Quantification of the mechanical strength of thermally reduced graphene oxide layers on flexible substrates
Introduction
The bonding properties between thin films play a critical role in the durability and reliability of micro and nano-fabricated devices. This is especially true when films are deposited using different methods like e-beam evaporation, sputter coating, drop casting, atomic layer deposition [1], chemical vapor deposition, etc. This mechanical durability is especially important in implantable biomedical devices.
Graphene is an incredibly promising material for a new generation of electronics due to its high elastic modulus and strength [2], [3], [4], [5] combined with beneficial electrical properties, thermal conductivity [4], [5], [6], [7], impermeability [8], and incredible conformity [9]. Exploration of these properties have been the focus of a tremendous amount of research into using graphene in nano-scale electronics [1], [10], [11], [12], [13], [14]. Reduced Graphene Oxide (rGO) shares many similar properties with graphene while simplifying device synthesis and enabling the creation of large-scale graphene-based electronics through the deposition of graphene oxide (GO) followed by reduction to interconnect the distinct precursor flakes [15], [16], [17], [18]. The mechanical properties of graphene and rGO make them well suited for flexible and stretchable electronics [16], [17], [18], [19], which are regularly created on polymeric substrates.
The adhesive energy of graphene-based materials to the substrates they are deposited upon and between graphene layers can have a large effect on the mechanical survivability of devices they compose. The effective contact area between graphene and substrate [11], the number of graphene layers [3], [20], and surface properties of the substrate [9], [21], [22] all affect the adhesion energy between graphene and substrate. Many studies have been performed using the bulge test [23] to identify the bonding energy of graphene and SiO2, modeling graphene sheets as a perfectly 2D film [24] and reporting adhesion energy in the range of 0.15–0.45 (J/m2) [3], [8], [20], [25], [26], [27]. Yoon et al determined the bonding energy of mono-layer graphene on Copper to have a value of 0.72 (J/m2) [28] based on double cantilever beam testing. A nonlinear decrease of adhesion energy was also demonstrated by increasing the number of graphene layers [20], [26]. Direct measurement of cleavage energy of graphite was done employing self-retraction phenomenon that led to cleavage energy of 0.37 (J/m2) [29].
Adaptations of tape based peel tests have been employed to evaluate the mechanical robustness of electrodes of flexible lithium-ion batteries and reach out the optimum electrode composition [30] and to measure bulk failure properties of rGO on various backings without differentiating failure modes [31].
The methods previously employed are inherently high precision but require expensive high precision equipment and have been limited to quasi-rigid substrates. Additionally, the potential for multi-layered rGO devices introduces multiple potential failure modes, with differing failure energies that previous studies did not address and cannot differentiate.
The objective of the work presented here was to measure the bonding strength of graphene-based structures on flexible substrates, accounting for multiple failure modes, and using commonly available equipment through conducting a controlled peel test. Tests were performed on specimens of reduced Graphene Oxide deposited on polyimide substrate quantifying the failure energy resulting from different failure modes, and preparation processes to identify mechanically strong structures.
Section snippets
Methods
This paper presents testing methods and results measuring the cohesive failure energy between layers of reduced graphene oxide (rGO) flakes as well as adhesive failure energy between rGO and flexible substrates mapped out in Fig. 1. Testing employed a highly controlled peel test combined with image processing data. Several samples of rGO were created by drop casting of precise amounts of GO solution with specific concentration (listed in Table 1), that had been sonicated for one hour, onto
Experimental results
Generally, variation of OCF ratio is corresponding to variation of amount of detached rGO such that an increased OCF would be corresponded to a weaker cohesive strength and consequently, smaller average peel force. Fig. 3(a) shows a sample with high OCF of 92.3% and a small average peel force around 0.1 N (represented by the red line in Fig. 3(b)). The profile of adhesive contribution (given from image processing) was represented by a blue line in Fig. 3(b) showing the correlation of 91% with
Conclusion
The improved 90-degree peel test was employed in this study in addition to an image processing technique to distinguish between failure modes of samples of rGO deposited on flexible polyimide (Kapton) substrates. Through determining the amount of visible rGO particles that were removed form a substrate during testing, the corresponding failure mode energies were obtained using a mathematical model. Using the model presented, the adhesive and cohesive failure energies were be extracted from a
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by The National Science Foundation under Grant No 1727846, titled “SNM: Customized Inkjet Printing of Graphene-Based Real-time Water Sensors”. DuPont Company provided Kapton samples. Special thanks to Shima Mehrvar for help in the image processing coding. The O2 plasma machine is owned by UWM Water Technology Accelerator.
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