Widely Dispersed Intermolecular Valence Bands of Epitaxially Grown

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Widely Dispersed Intermolecular Valence Bands of Epitaxially Grown Perfluoropentacene on Pentacene Single Crystals Yasuo Nakayama, Ryohei Tsuruta, Naoki Moriya, Masataka Hikasa, Matthias Meissner, Takuma Yamaguchi, Yuta Mizuno, Toshiyasu Suzuki, Tomoyuki Koganezawa, Takuya Hosokai, Takahiro Ueba, and Satoshi Kera J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b03866 • Publication Date (Web): 15 Feb 2019 Downloaded from http://pubs.acs.org on February 18, 2019

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The Journal of Physical Chemistry Letters

Widely

Dispersed

Intermolecular

Valence

Bands

of

Epitaxially

Grown

Perfluoropentacene on Pentacene Single Crystals

Yasuo Nakayama1*, Ryohei Tsuruta1, Naoki Moriya1, Masataka Hikasa1, Matthias Meissner2, Takuma Yamaguchi2,3, Yuta Mizuno4, Toshiyasu Suzuki2,3, Tomoyuki Koganezawa5, Takuya Hosokai6, Takahiro Ueba2,3, Satoshi Kera2,3,4

1

Department of Pure and Applied Chemistry, Tokyo University of Science, 2641

Yamazaki, Noda 278-8510, Japan 2

Institute for Molecular Science (IMS), National Institutes of Natural Sciences, 38

Nishigo-Naka, Myodaiji, Okazaki 444-8585, Japan 3 SOKENDAI,

38 Nishigo-Naka, Myodaiji, Okazaki 444-8585, Japan

4 Graduate School of Advanced Integration Science, Chiba University, 1-33 Yayoi-cho,

Inage-ku, Chiba 263-8522, Japan 5

Industrial Application Division, Japan Synchrotron Radiation Research Institute

(JASRI), SPring-8, 1-1-1 Kouto, Sayo, Hyogo 679-5198, Japan 6

National Metrology Institute of Japan, National Institute of Advanced Industrial

Science and Technology (AIST), 1-1-1 Umezono, Tsukuba 305-8568, Japan

* [email protected]

1 / 29 ACS Paragon Plus Environment

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Abstract Strong

intermolecular

electronic

coupling

and

well-ordered

molecular

arrangements enable efficient transport of both charge carriers and excitons in semiconducting π-conjugated molecular solids. Thus, molecular heteroepitaxy to form crystallized donor–acceptor molecular interfaces potentially leads to a novel strategy for creating efficient organic optoelectronic devices via the concomitance of these two requirements. In the present study, the crystallographic and electronic structures of a heteroepitaxial molecular interface, perfluoropentacene (PFP, C22F14) grown on pentacene single crystals (Pn-SCs, C22H14), were determined by means of grazing-incidence X-ray diffraction (GIXD) and angle-resolved ultraviolet photoelectron spectroscopy (ARUPS), respectively. GIXD revealed that PFP uniquely _ aligned its primary axis along the [110] axis of crystalline pentacene to form wellcrystallized overlayers. Valence band dispersion (at least 0.49 eV wide) was successfully resolved by ARUPS. This indicated a significant transfer integral between the frontier molecular orbitals of the nearest-neighbor PFP molecules.

TOC graphic


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The functionalities of semiconductor devices are generally based on electronic processes occurring at p–n junctions. In the case of organic (opto)electronics, where molecules with π-conjugation are used as the semiconductors, the heterointerfaces between two different molecular species of contrasting redox activities (i.e., donors and acceptors) play the important role of p–n junctions

1,2.

Therefore, sophisticated designs of molecular heterointerfaces

have advanced the evolution of organic electronic devices. The controlled intermixing of donors and acceptors has driven the development in organic photovoltaics (OPVs) forward 3–5 in virtue of the efficient intermolecular coupling and maximized optical response at the interfaces

6–9.

