A new application of MIES: monitoring the growth of oxide islands on Ni(100,111)

Abstract

The lateral electric field that is created on a surface near the boundary between areas of different work functions φ 1, φ 2 drags approaching metastable helium atoms — after conversion to He + ions — to the area of higher work function φ 2. This effect leads us to the concept of the capture range r capt which means that He ∗ atoms originally aimed at a point on the low φ 1 area within a distance from the boundary which is smaller than r capt will finally land on the high φ 2 area. We demonstrate the effect for oxide islands surrounded by chemisorbed oxygen on Ni surfaces. The oxide work function is roughly 1.5 eV below that of the chemisorbed oxygen. As a consequence we find that the oxide contribution is systematically underestimated in MIES. By combining with XPS data we are able to exploit this fact to determine the average radius of the oxide islands and the number of islands per cm 2. At present both quantities still depend on the size of r capt which serves as a scaling length. However, their relative values can be followed during oxide growth until closed oxide layers are completed. This novel method has the potential to test existing oxide growth models in a rather direct way. The model should be applicable to a large variety of problems. The method can be developed to give not only an absolute value for the basic scaling length r capt, but to derive information on island shapes.