Organic electronic devices that use graphene electrodes have received considerable attention because graphene is regarded as an ideal candidate electrode material. Transfer and lithographic processes during fabrication of patterned graphene electrodes typically leave polymer residues on the graphene surfaces. However, the impact of these residues on the organic semiconductor growth mechanism on graphene surface has not been reported yet. Here, we demonstrate that polymer residues remaining on graphene surfaces induce a stand-up orientation of pentacene, thereby controlling pentacene growth such that the molecular assembly is optimal for charge transport. Thus, pentacene field-effect transistors (FETs) using source/drain monolayer graphene electrodes with polymer residues show a high field-effect mobility of 1.2 cm2/V s. In contrast, epitaxial growth of pentacene having molecular assembly of lying-down structure is facilitated by π−π interaction between pentacene and the clean graphene electrode without po...

Surface-Directed Molecular Assembly of Pentacene on Monolayer Graphene for High-Performance Organic Transistors

Organic–organic heterojunctions (OOHs) are critical features in organic light‐emitting diodes, ambipolar organic field‐effect transistors and organic solar cells, which are fundamental building blocks in low‐cost, large‐scale, and flexible electronics. Due to the highly anisotropic nature of π‐conjugated molecules, the molecular orientation of organic thin films can significantly affect the device performance, such as light absorption and charge‐carrier transport, as well as the energy level alignment at OOH interfaces. This Feature Article highlights recent progress in the understanding of interface energetics at small molecule OOH interfaces, focusing on the characterization and fabrication of OOH with well‐defined molecular orientations using a combination of in situ low‐temperature scanning tunneling microscopy, synchrotron‐based high‐resolution ultraviolet photoelectron spectroscopy and near‐edge X‐ray absorption fine structure measurements. The orientation dependent energy level alignments at the OOH interfaces will be discussed in detail.

Organic–Organic Heterojunction Interfaces: Effect of Molecular Orientation

Hexagonal boron nitride monolayers on metal supports: Versatile templates for atoms, molecules and nanostructures

Orientation-dependent ionization potentials (IPs) of organic thin films have been investigated with the combination of synchrotron-based high-resolution photoemission spectroscopy and near-edge X-ray absorption fine structure measurements Organic thin films of copper(II) phthalocyanine (CuPc) and its fully fluorinated counterpart of copper hexadecafluorophthalocyanine (F16CuPc) with well-controlled orientation are used as model systems Both molecules lie flat on the graphite surface and stand upright on Au(111) terminated by self-assembled monolayer of octane-1-thiol The IP for the standing-up CuPc thin film (IP = 475 eV) is 040 eV lower than that of the lying-down film (IP = 515 eV) In contrast, the IP of the standing up F16CuPc (IP = 650 eV) is 085 eV higher than that of the lying-down film (IP = 565 eV) This reversed orientation dependence in IP is explained by the opposite surface dipoles in the standing-up CuPc and F16CuPc molecular layers originating from the different intramolecular dipo

Molecular Orientation-Dependent Ionization Potential of Organic Thin Films

The microstructure of pentacene films grown on the basal plane of graphite has been investigated. By combining various complementary techniques including scanning tunneling microscopy, atomic force microscopy, x-ray diffraction, thermal desorption spectroscopy, and x-ray absorption spectroscopy the molecular orientation, crystalline structure, and morphology of the films as well as their thermal stability have been characterized in detail as a function of the film thickness. Initial film growth leads to the formation of a commensurate monolayer consisting of flat-lying molecules while upon subsequent deposition epitaxially ordered (022)-oriented pentacene films are formed which adopt the Siegrist phase. The detailed analysis shows that this epitaxial growth of films with an essentially recumbent molecular orientation is brought about by a slight rotation of the molecules in the first layer around their long molecular axis upon deposition of overlying molecular layers. Such a structural modification is unusual and becomes possible by the rather weak adsorption energy on graphite. In contrast, a very different film structure including an upright orientation of molecules even in the first layer is found on nonperfect but rough graphite surfaces leading to the formation of (001)-oriented films which initially reveal the thin-film phase and continue to grow in the Campbell phase of pentacene.

Andrew Thye

Papers

Molecular orientation transition of organic thin films on graphite: the effect of intermolecular electrostatic and interfacial dispersion forces.