Electronic States of a Single Layer of Pentacene: Standing-Up and Flat-Lying Configurations † Maria Grazia Betti,* ,‡ Aloke Kanjilal, ‡,£ and Carlo Mariani ‡ Dipartimento di Fisica, UniVersita ` di Roma “La Sapienza”, Piazzale Aldo Moro 2, I-00185 Roma, Italy, and INFM-CNR Center on nanoStructures and bioSystems at Surfaces (S ∧ 3), Via G. Campi 231/A, I-41100 Modena, Italy ReceiVed: June 6, 2007; In Final Form: August 3, 2007 The electronic properties of a single layer (SL) of pentacene molecules are investigated by high-resolution UV photoemission and near-edge X-ray absorption spectroscopy in different configurations of the SL, either standing up on an aromatic self-assembled monolayer or planar on a bare Cu(001) substrate. The weakly interacting pentacene molecules in the standing-up SL present a semiconducting character, and the empty states distribution reflects that of gas-phase pentacene, while the planar pentacene-Cu system shows a metallic interface with redistribution of the empty molecular states. The highest-occupied molecular orbital lineshape in the weakly interacting SL shows a double structure, attributed to two nonequivalent molecules in the ordered configuration. Introduction In the rapidly expanding field of the electronics based on hybrid organic-inorganic systems, the design of new architec- tures of π-conjugated materials has stimulated an enormous interest in realizing prototype hybrid organic devices, 1-6 where the performance and success depend mainly on the molecular orientation and packing. 2,3,7-9 Recently, a high-mobility and low- voltage organic thin-film transistor (TFT) has been designed with a heterostructure constituted of standing-up pentacene molecules deposited on a buffer self-assembled monolayer (SAM), where the SAM has been used successfully as a dielectric layer instead of a thin oxide for reducing the operating voltages. 10-14 Pentacene (C 22 H 14 ) presents electron charge delocalized on the fused benzene rings supported by a highest- occupied molecular orbital (HOMO) and a lowest-unoccupied molecular orbital (LUMO). Taking into account the pentacene polymorphs 15,16 or other nanostructured architectures or single- layer (SL) phases 17 due to the different packing of pentacene molecules, the carrier mobility is basically governed by the strength of the electronic coupling between the HOMO levels (for hole transport) and the LUMO states (for electron transport) of adjacent molecules in a band-transport mechanism, 18,19 where the transfer integral (electronic coupling) has generally been invoked to quantify the mechanism. 19,20 The pentacene orientation and the molecule-substrate inter- action are crucial to understand the related electronic and transport properties of the organic devices. With the aim of studying those mechanisms that tune the transport properties of organic/inorganic interfaces, we have separately prepared ordered single layers of standing-up and flat-lying pentacene arrays, respectively, and measured and compared the molecular electronic states supporting the mobile charge carriers in the two structural configurations. The main objective is to clarify to what extent the HOMO and LUMO states are involved in the molecule-molecule and molecule-substrate interactions. Pentacene can be adsorbed as a single layer of standing-up molecules on a buffer SAM of benzenethiolate (C 6 H 5 S-, Bt) or a SL of flat-lying molecules directly on a bare Cu surface, as deduced by near-edge X-ray absorption fine structure (NEXAFS) at the C K-edge. While a SL of standing-up molecules weakly interacts with the underlying SAM, the strong redistribution of the LUMO-related final states for flat-lying pentacene on Cu is a sign of an electronic mixing between the molecular orbitals and the metal electronic states. The latter phenomenon can be accounted for by considering a partial LUMO occupation induced by the metal charge, enhancing the metallic character at the interface. This process can largely influence the charge injection barrier and the device perfor- mance. On the other side, the electronic-transport properties of the pentacene SL deposited on the SAM buffer are not affected by the substrate and preserve the semiconducting properties due to the π-π interaction of the pentacene standing-up floating layer. Experimental Section The HR-ARUPS experiments were performed at the LOTUS laboratory in Rome, in an ultrahigh-vacuum (UHV, base pressure 10 -8 Pa) environment. Details of the experimental approach have been given elsewhere. 13 HR-ARUPS data were acquired using a high-intensity He discharge lamp (He I R photons, hν ) 21.218 eV) with a 45° incidence angle, and the photoemitted electrons were analyzed in the plane of incidence with a hemispherical SCIENTA SES-200 analyzer, used with an energy resolution of 15 meV and an angular resolution of 0.18° and by keeping the integration angle of ( 8° with respect to the direction of normal emission for the angle-integrated measurements. The Cu(001) and Cu(119) single crystals were cleaned by repeated sputtering-annealing cycles, 13,21 and the Cu(001) was exposed to benzenethiol (C 6 H 5 SH) vapors in UHV † Part of the “Giacinto Scoles Festschrift”. * To whom correspondence should be addressed. Phone: +39 06 49914389. Fax: +39 06 4957697. E-mail: betti@roma1.infn.it. ‡ Universita ` di Roma “La Sapienza”. £ INFM-CNR Center on nanoStructures and bioSystems at Surfaces (S ∧ 3). 12454 J. Phys. Chem. A 2007, 111, 12454-12457 10.1021/jp074365i CCC: $37.00 © 2007 American Chemical Society Published on Web 11/08/2007