Spectroscopic Investigation of Unoccupied States in Nano- and

Article pubs.acs.org/JPCC

Spectroscopic Investigation of Unoccupied States in Nano- and Macroscopic Scale: Naphthalene Overlayers on Highly Oriented Pyrolytic Graphite Studied by Combination of Scanning Tunneling Microscopy and Two-Photon Photoemission Takashi Yamada,* Mio Isobe, Masahiro Shibuta, Hiroyuki S. Kato, and Toshiaki Munakata Department of Chemistry, Graduate School of Science, Osaka University, Toyonaka 560-0043, Japan S Supporting Information *

ABSTRACT: We have clarified the correlation between geometric and electronic structures of naphthalene ultrathin films on graphite using a combination of scanning tunneling microscopy (STM) and two-photon photoemission (2PPE) spectroscopy. Depending on the geometrical superstructure, as characterized by STM, shifts in the first image potential states are observed by STM-based local spectroscopy on the nanometer scale, which is consistent with coverage-dependent 2PPE spectra measured on the macroscale. An adsorption-induced unoccupied feature, which is specific to the (2√3×2√3) R30° superstructure, is detected at submonolayer coverages and is assigned to the lowest unoccupied molecular orbital (LUMO) derived level. It is interesting that the LUMO feature disappears for multilayer films. These behaviors indicate that a drastic change in electronic states occurs at the organic/metal interface associated with the change in the geometric structure.

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INTRODUCTION The interaction between organic molecules and solid surfaces has attracted wide interest not only for fundamental surface science but also for technological applications. An accurate and detailed knowledge of this interaction will help enhance applications for organic molecule-based electronic devices, such as light emitting devices, organic field effect transistors, and organic photovoltaic cells.1−3 A crucial feature is that electronic states at these interfaces are strongly influenced by the geometrical structures of adsorbed molecules and vice versa. Organic ultrathin films with monolayer or submonolayer thickness have attracted increasing attention wherein the geometry of molecules is governed by complicated interplay between substrate−molecule and intermolecular interactions.4 Therefore, it is important to clarify the correlation between adsorbed structures and their electronic structures by making one-to-one correspondence between them. In organic devices, charge injection/ejection barriers from electrodes to organic layers are one of the key parameters to determine the efficiency of performance.1−3 Therefore, it is a demanding task to carefully investigate both occupied and unoccupied states in the vicinity of the Fermi level (EF) at organic/metal interfaces. For this purpose, ultraviolet photoelectron spectroscopy (UPS) is often used to obtain fruitful information on the occupied states of organic thin films.4,5 In contrast, it is still a challenging task to measure unoccupied states. Combined with UPS, inverse photoemission spectroscopy (IPES) has been adopted as a useful tool to investigate the transport gap for organic thick films.1,5 However, this is not always suitable for organic ultrathin films, which often include nanometer scale structures. This is because nanostructures © 2013 American Chemical Society

formed by organic molecules are typically so weak that they are easily damaged by the probe electron beam of IPES. Recently, a pulsed laser-based technique, two-photon photoemission (2PPE) spectroscopy, has been applied to various molecular adsorption systems and organic ultrathin films due to its high energy resolution and nondestructive nature.6,7 In 2PPE, a femtosecond laser is focused on the sample to induce photoemission from occupied and unoccupied states, and time-resolved pump−probe techniques enable us to understand carrier dynamics at the organic/metal interfaces in real time. By reducing the diameter of the probe laser down to the diffraction limit, 2PPE can be used as a “microscope” by mapping photoemission intensity from occupied/unoccupied electronic states with spatial resolution on the order of submicrometers.8,9 Furthermore, by combining with photoelectron emission microscopy (2PPE-PEEM), one can directly know the correlation between adsorbed structures and their electronic states with lateral resolution better than 50 nm.10 As an extreme case, to minimize the analytical area down to the nanometer scale, scanning tunneling microscopy (STM) can be a powerful tool to investigate adsorbed structures in real space at a molecular and atomic level.11,12 As a local spectroscopic technique, scanning tunneling spectroscopy (STS) is often employed to measure the electronic states in the vicinity of the Fermi level. More recently, a distance− voltage measurement (zV spectroscopy) was successfully Received: October 2, 2013 Revised: December 16, 2013 Published: December 19, 2013 1035

