Browsing by Author "Ozel, T."

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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 Volkan
We 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.
Open Access
Blue quantum electroabsorption modulators based on reversed quantum confined Stark effect with blueshift
(AIP Publishing LLC, 2007-01-02) Sari, E.; Nizamoglu, S.; Ozel, T.; Demir, Hilmi Volkan
The authors present the design, growth, fabrication, experimental characterization, and theoretical analysis of blue quantum electroabsorption modulators that incorporate similar to 5 nm thick In0.35Ga0.65N/GaN quantum structures for operation between 420 and 430 nm. Growing on polar c plane on sapphire, they obtain quantum structures with zigzag potential profile due to alternating polarization fields and demonstrate that their optical absorption blueshifts with applied electric field, unlike the redshift of conventional quantum confined Stark effect. In InGaN/GaN quantum structures, they report the largest absorption change of 6000 cm(-1) for 6 V bias swing around 424 nm, holding promise for blue optical clock generation and injection directly into silicon chips.
Open Access
Comparative study of electroabsorption in InGaN/GaN quantum zigzag heterostructures with polarization-induced electric fields
(AIP Publishing LLC, 2008-05-19) Sari, E.; Ozel, T.; Koc, A.; Ju, J. W.; Ahn, H.-K.; Lee, I.-H.; Baek, J.-H.; Demir, Hilmi Volkan
We present a comparative study on InGaN/GaN quantum zigzag structures embedded in p-i-n diode architecture that exhibit blue-shifting electroabsorption in the blue when an electric field is externally applied to compensate for the polarization-induced electric field across the wells. With the polarization breaking their symmetry, the same InGaN/GaN quantum structures redshift their absorption edge when the external field is applied in the same direction as the well polarization. Both computationally and experimentally, we investigate the effects of polarization on electroabsorption by varying compositional content and structural parameters and demonstrate that electroabsorption grows stronger with weaker polarization in these multiple quantum well modulators.
Open Access
Highly efficient nonradiative energy transfer mediated light harvesting in water using aqueous CdTe quantum dot antennas
(Optical Society of America, 2010) Mutlugun, E.; Samarskaya, O.; Ozel, T.; Cicek, N.; Gaponik, N.; Eychmuller, A.; Demir, Hilmi Volkan
We present light harvesting of aqueous colloidal quantum dots to nonradiatively transfer their excitonic excitation energy efficiently to dye molecules in water, without requiring ligand exchange. These as-synthesized CdTe quantum dots that are used as donors to serve as light-harvesting antennas are carefully optimized to match the electronic structure of Rhodamine B molecules used as acceptors for light harvesting in aqueous medium. By varying the acceptor to donor concentration ratio, we measure the light harvesting factor, along with substantial lifetime modifications of these water-soluble quantum dots, from 25.3 ns to 7.2 ns as a result of their energy transfer with efficiency levels up to 86%. Such nonradiative energy transfer mediated light harvesting in aqueous medium holds great promise for future quantum dot multiplexed dye biodetection systems. (C) 2010 Optical Society of America.
Open Access
Hyperbolic metamaterials based on quantum-dot plasmon resonator nanocomposites
(Optical Society of America, 2014) Zhokovsky, S. V.; Ozel, T.; Mutlugun, E.; Gaponik, N.; Eychmuller, A.; Lavrinenko, A. V.; Demir, Hilmi Volkan; Gaponenko, S. V.
We theoretically demonstrate that nanocomposites made of colloidal semiconductor quantum dot monolayers placed between metal nanoparticle monolayers can function as multilayer hyperbolic metamaterials. Depending on the thickness of the spacer between the quantum dot and nanoparticle layers, the effective permittivity tensor of the nanocomposite is shown to become indefinite, resulting in increased photonic density of states and strong enhancement of quantum dot luminescence. This explains the results of recent experiments [T. Ozel et al., ACS Nano 5, 1328 (2011)] and confirms that hyperbolic metamaterials are capable of increasing the radiative decay rate of emission centers inside them. The proposed theoretical framework can also be used to design quantum-dot/nanoplasmonic composites with optimized luminescence enhancement. (C) 2014 Optical Society of America
Open Access
Non-radiative resonance energy transfer in bi-polymer nanoparticles of fluorescent conjugated polymers
(Optical Society of American (OSA), 2010) Ozel I.O.; Ozel, T.; Demir, Hilmi Volkan; Tuncel, D.
