Quasi-One-Dimensional K-O Chain in PTCDA Thin Films: Evidence from First-Principles Calculations Costantino Zazza Consorzio Interuniversitario per le Applicazioni di Supercalcolo Per Universita ` e Ricerca (CASPUR), Via dei Tizii 6b, 00185 Roma, Italy Dipartimento di Chimica, Ingegneria Chimica e Materiali, Universita ` de l’Aquila, via Vetoio (Coppito 1), 67010 l’Aquila, Italy Simone Meloni Consorzio Interuniversitario per le Applicazioni di Supercalcolo Per Universita ` e Ricerca (CASPUR), Via dei Tizii 6b, 00185 Roma, Italy Amedeo Palma * Istituto per lo Studio dei Materiali Nanostrutturati (ISMN), Via Salaria, Km 29.3, 00016 Monterotondo S. (Roma), Italy Martin Knupfer and Gonzalo G. Fuentes IFW Dresden, D-01171 Dresden, Germany Roberto Car Department of Chemistry and Princeton Institute for the Science and Technology of Materials (PRISM), Princeton University, 08544 New Jersey, USA (Received 26 September 2006; published 22 January 2007) Using density functional theory calculations we have found that K atoms in a PTCDA (3; 4:9; 10-perylenetetracarboxylic dianhydride) crystal form a quasi-one-dimensional (1D) K-O chain interacting with carboxylic oxygen of the terminal anhydride groups of PTCDA. The K-K distance in the chain (3.72 A ˚ ) is commensurate to the periodicity of the organic semiconductor. We obtain that the K-O structure is stabilized by charge transfer from K to PTCDA molecules, forming prevalently ionic bonds: the electronic density of the chemistry induced gap states is essentially delocalized on the perylene core of PTCDA, while potassium appears spoiled of its charge. Band dispersion along the direction of molecular stack is evaluated to be 0.2 eV in pure PTCDA crystal and 0.5 eVin the K-doped system, confirming that the interaction occurs between different molecular planes. DOI: 10.1103/PhysRevLett.98.046401 PACS numbers: 71.15.Mb, 71.20.Rv Organic molecular materials have attracted particular attention in science and technology since they have been successfully used in organic electronic or opto-electronic devices. In fact, due to their relatively open crystal struc- ture they can be easily doped by the incorporation of electron acceptors and donors which can help to optimize opto-electronic devices [1– 3]. In particular, it has been demonstrated that charge carrier mobility or conductivity can be considerably enhanced adding appropriate dopants [1,4,5]. Moreover, doped molecular crystals might show fundamentally new and intriguing physical properties. A prominent example is the observation of metallic, super- conducting, or insulating phases in the alkali metal doped fullerides depending on their stoichiometry [6 – 8]. More recently, interesting phenomena were observed in other alkali metal doped organic molecular crystals, among them evidence for a metallic potassium doped PTCDA phase [9]. The knowledge about the possible stable phases and the interaction between the organic molecules and the intercalated species is very important for a detailed micro- scopic understanding of the physical properties in doped molecular crystals. It has been shown recently that it is possible to prepare single-phase K 1 PTCDA thin films [10], which is clear evidence for the existence of stable phases in alkali doped PTCDA. In this Letter we address the possi- bility of quasi-one-dimensional O-K-O nanostructure for- mation inside empty channels along the PTCDA crystal structure, using first-principles calculations. This repre- sents indispensable information in regard of the micro- scopic understanding of these crystals and their physical properties. We model the thin films in terms of the bulk crystallographic structure of PTCDA [11,12]. Electronic and all internal atomic degrees of freedom are simulta- neously relaxed using a damped second order ab initio molecular dynamics [13] within the Car-Parrinello scheme [14,15]. The electronic structure calculation is based on density functional theory, using the Perdew-Wang gener- alized gradient approximation for exchange and correla- tion (PW91) [16,17]. Open shell systems are treated with spin unrestricted calculations. We adopt a plane-wave pseudopotential approach using ultrasoft [18] pseudopo- tentials (PP’s). This computational approach was already used in previous works [19 – 21]. The PTCDA crystal lattice parameters have been kept fixed at their experimen- PRL 98, 046401 (2007) PHYSICAL REVIEW LETTERS week ending 26 JANUARY 2007 0031-9007= 07=98(4)=046401(4) 046401-1  2007 The American Physical Society