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

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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. ? 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 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. ? 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 ?L aliquot of oleyamine was added, and the mixture was sonicated, resulting in dissolution of the Au precursor. A 300 ?L 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

Article pubs.acs.org/Langmuir ? XXXX American Chemical Society A DOI: 10.1021/acs.langmuir.5b01986

Langmuir XXXX, XXX, XXX?XXX 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 ?m) 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 ?m. 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