Twisting the layers in a semiconductor dramatically affects its electronic properties, creating the potential to simplify the structure of many common electronic components.
Moiré patterns occur when 2 regular patterns are overlaid at a slight angle, creating a large-scale interference pattern visible to the naked eye. A new picture emerges with a little twist.
Twistronics is an emerging field that examines what happens when you stack 2D atomic sheets on top of each other and then twist one layer out of alignment with the other. The twisting creates Moiré superlattices, giving rise to several emergent electronic properties, like ferroelectricity, that are not present in the individual layers of the materials themselves.
Ferroelectricity occurs in some materials in which they can have a built-in electric charge that can be flipped back and forth. Imagine a tiny magnet with a positive and negative side instead of north and south poles. These materials can switch the electric charge direction when you apply an electric field, similar to flipping a magnet’s poles with a magnetic field. This ability to switch their charge direction makes ferroelectric materials useful in various applications like memory devices and sensors.
Conventional ferroelectric materials are typically bulk materials. In contrast, 2D ferroelectric materials are fabricated by stacking atomically thin layers, and the electrical behaviour of these stacked semiconductors can be changed with a mere twist.
Monash University’s Dr Mark Edmonds and PhD student Thi Hai Yen (Emily) Vu explore the electrical properties of twisted transition metal dichalcogenide (TMD) semiconductors. While 2D ferroelectric materials are highly sought after, visualising and characterising these materials are challenging because they exhibit unique properties that are not easily observable, which requires sophisticated, high-resolution techniques to detect and measure.
Dr Edmonds and Ms Vu teamed up with Dr Hemayet Uddin at ANFF’s Melbourne Centre for Nanofabrication to adapt an atomic force scanning microscope to perform piezoresponse force microscopy (PFM) to visualise their twisted materials. PFM uses a tiny probe, not unlike a miniature pickaxe, to measure how materials change shape when an electric field is applied, helping visualise their surface structures at the nanoscale.
The resulting PFM technique can operate in air and at room temperature, making the process simpler and applicable to a wider range of materials.
PFM is the ideal methodology to study the effects of changing the twist angle on ferroelectric properties and the size of the ferroelectric domains in Moiré superlattices, which could lead to significant advancement in the development of these novel 2D ferroelectric materials.
Electronic circuits typically consist of various components like metallic conductors, insulators, semiconductors and magnetic elements, requiring the integration of multiple materials, which poses significant engineering challenges. However, a single twistronic device, capable of being locally ‘twisted’ to realise each of these components, could revolutionise engineering by simplifying material requirements.
Beyond practical applications, Moiré superlattices and the innovative instruments created to measure their properties provide a straightforward platform for investigating complex scientific phenomena, offering new insights into superconductivity, ferroelectricity and magnetism, thereby deepening our understanding of fundamental physics.
Published 15 October 2024 in ANFF’s 2024 Casebook ‘ANFF NEXT‘
Posted 11 June 2025
