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J. Phys. Chem. C 2009, 113, 6202–6207
Fluorine Substitution of Hexa-peri-hexabenzocoronene: Change in Growth Mode and Electronic Structure Shiro Entani,† Toshihiko Kaji,‡ Susumu Ikeda,† Tomohiko Mori,§ Yoshihiro Kikuzawa,§ Hisato Takeuchi,§ and Koichiro Saiki*,†,‡ Department of Complexity Science & Engineering, Graduate School of Frontier Sciences, The UniVersity of Tokyo, Kashiwanoha 5-1-5, Kashiwa, Chiba 277-8561, Japan, Department of Chemistry, School of Science, The UniVersity of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-0033, Japan, and Toyota Central Research and DeVelopment Laboratories, Inc., Nagakute, Aichi 480-1192, Japan ReceiVed: October 7, 2008; ReVised Manuscript ReceiVed: February 10, 2009
We have studied the effect of fluorine substitution on growth mode and electronic structure of hexa-perihexabenzocoronene (C42H18, HBC). Fluorinated-HBC, 2,5,8,11,14,17-hexafluoro-hexa-peri-hexabenzocoronene (C42F6H12, 6F-HBC), grew epitaxially on a Cu(111) single-crystal surface, similar to HBC. Higher crystalline films with rather flat surfaces were obtained for 6F-HBC than for HBC due to the attractive intermolecular C-H · · · F-C interaction. Electron spectroscopy measurements of HBC and 6F-HBC films indicated that fluorination causes energy shifts of molecular orbitals to higher binding energies and increase of the work function. Thickness dependence spectroscopic measurements revealed that the interface dipole and the ionization potential are drastically increased in 6F-HBC/Cu(111), which is favorable for n-type electric conduction through the decrease of electron injection barrier to LUMO. Introduction Hexa-peri-hexabenzocoronenes (HBCs) are polycyclic aromatic hydrocarbons and can be regarded as disk-shaped hydrogen-terminated graphene fragments. They have been extremely studied in the past decade, because they show novel electronic and optoelectronic properties.1-3 Most of these characteristics relate closely to the self-assembling nature of HBCs, which comes from the strong π-π stacking of the aromatic cores and helps formation of a well-ordered uniaxially stacked film.1-9 For example, Ruffieux et al. grew supramolecular columns of HBC by organic molecular beam epitaxy. In the substituted HBC compounds, however, the attached side chains also contribute to self-assembly in addition to the native aromatic core interaction. Hill et al. demonstrated that an amphiphilic HBC self-assembles to form graphitic discrete nanotubes.3 We have also observed a strong correlation between the configuration of side chains and film morphology of HBCs.10 The self-ordering behavior of HBCs possesses several advantages as a semiconductor component of organic field-effect transistors (OFETs).1,11,12 Recently, Mori and co-workers have reported that modification of the number and/or the position of the substituents affects the conducting pathway in the FETs, and accordingly changes the field-effect mobility.10,11 Plenty of HBC compounds, however, operate as a p-type semiconductor like most of the organic semiconductors. Considering the production of bipolar transistors and complimentary circuits, the n-type organic semiconductor is expected to have physical and electrical properties similar to those of its counterpart except for the type of carriers. One of the ways to synthesize an n-type organic semiconductor is attaching the strong electron* Corresponding author. Tel./fax: +81-4-7136-3903. E-mail: saiki@ k.u-tokyo.ac.jp. † Department of Complexity Science & Engineering, The University of Tokyo. ‡ Department of Chemistry, The University of Tokyo. § Toyota Central Research and Development Laboratories, Inc.
