Hexagonal Boron Nitride
Hexagonal boron nitride (h-BN) is a wide band-gap material with outstanding mechanical, electrical and optical properties. It consists of a layered structure that is very similar to graphite. We have achieved to fabricate free-hanging single layer hexagonal boron nitride membranes and detect individual boron and nitrogen atoms by aberration-corrected transmission electron microscopy. These single layers were formed from thin h-BN by sweeping away one layer at a time using the beam of the transmission electron microscope. Only one of the elements forms mono-vacancies that grown into beautiful triangle-shaped holes. The resulting shapes resemble Sierpinski-triangles with a minimum granularity given by atomic dimensions. 
Fig. 1 Hexagonal boron nitride structural model and atomically resolved image of an hBN membrane with irradiation-induced, atomically precise triangle-shaped holes.
 J.C. Meyer, A. Chuvilin, G. Algara-Siller, J. Biskupek, U. Kaiser
Selective sputtering and Atomic Resolution imaging of atomically thin Boron Nitride Membranes
Nano Letter, 9, 7, 2683 – 2689 (2009)