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

Abstract

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 τrise and a fall time τfall 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.