The structural, magnetic and optical properties of TMn@(ZnO)42 (TM = Fe, Co and Ni) hetero-nanostructure

Abstract

The magnetic transition-metal (TM) @ oxide nanoparticles have been of great interest due to their wide range of applications, from medical sensors in magnetic resonance imaging to photo-catalysis. Although several studies on small clusters of TM@oxide have been reported, the understanding of the physical electronic properties of TM_n@(ZnO)₄₂ is far from sufficient. In this work, the electronic, magnetic and optical properties of TM_n@(ZnO)₄₂ (TM = Fe, Co and Ni) hetero-nanostructure are investigated using the density functional theory (DFT). It has been found that the core-shell nanostructure Fe₁₃@(ZnO)₄₂, Co₁₅@(ZnO)₄₂ and Ni₁₅@(ZnO)₄₂ are the most stable structures. Moreover, it is also predicted that the variation of the magnetic moment and magnetism of Fe, Co and Ni in TM_n@ZnO₄₂ hetero-nanostructure mainly stems from effective hybridization between core TM-3d orbitals and shell O-2p orbitals, and a magnetic moment inversion for Fe₁₅@(ZnO)₄₂ is investigated. Finally, optical properties studied by calculations show a red shift phenomenon in the absorption spectrum compared with the case of (ZnO)₄₈.

Figure 1

The optimized geometries of TM_n@(ZnO)₄₂ core-shell nanostructure. The pink, purple and blue balls show the positions of O, Zn and TM atoms, respectively. The small or abnormal magnetic moment of TM atoms are shown by yellow balls. The numbers below the inner core configurations indicate the average bond lengths (Å) within 3.00 Å (see Supporting Information III and supporting information I for enlarged picture of core-shell structures).

Figure 2

The optimized geometries of Fe inner-core of each size (n = 6–13 and 15).

Figure 3

The second-order differences of total energies Δ ₂ E _n of TM_n@(ZnO)₄₂ nanostructure. It is noted that the largest Δ ₂ E _n are found at n = 13, 15 and 15 for TM = Fe, Co and Ni in TM_n@(ZnO)₄₂ core-shell structures, respectively, indicating that Fe₁₃@(ZnO)₄₂, Co₁₅@(ZnO)₄₂ and Ni₁₅@(ZnO)₄₂ are the most stable structure.

Figure 4

Plot of the 2D electron density difference and geometry configuration for (a) Fe₁₅@(ZnO)₄₂; (b) Co₁₅@(ZnO)₄₂ and (c) Ni₁₃@(ZnO)₄₂. The atom numbers which overlap with corresponding atom and the bond length (Å) are depicted on geometry configuration.

Figure 5

The partial DOS of (a) Fe₁₅@(ZnO)₄₂, (b) Co₁₅@(ZnO)₄₂ and (c) Ni₁₃@(ZnO)₄₂; and the total DOS of (d) M@ZnO; The dotted lines refer to the Fermi level. The unit is electrons/eV. The magnetic moment of some corresponding atom is also depicted inside each DOS and the unit is μB/atom.

Figure 6

The calculated optical absorption (a), and dielectric function: real part (b) and imaginary part (c) of M@ZnO (M = Fe,Co,Ni) core-shell structure and (ZnO)₄₈.