On the other hand, the

structural complexity tends to hinder the charge carrier transport due to inevitable violation of the long-range ordering and homogeneity. Indeed, the localization of charge carriers within individual molecules is a serious drawback when using such van der Waals molecular solids as semiconductors. The concomitance of efficient overlap between adjacent molecular orbitals and a well-ordered molecular arrangement on a crystal offers a potential solution for this problem by enabling the delocalization of charge carriers via the formation of widely dispersed electronic bands 10. In practice, this leads to a considerable increase in magnitude for charge carrier mobility

11,12

and long exciton diffusion lengths

about efficiency improvements in the case of OPVs 15–17.


13,14,

potentially bringing


Well-ordered molecular assemblies at the donor–acceptor interface can be produced by molecular heteroepitaxy 16–23. For instance, it has been revealed that ntype C60 and perfluoropentacene [PFP, C22F14; see Fig. S1(a)] molecules crystalize by aligning their molecular arrangements uniquely on a single-crystal surface of pentacene [C22H14; see Fig. S1(b)] 24,25. Since the pentacene single-crystal (Pn-SC) is a typical p-type organic semiconductor that exhibits considerable valence band dispersion

26,27,

such heteroepitaxial donor–acceptor interfaces potentially realize

the delocalization of both positive and negative charge carriers across the junction. The PFP/Pn-SC heteroepitaxial interface is the focus of the present study. After accurately determining its interface structures using atomic force microscopy (AFM) and grazing-incidence X-ray diffraction (GIXD), the electronic band structures of epitaxial PFP were elucidated by means of angle-resolved ultraviolet photoelectron spectroscopy (ARUPS). Plate-shaped pieces of single-crystal pentacene were prepared according to the literature and employed as substrates 24,27,28. The surface indices of the Pn-SC samples were determined by either GIXD patterns

27.

24

or low-energy electron diffraction

PFP was deposited on the Pn-SC surfaces under ultrahigh vacuum

conditions at an evaporation rate of 5–30 pm/s. All experiments, including the sample preparation and measurements, were performed at room temperature. The structures of the PFP/Pn-SC interfaces were studied by noncontact


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mode AFM and GIXD under ambient conditions. AFM measurements were conducted using an SPA-400 (SII Nanotechnology, Inc.). GIXD measurements were carried out at BL19B2, SPring-8 as follows 29. First, the lattice orientations of the PnSC substrates and structures of the PFP overlayers were roughly discerned on the basis of 2D-GIXD images taken by a 2D pixel detector (PILATUS 300K; Dectris). Then, the accurate crystallographic relationships at the interfaces were subsequently determined by diffraction spot profiles collected by a NaI scintillation counter through a soller slit. ARUPS measurements were performed using an electron spectrometer (A1; MB Scientific), and a monochromatized rare-gas discharge lamp, as reported previously

27.

In the present study, He-Iα (hν = 21.22 eV) was adopted as the

excitation light source. For the ARUPS experiments, the in-plane azimuthal orientations were decided according to the low-energy electron diffraction patterns of the Pn-SC substrate 27, and the photoemission measurements were conducted on PFP/Pn-SC samples without intentional exposure to the ambient conditions. For the sake of reducing the sample charging effects, the sample was illuminated by continuous-wave laser light (hν = 3.06 eV, 30 mW) during the ARUPS measurements. Figure 1(a) shows an AFM image of a Pn-SC sample covered with a 5 nm thick PFP film. The PFP overlayer was formed in flat-faced grains with individual



step heights of ca. 1.5 nm [Fig. 1(b)], implying growth in a layer-by-layer manner. The grains elongated predominately from the upper-left to the lower-right direction in this image. A similar anisotropic growth tendency for the PFP grains has also been reported elsewhere

30–32.

Increasing the PFP thickness to 20 nm led to a rougher

surface overall. This morphology closely resembles that of PFP films formed on pentacene thin films 32,33. Nevertheless, the regularity of the grains’ orientation was maintained, as seen in Fig. 1(c), whereas small needle-like grains of PFP grew in random directions on SiO2 [Fig. 1(d)]. In fact, the fast Fourier transformed (FFT) patterns of the AFM images of both the 5 nm and the 20 nm thick PFP films on PnSCs [Figs. S2(a) and S2(b)] exhibited strong intensities only in specific directions. This contrasts the isotropic FFT pattern found for PFP overlayers formed on a SiO2 substrate [Fig. S2(c)]. Thus, there was strong evidence that the PFP islands grow with defined directionality on a Pn-SC surface 34.


