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Image Potential State Mediated Excitation at Rubrene/Graphite Interface J. Park,† T. Ueba, R. Terawaki, T. Yamada, H. S. Kato, and T. Munakata* Department of Chemistry, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan ABSTRACT: We report an electronic excitation of adsorbed molecules mediated by the image potential state (IPS) on the substrate. Two-photon photoemission (2PPE) spectroscopy for rubrene films formed on a highly oriented pyrolytic graphite (HOPG) surface reveals a prominently enhanced peak due to an unoccupied molecular level (denoted as Ln), which is resonantly excited from the highest occupied molecular orbital (HOMO) derived level. The enhancement of the Ln peak becomes less significant at the coverage higher than 1 monolayer (ML), where the IPS peak on the substrate disappears. The resonance enhancement is moderate with s-polarization, by which the transition to IPS is completely suppressed. We ascribe that the excitation of the Ln level is mediated by the IPS on HOPG. Though the IPS wave function extends outside the molecules, it interacts with the unoccupied molecular orbital at the edges of molecular islands, causing the strong resonance enhancement of the Ln level. excitation is known for electron scattering.9,10 It is expected that an IPS on the substrate can significantly enhance the electronic excitation of adsorbed molecules. We report 2PPE spectroscopy for thin films of rubrene (5,6,11,12-tetraphenyltetracene, inset in Figure 1) formed on highly oriented pyrolytic graphite (HOPG) to reveal the IPS mediated molecular excitation.

1. INTRODUCTION Electronic excitation at the interface between an organic molecular film and a substrate is of general importance for the area of organic electronics and light conversion processes. Two types of molecular excitation mechanism at the interface are commonly discussed: (a) the direct transition by intramolecular excitation and (b) the indirect one by hot electron transfer from bulk substrate to molecules.1 As direct optical excitations, the transition between delocalized bands and also the transition between localized molecular-orbital derived levels are typically considered. The optical transition between the moleculederived level and the substrate band is sometimes thought to be small because overlapping between a localized molecular orbital and a delocalized substrate band cannot be large.2 On the other hand, the transition between a localized orbital and a delocalized band dominates over the hot electron mechanism in two-photon photoemission (2PPE) spectroscopy.3−5 Here, we demonstrate a novel process mediated by the image potential state (IPS) on the substrate as an unique electronic excitation caused by interactions among delocalized bands and localized molecular orbitals. IPS is the Rydberg-like unoccupied state supported in between the Coulombic image potential and the band gap of the substrate. IPS exhibits free electron like dispersion parallel to the surface. The interactions of IPS on or within molecular films with molecule-derived unoccupied levels play crucial roles on the electron dynamics at the interfaces.6−8 When a surface is partially covered with domains of molecular film, the IPS on the substrate and that on the film appear as separate peaks in 2PPE spectroscopy. In such a case, the role of the IPS on the substrate in the electronic excitation of the molecule has rarely been reported because an IPS orbital locating outside the molecules is not favorable to interact with molecules. Nevertheless, the role of IPS for molecular © 2012 American Chemical Society

Figure 1. 2PPE spectra for the clean HOPG (bottom) and the 0.8 ML rubrene film (top) measured at the photon energy of 4.43 eV. The photon energy, smaller than the work function of HOPG only by 0.02 eV, generates rather strong one-photon photoemission at around 0 eV. Except for the one-photon photoemission, the peak labeled Ln is prominently strong. Refer to the text for the other peaks. The molecular structure of rubrene is shown in the inset.

