Atom layers

Most metal surfaces have the same atomic structure as in the bulk, except that the interlayer spaciugs of the outenuost few atomic layers differ from the bulk values. In other words, entire atomic layers are shifted as a whole in a direction perpendicular to the surface. This is called relaxation, and it can be either inward or outward. Relaxation is usually reported as a percentage of the value of the bulk interlayer spacing. Relaxation does not affect the two-dimensional surface unit cell synuuetry, so surfaces that are purely relaxed have (1 x 1) synuuetry. 

In many materials, the relaxations between the layers oscillate. For example, if the first-to-second layer spacing is reduced by a few percent, the second-to-third layer spacing would be increased, but by a smaller amount, as illustrated in figure Al,7,31b). These oscillatory relaxations have been measured with FEED [4, 5] and ion scattering [6, 7] to extend to at least the fifth atomic layer into the material. The oscillatory nature of the relaxations results from oscillations in the electron density perpendicular to the surface, which are called Eriedel oscillations [8]. The Eriedel oscillations arise from Eenni-Dirac statistics and impart oscillatory forces to the ion cores. 

The coverage of adsorbates on a given substrate is usually reported in monolayers (ML). Most often, 1 ML is defined as the number of atoms in the outemiost atomic layer of the umeconstmcted, i.e. bulk-tenuinated, substrate. Sometimes, however, 1 ML is defined as the maximum iiumber of adsorbate atoms that can stick to a particular surface, which is temied the saturation coverage. The saturation coverage can be much smaller... 

Electrons are extremely usefiil as surface probes because the distances that they travel within a solid before scattering are rather short. This implies that any electrons that are created deep within a sample do not escape into vacuum. Any technique that relies on measurements of low-energy electrons emitted from a solid therefore provides infonuation from just the outenuost few atomic layers. Because of this inlierent surface sensitivity, the various electron spectroscopies are probably the most usefid and popular teclmiques in surface science. 

It is useful to define the tenns coverage and monolayer for adsorbed layers, since different conventions are used in the literature. The surface coverage measures the two-dimensional density of adsorbates. The most connnon definition of coverage sets it to be equal to one monolayer (1 ML) when each two-dimensional surface unit cell of the unreconstructed substrate is occupied by one adsorbate (the adsorbate may be an atom or a molecule). Thus, an overlayer with a coverage of 1 ML has as many atoms (or molecules) as does the outennost single atomic layer of the substrate. 

To obtain spacings between atomic layers and bond lengdis or angles between atoms, it is necessary to measure and analyse the intensity of diffraction spots. This is analogous to measuring the intensity of XRD reflections. 

Surface crystallography started in the late 1960s, with the simplest possible structures being solved by LEED [14]. Such structures were the clean Ni (111), Cu(l 11) and Al(l 11) surfaces, which are unreconstructed and essentially unrelaxed, i.e. very close to the ideal temrination of the bulk shown in figure B 1.211 a) typically, only one unknown structural parameter was fitted to experiment, namely the spacing between the two outennost atomic layers. 

Brongersma H H and Mul P 1973 Analysis of the outermost atomic layer of a surface by low-energy ion scattering Surf. Sc/. 35 393-412... 

C2.18.4.1 HOMOEPITAXY OF GALLIUM ARSENIDE BY ATOMIC LAYER EPITAXY... 

C2.18.4.2 DEPOSITION OF OXIDE FILMS BY ATOMIC LAYER PROCESSING... 

Ozeki M, Usui A, Yoshinobu A and Nishizawa J (eds) 1994 ALES Proc. 3rd Int. Conf on Atomic Layer Epitaxy (Sendai, Japan, May 1994) Appl. Surf. Sc/. 82/83... 

In the sputtering process, each surface atomic layer is removed consecutively. If there is no diffusion in the target, the composition of the vapor flux leaving the surface is the same as the composition of the bulk of the material being sputtered, even though the composition of the surface may be different from the bulk. This allows the sputter deposition of alloy compositions, which can not be thermally vaporized as the alloy because of the greatly differing vapor pressures of the alloy constituents. 

This reaction is catalyzed by iron, and extensive research, including surface science experiments, has led to an understanding of many of the details (72). The adsorption of H2 on iron is fast, and the adsorption of N2 is slow and characterized by a substantial activation energy. N2 and H2 are both dis so datively adsorbed. Adsorption of N2 leads to reconstmction of the iron surface and formation of stmctures called iron nitrides that have depths of several atomic layers with compositions of approximately Fe N. There is a bulk compound Fe N, but it is thermodynamically unstable when the surface stmcture is stable. Adsorbed species such as the intermediates NH and NH2 have been identified spectroscopically. 

In the discussion so far, an ideal termination of the bulk crystal has been assumed at the surface that is, the positions of atoms in the surface have been assumed to be the same as what they would have been in the bulk before the surface was created. This may not be true. Reconstruction, a rearrangement of atoms in the surfrce and near-surface layers, occurs frequently. It is caused by an attempt of the surface to lower its free energy by eliminating broken bonds. The atomic layers par-... 

---