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Photoexcited State Confinement in TwoDimensional Crystalline Anthracene Monolayer at Room Temperature Masahiro Shibuta,† Naoyuki Hirata,‡,§ Toyoaki Eguchi,‡,§,∥ and Atsushi Nakajima*,†,‡,§ †
Keio Institute of Pure and Applied Science (KiPAS), Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan Nakajima Designer Nanocluster Assembly Project, ERATO, Japan Science and Technology Agency (JST), 3-2-1 Sakado, Takatsu-ku, Kawasaki 213-0012, Japan § Department of Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan ‡
S Supporting Information *
ABSTRACT: Organic thin film electronics place a high demand on bottom-up technology to form a two-dimensionally (2D) functional unit consisting of a single molecular crystalline layer bound to a layered structure. As the strong interaction between a substrate and molecules makes it difficult to evaluate the electronic properties of organic films, the nature of electronic excited states has not been elucidated. Here, we study a 2D crystalline anthracene monolayer electronically decoupled by alkanethiolates on a gold substrate using scanning tunneling microscopy and time-resolved two-photon photoemission spectroscopy and unravel the geometric/electronic structures and excited electron dynamics. Our data reveal that dispersive 2D excited electrons on the surface can be highly coupled with an annihilation of nondispersive excitons that facilitate electron emission with vibronic interaction. Our results provide a fundamental framework for understanding photoexcited anthracene monolayer and show how the coupling between dispersive and nondispersive excited states may assist charge separation in crystalline molecular layers. KEYWORDS: self-assembled monolayer, two-photon photoemission, two-dimensional crystal, electron dynamics, excitons, organic devices rganic films have received significant attention because of their great potential in next-generation electronics for low-cost, flexible, and large-area devices.1−5 Organic electronic devices, such as light-emitting diodes, solar cells, and field-effect transistors, are usually composed of organic heterolayers, which govern their elemental functionalities. To increase the conversion efficiency for organic photovoltaic cells, it was recently suggested that electron delocalization can promote long-range charge separation.6−11 Employing such an effect requires an improved microscopic view of the evolution of excited states from dispersive to nondispersive behaviors, as well as the heterolayer formation. Since the organic heterolayers are generally fabricated by physical vapor deposition or spin-coating, it is difficult to control the crystallinity and molecular orientation in each organic layer. To this end, a bottom-up fabrication of organic layers utilizing a self-assembled monolayer (SAM) seems promising.12,13 A two-dimensional (2D) molecular layer consisting of alkanethiolate can be easily formed on a
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Au(111) substrate in a one-pot wet-chemical process (alkanethiolate-SAM), whose structure is secured by σ−σ interactions of alkyl chains anchored with Au−S chemical bonds.12−15 By substitution of the end hydrogen of an alkanethiolate-SAM directed to the vacuum with an organic unit, it is expected that desired surface functionalities in the 2D organic heterolayer can be appended without additional fabrication procedures. While the concept of substituted alkanethiolate-SAM has been applied so far in electrochemistry and biosensing,13,16−19 however, this technique has not yet been applied successfully for practical organic electronic devices. This is because a high degree of crystallinity of the substituted end groups, as well as the alkyl groups, which are crucial for a good carrier transport property in the 2D organic Received: March 2, 2017 Accepted: April 11, 2017 Published: April 11, 2017 4307
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reflecting the crystallinity of the 2D molecular heterolayer are evaluated using scanning tunneling microscopy (STM) and femtosecond time-resolved (TR-) two-photon photoemission (2PPE) spectroscopy, the latter of which detects photoexcited electrons or excitons as photoelectrons by a probe photon. The results reveal that an image potential state (IPS) and surface charge transfer excitons (S-CTEs) are formed at the atomically flat surface of Ant-C11-SAM, as well as an S1 exciton and free carrier electron in the lowest unoccupied molecular orbital (LUMO) confined in the 2D anthracene monolayer. The high crystallinity of the 2D layer allows the dispersive IPS electrons to be coupled with an annihilation of nondispersive S1 excitons, facilitating electron emission with vibronic interaction. We expect that the electronic characterization revealed here will contribute to reach ultimately designed organic electronic devices.
