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The transfer of energy from semiconductor Quantum Dots (QDs) to Graphene in hybrid structures provides a rich environment to explore excitation dynamics in heterogeneous hybrid systems The photo-physical properties of QDs can be predictably fine-tuned by size selection, and Graphene can also be doped, with the aim of producing well-defined conditions for photoexcitation and energy transfer. Additionally, both Graphene and QDs display superior photo-stability. For all these reasons, hybrid QD-Graphene superstructures have strong potential for use in a wide variety of photonics applications where a well-defined modulation of optical and spectroscopic properties are crucial [1]. For example, several works have already demonstrated the use of graphene as a substrate to manipulate and quench photoluminescence (PL) [2], to enhance resonance Raman signals [3], or to support saturated absorption mode-locking of ultrafast lasers [4].
Here we explore the changes in the PL emission of CdSe colloidal QDs deposited on CVD graphene as the Fermi-level of graphene is varied. In solution, QDs’ PL spectrum consists of a band centered at λ=590 nm and FWHM of about 40 nm. The emission band becomes considerably narrower as the doping level of graphene is increased. Furthermore, the emission maximum shifts to the blue when switching from p- to n-type doping of the graphene. We suggest that the reason is selective quenching of the emission of some QDs within the size distribution. Charge transfer from graphene to a QD via tunneling can be possible when the Fermi energy of graphene is higher than the first energy level of QD electrons. Shifting graphene Fermi level allows us to control the amount of charge carriers available to be transferred via tunnelling to a subset of QDs within the size distribution.
The theoretical part of this work is divided in two parts. The first one is dedicated to the evaluation of the tunneling transition probability using Bardeen’s. We found that the tunneling probability is quite low (~1 s-1) due to a large wave-vector mismatch between the QD electronic states (near the Γ point) and graphene’s Dirac point. Even though, QDs and graphene will come to an equilibrium within a few seconds after being put in contact. The second part focuses on the statistics of electrons in the system at equilibrium, which allows us to evaluate the occupation probability of the QD lowest level in the conduction band and the fraction of QDs that cannot be excited by light because of the Pauli blocking and, therefore, do not contribute to the PL emission. It explains the QD PL spectrum dependence on the graphene Fermi energy.