Figure 1. Molecular structures of HBC and 6F-HBC.
withdrawing element to the corresponding p-type semiconductor. For several organic substances such as pentacene, oligothiophene, and phthalocyanine, n-type conduction has been realized by substituting the hydrogens with fluoro or fluoroalkyl groups.13-16 A similar investigation of fluorine substitution has been reported also in HBCs.17-20 Recently, Kikuzawa et al. synthesized fluorinated-HBC, 2,5,8,11,14,17-hexafluoro-hexa-peri-hexabenzocoronene (6FHBC).18 The molecular structure is schematically shown in Figure 1. 6F-HBC works as an n-type transistor with a fieldeffect mobility of 1.6 × 10-2 cm2/V · s, which is comparable to the hole mobility of unsubstituted HBC (3.3 × 10-2 cm2/V · s).19 In the FET measurement of 6F-HBC, the electrodes were made of Ca/Al under the assumption that low work function materials are favorable for reducing the injection barrier of electrons to the LUMO of 6F-HBC. In our recent work, however, formation of dipole layer at the interface between an organic semiconductor and a metal electrode affects the energy alignment considerably and moreover determines the carrier polarity.21 Thus, the carrier reversal observed between HBC and 6F-HBC should be discussed from a viewpoint of energy level rearrangement at the interface. In this case, it is preferable to use a single metal as a reference electrode for both HBC and 6F-HBC, because the problem can be simplified to focus on the fluorination effect on the interface formation.
10.1021/jp808861y CCC: $40.75 2009 American Chemical Society Published on Web 03/23/2009
Fluorine Substitution of Hexa-peri-hexabenzocoronene
Figure 2. (a) RHEED images of (i, ii) a Cu(111) substrate, (iii, iv) 20 nm thick HBC/Cu(111), and (v, vi) 20 nm thick 6F-HBC/Cu(111). The electron incidences were parallel to the [11j0] (left column) and [112j] (right column), respectively. (b, c) Structure models of HBC6 and 6FHBC molecules grown on Cu(111).
Here, we report film growth and electronic properties of 6FHBC on Cu (111), as compared to those of HBC. We clarified the difference of orientation between HBC and 6F-HBC, which could be explained by the difference in intermolecular interaction. Fluorination of HBC caused the downward energy level shift by 0.4 eV, whereas the energy gap was almost unchanged. This energy shift is favorable for n-type operation of 6F-HBC FETs. Experimental Methods The organic films were deposited onto a Cu(111) substrate under ultrahigh vacuum (UHV) conditions by evaporating HBC or 6F-HBC molecules from Knudsen cells. Before deposition, a mechanically and electrochemically polished Cu(111) substrate was cleaned in UHV by repeated cycles of Ar+ sputtering and annealing at 830 K. After this cleaning procedure, a sharp streak pattern of a Cu(111) surface was observed by reflection high energy electron diffraction (RHEED), and no contamination was detected by Auger electron spectroscopy (AES). During film growth, the substrate temperature was kept at room temperature (RT). The deposition rate was monitored by a quartz crystal microbalance, and it was on the order of 0.5 nm per minute. Surface crystallinity and epitaxial orientation of the grown films were observed in situ by RHEED. Electronic structure and surface composition of the films were investigated by ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), and electron energy loss spectroscopy (EELS). Energy scan was done with a SPECS PHIBOS 100 hemispherical energy analyzer. The UV source was a SPECS UVS300 UV lamp equipped with a TMM 302 R2 toroidal mirror monochromator, and the X-ray source was a Thermo VG Scientific XR3E2. The electron source of EELS was a SPECS eq 22. After the measurements, the surface morphology was inspected by a JEOL JSPM-5200 atomic force microscope (AFM) in the atmosphere. Results A. Film Growth of HBC and 6F-HBC on Cu(111). The RHEED images of a Cu(111) surface, 5 nm thick HBC, and 6F-HBC films are shown in Figure 2a. The incident electron beam is parallel to [11j0] (i, iii, v) and [112j] (ii, iv, vi) azimuths of the Cu(111) substrate. Sharp streaks and low background intensity in Figure 2a (iii-vi) show evidence that HBC and 6F-
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Figure 3. AFM images of 20 nm thick HBC (a) and 6F-HBC (b) films and the line profiles measured along the black lines.
