Ligand‐Induced Control of Photoconductive Gain and Doping in a Hybrid Graphene–Quantum Dot Transistor

Recent advances in graphene-based electronics range from the discovery of fundamental physical phenomena to the development of new high-performance photosensitive devices. [ 1–5 ] These applications exploit not only the unique electronic properties of graphene but also additional functionalities that can be achieved by capping the graphene layer with another material or nanostructure, e.g., atomically thin fi lms, [ 6,7 ] carbon nanotubes [ 8 ] or inorganic nanoparticles. [ 8–10 ] Colloidal semiconductor quantum dots (QDs) are of particular interest as their optical properties can be fi ne-tuned by varying their size and/or composition. [ 11 ] In addition, colloidal synthesis enables QDs to be functionalized by doping [ 12 ] and/or surface encapsulation. [ 13 ] Recently, a photoresponsivity of 10 7 A W −1 was achieved by depositing QDs on graphene and explained in terms of trapping of photoexcited carriers on the QDs and charge transfer between them and the graphene layer. [ 9,10 ] This mechanism should be strongly dependent on the interface between the QDs and graphene. Furthermore, the ligands that encapsulate the QDs may provide a means of modifying the transfer of electronic charges and enhancing the electronic properties of the graphene layer. Here, we investigate the properties of single layer graphene (SLG) functionalized with an overlayer of near-infrared PbS colloidal quantum dots capped with thioglycerol/dithioglycerol or polyethylene glycol (PEG500 and PEG2000). We demonstrate that the polarity of the conductivity and the carrier concentration can be modifi ed, and photoresponsivity of SLG can be signifi cantly enhanced by the choice of ligands. By reducing the length of capping ligands, hence the thickness of the dielectric barrier between the QDs and the SLG, and by preserving the integrity of the ligand layer, we achieve the effi - cient transfer of photogenerated carriers from the QDs to the graphene before recombining, resulting in enhanced photoresponsivities of up to ≈10 9 A W −1 .