This work demonstrates the comparative studies of non-radiative resonance energy transfer in bi-polymer nanoparticles based on fluorescent conjugated polymers. For this purpose, poly[(9,9-dihexylfluorene) (PF) as a donor (D) and poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) as an acceptor (A) have been utilized, from which four different bi-polymer nanoparticle systems are designed and synthesized. Both, steady-state fluorescence spectra and time-resolved fluorescence measurements indicate varying energy transfer efficiencies from the host polymer PF to the acceptor polymer MEH-PPV depending on the D-A distances and structural properties of the nanoparticles. The first approach involves the preparation of PF and MEH-PPV nanoparticles separately and mixing them at a certain ratio. In the second approach, first PF and MEH-PPV solutions are mixed prior to nanoparticle formation and then nanoparticles are prepared from the mixture. Third and fourth approaches involve the sequential nanoparticle preparation. In the former, nanoparticles are prepared to have PF as a core and MEH-PPV as a shell. The latter is the reverse of the third in which the core is MEH-PPV and the shell is PF. The highest energy transfer efficiency recorded to be 35% is obtained from the last system, in which a PF layer is sequentially formed on MEH-PPV NPs. © 2010 Optical Society of America.
Open Access
Observation of selective plasmon-exciton coupling in nonradiative energy transfer: donor-selective versus acceptor-selective plexcitons
(American Chemical Society, 2013-06) Ozel, T.; Hernandez-Martinez, P. L.; Mutlugun, E.; Akin, O.; Nizamoglu, S.; Ozel, I. O.; Zhang, Q.; Xiong, Q.; Demir, Hilmi Volkan
We report selectively plasmon-mediated nonradiative energy transfer between quantum dot (QD) emitters interacting with each other via Forster-type resonance energy transfer (FRET) under controlled plasmon coupling either to only the donor QDs (i.e., donor-selective) or to only the acceptor QDs (i.e., acceptor-selective). Using layer-by-layer assembled colloidal QD nanocrystal solids with metal nanoparticles integrated at carefully designed spacing, we demonstrate the ability to enable/disable the coupled plasmon-exciton (plexciton) formation distinctly at the donor (exciton departing) site or at the acceptor (exciton feeding) site of our choice, while not hindering the donor exciton-acceptor exciton interaction but refraining from simultaneous coupling to both sites of the donor and the acceptor in the FRET process.. In the case of donor-selective plexciton, we observed a substantial shortening in the donor QD lifetime from 1.33 to 0.29 ns as a result of plasmon-coupling to the donors and the FRET-assisted exciton transfer from the donors to the acceptors, both of which shorten the donor lifetime. This consequently enhanced the acceptor emission by a factor of 1.93. On the other hand, in the complimentary case of acceptor-selective plexciton, we observed a 2.70-fold emission enhancement in the acceptor QDs, larger than the acceptor emission enhancement of the donor-selective plexciton, as a result of the combined effects of the acceptor plasmon coupling and the FRET-assisted exciton feeding. Here we present the comparative results of theoretical modeling of the donor- and acceptor-selective plexcitons of nonradiative energy transfer developed here for the first time, which are in excellent agreement with the systematic experimental characterization. Such an ability to modify and control energy transfer through mastering plexcitons is of fundamental importance, opening up new applications for quantum dot embedded plexciton devices along with the development of new techniques in FRET-based fluorescence microscopy.
Open Access
On the origin of high quality white light emission from a hybrid organic / inorganic light emitting diode using azide functionalized polyfluorene
(Royal Society of Chemistry, 2008) Huyal I.O.; Koldemir, U.; Ozel, T.; Demir, Hilmi Volkan; Tuncel, D.
