Excitation and Relaxation Dynamics of Two-Dimensional Photoexcited

Article pubs.acs.org/JPCC

Excitation and Relaxation Dynamics of Two-Dimensional Photoexcited Electrons on Alkanethiolate Self-Assembled Monolayers Masahiro Shibuta,† Naoyuki Hirata,‡,§ Ryo Matsui,‡ Masato Nakaya,§,‡ Toyoaki Eguchi,‡,§ and Atsushi Nakajima*,†,‡,§ †

Keio Institute of Pure and Applied Science (KiPAS) and ‡Department of Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan § Nakajima Designer Nanocluster Assembly Project, JST, ERATO, 3-2-1 Sakado, Takatsu-ku, Kawasaki 213-0012, Japan ABSTRACT: The electron dynamics in alkanethiolate selfassembled monolayers (Cn-SAMs; n = 6−18, where n is the number of alkyl carbons) formed on Au(111) surfaces has been investigated by time- and angle-resolved two-photon photoemission spectroscopy. The time evolution of photoexcited electrons flowing down into image potential states (IPSs) formed on standing-up structure of SAMs is resolved twodimensionally; the electron lifetime in the IPS increases with chain length, from sub-ps to 100 ps. The chain length dependence of the IPS lifetime is particularly marked at shorter chain lengths of n = 6−10, whereas it becomes milder at chain lengths above n = 10, whose alkyl layer thickness is ≈10 Å. This thickness dependence can be explained by two competitive channels for the decay of IPS electrons: one is electronic coupling of IPS with unoccupied bulk Au states and an interfacial state localized at the Au−S linkage, and the other is IPS electron decay to the Au substrate through a tunneling barrier of insulating alkyl chains. The former is most influential at shorter chain lengths, while the latter is solely dominant at longer chain lengths. In addition, the photon energy dependence of the IPS intensity revealed that electron injection into the IPS is mediated effectively by an electron excitation into interfacial resonance formed in the alkyl layer.

1. INTRODUCTION Charge transfer or separation is a very important phenomenon in chemistry, physics, and biology, enabling a broad array of technological applications such as photocatalysis,1−3 photoemitting diode,4 and solar cells.5−8 The charge transfer induced by photoirradiation at a surface is triggered by excitation of electrons and develops by subsequent relaxation processes within a time scale of femto- to picoseconds. In addition to the time evolution of the excited electron, the energetic and spatial distributions of normally unoccupied states must be known to understand the photochemistry and electron dynamics at surfaces and interfaces because excited electrons transiently occupy those vacant electronic states.5,9−14 Charge transport across insulator adlayers has been extensively investigated using well-defined surfaces consisting of physisorbed adsorbate matrices of rare gas or small molecule overlayers9−14 and self-assembled monolayers (SAMs) chemically bonded to gold (Au) and silver (Ag) metal substrates15−19 and evaluating their tunneling decay constants through the insulators. The former type of adsorbate layer was investigated by two-photon photoemission (2PPE) from electrons in the image potential states (IPS) ∼4 eV above the Fermi level (EF), while the latter type of SAMs were investigated by measure© 2015 American Chemical Society

ments of current density using junctions within a bias voltage of ±0.5 V around EF. The tunneling decay constants (β) obtained for argon (Ar) and alkanethiolate were evaluated to be 0.11 and 0.73−0.74 Å−1, respectively.12,19 Physisorbed adsorbate layers are good systems to use to better develop the photophysical description of ordered insulator surfaces, while alkanethiolate SAMs are good for understanding the charge dynamics in organic insulators and also are promising for technological applications as ultrathin insulating layers for surface nanotechnology.20−28 Since chemical Au−S bonds are formed to anchor the alkyl insulating chain of the SAM, the interfacial modification introduced by chemical states should be included in the description of the charge transport. In particular, highly photoexcited electrons located a few eV above the EF are directly caused by the presence of electronic states associated with the alkanethiolate chemisorption. Although direct observations of such nonequilibrium electron behavior remain challenging, time- and angle-resolved 2PPE (TR- and AR-2PPE) spectroscopy are among the most Received: July 8, 2015 Revised: September 10, 2015 Published: September 29, 2015 22945

