Flow-Directed Growth of Aligned Metal Nanowire Films: Toward Light

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Flow Directed Growth of Aligned Metal Nanowire Films: Towards Light Polarizing Transparent Conductors Muriel Layani-Tzadka, Lee-Or Barouch, Daniel Vestler, Ilana Aksenfeld, Daniel Azulai, Einat Tirosh, and Gil Markovich ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00424 • Publication Date (Web): 24 Apr 2019 Downloaded from http://pubs.acs.org on April 25, 2019

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ACS Applied Nano Materials

Flow Directed Growth of Aligned Metal Nanowire Films: Towards Light Polarizing Transparent Conductors Muriel E. Layani-Tzadka, Lee-Or Barouch, Daniel Vestler, Ilana Aksenfeld, Daniel Azulai, Einat Tirosh and Gil Markovich* School of Chemistry, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv University 6997801, Israel

ABSTRACT

We present a unique technique for the aligned growth of metal nanowires (NWs) in an aqueous environment under the shear-force of laminar flow of the aqueous solution applied during NW formation

on

a

substrate.

It

is

based

on

alignment

of

self-assembled

CTAB

(cetyltrimethylammonium bromide) surfactant template nanostructures formed around seed metal particles by the shear-force. The obtained silver-based NW films exhibit anisotropic sheet resistance and linear polarization of the transmitted light in the visible range, which is opposite to the polarization characteristics of regular wire-grid polarizers due to localized plasmon resonances in the visible range and near infrared. Nickel based NWs, on the other hand, produced partial

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transmitted light polarization which was consistent with standard wire grid polarizers. The polarization results are supported by Finite-Difference-Time-Domain (FDTD) simulations.

KEYWORDS: Metal nanowires, Aligned nanowires, Light polarization, Surface plasmons, Transparent electrodes

INTRODUCTION Alignment of elongated nanostructures on surfaces is a key topic in nanotechnology. The ability to obtain controlled assembly of NWs can be used in many applications such as nanoelectronic circuits, sensors, optoelectronics, and thermoelectric devices. There are different reported ways to align NWs. Fan et al. presented alignment using mechanically applied shear forces,1and Pevzner et al. demonstrated alignment by knocking-down vertically grown NWs.2 Other alignment techniques include electric field induced alignment,3,4 electrostatic interactions with patterned surfaces,5 magnetic fields,6,7 the oil–water–air interface,8 and a push–pull nozzle system to form aligned NWs by injecting and pumping the reactants through different nozzles.9 A frequently used NW alignment technique involves the use of shear forces in microfluidic systems. Shear flow induces stress over a finite distance from a stationary surface, where adjacent layers of fluid move parallel to each other at different velocities. Lieber and coworkers formed aligned arrays of InP NWs by passing suspensions of the NWs through channel structures formed between a polydimethylsiloxane (PDMS) mold and a flat substrate.10 Silver NWs were aligned by flowing their suspension through a capillary.11 Not only prefabricated NWs could be manipulated by flow, but also nanostructure preparation can be done under flow, for example; Puigmarti-Luis et al. used microfluidics-dictated self-


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assembly as means to influence the supramolecular construction of hybrid materials based on tetrathiafulvalene and gold.12 Many types of elongated inorganic nanostructures can be templated by surfactant molecule nanostructures. CTAB has been a widely used surfactant for the formation of nanostructures in general and for the formation of nanorods and nanowires in particular.13-16 We have previously reported the use of CTAB as a template for growing random networks of gold-silver NWs directly on substrates, where the growth process occurred in a thin film containing the growth solution, directly at the surface of the desired substrate.16-18 Such films may be used as transparent electrodes for various applications. The organization, size, and shape of surfactant mesostructures mostly depend on their molecular details and on the concentration of the surfactant, but can also be controlled by shear forces. Cappelaere et al. found two structural transitions in CTAB mesostructures as a function of flow rate; the first transition induced by the shear-force corresponds to a nematic liquid crystalline phase.19 The influence of different salts and surfactant additives on the rheological behavior of CTAB has also been investigated, and in particular the formation of structures due to the shear thickening effect.20-24 The use of microfluidics for the generation and investigation of CTAB shear-induced structures was also reported.25-28 The investigation of the alignment of CTAB worm-like micelles under flow using contraction slitchannel has been reported by Lutz-Bueno et al.29 Hillhouse et al. have reported on the preparation of hexagonal mesoporous silica films on glass substrates under continuous flow to induce a preferred orientation.30 Kim et al. reported the formation of flow-induced silica structures by insitu gelation in the presence of wormlike micelles of CTAB and a salicilate additive.31 The ability to align metal NWs on surfaces can promote the development of novel light polarizing films for different wavelength regimes. Wire grid polarizers have been frequently used


