Letter pubs.acs.org/macroletters
Microplasma-Induced in Situ Formation of Patterned, Stretchable Electrical Conductors Souvik Ghosh,† Erika Klek,‡ Christian A. Zorman,‡ and R. Mohan Sankaran*,† †
Department of Chemical and Biomolecular Engineering and ‡Department of Electrical Engineering and Computer Science, Case Western Reserve University, Cleveland, Ohio, United States S Supporting Information *
ABSTRACT: We report a microplasma-based process to fabricate stretchable, electrically conductive metal patterns from metal-cation containing polymers. The technique is compatible with prestraining strategies, allowing films to remain conductive with almost no drop in resistance up to 35% strain. We show that the stretchability of the films is related to uniform strain delocalization which is made possible by how the metallized layer is formed in situ, growing from within the polymer matrix rather than by deposition, to create a quasi-monolithic structure without a well-defined metal−polymer interfacial boundary.
temperature and film damage is minimized.20 Stretching studies show that the films exhibit negligible change in resistance up to 10% uniaxial strain and by prestraining, can be extended to more than to 35%. Scanning electron microscopy (SEM) analysis reveals that strain is highly delocalized, producing microcracks distributed evenly throughout the film without delamination of the metallized surface layer from the polymer substrate. We suggest that this is achieved by excellent interfacial adhesion uniquely created by the nature of the growth of the percolated metal layer from within the polymer itself. Electrically conductive patterns of metallic silver (Ag) were fabricated on a stretchable elastomer, polydimethylsiloxane (PDMS) by an atmospheric-pressure microplasma process (Figure 1a). A 50 μm thick poly(acrylic acid) (PAA) film containing Ag cations was deposited on PDMS by doctor’s blade from solutions of 0.4 g PAA (1.25 MM MW, SigmaAldrich) and 0.47 g AgNO3 mixed in 150 mL of 1:3 v/v water:ethanol to give a final metal loading of 30 wt %. The microplasma was formed in a flow of helium gas and powered by high voltage alternating current (AC) at a constant voltage and frequency of 1.5 kV peak-to-peak and 35 kHz, respectively. The microplasma was scanned across the films either asdeposited or under uniaxial strain, termed prestrain.21,22 Highly reflective, white-gray features were immediately observed as the microplasma interacted with the film surface in all cases, indicating reduction of the Ag+ in the PAA film to metallic Ag (Figure 1b, left). The crystallinity of the generated patterns was confirmed using X-ray diffraction (XRD) (Figure S1,
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atterned, stretchable electrical conductors are a critically important building block to emerging flexible electronic applications, requiring seamless integration of relatively stiff metals and much more flexible polymers.1 Subtractive approaches, which describe the deposition and selective removal of polycrystalline thin films of metals on polymer substrates by a combination of sputtering or evaporation and lithography, are capable of producing extremely high fidelity micron-scale features with electrical conductivities that are suitable for microelectronics applications.2−6 Interfacial stresses between the dissimilar materials limits mechanical strain in these systems to less than 3% before the metal cracks and conductivity is irreversibly lost,3,5 necessitating adhesion layers between the metal and polymer to delocalize the strain.4,7,8 Recently, additive approaches based on printing of ink solutions containing premade inorganic nanomaterials have allowed highly stretchable conductive patterns to be selectively deposited on elastomers.9−13 However, the presence of polymer binders and other organics in the inks14 hinder electrical conductivity; therefore, postdeposition heat treatment is often required, which is incompatible with temperaturesensitive materials.15 Techniques that can avoid high temperatures have also been reported, but have yet to demonstrate highly stretchable conductors.16−19 This communication presents a rapid, nonthermal, atmospheric-pressure microplasma-based process to fabricate patterned, stretchable, electrically conductive structures. Metalcation containing polymer films are exposed to a scanning microplasma jet which leads to in situ electrodiffusion, reduction which leads to in situ electrodiffusion, reduction, and percolation of the metal cations, forming a thin (
