Amorphous carbon interlayers for gold on elastomer stretchable conductors

Abstract

Gold on polydimethylsiloxane (PDMS) stretchable conductors were prepared using a novel approach by interlacing an hydrogenated amorphous carbon (a-C:H) layer between the deposited metal layer and the elastomer. AFM analysis of the a-C:H film surface before gold deposition shows nanoscale buckling, the corresponding increase in specific surface area corresponds to a strain compensation for the first 4-6 % of bi-axial tensile loading. Without this interlayer, the deposited gold films show much smaller and uni-directional ripples as well as more cracks and delaminations. With a-C:H interlayer, the initial electrical resistivity of the metal film decreases markedly (280-fold decrease to 8x10 -6 Ω.cm). This is not due to conduction within the carbon interlayer; both a-C:H/PDMS and PDMS substrates are electrically insulating. Upon cyclic tensile loading, both films become more resistive, but return to their initial state after 20 tensile cycles up to 60% strain. Profiling experiments using Secondary ion mass spectroscopy (SIMS) and X-ray photoelectron spectroscopy (XPS) indicate that the a-C:H layer intermixes with the PDMS, resulting in a graded layer of decreasing stiffness. We believe that both this graded layer and the surface buckling contribute to the observed improvement in the electrical performance of these stretchable conductors.

Stretchable conducting gold films prepared with composite MWNT/PDMS substrates

“…12 Away from such stiff platforms, these materials experience high tensile strains and rely on conduction through micro-cracked tracks. The work undertaken in this study, as well as the previous investigations in our laboratory 6,7 only confirms further the resilience of such thin film conductors. The reaction between the MWNT and the PDMS cure is demonstrated independently with swelling measurements as well as spectroscopic and thermal data.…”

“…5 We have generalized this method to bi-axial stretching using a photolithographic structuring of the gold film 6 and the insertion of a stiff carbon interlayer to generate a random buckling pattern. 7 We observed a stable electrical behaviour up to 35% strain for 115 cycles without failure 6 and a film resistivity around 8x10 −6 Ω.cm. 7 These two methods rely on creating a large gold specific surface area, producing conductors with improved electrical conduction at zero strain, while sustaining prolonged mechanical cycling.…”

“…7 We observed a stable electrical behaviour up to 35% strain for 115 cycles without failure 6 and a film resistivity around 8x10 −6 Ω.cm. 7 These two methods rely on creating a large gold specific surface area, producing conductors with improved electrical conduction at zero strain, while sustaining prolonged mechanical cycling. These materials are potentially useful for applications where measurements can be made at low strain as long as the device can survive high strain conditions, for instance for biomedical skin electrodes used in continuous monitoring.…”

“…Another is the dynamic nature of these boundary regions, which are less pronounced after cycling, as shown in figure 7. In some cases, 7 this is accompanied by a reduction in resistance at zero load. One tentative explanation is that, during cycling, Poisson stresses result in shear contact stresses on the sea-saw crack edges, shown in figure 9(b).…”

“…The combined effects are, therefore, difficult to predict, hence, in this study the gold film thickness was kept at 25 nm, a process parameter optimised in previous investigations. 6,7 Thermogravimetric analysis (TGA) was performed in ceramic crucibles using a ∼5 mg specimen at a heating rate of 10 • C/min. The MWNT samples were analysed in air to estimate the metallic residue.…”