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Posted: Feb 16, 2015

Printing graphene folds

(Nanowerk Spotlight) Folding a structure changes its form and functionality. From paper origami to the coiling of proteins, folding can transform relatively simple structures into complex shapes with new and distinct physical qualities.
In a similar manner, two-dimensional graphene is flexible and stable enough so that it can be manipulated into a third dimension by folding. Researchers have already demonstrated that folding may transform graphene into complex shapes with new and distinct properties, for instance modifying its band gap structure ("Multiply folded graphene").
"The ability to controllably form folds in graphene has significant research and technological applications," Toby Hallam, a Research Fellow at Trinity College Dublin, tells Nanowerk. "Induced folds have a sublithographic width and macroscopic length. They could be used as channel materials or interconnects in chips, and it has been shown that stable field emitters are formed by folded graphene. Besides the morphological characteristics, the electronic structure is also affected. Folding of graphene also causes rehybridization of the sp2 bond network leading to a more sp3-type character which allows for covalent chemistry to be carried out on the usually unreactive graphene surface."
In an effort to find a way to introduce folds or waves into graphene in a simple and large-scale way, Hallam and his collaborators have invented a rubber-stamp printing method to introduce waves into the graphene.
A paper in Nano Letters ("Controlled Folding of Graphene: GraFold Printing") details how the printing process works, using computer modelling to show the behavior of the graphene films on the stamp and substrate.
Wavy graphene has been produced before in a limited, small-scale environment, but the advantage of the controlled graphene folding process (GraFold) that Hallam has developed is that wavy graphene can be printed onto any type of surface allowing for more sophisticated investigations of its properties.
Schematic representation of the GraFold concept
Figure 1: Schematic representation of the GraFold concept. (A) A polymer supported graphene film is draped across a relief patterned stamp. (B) Polymer layer is dissolved. (C) The stamp is placed onto the substrate, and PDMS is separated from graphene by slow peeling. (D) Collapse of graphene loops creates free-standing folds. (Reprinted with permission by American Chemical Society) (click on image to enlarge)
GraFold is a transfer printing process. The excess graphene required for forming the folds is induced by using PDMS stamps with a relief pattern such that the graphene tension and adhesion is modulated across the stamp.
Initially the graphene is kept in a rigid planar structure by a polymer support layer (see Figure 1a). When the polymer layer is dissolved, the graphene is able to relax into the recessed features and partially adhere to the sidewalls (see Figure 1b).
The graphene inked stamp is then placed gently onto the destination substrate, and a conformal contact is made between the stamp, graphene, and substrate. The stamp is then slowly peeled away leaving the mechanically patterned graphene film attached to the substrate.
Beyond this, Hallam and his colleagues have high hopes that new stamp designs will allow for other GraFold structures such as fold intersections and pleats, both of which have their own specific electronic behaviors in graphene ("Geometry, Mechanics, and Electronics of Singular Structures and Wrinkles in Graphene").
"We use some advanced microscopy techniques to reveal the structure of the folds and spectroscopy to reveal the electronic and strain behavior of graphene that has been folded," says Hallam. "Finally, by way of a demonstrator of the influence GraFold processing has on graphene films we show that just by folding you can cause electrons to see a higher resistance in one direction (transport anisotropy)."
In previous work ("Field Emission Characteristics of Contact Printed Graphene Fins"), the researchers have shown that this technique can improve the field emission properties of graphene and it was this effect that led them to really try to understand what was going on in the GraFold process.
"But what is quite exciting now is really getting to grips with the new physics that can be explored with GraFold graphene," notes Hallam. "In upcoming work we are hoping to show that folding of graphene can lead to bandgap opening, and spin polarized currents ("Origami-based spintronics in graphene"). This is specifically to do with the fact that deforming the graphene lattice allows for a larger spin orbit coupling and hence magnetic interactions. Such an effect has been seen before in STM of strained graphene bubbles ("Strain-Induced Pseudo–Magnetic Fields Greater Than 300 Tesla in Graphene Nanobubbles")."
Looking further into the future the team is hoping to apply this technique to more 2D materials. The most explored of these materials – monolayer MoS2 – has a large optically active bandgap which can be modified by strain. This offers in plane quantum wells, pn junctions, solar cells and superlattices of the same. All this from folding and just one new material.
"I hope that the simplicity of this process advertises it to other researchers," concludes Hallam. "I am already providing GraFold films to other scientists with specialities different to my own. I hope that with time, GraFold process can become a generally accepted vector for manipulation of 2D materials."
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