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employed to measure the electronic states far from the Fermi level.13−16 In this context, application of STM to local spectroscopy is necessary to understand the correlation between geometrical structures and their electronic structures at a nanometer scale. However, in local spectroscopies, an “observer effect” is inevitable, such as the STM tip apex enhanced effect and/or the effect of the electric field between the tip and sample. By comparing with 2PPE spectroscopy, these effects should be carefully examined to establish the validity of local spectroscopies. In this study, we have investigated the correlation between adsorption geometries and their corresponding electronic structures using low temperature STM and 2PPE, for clarifying the one-to-one correspondence between geometric and electronic structures. Naphthalene overlayers on highly oriented pyrolytic graphite (HOPG) were used as a typical model for a weakly adsorbed system. In our previous STM measurements, we clarified two kinds of superstructures, for submonolayers and multilayers.17 For each superstructure characterized by STM, zV spectroscopy was performed to detect unoccupied electronic states on the nanometer scale. The combination of 2PPE and zV spectroscopy makes it possible to understand the correlation between geometric and electronic structures at the nano- and macroscopic scale.

Figure 1. (a) STM image at the initial stage of naphthalene adsorption. The HOPG substrate and island structure of naphthalene are imaged in one frame. Scan size is 30 × 30 nm2, taken at a sample bias (Vs) of −3.09 V and a tunneling current (It) of 100 pA, respectively. (b) A magnified image of a molecular island. A defect-free ordered layer is observed with a periodicity of (2√3×2√3) R30°. Carbon frameworks of naphthalene and a unit cell are shown in the image as a guide to the eye. 5.4 × 5.4 nm2, Vs = −3.18 V, It = 100 pA. (c) Large area STM image of a multilayer film. 164 × 164 nm2, Vs = −4.00 V, and It = 100 pA. (d) Magnified image taken on a wide ⎛ 3 − 1⎞ ⎟ is observed on terrace. Superstructure with a periodicity of ⎜ ⎝2 3 ⎠ each layer. 5.4 × 5.4 nm2, Vs = −3.40 V, It = 200 pA.

(2√3×2√3) R30°.17 For multilayer films, layer growth of naphthalene is confirmed as shown in Figure 1c. On a wide terrace of each layer, the superstructure described by the matrix ⎛ 3 −1⎞ ⎟ is observed, as shown in the magnified image in of ⎜ ⎝2 3 ⎠ Figure 1d. Molecular densities are 1.60 nm−2 for the submonolayer and 1.76 nm−2 for the multilayer film, respectively. In a series of STM measurements, superstructures with different rotational domains are observed. This is ascribed to the polycrystalline nature of the HOPG substrate. 3.2. Coverage-Dependent 1PPE and 2PPE. Figure 2 shows coverage-dependent 1PPE and 2PPE experiments conducted to explore the correlation between geometrical structures and corresponding electronic structures. Surface coverage is determined from the coverage-dependent change in the work function, wherein an ∼80 meV linear increase in the work function is observed, associated with an increase in coverage up to the saturation of the first layer.19 In 2PPE, electrons excited in the first image potential states (IPS(n = 1)) can be utilized as a sensitive probe to estimate the adsorbate coverage, where surface coverage can be estimated from the energy of the IPS(n = 1) peak on the molecular film.20−23 In this study, one monolayer (ML) coverage as estimated from the change in the work function is consistent with completion of the IPS(n = 1) on a monolayer film. Furthermore, a shift of the IPS(n = 1) position toward higher energy at the completion of the monolayer film is consistent both in 2PPE and in STMbased local spectroscopies. Thus, a common definition of the coverage can be used both in 2PPE and in STM experiments. It is noteworthy that the total amount of naphthalene dosage for completion of a monolayer film is nearly identical to the case on the Cu(111) substrate where the coverage is also titrated by the change in the work function and energy shift of the IPS(n = 1).18 In terms of the basic concepts of 2PPE excitation processes with respect to the Fermi level (EF = 0), a first photon (pump photon) excites an electron from an occupied state (Ei (0)). The second

