Search

Interpreting experimental spectra of thin films of organic semiconductors is challenging, and understanding the relationship between experimental data obtained by different spectroscopic techniques requires a careful consideration of the initial and final states for each process. The discussion of spectroscopic data is frequently mired in confusion that originates in overlapping terminology with however distinct meaning in different spectroscopies. Here, we present a coherent framework that is capable of treating on equal footing most spectroscopies commonly used to investigate thin films of organic semiconductors. We develop a simple model for the expected energy level positions, as obtained by common spectroscopic techniques, and relate them to the energies of molecular states. Molecular charging energies in photoionization processes, as well as adsorption energies and the screening of molecular charges due to environmental polarization, are taken into account as the main causes for shifts of the measured spectroscopic features. We explain the relationship between these quantities, as well as with the transport gap, the optical gap and the exciton binding energy. Our considerations serve as a model for weakly interacting systems, e.g., various organic molecular crystals, where wave function hybridizations between adjacent molecules are negligible.

Halogenation of conjugated molecules represents a powerful approach to tune the electronic structure of molecular thin films through inductive effects and long-range intermolecular electrostatic interactions. The mixing of halogenated molecules with their pristine counterparts has recently proven successful in altering the blend’s energy levels to adjust the open-circuit voltage of organic solar cells by the mixing ratio. Here, we show that the prevailing rationale for this effect is not equally valid for different molecular orientations. We provide a comprehensive experimental and theoretical analysis of the prototypical blend formed by pentacene and perfluoropentacene to relate structure with electronic properties. We find a mixed-stack structural motif in standing and lying orientations depending on the substrate nature. In the standing orientation, the ionization potential lies in between the values of the pure components, in line with the established picture of averaged molecular quadrupole moments. For the lying orientation, however, we experimentally observe an ionization potential lower than both pristine values, which seems at odds with this simple rationale. Electrostatic simulations based on the knowledge of the atomistic structure of the films capture the complex experimental scenario for both orientations. In particular, the ultralow ionization potential of films formed by lying molecules is identified as a signature of the monolayer structure, where quadrupolar interactions are responsible for a difference of ca. 0.4 eV in the highest occupied molecular orbital energy as compared to thicker films with the same molecular orientation.

Charge-carrier injection in organic electronics is critically influenced by the interface electronic states formed at the organic semiconducting thin films and metal electrode contacts. A practical solution for the issue is to form a charge-transfer (CT) state at the interfaces, which will produce a substantial density of gap states in the vicinity of the Fermi level of metal electrodes and thus reduce significantly the charge-injection barrier at the interfaces. In this study, we use ultraviolet photoelectron spectroscopy and demonstrate the formation of a CT state at diindenoperylene (DIP) and Ag (or Cu) electrode contacts. Remarkably, the CT state occurs irrespective of the electrode crystallinity, crystal grain orientations, or the molecular orientations in the DIP film, hence both the top- or bottom-metal contact conditions result in the formation of the CT state. Theoretical calculation simulations show the sign of strong interaction between those metal atoms and DIP leading to the CT state formation, analogous to the surface-induced aromatic stabilization proposed by Heimel et al. in a literature [Nat. Chem., 5(2013)187.]. Our results aid in understanding the mechanism of the CT state formation and propose the DIP-CT interlayers being novel candidates for the efficient hole-injection layers used in organic electronics.

We report on the growth and characterization of molecular mixed thin films of α-sexithiophene (6T), a well-known organic p-type semiconductor with high hole mobility, together with its perfluorinated counterpart, the so far rarely studied tetradecafluoro-α-sexithiophene (PF6T). Pure and blended thin films of these two molecules with different mixing ratios were grown on silicon oxide in ultrahigh vacuum by coevaporation. The effect of perfluorination and mixing on crystal structure, morphology, electronic, and optical properties was examined. The evolution of the PF6T crystal structure was followed in situ in real time by X-ray scattering. We found a new thin film structure different from the reported bulk phase with molecules either standing-up or lying-down depending on the growth temperature. The different morphologies of pure films and blends were investigated with atomic force microscopy. The impact of mixing on the core-levels and on the highest occupied molecular orbitals of 6T and PF6T is discussed.