heterolayer, is partially lost due to the steric constraint of the end moieties.13,16,18 Straight π-conjugated oligoacenes (e.g., anthracene, tetracene, and pentacene) are good candidates for appropriate end groups because their planar and long molecular frameworks favorably suit the lattice constant of the densely packed underlying alkyl chains and are prospective classes of semiconductors for organic electronic devices.20−25 Since a methyl group can be introduced to carbon #2 in anthracene without disturbing the intermolecular π−π interactions, that is, 2-methylanthracene,26 the alkyl chain substitution of anthracene combines the intermolecular overlaps of the planar π-system along the a−b plane with the molecularly ordered alkyl comb, producing a favorable situation for realizing a high mobility of charge carriers in the ultimately thin 2D organic crystal. Here, we report the 2D crystalline structure and ultrafast electron dynamics of photoexcited states at an anthracene monolayer that is fabricated on an insulating alkyl layer by dipping a Au(111) substrate into a solution of 11-(anthracene2-yl)undecane-1-thiol [(C14H9(CH2)11SH (Ant-C11, Figure 1a)].27 Owing to the chemical connection between the anthracene moiety and alkanethiolate, 2D crystalline anthracene monolayers are formed stably even at room temperature (RT), which have been spectroscopically evaluated using spatially averaged structure information.27 The geometric/ electronic structures and non-equivalent electron dynamics
RESULTS AND DISCUSSION Surface Structure. Figure 1b,c displays the STM images for Ant-C11-SAM, showing rectangular and hexagonal unit cells, sharing almost equally the surface area. The rectangular phase with arect = 0.75 nm and brect = 0.52 nm resembles the molecular arrangement in the a−b plane of an anthracene crystal (a = 0.856 nm and b = 0.604 nm),28 where the individual molecules adopt a face-on-edge herringbone 2D arrangement. Compared with bulk anthracene, the anthracene moieties in Ant-C11-SAM are more densely packed along the surface parallel direction, which would lead to an upright orientation compared to that in the bulk crystal. The hexagonal phase with ahex = 0.91 nm and bhex = 0.62 nm aligns along the ⟨112̅⟩ direction on Au(111). Comparing the packing densities of bulk anthracene [0.258 nm2/molecule (a−b plane)] and the rectangular phase (0.195 nm2/molecule), it suggests that each protrusion is composed of a pair of anthracene moieties, resulting in a packing density of 0.244 nm2/molecule. Figure S1 shows both 2D arrangements schematically. The molecular orientation of Ant-C11-SAM was also characterized spectroscopically by infrared reflection absorption spectroscopy (IRAS), supporting consistently the anthracene moiety being upright (see Supporting Information and Figure S2).27 Furthermore, the IRAS peaks show that the high 2D crystallinity of the underlying alkyl layer is preserved at RT. From the STM and IRAS results, it can be safely concluded that a chemically connected 2D anthracene and alkyl crystalline heterolayer is formed on Au(111), which can be regarded as ultimately thin semiconducting and insulating organic heterolayers on the metal substrate with double molecular selfassemblies of π−π and σ−σ interactions (Figure 1d). The heterolayers are firmly connected by a chemical bond that is markedly different from the interaction resulting from a stepby-step vacuum vapor deposition. In fact, temperatureprogrammed desorption (TPD) for Ant-C11-SAM shows that the parent Ant-C11 signals can be observed above 400 K along with its anthracene fragments (Figure S3). Without the chemical bond, it is necessary to cool the substrate to a low temperature (90 K) to adsorb free anthracene molecules on an alkanethiolate-SAM. Note that the Ant-C11-SAM is stable at RT and tolerates a series of experiments with exposure to ambient air, reflecting the high crystallinity of the 2D anthracene layer. Indeed, no contamination, including oxygen, was detected in X-ray photoemission spectrum (XPS) even after air exposure for 10 days (Figure S4).