DOI: 10.1021/acs.jpcc.5b06549 J. Phys. Chem. C 2015, 119, 22945−22953

Article

The Journal of Physical Chemistry C

MHz, ∼100 fs) was used. The fundamental wave (hν = 1.32− 1.63 eV) was partially converted to third harmonic (hν = 3.96− 4.89 eV) by a couple of β-BaB2O4 crystals and irradiated the sample as a pump photon. The remaining fundamental wave or its second harmonic (hν = 2.64−3.26 eV) was used as a probe photon. Incident pump and probe photons were focused (f = 400 mm) onto the sample in the UHV chamber at an incident angle of 55° from surface normal and were limited to 3 ps, the lifetimes of excited electrons in the first (n = 1) and second (n = 2) IPSs were evaluated to be 4.7 and 4.4 ps, respectively. IPS (n = 1) intensity increase just after excitation can be explained using data from TR- and AR-2PPE measurements. Figure 4a shows the angular distribution of IPS (n = 1)

Figure 3. Polarization dependence of 2PPE spectra for standing-up C8-SAM (hνpump = 4.48 eV and hvprobe = 1.49 eV, Δt = 75 fs, 90 K). (top) Spectra taken with p-polarized (blue line) and s-polarized (pink line) probe photons with fixed p-polarized pump photon. (bottom) Spectra taken with p-polarized (blue line) and s-polarized (green line) pump photons with fixed p-polarized probe photons. The ratio between the IPS intensity pumped with p- and s-polarized photons (Ip−p/Is−p) is denoted in the figure.

nature parallel to the surface. The bound states form a Rydberglike quantized energy set converging toward the vacuum level (Evac); En = Evac − 0.85/(n + a)2 eV, where n = 1, 2, 3, ... is the principal quantum number and a is the quantum defect. The IR is a bound state similar to the IPS, but formed in a thick dielectric layer. Image-like unoccupied resonances formed at the dielectric/metal interface first observed for rare gas adlayers on metal substrates, which were referred to as buried interface states or as interfacial resonant states, have been investigated in detail by Höfer’s group.12,49 The word “interfacial resonance” has been proposed by Zhu et al. for the similar states formed in the SAM system.34 Although the energy of the IR (EIR) is greatly affected by the dielectric constant (ε) and thickness of the film, it is typically a few hundred meV below the conduction band minima (ECBM) of dielectrics.12,49 In the present alkanethiolate SAM, exhibiting ECBM > Evac, the IPS and IR are located at 0.5 and 0.1 eV below Evac (EF + 4.2 eV), respectively, and the ECBM of the standing-up alkyl layer is 0.2 eV above Evac, taking from the electron affinity of a typical nalkane (+0.2 eV).29 Namely, the alkanethiolate SAM energetically satisfies the relationship of EIR > EIPS, where the EIPS is the energy of the IPS (n = 1). With increasing Δt, electrons excited to the IR are immediately quenched within a pump−probe cross-correlation, whose lifetimes are estimated to be less than 30 fs. On the other hand, the intensity of IPS (n = 1) gradually grows and reaches a maximum at Δt ≈ 1 ps, as shown in Figure 2a. Furthermore, the Δt ≈ 1 ps 2PPE spectra exhibits a weak structure at EF + 3.9 eV, which we attribute to the second (n = 2) IPS. Figure 2c shows the time evolutions of the IPS peak intensities, where a strong humped feature is observed for the IPS (n = 2) around zero delay, due to coherently excited bulk electrons and/or highly excited hot electrons showing cross-correlation of pump

Figure 4. Angle-resolved 2PPE for standing-up C8-SAM at 300 K (hνpump = 4.54 eV and hvprobe = 3.02 eV) at Δt = (a) 40, (b) 320, (c) 900, and (d) 2000 fs. In order to cover a wide photoemission angle, two spectra are connected around 17° in (a). Corresponding energies of the IR and Evac + hνprobe are marked by dashed horizontal lines in (a). The sample was biased with (a) 0 V and (b−d) −3 V. The spectra in (b−d) are represented with the same intensity scale to clearly show the intensity decay.

