Research

Methods

We combine optical spectroscopy and charge transport measurements at various conditions and temperatures. Spectroscopic techniques include low-frequency (Terahertz range) Raman scattering, reflectance and photoluminescence. Our setups are assembled in-house and are modified according to the specific requirements of each experiment.

Semiconductors

We investigate the electron-phonon (e-ph) interactions and carrier scattering in semiconductors that exhibit strongly anharmonic nuclear motion.

Materials of interest include three-dimensional and low-dimensional halide perovskites, organic crystals, and ionic crystals that favour six and eight-fold coordination structures (e.g. PbTe and TlBr, respectively).  

Standard theories in semiconductor-physics were developed primarily for tetrahedrally bonded (i.e. diamond and Zinc-Blende) elemental (e.g. Si, Ge) and binary (e.g. GaAs, InP and CdTe) semiconductors.

The structural dynamics of tetrahedrally bonded crystals is captured reasonably well by perturbative treatments.

Therefore, analytical models for e-ph interactions in (quasi-)harmonic crystals describe their electronic properties very well.

However, the structural dynamics of the semiconductors we study is strongly anharmonic. Therefore, standard theory of e-ph interactions does not capture the experimental measurement of carrier mobilities and optical properties. Our experimental work provides new design rules toward new semiconductors with desired functionalities.

Ionic conductors

Ionic conducting materials are of great importance for energy storage and conversion. Batteries, fuel cells and supercapacitors can all benefit greatly from the development of new and improved solid ionic conductors at room temperature.

In recent years, new ionic conductors demonstrated very promising high ionic conductivities.

However, mechanistic understanding of the ionic transport is still lacking, and current design innovations proceed by modifying already-known structures.

One proposed design rule is based on the fact that lattice motion is known to be an important factor in the conductivity of superionic conductors. Recent work in the field has proposed that the energy of the harmonic optical phonon frequency is an indicator of how conductive a material will be.

We explore the alternative hypothesis that large amplitude anharmonic lattice displacements in solid electrolytes improve the efficiency of ionic conduction.

Currently we are studying known ionic conductors (e.g. AgI) and our next step is to use our gained knowledge to design new solid electrolytes.through the material and experiences the complex interplay with a dynamical and responsive potential.

Crystallization pathways in amorphous solids

We aim to investigate crystallization and particle accretion pathways of ACO3 compounds where 'A' is a divalent metal cation (e.g. Ca+2, Mg+2).

This family of materials is ubiquitous in minerals, bio-minerals and industrial applications. They form an extraordinarily rich array of solid substances that differ, among other things, in shape, mechanical strength, optical reflectivity and chemical reactivity.

This variety of properties stems from the versatile crystalline (i.e polymorphs) and mesoscopic structures that carbonate-based solids can adopt.

Our hypothesis is that a crystallisation pathway requires anharmonic nuclear motion as the atom in the solid rearrange and change configurations to form different polymorphs. To that end, low-frequency Raman scattering is expected to be a powerful tool to probe the motion and rearrangement of the atoms in real-time while controlling a wide range of parameters such as temperature, pressure and humidity.