Novel Optical Properties Of Multiple Excitons In Semiconductor Quantum Dots

The radius of semiconductor QDs is generally about 1-10 nm with-100-100000 atoms. Because of the small size and the enhanced Coulomb interaction between electon and hole wave functions in semiconductor QDs, nonradiative Auger process. of charge carriers is extremely efficient, which induces a variety of intriguing optical properties, such as photoluminescence (PL), blinking of charged excitons and PL lifetime shortening of multiple excitons. Due to the Quantum confinement effect, the energy levels of semiconductor QDs are greatly quantified, which leads to the formations of both bright and dark states. Apart from the above characteristics, semiconductor QDs have many other interesting features, such as spectral blue shift, carrier multiplication and resonance energy transfer. With the in-depth studies of semiconductor QDs, their multiple excitons are unavoidably encountered in various device applications ranging from lasers, photodetectors, and solar cells to light-emitting diodes where high electrical currents are normally applied to dense NC films. Over the past few years, great progresses have been made in the optical studies of multiple excitons in semiconductor QDs, but there are still some important optical progresses yet to be investigated, such as including energy transfer (ET) and dark state recombination. In this thesis, we focus on the above two optical processes, hoping that the current work could stimulate intensive theoretical calculations and experimental measurements in future work to guide efficient applications of multiple excitons in potential optoelectronic devices.In chapter II, we studied the energy transfer process of biexcitons in single semiconductor QDs. We attached several acceptor dyes (ATTO647) to the surface of a donor QD (Dot605) to construct a single-particle ET system where the ET process could be turned off by simply bleaching the acceptor dyes. With the presence of both single excitons and biexcitons in the ET process, the donor QD possessed a PL decay with the fast and slow lifetimes arising from the Auger, ET, and radiative decays of biexcitons and the ET and radiative decays of single excitons, respectively. After subsequent bleaching of acceptor dyes, PL decay of the same single QD was still biexponential but with the biexciton and single-exciton ET components being removed from the fast and slow lifetimes, respectively. By comparing the above two PL decay dynamics, the ET lifetimes of both single excitons and biexcitons could be reliably obtained, the average ratio of which was larger than four from statistical measurements on a large number of single ET particles. Our current work has not only provided the first experimental characterization of the multiple-exciton ET process in single semiconductor QDs but also paved the way toward judicious collection of multiexcitons through the ET process in photovoltaic devices.In chapter Ⅲ, we focused on the optical properties of multiple-extion recombination in the dark state of semiconductor QDs. At room temperature (T= 295 k), we studied the optical properties of multiple excitons in the bright state of semiconductor QDs emitting at 605 nm and 655 nm. With the increasing laser excitation power, more multiple excitons were created in the semiconductor QDs with a decreased PL lifetime. At 4 K, we successfully resolved the dark-state PL from semiconductor QDs and an additional PL decay component emerged with the increase of the laser excitation power, which was attributed by us to be from the Auger recombination of dark-state multiple excitons. Furthermore, more dark-state PL from multiple excitons was induced with the increasing laser power, similar to what was observed in the bright-state PL measured at room temperature. We hope that our current work can stimulate more theoretical calculations and experimental measurements in the future to promote efficient applications of dark-state multiple excitons in practical optoelectronic devices.