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FIG. 1: (a) AFM image of a 5 nm thick PFP overlayer on a Pn-SC substrate. (b) Profiles along the cross-hair lines in Fig. 1(a), where the intersection point is defined as the origin. (c) AFM image of 20 nm thick PFP on Pn-SC. (d) AFM image of 20 nm thick PFP deposited on a Si wafer.

Figure 2(a) shows a two-dimensional GIXD (2D-GIXD) image of the Pn-SC sample covered with 20 nm thick PFP taken at ϕ = 0°, defined as the azimuthal angle at which a spot attributed to the (1, 0, 0) diffraction of Pn-SC appears at qxy = 10.1 nm−1 and qz ~ 0 nm−1. In-plane rotation of the sample by 42° counterclockwise (CCW) changed the 2D-GIXD pattern as shown in Fig. 2(b). Three spots emerged at

qxy = 11.0 nm−1 in a vertical interval of Δqz ~ 4 nm−1 that could not be assigned to any diffraction order of Pn-SC; however, these spots can be predicted by assuming that they belong to the (100) surface of the reported crystal structure of PFP 7 / 29 ACS Paragon Plus Environment

35,


where the (0, 0, 2), (1, 0, 2), and (2, 0, 2) spots are expected to appear at (qxy, qz) = (10.98 nm−1, 0 nm−1), (10.87 nm−1, 4.05 nm−1), and (10.76 nm−1, 8.10 nm−1), respectively. This indicated that PFP forms a crystal phase identical to its bulk structure in the a*-axis orientation. Furthermore, these PFP-derived diffraction spots appeared only at specific sample azimuthal angles. For instance, the PFP (0, 0, _ 2) and (0, 0, 2) spots appeared briefly around ϕ = 42° and 222° [Fig. S3(a)], respectively, indicating that the a*-orientated PFP crystallites grew epitaxially along a specific in-plane axis of the Pn-SC surface. Precise GIXD measurements using a scintillation counter indicated that the spot intensity of the PFP (0, 0, 2) diffraction signal was maximized at ϕ = 41.8° (±0.5°) [Fig. S3(b)]. This condition can be fulfilled by an interface structure where the a*-orientated PFP crystal is stacked on _ the Pn-SC(001) surface by rotating the primary axis (b-axis) of the PFP by 53.1° CCW from the a-axis of the Pn-SC, as indicated in Figs. S3(d) and S3(e). The resulting _ interlattice relationship between the epitaxial PFP overlayer and the Pn-SC(001) surface is shown in Fig. 2(c). The b-axis of PFP, in which the molecules pack most _ densely [Fig. S1(a)], aligns to the [110] direction of the Pn-SC surface.


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FIG. 2: (color online) (a) 2D-GIXD image of 20 nm thick PFP on Pn-SC taken at an inplane sample orientation of ϕ = 0°. (b) 2D-GIXD image of the same sample as in (a) taken at ϕ = 42°. (c) Schematic drawing of the interlattice relationship _ between the epitaxial PFP overlayers and the (001) surface of the Pn-SC, where inequivalent molecules of PFP and Pn in each unit cell are represented as filled or open circles and dark- and light-colored bars, respectively.

The degree of crystallinity can be estimated quantitatively from a sharpness



Page 10 of 34

of diffraction spots. In the present case, since the full-widths at half maxima (FWHMs) of the PFP (0, 0, ±2) and (0, ±1, ±2) diffraction spots of 0.08° - 0.09° (Supporting Information Fig. S4) were about ten-times broader than the instrumental resolution limit (0.009°

25),

the crystallographic coherent length

(mean crystallite size) of the PFP crystallites in the in-plane directions was evaluated to be 0.07 μm under an assumption of the Scherrer equation. This size is apparently smaller than the grain size observed in the AFM images, suggesting that, even though the crystal lattice was uniformly oriented, those PFP grains were not single crystalline but several structural discontinuities (e.g., anti-phase domain boundaries) were included in the grains. This is contrasting to the cases of PFP on Si3N4 33 and C60 on the Pn-SC surfaces 29, while the similar tendency was reported for the pentacene thin films formed on SiO2 substrates

36.

