Main research activities in the optoelectronic materials group during the past ten years
We have provided understanding of the nature and dynamics of charges and excited states (excitons) in organic materials, semiconductor quantum dots, and DNA. Charges and excitons are produced by (ultrashort) laser or high-energy electron pulses, and detected by time-resolved optical and microwave or THz conductivity techniques. During the past years he installed a picosecond pulsed electron accelerator with funding from an NWO/VICI grant. The combination with time-resolved AC conductivity detection is unique in the world. The experimental research is supported by quantum chemical electronic structure calculations, molecular dynamics simulations and Monte Carlo simulations of charge and exciton motion.
Using the electron accelerator facility the mobility of charges along isolated conjugated polymer chains in dilute solution could be measured. The intrachain charge mobility depends strongly on torsional disorder along a polymer chain. The charge mobility could be enhanced by formation of supramolecular ladder structures to achieve a record intrachain mobility as high as 600 cm2/Vs (Phys. Rev. Lett. 96, 146601, (2006))
We developed a method to calculate electronic couplings involved in charge transport through molecuar materials(J. Chem. Phys. 119, 9809 (2003); Int. Rev. Phys. Chem. 27, 87 (2008)). This method has been implemented in the Amsterdam Density Functional (ADF) program. It was used to provide explanations for measured effects of nucleobase sequence on charge transfer through DNA. One of our papers on this topic (JACS 127, 148094 (2005)) was recognized as a 'hot paper' in terms of immediacy of citations.
In recent years we extended our research to (ultrafast) laser spectroscopic studies on exciton dynamics in semiconductor quantum dots (QDs). Conclusive evidence for the production of two or more excitons by the absorption of a single photon in PbSe QDs has been provided (Nano Lett., 8,1713(2008)). The efficiency of this carrier multiplication (CM) process was an issue of hot debate in the literature, due to its importance for development of highly efficient solar cells. Our recent papers on charge carrier photogeneration and CM in thin films of QDs (Nature Phot. 6, 316 (2012); Nature Nanotech. 6, 733 (2011)) have been highlighted by others in Nature Materials (10, 808, (2011)) and Nature Photonics (6, 272, (2012)).