electrons obtained from for the standing-up structure of a C8SAM with Δt = 40 fs (hvpump = 4.54 eV and hvprobe = 3.02 eV), where the horizontal and vertical axes are the photoemission angle (θ) from the surface normal direction and the electron kinetic energy (Ek), respectively. In order to cover a wide emission angle, two spectral images were connected at θ = 17°. At Δt = 40 fs, the IPS signal shows a parabolic energy dispersion characteristic of its free-electron-like nature delocalized parallel to the surface. The effective mass (m*) of the IPS electron is evaluated to be 1.1me (me: mass of free electron) by fitting a parabolic function of Ek = E0 + ℏ2k∥2/2m*, where E0 is the kinetic energy of the IPS at a normal emission (θ = 0°). The IPS electrons show dynamical time and momentum evolution. Figures 4b−d show AR-2PPE spectra taken with Δt of 320, 900, and 2000 fs, respectively. With increasing Δt, the excited electron in the image potential band energetically relaxed toward the band minimum at the Γ̅ point (θ = 0°) through an intraband transition and then gradually decayed over a time scale of a few picoseconds. This intraband relaxation induces the initial increase of the 2PPE signal intensity observed in Figure 2a because the spectra are measured around θ = 0° with a limited acceptance angle (±4°). Here, we briefly discuss the electron dynamics in the other SAM morphology of the lying-down structure shown in Figure 22948

DOI: 10.1021/acs.jpcc.5b06549 J. Phys. Chem. C 2015, 119, 22945−22953

Article

The Journal of Physical Chemistry C

occupied molecular orbital and the lowest unoccupied molecular orbital (HOMO−LUMO gap) of ∼9 eV.50 Therefore, it can be concluded that the long lifetime of an IPS electron of the standing-up structure has been realized by inserting the electrically insulating organic layer. 3.3. Thickness Dependence of IPS Lifetimes. The previous section indicates that the standing-up structure of C8SAM acts as an organic insulator to effectively separate excited electrons in the IPS from the metal substrate. Thus, the IPS lifetime in the standing-up structure should depend on the thickness of the SAM, which can be controlled by the length of the alkyl chain. Figure 6 shows the time evolution of IPS (n = 1) intensity in various chain lengths (C6−C18) of SAMs with the standing-up

1b. In contrast to the standing-up structure, all spectral features observed form the lying-down structure of C8-SAM were quickly quenched within a time scale of a few hundred femtoseconds. Figure 5a shows normal emission 2PPE spectra

Figure 5. (a) 2PPE spectra for lying-down C8-SAM taken with various Δt at 90 K (hνpump = 4.54 eV and hvprobe = 1.51 eV). (b) Time evolutions of normalized intensities of the IPS (n = 1) and (n = 2) as well as the C state.

obtained from the lying-down structure of C8-SAM with various Δt (hvpump = 4.54 eV and hvprobe = 1.51 eV). The work function is evaluated to be 4.7 eV. At Δt = 0 fs, noticeable peaks appear at EF + 4.05 eV and EF + 4.50 eV. Since both peaks show a free-electron-like parabolic energy dispersion with respect to the surface parallel and require p-polarized probe photons (not shown), they can be attributed to the first (n = 1) and second (n = 2) IPSs, respectively. The shoulder-like feature labeled “C” at EF + 3.6 eV is assignable to an unoccupied state derived from Au−S bonds located at the interface between the SAM and Au substrate.32,40 From the time evolution of IPS intensities (Figure 5b), the IPS lifetimes in the lying-down structure of C8-SAM are less than 30 fs for the IPS (n = 1), which is below the limit of the experimental time resolution, and 47 fs for the IPS (n = 2). These values are much shorter than those in the standing-up structure by a factor of 10−2. The lifetime of excited electrons in the C state is also very short, 10 Å region are close to that reported for an insulating rare-gas film grown on a metal substrate; β = 0.11 Å−1 for Ar/Cu(111) (one magnitude per 6.7 ML of Ar),12,51 in which the ECBM of the Ar layer is located 0.25 eV above Evac, similar to our system. It is well-known that the atomically thin rare-gas films can effectively decouple the surface states in the vicinity of EF from the metal substrate owing to the 10−22 eV band gaps of rare-gas films and the proximity of the ECBM to the Evac.12 The IPS electron decays through the tunneling barrier in such films. Similar to the rare-gas system, we consider the tunneling decay process is dominant for SAMs thicker than 10 Å. On the other hand, β values of alkyl layers thinner than 10 Å are 1 order larger than those of thicker layers, indicating that the electronic coupling between the IPS and bulk or interfacial electronic states becomes dominant in the decay process oversimple tunneling. Extrapolating the simple fit to thinner films than those studied experimentally here, the decay rate

Figure 8. hν dependence of single-color 2PPE spectra for standing-up C8-SAM at 293 K. Spectra are taken with both p-polarized (red line) and s-polarized (purple line), which are normalized at the spectral onset of the C state around EF + 3.6 eV.

