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The photoelectron angular distribution of organic thin films is essential in the discussion of the (opto)electronic properties from the viewpoints of the spatial distribution of molecular orbitals and the interaction of molecular electronic states. We discuss the role of the weak intermolecular interaction in highly ordered aromatic molecules on the photoelectron angular distribution obtained by angle-resolved photoemission spectroscopy (ARPES). The experimental and theoretical investigation performed for superstructures of polycyclic aromatic hydrocarbons (PAHs) on Au(111) indicates that at least the intramolecular single-scattering process is necessary to be considered in simulation of the orbital tomography. Furthermore, ARPES spectra for various PAH monolayers reveal that the weak π-band dispersion is dependent on the surface Brillouin zone of the monolayer but not of the substrate. The long-range lateral intermolecular interaction in highly ordered aromatic molecules plays a main role in the π-band d...

The orbital-specific interfacial and intermolecular interactions in crystalline films of iron phthalocyanine (FePc) grown on an Au(111) surface were studied by means of angle-resolved photoemission spectroscopy (ARPES) with synchrotron radiation. The temperature-dependent ARPES measurement on the FePc monolayer reveals the formation of the molecule–metal hybrid state together with the Kondo resonance state below the Fermi level. These interface-specific states are derived from the electronic coupling of the Fe 3d orbital in the molecule with the Au(111) surface electron. For the flat-lying FePc multilayer on Au(111), the photon-energy-dependent ARPES spectra reveal the intermolecular valence-band dispersion for the C 2p-derived state, while the Fe 3d-derived state does not show such dispersion, indicating the localized Fe 3d spin state. These orbital-specific interactions play crucial roles in the unique electronic and magnetic properties of FePc.

The electronic structure of 7,7,8,8-tetracyanoquinodimethane (TCNQ) and 2,3,5,6-tetrafluoro-TCNQ (F4TCNQ) monolayers on Au(111) has been investigated by means of angle-resolved photoemission spectroscopy (ARPES) with synchrotron radiation. In contrast to the physisorbed TCNQ/Au(111) interface, the high-resolution core-level photoemission spectra and the low-energy electron diffraction at the F4TCNQ/Au(111) interface show evidence for the strong charge transfer (CT) from Au to F4TCNQ and for the Au atom segregation from the underlying Au(111) surface, suggesting a possible origin of the spontaneous formation of the two-dimensional F4TCNQ-Au network. The ARPES experiment reveals a low hole-injection barrier and large band dispersion in the CT-induced topmost valence level of the F4TCNQ-Au network with 260 meV bandwidth due to the adatom-mediated intermolecular interaction. These results indicate that strong electron acceptor molecules with metal adatoms can form high hole-mobility molecular layers by contro...

We present the electronic structure of various pentacene thin films grown on Au(1 1 1), Cu(1 1 1), Cu(1 0 0), and Cu(1 1 0) surfaces studied by angle-resolved ultraviolet photoemission spectroscopy using synchrotron radiation. A systematic variation of the metal surface such as the substrate metal and its surface symmetry allows a comprehensive discussion on the correlation between the electronic structure and the interface geometric structure. In the monolayer regime, we observed the evidence of the formation of the organic–metal interface state depending on the metal surface, i.e., the interface geometric structure. This evidence is explained by the different organic–metal and intermolecular interactions, which originate from the hybridization of the molecular orbitals with the metal wavefunction. These interface geometric and electronic phenomena can be a seed for the subsequent film growth and resultant films electronic structure.

We report the electronic structure and the charge transport mechanism of highly ordered films of pentacene on Cu(1 10) surface studied by angle-resolved ultraviolet photoemission spectroscopy using synchrotron radiation. For a flat-lying monolayer film, we observed the evidences of (i) formation of the interface states and (ii) two-dimensional intermolecular band dispersion of the resultant interface states, which may originate from the hybridization between the molecular orbitals and the wave function of the substrate. For an upright-standing multilayer film, we observed the two-dimensional intermolecular band dispersion, which originates from the intermolecular π-π interaction. The observed effective masses of the hole for different azimuths demonstrate the presence of the anisotropy of the hole mobility in pentacene crystals also at higher temperatures.

