Effect of molecule-substrate interaction on thin-film structures and molecular orientation of pentacene on silver and gold

Evaporated pentacene thin films with thicknesses from several nm to 150 nm on gold and silver substrates have been studied by ultraviolet photoelectron spectroscopy (UPS), near-edge X-ray absorption fine structure (NEXAFS), scanning tunneling microscopy (STM), and atomic force microscopy (AFM). It was found that pentacene thin-film structures, particularly their molecular orientations, are strongly influenced by the metal substrates. UPS measurements revealed a distinct change in the valence band structures of pentacene on Au compared to those on Ag, which is attributed to the different packing between adjacent molecules. Using NEXAFS, we observed 74+/-5 degrees and 46+/-5 degrees molecular tilt angles on Ag and Au, respectively, for all measured thicknesses. We propose that pentacene molecules stand up on the surface and form the "thin-film phase" structure on Ag. On Au, pentacene films grow in domains with molecules either lying flat or standing up on the substrate. Such a mixture of two crystalline phases leads to an average tilt angle of 46 degrees for the whole film and the change in valence band structures. STM and distance-voltage (z-V) spectroscopy studies confirm the existence of two crystalline phases on Au with different conducting properties. z-V spectra on the low conducting phase clearly indicate its nature as "thin-film phase".     

Studies of tetracene- and pentacene-based organic thin-film transistors fabricated by the neutral cluster beam deposition method

The neutral cluster beam deposition (NCBD) method has been applied to produce and characterize organic thin-film transistors (OTFTs) based upon tetracene and pentacene molecules as active layers. Organic thin films were prepared by the NCBD method on hexamethyldisilazane (HMDS)-untreated and -pretreated silicon dioxide (SiO2) substrates at room temperature. The surface morphology and structures for the tetracene and pentacene thin films were examined by atomic force microscopy (AFM) and X-ray diffraction (XRD). The measurements demonstrate that the weakly bound and highly directional neutral cluster beams are efficient in producing high-quality single-crystalline thin films with uniform, smooth surfaces and that SiO2 surface treatment with HMDS enhances the crystallinity of the pentacene thin-film phase. Tetracene- and pentacene-based OTFTs with the top-contact structure showed typical source-drain current modulation behavior with different gate voltages. Device parameters such as hole carrier mobility, current on/off ratio, threshold voltage, and subthreshold slope have been derived from the current-voltage characteristics together with the effects of surface treatment with HMDS. In particular, the high field-effect room-temperature mobilities for the HMDS-untreated OTFTs are found to be comparable to the most widely reported values for the respective untreated tetracene and pentacene thin-film transistors. The device performance strongly correlates with the surface morphology, and the structural properties of the organic thin films are discussed.     

Imaging of crystal morphology and molecular simulations of surface energies in pentacene thin films

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 (approximately 20 microm) 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).     

Molecular orientation of evaporated pentacene films on gold: alignment effect of self-assembled monolayer

Pentacene films deposited on self-assembled monolayers (SAMs) bearing different terminal functional groups have been studied by reflection-absorption IR, grazing angle XRD, NEXAFS, AFM, and SEM analyses. A film with pentacene molecules nearly perpendicularly oriented was observed on Au surfaces covered with an SAM of alkanethiol derivative of X-(CH2)(n)-SH, with X = -CH(3), -COOH, -OH, -CN, -NH(2), C(60), or an aromatic thiol p-terphenylmethanethiol. On the other hand, a film with the pentacene molecular plane nearly parallel to the substrate surface was found on bare Au surface. A similar molecular orientation was found in thinner ( approximately 5 nm) and thicker (100 nm) deposited films. Films deposited on different surfaces exhibit distinct morphologies: with apparently smaller and rod-shaped grains on clean bare Au surface but larger and islandlike crystals on SAM-modified surfaces. X-ray photoemission electron microscopy (X-PEEM) was used to analyze the orientation of pentacene molecules deposited on a SAM-patterned Au surface. With the micro-NEXAFS spectra and PEEM image analysis, the microarea-selective orientation control on Au was characterized. The ability to control the packing orientation in organic molecular crystals is of great interest in fabricating organic field effect transistors because of the anisotropic nature of charge transport in organic semiconducting materials.     

