Reducing threshold of biexciton formation in semiconductor nanocrystals through their self-assembly into nano-antennae

The reduction in the threshold of biexciton formation in CdTe semiconductor nanocrystals (NCs) was studied, as an approach towards reducing the optical-gain threshold and improving the lasing abilities of semiconductor nanocrystals.;CdTe quantum dots (QDs) were assembled into an antenna-like architecture. In this nanocomposite, an energy-acceptor CdTe QD was at the center and surrounded by several energy-donor CdTe QDs. The electronic structures of the involved QDs were tailored so that the donor QDs have larger band gap than the band gap of the acceptor QD, which directs the energy irreversibly from the donors to the acceptor through electronic energy transfer. Focusing the excitation energy at the center of this nano-antenna simply reduces the pump intensity required to excite the acceptor QD.;In order to realize the required band-gap gradient, small CdTe QDs (2.9 nm) were used as donors, and large CdTe QDs (3.9 nm) were used as acceptors. Deriving the self-assembly of CdTe QDs to form the desired nanocomposite was managed by electrostatic interaction between the ligands attached to the surface of the QDs. Donor QDs were synthesized with thioglycolic acid on their surface (TGA), and acceptor QDs were synthesized with 2-mercaptoethylamine (MA) on their surface. The electrostatic interaction between the positive charge of MA and negative charge of TGA in the neutral medium leads to the desired self-assembly of the donors and acceptors.;Occurrence of electronic energy transfer from the donors to the acceptors was investigated by analyzing the steady state photoluminescence (PL) spectra and the PL decay dynamics of the free and assembled donors and acceptors. Analyzing the PL decay dynamics of the free acceptors and comparing them with the dynamics of the acceptors in the nano-antenna shows reduction in the threshold of the biexciton formation by factor 3.6+/-2 times. This result is promising in the future reduction of the optical-gain threshold and so improving the lasing capabilities of semiconductor NCs.