrate. Figure 2(e-f) are the AFM images that map the height profile of the nanosheet,
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which determine the dimension of the nanosheet is 15.56um (length) × 1.42um (width) ×
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66nm (thickness). The maximum current density of the nanobelt is Jmax=4.7×106A∙cm–2 which
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is comparable to copper nanowires [20]. The effective conductivity was calculated as
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7.68×104–1∙m–1. Prior to fusing, the conductivity is stable throughout the material, denoting
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excellent thermal conductance in the WC nanobelt. The I-V characteristics of the nanowire
10
were further investigated as illustrated in Figure 2(g). A deep trench was fabricated using a
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focused ion beam (FIB) to create a sharp step. One end of the nanowire was welded as shown
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in Figure 2(h) and the other end was fixed on a W-tip, which is a convenient form for
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bending measurements. The obtained I-V curve is shown in Figure 2(i), and the calculated
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conductivity of the nanowire is ~7.86×103–1∙m–1. There is one point should be demonstrated.
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Although the device geometries for test are quite different between nanowire and nanosheet, it
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would contribute tiny influence to the results, considering the contacts are metal-metal type.
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On the other hand, the W-tip used for nanowire test is not fresh, which is covered by a layer
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of Pt-C composite due to repeated usage. Therefore, we believe the I-V data of nanowire and
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nanosheet are feasible to compare. Noticeably, the electrical conductivity of the nanobelt is
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significantly higher than that of the nanowire, even though they have the same composition
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and crystal structure. As previously being established, metallic, ionic and covalent bond
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coexist within the WC lattice. The significant difference in conductivity between the nanowire
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and nanobelt can be attributed to the anisotropic electron transport introduced by the different
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growth directions of the nanowire and nanosheet. Considering the diverse bonding
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characteristics in the lattice along the different crystal orientations, the metallic W-W bond is
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dominant in the basal planes, while the W-C chains generate covalent and ionic bonding along the [0001] direction. The HRTEM images and the electron diffraction patterns in Figure 1 indicate that the growth orientation of the nanowire is (0001), while the nanobelts expanded
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AUTHOR SUBMITTED MANUSCRIPT - NANO-114048.R2
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in basal plane. D.V. Suetin and coworkers reported the electronic structure of h-WC using
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first-principles calculations [21], demonstrating that the charge density distribution is
AUTHOR SUBMITTED MANUSCRIPT - NANO-114048.R2 Anisotropic electrical transport of flexible tungsten carbide nanostructures: towards nanoscale interconnects and electron emitters
significantly different along the two different crystal orientations; Liu and Li respectively
2
reported similar results [22-23]. In detail, the highly localized W d state orients to form bonds
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in the c direction, while there is little intraplanar directional bonding in the W planes.
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Therefore, free electrons are inclined to move in the basal plane rather than along the c-axis.
cri pt
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Generally, the Peierls-Nabarro barrier can be decreased or even eliminated due to size
6
effects, and active dislocation migration is possible in low-dimensional nanostructures,
7
implying the possibility to realize considerable flexibility even in hard and brittle materials
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[24-25]. High bending strength and even plastic behavior have been observed in Si, SiC, ZnO
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[26-28]. Herein, the flexibility of WC nanostructures was assessed by in situ bending tests
10
with W-tips linked to a micro-electric actuator attached in a crossed dual-beam platform.
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Notably, despite the rigid characteristics of the bulk material, both WC nanowires and
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nanosheets featured outstanding bendability, as shown in Figure 3. Bending strains as large as
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~7.5% were achieved in the WC nanowire, as shown in Figure 3(a1-a3).The strain value is
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calculated as
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(1)
dM
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where is the bending strain of the nanowire, r is the nanowire diameter and R is the
17
curvature radius of the bent nanowire. This means a nanowire 105nm in diameter can be bent
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into a circle with a maximum curvature of 0.71m–1, which is high enough for practical use in
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flexible devices; the nanowire did not exhibit any fracturing after being bent. A similar
20
manipulation was also used on 2D nanostructures, where remarkable strain was also achieved
21
in a nanobelt (with an aspect ratio of ~3) and a nanosheet. Figure 3(b1-b6) shows a
22
nanosheet folded in half. Although a precise value cannot be determined here, this strain
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exceeds the requirements of flexible devices. Noticeably, the folded nanosheet recovers
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completely after the W-tip moves away. It is necessary to combine two W-tips to carry out the
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manipulation, considering the interaction between the nanosheet and the substrate. Although a nanosheet adheres to the substrate tightly, as soon as a W-tip lever a corner, the combination become weaker (Figure 3(b2)), which seems impossible to rebuilt. The nanosheet would
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move away if it was bent with only one single W-tip. Therefore, another W-tip was used to
29
pin the nanosheet at a corner as shown in Figure 3. Due to the rigid characteristic of WC and
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Anisotropic electrical transport of flexible tungsten carbide nanostructures: towards nanoscale interconnects and electron emitters
the smooth surface of the substrate, the bent nanosheet has a large risk to bounce away in
2
vacuum chamber, which frequently occurred in the experiments. However, as long as the
3
nanosheet was bent successfully, it would be easy to recover if the W-tip withdrawn, because
4
there is no external force in normal direction to the substrate. Figure 3(c1-c6) display images
5
of another nanobelt undergoing a similar bending process. As shown in Figure 3(c5), the
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nanobelt remained intact even after being folded in three. The shape finally fractured when
7
the W-tip further increased the strain, as in Figure 3(c6). It is worth mentioning that residual
8
plastic deformation was observed in one bending region, instead of pure elastic recovery. The
9
possibility of elasticity could be excluded because the shape of the nanobelt remained
10
reversibly deformed even after long periods [29]. This phenomenon indicates that irreversible
11
deformation can occur in WC nanostructures under ultra-high strain, contradicting our
12
understanding that hard alloy materials are inclined to fracture even under very low strain. As
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shown in Figure 3(b2, b5, c5), alternating stripes with varying contrast were observed in the
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sample. This feature may be resulted from the local stress relation in the basal plane during
15
the bending operation. These WC nanostructures have great prospects for use in flexible
16
electronics, based on their novel electrical and mechanical properties.