HBC films grew epitaxially on Cu(111) with relatively good crystallinity. The streak interval in the RHEED images provides information on the distance between neighboring molecules, which were 14.1 ( 0.7 Å for HBC and 14.9 ( 0.7 Å for 6FHBC, respectively. The epitaxial growth of HBC film on a Cu(111) surface was studied by Ruffieux et al. using X-ray photoelectron diffraction, in which the HBC molecules formed ordered films with (31 × 31) R ( 8.95° superstructures (schematically represented in Figure 2b).6 In the present study, however, we can see the same streak interval independently of the incident electron azimuth (arrows in Figure 2a (iii and iv)). This means that each HBC domain rotates randomly around the surface normal and forms the (31 × 31) superstructured fiber texture on Cu(111). In contrast with HBC, a unique epitaxial orientation was observed for the 6F-HBC film with its crystallographic axes (arrows in Figure 2c) parallel to the [11j0] azimuth of a Cu(111) substrate. The streak patterns (Figure 2a (v and vi)) were observed until the thickness reached 30 nm. The crystal structure of the 6F-HBC film inferred from the RHEED measurements is schematically depicted in Figure 2c. The AFM images of HBC and 6F-HBC films are shown in Figure 3a and b, respectively. The thickness of both HBC and 6F-HBC films was 20 nm. The HBC film consists of plenty of grains whose diameters are less than 100 nm. The 6F-HBC film, however, has a rather flat surface, on which patch-like domains with a size of more than 200 nm are seen. The line profiles in Figure 3c and d clearly show that the surface roughness of 6FHBC film is much smaller than that of HBC. In ref 19, the authors observed a surface morphology of a 20 nm thick 6FHBC film grown on SiO2. The rectangular grains with an average size of 200 nm were formed, in close agreement with the present result. On the basis of the XRD results, they reported that 6F-HBC has a face-to-face structure, whereas HBC has a herringbone structure.19 Taking account of the fact that the order of stacking is higher in bulk 6F-HBC than in bulk HBC, it can be considered that 6F-HBC molecules assemble with a columnar structure on the Cu(111) substrate and form the nanometer-scale flat grains. B. Electronic Structure of HBC/Cu(111) and 6F-HBC/ Cu(111). The XPS spectra of 20 nm thick HBC and 6F-HBC films were measured using a Mg KR (1253.6 eV) source, as shown in Figure 4. The intense C 1s peak is observed at 284.4 eV for the HBC film, whereas an additional peak appears at 287.0 eV in the 6F-HBC spectrum. Because of the strong electrophilic nature of a fluorine atom, the electron density is reduced at the carbon atoms that are bonded to a fluorine atom,
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Figure 4. XPS spectra of HBC and 6F-HBC films in C 1s (left) and F 1s (right) core level regions taken using a Mg KR source. Figure 6. UPS spectra of HBC and 6F-HBC measured with a He I source (solid lines). Calculated DOS of HBC and 6F-HBC molecules are superimposed in the same figure (see text).
Figure 5. Thickness dependence of HBC (a) and 6F-HBC (b) films in the C 1s region.
which causes increase of the binding energy (BE) of the C 1s core level.22,23 We carried out spectral analysis and estimated the intensity ratio of the main peak (285.0 eV) to the additional peak (287.0 eV) for the 6F-HBC film. The ratio amounts to 6.1((0.5):1.0, supporting the interpretation that the peak at the higher BE side corresponds to the six carbon atoms bonded to fluorine atoms. In contrast with the C 1s peak, the F 1s peak of 6F-HBC consists of a single component (687.4 eV, Figure 4b), meaning a unique chemical environment of the carbon-bonded fluorine atoms. To understand the energy level shift at the interfaces, the C 1s core level spectra were measured for the HBC and 6F-HBC films with various thicknesses (Figure 5). No additional peak was observed even for the thinnest HBC films on Cu(111), indicating that HBC molecules did not interact with the Cu substrate at the interfaces. However, the BE shift of the core level peak was detectable with increasing film thickness. The C 1s peaks of both HBC and 6F-HBC films shifted to the higher BE side at the initial stage of film growth, and the observed energy shifts are ∼0.05 eV for HBC and 0.2 eV for 6F-HBC, respectively. These core level shifts could be explained by final state screening effects, in which photoholes are screened in the near presence of the metal substrate. For the 6F-HBC films thicker than 5 nm, the core level peaks shifted again to the lower BEs region (∼0.05 eV). This tendency observed at above 5 nm might be caused not by the final state effect but by the formation of an interface dipole layer between 6F-HBC and Cu(111), which will be discussed in detail in the following section. To investigate the valence states, UPS spectra were measured with a He I (21.2 eV) source, as seen in Figure 6. Occupied density of states (DOS) of HBC and 6F-HBC molecules, which were calculated by using B3LYP/6-31G(d) geometry optimization of the molecule, is included in the same figure.24 The
Figure 7. UPS spectra of various thicknesses of HBC and 6F-HBC films (right) and those measured with a specimen biased by 5.0 V (left).
