Browsing by Subject "Luminescence"

Now showing 1 - 20 of 23

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Bifunctional highly fluorescent hollow porous microspheres made of BaMoO4: Pr3+ nanocrystals via a template-free synthesis
(The Royal Society of Chemistry, 2011) Yang, X.; Zhou, Y.; Yu, X.; Demir, Hilmi Volkan; Sun, X. W.
We report a bifunctional hollow porous microsphere composed of single-component BaMoO4 : Pr3+ nanocrystals by a facile template-free synthesis. All the as-synthesized hollow microspheres are well-dispersed with a diameter of 2-4 mu m and the BaMoO4 : Pr3+ nanocrystals measure 30-60 nm in diameter. It is observed that there are a large amount of pores with an average diameter is 17.5 nm in the shell of these BaMoO4 : Pr3+ hollow microspheres, thereby exhibiting a great promise for drug delivery. Meanwhile, the strong, narrow-bandwidth red emission centered at 643 nm from these nanostructures can be efficiently excited from 430 nm to 500 nm. The combination of excellent luminescent properties and a hollow porous nanostructure suggest a great promise in the application of these nanostructures in lighting and displays, and in biomedicine such as targeted drug delivery, integrated imaging, diagnosis, and therapeutics. In addition, the template-free solution synthesis can be applied to the design and fabrication of other functional architectures.
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Bright White-Light Emitting Manganese and Copper Co-Doped ZnSe Quantum Dots
(Wiley, 2011) Panda, S. K.; Hickey, S. G.; Demir, Hilmi Volkan; Eychmuller, A.
Doubly doped quantum dots with highly efficient (17 %) white-light emission (WLE) have been directly synthesized using a one-pot hot-injection technique (see picture). The generation of WLE was due to the judicious manipulation of the synthesis strategy for the co-doping of the host material-ZnSe quantum dots-with Mn and Cu.
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Computational modeling of quantum-confined impact ionization in Si nanocrystals embedded in SiO2
(2007) Sevik, C.; Bulutay, C.
Injected carriers from the contacts to delocalized bulk states of the oxide matrix via Fowler-Nordheim tunneling can give rise to quantum-confined impact ionization (QCII) of the nanocrystal (NC) valence electrons. This process is responsible for the creation of confined excitons in NCs, which is a key luminescence mechanism. For a realistic modeling of QCII in Si NCs, a number of tools are combined: ensemble Monte Carlo (EMC) charge transport, ab initio modeling for oxide matrix, pseudopotential NC electronic states together with the closed-form analytical expression for the Coulomb matrix element of the QCII. To characterize the transport properties of the embedding amorphous SiO2, ab initio band structure and density of states of the α-quartz phase of SiO2 are employed. The confined states of the Si NC are obtained by solving the atomistic pseudopotential Hamiltonian. With these ingredients, realistic modeling of the QCII process involving a SiO2 bulk state hot carrier and the NC valence electrons is provided.
Open Access
Crystallization of Ge in SiO2 matrix by femtosecond laser processing
(American Vacuum Society, 2012-01-19) Salihoglu, O.; Kürüm, U.; Yaglioglu, H. G.; Elmali, A.; Aydınlı, Atilla
Germanium nanocrystals embedded in a siliconoxide matrix has been fabricated by single femtosecond laser pulse irradiation of germanium doped SiO2 thin films deposited with plasma enhanced chemical vapor deposition. SEM and AFM are used to analyze surface modification induced by laser irradiation. Crystallization of Ge in the oxide matrix is monitored with the optic phonon at 300 cm(-1) as a function of laser fluence. Both the position the linewidth of the phonon provides clear signature for crystallization of Ge. In PL experiments, strong luminescence around 600 nm has been observed.
Open Access
Determination of energy-band offsets between GaN and AlN using excitonic luminescence transition in AlGaN alloys
(American Institute of Physics, 2006) Westmeyer, A. N.; Mahajan, S.; Bajaj, K. K.; Lin J. Y.; Jiang, H. X.; Koleske, D. D.; Senger, R. T.
