1. Laboratory for the Research on the Structure of Matter University of Pennsylvania Philadelphia PA 19104 USA
2. Department of Electrical and Systems Engineering University of Pennsylvania Philadelphia PA 19104 USA
3. School of Electrical Engineering Bangor University Bangor LL57 1UT UK
4. Carl Zeiss Microscopy LLC Ion Microscopy Innovation Center Peabody MA 01960 USA
5. Cavendish Laboratory University of Cambridge Cambridge CB3 0HE UK