Browsing by Subject "Energy transfer."

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
Design strategies for chemosensors and their applications in molecular scale logic gates
(2013) Guliyev, Ruslan
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
Light harvesting and efficient energy transfer in boron dipyrrin dyads and derivatization for potential utility in dye-sensitized solar cells
(2008) Barın, Gökhan
In bichromophoric supramolecular systems light is harvested by antenna components and excitation energy is channeled into an acceptor component. We have designed and synthesized novel energy transfer cassettes which are based on boradiazaindacene (BODIPY) units. Facile synthesis of long wavelength absorbing distyryl BODIPY dyes has been applied successfully in this study. In the first part of the thesis, efficient energy transfer from energy donor BODIPYs to long wavelength absorbing distyryl BODIPY core was demonstrated. To observe the antenna effect quantitatively, we have designed the cassettes with an increasing number of energy donor components. Based on these observations, in the second part of the thesis, we have introduced a light-harvesting photosensitizer for dye-sensitized solar cell (DSSC) purposes. The target molecule absorbs in visible and near-IR region and energy transfer is demonstrated successfully. Our design appears to be highly promising for DSSC.
Open Access
Novel multichromophoric energy transfer cassettes based on functionalized BODIPY dyes
(2012) Çeltek, Gizem
Energy necessity is one of the leading problems in the world due to the developing technologies and strategies. There are many energy sources, which are being used for years, however; conversion and transfer of the energy is a problem in many fields due to energy loss. In this manner, the efficiency of energy transfer is very crucial. For this purpose, we have designed multichromophoric molecules, which can absorb the light with donor parts, then transfer the energy to the acceptor site. During this process, energy loss is tried to be prevented by lowering the distance between the donor and acceptor Boradiazaindacene (BODIPY) molecules. Three different energy transfer cassettes are synthesized and characterized. The design of the supramolecule, in means of spectral overlap and distance between the donor and the acceptor site are observed to affect the energy transfer efficiency. Through functional design, these molecules absorb and emit light in different wavelengths. Substation of distyryl and tetrastyryl groups to the acceptor BODIPY core changes the emission and absorption maxima. Increasing number of styryl groups attached to the molecule shifts the spectrum to the red part of the visible region. Through rational design, these molecules can be used in applications of energy transfer and broad spectrum absorber purposes.
Open Access
Novel nanocrystal-integrated LEDs utilizing radiative and nonradiative energy transfer for high-quality efficient light generation
(2011) Nizamoğlu, Sedat
To combat environmental issues escalating with the increasing carbon footprint, combined with the energy problem of limited resources, innovating fundamentally new ways of raising energy efficiency and level of energy utilization is essential to our energy future. Today, to this end, achieving lighting efficiency is an important key because artificial lighting consumes about 19% of total energy generation around the globe. There is a large room for improving lighting efficacy for potential carbon emission cut. However, the scientific challenge is to reach simultaneously high-quality photometric performance. To address these problems, we proposed, developed and demonstrated a new class of color-conversion light emitting diodes (LEDs) integrated with nanophosphors of colloidal quantum dots. The favorable properties of these semiconductor nanocrystal quantum dots, including size-tuneable and narrow-band emission with high photostability, have provided us with the ability of achieving highquality, efficient lighting. Via using custom-design combinations of such nanocrystal emitters, we have shown that targeted white luminescence spectra can be generated with desired high photometric performance, which is important for obtaining application-specific white LEDs, e.g., for indoors lighting, street lighting, and LED-TV backlighting. Furthermore, dipole-dipole coupling capability of these semiconductor nanocrystals has allowed us to realize novel device designs based on Förster-type nonradiative energy transfer. By mastering exciton-exciton interactions in color-conversion LEDs, we have demonstrated enhanced color conversion via recycling of trapped excitons and white light generation based on nonradiative pumping of nanocrystal quantum dots for color conversion. This research work has led to successful demonstrators of semiconductor nanocrystal quantum dots that photometrically outperform conventional rareearth phosphor powders in terms of color rendering, luminous efficacy of optical radiation, color temperature and scotopic/photopic ratio for the first time.
