Gate-Controlled Ultrafast Interlayer Carrier Flow in Gr/MoS Heterostructures.

The manipulation of interlayer and interfacial carrier transport in heterostructures represents a fundamental challenge in the design of next-generation optoelectronic devices. In this work, we employ ultrafast spectroscopy to investigate gate-tunable carrier dynamics in a prototypical van der Waals heterostructure: a vertically stacked molybdenum disulfide (MoS)/graphene (Gr) system. This material system has emerged as a promising platform for atomically thin optoelectronics due to its unique electronic properties. We fabricated a transparent field-effect transistor based on the Gr/MoS heterostructure and systematically studied the ultrafast charge transfer processes using complementary spectroscopic techniques: transient terahertz (THz) spectroscopy probes the photoconductivity dynamics in graphene, while transient absorption spectroscopy monitors the corresponding energy state evolution in MoS. Our findings demonstrate that both below- and above-bandgap excitations of the MoS layer yield gate-tunable THz photoconductivity responses in the heterostructure. Remarkably, we achieve picosecond-scale control over both the magnitude and sign (positive, negative, or zero) of the photoconductivity by modulating the graphene Fermi level and defect state occupation. This study provides fundamental insights into carrier dynamics in van der Waals heterostructures and establishes important design principles for developing advanced optoelectronic devices with tailored performance characteristics.