Browsing by Subject "Quantum dots."

Now showing 1 - 14 of 14

Open Access
Cascading and modifying nonradiative energy transfer mechanisms in strong coupling region of plasmons and excitons in semiconductor quantum dots
(2010) Akın, Onur
Nonradiative energy transfer finds important applications in nanophotonics and nanobiotechnology including nanoscale optical waveguiding and biological nanosensors. Various fluorophores can take part in such energy transfer interactions in close proximity of each other. Their emission kinetics can be strongly modified and controlled as a result. For example, colloidal semiconductor quantum dots, also known as nanocrystals, have widely been shown to serve as donors and acceptors among themselves or with other fluorescent species to transfer excitation energy nonradiatively. In their close proximity, emission characteristics of such fluorophores can also be altered when coupled with plasmonic structures, e.g., metal nanoparticles. One favored result of these plasmon-exciton interactions is the emission enhancement. In principle it is possible to plasmon-couple acceptor-donor pairs of nonradiative energy transfer to modify their transfer rate. Such plasmon-mediated energy transfer has been demonstrated, where both acceptor-donor pairs are plasmoncoupled. In these cases, however, the resulting plasmon-exciton interactions are not controlled to take place either at the donor site or the acceptor site but at both of the sites. Therefore, it has previously not been possible to identify the coupled interactions. In this thesis, we propose and demonstrate cascaded plasmonic - nonradiative energy transfer interactions that are controlled by selectively plasmon-coupling either only the donor quantum dots or only the acceptor quantum dots. For that, we designed a novel self-assembly architecture of our hybrid layered systems of semiconductor nanocrystals and metal nanoparticles in a bottom-up fashion through precise spatial and spectral control. This scheme uniquely allowed for the ability to spatially control plasmonexciton interactions to take place either at the “start” site (donors) or “finish” site (acceptors) of the energy transfer. This control was achieved by placing the plasmonic layer in the right proximity of the donors (for strong donor-exciton plasmon-coupling) while sufficiently being far away from the acceptors (for weak acceptor-exciton plasmon-coupling), or vice versa. Here we comparatively studied and analyzed consequent modifications of quantum dot emission kinetics in response to both cases of plasmon-coupling to only the donors and to only the acceptors through steady-state and time-resolved photoluminescence measurements, along with their lifetime and rate calculations. Such cascaded energy transfer interactions in the strong exciton-plasmon coupling region hold great promise for innovative near-field photonic devices and biological tags. system.
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
Layer-by-layer self-assembled semiconductor nanocrystal composites with nonradiative resonance energy transfer for innovative architectural precise color tuning and control
(2009-08) Çiçek, Neslihan
In recent years semiconductor quantum dot nanocrystals (NC) have attracted significant interest and have found numerous important optoelectronic device applications mainly because of their highly tunable optical properties. For example, precisely tuning shades of color chromaticity is critically important in solid state lighting to achieve ultra-efficient, application-specific, spectrallyengineered illumination. To date such color tuning and control of NC emitters have been investigated and demonstrated only based on their composition, shape, and size (using the quantum confinement effect). All of these parameters are, however, limited to be controlled and set during the synthesis process. As a post-synthesis alternative, we proposed and demonstrated the precise and broad control and tuning of color chromaticity by strongly modifying photoluminescence decay kinetics of NC emitters solely based on nonradiative Förster resonance energy transfer (FRET) in layer-by-layer self-assembled NC composite structures. Locating NC emitters in such a layered architecture with a targeted gradient of bandgap in the close proximity (<10 nm) of each other and spatially interspacing them at the nanoscale (with a precision of <1 nm) enabled us to fine-tune and master FRET at a desired efficiency level of nonradiative energy transfer from electronically excited donor NCs to luminescent acceptor iv NCs. These proof-of-concept experimental demonstrations, combined with our numerical modeling and simulation results, proved a highly sensitive tuning capability based on FRET to span a broad color area in Commission Internationale De L’Eclairage (CIE) chromaticity diagram in principle beyond the limits of each of the commonly used LED epitaxial material systems. This innovative architectural tuning opens up a new direction for the photometric engineering of color-conversion LEDs.
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
Novel ultraviolet scintillators based on semiconductor quantum dot emitters for significantly enhanced photodetection and photovoltaics
(2007) Mutlugün, Evren
Silicon photonics opens opportunities to realize optoelectronic devices directly on large-scale integrated electronics, leveraging advanced Si fabrication and computation capabilities. However, silicon is constrained in different aspects for use in optoelectronics. Such one limitation is observed in Si based photodetectors, cameras, and solar cells that exhibit very poor responsivity in the ultraviolet (UV) spectral range. Si CMOS photodetectors and CCD cameras cannot be operated in UV, despite the strong demand for UV detection and imaging in security applications. Also, although 95% of the photovoltaics market is dominated by Si based solar cells, silicon is not capable of using UV radiation of the solar spectrum for solar energy conversion, as required especially in space applications. In this thesis for the first time, we demonstrate novel UV scintillators made of semiconductor quantum dot emitters hybridized on Si detectors and cameras to detect and image in UV with significantly improved responsivity and on Si solar cells to generate electrical energy from UV radiation with significantly improved solar conversion efficiency. We present the device conception, design, fabrication, experimental characterization, and theoretical analysis of these UV nanocrystal scintillators. Integrating highly luminescent CdSe/ZnS core-shell nanocrystals, we demonstrate hybrid photodetectors that exhibit two-orders-of-magnitude peak enhancement in their responsivity. We also develop photovoltaic nanocrystal scintillators to enhance open-circuit voltage, short-circuit current, fill factor, and solar conversion efficiency in UV. Hybridizing CdSe/ZnS quantum dots on Si photovoltaic devices, we show that the solar conversion efficiency is doubled under white light illumination (Xe lamp). Such UV scintillator nanocrystals hold great promise to enable photodetection and imaging in UV and extend photovoltaic activity to UV.
