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".     

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.     

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.     

Spectroscopic analysis of small organic molecules: a comprehensive near-edge x-ray-absorption fine-structure study of C6-ring-containing molecules

We report high-resolution C 1s near-edge x-ray-absorption fine-structure (NEXAFS) spectra of the C6-ring-containing molecules benzene (C6H6), 1,3- and 1,4-cyclohexadiene (C6H8), cyclohexene (C6H10), cyclohexane (C6H12), styrene (C8H8), and ethylbenzene (C8H10) which allow us to examine the gradual development of delocalization of the corresponding pi electron systems. Due to the high experimental resolution, vibrational progressions can be partly resolved in the spectra. The experimental spectra are compared with theoretical NEXAFS spectra obtained from density-functional theory calculations where electronic final-state relaxation is accounted for. The comparison yields very good agreement between theoretical spectra and experimental results. In all cases, the spectra can be described by excitations to pi*- and sigma*-type final-state orbitals with valence character, while final-state orbitals of Rydberg character make only minor contributions. The lowest C 1s-->1pi* excitation energy is found to agree in the (experimental and theoretical) spectra of all molecules except for 1,3-cyclohexadiene (C6H8) where an energy smaller by about 0.6 eV is obtained. The theoretical analysis can explain this result by different binding properties of this molecule compared to the others.     

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.     

Thin film structure of tetraceno[2,3-b]thiophene characterized by grazing incidence X-ray scattering and near-edge X-ray absorption fine structure analysis

Understanding the structure-property relationship for organic semiconductors is crucial in rational molecular design and organic thin film process control. Charge carrier transport in organic field-effect transistors predominantly occurs in a few semiconductor layers close to the interface in contact with the dielectric layer, and the transport properties depend sensitively on the precise molecular packing. Therefore, a better understanding of the impact of molecular packing and thin film morphology in the first few monolayers above the dielectric layer on charge transport is needed to improve the transistor performance. In this Article, we show that the detailed molecular packing in thin organic semiconductor films can be solved through a combination of grazing incidence X-ray diffraction (GIXD), near-edge X-ray absorption spectra fine structure (NEXAFS) spectroscopy, energy minimization packing calculations, and structure refinement of the diffraction data. We solve the thin film structure for 2 and 20 nm thick films of tetraceno[2,3-b]thiophene and detect only a single phase for these thicknesses. The GIXD yields accurate unit cell dimensions, while the precise molecular arrangement in the unit cell was found from the energy minimization and structure refinement; the NEXAFS yields a consistent molecular tilt. For the 20 nm film, the unit cell is triclinic with a = 5.96 A, b = 7.71 A, c = 15.16 A, alpha = 97.30 degrees, beta = 95.63 degrees, gamma = 90 degrees; there are two molecules per unit cell with herringbone packing (49-59 degree angle) and tilted about 7 degrees from the substrate normal. The thin film structure is significantly different from the bulk single-crystal structure, indicating the importance of characterizing thin film to correlate with thin film device performance. The results are compared to the corresponding data for the chemically similar and widely used pentacene. Possible effects of the observed thin film structure and morphology on charge carrier mobility are discussed.     

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.     

Epitaxial growth of pentacene films on Cu110

The molecular structure of thin pentacene (C(22)H(14)) films grown on a Cu(110) surface has been studied by means of He atom scattering, low energy electron diffraction, thermal desorption spectroscopy, x-ray photoelectron spectroscopy, and x-ray absorption spectroscopy. Depending on the actual film thickness three different crystalline phases have been identified which reveal a characteristic reorientation of the molecular plane relative to the substrate surface. In the monolayer regime the molecules form a highly ordered commensurate (6.5x2) structure with a planar adsorption geometry. For thin multilayers (thickness <2 nm) a second phase is observed which is characterized by a lateral ((-0.65 5.69) ( 1.90 1.37)) structure and a tilting of the molecular plane of about 28 degrees around their long axis which remains parallel to the surface. Finally, when exceeding a thickness of about 2 nm subsequent growth proceeds with an upright molecular orientation and leads to the formation of crystalline films which are epitaxially oriented with respect to the substrate. The present study thus demonstrates that also on metal substrates highly ordered pentacene films with an upright orientation of the molecular planes can be grown. Photoelectron spectroscopy data indicate further that thick films do not grow in a layer-by-layer mode but reveal a significant degree of roughness.     