C state is mainly located at the interface containing the Au−S chemical bond, the electronic coupling between the IPS and C state becomes larger with thinner films. In fact, the IPS lifetime of an n-heptane/Au(111) system, which has no such interfacial electronic states, starts to increase from the first monolayer.53 As shown in Figure 7, when the film is thicker than 10 Å, the contribution of the electronic coupling to the decay process decreases, and the dominant decay process switches to tunneling. 3.4. Excitation Pathway of IPS Electrons. The long lifetime of IPS electrons for the standing-up structure indicates that inserting an insulating alkyl layer effectively separates the IPS wave function from the electronic states of the bulk and interfaces. Now we discuss how electrons can be efficiently excited to the IPS. If the electron transition probability to the IPS from a metal substrate depends on the extent of wave function penetration into the substrate, the IPS intensity from a thick alkyl layer SAM should be small. As mentioned in the previous section, in fact, the IPS intensity formed on noble 22950

DOI: 10.1021/acs.jpcc.5b06549 J. Phys. Chem. C 2015, 119, 22945−22953

Article

The Journal of Physical Chemistry C metals is drastically attenuated by few atomic layers of rare-gas film; the adsorption of a 0.304 nm thick Ar monolayer decreases the IPS intensity on Cu(111) by a factor of 10 or more.52,54 The polarization dependence of the IPS intensity shown in Figure 3b, however, suggests that the electrons are supplied from the bulk Au substrate; the ratio of the 2PPE yields with pand s-polarized pump photons, Ip−p/Is−p = 1.88 for IPS (n = 1), corresponds well to the ratio of the photoabsorption efficiencies of the Au substrate for both polarizations, Ap/As = 1.89, calculated with refractive indexes at 4.48 eV55 and an incidence angle of 55°. If the IPS electrons are resonantly excited from occupied states derived from Au surface or SAM film, the 2PPE yield would follow the electric field in vacuum near the substrate surface.56 The result implies the existence of an electron pathway to assist the electron population from occupied states in bulk Au to unoccupied IPS formed on SAM. The photon energy dependence of IPS intensity offers the key to understanding the process that generates IPS electrons. Figure 8 shows the photon energy dependence of single-color 2PPE spectra for the standing-up structure taken with a constant incident laser power of 10 pJ/pulse. Although this laser power is considerably higher than that used in the twocolor experiments above, it was carefully confirmed that no spectral degradation was observed during the photon energy scan. Incident photon energies are noted in the figure. The red and purple lines represent the spectra taken with p- and spolarized photons, respectively. The structure at EF + 3.6 eV originates from the C state,40 identical with the structure observed in lying-down structure (Figure 5a).32 In Figure 8, spectral intensities were normalized by the intensity of the C state, which has little polarization selectivity.40 Since the IPS located at EF + 3.7 eV can be detected only with p-polarized probe photon, the difference in the spectra with p- and spolarized pump photons arises from the contribution of IPS signals. As shown in Figure 8, the polarization dependence becomes prominent with increasing photon energy. The contribution of IPS signals is quite small for hν below 4.04 eV, although it is high enough to excite the electrons from bulk Au (EF = 0 eV) to IPS (EF + 3.7 eV). The results indicate that hν above ∼4.1 eV is required to generate IPS electrons. Based on the above results, a reasonable state that can populate a large amount of electrons into the IPS is an intermediate unoccupied state located spatially closer to the Au substrate and energetically higher than IPS. As discussed previously in Figure 2, the IR is formed in the film for the standing-up structures of alkanethiolate SAMs at EF + 4.1 eV,34 satisfying EIR > EIPS. Figures 9a−c illustrate schematic energy diagrams of (a) lying-down, (b) standing-up (thin), and (c) standing-up (thick) structures. In the case of a standing-up structure, the electrons excited from the substrate initially occupy the IR and then quickly relax to the IPS by an interband transition. Since the probability density maximum of the IR is near the metal substrate in the dielectric medium,12,49 electrons easily populate the IR rather than the IPS. Furthermore, the IR does not contribute to the relaxation of IPS electrons reversibly because the IR is energetically higher than the IPS. In fact, when terminal CH3 group is substituted by CF3 in C12-SAM, no IPS electrons were observed owing to upward shift in Evac with a terminal dipole, resulting in EIR < EIPS. When an overlayered dielectric medium has low ECBM, the IR should be located at an energy lower than IPS (EIR < EIPS). In such case, the IR would play a role as a decay channel of IPS