We report the determination of the energy-band offsets between GaN and AlN using the linewidth (full width at half maximum) of an extremely sharp excitonic luminescence transition in Alx Ga1-x N alloy with x=0.18 at 10 K. Our sample was grown on C -plane sapphire substrate by metal-organic chemical-vapor deposition at 1050 °C. The observed value of the excitonic linewidth of 17 meV is the smallest ever reported in literature. On subtracting a typical value of the excitonic linewidth in high-quality GaN, namely, 4.0 meV, we obtain a value of 13.0 meV, which we attribute to compositional disorder. This value is considerably smaller than that calculated using a delocalized exciton model [S. M. Lee and K. K. Bajaj, J. Appl. Phys. 73, 1788 (1993)]. The excitons are known to be strongly localized by defects and/or the potential fluctuations in this alloy system. We have simulated this localization assuming that the hole, being much more massive than the electron, is completely immobile, i.e., the hole mass is treated as infinite. Assuming that the excitonic line broadening is caused entirely by the potential fluctuations experienced by the conduction electron, the value of the conduction-band offset between GaN and AlN is determined to be about 57% of the total-band-gap discontinuity. Using our model we have calculated the variation of the excitonic linewidth as a function of Al composition in our samples with higher Al content larger than 18% and have compared it with the experimental data. We also compare our value of the conduction-band offset with those recently proposed by several other groups using different techniques.
Open Access
Distributed contact flip chip InGaN/GaN blue LED; comparison with conventional LEDs
(Elsevier, 2019) Genç, M.; Sheremet, Volodymyr; Elçi, M.; Kasapoğlu, A.; Altuntaş, İ.; Demir, İ.; Eğin, G.; İslamoğlu, Serkan; Gür, E.; Muzafferoğlu, N.; Elagöz, S.; Gülseren, Oğuz; Aydınlı, A.
This paper presents high performance, GaN/InGaN-based light emitting diodes (LEDs) in three different device configurations, namely Top Emitting (TE) LED, conventional Flip Chip (FC) and Distributed Contact (DC) FC. Series resistances as low as 1.1 Ω have been obtained from FC device configurations with a back reflecting ohmic contact of Ni/Au/RTA/Ni/Ag metal stack. A small shift has been observed between electroluminescence (EL) emissions of TE LED and the FC LEDs. In addition, FWHM value of the EL emission of DCFC LED has shown the minimum value of 160 meV (26.9 nm). Furthermore, DCFC LED configuration has shown the highest quantum efficiency and power output, with 330 mW at 500 mA current injection, compared to that of traditional wire bonded TE LEDs and the conventional FC LEDs.
Open Access
The effect of gadolinium doping on the structural, magnetic and photoluminescence properties of electrospun bismuth ferrite nanofibers
(Elsevier Ltd, 2015) George Philip G.; Senthamizhan, A.; Srinivasan Natarajan, T.; Chandrasekaran G.; Annal Therese H.
Gadolinium (Gd) doped Bismuth ferrite (BFO) nanofibers (Bi1-xGdxFeO3 (x=0.0, 0.05, 0.10, 0.15 and 0.20)) were synthesized via electrospinning. Scanning Electron Microscope (SEM) analysis showed that the diameter of the nanofibers ranged from 150 to 250 nm. X-Ray Diffraction (XRD) analysis revealed a structural phase transition with varying 'x', the compositions with x≤0.10 have crystal structures with space group R3c, while the compositions with x > 0.10 have crystal structures with space group Pnma. Vibrating Sample Magnetometer (VSM) analysis exhibited the weak ferromagnetic nature of the BFO nanofibers. However an increase in the saturated magnetic moment with increase in Gd dopant concentration was observed. The Photoluminescence (PL) spectra of the Bi:1-x :x nanofibers show enhanced Near Band Emission (NBE) intensity at x=0.10 due to the passivation of oxygen vacancies by Gd doping. © 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Open Access
Facile route to produce spherical and highly luminescent Tb3+doped Y2O3 nanophosphors
(Elsevier, 2017) Kumar, D.; Sharma, M.; Haranath, D.; Pandey, O. P.
