Tag: 2D materials

Optimization of 2D material-based devices

Optimization of 2D material-based devices

How to visualize electric fields in situ to boost the performance of tomorrow’s LEDs

2D materials are excellent candidates for light emission in LED-type components. Furthermore, combining several of these materials with different properties (metal, insulator, semiconductor) theoretically makes it possible to obtain complex components that combine these properties. To function, these components must be connected to electrodes. But where exactly should the electrical voltage be applied? 
To answer this question, a team from the Paris Institute of NanoSciences used the ANTARES beamline to probe operando the distribution of the electric field within a heterostructure composed of two semiconductors.

Two-dimensional (2D) materials, such as transition metal dichalcogenides (TMDs) (e.g., MoS₂, WSe₂, and their derivatives), exhibit strongly enhanced excitonic effects due to the robust Coulomb coupling between electron-hole pairs. This makes them outstanding candidates for light emission in devices such as LEDs. A second key advantage of this materials platform is the ability to assemble these materials without epitaxial constraints. In theory, this allows for the combination of materials with diverse properties—metals, insulators, and semiconductors with tunable bandgaps—to fabricate complex devices. The entire structure is ultimately connected to electrodes, which serve to inject charges or modulate the potential profile. However, a critical challenge remains: the voltage must be applied in the right location! In these structures, the energy landscape is influenced by edge effects, doping, flake thickness, defects, and above all interfaces. In this study, a team from INSP uses the ANTARES beamline to operando probe the electric field distribution within a heterostructure composed of two semiconductors.

In optoelectronic devices, electrodes are used to inject the current/energy necessary for device operation. In the context of LEDs, applying a bias is essential to inject holes into the valence band while electrons are resonantly injected into the conduction band. When these energy conservation rules are fulfilled, charges can be injected into the optically active semiconductor, enabling light emission. However, the turn-on voltage for an LED can be significantly larger than the material’s bandgap if an electric field is also applied to the intermediate material between the electrode and the optically active layer. This results in power efficiency losses, which must be mitigated. Therefore, the localization of the electric field is critical, and tools to measure the field distribution operando are essential.

Read more on the SOLEIL website

From Theory to Confidence: Building Trust in Twistronics Models

From Theory to Confidence: Building Trust in Twistronics Models

A UK collaboration explored the transformative impact of twist angles on the electronic structure of 2D materials

A single sheet of graphene, composed of a single layer of carbon atoms in a hexagonal pattern, is a semimetal. However, adding a second sheet of graphene, twisted at a slight angle to the first, can give rise to very different electronic properties, depending on the angle. At the ‘magic’ angle of about 1.1°, for example, a twisted bilayer sheet of graphene is a superconductor. The same effect is seen in other 2D materials, giving rise to a new field of study – twistronics – seeking to both understand and exploit the relationship between twist angles and novel electronic properties. In work recently published in Nano Letters, researchers from the University of Warwick and the National Graphene Institute at the University of Manchester used spatially-resolved angle-resolved photoemission spectroscopy (ARPES) on Diamond’s I05 beamline to study the twist-dependent band structure of twisted-bilayer, monolayer-on-bilayer, and double-bilayer graphene. Their results show good agreement between experimental measurements and theoretical simulations, confirming that the models can be used to explore the electronic band structure and emergent transport and optical properties of twisted-few-layer graphenes. 

Prof Neil Wilson at the University of Warwick opens by noting that;

Twistronics is a new concept in 2D materials, in condensed matter physics. When you have two atomically thin layers next to each other, their properties depend on the twist angle between them. This happens because of changes to the electronic structure, and there has been a huge amount of research on twistronics – putting two layers together at different twist angles and seeing what happens to the optical properties and electrical properties. You’re working with two very small pieces of 2D material stacked on top of each other, typically only a few micrometres across, which is fine for optical measurements and electrical transport measurements. However, that makes it extremely challenging to study the electronic structure directly.

To get a good look at the electronic structure of these exciting materials, Prof Wilson’s group at the University of Warwick worked with researchers from the National Graphene Institute and Diamond’s I05 beamline. Prof Roman Gorbachev’s group at the National Graphene Institute is a world leader in fabricating these complex samples.

Senior Beamline Scientist Matthew Watson explains;

Angle-resolved ARPES allows us to measure directly the electronic structure of the 2D materials. It allows us to determine both the energy and momentum of the electrons within the material, which gives us directly the electronic structure which underpins the optical properties and the transport properties. And the I05 nano-branch endstation delivers spatially-resolved ARPES from ultra-small spots, on the micrometre length scales we have in these 2D samples.

Read more on Diamond website

Image: Electronic structures of twisted double bilayer graphene at large (left) and “magic” (centre) twist angles, showing the emergence of a flat band at the top, which is at the heart of the various phenomena that emerge in this system. Data measured at I05 at Diamond and reported in Nunn et al.

Credit: Matthew Watson