Figure 9. Schematic diagrams of 2PPE process for (a) lying-down, (b) standing-up (l < 10 Å), and (c) standing-up (l > 10 Å) structures of SAMs.

electron rather than the excitation path. Thus, the energetics of the IR should be formed commonly in any dielectric media to govern the efficiency of photoinduced charge injection and isolation from a metal substrate.

4. CONCLUSIONS We employed 2PPE spectroscopy to study the dynamics of photoexcited electrons for alkanethiolate-SAMs. We found that electrons excited to an IPS of the standing-up structures have a long lifetime owing to the excellent insulating property of the film. Although the IPS is spatially well-separated from the Au bulk substrate by the organic insulating layer, the electrons can be injected efficiently from the substrate to the IPS via an IR state, which is located energetically higher than the IPS and also spatially bridges the IPS and Au substrate. The lower lying C state that arises at the anchoring Au−S interface provides a major relaxation pathways for the IPS electrons, but its chemical linkage contribution becomes negligible in films thicker than ≈10 Å. This result suggests that the IPS lifetime can be increased by lowering the energy position of the IPS below that of the C state, e.g., by modifying the surface to achieve a lower work function. This is because the IPS energy is pinned to the vacuum level, while the C state energy does not depend on the work function of the surface. These experimental findings provide fundamental knowledge for describing surface electron dynamics in chemisorbed monolayer films.

■

AUTHOR INFORMATION

Corresponding Author

*Tel +81-45-566-1712; Fax +81-45-566-1697; e-mail [email protected] (A.N.). Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work is partly supported by JSPS KAKENHI of Grant-inAids for Young Scientists (B) Grant Number 25810010 and for Scientific Research (A) Grant 15H02002. 22951