In 2PPE, a first photon (pump photon) excites an electron from an initial state Ei to an intermediated state Em of the adsorbate or the substrate. The second photon (probe photon) Received: December 12, 2011 Revised: February 10, 2012 Published: February 15, 2012 5821

dx.doi.org/10.1021/jp211938h | J. Phys. Chem. C 2012, 116, 5821−5826

The Journal of Physical Chemistry C

Article

Figure 2. (a) 2PPE spectra for the 0.4 ML film measured at the photon energies indicated on the right side. The 2PPE spectrum for clean HOPG is shown at the bottom. The orange and green traces are for the photon energies at the resonance and the highest peak intensity, respectively. (b) The intermediate energies of the peaks shown in part a are plotted against the photon energy. The different marks are for different coverages shown at the right bottom. The HOMO peak lies on the line of slope one (blue), while the other peaks are on the flat lines. The break point changing from the slope 1 line to the horizontal line is indicated by the vertical line at the photon energy of 4.38 eV. (c) Energy level diagram for the observed levels.

The light is focused onto the sample in UHV through a quartz window with a concave mirror of 40 cm focal length at the incident angle of 60°. The p-polarized light is used unless mentioned. No significant damage of the sample was detected. Photoelectrons emitted to the surface normal are detected with a hemispherical energy analyzer (VG-CLAM-4 with nine channeltrons detector) of 20 meV resolution. The analyzer is modified to limit the electron acceptance angle to be ±1°. A helium discharge lamp of 21.22 eV photon energy allows conventional UV photoemission spectroscopy (UPS) for the identical samples. All measurements were made at room temperature.

ejects the electron above the vacuum level. In the simple onecolor 2PPE process, the final energy of the photoelectron Ek is related to either Ei or Em by the equations Ek = 2hν + Ei or Ek = hν + Em, where hν is the photon energy. The energy levels refer to the Fermi level (EF). The former is the coherent 2PPE from the initial state Ei, and the latter, the one-photon photoemission from the intermediate state Em. When 2PPE spectra measured with different photon energies are put on the intermediate energy scale defined as Ek − hν, the photoemission peak from an intermediate state aligns at a fixed energy Em. The peak from an occupied initial state shifts by one-time of the photon energy increment. At the resonant photon energy, the two peaks merge together with enhanced intensity. We introduce rubrene/HOPG as a model system for an organic film/substrate interface. Many efforts have been devoted to improve the performance of a rubrene thin film transistor after high carrier mobility was achieved for rubrene single crystals.11 The reasons for the poor efficiency of the thin films are attributed to the molecular geometry on the surface.12 To understand the mechanisms of charge transportation for organic molecular devices, it is the primary step to unravel the molecular electronic structures of both occupied and unoccupied states at interfaces between the film and the substrate.

3. RESULTS AND DISCUSSION 3.1. Electronic Structure of Rubrene Thin Film on HOPG. 2PPE spectra for HOPG and the rubrene film of 0.8 monolayer (ML) coverage are shown in Figure 1. The photon energy of 4.43 eV is close to the work function of HOPG (4.45 eV). Except for the one-photon photoemission at around 0 eV, the peak labeled Ln stands out from other features. The Ln peak is stronger than that of the secondary electron in the low energy region. The prominently strong peak is quite unusual when compared with typical one-color 2PPE spectra for thin films.2,5 We focus on the origin of the strong enhancement of the Ln peak. The assignments of the 2PPE peaks are made by measuring the photon energy dependence as shown in Figure 2. The peak indicated IPS1 is commonly observed at 3.60 eV for both the clean HOPG and the 0.4 ML film. The peak is due to the IPS on bare HOPG.5,14 The intensity of the IPS1 peak is not strongly dependent on the photon energy because it is excited from the bulk occupied bands of HOPG.5 The intermediate energy of the peaks labeled as HOMO, Ln, IPS1, and IPS2 are plotted in Figure 2b against the photon energy. The IPS2 peak at the fixed intermediate energy of 3.89 eV is attributed to the IPS on the film. The assignment is confirmed by the polarization dependence (Figure 5) and a preliminary angle resolved

2. EXPERIMENTAL METHODS All the experiments were carried out in an ultrahigh vacuum (UHV) chamber of base pressure less than 1 × 10−10 Torr. HOPG was cleaved in air and heated in UHV at 670 K for more than 50 h.5,13,14 Purified rubrene was deposited on the HOPG surface by sublimation in an UHV preparation chamber with a rate of 0.1 Å/min as monitored by a quartz microbalance. No annealing was carried out. The light source is the third harmonic output (4.18−4.56 eV) of a wavelength tunable titanium sapphire laser operated at a repetition rate of 76 MHz and a pulse duration of 100 fs. The power of the incident light is controlled to be 0.20 nJ/pulse throughout the experiment. 5822