Core-shell photoabsorption and photoelectron spectra of gas-phase pentacene: experiment and theory

The C K-edge photoabsorption and 1s core-level photoemission of pentacene (C22H14) free molecules are experimentally measured, and calculated by self-consistent-field and static-exchange approximation ab initio methods. Six nonequivalent C atoms present in the molecule contribute to the C 1s photoemission spectrum. The complex near-edge structures of the carbon K-edge absorption spectrum present two main groups of discrete transitions between 283 and 288 eV photon energy, due to absorption to pi* virtual orbitals, and broader structures at higher energy, involving sigma* virtual orbitals. The sharp absorption structures to the pi* empty orbitals lay well below the thresholds for the C 1s ionizations, caused by strong excitonic and localization effects. We can definitely explain the C K-edge absorption spectrum as due to both final (virtual) and initial (core) orbital effects, mainly involving excitations to the two lowest-unoccupied molecular orbitals of pi* symmetry, from the six chemically shifted C 1s core orbitals.     

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.     

Functional K-doping of eumelanin thin films: density functional theory theory and soft x-ray spectroscopy experiments in the frame of the macrocyclic protomolecule model

We demonstrate the possibility to achieve the doping of eumelanin thin films through K(+) incorporation during the electrodeposition of the film. K-doping changes the optical properties of the eumelanin thin films, reducing the energy gap from 1.0 to 0.6 eV, with possible implications for the photophysical properties. We have identified the doping-related occupied and unoccupied electronic states and their spectral weight using resonant photoemission spectroscopy (ResPES) and x-ray absorption at the C and N K-edges (near edge x-ray absorption fine spectroscopy, NEXAFS). All data are consistently interpreted by ab initio calculations of the electronic structure within the frame of the macrocycle model developed for the eumelanin protomolecule. Our analysis puts in evidence the intercalation of K with one specific oligomer (a tetramer composed of one indolequinone and 3 hydroquinone monomers) in correspondence of the nitrogen macrocycle. The predicted variation of the tetramer spacing is also in agreement with the recent x-ray diffraction experiments. The charge donation from K to N and C atoms gives rise to new electronic states at the top of the valence band and in NEXAFS resonances of the unoccupied orbitals. The saturation of the tetramer macrocycles leaves an excess of K that bind to N and C atoms in alternative configurations, as witnessed by the occurrence of additional spectral features in the carbon-related ResPES measurements.     

Self-assembled carbon nanotubes on gold: polarization-modulated infrared reflection-absorption spectroscopy, high-resolution X-ray photoemission spectroscopy, and near-edge X-ray absorption fine structure spectroscopy study

Recently we reported noncovalent functionalization of nanotubes in an aqueous medium with ionic liquid-based surfactants, 1-dodecyl-3-methylimidazolium bromide (1) and 1-(12-mercaptododecyl)-3-methylimidazolium bromide (2), resulting in positively charged single-wall carbon nanotube (SWNT)-1,2 composites. Thiolation of SWNTs with 2 provides their self-assembly on gold as well as templating gold nanoparticles on SWNT sidewalls via a covalent -S-Au bond. In this investigation, we studied the electronic structure, intermolecular interactions, and packing within noncovalently thiolated SWNTs and also nanotube alignment in the bulk of SWNT-2 dried droplets and self-assembled submonolayers (SAMs) on gold by high-resolution X-ray photoemission spectroscopy (HRXPS), C K-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and polarization-modulated infrared reflection-absorption spectroscopy (PM-IRRAS). HRXPS data confirmed the noncovalent nature of interactions within the nanocomposite of thiolated nanotubes. In PM-IRRAS spectra of SWNT SAMs on gold, the IR-active vibrational SWNT modes have been observed and identified. According to PM-IRRAS data, the hydrocarbon chains of 2 are oriented with less tilt angle to the bare gold normal in a SAM deposited from an SWNT-2 dispersion than those of 1 deposited from an SWNT-1 dispersion on the mercaptoethanesulfonic acid-primed gold. For both the dried SWNT-2 bulk and the SWNT-2 SAM on gold, the C K-edge NEXAFS spectra revealed the presence of CH-pi interactions between hydrocarbon chains of 2 and the pi electronic nanotube structure due to the highly resolved vibronic fine structure of carbon 1s --> R*/sigma*C-H series of states in the alkyl chain of 2. For the SWNT-2 bulk, the observed splitting and upshift of the SWNT pi* orbitals in the NEXAFS spectrum indicated the presence of pi-pi interactions. In the NEXAFS spectrum of the SWNT-2 SAM on gold, the upshifted values of the photon energy for R*/sigma*C-H transitions indicated close contact of 2 with nanotubes and with a gold surface. The angle-dependent NEXAFS for the SWNT-2 bulk showed that most of the molecules of 2 are aligned along the nanotubes, which are self-organized with orientation parallel to the substrate plane, whereas the NEXAFS for the SWNT-2 SAM revealed a more normal orientation of functionality 2 on gold compared with that in the SWNT-2 bulk.     

