Browsing by Author "Gaponik N."
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Item Open Access Anisotropic Emission from Multilayered Plasmon Resonator Nanocomposites of Isotropic Semiconductor Quantum Dots(American Chemical Society, 2011-01-19) Ozel, T.; Nizamoglu, S.; Sefunc, M.A.; Samarskaya, O.; Ozel, I. O.; Mutlugun, E.; Lesnyak, V.; Gaponik N.; Eychmuller, A.; Gaponenko, S. V.; Demir, Hilmi VolkanWe propose and demonstrate a nanocomposite localized surface plasmon resonator embedded into an artificial three-dimensional construction. Colloidal semiconductor quantum dots are assembled between layers of metal nanoparticles to create a highly strong plasmon-exciton interaction in the plasmonic cavity. In such a multilayered plasmonic resonator architecture of isotropic CdTe quantum dots, we observed polarized light emission of 80% in the vertical polarization with an enhancement factor of 4.4, resulting in a steady-state anisotropy value of 0.26 and reaching the highest quantum efficiency level of 30% ever reported for such CdTe quantum dot solids. Our electromagnetic simulation results are in good agreement with the experimental characterization data showing a significant emission enhancement in the vertical polarization, for which their fluorescence decay lifetimes are substantially shortened by consecutive replication of our unit cell architecture design. Such strongly plasmon-exciton coupling nanocomposites hold great promise for future exploitation and development of quantum dot plasmonic biophotonics and quantum dot plasmonic optoelectronics.Item Open Access Bio-nanohybrids of quantum dots and photoproteins facilitating strong nonradiative energy transfer(Royal Society of Chemistry, 2013-05-21) Seker U.O.S.; Mutlugun, E.; Hernandez-Martinez, R. L.; Sharma, V. K.; Lesnyak, V.; Gaponik N.; Eychmuller, A.; Demir, Hilmi VolkanUtilization of light is crucial for the life cycle of many organisms. Also, many organisms can create light by utilizing chemical energy emerged from biochemical reactions. Being the most important structural units of the organisms, proteins play a vital role in the formation of light in the form of bioluminescence. Such photoproteins have been isolated and identified for a long time; the exact mechanism of their bioluminescence is well established. Here we show a biomimetic approach to build a photoprotein based excitonic nanoassembly model system using colloidal quantum dots (QDs) for a new bioluminescent couple to be utilized in biotechnological and photonic applications. We concentrated on the formation mechanism of nanohybrids using a kinetic and thermodynamic approach. Finally we propose a biosensing scheme with an ON/OFF switch using the QD-GFP hybrid. The QD-GFP hybrid system promises strong exciton-exciton coupling between the protein and the quantum dot at a high efficiency level, possessing enhanced capabilities of light harvesting, which may bring new technological opportunities to mimic biophotonic events.Item Open Access Colloidal nanocrystals embedded in macrocrystals: methods and applications(American Chemical Society, 2016) Adam, M.; Gaponik N.; Eychmüller A.; Erdem, T.; Soran-Erdem, Z.; Demir, Hilmi VolkanColloidal semiconductor nanocrystals have gained substantial interest as spectrally tunable and bright fluorophores for color conversion and enrichment solids. However, they suffer from limitations in processing their solutions as well as efficiency degradation in solid films. As a remedy, embedding them into crystalline host matrixes has stepped forward for superior photostability, thermal stability, and chemical durability while simultaneously sustaining high quantum yields. Here, we review three basic methods for loading the macrocrystals with nanocrystals, namely relatively slow direct embedding, as well as accelerated methods of vacuum-assisted and liquid-liquid diffusion-assisted crystallization. We discuss photophysical properties of the resulting composites and present their application in light-emitting diodes as well as their utilization for plasmonics and excitonics. Finally, we present a future outlook for the science and technology of these materials.Item Open Access Colloidal Nanocrystals Embedded in Macrocrystals: Robustness, Photostability, and Color Purity(American Chemical Society, 2012-09-14) Otto, T.; Mueller, M.; Mundra, P.; Lesnyak, V.; Demir, Hilmi Volkan; Gaponik N.; Eychmuller, A.The incorporation of colloidal quantum dots (QDs) into ionic crystals of various salts (NaCl, KCl, KBr, etc.) is demonstrated. The resulting mixed crystals of various shapes and beautiful colors preserve the strong luminescence of the incorporated QDs. Moreover, the ionic salts appear to be very tight matrices, ensuring the protection of the QDs from the environment and as a result providing them with extraordinary