Effects of substitutional doping on structural, electrical, and optical properties of nickel manganite NiMn2O4 films

Abstract

Nickel manganite (NiMn2O4), the most studied material for negative temperature coefficient thermistor applications, doped with one or more metal elements have been fabricated successfully to achieve a better performance. However, the influence of different doping elements on the distribution of manganese ions was rarely reported. We use X-ray photoelectron spectroscopy as an effective tool to study the influence of substitutional doping on the distribution of Mn ions with different oxidation states. NiMn2O4 films, doped with Mg, Zn, Co, and Cu, were systematically prepared on Al2O3 substrates by chemical solution deposition method. The crystalline structure and surface morphology were studied by X-ray diffraction and atomic force microscope. X-ray photoelectron spectroscopy shows that the doping of Mg and Zn makes the ratios of Mn3+/Mn4+ decrease, while Co and Cu make the ratio Mn3+/Mn4+ increase. The resistivity of films is inversely proportional to the concentration of Mn3+–Mn4+ pairs. Investigation of UV–Vis–NIR transmission spectroscopy shows that the band-gap of NiMn2O4 can be adjusted by the substitutional doping. The structural, electrical, and optical properties of nickel manganite NiMn2O4 films can be successfully modified with the partial substitution of Mg, Zn, Co, and Cu for Ni, which is valuable to the applications of negative temperature coefficient thermistors.