Facile synthesis of WS2 nanotubes by sulfurization of tungsten thin films: formation mechanism, and structural and optical properties

Abstract

While 2D layers of WS₂ have been extensively studied, there are very few investigations of WS₂ nanotubes. These have usually been grown via a 2-step process involving a WO_3−x intermediate. We report a simple process for the synthesis of WS₂ nanotubes via the sulfurization of tungsten films under appropriate conditions and present details of their structural and optical properties that help elucidate the formation mechanism. Electron-beam evaporated films of tungsten are sulfurized under flowing N₂ gas at 950–1000 °C temperature under atmospheric pressure to obtain WS₂ nanotubes. High-resolution scanning and transmission electron microscopy studies show that 2D WS₂ flakes curl up and wrap around themselves to form nanotubes. Interlayer spacings in both ‘a’ and ‘c’ directions are slightly smaller than the corresponding values in bulk and thin film WS₂. Micro-photoluminescence and micro-transmission studies show a resonance that seems intrinsic to the WS₂ nanotubes since it cannot be related to the known optical characteristics of WS₂ flakes.