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The switching of the peak intensity from the occupied peak to the unoccupied peak on crossing the resonant photon energy is known for several systems, for example, lead phthalocyanine (PbPc) film on HOPG5 and Si(100).19 The intensity switching is the general trend of the resonant excitation in 2PPE spectroscopy. The intensity switching is a key to understand the excitation mechanism as is discussed in section 3.3. The Ln peak energies are not well fitted on the flat line when compared with those of the IPS1 and IPS2. The deviation from the flat line is discussed in section 3.2. The Ln level is derived from an unoccupied molecular orbital higher than the lowest unoccupied molecular orbital (LUMO). Since the Ln level is resonantly excited from the HOMO, which is mainly derived from the π-orbital of the tetracene backbone, the Ln orbital should have the π*-character of the backbone. By comparing the experiment with a DFT molecular orbital calculation with B3LYP and 6-31G* basis, the LUMO (orbital index of 141) is located around 1 eV above EF. LUMO+1 and higher unoccupied orbitals up to LUMO+10 are located in the energy region between 2.5 and 3.5 eV with small energy separations. The change of the energy levels by the geometry of the molecule (planar or tilted tetracene backbone) is comparable to the energy level separations. The detailed comparison between the calculation and the experiment is left for further work, and the observed level is denoted as Ln. The peak denoted by Lm is also due to a molecule-derived unoccupied level, as confirmed by the photon energy dependence in Figure 2a. The energy levels are summarized in Figure 2c. We mention the definition of the coverage in this work. By increasing the thickness of the film (reading of the microbalance) up to 8 Å, the work function decreases by 70 meV from that of the clean HOPG. At the thickness of 13 Å and higher, the work function gradually increases. The IPS1 peak, which arises from the bare domains of HOPG, becomes weak as the thickness increases, and it becomes difficult to be identified at thicknesses higher than 8 Å (see Figure 3). Thus, we assume the thickness of 10 Å corresponds to the 1 ML coverage. We do not consider that the 1 ML film is perfect. Small areas of bare HOPG still exist on the surface, and small bilayer islands coexist in the film. 3.2. Electronic Excitation at the Rubrene/HOPG Interface. The origin of the strong enhancement of the Ln peak becomes clear from the coverage dependence shown in Figure 3. The spectra are measured at the photon energy of 4.43 eV, at which the significant enhancement of the Ln peak occurs. The Lm peak becomes intense rapidly as the coverage exceeds 1 ML. On the other hand, the Ln peak becomes most intense at the coverage of 0.8 ML, and it becomes weaker at higher coverage. The Ln peak is not standing out from other features at the 1.6 ML coverage. The strong enhancement occurs only for sub-ML films. The intensities of the HOMO and the Ln peaks are plotted against the photon energy in Figure 4a for different coverages. Interestingly, the enhancement of the Ln intensity does not come to a peak at the resonance energy of 4.38 eV (orange bar) but continues to increase up to the photon energy of 4.43 eV or higher (green bar). These results suggest that the Ln peak involves not only the molecular derived unoccupied level but also an unoccupied level which exists solely on the sub-ML films. The smooth intensity variation in Figure 4a indicates that the Ln level is mixing with the substrate unoccupied state. Because the IPS1 peak vanishes at coverage higher than 1 ML, it is straightforward to attribute the mixing state to the IPS1.

measurement which shows a parabolic dispersion (not shown). The appearance of the IPS2 peak indicates that rubrene molecules form well ordered islands on the surface at the coverage. Coverage dependence is shown in Figure 3. Since IPS1 peak for

