Observation of charge transfer in mixed-dimensional heterostructures formed by transition metal dichalcogenide monolayers and PbS quantum dots

We report an experimental study on charge transfer properties of mixed-dimensional heterostructures formed by zero-dimensional PbS quantum dots and two-dimensional transition metal dichalcogenides. Monolayers of MoS2 and MoS2 were fabricated by mechanical exfoliation and chemical vapor deposition techniques, respectively. PbS quantum dots with diameters of 2.3 and 5 nm were synthesized by a hot-injection method and characterized by optical absorption spectroscopy and ultraviolet photoemission spectroscopy. The quantum dots were deposited on the MoS2 and MoS2 monolayers to form heterostructures. Photoluminescence and transient absorption measurements were performed on the heterostructures as well as individual materials to reveal their photocarrier dynamics. We found that the holes excited in MoS2 can efficiently transfer to both 2.3- and 5-nm PbS quantum dots, while electrons in these quantum dots cannot transfer to MoS2. Similar charge transfer properties were observed between MoS2 and the 5-nm PbS quantum dots, while no charge transfer was observed between MoS2 and the 2.3-nm quantum dots. These results provide useful information for understanding the physical mechanism of charge transfer in mixed-dimensional heterostructures and for developing PbS quantum-dot-based mixed-dimensional materials.