Propriedades eletrônicas, magnéticas e estruturais de materiais 2D nano-estruturados

Abstract

Two-dimensional (2D) systems present electronic and magnetic properties which are suitable for nanoscale device applications. Besides the promisor 2D graphene material other 2D systems like ZnO graphitic sheet as well other III/V, II/VI, and IV/IV compounds which are originally in the wurtzite (WZ) structure, were predicted to transform into graphitic like structures when formed by few layers. Some of these 2D systems are intrinsically magnetic, like graphene and ZnO sheet, when in the nanoribbon form, due to the unsaturated edge bonds. However most of these 2D systems have to be doped to be magnetic. The control of charge and magnetic states in these 2D systems is a hard task. Doping with transition metals (TM) or sp elements is a natural choice. Although some sp elements can exhibit magnetism and they are easier to dope, they present smaller ferromagnetic (FM) stability than TM elements in 2D systems. On the other hand, TMs exhibit higher FM coupling, either in 0D,1D,or 2D systems. TMs doped bulk ZnO present a slightly antiferromagnetic (AFM) coupling, while thin films can exhibit ferromagnetism at room temperature, where the magnetic coupling can only be understood if ruled by co-defects. Although magnetism in nanostructures is potential for spintronic applications, those systems must be in contact with other material for practical applications. For example, the planar graphitic ZnO sheets are always onto a surface, which in general is a metallic surface like Ag(111). To grow few layers of ZnO onto a semiconductor material, like Si, is dificult. Usually the interface is not well defined, and a buffer between them has been used. In this way the understanding of the interface between 2D sheets and semiconductor surfaces is crucial for nanodevice applications. More than that, even though confined ZnO systems TM doped present strong FM coupling, it is not known the effects on the ferromagnetism when the ZnO is onto a metal or semiconductor surface. In this thesis, we first investigate the energetic stability of few layers of ZnO adsorbed onto a metal Ag(111) surface, a semiconductor Si(111) surface, graphite and graphene surface. Our results show that the ZnO layers are energetically more stable on nonreconstructed Si(111) surface, followed by ZnO on reconstructed Si(111)7x7, and finally on Ag(111) and graphite/graphene surfaces. TMs doped ZnO layers onto the subtrates present diferent properties for each MT studied. We investigate either the electronic and structural properties of interfaces between semiconductor (III/V, II/VI and IV/IV compounds) and graphene. Our results show that AlN, ZnO and h-BN sheets onto graphene present a interesting possibility to construct heterostructures keeping the graphene electronic propertiers like the high mobility around the k point.