Photoelectron Angular Distribution Induced by Weak Intermolecular

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Photoelectron Angular Distribution Induced by Weak Intermolecular Interaction in Highly-Ordered Aromatic Molecules Hiroyuki Yamane, and Nobuhiro Kosugi J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b08346 • Publication Date (Web): 30 Oct 2018 Downloaded from http://pubs.acs.org on November 3, 2018

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The Journal of Physical Chemistry

Photoelectron Angular Distribution Induced by Weak Intermolecular Interaction in Highly-Ordered Aromatic Molecules Hiroyuki Yamane*,†,‡,§ and Nobuhiro Kosugi†,‡,// †Institute

for Molecular Science, Myodaiji, Okazaki 444-8522, Japan.

‡SOKENDAI

(The Graduate University for Advanced Studies), Myodaiji, Okazaki 444-8522,

Japan. Corresponding Author * [email protected] ORCID Hiroyuki Yamane: 0000-0002-8023-7918 Present Addresses §RIKEN

SPring-8 Center, Sayo, Hyogo 679-5148, Japan.

//Institute

of Materials Structure Science, KEK, Tsukuba 305-0801, Japan.

ACS Paragon Plus Environment

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Author Contributions The manuscript was written through contributions of all authors.

ABSTRACT

The photoelectron angular distribution of organic thin films is essential in 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 singlescattering 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 dispersion. We also discuss the effect of these weak interactions on the orbital tomography.


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INTRODUCTION Organic semiconductors have been regarded as a promising nanomaterial for the nextgeneration (opto)electronics that can solve energy and environmental problems. Intermolecular interaction, such as electrostatic, exchange repulsion, polarization, and charge transfer or delocalization effects, is a fundamental and key parameter for (opto)electronic properties of organic solids and interfaces related to organic electronics, and is controllable by the design of molecules and its assembly.1 The intermolecular polarization energy for holes in organic solids (P+) was investigated, by comparing the ionization energies between gaseous molecules and its solids using photoemission spectroscopy, and was obtained as P+ = 1–3 eV depending on the molecule.2,3 At organic/metal interfaces, it is now well-understood that the formation of the electric dipole layer at the interface due to various interactions plays a crucial role in the energy-level alignment at the interface and in the resultant device performance.1,4-7 Some factors of the electric dipole layer have been demonstrated for the intentional control of the energy-level alignment at organic/metal interfaces.6,7 The recent progress in the electron energy analyzer, the photon source, and the sample preparation enables us to apply angleresolved photoemission spectroscopy (ARPES) to well-defined organic solids and interfaces.8 The ARPES experiments as a function of the photoemission angle (θ) or the photon energy (hv) give the wavevector (k)-resolved electronic structure of materials. The energy-vs.wavevector (E-k) relation has revealed the electronic band dispersion for ideal π-π stacking organic solids8-13 and for highly-ordered organic monolayers chemisorbed on metals,14-19


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originating from the van der Waals interaction and from the metal-mediated orbital hybridization, respectively. On the other hand, since the photoelectron intensity angular distribution (PIAD) reflects the Fourier-transformed spatial distribution of molecular orbitals, the intensity-vs.-wavevector (I-k) mapping by ARPES has recently been attracted much attention as the real-space visualization of molecular electronic states, so-called orbital tomography.16-18,20-32 This kind of advanced ARPES analysis has enabled precise physicochemical investigation of the electronic coupling in organic solids and interfaces; however, the theoretical treatment for ARPES on organic solids and interfaces is still under debate because of the presence of various multi-electronic interactions and electron scatterings in the photoelectron emission process. In the present work, we shed light on archetypal organic semiconductors of polycyclic aromatic hydrocarbon (PAH) molecules. It is known that the PAH molecules, perylene, coronene, and hexa-peri-hexabenzocoronene (HBC), assemble to form superstructures on Au(111).33-36 Systematic ARPES measurements for highly-ordered PAH monolayers physisorbed on Au(111) with different ordered parameters can be a pertinent approach to reveal electronic phenomena derived from the complex interplay of the weak intermolecular and interfacial interactions. For this purpose, we have prepared well-known and new PAH/Au(111) superstructures as confirmed by low-energy electron diffraction (LEED). The ARPES spectra for various PAH/Au(111) superstructures have shown a strong azimuthal anisotropy in the I-k relation. We have found that a simplified approximation based on the


