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Ge atoms segregating on zirconium diboride thin films grown on Ge(111) were found to crystallize into a two-dimensional bitriangular structure, which was recently predicted to be a flat band material through an embedded kagome lattice. Angle-resolved photoelectron emission experiments together with theoretical calculations pointed out the existence of a nearly flat band in spite of non-negligible in-plane long-range hopping and interactions with the substrate. This provides an experimental verification for the fact that a flat band can emerge from the electronic coupling between atoms and not from the geometry of the atomic structure.

The correlation between the $p$-type hole conduction and the electronic structures of Cr-deficient CuCr${}_{1\ensuremath{-}x}$O${}_{2}$ ($x=0\ensuremath{-}0.1$) compounds was investigated using O $K$-, Cu, and Cr ${L}_{3,2}$-edge x-ray absorption near-edge structure (XANES), scanning photoelectron microscopy, and x-ray emission spectroscopy measurements. XANES spectra reveal a gradual increase in the Cu valence from Cu${}^{1+}$ to Cu${}^{2+}$ with increasing Cr deficiency $x$, whereas, the valence of Cr remains constant as Cr${}^{3+}$. These results indicate that the $p$-type conductivity in the CuCr${}_{1\ensuremath{-}x}$O${}_{2}$ samples is enhanced by a Cu${}^{1+}$-O-Cu${}^{2+}$ rather than a Cr${}^{3+}$-Cr${}^{4+}$ or direct Cu${}^{1+}$-Cu${}^{2+}$ hole mechanism. Remarkable Cr-deficiency-induced changes in the densities of Cu 3$d$, Cu 3$d$-O 2$p$, and O 2$p$ states at or near the valence-band maximum or the Fermi level were also observed. In addition, a crossover of conduction mechanism from thermally activated (TA) hopping to a combination of TA and Mott's three-dimensional variable range hopping occurs around 250 K.

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.

We demonstrate spectroscopic evidence of $2t$ level splitting of the highest occupied molecular orbital (HOMO) state, which is characterized by twice the intermolecular transfer integral $t$, with ultraviolet photoelectron spectroscopy in a well-ordered bilayer of lead-phthalocyanine deposited on graphite. The HOMO state splits clearly by $2t=0.35\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ upon bilayer formation at $295\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and the splitting increases to $0.41\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ at $47\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The splitting is ascribed to dimer nanostructures with antiparallel molecular electric dipoles.

The electronic structure of pentacene/graphite, pentacene/Cu-phthalocyanine (CuPc)/graphite, and $\text{pentacene}∕\mathrm{Si}{\mathrm{O}}_{2}∕\mathrm{Si}(100)$ was studied by ultraviolet photoelectron spectroscopy as a function of the thickness of the pentacene film. We observed that the binding energy position of the highest occupied molecular orbital (HOMO) of pentacene becomes significantly lower on CuPc and $\mathrm{Si}{\mathrm{O}}_{2}$ surfaces than that on graphite. Furthermore, a splitting of the UPS band was observed only for the HOMO of thicker crystalline films on CuPc and $\mathrm{Si}{\mathrm{O}}_{2}$ surfaces, where the molecules are oriented with their long axis nearly perpendicular to the substrate surface. The splitting is ascribed to the intermolecular band dispersion, and the decrease in the threshold ionization energy on CuPc and $\mathrm{Si}{\mathrm{O}}_{2}$ surfaces originates from the HOMO-band dispersion as well as the increase in the relaxation/polarization energies, which may be caused by the better molecular packing structure with a nearly standing molecular orientation.

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 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.

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.

The electronic structure of a well-oriented perylene-3,4,9,10-tetracarboxylic acid-dianhydride multilayer prepared on ${\mathrm{MoS}}_{2}$ single crystal surface were studied by angle-resolved ultraviolet photoemission spectroscopy using synchrotron radiation. From the photon energy dependence of normal emission spectra, we observed an intermolecular energy-band dispersion of about 0.2 eV for the highest occupied molecular orbital (HOMO) band of single $\ensuremath{\pi}$ character. The observed energy-band dispersion showed a cosine curve, which originates from the intermolecular $\ensuremath{\pi}\ensuremath{-}\ensuremath{\pi}$ interaction. Analyses using the tight-binding model gave that the transfer integral of about 0.05 eV for the $\ensuremath{\pi}\ensuremath{-}\ensuremath{\pi}$ interaction, the effective mass of HOMO hole ${m}_{h}^{*}{=5.28m}_{0},$ and the hole mobility ${\ensuremath{\mu}}_{h}g3.8{\mathrm{cm}}^{2}/\mathrm{V}\mathrm{}\mathrm{s}.$ This is the first observation of the intermolecular energy-band dispersion of a conventional single-component organic semiconductor only with the weak intermolecular van der Waals interaction.