Electronic and magnetic properties of Ni nanoparticles embedded in various organic semiconductor matrices.

Ni nanoparticles with a size distribution from 2 to 6 nm, embedded in various organic matrices, were fabricated in ultrahigh vacuum. For this purpose metal free and Ni phthalocyanine, fullerene C(60), and pentacene were coevaporated with Ni. When coevaporated, Ni and H(2)Pc react, leading to the formation of NiPc and Ni nanoparticles. The molecular structure of the matrix was found to have negligible effect on the size of the nanoparticles but to influence the magnetic anisotropy of the nanoparticles: Ni nanoparticles formed in the buckyball matrix have a cubic symmetry, while nanoparticles formed in matrices consisting of planar molecules exhibit a uniaxial symmetry. After exposure to atmosphere, photoelectron spectroscopy investigations demonstrate the presence of metallic Ni nanoparticles accompanied by Ni oxide and the existence of a charge transfer from the organic matrix to the particles in all investigated systems. The oxidized Ni nanoparticles exhibit a larger magnetic anisotropy compared to the freshly prepared particles which show superparamagnetic properties above 17 K. Moreover, photoelectron spectroscopy was used to probe the oxidation process of the Ni nanoparticles in different organic matrices. It could thus be shown that a matrix consisting of spherical molecules like C(60) prevent the particles much better from oxidation compared to matrices of flat molecules.