2. EXPERIMENTAL SECTION All STM measurements were carried out in an ultra high vacuum (UHV) chamber equipped with a low temperature STM.18 The base pressure of the chamber was less than 1 × 10−10 Torr. All STM images were acquired in the constant current mode at 80 K with an electrochemically etched W tip. Photoemission experiments were performed in another UHV chamber with a base pressure less than 1 × 10−10 Torr, and all measurements were conducted at 90 K. For the 1PPE (UPS) experiments, He−I radiation (hν = 21.2 eV) was used as a light source. For 2PPE measurements, the light source was ppolarized third harmonic output of a titanium sapphire laser operated at a pulse duration of 100 fs and a repetition rate of 76 MHz, respectively. To avoid photoinduced desorption and destruction of adsorbates, the power of the incident light was suppressed to be less than 0.13 nJ/pulse. Emitted photoelectrons were detected with an angle-resolved hemispherical energy analyzer. In a series of experiments, the HOPG substrate was cleaved in air and heated in UHV at 670 K for 60 h. For clean surfaces, atomically resolved STM images were obtained, and photoemission spectra showed no trace of impurities with a work function of 4.45 eV.9,10 Gaseous naphthalene was deposited on clean HOPG surfaces via pulsed dosers at a substrate temperature of about 150 K.18 3. RESULTS AND DISCUSSION 3.1. Superstructures of Naphthalene Overlayers on HOPG. Figure 1a shows a large area STM image taken at the initial stage of naphthalene adsorption, showing that island structures formed by the molecules coexist with the bare HOPG substrate. Island formation at the initial stage is different from adsorption of naphthalene on the Cu(111) surface, where molecule−substrate interaction is stronger than that on HOPG and isolated single molecules are dispersed on the surface.18 In Figure 1b, an ordered superstructure within the molecular island is shown where molecules are imaged as bright protrusions. The ordered layer has a periodicity of 1036

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Figure 3. Intermediate energies for 2PPE spectroscopic features in Figure 2a are plotted as a function of photon energy. Blue ○ lie on the line of slope one and are assigned to an occupied state. Other features are independent of photon energy and originate from unoccupied states.

Figure 2. (a) Coverage-dependent 2PPE spectra taken at a photon energy of 4.33 eV, except for the topmost one at 4.54 eV. The horizontal axis indicates an intermediate energy with respect to the Fermi level. Dotted lines with open squares indicate the energy positions of IPS(n = 1). Other spectroscopic features indicated by open circles and diamonds are adsorption-induced states of naphthalene. (b) Coverage-dependent 1PPE spectra taken at a photon energy of 21.2 eV (UPS). At coverages above 1 ML, adsorptioninduced occupied states (blue ○) are observed on the higher energy side of the HOMO peak.

E = Evac −

⎛ ε − 1 ⎞2 0.85 ⎜ ⎟ (eV) ⎝ ε + 1 ⎠ (n + a)2

(1)

where n (=1, 2, 3...) is the quantum number of IPS and a (≥0) is the quantum defect parameter. For clean metal surfaces including HOPG, the IPS(n = 1) energy position is typically 0.85 eV below the vacuum level.6,7 On the surface with an adsorbed layer, it can be assumed that the dielectric medium (ε > 0) is sandwiched between the metal and vacuum, and this results in the shifting of the IPS energy position closer to the vacuum level. Electrons excited in the IPS (IPS electrons) are confined perpendicular to the surface but disperse parallel to the surface as nearly free electrons. Thus, IPS shows parabolic energy-momentum dispersion with an effective electron mass of meff = ∼me, where me indicates the mass of free electrons.6,7 In our 2PPE measurements, all assignments for IPS peaks are based on angle-resolved 2PPE measurements, as typically represented in Figure 4. In our coverage-dependent 2PPE measurements, shown in Figure 2a, IPS(n = 1) for the clean HOPG surface is observed at 3.60 eV above the Fermi level (EF + 3.60 eV) and is labeled as IPSclean. The measured work function of the clean HOPG surface is 4.45 eV; therefore, the IPS(n = 1) energy position is located 0.85 eV below the vacuum level. This is in agreement with the energy position estimated from eq 1 with quantum number n of 1 and quantum defect parameter a of 0. This is also in agreement with previous 2PPE and IPES studies.25−29 From 0.4 ML, an additional IPS(n = 1) peak, labeled IPS1ML energy position from that of IPS2ML. On the other hand, weak unoccupied peaks at EF + 3.5 eV to EF + 3.7 eV in the intermediate energy scale show dispersionless behavior, and can be assigned to unoccupied states induced by adsorbed molecules, rather than IPS related states, as discussed below. 1038