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in the infrared region and now extend into the visible regime. The most important feature in a wire grid polarizer is the metallic grid periodicity, which should be shorter than the polarized wavelengths. In a perfect wire grid polarizer, the electromagnetic wave with electric field component incident parallel to the wires is reflected or absorbed while the electric field in the perpendicular direction passes straight through the grid, making it highly transparent to this polarization. Wire grid polarizers based on metal NWs can even be used in the UV-visible range, as reported by Pelletier et al., who fabricated aluminum NWs with a periodicity of 33 nm (the sum of the NWs width and separation between NWs) aligned using a thin film of a self-assembling diblock copolymer as a template, that showed 20-50% polarization efficiency in the visible range.32 Shrestha et al. fabricated polarization-tuned color filters made of arrays of aluminum NWs in periods of 300-460 nm, which were fabricated by electron-beam lithography.33 Silver NWs based polarizers have also been studied recently: Wang et al. investigated the polarized absorption spectra of a glass slide containing embedded silver nanorods at low density, where the transmittance of the polarizing glass at 780 nm increased from 10 to 90% as the value of the polarization angle increases from 0° to 90° with respect to the nanorod alignment directions. This indicated that the longitudinal plasmon absorption of the silver nanorods is much stronger than the transverse plasmon absorption at 780 nm.34 The polarized transmission of pre-synthesized silver NWs (260 nm diameter) aligned by off-center spin-coating was investigated by Kang et al.35 They showed an opposite trend (relative to the nanorods of ref. 34) of polarized transmission of the two modes of surface plasmon resonances of silver NWs (the longitudinal mode and the transverse mode) when introduced to polarized light. In this work, we present the growth of aligned ultrathin gold-silver NWs in a laminar flow system via alignment of CTAB surfactant template mesostructures by the shear-force of the flow. The


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aligned NWs, coated with silver, display light polarization characteristics in the visible range that are strongly influenced by plasmonic effects and are opposite to the polarization characteristics of regular wire-grid polarizers. Changing the coating of the NWs to nickel produced regular wiregrid polarizer like polarization anisotropy.

EXPERIMANTAL SECTION Random and aligned networks of gold-silver NWs were produced directly on PET substrates through surfactant-based tubular-templates, where the growth process occurred in a thin solution film, either at static or flow conditions, respectively. Materials. The reagents used for the metal NW film preparation, i.e., CTAB, HAuCl4, AgNO3, sodium ascorbate, bovine serum albumin (BSA), citric acid, trisodium citrate, hydroquinone, nickel sulfate hexahydrate and borane dimethylamine complex were purchased from SigmaAldrich and used without further purification. The Au seed particles (Nanogold©) were purchased from Nanoprobes Inc. All water used was ultrapure (18

H cm), obtained from a Direct-Q

Millipore system. Laminar flow system. The system (see Figure 1b) includes a growth solution reservoir as a source for flowing the growth solution on top of our 10 10 mm2 PET substrate positioned in a laminar flow cell. The substrate is held in a tight-fit groove, by a vacuum suction hole at the bottom of the groove, with its top face levelled with the 1 mm deep flow channel bottom, to avoid disturbance of the laminar flow profile. The flow is driven by a small, Teflon-lined electric pump.36 The flow rate is controlled by two small flow restrictors which can be varied to control the tubes' resistance. The optimal flow rate used was in the range of 5-10 mm/s. Deposition of the NW Films.