Measuring molecular order in poly(3-alkylthiophene) thin films with polarizing spectroscopies

We measured the molecular order of poly(3-alkylthiophene) chains in thin films before and after melting through the combination of several polarized photon spectroscopies: infrared (IR) absorption, variable angle spectroscopic ellipsometry (SE), and near-edge X-ray absorption fine structure (NEXAFS). The data from the various techniques can be uniformly treated in the context of the dielectric constant tensor epsilon for the film. The combined spectroscopies allow determination of the orientation distribution of the main-chain axis (SE and IR), the conjugated pi system normal (NEXAFS), and the side-chain axis (IR). We find significant improvement in the backbone order of the films after recrystallization of the material at temperatures just below the melting temperature. Less aggressive thermal treatments are less effective. IR studies show that the changes in backbone structure occur without significant alteration of the structure of the alkyl side chains. The data indicate that the side chains exhibit significant disorder for all films regardless of the thermal history of the sample.     

Calibrated NEXAFS spectra of common conjugated polymers

Near edge x-ray absorption fine structure (NEXAFS) spectroscopy has evolved into a powerful characterization tool for polymeric materials and is increasingly being used to elucidate composition and orientation in thin films of relevance to organic electronic devices. For accurate quantitative compositional analysis, insight into the electronic structure and the ability to assess molecular orientation, reliable reference spectra with known energy resolution and calibrated energy scale are required. We report a set of such NEXAFS spectra from 23 semiconducting polymers and some related materials that are frequently used in organic device research.     

Dipole-induced structure in aromatic-terminated self-assembled monolayers: a study by near edge x-ray absorption fine structure spectroscopy

The structure of self-assembled monolayers presenting aromatic rings at a surface is studied by near edge x-ray absorption fine structure spectroscopy (NEXAFS). Fluorine substitution at asymmetric positions in the aromatic rings is used to generate a layer of dipoles at the surface of the monolayer. We find that fluorine substituted aromatic rings are more ordered than unsubstituted aromatic rings by a factor of two based on the polarization dependence of the lowest C 1s to pi* transition, which is associated with transitions from phenyl carbons attached to hydrogens. This result is consistent with the influence of dipole-dipole interactions and quadrupolar interactions between the aromatic groups due to the substitution of fluorine atoms. The work also serves to illustrate how subtle variations in the orientation of an end group of a self-assembled monolayer can be determined by using NEXAFS.     

Core level study of alanine and threonine

Core level X-ray photoemission spectra (XPS) and near edge X-ray absorption fine structure (NEXAFS) spectra of alanine and threonine in the gas phase have been measured at the carbon, nitrogen, and oxygen K edges and interpreted in the light of theoretical calculations. For the computations, a set of approximations is made which allows sufficiently accurate calculations of several conformers to be performed in reasonable computing time. The accuracy has been checked by comparing results obtained for proline to our previous, higher level calculations. The photoemission spectra at the carbon and oxygen edges are assigned and compared. The nitrogen 1s photoemission peaks show anomalous broadening which we relate to the populations and types of conformers. The carbon K-edge NEXAFS spectra of alanine and threonine are compared with our previous data on glycine and resonances assigned accordingly. The nitrogen K-edge NEXAFS spectra of alanine and threonine do not show measurable effects due to the population of conformers, in contrast to the photoemission results. At the oxygen K edge, the spectra of these amino acids are similar with two prominent peaks assigned to transitions of O 1s electrons from the oxo and hydroxyl groups to vacant pi* and sigma* orbitals and additional intensity for threonine due to the second OH group. Conformer effects are observable in photoemission but appear to be more difficult to resolve in photoabsorption. We explain this by energetic shifts of opposite sign for the core hole states and unoccupied orbitals, which causes partial cancelation in NEXAFS but not in photoemission.     

