Nano Lett. http://doi.org/nvr (2013)

Graphene has a variety of intriguing properties because of its honeycomb lattice. Other materials with such two-dimensional structures are known including hexagonal boron nitride and silicene. However, these materials, like graphene, are typically made of p-block elements. Yeliang Wang, Shengbai Zhang, Hong-Jun Gao and colleagues have now shown that two-dimensional honeycomb lattice structures can also be created using transition metal atoms.

The researchers — who are based at Beijing National Laboratory of Condensed Matter Physics, Jilin University and Rensselaer Polytechnic Institute — created a crystalline layer of hafnium on an iridium(111) surface. To prepare the layer, hafnium atoms were deposited on the surface under ultrahigh-vacuum conditions using an electron-beam evaporator. The hafnium initially forms nanoclusters on the surface, but after annealing a well-ordered honeycomb structure was observed using low-energy electron diffraction and scanning tunnelling microscopy.

The experimental results, together with charge-density calculations, suggest that the hafnium forms its own honeycomb lattice with direct hafnium–hafnium bonds. A second hafnium layer can also be formed on top of the first by increasing the hafnium coverage on the surface. Furthermore, calculations suggest that freestanding layers of the material would be ferromagnetic.