Van der Waals heterostructures are fabricated by layer-by-layer assembly of individual two-dimensional materials and can be used to create a wide range of electronic devices. However, current assembly techniques typically use polymeric supports, which limit the cleanliness—and thus the electronic performance—of such devices. Here, we report a polymer-free technique for assembling van der Waals heterostructures using flexible SiN_x membranes. Eliminating the polymeric supports allows the heterostructures to be fabricated in harsher environmental conditions (incompatible with a polymer) such as at temperatures of up to 600 °C, in organic solvents and in ultra-high vacuum. The resulting heterostructures have high-quality interfaces without interlayer contamination and exhibit strong electronic and optoelectronic behaviour. We use the technique to assemble twisted graphene heterostructures in ultra-high vacuum, resulting in a tenfold improvement in moiré superlattice homogeneity compared to conventional transfer techniques.

Based on this SiN_x stacking technique, we modified it into grid to fabricate pristine suspended 2D material samples suitable for STEM analysis directly. STEM is a powerful tool for characterizing the atomic structure of materials. However, its application to highly air-sensitive 2D materials such as 2D transition metal halides has been limited due to challenges in sample fabrication. This new technique enables hermetic encapsulation of extremely air-sensitive 2D materials by protecting them with graphene layers on both sides. The simple, fast and polymer free technique causes no mechanical damage to the sample, preventing degradation due to minor imperfections in the seal. Using this method, we structurally characterized transition metal dihalides (FeI₂, CoI₂, NiI₂, MnI₂) across various thicknesses, from bulk to monolayer structures for the first time. We observed a thickness dependent stacking transition in FeI₂, where the preferred arrangement changed from bulk 1T stacking to bilayer 3R stacking. Additionally, in monolayer FeI₂, we characterised and analysed the movement of individual halide vacancies. This work demonstrates thickness-dependent structural phase transitions in atomically thin transition metal dihalides and highlights the ability to track atomic-scale defects in monolayers. Our findings will allow use of STEM (and complementary techniques which require suspended samples) to investigate the atomic structure of highly air-sensitive thin 2D materials.

Biosketch

I hold a bachelor’s degree in applied physics and a master’s degree in electrical engineering, both from Xi'an Jiaotong University, where I developed a solid foundation in both theoretical and applied sciences. Building on this background, I pursued my PhD in Physics at the University of Manchester, focusing on condensed matter physics. Currently, I am a Postdoctoral Researcher at the University of Manchester, specializing in the study and fabrication of 2D materials and their functional devices. My research aims to explore the unique properties of these materials to develop next-generation technologies in areas such as electronics, sensors, and energy applications. Throughout my academic and research career, I have gained extensive experience in experimental techniques, material characterization, and device engineering, contributing to advancements in the rapidly growing field of nanotechnology.

Please contact phweb@ust.hk should you have questions about the talk.