Figure 3. 2PPE spectra for the different coverages shown on the right side. The photon energy is 4.43 eV. The traces for 1.3 and 1.6 ML coverage are demagnified by a factor of 0.6. The region around IPS1 is magnified by a factor of 5. The Ln peak becomes most intense at the coverage of 0.8 ML. At coverage higher than 1 ML, the Ln peak is no longer standing out from the other features. The IPS1 peak is quenched at coverage higher than 1 ML, while the IPS2 peak survives. The Lm peak becomes stronger as the coverage increases.

the sub-ML films shows no significant shift from that of the clean HOPG, the bare domains of HOPG is laterally separated from the islands of the molecular film. The IPS2 peak appears at the fixed intermediate energy with increasing intensity as the coverage increases up to 1 ML. The well-ordered islands occupy the larger area of the surface as the average coverage increases. The peak labeled HOMO arises from the highest occupied molecular orbital (HOMO) derived occupied level. The energy of the HOMO peak lies on the line of slope 1 (blue line) at the photon energy region between 4.23 and 4.38 eV. The slope 1 line passes the intermediate energy of 3.26 eV at the photon energy of 4.38 eV (the crossing point in Figure 2b). The initial energy determined from the slope 1 line is −1.12 eV, in agreement with the HOMO peak energy of our UPS result and also literature data.15−18 Our UPS result also shows that dispersion of the HOMO peak is negligibly small for sub-ML and 1 ML films, in contrast to the clear band dispersion for the single crystal.16 Taking 4.38 eV as the break point photon energy changing from the slope 1 line to the horizontal line (Figure 2b), the intermediate energy of the unoccupied Ln level is 3.26 eV. The break point is the resonance energy between the HOMO and the Ln levels. The intensity of the HOMO peak increases following the photon energy increase from 4.23 eV. At photon energy higher than 4.43 eV, the HOMO peak is not visible, and the Ln peak appears at the nearly fixed intermediate energy. The widths of the HOMO and Ln peaks are both about 0.15 eV. From a simple model of two energy levels, one may suppose that the HOMO (Ln) peak appears even at the photon energies above (below) the resonant photon energy of 4.38 eV. In reality, the HOMO (Ln) peak is not observed, for example, at the photon energy of 4.45 (4.30) eV. 5823



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between the Ln and Lm peaks with s-polarization is comparable to the ratio for the 1.6 ML film in Figure 3. Regarding the factor of 9 to the polarization dependence of the intramolecular transitions, the additional enhancement of the Ln peak is the result of a nonmolecular transition, which is allowed only for p-polarization. This indicates that the IPS1 is mixing with the Ln orbital and causes the strong enhancement with p-polarization. The mixed state is denoted as Ln* in Figure 4b. Contribution of the IPS2 is ruled out because the peak survives at coverage higher than 1 ML, as can be seen in Figure 3. Owing to the transition from the substrate occupied bands, the IPS1 contribution in the Ln* state causes the strong resonant enhancement of the Ln peak intensity. The formation of the Ln* state is further confirmed from the expanded spectra shown in Figure 6. The peak position slightly Figure 4. (a) Intensities of the HOMO peak and the Ln peak are plotted against the photon energy below and above the resonance (orange bar), respectively. Different marks are for different coverages shown at the left top. The Ln peak becomes most intense at the energy (green bar) slightly higher than the resonance. (b) Schematic illustration of the interaction between the IPS1 and the Ln orbital occurring at the edge of a molecular grain. Ln* represents the mixed state.

