VLEED

C.Q. Sun, Oxygen-reduced inner potential and work function in VLEED. Vacuum 48(10), 865-869 (1997)... 

H. Pfnur, M. Lindroos, D. Menzel, Investigation of adsorbates with low-energy electron-diffraction at very low energies (VLEED). Surf. Sci. 248(1-2), 1-10 (1991)... 

S.M. Thuigate, G. Hitchen, C.Q. Sun, Surface structural determination by VLEED analysis. Surface Science Principles and Applications, ed. by R.J. MacDonald, E.C. Taglauer, K.R. Wandelt. vol. 29 (Springer, Berlin, 1996)... 

The surface matrix in Fig. 3.1c shows the alternative sign of charge distribution both within the domains and in the domain boundaries. Therefore, the surface is fully covered with dipoles in a way that stabilizes not only the domains but also the domain boundaries. Hence, the surface undergoes the tensile stress due to electrostatic attraction among the charged bodies. The atomic valences of the copper atoms within the domain (0 Cu" or Cu" ) differ from that at the domain boundary (Cu ). The surface reaction in the -derived phase takes place without involvement of the second atomic layer at the precursor 0 stage. There are no atoms missing in the short-ordered c(2 x 2)-20 surface but only electron repopulation and polarization. VLEED optimization revealed an off-centered 0 pyramid with a position of the adsorbate (ZX) 0.40 A. DOj 0.18 A is about 5 % of the fourfold hollow dimension) with respect to the fourfold hollow [1]. 

VLEED (with E < 16 eV, or lower than the plasma excitation energy) spectral collects simultaneously comprehensive yet nondestructive information from the outermost two atomic layers about the bond geometry, surface potential barrier (SPB), valence electrons (DOS features), Brillouin zones, and work function, reaction dynamics [69, 70]. VLEED is very insensitive to dislocations of atoms in the third layer and below [69, 70]. In the VLEED, electron beams interact with the valence electrons of atoms in the outermost two layers and resonant within the barrier of surface potential. 

The inelastic damping predominates only in the topmost layer in the VLEED. Therefore, LEED at very low energies is the unique technique that collects nondestructive information from monolayer skin of a substance and its energy covers the valence band. 

Fig. 3.7 Azimuth-angular-resolved VLEED profiles collected from a Cu(OOl) surface exposed to 300-L oxygen at 69.0° incidence. There is a cross-point at about 45° azimuth. Two small sharp peaks (indicated by dotted lines) appear on each curve. The broaden fine-structure feature (dashed lines) splits when the azimuth moves away from the (11) direction...

S.2.4.2 Angular-resolved VLEED Brillouin Zones, Band Structure, and Bond Geometry... 

Decoding the angular-resolved VLEED profiles from the O-Cu(001) surface has yielded static information about the Brillouin zones, band stmcture, and bond geometry (Fig. 3.7 [71, 72]). Calculations also reveal the nonuniformity and anisotropy of the SPB, and the distribution of the DOS in the upper part of the valence band. Oxygen adsorption has reduced the inner potential constant (Vo) by 9.6 % (from 11.56 to 10.5 a.u.) and the work function by 1.2 eV from 5.0 to 3.8 eV. The oxygen-induced reduction in Vq is due to the transportation of atoms and valence electrons at the surface. The lowered work function arises from surface dipole formation that increases the local charge density. 

VLEED profiles. The VLEED I—E profiles were collected from a Cu(OOl) surface exposed to 300-L oxygen. For the symmetry consideration, angles from 18.5° to 63.5° with 5° increment are sufficient to represent the fiiU azimuth of the surface. The following feamres the VLEED fine-structure in Fig. 3.7 ... 

Accordingly, two-dimensional Brillouin zones can be constructed from the sharp-peak positions in the angular-resolved VLEED data, as summarized in Table 3.5. The location of a peak ), can be decomposed in fc-space as (in atomic units m = e = Jr = 1, 1 a.u. = 0.529 A and 27.21 eV) ... 

Table 3.5 Positions of the sharp peaks, Epi and Ep2, in the angular-resolved VLEED profiles of the O-Cu(001) surface. The incident angle keeps constant at 69.0°...

Fig. 3.8 The first two Brillouin zones (indicated by open circles and triangles) derived from the critical positions on the angular-resolved VLEED profiles, compared with the theoretical ones in solid lines, which results in the effective electron masses as an adjustable. Deformation of the first Brillouin zone near Y point corresponds to the DCUv in real lattice...

Fig. 3.9 Band structure extracted from the angular-resolved VLEED profiles [75], The boundary lines divide the VLEED energies into various bands as indicated. The VLEED window covers that of both STS and UPS in the valence band and above...

Angular-resolved VLEED analysis yields the change in energy states that contribute to the damping. Notable regions exhibiting DOS features ... 

The inner potential relates to the charge quantity and the work function depends on the density of the polarized electrons at the surface. The 3B model explains consistently the reduction both in the inner potential constant and the work function. With expansion of sizes and elevation of energy states, metal dipoles are responsible for the reduction in the local work function. Electron transportation due to reaction dominates the change of the inner potential constant. Besides, at very low incident beam energies, the exchange interaction between the energetic incident beams and the surface is insignificant, and hence, the VLEED is such a technique that collects nondestructive information from the skin of two-atomic-layer thick. 

Oxygen reduced Fq. Chemisorbed oxygen changes the Fq in a way that is much more complicated. The amount of the reduction varies with crystal structure. For a Cu(001)-c(2 X 2)-20 and a (2 2 x 2)R45°-20 structure, the reduction was found 1.21 and 2.15 eV, respectively [17]. Calculations of VLEED I-E curves with the CU3O2 bond configuration require a 1.06 eV (9.2 %) reduction in the Fq from the bulk value of Cu(OOl), 11.56 eV[70]. Pfniir et al. [77] used a step function to describe the reduction in the Fq of the O-Ru top layer. It is certain that oxygen chemisorption results in a pronounced reduction in the Fq. 

Oxygen-reduced >l(E). The is related to the valence DOS density n(E). As justified, the z-directional integration of the p x, y, z), from the second layer to infinitely far away of the surface, yields the local DOS n x, y) that contributes to the SPB as well. Since the VLEED integrates over large surface areas, aU the quantities depending on coordinate (jc, y) become E dependent. Therefore, at a certain energy, the VLEED integration results the n x, y) into n E) that relates to the occupied DOS and the local work function. 

The work function depends uniquely on the electron density at surface. However, the concept holds for large surface areas over which the VLEED... 

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