Dependence of the shape of graphene nanobubbles on trapped substance

Abstract

Van der Waals (vdW) interaction between two-dimensional crystals (2D) can trap substances in high pressurized (of order 1 GPa) on nanobubbles. Increasing the adhesion between the 2D crystals further enhances the pressure and can lead to a phase transition of the trapped material. We found that the shape of the nanobubble can depend critically on the properties of the trapped substance. In the absence of any residual strain in the top 2D crystal, flat nanobubbles can be formed by trapped long hydrocarbons (that is, hexadecane). For large nanobubbles with radius 130 nm, our atomic force microscopy measurements show nanobubbles filled with hydrocarbons (water) have a cylindrical symmetry (asymmetric) shape which is in good agreement with our molecular dynamics simulations. This study provides insights into the effects of the specific material and the vdW pressure on the microscopic details of graphene bubbles.

Strain Engineering of 2D Materials: Issues and Opportunities at the Interface

“…Many applications of 2D materials involve multiple transfer processes of 2D materials to a substrate, by which 2D material blisters, such as bubbles and tents, frequently form by trapping water, gas, or solid nanoparticles at the interface ( Figure ) . Initially, these circular blisters have been viewed as an inconvenience for device applications .…”

“…However, recent studies have shown considerable in‐plane strain associated with these out‐of‐plane bubbles and tents, which creates opportunities for the study of new fundamental physics and applications of 2D materials emerging at large strain level . Alternatively, many 2D material bubbles and tents are designedly created by confining interface liquid and prepatterning substrates (Figure b), respectively. Famous examples include the highly strained bubble‐like and tent‐like graphene, which have been shown to possess large pseudo‐magnetic fields, up to 300 T (Figure c), as well as 2D semiconductors draping over an array of micropillars, which can be applied as large‐scale quantum emitters (Figure d) …”

“…The maximal applicable strain to the 2D material hinges on the critical shear stress the vdW interface can undergo . In the out‐of‐plane deformation mode, the 2D material–substrate adhesion (also called interfacial toughness) is one of the crucial parameters that dominate the wrinkling/buckling behavior, determine the strain in bubbles and tents, and predict the conformability between the thin membrane and the patterned substrate . Such facts have motivated a lot of theoretical and experimental efforts into elucidating how these atomically thin 2D materials interact with their substrate.…”

“…More recently, Sanchez et al developed a theoretical framework for estimating the work of adhesion of various 2D material interfaces using the experimentally measured aspect ratios of 2D‐material bubbles (Figure c) . Moreover, as bubbles are almost inevitable when transferring 2D materials to a substrate in 2D material devices, a comprehensive adhesion map for various 2D interfaces may be constructed by “reading” bubble profiles (Figure c) . Knowing the interfacial adhesion of the 2D material to its substrate will be particularly convenient for the design of strain engineering devices of 2D materials through out‐of‐plane deformation modes.…”

Cleaning interfaces in layered materials heterostructures

“…Encapsulated SLG and other layered material heterostructures are assembled by first producing the individual layered materials on separate substrates, typically Si + SiO 2 19 , or polymers, such as Polymethyl methacrylate (PMMA) 18,30 , followed by transfer and stacking to achieve the desired heterostructure 18,19,30 . During stacking, contaminants such as hydrocarbons 31 , air 23 , or water 32,33 , can become trapped between the layers, aggregating into spatially localized pockets with typical lateral sizes from a few nanometers 34 up to micrometers 23 , known as blisters 23 or bubbles 19,20,31 , which form due to the interplay of the layered material elastic properties and van der Waals forces 34 . This aggregation of contaminants into blisters leaves the regions located between them with clean interfaces 31 , and devices can therefore be fabricated exploiting these areas 30 .…”