DOI: 10.1002/cphc.200900084 Molecular Dynamics Simulations for a Pentacene Monolayer on Amorphous Silica Raffaele Guido Della Valle,* [a] Elisabetta Venuti, [a] Aldo Brillante, [a] and Alberto Girlando [b] 1. Introduction In recent years, the promise of low-cost, flexible devices has stimulated the rapid growth of the field of organic electronics. Devices based on single crystals have been realized, [1, 2] but dramatic improvements have also been achieved in the field- effect mobility of organic thin-film transistors (OTFTs). [3–9] The present theoretical work focuses on the packing and stability of pentacene films deposited on amorphous silica (aSiO 2 ), to be compared with the corresponding bulk properties. These systems are widely investigated [9–22] because aSiO 2 is a com- monly used dielectric layer in OTFTs, while pentacene is a promising candidate for the semiconducting transport layer, since it has thin-film charge-carrier mobilities approaching the bulk-crystal value. Precise control and optimization of the film quality are essential for the performance of OTFTs, as the struc- ture and degree of ordering of the films have spectacular ef- fects on the transport properties. [9, 22–25] In fact, these properties depend strongly on the intermolecular overlap of electronic wave functions within the semiconductor layer and thus are very sensitive to the molecular packing. [4, 5, 12, 13, 26–28] For this reason, it is of interest to summarize the available experimental information on the structure of bulk [28–33] and thin-film [10–16, 22] forms of pentacene. Two bulk forms of crystalline pentacene have been experi- mentally identified. One is the low-temperature (LT) structure, the most stable and most commonly observed [28–31] poly- morph. The other is the high-temperature (HT) structure, the “lost and found” polymorph, [29] first observed in 1961 [32, 33] and not reproduced for several decades. An unexpected theoretical prediction [34] of the stability of both forms led to an intense ex- perimental search, which culminated in Raman [35] and powder X-ray [36] experiments confirming the existence of the two dis- tinct phases. The published [28, 30–33] lattice parameters of both forms were finally reproduced by single-crystal X-ray diffraction measurements. [29] The two bulk polymorphs are both triclinic, with space group P  1ðC 1 i Þ, and present a characteristic “herring- bone” arrangement [31] with two nonequivalent molecules per unit cell (Z = 2), twisted with respect to each other and sitting on layers parallel to the (001) plane. Since the molecules stand approximately normal to the layer, the layer thickness d, that is, the plane periodicity dACHTUNGTRENNUNG(001), is by far the largest packing dis- tance and is thus characteristic of the structure. [11–13] The LT polymorph [28] has d = 14.1 , whereas the HT polymorph [29] has d = 14.4 . At least four different multilayer thin-film forms, with d spac- ings of 14.1, 14.4, 15.0, and 15.4 , have been identified by X- ray diffraction. [11, 12, 30] The various spacings are caused by differ- ent tilt angles of the molecules with respect to the normal to the layers, [29] and thus indicate dissimilar packings of the pen- tacene molecules. The last two structures can only be ob- served for sufficiently thin films (below 50 nm) [11, 12] and thus are genuine thin-film phases, whereas the first two structures clearly coincide with the LT and HT bulk phases. [29] This coinci- dence was immediately recognized for the LT form, [30] but not for the HT form. [37] However, recent grazing incidence X-ray dif- fraction (GIXD) experiments [14] for the form with d = 14.4  con- firm that all lattice parameters, and not just the d spacing, really match those of the HT bulk polymorph, [29, 32, 33, 36] despite suggestions to the contrary. [37] The form with d = 15.4 , the lattice parameters [15, 16, 22] and molecular arrangement [15] of which have recently been meas- ured, is also found in ultrathin films [38, 39] and even in monolay- ers. [13] In fact, its in-plane lattice parameters (a,b,g) match those of the Z = 2 planar unit cell revealed by GIXD measure- ments [13] of a pentacene monolayer grown on aSiO 2 . Atomic Molecular dynamics simulations are presented for “bulklike” and “filmlike” monolayers of pentacene deposited on a slab of amorphous silica. The two simulated systems, which mainly differ in the tilt angle between the pentacene molecules and the silica surface, exhibit structural and energetic properties that match the available measurements. The bulklike monolay- er, the structure of which corresponds to that of the low-tem- perature polymorph of crystalline pentacene, is stable. The filmlike monolayer, in which the molecules are most closely normal to the surface, is instead thermodynamically metasta- ble, in agreement with the experimental evidence. [a] Prof. R. G. Della Valle, Dr. E. Venuti, Prof. A. Brillante Dipartimento di Chimica Fisica e Inorganica and INSTM-UdR Bologna Università di Bologna, Viale Risorgimento 4 40136 Bologna (Italy) Fax: (+ 39) 0512093690 E-mail : valle@aronte.fci.unibo.it [b] Prof. A. Girlando Dipartimento di Chimica G.I.A.F. and INSTM-UdR Parma Università di Parma, Parco Area delle Scienze 43100 Parma (Italy) ChemPhysChem 2009, 10, 1783 – 1788  2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1783