Directed Assembly of Ultrathin Gold Nanowires over Large Area by

Article pubs.acs.org/Langmuir

Directed Assembly of Ultrathin Gold Nanowires over Large Area by Dielectrophoresis R. Venkatesh,† Subhajit Kundu,† Avradip Pradhan,‡ T. Phanindra Sai,‡ Arindam Ghosh,‡ and N. Ravishankar*,† †

Materials Research Centre, and ‡Department of Physics, Indian Institute of Science, Bangalore 560012 India S Supporting Information *

ABSTRACT: Ultrathin Au nanowires (∼2 nm diameter) are interesting from a fundamental point of view to study structure and electronic transport and also hold promise in the field of nanoelectronics, particularly for sensing applications. Device fabrication by direct growth on various substrates has been useful in demonstrating some of the potential applications. However, the realization of practical devices requires device fabrication strategies that are fast, inexpensive, and efficient. Herein, we demonstrate directed assembly of ultrathin Au nanowires over large areas across electrodes using ac dielectrophoresis with a mechanistic understanding of the process. On the basis of the voltage and frequency, the wires either align in between or across the contact pads. We exploit this assembly to produce an array of contacting wires for statistical estimation of electrical transport with important implications for future nanoelectronic/sensor applications.

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INTRODUCTION Rapid growth and advancement of nanoelectronics requires devices that are not only smaller but also efficient, fast, and inexpensive.1 Nanomaterials of various dimensionalities are interesting because of their unique size and shape-dependent quantum confinement behavior and for their potential to be integrated into devices at the nanoscale. One-dimensional nanostructures are particularly useful in this regard.2 The successful synthesis of ultrathin Au nanowire (∼2 nm diameter) by simple wet chemical means3−10 has provided a big boost for the development of interconnects at the nanoscale with a variety of applications in the field of nanoelectronics and nanoelectromechanical systems (NEMS).3,5,11 Direct growth onto various substrates has been employed to fabricate devices out of these nanowires.12−14 An interesting electrical transport property with high sensivity to the change in local environment has been demonstrated.15,16 Such high sensivity to minor changes in the environment indicates potential in development of various chemical and biological sensors with high sensitivity.17,18 Recently a few successful demonstrations have been made in this regard which points toward the usefulness of the nanowires in future electronics.19,20 For efficient realization of the above-mentioned applications, it is critical to align the wires into parallel arrays between contact pads. Different strategies21−25 have been reported for assembly of nanomaterials including dielectrophoresis,26−30 Langumir−Blodgett technique,31 contact transfer,32 microfluidic flow,33 and nanocombing technique.34−36 Dielectrophoresis, using either direct or alternating electric fields, has been proven to be an efficient tool for rapid, reproducible, and © XXXX American Chemical Society

accurate assembly of nanoparticles, nanowires, and nanotubes over large areas.37 Alignment using the AC field helps avoid the electro-osmotic and electrolysis effect that are the main drawbacks in the case of DC dielectrophoresis. This method also has the advantage of morphology selectivity that enables alignment of particle-free nanowires. In this work, ultrathin Au nanowires of ∼2 nm diameter prepared by wet chemical synthesis have been shown to align in between and on the sides of the contact pads. An understanding of the optimized parameters for alignment of nanowires has been achieved based on control experiments using different conditions. The method has been extended to demonstrate assembly of nanowires forming an array of devices in a single experiment. Further, it has been used for statistical estimation of the electrical transport property of these ultrafine nanowires.

