Browsing by Subject "Light emission"

Now showing 1 - 18 of 18

Open Access
CdSe/CdSe1-xTex core/crown heteronanoplatelets: tuning the excitonic properties without changing the thickness
(American Chemical Society, 2017) Kelestemur Y.; Guzelturk, B.; Erdem, O.; Olutas M.; Erdem, T.; Usanmaz, C. F.; Gungor K.; Demir, Hilmi Volkan
Here we designed and synthesized CdSe/CdSe1-xTex core/crown nanoplatelets (NPLs) with controlled crown compositions by using the core-seeded-growth approach. We confirmed the uniform growth of the crown regions with well-defined shape and compositions by employing transmission electron microscopy, X-ray photoelectron spectroscopy, and X-ray diffraction. By precisely tuning the composition of the CdSe1-xTex crown region from pure CdTe (x = 1.00) to almost pure CdSe doped with several Te atoms (x = 0.02), we achieved tunable excitonic properties without changing the thickness of the NPLs and demonstrated the evolution of type-II electronic structure. Upon increasing the Te concentration in the crown region, we obtained continuously tunable photoluminescence peaks within the range of ∼570 nm (for CdSe1-xTex crown with x = 0.02) and ∼660 nm (for CdSe1-xTex crown with x = 1.00). Furthermore, with the formation of the CdSe1-xTex crown region, we observed substantially improved photoluminescence quantum yields (up to ∼95%) owing to the suppression of nonradiative hole trap sites. Also, we found significantly increased fluorescence lifetimes from ∼49 up to ∼326 ns with increasing Te content in the crown, suggesting the transition from quasi-type-II to type-II electronic structure. With their tunable excitonic properties, this novel material presented here will find ubiquitous use in various efficient light-emitting and -harvesting applications.
Open Access
Colloidal nanophotonics: The emerging technology platform
(OSA - The Optical Society, 2016) Gaponenko S.; Demir, Hilmi Volkan; Seassal C.; Woggon U.
Dating back to decades or even centuries ago, colloidal nanophotonics during the last ten years rapidly extends towards light emitting devices, lasers, sensors and photonic circuitry to manifest itself as an emerging technology platform rather than an entirely academic research field. ©2016 Optical Society of America.
Open Access
Colloidal quantum dot light-emitting diodes employing phosphorescent small organic molecules as efficient exciton harvesters
(American Chemical Society, 2014) Mutlugun, E.; Guzelturk, B.; Abiyasa, A. P.; Gao, Y.; Sun X. W.; Demir, Hilmi Volkan
Nonradiative energy transfer (NRET) is an alternative excitation mechanism in colloidal quantum dot (QD) based electroluminescent devices (QLEDs). Here, we develop hybrid highly spectrally pure QLEDs that facilitate energy transfer pumping via NRET from a phosphorescent small organic molecule-codoped charge transport layer to the adjacent QDs. A partially codoped exciton funnelling electron transport layer is proposed and optimized for enhanced QLED performance while exhibiting very high color purity of 99%. These energy transfer pumped hybrid QLEDs demonstrate a 6-fold enhancement factor in the external quantum efficiency over the conventional QLED structure, in which energy transfer pumping is intrinsically weak.