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Based on the outstanding flexibility and novel electron-transport properties of the structures,
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we investigated the I-V characteristics of a WC nanowire under moderate bending strain in
19
order to evaluate theirfuture behavior in nanoscale flexible conductorsor other integrated
20
devices. The test configuration is as illustrated as Figure 2(e), where the strain was applied
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using the connected W-tip. Noticeably, the specific strain could not be precisely determined in
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this way, as the nanowire was operated suspended without a rigid plane to restrict it. Here,
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two different nanowires were investigated as shown in Figure 4. The I-V curves of each
24
nanowire were measured under different bending strains and were well-matched to each other,
25
indicating that the nanowires have considerable stable electrical transport under moderate
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deformation. Even under a significantly complex 3D bending strain with an obvious larger strain (Figure 4(b)), the discrepancy among the three curves is still less than 10%. The high conductivity and stable I-V performance represent an advantage of WC nanowires for use as
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AUTHOR SUBMITTED MANUSCRIPT - NANO-114048.R2
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flexible nanoscale interconnects. Field emission is an important feature of nanostructures, and greatly expands the possible
AUTHOR SUBMITTED MANUSCRIPT - NANO-114048.R2 Anisotropic electrical transport of flexible tungsten carbide nanostructures: towards nanoscale interconnects and electron emitters
application space in field electron displays and vacuum microelectronic devices. There are
2
numerous works regarding field emission properties of various low-dimensional
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nanomaterials [30-31]. Based on theoretical analysis and experimental investigation, there are
4
several factors that influence field emission performance: (1) work function of the emitter
5
surface, (2) the radius of curvature of the emitter apex, (3) the effective emission area and (4)
6
the electric pathway of the emitter. The emission properties will be limited if any of these
7
factorsare insufficient. WC is intrinsically expected to be a field emission material with
8
metallic conductivity behavior. However, its field emission behavior has not been widely
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conducted due to the difficulty of synthesizing low-dimensional WC nanostructures. Here, we
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carried out field emission characterization of the as-synthesized samples after successfully
11
obtaining low-dimensional WC nanostructures.
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The field emission J-V characteristic of the as-synthesized sample was measured in a
13
vacuum chamber at a pressure of 10–7 Torr at room temperature. Column-shaped
14
stainless-steel probes were used as the anode. The fresh sample used as the cathode was
15
fastened to a Cu plate using double-sided conductive carbon tape [32-33]. In the measuring
16
circuit, the emission current was directly determined using a Keithley 6485 apparatus. The
17
distance between the tip and the cathode was controlled by a digital micrometer controller.
18
The J-E plots are shown in Figure 5(a), from which the turn-on field and threshold field can
19
be determined as 13.5V∙m–1 and 24.5V∙m–1 respectively. Over the provided voltage range,
20
we recorded current density values as high as 80 mA∙cm–2, which could be further improved
21
under higher electric fields. This takes advantage of the excellent electric pathway during
22
electron emission. According to the typical Fowler-Nordheim (FN) theory, the emission
23
current J can be expressed as [34]:
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an
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3 J A E / exp B 2 / E 2
2
where A=1.54×10–10 in units of A (eV) V–2, B=6.83×107 in units of (eV)
(2) −3/2
V∙cm−1, is the
field enhancement factor, E is the applied field (E=V/d), and is the effect work function of the emission tip, which is 4.9 eV for WC [35]. By plotting ln(J/E2) versus 1/E, traditional FN
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plots can be obtained (Figure5(b)). The well-fitting linear FN relationship indicates the
29
electric field induced electron emission dominates, with no contribution from thermionic
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Anisotropic electrical transport of flexible tungsten carbide nanostructures: towards nanoscale interconnects and electron emitters
effect. According to the above parameters, is calculated to be 120. This results in a higher
2
threshold field (24.5 V∙m–1) than being expected, owing to restrictions from the density of
3
emission sites and the value. Comparing to tungsten (W) and LaB6 nanostructures, WC
4
low-dimensional nanostructures displays higher turn-on fields. However, it seems
5
advantageous in large-current emission performance. For example, Bando et al. reported that
6
W whiskers exhibits turn-on field of 4.0V/um [36]. However, the maximum emission current
7
was lower than 2.5mA/cm2. Lee et al. and Baek et al. reported the emission performance of
8
W respectively, which provided turn-on field of 4.2V/um and 6.2V/um [37-38]. However, the
9
maximum current obtained is also far lower than the threshold value we defined here
10
(10mA/cm2). For LaB6 nanowires, Qin et al investigated the emission performance of a
11
140nm thick nanowire, obtaining maximum emission current of 30-35nA [39]. Zhai et al
12
fabricated LaB6 nanowires array, and reported the turn-on field as low as 1.82 V/um with
13
maximum current below 10mA/cm2 [40]. Field emission applications would benefit greatly
14
from improvements to this value.