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EXPERIMENTAL SECTION

The ultrathin gold nanowires were synthesized by a literature method6,12 that used n-hexane as the solvent for nanowire growth. For synthesis of Au nanowires, 6 mg of HAuCl4 was taken in 5 mL of hexane. A 200 μL aliquot of oleyamine was added, and the mixture was sonicated, resulting in dissolution of the Au precursor. A 300 μL aliquot of triisopropylsilane was added. The solution was aged for 1 day to obtain a dark purple dispersion containing ultrathin Au nanowires. The solution containing the nanowires was then mixed with toluene as it has a lower vapor pressure as compared to n-hexane; Received: May 30, 2015 Revised: August 7, 2015

A

DOI: 10.1021/acs.langmuir.5b01986 Langmuir XXXX, XXX, XXX−XXX

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Figure 1. (a) Schematic diagram of dielectrophoresis experiment with nanowire solution drop casted in between the contact pads where the AC field is applied; (b) lower magnification SEM image of aligned nanowires in between contact pads with a gap of ∼3 μm; (c) higher magnification image of the gap between the pads. this provides sufficient time (∼40 s in closed environment) for the alignment of the wires before the solvent evaporates. The solution is diluted with toluene until the dark purple color reaches a pale purple color. Low magnification SEM images of dropcasted nanowires (without field), shown in the Supporting Information (Figure S2) represent the typical ratio of the different morphologies present in the Au nanowire solution used for dielectrophoresis experiments. Trace amounts of nanoparticles are present as a byproduct of synthesis as indicated in those images. A two-probe device with prepatterned Cr (5 nm)/Au (50 nm) contact pads on SiO2 (300 nm) over Si (1 μm) substrate were fabricated using photolithography. A schematic diagram of the parallel probes is shown in Figure 1a. Bias voltage is applied across the probes for aligning the nanowires simultaneously across all the contact pads. Figure 1, panels b and c show scanning electron microscopy (SEM) images of such ultrathin Au nanowires aligned using AC-dielectrophoresis in between contact pads separated by 3 μm. Since the e-beam lithography of a large network shown in Figure 6 is time-consuming, expensive, and not fit for optimization, we did initial experiments as shown in Figures 1−3 in pads as shown in Figure 1a. After optimization of this approach in several pads and when it became reproducible, we moved on to make a wafer scale circuit as shown in Figure 6 for a concise approach which can give a geater number of nanowire devices that is very much needed for getting a statistical output for electrical properties

Figure 2. SEM images of nanowires aligned across contact pads having a spacing of ∼400 nm.

between the contact pads and along the side of the probe is obtained only when both the voltage and frequency are increased to 5 V and 10 MHz, respectively, as shown in Figure 2c. When a voltage of 10 V (peak-to-peak) was applied at 5 MHz frequency, the alignment of nanoparticles was very fast leading to shortening of the probes as shown in Supporting Information Figure S3a and alignment of wires along the sides as shown in Figure S3b. The same trend continued for 10 VPP and 7 MHz as shown in Figure S4a and the alignment in the sides can be seen in Figure S4b. A reasonable alignment of wires in between the gaps could be obtained only when the applied frequency was more than the applied voltage. As an example we have provided the alignment of nanowires between the probes as well as along the sides of the probes in 10 VPP and 15 MHz as shown below in Figure S5 panels a and b. It can be clearly seen that the number of wires between the gaps has reduced considerably in 10 VPP; 15 MHz case when compared to the 5 VPP; 10 MHz range as given in Figure 2c. Figure 2d shows an interesting case when 10 V and 20 MHz is applied; in

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RESULTS AND DISCUSSION Figure 2 shows SEM images of alignment of ultrathin nanowires in between contact pads separated by ∼400 nm under different frequencies and applied voltages. A balance between the time of aligning the nanowires between the contact pads and time within which the solution containing the nanowires dries is determined by optimizing the frequency and magnitude of applied ac-voltage. An increase in the voltage decreased the time of alignment of nanowires but also led to the attraction of larger Au nanoparticles forming a polycrystalline nanowire between the contact pads.38 Figure 2a shows the case where a peak-to-peak AC voltage of 2 V and frequency 2 MHz is applied; there is no alignment of wires in this case. Nanowire alignment in 2 V and 5 MHz is given in Figure 2b where the yield of nanowires in between the two probes is found to be minimal. The maximum yield of nanowires B