Thickness and substrate dependence of film growth, morphology, unit-cell structure, and electronic structures was thoroughly investigated for picene, the zigzag connected 5-ring molecule, by employing complementary techniques of in situ real-time X-ray reflectivity/diffraction, in situ electron spectroscopies, and atomic force microscopy. A different kind of thickness dependent structural transition was observed on SiO2 and graphite, resulting in a distinct electronic structure. On SiO2 picene films with 3D crystalline domains are formed with nearly upright molecular orientation from the initial growth stage. With increasing the film thickness the in-plane dimensions of the unit cell in the initially grown domains become smaller (in other words, more compressed), and, at the same time, crystalline domains with a more relaxed structure are nucleating on top of the compressed domains. In spite of such structural changes, the electronic structure, namely energy position of the highest occupied molecular orbi...

The dependence of substrate work function (WF) on organic–organic heterojunction (OOH) interface was investigated using ultraviolet and X-ray photoelectron spectroscopy. We studied the interface of boron subphthalocyanine chloride (SubPc)/α-sexithiophene (6T) deposited on MoO3, SiO2, Cs2CO3. We observed that MoO3 and Cs2CO3 induce a p-doping and n-doping, respectively, due to the WF position, that can generate charge transfer at the OOH interface. However, the same effect was not observed after annealing the organic layers. Using scanning transmission X-ray microscope combining with near-edge X-ray absorption fine structure, we could observe that SubPc film became well-ordered after annealing the thin film. Thus, we suggested that the control of charge transfer arises from the reduction of the molecules misorientation on the film that induces a reduction in the density of gap states.

The structural order of C60 thin films is shown to be significantly improved by inserting a templating layer of diindenoperylene (DIP) between the SiO2 substrate and C60. In contrast to growth on an amorphous substrate like SiO2, C60 grown on DIP exhibits alignment of fcc-domains with the (111) plane parallel to the substrate and a significant increase of the coherent in-plane island size by a factor of ∼4. Modification of the structural quality of the DIP bottom layer leads to a change in structural order in the C60 top layer. In addition, ultraviolet photoelectron spectroscopy data from templated and nontemplated C60 films are discussed. In contrast to other anisotropic organic molecules, for C60 the spectral broadening and density of states of the highest occupied molecular orbital region do not depend significantly on the structural order in the C60 film, which can be rationalized by the isotropic shape of the C60 molecule.

We investigated the structural and electronic properties of vacuum sublimed 7,8,15,16-tetraazaterrylene (TAT) thin films on Au(111), Ag(111), and Cu(111) substrates using inverse photoemission spectroscopy, ultraviolet photoelectron spectroscopy (UPS), x-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED), and the x-ray standing wave (XSW) technique. The LEED reveals a flat adsorption geometry of the monolayer TAT on these three substrates, which is in accordance with the XSW results. The molecules are slightly distorted in monolayers on all three substrates with the nitrogen atoms having smaller averaged bonding distances than the carbon atoms. On Ag(111) and Cu(111), chemisorption with a net electron transfer from the substrate to the adsorbate takes place, as evidenced by UPS and XPS. Combining these results, we gain full insight into the correlation between electronic properties and interface geometry.

Tetraphenyldibenzoperiflanthene (DBP) is a promising candidate as a component of highly efficient organic photovoltaic cells and organic light-emitting diodes. The structural properties of thin films of this particular lander-type molecule on Ag(111) were investigated by complementary techniques. Highly ordered structures were obtained, and their mutual alignment was characterized by means of low-energy electron diffraction (LEED). Scanning tunneling microscopy (STM) images reveal two slightly different arrangements within the first monolayer (ML), both describable as specific herringbone patterns with two molecules per unit cell whose dibenzoperiflanthene framework is parallel to the surface. In contrast, single DBP molecules in the second ML were imaged with much higher intramolecular resolution, resembling the shape of the frontier orbitals in the gas phase as calculated by means of density functional theory (DFT). Further deposition leads to the growth of highly ordered bilayer islands on top of the first ML with identical unit cell dimensions and orientation but slightly inclined molecules. This suggests that the first ML acts as a template for the epitaxial growth of further layers. Simultaneously, a significant number of second-layer molecules mainly located at step edges or scattered over narrow terraces do not form highly ordered aggregates.