an
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We also investigated the FE performance of WC nanowires and nanosheets based on a
16
single nano-tip emitter. These experiments were carried out in high-vacuum SEM chamber
17
using a W-tip as the electron collector, as illustrated in Figure 5(c). A Keithley 2634B served
18
as the two-channel voltage source and amperemeter. The W-tip was first contacted to the
19
nanowire to determine the resistance of the circuit, after which a typical I-V curve was
20
recorded and the resistance of the circuit was determined as 10 k. This process also removes
21
any oxidation layer via heating. The W-tip was then retracted from the nanowire and the
22
tunneling current was recorded over a voltage sweep. Several different nanowires were tested
23
and two typical curves are provided in Figure 5(d); both curves indicate micro-ampere
24
emission current levels on the same order of magnitude as individual molybdenum
25
nanoscrews [41]. The current increased sharply when the voltage reached 104V and was
27 28
pte
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2.5A at voltages ~3A before fusing, owing to its
18
good conductivity and thermal conductivity. These properties and their outstanding stability
19
make such WC nanostructures worthy candidate as cold cathode electron emitters.
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Anisotropic electrical transport of flexible tungsten carbide nanostructures: towards nanoscale interconnects and electron emitters
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Fracture-ResistanceInvestigation of Tungsten Carbide Nanowires small Small DOI:
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AUTHOR SUBMITTED MANUSCRIPT - NANO-114048.R2 Anisotropic electrical transport of flexible tungsten carbide nanostructures: towards nanoscale interconnects and electron emitters
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Nanothorn Arrays on WO3 Nanowhiskers Adv. Mater. 18 3105-3110
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Hexaboride Nanowire Arrays and Investigation of their Field-Emission Behaviors NPG Asial
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[41]Shen Y, Xu N, Deng S, Zhang Y, Liu F, and Chen J 2014 A Mo nanoscrew formed by
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crystalline Mo grains with high conductivity and excellent field emission properties
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Nanoscale 6 4659–68
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Acknowledgements This work was supported by the NSF of China (Grant No.U1401241 and 51502353). We acknowledge the technical supports from Phd. Cairong Ding (State Key Laboratory of Optoelectronic Materials and Technologies, Sun Yat-sen Univ.).
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Anisotropic electrical transport of flexible tungsten carbide nanostructures: towards nanoscale interconnects and electron emitters
Figure captions
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Figure 1. HRTEM characterization of WC nanowires and nanosheets. (a) Lattice diagram of
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h-WC. (b) TEM image of a WC nanosheet. (c) HRTEM image of a WC nanosheet. (d) SAED
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pattern of the nanosheet. (e) TEM image of a WC nanowire. (f) HRTEM image of a WC
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nanowire. (g) SAED pattern of a WC nanowire. The scale bars in (c, f) is 1nm.
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Figure 2.Electric transport properties of WC nanowires and nanosheets. (a) Test configuration
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of WC nanosheet. (b) Double-electrode device utilizing a nanosheet. (c) I-V characteristic of
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the nanosheet. (d) SEM image of the nanosheet after fusing. (e) AFM image of the nanosheet
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after fusing. (f) The fine height profile of piece of the nanosheet. (g) the schematic description
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of the test configuration of a nanowire. (h) the real contacts for test. (i)TheI-V curve of the
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nanowire.
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Figure 3. Flexibility investigations of the WC nanowire and nanosheet. (a1-a3) Bending
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process of a WC nanowire.The scale bar is 1um. (b1-b6) Bending process of a WC nanosheet
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with elastic behavior. (c1-c6) Flexibility of a nanosheet folded in three. The scale bars in
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(b1-c6) are 2 um.
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Figure 4. Flexible WC nanoscale conductor. (a,b)I-V investigation of two WC nanowires
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under moderate strain,with SEM images of the nanowires at different bending states. The
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scale bars 1um.
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Figure 5. FE performance of low-dimensional WC nanostructures. (a) J-E dependence of WC
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nanosheet. (b) FN curve of the emission characteristics. (c) Schematic of the FE investigation
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method using a single emission tip. (d) Emission performance of nanowires, with the relation
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between I/V2 and 1/V in the inset. (e) Emission performance of WC nanosheet, with the
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relation between I/V2 and 1/V in the inset.
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Figure 1
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Anisotropic electrical transport of flexible tungsten carbide nanostructures: towards nanoscale interconnects and electron emitters
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Anisotropic electrical transport of flexible tungsten carbide nanostructures: towards nanoscale interconnects and electron emitters
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Download details: IP Address: 132.174.250.220 This content was downloaded on 30/08/2017 at 15:04 Manuscript version: Accepted Manuscript sun et al To cite this article before publication: sun et al, 2017, Nanotechnology, at press: https://doi.org/10.1088/1361-6528/aa87c3 This Accepted Manuscript is: © 2017 IOP Publishing Ltd During the embargo period (the 12 month period from the publication of the Version of Record of this article), the Accepted Manuscript is fully protected by copyright and cannot be reused or reposted elsewhere. As the Version of Record of this article is going to be / has been published on a subscription basis, this Accepted Manuscript is available for reuse under a CC BY-NC-ND 3.0 licence after the 12 month embargo period. After the embargo period, everyone is permitted to copy and redistribute this article for non-commercial purposes only, provided that they adhere to all the terms of the licence https://creativecommons.org/licences/by-nc-nd/3.0 Although reasonable endeavours have been taken to obtain all necessary permissions from third parties to include their copyrighted content within this article, their full citation and copyright line may not be present in this Accepted Manuscript version. Before using any content from this article, please refer to the Version of Record on IOPscience once published for full citation and copyright details, as permission will likely be required. All third party content is fully copyright protected, unless specifically stated otherwise in the figure caption in the Version of Record. When available, you can view the Version of Record for this article at: http://iopscience.iop.org/article/10.1088/1361-6528/aa87c3
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Anisotropic electrical transport of flexible tungsten carbide nanostructures: towards nanoscale interconnects and electron emitters
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Anisotropic electrical transport of flexible tungsten carbide nanostructures: towards nanoscale interconnects and electronemitters
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Bo Sun, Yong Sun* and Chengxin Wang*
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State Key Laboratory of Optoelectronic Materials and Technologies, School of Materials and Engineering, The Key Laboratory of Low-Carbon Chemistry & Energy Conservation of Guangdong Province, Sun Yat-Sen (Zhongshan)University, Guangzhou 510275, China
*Correspondence and requests for materials should be addressed to C. X. Wang and Y. Sun.