high photo- and chemical stability. A prototype of a white light-emitting diode (WLED) with a color conversion layer consisting of this kind of mixed crystals is demonstrated. These materials may also find applications in nonlinear optics and as luminescence standards.Item Open Access Flexible and fragmentable tandem photosensitive nanocrystal skins(Royal Society of Chemistry, 2016) Akhavan S.; Uran, C.; Bozok, B.; Gungor K.; Kelestemur Y.; Lesnyak, V.; Gaponik N.; Eychmüller A.; Demir, Hilmi VolkanWe proposed and demonstrated the first account of large-area, semi-transparent, tandem photosensitive nanocrystal skins (PNSs) constructed on flexible substrates operating on the principle of photogenerated potential buildup, which avoid the need for applying an external bias and circumvent the current-matching limitation between junctions. We successfully fabricated and operated the tandem PNSs composed of single monolayers of colloidal water-soluble CdTe and CdHgTe nanocrystals (NCs) in adjacent junctions on a Kapton polymer tape. Owing to the usage of a single NC layer in each junction, noise generation was significantly reduced while keeping the resulting PNS films considerably transparent. In each junction, photogenerated excitons are dissociated at the interface of the semi-transparent Al electrode and the NC layer, with holes migrating to the contact electrode and electrons trapped in the NCs. As a result, the tandem PNSs lead to an open-circuit photovoltage buildup equal to the sum of those of the two single junctions, exhibiting a total voltage buildup of 128.4 mV at an excitation intensity of 75.8 μW cm-2 at 350 nm. Furthermore, we showed that these flexible PNSs could be bent over 3.5 mm radius of curvature and cut out in arbitrary shapes without damaging the operation of individual parts and without introducing any significant loss in the total sensitivity. These findings indicate that the NC skins are promising as building blocks to make low-cost, flexible, large-area UV/visible sensing platforms with highly efficient full-spectrum conversion.Item Open Access Implementation of high-quality warm-white light-emitting diodes by a model-experimental feedback approach using quantum dot-salt mixed crystals(American Chemical Society, 2015) Adam, M.; Erdem, T.; Stachowski, G.M.; Soran-Erdem Z.; Lox, J. F. L.; Bauer, C.; Poppe, J.; Demir, Hilmi Volkan; Gaponik N.; Eychmüller A.In this work, a model-experimental feedback approach is developed and applied to fabricate high-quality, warm-white light-emitting diodes based on quantum dots (QDs) as color-conversion materials. Owing to their unique chemical and physical properties, QDs offer huge potential for lighting applications. Nevertheless, both emission stability and processability of the QDs are limited upon usage from solution. Incorporating them into a solid ionic matrix overcomes both of these drawbacks, while preserving the initial optical properties. Here borax (Na2B4O7·10H2O) is used as a host matrix because of its lower solubility and thereby reduced ionic strength in water in comparison with NaCl. This guarantees the stability of high-quality CdSe/ZnS QDs in the aqueous phase during crystallization and results in a 3.4 times higher loading amount of QDs within the borax crystals compared to NaCl. All steps from the synthesis via mixed crystal preparation to the warm-white LED preparation are verified by applying the model-experimental feedback, in which experimental data and numerical results provide feedback to each other recursively. These measures are taken to ensure a high luminous efficacy of optical radiation (LER) and a high color rendering index (CRI) of the final device as well as a correlated color temperature (CCT) comparable to an incandescent bulb. By doing so, a warm-white LED with a LER of 341 lm/Wopt, a CCT of 2720 K and a CRI of 91.1 is produced. Finally, we show that the emission stability of the QDs within the borax crystals on LEDs driven at high currents is significantly improved. These findings indicate that the proposed warm-white light-emitting diodes based on QDs-in-borax hold great promise for quality lighting. © 2015 American Chemical Society.Item Open Access Liquid-liquid diffusion ‐ assisted crystallization: a fast and versatile approach toward high quality mixed quantum dot ‐ salt crystals(Wiley-VCH Verlag, 2015) Adam, M.; Wang, Z.; Dubavik, A.; Stachowski, G. M.; Meerbach, C.; Soran-Erdem, Z.; Rengers, C.; Demir, Hilmi Volkan; Gaponik N.; Eychmuller, A.Here, a new, fast, and versatile method for the incorporation of colloidal quantum dots (QDs) into ionic matrices enabled by liquid-liquid diffusion is demonstrated. QDs bear a huge potential for numerous applications thanks to their unique chemical and physical properties. However, stability and processability are essential