Interaction between individual semiconductor Quantum Dots and Graphene monolayers: optical studies and electrical control of energy and charge transfer. – QuanDoGra

Light emission by a quantum system is dramatically affected near an interface. Such phenomena, which have been studied since the 1960’s are of prime importance in nano-scaled systems, where size, shape, surface chemistry and the local environment govern the physical properties. The example of a single atom, molecule or quantum dot at the vicinity of a metallic surface is a textbook case. In this situation resonant energy transfer or photoinduced charge transfer from the emitter (donor) to the metal (acceptor) may occur. These processes that both involve a donor-acceptor pair may have either positive or negative consequences for applications in nanophotonics, optoelectronics and photovoltaics. As such, energy and charge transfer deserve quantitative fundamental studies, as well as “proof of concept” experiments in model devices.
The QuanDoGra project proposes a set of original experiments that aim at studying energy and charge transfer between colloidal semiconductor quantum dots (CQDs) and graphene monolayers, two of the most promising nano-materials for industrial applications. Graphene is an atomically thin semi-metal, composed of sp2 carbon atoms arranged in a honeycomb lattice. As such, this highly conductive, yet optically transparent material is an ideal transparent electrode. Owing to their broadband absorption and size tunable optical properties, CQDs hold great promise as active media in next generations of solar cells, photodetectors and light emitting diodes and lasers.
We will study CQDs as “0D” light absorbers and energy and/or charge donors, while graphene will be investigated as a “2D” acceptor. The consequences of energy and charge transfer in the CQD-graphene system will be thoroughly analyzed through changes of the luminescence of individual CQDs that are physisorbed on graphene. We will take advantage of the unique controllability of CQDs to utilize specific core/shell structures that will favor energy transfer over charge transfer, and “core only” structures that facilitate charge transfer. We thus aim at realizing graphene-based molecular rulers based on energy transfer (with core/shell CQDs), and demonstrating the possibility of photo-induced charge separation assisted by graphene (with “core only” CQDs). Next, the purpose of our project consists in integrating CQDs in graphene transistors with a transparent top gate. Such devices offer a unique control of the Fermi level of graphene, and thus of its interaction with CQDs. First, we plan to achieve electrical control of energy transfer rate, and hence of light emission, at the single CQD level. Second, we will carefully study the photoresponse of the graphene-CQD hybrid device, a particularly relevant aspect for photovoltaic applications.