Ankara : The Department of Electrical and Electronics Engineering and the Graduate School of Engineering and Science of Bilkent Univ., 2013.Thesis (Master's) -- Bilkent University, 2013.Includes bibliographical references leaves 83-95.Semiconductor nanocrystal quantum dots (QDs) have been found to be very
promising for important application areas in optoelectronics and photonics.
Their energy band-gap tunability, high performance band-edge emission, decent
temperature stabilities, and easy material processing make the QDs attractive for
these applications ranging from photovoltaic devices to photodetectors and
lasers to light-emitting diodes. For these QDs, the concepts of multi exciton
generation (MEG) and recombination (MER) have recently been shown to be
important especially because they possibly enable efficiency levels exceeding
unity using these QDs in various device configurations. However, understanding
multi exciton kinetics in QD solids has been hindered by the confusion of MER
with the recombination of carriers in charged QDs. This understanding lacks to
date and the spectral-temporal aspects of MER still remain unresolved in solid
QD ensembles. In this thesis, we reveal the spectral-temporal behavior of
biexcitons (BXs) in the presence of photocharging using near-unity quantum
yield core/shell CdSe/CdS QDs. The spectral behavior of BXs and that of
excitons (Xs) were obtained for the QD samples with different core sizes,
exhibiting the strength-tunability of the X-X interaction energy in these QDs.
The extraction of spectrally resolved X, BX, and trion kinetics, which would be
spectrally unresolved using conventional approaches, is enabled by our approach
introducing the integrated time-resolved fluorescence. Besides the fundamental understanding of MEG and MER concepts, we also explored the possibility of
utilizing multi excitons in these QDs for optical gain. In this part of the thesis,
tunable, high performance, two-photon absorption (TPA) based amplified
spontaneous emission (ASE) from the same QDs is presented. Here, for the first
time, in addition to the absolute spectral tuning of the ASE, on the single
material system of CdSe/CdS, the relative spectral tuning of ASE peak with
respect to spontaneous emission was demonstrated. With the core and shell size
adjustments, it was shown that Coulombic X-X interactions can be tuned to be
either attractive leading to the red-shifted ASE peak or repulsive leading to the
blue-shifted ASE peak and that non-shifting ASE can be achieved with the right
core-shell combinations. It was further found here that it is possible to obtain
ASE at a specific wavelength from both Type-I-like and Type-II-like CdSe/CdS
QDs. In addition to the CdSe/CdS QDs, we showed ASE and Type-tunability
features on CdSe/CdS nanorods (NRs), which are particularly promising with
their extremely high TPA cross-sections and independent emission/absorption
tunabilities. In the final part of the thesis, we report the observation of MEG on
CdHgTe QDs, for the first time in the literature, and a novel application of MEG
concept in a photosensor device, one of the first examples of real-life
photosensing application of MEG concept. We believe that the results provided
in this thesis do not only contribute to the fundamental understanding of MEG
and MER concepts in the QDs, but also pave the way for the utilization of these
concepts in the QD-based lasers, photodetectors and photovoltaic devices.Cihan, Ahmet FatihM.S