Structural Characterization of a Pentacene Monolayer on an Amorphous SiO₂ Substrate with Grazing Incidence X-ray Diffraction. Pentacene, a simple compound consisting of five fused benzene rings, has emerged as a viable candidate for the semiconducting transport layer in organic field effect transistors (OTFTs). These devices have substantial promise for low-cost flexible electronics, with applications ranging from inexpensive radiofrequency identification tags that can improve inventory control and security to electronic newspapers. OTFTs rely on the efficient transport of charge between two electrodes - designated "source" and "drain" -when the pentacene film is activated by a "gate" electrode beneath the film. This activation, which creates charge carriers in the film ("holes" in the case of pentacene), effectively serves as a switch that turns the OTFT on and off. Because these charge carriers are generated just above the gate electrode, the structure of the first layer of pentacene molecules is crucial to the operation of these devices. Surprisingly, little is known about the detailed crystal structure of the active transport layers in pentacene OTFTs.

Addressing the need to characterize the structure of the active transport layers in pentacene OTFTs, graduate students Sandra Fritz and Steven Martin, with MRSEC investigators Daniel Frisbie and Michael Ward and collaborator Michael Toney of the Stanford Synchotron Radiation Laboratory, used grazing incidence X-ray diffraction to determine the structure of the first layer of pentacene molecules above the gate electrode. The pentacene monolayer was prepared on amorphous silicon dioxide (a-SiO₂), the actual dielectric layer in OTFTs. The X-ray diffraction data, acquired by directing X-rays onto the sample at a nearly horizontal angle, demonstrate that the pentacene monolayer is crystalline. This characteristic, which was previously unknown, is crucial for transport as crystalline layers are thought to have high mobilities than their corresponding amorphous films. Crystallinity in the first a-SiO₂ layer is also important for establishing the orientation and crystallinity of subsequent layers in thicker films. Furthermore, the structure of the pentacene monolayer is different from that of bulk pentacene. Because the structure affects the bandwidth and mobility of charge carriers within the layer, this discovery has important implications for carrier transport in pentacene-based OTFTs. Band structure calculations are now being performed to assess the impact of these structural differences is. [Sandra E. Fritz, Stephen M. Martin, C. Daniel Frisbie, Michael D. Ward, and Michael F. Toney, University of Minnesota MRSEC J. Am. Chem. Soc. 2004, 126, 4084]

Figure 2. GIXD pattern (bottom) for a pentacene monolayer and a diffraction pattern (top) calculated for an energy-minimized crystal structure model based on the GIXD lattice parameters and the (001) layer motif of bulk pentacene as starting point.

Figure 3. Normal views of the abplanes of bulk pentacene and the model monolayer structures (left) and the respective side views (center, right) The z-axis is the normal to the ab plane.