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photoelectron scattering by each atomic hole potential is insufficient to obtain the reliable PIAD map of organic thin films. The intramolecular scattering at least must be considered for the better description of the PIAD map of organic thin films. Furthermore, we have succeeded in observation of the quite weak but non-negligible π-band dispersion in the flat-lying PAH/Au(111) superstructures. Although no direct intermolecular π-π coupling exists at the PAH/Au(111) superstructures, the energy width and k periodicity in the π-band dispersion are dependent on the ordered structure of PAH molecules. The observed π-band dispersion can be ascribed not to the metal-mediated electronic coupling but to the long-range lateral intermolecular interaction, which also affects the PIAD map of molecular orbitals (MOs). EXPERIMENTAL The present experiment was performed at a highly-brilliant in-vacuum undulator beamline BL6U of the UVSOR-III Synchrotron (Okazaki, Japan). The BL6U covers the hv range between 40 eV and 800 eV. The ARPES endstation consists of the measurement, preparation, and loadlock chambers. The ARPES spectra were acquired from the energy-vs.-angle image by using a micro-channel-plate (MCP) detector of a hemispherical electron energy analyzer (MB Scientific A-1). All spectra in the present work were measured at 20 K by using the liquid Heflow cryostat in the six-axes [x, y, z, θ, and azimuth (ϕ)] manipulator. A clean Au(111) single crystal substrate (5N purity) was obtained by repeated cycles of the Ar+ sputtering and the subsequent annealing at 700 K. The cleanliness of the Au(111) surface was


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confirmed by core-level and valence-band ARPES and MCP-LEED (OCI BDL800IR-MCP). Highly-ordered monolayers of PAHs (perylene, coronene, and HBC) were prepared by the vacuum deposition of highly purified powders onto the clean Au(111) substrate. The deposition rate (< 1 Å/min) was measured using a quartz crystal microbalance. The lateral ordered structure of the PAH monolayers was characterized by MCP-LEED. Simulation of the PIAD pattern was carried out using the independent-atomic-center approximation combined with the MO calculation (IAC/MO). In the IAC approximation, a molecule is regarded as a set of independent atomic emitters, and the photoelectron scattering by each atomic hole potential is considered. In the present work, we used the IAC/MO and the single-scattering approximation (SS/MO), where the interference between the IAC waves and the single-scattering waves from the atoms surrounding the IAC atoms is considered in the calculation of the final states. We employed the wave functions calculated by the B3LYP/STO-6G basis set for the initial state in the IAC/MO and SS/MO calculations. The twodimensional PIAD map was obtained as a function of θ and ϕ by considering the p-polarized light with hv = 45 eV. Details of the IAC/MO and SS/MO approximations are described elsewhere.37,38 RESULTS AND DISCUSSION


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Figure 1. LCAO pattern of the valence MOs for isolated molecules and the corresponding PIAD map by IAC/MO and SS/MO; (a) au orbital of perylene, (b) e2u orbital of coronene, and (c) e1g and (d) a1u orbitals of HBC. The center position and the outermost circumference in the PIAD map correspond to θ = 0° and 90°, respectively. In order to account for the film symmetry on Au(111), six-fold rotational and mirror symmetries were applied in the far-right PIAD map, which is shown with MDCs at ϕ = 0° and 90°.

Prior to discussion of the electronic structure of the PAH monolayers on Au(111), we discuss the simulated PIAD maps of perylene, coronene, and HBC. Figure 1 shows the linear combination of atomic orbital (LCAO) patterns for the isolated molecule and the corresponding PIAD map by IAC/MO and SS/MO at hv = 45 eV, calculated for the valence MOs of (a) perylene, (b) coronene, and (c, d) HBC with the flat-lying configuration. The center position and the outermost circumference in the PIAD map correspond to the normal emission (θ = 0°) and θ = 90°, respectively. The LCAO pattern for the highest-occupied MO (HOMO) of


