Article pubs.acs.org/JPCC
Fabrication of Invisible Ag Nanowire Electrode Patterns Based on Laser-Induced Rayleigh Instability Harim Oh, Jeeyoung Lee, Jin-Hoon Kim, Jin-Woo Park, and Myeongkyu Lee* Department of Materials Science and Engineering, Yonsei University, Seoul 120-749, Korea S Supporting Information *
ABSTRACT: We here present a laser-patterning method that may solve the visibility problem associated with silver nanowire (AgNW) transparent electrodes. In conventional methods, AgNW electrodes are patterned on a transparent substrate by either selective removal or deposition of the material. Therefore, the fabricated pattern becomes visible due to the difference in transmittance/reflectance between the regions with and without AgNWs. Our approach is to disconnect the AgNW networks in selective areas by a laser and thus make the irradiated areas electrically insulating. This method is fundamentally based on Rayleigh instability. Given that the laser-cut nanowires remain on the substrate, the electrode pattern fabricated by the method can be invisible. The feasibility of the presented approach is demonstrated by fabricating organic light-emitting diodes using patterned AgNW electrodes and then characterizing their performance capabilities.
1. INTRODUCTION Transparent conducting electrode (TCE) is an essential component in a variety of optoelectronic devices, such as light-emitting diodes (LEDs), liquid-crystal displays, solar cells, and touch-screen panels. Although the most common TCE material to date is indium tin oxide (ITO) film, the intrinsic brittleness of ITO makes this material unsuitable for future flexible devices. Several other materials have thus been intensively investigated as replacements for ITO, including carbon nanotubes,1,2 graphene,3,4 conducting polymers,5,6 metal meshes,7−9 and silver nanowires (AgNWs).10−13 Carbon-based TCE materials and polymers are highly flexible, but their conductivities remain very low. Among these alternatives, AgNWs are particularly attractive because a random network of AgNWs prepared by a simple solution process can provide high conductivity and flexibility. Because of the substantial progress made over recent years, the original limitations associated with AgNWs, such as high surface roughness, low long-term stability, and poor adhesion to the substrate, have mainly been overcome.14−19 However, solving the visibility issue remains a challenge, which restricts the wider use of these materials in actual devices. While AgNWs coated onto a substrate should be patterned for device integration, the patterned structure can be visible to the naked eye due to the difference in transmittance/reflectance between the regions with and without AgNWs. Here, we present a patterning method that enables this visibility problem to be overcome. Our approach is to disconnect the AgNW networks in selective areas by a laser and thus make the irradiated areas electrically insulating. Since the laser-cut nanowires remain on the © XXXX American Chemical Society
substrate, the electrode pattern fabricated by this method can be invisible. Lasers can be effectively utilized to manipulate nanomaterials, which include the laser-induced synthesis of nanoparticles and others in liquids,20−23 the shape and size control of these materials,24−27 the fabrication of core−shell nanostructures from thin films,28 the alloying of colloidal mixtures,29 and the direct writing of two- and three-dimensional nanostructures.30,31 Useful metal nanostructures can be fabricated by a number of different techniques, including conventional lithography and chemical processes based on self-assembly. A simple alternative method to form nanostructures is based on the dewetting of metal thin films on an inert substrate. Dewetting is a spontaneous physical phenomenon describing the rupture of a thin liquid film on the substrate and the formation of droplets or other forms of nanostructures. Since the high melting temperatures of most metals are not easily reached by conventional heating, energetic sources such as laser are required for liquid dewetting. Pulsed laser melting and dewetting, especially using nanosecond laser pulses, have been the subject of tremendous theoretical and experimental investigations.32−36 Simulations32,33 showed that temperatures up to 2000 K can be achieved in Co and Ni films by a single nanosecond laser pulse at fluence levels of 100−300 mJ/cm2. It was also found that the characteristics of dewetting are influenced by many parameters, including the physical and Received: August 9, 2016 Revised: August 24, 2016
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DOI: 10.1021/acs.jpcc.6b08019 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
