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Liquid Deposition Patterning of Conducting Polymer Ink onto Hard and Soft Flexible Substrates via Dip-Pen Nanolithography Hiroshi Nakashima,*,†,‡ Michael J. Higgins,† Cathal O’Connell,† Keiichi Torimitsu,‡ and Gordon G. Wallace*,† †
ARC Centre of Excellence for Electromaterials Science, Intelligent Polymer Research Institute, AIIM Facility, Innovation Campus, University of Wollongong, Wollongong, NSW 2522, Australia ‡ Materials Science Research Laboratory, NTT Basic Research Laboratories, NTT Corporation, 3-1, Morinosato Wakamiya, Atsugi, Kanagawa 243-0198, Japan
bS Supporting Information ABSTRACT: Ink formulations and protocols that enable the deposition and patterning of a conducting polymer (PEDOT: PSS) in the nanodomain have been developed. Significantly, we demonstrated the ability to pattern onto soft substrates such as silicone gum and polyethylene terephthalate (PET), which are materials of interest for low cost, flexible electronics. The deposition process and dimensions of the polymer patterns are found to be critically dependent on a number of parameters, including the pen design, ink properties, time after inking the pen, dwell time of the pen on the surface, and the nature of material substrate. By assessing these different parameters, an improved understanding of the ability to control the dimensions of individual PEDOT:PSS structures down to 600 nm in width and 10 80 nm in height within patterned arrays was obtained. This applicability of DPN for simple and nonreactive liquid deposition patterning of conducting polymers can lead to the fabrication of organic nanoelectronics or biosensors and complement the efforts of existing printing techniques such as inkjet and extrusion printing by scaling down conductive components to submicrometer and nanoscale dimensions.
’ INTRODUCTION Organic conducting polymers (OCPs) such as polypyrrole and poly(3,4-ethylenedioxythiophene) (PEDOT) are increasingly being integrated into electronic devices.1 Their ability to undergo electrically induced reversible color changes, conductive and surface energy states, expansion/contraction, and biofunctionality adds new dimensions to existing traditional electronic components (e.g., metal electrodes)2 and is currently being employed to enhance capabilities in applications such as electrochromics, actuating devices, biosensors, organic microelectronics, and implantable electrode arrays.2,3 Many of these current technologies are demanding that the electronic components, particularly individual electrodes within arrays, be scaled down and patterned in the nanodomain to improve spatial resolution and signal-to-noise ratio. These demands are challenging the ways in which OCP electrodes are typically fabricated, for example, the electrochemical deposition from a monomer solution that results in the formation of a thin OCP film on a macro to meso-sized electrode. Reproducing this electrochemical deposition on submicrometer or nanoscale electrodes is possible;4 however, the ability to control the OCP structure is difficult due to deviations from simple mass transfer processes (i.e., diffusive losses of monomer or reactive intermediates) which have been shown to occur on these smaller length scales.5 Therefore, r 2011 American Chemical Society
the fabrication of nanopatterned structures from electrochemically deposited OCP has primarily been done using top-down lithography to manipulate macroscale OCP thin films covering larger area electrodes.6 Beautiful OCP line patterns with individual widths down to 80 nm have been achieved using such lithographic approaches7 and demonstrate the ability for precise control and high resolution for nanopatterning. However, the use of electrochemical deposition as a starting point in the fabrication process still puts constraints on the diversity of material compositions/formulations and supporting substrates that can be employed within a patterned area or array. More fragile organic structures or biological constituents that are commonly incorporated into OCP as dopants for added biofunctionality may also be incompatible with conventional lithography. To enable greater flexibility over the deposition process and types of available OCP materials, the use of solution processable OCP coupled to fabrication techniques (e.g., extrusion printing, inkjet printing, microcontact printing, and microplotting) wellmatched for patterning “liquid” material dispersions (or inks) is increasingly being used in electronics fabrication research8 and Received: August 26, 2011 Revised: November 9, 2011 Published: November 21, 2011 804
dx.doi.org/10.1021/la203356s | Langmuir 2012, 28, 804–811
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on soft, flexible substrates through conventional DPN strategies still presents significant challenges.23,24 In this study, we investigated the versatility of patterning a commercial conducting polymer ink optimized for other printing techniques currently used in industry (e.g., inkjet printing). It is not known whether such inks are in fact capable of deposition when operating on a nanometer confined scale using DPN. We specifically assessed the effect of the probe design, stability of the ink, and importantly the ability to pattern on flexible substrates such as silicone gum, polyethylene terephthalate (PET), and polydimethylsiloxane (PDMS).
