ATM synthesis and spin coating. Ammonium thiomolybdate (ATM) films were prepared similar to the procedure reported by Müller et al. 4g of (NH 4 ) 6 Mo 7 O 24 •4H 2 O were dissolved in 20 mL deionized water. A solution of ammonium polysulfide (NH 4 ) 2 S 3 (120 mL, 8%, Fisher Scientific) was added and the mixture was heated under reflux overnight (90 °C). The dark-red precipitate (NH 4 ) 2 Mo 3 S 13 was filtered and washed with ethanol and water. Subsequently, the powder was heated at 80 °C in 50 mL toluene three times for about 2 h. After filtering and drying, the thiomoybdate precursor material (5.2 g, 93% yield) was obtained as a dark red powder (Supplementary Fig. ).