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and they were also observable by using s-polarized light (see the Supporting Information for details). Furthermore, the peak intensities of the LUMO for the (2√3×2√3) R30° phase rapidly increased at higher photon energies. Judging from the experimental results mentioned above, excitations to the LUMOs are mainly by intramolecular excitations, not attributed to photoinduced electron transfer from the HOPG substrate or charge transfer excitons reported on other systems.34,35 It should also be noted that the LUMO for multilayer film is composed of some smaller peaks, similar to the case of the LUMO for the (2√3×2√3) R30° phase. This is also indicative of vibrational progressions by the LUMO for the multilayer film, because peak energy interval is comparable to the LUMO for the (2√3×2√3) R30° phase. 3.3. Local Spectroscopy on Nanometer Scale. To understand the electronic structures on the atomic and molecular scale, local spectroscopy is employed using STM. With its high spatial resolution, STM is often used as a powerful spectroscopic tool to evaluate the electronic structures on the nanometer scale.12 In this study, our aim was not only to clarify the one-to-one correspondence with adsorbed superstructures but also for comparison with the results of 2PPE to comprehend unoccupied states probed on the macroscale. As one of the local spectroscopic methods, STS is often employed to measure the electronic states in the vicinity of the Fermi level. Under the operation of general STS, the change in the tunneling current is recorded as differential conductance (dI/dV) at constant tip−sample distance. To detect a slight change in the tunneling current, sufficient amount of current on the order of nA is often required. During the STS measurements, the feedback loop needs to be turned off, causing difficulty to keep a constant tip−sample distance. STS is generally known as a powerful method for local spectroscopy; however, it is not always suitable to measure the energy range away from the Fermi level.16 This is because of high tunneling current flow under high bias voltage, as predicted by the Ohmic law. In particular, STS is hardly applied for weakly physisorbed systems because the tunneling current sometimes induces a change in the tip apex, destruction of superstructures, change in molecular configuration, diffusion, and dissociation of adsorbed molecules. To avoid the unintentional effects mentioned above, zV spectroscopy has been successfully employed to measure the electronic states located away from the Fermi level.13−16 In this spectroscopic mode, the tip−sample distance (z) is measured as a function of the sample bias voltage (V) with the feedback loop engaged. Under the operation of zV spectroscopy, the STM tip retracts away from the surface with increasing sample bias. When electron tunneling occurs resonantly through specific electronic states, the increase in the tip−sample distance is observed to maintain constant current under feedback operation. With a small cost of the spatial resolution of STM, zV spectroscopy can be successfully employed to avoid a change in the tip apex and high tunneling current flow, which induce the unexpected effect. To confirm the validity of our experimental setup, zV spectroscopy was conducted on the HOPG surface. Figure 5a shows a set of spectra, which record the tip sample distance as a function of sample bias voltage, and the red line shows a spectrum averaged over some tens of raw data. During zV spectroscopy, a change in the STM image was not observed after the sample bias sweep. This guarantees that any change in the tip apex is negligible in our experimental setup. The

Figure 5. STM zV spectroscopy performed on a clean HOPG surface. (a) Distance between the tip and sample (z) is recorded as a function of sample bias voltage (V) with the feedback loop engaged. Tunneling current is set to 20 pA. The red line is an average of several tens of raw data (black lines), and its numerical differentiation (dz/dV) is shown in (b). (c) dz/dV spectra taken with different tunneling currents. IPS(n = 2) is remarkably influenced by the electronic fields applied between the tip and sample as well as the tip apex. (d) Schematic description of an STM tip in a tunnel contact with a HOPG surface at positive sample voltage of V, where resonant electron tunneling occurs through higher IPSs (dotted red arrow). The energy positions of IPS(n ≥ 2) are influenced more by the electric field, because higher members of IPS levels are closer to the upper part of the potential.

reproducibility of the data was also confirmed by checking zV traces obtained by decreasing the sample bias voltage. Numerical differentiation of the averaged data was conducted and is shown in Figure 5b. A sharp peak at 4.2 eV in the dz/dV spectra can be assigned to IPS(n = 1), and other higher members of IPS(n ≥ 2) are observed on the higher energy sides of IPS(n = 1). In Figure 5c, the effect of the electric field applied between the tip and the sample is examined by changing the tunneling current. The distance between the tip and sample becomes close at a higher tunneling current condition, and this results in a worse signal-to-noise ratio. It should also be mentioned that IPS(n ≥ 2) is largely influenced not only by the electronic fields applied between the tip and sample, but also by the day-to-day fluctuation of the tip apex condition as reported by Dougherty et al.14 As shown schematically in Figure 5d, higher members of IPS(n ≥ 2) are close to the vacuum level as compared to the IPS(n = 1). Therefore, IPS(n ≥ 2) is more remarkably influenced by the electrostatic potentials formed between the tip and the sample surface. 3.4. Comparison between 2PPE and STM-Based Local Spectroscopy. For each of the superstructures assigned in Figure 1, local zV spectroscopy is performed as shown in Figure 6. In a series of experiments, the tunneling current was set to 20 pA to minimize the effect of the electric field applied between the tip and the sample, and it was confirmed the zV spectroscopy is nondestructive even for the naphthalene films. Figure 6a and b shows the dz/dV spectra of unoccupied states taken on the HOPG substrate and on a large island of the (2√3×2√3) R30° phase, respectively. In Figure 6b, two peaks derived from the unoccupied states appear at 3.4 and 4.35 V above the Fermi level. By comparing the results of 2PPE, the peak observed at 3.4 V can be assigned to the LUMO for the 1039