The energy-level alignment of vacuum-sublimed thin films of a series of differently alkyl-substituted sexithiophenes (6T) ( α , ω -didecylsexithiophene (DD6T), α , ω -diethylsexithiophene (DE6T) and 3′,4′-dimethyl- α , ω -dihexylsexithiophene (Me-DH6T)) on Ag(1 1 1) substrates was investigated by means of ultraviolet photoelectron spectroscopy (UPS). The orientation dependent ionization energy of the 6T derivatives allowed determining the thin film growth mode of the molecules, which varied with the length and substitution-position of the alkyl chains on the conjugated 6T backbone. DE6T and Me-DH6T are lying almost flat in the monolayer and in multilayers, whereas DD6T undergoes an orientational transition from lying to standing from the second molecular layer onwards. Additional UPS experiments of DD6T on a high work function substrate, i.e., Ag(1 1 1) pre-covered with a monolayer tetrafluoro-tetracyanoquinodimethane, illustrate how a slight shift of the center-of-mass of the molecule’s electron density might lead (in contrast to a charge transfer) to interface dipoles at organic–organic interfaces and thus to Fermi-level pinning at such interfaces.

We investigate the energy level alignment and the Fermi level pinning mechanism at organic donor–acceptor heterojunctions interfaces by using the model organic–organic heterojunctions (OOHs) with well-defined molecular orientation of the standing copper (II) phthalocyanine (CuPc) and zinc phthalocyanine (ZnPc) films on the standing copper-hexadecafluoro-phthalocyanine (F16CuPc) thin films on SiO2. We identify two distinct regions for the energy level alignment by in situ ultraviolet photoelectron spectroscopy investigation. In region (I) where the work function (WF) of the underlying substrate is larger than the ionization potential (IP) of the top organic layers, the substrate Fermi level is pinned at the leading edge of the HOMO peak accompanied by a decreasing of the WF; in region (II) where the WF is smaller than the IP of the top organic layers, a downward shift of both the HOMO and vacuum level is observed. In connection with the defect induced gap states, we provide a detailed explanation for this thickness dependent energy level alignment and Fermi level pinning mechanism at the organic donor–acceptor OOH interface.

We studied the thickness dependence of electronic structure and molecular orientation of neutral, square planar, diamagnetic diradical metal complex, bis( o -diiminobenzosemiquinonate) nickel(II) complexes (Ni(DIBSQ) 2 ), thin films grown on a SiO 2 substrate by using ultraviolet photoelectron spectroscopy in combination with metastable atom electron spectroscopy. We observed that the small hole-injection barrier of 0.46 eV for the monolayer film, and the barrier increases continuously with the film thickness and becomes larger than the electron-injection barrier. We further found that molecules are oriented with their long-axes nearly perpendicular to the surface both in the monolayer and multilayer films independent of the film thickness.

We have studied the vibrational properties of perfluoropentacene (PFP) thin films on highly oriented pyrolytic graphite (HOPG) substrates by high-resolution electron energy loss spectroscopy (HREELS) and Raman spectroscopy. The HREELS spectra showed slight but clear increase in the vibrational energies with increasing film thickness as well as with decreasing temperature of the multilayer films. In the polarization-dependent Raman spectra of a multilayer film, the depolarization ratios of all detected vibrations with the Ag irreducible representation were found to be larger than 1, indicating that the molecular structure is distorted in the multilayer. Both results suggest that there is a fairly strong intermolecular interaction in PFP multilayer films.

We summarize both historical and recent challenges on angle-resolved and high-energy resolution ultraviolet photoelectron spectroscopy (UPS) of organic thin films. Topics selected for this article are mainly on electron spectroscopic study of the electronic states in relation to charge mobility of organic molecular thin films, especially of weakly interacting organic molecular solids. We describe intramolecular band dispersion in a quasi-one-dimensional molecular chain and intermolecular band dispersion measured with angle-resolved UPS. The latter offers a spectroscopic estimation of the drift hole mobility in organic semiconductors. Furthermore we describe briefly hole-vibration coupling in organic ultrathin films, which dominates the hopping hole mobility and has been recently measured with high resolution UPS. These experiments are thus considered to be a kind of the first-principle measurement of the mobility of organic thin films, which have not yet been realized with electrical measurements. Conduction band dispersion studied with low-energy electron transmission, which is needed in analyzing angle-resolved UPS, ultrafast phenomena appearing in conventional UPS measurements and other interesting work are also introduced.

