Estimation of ad-vacancy formation energy on the Pt(111) surface by using reflection electron microscopy

The quenching technique which is usually employed for studying lattice defects in metals was applied to estimate the formation energy of an ad-vacancy on the Pt(111) surface. These ad-vacancies coagulate in clusters, forming monoatomic step depression islands during cooling. They can be imaged by reflection electron microscopy as atomic step loops. The concentration of ad-vacancies at high temperatures could be measured from the total area of the atomic step loops covering the surface. The obtained value of ad-vacancy formation energy is about 0.62 eV on the assumption that the relation for the massive crystal can also be applied to the crystal surface phenomena

Observation of surface crystallography by reflection electron microscopy

Surface crystallography of (111) and (100) facets on spherical single crystals of Pt and Au was studied by reflection electron microscopy. Effects of surface channelling allow a very accurate determination of crystallographic directions. The facets were cleaned by Ar ion bombardment and smoothened by electron beam heating. In order to obtain atomically flat surfaces the specimens had to be cooled down very slowly after annealing. By such treatments the (100) surfaces of Au and Pt and also the (111) surface of Au are reconstructed with certainty. The direct electron microscopic observation of these reconstructions is very difficult or impossible after the transfer through the atmosphere to a conventional electron microscope. However, modifications of the surface topography controlled by the reconstruction can be observed. As an example the edges of the (100) facets at the starting curvature of the crystal sphere are shown. Here sequences of monatomic steps are formed, following the directions of the (100) surface reconstruction, which is written in matrix form as N1-15 with N = 12–14. Reconstructed domains on (100) surfaces of Pt and Au could be imaged. As another example for surface crystallography the observation of screw dislocations starting a monatomic step and edge dislocations forming a low-angle grain boundary are shown

REM studies of surface crystallography

Reflection electron microscopy (REM) allows a very accurate determination of crystallographic directions on a surface. The lack of resolution in REM due to foreshortening is compensated by enhanced sensitivity for measuring angular differences of these directions. This is shown by sequences of atomic steps on Pt(100). The lattice of reconstruction on (100) and (111) surfaces of gold could directly be imaged with a separation of approximately 4.0 nm and 6.7 nm, respectively

METHOD OF DIRECTLY IMAGING THE RECONSTRUCTION ON AU(111) AND AU(100) BY REFLECTION ELECTRON-MICROSCOPY

The reconstruction of the (100) and (111) surfaces on gold could be directly imaged in a conventional electron microscope with a double-tilt heating goniometer in combination with the minimum-exposure technique. On Au(100) superlattice fringes of 1.4 and 4.0 nm could be observed. They correspond to the {{27, 2}, {-1, 10}} reconstruction. On Au(111) the 6.6 nm fringes of a complicated 23 x 1 superstrurture could also be seen. The visibility of the reconstruction disappeared during observation within several seconds. The preheating of the gold crystal up to approximately 180-degrees-C without the electron beam was essential for the observation of the reconstruction

CONTRAST VARIATION OF AU(111) RECONSTRUCTED SURFACES WITH DEFOCUS IN REM

Reconstruction on a clean Au(111) surface shows complicated contrast in reflection electron microscopic images using many diffracted beams originating from the superstructure. From an analysis of contrast variations with focusing conditions, we deduce that the image consists of phase and amplitude contrast. This contrast is produced by the difference between electron waves scattered from the coincidence and non-coincidence regions on the surface of the Au(111) reconstruction

In-situ observation of the phase transition on Au(100) surfaces

Nucleation and growth of the Au(100) reconstruction were observed by means of reflection electron microscopy during the phase transition from nonreconstructed to reconstructed surface. In the early stage of the reconstruction nucleation, reconstructed areas were imaged as stripe domains which have certain orientations. These domains grew only in width at about 160°C. At 250°C, the Au(100) surface was covered completely by the reconstruction which could be destroyed by a high-energy electron beam. The destroyed reconstruction might recover subsequently. A contracted and twisted dislocation core structure was proposed to explain the formation of the stripe reconstruction domains

Reflection electron microscopy of the catalytic etching of Pt single-crystal spheres in CO + O₂

The influence of the catalytic oxidation of CO on structural changes of a Pt catalyst was studied with a Pt single-crystal sphere in the 10−4 Torr range at 200 °C sample temperature. Reflection electron microscopy (REM) served to follow the structural changes which are induced by the catalytic reaction on the Pt(100) and Pt(111) facets. Using an intensity-enhanced Bragg reflection for imaging the surface, the diffraction contrast allows one to resolve atomic steps. Both surfaces are homogeneously roughened by the catalytic reaction, but the Pt(100) surface is much more strongly attacked than Pt(111). Small positive islands develop on an initially flat (100) terrace which are preferentially oriented in the directions of the unit cell of the hex reconstruction. The mechanism of the roughening process on Pt(100) is traced back to the adsorbate-induced 1 × 1 ⇌ hex phase transition which can strongly enhance the mass transport of Pt atoms under reaction conditions. Oxide formation or mass transport via volatile Pt compounds is considered to play a negligible role under the conditions applied here

Interpretation of RHEED oscillations during MBE growth

The intensity oscillations of RHEED reflections during molecular beam epitaxial growth are explained in terms of refraction and total reflection. Experimental observations of integrated intensity oscillations on silicon are in good agreement with calculations based on the simple model