Browsing by Author "Erdem, Talha"
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Item Open 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.Item Open Access Color science and technology of novel nanophosphors for high-efficiency high-quality LEDs(Bilkent University, 2011) Erdem, TalhaToday almost one-fifth of the world‟s electrical energy is consumed for artificial lighting. To revolutionize general lighting to reduce its energy consumption, high-efficiency, high-quality light-emitting diodes (LEDs) are necessary. However, to achieve the targeted energy efficiency, present technologies have important drawbacks. For example, phosphor-based LEDs suffer from the emission tail of red phosphors towards longer wavelengths. This deep-red emission decreases substantially the luminous efficiency of optical radiation. Additionally, the emission spectrum of phosphor powders cannot be controlled properly for high-quality lighting, as this requires careful spectral tuning. At this point, new nanophosphors made of colloidal quantum dots and crosslinkable conjugated polymer nanoparticles have risen among the most promising alternative color convertors because they allow for an excellent capability of spectral tuning. In this thesis, we propose and present high-efficiency, highquality white LEDs using quantum dot nanophosphors that that exhibit luminous efficacy of optical radiation ≥380 lm/Wopt, color rendering index ≥90 and correlated color temperature ≤4000 K. We find that Stoke‟s shift causes a fundamental loss >15%, which limits the maximum feasible luminous efficiency to 326.6 lm/Welect. Considering a state-of-the-art blue LED (with 81.3% photon conversion efficiency), this corresponds to 265.5 lm/Welect. To achieve 100 and 200 lm/Welect, the layered quantum dot films are required to have respective quantum efficiencies of 39 and 79%. In addition, we report our numerical modeling and experimental demonstrations of the quantum dot integrated LEDs for the different vision regimes of human eye. Finally, we present LEDs based on the color tuning capability of conjugated polymer nanoparticles for the first time. Considering the outcomes of this thesis, we believe that our research efforts will help the development and industrialization of white light emitting diodes using nanophosphor components.Item Open Access Colorimetry for LED lighting(Springer, 2019-01) Erdem, Talha; Demir, Hilmi Volkan; Erdem, Talha; Demir, Hilmi VolkanIn this Chapter, we explain the basics of colorimetry and introduce the colorimetric tools useful for designing light sources.Item Open Access Common white light sources(Springer, 2019-01) Erdem, Talha; Demir, Hilmi Volkan; Erdem, Talha; Demir, Hilmi VolkanIn this Chapter we describe the features of common light sources. We first present the spectral features of the sun and discuss its colorimetric properties. Next, we summarize the properties of traditional light sources including incandescent lamps, fluorescent lamps, and high-pressure sodium lamps. Subsequently, we discuss the white light-emitting diodes of various types.Item Open Access Future outlook(Springer, 2019-01) Erdem, Talha; Demir, Hilmi Volkan; Erdem, Talha; Demir, Hilmi VolkanIn this final Chapter, we present a future perspective for the light source design. We discuss the existing problems and briefly introduce new material systems and device architectures that can overcome the current issues.Item Open Access How to design quality light sources with discrete color components(Springer, 2019-01) Erdem, Talha; Demir, Hilmi Volkan; Erdem, Talha; Demir, Hilmi VolkanWhite light sources using discrete emitters require careful design and optimization. The first step of the design should be determining the intended use of the light source so that application specific requirements can be addressed. Subsequently, optimal designs made of discrete emitters should be determined and finally, experimental implementation of the light source should be carried out. In this Chapter of the brief, we limit ourselves to the use of discrete emitters for indoor and outdoor lighting together with display backlighting applications. For each application, we summarize the requirements that need to be satisfied and present design guidelines to implement quality light sources made of discrete emitters.Item Open Access Large-area (> 50 cm × 50 cm), freestanding, flexible, optical membranes of Cd-free nanocrystal quantum dots(IEEE, 2012) Mutlugün, Evren; Hernandez Martinez, Pedro L.; Eroğlu, Cüneyt; Coşkun, Yasemin; Erdem, Talha; Sharma, Vijay K.; Ünal, Emre; Panda, S. K.; Hickey, S. G.; Gaponik, N.; Eychmuller, A.; Demir, Hilmi VolkanColloidal semiconductor quantum dots (QDs) have been extensively