Variations of the lattice parameter can significantly change the properties of a material, and, in particular, its electronic behaviour. In the case of graphene, however, variations of the lattice constant with respect to graphite have been limited to less than 2.5% due to its well-established high in-plane stiffness. Here, through systematic electronic and lattice structure studies, we report regions where the lattice constant of graphene monolayers grown on copper by chemical vapour deposition increases up to similar to 7.5% of its relaxed value. Density functional theory calculations confirm that this expanded phase is energetically metastable and driven by the enhanced interaction between the substrate and the graphene adlayer. We also prove that this phase possesses distinctive chemical and electronic properties. The inherent phase complexity of graphene grown on copper foils revealed in this study may inspire the investigation of possible metastable phases in other seemingly simple heterostructure systems.

As a promising thermoelectric material, BiCuSeO is of great interest for energy conversion. A higher figure of merit in n-type BiCuSeO than that in the p-type was predicted from theory, suggesting a need of in-depth investigations on the doping effects. In this work, the influences of group IV elements (Si, Ge, Sn, and Pb) on the electronic structures of BiCuSeO are studied from first principles. Despite the similar electronegativities of the group IV elements, Si is found to be an n-type dopant, being distinctly different from Ge, Sn, and Pb, which exhibit typical p-type behaviors. Detailed analysis on the doping effects is performed based on a recently developed band unfolding technique. Furthermore, Si-doped BiCuSeO is shown to have a higher power factor than p-type BiCuSeO from the Boltzmann transport theory.

The doping effects of the boron group (Al, Ga, In and Tl) and the nitrogen group elements (As and Sb) on BiCuSeO are studied combining a band unfolding technique and density functional theory. Substitutional site preferences of these dopants are predicted based on the formation energy. It is found that significant resonant states near the conduction band minimum and valence band maximum of BiCuSeO are induced by In and Tl, respectively, providing a guideline for the enhancement of thermoelectric efficiency through band engineering. Effective band structures of the doped systems have been obtained for direct examination of the resonant states. Arsenic tends to substitute either Bi or Se atoms, while the electronic structure strongly depends on the substitutional sites. The decomposed density of states and charge densities are also calculated to unveil the origins of the resonant effects.