The interaction occurs at the edge of molecular islands, as schematically shown in Figure 4b. The interaction between the Ln and the IPS1 is also confirmed by the polarization dependence shown in Figure 5. The

Figure 6. Expanded 2PPE spectra measured with the photon energies indicated by 1 to 3. The coverage is 0.5 ML. The peak position of IPS2 does not depend on the photon energy, while the Ln peak and the rising edge at around 3.4 eV shift to higher energy as the photon energy increases.

changes with the photon energy. The shift of the peak energy is seen as the deviation of the data points from the horizontal line in Figure 2b. The high-energy rising tail at around 3.4 eV also shifts to higher energy as the photon energy increases. The shifts indicate that the Ln peak contains at least two components. At the photon energy of 4.38 eV, the molecule-derived Ln level is substantially excited. As the photon energy increases, both the Ln and the mixed Ln* state are excited. The Ln* state is observed as the strongly enhanced peak. The peak shift is in parallel with Figure 4a, where the intensity maximum occurs at a photon energy (4.43 eV or higher) slightly above the resonance energy (4.38 eV). At coverage higher than 1 ML, no bare HOPG domain remains and the mixed state Ln* is not formed. Then the strong resonant enhancement vanishes. The HOMO band in UPS is reported to exhibit the vibrational satellites for the molecular ions of tilted and planar tetracene backbone configurations.18 Because the final state of the Ln peak is the same as the HOMO band in UPS, the Ln peak is expected to be accompanied with the vibrational satellite. But no such satellite is seen on the Ln peak. The Ln peak width in Figure 6 is less than 140 meV, which is narrower than the HOMO peak with the vibrational progression in ref 18. The narrow peak width indicates that the vibrational satellite is not excited in our resonant 2PPE. Similar suppression of the vibrational satellite was reported for the resonant 2PPE of PbPc films.13 The suppressed vibrational satellite is a result of the nuclear motion of the molecule in the intermediate state. By electron excitation from the bonding HOMO to the antibonding Ln level, the equilibrium geometry of the excited molecule becomes closer to the positive ion. The deformation of the excited

Figure 5. Polarization dependence of the 2PPE spectra measured for the 0.4 ML film. Black and blue lines are for the p- and s-polarization, respectively. The IPS1 and IPS2 peaks completely disappear with s-polarization (s-pol). The s-pol spectrum is magnified by 9 times to estimate the contribution of the intramolecular transition probability. The Lm peak and the background around 2 eV of the magnified spectrum overlap with the p-polarization (p-pol) spectrum, while the Ln peak for p-pol is about 10 times stronger than the magnified one for s-pol. The intensity ratio between the Ln and the Lm peaks for s-pol is rather similar to that for the 1.6 ML film in Figure 3.

IPS1 and IPS2 peaks disappear with s-polarization, in accordance with the selection rule. Because IPS is a free electron along the surface parallel directions, optical transitions of pump and probe processes are forbidden for s-polarization. Both the Lm and Ln peaks become weak with s-polarization but do not disappear, as can be seen in the magnified spectrum by a factor of 9. While the Lm peak intensity and the lower energy background in the magnified spectrum overlap with those of the p-polarization spectrum, the Ln peak intensity for p-polarization is nearly 1 order of magnitude higher than that of the magnified s-polarization spectrum. The intensity ratio 5824



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standing in thick films. Tilting configurations are known on several meal surfaces.12,22,23 The configuration may have some relation to the interaction between the IPS plane wave and the π-orbital, though the mechanism requires further considerations. The situation is different from that of C6F6/Cu(111), in which only an unoccupied σ-orbital of the flat lying molecule can couple with IPS of the substrate.8 We focused only on the formation of the Ln* state, while the IPS2 orbital, formed on the film, also interacts with the molecular film. The interaction is reflected in the energy separation of 0.3 eV between IPS1 and IPS2 levels. The effect of the interaction on the electronic excitation process is not identified in our photon energy region. It is interesting that the mixed state Ln* and IPS2 appear at different energies. There should be several modes of interaction between the molecular film and the IPS. Detailed discussions on the molecular configuration and the mechanism of the IPS interaction require for further investigations.

