Colloquium: Dr. Cem Sevik, Eskisehir Tech University

Extraordinary negative thermal expansion of two-dimensional honeycomb materials

ABSTRACT:

The negative thermal expansion nature of the graphite is more pronounced in the two-dimensional limit, so-called graphene. This exciting change - mainly attributed to the enhanced anharmonicity and quadratic behavior of the out-of-plane acoustic mode - is validated via both First-principles Quasiharmonic theory calculations and experimental measurements. However, the notable difference between the different experimental studies and quasi-harmonic theory is still unclear. On the other hand, the extend of negative thermal expansion of other honeycomb structures is also an interesting research subject. In this respect, we investigated the Thermal expansion behavior of two-dimensional nitrides and graphene by ab initio molecular dynamics simulations as well as a quasiharmonic approximation. We predicted that anharmonicity of the acoustic phonon modes is related to the unusual negative thermal expansion behavior of the nitrides. Our results also hint that direct ab initio MD simulations are a more elaborate method to investigate the thermal expansion behavior of 2D materials than the QHA. Nevertheless, we show that h-GaN and h-AlN possess giant NTE coefficients within the covered temperature range 100–600 K regardless of the chosen computational method. This unusual NTE of 2D nitrides is reasoned with the out-of-plane oscillations related to the rippling behavior of the monolayers.