Browsing by Subject "Quantum Dots"

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Open Access
Color science of nanocrystal quantum dots for lighting and displays
(De Gruyter, 2013-02) Erdem, T.; Demir, Hilmi Volkan
Colloidal nanocrystals of semiconductor quantum dots (QDs) are gaining prominence among the optoelectronic materials in the photonics industry. Among their many applications, their use in artificial lighting and displays has attracted special attention thanks to their high efficiency and narrow emission band, enabling spectral purity and fine tunability. By employing QDs in color-conversion LEDs, it is possible to simultaneously accomplish successful color rendition of the illuminated objects together with a good spectral overlap between the emission spectrum of the device and the sensitivity of the human eye, in addition to a warm white color, in contrast to other conventional sources such as incandescent and fluorescent lamps, and phosphor-based LEDs, which cannot achieve all of these properties at the same time. In this review, we summarize the color science of QDs for lighting and displays, and present the recent developments in QD-integrated LEDs and display research. First, we start with a general introduction to color science, photometry, and radiometry. After presenting an overview of QDs, we continue with the spectral designs of QD-integrated white LEDs that have led to efficient lighting for indoor and outdoor applications. Subsequently, we discuss QD color-conversion LEDs and displays as proof-of-concept applications - a new paradigm in artificial lighting and displays. Finally, we conclude with a summary of research opportunities and challenges along with a future outlook.
Open Access
Excitonics of semiconductor quantum dots and wires for lighting and displays
(Wiley-VCH Verlag, 2013) Guzelturk, B.; Martinez, P. L. H.; Zhang, Q.; Xiong, Q.; Sun, H.; Sun, X. W.; Govorov, A. O.; Demir, Hilmi Volkan
In the past two decades, semiconductor quantum dots and wires have developed into new, promising classes of materials for next-generation lighting and display systems due to their superior optical properties. In particular, exciton-exciton interactions through nonradiative energy transfer in hybrid systems of these quantum-confined structures have enabled exciting possibilities in light generation. This review focuses on the excitonics of such quantum dot and wire emitters, particularly transfer of the excitons in the complex media of the quantum dots and wires. Mastering excitonic interactions in low-dimensional systems is essential for the development of better light sources, e.g., high-efficiency, high-quality white-light generation; wide-range color tuning; and high-purity color generation. In addition, introducing plasmon coupling provides the ability to amplify emission in specially designed exciton-plasmon nanostructures and also to exceed the Forster limit in excitonic interactions. In this respect, new routes to control excitonic pathways are reviewed in this paper. The review further discusses research opportunities and challenges in the quantum dot and wire excitonics with a future outlook.
Open Access
Localized plasmon-engineered spontaneous emission of CdSe/ZnS nanocrystals closely-packed in the proximity of Ag nanoisland films for controlling emission linewidth, peak, and intensity
(Optical Society of America, 2007) Soganci, I. M.; Nizamoglu, S.; Mutlugun, E.; Akin, O.; Demir, Hilmi Volkan
Using metallic nanoislands, we demonstrate the localized plasmonic control and modification of the spontaneous emission from closely-packed nanocrystal emitters, leading to significant changes in their collective emission characteristics tuned spectrally and spatially by plasmon coupling. Using randomly-distributed silver nanoislands, we show that the emission linewidth of proximal CdSe/ZnS core-shell quantum dots is reduced by 22% and their peak emission wavelength is shifted by 14nm, while their ensemble photoluminescence is enhanced via radiative energy transfer by 21.6 and 15.1 times compared to the control groups of CdSe/ZnS nanocrystals with identical nano-silver but no dielectric spacer and the same nanocrystals alone, respectively. (C) 2007 Optical Society of America.
Open Access
Phonon-assisted exciton transfer into silicon using nanoemitters: the role of phonons and temperature effects in förster resonance energy transfer
(American Chemical Society, 2013) Yeltik A.; Guzelturk, B.; Hernandez-Martinez, P. L.; Govorov, A. O.; Demir, Hilmi Volkan
We study phonon-assisted Forster resonance energy transfer (FRET) into an indirect band-gap semiconductor using nanoemitters. The unusual temperature dependence of this energy transfer, which is measured using the donor nanoemitters of quantum dot (QD) layers integrated on the acceptor monocrystalline bulk silicon as a model system, is predicted by a phonon-assisted exciton transfer model proposed here. The model includes the phonon-mediated optical properties of silicon, while considering the contribution from the multimonolayer-equivalent QD film to the nonradiative energy transfer, which is derived with a d(-3) distance dependence. The FRET efficiencies are experimentally observed to decrease at cryogenic temperatures, which are well explained by the model considering the phonon depopulation in the indirect band-gap acceptor together with the changes in the quantum yield of the donor. These understandings will be crucial for designing FRET-enabled sensitization of silicon based high-efficiency excitonic systems using nanoemitters.
