Browsing by Subject "LED"

Filter results by typing the first few letters
Now showing 1 - 8 of 8
  • Thumbnail Image
    ItemOpen Access
    Brightly luminescent Cu-Zn-In-S/ZnS Core/shell quantum dots in salt matrices
    (De Gruyter, 2019) Lox, J. F. L.; Eichler, F.; Erdem, Talha; Adam, M.; Gaponik, N.; Demir, Hilmi Volkan; Lesnyak, V.; Eychmüller, A.
    In the past decades cadmium-free quantum dots (QDs), among which are quaternary colloidal Cu-Zn-In-S/ZnS (CZIS/ZnS) core/shell nanocrystals (NCs), have attracted great scientific interest. Particularly, their low toxicity and the possibility to tune their photoluminescence (PL) properties by varying the composition in the multicomponent system make them highly attractive for applications in light-emitting diodes (LEDs). Thus, the demands for high quality CZIS/ZnS QDs and methods to process them into bulk materials stimulate investigations of these nanomaterials. Herein, we demonstrate the synthesis of CZIS/ZnS core/shell NCs via a surfactant induced nucleation process, which emit in various colors covering the range from 520 nm to 620 nm possessing high photoluminescence quantum yields (PLQYs) up to 47%. Furthermore, the as synthesized NCs were successfully integrated into two different salt matrices [Na2B4O7 (Borax) and LiCl] using two different approaches. The commonly used incorporation of the NCs into Borax salt led to salt crystals emitting from 540 nm to 600 nm with PLQYs up to 24%. By encapsulating the QDs into LiCl, brightly emitting NCs-in-LiCl powders with the PL covering a range from 520 nm to 650 nm with PLQYs of up to 14% were obtained. As a proof of concept, the fabrication of a color conversion LED using NCs encapsulated into LiCl demonstrated the applicability of the encapsulated NCs.
  • Thumbnail Image
    ItemOpen Access
    Distance and position estimation in visible light systems with RGB LEDs
    (Elsevier, 2022-01-17) Demirel, I.; Gezici, Sinan
    In this paper, distance and position estimation problems are investigated for visible light positioning (VLP) systems with red-green-blue (RGB) light emitting diodes (LEDs). The accuracy limits on distance and position estimation are calculated in terms of the Cramér-Rao lower bound (CRLB) for three different scenarios. Scenario 1 and Scenario 2 correspond to synchronous and asynchronous systems, respectively, with known channel attenuation formulas at the receiver. In Scenario 3, a synchronous system is considered but channel attenuation formulas are not known at the receiver. The derived CRLB expressions reveal the relations among distance/position estimation accuracies in the considered scenarios and lead to intuitive explanations for the benefits of using RGB LEDs. In addition, maximum likelihood (ML) estimators are derived in all scenarios, and it is shown that they can achieve close performance to the CRLBs in some cases for sufficiently high source optical powers. © 2022 Elsevier Inc.
  • Thumbnail Image
    ItemOpen Access
    High-quality InP/ZnS nanocrystals with high photometric performance and their application to white quantum dot light-emitting diodes
    (IEEE, 2012) Yang, X.; Tan, S. T.; Demir, Hilmi Volkan; Sun, X. W.
    Full visible range covering InP/ZnS core-shell nanocrystals with high photometric performance have been prepared. Making use of these nanocrystals, we demonstrate a white quantum dot LED with a high color rendering index of 91. © 2012 IEEE.
  • Thumbnail Image
    ItemOpen Access
    Novel nanocomposite coatings of nanoparticles
    (2011) Toru, Refik Sina
    Incorporating nanoparticles into nanocomposite thin-films enables coatings with multi-functionality depending on the particle type and size, and the film morphology. These multiple functions may include, for example, combinations of photocatalysis, hydrophobicity, scratch resistance, and antibacterial property. Here we proposed and demonstrated a new encapsulation nanocomposite with controllable refractive index and potentially additional functional properties for coating photonic devices, for instance, light-emitting diodes (LEDs). To design and implement this nanocomposite coating with tunable refractive index, we employed TiO2 nanoparticles of various diameters because of their relatively high refractive index. We embedded these nanoparticles in our encapsulation sol-gel material during synthesis. In addition, we incorporated several polymerforming chemicals during synthesis to control additional functions such as hydrophobicity and scratch resistance. We used characterization tools of atomic force microscopy, refractometry, contact angle measurement, and scanning electron microscopy to study material properties.
