Electronic and vibronic interactions at weakly bound organic interfaces: The case of pentacene on graphite

Pavel B. Paramonov, Veaceslav Coropceanu, Jean-Luc Brédas
2008 Physical Review B  
The electronic and vibronic processes at the interface between a pentacene monolayer and a graphite surface were characterized using a combination of density-functional theory ͑DFT͒ and dynamic vibronic coupling simulations. The electronic interactions were evaluated at the DFT level first between the highest occupied states of pentacene and the graphite surface, as well as among the pentacene molecules within a monolayer. The former are found to be ca. four times stronger than the latter for a
more » ... an the latter for a parallel molecule/surface geometry. A dynamic vibronic model was used to analyze the interplay between the electronic and electron-vibration couplings and their effects on spectroscopic characteristics. The agreement between the simulated and experimental photoelectron spectra underlines the importance of weak electronic interactions on the vibronic coupling at the interface. Understanding of electronic processes at the interfaces between conjugated organic compounds and electrically conducting surfaces is of considerable interest in the fields of organic electronics and optoelectronics. 1,2 While chemically bound interfaces such as those formed by self-assembly of organic thiolates on the gold surface 3,4 have been studied at the atomistic level, 5-7 interfaces characterized by weak longrange attractions between molecules and a surface have not yet received the attention they deserve from the theoretical standpoint. Recently, it was shown that the interaction between an organic monolayer and a substrate can result in a strong substrate-mediated electronic coupling between the molecules within the monolayer. 8 High-resolution ultraviolet photoelectron spectroscopy ͑UPS͒ measurements of pentacene molecules physisorbed on the graphite surface reveal that the monolayer-substrate interactions can also affect the vibrational structure of the ionization spectrum. 9 In this regard, it is useful to recall that the line shape of the first ionization band can be directly related to the polaron binding energy ͑reorganization energy͒, a key ingredient in the description of charge transport ͑hole transfer͒. 10-12 The analysis of the UPS data suggests an increase of about 20% in the reorganization energy ͑hole-vibrational coupling͒ of pentacene at the interface compared with that estimated from the pentacene gas-phase UPS spectrum. 9 In this Rapid Communication, we address the nature of the electronic and electron-vibration interactions and their impact on the UPS spectrum and reorganization energy for a weakly bound interface formed by pentacene molecules physisorbed on the graphite surface. Our first goal is to develop a methodology capable of describing electronic coupling in weakly bound systems, based on a molecular picture. In addition, we study the interplay between electronic and electron-vibration interactions as reflected in the vibrational fine structure of the first ionization. Using our methodology, we simulate the UPS spectrum for the pentacene/graphite interface and find excellent agreement with the experimental data of Yamane et al. 9 One of the challenges for structural predictions of weakly bound systems lies in accounting for long-range electron correlation. 13,14 High-level correlated methods such as coupled cluster calculations provide accurate energetics and geometry for weakly bound systems 15 but at a very high computational cost. Therefore, the use of empirical potentials derived from applications of such methods to small-size systems is an efficient means of geometry predictions for the larger systems of practical interest. Importantly, we note that while less expensive density-functional theory ͑DFT͒ methods may give inaccurate geometry predictions for weakly bound complexes, they can provide a reasonable picture regarding state mixing and electronic couplings. 12
doi:10.1103/physrevb.78.041403 fatcat:iyounwls5fbnlgw7jymbmjdkle