When Your CVD Reactor Runs for 100 Hours: What Actually Dictates Wafer-Scale MoS₂ Uniformity
You have spent three days watching a CVD reactor hum. The furnace has been at 850°C for 100 hours straight. Your precursor boat — MoO₃ and sulfur — has been replenished twice. And when you pull out the 4-inch sapphire wafer, the result is not a uniform sheet of MoS₂. It is a patchwork: monolayer here, bilayer there, a 5-layer island near the gas inlet, and bare substrate near the edges. What happened? This is the central issue of scalable 2D material synthesis. Short runs — 10 minutes, 1 hour — produce beautiful flakes. But the moment you stretch to 100 hours for wafer-volume coverage, every imperfection in your setup becomes a showstopper. Temperature gradients, precursor depletion, nucleation density variations — they all compound. And the literature often glosses over this. Papers show perfect 4-inch films but rarely discuss the yield or the edge cases.