Open Access
Color generation and enhancement using large-scale compatible metamaterial design architectures
(2022-01) Köşger, Ali Cahit
Metamaterials are a type of artificial matt that can impose exotic functionalities beyond natural materials. These specifically designed sub-wavelength structures acquire these functionalities from their collective geometric arrangement rather than their individual single-unit properties. As a result, metamaterials have shown promising applications, including negative refraction, artificial magnetism, asymmetric transmission, lasing, and cloak of invisibility. Among all these applications, the concept of color generation and enhancement using metamaterial designs have attracted much attention in recent years. We can achieve color generation from two primary sources: i) filtering white light, and ii) generating light from emitting materials such as quantum dots. In color generation using white light, a metamaterial design reflects or transmits a narrow portion of the incident spectrum. Thus, the design acts as a color filter. However, the source is already a narrowband color light in the second category. Thus metamaterials merely amplify the color intensity rather than manipulate its spectral response. In this thesis, metamaterial structures are designed, fabricated, and characterized in both categories mentioned above; The content of this thesis consists of two parts; i) In the first part, we generated additive red-green-blue (RGB) colors in reflectance mode with near-unity amplitude. For this purpose, we designed a multilayer structure made of metal-insulator-metal-semiconductor-insulator (MIMSI) stacks to achieve >0.9 reflection peaks with full-width-at-half-maximum (FWHM) values <0.3λpeak. The proposed design also shows near-zero reflection in off-resonance spectral ranges, which, in turn, leads to high color purity. Finally, we fabricated the optimized designs and verified the simulation and theoretical results with characterization findings. This work demonstrates the potential of multilayer tandem cavity designs in realizing lithography-free large-scale compatible functional optical coatings. ii) In the second part, we utilized a large-scale compatible plasmonic nanocavity design platform to achieve almost an order of magnitude photoluminescence enhancement from light-emitting quantum dots. The proposed design is multi-sized/multi-spacing gold (Au) nano units that are uniformly wrapped with thin aluminum oxide (Al2O3) layer as a foreign host to form a metal-insulator-semiconductor (MIS) cavity, as we coated them with semiconductor quantum dots (QDs). Our numerical and experimental data demonstrate that, in an optimal insulator layer thickness, the simultaneous formation of broadband Fabry-Perot (FP) resonances and plasmonic hot spots leads to enhanced light absorption within the QD unit. This improvement in absorption response leads to the PL enhancement of QDs. This work demonstrates the potential and effectiveness of a host comprised of random plasmonic nanocavities in the realization of lithography-free efficient emitters. Overall, this thesis presents an alternative perspective on applying large-scale compatible metamaterials in color generation. Furthermore, the proposed designs and routes can be extended toward other functional photoelectronic designs, where high performances can be acquired in scaleable architectures.
Open Access
Controlled growth and characterization of epitaxially-laterally-overgrown InGaN/GaN quantum heterostructures
(IEEE, 2010) Sarı, Emre; Akyuz, Özgün; Choi, E. -G.; Lee I.-H.; Baek J.H.; Demir, Hilmi Volkan
Crystal material quality is fundamentally important for optoelectronic devices including laser diodes and light emitting diodes. To this end epitaxial lateral overgrowth (ELO) has proven to be a powerful technique for reducing dislocation density in GaN and its alloys [1,2]. Implementation and design of ELO process is, however, critical for obtaining high-quality material with high-efficiency quantum structures for light emitters [3]. ©2010 IEEE.
Open Access
Enhanced optical characteristics of light emitting diodes by surface plasmon of Ag nanostructures
(SPIE, 2011) Jang L.-W.; Ju J.-W.; Jeon J.-W.; Jeon, D.-W.; Choi J.-H.; Lee, S.-J.; Jeon, S.-R.; Baek J.-H.; Sarı, Emre; Demir, Hilmi Volkan; Yoon H.-D.; Hwang, S.-M.; Lee I.-H.
We investigated the surface plasmon coupling behavior in InGaN/GaN multiple quantum wells at 460 nm by employing Ag nanostructures on the top of a roughened p-type GaN. After the growth of a blue light emitting diode structure, the p-GaN layer was roughened by inductive coupled plasma etching and the Ag nanostructures were formed on it. This structure showed a drastic enhancement in photoluminescence and electroluminescence intensity and the degree of enhancement was found to depend on the morphology of Ag nanostructures. From the time-resolved photoluminescence measurement a faster decay rate for the Ag-coated structure was observed. The calculated Purcell enhancement factor indicated that the improved luminescence intensity was attributed to the energy transfer from electron-hole pair recombination in the quantum well to electron vibrations of surface plasmon at the Ag-coated surface of the roughened p-GaN. © 2011 SPIE.