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perylene (au), coronene (e2u), and HBC (e1g) [Figure 1(a)–(c)] is distributed throughout the molecular frame, while the HOMO-1 of HBC (a1u) [Figure 1(d)] is localized at the outer benzene rings in the molecule. The difference in the in-plane rotational symmetry in MOs is reflected in the PIAD map. For both the IAC/MO and SS/MO, the PIAD maps for the HOMO of the isolated perylene, coronene, and HBC molecules have a two-fold symmetry; on the other hand, the PIAD map for the HOMO-1 of the isolated HBC molecule has a six-fold symmetry. The obvious difference between IAC/MO and SS/MO is found in the intensity at the outer (high-θ) region in the PIAD map, where the stronger photoemission signal is clearly observable in the SS/MO. The intramolecular SS/MO approximation is at least required to reproduce the experimental PIAD pattern since the ARPES signal at the high-θ region is nonnegligible as discussed below. Since the PAH molecules of perylene, coronene, and HBC assemble to form single-domain hexagonal superstructures on Au(111),33-36 the six-fold rotational and mirror symmetries must be applied to the simulated PIAD map, which is displayed in the far-right PIAD map together with the momentum distribution curve (MDC) in Figure 1. The PIAD maps for the HOMO of perylene, coronene, and HBC on the (111) surface show the six-equivalent prominent PIAD peaks at θ = 29°, which corresponds to the lateral wavevector (k||) of 1.54 Å−1 in MDC. The prominent PIAD peak at θ = 29° (k|| = 1.54 Å−1) shows a clear anisotropy depending on ϕ, where the maximum and minimum intensities appear at ϕ = 60n° and (60n+30)° (n = integer), respectively. On the other hand, the PIAD map for the HOMO-1 of HBC is different from the


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PIAD map for the HOMO of perylene, coronene, and HBC, and does not show a clear anisotropy between ϕ = 60n° and (60n+30)° but the twelve-equivalent prominent PIAD peaks at ϕ = (60n±15)°. These characteristics in the simulated PIAD map based on SS/MO are observable in the experimental ARPES spectra as shown below.

Figure 2. Electronic structure of coronene/Au(111) at 20 K along the Γ-K-M direction. (a) θ-dependent ARPES spectra at hν = 45 eV, shown with 1° step; (Upper panel) Coronene/Au(111) and the calculated DOS. (Lower panel) Clean Au(111) surface. (b) E-

k|| maps of coronene/Au(111) and HOPG obtained from the ARPES spectra.

Figure 2(a) shows the θ-dependent ARPES spectra (hv = 45 eV) for Au(111) and coronene/Au(111) at 20 K along the Γ-K-M high-symmetry direction corresponding to ϕ = 0° in Figure 1. The clean Au(111) surface shows the well-known electronic band dispersion for the Shockley state (labeled S) around θ = 0°, sp band around θ = 20°, and 5d band in the full-θ


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region. Upon adsorption of coronene, the Shockley state is weakened and is shifted to the lower binding-energy (Eb) side. The modification of the Shockley state with the small Eb shift can be ascribed to the exchange repulsion of surface electron systems in the weak physisorption of adsorbates,39 as discussed below. Furthermore, the weak physisorption at coronene/Au(111) is also suggested by the core-level photoemission experiment in the Supporting Information. At the physisorbed coronene/Au(111) interface, the HOMO-derived peak at Eb = 1.58 eV shows the first and second intensity maxima around θ = 30° and 45°, respectively. Moreover, the HOMO-derived peak exhibits the high-Eb satellite structure originating from the electron-phonon coupling,40 suggesting again the weak physisorption of coronene on Au(111). The deeper lying π-derived molecular electronic states (labeled MO1, MO2, MO3, and MO4) show the maximum intensity at the different θ, as indicated by dashed squares. The relative Eb position of the observed molecular electronic states corresponds to that of the calculated and experimental density-of-states (DOS) for the isolated coronene molecule.41 The E-k|| map of the electronic structure at the coronene/Au(111) interface is shown in Figure 2(b) as obtained using the relation of

𝒌∥ =

2𝑚0(ℎ𝜈 ― 𝐸b ― 𝛷)sin 𝜃/ℏ,

where m0 and Φ are the free-electron mass and the work function, respectively. The modified Shockley state (labeled S’) exhibits the well-known free-electron-like dispersion around k|| =


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0 Å−1. The maximum intensity for the π-derived MOs of coronene (HOMO and MO1–MO4) appears parabolically in the E-k|| map, where the π-derived MO with the higher Eb gets intense at the lower k|| position, i.e., HOMO at k|| ≈ 1.6 Å−1, MO1 at 1.2 Å−1, MO2 at 0.9 Å−1, MO3 at 0.6 Å−1, and MO4 at 0 Å−1. As shown in Figure 2(b), the E-k|| relation of the π-derived MOs of the coronene/Au(111) traces the π-band dispersion of the highly oriented pyrolytic graphite (HOPG). The observed correspondence is explained as the intramolecular band dispersion, which can be regarded as the pseudo π band of the graphene quantum dots.25 Furthermore, the HOMO-derived peak exhibits the first and second intensity maxima at k|| ≈ 1.6 Å−1 and 2.4 Å−1, respectively, which agrees well with the simulated PIAD map based on SS/MO. The observed agreement between the experimental and simulated I-k|| relations indicates the importance of the SS/MO approximation for the reliable orbital tomography.


