Research
Our current research is focussed on the theory of electronic excitations in complex materials with applications to nanomaterials, such as metallic nanoparticles or two-dimensional materials. Below you can read about a few of our recent research highlights.
Hot electrons in metallic nanoparticles
Metallic nanoparticles are highly efficient light absorbers because of their localized surface plasmon resonances which are collective oscillations of the conduction electrons. When these plasmons decay, highly energetic or "hot" electrons and holes are generated which can be extracted and harnessed for applications in solar energy conversion or sensing.
To guide experimental efforts towards highly efficient hot carrier devices, we have recently developed a novel atomistic modelling approach which allows us to describe hot carrier generation and thermalization in large nanoparticles containing millions of atoms (PRX Energy 1, 013006, 2022). We have used this approach to explain why gold nanoparticles with certain shapes (such as cubes or octahedra) are better photocatalysts for the reduction of carbon dioxide than others (such as dodecahedra), see arXiv:2310.15120. We were also able to explain why antenna-satellite nanoarchitectures of Au and Pd nanoparticles are better photocatalysts for the generation of hydrogen from formic acid than core-shell systems (ACS Photonics 10, 3629, 2023). Very recently, we used our new technique to predict how hot-carrier properties of Au-Ag alloy nanoparticles depend on the alloy composition (arXiv:2402:05292).
Hot carrier generation in a metallic nanoparticle illuminated by light.
Modelling twisted bilayers of two-dimensional materials
Fascinating new phenomena emerge when two (or more) layers of two-dimensional materials are stacked on top of each other and then rotated creating a moire material. Famously, twisted bilayer graphene becomes a insulator and a superconductor at a magic rotation (or twist) angle of 1.3 degree, while the monolayer is a semimetal.
We have pioneered the atomistic modelling of twisted bilayer (and multilayer) materials. This is highly challenging because the twist gives rise to very large unit cells that often contain thousands of atoms. For example, we predicted that the strength of electron-electron interactions can be tuned by changing the thickness of the insulating spacer layer that separates the bilayer from the metallic gate. In particular, the correlated insulator state of twisted bilayer graphene will be destroyed if the spacer layer is very thin. We tested this prediction in collaboration with the experimental group of Prof. Efetov (Barcelona) which resulted in joint paper in Nature (Nature 583, 375, 2020).
More recently, our work has focused on twisted bi- and trilayer of semiconducting transition metal dichalcogenides (TMDs). These materials exhibit fascinating optical properties that can be controlled via the twist angle. For example, we demonstrated the emergence of excitonic Mott insulators in collaboration with the experimental group of Prof. Sufei Shi from Carnegie Mellon University (Nature Physics 20, 34 (2024)) and explained the sensitivity of the optical properties to vertical electric fields observed by the group of Prof. Brian Gerardot at Heriot-Watt University (Nature Physics 2024).
Top view of a twisted bilayer of transition metal dichalcogenides.
Shedding light on core-electron spectroscopy
X-ray photoemission spectroscopy is a widely used experimental technique to characterize the electronic and chemical properties of materials and their surfaces. In this technique, the photoelectric effect is used to "kick" core electrons out of the material. The energy that is required to do this gives us valuable information about the chemical environment of the core electron in the material. In practice, this analysis of X-ray photoelectron spectra is highly challenging and insights from modelling are needed to guide their interpretation.
To address this challenge, we have recently developed a new approach to calculate the binding energies of core electrons in materials (Phys. Rev. Materials 3, 100801(R), 2019). This approach allows for the first time the accurate calculation of absolute binding energies for molecules, bulk materials and also surfaces from first-principles. We are currently using this technique to understand the mechanism of gas sensing on SnO2 surfaces and the binding of oxygen to hematite.
Schematic of the photoemission process.