Molecular-Scale Structure of Pentacene Interfaces with Si (111) (original) (raw)
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Surface Science, 1998
Semi-empirical molecular orbital calculations reveal the local surface density of states for the adsorbed molecules on the Si surface. The organic molecules adenine, thymine, cytosine, and pentacene, which are adsorbed on Si(100)2×1 surfaces have been imaged by scanning tunneling microscopy (STM). The molecular images obtained by STM exhibit distinct shapes corresponding to the expected shapes for adsorption configurations. The energy level diagrams of the molecular orbitals (MOs) of the Si cluster on which the molecules are adsorbed are shown. The calculated MOs for adenine and thymine are in good agreement with the molecular images observed in STM. The bias dependence image of adsorbed cytosine is also explained by the calculated MOs of the molecule.
Scanning Tunneling Microscopy and Spectroscopy of Pentacene films Deposited on SiC
2007
Among various organic semiconductors, pentacene (Pn) has attracted much attention because of its ability of form ordered structures and its relatively high electron and hole mobilities. We have used SiC surfaces etched at 1600 C in 1 atm of hydrogen to form atomically flat substrates for Pn deposition. Oxidizing these substrates prior to Pn deposition electronically decouples the molecular films from the substrate. Scanning tunneling microscopy (STM) and spectroscopy (STS) was performed at room temperature on in-situ deposited Pn films. STM reveals a dendritic morphology of the films, consistent with prior reports [1]. We find a step height of 1.43±0.10 nm indicating that the Pn molecules are standing up, confirming the relatively weak interaction between the substrate and the film. STS reveals a band gap of about 2.0 eV, which is attributed to the edges of HOMO and LUMO bands of the molecules. Measurements over a wide range of tunnel currents are in progress, in an effort to deduce any transport limitations in the films. Supported by NSF. [1] F.-J. Meyer zu Heringdorf et al., Nature 412, 517 (2001)
Surface Science, 2003
Scanning tunneling microscopy has been used to study the ordering of pentacene (C 22 H 14 ) molecules on the Ag/ Si(1 1 1)-( p 3 Â p 3)R30°surface at room temperature. Two solid phases, S1 and S2, are observed at coverages of 0.35monolayer(ML)and0.35 monolayer (ML) and 0.35monolayer(ML)and1.0 ML respectively. It is shown that the solid phase S1 has a high-order commensurate lattice, Ag/Si(1 1 1)-(25 Â 25)-pentacene, containing 75 molecules. The structure of this phase is determined from STM measurements at very low coverages where it is possible to image both the pentacene molecules and the structure of the Ag/ Si(1 1 1) substrate. Two adsorption sites are identified, a three-fold hollow site at the centre of a Ag-trimer (CA-site) and a six-fold hollow site at the centre of the hexagonal arrangement of silver atoms (CB-site). A higher pentacene coverage of $1 ML lead to a molecular reorganization and forms a new commensurate structure Ag/Si(1 1 1)-(2 Â 3)-pentacene, containing two molecules per unit cell. Because low energy electron diffraction patterns were not obtainable for this system, the structure of this second phase is determined by using the bias voltage as a tunable parameter to ''focus'' on either the molecular film or on the substrate. In this phase adsorption takes place exclusively on the Ag-trimer (CA) site and the CB-site is lost because of strong lateral molecule-molecule interactions. The role of competition between intermolecular and molecule-substrate interactions and the nature of the adsorption sites in determining the structure of the pentacene layers is discussed.
Imaging of Crystal Morphology and Molecular Simulations of Surface Energies in Pentacene Thin Films
The Journal of Physical Chemistry B, 2006
We have investigated the crystal growth of the organic semiconductor pentacene by complementing molecular simulations of surface energies with experimental images of pentacene films. Pentacene thin films having variations in thickness and grain size were produced by vacuum sublimation. Large (∼20 µm) faceted crystals grew on top of the underlying polycrystalline thin film. The films were characterized using optical microscopy (OM), X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Single crystals most commonly grew in a truncated diamond shape with the largest crystal face, (001), growing parallel to the substrate. Crystal morphologies and surface energies were calculated using force field-based molecular simulations. The (001) surface was found to have the lowest energy, at 76 mJ/ m 2 , which was consistent with experimental observations of crystal face size. It was demonstrated that the morphology of the large faceted crystals approached the equilibrium growth shape of pentacene. From contact angle measurements, the critical surface tension of textured pentacene thin films in air was determined to be 34 mJ/m 2 .