Terbium doped yttrium oxide (Y2O3:Tb3+) nanophosphor has been synthesized via a facial yet modified co-precipitation method. To get maximum luminescence output from Y2O3:Tb3+nanophosphors, surfactants namely, Cetyl trimethylammonium bromide (CTAB) and Trioctylphosphine oxide (TOPO) were added during synthesis. Further, it has been observed that combined addition of surfactant (CTABï¿½+ï¿½TOPO) at the time of synthesis has resulted in nearly spherical morphology of the nanophosphor. Furthermore, these optimized material are observed to have enhanced integrated photoluminescence (PL) intensity of ∼23% as compared to the one synthesized without the addition of any surfactant. The results are further supported by detailed structural and optical studies. Optimum use of surfactants during synthesis shows for the first time that both nano-sized distribution and high crystallinity can be achieved simultaneously which has resulted in bright green emission in Tb3+doped Y2O3nanophosphors.
Open Access
Facile synthesis of luminescent AgInS2–ZnS solid solution nanorods
(Wiley-VCH Verlag, 2013-04-16) Yang, X.; Tang, Y.; Tan S.T.; Bosman, M.; Dong, Z.; Leck K.S.; Ji Y.; Demir, Hilmi Volkan; Sun, X. W.
Highly luminescent semiconducting AgInS2–ZnS solid solution nanorods are successfully prepared by a facile one-pot solvothermal method. The resulting solid solution nanorods with length of 32 ± 5 nm are formed by fast growth of the AgInS2-rich solid solution head, followed by slow growth of the ZnS-rich solid solution tail. Photoluminescence studies on the solid solution nanorods reveal strong photoluminescence with peak emission wavelengths tunable from 650 to 700 nm.
Open Access
Free-standing ZnO-CuO composite nanowire array films and their gas sensing properties
(IOP Publishing, 2011-07-19) Wang, J. X.; Sun, X. W.; Yang, Y.; Kyaw, A. K. K.; Huang, X. Y.; Yin, J. Z.; Wei, J.; Demir, Hilmi Volkan
A modified hydrothermal method was developed to synthesize ZnO-CuO composite nanostructures. A free-standing film made of ZnO-CuO nanostructures was assembled on the surface of the hydrothermal solution with a smooth surface on one side and a spherical surface on the other side. The structure, growth mechanism and the optical properties of the composite nanostructures were studied. Structural characterizations indicate that the composite nanostructure mainly consisted of two single-crystal phases of CuO and ZnO. The sensitivity for CO gas detection was significantly improved for the composite CuO-ZnO nanostructure film. This method offers a possible route for the fabrication of free-standing nanostructure films of different functional composite oxides.
Open Access
Localized plasmon-engineered spontaneous emission of CdSe/ZnS nanocrystals closely-packed in the proximity of Ag nanoislands
(2007) Soğancı, İbrahim Murat; Nizamoğlu, Sedat; Mutlugün, Evren; Demir, Hilmi Volkan
As a proof-of-concept demonstration, we show that the localized plasmons of metal nanoisland films provide the ability to modify and control the collective spontaneous emission of nanocrystals in their proximity (including emission peak wavelength and linewidth in addition to intensity). Using randomly-distributed Ag nanoparticles, we demonstrate plasmonic resonance tuned for the proximal CdSe/ZnS NC emitters to shift PL peak wavelength (by 14 nm) and reduce the FWHM (by 10 nm), while enhancing PL intensity by 15.1 and 21.6 times compared to the control groups of nanocrystals alone and those with nanoAg but no dielectric spacer, respectively.
Open Access
Luminescence of Gold Nanorod-Quantum Dots Complexes
(World Scientific Publishing Company, 2019) Trotsiuk, L. L.; Muravitskaya, A. O.; Kulakovich, O. S.; Gaponenko, S. V.; Demir, Hilmi Volkan
The photoluminescence of gold nanorod-quantum dots complexes was investigated in order to find optimal conditions for the luminescence enhancing. The number of quantum dots and polyelectrolyte layers on the gold nanorod surface in these complexes were considered as main factors in the enhancement.
