Article pubs.acs.org/JPCC
Cite This: J. Phys. Chem. C 2017, 121, 27399−27405
Energy Level Alignment of Organic Molecules with Chemically Modified Alkanethiolate Self-Assembled Monolayers Masahiro Shibuta,† Munehisa Ogura,‡ Toyoaki Eguchi,‡,§ and Atsushi Nakajima*,†,‡ †
Keio Institute of Pure and Applied Sciences (KiPAS), and ‡Department of Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan S Supporting Information *
ABSTRACT: We have employed two-photon photoemission spectroscopy to nondestructively resolve the unoccupied energy levels of fullerene C60 molecules deposited on alkanethiolate selfassembled monolayers (SAMs). By fluorine substitution of the hydrogen atoms in the alkyl chain, the work function (WF) increased from 4.3 eV for the alkanethiolate-SAM (H-SAM) to 5.7 eV for the fluorine-substituted SAM (F-SAM), owing to the formation of surface dipole layers. When C60 is deposited on the H-SAM and F-SAM, the energy positions of the unoccupied/ occupied levels of C60 are pinned to the vacuum level (Fermi level (EF) + WF). As a result of the energy level alignment, on the F-SAM, the relative energy from EF of the highest occupied molecular orbital of C60 almost equals that of the lowest unoccupied molecular orbital, implying that the C60 film on the F-SAM exhibits both p- and n-type (ambipolar) charge transport properties, while C60 is known as a typical n-type semiconductor. The energetics are preserved even with multilayered C60 films at least up to ∼5 nm in thickness, showing that the dipole layers induced by SAMs are robust against molecular overlayers. Such a spectroscopic study on the energy levels for organic films will be of importance for further development of organic thin film devices.
1. INTRODUCTON Self-assembled monolayers (SAMs) are one of the promising nanoscale functional films because the ultimately thin and molecularly ordered monolayer films fabricated with a one-pot chemical process show freedom for chemical modifications.1−3 Since interfacial electronic properties and the strength of chemical interactions can be widely designed by the substitution or functionalization of the hydrogen atoms comprising the SAMs, the entire class of SAMs has been suggested to be applicable to organic electronic devices as well as to biosensing.4−10 The most fundamental SAMa are the alkanethiolate-SAMs, whose sulfur atoms strongly anchor onto the Au(111) surface while directing the alkyl chain to vacuum.1−3,11 Owing to the insulating effect of the alkyl comb, alkanethiolate-SAMs have been suggested to be applicable to organic electronic devices, especially as gate insulators of organic field effect transistors (OFETs),7−10 which are fabricated by deposition of active organic semiconducting layers and metallic source/drain electrodes onto the SAMs. In OFETs, when a hole or electron is injected into the active layer, it transfers between the source and drain electrodes, through the occupied and unoccupied levels of the organic molecules (e.g., the highest occupied molecular orbital (HOMO) or the lowest unoccupied molecular orbital (LUMO)). Therefore, the functionality of OFETs is governed by energy positions with respect to the Fermi level (EF), where the OFET functionality is generally tuned by a selection of organic molecular species © 2017 American Chemical Society
(p-type (pentacene, rubrene, etc.) or n-type (C60 fullerene, TCNQ, etc.)). Furthermore, it has been demonstrated that charge transport properties of organic active layers (e.g., p-type or n-type in OFETs) can be tuned by the chemical modification of the alkyl chains, which changes the surface dipole moment of SAMs. 12−14 In order to understand and control the functionalities of nanodevices with SAMs, including OFETs, it is important to spectroscopically investigate both the occupied and unoccupied energy levels of organic molecules deposited upon SAMs: HOMO and LUMO located below and above the EF, respectively. The occupied levels of organic films have been extensively studied by ultraviolet photoelectron spectroscopy (UPS),15−18 while unoccupied levels have generally been identified by inverse photoemission spectroscopy (IPES),15−22 which probes photoluminescence by the relaxation of electrons irradiated into the unoccupied levels of surfaces. Although IPES measurements for organic films are accompanied by sample degradation due to electron irradiation20−22 and limited energy resolutions of about 0.1 eV, experimental difficulties in IPES have noticeably been improved.21,22 However, there still are fewer examples of IPES studies for unoccupied states of organic films, compared to the well-established UPS. Received: August 10, 2017 Revised: November 14, 2017 Published: November 14, 2017 27399
