Electrical Field-Induced Alignment of Nonpolar Hexabenzocoronene Molecules into Columnar Structures on Highly Oriented Pyrolitic Graphite Investigated by STM and SFM Anna Cristadoro, ² Min Ai, ‡ Hans Joachim Ra 1 der, ² Ju 1 rgen P. Rabe,* ,‡ and Klaus Mu 1 llen* ,² Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany and Department of Physics, Humboldt UniVersity Berlin, Newtonstrasse 15, D-12489 Berlin, Germany ReceiVed: December 13, 2007; In Final Form: January 23, 2008 Scanning tunneling microscopy (STM) and scanning force microscopy (SFM) have been used to study ultrathin films of a kind of nanographene, hexa(p-n-dodecylphenyl)hexabenzocoronene (HBC-PhC 12 ), on a highly oriented pyrolitic graphite (HOPG) surface. An electrical field, applied parallel to the substrate surface during adsorption from solution and subsequent drying, has been found to align columns of HBC-PhC 12 molecules with their long axes perpendicular to the field direction, independently from the HOPG lattice orientation underneath. The molecules, under field influence, adopt a tilted edge-on arrangement on the HOPG surface. On the contrary, the corresponding drop-cast films, dried in the absence of an electrical field, shows a face- on arrangement of the extended aromatic π-system of the HBC-PhC 12 discs, and no columnar structures ordered parallel to the graphite surface are recognized. Formation of unidirectionally aligned columns of HBC-PhC 12 molecules is, therefore, related to the influence of the electrical field during self-assembly of HBC-PhC 12 molecules on top of the conducting substrate. The electrical field competes favorably with the strong intermolecular interactions between HOPG and HBC, avoiding the epitaxial growth of thin films with the molecules in face-to-face arrangements parallel to the substrate. Our results constitute an important step toward control of the order and arrangement of functional conjugated molecules in ultrathin layers using electrical fields. Introduction During recent years organic-based electronic devices, such as photodiodes, organic field effect transistors, and organic light emitting transistors, have been the subject of numerous investigations. 1-4 Solution processing and deposition techniques have been developed to improve the molecular order in organic thin films 5-7 since it has been clearly demonstrated that electronic performance is morphology dependent. 8 Molecules, which form discotic mesophases with significant long-range order and anisotropic electrical and optical properties, continue to be sought for electronic applications. Side-chain-modified hexabenzocoronenes (HBC) are an example of soluble graphene- type molecules that form columnar assemblies exhibiting high charge mobility along the columns, 9 which therefore can be considered as self-assembled nanowires with strong π-π interactions. In particular, the good solubility and high aggrega- tion tendency of hexa(p-n-dodecylphenyl)hexabenzocoronene (HBC-PhC 12 ) molecules have made them suitable candidates for novel electronic devices. 1 Recently, highly ordered HBC- PhC 12 films have been obtained by means of electrical fields. 10 The field was used to induce a dipole moment along the planar core of the HBC-PhC 12 molecules. As a consequence, the HBC- PhC 12 molecules oriented their aromatic discs along the direction of the field lines and, due to the strong aggregation tendency, created columns perpendicularly aligned to the external electrical field. The HBC-PhC 12 cores, assembling in columnar aggregates, created an angle with the surface of 90°, defined as edge-on arrangement, which seems to be, independently from the presence of an external electrical field, the most stable HBC- PhC 12 packing on a glass surface. 10 Here we present a combined STM and SFM study of HBC- PhC 12 molecules (Chart 1) deposited on a conducting surface (HOPG) and dried in the presence of an electrical field. At solid-liquid interfaces, STM studies have shown symmetrically substituted and parent HBC molecules with a face-on arrange- ment on HOPG, 11-13 defined as the arrangement of the molecules with their aromatic disc lying flat on the graphitic surface. So far, only nonsubstituted HBC molecules deposited from the gas phase on HOPG via soft-landing mass spectrometry showed a different behavior. 14-15 In this case, the molecules adopt an edge-on arrangement, where the aromatic discs are nearly oriented perpendicularly to the surface plane. During a soft-landing deposition, HBC ions are formed by a matrix- assisted laser desorption ionization (MALDI) source, acceler- ated, separated, and then decelerated and focused on a con- ducting surface by means of electrical fields. Most likely dipole moments are induced in the molecules, which orient their planar cores along the electrical field lines. Hypothesizing that this mechanism allows the molecules to adopt an edge-on arrange- ment on the HOPG surface, electrical fields could be used to increase the order and control the arrangement of the molecules on the surface. Thus, the field influence on the assembly of HBC-PhC 12 molecules is investigated. Additionally, a compara- tive STM study on HBC-PhC 12 films drop cast in the absence of an electrical field at the solid-liquid interface and solid-air interface is conducted. * To whom correspondence should be addressed. K.M.: fax, (+49)- 6131-379350; e-mail, muellen@mpip-mainz.mpg.de. J.P.R.: fax, (+49)- 30-20937632; e-mail, rabe@physik.hu-berlin.de. ² Max Planck Institute for Polymer Research. ‡ Humboldt University Berlin. 5563 J. Phys. Chem. C 2008, 112, 5563-5566 10.1021/jp711707w CCC: $40.75 © 2008 American Chemical Society Published on Web 03/19/2008