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Step 1: O2 plasma cleaning and surface activation was performed on the PET substrates prior to the NWs deposition process. Step 2: Metal seed particle solution was printed on the desired area of the PET substrate. Au seed nanoparticles of size ~1.4 nm (~55 gold atoms) were used. A 10M- M solution of seeds in a mixture of 50:50% water/ethanol by volume was used for printing the seed pattern, where the growth of the NW film is desired, on the PET substrates. Inkjet printing of seed solution was done using a MicroFab Jetlab-4 system, with an 80 O internal diameter piezo-actuated jetting device.18 Step 3: Ultrathin NWs were grown by bringing the growth solution in contact with the substrate. At this stage, the growth solution was flown laminarly above the PET substrate to create aligned ultrathin gold-silver NWs with typical diameter of ~2-4 nm. The NW film grows only in areas where the seed particles were deposited.18 The CTAB growth solution was prepared by mixing aqueous solutions of CTAB (0.25 M, 20 mL), HAuCl4 (0.025 M, 1000 O 4# AgNO3 (0.1 M, 500 O 4# and sodium ascorbate (1.8 M, 850 O 4 sequentially at 35 °C.18 The growth solution was put in the designated reservoir vessel of the flow system. Then the pump was turned on and the 3-way valve opened to allow the growth solution to flow through the flow cell at the velocity range of 5-10 mm/s for 5 min. Finally, the substrate was washed by flowing 70% ethanol/water through the flow cell for 1 min. A control sample (without flow) was prepared using the same growth solution, which was statically contacted with the seed-coated substrate for 5 min and washed with 70% ethanol/water for 1 min. Step 4: In order to enhance the stability of the NWs and their conductivity we performed metal plating that thickens the flow-aligned (or unaligned) ultrathin NWs. This step has to be selective


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to thicken the NWs while avoiding random metal deposition on the substrate. This step was achieved by two different plating baths: 1. Selective catalytic electroless silver-plating process: The silver-plating solution was prepared by mixing aqueous solutions of BSA (0.125% w/v, 200 mL), AgNO3 (0.1 M, 1.11 mL), citric acid and sodium citrate buffer (1.2 and 1.6 M, respectively, 11.1 mL), and hydroquinone (0.3 M, 33 mL) sequentially at 26 °C. The substrates with the aligned ultrathin NW films were dipped for 26 min in the silver-plating solution. Then, the samples were washed for 1 min in 70% ethanol and 1 min in water.18 2. Selective auto-catalytic electroless nickel-plating process:37 The nickel-plating solution was prepared by mixing aqueous solutions of nickel sulfate (0.06 M, 10 mL) and borane dimethylamine (0.13 M, 10 mL) sequentially at 45°C. The substrates with the aligned NW films were dipped for 1 min in the nickel-plating solution. Then, the samples were washed for 1 min in 70% ethanol and 1 min in water. Sheet resistance measurements. These measurements were done by applying silver paint dots across the area to be measured along the two directions: R (=1/G ) was measured along the NW alignment direction, and R (=1/G ) was measured perpendicular to this direction. The sheet resistance was roughly estimated by multiplying the measured resistance by the dot width and dividing by the separation between the silver paint dots. Polarized transmission measurements. To measure the polarization of the aligned NW samples we used a Zeiss Axio microscope based setup where white light passed through a rotatable linear polarizer, to control its polarization angle with respect to the alignment direction of the NWs. Then the polarized light passed through the sample, then through a 100 objective lens, and was finally collected into an optical fiber attached to the camera port, which transfered the collected light to