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Figure 3. Lateral ordered structure determined from LEED and PIAD (left top), corresponding SBZ and LEED (left bottom), and E-k|| map (right), measured for the superstructures

of

(a)

coronene(4×4)/Au(111),

(b)

perylene(4×4)/Au(111),

(c)

HBC(5×5)/Au(111), and (d) HBC(3√3×3√3)R30/Au(111) at 20 K. The red dot in the LEED image corresponds to the diffraction spot of the substrate. The SBZs of the substrate and the monolayer, determined from LEED, are shown by thick-black and thin-blue hexagons, respectively. The red line in SBZ indicates the scanned region in the E-k|| map. The EDC curves at the high-symmetry points in the SBZ of the monolayer are shown in the right-hand side of the E-k|| map (Γm: red, Km or Mm: black).

In addition to the I-k|| relation, we discuss the E-k|| relation of the HOMO-derived peak, which shows a quite weak dispersion as shown in Figure 3(a) in more detail. The lateral electronic band dispersion for flat-lying organic monolayers has been observed for strongly chemisorbed molecules on metals; pentacene on Cu,14,24 cobalt- and iron-phthalocyanine (CoPc and FePc) on Au,15 perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) and naphthalene-1,4,5,8tetracarboxylic dianhydride (NTCDA) on Ag and/or Cu,16-18 and 2,3,5,6-tetrafluoro-7,7,8,8tetracyanoquinodimethane (F4TCNQ) on Au19 with the 0.2–1.0 eV dispersion width, which is much larger than the present dispersion with a few tenth meV scale. Lateral band dispersions in chemisorbed organic monolayers have been understood to originate from the metalmediated electronic coupling due to the hybridization between MOs and metal wavefunctions. On the other hand, as judged from the present spectroscopic evidences, the interaction at the


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coronene/Au(111) interface is dominated by the weak physisorption, and the strong orbital hybridization at the interface cannot be expected. To clarify the origin of the E-k|| dispersion for the HOMO of the coronene monolayer on Au(111), we performed ARPES experiments on various PAH monolayers physisorbed on Au(111) from the viewpoints of the intermolecular space and the interaction with the substrate. It is known that the coronene and perylene molecules assemble to form a (4×4) superstructure on Au(111).33 Since the molecular size of coronene is larger than that of perylene, the intermolecular space at coronene(4×4)/Au(111) is smaller than that at perylene(4×4)/Au(111). On the other hand, as shown in Figure 3(c) and 3(d), we succeeded in the preparation of the different HBC superstructures; HBC(5×5)/Au(111) and HBC(3√3×3√3)R30/Au(111) (R0 and R30 phase, respectively).34-36 To the best of our knowledge, the HBC R0 phase is a newlyfound superstructure, which can be assembled by controlling the substrate temperature during the film growth; 480 K and 310 K were applied to obtain the R0 and R30 phases, respectively. The different superstructures of the same molecule enable us to discuss the azimuthal anisotropy in the electronic coupling with the substrate. Figure 3 shows the ordered structure, the surface Brillouin zone (SBZ) map, the LEED image, and the E-k|| map, obtained for (a) coronene(4×4)/Au(111), (b) perylene(4×4)/Au(111), (c) HBC(5×5)/Au(111), and (d) HBC(3√3×3√3)R30/Au(111) at 20 K. The E-k|| maps are shown with the selected energy distribution curves (EDCs) at the high-symmetry points in the SBZ of the monolayer (Γm: red curve, Km or Mm: black curve). In the LEED image, the Au(111)-


