1. Department of Physics; University of the Western Cape; Robert Sobukwe Road, Private Bag X17 Bellville 7535 South Africa
2. DST/CSIR National Centre for Nano-Structured Materials; Council for Scientific and Industrial Research; Pretoria 0001 South Africa
3. Electron Microscopy Unit; University of the Western Cape; Private Bag X17 Bellville 7535 South Africa
4. National Metrology Institute of South Africa; Private Bag X34, Lynwood Ridge Pretoria 0040 South Africa