Tunable resistivity in ink-jet printed electrical structures on paper by plasma conversion of particle-free, stabilizer-free silver inks

Abstract

Printable metal inks are typically composed of premade nanoparticles that require postdeposition thermal sintering to produce crystalline, electrically conductive features. In this paper, it is shown that particle-free Ag inks made from simple, water-soluble metal salts such as silver nitrate can be ink-jet printed and converted into electrical features with tunable resistivity at low temperature (<100 °C) by exposure to a pure argon plasma. X-ray diffraction confirms that the converted inks are crystalline, and four-point probe electrical measurements show that the sheet resistances are a function of the pressure and power in the plasma. From cross-sectional scanning electron microscopy analysis, it is found that the morphology of the converted silver layer becomes increasingly dense with increasing plasma treatment time, which explains the measured changes in sheet resistance, and that the thickness of the layer is ∼1.5 μm, which yields a minimum resistivity of ∼6 × 10−8 Ω m, approximately 3.8 times higher than bulk resistivity of silver. Interestingly, the resistivity can be varied over a span of 6 orders of magnitude which allows resistor–capacitor filter devices to be fabricated exhibiting varying cut-off frequencies from a single material and geometry.

Plasmas for additive manufacturing

“…Another study showed that the conductivity of geometrically identical printed Ag structures could be systematically varied over a span of six orders of magnitude by adjusting plasma conductions such as power, background gas pressure, and duration. [ 28 ] The tunable resistance is attractive for small area resistors where changing the dimensions of the structure to achieve different resistances is challenging. Recently, the plasma‐activated conversion of printed ionic compounds was successfully extended to a large set of aqueous metal salt precursors, establishing a new class of particle‐free inks.…”

Fabrication of a Silver-Based Thermistor on Flexible, Temperature-Sensitive Substrates Using a Low-Temperature Inkjet Printing Technique

“…In practice, to achieve high sensitivity, the resistance of the thermistor at the reference temperature must be large. With the inorganic Ag-salt ink, in addition to the serpentine design, the resistance of the printed Ag line can be tuned by varying the plasma treatment time [16]. We found that for a line thickness of ~1 μm, plasma treatment at 150 W for 10 min yields Ag with a resistivity of 2 × 10 −6 Ω•m.…”

“…The thermistor was fabricated by the inkjet printing and plasma treatment process illustrated in Fig. 1(a) and first described in [16]. The Ag ink was prepared by dissolving 0.5 M (1.7 g) silver nitrate (AgNO 3 , 98% purity, Alfa Aesar) in a mixture of 4 ml de-ionized water (Millipore), and 16 ml ethylene glycol (Fisher Scientific).…”

“…In this letter, we report on the development and preliminary testing of a silver-based thermistor on flexible, degradable, and temperature-sensitive polymer substrates using a low-temperature inkjet printing technique. The printing technique utilizes a unique Ag saltbased ink and a plasma reduction method to convert the ink to Ag [16]. The highest substrate temperature in our printing process was 42 °C, which is significantly lower than the annealing temperature (~150 °C) used to fabricate thermistors from nanoparticle inks [7], [8].…”

A New Class of Low‐Temperature Plasma‐Activated, Inorganic Salt‐Based Particle‐Free Inks for Inkjet Printing Metals

“…Here, we report a new approach for inkjet printing based on particle‐free inks composed of inorganic metal salts and their conversion by a non‐equilibrium plasma to produce metallic structures at low temperatures (<138 °C). Plasmas have been previously applied to reduce metal salts to common noble metals such as Au, Ag, Pd, and Pt . Our study expands this technology to enable a library of metals, including both noble metals such as Au, Ag, Pd, and Pt and non‐noble metals such as Cu, Pb, Bi, and Sn, to be printed by a single process that involves plasma reduction of the printed structure.…”