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Tel & Fax: +86-20-84113901; E-mail: [email protected]; [email protected]
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Abstract: Due to the coexistence of metal- and ionic-bonds in hexagonal tungsten carbide (WC)
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plane-expanded
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lattice, the disparate electrons behaviors were found in basal plane and along c-axial direction, which may create interesting anisotropic mechanical and electrical performance. To demonstrate this topic, the low-dimensional nanostructures like nanowires and nanosheets are suitable for investigation, because they usually grow in single crystals with special orientations. Herein, we report the experimental research about the anisotropic conductivity of [0001] grown WC nanowires and basal
7.86×103–1∙m–1 and 7.68×104–1∙m–1 respectively. This conforms to the fact that the highly localized W d state align along the c direction, while there is little intraplanar directional bonding in the W planes. With which
resulted
in
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nanosheets,
conductivity
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advanced micro-manipulation technology, the conductivity of a nanowire was tested approximately constant even under considerable bending state. Moreover, the field electron emission of WC was evaluated based on large area emission and single nanowire (nanosheet) emission. A single nanowire exhibits stable electrons emission performance, which can output emission currents > 3uA before fusing. These results provide useful references to assess low-dimensional WC nanostructures as
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electronic materials in flexible devices, such as nanoscale interconnects and electron emitters.
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AUTHOR SUBMITTED MANUSCRIPT - NANO-114048.R2 Anisotropic electrical transport of flexible tungsten carbide nanostructures: towards nanoscale interconnects and electron emitters
1
Introduction Tungsten carbide (WC) alloys see a wide range of use in different fields as special
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ceramics, hard alloys, metallic carbides, and other settings that utilize their favorable
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mechanical properties [1-2]. The hard and brittle characteristics of WC lead to its frequent use
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in cutting and drilling tools [3-4], and WC is also useful as a protective coating as it is an acid
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and alkali resistant and antioxidant material [5]. As discovered recently, it also has good
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performance as a catalyst in electrocatalysis field, owing to the Pt-like electron behavior
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arising from the contraction of the d-band electrons [6], making it a possible substitute for Pt
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in such processes. Metallic bonding in the basal plane also endows the material with excellent
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conductivity.
In terms of crystallography, h-WC can be considered as stacked regular hexahedrons,
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with three adjacent W atoms composing the base plane and C atoms occupying the two vertex
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position along the [0001] direction, or as sandwich stacking layers of -W-C-W- [7]. Therein,
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anisotropic atomic bonds are dominant, i.e., metallic W-W bonds in the base plane and ionic
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W-C bonds perpendicular to those. As such, the electric properties of WC are unique
16
compared to most other carbides [8]. However, these properties have rarely been investigated
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in WC materials, especially for low-dimensional nanostructures.
dM
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Here, we focus on the electronic properties of low-dimensional WC nanostructures for
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potential use in nanoscale electronics [9-10]. Besides their high conductivity (5.21×106Ω-1m-1),
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WC materials have other advantages such as outstanding physical and chemical stability and
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robust mechanical properties, implying that WC nanostructures can be used in complex and
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hard environments as interconnectors of nanoscale devices. On the other hand, flexibility is
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also present in their low-dimensional nanostructures, as hard and brittle materials can show
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novel mechanical performance including robust elasticity and plasticity. In this case, WC
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nanowires and nanobelts could be utilized for their high stability, excellent hardness, high
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Young’s modulus (450-650GPa) [11-12], considerable flexibility and high conductivity. Therefore, for flexible devicesthat undergo mechanical deformation (bending, stretching), the reversible elastic behavior of WC would be dominantover a wider strain amplitude
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(maximally tested ~20% of nanowires) [11] compared to pure metals such as Au, Ag, and Cu,
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owing to its large Young’s modulus and yield strength (~6GPa) [13-14]; irreversible plastic
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Anisotropic electrical transport of flexible tungsten carbide nanostructures: towards nanoscale interconnects and electron emitters
deformation and lattice destruction [15-16] that would degrade device performance can be
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avoided in this way. In this case, low-dimensional WC nanostructures are more stable under
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mechanical deformation in flexible devices, and both W and C are more widely available
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compared to the noble metals that often serve similar functions. Noticeably, the mechanical
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properties and antioxidant ability of WC is more prominent than that of tungsten [17], which
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itself is widely used in industrial electronemitters [18]. From this perspective, WC is also
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promising in vacuum micro-nano electronics as a cold cathode electron emitter [19].
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Herein, we investigated the electric properties of single WC nanostructures using advanced
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micro-nano manipulation technology. Conductivity, flexibility performance and field emission
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(FE) properties of single nanowires and nanosheets were examined. Remarkable anisotropic
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conductivity was found on WC nanostructures grown in different directions. Considerable
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large-current FE characteristics (~80mA/cm2) were also demonstrated. Moreover, both the
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nanosheets and nanowires exhibit outstanding bendability and fracture resistance, deviating
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from our common understanding of hard and brittle bulk WC. These findings can be used to
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assess the use of WC in electronics, especially for nanoscale flexible conductors and large
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current electronemitters.