A high-resolution near-edge x-ray absorption fine structure investigation of the molecular orientation in the pentacene/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) pentacene/system

We present x-ray photoemission spectroscopy and highly resolved near-edge x-ray absorption fine structure spectroscopy measurements taken on pentacene thin films of different thicknesses deposited on a spin coated poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) substrate. Thin films of pentacene were prepared by using organic molecular beam deposition in situ using strictly controlled evaporation conditions. Our investigations show that pentacene thin films on PEDOT:PSS are characterized by upright standing molecules. Due to the strong dichroic behavior, the calculated values of the molecular orientation give a clear indication not only of the real molecular arrangement in the films but also of a high orientational order. This high degree of molecular orientation order is a characteristic already of the first layer. The films show the tendency to grow on the PEDOT:PSS substrate following an island-fashion mode, with a relatively narrow intermixing zone at the interface between the pentacene and the polymer blend. The peculiarity of the growth of pentacene on PEDOT:PSS is due to the fact that the substrate does not offer any template for the nucleated films and thus exerts a lateral order toward the crystal structure arrangement. Under these conditions, the upright orientation of the molecules in the films minimizes the energy required for the system stability.     

Highly ordered thin films of 5,5'-bis(3'-fluoro-biphenyl-4-yl)-2,2' : 5',2'-terthiophene with two meso-phases

The growth process and phase state of 5,5'-bis(3'-fluoro-biphenyl-4-yl)-2,2'?:?5',2'- terthiophene (m-F2BP3T) thin films were investigated by atomic force microscopy (AFM), in-plane and out-of-plane X-ray diffraction (XRD), and selected area electron diffraction (SAED). Two meso-phases (thin film phases) of m-F2BP3T films on SiO(2) surface were obtained in the early stages. The m-F2BP3T films initially exhibited two-dimensional (2D) layers (≤4 ML) followed by three-dimensional (3D) island growth. The film structure evolved two thin film phases in the first four layers and the bulk phase was formed from the fifth layer, which occurred concomitantly with the change of the growth mode. Moreover, the variation of weak epitaxy growth behavior of ZnPc from 2D to 3D growth further reflects that the phase state of the first three layers is different from that of the fourth layer, in spite of ZnPc crystals showing just one orientation corresponding to commensurate epitaxy. The novel phase behavior is closely related to the synergistic effects of the outstanding soft matter properties, limited elasticity of organic molecules, and strain originating from the SiO(2) substrate. This study investigates novel phase behavior in organic thin films and provides significant insight into the mechanism of the phase transition.     

Structure versus electron effects in the growth mode of pentacene on metal-induced Si(111)-square root(3) x square root(3) surfaces

The growth of pentacene films on different metal (Ga, Pb, Bi, Ag) induced Si(111)-(square root(3) x square root(3))R30 degrees surfaces is investigated by scanning tunneling microscopy. On surfaces with high atomic surface roughness, such as GaSi-square root(3), beta-PbSi-square root(3), and alpha-BiSi-square root(3), pentacene forms an initial disordered wetting layer followed by the growth of crystalline thin films. The growth behavior is independent of the metallicity of the substrate surface in this regime. On the other hand, on surfaces with low adatom surface roughness, pentacene molecules form self-organized structures without forming a wetting layer. Moreover, the molecular orientation is critically dependent on the surface metallicity. This work reveals that the growth mode of pentacene on solid surfaces is determined by the combined effects of structural and electronic properties of the substrate.     

High-performance organic semiconductors: asymmetric linear acenes containing sulphur

Two new linear acenes with fused thiophene units have been synthesized. These acenes have conjugation lengths between anthracene and pentacene. Thin films of these linear molecules were characterized by ultraviolet spectroscopy, X-ray diffraction, atomic force microscopy (AFM), and field-effect transistor measurements. Submonolayer AFM studies show growth that greatly resembles pentacene, while thin-film growth is dendritic. Mobilites as high as 0.47 cm2 V-1 s-1 have been found for the tetraceno[2,3-b]thiophene and are as high as 0.15 cm2 V-1 s-1 for anthra[2,3-b]thiophene.     

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.     

Synthesis,assembly, and thin film transistors of dihydrodiazapentacene: an isostructural motif for pentacene

The study below details the synthesis, assembly, and thin film transistors from dihydrodiazapentacenes. These molecules have the same molecular shape as pentacene but are much easier to prepare and have much greater environmental stability. Thin films made from the dihydrodiazapentacene behave as field effect transistors with mobilities and on/off ratios high enough to be useful in certain applications. X-ray diffraction and AFM experiments on these films show that the molecules stack in layers with their long axis upright from the surface. Some of the derivatives synthesized for this study have unexpectedly high solubility in polar solvents such as DMF and DMSO. The crystal structure from DMF reveals self-assembled channels with each of the aniline functionalities forming a hydrogen bond with solvent. In more nonpolar solvents, the solid-state assembly switches to a herringbone motif characteristic of the linear acenes.     