Open Access
Multi-layered CdSe/ZnS/CdSe heteronanocrystals to generate and tune white light
(2008-11) Nizamoğlu, Sedat; Mutlugün, Evren; Özel, Tuncay; Demir, Hilmi Volkan; Sapra, S.; Gaponik, N.; Eychmüller, A.
In this study, tuneable white light generation by controlling CdSe/ZnS/CdSe core/shell/shell heteronanocrystals integrated on InGaN/GaN light emitting diodes was presented. These multilayered quantum dots, also known as onion-like heterostructures, were designed and synthesized to emit in red (around 600 nm) from the CdSe core and in green (around 550 nm) from the CdSe shell. By designing and hybridizing these red-green emitting heterostructures on blue emitting LEDs, an integrated WLEDs on a single chip was demonstrated. By controlling the number of integrated heteronanocrystals, their (x,y) tristimulus coordinates were tuned from (0.26,0.23) to (0.37,0.36), along with their corresponding correlated color temperature tuned from 27413 K to 4192 K and the luminous efficacy of their optical radiation (the ratio of the emitted luminous flux to the radiant flux) tuned from 258 lm/W to 375 lm/W. Further investigation on the change of in-film optical properties of these heteronanocrystals with respect to their in-solution emission was performed.
Open Access
Nanosecond pulsed laser ablated sub-10 nm silicon nanoparticles for improving photovoltaic conversion efficiency of commercial solar cells
(Institute of Physics Publishing Ltd., 2017) Rasouli, H. R.; Ghobadi, A.; Ghobadi, T. G. U.; Ates, H.; Topalli, K.; Okyay, Ali Kemal
In this paper, we demonstrate the enhancement of photovoltaic (PV) solar cell efficiency using luminescent silicon nanoparticles (Si-NPs). Sub-10 nm Si-NPs are synthesized via pulsed laser ablation technique. These ultra-small Si nanoparticles exhibit photoluminescence (PL) character tics at 425 and 517 nm upon excitation by ultra-violet (UV) light. Therefore, they can act as secondary light sources that convert high energetic photons to ones at visible range. This down-shifting property can be a promising approach to enhance PV performance of the solar cell, regardless of its type. As proof-of-concept, polycrystalline commercial solar cells with an efficiency of ca 10% are coated with these luminescent Si-NPs. The nanoparticle-decorated solar cells exhibit up to 1.64% increase in the external quantum efficiency with respect to the uncoated reference cells. According to spectral photo-responsivity characterizations, the efficiency enhancement is stronger in wavelengths below 550 nm. As expected, this is attributed to down-shifting via Si-NPs, which is verified by their PL characteristics. The results presented here can serve as a beacon for future performance enhanced devices in a wide range of applications based on Si-NPs including PVs and LED applications.
Open Access
Near-unity emitting copper-doped colloidal semiconductor quantum wells for luminescent solar concentrators
(Wiley-VCH Verlag, 2017) Sharma, M.; Gungor K.; Yeltik A.; Olutas M.; Guzelturk, B.; Kelestemur Y.; Erdem, T.; Delikanli S.; McBride, J. R.; Demir, Hilmi Volkan
Doping of bulk semiconductors has revealed widespread success in optoelectronic applications. In the past few decades, substantial effort has been engaged for doping at the nanoscale. Recently, doped colloidal quantum dots (CQDs) have been demonstrated to be promising materials for luminescent solar concentrators (LSCs) as they can be engineered for providing highly tunable and Stokes-shifted emission in the solar spectrum. However, existing doped CQDs that are aimed for full solar spectrum LSCs suffer from moderately low quantum efficiency, intrinsically small absorption cross-section, and gradually increasing absorption profiles coinciding with the emission spectrum, which together fundamentally limit their effective usage. Here, the authors show the first account of copper doping into atomically flat colloidal quantum wells (CQWs). In addition to Stokes-shifted and tunable dopant-induced photoluminescence emission, the copper doping into CQWs enables near-unity quantum efficiencies (up to ≈97%), accompanied by substantially high absorption cross-section and inherently step-like absorption profile, compared to those of the doped CQDs. Based on these exceptional properties, the authors have demonstrated by both experimental analysis and numerical modeling that these newly synthesized doped CQWs are excellent candidates for LSCs. These findings may open new directions for deployment of doped CQWs in LSCs for advanced solar light harvesting technologies.