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suspended in the medium are attracted toward the region of high field gradient, that is, between the probes; otherwise they are repelled away from high field gradient region and settle in low field gradient regions (negative dielectrophoresis). K(ω) determines whether dielectrophoresis is positive or negative. A spherical nanoparticle experiences isotropic dielectric force in all directions, and hence the frequency dependence of Re[K(ω)]Particle can be explained on the basis of eq 2. In the case of nanowires, the dielectrophoresis force is anisotropic and hence the Re[K(ω)] must be separated into two components, that is, force along the length of nanowire (Re[K(ω)]length) and force along the width of nanowires (Re[K(ω)]width),26 which are given by the relation.

this case gold nanoparticles are observed to be trapped between the probes whereas the nanowires are aligned only at the sides of the probes and not between the contact pads. Alignment at these parameters has the advantage of separation of Au nanowires from nanoparticles. Figure 3 compares the alignment of nanowires at optimized frequencies for various spacing between the probes. The images

ReK(ω)length = 2

ReK(ω)width =

(εP* − εm*) εm*

(4)

(5)

[(Re[K(ω)]length) aligns the nanowire along the electric field lines and (Re[K(ω)]width) is responsible for the translational dielectrophoresis force which brings the nanowire across the electric field gradient which is perpendicular to the contact pads. A balance between these two forces using applied frequency is necessary to get the maximum yield of nanowires. A theoretical calculation of the real part of the Clausius− Mossotti relation is attempted for which we have calculated the Re[K(ω)] for the gold nanoparticles and nanowires, suspended in toluene medium. The rounded constants were taken from ref 28 : εP = permittivity of particle (Au), 7; εm = permittivity of medium (toluene), 2; σP = conductivity of particle (Au), 4.5 × 1011 μS/cm; σm = conductivity of medium (toluene), 9 × 10−5 μS/cm. By substituting in eq 2 for gold nanoparticles in toluene medium and eq 4 and eq 5 for nanowires, Re[K(ω)] is found to be always positive and hence positive dielectrophoresis is expected irrespective of frequency in all the cases. The calculated value of Re[K(ω)] is plotted in Figure 4 which shows that the (Re[K(ω)]width) responsible for the movement of nanowires along the field gradient is found to be independent of frequency. (Re[K(ω)]length) responsible for

Figure 3. SEM images of nanowires aligned (a) between contact pads {5 VPP, 10 MHz} and (b) along the sides {10 VPP, 20 MHz} at the optimized ac voltage and frequency.

of the aligned nanowires at different contact pad spacing (400, 600, 800, and 1000 nm respectively) are shown in Figure 3a. At a parameter of 5 V and 10 MHz, the number of nanowires aligned, at the sides and in between the contact pads, increases monotonically on decreasing the spacing between the probes. On application of 10 V and 20 MHz for the case of 1000 and 800 nm devices, it is observed (Figure 3b) that the nanowires are aligned only at the sides of the contact pads and not in between the gaps. For the case of 600 and 400 nm (Figure 3b) probe spacing, only nanoparticles are attracted between the contact pads that form polycrystalline pearl-chain-like structure. Electrical measurement of this device shows that it is open, which indicates that the particles arranged in chain-like order may not be in contact. A large number of nanowires are observed to be at the sides of the contact pads under these parameters. We have used this strategy for ultrafast assembly of nanowires to form a large number of devices in a single experiment which has been demonstrated in the following sections. The dielectrophoresis force for a nanoparticle in aqueous medium28 based on the effective dipole moment (EDM) theory is given by the relation ⃗ (t )⟩ = πεma3Re[K (ω)]∇|Erms ⃗ |2 ⟨FDep

(εP* − εm*) (εP* + εm*)

(1)

where

K(ω) = 2

(εP* − εm*) (εP* + 2εm*)

(2)

and σ (3) ω where K(ω) is the Clausius−Mossotti factor, εP is the dielectric permittivity of the particle, εm is the permittivity of medium, σP is the conductivity of particle, and σm is the conductivity of the medium. In the case of positive dielectrophoresis, particles ε* = ε − j

Figure 4. Frequency dependence of Re[K(ω)] for nanoparticle and nanowire with the magnitude of applied ac voltage for different frequencies inset: logarithmic plot of the MHz frequency range where the alignment of ultrathin nanowires was possible. C


