Symposium Organizers
Vokmar Dierolf Lehigh University
WojciechM. Jadwisienczak Ohio University
Yasufumi Fujiwara Osaka University
Tom Gregorkiewicz University of Amsterdam
V2: ZnO, GaN, Phosphors
Session Chairs
Thursday PM, April 28, 2011
Room 3012 (Moscone West)
2:30 PM - **V2.1
Luminescence Properties of Eu-doped ZnO Grown by Sputtering-assisted Metalorganic Chemical Vapor Deposition.
Yoshikazu Terai 1 , Takahiro Tsuji 1 , Muhammad hikim Kamarudin 1 , Yasufumi Fujiwara 1
1 Graduate School of Engineering, Osaka university, Suita Japan
Show AbstractRare-earth (RE) doped semiconductor is one of the promising materials for an active layer in light-emitting devices. As a new candidate of RE-doped semiconductor, ZnO has a potential for the active layer in visible light range due to the large band-gap of ∼3.37 eV. However, there are few reports of RE-doped ZnO thin films, and the optical properties are also unclear. Recently, we have developed a sputtering-assisted metalorganic chemical vapor deposition (SA-MOCVD) technique for the growth of RE-doped ZnO and succeeded in the growth of Eu-doped ZnO (ZnO:Eu). In this contribution, we have investigated photoluminescence (PL) properties of the ZnO:Eu in order to clarify the energy-transfer mechanism from the ZnO host to Eu ions.ZnO:Eu with a thickness of ∼700 nm was grown on c-plane sapphire substrates at 700 °C by the SA-MOCVD technique. ZnO host was grown by a thermal reaction of diethylzinc (DEZ) and O2. During the ZnO growth, Eu was doped by RF magnetron sputtering of a Eu2O3 sintered target which was fixed over the top of the substrate. In this study, the Eu2O3 target was sputtered at RF power of 30 W during the ZnO growth. XRD and EDX measurements showed that the Zn1-xEuxO film with x < 0.05% was grown with c-axis orientation. The as-grown ZnO:Eu was annealed at 600 °C for 0.5 h in O2 ambient.In PL spectra at room temperature under the excitation wavelength of 266 nm which is shorter than the band-gap of ZnO (∼370 nm), the as-grown sample showed only band-edge PL of ZnO. On the other hand, the annealed one showed both the band-edge PL and sharp red luminescence originating from intra-4f shell transition of 5D0–7F2 in Eu3+. The result revealed that the luminescent Eu3+ centers were formed after the thermal annealing. In the PLE spectra of 5D0–7F2 transition at 77 K, remarkable increases of the PL intensity were observed at 375 nm, 402 nm and 465 nm. The excitation wavelengths of 402 nm and 465 nm almost coincide with the transition energies of the 7F0–5D3 (410 nm) and 7F0–5D2 (464 nm) in Eu3+ ions. The excitation wavelength of 375 nm corresponds to the band-gap of ZnO (~370 nm at 77 K). In addition, the high PL intensity was obtained under the excitation wavelengths of 337.1 nm and 266 nm which are not the resonant excitation wavelength of Eu3+-related transitions. These results indicate that the band-gap excitation of ZnO host results in the indirect excitation of Eu3+ ions due to an energy transfer from ZnO to Eu3+.When the Eu ions replace the Zn site, the charge state of the Eu ions should be divalent which is equivalent with the Zn cation in the ZnO. The Eu2+ ions change to trivalent charge state during the thermal annealing because Eu2+ ions can be easily oxidized. As a result, the luminescent Eu3+ centers are considered to be formed in the annealed ZnO:Eu.
3:00 PM - V2.2
Electron Microscopy and Electron Energy-loss Spectroscopy Study of Yb and Li Co-doped ZnO.
Nan Jiang 1
1 Physics, Arizona State Univ., Tempe, Arizona, United States
Show AbstractZinc oxide is a fascinating material with extraordinary electric and optical properties, with a direct band gap of 3.4eV. It is also one of the highly efficient luminescence materials. Various emissions can be obtained when ZnO is doped with different activators such as rare-earth (RE) ions. The potential applications in optoelectronics vary from multicolor emission for flat panel displays to solar cells. However, one of the key factors preventing RE-doped ZnO from practical applications was its low luminescence efficiency, which was mainly attributed to inefficient energy transfer from the ZnO host to RE dopants. X-ray diffraction measurements showed that the lattice parameters of ZnO did not change before and after RE dropping. Although it is rather subjective, the low solubility of RE in ZnO was traditionally interpreted as due to the large mismatching of atom radii and electric charges between RE3+ and Zn2+. Co-doping RE with Li in ZnO can greatly enhance the RE3+ emission intensities. One hypothesis was that the co-doping of Li might enhance the solubility of RE in the ZnO. However there was no evidence for the lattice parameter changes after co-doping Li. This indicates that the limitation of solubility of RE in ZnO may not be responsible for the low luminescence efficiency. Recently, a new phase, LiYbO2, has been discovered in our study of the Yb and Li co-doped ZnO. These LiYbO2 phases are embedded in the ZnO particles. The optical spectroscopy study clearly shows that the ZnO-LiYbO2 can harvest the energy from near-UV on a broad wavelength region and effectively convert them into the wavelength region that Si PV device exhibits the maximum spectral response. The detailed electron microscopy and electron energy-loss spectroscopy studies indicate that the highly efficient energy transfer may benefit from the inter-diffusion of Li, Yb and Zn in the interfacial region due to the strong interactions between ZnO and Yb.