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derived spots are indicated by the red dots, which have the small sub-spots due to the 22×√3 herringbone surface reconstruction. Upon adsorption of coronene [Figure 3(a)], the coronenederived LEED spots appear as the 1/4 periodicity with respect to the Au(111)-derived LEED spots due to the formation of the (4×4) superstructure. The E-k|| map of coronene(4×4)/Au(111) exhibits the HOMO-derived peak at Eb ~ 1.6 eV. Although no direct intermolecular π-π overlap exists in the flat-lying coronene monolayer, the HOMO-derived peak shows a quite weak but non-negligible dispersion. The HOMO-derived peak is shifted to the lower Eb side from the Γm point, and the total dispersion width is 18 meV along the Γ-K direction (k|| = kΓK), where the HOMO-derived peak is observable at Eb = 1.577 eV when kΓK = 1.09 Å−1 (Γm) and at

Eb = 1.559 eV when kΓK = 1.45 Å−1 (Km). The HOMO-derived peak is weakened along the Γ-M direction (k|| = kΓM) as predicted in Figure 1. The EDC analysis yields the dispersion width by 27 meV along the kΓM direction with the inflection point at kΓM = 1.26 Å−1 (Γm) and 1.57 Å−1 (Mm). The anisotropic E-k|| relations for the molecular electronic states are also observable for the other PAH monolayers on Au(111), as summarized in Table 1.

Table 1. Total Eb shift (meV scale) of the π-band dispersion from the Γm point to the next high-symmetry point (Km or Mm) in the PAH monolayers on Au(111) at 20 K. coronene

perylene

HBC R0

HBC R30

HOMO

kΓK

−18

−14

14

8

HOMO

kΓM

−27

~0

7

18


14

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HOMO-1

kΓK

***

***

−13

−31

HOMO-1

kΓM

***

***

−25

−14

The ordered structure, the SBZ map, the LEED image, and the E-k|| map of perylene(4×4)/Au(111) are shown in Figure 3(b). The HOMO-band dispersion at perylene(4×4)/Au(111) is observable only along the kΓK direction, and is narrower than that at coronene(4×4)/Au(111). The different dispersion width can be explained by the larger intermolecular space in perylene(4×4)/Au(111) than that in coronene(4×4)/Au(111). Figures 3(c) and 3(d) show the E-k|| map of HBC(5×5)/Au(111) and HBC(3√3×3√3)R30/Au(111), respectively, together with the ordered structure, the SBZ map, and the LEED image. In contrast to coronene and perylene, the HOMO-derived peak of HBC/Au(111) shows the dispersion to the higher Eb side from the Γm point, which is dominated by the bonding and antibonding characters at the high-symmetry point in overlapped MOs. The HOMO-derived peak in the R0 phase shows dispersion by 14 meV and 7 meV along the kΓK and kΓM directions, respectively. The azimuthal anisotropies in the dispersion width and the photoemission intensity are flipped between the R0 and R30 phases; the larger dispersion width and photoemission intensity are observable along the kΓM direction for the R30 phase. Judging from the simulated PIAD pattern in Figure 1, the molecular arrangement of HBC with respect to the crystallographic direction of Au(111) is different between the R0 and R30 phases as shown in Figures 3(c) and 3(d). It is known that the appearance of the metal-mediated π-band dispersion in organic monolayers is dominated by the substrate metal-row direction, which plays a crucial


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role in the interfacial orbital hybridization.24 If this is the case, the larger dispersion width should appear for the same azimuth with respect to the SBZ of the substrate. As demonstrated in Figure 3(c) and 3(d), the anisotropic dispersion width for the HOMO is dependent on the SBZ of the monolayer but not on the SBZ of the substrate. Since the dispersion periodicity, width, and its azimuthal anisotropy are dominated by the intermolecular space and the SBZ of the monolayer, the possible origin of the lateral π-band dispersion is the inherent long-range intermolecular interaction in highly-ordered PAH monolayers. This attribution is supported by the HOMO-1 dispersion of HBC/Au(111). For both the R0 and R30 phases, the dispersion width of the HOMO-1 is larger than that of the HOMO. Since the HOMO-1 of HBC is distributed over the outer benzene rings in the molecule as shown in Figure 1(d), the HOMO-1 of the HBC molecule can effectively interact with the HOMO-1 of the nearest neighbor molecules, resulting in the less anisotropy in both the E-k|| and I-k|| relations. Furthermore, the recent theoretical calculations predicted the presence of the lateral π-band dispersion in free-standing monolayers of pentacene and naphthalene with a few tenth meV, which can be enhanced by the hybridization with the substrate band.24,42 The other possible origin of the lateral π-band dispersion is the intermolecular interaction with a jellium, based on the homogeneous electron gas model and akin to the substratemediated interaction, under the assumption that the PAH molecules and the surface symmetry of Au(111) are nearly isotropic. In this scenario, the symmetry of the molecular superstructure only exists. It is known that the interfacial orbital hybridization exists at CoPc/Au(110) and


