Zn vacancy ferromagnetism in ZnS nanocrystals

Abstract

To explain three order of magnitude discrepancy between the experimental and calculated magnetic moments due to Zn vacancies in ZnS nanocrystals we study the effect of Zn vacancy distribution in a nanocrystal (NC). We consider small, intermediate, and large quantum dots. For the latter we employ the surface-bulk model. To study Zn vacancy distributions we choose aggregates and uniformly distributed gas in an NC core, on a surface, and both core+surface. From the investigation we find that the uniform distribution of vacancies on a surface is the most favorable configuration for quantum dots of all sizes. Then we investigate the detailed vacancy arrangement on an NC surface. We find that the most likely arrangement if the vacancies are placed along the diagonal of the unit cell square rather than along the square side. The former has the zero magnetic moment what explains the huge reduction in the total magnetization. The small nonvanishing magnetization is probably due to the presence of a small amount of Zn vacancies in the side configuration in a metastable state.