It should also be

mentioned that, as seen in Figs. 2(a) and 2(b), a weak-intensity 2D-GIXD signal was persistently detected at qz ~ 17.8 nm−1, which corresponds to (h, ±1, ±2) diffraction of PFP, independent of the ϕ angle. This implies that the epitaxial alignment of PFP on the Pn-SC surface was not perfect and that randomly oriented crystallites coexisted. Nevertheless, the PFP molecules predominantly pointed their principal axes to the surface normal direction on the Pn-SC surface regardless the existence of disordering for in-plane lattice orientations. A UPS spectrum of a Pn-SC sample covered with a 4 nm thick PFP overlayer


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is shown in Fig. 3(a). This spectrum was obtained from the integration of photoelectrons emitted in a range of ±18° from the surface normal. Thus, it roughly replicates the density of states averaged over the Brillouin zone. The major structure appearing in the ionization energy (IE) region near 6.5–7.5 eV corresponds to the highest occupied molecular orbital (HOMO) of PFP. The weak signal at IE = 5-6 eV is attributed to photoemission from the Pn-SC substrate beneath the PFP layers, and thus the spectrum of PFP/Pn-SC is regarded as a superposition of the photoemission from the PFP overlayer and the Pn-SC. Actually, the form of this weak-intensity structure was similar to that of the UPS spectra of the Pn-SC before PFP deposition, as shown in Supporting Information Fig. S5(a). This certified that the photoemission contribution of the Pn-SC (shaded area in Fig. 3(a)), overlapping to that of the PFP HOMO, was relatively minor and featureless.



FIG. 3: (color online) (a) UPS spectrum of 4 nm thick epitaxial PFP overlayers on a Pn-SC substrate. The spectrum was obtained by integrating the electron emission angle (θ) of the ARUPS spectra over ±18° from the sample normal. A UPS spectrum of the Pn-SC substrate, which is normalized with the HOMO region intensities of Pn, is presented as a shaded curve. The abscissae (top scale) of these spectra are aligned with respect to each vacuum level position. The threshold IE of the PFP overlayer is indicated as a downward wedge mark. (b) ARUPS spectra of the PFP/Pn-SC sample taken in the Γ-Y direction of PFP. The spectra are displayed with respect to the Fermi level (bottom axis), where the top and the bottom axes are aligned, taking the work function (4.50 eV) of the PFP/Pn-SC sample into consideration. Inset: SBZ of the PFP(100) surface. 12 / 29 ACS Paragon Plus Environment

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It was also revealed that the coverage of PFP over the Pn-SC surface lifted the vacuum level upward away from the Fermi level as represented in an energy shift of the secondary electron cutoff (SECO) positions (Supporting Information Fig. S5(b)). This suggests the formation of an electric double layer at the PFP/Pn-SC interface, where the PFP side charged negatively with respect to the Pn-SC side, and/or enhanced photovoltaic effects, as the UPS measurements were conducted under the laser light illumination as mentioned above, by the presence of donoracceptor heterojunctions in comparison to the donor-only cases 37. The threshold IE of the Pn-SC HOMO increased to 5.0 eV by the deposition of PFP, as shown in Fig. S5(a), presumably due to the presence of the upward vacuum level shift across the PFP/Pn-SC interface. On the other hand, the spectral contribution of PFP emerged at IE = 6.4 eV (±0.1 eV), which corresponds to the threshold IE of the epitaxial PFP crystallites. This value agrees fairly well with previous reports on polycrystalline PFP films of upright standing molecular orientation

38,39.

From these values, an

energy offset of the hole transporting levels at this well-defined head-on PFP/Pn-SC interface was directly derived to be 1.4 (±0.1) eV. Figure 3(b) shows the ARUPS spectra taken in an azimuthal angle of the sample set at 48° clockwise from the Γ-X direction of the Pn-SC (001) surface. This approximately corresponds to the Γ-Y direction of the epitaxial PFP crystallites (Fig.



