1. School of Physics, Trinity College Dublin, College Green , Dublin D02 PN40 , Ireland
2. Centre for Doctoral Training in the Advanced Characterisation of Materials, AMBER Centre, Trinity College Dublin , Dublin D02 PN40 , Ireland
3. Department of Materials, University of Oxford , Parks Road, Oxford OX1 3PH , UK
4. Advanced Microscopy Laboratory, Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN), Trinity College Dublin , Dublin D02 PN40 , Ireland