Langmuir 1998, 14, 1355-1360
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Applications of High-Resolution Electron Energy Loss Spectroscopy to Technical Surfaces L. L. Kesmodel Department of Physics, Indiana University, Bloomington, Indiana 47405 Received July 3, 1997. In Final Form: September 24, 1997 Within the past several years, advances in high-resolution electron energy loss spectroscopy (HREELS) have permitted the application to increasingly complex systems, “technical surfaces”, which depart from well-characterized, smooth, single-crystal surfaces. These developments have tracked a series of improvements in instrumentation, with present generation spectrometers exhibiting limiting energy resolution below 1 meV at relatively high detector currents (10 pA, direct beam). This paper addresses recent progress in two of these areas: polymer surfaces and model-supported catalysts.
Introduction In recent years, high-resolution electron energy loss spectroscopy (HREELS) has become widely employed as a surface vibrational spectroscopy.1 The method involves the scattering of a relatively low-energy electron beam (1-300 eV) from a specimen surface in high vacuum or ultrahigh vacuum (UHV) conditions in which the quantized energy losses (pωi ) of the beam are detected, thereby leading to molecular level information (e.g., vibrational mode frequencies ωi ) on adsorbates as well as intrinsic substrate information. Because of the short penetration depth (5-10 Å) of electrons in this energy range, the method exhibits higher surface sensitivity than optical methods, and the spectral range can be easily selected to 200-5000 cm-1 and higher by simply changing the spectrometer scan energy. With the continuing improvements in instrumentation, it is now common to achieve 2-4 meV (16-32 cm-1) energy resolution, and even better performance has been achieved under optimal conditions as noted later. HREELS has been applied to a wide range of materials, including metals, semiconductors, and insulators. There is already a very large literature associated with the adsorption of atomic and molecular species on singlecrystal metal samples as studied by the technique. In this paper we therefore emphasize systems of a more technical nature, which are likely to be encountered outside of the university research laboratory, but nevertheless are quite amenable for study by HREELS. These systems tend to place challenging demands on instrumentation so we begin with an overview of the apparatus and recent optimizations. Experimental Apparatus HREELS experiments are typically carried out in UHV in conjunction with a variety of other surface analysis methods that would probe different aspects of the specimen surface; auger electron spectroscopy (AES) for chemical composition, X-ray photoelectron spectroscopy (XPS) for chemical analysis, low-energy electron diffraction (LEED), or scanning tunneling microscopy (STM) for surface structural information are among the most commonly employed methods. Instrumentation for HREELS has therefore been developed to conveniently interface with these other important probes. The electrostatic energy (1) See, for example, Ibach, H.; Mills, D. L. Electron Energy Loss Spectroscopy and Surface Vibrations; Academic: New York, 1982.
analyzer has thus far been the almost universal method of achieving and analyzing the monochromatic electron beams used in HREELS spectrometers and, not surprisingly, is the basis for most AES and XPS systems. We will, therefore, discuss the essential features of electrostatic analyzers with particular emphasis on elements of importance in HREELS. A schematic diagram of an HREELS spectrometer2 is given in Figure 1. The electrons originate in the gun (G) region from a hot tungsten or lanthanum hexaboride (LaB6 ) source. This beam is then focused at the entrance slit of a premonochromator (M1). The beam from the source will have a typical energy spread of 0.3 eV. The premonochromator, however, as described later, acts to select a fraction of these electrons with a much narrower energy distribution (perhaps 50 meV or less) and, in combination with the intermediate lens system (IL), focuses the beam on the entrance slit of the monochromator (M2). The monochromator is used to further narrow the energy spread in modern instruments to as low as 0.5 meV. Because this energy selection is performed at low electron kinetic energies (∼1 eV), whereas the experimenter desires a range of kinetic energies of 1-300 eV at the sample (S), a rather versatile zoom lens (ZL) system is required to focus and accelerate the beam emerging from M2. A symmetric lens system is employed to focus and decelerate the scattered beam onto the entrance slit of the analyzer (AN), which is typically identical in form to the monochromator M2. After passage through the exit slits (ES), the electrons are detected by a channeltron electron multiplier (D) and sent as pulses to a preamplifier for further processing by standard pulse-counting methods. The elements of the monochromator (M1, M2) and analyzer (AN) are examples of electrostatic energy analyzers very commonly employed in surface analysis applications. A complete description of these devices and their optimization for HREELS is beyond the scope of this article. The interested reader is referred to the review by Roy and Carrette3 for a general introduction to design considerations and to a recent monograph by Ibach4 concerning optimization of these devices for HREELS, especially for cylindrical deflection analyzers (CDA). (2) LK Technologies, Inc., Bloomington, Indiana; model LK2000. (3) Roy, D.; Carette, J. D. Electron Spectroscopy for Surface Analysis; Ibach, H., Ed., Springer: Berlin, 1977. (4) Ibach, H. Electron Energy Loss Spectrometers; Springer: Berlin, 1991.
S0743-7463(97)00729-4 CCC: $15.00 © 1998 American Chemical Society Published on Web 01/31/1998
1356 Langmuir, Vol. 14, No. 6, 1998
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Figure 1. Schematic of a high-resolution electron energy loss spectrometer2 that employs a double-pass monochromator (M1,M2) and rotatable analyzer (AN). (See text for details.)
Energy analyzers employed in HREELS have commonly been of the spherical (SDA-180°) or CDA-127° types, where the number in degrees refers to the angle at which a focused beam returns to focus after passing through the analyzer. An approximate equation governing the resolving power of the CDA-127° with mean radius R0 may be written as follows3:
δE δS R2 β2 + ) + E0 R0 3 4 where δS is the slit width, E0 is the spectrometer pass energy (kinetic energy of the electron beam while passing through the sectors), and R and β are semiangular divergences of the beam in the radial and axial planes, respectively. Here, δE is the full width half-maximum (fwhm) of the monochromatic beam. Similar equations apply to the SDA-180° and other geometries. The essential point here is that given practical limitations on the righthand side of the equation to ∼0.2% (i.e., slit width of 0.1 mm and mean radius of 50 mm), one requires a pass energy of 1 eV to obtain a fwhm of 2 meV and semiangular divergences of ∼2°. This pass energy is, of course, selected by the proportionate difference potential applied to the inner and outer sectors of the analyzer. These considerations mean that one is dealing with very low energy electron beams that will then be sensitive to magnetic fields, stray electric fields, and surface work function inhomogeneities present in the apparatus. For these reasons, commercial instruments typically have a carefully designed magnetic shielding to reduce the ambient field to