Open Access
Optical transitions of Tm3 + ions for amplifiers: how the local structure works in (1-x) TeO2 + (x) M (where M = LiCl, CdCl2, WO3) glass
(OSA, 2002) Aydınlı, Atilla; Ay, Feridun; Sennaroǧlu, A.; Cenk, S.; Özen, G.
Optical transitions of Tm3+ ions for amplifiers was presented. Stimulated emission cross-section at the peak wavelength of the emission bands was determined. It was concluded that Tm3+ doped binary tellurite glasses are promising materials for the infrared amplifiers.
Open Access
pH-responsive near-infrared emitting conjugated polymer nanoparticles for cellular imaging and controlled-drug delivery
(John Wiley and Sons Inc., 2014) Pennakalathil, J.; Özgün, A.; Durmaz, I.; Cetin Atalay, R.; Tuncel, D.
In this article, pH-responsive near-infrared emitting conjugated polymer nanoparticles (CPNs) are prepared, characterized, and their stabilities are investigated under various conditions. These nanoparticles have capacity to be loaded with water insoluble, anticancer drug, camptothecin (CPT), with around 10% drug loading efficiency. The in vitro release studies demonstrate that the release of CPTs from CPNs is pHdependent such that significantly faster drug release at mildly acidic pH of 5.0 compared with physiological pH 7.4 is observed. Time and dose-dependent in vitro cytotoxicity tests of blank and CPT-loaded nanoparticles are performed by realtime cell electronic sensing (RT-CES) assay with hepatocellular carcinoma cells (Huh7). The results indicate that CPNs can be effectively utilized as vehicles for pH-triggered release of anticancer drugs.
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
Quantum dots on vertically aligned gold nanorod monolayer: plasmon enhanced fluorescence
(Royal Society of Chemistry, 2014) Peng, B.; Li, Z.; Mutlugun, E.; Martinez, P. L. H.; Li, D.; Zhang, Q.; Gao, Y.; Demir, Hilmi Volkan; Xiong, Q.
CTAB-coated Au nanorods were directly self-assembled into a vertically aligned monolayer with highly uniform hot spots through a simple but robust approach. By coupling with CdSe/ZnS quantum dots, a maximum enhancement of 10.4 is achieved due to: increased excitation transition rate, radiative rate, and coupling efficiency of emission to the far field.
Open Access
Seed layer assisted hydrothermal deposition of low-resistivity ZnO thin films
(Materials Research Society, 2017) Chubenko, E.; Bondarenko, V.; Ghobadi, Amir; Ulusoy, Gamze; Topallı, Kağan; Okyay, Ali Kemal
In this work, we describe the combination of hydrothermal and atomic layer deposition (ALD) for growing low-resistivity ZnO polycrystalline continuous films. The effect of the thickness of ALD seed layers on the morphology of the hydrothermal ZnO films was studied. It was shown that ZnO films hydrothermally deposited on very thin seed layer consist of separate nanorods but in the case of 20 nm seed layer ZnO films transform to uniform continuous layers comprising of closely packed vertically aligned crystallites. Photoluminescence spectra were shown to exhibit broad band behavior in the visible range, corresponding to radiative recombination processes via oxygen defects of ZnO crystalline lattice, and narrow band in the UV region, associated with band-to-band recombination processes. It was shown that the resistivity of the obtained ZnO films is decreased gradually with the increase of ZnO films thickness and determined by the presence of crystal lattice defects in the seed layer.