Defect Engineering Graphene for Dynamic Reliability

The ability to withstand high tensile strain as a result of strong in-plane sp² covalent bonding, low bending stiffness, and a wide variety of tunable electrical properties has led to wide interest in the development of 2D materials-based flexible devices such as flexible electronics, nanocomposites, and multifunctional coatings. Despite exceptional intrinsic mechanical, electrical, and optical properties, challenges remain in the realization of 2D materials in their functional device applications. The interface between 2D materials and soft stretchable substrates is a governing parameter in proposed 2D materials-based flexible devices. However, the same interface is dominated by van der Waals (vdW) forces and the mismatch in elastic constants between the contact materials leads to very low adhesion energy (also referred as interface toughness, work of separation, or work of adhesion). For example, the adhesion energy between monolayer graphene and silicon oxide is estimated to be ∼450 mJ.m^-2 (<i>1</i>), while the adhesion energy between monolayer graphene and polydimethylsiloxane (PDMS), a soft and stretchable elastomeric substrate, is estimated to be only ∼7 mJ.m^-2 (<i>2</i>). When the 2D material/stretchable substrate system is subjected to cyclic loading by applying continuous tension and compression, the low adhesion leads to decoupling, slippage, and extensive damage propagation in the 2D lattice (<i>3</i>). With only 10 cycles of cyclic loading of graphene on PDMS at ε = 20% strain range, ∼30% of a graphene flake’s area is catastrophically damaged (<i>3</i>).<br/><br/>We reveal that through careful defect engineering of graphene, the contact interface between graphene and PDMS can be modified to increase the adhesion energy to ∼40 mJ.m^-2, which is characterized with an atomic force microscope (AFM) by employing buckling-based metrology (<i>4</i>). The defect engineering consists of exposing a single side of a graphene flake to controlled and mild oxygen plasma, and then placing the flake's functionalized side in direct contact with PDMS. The type and density of defects are characterized using X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The defects present are revealed to be a low density of sp³-type defects, mainly oxygen functional groups. In situ micro tensile testing under an optical microscope and optical image processing reveals that the increased adhesion in defect engineered graphene/PDMS systems drastically inhibits damage propagation and improves fatigue life. Additionally, insight into the role of each functional group in relation to the change in adhesion is investigated using molecular dynamics (MD) simulations. This work could be extended to other 2D material/stretchable substrate systems and offers insight into achieving dynamically reliable contacts, which could facilitate the development of 2D materials-based flexible devices.<br/><br/>1. S. P. Koenig, N. G. Boddeti, M. L. Dunn, J. S. Bunch, Ultrastrong adhesion of graphene membranes. <i>Nature Nanotechnology</i>. <b>6</b>, 543–546 (2011).<br/>2. S. Scharfenberg, D. Z. Rocklin, C. Chialvo, R. L. Weaver, P. M. Goldbart, N. Mason, Probing the mechanical properties of graphene using a corrugated elastic substrate. <i>Applied Physics Letters</i>. <b>98</b>, 1–4 (2011).<br/>3. T. Cui, K. Yip, A. Hassan, G. Wang, X. Liu, Y. Sun, T. Filleter, Graphene fatigue through van der Waals interactions. <i>Science Advances</i>. <b>6</b> (2020), doi:10.1126/sciadv.abb1335.<br/>4. C. J. Brennan, J. Nguyen, E. T. Yu, N. Lu, Interface Adhesion between 2D Materials and Elastomers Measured by Buckle Delaminations. <i>Advanced Materials Interfaces</i>. <b>2</b>, 1–7 (2015).