LETTER pubs.acs.org/Langmuir
Synthesis of Nickel Nanowires via Electroless Nanowire Deposition on Micropatterned Substrates Zhiwei Shi and Amy V. Walker* Department of Materials Science and Engineering, University of Texas at Dallas, 800 W. Campbell Road RL10, Richardson, Texas 75080, United States
bS Supporting Information ABSTRACT: Electroless nanowire deposition on micropatterned substrates (ENDOM) is a promising new technique by which to direct the synthesis and precise placement of metallic nanowires. ENDOM is generally applicable to the preparation of metallic, semiconducting, and even insulating nanowires on technologically relevant substrates, is inexpensive, and can achieve high growth rates. The deposited nanowires are ultralong (centimeters) and can be patterned in arbitrary shapes. We demonstrate ENDOM using the growth of nickel nanowires. By controlling the deposition time, the width of the nanowires can be varied from 200 to 1000 nm and the height can be varied from 7 to 20 nm.
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etallic nanowires have properties that differ significantly from those of their bulk counterparts1 and potential technological applications in electronics,1 optoelectronics,1 and sensors.2 A key challenge in the practical use of metallic nanowires is their effective integration into devices through assembly, patterning, and alignment on technologically relevant substrates including semiconductors such as Si, InAs, and GaAs. Current techniques for the synthesis and placement of nanowires include template synthesis,1 step-edge decoration,2 vaporliquid solid (VLS) growth,1 and lithographically patterned nanowire deposition.3 In some nanowire synthesis methods, such as VLS growth or template synthesis, the nanowires are grown ex situ, and a second processing step, such as optical trapping,4 magnetic trapping,5 electrostatic trapping,6 dielectrophoresis,7 or microfluidic flow,8 is required to place and orient them precisely on the substrate. Directed growth methods based on chemical vapor deposition (CVD)9 or electrodeposition3 in prepatterned structures prepare the nanowires in situ and do not require subsequent placement. However, these techniques can be complex, involving multiple lithographic and deposition steps,3 and the growth of the nanowires can be very slow, less than 1 μm/min.9 In this letter, we introduce a promising new technique, electroless nanowire deposition on micropatterned substrates (ENDOM), by which to direct the growth of metallic nanowires. The deposited nanowires are ultralong (centimeters) and can be made in arbitrary shapes. In this method, a micropatterned multifunctional surface is employed to direct the growth of metallic nanowires. Although selective deposition on micropatterned surface has been extensively studied,10,11 with only a few prior exceptions11 all prior applications have yielded correspondingly large deposits. The deposition method used is seedless electroless deposition (ELD). In ELD, a redox reaction is employed to deposit overlays including metals, alloys, and semiconductors.12 At short deposition times, a nanowire will form at the interface of r 2011 American Chemical Society
two dissimilar materials if the following conditions are met. First, metal deposition is kinetically favored on one of the materials because of the preferential adsorption of the reducing agent. Second, the transport of the reactants must be slower to the surface with the faster electroless deposition rate. We illustrate the ENDOM process using the formation of nickel nanowires. A schematic of the method is shown in Figure 1. We start with an OH-terminated alkanethiolate SAM (SAM 1) adsorbed on Au. An image in SAM 1 is created using UV light shining through a mask (UV photopatterning) (step 1), and a second CH3-terminated SAM (SAM 2) is adsorbed where SAM 1 has been photooxidized and removed (step 2). A micrometer-scale image has thus been created on the surface. SAMs are ideal for the construction of nanostructures because their surface chemistry can be easily tuned and they can be formed on a wide range of substrates, including metals, semiconductors, and polymers.13,14 Furthermore, the micropatterning of SAMs does not require a clean room or expensive lithographic tools. Seedless, selective ELD is then used to deposit nickel nanowires at the interface of SAMs 1 and 2 (steps 3 and 4) by careful control of the reaction conditions. Additional patterning/metal deposition cycles can then be used to assemble complex devices. Figures 2 and 3 display SEM and AFM images, respectively, of Ni nanowires formed on micropatterned CH3/OH-terminated SAM surfaces. In Figure 2, it can be clearly seen that the Ni nanowires are continuous, are well-formed, and can follow complex paths, such as a right-angled turn. The uniformity of the deposited nanowires can be measured using AFM cross sections. Along the wire, it can be seen that both the height and width are constant to (15% (Figure 3 and Supporting Information). Received: July 7, 2011 Revised: August 21, 2011 Published: August 23, 2011 11292
dx.doi.org/10.1021/la2025878 | Langmuir 2011, 27, 11292–11295
Langmuir
LETTER
Figure 1. Schematic of electroless nanowire deposition on micropatterned substrates (ENDOM). (1) A micropattern is created in SAM 1 using UV photopatterning. (2) In the photooxidized areas, the second SAM (SAM 2) is adsorbed. (3) The entire sample is immersed in an electroless deposition bath containing nickel ions (Ni2+); sodium pyrophosphate, the complexing agent; and dimethylamine borane (DMAB), the reducing agent. The electroless deposition process leads to the production of nanowires at the interface of SAM 1/SAM 2 (step 4).
Figure 2. SEM images of Ni nanowires synthesized at the junction of CH3 (hexadecanethiol, HDT) with OH (mercaptohexadecanol, MHL)-terminated SAMs. Deposition conditions: 0.08 M NiSO4 3 6 H2O, 0.14 M Na4P2O7 3 10H2O (complexing agent), 0.07 M (CH3)2NHBH3 (reducing agent), pH 10, 45 °C, deposition time 10 min.
By controlling the deposition time, the width of the nanowires can be varied from 200 to 1000 nm. The AFM data also indicate that the height of the nanowires varies from approximately 7 to 20 nm. The height of the nanowire appears to be linearly dependent on its width (Supporting Information). Because the rate of Ni deposition is dependent on the concentration of the reactants, the bath pH, and the deposition temperature,15,16 the size of the nanowires is also very sensitive to these experimental variables. Furthermore, because Ni ELD is an autocatalytic process the width and height of the nanowires increase nonlinearly with deposition time. Thus, there is a short time between the initial deposition of Ni and the formation of a Ni overlayer form over the entire CH3-terminated SAM surface (
