S powder

WS₂ monolayers were grown by CVD on a SiO₂ (300 nm)/Si substrate. Ammonium metatungstate hydrate ((NH₄)₆H₂W₁₂O₄₀·xH₂O (Sigma-Aldrich, ≥ 66.5% (W)) was used as a W source and sodium cholate hydrate (Sigma-Aldrich) was used as a promoter, which were dissolved in DI water separately. Then, they were well-mixed with a medium solution (OptiPrep). After dropping the mixed solution onto the Si/SiO₂ substrate, we conducted spin-coating process to spread the solution uniformly (Jeong et al., 2017 ; Yun et al., 2015 (link)). S powder (Sigma Aldrich, ≥ 99.95 %) was supplied continuously during the CVD process. A two-zone furnace was separately heated with a steady flow of Ar gas, 500 standard cubic centimeters per minute (sccm), to the set temperatures with a ramping rate: 200°C at 33°C/min for the S source and 770°C with 128°C/min for the SiO₂/Si substrate with W. The temperature was maintained at the set temperature for 12 min, and afterward, the furnace was gradually cooled to room temperature.

Preparation of precursor: 20.6 mg Na ${}_2$ MoO ${}_4$ (99 $\%$ , Aladdin) or 88.2 mg Na ${}_2$ WO ${}_4$ (98 $\%$ , Adamas) dissolved into 10 mL DI water to make precursor solution, respectively. Substrate rinsed by O ${}_2$ plasma (CIF Tech Co., Ltd. CPC-C-40 kHz) to improve the hydrophilicity. The substrate first coated 0.01 M Na ${}_2$ MoO ${}_4$ /0.03 M Na ${}_2$ WO ${}_4$ solution was then placed into the center of the furnace. Crucible with 80 mg S powder (99.5 $\%$ , Sigma Aldrich) 10 cm upstream the heating center. Before heating, the tube was flushed with 300 sccm Ar for 10 min to keep an inert environment. Then the furnace was heated to 850  ${}^{\circ }$ C in 45 min and was kept at this temperature for 40 min with 280 sccm Ar to realize a molten liquid precursor state. Finally, the reaction was finished within 2 min by introducing the sulfur powder into the heating zone for the growth of MoS ${}_2$ . For the growth of WS ${}_2$ , 270 sccm Ar and 30 sccm H ${}_2$ mixture gases were introduced to convey the sulfur vapor and other growth conditions were same as that of MoS ${}_2$ . After growth, the furnace was naturally cooled down to room temperature.

The Li rod (99.9%, trace
metals basis) and Na cubes stored in a mineral oil (99.9%, trace metals
basis) were purchased from Sigma-Aldrich and were stored in a glovebox
under an Ar atmosphere. S powder (99.998%, trace metals basis, Sigma-Aldrich)
was dried in a vacuum oven at 100 °C overnight before use. Li
and Na metals were wiped with a tissue paper to remove the residual
oil and freshly cut with stainless-steel blades. Subsequently, they
were sandwiched between two Celgard separators and roll-pressed to
approximately the same thickness (0.15 mm). The Celgard separator
was used during pressing since Li and Na adhere strongly to the stainless-steel
pressing plate. After the Li(Na) pressing procedure, discs with a
diameter of 10 mm were cut and attached to stainless steel discs with
a diameter of 18 mm. The stainless-steel discs act as a supporting
substrate since the alkali metals easily lose their form. The so-prepared
electrodes were placed in homemade glass ampoules with a length of
600 mm together with the S powder. The glass ampoule was placed in
an oven and heated at different temperatures (in the temperature range
from 298 to 433 K). All processes were performed in a glovebox (both
O₂ and H₂O levels were below 0.1 ppm). A schematic
diagram of the Li_xS_y(Na_xS_y) synthesis is shown in Figure 1.

A horizontal tube furnace was used for the growth of MoS₂. S powder (100 mg, 99.5%, Sigma-Aldrich) was loaded upstream at 220°C. MoO₃ powder (10 mg, 99.5%, Sigma-Aldrich) was placed in the center of the furnace. SiO₂/Si (Si substrate with a 300 nm thick thermally grown oxide) was used as growth substrate and placed on top of MoO₃ powder with the oxide layer facing down. The temperature was increased to the growth temperature of 700°C at a rate of 50°C min⁻¹. During growth, Ar was used as the carrier gas with a flow rate of 80 sccm at ambient pressure, and the growth lasted for 10 min.

The SiO₂/Si (270 nm SiO₂) substrates were purchased from Suzhou Ruicai Semiconductor Co., Ltd.; MoO₃ (≥99.5%) and S powder (≥99.5%) were purchased from Sigma-Aldrich (Shanghai) Trading Co., Ltd.; NaCl (≥99.5%), chloroplatinic acid and sodium molybdate (99.0%) were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.