calculated DOS was obtained by summing up the contributions from the discrete energy levels with a Gaussian broadening of 0.8 eV. The correspondence between the measured spectrum and calculated DOS is fairly good. The observed features can be assigned as follows. The three peaks at the low BE region (up to 3.7 eV) in both spectra are ascribed to π bands, and the peaks between 4.7 and 10 eV are ascribed to σ-π mixed bands. For the 6F-HBC film, the σ (C-F) state is observed at 6.7 eV, and lone pair orbitals of the fluorine atoms are concentrated around 8.6 eV. When we compare the spectral feature of 6FHBC with that of HBC, the energy stabilization by fluorination can be recognized. The energy shift of the highest occupied molecular orbital (HOMO) from HBC to 6F-HBC is approximately 0.4 eV, which is comparable to 0.6 eV observed in the energy shift of the C 1s peak (shown in Figure 4). Thickness dependence of the UPS spectra of HBC and 6FHBC is shown in Figure 7. For the thinnest film (∼1 nm) on Cu(111), the spectra of HBC and 6F-HBC are considered as a superposition of the organic layer and the Cu substrate. With increasing film thickness, the contribution from Cu decreases gradually. For the 5 nm thick 6F-HBC film, however, a finite shoulder at the Fermi level (EF) coming from Cu is still recognized. In contrast, there is no indication of Cu signal for the HBC film with the same thickness. It seems contradictory to the AFM result, which showed that the surface flatness is higher for the 6F-HBC film than for the HBC film. This could be explained by the difference in growth mode. According to the crystal structure, 6F-HBC molecules stack with the aromatic
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TABLE 1: Changes in Work Function and HOMO Threshold for Various Thicknesses of HBC and 6F-HBC Films material (thickness) HBC work function (eV) EHOMOth (eV)
6F-HBC
1 nm
5 nm
20 nm
1 nm
5 nm
20 nm
5.1 0.66
4.9 0.83
4.7 0.94
5.4 1.59
5.7 1.66
5.5 1.37
cores faced to each other and aggregate in a columnar stacking fashion like a so-called island growth mode. This results in delay of complete covering of the substrate, which will be discussed again from a viewpoint of intermolecular interaction in the following section. We evaluated the energy shifts of HOMO threshold (EHOMOth) by linearly extrapolating the slope of the HOMO peak to the plateau region below the EF of the Cu substrate. The work function was also evaluated from the low kinetic energy cutoff regime of the spectrum. To decide the cutoff of the spectrum precisely, the UPS spectrum was measured with a bias voltage of 5.0 V (the left part of Figure 7). The evaluated EHOMOth and work function are summarized in Table 1. Although the EHOMOth and work function of HBC shift monotonously with increasing film thickness, those values of 6F-HBC show complicated thickness dependence, just similar to the energy shifts observed in the XPS spectra. To know electronic levels other than the occupied states, the EEL spectrum was measured using 60 eV electrons (Figure 8). Three energy-loss structures are observed around 3.6, 5.9, and 7.6 eV for HBC, and 3.7, 6.2, and 7.8 eV for 6F-HBC, respectively. The peaks around 3.6-3.7 and 5.9-6.2 eV are assigned to the π f π* transitions and π plasmon excitations.25 Taking account of the outcrop of the substrates even for the thicker films, the feature around 7.6-7.8 eV could be attributed to the ionization loss and/or plasmon excitations from the Cu substrate.26 The HOMO-LUMO gap (Egap) is obtained from the threshold of the energy-loss structure. Egap is 2.5 eV both for HBC and for 6F-HBC, which is comparable to the previous value of 2.7 eV obtained from UV-vis measurements.18 It should be noted that Egap is unchanged by fluorination in contrast with EHOMOth. Discussion On the basis of the structural and spectroscopic results, the effect of fluorination on HBC will be discussed in the following. First, we consider the difference in crystal structure and growth mode between HBC and 6F-HBC. The RHEED and AFM results in Figures 