Figure 1. (a) Molecular structure of 11-(anthracene-2-yl)undecane1-thiol [C14H9(CH2)11SH: Ant-C11]. (b,c) STM images of AntC11-SAM; (b) rectangular phase (tip bias, Vt = +3.0 V, tunneling current, It = 5 pA) with arect = 0.75 nm and brect = 0.52 nm; (c) hexagonal phase (Vt = +2.5 V, It = 3 pA) with ahex = 0.91 nm and bhex = 0.62 nm. (d) Schematic drawing of semiconducting and insulating heterolayer with Ant-C11-SAM fabricated on Au(111). The photoexcited electronic states observed with 2PPE are also indicated. 4308
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Figure 2. (a) Single-color 2PPE spectra with various hν1 values (4.47−5.17 eV). The spectra are aligned with an excited state energy relative to EF. (b) Polarization dependence of 2PPE spectra at hν1 = 5.03 eV. The p-polarized photon has a magnetic field perpendicular to the surface (inset). (c) Angle-resolved 2PPE with hν1 = 5.03 eV, where the photoemission angles are indicated from the surface normal. The IPS shows clear band dispersion with an effective mass of the IPS electron of 1.1me for the lateral direction (me is the mass of free electron), whereas SCTEs have little dispersion. (d) Single-color 2PPE spectra with hν2 values of 2.9−3.5 eV. Magnified spectra at higher energy region (inset) show a peak at EF +2.5 eV with hν > 3.1 eV, where the colors of each spectrum correspond to those in the main panel. (e) Total 2PPE yield versus hν obtained from (d). The first and second intensity maxima originate from resonances of S0−S1 electrovibronic transitions of 0−0 and 0−1. The photoabsorption spectrum of Ant-C11 toluene solution is overwritten, exhibiting a similar electrovibronic feature with the 2PPE yield at a 0.2 eV blue shift. (f) Energy diagram of Ant-C11-SAM. The photoexcitation scheme of single-color (i and ii) and dual-color (iii) 2PPE measurements are indicated as vertical arrows.
Electronic Structures. The electronic states of the AntC11-SAM were characterized with 2PPE. Figure 2a shows single-color 2PPE spectra with various photon energies (hν) ranging from hν1 = 4.47 to 5.17 eV. A peak at 3.84 eV above the Fermi level (EF) originates from the first IPS formed outside the surface showing selectivity for p-polarized incident photons (Figure 2b) and a 2D-free electron-like dispersive nature parallel to the surface (Figure 2c).29,30 The IPS observation also guarantees the molecular uniformity from the viewpoint of electronic structures because IPS is observable only on an atomically flat surface.31,32 At hν1 > 4.6 eV, another peak strongly appears at EF +3.17 eV, and it is maximized at hν1 = 4.90 eV, implying a resonance electron−hole excitation involving occupied and unoccupied states. In fact, an ultraviolet photoelectron spectrum (UPS, Figure S5) shows a peak at EF −1.7 eV assignable to the highest occupied molecular orbital (HOMO) of the anthracene moiety,27 and therefore, it is reasonable that the peak at EF +3.17 eV appears from the resonance electron excitation from the HOMO with hν1 = 4.90 eV (≅3.17 + 1.7 eV). In addition, the peak shows little angular dispersion, indicating that the excited electron is localized parallel to the surface. It has been reported that very small energy dispersions of HOMO for πstacked oligoacene films are observed by angle-resolved UPS at
photoemission angles greater than 20° from the surface normal.33,34 Therefore, the Coulomb potential of a cogenerated hole in the HOMO in the anthracene layer would cause the localized characteristic of an excited electron in the unoccupied state at EF +3.17 eV. From the above results, the peak at EF +3.17 eV is assignable to a surface charge transfer exciton (S-CTE) formed on the 2D anthracene crystal, where the series of quantum states are formed by a Coulombic interaction between photoexcited electron−hole pairs separated by the anthracene−vacuum interface. Such S-CTEs have been identified by Zhu and coworkers in pentacene deposited on Bi(111) as CTE.35−37 At hν1 > 5 eV in this study, another peak is indeed observed at EF +3.49 eV, which is assignable to the second S-CTE (S-CTE2). The polarization dependence indicates that both of the electrons in S-CTE and S-CTE2 are bound in a vertical direction (Figure 2b), suggesting that they are attributable to 1s and 2s quantum states, respectively. Note that “S-CTE” is used here to emphasize that electrons are transiently bound by molecular holes at the surface as well as by the polarization of the organic surface, which is more specifically expressed by the basic concept of CTE.38 The energy of S-CTE from the vacuum level, −1.14 eV, is higher than that of pentacene or tetracene films (approximately 4309