explored for numerous applications ranging from optoelectronics to biotechnology. This strong demand for the colloidal QDs arises because of their favorable optical and electronic properties. From the application points of view, QDs typically need to be used in their solid form, as opposed to their as-synthesized dispersion form. For immobilization of QDs and homogeneity of their films, various polymers have been used to host QDs within solid media. However, the integration of QDs into a polymeric medium is commonly complex, which requires a high level of understanding to provide optical quality. © 2012 IEEE.Item Open Access Light stimulus and human eye(Springer, 2019-01) Erdem, Talha; Demir, Hilmi Volkan; Erdem, Talha; Demir, Hilmi VolkanIn this Chapter, we summarize the structure of the human eye and introduce the sensitivity functions of various photoreceptors and present the visual regimes and corresponding eye sensitivity functions.Item Open Access Metrics for light source design(Springer, 2019-01) Erdem, Talha; Demir, Hilmi Volkan; Erdem, Talha; Demir, Hilmi VolkanIn this part of this brief, we summarize the metrics that need to be considered for designing light sources. We start with metrics on the shade of color and then continue with color rendering and photometry.Item Open Access Non-radiative energy-transfer-driven quantum dot LEDs(IEEE, 2010) Güzeltürk, Burak; Erdem, Talha; Ünal, Emre; Nizamoğlu, Sedat; Tuncel, Donus; Demir, Hilmi VolkanItem Open Access Novel light-emitting devices of semiconductor quantum dots and conjugated polymer nanoparticles(Bilkent University, 2016-08) Erdem, TalhaStarting with the modern times, lighting has become an essential part of our lives. Today, its share of the total energy consumption reaching 15% should not surprise us. This share further increases when the energy demand for display backlighting is taken into account. Therefore, increasing the effciency of the lighting sources is of significant importance for decreasing the carbon footprint for a sustainable environment. At this point, light-emitting diodes (LEDs) step forward as the most important candidate for revolutionizing the existing lighting systems; however, the current conventional technologies, which typically employ rare-earth ion based broad-band emitters, are plagued with low photometric effciency, lack of light quality, and incapability of the spectrum design for application-specific performance. As a remedy to these problems, in this thesis we study light-emitting diodes of quantum dots that are effcient narrow-band emitters as opposed to phosphors. These colloidal quantum dots allow for the achievement of the light source performance specific to each application. By employing this strength, we first present our design of quantum dot integrated LED display backlight for reducing the adverse effects of the displays on the human biological rhythm while maximizing the color definition. Here we also addressed the need for light sources exhibiting polarization anisotropy for display backlights by hybridizing self-assembled magnetic nanowires and quantum dots. To solve the emission stability problem of the quantum dots in solid-films, we demonstrated the incorporation of the quantum dots within crystalline matrices that act as a barrier against oxygen and humidity and substantially increase their emission stability. Another important strength of this technique has been the preservation of the dispersion quantum effciencies of the quantum dots in powder form and in solid-films. By employing these material systems, we designed and successfully demonstrated a warm white LED exhibiting successful color rendition capability and large spectral overlap with the human eye sensitivity function. We also showed that embedding quantum dots into crystalline matrices offers a robust platform to study the excitonic and plasmonic interactions, both of which we utilized for increasing the effciencies of the quantum dots in crystalline matrices. To meet the need for non-toxic color converter enabling color tuning, we also employed conjugated polymer nanoparticles and studied their near-field interaction with epitaxially grown quantum well nanopillars to boost their emission intensity. We believe that the materials and light sources that we presented in this thesis will enable to reach the targets for realizing high-effciency but also high-quality light sources for general lighting and displays.Item Open Access On-Chip integration of functional hybrid materials and components in nanophotonics and optoelectronics(John Wiley & Sons, 2011) Erdem, Talha; Demir, Hilmi Volkan; Singh, M.; Ohji, T.; Asthana, R.; Mathur, S.Incorporation of custom - designed