2 and 3 show that crystallinity and flatness of 6F-HBC films are higher than those of HBC films. To obtain a well-ordered uniaxially stacked structure of HBC, an elaborated method was proposed: a monolayer of HBC is prepared by annealing the multilayered film, on which additional HBC layers are deposited.6 In the case of the 6F-HBC, however, the grown film shows higher crystallinity and flatness without use of such a complicated method. What brings about such a change in the self-assembling process of HBCs? 6F-HBC is a HBC derivative in which the six outermost hydrogen atoms are substituted with fluorine atoms. Introduction of fluorine atoms is considered to induce intermolecular attractive C-H · · · F-C interaction (hydrogen bond).27,28 This interaction helps formation of an inplane ordered structure on Cu(111) as shown in Figure 2c. The resemblance in interlayer distance between HBC and 6F-HBC19 suggests that hydrogen bond has less effect on the intermolecular interaction along the stacking direction (out-of-plane direction).
Figure 8. Thickness dependences of the EEL spectra of HBC and 6F-HBC measured by 60 eV electrons.
Although a detailed mechanism is not clear at the present stage, an in-plane hydrogen bond causes columnar stacking structure in 6F-HBC and helps in the formation of films with long-range order and fewer defects than the HBC films. Next, we will discuss the changes in the electronic structure of HBCs caused by fluorination, also referring to the case of copper phthalocyanines (CuPcs).23 The energy shifts of the C 1s core level and the valence band and the energy gap will be noticed. At first the energy shifts of the C 1s core level, which was shown in Figure 4, are discussed. After the fluorine substitution, an additional peak is observed at 2.0 eV higher BE than the intense C 1s main peak. In the above section, this additional peak was assigned as the emission from fluorinebonded carbon atoms. Because of the strong electrophilic nature of a fluorine atom, the electron density is reduced at the carbon atoms that are bonded to a fluorine atom, resulting in an increase of the BE of the C 1s level. A similar difference between the main C 1s peak and the additional peak was reported in the fluorinated CuPcs: 2.0 eV for CuPcF4 and 2.1 eV for CuPcF16.23 Furthermore, we found that the main C 1s peak, which is not bonded to a fluorine atom, was also shifted by 0.6 eV to the higher BE region after fluorination. The strong electronegative nature of the fluorine atom reduces the electron density in the aromatic core and increases the BE of its whole electronic state. The energy shift of 0.6 eV is larger than that of CuPcF16, which is 0.2 eV. In the case of unsubstituted molecule, C 1s of HBC has lower BE than that of CuPc due to the graphene-like nature of all-benzenoid aromatic hydrocarbons. After fluorination, however, the C 1s’s are similar to each other between 6F-HBC (285.0 eV) and CuPcF16 (284.9 eV). Next, we will discuss the influence of fluorine substitution upon the spectral feature of the valence band and Egap. In contrast with the energy shifts of the C 1s level, the upper valence band (BE < 5 eV, in Figure 6) and Egap (EELS in Figure 8) showed little change. The electronic states derived from the fluorine atom appear in the higher BE region above 5 eV; the σ (C-F) state at 6.7 eV and the lone pair orbitals concentrated around 8.6 eV. This is comparable with the previous report of the fluorinated CuPcs.23 These results indicate that π and π* (LUMO) orbitals are hardly influenced by the fluorination, and the effect of fluorination appears only as the relative shift of energy levels. We proceed to the topic of energy level alignment at the interface. According to the present spectroscopic results on the 20 nm thick films, the energy level diagrams could be depicted for the HBC and 6F-HBC, as shown in Figure 9. The LUMO
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Figure 10. Energy shifts of work function (upper) and HOMO threshold (lower) for HBC/Cu(111) and 6F-HBC/Cu(111).
