One-dimensional nanostructured metals play important roles as interconnects and nanoscale electronic devices. Silver nanowire is especially attractive due to its extremely high electric and thermal conductivities 1. Template-directed synthesis and the solution-phase based approach have been widely used to produce Ag nanowires 234. However, pure silver nanowires are limited for metallic interconnection applications as its resistivity increase largely with the dimension decreasing due to its increased electron scattering. On the other hand, carbon nanotubes [CNTs] are known for having long mean free path (order of several microns), extremely high current densities (> 109 A/cm2 at 25°C), and high aspect ratio. The bulk composites made of metal matrix with CNTs as the reinforcement component for augmenting the metal conductivity have been extensively investigated 567. However, how to fabricate and to characterize a single one-dimensional nano-composite with embedding a single CNT within the silver matrix are difficult 89, and electroless silver plating process has been used to deposit silver onto multiwalled carbon nanotube [MWCNT] 10. In this paper, one-dimensional Ag/MWCNT nano-composite was prepared by coating the MWCNT with relatively uniform nano-crystalline silver layer using the electroless silver plating process. The electrical properties of one-dimensional Ag/MWCNT nano-composite were investigated using two terminal conductance measurements to demonstrate the resistivity and the current carrying capacity.
Results and discussion
Transmission electron microscopy observation shows that the one-dimensional Ag/MWCNT nano-composite has a relatively uniform silver coverage as shown in Figure 1a. The diameter is about 160 nm, with a deviation of about 20 nm, and the length is about 3.2 μm. The inset of Figure 1c is the selected area electron diffraction [SAED] pattern obtained from a massive bundle of one-dimensional Ag/silver nano-composites. Several continued rings and some diffraction spots that were observed in the SAED pattern indicate that the Ag/MWCNT nano-composites have a polycrystalline structure.
The distances of the lattice planes are measured using a high-resolution transmission electron microscopy [HRTEM] and shown in Figure 1d. Interplanar distance of the (111) planes which is 0.234 nm indicates that the nano-crystal silver coating possesses a face-centered cubic [FCC] structure, which are consistent with the standard lattice constant of the Ag crystal structure 14. Lattice fringes with spacing of 0.2046 nm which are also observed corresponds to the (200) planes in FCC crystalline silver. It can be seen that the silver coating layer is nano-crystallized without any amorphous structure 15.
Figure 2 gives the X-ray diffraction [XRD] patterns of the nano-crystalline Ag/MWCNT nano-composites. One weak peak at about 2θ = 26.00 is from MWCNTs and the peaks at 38.2°, 44.3°, 64.4° and 77.4° can be assigned, respectively, to <111>, <200>, <220>, and <311> crystalline planes of the silver coating. It is consistent with the crystalline silver measured in HRTEM. It is further observed that the nano-crystalline silver coating has a strong (111) orientation along the nanotube's axial direction because the specific free energy of silver is minimum on (111) planes of the face center cubic structure 16.
<p>Figure 2</p>XRD patterns of one-dimensional Ag/MWCNT nano-composite
XRD patterns of one-dimensional Ag/MWCNT nano-composite.
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A single, one-dimensional nano-crystalline Ag/MWCNT nano-composite wire was placed onto an electrically isolated wafer substrate, and two electrodes were made by liftoff (10 nm Cr/170 nm Au), as shown in Figure 3 (left). The gap between the two electrodes is about 500 nm and the diameter of the nano-crystalline Ag/MWCNT nano-composite is about 160 nm. Resultant electric currents were recorded under applied bias voltages, ranging from -100 to 100 mV, to the two electrodes.
<p>Figure 3</p>SEM image of the setup of electrical characterization
SEM image of the setup of electrical characterization. Inset is the magnified image of the connection (left) and the I-V curve of Ag/MWCNT nano-composite wire. The straight line indicates a resistive nature, and no semiconductive characteristics as in silver oxide can be identified.
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Figure 3 (right) displays the typical linear I-V curves measured on the single, one-dimensional nanocrystalline Ag/MWCNT nano-composite wire at room temperature. More than five measurements were carried out on the sample. The resistance was calculated (based on the applied voltages and the resultant currents). The average resistance of the Ag/MWCNT is at about 75.7 Ω, and the variation among different measurements is less than 2%. Therefore, the resistivity of the one-dimensional Ag/MWCNT nano-composite is approximately 3.0 × 10-6 Ωm. It is reasonable that the resistivity of the nanocrystalline Ag/MWCNT nano-composite is higher than the bulk silver (1.5 × 10-8 Ωm) because the surface scattering of electron increases largely when the diameter is as smaller as the mean free path of the silver 16. Since the I-V curve is a straight line, therefore, no oxidation effects can be identified. This also agrees with the composition analysis in which no oxygen was detected. It was indicated that the resistivity of a DNA-templated silver nanowire measured by SH Park et al. was about 4 × 10-6 Ωm 17. It can be deduced that the one-dimensional Ag/MWCNT nano-composites have better conductivity than the DNA-coated silver nanowire. However, the resistivity of Ag/MWCNT nanowire is greater than that of a pure single crystal silver nanowire which has the resistivity of about 1.87 × 10-6 Ωm 15. The increased resistivity of Ag/MWCNT could be due to the multigrained structure as well as the contact resistance as well. Based on the free electron theory, the electric conductivity of metallic conductors can be described as 518:
ρ
=
n
e
2
λ
̄
m
e
v
F
through the free electron gas theory 5, where n is the electron density of state,
λ
̄
is the mean free path, me is the mass of electron and VF is the Fermi velocity. The electrical conductivity is proportional to the mean free path, i.e., the longer the mean free path, the higher the conductivity will be. The silver coating on the sidewalls of MWCNTs forms composite couplings to electronic states through the charge transfer interaction from the silver ions to MWCNTs 19. The metallic MWCNTs can be ballistic conductors and have a mean free path of 0.5 μm equaling with the length of the tube 2. The embedding MWCNT in a silver matrix facilitates electron transfer along with the ballistic transport from end to end, thereby prolonging the electron's mean free path of nano-composite ten times of the silver 20. The low resistivity of one-dimensional Ag/MWCNT nano-composite silver nanowire can attribute to the longer mean free path of MWCNT. The current density t was calculated to be about 6.56 × 107 A/cm2 at the voltage of 100 mV through the t = I/S, where S is the cross sectional area and I is the current. The current can be kept stable during the 20 min measurement under the constant voltage of 100 mV. Since electromigration occurring in Ag interconnects at the current density of 106 to 107A/cm2 21, and the metallic MWCNT can stand electric current density (along axis) more than 1,000 times greater than that of silver; therefore, it is clear that integration of MWCNTs in nano-composites would be good to counter electromigration.