Directed Energy Transfer from Monolayer $WS_{2}$ to NIR Emitting PbS-CdS Quantum Dots.

Heterostructures of two-dimensional (2D) transition metal dichalcogenides (TMDs) and inorganic semiconducting zero-dimensional (0D) quantum dots (QDs) offer unique charge and energy transfer pathways which could form the basis of novel optoelectronic devices. To date, most has focused on charge transfer and energy transfer from QDs to TMDs, i.e. from 0D to 2D. Here, we present a study of the energy transfer process from a 2D to 0D material, specifically exploring energy transfer from monolayer tungsten disulphide ($WS_{2}$) to near infrared (NIR) emitting lead sulphide-cadmium sulphide (PbS-CdS) QDs. The high absorption cross section of $WS_{2}$ in the visible region combined with the potentially high photoluminescence (PL) efficiency of PbS QD systems, make this an interesting donor-acceptor system that can effectively use the WS2 as an antenna and the QD as a tuneable emitter, in this case downshifting the emission energy over hundreds of meV. We study the energy transfer process using photoluminescence excitation (PLE) and PL microscopy, and show that 58% of the QD PL arises due to energy transfer from the $WS_{2}$. Time resolved photoluminescence (TRPL) microscopy studies show that the energy transfer process is faster than the intrinsic PL quenching by trap states in the $WS_{2}$, thus allowing for efficient energy transfer. Our results establish that QDs could be used as tuneable and high PL efficiency emitters to modify the emission properties of TMDs. Such TMD-QD heterostructures could have applications in light emitting technologies, artificial light harvesting systems or be used to read out the state of TMD devices optically in various logic and computing application