Imagine being able to follow individual molecules as they are deposited on a surface. Now, Koichiro Saiki at the University of Tokyo and colleagues1 have developed a new in situ approach that allows the growth of ultra-thin pentacene films at silica–gold interfaces to be monitored in real time. “The essential point of our work is that we have succeeded in observing the nucleation process of pentacene at the very initial stage of growth,” says Saiki.

Pentacene is an organic compound that exhibits exceptional charge-carrier mobility — an attractive feature for field-effect transistor applications. However, the formation of grooves in pentacene films results in poor contact at the channel–electrode boundary, seriously affecting the charge transfer properties of the devices.

The substrate was prepared by depositing round gold electrodes on the native silica (SiO2) layer on the surface of silicon wafers. Ultra-thin films of pentacene were then evaporated onto the substrate. Using photoemission electron microscopy, the researchers monitored the film growth and found the appearance of grooves around the gold electrodes within the deposition time of the first pentacene monolayer2.

“Most researchers in this field found the discontinuity in pentacene films at the boundary between the gold electrode and the silica channel using atomic force microscopy after film growth was completed,” says Saiki. “However, the origin of groove formation was not known.”

Fig. 1: Schematic illustration showing a decrease in pentacene density around the gold electrodes (discs), but not in the flat region, causing grooves to form. A 1.9 molecular layer (ML) pentacene film grows in the flat region, but only 1.6 ML grows in the electrode region, suggesting that 0.3 ML equivalent of pentacene molecules are incorporated onto the gold surface.

Experiments showed that the grooves originate from differences in the molecular orientations of pentacene on the silica and gold substrates. Pentacene molecules lie down on gold surfaces, but stand parallel to each other on silica surfaces. These differing arrangements create significant molecular flow from the silica to the gold electrodes, preventing pentacene growth at the silica–gold interface (Fig. 1).

Coating the gold electrodes with organic self-assembled monolayers (SAMs) significantly enhanced pentacene growth around the electrodes. According to Saiki, the organic SAMs reduced the surface energy of the gold electrode, causing the grooves to disappear.

The team is currently investigating the growth of the second pentacene monolayer by additional electron microscopy measurements. Their goal is to understand the growth mechanism by relating the crystallographic orientations of the first and second pentacene layers.