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index surfaces of noble metals.14 In contrast, the difference in energy between two spectroscopies is smaller (∼0.35 eV) in the case of C6F6 on Cu(111).38,39 In zV spectroscopy for adsorbed layers, the degree of perturbation in the electric field presumably depends on the materials in a tunnel contact, screening effect by dielectric adsorbed layer,40 and so on. The quantitative understanding of the shift should be the subject of future study. As discussed in the previous studies,13,14 the presence of an electric field applied between the tip and the sample can be the main reason for the difference in measured energy. It is noteworthy that the relative peak positions of IPS(n = 1) on clean HOPG, the (2√3×2√3) R30° phase, and the multilayer film are reproduced well between 2PPE and STM-based spectroscopies. Furthermore, the LUMO peak appears only on the (2√3×2√3) R30° phase and not on multilayer film, which is consistently observed in both spectroscopies. As discussed above, we cannot determine the absolute energy values only from STM-based spectroscopy; however, the “observer effect” in STM-based local spectroscopy can be compensated by carefully comparing the 2PPE. It must be stressed that the combination of 2PPE and STM-based spectroscopy enables us to directly compare the geometrical structures and electronic states from a molecular or atomic viewpoint. Last, with regard to the difference in the IPS peak intensity between the 2PPE and zV spectroscopies, it should be pointed out that the IPS peaks are enhanced more than other adsorption-induced peaks in zV spectroscopy. This trend was also observed for the self-assembled monolayers on Au(111)16 and shows a marked contrast to the IPS peak intensities measured by 2PPE. This is because the wave function of the IPS electrons spill over toward the vacuum side; therefore, the IPS peaks are observed with enhanced intensity by the STM tip, which is probed from the vacuum side.

Figure 6. dz/dV spectra recorded on a (a) HOPG clean surface domain, (b) (2√3×2√3) R30° phase, and (c) multilayer phase, respectively. Tunneling current is set to 20 pA for all measurements. The scale bars in the STM images indicate 1 nm.

(2√3×2√3) R30° phase. The peak at 4.35 V is observed with a shoulder on the higher sample bias side, and assigned to IPS(n = 1) for the (2√3×2√3) R30° phase. The origin of this shoulder is not clear at present, but it can be related to the appearance of the IPS>1ML peak on the higher energy side of IPS1ML by 2PPE, but this is not clear from zV spectroscopy. To comprehend the unoccupied electronic structures measured on the nano- and macroscale, a direct comparison between the 2PPE and zV spectroscopies is necessary.13,14 To achieve this purpose, one should take into account the fact that both spectroscopies are conducted under different measurement conditions; STM-based local spectroscopy is conducted under a strong electric field applied between the tip and the sample on the order of 1 V/nm, that is, 109 V/m. Figure 7 summarizes the unoccupied peak positions observed in this study. The absolute values of the peak positions are given with respect to the Fermi level, and the energy positions are typically different, on the order of 0.6−0.8 eV, between the two spectroscopies. One might feel that this difference is large; however, this is comparable to the differences reported on low

4. CONCLUSIONS In this study, we have clarified the unoccupied electronic structures of specific geometrical structures by a combination of 2PPE, STM, and local spectroscopy using STM. Depending on the superstructures characterized by STM, local spectroscopy was carried out on the nanometer scale. Coverage-dependent shifts of IPS and adsorption-induced unoccupied states were detected by zV spectroscopy, which is also supported by coverage-dependent 2PPE measurements. The combination of both spectroscopic techniques enables us to elucidate one-toone correspondence between geometric and electronic structures on the nanoscale. This information will help us obtain a detailed description of the metal/organic interfaces, which should help improve the efficiency of organic moleculebased devices in the future.

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ASSOCIATED CONTENT

S Supporting Information *

Photon energy- and polarization-dependent 2PPE spectra, to discuss the excitation mechanism in 2PPE. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

Figure 7. Summary of energy positions of unoccupied states observed in this study shown with respect to the Fermi level. Differences of the order of 0.6−0.8 eV are observed between 2PPE and zV spectroscopy.

Notes

The authors declare no competing financial interest. 1040

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ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for Young Scientists (A) from MEXT Grant no. 24685004, and Grantin-Aids for Challenging Exploratory Research from MEXT Grant nos. 24656036 and 25600004.

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dx.doi.org/10.1021/jp4097875 | J. Phys. Chem. C 2014, 118, 1035−1041