Wrinkle networks in exfoliated multilayer graphene and other layered materials

Abstract

We describe a method to obtain networks of wrinkles in multilayer graphene flakes (and other layered materials) by thermal contraction of the underlying PDMS substrate they are deposited on. The exfoliated flakes on PDMS are dipped into liquid nitrogen and after removal networks of wrinkles are found. The density of wrinkles can be controlled to some degree by sequential dipping into liquid nitrogen. Atomic force microscopy shows that wrinkles form preferentially along the armchair direction of the graphene lattice in such multilayer graphene platelets. Raman spectra show that the interlayer coupling at a wrinkle in multilayer graphene differs from, and is weaker than, that in undeformed regions. High resolution transmission electron microscopy measurements show that the interlayer distance increases in strained regions, which results in the interlayer coupling being decreased in particular regions of the wrinkles in these multilayer graphenes.

Introduction

It has been observed that when materials sizes are reduced to micro- and nano-meter scales, anomalous stress-strain behaviors such as static and dynamic negative compressibility emerge [1,2]. In particular, two-dimensional (2D) materials, such as graphene, MoS₂, and WSe₂, have been reported to show unusual physical and chemical properties under stress and strain [[3], [4], [5], [6], [7], [8], [9]]. Due to the layered nature of 2D materials, wrinkles can be formed by applying uniaxial or biaxial stress and are known to significantly alter the materials' properties [10], for example, previous studies have shown that giant pseudomagnetic fields can be realized on strained graphene [11,12] and that optical absorption and photoluminescence properties in MoS₂ and other layered semiconductor materials can be tuned by strain [13,14]. Understanding how strain affects graphene's electronic and optical properties is important for science and also for technological applications. For instance, in wearable electronics, tailoring of wrinkle structures has been reported to enhance the fatigue strength of the material [15]. Thus, “straintronics” or strain-based electronics where electronic properties are modified by strain engineering has emerged as a new area of research in this field [[3], [4], [5],9,16]. Creating or designing wrinkles can be potentially used to create controlled strain in the material and also allows to systematically study the mechanical behavior of wrinkled graphene.

When graphene films are exfoliated from graphite, wrinkles can be created by localized stress [4,17,18], which perturbs the crystal lattice of graphene [4]. Wrinkles are often found on exfoliated samples, but are also common in chemical vapor deposition (CVD)-grown graphene both after cooling down and during transfer to other substrates [6,19,20]. Previous studies have shown that graphene wrinkles can be generated in a controlled way to make nanoelectromechanical systems and for creating patterned graphene nanostructures [20,21]. Wrinkles have also been studied in other 2D materials including hexagonal boron nitride (h-BN), transition metal dichalcogenides (TMDCs), silicene, monochalcogenide monolayers [22,23], and on multi-layers/superlattices of heterostructures. Although there have been some studies of wrinkling in 2D materials, such as high anisotropy in wrinkled h-BN [24], and band structure change in wrinkled MoS₂ [25], there is still much to explore, both in the creation of wrinkles and in elucidating the effect of wrinkles on material properties.

We report here an efficient method for creating wrinkles on multilayer graphene and other layered materials. Due to the large thermal expansion coefficient difference, bi-axial compression can be applied to multilayer graphene supported on a PDMS substrate during “shock” cooling in liquid nitrogen to generate wrinkles within a few seconds. Different from parallel wrinkles generated by uniaxial compression, biaxial compression can generate wrinkle networks containing junctions. Our wrinkles were studied by atomic force microscopy (AFM) and Raman spectroscopy. Hydrogen plasma etching and AFM characterization results indicated that such wrinkles are preferentially generated along the armchair direction. Raman spectroscopy showed that the 2D band is more sensitive to wrinkles than the G band; the 2D⁻ peak increases while the 2D⁺ peak decreases on top of a wrinkle, suggesting that interlayer coupling could be weaker on a wrinkle in multilayer graphene. It is likely that this thermal shock method will work for many and perhaps any other 2D materials, such as GaS, MoS₂, and WSe₂.