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Thin films of molecular organic semiconductors present the tantalizing opportunity to develop thin film materials with optimized properties for applications in optoelectronics and photovoltaics.  Small-molecule semiconductors transport charge over relatively long distances with high carrier mobilities.  These high mobilities in turn result in relatively long diffusion lengths for the excitons created by the absorption of light.  It has long been recognized, however, that the mobility of carriers in thin films is far lower than in single crystals of the same materials.  For molecules such as pentacene the difference can be an order of magnitude or more.  A commensurate increase in the exciton diffusion length would be extremely valuable in light-harvesting applications.  We have begun a comprehensive study of the structural defects that limit in the mobility of carriers in these thin film structures.  The first element of our study involves probing the defects that arise in the formation of thin films of the small-molecule semiconductor pentacene.  Here we have probed the structural relaxation of the pentacene lattice near vacancies and identified a variety of previously unreported defects.  Defect structures at interfaces pose a more complicated structural problem that we have begun to approach by studying the interface between pentacene and the dipolar molecule nitrobenzene.

Vacancies

Depositing pentacene molecules on Si surfaces terminated by a monolayer of styrene molecules pentacene crystal that are structurally similar to those grown on insulating substrates, but which are sufficiently smooth and conductive for STM studies.  Features in scanning tunneling microscopy (STM) images of these crystals correspond to the exposed terminal atoms of molecules, as in Figure 1(a).  A surprisingly high concentration of vacancies occurs in these  pentacene layers, with approximately 1% of the molecular sites occupied by vacancies.  This concentration is far higher than would be expected based on thermodynamic arguments.  The (001) and (00-1) surfaces of pentacene are distinguishable, which allows for the identification of the absolute orientation of crystals and for the unambiguous assignment of the position of molecules relative to each vacancy.  For vacancies in each of the two molecular basis site of the pentacene (001) surface, shown in Figure 1(b) and (c), the image feature associated with one molecular nearest neighbor is displaced by significantly more than other molecules.

Figure 1 (a) STM images of vacancies within the (001) surface of a crystalline pentacene island.  (b) and (c) show larger images of  the areas within the solid and dashed boxes, after ref [1].

Extended structural defects

The growth of pentacene thin films also results in a variety of extended structural defects including dislocations, grain boundaries, and stacking faults.  We have used STM to probe the molecular-scale structure of grain boundaries and stacking faults in a pentacene thin film on the styrene-modified Si (001) surface.  STM images show two types of grain boundaries: in-plane high-angle tilt grain boundaries at the junctions between pentacene islands, and twist boundaries between molecular layers.  Segments of the tilt grain boundaries are faceted along low-energy crystallographic directions.  Stacking faults, as shown in the image and line scan in Figure 2, are occur within planes of molecules inside individual pentacene grains.  Two rows of molecules near the stacking fault are shifted along the surface normal by 60 pm.  Previous theoretical studies of the electronic properties of pentacene crystals have found that this type of distortion produces energy levels within the pentacene energy gap.  Electronically relevant trap states may thus be associated with stacking faults in pentacene thin films, which can be an important limitation to the carrier mobility.

Figure 2 (a) STM image of a stacking fault in a pentacene crystal.  (b) Cross section showing the height difference of 60 pm between molecules at the stacking fault and molecules in the remainder of the pentacene island.

As part of the preparation of the silicon surfaces for these studies of pentacene crystals, we have observed the formation of a new class of nanostructures on silicon-on-insulator surfaces.  These structures are stepped rectangular truncated pyramids, with lateral dimensions of tens of nanometers, that form on the outer Si layer of ultra-thin (001)-oriented SOI during heating in ultrahigh vacuum (Figure 3).  The edges of the pyramids are bounded by doubled atomic steps, with corners consisting of a complex series of single-layer steps.

Figure 3 (a) A corner of a nanostructure formed by annealing a SOI(001) template layer at 960 °C for 18 s and cooling at 1 °C/s. (b) Schematic diagram illustrating the arrangement of steps. Lines 1 and 3 indicate the sequence of double-height steps from the top terrace along its edges. Line 2 shows the sequence of single-height steps at the corner.

Conclusion and Future Directions

We are now beginning to study the formation of interfaces using this approach, beginning with the interface between pentacene and the polar surface passivation layer nitrobenzene.  The identification of defects associated with these interfaces and of the structure of the interfaces themselves promises to be an important step in the development of organic semiconductor devices incorporated layered small molecule structures.