DOI: 10.1021/acs.jpcc.5b06549 J. Phys. Chem. C 2015, 119, 22945−22953

Article

The Journal of Physical Chemistry C

■

(22) Smith, R. K.; Lewis, P. A.; Weiss, P. S. Patterning SelfAssembled Monolayers. Prog. Surf. Sci. 2004, 75, 1−68. (23) Schreiber, F. Self-Assembled Monolayers: from ‘Simple’ Model Systems to Biofunctionalized Interfaces. J. Phys.: Condens. Matter 2004, 16, R881−R900. (24) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Self-Assembled Monolayers of Thiolates on Metals as a Form of Nanotechnology. Chem. Rev. 2005, 105, 1103−1170. (25) Vericat, C.; Vela, M. E.; Benitez, G. A.; Martin Gago, J. A.; Torrelles, X.; Salvarezza, R. C. Surface Characterization of Sulfur and Alkanethiol Self-Assembled Monolayers on Au(111). J. Phys.: Condens. Matter 2006, 18, R867−R900. (26) Maksymovych, P.; Voznyy, O.; Dougherty, D. B.; Sorescu, D. C.; Yates, J. T., Jr. Gold Adatom as a Key Structural Component in Self-Assembled Monolayers of Organosulfur Molecules on Au(111). Prog. Surf. Sci. 2010, 85, 206−240. (27) Halik, M.; Klauk, H.; Zschieschang, U.; Schmid, G.; Dehm, C.; Schütz, M.; Maisch, S.; Franz, E.; Brunnbauer, M.; Stellacci, F. LowVoltage Organic Transistors with an Amorphous Molecular Gate Dielectric. Nature 2004, 431, 963−966. (28) DiBenedetto, S. A.; Facchetti, A.; Ratner, M. A.; Marks, T. J. Molecular Self-Assembled Monolayers and Multilayers for Organic and Unconventional Inorganic Thin-Film Transistor Applications. Adv. Mater. 2009, 21, 1407−1433. (29) Lingle, R. L., Jr.; Ge, N.-H.; Jordan, R. E.; McNeill, J. D.; Harris, C. B. Femtosecond Studies of Electron Tunneling at Metal-Dielectric Interfaces. Chem. Phys. 1996, 205, 191−203. (30) Ge, N.-H.; Wong, C. M.; Lingle, R. L., Jr.; McNeill, J. D.; Gaffney, K. J.; Harris, C. B. Femtosecond Dynamics of Electron Localization at Interfaces. Science 1998, 279, 202−205. (31) Petek, H.; Ogawa, S. Femtosecond Time-Resolved Two-Photon Photoemission Studies of Electron Dynamics in Metals. Prog. Surf. Sci. 1997, 56, 239−310. (32) Lindstrom, C. D.; Muntwiler, M.; Zhu, X.-Y. Electron Transport Across the Alkanethiol Self-Assembled Monolayer/Au(111) Interface: Role of the Chemical Anchor. J. Phys. Chem. B 2005, 109, 21492− 21495. (33) Dutton, G.; Quinn, D. P.; Lindstrom, C. D.; Zhu, X.-Y. Exciton Dynamics at Molecule-Metal Interfaces: C60/Au(111). Phys. Rev. B: Condens. Matter Mater. Phys. 2005, 72, 045441. (34) Muntwiler, M.; Lindstrom, C. D.; Zhu, X.-Y. Delocalized Electron Resonance at the Alkanethiolate Self-Assembled Monolayer/ Au(111) Interface. J. Chem. Phys. 2006, 124, 081104. (35) Schwalb, C. H.; Sachs, S.; Marks, M.; Schöll, A.; Reinert, F.; Umbach, E.; Höfer, U. Electron Lifetime in a Shockley-Type Metal− Organic Interface State. Phys. Rev. Lett. 2008, 101, 146801. (36) Stähler, J.; Meyer, M.; Kusmierek, D. O.; Bovensiepen, U.; Wolf, M. Ultrafast Electron Transfer Dynamics at NH3/Cu(111) Interfaces: Determination of the Transient Tunneling Barrier. J. Am. Chem. Soc. 2008, 130, 8797−8803. (37) Yamamoto, I.; Mikamori, M.; Yamamoto, R.; Yamada, T.; Miyakubo, K.; Ueno, N.; Munakata, T. Resonant Two-Photon Photoemission Study of Electronically Excited States at the Lead Phthalocyanine/graphite Interface. Phys. Rev. B: Condens. Matter Mater. Phys. 2008, 77, 115404. (38) Shibuta, M.; Miyakubo, K.; Yamada, T.; Munakata, T. Angleand Time-Resolved Two-Photon Photoemission Spectroscopy for Unoccupied Levels of Lead Phthalocyanine Film. J. Phys. Chem. C 2011, 115, 19269−19273. (39) Shibuta, M.; Hirata, N.; Matsui, R.; Eguchi, T.; Nakajima, A. Charge Separation at the Molecular Monolayer Surface: Observation and Control of the Dynamics. J. Phys. Chem. Lett. 2012, 3, 981−985. (40) Hirata, N.; Shibuta, M.; Matsui, R.; Nakajima, A. Electronic States of Alkanethiolate Self-Assembled Monolayers on Au(111) Studied by Two-Photon Photoemission Spectroscopy. J. Phys. Chem. C 2012, 116, 13623−13628. (41) Varene, E.; Bogner, L.; Bronner, C.; Tegeder, P. Ultrafast Exciton Population, Relaxation, and Decay Dynamics in Thin Oligothiophene Films. Phys. Rev. Lett. 2012, 109, 207601.