  • Thumbnail Image
    ItemOpen Access
    Possible plasmonic acceleration of LED modulation for Li-Fi applications
    (Springer New York LLC, 2018) Guzatov, D. V.; Gaponenko, S. V.; Demir, Hilmi Volkan
    Emerging LED-based wireless visible light communication (Li-Fi) needs faster LED response to secure desirable modulation rates. Decay rate of an emitter can be enhanced by plasmonics, typically by an expense of efficiency loss because of non-radiative energy transfer. In this paper, metal-enhanced radiative and non-radiative decay rates are shown to be reasonably balanced to get with Ag nanoparticles nearly 100-fold enhancement of the decay rate for a blue LED without loss in overall efficacy. Additionally, gain in intensity occurs for intrinsic quantum yield Q0 < 1. With silver, rate enhancement can be performed through the whole visible. For color-converting phosphors, local field enhancement along with decay rate effects enable 30-fold rate enhancement with gain in efficacy. Since plasmonics always enhances decay rate, it can diminish Auger processes thus extending LED operation currents without efficiency droop. For quantum dot phosphors, plasmonic diminishing of Auger processes will improve photostability.
  • Thumbnail Image
    ItemOpen Access
    The role of ITO resistivity on current spreading and leakage in InGaN/GaN light emitting diodes
    (Academic Press, 2017) Sheremet, Volodymyr; Genç, M.; Elçi, M.; Sheremet, Nina; Aydınlı, Atilla; Altuntaş, İsmail; Ding, Kai; Avrutin, Vitaliy; Özgür, Ümit; Morkoç, Hadis
    The effect of a transparent ITO current spreading layer on electrical and light output properties of blue InGaN/GaN light emitting diodes (LEDs) is discussed. When finite conductivity of ITO is taken into account, unlike in previous models, the topology of LED die and contacts are shown to significantly affect current spreading and light output characteristics in top emitting devices. We propose an approach for calculating the current transfer length describing current spreading. We show that an inter-digitated electrode configuration with distance between the contact pad and the edge of p-n junction equal to transfer length in the current spreading ITO layer allows one to increase the optical area of LED chip, as compared to the physical area of the die, light output power, and therefore, the LED efficiency for a given current density. A detailed study of unpassivated LEDs also shows that current transfer lengths longer than the distance between the contact pad and the edge of p-n junction leads to increasing surface leakage that can only be remedied with proper passivation.
  • Thumbnail Image
    ItemOpen Access
    Ultraviolet-visible nanophotonic devices
    (2010) Bütün, Bayram
    Recently in semiconductor market, III-Nitride materials and devices are of much interest due to their mechanical strength, radiation resistance, working in the spectrum from visible down to the deep ultraviolet region and solar-blind device applications. These properties made them strongest candidates for space telecommunication, white light generation, high power lasers and laser pumping light emitting diodes. Since, like other semiconductors, there have been material quality related issues, ongoing research efforts are concentrated on growing high quality crystals and making low p-type ohmic contact. Also, in light emitting device applications, similar to the visible and infrared spectrum components, there are challenging issues like high extraction efficiency and controlled radiation. In this thesis, we worked on growth and characterizations of high quality (In,Al)GaN based semiconductors, fabricating high performance photodiodes and light emitting diodes. We studied different surface modifications and possibilities of obtaining light emitting diode pumped organic/inorganic hybrid laser sources
  • Thumbnail Image
    ItemOpen Access
    White light generation with CdSe/ZnS core-shell nanocrystals and InGaN/GaN light emitting diodes
    (IEEE, 2006) Nizamoğlu, Sedat; Özel, Sedat; Sarı, Emre; Demir, Hilmi Volkan
    We present hybrid white light sources that integrate CdSe/ZnS core-shell nanocrystals on blue InGaN/GaN light emitting diodes (LED). We report on the demonstrations of white light generation using yellow nanocrystals (λPL=580 nm) hybridized on a blue LED (λEL= 440 nm) with tristimulus coordinates of x=0.37 and y=0.25, correlated color temperature of Tc=2692 K, and color rendering index of R a=14.6; cyan and red nanocrystals (λPL=500 nm and 620 nm) on a blue LED (λEL=440 nm) with x=0.37, y=0.28, T c=3246 K, and Ra=19.6; and green, yellow, and red nanocrystals (λPL=540 nm, 580 nm, and 620 nm) on a blue LED (λEL=452 nm) with x=0.30, y=0.28, Tc =7521 K, and Ra=40.9. © 2006 IEEE.