Crystalline Pentacene Research Articles

A computational study of surface diffusion of C 60 on pentacene

The morphology of the C 60/pentacene heterojunction is of interest for organic photovoltaic applications, yet is not well characterized. With that in mind, all-atom molecular dynamics simulation techniques were used to elucidate the diffusional behavior of small numbers of C 60 molecules on the surface of crystalline pentacene as a probe of the molecular-level interactions between C 60 and pentacene. The ultimate molecular probe of the pentacene surface, a single C 60 admolecule, exhibited an anisotropic diffusion pattern that lingered in energetically preferred sites in the [ 1 1 ¯ 0 ] direction, intercepting the (0, 1/2, 0) point in the unit cell. An Arrhenian analysis of this diffusion data gave estimates for the prefactor, D 0, and energy barrier, E a, of 2 × 10 −3 cm 2/s and 0.1 eV, respectively. Surface diffusion of one C 60 molecule on pentacene is significantly more rapid (by about 1–2 orders of magnitude) than if even one additional C 60 admolecule is present, implying that the C 60–C 60 cohesion interaction is stronger than the C 60–pentacene adhesion interaction. Simulations with up to four C 60 molecules, the practical limit of an all-atom approach, reinforced the suggestion that C 60 likes to dewet a pentacene surface and will show a preference for forming small 3D nuclei.

Effect of modifying a methyl siloxane-based dielectric by a polymer thin film for pentacene thin-film transistors

We report on the fabrication of pentacene thin-film transistors (TFTs) utilizing a spun methyl siloxane-based spin-on-glass (SOG) dielectric and show that these devices can give a similar electrical performance as achieved by using pentacene TFTs with a silicon dioxide (SiO 2) dielectric. To improve the electrical performance of pentacene TFTs with the SOG dielectric, we employed a hybrid dielectric of an SOG/cross-linked poly-4-vinylphenol (PVP) polymer. The PVP film was deposited onto the spun SOG dielectric prior to pentacene evaporation, resulting in an improvement of the saturation field effect mobility ( μ sat) from 0.01 cm 2/(V s) to 0.76 cm 2/(V s). The good surface morphology and the matching surface energy of the SOG dielectric that was modified with the polymer thin film allow the optimized growth of crystalline pentacene domains whose nuclei are embedded in an amorphous phase.

Oriented Growth of Crystalline Pentacene Films with Preferred a−b in-Plane Alignment on a Rubbed Crystalline Polymethylene Surface

We have achieved a growth of highly oriented crystalline pentacene thin films, with preferred a-b in-plane orientation with respect to the rubbing direction of a rubbed polymethylene surface. The polymethylene thin film, generated on a gold surface by gold-catalyzed decomposition of diazomethane, was annealed and gently rubbed in a fixed direction by a flannelette cloth to serve as an alignment layer during the deposition of pentacene molecules. Various surface analysis techniques, including reflection absorption IR spectroscopy (RAIRS), near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, grazing incidence X-ray diffraction (GIXD), and atomic force microscopy were used to elucidate the structural details of the polymethylene and the pentacene thin films deposited on it. Two crystalline morphologies of pentacene thin film were observed: the minor one of rod-like molecular crystals having their long axes of the crystals perpendicular to the rubbing direction, and the dominant one of platelet-like and layered crystals having the molecular axes stand near vertical to the surface. Moreover, GIXD revealed that the rubbing on polymethylene indeed induced a preferential azimuthal alignment of pentacene crystallites. The deposition of pentacene at 25 degrees C led to a twin growth of crystallites with the [110] direction predominately aligned perpendicular to the rubbing direction. In contrast, the pentacene deposition at 50 degrees C produced twinned crystallites of lower twin angle and the [120] direction aligned parallel to the rubbing direction.