REFERENCES

(1) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Applications of Semiconductor Photocatalysis. Chem. Rev. 1995, 95, 69−96. (2) Tachikawa, T.; Fujitsuka, M.; Majima, T. Mechanistic Insight into the TiO2 Photocatalytic Reactions: Design of New Photocatalysts. J. Phys. Chem. C 2007, 111, 5259−5275. (3) Akimov, A. V.; Neukirch, A. J.; Prezhdo, O. V. Theoretical Insights into Photoinduced Charge Transfer and Catalysis at Oxide Interfaces. Chem. Rev. 2013, 113, 4496−4565. (4) Kuik, M.; Wetzelaer, G.-J. A. H.; Nicolai, H. T.; Craciun, N. I.; De Leeuw, D. M.; Blom, P. W. M. Charge Transport and Recombination in Polymer Light-Emitting Diodes. Adv. Mater. 2014, 26, 512−531. (5) Linsebigler, A. L.; Lu, G.; Yates, J. T., Jr. Photocatalysis on TiO2 Surfaces: Principles, Mechanisms, and Selected Results. Chem. Rev. 1995, 95, 735−758. (6) Brédas, J.-L.; Norton, J. E.; Cornil, J.; Coropceanu, V. Molecular understanding of organic solar cells: the challenges. Acc. Chem. Res. 2009, 42, 1691−1699. (7) Zhu, X.-Y.; Yang, Q.; Muntwiler, M. Charge-Transfer Excitons at Organic Semiconductor Surfaces and Interfaces. Acc. Chem. Res. 2009, 42, 1779−1787. (8) Clarke, T. M.; Durrant, J. R. Charge Photogeneration in Organic Solar Cells. Chem. Rev. 2010, 110, 6736−6767. (9) Harris, C. B.; Ge, N.-H.; Lingle, R.-L., Jr.; McNeill, J. D.; Wong, C. M. Femtosecond Dynamics of Electrons on Surface and Interfaces. Annu. Rev. Phys. Chem. 1997, 48, 711−744. (10) Zhu, X. Y. Electronic Structure and Electron Dynamics at Molecule−Metal Interfaces: Implications for Molecule-Based Electronics. Surf. Sci. Rep. 2004, 56, 1−83. (11) Echenique, P. M.; Berndt, R.; Chulkov, E. V.; Fauster, Th.; Goldmann, A.; Höfer, U. Decay of Electronic Excitations at Metal Surfaces. Surf. Sci. Rep. 2004, 52, 219−317. (12) Güdde, J.; Höfer, U. Femtosecond Time-Resolved Studies of Image-Potential States at Surfaces and Interfaces of Rare-Gas Adlayers. Prog. Surf. Sci. 2005, 80, 49−91. (13) Hotzel, A. Electron Dynamics of Image Potential States in Weakly Bound Adsorbate Layers: A Short Review. Prog. Surf. Sci. 2007, 82, 336−354. (14) Dynamics at Solid State Surfaces and Interfaces; Bovensiepen, U., Petek, H., Wolf, M., Eds.; Wiley-VCH Verlag GmbH and Co. KGaA: Weinheim, 2010; Vol. 1. (15) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. Formation of Monolayer Films by the Spontaneous Assembly of Organic Thiols from Solution onto Gold. J. Am. Chem. Soc. 1989, 111, 321−335. (16) Scaini, D.; Castronovo, M.; Casalis, L.; Scoles, G. Electron Transfer Mediating Properties of Hydrocarbons as a Function of Chain Length: A Differential Scanning Conductive Tip Atomic Force Microscopy Investigation. ACS Nano 2008, 2, 507−515. (17) Beebe, J. M.; Kim, B.; Frisbie, C. D.; Kushmerick, J. G. Measuring Relative Barrier Heights in Molecular Electronic Junctions with Transition Voltage Spectroscopy. ACS Nano 2008, 2, 827−832. (18) Dickey, M. D.; Chiechi, R. C.; Larsen, R. J.; Weiss, E. A.; Weitz, D. A.; Whitesides, G. M. Eutectic Gallium-Indium (EGaIn): A Liquid Metal Alloy for the Formation of Stable Structures in Microchannels at Room Temperature. Adv. Funct. Mater. 2008, 18, 1097−1104. (19) Baghbanzadeh, M.; Simeone, F. C.; Bowers, C. M.; Liao, K.-C.; Thuo, M.; Baghbanzadeh, M.; Miller, M. S.; Carmichael, T. B.; Whitesides, G. M. Odd−Even Effects in Charge Transport across nAlkanethiolate-Based SAMs. J. Am. Chem. Soc. 2014, 136, 16919− 16925. (20) Ulman, A. Formation and Structure of Self-Assembled Monolayers. Chem. Rev. 1996, 96, 1533−1554. (21) Cygan, M. T.; Dunbar, T. D.; Arnold, J. J.; Bumm, L. A.; Shedlock, N. F.; Burgin, T. P.; Jones, L., II; Allara, D. L.; Tour, J. M.; Weiss, P. S. Insertion, Conductivity, and Structures of Conjugated Organic Oligomers in Self-Assembled Alkanethiol Monolayers on Au(111). J. Am. Chem. Soc. 1998, 120, 2721−2732. 22952