This article describes the origins of the width of the highest occupied molecular orbital (HOMO) state observed in the ultraviolet photoemission spectra (UPS) of thin organic semiconductor films. Although much research has been performed on the electrical properties of organic devices, a lot of crucial problems still remain. Among these problems, the charge mobility in organic semiconductor systems is one the most important subjects to be elucidated. In order to discuss the mobility, it is essential to understand both the intermolecular interaction and the electron-molecular vibration coupling. Experimental measurements of the energy band dispersion give information about the intermolecular interaction, and experimental detection of the HOMO hole-vibration coupling is indispensable to comprehend impacts of the electron-vibration coupling on the hole transport. Since most of the information is hidden behind the finite bandwidth of the HOMO, only careful UPS measurements can provide information on these important phenomena related to charge carrier dynamics. In this article, we summarize our recent challenges on UPS measurements of organic thin films, which give the band dispersion of the HOMO and the HOMO hole-vibration coupling, and discuss the origins of the UPS bandwidth that relates to the charge carrier dynamics.

We have investigated seven samples of radio frequency magnetron-sputtered TiO 2 thin films by using near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD). A detailed analysis of the Ti 2p and O 1s NEXAFS edges allows determining the degree of order and the polymorphs of the investigated samples. We have found that thin films grown by using a Ti target and lower deposition rates have a rutile character, while, by using a TiO 2 target and higher deposition rates, amorphous/mixed thin films are obtained. We have also found that this correlation between phase and preparation conditions is perfectly reproducible leading to films with equal characteristics for the same preparation conditions. The NEXAFS results are supported by complementary XRD results.

Interchain interaction, i.e., pi-pi stacking, can benefit the carrier transport in conjugated regio-regular poly(3-hexylthiophene) (P3HT) thin films. However, the existence of the insulating side hexyl chains in the surface region may be detrimental to the charge transfer between the polymer backbone and overlayer molecules. The control of the molecular orientation in the surface region is expected to alter the distribution of the pi electron density at the surface to solve such problems, which can be achieved by controlling the solvent removal rate during solidification. The evidence that the pi-electron density distribution at the outermost surface can be controlled is demonstrated by the investigation using the powerful combination of near edge X-ray absorption fine structure spectroscopy, ultraviolet photoelectron spectroscopy, and the most surface-sensitive technique: Penning ionization electron spectroscopy. From the spectroscopic studies, it can be deduced that the slower removal rate of the solvent makes the polymer chains even at the surface have sufficient time to adopt a more nearly equilibrium structure with edge-on conformation. Thus, the side hexyl chains extend outside the surface, which buries the pi-electron density contributed from the polymer backbone. Contrarily, the quench of obtaining a thermo-equilibrium structure in the surface region due to the faster removal of the solvent residual can lead to the surface chain conformation without persisting to the strong bulk orientation preference. Therefore, the face-on conformation of the polymer chain at the surface of thin films coated with high spin coating speed facilitate the electron density of the polymer backbone exposed outside the surface. Finally, thickness dependence of the surface electronic structure of P3HT thin films is also discussed.

We report the surface electronic structures of polythiophene derivatives (including polythiophene, poly(3-butylthiophene), poly (3-hexylthiophene)(P3HT)) thin films observed by ultraviolet photoelectron spectroscopy and Penning ionization electron spectroscopy. He*-impact-surface-modification was applied on the P3HT surface to control the distribution of the electron density.

The new type doping method, VUV-induced doping, has been applied to copper phthalocyanine (CuPc) thin film. The highest occupied molecular orbital (HOMO) peak in the ultraviolet photoelectron spectrum of CuPc film was gradually shifted toward high binding energy side upon the irradiation of He Iα radiation. Furthermore, the radiation induced gap states were observed up to the Fermi level. The shift of the HOMO peak in the low-dose region depends in a semi-logarithmic fashion on the irradiation time, indicating that the intentional n-type doping of CuPc film can be realized by VUV irradiation.

