Letter pubs.acs.org/JPCL
Charge Separation at the Molecular Monolayer Surface: Observation and Control of the Dynamics Masahiro Shibuta,†,‡ Naoyuki Hirata,‡ Ryo Matsui,‡ Toyoaki Eguchi,†,‡ and Atsushi Nakajima*,†,‡ †
Nakajima Designer Nanocluster Assembly Project, JST, ERATO, 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: Charge separation dynamics relevant to an electron transfer have been revealed by time- and angle-resolved two-photon photoemission spectroscopy for an nalkanethiolate self-assembled monolayer (SAM) on a Au(111) surface fabricated by a chemical-wet process. The electron was photoexcited into an image potential state located at 3.7 eV above the Fermi level (EF), and it survived well for more than 100 ps on dodecanethiolate (C12)-SAM. The degree of electron separation is precisely controlled by selecting the length of the alkyl chain (C10−C18). We have also evaluated molecular conductivity at the specific electron energy of EF + 3.7 eV. The tunneling decay parameter, β, was fitted by β90 K = 0.097 Å−1 and βRT = 0.13 Å−1. These values were one order smaller than that at around EF by conventional contact probe methods.
SECTION: Surfaces, Interfaces, Porous Materials, and Catalysis
T
electron energy without any destructive effect, instead of contact probe methods.8,9 In order to probe the charge separation dynamics, here, we employed two-photon photoemission (2PPE) spectroscopy in which an electron is pumped by a first photon (hνpump) from an occupied state to an unoccupied state and is then photoemitted from the unoccupied state by a second probe photon, (hνprobe) (Figure 1a).5,10 By scanning the delay between the pump and probe pulses, the time evolution of the excited electron is detected with femtosecond time resolution (time-resolved (TR)-2PPE). Figure 1b shows the TR-2PPE spectra for a C12SAM (see the Supporting Information for molecular ordering of the sample) with various time delays. At zero delay, a small unoccupied state was observed at 3.7 eV above the Fermi level (EF), coincident with the 2PPE signals from a hot electron below the accessible intermediate energy of EF + hνpump. With a nonzero time delay, the unoccupied state grew significantly and achieved a maximum intensity at about 4 ps. Moreover, the unoccupied state was observed even at 200 ps after the pump pulse. These results indicate that the photoexcited electron is trapped in the unoccupied state and is effectively isolated from the metal substrate. The character of the unoccupied state that we obtained in Figure 1b can be examined by its polarization dependence (Figure 2); a p-polarized probe pulse is definitely required to
he electron dynamics of charge separation from a metal substrate to the surface and interface is an important issue to create organic thin-film devices, such as gas sensors, solar cells, and transistors.1−4 The surface reactivity or functionality is triggered by injection of the excited electron into an unoccupied electronic state at the active site. Their quantum efficiencies are dominated by a degree of charge separation; nevertheless, the excited electron on the surface immediately relaxes to the metal substrate within the order of femtoseconds (fs = 10−15 s).5 To overcome this challenge, an n-alkanethiolate self-assembled monolayer (Cn-SAM, where n is a number of carbon atoms) has attracted attention; the layer made of an alkyl chain is used as a thin insulator or inactivator.1−4 It is well recognized that the linkage arrangement with a strong Au−S bond forms a close-packed “standing-up” structure (Figure 1a) in which all alkyl chains are directed toward the vacuum.1,6,7 Owing to its high molecular packing and ordering, the alkyl layer of the SAM is considered as an efficient insulating molecular layer even at room temperature (RT). Such an insulating effect relevant to the charge separation dynamics in the SAM, however, has not been clarified in real time. In this study, we have tracked the highly separated photoexcited electron whose lifetime is close to 100 ps (ps = 10−12 s) for the SAM on Au(111) prepared by a chemical-wet process, demonstrating that the degree of electron separation can be precisely controlled by the length of the alkyl chain. We also discuss the tunneling decay process relevant to the electron conductance for the insulating molecular layer at the specific © 2012 American Chemical Society
Received: March 3, 2012 Accepted: March 22, 2012 Published: March 27, 2012 981
dx.doi.org/10.1021/jz3002579 | J. Phys. Chem. Lett. 2012, 3, 981−985
The Journal of Physical Chemistry Letters
Letter
(hνpump = hνprobe = 4.23 eV) at a slightly lower binding energy than that of the IPS (see the Supporting Information). Although the σ* state and the IPS have not been observed simultaneously, they can be distinguished clearly from their spectral and temporal features. The σ* state is observed independently of the light polarization and has a very short lifetime (< 20 fs) and very small energy dispersion.14 The IPS observed in the present study, on the other hand, shows distinct polarization dependence, as indicated in Figure 2, and has a long lifetime (∼100 ps) and clear energy dispersion, which are discussed in detail below. It is known that the IPS has a two-dimensional free-electronlike nature.11−13 Figure 3a and b shows the angle-resolved
Figure 1. (a) Schematic description of the close-packed standing-up structure of Cn-SAM and the 2PPE process. (b) TR-2PPE spectra (surface normal emission) for a C12-SAM on Au(111) at 90 K. Photon energies were hνpump = 4.23 eV and hνprobe = 1.41 eV. The horizontal axis is the intermediate energy relative to EF. Time delays between both photons are noted on the right-hand side.