Page 14 of 34

S6). The spectral profiles in the binding energy from 2.2 to 3.2 eV and from 3.5 to 4.5 eV varied depending on the photoelectron emission angle (θ). While an energy shift of the PFP molecular orbitals by 0.6 eV due to a variation in the directions of the polarized intra-molecular C-F bonds was previously reported

40,

such

electrostatic effects can be excluded from possible origins of the present θdependent energy shifts because the PFP molecules were uniformly oriented approximately along the surface normal direction. Also, since the principal axis of the PFP molecules was always perpendicular to the θ-scan directions, an intramolecular dispersion of the π-orbital 41,42 is ruled out in the present case. Instead, the θ-dependence of the spectral profiles indicated that the electrons in these states changed their energy as a function of the surface parallel component of the momentum (i.e., inter-molecular electronic bands with considerable energymomentum dispersion were formed). As shown in Supporting Information Fig. S7, the spectral profiles also exhibited a clear variation by changing θ along the diagonal (Γ-C) direction in the surface Brillouin zone (SBZ), while no significant θdependence was found in the perpendicular (Γ-Z) direction to the Γ-Y. The photoelectron emission angle θ can be converted into the electron wavenumber in the surface parallel directions K|| as a function of photoelectron kinetic energy Ek as ħK|| = (2m0Ek)1/2 sin θ, where ħ is the reduced Planck constant and m0 is the electron rest mass. Based on this relation, the ARUPS data taken in


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three symmetry directions in the SBZ of PFP were replotted in E-K|| planes, as shown in Fig. 4. In the Γ-Y direction, both the first and the second valence bands derived from the HOMO and HOMO−1 of PFP, respectively, dispersed upward in the SBZ, moving from the center (Γ-point) to the boundary (Y-point). This behavior is exhibited more clearly in the second derivative intensity mapped in the E-K|| plane as shown in Supporting Information Fig. S8. Whereas the similar behavior was also observed in the Γ-C direction, the presence of E-K|| dispersion was not clear in the Γ-Z direction. These trends are consistent with the band calculation results reported by Yoshida and coworkers

43.

Even though the crystallographic orientation of the

present PFP overlayers was not perfectly uniform, as discussed above, the electronic band structures of crystalline PFP were successfully resolved by ARUPS.



FIG. 4: (color online) ARUPS spectral images of the PFP/Pn-SC sample taken in the Γ-Y (left), Γ-C (middle), and Γ-Z (right) directions plotted on the E-K|| planes. The theoretical band structures predicted by Yoshida et al.43 are plotted as circles.

The E-K|| dispersion relations of the epitaxial PFP were derived through curve fitting of the individual UPS spectra corresponding to the respective θ (the detailed procedures are explained in Fig. S9). For each spectrum, the peak derived from the PFP HOMO can be separated into two components, and the peak positions are plotted as a function of the corresponding K|| in Fig. 5, where the size of the symbol represents the intensity of each component. Judging from a narrow energy split between the HOMO-derived twin bands, as predicted by the band calculation, these two spectral components are unlikely to result from two inequivalent PFP 16 / 29 ACS Paragon Plus Environment

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molecules in the unit cell. The persistent existence of these two components at any

K|| is presumably ascribed to scattered photoelectrons losing information of their initial momentum, which were also observed in previous ARUPS works on organic molecular single crystals 10,26,44–46, and/or disordering of the lattice orientation of a part of PFP crystallites. Either of these two components was assigned as the main feature for each spectrum, as indicated by the outlined symbols in Fig. 5. Even though the E-K|| dispersion relations of the main feature exhibited gaps in the center of the BZ presumably due to the hole-phonon coupling 46, the dispersion relations could be reproduced fairly well by a simple 2D tight-binding (TB) model only considering inequivalent nearest-neighbor molecules [inset of Fig. 5(c)]

44.

In the

2D-TB model, three inequivalent intermolecular transfer integrals—tb, tc, and |td| (=|td’| by symmetry)—and the energy difference |ΔE| between two inequivalent PFP molecules are derived as fitting parameters (Table 1). From this fitting result, the total width (W) of the valence band dispersion and the hole effective mass (mh*) at the Y-point in the Γ-Y direction were determined to be 0.49 eV and 0.98 m0, respectively. The former value was equivalent to or even greater than the value of

W of rubrene or pentacene reported thus far 10,26,44.



