Selective Nucleation of Organic Single Crystals from Vapor Phase on Nanoscopically Rough Surfaces

Organic field-effect transistors (OFETs) are attractive for microelectronic applications such as sensor arrays or flexible displays, due to their adequate performance and relatively low production costs. Organic single-crystal transistors have emerged as benchmark devices for studying the intrinsic charge-transport properties in organic semiconductor materials. Conventional approaches for growing organic single crystals result in uncontrollable dimensions and the formation of extremely fragile crystals. In addition, the hand-selection and placement of individual crystals on a device structure represents a severe limitation for producing arrays of single-crystal transistors with high density and reasonable throughput. As a result, the application of organic single-crystal transistors has been restricted to fundamental charge transport studies, with their commercial application not yet realizable. We recently reported a materials-general method of fabricating large-area arrays of patterned organic single crystals. Microcontact-printed octadecyltriethoxysilane (OTS) film domains on smooth, inert substrates were found to act as preferential nucleation sites for single crystals for a broad range of organic semiconductor materials, such as pentacene, tetracene, rubrene and C60. In order to understand the underlying mechanism of preferential nucleation, the stamped OTS domains and the contact plane between the OTS domains and the organic crystals were inspected by atomic force microscopy (AFM) and optical microscopy. Our analysis suggests that crystals nucleate at the base of tall OTS pillars that form the significantly rough surface in the stamped domains. The selective nucleation inside the rough surface regions is discussed by means of a rate-equation model of the growth process.