Figure 3. AR-2PPE image (hνpump = 4.23 eV, hνprobe = 2.82 eV) of a C12-SAM taken at delay times of 0.27 (a) and 10 ps (b). A freeelectron-like dispersion is observed in (a) with an effective mass of 1.6 me. The electrons excited below EF + hνpump (dotted line in (a)) subsequently relax toward the band minimum (k∥ = 0), as shown in (b).
(AR)-2PPE spectra measured at delays of 0.27 and 10 ps, respectively. The horizontal and vertical axes are the emission angle (θ) with respect to the surface normal and kinetic energy (Ek) of the photoelectron, respectively. At 0.27 ps, a freeelectron-like dispersion was observed. The surface parallel momentum of the photoelectron (k∥) is connected to the energy by k∥ = (2meEk)1/2 sin θ/ℏ, where me is the mass of a free electron of 1 me. By fitting a parabolic function of Ek = E0 + ℏ2k∥2/(2m*), the effective mass (m*) of the excited electron is evaluated to be 1.6 me. This value is slightly heavier than the free electron. The effective mass of the IPS is affected by hybridization with localized states such as molecular unoccupied states or electron affinity5 and surface lateral inhomogeneity.15 In the SAM case, the former is not so predominant because of the wide band gap in the alkyl chain layer. The intrinsic defect's so-called “etch pit”7 (see also the Supporting Information) may affect the heavier m*. At a time delay of 10 ps (Figure 3b), the 2PPE signal is strongly enhanced near normal emission (k∥ = 0). This result shows that the electrons are initially populated at the image potential band with all of the parallel momenta (Figure 3a) and are subsequently relaxed toward the band minimum (k∥ = 0) through an intraband transition. This intraband relaxation
Figure 2. Polarization dependence of the 2PPE spectra at a 4 ps delay. The inset illustrates the light polarization configurations of both hνpump and hνprobe.
excite a photoelectron (red and orange lines). The polarization selectivity for the probe pulse indicates that the photoexcited electron in the unoccupied state is bound to the surface normal direction but not to the surface parallel.5,10 Considering the work function of the SAM (4.25 ± 0.02 eV), the unoccupied state is located at about 0.55 eV below the vacuum level. From the above results, we have attributed the unoccupied state to the first image potential state (IPS)11−13 formed on the SAM. Lindstrom et al. observed the IPS formed on the C6-SAM but only for the “lying-down” phase and not for the standing-up phase in ref 14. In the same paper, they also reported about the σ* state derived from Au−S chemical bonds for both phases. In our case, the σ* state can be observed with single-color 2PPE 982
dx.doi.org/10.1021/jz3002579 | J. Phys. Chem. Lett. 2012, 3, 981−985
The Journal of Physical Chemistry Letters
Letter
(hνpump = 4.65 eV, hνprobe = 1.55 eV), we may say that the electrons excited from the substrate initially occupy the IR and then immediately transfer to the IPS by interband transition. Because the IR is spatially located at the metal−SAM interface, it is possible to populate more electrons to the IR than the IPS. It seems that almost all of the electron excitation to IPS through IR occurs in a relatively short time just after the pumping because the integrated intensities of the 2PPE spectra shown in Figure 3a and b are much the same (Figure 3b/a = 1.08). It is consistent with a short lifetime (