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1. Experimental section
The synthesis of WC low-dimensional nanostructures: WC nanostructures were grown on
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tungsten substrate at 1310℃ with CH4 and H2 at flow of 5 sccm and 100 sccm respectively.
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The equipment used is home-made high temperature and vacuum horizontal furnace. The
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detailed introduction can be found in our previous reports [7]. Simply, Al source was used to
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alloy with tungsten at low temperature (the melting points of Al, ~660℃). After carbon being
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incorporated in at higher temperature (above 1000℃), W-Al-C ternary liquid alloy would
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form. The WC nanostructures grew via novel eutectic precipitation. The structural analysis
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was employed in HRTEM (FEI, F30 300 kV).
The in situ manipulation and test of nanowires: SEM, EBID (electron beam induced
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deposition), micro-manipulation and electrical measurement were carried out in dual-beam
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SEM/FIB working platform (Carl Zeiss, Auriga) integrated with GIS (gas injection system)
AUTHOR SUBMITTED MANUSCRIPT - NANO-114048.R2 Anisotropic electrical transport of flexible tungsten carbide nanostructures: towards nanoscale interconnects and electron emitters
and 3D moving W-probes. During the experiments, SE2 imaging mode was applied for SEM
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observation, including the content in Figure 2, 3, and 4. The vacuum degree of the chamber
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reached the level of ~10-7 torr. For Pt/C deposition, electron beam was accelerated by a 5kV
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voltage in high-current mode. As soon as the organo platinum gas released, the vacuum raised
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to 2×10-5 torr. The manipulation of a nanowire and nanosheet was processed using one or two
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W-probes linked to micro electric actuator, which can move along three dimensions with
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minimum step-size less than 100nm. The electrical test of two-electrode device was employed
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using two W-probes connected to a dual-channel system source meter (Keithley, 2634B).
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2. Results and discussion
Figure 1 shows high-resolution transmission electron microscopy (HRTEM) images of a
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nanowire and a nanosheet. Figure 1(a) shows the simulative lattice configuration as described
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previously, while a typical TEM image of a nanosheet is shown in Figure 1(b). Figure 1(c)
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and (d) correspond to the HRTEM image and selected area electron diffraction (SAED)
14
pattern of a select nanosheet region, respectively. As evidenced in Figure 1(c), the nanosheet
15
expanded in the basal plane according to the growth characteristics of hexagonal crystals,
16
where the hexagonal lattice can be distinguished as marked using red hexagons. The grey and
17
blue spheres represent C and W sites respectively, as shown in the inset of Figure 1(c). The
18
typical indices of the crystal plane are marked in (d), with zone axis of [0001]. Noticeably, all
19
of the 2D WC nanostructures possess the same growth orientation although their shapes are
20
distinctly different. Figure 1(e-g) show the lattice analysis of a nanowire grew along the
21
c-axis direction with zone axis of [1010]. Figure 1(f) is the corresponding HRTEM image,
22
where only one type of …ABAB… stacking layers could be distinguished. Figure 1(g) is the
23
SAED pattern of the nanowire, indicating very high quality.
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A double-electrode configuration based on a nanobelt was used to test the electrical
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10
properties of these materials, as illustrated in Figure 2(a). Ion-beam induced deposition (IBID) was used to deposit a layer of Pt/C at both ends (Figure 2(b)). Figure 2(c) shows the I-V curve obtained from in situ measurements with two W-tips as electrodes. There is an abrupt
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28
increase of conductivity at a voltage of 0.8V, which was attributed to removal of the oxide
29
layer due to heating, as found in our previous research [7]. The inset shows the I-V curve
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Anisotropic electrical transport of flexible tungsten carbide nanostructures: towards nanoscale interconnects and electron emitters
measured in the low-voltage range, showing non-ohmic contact. Fusing of the nanobelt
2
occurred when the current reached approximately 5mA. Figure 2(d) shows a SEM image of
3
the device after fusing, in which one can see the broken site points to the locations untight to
4
the substrate. Figure 2(e-f) are the AFM images that map the height profile of the nanosheet,
5
which determine the dimension of the nanosheet is 15.56um (length) × 1.42um (width) ×
6
66nm (thickness). The maximum current density of the nanobelt is Jmax=4.7×106A∙cm–2 which
7
is comparable to copper nanowires [20]. The effective conductivity was calculated as
8
7.68×104–1∙m–1. Prior to fusing, the conductivity is stable throughout the material, denoting
9
excellent thermal conductance in the WC nanobelt. The I-V characteristics of the nanowire
10
were further investigated as illustrated in Figure 2(g). A deep trench was fabricated using a
11
focused ion beam (FIB) to create a sharp step. One end of the nanowire was welded as shown
12
in Figure 2(h) and the other end was fixed on a W-tip, which is a convenient form for
13
bending measurements. The obtained I-V curve is shown in Figure 2(i), and the calculated
14
conductivity of the nanowire is ~7.86×103–1∙m–1. There is one point should be demonstrated.
15
Although the device geometries for test are quite different between nanowire and nanosheet, it
16
would contribute tiny influence to the results, considering the contacts are metal-metal type.