DOI: 10.1021/acs.jpcc.7b07955 J. Phys. Chem. C 2017, 121, 27399−27405
Article
The Journal of Physical Chemistry C
standing-up alkanethiolate-SAMs formed on the Au(111) single crystal without penetration.37,38 Because of the formation of molecularly ordered H- and F-SAM, in the present study, it is reasonably considered that the C60 molecules stay on top of the surfaces, although they might penetrate into the SAM when a less-ordered SAM is used.39 2.2. 2PPE Measurements. In 2PPE measurements, a frequency-tripled femtosecond titanium sapphire laser (COHERENT Mira-900F; 76 MHz, 150 fs, p-polarized) was used as an excitation source, where the photon energy (hν) was tunable from 3.96 to 5.17 eV. The laser was focused by a concave mirror (f = 400 mm) onto the sample in the UHV chamber through a MgF2 window. Note that, in the present 2PPE experiment, single-color 2PPE was employed, where the incident photon works as both pump and probe photons. The power of the incident laser was carefully controlled to be less than 0.13 nJ/pulse to avoid the degradation of samples; indeed, no spectral changes were recognized during energy scans. Photoelectrons emitted from sample surfaces were detected by a commercial hemispherical electron energy analyzer (VG SCIENTA, R-3000). UPS measurements were also performed by a He discharge lamp (He Iα, hν = 21.22 eV). The total energy resolutions in both 2PPE and UPS measurements were better than 30 meV. The sample temperature was kept to 293 K during all measurements.
Along this context, two-photon photoemission (2PPE) spectroscopy is a versatile method to resolve the unoccupied levels of organic films in a nondestructive manner with higher energy resolution (99%, Figure 1a) or 1H1H2H2H-
3. RESULTS AND DISCUSSION 3.1. Electronic Structures of the H-SAM and the FSAM. Figure 2 shows the UPS spectra for bare Au(111), the H-
Figure 1. Molecular structures of (a) decanethiol (CH3(CH2)9SH) and (b) 1H1H2H2H-perfluoro decanethiol (CF3(CF2)7(CH2)2SH). Schematic surface structures of (c) H-SAM and (d) F-SAM.
perfluoro decanethiol (CF3(CF2)7(CH2)2SH; Aldrich, >99.0%, Figure 1b) for about 20 h to form the H-SAM (Figure 1c) or the F-SAM (Figure 1d), respectively. The samples were rinsed by ethanol and introduced into the UHV chamber through a load lock for the 2PPE and UPS measurements. The qualities of the samples were evaluated by X-ray photoelectron spectroscopy (XPS), which detected no unexpected contaminations on both SAMs (see Supporting Information Figure S1). It is known that the molecularly ordered structures of both the H-SAM and the F-SAM can be obtained with the same procedures, which have been evaluated by scanning probe microscopy as well as various spectroscopic methods;32−36 in this study, the formation and the cleanliness of the SAMs were confirmed through 2PPE measurements as mentioned below. C60 molecules (Aldrich, sublimed, 99.9%) were deposited in an UHV environment using a quartz effusion cell. The deposition rate (0.07 monolayer-equivalent (MLE)/min) was monitored by a quartz microbalance. It is known that organic molecules and organometallic complexes physisorb on the well-packed
Figure 2. UPS spectra for bare Au(111) (bottom), H-SAM (middle), and F-SAM (top). The structures derived from the Au 5d-band attenuate by the formation of the H-SAM or the F-SAM. In contrast, the sigma bands due to alkyl chains appear from 3 eV (H-SAM) and 7 eV (F-SAM). The low-energy cutoff region is magnified on the left, showing a decrease (4.3 eV, H-SAM) and an increase (5.7 eV, F-SAM) of the WF from that of bare Au(111) (5.5 eV).