nanomaterial into nanophotonic devices and components in optoelectronic systems enables the realization of optical functionalities favorably controlled with external optical and electrical effects. To this end, innovative nanophotonic devices and optoelectronic systems are developed using multiple combinations of nanostructures (epitaxially grown, chemically synthesized, deposited, plated, etc.) that are embedded in hybrid architectures and on - chip integration of components for a variety of applications including light generation, displays, modulation, sensing, imaging, and communications in a wide spectral range from ultraviolet (UV) to visible to infrared.Item Open Access Power conversion and luminous efficiency performance of nanophosphor quantum dots on color-conversion LEDs for high-quality general lighting(SPIE, 2012) Erdem, Talha; Nizamoglu, Sedat; Demir, Hilmi VolkanFor high-quality general lighting, a white light source is required to exhibit good photometric and colorimetric performance along with a high level of electrical efficiency. For example, a warm white shade is desirable for indoors, corresponding to correlated color temperatures ≥4000 K, together with color rendering indices ≥90. Additionally, the luminous efficacy of optical radiation (LER) should be high, preferably ≥380 lm/W opt. Conventional white LEDs cannot currently satisfy these requirements simultaneously. On the other hand, color-conversion white LEDs (WLEDs) integrated with quantum dots (QDs) can simultaneously reach such high levels of photometric and colorimetric performance. However, their electrical efficiency performance and limits have been unknown. To understand their potential of luminous efficiency (lm/Welect), we modeled and studied different QD-WLED architectures based on layered QD films and QD blends, all integrated on blue LED chips. The architecture of red, yellow and green emitting QD films (in this order from the chip outwards) is demonstrated to outperform the rest. In this case, for photometrically efficient spectra, the maximum achievable LE is predicted to be 327 lm/W elect. Using a state-of-the-art blue LED reported with a power conversion efficiency (PCE) of 81.3%, the overall WLED PCE is shown to be 69%. To achieve LEs of 100, 150 and 200 lm/Welect, the required minimum quantum efficiencies of the color-converting QDs are found to be 39, 58 and 79%, respectively. © 2012 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE).Item Open Access Superior warm-White light-emitting diodes integrated with quantum dot nanophosphors for high luminous efficacy and color rendering(Optical Society of America, 2011) Nizamoğlu, Sedat; Erdem, Talha; Sun, X. W.; Demir, Hilmi VolkanQuantum dot nanophoshor hybridized warm-white LEDs are reported to exhibit high photometric performance of luminous efficacy exceeding 350 lm/Wopt and color rendering index close to 90 at correlated color temperatures <3000 K. ©2011 Optical Society of America.Item Open Access Ultrathin highly luminescent two-monolayer colloidal CdSe nanoplatelets(WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, 2019) Delikanlı, Savaş; Yu, G.; Yeltik, Aydan; Bose, S.; Erdem, Talha; Yu, J.; Erdem, Onur; Sharma, Manoj; Sharma, Vijay Kumar; Quliyeva, Ulviyya; Shendre, S.; Dang, C.; Zhang, D.; Sum, T.; Fan, W.; Demir, Hilmi VolkanSurface effects in atomically flat colloidal CdSe nanoplatelets (NLPs) are significantly and increasingly important with their thickness being reduced to subnanometer level, generating strong surface related deep trap photoluminescence emission alongside the bandedge emission. Herein, colloidal synthesis of highly luminescent two‐monolayer (2ML) CdSe NPLs and a systematic investigation of carrier dynamics in these NPLs exhibiting broad photoluminescence emission covering the visible region with quantum yields reaching 90% in solution and 85% in a polymer matrix is shown. The astonishingly efficient Stokes‐shifted broadband photoluminescence (PL) emission with a lifetime of ≈100 ns and the extremely short PL lifetime of around 0.16 ns at the bandedge signify the participation of radiative midgap surface centers in the recombination process associated with the underpassivated Se sites. Also, a proof‐of‐concept hybrid LED employing 2ML CdSe NPLs is developed as color converters, which exhibits luminous efficacy reaching 300 lm Wopt−1. The intrinsic absorption of the 2ML CdSe NPLs (≈2.15 × 106 cm−1) reported in this study is significantly larger than that of CdSe quantum dots (≈2.8 × 105 cm−1) at their first exciton signifying the presence of giant oscillator strength and hence making them favorable candidates for next‐generation light‐emitting and light‐harvesting applications.