Crystallographic and morphological characterization of thin pentacene films on polycrystalline copper surfaces

The degree of crystallinity, the structure and orientation of crystallites, and the morphology of thin pentacene films grown by vapor deposition in an ultrahigh vacuum environment on polycrystalline copper substrates have been investigated by x-ray diffraction and tapping-mode scanning force microscopy (TM-SFM). Depending on the substrate temperature during deposition, very different results are obtained: While at 77 K a long-range order is missing, the films become crystalline at elevated temperatures. From a high-resolution x-ray-diffraction profile analysis, the volume-weighted size of the crystallites perpendicular to the film surface could be determined. This size of the crystallites increases strongly upon changing temperature between room temperature and 333 K, at which point the size of individual crystallites typically exceeds 100 nm. In this temperature region, three different polymorphs are identified. The vast majority of crystallites have a fiber texture with the (001) net planes parallel to the substrate. In this geometry, the molecules are oriented standing up on the substrate (end-on arrangement). This alignment is remarkably different from that on single-crystalline metal surfaces, indicating that the growth is not epitaxial. Additionally, TM-SFM images show needlelike structures which suggest the presence of at least one additional orientation of crystallites (flat-on or edge-on). These results indicate that properties of thin crystalline pentacene films prepared on technologically relevant polycrystalline metal substrates for fast electronic applications may be compromised by the simultaneous presence of different local molecular aggregation states at all temperatures.

Orientation Control of Standing Epitaxial Pentacene Monolayers Using Surface Steps and In-plane Band Dispersion Analysis by Angle Resolved Photoelectron Spectroscopy

ABSTRACTWe prepared quasi single crystalline pentacene monolayer films on Bi-terminated Si(111) by using bunched steps on vicinally-cut surfaces as an orientation template. Band dispersion in the conduction plane of pentacene was clearly observed by angle-resolved photoelectron spectroscopy.

Shallow trap states in pentacene thin films from molecular sliding

Petacene is one of the most promising organic semiconductors for thin-film transistors. Transport measurements in the past have established the presence of shallow traps but their origins have remained a mystery. Here we show that shallow traps in vapor-deposited crystalline pentacene thin films are due to local defects resulting from the sliding of pentacene molecules along their long molecular axis, while two-dimensional crystalline packing is maintained. Electronic structural calculation confirms that these sliding defects are shallow-charge traps with energies ⩽100meV above (below) the valence band maximum (conduction band minimum).

Exploring the polymorphism of crystalline pentacene

Starting from each of the five complete X-ray structures published so far for crystalline pentacene, we have computed the structures of minimum potential energy, and obtained two local minima of the potential energy, i.e., two different “inherent structures” of mechanical equilibrium. This behavior, which has been found to be independent of the details of the potential model, indicates that there are at least two different single crystal polymorphs of pentacene. One of the two polymorphs corresponds to the structure originally determined by Campbell et al. [Acta Cryst. 14 (1961) 705]. The other polymorph corresponds to the structure found in all more recent measurements. The calculations predict significant differences between the corresponding Raman spectra of the lattice phonons, which we have checked experimentally, confirming the existence of two polymorphs. The correct identity of the samples, initially assigned only by matching experimental and calculated spectra, has been verified directly with X-ray diffraction measurements. Finally, we have obtained theoretical information on the global stability of the minima by systematically sampling the potential surface of crystalline pentacene, and found that the two polymorphs correspond to the two deepest minima. Further deep minima with layered structures, which might correspond to the thin film polymorphs found to grow on substrates, are also predicted.

Exploring the polymorphism of crystalline pentacene

Starting from each of the five complete X-ray structures published so far for crystalline pentacene, we have computed the structures of minimum potential energy, and obtained two local minima of the potential energy, i.e., two different “inherent structures” of mechanical equilibrium. This behavior, which has been found to be independent of the details of the potential model, indicates that there are at least two different single crystal polymorphs of pentacene. One of the two polymorphs corresponds to the structure originally determined by Campbell et al. [Acta Cryst. 14 (1961) 705]. The other polymorph corresponds to the structure found in all more recent measurements. The calculations predict significant differences between the corresponding Raman spectra of the lattice phonons, which we have checked experimentally, confirming the existence of two polymorphs. The correct identity of the samples, initially assigned only by matching experimental and calculated spectra, has been verified directly with X-ray diffraction measurements. Finally, we have obtained theoretical information on the global stability of the minima by systematically sampling the potential surface of crystalline pentacene, and found that the two polymorphs correspond to the two deepest minima. Further deep minima with layered structures, which might correspond to the thin film polymorphs found to grow on substrates, are also predicted.

