dr.ir. P. Bampoulis (Pantelis)

• To uncover the principles underpinning charge and spin transport in QSH insulators.
• To identify the quantum mechanisms causing deviations from ideal, dissipationless transport.
• To engineer novel quantum states by controlling electron-electron interactions
• To study topological phase transitions (topological to trivial, 2D to 1D) by controlling their size or applying an electric field.

Quantum spin Hall (QSH) insulators have an insulating interior, but metallic edges that conduct current without dissipating energy. As a result, these materials have received a great deal of interest for the development of novel low-power electronic and spintronic devices. However, despite their promising features, such devices have yet to be experimentally realized. This project will close this gap by developing a concept topological field-effect transistor, based on the two-dimensional material germanene, that operates at room temperature. This revolutionary type of transistor operates by using an electric field to turn ‘ON’ and ‘OFF’ the dissipationless metallic edge channels. In order to achieve our goal, we need to demonstrate that the electronic properties of germanene can be tuned with an electric field at room temperature, and that current flows along the edges of germanene without energy losses. Successful demonstration of a working topological transistor will pave the way for the development of novel low-power electronic and spintronic devices, which could be a potential answer to the increasing challenge of energy wasted in modern computing.

Transition Metal Dichalcogenides (TMDs) are a class of layered 2D materials with the general formula MX₂, where M represents a transition metal (e.g., Mo, W) and X represents a chalcogen (e.g., S, Se, Te). These materials have garnered significant interest due to their unique optoelectronic properties. Excitons in these materials are electron-hole pairs. Because of the reduced dimensionality and Coulomb interaction in TMDs, these excitons have high binding energies and distinct optical properties. They can be broadly categorized into intralayer excitons (formed by an electron and a hole in the same layer) and interlayer excitons (formed by an electron and a hole in different layers). The spatial separation of interlayer excitons makes them long-lived and gives them unique properties.

Moiré and Interlayer Excitons: When two TMD layers are stacked with a small twist angle, a periodic potential landscape known as a moiré pattern emerges due to the interference between the two atomic lattices. This moiré superlattice potential can trap interlayer excitons, leading to the formation of moiré exciton states. These states can exhibit unique photoluminescence properties, which can be tuned by changing the twist angle.

Defect Bound Excitons: Defects in TMDs, such as vacancies or impurities, can trap excitons, leading to the formation of defect-bound excitons. These excitons have distinct energy levels and can be identified through their unique photoluminescence peaks. Understanding the nature of these defects and their interaction with excitons is crucial for device applications.

Photoconductive Atomic Force Microscopy (pc-AFM) is an advanced AFM technique that measures the local photoconductivity of a material. When combined with optical excitation, pc-AFM can be used to:

• Map the spatial distribution of excitons.
• Identify the origin and nature of defect-bound excitons.
• Investigate the dynamics of interlayer excitons in Moiré superlattices.

By studying the local variations in photoconductivity, pc-AFM can provide insights into the origins and properties of excitons in TMDs. This can be especially useful for understanding the role of defects, interfaces, and stacking configurations on exciton behavior.

Comprising a single layer of atoms 2D materials have unique electronic, optical, and mechanical properties distinct from their bulk counterparts. One of the most notable developments in this domain is the ability to "print" these 2D materials, heralding a new era in device fabrication and application. Printing 2D materials is not about traditional ink-on-paper methods. Instead, it is about depositing ultra-thin layers of materials like graphene, transition metal dichalcogenides (TMDs), and hexagonal boron nitride (hBN) onto substrates. Various techniques, such as inkjet printing, roll-to-roll processing, and stamping, have been adapted to handle the unique properties of 2D materials.

The ability to print 2D materials offers numerous advantages: (i) Printable 2D materials can be deposited on a range of substrates, including flexible ones, paving the way for bendable electronics and wearables. (ii) Printing techniques are inherently scalable, allowing for the mass production of devices or coatings with 2D materials. (iii) Printing allows for the precise placement of 2D materials, essential for creating device architectures.

While the promise of printed 2D materials is vast, challenges remain. Ensuring the uniformity and quality of printed layers, integrating with other materials, and scaling production techniques to industrial levels are areas of active research. We are focusing on a novel printing technique in close collaboration with Actega to rapidly advance both materials science and printing technology.