C. Hohmann (Nanosystems Initiative Munich, NIM)
Artists view of freely suspended organic semiconductor thin film (selected front cover in Advanced Materials).
In recent years, interest in the investigation of several organic semiconductor materials has steadily increased. These compounds promise easier and cheaper fabrication without the need for scarce elements, as well very favorable electrical switching characteristics and correspondingly high performance. They are also amenable to large-scale production techniques, including printing processes. Their inherent properties, such as their flexibility, are compatible with applications in new electronic devices, such as displays and sensors.
Up to now, precise characterization of the properties of these materials has been difficult. This is in part because organic semiconductors have so far been mounted on top of a supporting substrate. The substrate, as well as the resulting interfaces, inevitably have an impact on, for example, chargecarrier mobility, and could therefore influence and distort the natural processes inherent to the material.
Focusing on this issue, Professor Thomas Weitz and Professor Achim Hartschuh have developed a way to fabricate freely suspended, ultrathin, organic semiconductor films.
Only two semiconducting layers
In their experiments, the physicists were able to fabricate organic semiconducting films consisting of only two to three layers of material but spanning several hundreds of nanometers (channel lengths of 100-450 nm and widths of 0.5-1 µm) without a supporting substrate.
In contrast to inorganic semiconductors, the crystal structure of their organic counterparts is stabilized by van der Waals forces. Such bonds are at least two orders of magnitude weaker than covalent bonds. “Nevertheless, our measurements indicate that our suspended semiconductor thin films exhibit high crystal quality,” stresses Thomas Weitz, “and the material has excellent stability and electrical properties.”
Freely suspended and electronically decoupled films
The aim of the project was to synthesize highly ordered and electronically active organic films, which are as thin as possible and are completely isolated from their environment. Temperature-dependent charge-transport measurements confirmed that the suspended films were indeed electrically decoupled.
Up to now, freely suspended, few-nm-thin semiconductor films with purely van der Waals bond-mediated stability have not been realized and hence could not be studied. Those now described by Weitz and colleagues are the first of their kind, and will permit detailed study of the charge transport processes in such films.