The hole-vibration coupling of the highest occupied state in pentacene thin films on graphite was studied by high-resolution ultraviolet photoemission spectroscopy. It was found that vibration satellites in the film are more intense than that in the gas phase and the vibrational energy in the film is slightly lower than that in the gas phase. This demonstrates that the reorganization energy (hole mobility) in the pentacene thin film is slightly larger (smaller) than that expected from the photoemission spectrum of free pentacene molecules.

The impact of potassium (K) doping on the electronic structure of single crystalline (SC) pentacene (Pn) thin film grown on Cu(110) was investigated by high-sensitivity angle-resolved ultraviolet photoemission spectroscopy for K x Pn1 (0 ≤ x ≤ 1). At room temperature (293 K), we observed that (i) Pn electronic states shift towards high binding side with progressive growth of a gap state with the K-doping level, (ii) the HOMO band dispersion of the pristine Pn SC multilayer survives even at x = 1, and (iii) no dispersion occurs for the K-induced gap state. At 50 K, on the other hand, we found a small dispersion (width ~0.08 eV) of the K-induced gap state for K1Pn1 SC multilayer. This finding demonstrates that the K-induced gap state originates from the Mott–Hubbard insulator formed by electron transfer from K atom to Pn LUMO. The impact of K doping on the energy level alignment was discussed in terms of coexistence of K1Pn1 Mott–Hubbard insulator and Pn SC film.

The study of charge mobility in organic semiconductor systems has been one of the most important subjects that has remained a puzzle for many years. It is essential to quantitatively understand conduction charge-molecular vibration coupling as well as the intermolecular interaction to discuss mobility. This chapter describes recent successes with direct measurements of valence hole-vibration coupling in ultrathin films of organic semiconductors by using ultraviolet photoelectron spectroscopy (UPS), which can be used to experimentally study charge mobility based on energy and momentum conservation rules. The method may thus be categorized as a first-principles study of charge mobility. The detection of hole-vibration coupling of the highest occupied molecular orbital (HOMO) state in a thin film by UPS is essential to comprehend hole-hopping transport and polaron-related transport in organic semiconductors. Only careful measurements can attain the high-resolution spectra and provide these key parameters in hole-transport dynamics. A key method in achieving such high-resolution UPS measurements is also described.

Anisotropic surface diffusion of In atoms was observed by photoelectron emission microscopy (PEEM) when a square-shaped microstructure of In metal was deposited onto thin films of perylene-3,4,9,10-tetracarboxyric acid-dianhydride (PTCDA) grown on cleaved MoS 2 and GeS surfaces. For In microstructure on PTCDA/MoS 2 system, the PEEM images showed a triangle pattern, although the deposited shape of the In microstructure was square, indicating that In atoms diffuse to three directions on PTCDA/MoS 2 . Such triangle pattern was not observed on PTCDA/GeS but an oval structure was observed, revealing that In atoms diffuse to two directions on PTCDA/GeS with different diffusion constant. The two directions correspond well with surface crystal axes of GeS. Since the unit cells of PTCDA adsorbed on these two crystal surfaces are nearly the same, it is suggested that different anisotropic diffusion of In in these systems is related to strong In-PTCDA interaction which may be affected by the surface structure of underlying substrate.

A diffusion-induced changes of indium microstructure was observed by photoelectron emission microscopy (PEEM) and scanning electron microscopy (SEM) for evaporated microstructure of In metal on a pelylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) thin film on MoS2 substrate. The PEEM and SEM images show a periodic triangle pattern, although deposited shape of the In microstructure was square. In contrast such an unusual images originating from anisotropic diffusion was not observed when deposition metals or substrate materials were changed. These results suggest that the peculiar phenomenon originates from (1) chemical reaction between PTCDA and In, and (2) the molecular packing structure which depends on substrate materials.