DOI: 10.1021/acs.jpcc.5b06549 J. Phys. Chem. C 2015, 119, 22945−22953

Article

The Journal of Physical Chemistry C (42) Chan, W.-L.; Ligges, M.; Zhu, X.-Y. The Energy Barrier in Singlet Fission can be Overcome through Coherent Coupling and Entropic Gain. Nat. Chem. 2012, 4, 840−845. (43) Ueba, T.; Terawaki, R.; Morikawa, T.; Kitagawa, Y.; Okumura, M.; Yamada, T.; Kato, H. S.; Munakata, T. Diffuse Unoccupied Molecular Orbital of Rubrene Causing Image-Potential State Mediated Excitation. J. Phys. Chem. C 2013, 117, 20098−20103. (44) Muller, E. A.; Strader, M. L.; Johns, J. E.; Yang, A.; Caplins, B. W.; Shearer, A. J.; Suich, D. E.; Harris, C. B. Femtosecond Electron Solvation at the Ionic Liquid/Metal Electrode Interface. J. Am. Chem. Soc. 2013, 135, 10646−10653. (45) Shibuta, M.; Hirata, N.; Matsui, R.; Eguchi, T.; Nakajima, A. Probing of an Adsorbate-Specific Excited State on an Organic Insulating Surface by Two-Photon Photoemission Spectroscopy. J. Am. Chem. Soc. 2014, 136, 1825−1831. (46) Kato, H. S.; Murakami, Y.; Kiriyama, Y.; Saitoh, R.; Ueba, T.; Yamada, T.; Ie, Y.; Aso, Y.; Munakata, T. Decay of the Exciton in Quaterthiophene-Terminated Alkanethiolate Self-Assembled Monolayers on Au(111). J. Phys. Chem. C 2015, 119, 7400−7407. (47) Nakaya, M.; Shikishima, M.; Shibuta, M.; Hirata, N.; Eguchi, T.; Nakajima, A. Molecular-Scale and Wide-Energy-Range Tunneling Spectroscopy on Self-Assembled Monolayers of Alkanethiol Molecules. ACS Nano 2012, 6, 8728−8734. (48) Sharma, M.; Komiyama, M.; Engstrom, J. R. Observation from Scanning Tunneling Microscopy of a Striped Phase for Octanethiol Adsorbed on Au(111) from Solution. Langmuir 2008, 24, 9937−9940. (49) Rohleder, M.; Berthold, W.; Güdde, J.; Höfer, U. Time-Resolved Two-Photon Photoemission of Buried Interface States in Ar/Cu(100). Phys. Rev. Lett. 2005, 94, 017401. (50) Boulas, C.; Davidovits, J. V.; Rondelez, F.; Vuillaume, D. Suppression of Charge Carrier Tunneling through Organic SelfAssembled Monolayers. Phys. Rev. Lett. 1996, 76, 4797−4800. (51) Berthold, W.; Rebentrost, F.; Feulner, P.; Höfer, U. Influence of Ar, Kr, and Xe layers on the Energies and Lifetimes of Image-Potential States on Cu(100). Appl. Phys. A: Mater. Sci. Process. 2004, 78, 131− 140. (52) Fauster, Th.; Steinmann, W. Photonic Probes of Surfaces, Electromagnetic Waves: Recent Developments in Research; Halevi, P. Ed.; North-Holland: Amsterdam, 1995; Vol. 2, Chapter 8. (53) Lindstrom, C. D.; Quinn, D.; Zhu, X.-Y. Using Image Resonances to Probe Molecular Conduction at the n-heptane/ Au(111) Interface. J. Chem. Phys. 2005, 122, 124714. (54) Damm, A.; Schubert, K.; Güdde, J.; Höfer, U. Observation of the Transition from Image-Potential States to Resonances on ArgonCovered Cu(111) and Ag(111) by Time-Resolved Two-Photon Photoemission. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 80, 205425. (55) Johnson, P. B.; Christy, R. W. Optical Constants of the Noble Metals. Phys. Rev. B 1972, 6, 4370−4379. (56) Wolf, M.; Hotzel, A.; Knoesel, E.; Velic, D. Direct and Indirect Excitation Mechanisms in Two-Photon Photoemission Spectroscopy of Cu(111) and CO/Cu(111). Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 59, 5926−5935.

22953

DOI: 10.1021/acs.jpcc.5b06549 J. Phys. Chem. C 2015, 119, 22945−22953