High quality white light generation with high colour rendering index (CRI) was achieved by integrating a cross-linkable azide functionalized polyfluorene derivative, namely poly[(9,9-dihexylfluorene)-co-alt-(9,9-bis(6-azidohexyl) fluorene)] (PFA), as a down-converting fluorescent material on the inorganic n-UV InGaN/GaN LED platform. For comparison, two other polyfluorene based polymers, namely poly[(9,9-dihexylfluorene)-co-alt-(9,9-bis(6-bromohexyl) fluorene)] (PFB) and poly[9,9-dihexyl-9H-fluorene] (PF), were tested for white light generation. While PFA and PF both led to white light generation, PFB fell out of the white region on the chromaticity diagram. Compared to PFA, both of the control groups (PF and PFB) exhibited much lower CRI. To gain a better insight into the mechanisms playing a key role for the generation of such high quality white light in PFA, all of these polymers were further subjected to a series of experiments such as controlled exposure to heat at 220 °C for 2 h under Ar and in air. The polymers PFA and PFB, which include cross-linkable groups, produced broad emission spectra in the region of 430-650 nm upon annealing in the absence of oxygen under Ar atmosphere while almost no change was observed in the emission spectrum of PF without any cross-linkable groups. PFA undergoes cross-linking through the decomposition of azide leading to reactive nitrene species, whereas in PFB cross-linking probably occurs via debromination. This result clearly proved that the broadening can not be attributed only to photo or thermal oxidation, but it is also due to cross-linking. PFA was also exposed to n-UV light from the InGaN/GaN LED to investigate its photostability. In these experiments, the spectral changes in absorbance and emission properties and thermal transitions of these polymers were monitored by FT-IR, UV-Vis and fluorescent spectrometry, and differential scanning calorimetry (DSC). These experiments indicated that PFA provides high quality white light opportunely via cross-linking and remains stable once cross-linking is formed in a solid film.
Open Access
Peptide-mediated constructs of quantum dot nanocomposites for enzymatic control of nonradiative energy transfer
(American Chemical Society, 2011) Seker U.O.S.; Ozel, T.; Demir, Hilmi Volkan
A bottom-up approach for constructing colloidal semiconductor quantum dot (QDot) nanocomposites that facilitate nonradiative Förster-type resonance energy transfer (FRET) using polyelectrolyte peptides was proposed and realized. The electrostatic interaction of these polypeptides with altering chain lengths was probed for thermodynamic, structural, and morphological aspects. The resulting nanocomposite film was successfully cut with the protease by digesting the biomimetic peptide layer upon which the QDot assembly was constructed. The ability to control photoluminescence decay lifetime was demonstrated by proteolytic enzyme activity, opening up new possibilities for biosensor applications.
Open Access
Quantum efficiency enhancement in film by making nanoparticles of polyfluorene
(Optical Society of America, 2008) Huyal, I. O.; Ozel, T.; Tuncel, D.; Demir, Hilmi Volkan
We report on conjugated polymer nanoparticles of polyfluorene that were formed to exhibit higher fluorescence quantum efficiency in film (68%) and reduce undesired emission peak wavelength shifts in film (by 20 nm), compared to the solid-state polymer thin film made directly out of the same polymer solution without forming nanoparticles. Using the facile reprecipitation method, solutions of poly[9,9-dihexyl-9H-fluorene] in THF were added at different volume ratios to obtain different size distributions of nanoparticle dispersions in water. This allowed us to control the sizedependent optical emission of our polyfluorene nanoparticles. Such organic nanoparticles hold great promise for use as efficient emitters in optoelectronic device applications. (C) 2008 Optical Society of America.
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 Volkan
We 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.
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 Volkan
We 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.
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.
Open Access
Violet to deep-ultraviolet InGaN∕GaN and GaN∕AlGaN quantum structures for UV electroabsorption modulators
(AIP Publishing LLC, 2007) Ozel, T.; Sari, E.; Nizamoglu, S.; Demir, Hilmi Volkan
In this paper, we present four GaN based polar quantum structures grown on c-plane embedded in p-i-n diode architecture as a part of high-speed electroabsorption modulators for use in optical communication (free-space non-line-of-sight optical links in the ultraviolet) UV: the first modulator incorporates 4 – 6 nm thick GaN/AlGaN quantum structures for operation in the deep-UV spectral region and the other three incorporate 2 – 3 nm thick InGaN/GaN quantum structures tuned for operation in violet to near-UV spectral region. Here, we report on the design, epitaxial growth, fabrication, and characterization of these quantum electroabsorption modulators. In reverse bias, these devices exhibit a strong electroabsorption (optical absorption coefficient change in the range of 5500– 13 000 cm−1 with electric field swings of 40– 75 V/um at their specific operating wavelengths. In this work, we show that these quantum electroabsorption structures hold great promise for future applications in ultraviolet optoelectronics technology such as external modulation and data coding in secure non-line-of-sight communication systems.
