Abstract

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 .