3:15 PM - V2.3
Metal Oxides for Efficient Infrared to Visible Upconversion.
Isabelle Etchart 1 , Anthony Cheetham 1
1 Materials Science and Metallurgy, University of Cambridge, Cambridge United Kingdom
Show AbstractUpconversion phosphor materials are attracting considerable attention for their possible applications in solar cells with improved efficiency, water splitting, nanomaterials for bio-imaging, and novel display technologies. Upconversion materials, usually consisting of crystals doped with rare-earth ions, can convert low-energy incident light into a higher energy emitted light. Several mechanisms are involved, including multiple photon absorption and energy transfers between dopants. Up to now, reported upconversion efficiencies have been quite low and the phosphors usually present poor chemical stability, limiting their industrial applicability. Upconversion luminescence characteristics of a new rare-earth-doped Y2BaZnO5 host lattice have been investigated for potential use as a near-infrared to visible upconverter. A variety of dopants, including Yb3+, Er3+, Ho3+ and Tm3+, were embedded in the host lattice, resulting in interesting red, green, blue and white light emissions under 980 nm excitation. Upconversion efficiencies up to 5.2% were obtained in the best system, which we believe is the highest reported efficiency to date. The upconversion mechanisms were determined by means of detailed spectroscopic investigations, including concentration and power dependence studies associated with lifetime measurements. Work was also initiated in order to shape the phosphors for potential applications. The presentation will focus primarily on results obtained for the Y2BaZnO5: Yb3+,Er3+ system.
4:00 PM - **V2.4
Rare-earth Doped Phosphor Materials for Green Technologies.
Setsuhisa Tanabe 1 , Jumpei Ueda 1
1 Graduate School of Human and Environmental Studies, Kyoto University, Kyoto Japan
Show Abstract Among many technologies that are stated to be “sustainable”, there seems little room for controversial discussion in the statement that increased installation of photovoltaic systems and adoption of energy-efficient LEDs in many lighting fields are no doubt the trend toward sustainability. Rare-earth doped phosphors are playing crucially important roles in the phosphor-converting white LED (pc-wLED), where they convert blue light from an InGaN-based LED into white spectra with good color-rendering and without any UV and infrared light, unlike incandescent light bulbs, being on its way out after 130-years run due to government restrictions or efficiency regulations. Since 2004, we have developed glass ceramic phosphors based on Ce3+:YAG or various Eu2+ silicate crystals, proposing “all inorganic solution” for the solid-state lighting, especially high-power illumination. The broadband photoluminescence by the Laporte-allowed 5d→4f transitions in the lanthanide (Ln) enables efficient and reasonable conversion of near-UV or blue light into visible white in the pc-wLEDs. Device forms of glass ceramics in the pc-wLED have many advantages of formability, thermal resistance and no need of packaging powder phosphors. Recently, we are also developing “quantum-cutting” (QC) phosphors for photovoltaic (PV) applications to improve solar cell efficiency by modifying solar spectrum. The solar spectrum is very broad having a peak around 500nm (2.4eV). Solar cells, on the other hand, create one electron-hole pair most effectively with a photon of energy just above the semiconductor band gap, i.e., 1.1eV for the c-Si. The mismatch between the incident solar spectrum and the spectral response of semiconductors is one of the main reasons to limit the efficiency of single-junction cells. The efficiency limit of the c-Si has been estimated to be 29%. However, this limit is estimated to be improved up to 38.4% by modifying the solar spectrum by a QC down-converting phosphor that converts one photon of high energy into two photons of lower energy. The Yb3+ ion has a simple energy-level structure having only one excited level, 2F5/2 at 1.2eV. There are several combinations of Ln and Yb in glass and ceramics, by which Ln3+ ions can sensitize the Yb3+ emission at 1μm. Some of the combinations exhibit the QC after optical absorption of uv and blue photon. Since the Yb3+: 2F5/2→ 2F7/2 emission and sensitivity peak of the c-Si solar cell overlap each other, the QC materials which convert one UV or blue photon into two 1.2eV photons would be an ideal phosphor for the PV generation. In this talk, I will review the glass ceramic phosphors for solid-state lighting and also discuss recent studies on the QC phosphors of Ln3+ and Yb3+ ions codoped ceramics and glasses for efficient PV generation.