17
On the other hand, the W-tip used for nanowire test is not fresh, which is covered by a layer
18
of Pt-C composite due to repeated usage. Therefore, we believe the I-V data of nanowire and
19
nanosheet are feasible to compare. Noticeably, the electrical conductivity of the nanobelt is
20
significantly higher than that of the nanowire, even though they have the same composition
21
and crystal structure. As previously being established, metallic, ionic and covalent bond
22
coexist within the WC lattice. The significant difference in conductivity between the nanowire
23
and nanobelt can be attributed to the anisotropic electron transport introduced by the different
24
growth directions of the nanowire and nanosheet. Considering the diverse bonding
25
characteristics in the lattice along the different crystal orientations, the metallic W-W bond is
27 28
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1
dominant in the basal planes, while the W-C chains generate covalent and ionic bonding along the [0001] direction. The HRTEM images and the electron diffraction patterns in Figure 1 indicate that the growth orientation of the nanowire is (0001), while the nanobelts expanded
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AUTHOR SUBMITTED MANUSCRIPT - NANO-114048.R2
29
in basal plane. D.V. Suetin and coworkers reported the electronic structure of h-WC using
30
first-principles calculations [21], demonstrating that the charge density distribution is
AUTHOR SUBMITTED MANUSCRIPT - NANO-114048.R2 Anisotropic electrical transport of flexible tungsten carbide nanostructures: towards nanoscale interconnects and electron emitters
significantly different along the two different crystal orientations; Liu and Li respectively
2
reported similar results [22-23]. In detail, the highly localized W d state orients to form bonds
3
in the c direction, while there is little intraplanar directional bonding in the W planes.
4
Therefore, free electrons are inclined to move in the basal plane rather than along the c-axis.
cri pt
1
Generally, the Peierls-Nabarro barrier can be decreased or even eliminated due to size
6
effects, and active dislocation migration is possible in low-dimensional nanostructures,
7
implying the possibility to realize considerable flexibility even in hard and brittle materials
8
[24-25]. High bending strength and even plastic behavior have been observed in Si, SiC, ZnO
9
[26-28]. Herein, the flexibility of WC nanostructures was assessed by in situ bending tests
10
with W-tips linked to a micro-electric actuator attached in a crossed dual-beam platform.
11
Notably, despite the rigid characteristics of the bulk material, both WC nanowires and
12
nanosheets featured outstanding bendability, as shown in Figure 3. Bending strains as large as
13
~7.5% were achieved in the WC nanowire, as shown in Figure 3(a1-a3).The strain value is
14
calculated as
an
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(1)
dM
15
where is the bending strain of the nanowire, r is the nanowire diameter and R is the
17
curvature radius of the bent nanowire. This means a nanowire 105nm in diameter can be bent
18
into a circle with a maximum curvature of 0.71m–1, which is high enough for practical use in
19
flexible devices; the nanowire did not exhibit any fracturing after being bent. A similar
20
manipulation was also used on 2D nanostructures, where remarkable strain was also achieved
21
in a nanobelt (with an aspect ratio of ~3) and a nanosheet. Figure 3(b1-b6) shows a
22
nanosheet folded in half. Although a precise value cannot be determined here, this strain
23
exceeds the requirements of flexible devices. Noticeably, the folded nanosheet recovers
24
completely after the W-tip moves away. It is necessary to combine two W-tips to carry out the
26 27
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manipulation, considering the interaction between the nanosheet and the substrate. Although a nanosheet adheres to the substrate tightly, as soon as a W-tip lever a corner, the combination become weaker (Figure 3(b2)), which seems impossible to rebuilt. The nanosheet would
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Page 6 of 20
28
move away if it was bent with only one single W-tip. Therefore, another W-tip was used to
29
pin the nanosheet at a corner as shown in Figure 3. Due to the rigid characteristic of WC and
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Anisotropic electrical transport of flexible tungsten carbide nanostructures: towards nanoscale interconnects and electron emitters
the smooth surface of the substrate, the bent nanosheet has a large risk to bounce away in
2
vacuum chamber, which frequently occurred in the experiments. However, as long as the
3
nanosheet was bent successfully, it would be easy to recover if the W-tip withdrawn, because
4
there is no external force in normal direction to the substrate. Figure 3(c1-c6) display images
5
of another nanobelt undergoing a similar bending process. As shown in Figure 3(c5), the
6
nanobelt remained intact even after being folded in three. The shape finally fractured when
7
the W-tip further increased the strain, as in Figure 3(c6). It is worth mentioning that residual
8
plastic deformation was observed in one bending region, instead of pure elastic recovery. The
9
possibility of elasticity could be excluded because the shape of the nanobelt remained
10
reversibly deformed even after long periods [29]. This phenomenon indicates that irreversible
11
deformation can occur in WC nanostructures under ultra-high strain, contradicting our
12
understanding that hard alloy materials are inclined to fracture even under very low strain. As
13
shown in Figure 3(b2, b5, c5), alternating stripes with varying contrast were observed in the
14
sample. This feature may be resulted from the local stress relation in the basal plane during
15
the bending operation. These WC nanostructures have great prospects for use in flexible
16
electronics, based on their novel electrical and mechanical properties.
