Transforming layered MoS2 into functional MoO2 nanowires

Abstract

A new in situ synthesis method for the growth of MoO₂ nanowires via controlled thermal oxidation of MoS₂ flakes is presented, going from a 2D transition metal chalcogenide to a transition metal oxide nanostructure. The wire growth is performed under an optical microscope using a heating stage with adjustable atmospheric conditions. In contrast to prevalent syntheses, this templated growth leads to highly directional wires along defined MoS₂ crystallographic directions. We examine the growth kinetics of the wires in dependence of the process temperature. In the temperature regime from 650 °C to 710 °C high quality MoO₂ nanowires are formed in a reaction-limited growth process with an activation energy of 596 kJ mol⁻¹. The functional properties of the nanowires are studied by a combination of in situ electron microscopy techniques. Four point measurements in an SEM reveal outstanding metal-like behavior of the nanowires with resistivity values as low as 3.5 × 10⁻⁶ Ω m. Surprisingly, junctions between intergrown nanowires show hardly any increase in resistivity which can be attributed to the well-defined orientational relationship of the nanowires resulting from their templated growth on MoS₂. Elastic properties of the nanowires are studied by complementary in situ bending and resonance measurements in SEM yielding consistent values of 383 GPa for the Young's modulus. Finally, field emission of single MoO₂ nanowires is studied in situ inside the TEM, and an emission current of 500 nA is achieved. The combination of simple synthesis route with outstanding functional properties make the MoO₂ nanowires promising candidates for functional devices in the field of novel 1D-oxide/2D-chalcogenide hybrids. The presented synthesis can be generalized and applied to other metal chalcogenides such as WS₂, as well.