Thickness dependent cracking in layered 2-D system for strain sensing application*

We study crack propagation in a layered 2D material, reduced graphene oxide, on a flexible substrate under applied strain. This problem is interesting since the crack formation strongly modulates the electrical transport in this system. We study how film cracking is affecting the electrical transport in strained rGO films. It is observed that the change in fractional resistance at 5% strain varies from 3 times to 12 times the resistance at unstrained condition, as the film thickness is increased. Parallel and qausi periodic cracks are observed in rGO films upon straining, which are found to have strong dependence on the rGO film thickness. Crack density and crack width show contrasting trend as the film thickness is increased, and the observations are described by sequential cracking model. It is found that by variation in the film thicknesses, the films can be tuned from being strain resistant to strain responsive, and the former is attributed to a favorable combination of crack density and crack width. When we tune the thickness of the film for strain responsive behavior, strain sensors with gauge factors of upto 370 are realized. While, for the thickness corresponding to strain resistant film, stable stretchable rGO-TiO₂ UV photodetector is realized.