Incommensurability and negative thermal expansion of single layer hexagonal boron nitride

The emerging field of straintronics, i.e., the control and utilization of the strain state of 2D-materials, is of great importance for their technological development, specifically in view of their future incorporation into van der Waals heterostructures. To gain fundamental insight into structural peculiarities of two-dimensional systems, single layer hexagonal boron nitride (hBN) grown on Ir(1 1 1) by chemical vapor deposition was used as a prototypical model system: High-resolution reciprocal space mapping reveals the incommensurate nature of the material system by measuring the hBN in plane lattice parameter with high precision, facili-tated by the moire magnification effect in electron diffraction. In a growth temperature (T-g) regime of 700-1150 degrees C an average lattice parameter of 2.496 +/- 0.006 angstrom was found. Eventually, careful disentanglement of the hBN's and substrate's behavior for rising Tg allowed the determination of a negative thermal expansion coefficient of alpha(hBN) =-2.4 +/- 1.2 x 10(-6) K-1 for free-standing hBN.