Browsing by Subject "Nanocrystals--Optical properties."

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 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
Germanium alloys for optoelectronic devices
(2008) Erbil, Ayşe
Silicon has been the backbone of the mainstream electronics of the last fifty years. It is however, used in conjunction with other matierals, mainly with its oxides and nitrides. Germanium, on the other hand, is also a group IV element and has been used in the early stages of transistor and detector development. In addition to Si/Ge heterojunctions, bandgap engineering through SiGe alloys has also been used in photodetectors. Recent progress in light emitting devices utilizing Si nanocrystals suggest the use of Ge1-xNx layers as barriers due to its suitable band offsets [1]. Experiments have shown that Ge1-xNx is also a promising material for applications in photodiodes, amplifiers, optic fibers, protective coatings, etc [1]. Both Si and Ge are, however indirect bandgap semiconductors, lacking efficient light emission. On the other hand, strong light emission observed in Si nanocrystals has made the study of semiconductor nanocrystals an expanding field of interest due to potential applications in novel optoelectronic devices [2]. These nanocrystals exhibit strong luminescence and nonlinear optical properties that usually do not appear in the bulk materials [2]. SiGe nanocrystals attract attention due to the possibility of a tunable band gap with composition. In this study, formation of Ge1-xNx thin films and SiGe nanocrystals by plasma enhanced chemical vapor deposition (PECVD) reactor has been studied. We present the growth conditions and experimental characterization of the resulting thin films and nanocrystals. We used ellipsometry, Raman Spectrometry, Fourier Infrared Spectrometry (FTIR) and X-ray photoelectron Spectroscopy (XPS). For SiGe nanocrystals, 4 peaks in the Raman Spectra were observed around 295 cm-1, 400 cm -1, 485 cm-1 and 521 cm-1. These peaks are assigned to the Ge-Ge, Si-Ge, local Si-Si and crystalline Si-Si vibrational modes, respectively [3]. For the Ge1-xNx thin films FTIR spectrum showed the existence of the Ge-N bonds and its band offsets determined by XPS confirm its suitability for optoelectronic devices.
Open Access
Low dimensional structures for optical and electrical applications
(2008) Akça, İmran
Low dimensional structures such as quantum dots have been particularly attractive because of their fundamental physical properties and their potential applications in various devices in integrated optics and microelectronics. This thesis presents optical and electrical applications of low dimensional structures. For this purpose we have studied silicon and germanium nanocrystals for flash memory applications and InAs quantum dots for optical modulators. As a quantum dot, nanocrystals can be used as storage media for carriers in flash memories. Performance of a nanocrystal memory device can be expressed in terms of write/erase speed, carrier retention time and cycling durability. Charge and discharge dynamics of PECVD grown nanocrystals were studied. Electron and hole charge and discharge currents were observed to differ significantly and strongly depend on annealing conditions chosen for the formation of nanocrystals. Our experimental results revealed that, discharge currents were dominated by the interface layer acting as a quantum well for holes and route for direct tunneling for electrons. On the other hand, possibility of obtaining quantum dots with enhanced electro-optic and/or electro-absorption coefficients makes them attractive for use in light modulation. Therefore, waveguides of multilayer InAs quantum dots were studied. Electro-optic measurements were conducted at 1.5 µm and clear Fabry-Perot resonances were obtained. The voltage dependent Fabry-Perot measurements revealed that 6 V was sufficient for full on/off modulation. Electroabsorption measurements were conducted at both 1.3 and 1.5 µm. Since the structure lases at 1285 nm, high absorption values at 1309 nm were obtained. The absorption spectrum of the samples was also studied under applied electric field. Absorption spectra of all samples shift to lower photon energies with increasing electric field.
Open Access
Macrocrystals of semiconductor nanocrystals for light emitting diodes
(2013) Akcalı, Halil
Worldwide energy consumption is rapidly increasing and general lighting constitutes an important portion of it. By considering most of the lighting sources to rely on fluorescent lamps today, solid-state lighting has a great potential especially with recent advances on efficiency and color quality of white light-emitting diodes (LEDs). One of the most promising approaches for the current white LED technology is based on the use of color-conversion materials. In recent years semiconductor nanocrystal quantum dots (NQDs) have arisen as an important class of color-convertors because of their tunable and superior optical properties and today there are various prototypes and commercial products. Recently, a new material system involving macrocrystals of NQDs has been introduced. In this thesis, different types of NQD macrocrystals are developed and studied for the white LED application. These macrocrystals of NQDs are produced by forming ionic salts in the presence of water soluble NQDs. This enhances the stability of NQDs wrapped in salt solids and allows for high integration capability in powder form to current LED technology employing commercial encapsulants. This thesis also includes a colorimetric study of white LEDs using such macrocrystals of NQDs to demonstrate high-quality white light with high color rendering index, low correlated color temperature, and high luminous efficacy of optical radiation.
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
Targeted self-assembly of nanocrystal quantum dot emitters using smart peptide linkers on light emitting diodes
(2008) Zengin, Gülis
Semiconductor nanocrystal quantum dots find several applications in nanotechnology. Particularly in device applications, such quantum dots are typically required to be assembled with specific distribution in space for enhanced functionality and placed at desired spatial locations on the device which commonly has several diverse material components. In conventional approaches, self-assembly of nanocrystals typically takes place nonspecifically without surface recognition of materials and cannot meet these requirements. To remedy these issues, we proposed and demonstrated uniform, controlled, and targeted self-assembly of quantum dot emitters on multi-material devices by using cross-specificity of genetically engineered peptides as smart linkers and achieved directed immobilization of these quantum dot emitters decorated with peptides only on the targeted specific regions of our color-conversion LEDs. Our peptide decorated quantum dots exhibited 270 times stronger photoluminescence intensity compared to their negative control groups.