Condensed-Matter Physics (1986) / Chapter Skim
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7 Surfaces and Interfaces
7 Surfaces and Interfaces
Pages 144-163
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From page 144... ...
Numerous examples now exist where important new conclusions have followed from the interplay between theory and experiment. In addition, we see substantial progress in the development of both ab initio descriptions of the electronic structure of clean and adsorbate-covered surfaces and of the dynamics of crystal surfaces.
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From page 145... ...
S UREA CES A ND I N TERFA CES 145 TABLE 7.1 Experimental Techniques Used in the Study of Physical Properties of Surfaces and Interfaces Physical Property Studied Elementary Interface Excitations Atoms and Between on the Molecules on Solids and Experimental Technique Structure Surface the Surface Dense Media Ion beams Raman spectroscopy Scanning vacuum tunneling X X X X X microscope Synchrotron radiation X X Electron energy-loss X X spectroscopy Electron microscopy X Atom/surface scattering X X X Low-energy electron X X diffraction Neutron scattering X X X X rays X Infrared spectroscopy X X X Spin-dependent electron X scattering Brillouin scattering X X Diffusion of adsorbed X species Molecular beams X Laser-induced desorption X or fluorescence Inelastic electron-tunneling X spectroscopy This chapter is concerned primarily with the physics of the outermost atomic layer or two of single crystals in an ultrahigh-vacuum environment, along with that of monolayer quantities of adsorbate atoms or molecules upon it. The adsorbates may be chemisorbed, i.e., bound to the surface tightly via chemical bonds similar to those encountered in molecules, or physisorbed, where only a much weaker van der Waals attraction traps the adsorbate near the surface.
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From page 146... ...
The environment of a chemisorbed species differs greatly from that in an isolated molecule, and this leads to new electronic configurations. At finite coverage, adsorbates may interact directly via the overlap of their wave functions, indirectly via the perturbation of the electronic structure of the substrate, or by local strains induced by chemisorption.
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From page 147... ...
Thus, research in surface physics has a direct impact on this exciting new area of materials science. THE STRUCTURE OF THE CRYSTAL SURFACE If one is to understand properties of, and bonding to, the outermost layer of a crystal, a first step is the elucidation of the geometrical arrangement of the constituents.
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From page 148... ...
Both have a deBroglie wavelength comparable to lattice spacings, or bond lengths, and thus serve as sensitive probes of microscopic aspects of surface geometry. Of the electron spectroscopies.
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From page 149... ...
As remarked earlier, low-energy atoms have a de Broglie wavelength comparable to crystalline lattice constants, and such atoms are backscattered from the outermost portions of the electron chargedensity contours. Recently there has been a great improvement in our ability to prepare monoenergetic beams of light atoms, such as He.
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From page 150... ...
These are sensitive to the details of the potential, including its variation in the two dimensions parallel to the sample surface. New high-energy ion backscattering studies place important constraints on surface geometry and provide quantitative information on whether surface atoms are shifted off high symmetry sites or if there is a contraction or expansion of the distance between outermost layers of the crystal.
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From page 151... ...
Electron microscopy will become an important tool for exploring a range of issues such as the nature of defects on the surface, dynamical aspects of the surface environment, and also microscopic aspects of surface structure. A final probe that is being used extensively in surface studies is the field ion microscope.
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From page 152... ...
During the past 2 years, two methods-inelastic scattering of helium atoms and electron energy-loss spectroscopy have been developed to the point where they can now be used for detailed measurements of surface phonon dispersion curves. In the previous section we noted that we now have in hand highly monoenergetic beams of slow, neutral He atoms.
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From page 153... ...
We thus have two methods that may be utilized to explore surface phonon dispersion curves, and the coming years should prove to be an exciting era in the spectroscopy of surface vibrations. It should be remarked that the two methods discussed above will surely emerge as complementary approaches to the problem.
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From page 154... ...
Since infrared spectroscopy is at the moment the only experimental method that has provided data on the intrinsic linewidths of simple adsorbates on single-crystal surfaces, further application of these new approaches to a broader class of systems is of paramount importance. Great excitement was generated a few years ago by the discovery of giant Raman signals from adsorbed molecules.
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From page 155... ...
Systematic control of the enhancement effect can be achieved utilizing a surface on which a diffraction grating is present. The resulting enhanced fields can be exploited to enhance a wide variety of linear and nonlinear optical interactions near surfaces and interfaces, in addition to the Raman effect.
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From page 156... ...
Much attention has been devoted to the phase diagram of monolayer or near-monolayer quantities of rare-gas adsorbates, in the limit where the influence of the corrugations in the substrate potential may be ignored. The phase diagram of chemisorbed systems has also been studied both experimentally and theoretically, and the strengths of the various lateral interactions deduced from it.
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From page 157... ...
A non-Boltzmann distribution is frequently found, and there has been considerable success in the comparison between such data and theory based on computer simulations. THE INTERFACE BETWEEN SOLIDS AND DENSE MEDIA In many instances, one is interested in surfaces placed in a high-pressure environment or in the interface between a surface and a dense medium such as a liquid.
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From page 158... ...
In an earlier section we mentioned that, under certain circumstances, the Raman signal from adsorbed molecules may be enhanced enormously over that appropriate to the gas phase. However, since surface roughness combined with the presence of long-lived surface resonances is necessary to realize these large signals, the effect is of limited utility for the study of single crystals in high vacuum.
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From page 159... ...
THEORY Advances in experimental techniques for the study of structural and dynamical properties of crystal surfaces, particularly through the use of external probes such as electrons and rare-gas atoms, require parallel theoretical efforts for their interpretation owing to the strong interaction of these probes with the system being studied. The experimental advances of the past decade described in the preceding four sections have been accompanied by significant theoretical achievements.
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From page 160... ...
OPPORTUNITIES The achievements of the past decade provide some indications of the areas in which research in surface physics will be carried out in the next decade. Surface Brillouin spectroscopy, the emergence of Raman spectroscopy as a surface-sensitive probe, and the use of field enhancement in a variety of optical interactions near surfaces and interfaces constitute
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From page 161... ...
In spin-dependent scattering of electrons from a surface we have a new surface probe that for the first time can be used to probe magnetism in the outermost atomic layers of crystals. For example, antiferromagnets should be readily studied by this technique, since new Bragg beams will appear if the surface orders in such a manner that the appropriate two-dimensional unit cell increases in size.
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From page 162... ...
A complete understanding of the photoemission process requires knowledge not only of the electron energy states, their wave functions, and the sensitivity of both to surface structure, but it also requires knowledge of the electromagnetic field of the incoming photon in the near vicinity of the surface. This is an area where further theoretical understanding is both required and will prove fundamental not only to photoemission spectroscopy but to other surface spectroscopies addressed in this report.
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From page 163... ...
Since these interactions play a key role in stabilizing the various surface phases encountered in chemisorbed systems, and control the degree of short-range order present in a disordered overlayer, a more complete understanding of the underlying physics that controls their strength and magnitude is important to have. The study of the interaction of small molecules with surfaces, with emphasis on the interchange of vibrational or rotational energy, is expected to be a lively and active area in the coming years.
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