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an Ocean Optics S2000 spectrometer to measure the transmission spectrum in the visible range. Each polarization state was normalized to a blank PET substrate measurement (see Supporting Information, Figure S1). The light was collected from a field of view of about 100 m radius. Hence the polarization effect was averaged over a ~0.03 mm2 area. Scanning Electron Microscopy (SEM). SEM imaging was carried out in a Quanta200 field emission gun environmental SEM using the FEI wet-STEM detector. To examine the NW films deposited on non-conducting substrates (PET) we used water vapor environment (low vacuum). Image analysis was performed using ImageJ (NIH), where , after noise filtering and image binarization, the particle analysis feature was used to obtain the angular distribution of the NWs identified as high aspect ratio (>3) features in the images. Transmission Electron Microscopy (TEM). NW film samples were deposited on carbon-coated copper grids (SPI). Images were recorded using a Philips FEI Tecnai F20 TEM. Atomic Force Microscopy (AFM). Topography imaging was done using a NT-MDT P47 AFM using standard silicon tips (HQ-NSC35, MikroMasch) in semi-contact mode. Image analysis was performed by the WSXM 4.0 software. Finite-Difference-Time-Domain (FDTD) simulations. A commercial-grade simulator based on the finite-difference time-domain method was used to perform the calculations, “FDTD Solutions” by Lumerical Inc. Silver, gold and nickel optical constants were used as reported by Palik.38 Boundary conditions for all simulations consisted of a Perfectly Matched Layer (PML) of 8 layers in the direction of radiation propagation and periodic boundary was applied to the other simulation dimensions, i.e. the NWs' length and periodicity were infinite. We applied a function of varying random amplitude to the NW surface features to simulate NW roughness (software built-in). Conformal variant 1 mesh refinement was utilized as metals are involved and the simulation was


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allowed to reach auto-shutoff criteria (where all fields left the simulation region). Plane wave illumination was used in all simulations and mesh refinement was applied in the region around the NW.

RESULTS AND DISCUSSION We were able to prepare aligned silver NW arrays by executing the ultrathin Au/Ag NW growth step (step 3 of deposition of NW films) in the flow system presented in Figure 1. When the laminar flow of the growth solution, containing the CTAB surfactant, the gold and silver ions and the reducing agent, meets the preprinted seeds on the PET substrate, the CTAB self assembles into long tubules (with embedded seed particles, see Figure 1a) attached to the surface. These tubules serve as the templates for the growth of the NWs, as was previously described. 16-18 In parallel, the flow-induced shear forces applied to the CTAB tubules tend to align the tubular structures along the flow direction as the NWs grow within the tubules. The alignment obtained at this stage is sustained during the washing steps, and during the electroless silver/nickel plating step. SEM images of the aligned and unaligned NWs (random mesh of silver NWs prepared under static growth conditions16-18) are presented in Figure 2. Also presented in the inset of Fig. 2a is the result of SEM image analysis of the angular distribution of the NWs around the flow direction, which is of the order of ±10 . An AFM image of a small part of the aligned NW film shows that the aligned NWs form thicker NW bundles, of the order of ~50-100 nm high (see lower AFM line profile), compared with 30-40 nm high NWs formed under similar Ag plating conditions for the unaligned film.18 A careful inspection of the SEM and AFM images of the aligned sample reveals some thinner, unaligned NWs (see upper AFM line profile) that branch out of the thicker aligned


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are grown, a growth solution reservoir, an additional wash solution reservoir, a small Teflon-lined electric pump and two small flow restrictors which can be varied to control the tubes' resistance. (c) The details of the flow cell: The cell is constructed from a plastic bottom with a machined groove (1 mm deep) in which the growth solution flows laminarly and a transparent plexiglass cover, which seals the flow channel by pressing a silicone gasket to the circumference of the channel. The growth solution is flown in and out of the channel through vertical holes in the cover. The 10 10 mm2 substrate is held by vacuum suction in the tight fit groove with its top face leveled with the flow channel's floor. The optimal flow rate used was in the range of 5-10 mm/s.

Typical flow velocities used in the successful aligned NW growth experiments were in the range of 5-10 mm/s. Those were the lowest controllable velocities achievable in our system. Higher velocities resulted in lower NW densities due to drifting away of seed particles and growing NWs. 5-10 mm/s flow velocities correspond to low Reynolds numbers (

Flow-Directed Growth of Aligned Metal Nanowire Films: Toward Light

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