Browsing by Author "Cihan, Ahmet Fatih"
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Item Open Access Blue-and red-shifting amplified spontaneous emission of CdSe/CdS core/shell colloidal quantum dots(IEEE, 2013) Kelestemur, Yusuf; Cihan, Ahmet Fatih; Güzeltürk, Burak; Yerli, Ozan; Kurum, U.; Yaglioglu H.G.; Elmali, A.; Demir, Hilmi VolkanWe report blue- and red-shifting amplified spontaneous emission of CdSe/CdS quantum dots, controlled by varying core/shell dimensions and modifying exciton-exciton interactions, with low optical gain threshold of two-photon absorption pumping. © 2013 The Optical Society.Item Open Access Multi exciton generation and recombination of semiconductor nanocrystals : fundamental understanding and applications(Bilkent University, 2013) Cihan, Ahmet FatihSemiconductor 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.Item Open Access Observation of biexcitons in the presence of trions generated via sequential absorption of multiple photons in colloidal quantum dot solids(IEEE, 2012) Cihan, Ahmet Fatih; Hernandez-Martinez Pedro L.; Kelestemur, Yusuf; Demir, Hilmi VolkanMulti exciton generation (MEG) and multi exciton recombination (MER) in semiconductor quantum dots (QDs) have recently attracted significant scientific interest as a possible means to improve device efficiencies [1-5]. Convenient bandgap tunability, easy colloidal synthesis, and solution-based processability of these QDs make them further attractive for such device applications using MEG and MER. For example, recent theoretical and experimental studies have shown that MEG enables >100% peak external quantum efficiency where the generated multi excitons (MEs) are collected in a simple QD solar cell structure [1]. Furthermore, MEG has also been shown in QD photodetectors exhibiting substantially increased photocurrent levels [2]. Another promising application for MEs is the use of QDs as an alternative gain medium based on MER for lasers. Although MEG is very promising and supported with quite persuasive reports, there are still some debatable issues that need to be clarified. One of the issues that have generated great debates in the field has been the confusion of MER with the recombination of trions, which takes place in photocharged QDs. To utilize MEG and MER in practical devices such as QD solar cells and QD lasing devices, these phenomena need to be well understood. Here, we showed distinct spectrally-resolved temporal behavior of biexciton (BX), single exciton (X) and trion radiative recombinations in near unity quantum yield (QY) quasi-type II CdSe/CdS core/shell nanocrystal QDs. Upon sequential absorption of multiple photons, the extraction of Xs, BXs, and trions were achieved using time correlated single photon counting (TCSPC) measurements performed on low concentration thin film samples of these QDs at different emission wavelengths. The QDs were embedded in PMMA medium to obtain homogeneous samples and avoid Förster-type nonradiative energy transfer (NRET) between them. Here to extract Xs, BXs, and trions, we devised a new analysis approach for the time decays of the QDs that allowed us to attribute the physical events to their corresponding time decay terms, which were further verified with their excitation intensity dependencies [6]. © 2012 IEEE.Item Open Access Type-tuning of quasi-type-II CdSe/CdS seeded core/shell nanorods: Type-I vs. type-II(IEEE, 2013) Cihan, Ahmet Fatih; Kelestemur, Yusuf; Güzeltürk, Burak; Demir, Hilmi VolkanWe present tuning of quasi-type-II CdSe/CdS core/shell nanorods between type-I-like and type-II-like behavior in their amplified spontaneous emission pumped by 2-photon excitation, with the type attributions verified by time-resolved emission kinetics. © OSA 2013.