One-step gold line fabrication from particle-free inorganic salt-based ink via atmospheric pressure nonequilibrium plasma-assisted inkjet printing

Highlights

• One-step printing of a gold line from an aqueous solution via a plasma–assisted inkjet printing process was demonstrated.

• The substrate temperature was expected to be less than 70 °C.

• The time required to reach the gold line without any salt footprints was less than 1 min.

• The applicability of the printed gold as a SERS-sensing platform was confirmed.

Abstract

With the growing demand for printed electronics, the printing technology using metals has become increasingly crucial. To reduce the thermal damage to printed electronic devices, it is essential to produce the metallic pattern in a short time and at a low temperature. Furthermore, to prevent nozzle clogging in the inkjet printing process, solution-based inks rather than particle-dispersed inks are desired. Here, we demonstrate the one-step printing of a gold line from an aqueous chloroauric acid solution via a nanosecond-pulsed atmospheric pressure (NSAP) nonequilibrium plasma-assisted inkjet printing process. The time required to reach the gold line without any salt footprints was less than 1 min. The substrate temperature was expected to be less than 70 °C, which is lower than that observed for conventional processes. The width of the drawn line changed according to the power input to the plasma. The applicability of the printed gold as a sensing platform for surface-enhanced Raman scattering was confirmed by detecting low concentrations of rhodamine 6G molecules. High-density reactive species in NSAP nonequilibrium plasma are expected to promote rapid and low-temperature fabrication of a metallic pattern from an inorganic metal salt solution, leading to the sophistication and diversification of future printed electronics.

Introduction

Since its introduction in the nineteenth century, inkjet printing technology has become a commonly used printing technique in the publishing and graphics industries [1]. In recent years, this technology has generated a great deal of interest as a technique for manufacturing various electronic devices such as organic electronic displays, transistors, gas-sensing devices, and photodiodes [[2], [3], [4]]. Different inks, based on metals, metal oxides, carbides, polymers, and low-molecular-weight organic compounds, and their fabrication processes have been developed to manufacture various printed devices. Metals are indispensable for developing printed devices as they display high electrical conductivity in the bulk state and excellent malleability and ductility. In addition to being highly conductive for wiring, metals, particularly those with nanostructures such as nanoparticles or nanopores, possess unique optical and electrical features. Thus, these metals are extensively utilized in applications such as surface-enhanced Raman scattering (SERS), catalysts, and sensing devices [[5], [6], [7]].

For printing metal patterns, metal nanoparticle-dispersed inks, produced by dispersing metal nanoparticles at high concentrations in water or an organic solvent such as toluene are commonly used [8]. The prepared pattern exhibits a relatively high electrical resistivity of approximately 10⁻⁸−10⁻⁴ Ωm. However, these inks are prone to particle agglomeration due to the high particle densities and high surface energy of metal nanoparticles, thus clogging the printer nozzle. For a smooth ejection of ink from the nozzle, metal nanoparticles are often coated with an organic dispersant for their stable dispersion in the solvent [9]. Therefore, after the printing process, it is necessary to remove the organic dispersant that would otherwise impede electrical conduction and to make necking between the particles by high-temperature sintering. Generally, the substrate is subjected to high temperatures (several hundred °C) for several hours, and hence some heat-sensitive substrates cannot be used [10].

To eliminate the possibility of particle agglomeration described above, an alternative to nanoparticle inks, namely fully soluble molecular precursor ink, also known as particle-free ink, can be used [11]. Metal-organic decomposition (MOD) inks, composed of an organic metal solution, are typical particle-free inks [12,13]. The metal ions in the organometallic solution are reduced via heat treatment or light irradiation to form a metal pattern. However, there is still a concern that these compounds are not readily available and must be carefully designed and synthesized so that they are stable before and during printing under ambient temperatures, while remaining capable of being decomposed to produce metallic structures at slightly elevated temperatures [14]. Furthermore, although the temperature required to produce the metallic pattern is lower than that required in post-treatment with particle-dispersed inks, in many cases, heat treatments at approximately 100–200 °C are required to decompose and evaporate the organic compounds, and hence, still impose a limit on the type of substrate that can be used [15].

Other particle-free inks made from inorganic metal-salt solutions, such as a metal chloride or a metal nitrate compound, have also been reported [[16], [17], [18]]. As this ink is solution-based, similar to the MOD ink, there is a slight concern regarding particle agglomeration. Furthermore, inorganic metal salts can be synthesized and obtained relatively easily and are chemically stable over both a wide temperature range and a long period. As inorganic metal salts generally have more stable chemical bonds compared with MOD inks, heat treatments using higher temperatures are required to reduce the metal ions. To lower the process temperature, methods other than heat treatments were used. Sui et al. obtained metal patterns by irradiating a printed inorganic metal salt with low-pressure (LP) nonequilibrium plasma containing highly energetic electrons and ions [18]. Nonequilibrium plasmas, which display relatively low temperatures and high reactivity, could decompose inorganic metal salts at a low temperature of approximately 70–135 °C. As the plasma-irradiated patterns can have large surface areas, the sensing devices made from these patterns exhibit enhanced sensitivity compared with analogs made using the conventional method. However, the LP plasma process generally requires a vacuum system, which increases the number of process steps and operation cost, thus slightly impairing the simplicity of the original inkjet printing process.

Recently, nanosecond-pulsed atmospheric pressure (NSAP) nonequilibrium plasma has increasingly attracted attention; this plasma has a very small discharge time, of the order of nanoseconds, and thus, the gas temperature rise is relatively low even in a high-density medium [19]. High-density reactive species (electrons, ions, radicals, etc.) from the NSAP nonequilibrium plasma can promote the decomposition of the inorganic salt, and a recombination of radicals on the surface as well as a local current and associated local heating in the pattern, are expected to enhance the reactions. Furthermore, by using NSAP nonequilibrium plasma, an open-air process is possible without using a vacuum device. In this study, gold patterns were produced from inorganic metal–salt solutions via plasma-assisted inkjet printing [20,21] with NSAP nonequilibrium plasma, leading to high-speed and low-thermal-damage printing of gold in one step.