Open Access
White emitting polyfluorene functionalized with azide hybridized on near-UV light emitting diode for high color rendering index
(Optical Society of America, 2008) Huyal, I. O.; Ozel, T.; Koldemir, U.; Nizamoglu, S.; Tuncel, D.; Demir, Hilmi Volkan
We develop and demonstrate high-quality white light generation that relies on the use of a single-type simple conjugated polymer of polyfluorene functionalized with azide groups (PFA) integrated on a near-UV LED platform. The high-quality white emission from the polyfluorene is achieved by using the azide functionalization to facilitate cross-linking intentionally when cast into solid-state form. Hybridized on n-UV InGaN/GaN LED at 378 nm, the PFA emitters collectively generate a very broad down-converting photoluminescence at longer wavelengths across the entirety of the visible spectrum, yielding high color rendering indices up to 91. (c) 2008 Optical Society of America.
Open Access
White light generation tuned by dual hybridization of nanocrystals and conjugated polymers
(Institute of Physics Publishing Ltd., 2007) Demir, Hilmi Volkan; Nizamoglu, S.; Ozel, T.; Mutlugun, E.; Huyal I.O.; Sari, E.; Holder, E.; Tian, N.
Dual hybridization of highly fluorescent conjugated polymers and highly luminescent nanocrystals (NCs) is developed and demonstrated in multiple combinations for controlled white light generation with high color rendering index (CRI) (> 80) for the first time. The generated white light is tuned using layer-by-layer assembly of CdSe/ZnS core-shell NCs closely packed on polyfluorene, hybridized on near-UV emitting nitride-based light emitting diodes (LEDs). The design, synthesis, growth, fabrication and characterization of these hybrid inorganic-organic white LEDs are presented. The following experimental realizations are reported: (i) layer-by-layer hybridization of yellow NCs (λPL = 580nm) and blue polyfluorene (λPL = 439 nm) with tristimulus coordinates of (x, y) = (0.31, 0.27), correlated color temperature of Tc = 6962 K and CRI of R a = 53.4; (ii) layer-by-layer assembly of yellow and green NCs (λPL = 580 and 540 nm) and blue polyfluorene (λPL = 439 nm) with (x, y) = (0.23, 0.30), Tc = 14395 K and Ra = 65.7; and (iii) layer-by-layer deposition of yellow, green and red NCs (λPL = 580, 540 and 620 nm) and blue polyfluorene (λPL = 439 nm) with (x, y) = (0.38, 0.39), T c = 4052 K and Ra = 83.0. The CRI is demonstrated to be well controlled and significantly improved by increasing multi-chromaticity of the NC and polymer emitters.
Open Access
White light generation using CdSe/ZnS core–shell nanocrystals hybridized with InGaN/GaN light emitting diodes
(Institute of Physics Publishing Ltd., 2007) Nizamoglu, S.; Ozel, T.; Sari, E.; Demir, Hilmi Volkan
We introduce white light generation using CdSe/ZnS core-shell nanocrystals of single, dual, triple and quadruple combinations hybridized with InGaN/GaN LEDs. Such hybridization of different nanocrystal combinations provides the ability to conveniently adjust white light parameters including the tristimulus coordinates (x,y), correlated colour temperature (Tc) and colour rending index (Ra). We present the design, growth, fabrication and characterization of our white hybrid nanocrystal-LEDs that incorporate combinations of (1) yellow nanocrystals (λPL = 580 nm) on a blue LED (λEL = 440 nm) with (x,y) = (0.37,0.25), T c = 2692 K and Ra = 14.69; (2) cyan and red nanocrystals (λPL = 500 and 620 nm) on a blue LED (λEL = 440 nm) with (x,y) = (0.37,0.28), Tc = 3246 K and Ra = 19.65; (3) green, yellow and red nanocrystals (λPL = 540, 580 and 620 nm) on a blue LED (λEL = 452 nm) with (x,y) = (0.30,0.28), Tc = 7521 K and Ra = 40.95; and (4) cyan, green, yellow and red nanocrystals (λPL = 500, 540, 580 and 620 nm) on a blue LED (λEL = 452 nm) with (x,y) = (0.24,0.33), Tc = 11 171 K and Ra = 71.07. These hybrid white light sources hold promise for future lighting and display applications with their highly adjustable properties.