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FePc/Au(110), resulting in the E-k|| dispersion of the interface-specific hybrid states.15 Similar interface-specific hybrid states are known for CoPc/Au(111) and FePc/Au(111).43,44 If the jellium model plays a major role on the Au(111) surface, the interface-specific hybrid states at CoPc/Au(111) and FePc/Au(111) should show some dispersive behaviors, even though it might not show the clear E-k|| curve. Since we could not detect the dispersive behavior in the highresolution ARPES measurements for CoPc/Au(111) and FePc/Au(111),43,44 the interaction with the jellium plays a minor role, even if it exists at PAHs/Au(111).

Figure 4. E-k|| map of the Shockley state for the clean Au(111) surface and the monolayers of coronene, perylene, HBC R0, and HBC R30 on Au(111) at 20 K. The EDCs at 0 Å−1 are shown in the bottom panel. In the E-k|| map, the best-fit result for the Shockley state of the clean Au(111) surface is indicated by the dashed curves.


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On the other hand, only for coronene(4×4)/Au(111), the free-electron-like parabolic bands appear around the Γm point (kΓK = 1.09 Å−1 and kΓM = 1.26 Å−1) at Eb = 0–0.4 eV and 1.7–2.1 eV, both of which are not observable for the clean surface (see, Supporting Information). These interface-specific states originate from the superstructure-induced backfolding of the Shockley- and Tamm-type surface states of Au(111) at the Γ and M points, respectively. Such backfolded bands are not observable for other monolayers of perylene and HBC due to the lack of the crystalline-domain size as judged from the sharpness of the LEED spots, whereas the original Shockley state at k|| = 0 Å−1 is observable for all the PAH monolayers with modified dispersion parameters. To understand the effect of the molecule-substrate interaction on the HOMO-band dispersion at PAHs/Au(111) in more detail, we discuss the modification of the Shockley state at PAHs/Au(111). Figure 4 shows the E-k|| map of the Shockley state and its EDC at kΓK = 0 Å−1 for the clean Au(111) surface and the highly-ordered monolayers of coronene, perylene, HBC R0, and HBC R30 at 20 K. The Shockley state of the clean Au(111) surface shows a parabolic dispersion with the Rashba spin-orbit splitting. A least-squares fitting using two free-electronlike parabolas gives the effective mass (m*) of 0.27m0, the surface state energy at 0 Å−1 (E0) of 465 meV, and the Rashba momentum offset (ΔkR) of ±0.013 Å−1. Upon adsorption of the monolayer, the Shockley state appears at the lower Eb side for all PAHs. The observed Shockley state at PAHs/Au(111) can be fitted by modified dispersion parameters of the increased m* and the upshifted E0, while ΔkR is unchanged, with respect to the original Au(111) Shockley state.


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According to the ARPES studies on the Shockley state at rare-gas/Au(111) and organic/Au(111), the exchange repulsion of surface electron systems by adsorbates modifies the Shockley state with the weak upshift in E0.39 It was demonstrated that the shift in E0 (ΔE0) in the modified Shockley state gives an interfacial adsorption energy (Ea), as expressed by Ea = 0.106ΔE0.39 In the present work, since the size of the PAH molecules is enough large with respect to that of the underlying metal-atom unit cell, the relation of Ea = 0.106ΔE0 is applicable, even if the molecules form the commensurate overlayer on specific adsorption sites. As summarized in Table 2, the strongest Ea of 20.8 meV/Å2 and the weakest Ea of 13.6 meV/Å2 are determined for perylene and coronene, respectively. If the present HOMO-band dispersions at PAHs/Au(111) originate from the metal-mediated electronic coupling due to the interfacial orbital hybridization, the strongest dispersion should appear for perylene(4×4)/Au(111) with the strongest Ea, which is contrary to the present observation. Moreover, the modification of the Shockley state has been observed also for strongly chemisorbed interfaces such as pentacene/Cu(110)

with

the

commensurate

structure.

