Two-dimensional (2D) materials remain in the focus of intensive research directed towards their applications in future spintronics. It was shown theoretically that in order to inject highly spinpolarized current from ferromagnetic metal into graphene implementation of an insulating tunnel barrier is needed [1]. Nowadays, the most promising insulating layer for this purpose is an ultrathin hexagonal boron nitride (h-BN).
To date, the most notable heterostructure where the tunnel spin injection into graphene was directly demonstrated is based on the h-BN/Co interface [2]. Production of such heterostructures involves transfer of CVD-grown graphene on top of h-BN/Co system. During this process and further using the system inevitably gets in contact with molecular oxygen, so it is important to know how O2 molecules affect the h-BN/Co interface. Currently, interaction of molecular oxygen with h-BN/Co system remains completely unexplored.
Thus, in this work we demonstrate a comprehensive study of molecular oxygen reaction with hBN on Co and Au surfaces, which possess different chemical activity and interaction with h-BN. For this purpose, two samples of h-BN were synthesized on Co(0001) substrate. Afterwards, Au intercalation was carried out under one of them. Both samples were annealed in oxygen atmosphere at temperature of 300C; after each stage of annealing XPS and NEXAFS spectra were measured. Data analysis shows that h-BN reacts with oxygen on both surfaces, but in different ways. Comparison of experimental data with DFT calculations allows us to propose following mechanism of h-BN oxidation on cobalt. Oxygen molecules penetrate between h-BN and Co, where they dissociate and active O atoms are further incorporated into h-BN lattice with substitution of N atoms.
In the case of h-BN/Au oxidation occurs much more slowly. Presence of Au monolayer under hBN leads to significant decreasing of oxygen dissociation rate that results in low reactivity. Prolonged annealing leads to etching of h-BN. NEXAFS map at the edge of Co absorption shows forming of islands of cobalt oxide on the whole area in the field of view. We suppose that different mechanism of h-BN oxidation takes place; O atoms do not substitute N atoms inside h-BN lattice. We believe that in the case of h-BN/Au reaction proceeds mainly along the borders of h-BN layer. We suppose that at the areas of surface, where h-BN was etched, Au atoms form clusters and open the path to Co
surface for oxygen.
This work was supported by Saint Petersburg State University Grant 11.65.42.2018 and RFBR Grant No. 17-02-00427 А.