Open Access
A photometric investigation of ultra-efficient LEDs with high color rendering index and high luminous efficacy employing nanocrystal quantum dot luminophores
(Optical Society of America, 2009-12-24) Erdem, T.; Nizamoglu, S.; Sun, X. W.; Demir, Hilmi Volkan
We report a photometric study of ultra-efficient light emitting diodes (LEDs) that exhibit superior color rendering index (CRI) and luminous efficacy of optical radiation (LER) using semiconductor quantum dot nanocrystal (NC) luminophores. Over 200 million systematically varied NC-LED designs have been simulated to understand feasible performance in terms of CRI vs. LER. We evaluated the effects of design parameters including peak emission wavelength, full-width-at-half-maximum, and relative amplitudes of each NC color component on LED performance. Warm-white LEDs with CRI >90 and LER >380 lm/W at a correlated color temperature of 3000 K are shown to be achieved using nanocrystal luminophores. (C) 2009 Optical Society of America
Open Access
Photovoltaic nanocrystal scintillators hybridized on Si solar cells for enhanced conversion efficiency in UV
(Optical Society of American (OSA), 2008) Mutlugun, E.; Soganci I.M.; Demir, Hilmi Volkan
We propose and demonstrate semiconductor nanocrystal based photovoltaic scintillators integrated on solar cells to enhance photovoltaic device parameters including spectral responsivity, open circuit voltage, short circuit current, fill factor, and solar conversion efficiency in the ultraviolet. Hybridizing (CdSe)ZnS core-shell quantum dots of 2.4 nm in diameter on multi-crystalline Si solar cells for the first time, we show that the solar conversion efficiency is enhanced 2 folds under white light illumination similar to the solar spectrum. Such nanocrystal scintillators provide the ability to extend the photovoltaic activity towards UV. © 2008 Optical Society of America.
Open Access
Quantum dot light-emitting diode with quantum dots inside the hole transporting layers
(American Chemical Society, 2013) Leck K.S.; Divayana, Y.; Zhao, D.; Young, X.; Abiyasa, A. P.; Mutlugun, E.; Gao, Y.; Liu, S.; Tan S.T.; Sun, X. W.; Demir, Hilmi Volkan
We report a hybrid, quantum dot (QD)-based, organic light-emitting diode architecture using a noninverted structure with the QDs sandwiched between hole transporting layers (HTLs) outperforming the reference device structure implemented in conventional noninverted architecture by over five folds and suppressing the blue emission that is otherwise observed in the conventional structure because of the excess electrons leaking towards the HTL. It is predicted in the new device structure that 97.44% of the exciton formation takes place in the QD layer, while 2.56% of the excitons form in the HTL. It is found that the enhancement in the external quantum efficiency is mainly due to the stronger confinement of exciton formation to the QDs.
Open Access
Solution Processed Tungsten Oxide Interfacial Layer for Efficient Hole-Injection in Quantum Dot Light-Emitting Diodes
(Wiley-VCH Verlag, 2014) Yang, X.; Mutlugun, E.; Zhao, Y.; Gao, Y.; Leck, K. S.; Ma, Y.; Ke, L.; Tan, S. T.; Demir, Hilmi Volkan; Sun, X. W.
A highly efficient and stable QLED using an inorganic WO3 nanoparticle film as a hole injection layer is demonstrated.The resulting WO3 nanoparticle-based QLEDs also exhibit superior performance compared to that of the present PEDOT:PSS-based QLEDs. The results indicate that WO3 nanoparticles are promising solution-processed buffer layer materials and serve as a strong candidate for QLED technology towards the practical applications in the next-generation lighting and displays. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Open Access
Solution-processed nanostructures and devices for highly polarized light generation, scattering and sending
(2014) Uran, Can
Recent advancements in photonics have facilitated robust and reliable light sources, displays and photosensors with relatively long lifetimes and high energy efficiency in their classes. However, developing intrinsically polarization selective photonic devices still remains a challenge, although polarization sensitivity is essential to various advanced functions and/or improved performance. One of the main difficulties in making such devices emerges from the compromise on the efficiency while striving to reach high polarization contrast levels. For instance, commercially available birefringent structures including those integrating liquid crystals suffer from major transmission losses. On the other hand, solutionprocessed, high aspect-ratio nanostructures may offer power efficient platforms with high polarization contrasts via selection of the polarization in a preferred direction during emission, absorption and/or scattering process(es) while suppressing efficiency of the relevant ones in the other polarization. In this thesis, we present solution-processed metal and semiconductor nanostructures and optoelectronic devices made from them for highly polarized light generation and enhanced photosensing. Here we developed and demonstrated in-template fabricated suspended arrays of plasmonic thin nanodiscs with tunable disc-heights and gap-widths tailoring absorption and scattering properties for applications ranging from polarized light scattering to photodetection. Also, we proposed and showed highly polarized light emission in coupled thin films of magnetically aligned multisegmented nanowires and colloidal nanocrystals for polarized color enrichment in displays. Here well-controlled in-template synthesis of these nanowires together with their alignment under magnetic field allows for highly parallel orientation of the nanowires in massive numbers over large-area thin films. Integrating with color-enriching nanocrystals, this enabled a record high polarization contrast over 15:1 for the isotropic nanocrystals in the visible range. We believe that such hybrid assemblies of solution-processed nanostructures integrated into optoelectronic devices hold great promise for advanced functions in photonics.
Open Access
Stimulated emission and lasing from CdSe/CdS/ZnS core-multi-shell quantum dots by simultaneous three-photon absorption
(Wiley - V C H Verlag GmbH, 2014) Wang Y.; Ta, V. D.; Gao, Y.; He, T. C.; Chen R.; Mutlugun, E.; Demir, Hilmi Volkan; Sun, H. D.
Three-photon pumped stimulated emission and coherent random lasing from colloidal CdSe/CdS/ZnS core-multishell quantum dots are achieved for the first time. These results can offer new possibilities in biology and photonics, as well as at their intersection of biophotonics.