A combined metastable electron emission microscopy (MEEM) and photoelectron emission microscopy (PEEM) study has been performed for copper phthalocyanine (CuPc) thin films vacuum deposited on Si(100) with a lithographic SiO2 pattern. The MEEM image of SiO2 pattern edges changed considerably upon deposition of a very small amount of CuPc on the SiO2/Si(100) substrate, whereas no such changes were observed in the PEEM images. The different contrast features in MEEM and PEEM can be the result of different surface sensitivities of these microscopy techniques. The MEEM contrast characteristic for the pattern edges after the CuPc deposition may be ascribed to the difference of the molecular orientation of CuPc which cannot be observed by PEEM.

Using metastable atom electron spectroscopy (MAES), changes of the outermost electronic states of organic thin films were measured for an OTi-phtalocyanine (OTiPc) film and a 8-bromooctanehtiol (8BrT) self-assembled monolayer (SAM) during the impact of slow He* (23S). Removal of the outermost atoms, i.e., oxygen atom for OTiPc film and for Br atom for 8BrT self-assembled monolayer (SAM), whose molecular orbitals spread to the outside of the film, can be successfully realized. In the case of 8BrT SAM, we also observed a new band related to the formation of a C=C double bond at the surface. The observed spectral changes in MAES indicate that the surface reaction can be realized by the selective ionization of the outermost surface with the impact of slow metastable atoms.

In low-energy-electron transmission spectra of monolayer films of various organic-semiconductor molecules deposited on MoS2 and graphite surfaces, we found that the energy positions of spectral minima are proportional to (n+1/2)2, where n is positive integer and 0, independent of molecules and substrates. Despite the complex structure of each molecule, the (n+1/2)2 rule can be simply explained by the interference of an excess electron passing through the potential of the monolayer on the substrate. Using these results, we estimated the effective potential, the potential width and depth, of the monolayer felt by the injected excess electron.

Gap states in organic semiconductors play a crucial role in determining Energy-Level Alignment and in many cases they act as charge trapping centers to result in serious lowering of charge mobility. Thus origin of gap states has gained increasing attention in order to realize higher mobility organic devises [1-4]. Bussolotti et al. have demonstrated recently that gap states in a pentacene thin film increase even by exposing the film to inert gas and confirmed that the gas exposure mediates structural defects in the film thus gap states [4]. The results have also indicated that preparation of highly-ordered organic thin film is necessary to improve the device performance, namely to decrease trapping states. To improve the ordering of molecule in the film, deposition of a template molecular underlayer is one of the simplest methods to increase the domain size of overlayer film and its crystallinity, and thus we expect improvement of the charge mobility [5]. Hinderhofer et al. reported recently that diindenoperylene (DIP; Figure 1a) could be used as a template layer to grow highly ordered and oriented C60 film with its (111) plane parallel to the SiO2 substrate [6]. Considering the hole mobility of DIP single crystal, which is quite low (~0.005 cm2 V-1S-1 at room temperature [7]), it is expected for the DIP template C60 thin film system that lower drain current would be achieved to improve the on/off ratios based on n type C60 transistor and its electron mobility (especially on the negative Vgs region, compared to PEN modified C60 transistors [8]).

Metastable-atom electron spectroscopy (MAES) and ultraviolet photoelectron spectroscopy (UPS) were used to study the outermost surface of thin films of pendant group polymers: polystyrene, poly(2-vinylnaphthalene), and poly(9-vinylcarbazole). MAES is selectively sensitive to the outermost surface, and indicated that the surfaces of the polymer films were very clean, even though they were prepared by spin-casting in a room atmosphere. By comparison with gas-phase spectra and molecular orbital calculations of model molecules with pendant groups, it was confirmed that the principal constituent at the outermost surface of these polymer films is the pendant groups. Furthermore, it was observed that the intensity for σ(C–H) states of the pendant group is stronger in MAES spectra than in UPS spectra, indicating many pendant groups are inclined at large tilt angles.

Diffusion of a large organic semiconductor molecule, chloroaluminum phthalocyanine (ClAlPc), on a cleaved MoS2 surface was detected using photoemission electron microscopy (PEEM) and metastable electron emission microscopy (MEEM). The PEEM and MEEM images showed that a micropattern of ClAlPc ultrathin film prepared on the MoS2 surface by vacuum deposition shrinks with time and finally disappears even at room temperature at which the molecules do not evaporate. The results indicate that control of molecular diffusion is necessary for the preparation of stable micro or nanostructure of organic thin films.