Scanning tunneling microscopy of cyclic unsaturated organic molecules on Si(001)
Applied Physics A: Materials Science & Processing, 1998
Scanning tunneling microscopy and optical spectroscopy techniques have been utilized to investigate the formation of ordered organic monolayer films on the (001) face of silicon. While norbornadiene produces only disordered films, cyclopentene and 1,5-cyclooctadiene both produce monolayer films that are ordered translationally and rotationally. The rotational orientations of the molecules arise from the directional interaction of the π orbitals of the starting alkene with the π orbital of the dimers comprising the reconstructed Si(001) surface, with the Si(001) surface acting as a template for determining the directionality of molecules in the subsequent organic film. Using single-domain Si(001) samples, it is shown that the molecular films also exhibit anisotropy in optical properties when measured on centimeter length scales. Chemisorption of organic molecules on Si(001) surfaces typically involves fragmentation of the molecule. In the specific case of unsaturated organic molecules, however, another mode of bonding is possible. As depicted in Fig. 1, alkenes
Molecular-scale structural distortion near vacancies in pentacene
Applied Physics Letters, 2008
Molecular vacancies form in both of the crystallographic basis sites of thin pentacene crystals. Features in scanning tunneling microscopy images of these crystals correspond to the exposed terminal atoms of molecules. The (001) and (001¯) surfaces of pentacene are distinguishable, which allows for the identification of the absolute orientation of crystals and for the unambiguous assignment of the position of molecules relative to each vacancy. For vacancies in each molecular basis site of the pentacene (001) surface, the image feature associated with one molecular nearest neighbor is displaced by significantly more than other molecules.
Interface mediated growth of thin pentacene films on the silicon substrates
Annealed, well ordered Bi(001)/Si(111) films have been used as the substrates for the growth of pentacene (Pn). We determined using low energy electron microscopy (LEEM) and scanning tunneling microscopy (STM) that the pentacene molecules "stand up" on the Bi(001) surface, growing in a bulk-like structure directly from first layer, and that they are very well-ordered. Sharp low energy electron diffraction (LEED) patterns and STM observation of the onedimensional Moiré fringes indicate that the Pn layer is aligned with the Bi(001) substrate having "point-on-line" commensurate relation along <100> directions. By analyzing the contrast modulation in the STM images we determined the commensurate epitaxial relation between Pn film and Bi(001) substrates.
Journal of Nanoscience and Nanotechnology, 2001
Atomic force microscopy was used to study the growth modes (on SiO 2 , MoS 2 , and Au substrates) and the current-voltage (I-V) characteristics of organic semiconductor pentacene. Pentacene lms grow on SiO 2 substrate in a layer-by-layer manner with full coverage at an average thickness of 20 Å and have the highest degree of molecular ordering with large dendritic grains among the pentacene lms deposited on the three different substrates. Films grown on MoS 2 substrate reveal two different growth modes, snow ake-like growth and granular growth, both of which seem to compete with each other. On the other hand, lms deposited on Au substrate show granular structure for thinner coverages (no crystal structure) and dendritic growth for higher coverages (crystal structure). I-V measurements were performed with a platinum tip on a pentacene lm deposited on a Au substrate. The I-V curves on pentacene lm reveal symmetric tunneling type character. The eld dependence of the current indicates that the main transport mechanism at high eld intensities is hopping (Poole-Frenkel effect). From these measurements, we have estimated a eld lowering coef cient of 9077 10 ƒ6 V ƒ1=2 m 1=2 and an ideality factor of 18 for pentacene.
Scanning tunneling spectroscopy on organic semiconductors: Experiment and model
Physical Review B, 2004
Scanning-tunneling spectroscopy experiments performed on conjugated polymer films are compared with three-dimensional numerical model calculations for charge injection and transport. It is found that if a sufficiently sharp tip is used, the field enhancement near the tip apex leads to a significant increase in the injected current, which can amount to more than an order of magnitude and can even change the polarity of the predominant charge carrier. We show that when charge injection from the tip into the organic material predominates, it is possible to probe the electronic properties of the interface between the organic material and a metallic electrode directly by means of tip height versus bias voltage measurements. Thus, one can determine the alignment of the molecular orbital energy levels at the buried interface, as well as the single-particle band gap of the organic material. By comparing the single-particle energy gap and the optical absorption threshold, it is possible to obtain an estimate of the exciton binding energy. In addition, our calculations show that by using a one-dimensional model, reasonable parameters can only be extracted from z-V and I-V curves if the tip apex radius is much larger than the tip height. In all other cases, the full three-dimensional problem needs to be considered.
Physical Review B, 2008
A single layer ͑SL͒ of pentacene molecules deposited on the Cu͑119͒ surface and on an organic selfassembled monolayer ͑SAM͒ has been investigated by near-edge x-ray absorption fine structure ͑NEXAFS͒ at the C K edge, and by photoemission at the C 1s core level. The lowest unoccupied molecular orbitals ͑LU-MOs͒ of the pentacene SL on the SAM are basically unaffected with respect to that of pentacene in the gas phase, indicating a weak interaction of pentacene with the SAM, while a strong redistribution of the LUMOrelated final states is observed when the molecules are deposited on the Cu͑119͒ substrate, sign of an electronic mixing between molecular and metal electronic states, in agreement with recent theoretical predictions. The strong dichroic response of the NEXAFS signal indicates upstanding pentacene molecules oriented 16°off normal for the organic-organic heterostructure. The rehybridization of pentacene orbitals with Cu states at the pentacene/Cu interface hinders an accurate determination of the molecule orientation, which is, however, compatible with molecules lying flat on the Cu͑119͒ terraces.