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
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.