Table 1. Surface Properties of Various Substrates and the Size of the Polymer Dots Patterned on the Substrates static water contact substrate
rms
dot
roughnessg diameterh (μm)
dot heighth
angle (deg)
(nm)
a
(nm)
Si
24.6
0.6
1.09 ( 0.14 15.9 ( 3.2
SiO2a
22.6
3.7
1.24 ( 0.19
8.5 ( 3.5
ITOb,c
21.2
2.1
2.64 ( 0.58
8.0 ( 3.5
Goldb,d
88.5
2.2
1.96 ( 0.47 27.5 ( 5.6
PET filme
100.4
15.0
1.15 ( 0.27 81.9 ( 16.7
Silicone gume
114.2
21.0
0.62 ( 0.15 53.1 ( 38.1
PDMS (hydrophobic)e
117.7
4.0
PDMS (hydrophilic)f
12.9
2.2
’ EXPERIMENTAL SECTION Conducting Polymer Ink for DPN. An aqueous solution of poly(3,4-ethylene-dioxythiophene):poly (styrenesulfonate) (PEDOT: PSS) (CLEVIOS P Jet HC provided by H. C. Stark, solid content 0.6 1.0%, viscosity max 20 mPas) was filtered with a 1.2 μm pore-size filter and then diluted with methanol (Sigma Aldrich) (ink/methanol = 1/1 1/4 (v/v)). The diluted polymer solution was used as a PEDOT: PSS ink in all DPN experiments. Pretreatment of Pen and Substrate for DPN. We used two different types of AFM probes (termed pen for DPN). A single pen composed of a Si3N4 cantilever with a sharpened pyramidal tip (Type PNP DB provided by Nanoworld AG, 200 μm long, spring constant = 0.06 N/m) and a multiple arrayed pen composed of twelve Si3N4 cantilevers with sharpened pyramidal tips and an ink reservoir at the base of the tips (Type M provided by Nanoink, Inc., M 1 rectangular 150 μm long, spring constant = 0.4 N/m). The pens and silicon substrate were cleaned carefully in piranha solution (H2SO4/30% H2O2 = 3/1 (v/v)) at room temperature for 20 30 min (Caution: Piranha solutions react violently with organic compounds and should be used with extreme care). The pens and silicon substrate were washed several times with deionized water and methanol and dried with N2 flow. They were then placed in a plasma cleaning system (Harrick Plasma, PDC 002) for 3 min at a pressure of 1100 mTorr. These cleaning methods remove contaminants and increase the hydrophilicity of pens and silicon substrate, thus ensuring good affinity between the ink and the pen/substrate surfaces. Water contact angle measurements of the cleaned silicon substrate were performed with a goniometer (Dataphysics OCA 20 contact angle measurement system) by depositing small sessile droplets (volume of 0.5 μL) of deionized water onto the substrate. The mean value of static contact angle of water on the silicon substrate was 24.6°. We also used other substrates for DPN patterning such as SiO2 (slide glass), ITO (indium tin oxide layer (4 nm) deposited on SiO2), gold (Au (27 nm)/ Cr (3 nm) deposited on Si), PET (polyethylene terephthalate) film, and silicone gum. Precleaning methods and surface properties (contact angle and roughness) of these substrates are summarized in Table 1. DPN Process. The pens were directly dipped into the PEDOT: PSS/methanol ink for 10 20 s and then carefully dried with N2 flow. This inking method was repeated twice (double inking), and occasionally, the pens were subsequently rinsed in methanol after the inking procedure and sufficiently dried. DPN patterning was carried out using an Nscriptor system (Nanoink, Inc., Skokie, IL) equipped with a 90 μm closed loop scanner and commercial lithography software (InkCAD, Nanoink, Inc.). All procedures of the DPN patterning and characterization were performed under ambient conditions at 23 °C. AFM Imaging. AFM topographic imaging of the polymer patterning was done using the Nscriptor AFM or Asylum Research MFP 3D AFM in tapping mode. Conductive AFM was performed on the Asylum Research MFP 3D AFM using a PtIr 5 coated conductive probe (Nanoworld, EFM, spring constant: 2.8 N/m) in contact mode. Reflection Microscope Imaging. A Carl Zeiss Axio Imager microscope (equipped with EC Epiplan Plan 50 objective lens) was