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Figure 5. Schematic diagram for large scale assembly of nanowires using 10 VPP and 20 MHz.

aligning the wires along the electric field lines and (Re[K(ω)]particle) is strongly dependent on frequency of the applied electric field. At lower frequencies (LF) range ( T in all the cases but not too much greater than the temperature in which resistivity measurements were made (90 K to 300 K), and hence the temperature independent elastic tunneling is not possible. Similarly measured R(T) does not favor a thermally activated behavior. This type of thermal Table 1. Room Temperature Resistance, Characteristic Temperature (T0), T1/T0, and Barrier Height and Width Calculated Based on FIT Model

(6) E

no.

device

resistance (MΩ)

T0 (K)

T1/T0

V0 (mev)

W (nm)

1 2 3 4 5 6

1−2 2−3 8−9 9−10 20−21 28−1

18 14 31 25 5 33

601 521 507 435 460 891

13.6 16.1 13.6 14.4 18.1 15.7

235 261 219 215 264 308

1.6 1.9 1.74 1.88 2.12 1.7


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Langmuir fluctuation-induced tunneling behavior was previously observed in semiconducting RuO2 nanowires.44 Our earlier work on electrical properties of substrate adherent pristine ultrathin gold nanowires14 showed power law behavior based on the Tomonaga−Luttinger liquid (TLL) model in which the sample is free of localization except a barrier through which the electron must tunnel.16 In the ultrathin nanowires investigated in this work, variable range hopping model (VRH),45 activated behavior46 and power law fit47 were inadequate to explain the temperature-dependent electrical transport behavior as shown in Figure 7b,c while it is in accordance with thermal fluctuationinduced tunneling model. In our earlier work on ultrathin gold nanowires, it was proven using AFM measurements that the conduction property can change from a Luttinger liquid mechanism to a hopping mechanism based on the height of the nanowire from the substrate.15 In this present work Au nanowires have a monolayer of amine as demonstrated by FTIR measurements by Feng et al.6 Nanowires have been cleaned using the nonpolar solvent toluene which cannot remove oleylamine completely,12 and hence R(T) shows the thermal fluctuation-induced tunneling [FIT] mechanism. Electrical transport properties of assembled ultrathin nanowire bundles are recently reported to be in accordance with the FIT model because of the presence of a monolayer of oleylamine interface formation.48 The calculated values of T1/T0 are shown in Table 1 along with the barrier height and barrier width. Larger values of T1/T0 signify the characteristic behavior of the interface formed by oleylamine. Large values of T1/T0 observed in our assembled nanowires as shown in Table 1 are not in accordance with the study by Pud et al. but are similar to the value observed in carbon nanotube bundles49 assembled using ac dielectrophoresis. Though the nanowires with an oleylamine monolayer can be considered as coaxial cables, the large values of T1/T0 suggest that apart from the interface formed between the metal electrode and nanowire, the charge transport may also occur through the contact formed between the nanowires within a bundle separated by a monolayer of oleylamine.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Author R.V. would like to thank Dr. Mitali Banerjee, Mr.Varatharaja Perumal for their help in different stages of experiments. N.R. acknowledges financial support from the Thematic Unit of Excellence (TUE) and the Swarnajanti fellowship of the Department of Science and Technology, Government of India.

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CONCLUSION Directed assembly of ultrathin Au nanowires using ac dielectrophoresis has been demonstrated. On the basis of the voltage and frequency, the wires either align between or at the sides of the contact pads. Alignment of nanowires at the sides of the contact pads were exploited to show fabrication of a large number of devices in a single dielectrophoresis experiment. The electrical transport property of the aligned nanowires has been studied which gives insight into the potential of these nanowires as interconnect and sensor materials.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b01986. SEM and TEM images of the synthesized Au nanowires. Additional SEM images of assembled nanowires by dielectrophoresis (PDF)

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REFERENCES

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Corresponding Author

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Directed Assembly of Ultrathin Gold Nanowires over Large Area by

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