The thermal stability and rearrangement of field-evaporated tungsten surfaces

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Abstract
The changes that occurred on heating a field-evaporated tungsten surface were investigated at the atomic level by means of field ion microscopy. The tungsten surfaces were cleaned by heating before field evaporation, so that surface rearrangement observed was due only to the movement of tungsten atoms. It is shown that because of the imaging conditions, only displacements of atoms from one surface site to another of at least equal binding energy were directly observable. Apart from small changes at 300 $^\circ$K, which may have been due to the elimination of defects from the edges nearer to (001) of the (123) and (112) planes, the first observable changes were displacements of atoms from one kink to the next, in the edges of lattice planes. At 500 $^\circ$K atoms were displaced along the edges of (011) terraces from one kink site of (111) type to the next, with an activation energy of 1.8 eV, while displacement from the (001)-like kinks occurred at a slightly higher temperature, with an activation energy of 2.1 eV. Similar displacements along the edges of (111) and (001) terraces occurred at 600 to 700 $^\circ$K, with activation energies of 2.2 and 2.3 eV respectively. The difference between these activation energies and the higher values calculated for the particular atom displacements from the simple pairwise interaction model of surface forces is discussed. At temperatures above 850 $^\circ$K, surface rearrangement with a consequent lowering of surface energy was extensive. Enlargement of the (112) and (011) planes occurred by removal of their topmost layers by surface diffusion processes. Changes in local surface topography that were observed include, the extension of unbroken rows of close-packed atoms where these formed the edges of lattice planes, the formation of surface steps several atom layers high, and local roughening due to the random movement of surface atoms.

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