Hosted by: Qiang Li

Composition tuning in compound semiconductors allows for band structure engineering, which opens many avenues in fundamental and applied research. Here, I will show the application of band structure engineering in optimizing the electronic properties of bulk thermoelectric materials as well as probing the surface transport of topological insulator thin films. The main motivation of current thermoelectric research is to optimize the figure of merit ZT, so that the efficiency can be improved in (i) recovering waste industrial heat as well as (ii) providing local cooling of electronic devices. However, it is a challenge to independently tune the thermoelectric parameters that lead to high ZTs. We have found that Mg2Si1-xSnx thermoelectric material not only shows excellent thermoelectric performance but also exhibits decoupled Seebeck coefficient (α) and electrical conductivity (σ). We discover the convergence of two conduction bands in this material, which enhances the density-of-states effective mass and α in a large temperature range, without any detrimental effect on σ. Thus, a significantly improved power factor (PF = α2σ) is achieved, which results in a record high ZT in this material. Another material example with interesting consequences of band structure engineering is (Bi1-xSbx)2Te3. This material has become a mainstream material in the research of 3D topological insulators due to their potential to obtain surface transport protected by time-reversal-symmetry. I will show that using molecular beam epitaxy technique we can (i) tune the Bi/Sb ratio and (ii) control the thickness (as thin as several quintuple layers), which allow the observation of a dramatic distinction in electronic transport between high quality films and their bulk counterparts. An insulating-like regime has been obtained, which has enabled studies of magneto-resistance and thermoelectric properties associated with surface states.