Browsing by Subject "Exciton theory."

Now showing 1 - 8 of 8

Open Access
Computational study of excitons and biexcitons in semiconductor core/shell nanocrystals of type I and type II
(2013) Yerli, Ozan
In this thesis, we studied electronic structure and optical properties of Type-I, Type-II, and quasi Type-II semiconductor nanocrystals (also known as colloidal quantum dots). For a parametric study, we developed quantum mechanical models and solved them using both analytical and numerical techniques. The simulation results were compared to the experimental findings. We showed that charge carrier localization at di↵erent spatial locations could be tuned by controlling size parameters of the core and shell. While tuning charge localization, we also predicted photoluminescence peaks of these core/shell nanocrystals using our theoretical and numerical calculations. We demonstrated that Type-II nanocrystals exhibit di↵erent tuning trends compared to the Type-I ones. We also investigated biexcitonic properties of nanocrystals using quantum mechanical simulations, which are important especially in lasing applications. We showed that two-photon absorption mechanism can be tuned by changing the core and shell size in quantum dots. We calculated at which core and shell sizes biexcitons in quantum dots show attractive or repulsive interaction. The computational studies presented in this thesis played an important role in the experimental demonstrations and understanding of controlling excitonic features of core/shell nanocrystals.
Open Access
Exciton harvesting systems of nanocrystals
(2011) Mutlugün, Evren
Semiconductor nanocrystals, also known as colloidal quantum dots, have gained substantial scientific interest for innovative light harvesting applications including those in biolabeling. Organic dyes and fluorescent proteins are widely used in biotargeting and live cell imaging, but their intrinsic optical properties, such as narrow excitation windows, limit their potential for advanced applications, e.g., spectral multiplexing. Compared to these organic fluorophores, favorable properties of the quantum dots including high photoluminescence quantum yields together with tunable emission peaks and narrow spectral emission widths, high extinction coefficients, and broad absorption bands enable us to discover and innovate light harvesting composites. In such systems, however, the scientific challenge is to achieve high levels of energy transfer from one species to the other, with additional features of versatility and tunability. To address these problems, as a conceptual advancement, this thesis proposes and demonstrates a new class of versatile light harvesting systems of semiconductor nanocrystals mediated by excitonic interactions based on Förstertype nonradiative energy transfer. In this thesis, we synthesized near-unity efficiency colloidal quantum dots with as-synthesized photoluminescence quantum yields of >95%. As proof-of-concept demonstrations, we studied and achieved highly efficient exciton harvesting systems of quantum dots bound to fluorescent proteins, where the excitons are zipped from the dots to the proteins in the composite. This led to many folds of light harvesting (tunable up to 15 times) in the case of the green fluorescent protein. Using organic dye molecules electrostatically interacting with quantum dots, we showed high levels of exciton migration from the dots to the molecules (up to 94%). Furthermore, we demonstrated stand-alone, flexible membranes of nanocrystals in unprecedentedly large areas (> 50 cm × 50 cm), which paves the way for highend, large-scale applications. In the thesis, we also developed exciton-exciton coupling models to support the experimental results. This thesis opens up new possibilities for exciton-harvesting in biolabeling and optoelectronics.
Open Access
Exciton transfering macrocrystals of colloidal quantum dots
(2013) Akcalı, İbrahim
For nonradiative energy transfer (NRET) in the field of medicine and biology as well as optoelectronics, recent advances in the fluorophores, and optical techniques and devices have led to greatly increased interest in applications employing NRET in the past decade. Replacing traditional fluorophores, colloidal quantum dots have flourished the fluorescence properties of NRETbased applications. This has also given rise to working with narrower tunable emission at a higher quantum yield with broadband absorption, and easier handling and fabrication compared to those of traditional fluorophores. A newly discovered technique, QD incorporation into macrocrystals of various salts, has enhanced the processability, photostability and robustness of these colloidal QDs. To benefit from these enhanced properties for NRET, this thesis proposed and studied macrocrystals for exciton transfer via NRET and fabricated those considering NRET mechanism. The design of these QD-embedded macrocrystal structures has enabled strong energy transfer. The experimentally measured energy transfer reached ~51%, which was obtained with careful optimization. Moreover, these hybrid structures have allowed for the observation of the QD distribution dependence of the transfer efficiency for the QDs wrapped inside macrocrystals. The steady state and time-resolved measurements in this thesis revealed that QD-incorporated macrocrystals can possibly take place of QDs in various NRET-related applications.
Open Access
Multi exciton generation and recombination of semiconductor nanocrystals : fundamental understanding and applications
(2013) Cihan, Ahmet Fatih
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.