Open Access
Optical near field interaction of spherical quantum dots
(2012) Amirahmadov, Togay
Nanometer-sized materials can be used to make advanced photonic devices. However, as far as the conventional far-field light is concerned, the size of these photonic devices cannot be reduced beyond the diffraction limit of light, unless emerging optical near-fields (ONF) are utilized. ONF is the localized field on the surface of nanometric particles, manifesting itself in the form of dressed photons as a result of light-matter interaction, which are bound to the material and not massless. In this thesis, we theoretically study a system composed of differentsized quantum dots involving ONF interactions to enable optical excitation transfer. Here this is explained by resonance energy transfer via an optical nearfield interaction between the lowest state of the small quantum dot and the first dipole-forbidden excited state of the large quantum dot via the dressed photon exchange for a specific ratio of quantum dot size. By using the projection operator method, we derived the formalism for the transfered energy from one state to another for strong confinement regime for the first time. We performed numerical analyses of the optical near-field energy transfer rate for spherical colloidal quantum dots made of CdSe, CdTe, CdSe/ZnS and PbSe. We estimated that the energy transfer time to the dipole forbidden states of quantum dot is sufficiently shorter than the radiative lifetime of excitons in each quantum dot. This model of ONF is essential to understanding and designing systems of such quantum dots for use in near-field photonic devices.
Open Access
Selective plasmonic control of excitons and their non-radiative energy transfer in colloidal semiconductor quantum dot solids
(2009) Özel, Tuncay
To date extensive research has proved that semiconductors and metals exhibit extraordinary optical properties in nano-dimensions compared to their bulk counterparts. For example, an interesting effect is observed in metal nanostructures/nanoparticles (NPs) that we form to obtain localized plasmons, with their optical response highly tuneable using the size effect. Another field of interest at the nanoscale is the investigation of light generation and harvesting using colloidal semiconductor quantum dot nanocrystals (NCs) that we synthesize in few nanometers, with their emission and absorption excitonic peaks conveniently tuneable using the size effect. In this thesis, we proposed and demonstrated the first accounts of selectively plasmonically-controlled colloidal quantum dot emitters assembled in innovative architectures, with a control achieved either through spatial selection or spectral selection. In the first set of designs, we developed for the first time plasmonic NC-composites that rely on spatially-selected plasmon-coupled CdTe NC-monolayers interspaced with respect to Au NP-monolayers in a repeating three-dimensional layer-by-layer architecture. In these bottom-up designs of hybrid nanocomposites, the photoluminescence kinetics is strongly modified and a record quantum efficiency of 30% is achieved for such CdTe NC solids. In the second set of designs, we showed the first spectrally-selected plasmon-coupling of surfaceemitting CdS NCs using optimized Ag NP deposits. This architecture allowed for the surface-state emission to be selectively enhanced while the interband emission is simultaneously suppressed in the same plasmon-coupled NCs, leading to the strongest surface-state emission from such CdS NCs reported with respect to their interband emission (with a >12-fold enhancement). Yet another important proximity phenomenon effective among quantum dot emitters is the Förster-type non-radiative resonance energy transfer (ET), in which excitonic excitation energy of the donor-NCs is non-radiatively transferred to the acceptor-NCs via dipole-dipole coupling. In the third set of our designs, we combined two fundamental proximity mechanisms of plasmon coupling and non-radiative energy transfer in the same NC solids. In plasmonic ET, we reported for the first time selectively plasmon-coupling of NC-acceptors and then that of NC-donors in the ET pair, both of which result in substantial enhancement of the acceptor emission with respect to ET with no plasmon coupling (with a maximum of 2-fold enhancement) as verified by their steadystate and time-resolved photoluminescence. This concept of spectrally/spatiallyselective plasmon coupling in quantum dots paves a new path for devices and sensors in nanophotonics.
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
Synthesis and characterization of highly efficient CdSe/CdS core/shell nanocrystals with silar technique
(2012) Keleştemur, Yusuf
Owing to their size tunable electronic structure and optical properties, semiconductor nanocrystal quantum dots (NQDs) have become attractive for a wide range of device applications ranging from life sciences to electronics in the last two decades. However, highly efficient and stable NQDs are essential to reaching high performance with these devices utilizing NQDs. In this thesis, to meet these requirements, a new class of CdSe/CdS core/shell NQDs are studied including their colloidal synthesis and nanocharacterization. In this work, CdSe/CdS core/shell NQDs were synthesized with successive ion layer adsorption and reaction (SILAR) technique, which enabled highly precise shell thickness control and uniform coating of the shell material. When compared to the most commonly used CdSe/ZnS core/shell NQDs, CdSe/CdS core/shell NQDs were found to provide important advantages. First, the lattice mismatch within CdSe and CdS (3.9%) is lower than that within CdSe and ZnS (12%), which was very critical for obtaining highly efficient NQDs. Second, as a result of having lower bandgap in CdS, great enhancement in absorption cross section was achieved with more red-shifted emission, which is not possible with CdSe/ZnS core/shell NQDs. Moreover, suppression of Auger recombination was successfully observed with the partial separation of electron and hole wavefunctions in the synthesized CdSe/CdS core/shell NQDs. With all these attractive properties that were experimentally measured, CdSe/CdS core/shell NQDs were found to make better alternatives to CdSe/ZnS core/shell for numerous applications.
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.