Exploration of hole-doping-induced ferromagnetism in two-dimensional materials

Promovendus/a: Ruishen Meng

Promotor(en): Prof. dr. Michel Houssa

Since the discovery of graphene in 2004, two-dimensional (2D) materials have triggered a lot of research interest, both scientifically and technologically. Their versatile electronic properties make them interesting candidate materials to be integrated into future nanoelectronic devices. 2D magnetic materials have also received increasing research attention in the past few years, for their possible use in spintronic devices. However, the amount of current existing 2D ferromagnetic materials are relatively rare and their Curie temperatures are relatively low, it is imperative to find more 2D ferromagnetic materials with high Curie temperatures.

This dissertation aims to search for the 2D non-magnetic materials which will show non-magnetic to ferromagnetic transition after they are hole doped, using density functional theory calculations. More than one hundred candidate 2D materials with high stability and moderate Curie temperatures upon hole doping are identified, in which almost half of them are 2D metal halides, next to chalcogenides, oxides, and nitrides. The possible magnetic exchange coupling mechanism is discussed and we found that the hybridization between the metal d orbitals and the anion p orbitals in these 2D compounds plays an importantt role in promoting the Curie temperatures. In addition, the effects of intrinsic and extrinsic point defects on the electronic and magnetic properties of a few selected candidate materials are also studied. The results indicate that p-type doping of 2D nitrides (AlN, GaN, InN), sulfides (ZnS, SnS2) and oxides (Ga2O3) is very unlikely, most of the acceptor type defects producing deep states in their energy band-gap, rather behaving as trapping centers for charge carriers. On the other hand, more promising results are obtained for the considered 2D metal halides, namely PbBr2 and HgBr2. As a matter of fact, both metal vacancies and doping impurities, such as S, Se and Li, have relatively small formation energies, and produce shallow acceptor levels, typically less than 0.2 eV above the valance band edge.