a
Substrates were washed in piranha solution for 30 min, and then washed with water and methanol. b Substrates were washed with ethanol, methanol, and water, and then cleaned with UV Ozone cleaner for 3 min. c ITO layer was coated on SiO2 with a 40 ( 8 Å thickness. The conductivity of the ITO substrate was 5k/Ω0. d Gold substrates were fabricated by the deposition of Cr (3 nm, first layer) and Au (27 nm) on a Si surface. e Substrates were washed with ethanol, methanol, and water only. f Hydrophobic PDMS was treated with plasma cleaner for 1 min under oxygen at 1100 mTorr. g rms roughness was estimated to be 20 μm2. h All polymer dots were fabricated with a dwell time of 1 s in 20 μm2 on each substrate.
having an impact on the development of low cost, flexible electronics.9 These techniques importantly allow a wide variety of OCP ink formulations to be deposited on substrates other than hard inorganic substrates. For example, new, printable OCP inks are continually being developed,10 and techniques such as inkjet printing have demonstrated the ability to pattern allpolymer OCP electrode transistors.11 Simultaneous deposition of multiple inks, termed multiplexing, is also possible within a defined patterning area. The challenge in scaling down the deposition of inks to submicrometer and nanoscale length scales still remains, as many of the above liquid dispensing techniques operate within the limit of tens of micrometer resolution. One avenue to address this challenge has been the use of atomic force microscope (AFM)based probe designs with integrated reservoirs and channels to dispense subpicoliter volumes of liquid.12,13 The ink used consists of a carrier solvent to assist in the transport of a particulate material, and together, they deposit onto the substrate via physioadsorption processes. This AFM-based liquid deposition process is generally referred to as dip-pen nanolithography (DPN), but differs from the original descriptor of DPN used to define the diffusion transfer of molecules or particles through a water meniscus formed between the AFM tip and the substrate.14 16 The liquid deposition of OCP using DPN has been limited most likely due to loss of ink-polymer mobility during solvent evaporation and/or other difficulties associated with the high molecular weight, poor dispersive properties, and stability of OCP solutions. To our knowledge, there have only been a few previous reports on the DPN liquid deposition approach of OCP, including studies on a commercial PEDOT:PSS17,18 ink and a combined pyrrole:tetrahydrofuran ink.19 Several researchers have briefly explored the effects of critical parameters such as the AFM probes used for the deposition and ink properties;20 22 however, the controlled, precise patterning of nanomicrostructures of OCP ink 805
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resulted in uncontrolled “bleeding” of large amounts of ink on the Si substrate, suggesting that, even after the drying step, an excess amount of aqueous ink with unsuitable properties remained on the tip. This impeded the ability to produce patterns of uniform size via physioadsorption, as the ink uncontrollably flowed onto the substrate. In the one other report on using DPN to deposit liquid PEDOT:PSS, this problem appears to have been overcome by using higher ink concentrations (3% PEDOT:PSS) that may preferentially alter the surface tension and/or increase the viscosity of the ink. The higher concentrated PEDOT:PSS commercial ink was used to successfully pattern ∼500 1000-nm-wide and 100-nm-high conducting polymer lines on piranha cleaned/plasma treated silicon substrates.17,18 In contrast, our as-received commercial ink had a PEDOT:PSS concentration ranging between 0.6% and 1% (solid content) and viscosity of
