Ultrathin ZnS single crystal nanowires: controlled synthesis and room-temperature ferromagnetism properties.

Highly uniform single crystal ultrathin ZnS nanowires (NWs) with 2 nm diameter and up to 10 μm length were fabricated using a catalyst-free colloidal chemistry strategy. The nanowires crystallized in hexagonal phase structure with preferential growth along the direction of the (001) basal plane. The strong polarity of the (001) plane composed of Zn cations or S anions drives the oriented attachment of ZnS nanocrystals (NCs) along this direction via electrostatic (or dipole) interaction. The ultrathin ZnS nanowires show intrinsic ferromagnetism at room temperature and other unusual properties related to its unique nature, such as large anisotropic lattice expansion, large blue-shift of UV-vis absorption band of the excition, and photoluminescence spectrum of the exciton band edge. First-principles DFT computation results show that Zn vacancies can induce intrinsic ferromagnetism in these undoped ZnS NWs. The main source of the magnetic moment arises from the unpaired 3p electrons at S sites surrounding the Zn vacancies carrying the magnetic moment ranging from 0.26 to 0.66 μ(B). Calculated results indicate that the magnetic moment of the ultrathin ZnS NWs can be increased by increasing the Zn vacancy concentration without significant energy cost. The calculated magnetization value (1.96 or 0.40 emu/g for Zn vacancies on the surface of NWs or inside, respectively) by Zn(53)S(54) supercell model is larger than our experimental value (0.12 emu/g at 1.8 K and 0.05 emu/g at 300 K), but the ferromagnetic result is qualitatively in agreement.