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Seminar Series – Zeger Hens, Ph.D.
April 5 @ 12:50 pm - 2:00 pm
The Chemistry and Physics of Nanoscale Surfaces
Reducing the size of materials down to a few nanometer is a powerful approach to control material properties by design. A case in point are semiconductors, where size quantization leads to a size- and shape-dependent band gap once crystal dimensions become comparable or smaller than the excitonBohr radius; an observation first made almost 40 years ago. Much more recent, however, is the idea that nanocrystal properties can be widely tweaked by adjusting nanocrystal surfaces. This talk starts from the notion that about 50% of the atoms making up a 3 nm large crystallite are surface atoms. Especially in the case of colloidal nanocrystals, these surface atoms are readily accessible and standard analysis methods can give deep insight in the composition of nanocrystal surfaces and the impact of the surface on the chemical and physical characteristics of nanocrystals. In a first part, we will highlight the main findings of about 10 years of research at Ghent University on the chemistry of nanocrystal surfaces. Here, the combination of elemental analysis and NMR spectroscopy has led to a detailed description of the nanocrystal surface chemistry, which has made that nanocrystals are currently classified by means of their surface termination. These insights have made that a convincing connection has been developed more recently between experimental findings and the modeling of nanocrystal surfaces by means of density functional theory and molecular dynamics. Taking metal chalcogenide nanocrystals as an example, we will show that this has resulted in a detailed, atomistic description of the binding and packing of ligands on nanocrystal surfaces and the formation of surface defects that act as electronic trap states. In a final part of the presentation, we will bring these elements together and explain how surface trap states can be used in the case of HgTeQDs to achieve almost thresholdless optical gain in the near infrared under CW optical pumping. This example is unique as it shows a research direction in which surface properties are not seen as an issue to be addressed, but as an immense source of novel and unmatched material properties that is waiting to be tapped.
Prof. Dr. Ir. Zeger Hens is full professor at Ghent University. He is an applied physicist by training and received his Ph.D. in Applied Physics from Ghent University, Belgium in 2000. After a postdoctoral stay at Utrecht University, he was appointed as a professor at Ghent University in 2002. Upon his appointment, Zeger Hens started the research group Physics and Chemistry of Nanostructures on colloidal nanocrystals as a new research line within the Department of Inorganic and Physical Chemistry. From the very beginning, the group targeted research along the complete knowledge chain of colloidal nanocrystals, ranging from the development of new materials to the exploration of applications. This led to internationally recognized contributions at the level of synthesis of III-V nanocrystals, the understanding of nanocrystal surfaces and the quantitative analysis of linear and non-linear optical properties of nanocrystals. Moreover, thanks to a broad network of collaborators, the group actively pursues the application of colloidal nanocrystals as light absorbing or light emitting material for lighting and display, microlasersand infrared sensing. Zeger Hens has published over 230 scientific papers (h-index 44, >7500 citations), has been promoter of more than 20 Ph.D. students and currently (co)supervises 15 Ph.D.s.