dM
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Based on the outstanding flexibility and novel electron-transport properties of the structures,
18
we investigated the I-V characteristics of a WC nanowire under moderate bending strain in
19
order to evaluate theirfuture behavior in nanoscale flexible conductorsor other integrated
20
devices. The test configuration is as illustrated as Figure 2(e), where the strain was applied
21
using the connected W-tip. Noticeably, the specific strain could not be precisely determined in
22
this way, as the nanowire was operated suspended without a rigid plane to restrict it. Here,
23
two different nanowires were investigated as shown in Figure 4. The I-V curves of each
24
nanowire were measured under different bending strains and were well-matched to each other,
25
indicating that the nanowires have considerable stable electrical transport under moderate
27 28
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deformation. Even under a significantly complex 3D bending strain with an obvious larger strain (Figure 4(b)), the discrepancy among the three curves is still less than 10%. The high conductivity and stable I-V performance represent an advantage of WC nanowires for use as
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29 30
flexible nanoscale interconnects. Field emission is an important feature of nanostructures, and greatly expands the possible
AUTHOR SUBMITTED MANUSCRIPT - NANO-114048.R2 Anisotropic electrical transport of flexible tungsten carbide nanostructures: towards nanoscale interconnects and electron emitters
application space in field electron displays and vacuum microelectronic devices. There are
2
numerous works regarding field emission properties of various low-dimensional
3
nanomaterials [30-31]. Based on theoretical analysis and experimental investigation, there are
4
several factors that influence field emission performance: (1) work function of the emitter
5
surface, (2) the radius of curvature of the emitter apex, (3) the effective emission area and (4)
6
the electric pathway of the emitter. The emission properties will be limited if any of these
7
factorsare insufficient. WC is intrinsically expected to be a field emission material with
8
metallic conductivity behavior. However, its field emission behavior has not been widely
9
conducted due to the difficulty of synthesizing low-dimensional WC nanostructures. Here, we
10
carried out field emission characterization of the as-synthesized samples after successfully
11
obtaining low-dimensional WC nanostructures.
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1
The field emission J-V characteristic of the as-synthesized sample was measured in a
13
vacuum chamber at a pressure of 10–7 Torr at room temperature. Column-shaped
14
stainless-steel probes were used as the anode. The fresh sample used as the cathode was
15
fastened to a Cu plate using double-sided conductive carbon tape [32-33]. In the measuring
16
circuit, the emission current was directly determined using a Keithley 6485 apparatus. The
17
distance between the tip and the cathode was controlled by a digital micrometer controller.
18
The J-E plots are shown in Figure 5(a), from which the turn-on field and threshold field can
19
be determined as 13.5V∙m–1 and 24.5V∙m–1 respectively. Over the provided voltage range,
20
we recorded current density values as high as 80 mA∙cm–2, which could be further improved
21
under higher electric fields. This takes advantage of the excellent electric pathway during
22
electron emission. According to the typical Fowler-Nordheim (FN) theory, the emission
23
current J can be expressed as [34]:
25 26 27
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an
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3 J A E / exp B 2 / E 2
2
where A=1.54×10–10 in units of A (eV) V–2, B=6.83×107 in units of (eV)
(2) −3/2
V∙cm−1, is the
field enhancement factor, E is the applied field (E=V/d), and is the effect work function of the emission tip, which is 4.9 eV for WC [35]. By plotting ln(J/E2) versus 1/E, traditional FN
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Page 8 of 20
28
plots can be obtained (Figure5(b)). The well-fitting linear FN relationship indicates the
29
electric field induced electron emission dominates, with no contribution from thermionic
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Anisotropic electrical transport of flexible tungsten carbide nanostructures: towards nanoscale interconnects and electron emitters
effect. According to the above parameters, is calculated to be 120. This results in a higher
2
threshold field (24.5 V∙m–1) than being expected, owing to restrictions from the density of
3
emission sites and the value. Comparing to tungsten (W) and LaB6 nanostructures, WC
4
low-dimensional nanostructures displays higher turn-on fields. However, it seems
5
advantageous in large-current emission performance. For example, Bando et al. reported that
6
W whiskers exhibits turn-on field of 4.0V/um [36]. However, the maximum emission current
7
was lower than 2.5mA/cm2. Lee et al. and Baek et al. reported the emission performance of
8
W respectively, which provided turn-on field of 4.2V/um and 6.2V/um [37-38]. However, the
9
maximum current obtained is also far lower than the threshold value we defined here
10
(10mA/cm2). For LaB6 nanowires, Qin et al investigated the emission performance of a
11
140nm thick nanowire, obtaining maximum emission current of 30-35nA [39]. Zhai et al
12
fabricated LaB6 nanowires array, and reported the turn-on field as low as 1.82 V/um with
13
maximum current below 10mA/cm2 [40]. Field emission applications would benefit greatly
14
from improvements to this value.
an
us
cri pt
1
We also investigated the FE performance of WC nanowires and nanosheets based on a
16
single nano-tip emitter. These experiments were carried out in high-vacuum SEM chamber
17
using a W-tip as the electron collector, as illustrated in Figure 5(c). A Keithley 2634B served
18
as the two-channel voltage source and amperemeter. The W-tip was first contacted to the
19
nanowire to determine the resistance of the circuit, after which a typical I-V curve was
20
recorded and the resistance of the circuit was determined as 10 k. This process also removes
21
any oxidation layer via heating. The W-tip was then retracted from the nanowire and the
22
tunneling current was recorded over a voltage sweep. Several different nanowires were tested
23
and two typical curves are provided in Figure 5(d); both curves indicate micro-ampere
24
emission current levels on the same order of magnitude as individual molybdenum
25
nanoscrews [41]. The current increased sharply when the voltage reached 104V and was
27 28
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dM
15
2.5A at voltages ~3A before fusing, owing to its
18
good conductivity and thermal conductivity. These properties and their outstanding stability
19
make such WC nanostructures worthy candidate as cold cathode electron emitters.
dM
pte ce
20
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10
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Anisotropic electrical transport of flexible tungsten carbide nanostructures: towards nanoscale interconnects and electron emitters
References
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Acknowledgements This work was supported by the NSF of China (Grant No.U1401241 and 51502353). We acknowledge the technical supports from Phd. Cairong Ding (State Key Laboratory of Optoelectronic Materials and Technologies, Sun Yat-sen Univ.).