Open Access
Fluorescent Si QD decoration onto a flexible polymeric electrospun nanofibrous mat for the colorimetric sensing of TNT
(Royal Society of Chemistry, 2017) Arslan, O.; Aytac Z.; Uyar, Tamer
UV range light was used for the facile, effective and large-scale synthesis of visible light emitting, surface-protected silicon quantum dots (Si QDs) starting from an amine-functionalized alkoxy silane precursor. Within mild and easy hydrolysis/condensation environments, the use of an amine-functionalized precursor together with a reducing agent resulted in a bright visible green light that could be used for fluorescent analytical detection systems. Visible light emitting Si QDs were investigated and it was found that their emission character depends on the illumination time, hydrolysis/condensation conditions and pretreatments for the silane coupling agents. A Nylon 6,6 electrospun nanofibrous mat was selected as a substrate for decoration by the Si QDs in order to fabricate a flexible and free-standing polymeric nanofibrous mat posessing a visible light emitting character so that it could act as a visible colorimetric sensor. The visible light emitting Si QDs were decorated onto the Nylon 6,6 nanofibrous mats via covering the surfaces as a ‘nanodress’ by a simple impregnation/dip-coating and heat-curing methods. The analytical results revealed that the Si QDs decorated flexible polymeric nanofibrous mats could be utilized for colorimetric trinitrotoluene (TNT) detection in low concentrations.
Open Access
Graded-host phosphorescent light-emitting diodes with high efficiency and reduced roll-off
(A I P Publishing LLC, 2012) Liu, S. W.; Sun, X. W.; Demir, Hilmi Volkan
We demonstrated graded-host phosphorescent organic light-emitting diodes with high efficiency and reduced efficiency roll-off. The emissive layer of the graded host device consists of both electron and hole transport type hosts, 1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBI) and 4,4-,4- tris(Ncarbazolyl) triphenylamine, respectively, with graded composition, and the phosphorescent red emitter bis(2-phenylquinoline) (acetylacetonate) iridium(III), which wasuniformly doped into the graded hostmatrix. The graded host device shows improved quantum efficiency and power efficiency with significantly reduced efficiency rolloff as compared to the unipolar-host and double layer heterojunction host devices.
Open Access
High-voltage optical emission in binary gaseous mixtures of N2
(Elsevier, 1995) Zengin, V.; Gökmen, A.; Dinçer, S.; Süzer, Ş.
Optical emission in 1:1 binary gaseous mixtures under high voltage displays a varying character depending on pressure, applied voltage polarity, and chemical nature of the mixture. Under negative polarity, in pure N2 and 50% mixture of Ar, O2 and CO2, emission stemming from N2 + is enhanced relative to emission from neutral N2. In mixtures of N2 with gases containing halogens (CCl4, CHCl3, CH2Cl2, CF3H, CF2Cl2 and SF6) a reversal is observed, i.e. the N2 + emission is suppressed. An enhancement factor is defined as the ratio of the emission under negative polarity to positive polarity to quatify this polarity dependence. This enhancement factor varies between 0.01 and 50 depending on the second component in the mixture © 1995 Elsevier Science B.V.
Open Access
Implementation of graphene multilayer electrodes in quantum dot light-emitting devices
(Springer Verlag, 2015) Wolff, S.; Jansen, D.; Terlinden H.; Kelestemur, Y.; Mertin W.; Demir, Hilmi Volkan; Bacher G.; Nannen, E.
Graphene is a highly attractive candidate for implementation as electrodes in next-generation large-area optoelectronic devices thanks to its high electrical conductivity and high optical transparency. In this study, we show all-solution-processed quantum dot-based light-emitting devices (QD-LEDs) using graphene mono- and multilayers as transparent electrodes. Here, the effect of the number of graphene layers (up to three) on the QD-LEDs performance was studied. While the implementation of a second graphene layer was found to reduce the turn-on voltage from 2.6 to 1.8 V, a third graphene layer was observed to increase the turn-on voltage again, which is attributed to an increased roughness of the graphene layer stack. © 2015, Springer-Verlag Berlin Heidelberg.
Open Access
Infrared photoluminescence from TlGaS2 layered single crystals
(Wiley - V C H Verlag GmbH & Co., 2004) Yuksek, N. S.; Gasanly, N. M.; Aydınlı, Atilla; Ozkan, H.; Acikgoz, M.
