Pseudospirals, imperfect structures and crystal habit produced by field evaporation of metal crystals

Abstract

Spiral-like structures are frequently observed when the tip of a field emission microscope is subjected to field evaporation at elevated temperatures. Close inspection of tips of various metals with the field ion microscope reveals that these structures are pseudo spirals, appearing as a result of eccentric bunching of complete low index net planes, without requiring the presence of a screw dislocation. The extremely large stress of the field causes various dislocations to move towards and to pile up at the surface, where they can be seen in the ion image. A picture is shown of the atomic arrangement at a low angle twin boundary in a tungsten crystal parallel to the [100]-zone. Field evaporation carried out at temperatures below the range where imperfections can move to the surface may yield perfect surfaces. Their habit is characterized by a rounded-off shape, with the radius of curvature varying in the same sense as the work function of the particular crystallographic region. The field strength for evaporation of tungsten at practically zero temperature is measured to be 1 × 10 9 V/ cm on the 111 plane and 8 × 10 8 V/ cm on the 011 plane, in agreement with the theory. Crystal surfaces, as obtained by low temperature evaporation at slightly lower field strengths, are also shown for platinum, iridium and rhenium.