In all modern electronics, solar cells, and solid-state lighting, we use materials that are called «semiconducting». Compared to metals, these semiconductors are very poor at conducting electrical current, but the benefit is that we can control the current in a much better way. For example, we can make devices where the current can flow only in one direction, we can tailor semiconductors to harvest energy from sunlight, as in solar cells, or to provide light as in LEDs. The most common semiconducting material is silicon, which is used in nearly all consumer electronics and solar cells, but materials such as gallium arsenide and gallium nitride are also common.
To explain the properties of semiconductors we need to use quantum mechanics, which tells us that when atoms are arranged periodically, as in a crystal, the outer electron shells merge to form what we call energy bands. The structure of these energy bands determines which energy an electron can have, and how it is allowed to move in different directions. To control the properties of the material, we therefore need to understand its band structure. Unfortunately, there are very few ways to measure the band structure, and we are often left only with indirect methods and theoretical calculations. In this project, we will develop a new method to study the band structure directly using electron microscopy and spectroscopy. This will allow us to «see» the band structure of a material down to the nanometer scale and will provide a new tool for scientists working to improve the properties of semiconducting materials.
The ability to tailor materials’ properties by controlling their nanometer-scale structure and composition is defining for the new era of nanoscience: controlled alloying of semiconductors or introduction of quantum well structures can dramatically modify electronic structure, and thereby the optical and electronic properties of a material or device. For 2D materials and associated heterostructures, accurate control of nano-scale structure and chemistry will allow for their rich physical properties to be optimized for implementation in novel devices. Here, fundamental insight into many-body effects and the role of dielectric screening is required to set the stage for systematic engineering of electronic and excitonic states. However, observing and measuring the materials’ properties at the relevant length scales is one of the key limiting factors for further developments.
In this project we will develop methods to study the electronic band structure and excitons dispersions through momentum resolved EELS in (S)TEM. We will combine state of the art instrumentation with recent computational advances to arrive at a joint description of the electronic band structure, which for selected systems will be corroborated and combined with ARPES data. This approach will be used to provide insight into how heavy doping affects the electron band structure of wide-band gap semiconductors, and map their electron band structure with exceptional spatial resolution. Such information is essential for engineering of transparent conductive oxide semiconductors (TCOs), with applications in areas such photovoltaic modules, transparent electronics, and display technology. Furthermore, we will use q-EELS to determine the nonlocal dielectric response of 2D materials and related heterostructures. This will provide fundamental insights into the relationship between material composition, exciton dispersion, and the dielectric environment.