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Upper panel: UPS spectra of ClAlPc(0.8 ML)/HOPG at 300 K (a), Pn(0.8 ML)/ClAlPc(0.8 ML)/HOPG at 300 K (b1), 110 K (b2), and 60 K (b3), and Pn(3 ML)/ClAlPc(0.8 ML)/HOPG at 297 K (c) with the HOPG spectrum. Lower panel: The spectra near on a log intensity scale. FDD curve at each temperature convoluted with the instrumental resolution is also shown. The intensity of ClAlPc(0.8 ML)/HOPG substrate (◻) is normalized to that of Pn(0.8 ML)/ClAlPc(0.8 ML)/HOPG (○) near , where the substrate spectra were measured after evaporation of Pn by appropriate heating. The expected molecular packing structure is illustrated on the upper panel (right).
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UPS difference spectra between Pn(0.8 ML)/ClAlPc(0.8 ML)/HOPG and ClAlPc(0.8 ML)/HOPG at 300, 110, and 60 K. The substrate spectra were subtracted for the 0.8-ML Pn spectra (see also Fig. 1 caption). The UPS spectrum of Pn(3 ML)/ClAlPc(0.8 ML)/HOPG at 297 K is also plotted for comparison. In main panel, solid curves: Convoluted curves with Gaussian functions. Dotted curves: The Gaussian function located at the lowest in the convolution. Dashed lines: Guide for an exponential-type distribution of the gap states. The arrows correspond to boundaries of the distributions of the gap states. Inset shows fitting results of the difference spectrum with an exponential function and an arbitrary constant background (Dashed lines) for 0.8-ML Pn at 60 K and 3-ML Pn. The DOS values of the exponential tail function were evaluated, where the UPS intensities were normalized to the most intense point of the Pn HOMO band and the molecular packing density of the thin-film phase of Pn is assumed. The values are estimated to be , , and (at 300, 110, and 60 K), , , (at 300, 110, and 60 K), and , 0.32, 0.28 (at 300, 110, and 60 K) for the 0.8-ML Pn, respectively, and , , and for the 3-ML Pn.
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The energy diagram of the Pn/ClAlPc/HOPG interfaces. (a) Energy levels of ClAlPc(0.8 ML)/HOPG and Pn before contact, where the oriented 0.8 ML ClAlPc forms the dipole layer to give a high work function substrate due to an increase of the work function . (b) Observed energy levels of Pn(0.8 ML) and Pn(3 ML) overlayers on ClAlPc(0.8 ML)/HOPG at thermal equilibrium after contact, where electrons in the Pn are transferred to the HOPG to locate the near the HOMO. The transferred electrons come from gap states in the Pn. Energies are given in eV.
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We detected a very low density of electronic states in the band gap of a nondoped pentacene thin film by using ultraviolet photoelectron spectroscopy with ultrahigh sensitivity and ultralow background. The gap states, which may originate from the highest occupied molecular orbital (HOMO) state in imperfect molecular packing regions, are distributed exponentially up to the Fermi level and control the Fermi level relative to the HOMO band.
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