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Adv. Mater. 18 87–91
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Anisotropic electrical transport of flexible tungsten carbide nanostructures: towards nanoscale interconnects and electron emitters
Figure captions
2
Figure 1. HRTEM characterization of WC nanowires and nanosheets. (a) Lattice diagram of
3
h-WC. (b) TEM image of a WC nanosheet. (c) HRTEM image of a WC nanosheet. (d) SAED
4
pattern of the nanosheet. (e) TEM image of a WC nanowire. (f) HRTEM image of a WC
5
nanowire. (g) SAED pattern of a WC nanowire. The scale bars in (c, f) is 1nm.
6
Figure 2.Electric transport properties of WC nanowires and nanosheets. (a) Test configuration
7
of WC nanosheet. (b) Double-electrode device utilizing a nanosheet. (c) I-V characteristic of
8
the nanosheet. (d) SEM image of the nanosheet after fusing. (e) AFM image of the nanosheet
9
after fusing. (f) The fine height profile of piece of the nanosheet. (g) the schematic description
10
of the test configuration of a nanowire. (h) the real contacts for test. (i)TheI-V curve of the
11
nanowire.
12
Figure 3. Flexibility investigations of the WC nanowire and nanosheet. (a1-a3) Bending
13
process of a WC nanowire.The scale bar is 1um. (b1-b6) Bending process of a WC nanosheet
14
with elastic behavior. (c1-c6) Flexibility of a nanosheet folded in three. The scale bars in
15
(b1-c6) are 2 um.
16
Figure 4. Flexible WC nanoscale conductor. (a,b)I-V investigation of two WC nanowires
17
under moderate strain,with SEM images of the nanowires at different bending states. The
18
scale bars 1um.
19
Figure 5. FE performance of low-dimensional WC nanostructures. (a) J-E dependence of WC
20
nanosheet. (b) FN curve of the emission characteristics. (c) Schematic of the FE investigation
21
method using a single emission tip. (d) Emission performance of nanowires, with the relation
22
between I/V2 and 1/V in the inset. (e) Emission performance of WC nanosheet, with the
23
relation between I/V2 and 1/V in the inset.
us
an
dM
pte
ce
24
cri pt
1
Ac
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
AUTHOR SUBMITTED MANUSCRIPT - NANO-114048.R2
AUTHOR SUBMITTED MANUSCRIPT - NANO-114048.R2 Anisotropic electrical transport of flexible tungsten carbide nanostructures: towards nanoscale interconnects and electron emitters
1 2
(a)
(b)
cri pt
3
(d)
(c)
-
[1010]
-
1um
b
(f)
(g)
an
(e)
a
us
[0110]
dM
c
ce
pte
200nm
Ac
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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Figure 1
[0001]
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Anisotropic electrical transport of flexible tungsten carbide nanostructures: towards nanoscale interconnects and electron emitters
1 2 3 (b)
(i)
5000
6
Current (A)
3000 2000 1000
2um
0 0
-2
4.7×10 A cm
4000
1
2
cri pt
(a)
3
4
Voltage (V)
(c)
(d)
101.5nm
(e)
268nm
66nm
2um -25.5nm
(g)
(h)
pte
an Figure 2
ce
-118nm
(f)
dM
1um
us
66nm
Ac
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
AUTHOR SUBMITTED MANUSCRIPT - NANO-114048.R2
AUTHOR SUBMITTED MANUSCRIPT - NANO-114048.R2 Anisotropic electrical transport of flexible tungsten carbide nanostructures: towards nanoscale interconnects and electron emitters
1 2 3
5
(c6)
cri pt
4
(a1)
(a2)
6 7
(b1)
(b2)
(b6)
(b5)
an
(b4)
dM
(b3)
us
52nm
(c1)
(c2)
Figure 2
(c4)
ce
pte
(c3)
Ac
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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Figure 3
c5)
(a3)
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Anisotropic electrical transport of flexible tungsten carbide nanostructures: towards nanoscale interconnects and electron emitters
1 2
0.10
(a)
0.0 0.00
0.02 0.00
0.06
ce
pte
dM
0.02 0.04 Voltage (V)
0.04
Initial Bending 1 Bending 2
an
Initial Bending 1 Bending 2
Current (A)
0.1
0.06
us
0.08
0.2
Current (A)
(b)
cri pt
3
Ac
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
AUTHOR SUBMITTED MANUSCRIPT - NANO-114048.R2
Figure 4
0.00
0.05 0.10 Voltage (V)
0.15
0.20
AUTHOR SUBMITTED MANUSCRIPT - NANO-114048.R2 Anisotropic electrical transport of flexible tungsten carbide nanostructures: towards nanoscale interconnects and electron emitters
1 2
cri pt
3
-2
(a)
80
(c)
(b)
60
-6
2
ln(J/E )
2
J (mA/cm )
-4
40 20
-8 -10
15
35
40
6
2
ln(I/V )
-23 -24
-26 0.008
0.009
1/V
0.010
0.011
1.5 1.0 0.5 0.0 100
110
120
Voltage (V)
pte
Figure 5
ce
-23
5
-24 -25
4
-26 -27
0.007
0.0081/V 0.009
0.010
3 2 1 0
dM
90
(e)
-22
an
Current (A)
2.0
0.08
2
(d)
-25
2.5
0.07
-21
-22
3.5 3.0
30
ln(I/V )
4.0
20 25 E (V/um)
Emission Current (A)
10
Adj. R-Square 0.94766 -12 0.02 0.03 0.04 0.05 0.06 1/E (um/V)
us
0
Ac
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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100
110
120
130
Voltage (V)
140
150
160
