All spectroscopic measurements were performed on a home-built reflection microscope system based on a liquid-helium recirculating optical cryostat (Montana Instruments Fusion/X-Plane) with a 100×, NA 0.75 objective (Zeiss LD EC Epiplan-Neofluar 100×/0.75 HD DIC M27). The temperature of the sample stage could be varied between 3 and 350 K. In all experiments presented in this study, the TMDC heterobilayer and monolayer samples were at 4 K in a vacuum (<10−6 torr) environment, unless otherwise noted.

In steady-state PL measurements, a CW laser (532 nm) was focused by the objective to a diffraction-limited spot on the sample. The excitation power was measured by a calibrated power meter (OPHIR StarLite) with broad dynamic range. The PL light was collected by the same objective, spectrally filtered, dispersed by a grating, and detected by an InGaAs photodiode array (PyLoN-IR, Princeton Instruments). The wavelength was calibrated by neon-argon and mercury atomic emission sources (IntelliCal, Princeton Instruments). The intensity was calibrated by three independent NIST traceable light sources: a 400 to 1050-nm tungsten halogen lamp (StellarNet SL1-CAL), a 250 to 2400-nm quartz tungsten halogen lamp (Oriel 63355), and a 425 to 1000-nm LED (light-emitting diode) (IntelliCal, Princeton Instruments).

In TRPL measurements, the pulsed excitation light (hν = 1.82 eV; pulse duration, 150 fs) was from a wavelength tunable output of an visible optical parametric amplifier (Coherent OPA 9450) pumped by a Ti:sapphire regenerative amplifier (Coherent RegA 9050, 250 kHz, 800 nm, 100 fs). The interlayer PL emission in the 900 to 1000-nm region was selected and focused onto a single-photon avalanche photodiode (IDQ ID100-50). The TRPL trace was collected with a time-correlated single-photon counting module (Becker & Hickl GmbH SPC-130). The instrument response function, determined by collecting scattered laser light, has an FWHM of 100 ps (fig. S5). The time resolution of TRPL was estimated at ~20% of the FWHM, i.e., ~20 ps.

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