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Figure 3. (a) TR-2PPE spectra using hν1 = 4.77 eV and hν2 = 3.18 eV, where the delay times are shown in the left-hand axis. At the positive (negative) delay, the photoemission is induced with hν2 (hν1) after the irradiation of hν1 (hν2). The bottom energy axis represents the excited energy with hν1, whereas the top is that with hν2. (b) Intensity traces at the energy of bottom axis in (a), EF +3.84 (IPS), 3.6 (EAAI-IPS), and 3.17 eV (S-CTE and S1) as a function of delay time. While the intensity evolution for IPS shows rise (250 fs) and decay (1100 fs) profiles, the lifetimes of S-CTE and S1 excitons are 110 and 2800 fs, respectively, assuming a single exponential decay function. Intensity profiles at the energy from IPS and S-CTE (S1) involving EAAI-IPS show the same decay dynamics in both positive and negative delay, which can be interpreted by autoionization of IPS electrons associated with the S1 annihilation (EAAI process).
0.9 eV),37 which is caused by the smaller dielectric constant of an anthracene crystal.39 The observation of S-CTEs is very specific to the crystallinity of the 2D anthracene monolayer on the insulator because it is hardly observed for anthracene molecules directly deposited on a graphite substrate (Figure S6). The excitations of IPS and S-CTE are schematically illustrated in Figure 1d. In addition to such surface-specific electronic states, molecular excitons localized in a 2D anthracene crystal could be identified by 2PPE using lower hν values (hν2 = 2.9−3.5 eV) (Figure 2d). The 2PPE intensity drastically increases by >100 times sharply at hν2 > 3.1 eV, where the energy is in agreement with the optical absorption threshold of an anthracene crystal.40,41 Furthermore, the plot of total 2PPE yield versus hν2 (Figure 2e) is highly correlated with the optical absorption spectrum of an anthracene crystal, where the peaks originate from the S0−S1 (ν″ = 0 to ν′ = 0, 1; (0−0), (0−1)) transition.40,41 The results show that the intense 2PPE signal at hν2 > 3.1 eV is attributable to the photogeneration of a S1 exciton, where an electron−hole pair in the anthracene moiety is formed by their Coulombic interaction, as illustrated in Figure 1d. The energy correspondence also confirms the 2D crystallization of anthracene at RT because the optical absorption in an organic crystal generally exhibits a red shift toward that in solution (shown in Figure 2e).40 With hν2 adequate to excite S1, a spectral feature appears at EF +2.5 eV (Figure 2d, inset), which is 100 times weaker than that of the S1 signal. Comparing the inverse photoemission for an anthracene film,42 it is assignable to the LUMO localized at an anthracene moiety (see Figure 1d),27 where an electron transiently occupies the LUMO (a negative polaron). Although the LUMO electron should be supplied from the photoexcitations of a substrate or overlayered molecule, the electron injection from the Au substrate into the LUMO is largely hindered by the insulating alkyl layer in this study. Since the LUMO
electrons are observed along with the S1 excitation, the free carrier electron would be generated by a self-dissociation of a hot exciton (Sn) resonantly formed via the S1 exciton with a two-photon process. Although the LUMO electron signal is weak due to the two-photon process, the hot exciton dissociation is representative of the photocarrier generation in a fullerene film observed by 2PPE.43 Consistently, LUMO electrons are clearly observed at a similar energy (EF +2.4 eV) (Figure S6) when anthracene molecules are directly deposited on graphite; electrons are efficiently injected from the substrate. The energy diagram of the