Photophysical Dynamics in Semiconducting Graphene Quantum Dots Integrated with 2D MoS2 for Optical Enhancement in the Near UV

The hybrid structure of zero-dimensional (0D) graphene quantum dots (GQDs) and semiconducting two-dimensional (2D) MoS₂ has been investigated, which exhibit outstanding properties for optoelectronic devices surpassing the limitations of MoS₂ photodetectors where the GQDs extend the optical absorption into the near-UV regime. The GQDs and MoS₂ films are characterized by Raman and photoluminescence (PL) spectroscopies, along with atomic force microscopy. After outlining the fabrication of our 0D-2D heterostructure photodetectors comprising GQDs with bulk MoS₂ sheets, their photoresponse to the incoming radiation was measured. The hybrid GQD/MoS₂ heterostructure photodetector exhibits a high photoresponsivity R of more than 1200 A W^-1 at 0.64 mW/cm² at room temperature T. The T-dependent optoelectronic measurements revealed a peak R of ∼544 A W^-1 at 245 K, examined from 5.4 K up to 305 K with an incoming white light power density of 3.2 mW/cm². A tunable laser revealed the photocurrent to be maximal at lower wavelengths in the near ultraviolet (UV) over the 400-1100 nm spectral range, where the R of the hybrid GQDs/MoS₂ was ∼775 A W^-1, while a value of 2.33 × 10¹² Jones was computed for the detectivity D* at 400 nm. The external quantum efficiency was measured to be ∼99.8% at 650 nm, which increased to 241% when the wavelength of the incoming laser was reduced to 400 nm. Time-resolved measurements of the photocurrent for the hybrid devices resulted in a rise time τ_rise and a fall time τ_fall of ∼7 and ∼25 ms, respectively, at room T, which are 10× lower compared to previous reports. From our promising results, we conclude that the GQDs exhibit a sizable band gap upon optical excitation, where photocarriers are injected into the MoS₂ films, endowing the hybrids with long carrier lifetimes to enable efficient light absorption beyond the visible and into the near-UV regime. The GQD-MoS₂ structure is thus an enabling platform for high-performance photodetectors, optoelectronic circuits, and quantum devices.