Synthesis of atomically thin WO3 sheets from hydrated tungsten trioxide

Atomically thin WO3 is fabricated using a three-step process, involving wet-chemical synthesis of hydrated-WO3, mechanical exfoliation of fundamental layers and dehydration by annealing at 300 °C. Atomic force microscopy reveals that the minimum resolvable thickness of the hydrated flakes to be ~1.4 nm, which corresponds to the unit-cell height. The subsequent annealing temperature determines the degree of dehydration and thereby the crystallographic structure of the resultant WO3. The hydrated and dehydrated flakes are characterized by Raman spectroscopy to reveal spectroscopic fingerprints of flake thickness. Thin flakes are also shown to have higher propensity for lithium intercalation, which is also evidenced in Raman planar peak intensity.