Angle-resolved photoelectron spectra were measured for a well-ordered monolayer film of titanyl phthalocyanine (OTiPc) prepared on a cleaved graphite surface using synchrotron radiation. The observed photoelectron angular distributions were analyzed by a calculation using the single-scattering approximation combined with molecular orbital calculation. The molecular orientation of the OTiPc monolayer on graphite was estimated quantitatively.

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.

Angle-resolved photoelectron spectra were measured for a single-domain monolayer film of naphthalene-1,4,5,8-tetracarboxylic dianhydride (NTCDA) prepared on a cleaved GeS(0 0 1) surface using synchrotron radiation. The observed photoelectron angular distributions were analyzed by a calculation using the single-scattering approximation combined with molecular orbital calculation. With the help of the low-energy electron diffraction pattern, the structure of the NTCDA monolayer on GeS(0 0 1) was estimated quantitatively.

Ultraviolet photoelectron spectra were measured for ultrathin films of OTi-phthalocyanine (OTiPc), which has an electric dipole perpendicular to the molecular plane, prepared on highly oriented pyrolytic graphite in order to study effects of the molecular orientation and the electric dipole layer on the organic electronic states. For the as-grown films, the observed highest occupied molecular orbital (HOMO) band consists of two prominent peaks that can be assigned to different molecular orientations. For the oriented monolayer obtained by annealing the as-grown film, we detected a very sharp HOMO band at 290 K. The binding-energy (EB) difference between the HOMO bands of the as-grown and annealed films was found to agree with the shift in the vacuum level. For the oriented monolayer, the observed sharp HOMO band involves at least four components that are ascribed to the coupling between the HOMO hole and the molecular vibration. Upon cooling the sample to 95 K, the HOMO bandwidth became sharper than that at 290 K. From the peak fitting using Voigt function, additional components are expected in the HOMO band at 95 K. Moreover, we detected the EB shift in the HOMO band for the oriented monolayer upon cooling, which can be originated from decrease in the HOMO-hole screening due to the change in the film structure and/or the molecule-substrate interaction.

Metastable atom electron spectroscopy was used to characterize monolayer formation of chloroaluminum phthalocyanine (ClAlPc) prepared on graphite. For as-grown film, molecules form island structure of staggered doublelayers on the substrate. By annealing the film, molecules diffuse to form a uniform monolayer where all the molecules are oriented flat with Cl atom directed to the vacuum. After the confirmation of the oriented monolayer formation, high-resolution ultraviolet photoelectron spectra were measured to study effects of the molecular orientation on the energy levels. ClAlPc has an electric dipole perpendicular to the molecular plane, hence a well-defined electric dipole layer could be intentionally prepared by using the oriented monolayer. Difference of binding energies of HOMO bands between the oriented monolayer and the doublelayer was found to agree with the vacuum level shift, leading to a conclusion that the molecular energy level with respect to the substrate Fermi level is changed when the molecule is in the dipole-layer field.

Ultraviolet photoelectron spectra were measured for titanyl- and lead-phthalocyanine ultrathin films prepared on graphite in order to study effects of the electric dipole layer on the organic energy levels. Each of these molecules has an electric dipole perpendicular to the molecular plane, and hence a well-defined electric dipole layer could be intentionally prepared by using oriented monolayer of these molecules. For as-grown films, the observed highest occupied molecular orbital (HOMO) band consists of many peaks that could be assigned to different molecular orientations/aggregations. For well-oriented monolayer films obtained by annealing the as-grown films, we observed a very sharp HOMO band with two satellites for both molecules as for copper phthalocyanine. Difference of binding energy of HOMO bands between the oriented monolayer and the doublelayer in which molecular electric dipoles are cancelled was found to agree with the vacuum level shift for both molecules, leading to important conclusions that (1) the molecular energy level with respect to the substrate Fermi level is changed when the molecule is in the dipole layer and (2) the binding-energy shift corresponds with the vacuum level shift.

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.

We observed a very narrow bandwidth of the highest occupied molecular orbital (HOMO) state in ultraviolet photoemission spectra (UPS) of copper-phthalocyanine monolayer deposited on graphite. The HOMO band in UPS consists of three components which may originate from the vibrational coupling. The full width at half maximum of each component was found to be ∼150 meV at 295 K. This HOMO-bandwidth leads to an estimation that the lifetime of the HOMO hole should be at least longer than 2.2 fs, which may be dominated by the electron transfer rate from the substrate to the molecule.