Surface images of a chloroaluminum phthalocyanine (ClAlPc) film, which consists of monolayer and doublelayer regions, were observed by two surface sensitive microscopy, photoelectron emission microscopy (PEEM) using photons of the ionization threshold energy of the film and metastable electron emission microscopy (MEEM) using metastable He. In PEEM, an image contrast, which directly reflects slight difference of ionization threshold energies between monolayer and doublelayer regions, was obtained, while in MEEM little contrast was detected because of the use of all emitted electrons for the imaging. The PEEM result indicates that the ionization threshold energy of the film depends on the film thickness. Using PEEM, we can map the slight difference of electronic states due to the thickness difference of an organic thin film.

High-resolution electron energy loss spectra (HREELS), low-energy electron transmission spectra (LEET) and Penning ionization electron spectra (PIES) were measured for thin films of chloroaluminum phthalocyanine (ClAlPc) deposited on a cleaved MoS 2 surface in order to investigate the molecular orientation and aggregation in the monolayer range. It was found that the as-grown film of monolayer equivalence consists of ClAlPc islands and the substrate surface is not fully covered by the molecules. Upon heating the film, the molecules spread over the substrate surface to form a uniform monolayer with a flat-lie orientation, where the Cl atoms protrude outside the film.

The hole–phonon coupling of a rubrene monolayer on graphite is measured by means of angle resolved ultraviolet photoelectron spectroscopy. Thus, the charge reorganization energy λ and the small polaron binding energy is determined, which allows insight into the nature of charge transport in condensed rubrene.

We observed very small electronic density of states in the band gap of pentacene thin films deposited on inert surfaces using ultraviolet photoelectron spectroscopy (UPS) with ultrahigh sensitivity. We found, furthermore, that a pentacene film with less density of gap states gives a splitting of the HOMO band in UPS spectra with energy separation of about 0.45 eV due to the band dispersion even for ultrathin polycrystalline films. The results indicate that the gap states do not originate from electronic interaction between pentacene and the substrate surface but from imperfect molecular orientation/packing structure. We confirmed that the Fermi level pinning in the pentacene films originates from the intrinsic gap states depending on their density and energy distribution. The Fermi level position as well as appearance of the band dispersion in pentacene thin films therefore depends sensitively on perfectness of the molecular packing structure in each crystal grain.

The polarization dependence of F K‐edge X‐ray absorption near edge structure (XANES) spectra of highly‐oriented thin‐film of polytetrafluoroethylene (PTFE) has been analyzed by using multiple scattering theory. The spectra show clear polarization dependence due to the highly‐oriented structure. The multiple scattering calculations reflects a local structure around an absorbing atom. The calculated results obtained by considering intermolecular‐interactions are in good agreement with the observed polarization‐dependence. We have also analyzed structural models of the radiation damaged PTFE films.

The hole-vibration coupling of the highest occupied state in pentacene thin films on graphite is studied by high-resolution ultraviolet photoelectron spectroscopy. Vibration satellites in the film show a take-off-angle dependence, indicating that the Franck-Condon principle is not strictly satisfied in this system. They are more intense than in the gas phase and the vibrational energy in the film is slightly lower than that in the gas phase. This demonstrates that the reorganization energy in pentacene thin films is slightly larger than that estimated from the photoelectron spectrum of free pentacene molecules. Furthermore, it is pointed out that the electron hopping in the low-temperature film may occur in the femtosecond scale before the electronic polarization of the surrounding medium is completed. © 2005 The American Physical Society.

This article introduces our recent study on solid surfaces by metastable electron emission microscopy (MEEM) which is in principle selectively sensitive to outermost-surface electronic states. Examples of surface images by MEEM are shown for a SiO2 pattern on Si(100) and a microstructured thin film of organic semiconductor, chloroaluminum phthalocyanine (ClAlPc), on a MoS2 surface. The latter result demonstrates that very large organic molecule can diffuse on the surface. For the ClAlPc film, the results obtained by photoemission electron microscopy (PEEM) using photons near the threshold ionization energy of the target are also shown.

