Thermal surface reconstructions

Figure 8.1.1 The onset temperatures of thermally induced phenomena scaled to the bulk melting temperature, Tm- This generic scale is an approximation since some of the mentioned processes, for example, thermal surface reconstructions, may not exist for...

To take place, these surface reconstructions may require a certain temperature. One distinguishes two cases. In the first, the surface reconstruction appearing above the transition temperature remains stable all the way down to low temperature, whereas in the second, there is a reversible first-order phase transition between two surface terminations. In the first case, the surface needs to overcome an activation barrier to get out of its metastable state and achieve the reconstruction, which is the thermodynamic ground state down to low T. The thermal evolution is dominated by kinetic barriers, but not by minimization of the surface free energy. In the second case, the higher temperature phase is a thermal surface reconstruction, which appears when the surface is annealed beyond the transition temperature and disappears once it is cooled below. We discuss one example for the first case of an irreversible transition and two for the second case of thermal surface reconstructions, which represent true phase transitions. 

The thermal surface reconstructions of Si(lll) and Pt(lll) take place at similar relative transition temperatures, rc/Tm(Si) = 1200K/1683 K = 71%, versus Tc/Tm(Pt) = 1330 K/2045 K = 65%. The latter value has been used to situate this phase transition in Figure 8.1.1. 

Surface reconstruction has been earlier observed and reported in the literature [116]. Sequential reductive and oxidative thermal treatment usually leads to bulk transition from CoOx + La203 to LaCo03, respectively. On the other hand, the restoration of the perovskite structure is not observed under severe conditions at higher temperature. In those temperature conditions, the sintering of Co crystallites leads to irreversible redox cycle with the preferential formation of Co304 under lean conditions. 

It is now well established that the structure of a thermally equilibrated surface may be quite different from that obtained by simply truncating a solid.10 Surface atoms may rearrange to form a structure according to the lowest surface free energy of that surface. The determination of the atomic structure of solid surfaces is one of the important endeavors in surface science, as the atomic structure will decide both the physical and the chemical properties of the surfaces. Surface reconstruction will also play an important role in the crystal growth and epitaxy of thin films. 

Shimazaki, J., Shimmura, S., Tsubota, K. Donor source affects the outcome of ocular surface reconstruction in chemical or thermal burns of the cornea. Ophthalmology 111, 38-44 (2004)... 

Shimazaki, J., Yang, H.Y., Tsubota, K. Amniotic membrane transplantation for ocular surface reconstruction in patients with chemical and thermal burns. Ophthalmology 104, 2068-2076 (1997)... 

Excitation spectroscopy Monitoring of the surface emission allows one to discriminate the upper excited surface states and their relaxation dynamics. Problems such as surface reconstruction, or quantum percolation of surface excitons upon thermal and static disorder, are connected with high accuracy to changes of the exciton spectra.61118,119,121... 

High-energy ions, above 500 KeV, are scattered off of a single crystal surface. The "channeling" and "blocking" of scattered ions within the crystal can be used to triangulate deviations from the bulk structure. HEIS has been used in particular to study surface reconstructions and the thermal vibrations of surface atoms (see also MEIS, ISS)... 

The reason for surface relaxation and surface reconstruction is the minimization of the specific surface energy of the system. Surface reconstruction can be induced either thermally or by the electrode potential. 

Surface relaxation and surface reconstruction phenomena can be lifted thermally, by adsorption of foreign species, or by changing the electrode potential in the case of electrochemical systems [2.10]. In the case of AuQikl) surfaces, the influence of the electrode potential on the structure of the surface top layer is illustrated in Fig. 2.7. The transition from the reconstructed to the unreconstructed surface top layer represents a first order phase transition process in many cases. 

Figure 2.6 In situ STM images of a freshly prepared Au(lll) substrate showing the initial thermally induced reconstruction rows (visible as stripes) for different surface areas [2.10]. System Au(lll)/ 10 M H2SO4 at = - 150 mV vs. SCE and T = 298 K. (a) top view of an atomically smooth surface, and (b) 3D representation of a face with a monatomic step. Reprinted by permission of Kluwer Academic Publishers.

Low-energy boron bombardment of silicon has been simulated at room temperature by MD. Tersoff potential T3 was used in the simulation and smoothly linked up with the universal potential. The boron-silicon interaction was simulated according to Tersoff potential for SiC but modified to account for the B-Si interaction. Silicon crystal (Si-c) in the (001) direction, with (2x1) surface reconstruction, was bombarded with boron at 200 and 500 eV. Reasonably good statistics are obtained with 1000 impact points uniformly distributed over a representative surface area. The simulation size was 16x 16x 14 unit cells. Periodic boundary conditions were applied laterally. The temperature was kept at 300 K with a thermal bath applied to the more external cells in the crystal except the top surface. In these conditions the crystal was relaxed during 19 ps. In order to avoid direct channeling, the incidence was inclined 7° out of the normal, as usual in experiments, with random azimuthal direction. 

The surface structure of single-crystal faces of noble metals such as KgQtkl) and AvL hkJ) were found to be unreconstructed under defined electrochemical conditions. For example, a unreconstructed Ag(lll) surface domain on a terrace in contact with perchloric acid solution is shown in Fig. 1. Surface reconstruction changing the interatomic distance and symmetry of surface atoms can be induced thermally or by potential, as observed by other authors [12]. This phenomenon can be lifted by potential or by adsorption processes. Surface defects such as monatomic steps and... 

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