Ultraviolet photoelectron spectra were measured for vanadyl phthalocyanine (VOPc) ultrathin films prepared on graphite to study effects of the molecular orientation and the electric dipole layer on the organic electronic states. VOPc has a permanent electric dipole perpendicular to the molecular plane, hence a well-defined electric dipole layer could be intentionally prepared by using the oriented monolayer. The observed binding-energy difference of the highest occupied molecular orbital (HOMO) bands between the oriented monolayer and the double layer was found to agree with the vacuum level shift, leading to a conclusion that the molecular energy level with respect to the substrate Fermi level is changed when the molecule is in the electric dipole layer.

Changes in the work function (WF) of phthalocyanine-graphite systems were investigated as functions of phthalocyanine coverage and the value of the molecular dipole moment by using ultraviolet photoelectron spectroscopy. The dipoles in the film were found to be perpendicular to the graphite surface, and the coverage dependence of the WF of each dipole-phthalocyanine--graphite system clearly exhibited a sudden increase before the monolayer formation depending on the value of the dipole moment. The results show that the dipole-dipole repulsive interaction impacts the growth and structure of monolayer systems with weak intermolecular and molecule-substrate electronic coupling. The electric dipole moment and polarizability of each phthalocyanine in a monolayer was estimated by analyzing the observed coverage dependence of the WF using the Topping model (nanoscale method). Furthermore, it was demonstrated that the dielectric constant obtained with the nanoscale method and that estimated using the Helmholtz equation (macroscopic method) were almost the same. This agreement substantiates the application of the macroscopic model to an organic monolayer that is not a continuum medium.

The electronic structure at highly ordered pentacene monolayer prepared on Cu(110) substrate was studied by angle-resolved ultraviolet photoemission spectroscopy. The valence-level photoemission line shape showed the evidences of (i) formation of the interface states and (ii) two-dimensional energy-band dispersion of the resultant interface states. The lattice constant deduced from the observed energy-band dispersion is consistent with the reported one based on the low-energy electron diffraction experiments. Thus, the observed energy-band dispersion can be ascribed to the in-plane intermolecular energy-band dispersion in the pentacene monolayer on Cu(110). These phenomena may originate from the hybridization between the molecular orbital and the wave function of the substrate surface. Furthermore, work-function change of about $\ensuremath{-}0.9\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ by adsorption of pentacene was observed from the shift of the secondary-electron cutoff. Such a decrease of the work function indicates the formation of a dipole layer at the interface with the molecule positively charged. This direction is opposite to the naive expectation from the electron transfer from the substrate to the molecule, which was suggested from the previous work of core-level photoemission spectroscopy [McDonald et al., Surf. Sci. 600, 1909 (2006)]. This unexpected result may originate from the charge redistribution at the interface due to the induced image charge in the metal and the push back of electrons spilled out from the metal surface by the adsorbed molecules, which may overwhelm the effect of electron transfer.

Changes in the work function (WF) and binding energy of the highest occupied molecular orbital (HOMO) level in a titanyl phthalocyanine-(OTiPc) graphite system were studied as a function of OTiPc coverage by ultraviolet photoelectron spectroscopy. We observed that the WF increases with the coverage of the oriented OTiPc until the formation of the monolayer, while the energy position of the HOMO level stays almost unchanged with respect to the Fermi level. By analyzing the observed coverage dependence of the WF using the Topping model, we estimated the electric dipole moment and polarizability of OTiPc in a monolayer.

The effect of an interface dipole layer on the energy level alignment at organic-conductor interfaces is studied on a copper phthalocyanine (CuPc) monolayer/electric dipole layer/graphite system via ultraviolet photoemission spectroscopy (UPS) and metastable atom electron spectroscopy. An oriented monolayer of the OTi-phthalocyanine molecule, which has an electric dipole moment, is grown on graphite to yield a well-defined dipole layer with the vacuum side negatively charged. The CuPc monolayer is sequentially deposited on the dipole layer kept at $123\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. This weakly interacting system made of a very thin organic layer on top of a very thin dipole layer is in thermodynamic equilibrium. The UPS data from the system grown with and without the interface dipole layer show that the binding energy of the highest occupied state of the CuPc monolayer decreases when the dipole layer is inserted. The binding energy shift is in excellent agreement with the increase in vacuum level energy of the graphite substrate upon deposition of the dipole layer. The results show that the Fermi level of the CuPc shifts toward the valence states when the interface dipole layer is inserted.