Figure 9. Energy level diagram of HBC/Cu(111) and 6F-HBC/Cu(111) systems for the 20 nm thick films.
level was estimated from the equation: EHOMOth + Egap, where Egap is the HOMO-LUMO gap obtained by the EELS measurement. It has been reported that the energy gap that is measured using optical or excitation techniques is smaller than that of the transport gap estimated from both UPS and inverse photoelectron spectroscopy (IPES).29,30 Therefore, the LUMO level in Figure 9 might be rather lower than the real position. We note here that the Egap or HOMO-LUMO distance is unchanged by fluorination. However, the work function increases drastically and the EF moves relatively upward within the HOMO-LUMO gap. The separations of LUMO level from EF of Cu(111) were estimated to be 1.6 eV for HBC and 1.2 eV for 6F-HBC. The difference in work function and the relative EF position contribute to the raising of ionization potential (IP ) work function + EHOMOth) from 5.6 to 7.0 eV. In ref 18, the authors pointed out the increase of IP from 5.4 eV (HBC) to 5.9 eV (6F-HBC) by fluorination. The much larger IP increase of the present work can be ascribed to the difference in the measurement condition. The IP was estimated from the photoemission measurement under UHV conditions in the present study, whereas in the previous research it was done in the atmosphere. In our previous work, the surface condition easily modifies the energy level alignment at the interface, and thus the energy levels should be investigated in a well-controlled environment.21 In the present study, IP was determined more accurately under the conditions in which extrinsic factors could be removed as much as possible. As shown in ref 19, 6F-HBC works as an n-type transistor. For the n-type operation, electrons are injected into or ejected from the LUMO level; therefore, a small difference between LUMO and EF is favorable. When we discuss the carrier injection at the organic/metal interface, it is important to take account of the interface nature in addition to bulk electronic structure, because an electric dipole layer is formed at that interface.21 Although the mechanism of dipole formation is still under discussion,31,32 we should consider the electronic structure of the ultrathin HBCs films on Cu(111) for the comprehensive understanding of the charge injection phenomenon at the HBCs/ metal interfaces. In the following, we will discuss the energy level shift during the growth of 6F-HBC on Cu(111) and HBC/ Cu(111).
Thickness dependences of work function and EHOMOth of HBC and 6F-HBC/Cu(111) are depicted in Figure 10 according to the UPS measurements. At the earlier stage of the film growth, the work functions show an upward energy shift and gradually decrease with increasing film thickness. The upward shift corresponds to the formation of dipole layer. The estimated interface dipole, ∆, is drastically changed from +0.2 to +0.9 eV by fluorination. To our knowledge, larger values of the positive ∆ have been observed only in the electron acceptor/ metal interfaces, such as the C60/metal system.33 The heightening of ∆ by fluorination has been reported in CuPcs on a gold surface.23 The ∆ of fluorinated CuPcs increases according to the number of attached fluorine atoms, and it shows the positive value in CuPcF16.34 The effective thickness of the dipole layer in HBCs/Cu(111) could be estimated from the width of the transition region in which energy level shift is observed. From the energy shifts of work function in Figure 10, the effective thicknesses of the dipole layer in HBC and 6F-HBC are approximately 1 and 5 nm, respectively. The relatively larger thickness observed in 6F-HBC/Cu(111) seems to relate to its film growth mode. As discussed above, 6F-HBC molecules aggregate via strong attractive intermolecular force, and the completion of full coverage is delayed as compared to the HBC film. In this case, the electrons emitted from the interface continue to be detected until the substrate is completely covered by the overlayer. Indeed, the film thickness at which the substrate is completely covered (judged from UPS) agrees with the effective thickness of the dipole layer. With further