Two-dimensional self-assembly of 1-pyrylphosphonic acid: transfer of stacks on structured surface

Strong hydrogen bonding and pi-pi stacking between 1-pyrylphosphonic acid (PYPA) molecules were exploited to create self-assembled two-dimensional supramolecular structures. Polycrystalline films of these laminate crystalline PYPA bilayers were easily deposited onto the solid supports through a simple spin-coating technique. Atomic force microscopy (AFM), scanning tunneling microscopy (STM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), UV-vis absorption, and fluorescence spectroscopy reveal that processing parameters, such as solvent, concentration, and surface of the substrate, are critical factors in determining the final morphology of the stacked film. Robust laminate structures could be obtained only when short alkyl chain protic solvents (methanol or ethanol) and a nonhydrophobic substrate surface were used. Polycrystalline films were formed through the nucleation and growth of PYPA molecules into laminate structures at the air/solvent interface before they land on the substrate during the spin-coating process. These films possess good mechanical properties and were easily transferred onto a SiO2/Si substrate that was patterned with Au electrodes without breaking their crystalline structures. The successful transfer of the laminate crystals allows us to probe their electrical properties through a field effect transistor device. A gating effect on the charge transport of the stacked films indicates that PYPA laminate crystal possesses p-typed semiconductor characteristics.     

Pentacene films on Cu(119)

The molecular structure of thin pentacene film grown on a Cu(119) surface has been studied by near-edge X-ray absorption fine structure spectroscopy and scanning tunneling microscopy. The interaction between the π-molecular orbitals delocalized on the aromatic rings and the underlying copper substrate was deduced from XAS spectra. Pentacene molecules arrange with the main axis almost parallel with the Cu terraces according to the measured polarization dependence of the C 1s absorption spectra. For thickness exceeding 4 nm an upright arrangement of the molecules was observed with a dense herringbone-like ordering. The present study thus demonstrates that highly ordered pentacene films can be obtained on a Cu(119) vicinal surface both in a flat orientation for low coverages and in a bulk-like herringbone orientation for higher coverages.     

Conducting AFM and 2D GIXD studies on pentacene thin films

Among all organic semiconductors, pentacene has been shown to have the highest thin film mobility reported to date. The crystalline structure of the first few pentacene layers deposited on a dielectric substrate is strongly dependent on the dielectric surface properties, directly affecting the charge mobility of pentacene thin film OTFTs. Herein, we report that there is a direct correlation between the crystalline structure of the initial submonolayer of a pentacene film and the mobility of the corresponding 60-nm-thick films showing terrace-like structure, as confirmed by 2D grazing-incidence X-ray diffraction and atomic force microscopy. Specifically, multilayered pentacene films, grown from single crystal-like faceted islands on HMDS-treated surface, have shown much higher charge mobility (mu = 3.4 +/- 0.5 cm2/Vs) than those with polycrystalline dendritic islands (mu = 0.5 +/- 0.15 cm2/Vs) on OTS-treated ones.     

Effect of the phase states of self-assembled monolayers on pentacene growth and thin-film transistor characteristics

To investigate the effects of the phase state (ordered or disordered) of self-assembled monolayers (SAMs) on the growth mode of pentacene films and the performance of organic thin-film transistors (OTFTs), we deposited pentacene molecules on SAMs of octadecyltrichlorosilane (ODTS) with different alkyl-chain orientations at various substrate temperatures (30, 60, and 90 degrees C). We found that the SAM phase state played an important role in both cases. Pentacene films grown on relatively highly ordered SAMs were found to have a higher crystallinity and a better interconnectivity between the pentacene domains, which directly serves to enhance the field-effect mobility, than those grown on disordered SAMs. Furthermore, the differences in crystallinity and field-effect mobility between pentacene films grown on ordered and disordered substrates increased with increasing substrate temperature. These results can be possibly explained by (1) a quasi-epitaxy growth of the pentacene film on the ordered ODTS monolayer and (2) the temperature-dependent alkyl chain mobility of the ODTS monolayers.     

Electrochemically induced atom-by-atom growth of ZnS thin films: a new approach for ZnS co-deposition

Ultrathin films of ZnS were grown on Au (111) substrates using a novel, simple co-deposition method and characterized using X-ray diffraction (XRD), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and UV-visible spectroscopy. Cyclic voltammograms were used to determine approximate deposition potentials for co-deposition. XRD shows that the material growth is highly preferential with (111) orientation. Both AFM and XRD data indicate that the ZnS growth mechanism starts by the formation of rounded nanoparticles at the surface and then continues by lateral and vertical growth to form flat square crystallites of ZnS. UV-vis spectra taken for the ZnS thin films with various thicknesses, which is related to deposition time, shows that the band gap of the ZnS decreases as the film thickness increases.     

