Ni3S2 nanowires filled carbon nanotubes of ultra-high quality: Synthesis methods, structure, and electrical properties

The synergy between the host carbon nanotube (CNT) and the guest filler distinguishes the filled CNTs as hybrid nanomaterials with unique properties. Many materials have been filled inside the CNT cores using several strategies; however, a reliable filling method to encapsulate transition metal sulfide nanowires inside the CNTs is still lacking. An in situ method is developed for the first time to synthesize the Ni3S2 nanowires filled CNTs (Ni3S2@CNTs), and their fundamental properties are investigated. Electron microscopy results demonstrate a core-shell structure of Ni3S2@CNT, i.e., a CNT filled with a continuous and single-crystalline, rhombohedral Ni3S2 nanowire up to several micrometers in length. An exceptionally low value of the ratio between the intensities of D and G bands (ID/IG = 0.26) in the Raman spectroscopy measurements suggests that Ni3S2@CNTs are very well graphitized, possessing an ultra-high quality. A mean resistivity of 6.1 × 10−5 Ωm is estimated using both two probe and four-point probe measurements on individual Ni3S2@CNTs. Also, a diameter selective electrical breakdown or emptying of the filled nanotube channel is observed as higher currents pass through an individual Ni3S2@CNT.