SAM, and the F-SAM, for which the electron binding energy is indicated with respect to the EF. For the Au(111) substrate, spectral features derived from the Au 5d-band are observed at around 3−6 eV. In the left panel of Figure 2, the spectra show low-energy cutoffs at around 15.7 eV of the electron binding energy, which corresponds to the WF. Although there are some reports of a slightly lower WF (5.3 eV),40 the WF for bare Au(111) is 5.5 eV, which is in agreement with the literature for 27400
DOI: 10.1021/acs.jpcc.7b07955 J. Phys. Chem. C 2017, 121, 27399−27405
Article
The Journal of Physical Chemistry C a clean Au(111) surface,41−43 exhibiting a clear Shockley surface state, which is characteristic of noble metal (111) surfaces.44 By the formation of the H-SAM or the F-SAM, the spectral features originating from the Au 5d-band completely attenuate, indicating that densely packed molecular SAMs are formed on the Au(111)surfaces. For the H-SAM, a broad σband of the alkyl chains appears at a threshold energy of ∼4 eV, whereas the corresponding band shifts to a higher binding energy of ∼7 eV for the F-SAM. The energy shift suggests that the electrons in the σ-band of the alkyl chains are drawn in by substituted fluorine atoms, due to their high electron negativity.35,45 Furthermore, compared to the WF of the Au(111) surface obtained in the present study (5.5 eV), the WFs are largely changed by SAM formations (Figure 2 left); the WF decreases by −1.2 eV (WF = 4.3 eV) in the H-SAM, while it increases by +0.2 eV (WF = 5.7 eV) in the F-SAM. Although the amount of WF change seemingly depends on the kind of Au substrates (bulk35,45,46 or deposited film47,48), the WF change originates from the formation of surface dipole layers, induced by the upright molecular arrangements of SAMs. In the H-SAM, the WF decreases due to a upward surface dipole, whereas in the F-SAM, the WF increases due to a downward surface dipole, as indicated in Figures 1c and 1d. The WF difference between the H- and F-SAMs is 1.4 eV in total, which is consistent with the previous work, which uses the Kelvin probe method.49 In addition to the occupied electronic states and WFs, the unoccupied electronic states of SAMs are identified by 2PPE. Figure 3 compares the 2PPE spectra (hν = 4.33 eV) for bare
the occupied states derived from the Au 5d-band components.42,43 The unoccupied states of the H-SAM have already been extensively studied with 2PPE;29,30,43 thus, the 2PPE results are briefly summarized here: as seen in the middle of Figure 3, there are two unoccupied states at EF + 3.6 and 3.8 eV, labeled C and IPS respectively.30,43 The C state has been assigned to an electronic state derived from the Au−S chemical bond, whose wave function is localized predominantly at the interface. Therefore, the appearance of the C state guarantees that the sulfur atom in the alkanethiolate has anchored upon the topmost Au atom with strong chemical interactions. The IPS is assignable to the first (n = 1) image potential state formed on the SAM, where the bound state is formed by a Coulomb interaction between a photoexcited electron and its positive image charge. Note that both the IPS and C states can be identified from the angular dispersion results obtained by 2PPE and light polarization dependence shown in the Supporting Information (Figures S2 and S3).29,30,43 Since the IPS is observed only at atomically flat surfaces,21 the observation of the IPS indicates that the H-SAM on the Au(111) surface is atomically flat. In the case of the F-SAM (top in Figure 3), a peak appears at EF + 3.6 eV, whose energy position is identical to that of the C state observed with the H-SAM. Moreover, from the hνdependent 2PPE spectra (see Supporting Information, Figure S4), the peak is assignable to an unoccupied state. Since the Au−S chemical bond is commonly formed regardless of fluorine substitution in the alkyl chains, the peak at EF + 3.6 eV originates from the C state formed in the F-SAM. Note that the first and second neighboring alkyl carbons from the thiolate sulfur are “not” fluorinated (see Figure 1b). In fact, similar to the C state for the H-SAM, the peak is insensitive to the polarization of the incident photon (Supporting Information, Figure S4). Judging from the formation of the C state and the increase of WF, it can be safely concluded that a molecularly ordered F-SAM was prepared. Furthermore, the absence of the IPS for the F-SAM is reasonable because the energy of the IPS is pinned to the vacuum level, Evac (= EF + WF);25 the IPS on the F-SAM is expected to be located at EF > + 5 eV, which is inaccessible with the present hνs. 3.2. Energy Levels of C60. With the C60 deposition on the H-SAM and the F-SAM, the occupied and unoccupied energy levels of the deposited C60 are investigated by UPS and 2PPE measurements. In general, the energy positions are governed by the characteristics of the overlayer organic molecules: their electron transport (n-type) or hole transport (p-type) properties. Figure 4 shows the UPS spectra magnified below EF for the H-SAM and the F-SAM, before and after C60 depositions. The C60 deposition amount is 0.5 MLE. By C60 deposition, two notable peaks appear respectively at EF − 2.7 and − 4.0 eV for the H-SAM and at EF − 1.3 and − 2.7 eV for the F-SAM, where the WFs of both the H-SAM and the F-SAM are almost unchanged from the original positions (±
