Tuncay Ozel

White light generation using CdSe/ZnS core?shell nanocrystals hybridized with InGaN/GaN light emitting diodes

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 (?PL = 580?nm) on a blue LED (?EL = 440?nm) with (x,y) = (0.37,0.25), Tc = 2692?K and Ra = 14.69; (2) cyan and red nanocrystals (?PL = 500 and 620?nm) on a blue LED (?EL = 440?nm) with (x,y) = (0.37,0.28), Tc = 3246?K and Ra = 19.65; (3) green, yellow and red nanocrystals (?PL = 540, 580 and 620?nm) on a blue LED (?EL = 452?nm) with (x,y) = (0.30,0.28), Tc = 7521?K and Ra = 40.95; and (4) cyan, green, yellow and red nanocrystals (?PL = 500, 540, 580 and 620?nm) on a blue LED (?EL = 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.

Dual-color emitting quantum-dot-quantum-well CdSe-ZnS heteronanocrystals hybridized on InGaN/GaN light emitting diodes for high-quality white light generation

We report white light generation by hybridizing green-red emitting (CdSe)ZnS∕CdSe (core)shell/shell quantum-dot-quantum-well heteronanocrystals on blue InGaN∕GaN light emitting diodes with the photometric properties of tristimulus coordinates (x,y)=(0.36,0.30), luminous efficacy of optical radiation LE=278lm∕W, correlated color temperature CCT=3929K, and color-rendering index CRI=75.1. We present the photometric analysis and the quantum mechanical design of these dual-color emitting heteronanocrystals synthesized to achieve high-quality white light when hybridized on light emitting diodes. Using such multicolor emitting heteronanocrystals facilitates simple device implementation while providing good photometric properties.

Anisotropic emission from multilayered plasmon resonator nanocomposites of isotropic semiconductor quantum dots.

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.

Observation of Selective Plasmon-Exciton Coupling in Nonradiative Energy Transfer: Donor-Selective versus Acceptor-Selective Plexcitons

We report selectively plasmon-mediated nonradiative energy transfer between quantum dot (QD) emitters interacting with each other via Förster-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 complementary 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.

Selective enhancement of surface-state emission and simultaneous quenching of interband transition in white-luminophor CdS nanocrystals using localized plasmon coupling

We propose and demonstrate the controlled modification and selec- tive enhancement of surface-state emission in white-luminophor CdS nanocrys- tals (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 experimen- tally 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 specifi- cally 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.

White light generation tuned by dual hybridization of nanocrystals and conjugated polymers

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 (?PL=580?nm) and blue polyfluorene (?PL=439?nm) with tristimulus coordinates of (x, y)=(0.31, 0.27), correlated color temperature of Tc=6962?K and CRI of Ra=53.4; (ii) layer-by-layer assembly of yellow and green NCs (?PL=580 and 540?nm) and blue polyfluorene (?PL=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 (?PL=580, 540 and 620?nm) and blue polyfluorene (?PL=439?nm) with (x, y)=(0.38, 0.39), Tc=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.

Blue quantum electroabsorption modulators based on reversed quantum confined Stark effect with blueshift

The authors present the design, growth, fabrication, experimental characterization, and theoretical analysis of blue quantum electroabsorption modulators that incorporate ∼5nm thick In0.35Ga0.65N∕GaN quantum structures for operation between 420 and 430nm. 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 6000cm−1 for 6V bias swing around 424nm, holding promise for blue optical clock generation and injection directly into silicon chips.

Structural tuning of color chromaticity through nonradiative energy transfer by interspacing CdTe nanocrystal monolayers

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.

Peptide-mediated constructs of quantum dot nanocomposites for enzymatic control of nonradiative energy transfer.

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.

Quantum efficiency enhancement in film by making nanoparticles of polyfluorene

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.