mm) was employed as the laser source. The output laser beam was made incident into a galvanometric scanner and an F-theta lens (focal length = 205 mm) combined with the galvanometric scanner was used to steer the laser beam and maintain a uniform spot size on the sample surface. The sample was stationed on a z-translation stage, and the stage was vertically displaced 50 mm from the focal plane to have a defocused beam of 0.5 mm size on the sample surface. The laser beam was automatically scanned by computer software which controlled the galvanometric system. The power and scan rate of the beam were independently varied. The sheet resistances of the AgNW films were measured using a four-point probe. Transmission spectra were measured by a UV−vis spectrophotometer, and structural analyses were carried out using a field-emission scanning electron microscope (SEM, Model: JSM-7001F, JEOL Inc., 15 kV). White LEDs were attached to the patterned AgNW electrodes using a silver-filled electrically conductive adhesive (ELCOAT P-100, Cans). OLEDs were fabricated employing a common OLED structure. AgNW film was coated onto a glass substrate, and this film was laser-patterned to have two rectangular conducting electrodes. Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS, Clevios PVP AI 4083) was mixed with isopropyl alcohol (volume ratio = 1:1) and then vigorously stirred for 2 h at room temperature. A 200 μL drop of the mixture solution was spin-coated on the patterned AgNW anodes at 1000 rpm for 60 s and then dried for 1 min at 100 °C. The same coating and drying process was repeated six times, and the overall layer was finally dried for 10 min at 100 °C. A light-emissive polymer, SY-PPV (Super-Yellow by Merck), was dissolved in toluene (0.52 wt %), and the mixture was stirred for 8 h at room temperature. A 200 μL drop of this “super-yellow“ solution was spin-coated over the PEDOT:PSS layer at 1500 rpm for 30 s inside a glove box and then dried for 5 min at 100 °C. Cs2CO3 powders (0.5 wt %) dissolved in 2ethoxyethanol were ultrasonicated for 10 min at 85 °C and then vigorously stirred for 8 h at the same temperature. A 200 μL drop of the Cs2CO3 solution was then spin-coated onto the super-yellow emissive layer at 5000 rpm for 30 s inside a glove box, followed by drying for 5 min at 100 °C. Finally, Al cathodes (150 nm) were deposited on the Cs2CO3 layer by thermal evaporation using a shadow mask. A conductive silver adhesive was applied to the edges of the AgNW and Al electrodes to form an electrical connection. The characteristics of the OLEDs were measured with a Keithley 2400 source meter and a Konica-Minolta CS200 chroma-luminance meter.
thermodynamic properties of metals, the initial thickness and surface roughness of the metal film, and the thermal conductivity of the substrate. The laser−nanomaterial interaction suggests a possible way of solving the visibility problem of AgNW electrode pattern. In the current study, AgNW film was space-selectively irradiated using a nanosecond pulsed laser. This made it possible to obtain a patterned structure consisting of conducting and insulating regions. The electrical insulation in the irradiated areas was attributed to the disconnection of AgNWs caused by Rayleigh instability. The applicability of the presented method is demonstrated by fabricating organic lightemitting diodes (OLEDs) using patterned AgNW electrodes and then characterizing their performances.
2. EXPERIMENTAL SECTION The AgNWs used in this study were supplied from NANOPYXIS Inc. The as-received product was a 1 wt % solution of AgNWs dispersed in ethanol, where the wire length and diameter are 25 ± 5 μm and 32 ± 5 nm, respectively. The asreceived 1 wt % solution was further diluted to 0.3 or 0.1 wt %. Except for those in Figure 1, all data presented here were
3. RESULTS AND DISCUSSION The cause of the visibility is quite clear. The AgNW electrode can be patterned by a number of different methods, including etching,37,38 lift-off,39 printing,40,41 and laser ablation.42−44 In these conventional methods, the patterned structure is formed by either the selective removal or deposition of the material. Regardless of which method is utilized, the fabricated pattern can become visible due to the difference in transmittance/ reflectance between the regions with and without AgNWs. Light scattering by AgNWs also makes a contribution to the visibility of the pattern. This visibility issue is common to other metal-based TCEs, such as metal meshes. Images displayed from a transparent device need to be clearly discernible, but the electrode pattern should be invisible. The transparency of AgNW film decreases as its conductivity increases. Therefore, the electrode pattern formed on a transparent substrate is easily
Figure 1. (a) Transmission spectra of AgNW films coated using different solution concentrations. (b) Digital camera images of the glass substrates (2 cm × 2 cm) containing AgNW films. In the lower row of images, AgNW film is coated onto part of the substrate using a mask.
obtained from AgNW films coated using a 0.3 wt % solution. A 200 μL drop of the solution was spin-coated onto a slide glass substrate (2 cm × 2 cm) at 1000 rpm for 1 min. The coated film was then dried at 100 °C for 5 min. A nanosecond-pulsed ultraviolet (UV) laser (Coherent AVIA 355-5; wavelength = 355 nm, pulse width
