The areas of flexible electrodes and strain sensors have seen a lot of growth in recent years, especially as materials with a flexible nature have become more commercialised. One such material used in these applications is reduced graphene oxide (rGO), and a team of researchers from India have now determined the crack dynamics involved when a sheet of rGO is uniaxially strained on a flexible substrate.
There are currently two common methods used to determine the crack related properties of materials. These are: binding the material to a substrate as a film and using a flexible composite film. The researchers in this recent work opted for the former.
The researchers took a reduced graphene oxide (rGO) suspension and spin coated it onto a Polydimethylsiloxane (PDMS) substrate and modified the surface through oxygen plasma treatments, so that it became hydrophilic. The amount of rGO layers on the substrate was determined through the amount of spin coating (followed by annealing) steps the researchers undertook.
rGO is a highly oxygenated (functionalised) version of graphene oxide and is commonly produced through chemical, thermal or solar exfoliation methods. Graphene oxide itself contains a high number of oxygen moieties compared to ‘pure’ (or CVD-grown) graphene, and rGO contains even more still.
The functionalisation of the surface in both graphene oxide and rGO allows the material to be hydrophilic in nature compared to pure graphene sheets.
The researchers characterised the sheets through Raman spectroscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), optical microscopy (Leica Stereozoom M205 microscope), Optical Surface profilometry (Bruker, Non-contact), transmission electron microscopy (TEM, (JEOL 2100), scanning electron microscopy (SEM, FEI Quanta FEG 200), electron beam lithography (Raith 150).
The electrical measurements were performed using 4 terminal in-line contacts, a lock in amplifier (SR830 DSP) and an Agilent E4980A LCR frequency dependent meter. The strain measurements were performed through mounting using a uniaxial strain and optoelectronic properties were taken from exposure to light and dark conditions with a UV-lamp and micromanipulator.
The researchers found that crack propagation in the uniaxially strained rGO is largely dependent upon the film thickness. Upon strain, the suspended rGO sheet was found to produce quasi-periodic cracks, with the crack density and crack width following contrasting trends as the film thickness was increased. The researchers determined the trends to follow a sequential cracking model.
The cracks were also found to relax when the strain was removed from the sample and these features were reflected during the strain-dependent electrical dc and ac conductivity studies.
The researchers tested between 1 and 6 coats and the optimal thickness was found to be 3 coats (spin coats) of the graphene-based solution, where the films were found to behave as strain-resistant. All of the other thicknesses of rGO were found to be strain-responsive and is attributed to favourable combinations between the crack density and width. In the case of flexible electronic applications, strain-resistant properties are favourable.
By tuning the thickness of the sheet, the researchers showed that the graphene sheet retained its excellent conductivity properties under 5% strain- a value much higher than the limit for conventional flexible electrode materials. Conventional, inorganic-based, flexible electrodes in use today have not been able to withstand more than 1.75% strain, where any of the cracks produced under this strain led to complete material failure. So, the findings in this research present a significant enhancement on commercial materials.
The researchers also produced a primitive device composed of TiO2 nanoparticles and an rGO coated electrode (1 coat) on a PDMS substrate. The researchers tested the optoelectronic device in light and dark conditions. Both the resistance under dark and illuminated conditions produced a much weaker dependence on strain and the change in resistance was directly attributed to the photodetection mechanism.
The research into the thickness dependency and crack propagation mechanism of substrate-bound rGO sheets allows for the rationalisation of graphene-based systems for flexible optoelectronic and strain sensing applications.
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“Thickness-dependent Crack Propagation in Uniaxially Strained Conducting Graphene Oxide Films on Flexible Substrates”- Sakorikar et al, Scientific Reports, 2017, DOI:10.1038/s41598-017-02703-2