for their successful use in these applications. Incorporating QDs into a tight and chemically robust ionic matrix is one possible approach to increase both their stability and processability. With the proposed liquid-liquid diffusion-assisted crystallization (LLDC), substantially accelerated ionic crystallization of the QDs is shown, reducing the crystallization time needed by one order of magnitude. This fast process allows to incorporate even the less stable colloids including initially oil-based ligand-exchanged QDs into salt matrices. Furthermore, in a modified two-step approach, the seed-mediated LLDC provides the ability to incorporate oil-based QDs directly into ionic matrices without a prior phase transfer. Finally, making use of their processability, a proof-of-concept white light emitting diode with LLDC-based mixed QD-salt films as an excellent color-conversion layer is demonstrated. These findings suggest that the LLDC offers a robust, adaptable, and rapid technique for obtaining high quality QD-salts.Item Open Access Macrocrystals of colloidal quantum dots in anthrancene: exciton trannsfer and polarized emission(American Chemical Society, 2015) Soran-Erdem, Z.; Erdem, T.; Hernandez-Martinez, P. L.; Akgül, M. Z.; Gaponik N.; Demir, Hilmi VolkanIn this work, centimeter-scale macrocrystals of nonpolar colloidal quantum dots (QDs) incorporated into anthracene were grown for the first time. The exciton transfer from the anthracene host to acceptor QDs was systematically investigated, and anisotropic emission from the isotropic QDs in the anthracene macrocrystals was discovered. Results showed a decreasing photoluminescence lifetime of the donor anthracene, indicating a strengthening energy transfer with increasing QD concentration in the macrocrystals. With the anisotropy study, QDs inside the anthracene host acquired a polarization ratio of similar to 1.5 at 0 degrees collection angle, and this increases to similar to 2.5 at the collection angle of 60 degrees. A proof-of-concept application of these excitonic macrocrystals as tunable color converters on light-emitting diodes was also demonstrated.Item Open Access Multiexciton generation assisted highly photosensitive CdHgTe nanocrystal skins(Elsevier Ltd, 2016) Akhavan S.; Cihan, A. F.; Yeltik A.; Bozok, B.; Lesnyak, V.; Gaponik N.; Eychmüller A.; Demir, Hilmi VolkanMultiexciton Generation (MEG) enabled by the photogeneration of more than one electron-hole pairs upon the absorption of a single photon observed in colloidal semiconductor nanocrystals (NCs) is an essential key to high efficiency when operating in large enough photon energy regimes. Here, we report a newly designed class of solution-processed highly sensitive MEG-assisted photosensors of CdHgTe NCs, in which the charge accumulation is dramatically enhanced for photon energies greater than two times the bandgap of the employed NCs. We fabricated and comparatively studied five types of devices based on different NC monolayers of selected quantum-confined bandgaps resulting in different levels of photovoltage buildup readouts. Among these photosensitive platforms, MEG is distinctly observed for CdHgTe NCs, as the number of electrons trapped inside these NCs and the number of holes accumulating into the interfacing metal electrode were increased beyond a single exciton per absorbed photon. Furthermore, we conducted time-resolved fluorescence measurements and confirmed the occurrence of MEG in the CdHgTe NC monolayer of the photosensor. These findings pave the way for engineering of multiexciton kinetics in high-efficiency NC-based photosensors and photovoltaics.Item Open Access Quantum dot integrated LEDs using photonic and excitonic color conversion(Elsevier, 2011-11-23) Demir, Hilmi Volkan; Nizamoglu, S.; Erdem, T.; Mutlugun, E.; Gaponik N.; Eychmüller A.This review summarizes advantages, recent progress and challenges related to the quickly evolving research field of colloidal quantum dot integrated LEDs based on color conversion. We start with presenting a short general introduction to the field of solid state lighting and color conversion phenomena, which are essential for defining the requirements for high-quality general lighting. Subsequently we continue with a brief description of the synthesis of nanocrystal quantum dots and their optical properties together with the advantages of utilizing them in LEDs as color convertors. Following this basic background, we review the recent efforts on quantum dot integrated color-conversion LEDs, Förster resonance energy transfer (FRET) converted LEDs and FRET-enhanced LEDs. Finally, we conclude with a future outlook on semiconductor lighting and quantum dot integrated LEDs.Item Open Access Selective enhancement of surface-state emission and simultaneous quenching of interband transition in