Photolimuniscence (PL) spectra of TlGaS2 layered crystals were studied in the wavelength region 500-1400 nm and in the temperature range 15-115 K. We observed three broad bands centered at 568 nm (A-band), 718 nm (B-band) and 1102 nm (C-band) in the PL spectrum. The observed bands have half-widths of 0.221, 0.258 and 0.067 eV for A-, B-, and C-bands, respectively. The increase of the emission band half-width, the blue shift of the emission band peak energy and the quenching of the PL with increasing temperature are explained using the configuration coordinate model. We have also studied the variations of emission band intensity versus excitation laser intensity in the range from 0.4 to 19.5 W cm-2. The proposed energy-level diagram allows us to interpret the recombination processes in TlGaS2 crystals.
Open Access
Nanocrystal integrated light emitting diodes based on radiative and nonradiative energy transfer for the green gap
(IEEE, 2009) Nizamoğlu, Sedat; Sarı, Emre; Baek J.-H.; Lee I.-H.; Demir, Hilmi Volkan
Recently the photometric conditions for ultra-efficient solid-state lighting have been discussed [1-2]. These studies show that a luminous efficacy of optical radiation at 408 lm/Wopt and a color rendering index (CRI) of 90 at a correlated color temperature (CCT) of 3000 K are achievable at the same time. For this purpose light emitting diodes (LEDs) emitting in blue, green, yellow, and red colors at 463, 530, 573, and 614 nm with relative optical power levels of 1/8, 2/8, 2/8, and 3/8, are required, respectively [1-2]. Although InxGa1-xN material system is capable to cover the whole visible by changing the In composition (x), it is technically extremely challenging to obtain efficient green/yellow light emitting diodes especially at those wavelengths (i.e., at 530 nm and 573 nm, respectively) due to reduced internal quantum efficiency [2-4]. Furthermore, by using the (Al xGa1-x)1-yInyP quaternary alloy it is also possible to cover from 650 nm to 580 nm. However, the efficiencies significantly decrease towards green. Therefore, there exists a significant gap in the green-yellow spectral regions (known as "the green gap") to make efficient light emitting diodes. To address this green gap problem, we propose and demonstrate proof-of-concept nanocrystal (NCs) hybridized green/yellow light emitting diodes that rely on both radiative energy transfer and nonradiative energy transfer (i.e., FRET-Förster resonance energy transfer) for color conversion on near-ultraviolet (near-UV) LEDs.
Open Access
Polarization of radiation in multipole Jaynes-Cummings model
(Taylor & Francis Ltd, 2002) Can, M.A.; Shumovsky, A.S.
We discuss the spatial properties of quantum radiation emitted by a multipole transition in a single atom. It is shown that the polarization of multipole radiation and quantum fluctuations of polarization change with distance from the source. In the case of a transition specified by a given quantum number m, the quantum noise of polarization contains contributions coming from the modes with m′ ≠ m as well.
Open Access
Singlet and Triplet Exciton Harvesting in the Thin Films of Colloidal Quantum Dots Interfacing Phosphorescent Small Organic Molecules
(American Chemical Society, 2014) Guzelturk, B.; Hernandez Martinez P.L.; Zhao, D.; Sun X.W.; Demir, Hilmi Volkan
Efficient nonradiative energy transfer is reported in an inorganic/organic thin film that consists of a CdSe/ZnS core/shell colloidal quantum dot (QD) layer interfaced with a phosphorescent small organic molecule (FIrpic) codoped fluorescent host (TCTA) layer. The nonradiative energy transfer in these thin films is revealed to have a cascaded energy transfer nature: first from the fluorescent host TCTA to phosphorescent FIrpic and then to QDs. The nonradiative energy transfer in these films enables very efficient singlet and triplet state harvesting by the QDs with a concomitant fluorescence enhancement factor up to 2.5-fold, while overall nonradiative energy transfer efficiency is as high as 95%. The experimental results are successfully supported by the theoretical energy transfer model developed here, which considers exciton diffusion assisted Förster-type near-field dipole-dipole coupling within the hybrid films. © 2014 American Chemical Society.
Open Access
Superradiant lasing from J-aggregated molecules adsorbed onto colloidal silver
(American Institute of Physics, 1998) Özçelik, S.; Özçelik, I.; Akins, D. L.