molecule within the lifetime enhances the transition probability between the vibrational ground states. 3.3. IPS Mediated Excitation of Ln. The IPS1 on HOPG is populated by the indirect transition in which the electron momentum is not conserved.5 Thus, the intensity of the peak is weaker than that on noble metal surfaces such as Cu(111). Our preliminary angle resolved measurement shows that the dispersion of the Ln peak is not significant. Then transition to the IPS1 component in the Ln* state is allowed at any electron momentum. Released from the constraint of the indirect transition, the transition probability to the IPS1 component in the Ln* state becomes higher than that for the pure IPS1. As a result, the Ln peak becomes stronger than the IPS1 peak. The asymmetric intensity variation of the HOMO and Ln peaks around the resonant photon energy is denoted as intensity switching in section 3.1. Similar asymmetric behavior at the Si(001) surface was recently analyzed by two-state double-continuum Fano resonance.19 It is mentioned in ref 19 that the intensity switching for a PbPc film on HOPG5 can be described by the model. Taking the analysis into the present case, the HOMO level degenerates with the continuous valence band of the substrate and both the Ln level and the IPS1 degenerate with the conduction band of the substrate. The asymmetric intensity variation is the result of the quantum interference among the transition pathways. According to the model, the optical transitions from the HOMO-derived level to the Ln and Ln* levels should be coherent. Incoherent electron transfer from IPS1 to Ln cannot explain the intensity switching. The model also suggests that only the occupied substrate band degenerating with the HOMO level contributes to the Ln enhancement. Thus, the Ln peak does not appear at a photon energy largely deviated from the HOMO-Ln resonance. The Ln peak is really absent in the spectrum with the photon energy of 4.23 eV. Though the HOMO and Ln levels are within the band gap of the band structure of graphite at the Γ-point,5 they can degenerate with the substrate bands. Because the levels have no dispersion, they can interact with substrate states of any electron momentum. The interaction of the IPS on the substrate with molecular unoccupied levels drastically changes in molecular systems. In the sub-ML films of PbPc on HOPG, molecules are randomly distributed without forming molecular crystallites and scatter the IPS1 electrons. The mean free path of the IPS1 electron becomes short, resulting in the broadening of the IPS1 peak. When a well-ordered 1 ML film is formed, the IPS1 peak is quenched and the sharp IPS peak on the film is observed.20 In the rubrene case, molecules do not simply scatter the IPS1 electrons but form the mixed state. As the plausible scenario shown in Figure 4b, the IPS1 wave function is penetrating into the islands of molecules forming the mixed state. The reason for the difference between PbPc and rubrene is not clear at the present stage. The p-to-s intensity ratio in Figure 5 is as large as a factor of 9. The ratio is typically a factor of 3 to 4 when the transition dipole moment is not aligned.21 The high intensity ratio indicates that the HOMO-to-Ln transition dipole is well aligned perpendicular to the surface. Assuming that the excitation is derived from the π−π* transition of the tetracene backbone of rubrene, the polarization dependence suggests that the tetracene backbone is aligned in a tilted configuration. In a preliminary experiment for films thicker than 2 ML, the p-to-s ratio further increases with the thickness. The increasing ratio suggests that the tilted molecule in the 1 ML film becomes

4. CONCLUSION In conclusion, we have shown that the IPS1 on the substrate enhances the electronic excitation of adsorbed molecules by an order of magnitude. The excitation process is not limited to the present rubrene/HOPG case. By clarifying the mechanism, the excitation process is expected to be useful to highly enhance the efficiency of organic molecular devices and light conversion processes. The energy of IPS is generally governed by the work function. It may be possible to tune the IPS level nearly resonant to an unoccupied level of organic films. This provides a way to tailor the electronic excitation efficiency.

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AUTHOR INFORMATION

Corresponding Author

*E-mail address: [email protected]. Phone: +81(0)6 6850 6082. Fax: +81(0)6 6850 5779. Present Address

† On visit from Department of Chemistry, Seoul National University, Seoul 151-747, Korea.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge J. Takeya of Osaka University for supplying the rubrene sample. The DFT calculation was performed by Y. Kitagawa and M. Okumura of Osaka University. This work was partly supported by a Grant-in-Aid for Scientific Research from JSPS (22350009). J.P. is grateful to the GCOE program of Osaka University and to S. K. Kim of Seoul National University for giving her the chance to join this work.

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REFERENCES

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