ReviewTime- and angle-resolved two-photon photoemission studies of electron localization and solvation at interfaces
Introduction
The study of the electronic properties of interfaces between dissimilar materials constitutes a major research area of fundamental interest and technological significance. Chemical and physical processes occurring at interfaces may be understood on a microscopic level in terms of the associated energy levels and dynamics of the interfacial electrons. Hot electrons, for example, can induce surface chemical reactions or desorption of adsorbates [1]. Molecular anion formation via electron transfer from the metal substrate has been implicated as the initial step in these processes [2], [3]. Electron trapping and scattering at the interface can drastically affect carrier transport properties and the performance of devices [4], [5].
The sensitivity of interfacial electrons to the composition of the interface may also be exploited to probe the dynamics of adsorbed molecules. A strong electron–adsorbate interaction can cause an electron-induced adsorbate reorganization [6], [7], [8] similar to electron solvation in liquids [9], [10]. Studies of solvation at an intrinsically asymmetric environment such as a two-dimensional interface is particularly interesting because the reduced dimensionality and hindered solvent motion can result in electron dynamics distinct from those in the isotropic material [11], [12], [13], [14]. Characterizing the energy levels of interfacial electrons as a function of the adsorbate reorganization time can elucidate the time scales and mechanisms of both the electrons’ response as well as the molecular motions of the adsorbate overlayer. To address these issues, an investigation of the properties associated with films of molecular thickness in a well-controlled fashion is necessary.
Much of the research on electrons at interfaces has involved the study of image-potential states (IPS). The IPS are bound by the interaction between an electron outside of a surface and the polarization it induces at that surface (Fig. 1). The polarization can be treated formally by replacing it with the oppositely-charged image of the electron reflected across the surface plane, a configuration that reproduces the boundary conditions of the original problem [15], [16]. The resultant one-dimensional Coulombic potential, the image potential (IP), supports an infinite series of hydrogenic states. For metal surfaces, the energies of IPS in electron volts are given bywhere n represents the principal quantum number and a is the quantum defect parameter [17], [18], [19]. The series converges to V0, which is equal to the vacuum energy for a uniform surface. Image-potential state electrons reside only a few angstroms outside of the interface. The hydrogenic model [20] gives an expectation value of 〈z〉 = 6a0n2, where a0 is the Bohr radius [21]. This distance is roughly 3 Å and 12 Å for n = 1 and n = 2 IPS, respectively, making them sensitive to changes in the interfacial electrostatic potential. It has been shown that the surface electronic structure modified by the overlayer also changes the energy and lifetime of the IPS [22]. For a more detailed description of the properties of IPS and calculations of their energies, the reader is referred to review articles by Memmel [19], Fauster and Steinmann [17], Harris et al. [22], and Osgood and Wang [23].
It is the purpose of the present review to focus on recent work using two-photon photoemission (2PPE) to study electron localization, both static and dynamic, and the related process of electron solvation at dielectric/metal interfaces. Both IPS and the molecular orbitals (MO) of adsorbed molecules will be discussed. The angle- and time-resolved 2PPE technique will be discussed in Section 2, followed by a review of structurally-induced localization at interfaces (Section 3). Dynamic localization of electrons by nonpolar (Section 4) and polar (Section 5) adsorbates are covered separately, where the distinction is due to the significant dynamic energy relaxation caused by the latter. Section 5 also compares the localization and solvation that occur at interfaces to the analogous phenomena experienced by excess electrons in bulk liquids. Finally, a discussion of attempts to determine the spatial extent of localization parallel to the interface is presented in Section 6.
Section snippets
Experimental methods
Two-photon photoemission (2PPE) is a pump–probe spectroscopic technique for exploring interfacial electronic states. Unlike conventional photoemission, which uses one-photon processes, 2PPE is capable of examining initially unoccupied as well as initially occupied states. Initially unoccupied states, such as IPS, as studied by 2PPE are illustrated in Fig. 2a. Electrons from below the Fermi level (EF) of a metal substrate (or from an initially occupied surface [17] or adsorbate [24] state) are
Qualitative scaling arguments
Benzene, methylthiolate, and C60 each represent a case where the static structure of the layer dictates the spatial extent of the interfacial electrons. Nevertheless, electrons may localize in a dynamic process, where the mutual interaction of the electron and the layer changes the spatial extent of the electron over time.
Fig. 7 shows the dispersion of the n = 1 state of 2 ML of n-heptane/Ag(1 1 1). At 0 fs, the n = 1 IPS is delocalized in the plane of the surface, with an effective mass of m* = 1.2. At
Introductory remarks and background
The observation of dynamic localization at polar interfaces shows that the coupling of the electron to the molecular modes of an adsorbate can influence critically the dynamics of the electron. The coupling to these modes can, however, do more than simply alter the electron’s spatial extent. Polar adsorbates interact more strongly over longer ranges, and their motion causes dynamic changes in the energy of the electron through the process of solvation.
The topic of solvation dynamics is easy to
Spatial extent of localized interfacial states
In the study of localization at interfaces, the question of the spatial extent of localization arises. While estimates for higher-energy, delocalized electrons in bulk liquids have been made through modeling the reactions of the electrons with other solutes [139] and measurements of electron–hole recombination rates [140], localized states have been limited to quantum–mechanical calculations of the radius of gyration [130], [131], [132]. At interfaces, the nature of the electronic states are
Conclusions
Since the original studies of IPS at vacuum/metal interfaces, 2PPE has established itself as a valuable technique for determining the electronic properties of a variety of interfaces as a function of composition and structure. With recent demonstrations of the ability to measure the dynamics, energetics, and spatial extent accompanying electron localization, the utility of 2PPE as a probe of electron dynamics is greatly extended. Further experimental and theoretical work will likely expand the
Acknowledgement
The authors would like to thank T. Heinz for helpful discussions. This work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Chemical Sciences Division of the US Department of Energy, under Contract No. DE–AC03–76SF00098. The authors acknowledge the National Science Foundation support for specialized equipment used in the experiments described herein.
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Present address: Department of Chemistry, Brookhaven National Laboratory, Upton, NY 11973, United States.