observed excited states are summarized in Figure 2f. As illustrated in Figure 1d, the photoexcited states can be categorized into two states; (1) excited electrons are located on the surface (IPS and S-CTE) or in the anthracene 2D crystal moiety (S1 and LUMO), and (2) the excited electrons are bound with the simultaneously excited hole in HOMO (S-CTE and S1) or not (IPS and LUMO). Note that the T1 exciton formed by intersystem crossing, which should energetically distribute at 1.7 eV above the HOMO of anthracene,44 is not resolved here probably because the decay path of S1 is dominated by a much faster process of energy transfer to the substrate as discussed below. Photoexcited Electron Dynamics. To discuss the electron dynamics in a 2D anthracene crystal, Figure 3a shows the dual-color TR-2PPE spectra with various time delays between two photons of hν1 = 4.77 eV and hν2 = 3.18 eV. The hν1 excites to the IPS and S-CTE, whereas hν2 excites to the S1 exciton (Figure 2f(iii)). Then, the time evolution for these excited states appears in both positive (hν1 pump and hν2 probe for the IPS and S-CTE) and negative (hν2 pump and hν1 probe for the S1 exciton) delays. At the positive delay, the IPS and S-CTE appear at the energies obtained in Figure 2a. The intensity profiles (Figure 3b) are fitted by an IPS lifetime of 1100 fs. In the case of alkanethiolate-SAMs, namely, having no anthracene terminals, the IPS lifetime is very long [23.2 ps for dodecanethiolate 4310
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ACS Nano (CH3-C11-SAM)], owing to the insulating properties of alkyl chains.29,30 The shortened IPS lifetime of Ant-C11-SAM shows that the IPS electron relaxes to lower-lying electronic states in the 2D anthracene crystal. It seems reasonable that IPS electrons are accepted by LUMO+n in anthracene, leading to a LUMO at EF +2.5 eV (Figure 2b, inset), because anthracene molecules have a positive electron affinity (∼0.5 eV)26 and the IPS wave function partially penetrates into the anthracene layer.45 In fact, the LUMO electrons survive with a lifetime of 64 ps (Figure S7), which is comparable to IPS lifetimes for CH3-C11-SAM (23.2 ps) and CH3-C17-SAM (50.4 ps).29,30 A longer lifetime of the LUMO electrons in Ant-C11-SAM might be caused by electron stabilization by the polarization of anthracene moieties. On the other hand, the lifetime of S-CTE (110 fs) is much shorter than that of IPS, showing that such a high energy exciton quickly relaxes into lower-lying Sn, including S1. At the negative delay, a prominent structure is observed at around EF +1.4 eV (top axis in Figure 3a), which is assignable to the S1 exciton excited with hν2. The lifetime of S1 is 2.8 ps as evaluated by the single-color TR-2PPE with hν = 3.18 eV (Figure S7). Because the S1 lifetime in an anthracene crystal is much longer (∼30 ns),46 the decay mechanism of S1 in the 2D crystalline monolayer seems different from that in the bulk crystal. Generally, exciton decay when there is a thin separating insulating layer can be explained by an energy transfer to the substrate,47,48 where the decay is accelerated for a thinner spacer layer. In fact, the S1 lifetime is decreased to 1.7 ps in a shorter alkyl chain, Ant-C10-SAM (Figure S8). Note that the chemical bond between anthracene and alkanethiolate layers hardly affects the S1 lifetime. When anthracene molecules are physically deposited on CH3-C11-SAM, the S1 exciton is similarly observable with a lifetime of 3.4 ps (Figure S9), showing that the S1 lifetime is comparable to that for Ant-C11SAM irrespective of disordered molecular orientations. Importantly, electron dynamics specific to the 2D anthracene crystal appears in the dual-color 2PPE. At the energy region between IPS and S1 (or S-CTE), a spectral feature involving some fine structures is observed, which is not detected in the single-color experiment (Figure 2a). In positive and negative delays, the spectral feature appears at the constant energy but shows different time evolutions. In addition, its kinetic energy is independent of hν2 (Figure 4a), where the 2PPE spectra acquired with various hν2 values at the fixed delay of +1200 fs are indicated in