Step Morphology of Liquid Phase Epitaxial GaAs Determined by Atomic Force Microscopy

Single, double and triple growth spirals were observed on liquid phase epitaxy(LPE) grown layers using atomic force microscopy (AFM). Spiral growth arises from the motion of growth steps in a spiral fashion. The LPE grown GaAs Layers show quite a different step morphology as compared with cleaved GaAs crystals. The step treads are atomically flat planes in the cleaved crystals. but are generally not atomically flat planes and rather bow upwards in the LPE layers. The mechanism arousing this difference is unclear.     

Investigation on the orderly growth of thick zinc phthalocyanine films on Ag(100) surface

The growth of zinc phthalocyanine (ZnPc) on Ag(100) surface from monolayer to multilayer was investigated by low-energy electron diffraction, x-ray diffraction, and high-resolution electron energy loss spectroscopy (HREELS). At monolayer coverage, ZnPc molecules form an ordered film with molecular planes parallel to the substrate. The same structure is maintained as the film thickness increases. HREELS analysis shows that intermolecular π-π interaction dominates during the film growth from monolayer to multilayer. The π-d interaction between the adsorbates and the substrate is only applicable in the first adlayer. Stronger intermolecular-layer interaction is observed at higher coverages.     

Substituent effects on packing entropy and film morphologies in the nucleation of functionalized pentacenes on SiO(2) substrate: Molecular dynamics simulations

The changes in structural ordering, packing entropy, free energy, and film morphologies in the initial nucleation processes of pentacene (Pn), 6, 13-bis(t-butylethynyl) pentacene (t-Bu Pn), and 6, 13-bis(triisopropylsilylethynyl) pentacene (TIPS Pn) on the SiO(2) substrate were investigated, by using the molecular dynamics simulations. During the nucleation, the rod-like Pn molecules tend to diffuse rapidly and have different orientations on the SiO(2) surface. At the low coverage, the t-Bu Pn and TIPS Pn molecules with the branched topological structures almost lie flat on the substrate. On the basis of statistical distribution of various packing configurations of the functionalized Pn pairs, the packing entropy is estimated according to the Boltzmann formula. The packing entropy abruptly decreases in the early stage of deposition. Once the critical nucleus size is reached, the packing entropy converges to a constant value. As the coverage increases, the monolayer films of Pn and its branched derivatives become more ordered. The TIPS Pn with the relatively larger molecular area would occasionally stand on the surface during the nucleation, resulting in the dramatic changes in free energy. In the monolayers, the functionalized Pn molecules are packing more orderly than those in amorphous solids, but less orderly than those in crystals. The degree of order of these monolayers increases as the size of the substituents increases. The understanding of substituent effects on nucleation processes and packing structures is helpful to fabricate organic thin films with well-predefined molecular orientations.     

Langmuir-Blodgett films of a Mo-blue nanoring [Mo(142)O(429)H(10)(H(2)O)(49)(CH(3)CO(2))(5)(CH(3)CH(2)CO(2))](30)(-) (Mo(142)) by the semiamphiphilic method

Langmuir monolayers and LB films of the ring-shaped mixed-valence polyoxomolybdate [Mo142O429H10(H2O)49(CH3CO2)5(CH3CH2CO2)](30-) (Mo142) dissolved in the aqueous subphase have been successfully fabricated by using the adsorption properties of a DODA monolayer. Infrared and ultraviolet-visible spectroscopy of the LB films indicates that Mo142 and DODA molecules are incorporated within these LB films. X-ray reflectivity experiments indicate that the LB films exhibit a well-defined lamellar structure formed by bilayers of DODA molecules alternating with monolayers of Mo142. Using behenic acid-modified hydrophobic quartz substrate is critical for the formation of the well-defined lamellar structure. From the values of the periodicity obtained by these experiments it is clear that the Mo142 clusters lie flat along the charged organic layers. AFM images also showed the flat and homogeneous films on the quartz substrates treated with behenic acid. Cyclic voltammograms of Mo142-LB films deposited on ITO substrates showed quasi-reversible oxidation/reduction waves with positive shift of the potential compared to the case of solution.     

酞菁铜阴阳离子分子沉积(MD)膜润湿性研究

酞菁及其衍生物等大环平面分子具有良好的化学稳定性和优异的光电性能 [1,2 ] ,利用酞菁进行模拟生物光合作用的研究 ,以期实现新的光电转换技术 ,近年来已引起人们的广泛重视[3～ 5] .分子沉积(MD)超薄膜[6～ 8] 是以阴阳离子间静电相互作用为成膜驱动力 ,通过相反离子体系的单层交替沉积制备的层状有序超薄膜 .分子沉积技术广泛应用在固体表面改性和生物传感技术等领域 ,沉积分子的物理化学性质是影响其应用的主要因素 .本文通过紫外 -可见光谱跟踪了酞菁铜阴阳离子 MD膜的均匀沉积 ,并根据酞菁 MD膜表面接触角的变化 ,探讨分子端基的…