H2wo4

Commercial WO₃ (0.3 g, 99.9%, Sigma Aldrich), H₂WO₄ (0.3 g, 99.9%, Sigma Aldrich) and NaCl (0.3 g, ≥99.5%, Sigma Aldrich) powders were loaded in an alumina boat placed in the center of a 2 inch-diameter horizontal quartz tube CVD furnace. While an alumina boat containing sulfur powders (0.6 g, ≥99.5%, Sigma Aldrich) was loaded in the upstream zone of the tube, whose temperature was independently controlled by a different heater. The growth substrates were Si wafers (500 microns) with on top 285 nm of SiO₂ thermally deposited. The substrates were sequentially cleaned for 15 min in acetone, isopropanol and deionized water in a sonicator, followed by dipping in H₂SO₄/H₂O₂ (3:1) for two hours and rising in deionized water. They were then blow dried with nitrogen gas, cleaned with O₂ plasma for 5 min and loaded into the downstream zone of the furnace. The CVD growths were then performed at low pressure (~10⁻¹ mbar) and under flow of high purity Ar gas (flow rate of 100 sccm). The furnace was heated to 750–950 °C with a ramp rate of 25 °C min⁻¹, kept at the growth temperatures for 15 min and then naturally cooled down to room temperature. The sulfur powder was independently heated to 125 °C with a ramp of 5 °C min⁻¹, kept at this temperature for 15 min and naturally cooled down.