Pentacene growth on graphite investigated by low-energy electron microscope H.W. Liu a,n , A. Al-Mahboob a , Y. Fujikawa b , N. Fukui a , T. Hitosugi a , T. Hashizume a,c,d , Q.K. Xue a,e , T. Sakurai a a WPI-Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan b Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan c Advanced Research Laboratory, Hitachi Ltd., Hatoyama, Saitama 350-0395, Japan d Department of Physics, Tokyo Institute of Technology, Meguro, Tokyo 152-8551, Japan e Department of Physics, Tsinghua University, Beijing 100084, China article info Article history: Received 16 July 2009 Received in revised form 3 October 2009 Accepted 28 December 2009 Communicated by M. Uwaha Available online 4 January 2010 Keywords: A1: Crystal structure A2: Growth from vapor A2: Single crystal growth B1: Organic compounds abstract The growth of pentacene (Pn) on highly ordered pyrolytic graphite (HOPG) has been investigated by low-energy electron microscope (LEEM). Initially an ordered Pn layer with lying-down molecular orientation is formed on HOPG. The in-plane structure of the first Pn layer corresponds to a modified ac-plane with two different epitaxial orientations and their twining. No island nucleation could be detected in real time LEEM for the epitaxially ordered first layer, which corresponds to the wetting layer. Subsequently, separated nucleations of Pn with both lying-down and standing-up geometries occur on the top of the wetting layer. The lying-down overlayer has the in-plane structure of Pn ac-plane. The lattice parameters and symmetry of the overlayer, which are obtained from in-situ diffraction patterns, reveal a structural difference as compared to the first layer of pentacene. & 2009 Elsevier B.V. All rights reserved. 1. Introduction Organic semiconductors are presently receiving a large amount of attention with regard to their use as active layers in electronic devices such as organic field-effect transistors [1]. Among currently investigated molecules, pentacene (C 22 H 14 , Pn) is one of the promising candidates due to its significantly higher charge-carrier mobility [2]. Carrier mobility and injection in pentacene organic crystals depend strongly on molecular orienta- tion and packing [3]. Thus, in order to improve the performance of pentacene devices, monitoring and controlling the structural growth of pentacene from submonolayer becomes crucial for its applications. The low-energy electron microscope (LEEM) technique [4] shows a powerful capability in revealing film growth processes in real time. Moreover, the use of low energy electrons minimizes damage to the organic films. The Pn grown on various substrates such as metallic, semiconducting and modified ones [5–8] has been investigated by LEEM. In anisotropic systems like Pn, the molecular orientation and crystalline structure critically depend on the competition of the intermolecular and the molecule– substrate interactions. From this point of view, the surface localized density of states [7] of the substrate is decisive to determine whether the Pn molecules lie down or stand up on a surface. Most recently, we studied Pn growth on fullerene (C 60 ) [8] by LEEM as a model system for heterojunction organic devices. We observed a competitive growth, which is tunable with the substrate temperature, between Pn (0 0 1) with tilted-stand- ing-up geometry and a new orientation characterized by the long molecular axis aligned parallel to surface. To further understand the structural properties of pentacene/aromatic hydrocarbon systems, in this work we select carbon-upper- most highly ordered pyrolytic graphite (HOPG) as the sub- strate. Shionoiri et al. [9] reported a similar approach of Pn growth on graphite by a technique close to photoemission electron microscopy, and speculated its mechanism from the analysis of ultraviolet photoelectron spectroscopy (UPS) spec- tra. However, it is still unclear whether a wetting layer of Pn is present at the Pn/HOPG interface and what the structure of the wetting layer is, which is important to understand how the film grows in the very initial stage. In this paper, we report in-situ LEEM observations of Pn growth on HOPG. Our in-situ diffraction study demonstrates the existence ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jcrysgro Journal of Crystal Growth 0022-0248/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2009.12.066 n Corresponding author. Tel.: + 81 22 2175948; fax: + 81 22 2175943. E-mail address: liu@wpi-aimr.tohoku.ac.jp (H.W. Liu). Journal of Crystal Growth 312 (2010) 967–970