white-luminophor CdS nanocrystals using localized plasmon coupling(Institute of Physics Publishing Ltd., 2008) Ozel, T.; Soganci, I. M.; Nizamoglu, S.; Huyal I.O.; Mutlugun, E.; Sapra, S.; Gaponik N.; Eychmüller A.; Demir, Hilmi VolkanWe propose and demonstrate the controlled modification and selective enhancement of surface-state emission in white-luminophor CdS nanocrystals (NCs) by plasmon-coupling them with proximal metal nanostructures. By carefully designing nano-Ag films to match their localized plasmon resonance spectrally with the surface-state emission peak of CdS NCs, we experimentally show that the surface-state emission is substantially enhanced in the visible wavelength, while the interband (band-edge) transition at the shorter wavelength far away from the plasmon resonance is simultaneously significantly suppressed. With such plasmon tuning and consequent strong plasmon coupling specifically for the surface-state transitions, the surface-state emission is made stronger than the band-edge emission. This corresponds to an enhancement factor of 12.7-fold in the ratio of the surface-state peak emission to the band-edge peak emission of the plasmon-coupled film sample compared with that in solution. Such a plasmonic engineering of surface-state emission in trap-rich CdS white nanoluminophors holds great promise for future solid-state lighting.Item Open Access Stable and efficient colour enrichment powders of nonpolar nanocrystals in LiCl(Royal Society of Chemistry, 2015) Erdem, T.; Soran-Erdem Z.; Sharma, V. K.; Keleştemur, Y.; Adam, M.; Gaponik N.; Demir, Hilmi VolkanIn this work, we propose and develop the inorganic salt encapsulation of semiconductor nanocrystal (NC) dispersion in a nonpolar phase to make a highly stable and highly efficient colour converting powder for colour enrichment in light-emitting diode backlighting. Here the wrapping of the as-synthesized green-emitting CdSe/CdZnSeS/ZnS nanocrystals into a salt matrix without ligand exchange is uniquely enabled by using a LiCl ionic host dissolved in tetrahydrofuran (THF), which simultaneously disperses these nonpolar nanocrystals. We studied the emission stability of the solid films prepared using NCs with and without LiCl encapsulation on blue LEDs driven at high current levels. The encapsulated NC powder in epoxy preserved 95.5% of the initial emission intensity and stabilized at this level while the emission intensity of NCs without salt encapsulation continuously decreased to 34.7% of its initial value after 96 h of operation. In addition, we investigated the effect of ionic salt encapsulation on the quantum efficiency of nonpolar NCs and found the quantum efficiency of the NCs-in-LiCl to be 75.1% while that of the NCs in dispersion was 73.0% and that in a film without LiCl encapsulation was 67.9%. We believe that such ionic salt encapsulated powders of nonpolar NCs presented here will find ubiquitous use for colour enrichment in display backlighting. © The Royal Society of Chemistry 2015.Item Open Access Stable dispersion of iodide-capped PbSe quantum dots for high-performance low-temperature processed electronics and optoelectronics(American Chemical Society, 2015) Sayevich, V.; Gaponik N.; Plötner, M.; Kruszynsk, M.; Gemming, T.; Dzhagan, V. M.; Akhavan S.; Zahn, D. R. T.; Demir, Hilmi Volkan; Eychmüller, A.Here, we present a ligand exchange of long insulating molecules with short, robust, and environmentally friendly iodide ions via a mild flocculation of PbSe nanocrystals (NCs). This ligand exchange leads to the formation of stable colloidal solutions in various polar solvents and in a broad concentration range via electrostatic repulsion. The iodide capping ligands preserve the electronic structure and maintain the optical properties of the PbSe NCs, both in solution and in the form of solid films. The spin-coated PbSe NC solids exhibit good transport characteristics with electron mobilities in the linear and saturation regimes reaching (2.1 ± 0.3) cm2 /(V•s) and (2.9 ± 0.4) cm2 /(V•s), respectively. This opens up opportunities for the low-cost and low-temperature fabrication of NC thin films being attractive for applications in the fields of electronics and optoelectronics.Item Open Access Structural tuning of color chromaticity through nonradiative energy transfer by interspacing CdTe nanocrystal monolayers(American Institute of Physics, 2009-02-09) Cicek, N.; Nizamoglu, S.; Ozel, T.; Mutlugun, E.; Karatay, D. U.; Lesnyak, V.; Otto, T.; Gaponik N.; Eychmuller, A.; Demir, Hilmi VolkanWe proposed and demonstrated architectural tuning of color chromaticity by controlling photoluminescence decay kinetics through nonradiative Forster resonance energy transfer in the heterostructure of layer-by-layer spaced CdTe nanocrystal (NC) solids. We achieved highly sensitive tuning by precisely