The

Shockley

state

at

pentacene/Cu(110) is known to show the downshift since the contribution of the orbital hybridization (i.e., attractive force) is larger than that of the exchange interaction (i.e., repulsive force).45 The upshift in the Shockley state at PAHs/Au(111) indicates the less contribution of the attractive substrate-mediated interaction, including the jellium picture, on the lateral π-band dispersion, which supports the presence of the inherent intermolecular interaction in the PAH/Au(111) superstructures.


19


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Evidence for the lateral intermolecular interaction was reported also for the PTCDAcoronene/Au(111) binary interface studied by scanning tunneling microscopy, where the intermolecular overlap of π orbitals forms a one-dimensionally delocalized HOMO state through a surface-confined pseudodihydrogen bond, indicating the importance of the intermolecular H-H distance.46 For more detailed discussion on the lateral intermolecular interaction, the state-of-the-art band calculation for the free-standing monolayer is required. In addition to the electronic band dispersion, the advanced ARPES experiment for the orbital tomography has been performed for adsorption-induced electronic states at organic/metal interfaces. To retrieve MOs from PIAD precisely, various parameters related to the electronic coupling and the photoelectron scattering must be optimized. Among them, the intramolecular

and

intermolecular

photoelectron

scatterings

realize

quantitative

determination of the orientation and arrangement angles for adsorbed molecules.20 Note, however, that the lack of the electronic coupling in the usual theoretical approximation precludes the quantitative data analysis on the I-k|| relation since the high-resolution PIAD map can detect the electronic band dispersion, even if the dispersion width is small. Therefore, the presence or absence of the electronic band dispersion, and its origin, if present, is one of the important issues for the orbital tomography. The present observations suggest that the weak lateral intermolecular interaction is non-negligible and must be considered in the theoretical calculation towards the precise visualization of MOs, which enables the quantitative determination of the anisotropic hopping charge-transport property.40


20

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Table 2. Dispersion parameters in the Shockley state and the determined adsorption energy (Ea) obtained for the clean Au(111) surface and the monolayers of coronene, perylene, HBC R0, and HBC R30 on Au(111) at 20 K. m*/m0

E0 (meV)

ΔkR (Å−1)

Ea (meV/Å2)

Au(111)

0.27

465

0.013

0

Coronene

0.29

337

0.013

13.6

Perylene

0.29

269

0.013

20.8

HBC R0

0.29

306

0.013

16.9

HBC R30

0.29

304

0.013

17.1

CONCLUSION We studied the I-k|| and E-k|| relations in ARPES measurements for the π-derived electronic states of the highly-ordered monolayers of PAHs (coronene, perylene, and HBC) on Au(111) from the viewpoints of the intermolecular space, the underlying metal-atom row, and the molecule-substrate interaction. In the E-k|| relation, we succeeded in observation of the lateral π-band dispersion of a few tenth meV scale. The anisotropic dispersion width and its magnitude are mainly dominated by the intermolecular space and the SBZ of the monolayer but not by the molecule-substrate interaction or by the SBZ of the substrate. This is the experimental proof of the presence of the non-direct lateral intermolecular interaction as the origin of the weak π-orbital delocalization in the highly-ordered PAH monolayers on Au(111).


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Furthermore, the π-derived bands for the highly-ordered PAH monolayers on Au(111) showed the clear anisotropy in the I-k|| relation. Based on the present experimental and theoretical studies, we found that the weak intermolecular interaction is a non-negligible parameter for the quantitative analysis on the PIAD map of highly-ordered molecules, in addition to the intramolecular and intermolecular photoelectron scatterings. ASSOCIATED CONTENT Supporting Information. The supporting information is available free of charge on the ACS Publications website. Core-level photoemission data at coronene/Au(111). ARPES data for the clean Au(111) surface and the R0 and R30 coexistent phase of HBC/Au(111) at 20 K (PDF). Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors would like to thank the staff of the UVSOR-III Synchrotron for their kind supports in the present ARPES experiment. Funding Sources Grant-in-Aid for Scientific Research on Innovative Areas (π-System Figuration) JP17H05167 and Grant-in-Aid for Scientific Research (C) JP17K05767 from Japan Society for the Promotion of Science (JSPS).


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Figure 1 112x74mm (300 x 300 DPI)


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Figure 2 144x73mm (300 x 300 DPI)



Figure 3 177x90mm (300 x 300 DPI)


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Figure 4 82x87mm (300 x 300 DPI)



TOC graphic 60x40mm (300 x 300 DPI)


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Photoelectron Angular Distribution Induced by Weak Intermolecular

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