the final state energy (bottom axis). The most plausible explanation is that the signal originates from an autoionization of IPS electrons induced by an annihilation of S1 excitons into S0 states. In fact, the energy intervals and the relative intensities of the peaks composing the signal can be reproduced well with the vibronic structure in the photoluminescence spectrum for anthracene crystal at ambient pressure,41 whose first four peaks are derived from ν′ = 0 in S1 to ν″ = 0, 1, 2, and 3 in S0 (labeled as 0−0 to 0−3) (Figures 4b and S10). Moreover, the highest energy peak is located at +3.08 eV relative to EIPS (top axis in Figure 4a), which is in good agreement with the hν to maximize the 2PPE yield via the S0−S1 (0−0) transition (Figure 2e). In the exciton/annihilation induced autoionization (EAAI) process proposed here, the IPS electron excited with hν1 is emitted by the energy transfer from the annihilation of S1 exciton generated with hν2 at the positive delays, whereas, at the negative delays, the S1 exciton with hν2 is waiting for the excitation of IPS electrons with hν1. Therefore, the lifetime of the IPS electrons emitted by the EAAI process
Figure 4. (a) TR-2PPE spectra using various hν2 values from 3.02 to 3.35 eV. The delay between hν1 and hν2 is fixed to +1200 fs. Bottom and top energy axes represent the kinetic energy of emitted electrons with respect to EF and relative energy from EIPS, respectively. The peak of photoemitted IPS electrons shifts toward higher kinetic energy as EIPS + hν2, whereas the spectral feature denoted as EAAI-IPS stays at a constant energy regardless of hν2. (b) Schematic image of the EAAI process. IPS electrons are emitted using the de-excitation energy of the S1 exciton into S0.
(EAAI-IPS) at the positive (negative) delays has a strong correlation with the lifetime of IPS (S1) excited with hν1 (hν2), as can be seen in Figures 3b and S10. Furthermore, it is that the probability of the EAAI process should rely on both electron population in IPS and density of S1 exciton, which increases linearly with the pump powers (hν1 for IPS and hν2 for S1). The laser power dependence of dual-color 2PPE indeed shows that the signal intensity of EAAI-IPS increases linearly with the powers of hν1 and hν2, similarly to those of S1 and IPS (Figure S11). Since the IPS electron has a 2D-free electron-like dispersive nature, the occurrence of the EAAI process suggests that the 2D crystallinity might make the excitonic S1 state spread out within the anthracene layer. In general, S1 exciton localizes at an individual molecule and has a nondispersive nature when it is formed in the gas phase, solution, and randomly organized system. In the bulk anthracene crystal, however, it has been considered that the S1 exciton delocalizes over about 10 molecules within the π-stacked layer (a−b plane), roughly corresponding to the 1.5 nm2 (∼2a × ∼ 2b).49,50 In the 2Dcrystallized anthracene layer, therefore, the S1 exciton would spread two-dimensionally confined in the single molecular layer, which allows the S1 exciton to encounter the dispersive IPS electron partially penetrating into the anthracene layer and to transfer an amount of energy for emitting the IPS electron into the vacuum before decaying into LUMO+n (lifetime of 1100 fs). The appearance of the EAAI-IPS conclusively indicates a photochemical insight specific to the coupling between dispersive and nondispersive electronically excited states, which is highlighted in the surface system of the 2D crystalline anthracene monolayer on the insulator. The S1 exciton that emits light rarely dissociates into free electron and holes due to an attractive Coulombic interaction. As demonstrated by the EAAI process, when the S1 exciton is coupled with dispersive electronic states such as an IPS state, the coupling contributes to generate a free electron with S1 annihilation. A singular feature of excitons is that their spatial 4311
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(hν = 21.22 eV) for UPS and Mg Kα line (hν = 1253.6 eV) for XPS. The sample temperature was kept at RT in the 2PPE measurement, apart from the comparison of the temperature dependence.