increasing film thickness, the energy shift changes its direction and decreases slightly. This could be ascribed to the downward band bending, which results from the Fermi level alignment in the HBCs/Cu(111) system. With respect to this band bending, no obvious difference is observed between HBC and 6F-HBC. Although EHOMOth shows a dependence similar to that of the work function, there are two apparent differences in Figure 10. The thickness dependence of the EHOMOth of 6F-HBC deviates from that of the work function in the ultrathin film region, as though the EHOMOth of the 1 nm film seems pushed to the lower BEs (upward). We think that this upward shift in the ultrathin region could be explained by the final state screening effect, corresponding to a similar behavior observed in XPS spectra (shown in Figure 5). The screening effect of the photohole has been often reported in the valence band spectra of organic/metal systems.23,35 As was discussed in the film growth mode, a wellordered interface is formed at 6F-HBC/Cu(111) in which the molecular plane is parallel to the Cu(111) surface. Therefore,
Fluorine Substitution of Hexa-peri-hexabenzocoronene the aromatic core of 6F-HBC lies flat on the metal surface, meaning a short distance between the molecule and the substrate. It is suggested that the screening effect of EHOMOth of 6F-HBC is enhanced at the interface, because the screening effect by the metal substrate is in proportion to 1/r (r, distance from the metal surface). Therefore, the upward shift in the ultrathin region is clearly seen at 6F-HBC/Cu(111) rather than at HBC/ Cu(111). Another difference in thickness dependence between the work function and EHOMOth relates to the width of transition region. For instance, the EHOMOth of 6F-HBC/Cu(111) continues to increase until the thickness of 9 nm, whereas the work function saturates around 4 nm. This would be caused by the difference in sensitivity to morphology between secondary electrons and photoelectrons. Photoelectrons contain all information within the regions of probing depth, whereas the low kinetic energy cutoff of the secondary electrons has higher surface sensitivity. Thus, the change of EHOMOth remains for the thicker films rather than that of the work function as shown in Figure 10. From all of the above discussions, we found that fluorination mainly changes IP and ∆ in HBCs. In other words, fluorination can reduce only the electron injection barrier in HBCs/Cu(111), without changing its fundamental electronic structures. It is worthy to note that the electron injection barrier can be further decreased by modifying ∆, which is as high as 0.9 eV in the present case. There is a possibility that the effective ∆ of the 6F-HBC heterojunction can be reduced by its film formation on an organic/metal heterostructure with small ∆ at the interface.36,37 Therefore, it seems that 6F-HBC will become one of the most promising candidates for n-type organic semiconductor if ∆ would be successfully controllable. Conclusions The electronic structure of 6F-HBC grown on Cu(111) was investigated by comparison with that of unsubstituted HBC. The large intermolecular interaction in 6F-HBC films due to a hydrogen bond contributed to the increase in film crystallinity and surface flatness as compared to the HBC film. Fluorination of HBC caused energy shifts of molecular orbitals to higher binding energies and increased the work function, keeping the energy gap unchanged. Precise thickness dependence measurements of energy levels revealed that the interface dipole and the ionization potential drastically increased in 6F-HBC/ Cu(111), which is favorable for n-type electric conduction through the decrease of electron injection barrier to LUMO. Acknowledgment. This work was supported by a Grant-inAid from the Ministry of Education, Culture, Sports, Science, and Technology of Japan (14GS0207). References and Notes (1) van de Graas, A. M.; Warman, J. M.; Mu¨llen, K.; Geerts, Y.; Brand, J. D. AdV. Mater. 1999, 11, 1469. (2) Schmidt-Mende, L.; Fechtenko¨tter, A.; Mu¨llen, K.; Moons, E.; Friend, R. H.; MacKenzie, J. D. Science 2001, 293, 1119.
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