FIG. 5: (color online) Energy positions of the spectral components (upward and downward triangles) of the ARUPS spectra in the (a) Γ-Y, (b) Γ-C, and (c) ΓZ directions plotted as a function of K||. 2D-TB fitting results are indicated as thick curves. The inset shows a schematic drawing of the molecular arrangement of the PFP surface unit cell indicating transfer integrals for all four combinations of adjacent molecules.


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Table 1: Coupling parameters (tb, tc, |td|, and |ΔE|) of adjacent PFP molecules derived from 2D-TB fitting of the present experimental band structures and previous band calculation results 43.

tb

tc

|td| (= |td’|)

|ΔE|

experimental (this study)

−0.093 (±0.013)

+0.004 (±0.004)

0.036 (±0.018)

0.06 (±0.06)

band calc. (Yoshida et al.43)

−0.116 (±0.004)

+0.001 (±0.004)

0.019 (±0.007)

0.02 (±0.02)

units: eV

While conduction band structures are of the most interest from a practical point of view for PFP as an n-type semiconducting molecule, ARUPS typically provides information on the electron energy and momentum of the occupied states. Nevertheless, the present results still provide useful insights into the electron conduction properties of PFP when also taking the band calculation results 43 into consideration. The calculated valence band structures in the Γ-Y, Γ-C, and Γ-Z directions could be approximated by the 2D-TB model, yielding the intermolecular transfer integral values listed in Table 1. This indicates that, for the valence bands, the predicted intermolecular coupling strength is likely accurate using the present



experimental results. For the conduction bands, the same band calculation predicted

W to be 0.308 eV, which corresponds to a tb value of approximately +0.07 eV. Hence, the present results suggest that the strong intermolecular coupling in the epitaxial PFP crystals leads to an equivalent transfer integral value in the lowest unoccupied molecular orbital to that of the HOMO of Pn-SC

26.

Further improvements in the

crystallinity of the interface by the optimization of the growth conditions (e.g., growth temperature 47) may enable efficient band-like transport on both sides of the donor–acceptor molecular interface. In summary, the growth manner, crystallographic structures, and valence band dispersion of PFP epitaxially grown on Pn-SC substrates were demonstrated by means of AFM, GIXD, and ARUPS, respectively. Well-ordered overlayers of PFP were grown in one specific direction in a layer-by-layer manner, as revealed by AFM. An interlattice relationship between the PFP crystallites and Pn-SC, where the _ nearest-neighbor direction of the PFP molecules was arrayed along the [110] axis of the Pn-SC surface, was uniquely determined on the basis of the GIXD results. ARUPS measurements on the epitaxial PFP clearly demonstrated the valence bands with wide energy dispersion (W = 0.49 eV). This proved the strong intermolecular coupling between adjacent PFP molecules, for example, the transfer integral of −0.093 eV (±0.013 eV) for the HOMO state, ensuring the presumable occurrence of efficient band-like transport in the epitaxial PFP crystallites.


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Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Structures of PFP and pentacene; Regularities of the domain orientations of PFP; Azimuthal analyses of the PFP/Pn-SC heterointerface; Estimation of the in-plane coherent length (τ) of the PFP crystallites; Azimuthal relationship between the real and reciprocal lattices of the PFP/Pn-SC heterointerface; Shift in the vacuum level and Pn HOMO level at the PFP/Pn-SC interface; ARUPS spectra of PFP/Pn-SC to the Γ-C and Γ-Z directions; Second derivative of the ARUPS image in the Γ-Y direction; Procedures for peak separation of the ARUPS spectra

Acknowledgements The authors would like to thank Prof. Hisao Ishii for supporting the AFM experiments. Parts of this work were done as Joint Research of IMS [29-604 and 30205] and under approval of JASRI [2016A1676]. This work was financially supported by JSPS-KAKENHI Grant Nos. JP15H05498, JP16K14102, and JP18H03904, The Futaba Foundation, The Precise Measurement Technology Promotion Foundation, Iketani Science and Technology Foundation, and Murata Science Foundation.



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Nakayama, et al. Figure 1 78x88mm (300 x 300 DPI)

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Widely Dispersed Intermolecular Valence Bands of Epitaxially Grown

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