Open Access
Novel light-sensitive nanocrystal skins
(2013) Akhavan, Shahab
Light sensing devices traditionally made from crystalline or amorphous silicon, operating at the visible and near-infrared wavelengths, have led to a multibillion-dollar annual market. However, silicon faces various limitations including weak detection at long wavelengths (insufficient beyond 1.1 µm) with a cut-off at short wavelengths (in the ultraviolet) and small-area applications. On the other hand, solution-processed semiconductor nanocrystals (NCs), also known as colloidal quantum dots, offer large-area light sensing platforms with strong absorption cross-section. In this thesis we propose and demonstrate a new class of large-area, semi-transparent, light-sensitive nanocrystal skin (LS-NS) devices intended for large-surface applications including smart transparent windows and light-sensitive glass facades of smart buildings. These LS-NS platforms, which are fabricated over areas up to many tens of cm2 using spraycoating and several cm-squares using dip-coating, are operated on the basis of photogenerated potential buildup, as opposed to conventional charge collection. The close interaction of the monolayer NCs of the LS-NS with the top interfacing metal contact results in highly sensitive photodetection in the absence of external bias, while the bottom side is isolated using a high dielectric spacing layer. In operation, electron-hole pairs created in the NCs of the LS-NS are disassociated and separated at the NC monolayer - metal interface due to the difference in the workfunctions. As a result, the proposed LS-NS platforms perform as highly sensitive photosensors, despite using a single NC monolayer, which makes the device semi-transparent and reduces the noise generation Furthermore, because of the band gap tunability, it is possible to construct cascaded NC layers with a designed band gap gradient where the NC diameters monotonically change. Here we present the first account of exciton funneling in an active device, which leads to significant performance improvement in the device. We show highly photosensitive NC skins employing the exciton funneling across the multiple layers of NC film. To further enhance the device photosensitivity performance, we demonstrate embedding plasmonic nanoparticles into the light-sensitive skins of the NCs. In addition, we exhibit the LS-NS device sensitivity enhancement utilizing the device architecture of semi-transparent tandem skins, the addition of TiO2 layer for increased charge carrier dissociation, and the phenomenon of multiexciton generation in infrared NCs. With fully sealed NC monolayers, LS-NS is found to be highly stable under ambient conditions, promising for low-cost large-area UV/visible sensing in windows and facades of smart buildings. We believe the findings presented in this thesis have significant implications for the future design of photosensing platforms and for moving toward next generation large-surface light-sensing platforms.
Open Access
Semiconductor quantum dots driven by radiative and nonradiative energy transfer for high-efficiency hybrid LEDs and photovoltaics
(2011) Güzeltürk, Burak
Today the world energy demand has overtaken unprecedented consumption levels, which have never been reached before in the history of the world. The current trends indicate that the increasing demand for energy will tend to continue at an increasing pace in the coming decades due to worldwide globalization and industrialization. Scientific community is challenged to devise and develop fundamentally new technologies to cope with the energy problem of the world. To this end, optoelectronics can offer several solutions for energy efficiency both in light harvesting and generation. In this thesis, we propose and demonstrate enhanced light generation and harvesting by utilizing both radiative and nonradiative energy transfer capabilities of semiconductor nanocrystal quantum dots, which are profited for the development of novel hybrid devices combining superior properties of the constituent material systems. One of our proposals in this thesis relies on grafting nanostructured light emitting diodes with nanocrystal quantum dots to realize highly efficient color conversion. To the best of our knowledge, we report the highest nonradiative energy transfer efficiency of 83% obtained at room temperature for this type of colorconversion light emitting diodes owing to the architectural superiorities of their nanostructure. In another proposal, we addressed charge injection problems of electrically pumped nanocrystal-based light emitting diodes. We proposed and demonstrated the utilization of novel excitonic injection scheme to drive such LEDs of nanocrystals, which may become prominent especially for the display technology. Finally, we proposed and implemented quantum dot downconversion layers in nanostructured silicon solar cells to benefit the advantages of their nanostructured architecture. We have shown that nanostructured silicon solar cells lead to stronger enhancements compared to the planar counterparts.
Open Access
Spectroscopic and structural properties of TTBC J-aggregates
(2002) Birkan, Burak
The aim of this thesis is to investigate the spectroscopic and structural properties of the TTBC J-aggregates. Absorption, emission and excitation spectroscopy techniques are used to study the properties of the J-aggregates of 1,1’,3,3’-tetrachlorobenzimidazolocarbocyanine (TTBC). The dependence of absorption spectra on dye and ion concentration was investigated. A model is presented to explain the asymmetrical splitting. The interaction between molecules was found to be affected by the intermolecular distance, the orientation of the molecules and the size of the aggregate chain.
Open Access
Tuning the exciton-plasmon coupling
(2012) Ateş, Simge
Exciton-plasmon coupling has recently drawn much interest. In this work, FDTD simulations of exciton-plasmon coupling in plasmonic cavity structures with corrugation patterns are investigated. Excitonic modes are obtained from a Lorentz absorber modeling of a J-aggregate organic dye. The coupling of these excitonic and plasmonic modes on Ag thin films is demonstrated. Rabi splitting due to coupling was clearly observed. Flat metallic surfaces, uniform gratings and Moiré surfaces are used in simulations as corrugation patterns. Metal film thickness and dye concentration dependence of Rabi splitting via exciton-plasmon coupling was also observed on thin flat Ag films. We show that Rabi splitting occurs even at low dye concentrations, and the magnitude of splitting increases as dye concentration increases. A new state in the band gap is observed when the total oscillator strength is increased. Large Rabi splitting is observed when plasmon damping is modulated. Exciton-plasmon coupling on uniform gratings is studied as a function of cavity size, corrugation periodicity and depth. Q factor and Rabi splitting behavior of excitonplasmon coupling on Moiré cavities are investigated as a function of cavity size. Strong anti-crossing is observed when the excitonic absorption matches with the cavity state.