Gap states in organic semiconductors: Hydrogen- and oxygen-induced states in pentacene

First-principles pseudopotential density functional calculations are reported for hydrogen- and oxygen-related defects in crystalline pentacene $({\mathrm{C}}_{22}{\mathrm{H}}_{14}).$ It is shown that defects that perturb a carbon atom so as to remove its ${p}_{z}$ orbital from participation in the \ensuremath{\pi} system give rise to a state in the gap. One such defect is formed by adding a H atom to create a ${\mathrm{C}}_{22}{\mathrm{H}}_{15}$ molecule with one fourfold C atom. In the neutral-charge state a single electron occupies a \ensuremath{\pi} orbital having reduced amplitude on the perturbed carbon atom. Charged defects correspond to addition or removal of an electron from this orbital. The possibility that these defects give rise to the bias-stress effect in pentacene-based thin film transistors is discussed.

Ab initiocalculation of the electronic and optical properties of solid pentacene

The optical and electronic properties of crystalline pentacene are studied, using a first-principle Green's-function approach. The quasiparticle energies are calculated within the GW approximation and the electron-hole excitations are computed by solving the Bethe-Salpeter equation. We investigate the role of polymorphism on the electronic energy gap and linear optical spectrum by studying two different crystalline phases: the solution-phase structure and the vapor-phase structure. Charge-transfer excitons are found to dominate the optical spectrum. Excitons with sizable binding energies are predicted for both phases.

Inherent structures of crystalline pentacene

Using a quasi-Monte Carlo scheme, we search the potential energy surface of crystalline pentacene to sample its local minima, which represent the “inherent” structures, i.e., the possible configurations of mechanical equilibrium. The system is described in terms of rigid molecules interacting through a standard atom–atom potential model. Several hundreds of distinct minima are encountered, with a surprising variety of structural arrangements. We find that deep minima are easily accessible because they exhibit a favorable energy distribution and their attraction basins tend to be wide. Thanks to these features of the potential surface, the localization the global minimum becomes entirely feasible, allowing reliable a priori predictions of the crystallographic structures. The results for pentacene are very satisfactory. In fact, the two deepest minima correspond to the structures of the two known experimental polymorphs, which are described correctly. Further polymorphs are also likely to exist.

Temperature evolution of pentacene crystal structure and phonon dynamics

AbstractWe investigate the relationships among all currently known X-ray structures of crystalline pentacene by calculating their “inherent” structures of minimum potential energy. We are thus able to show that two distinct bulk crystalline phases of pentacene exist, with very subtle but clear di.erences. We then assess the effects of temperature on the crystal structures, by including both inter- molecular and low-frequency intra-molecular phonons in the framework of quasi harmonic lattice dynamics methods. In this way we properly reproduce the experimental thermal expansion, and obtain a reliable description of the phonon dynamics and of its temperature dependence. The calculated phonon frequencies compare well with the experimental Raman spectrum.

Singlet excitons in crystalline naphthalene, antracene, tetracene and pentacene

The energies and oscillator strengths of exciton transitions in crystalline naphthalene, anthracene, tetracene and pentacene are calculated using second quantized boson theory. The lattice sums of Coulomb exciton transfer interactions consist of an Ewald sum of molecular point dipole-dipole interations and a direct sum of nondipolar interactions calculated from PPP wavefunctions using the atomic—multipole representation of transition charge densities. The calculated exciton energies and oscillator strengths are compared with available experimental data. For anthracene, inclusion of the nondipolar interactions leads to substantially better agreement between theory and experiment. For tetracene and pentracene, the factor group splittings of the lowest transition are determined primarily by crystal induced mixing with higher transitions.