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
Optimization of orthogonal reactions on bodipy dyes for one-pot synthesis of light harvesting dendrimers
(2013) Bekdemir, Ahmet
For more than a decade, synthetic organic chemistry has dealt with focusing on highly selective and efficient reactions that can proceed under mild conditions which would then be categorized under the term “orthogonal click chemistry”. These types of reaction have served number of applications for years as in peptide synthesis, homogeneous catalysis and development of supramolecular systems. On the other side, after a partial understanding of how photosynthetic bacteria and plants harvest solar radiation in order to carry their necessary carbon dioxide reduction reaction by converting light to chemical energy, artificial light harvesting systems have captivated a lot attention of scientists. Because today’s one of the biggest and inevitable problems is to discover/invent alternative energy sources/devices for future demands, these artificial light harvesting and solar concentrator systems are highly open for further development and optimization. However, like most other macromolecular systems, synthesis of these kind of devices should be straightforward so as to decrease the cost and to increase the efficiency. At this point, orthogonal click reactions, being mild and efficient synthetic models, can undoubtedly be worthwhile to consider as proper tools for easy preparation of light harvesting molecules. Here we propose a synthesis of thiol, Michael accepting groups, amine and isothiocyanate modified BODIPY dyes for light harvesting cascade preparation. Moreover, the optimization of Michael addition type thiol – ene reaction of these functionalized dyes has been discussed. Among methyl methacrylate, cyanoacetic acid and nitroolefin functionalizations, it was found that nitroolefin attached BODIPY dyes are the most reactive one. The achieved product has been investigated in terms of fluorescence and energy transfer.
Open Access
Physical integration of chemical logic gates
(2012) Öztürk, Şeyma
Recent research in molecular logic gates produced molecular equivalence of highly complex digital designs. Advanced data processing at the molecular level requires a considerable degree of integration (concatenation) between molecular logic gates. So far, almost all the integration reported in the literature has been “virtual”, meaning that the outputs at various channels are determined first and then an integrated set of logic gates is proposed to be operating on inputs to produce those outputs. Nevertheless, there is no doubt that at some point there has to be methods to physically connect one molecular logic gate to the other one, for a rational design and implementation. In this study, we synthesized a few derivatives of the well known fluorophore “Bodipy” and then proposed two methodologies to concatenate separately existing and functioning Bodipy-based chemical logic gates. In one instance, we coupled a photochromicity-based AND gate to an ion-responsive Bodipy-based AND gate, making use of the modulation of inner filter effect. In the other example, we coupled two ion-responsive Bodipy-based AND gates through the increased efficiency of energy transfer and “click” chemistry. We are certain that these methodologies are highly promising and our studies are in progress to demonstrate more complex examples of physical integration.
Open Access
Rational design and synthesis of bodipy dyes for molecular sensing, light harvesting and photodynamic applications
(2014) Kütük, Tuğba Özdemir
BODIPY dyes have been addressed in many applications due to highly important features. These unique properties can be summarized as high photostability, high extinction coefficient, easy functionality, etc. Thus, tremendous studies have been published and, ion sensing, photodynamic therapy, dye-sensitized solar cells and light harvesting are some of the areas that BODIPY dyes have been utilized. In this thesis, BODIPY dyes were functionalized to be used for different concepts. In the first study, the main purpose was to seek for ion signaling differences of two isomeric tetra-styryl BODIPY dyes with charge donor ligand located at 1,7 versus 3,5 positions. Second work focuses on the light harvesting concept with the use of tetra-styryl BODIPY derivatives. Third study describes the coupling of energy transfer with internal charge transfer to monitor the alterations in intensity ratios, so, dynamic range of the fluorescent probe is improved. Design and synthesis of BODIPY dyes for detection of biological thiols in aqueous solution both chromogenically and fluorogenically was given in fourth study. Another biologically important molecule, hydrogen sulfide is selectively detected via BODIPY-based probe and depicted in the fifth study. In the sixth work, persistent luminescent nanoparticles are attached to BODIPY-based photosensitizer to activate the photodynamic action.
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 near-ir emissive tetra styryl-BODIPY based light harvesting energy transfer cassettes
(2011) Köstereli, Ziya
Light harvesting antenna systems are being used to harvest light through its antenna units. Using these systems, light is channeled into an acceptor chromophore and much more concentrated energy is obtained in acceptor unit with a specific wavelength. In this study, we have rationally designed and synthesized two different novel Förster-type light harvesting energy transfer cassettes which have large stokes shifts and emit in near-IR region. The first cassette has four boradiazaindacene (BODIPY) as donor groups and one tetrastyryl-BODIPY as an acceptor group. The second cassette has four distyryl-BODIPY units as donor groups and one tetrastyryl-BODIPY as an acceptor group. Click chemistry is successfully used to combine donor and acceptor groups to each other. Efficient energy transfer from donor groups to acceptor group in both cassettes was observed and characterized using emission spectrum, quantum yields and lifetimes. Energy transfer efficiencies and rate of energy transfer were calculated and it is demonstrated that there is more efficient energy transfer in cassette that has better overlap in donor emission and acceptor absorption spectrum which is in accordiance with expected behaviour for Förster-type of energy transfer cassettes.