The picosecond time-resolved emission spectrum of the cyanine dye 1,18-diethyl-3,38- bis-~3-sulfopropyl!-5,58,6,68-tetrachlorobenzimidazolocarbocyanine ~also known as BIC! adsorbed onto colloidal silver was examined as a function of laser pulse energy at room temperature. BIC is found to aggregate on colloidal silver, and the number of coherently responding molecules involved in the one-exciton state ~i.e., the coherence length! was estimated to involve 8–9 molecules. Lasing at a remarkably low incident pulse energy threshold was found for this system and explained in terms of a mechanism involving superradiant states created in coherently coupled adsorbed molecules that emit photons which stimulate emission from other spatially distributed superradiant states. © 1998 American Institute of Physics.
Open Access
TL/OSL studies of Li2B4O7:Cu dosimetric phosphors
(2013) Aydin, T.; Demirtaş H.; Aydin, S.
Dosimetric phosphors of Cu-doped lithium tetraborate (Li2B 4O7:Cu) were produced using a sintering technique in a laboratory environment and characterized using Scanning Electron Microscopy (SEM) and X-ray Diffractometry (XRD). The thermoluminescence (TL) and optically stimulated luminescence (OSL) properties of powdered (Li2B 4O7) phosphor doped with copper at different concentrations (0.020-0.025 wt %) were studied. The Cu-doped Li 2B4O7 phosphor material has two dominant TL glow peaks, and the maximum TL responses of the peaks are at 115 C and 243 C in the range of 0 C-310 C. The TL response of the Cu-doped lithium tetraborate is approximately 900 times more sensitive than undoped lithium tetraborate. The TL and OSL signal intensities increase as the beta radiation doses increase up to approximately 150.00 Gy and 76.50 Gy, respectively. The OSL dose-response curve is linear up to a dose range of 12.00 Gy for Cu-doped Li2B 4O7 dosimetric phosphors. The time-dependent fading behavior of the Cu-doped lithium tetraborate was found to be quite stable over long time durations. In addition, the repeatability of the OSL dose measurements were determined to be 2/3 lower compared to the TL measurements. The reproducibility of the OSL measurements was approximately 5%. Based on the TL and OSL results, the prepared phosphors can be used to measure beta doses ranging from 10 μGy to 150.00 Gy and 76.50 Gy, respectively, by using the TL and OSL techniques, with confidence limits of approximately 7% and 3-4%, respectively. © 2013 Elsevier Ltd. All rights reserved.
Open 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.
Open Access
White-emitting conjugated polymer nanoparticles with cross-linked shell for mechanical stability and controllable photometric properties in color-conversion LED applications
(2011) Park, Eun-Ju; Erdem, T.; Ibrahimova, V.; Nizamoglu, S.; Demir, Hilmi Volkan; Tuncel, D.
We report on the synthesis and characterization of water-dispersible, mechanically stable conjugated polymer nanoparticles (CPNs) in shelled architecture with tunable emission and controllable photometric properties via cross-linking. Using a reprecipitation method, whiteemitting polymer nanoparticles are prepared in different sizes by varying the concentration of polymer; the emission kinetics are tuned by controlling the shell formation. For this purpose, polyfluorene derivatives containing azide groups are selected that can be decomposed under UV light to generate very reactive species, which opportunely facilitate the inter- and intra-cross-linking of polymer chains to form shells. Nanoparticles before and after UV treatment are characterized by various techniques. Their size and morphologies are determined by using dynamic light scattering (DLS) measurements and imaging techniques including scanning electron microscopy (SEM) and atomic force microscopy (AFM). For optical characterization, UV vis and steady-state and timeresolved fluorescent spectroscopies are performed. Solid-state behaviors of these CPNs are also investigated by forming films through drop-casting. Moreover, the photometric calculations are also performed for films and dispersions to determine the color quality. A device has been constructed to show proof-of-principle white light generation from these nanoparticles. Additionally, mechanical stability studies are performed and demonstrated that these nanoparticles are indeed mechanically stable by removing the solvent after cross-linking using a freeze-dryer and redispersing in water and THF. Optical and imaging data confirm that the redispersed particles preserve their shapes and sizes after cross-linking.