Heterostructures of graphene and other 2D or ultrathin materials have potential applications in sensing, electronics, and battery technology. For example, graphene transistors encapsulated with alumina (Al2O3) are of interest for flexible electronics, and graphene/metal oxide heterostructures are widely used in lithium ion batteries. The mechanical strength, properties and stability of these structures is important for applications that make use of flexibility, or for applications that put to test the mechanical robustness of the materials, such as in high-performance electrochemical cells. Nevertheless, the mechanical properties of graphene heterostructures have not been widely and carefully investigated.
Now, an international team from the US, Germany and Spain have performed careful tests of graphene/alumina heterostructures for varying thickness of the alumina layer. The research revealed that graphene enhances the stiffness (Young’s modulus) compared to bare alumina, and that the alumina film strengthens the resistance of graphene to fracture under load. These findings indicate that such heterostructures have good mechanical strength and can thus be utilized in many devices. The measured values of stiffness and breaking strength add to the expanding body of knowledge of mechanical properties of graphene-related materials.
The method presented in the paper, published in the journal Nanotechnology, blends state-of-the-art fabrication, characterization, and calculation. The basis of the heterostructures is graphene on TEM grids, a single atomic layer of graphene deposited on a grid of circular holes. The monolayer graphene is thus suspended over the holes, making membranes with a diameter of two micrometers. Alumina is deposited on top of the graphene with atomic layer deposition (ALD), with thickness ranging from 1.5nm to 4.5nm. The mechanical properties are tested with atomic force microscopy, by landing an extremely sharp tip on top of the membrane, pushing on the membrane and studying the deflection. In strength tests, the membrane is pushed until it ruptures under load. The resulting force-distance curves are compared to results of finite element numerical calculations, yielding a quantitative measure of the mechanical properties. The calculations further revealed a nonintuitive shear stress distribution which indicates a maximum shear away from the line of symmetry and closer to the point of contact of diamond tip and the film, which is a key finding for reliable mechanical performance of the composite devices. Additionally, these findings illustrate the versatility of ALD techniques for use in heterostructure fabrication, and ease of implementation of graphene into thin-film hybrid structures in order to take advantage of its superior mechanical properties.