adjusting the energy transfer efficiency from donor NCs to acceptor NCs via controlling interspacing between them at the nanoscale. By modifying decay lifetimes of donors from 12.05 to 2.96 ns and acceptors from 3.68 to 14.57 ns, we fine-tuned chromaticity coordinates from (x,y)=(0.575,0.424) to (0.632, 0.367). This structural adjustment enabled a postsynthesis color tuning capability, alternative or additive to using the size, shape, and composition of NCs.Item Open Access Sweet plasmonics: sucrose macrocrystals of metal nanoparticles(Tsinghua University Press, 2015-03) Erdem, T.; Soran - Erdem, Z.; Hernandez - Martinez, P. L.; Sharma, V. K.; Akçalı, H.; Akçalı, I.; Gaponik N.; Eychmuller, A.; Demir, Hilmi VolkanThe realization of plasmonic structures generally necessitates expensive fabrication techniques, such as electron beam and focused ion beam lithography, allowing for the top-down fabrication of low-dimensional structures. Another approach to make plasmonic structures in a bottom-up fashion is colloidal synthesis, which is convenient for liquid-state applications or very thin solid films where aggregation problems are an important challenge. The architectures prepared using these methods are typically not robust enough for easy handling and convenient integration. Therefore, developing a new plasmonic robust platform having large-scale dimensions without adversely affecting the plasmonic features is in high demand. As a solution, here we present a new plasmonic composite structure consisting of gold nanoparticles (Au NPs) incorporated into sucrose macrocrystals on a large scale, while preserving the plasmonic nature of the Au NPs and providing robustness in handling at the same time. As a proof of concept demonstration, we present the fluorescence enhancement of green CdTe quantum dots (QDs) via plasmonic coupling with these Au NPs in the sucrose crystals. The obtained composite material exhibits centimeter scale dimensions and the resulting quantum efficiency (QE) is enhanced via the interplay between the Au NPs and CdTe QDs by 58% (from 24% to 38%). Moreover, a shortening in the photoluminescence lifetime from 11.0 to 7.40 ns, which corresponds to a field enhancement factor of 2.4, is observed upon the introduction of Au NPs into the QD incorporated macrocrystals. These results suggest that such "sweet" plasmonic crystals are promising for large-scale robust platforms to embed plasmonic nanoparticles.Item Open Access Tuning shades of white light with multi-color quantum-dot-quantum-well emitters based on onion-like CdSe-ZnS heteronanocrystals(Institute of Physics Publishing Ltd., 2008) Demir, Hilmi Volkan; Nizamoglu, S.; Mutlugun, E.; Ozel, T.; Sapra, S.; Gaponik N.; Eychmüller A.We present white light generation controlled and tuned by multi-color quantum-dot-quantum-well emitters made of onion-like CdSe/ZnS/CdSe core/shell/shell heteronanocrystals integrated on InGaN/GaN light-emitting diodes (LEDs). We demonstrate hybrid white LEDs with (x, y) tristimulus coordinates tuned from (0.26, 0.33) to (0.37, 0.36) and correlated color temperatures from 27 413 to 4192 K by controlling the number of their integrated red-green-emitting heteronanocrystals. We investigate the modification of in-film emission from these multi-layered heteronanocrystals with respect to their in-solution emission, which plays a significant role in hybrid LED applications. Our proof-of-principle experiments indicate that these complex heteronanocrystals hold promise for use as nanoluminophors in future hybrid white LEDs.Item Open Access White emitting CdS quantum dot nanoluminophores hybridized on near-ultraviolet LEDs for high-quality white light generation and tuning(Institute of Physics Publishing Ltd., 2008) Nizamoglu, S.; Mutlugun, E.; Akyuz, O.; Perkgoz, N. K.; Demir, Hilmi Volkan; Liebscher, L.; Sapra, S.; Gaponik N.; Eychmüller A.To generate white light using semiconductor nanocrystal (NC) quantum dots integrated on light emitting diodes (LEDs), multiple hybrid device parameters (emission wavelengths of the NCs and the excitation platform, order of the NCs with different sizes, amount of the different types of NCs, etc) need to be carefully designed and properly implemented. In this study, we introduce and demonstrate white LEDs based on simple device hybridization using only a single type of white emitting CdS quantum dot nanoluminophores on near-ultraviolet LEDs. Here we present their design, synthesis-growth, fabrication and characterization. With these hybrid devices, we achieve high color rendering index (>70), despite using only a single NC type. Furthermore, we conveniently tune their photometric properties including the chromaticity coordinates, correlated color temperature, and color rendering index with the number of hybridized nanoluminophores in a controlled manner.