Probing weak molecular orbital interactions in non-conjugated diene molecules by means of near-edge X-ray absorption spectroscopy

Carbon and oxygen near-edge X-ray absorption fine structure (NEXAFS) spectra of 1,4-cyclohexadiene, p-benzoquinone, norbornadiene, norbornadienone, and cis-cis-[4,4,2]propella-3,8-diene-11,12-dione were calculated by means of Hartree-Fock and hybrid density functional theory using the static-exchange (STEX) approximation. The NEXAFS spectra are used as a probe to identify weak molecular interactions between the two non-conjugated ethylenic pi* orbitals present in these molecules. We show that the X-ray absorption spectrum of 1,4-cyclohexadiene exhibits some particular spectral structures in the discrete energy region that evidence diene through-bond orbital interaction, whereas absorption peaks are identified in the norbornadiene and norbornadienone spectra that indicate effective through-space orbital interactions. The molecular structure of the cis-cis-[4,4,2]propella-3,8-diene-11,12-dione isomer is such that the indirect through-bond or through-space diene orbital interactions are too weak to be assigned by its C1s NEXAFS spectrum.     

Thickness dependence of microstructure in semiconducting films of an oligofluorene derivative

The measurement and optimization of microstructure development in organic semiconductor films is valuable because microstructure in many cases critically impacts electronic performance. We demonstrate a general method to measure microstructure thickness dependence in thin films using surface-sensitive near edge X-ray absorbance fine structure (NEXAFS) spectroscopy. The method is applied to an oligofluorene derivative DDFTTF, which consists of a fluorene-bithiophene-fluorene core that is end-substituted with linear dodecyl groups. The substrate-relative orientations of the aromatic core and the aliphatic end chains are independently determined, and comparing these orientations to terrace heights from atomic force micrographs proves that the end chains are interdigitated or folded. By measuring microstructure development from 6 to 150 nm, we find that DDFTTF exhibits two different preferential microstructures: one with large terraces within which molecules exhibit a strongly vertical orientation, and one with much smaller domains within which molecules exhibit a mildly horizontal orientation. The relative distribution of these two preferential microstructures depends on the distance of the domains from the substrate and the substrate temperature during deposition. The utility of this method is tested using a lamination technique to measure the saturation hole mobility at the top and bottom interface of DDFTTF films. We find that local microstructures with greater pi orbital alignment in the source-drain plane correlate directly to better local saturation hole mobilities.     

Molecular orientation and film morphology of pentacene on native silicon oxide surface

The growth morphology and mechanism of pentacene films on native Si oxide surface have been studied by using high-resolution electron energy loss spectroscopy (HREELS), X-ray diffraction (XRD), and atomic force microscopy (AFM). Despite the good agreement between our own and the reported XRD results, the previous XRD interpretation that the pentacene molecules are tilt-standing on the substrate cannot explain our HREELS data. The HREELS results show that a substantial portion of the first two layers of pentacene molecules are tilted-standing or randomly oriented, whereas the upper-layer molecules are mostly lying flat to the substrate. AFM reveals that the first two layers of molecules form a flat and smooth surface, but the upper layers show a rough terrace structure with a mean-square roughness equal to the average thickness (without counting the first two layers). This relationship is explained by a theoretical model which assumes the pentacene molecules to remain on a particular molecule layer after arrival. The observed film growth morphology may have significant implication on the performance of electronic devices based on pentacene thin films. A plausible explanation was proposed for the discrepancy between the HREELS-indicated and the XRD-derived molecular orientations.     

Characterization of protein immobilization at silver surfaces by near edge X-ray absorption fine structure spectroscopy

Ribonuclease A (RNase A) is immobilized on silver surfaces in oriented and random form via self-assembled monolayers (SAMs) of alkanethiols. The immobilization process is characterized step-by-step using chemically selective near-edge X-ray absorption fine structure spectroscopy (NEXAFS) at the C, N, and S K-edges. Causes of imperfect immobilization are pinpointed, such as oxidation and partial desorption of the alkanethiol SAMs and incomplete coverage. The orientation of the protein layer manifests itself in an 18% polarization dependence of the NEXAFS signal from the N 1s to pi* transition of the peptide bond, which is not seen for a random orientation. The S 1s to C-S sigma* transition exhibits an even larger polarization dependence of 41%, which is reduced to 5% for a random orientation. A quantitative model is developed that explains the sign and magnitude of the polarization dependence at both edges. The results demonstrate that NEXAFS is able to characterize surface reactions during the immobilization of proteins and to provide insight into their orientations on surfaces.