Electron Transfer Reactions at Interfaces

Electron injection from a molecular donor orbital into empty electronic acceptor states of a solid is often referred to as heterogeneous electron transfer (HET). HET is of great significance in many different contexts ranging from condensed matter physics to biology. Since more than two decades there has been a continuing effort towards developing the field of molecular electronics, where HET will play a key role. HET has been studied in nano-hybrid systems aiming at practical applications like solar-cells. We are employing and developing ultrafast time-resolved techniques to study HET in real time and gain direct access to parameters governing the transfer reaction. Recently, we used our newly developed pump-DFWM technique to gain insight into porphyrinoid photodynamics within the first picosecond before S₂-S₁ internal conversion takes place. Measurements on zinc(II)-tetraphenylporphyrin in solution revealed new information on the vibrational dynamics of the excited S₂ state. The measurements show two major Raman active modes in the first several hundred femtoseconds after excitation of the Soret band. They can be attributed to stretching motions at the metal center (390 cm^-1) and aromatic carbon stretching around the porphyrin perimeter (1350 cm^-1). Each peak exhibits a rapid rise in amplitude in 300 fs. All modes are red shifted immediately after excitation with respect to their ground state position, followed by a blue shift over the course of relaxation. An 11 cm^-1 shift is observed for the perimeter stretch mode on the same time scale.

Charge Carrier Dynamics in Nanomaterials

Measurements that are performed on a large ensemble of particles demand a high degree of homogeneity to gain meaningful results. A better way of reducing the effect of broad distributions in sample properties is by monitoring single particles, or measuring single molecules. To achieve this, we developed an ultrafast time-resolved Kerr-gated fluorescence microscope. Application of this technique to charge carrier dynamics in CdSSe semiconductor nanowires let to a detailed understanding of the underlying dynamics that was formerly hidden in the ensemble average. We are improving and expanding this technique and applying it to a wide range of technological important materials like chalcogenides and metal oxides. Recently we succeeded in synthesizing new tree-like ZnO/CdSSe nanocomposites with CdSSe branches grown on ZnO nanowires prepared via two-step chemical vapor deposition. The nanotrees are vertically-aligned on the substrate. The CdSSe branches result in strong visible light absorption and form a type-II heterojunction with the ZnO stem that facilitates efficient electron transfer. A combination of PL spectroscopy and ns-PL lifetime measurements showed band-to-band PL across the interface and indicated that the nanotrees are promising materials for applications that benefit from a Z-scheme charge transfer mechanism.