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| Book/Report | FZJ-2018-03511 |
1991
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
Jülich
Please use a persistent id in citations: http://hdl.handle.net/2128/18907
Report No.: Juel-2480
Abstract: The thermal behavior of surface structures of single crystals and the relation between the surface structure and the directionaf and temperature dependent anisotropy of the surface free energy $\gamma(\theta$, T) ($\theta$: angle of orientation in a particular cristallographic zone, T: temperature) has been investigated. The shape of one dimensional periodic surface profiles with periodicities of 5-7 $\mu$m and amplitudes of 0.05-0.2 $\mu$m on low index single crystals of Au has been measured by faser beam diffraction, optical interference-, scanning eiectron- and tunneling-microscopy to study the anisotropy of $\gamma$, while x-ray photoelectron diffraction has been used to investigate the thermal behavior of Pb(110) up to the melting point. Au(111) profiles in the <112> zone show extended flat regions with (111) orientation in the temperature range 1023-1073 K resulting from a cusp in $\gamma(\theta$) at this orientation. In addition, one finds a flat region on the sfoping part of the profile, $\simeq$ 2° from the (111) orientation. This additional fiat region corresponds to a vicinal surface with terraces of 22-23 atoms in width and monoatomic steps. This terrace width fits well with the longer side of the unit cell of the (23 x $\sqrt{3}$) reconstruction of Au(111). Thus, the reconstruction of Au(111) stabilizes this particular vicinal surface. As a result, $\gamma(\theta$) should have a focal minimum within $\simeq$ 2° of the (111) orientation in the <112> zone. Au(100) profiles in the <110> zone show extended flat regions with (100) orientation between 1023 and 1173 K resulting from a cusp in $\gamma(\theta$) at this orientation. In addition the profiles reveal several fiat sections in the sloped region indicating the predominance of an orientation of $\simeq$ 4.8 ° inclined relative to (100). This particular stable orientation in the vicinity of (100) could be the (17,1,1) orientation, also stabilizedby reconstruction of the Au(100) surface which shows a periodicity of 5 atoms along the <110> direction. Thus a stepped (100) surface would easily accomodate the reconstruction when the terrace width exceeded 6 atomic rows holding, e. g., for the (17,1,1) surface. Determing the profile shape by laser beam diffraction yields maximum anisotropies of $\gamma(\theta$) in the investigated temperature range of 6-8% for Au(111) in the <112> zone and 0.5-1 % for Au(100) in the <110> zone, respectively. In both cases y decreases approximately linearly with increasing temperature. The magnitude of the anisotropy of $\gamma$ clearfy demonstrates that the Au(111) and Au(100) surfaces are far below their roughening temperatures in the investigated temperature range. The shape of Au(110) profiles in the <100> zone is approximately sinusoidal above 973 K indicating an almost isotropic $\gamma(\theta$) in the vicinity of the (110) orientation. Profiles on Au(110) along the <110> zone show no flat regions with (110) orientation, but a flat region with (111) orientation in the sloping part of the profile. The almost isotropic $\gamma(\theta$) in the vicinity of the (110) orientation indicates that Au(110) is close to or slightly above its roughening temperature above 973 K. The polar angle distributions of Pb4f$_{7/2}$ core-level photelectrons at 1345 eV kinetic energy reveal enhanced intensity modulations for Pb(110) along the [1$\overline{1}$0], [001] and [1$\overline{1}$2] azimuths in the temperature range of 208-599.3 K, resulting from forwardscattering events along the particular bulk crystallographic directions. The forward scattering enhancement peaks, as a measure of crystalline order near the surface, decrease slowly up to about 500 K. Above this temperature a much more pronounced decrease is observed, indicative of a disordering of the Pb(110) surface. In addition the oft-normal forward-scattering intensities along the [1$\overline{1}$0] athimuth already reveal a small intensity drop above $\simeq$ 350 K. Relating the decrease in the forward-scattering intensity above 500 K to the orderparameter function in a Landau theory indicates anisotropic surface melting of Pb(110) above $\simeq$ 580K with a quasi-liquid surface layer growing logarithmically with increasing temperature. The anisotropy of surface melting results in a faster decrease of surface order in the [1$\overline{1}$0] than in the [001] direction. In the quasi-liquid layer, the forward-scattering intensities are predominantly attenuated by elastic multiple scattering events, which is demonstrated in an additional experiment with deposited K overlayers on Pb(110). In connection with other experiments and a theory for anisotropic surface roughening of fcc(110) surfaces, the features of the forward-scattering intensities in the temperature range of 350-570K can be explained. The srnall drop in intensity for the offnormal emission along the <110> azimuth above $\simeq$ 350K is indicative of an enhanced thermal generation of steps parallel to this azimuth. In addition, above $\simeq$ 500 K an enhanced thermal generation of steps parallel to the <100> azimuth occurs. In experiments determing the equilibrium shape of small Pb crystals, a roughening transition of Pb(110) is found at $\simeq$ 570K.
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