size can be enlarged by coherent excitations among uniform surroundings.51 In other words, the crystallinity of the organic monolayer preserves molecular orderings not only to enhance the spatial size of the exciton but also to suppress reorganization of organic moieties. With the support of delocalized wave functions, the higher probability of a process such as the EAAI may assist charge separation in molecular crystalline layers.
ASSOCIATED CONTENT S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b01506. Molecular arrangements (Figure S1), IRAS (Figure S2, Note S1, and Table S1), TPD (Figure S3), XPS (Figure S4), UPS (Figure S5), and additional 2PPE data for AntC11-SAM (Figures S7, S10, and S11), 2PPE of anthracene deposited on HOPG (Figure S6), Ant-C10SAM (Figure S8), and anthracene-deposited CH3-C11SAM (Figure S9) (PDF)
CONCLUSIONS We have fabricated a 2D crystal consisting of a molecularly πstacked oligoacene monolayer chemically supported on an alkanethiolate-SAM at RT. Our experimental results reveal that the ultimately thin molecular monolayer possesses highly robust crystallinity owing to the double self-assemblies of the constituents. The decoupling of functional molecules from a substrate by an alkyl-SAM facilitates the electronic characterization of the isolating heterolayers: the electronic structure and dynamics are characteristic, reflecting the nature of a 2D crystalline organic semiconductor on an insulator, exhibiting electron emission caused by an EAAI process as well as electronic states assignable to IPS, S-CTE, S1 exciton, and LUMO. The finding of the coupling between dispersive and nondispersive electronically excited states will strengthen fundamental design efforts to develop extremely small and efficient organic devices.
AUTHOR INFORMATION Corresponding Author
*Tel: +81-45-566-1712. Fax: +81-45-566-1697. E-mail:
[email protected]. ORCID
Atsushi Nakajima: 0000-0003-2650-5608 Present Address ∥
Department of Physics, Graduate School of Science, Tohoku University, 6-3 Aramaki Aza-Aoba, Aoba-ku, Sendai 980-8578, Japan.
METHODS
Notes
The authors declare no competing financial interest.
