Observation of Unusual Homoepitaxy in Ultrathin Pentacene Films and Correlation with Surface Electrostatic Potential

Identifying specific microstructure-property relationships in polycrystalline organic semiconductor films is a key goal for the field of organic electronics. Using transverse shear microscopy, we have established for the first time the presence of both epitaxial and non-epitaxial domains in ultrathin polycrystalline layers of pentacene grown on SiO2. The microstructure of pentacene films is particularly important, as pentacene is a benchmark semiconductor for organic field effect transistors (OFETs). Epitaxial domains in the second pentacene molecular layer exhibit unusual type-II coincidence with respect to the first pentacene monolayer, while the third and subsequent layers show commensurism with their respective underlayers. In addition, Kelvin probe force microscopy (KFM) reveals that the epitaxial domains have significantly more positive surface potential than the non-epitaxial domains. Collectively, these findings establish a previously unrecognized link between epitaxial order in organic semiconductor films and a well-defined electrical property, the surface potential, which is known to influence charge carrier transport parallel to the pentacene/SiO2 interface in OFETs. Recent developments in the field of organic electronics have generated widespread interest in identifying structure-property relationships for molecular films that form the active layers in devices such as organic field effect transistors (OFETs), organic light emitting diodes, organic memories, and organic solar cells. It is generally appreciated that film microstructure is critically important to the performance of organic semiconductors in devices and there are good examples in the literature in which clear structure-property correlations have been made. However, for all classes of organic semiconductors there remain significant open questions concerning the precise influences of microstructure on electrical performance. In the case of polycrystalline films, for example, the basic role of grain size on field effect mobility is still not conclusively established, with some reports describing a significant decrease in carrier mobility with smaller grain sizes, and others reporting no significant dependence. Much of the confusion in the literature likely results from differences in the specifics of experiments done in separate laboratories (e.g., substrate pretreatments). Still, the essential point is that fundamental understanding of microstructureproperty relationships for organic semiconductors is only just emerging, and much better comprehension is highly desirable for the continued improvement of organic devices. In this article, we describe a detailed scanning probe microscopy investigation of microstructure and properties for the first few crystallinemolecular layers of the benchmark organic semiconductor pentacene (C22H14, Fig. 1A) vapor deposited onto amorphous SiO2 substrates. The pentacene/SiO2 system is a model crystalline organic semiconductor/insulator interface that is widely studied and used in OFETs, and thus insights into the structure and properties of this interface have the potential to impact understanding of OFET operation. Furthermore, we believe that findings on this model interface can generally be taken as a qualitative guide for the types of issues that are at play in other crystalline organic semiconductor/insulator systems. We describe two major findings in this report. First, we have observed a type of thin film homo-epitaxial growth in pentacene layers that has not been reported previously for pentacene or for any other molecular system. Using transverse shear microscopy (TSM), we show that crystalline, vapor deposited films of pentacene on SiO2 substrates have a specific orientation between the pentacene layers. The second pentacene layer exhibits an unusual coincidence-II type epitaxy on the first layer, while the third and subsequent layers show commensurism with their underlayers. We propose a 4 4 supercell structure for the pentacene second layer on the first layer, as deduced from the elements of the transformationmatrix that defines the orientation between overlayer and underlayer. Our second finding is that a given pentacene overlayer has both epitaxial and non-epitaxial domains that manifest themselves in surface electrostatic potential differences. The one-to-one correspondence between epitaxy and surface electrostatic potential illustrates that strong coupling between microstructure and electrical properties is inherent to molecular thin films, and it provides a potentially critical, but so far unrecognized, link between structure and electrical transport behavior at pentacene/SiO2 interfaces. In our previous reports, we have demonstrated that TSM produces striking images of grain size, shape, and crystallographic orientation in single monolayers of pentacene on