Angle-resolved UV photoelectron spectra were measured for thin films of chloroaluminum phthalocyanine deposited on cleaved MoS2 surfaces. The take-off angle (θ) dependence of the photoelectron intensity of the highest π band showed a remarkable sharpening upon cooling the film, indicating that thermal excitation of molecular vibrations gives a considerable broadening of the photoelectron angular distribution. The θ dependence observed at ∼120 K agrees well with that calculated.

The electron affinity of pentacene thin films has been evaluated during the last decades, but it is still under controversial due to varieties of film quality and radiation damages of the films introduced during inverse photoemission spectroscopy (IPES) experiment together with insufficient energy resolution of the instruments. We employed the near-ultraviolet IPES with a better energy resolution 0.27 ∼ 0.32 eV and using lower energy electron beams (0 eV ≤ Ei ≤ 4.9 eV) to study the unoccupied states of pentacene thin film. Due to a large mean-free-path of the electron in this energy region, the threshold electron affinity of the bulk of pentacene film was precisely determined to be 2.70 ± 0.03 eV. Using the threshold ionization energy of 4.90 ± 0.05 eV determined by ultraviolet photoemission spectroscopy, the band-gap energy of the pentacene film is obtained to be 2.20 ± 0.06 eV.

We applied in situ x-ray reflectivity and ultraviolet photoelectron spectroscopy to study the impact of annealing on low temperature (200 K) deposited organic thin films of diindenoperylene (DIP) on SiO2 and indium tin oxide (ITO). At 200 K, DIP is crystalline on SiO2 and amorphous on ITO. Upon heating to room temperature, the roughness of DIP is reduced on both substrates, from 1.5 nm to 0.75 nm (SiO2) and from 0.90 nm to 0.45 nm (ITO). The smoothing is accompanied by crystallization of the surface molecules, whereas the bulk structure of the films does not strongly reorganize.

The impact of 1 atm N2 gas exposure on the electronic states of copper phthalocyanine thin films was investigated using ultrahigh-sensitivity ultraviolet photoelectron spectroscopy. The highest occupied molecular orbital band of the film showed a drastic reversible change in the bandwidth and band shape as well as in the energy position upon repeated cycles of N2 exposure and subsequent annealing. Furthermore, two types of gap-state densities with Gaussian and exponential distributions appeared after the exposure and disappeared due to the annealing. These changes are ascribed to a weak disorder in the molecular packing structure induced by N2 diffusion into the film.

Metastable atom electron spectroscopy and ultraviolet photoelectron spectroscopy were used to characterize orientation of bisazomethine dye molecules (DE2) in thin films deposited on graphite and ITO. We found that the molecular orientation of DE2 in multilayers is largely different on these substrates. On graphite, the molecules lie flat in a monolayer and become standing up with increasing the film thickness, while on ITO they lie nearly flat independent of the film thickness.

The effect of the molecular orientation on the molecular electronic structure is studied on the Cu-phthalocyanine∕graphite system via film thickness dependences of metastable-atom electron spectra and ultraviolet photoelectron spectra. We observed a decrease in the vacuum-level position and a corresponding band-bending-like shift in the highest occupied state only for thick films where the molecular tilt angle increases gradually with the film thickness. These shifts are explained by electric dipoles produced in the film by a gradient of the intermolecular electronic interaction along the surface normal due to the continuous increase in the molecular tilt angle. The result indicates that the change in the molecular orientation is an important origin of the band-bending-like shift in the molecular electronic states even if the molecule has no permanent electric dipole.

The surface electronic structures of conjugated regio regular and regio random poly (3- hexylthiophene) (rr-P3HT and rra-P3HT) thin films were studied by near edge X-ray absorption fine structure spectroscopy, ultraviolet photoelectron spectroscopy and Penning ionization electron spectroscopy (PIES). The distribution of the surface electronic states was controlled on rr-P3HT and rra-P3HT thin films with different molecular ordering by varying the coating process and PIES was adopted to observe the electronic states existing outside the surface.