Sample Preparation. SAM of Ant-C11 (purchased from Tokyo Chemical Industry Co., Ltd.) was formed from liquid phases onto a single-crystal Au(111) substrate. The substrates were cleaned by repeated cycles of sputtering with 0.6 kV Ar+ ions followed by annealing at 720 K in ultrahigh vacuum (UHV) prior to the formation of SAMs. The cleanness of surface was confirmed with 2PPE measurement, yielding a work function of 5.5 eV and a sharp Shockley surface state at 0.4 eV below EF. The Ant-C11-SAM was prepared by dipping the substrate into an ethanol solution including Ant-C11 with a concentration of 0.2 mM at RT for 36−40 h. The dipped substrate was rinsed thoroughly with ethanol and dried in air at atmospheric pressure. The sample was then introduced into the UHV chamber for the STM, IRAS, and 2PPE measurements. The sample was heated at 370 K for several minutes to remove multilayered molecules and to enhance the ordering of Ant-C11-SAM, where the heating temperature was determined based on the TPD measurement (Figure S3). STM Measurements. All STM experiments were performed at RT in a custom-built UHV chamber with a base pressure lower than 1.0 × 10−8 Pa, which houses a commercial STM unit (Omicron VT-AFMXA50/500). Electrochemically etched W wires were used for the STM probe. STM images were analyzed using WSxM.52 2PPE Measurements. In the single-color 2PPE, the second (2.63−3.44 eV) or third harmonics (3.96−5.17 eV) of a tunable titanium sapphire laser (COHERENT: Mira, ∼100 fs pulse width and 76 MHz repetition rate) were used as hν2 or hν1. The light sources were focused by an f = 400 mm concave mirror onto the sample surface with an incident angle of 55°. Photoelectrons emitted along the surface normal were detected with a hemispherical energy analyzer (VGSCIENTA: R-3000). In the dual-color 2PPE, both hν1 and hν2 were co-introduced through an optical delay stage with a pump−probe configuration. To avoid radiation damage to the sample, the incident laser powers of hν1 and hν2 were reduced to below 0.5 and 0.2 nJ/ pulse, respectively. The total energy and time resolutions of the 2PPE setup were about 20 meV and 30 fs, respectively. All polarization of incident photons were set to p-polarization except for the polarization dependence measurement. UPS and XPS were also conducted for the same sample by changing the light sources to the HeI resonance line
ACKNOWLEDGMENTS This work is partly supported by the Science Research Promotion Fund from the Promotion and Mutual Aid Corporation for Private Schools of Japan, and by JSPS KAKENHI Grant-in-Aid for Young Scientists (B) Grant Number 25810010 and for Scientific Research (A) Grant Number 15H02002. REFERENCES (1) Forrest, S. R. The Path to Ubiquitous and Low-Cost Organic Electronic Appliances on Plastic. Nature 2004, 428, 911−918. (2) Street, R. A. Thin-Film Transistor. Adv. Mater. (Weinheim, Ger.) 2009, 21, 2007−2022. (3) Klauk, H. Organic Thin-Film Transistors. Chem. Soc. Rev. 2010, 39, 2643−2666. (4) Wang, C.; Dong, H.; Hu, W.; Liu, Y.; Zhu, D. Semiconducting πConjugated Systems in Field-Effect Transistors: A Material Odyssey of Organic Electronics. Chem. Rev. 2012, 112, 2208−2267. (5) Mei, J.; Diao, Y.; Appleton, A. L.; Fang, L.; Bao, Z. Integrated Materials Design of Organic Semiconductors for Field-Effect Transistors. J. Am. Chem. Soc. 2013, 135, 6724−6746. (6) Bakulin, A. A.; Rao, A.; Pavelyev, V. G.; van Loosdrecht, P. H. M.; Pshenichnikov, M. S.; Niedzialek, D.; Cornil, J.; Beljonne, D.; Friend, R. H. The Role of Driving Energy and Delocalized States for Charge Separation in Organic Semiconductors. Science 2012, 335, 1340−1344. (7) Barker, A. J.; Chen, K.; Hodgkiss, J. M. Distance Distributions of Photogenerated Charge Pairs in Organic Photovoltaic Cells. J. Am. Chem. Soc. 2014, 136, 12018−12026. (8) Gélinas, S.; Rao, A.; Kumar, A.; Smith, S. L.; Chin, A. W.; Clark, J.; van der Poll, T. S.; Bazan, G. C.; Friend, R. H. Ultrafast Long-Range Charge Separation in Organic Semiconductor Photovoltaic Diodes. Science 2014, 343, 512−516. (9) Kaake, L. G.; Zhong, C.; Love, J. A.; Nagao, I.; Bazan, G. C.; Nguyen, T.-Q.; Huang, F.; Cao, Y.; Moses, D.; Heeger, A. J. Ultrafast 4312
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