4:30 PM - V2.5
Toward Light Conversion Phosphors with Optimized Microstructure for GaN White LEDs.
Thierry Gacoin 1 , Amelie Revaux 1 , Geraldine Dantelle 1 , Jean-Pierre Boilot 1
1 , CNRS - Ecole Polytechnique, Palaiseau France
Show AbstractRare-earth doped oxides are well-known for their applications in light emitting devices. Most systems such as white GaN LEDs rely on the light conversion from a UV or blue source of excitation into visible light within films of phosphors deposited on a substrate. The external efficiency of these conversion layers depends on two parameters: first one is the structure of the luminescent phosphor particles (including the size, shape and volumic fraction), which determines the internal emission yield, absorbance and diffusion properties of light within the material. Second one is the microstructure of the host matrix (including surface geometry, porosity or diffusive inclusions), which also determines the transport properties of the emitted light in the layer and its coupling to the outside of the film. The optimization of both contributions is crucial to obtain the most efficient conversion materials, but this issue has been poorly investigated mainly because the phosphor particles have an intrinsic microstructure related to their synthesis as micronic powders. In order to study the influence of microstructural effects, we developed a strategy that aims in suppressing any effect resulting from the microstructure of the phosphor itself. In a first approach, we used a molecular rare-earth chelate as light emitters, which do not induce any microstructure effects and allow us to focus on the structure of the film itself. We were thus able to investigate sol-gel films which surface were periodically textured by soft nanoimprint lithography. Enhancement of external luminescence yield was observed due to extraction from the 2D photonic crystal and the angular distribution of the emitted light was shown to be controlled depending on the chosen geometry.The second part of our work was devoted to the application of the above strategy using YAG:Ce3+ phosphor nanoparticles as the emitter, considering that this compound is more appropriate for the application. YAG:Ce particles were synthesized by a glycothermal method in static autoclave at low temperature (300°C). Pure Y3Al5O12 garnet phase was obtained, as verified by Rietveld analysis. The optical properties of the YAG nanoparticles were compared to those of bulk YAG. The quantum yield of nanoparticles was 50%, which is lower than the one of bulk crystal (85%) but still relatively high. The colloidal nanoparticles were finally incorporated into a sol-gel matrix of TiO2. Transparent thin films are obtained, offering the opportunity to study the extraction of the guided light with photonic crystal patterned by nanoimprint lithography. Emission properties of this device are discussed in relation with microstructural effects.
4:45 PM - V2.6
Morphologically Controlled Synthesis of Colloidal Upconversion Nanophosphors and Their Shape-Directed Self-Assembly.
Xingchen Ye 1 , Joshua Collins 2 , Yijin Kang 1 , Jun Chen 3 , Christopher Murray 1 3
1 Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania, United States, 2 , Intelligent Material Solutions, Inc., Princeton, New Jersey, United States, 3 Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania, United States
Show AbstractLanthanide-doped nanophosphors are an emerging class of optical materials. These nanocrystals (NCs) often possess “peculiar” optical properties (e.g., quantum cutting and photon upconversion), allowing the management of photons that could benefit a variety of areas including biomedical imaging and therapy, photovoltaics , solid state lighting, and display technologies. Colloidal upconversion nanophosphors (UCNPs) are capable of converting long-wavelength near-infrared excitation into short-wavelength visible emission through the long-lived, metastable excited states of the lanthanide dopants. In contrast to the Stokes-shifted emissions from semiconductor NCs or organic fluorophores and the multiphoton process employing fluorescent dyes, UCNPs offer several advantages including narrow emission bands tunable through the choice of dopants. With non-blinking emission and remarkable photostablity, good brightness under low power continuous-wave laser excitation, low autofluorescence background and deep penetration lengths in biological systems, these materials are very attractive probes for bioimaging applications. The hexagonal phase of NaYF4 (β-NaYF4) is one of the best host materials for upconversion due to its low phonon energies, being several orders of magnitude more efficient than the cubic, α-NaYF4 phase. We report a one-pot chemical approach for the synthesis of highly monodisperse colloidal nanophosphors displaying bright upconversion luminescence under 980nm excitation. This general method optimizes the synthesis with initial heating rates up to 100°C/minute generating a rich family of nanoscale building blocks with distinct morphologies (spheres, rods, hexagonal prisms and plates) and upconversion emission tunable through the choice of rare earth dopants. Furthermore, we employ an interfacial assembly strategy to organize these NCs into superlattices over multiple length scales (from nanometer to submillimeter scale) facilitating the NC characterization and enabling systematic studies of shape-directed assembly. The global and local ordering of these superstructures is programmed by the precise engineering of individual NC’s size and shape. This dramatically improved nanophosphor synthesis together with insights from shape-directed assembly will advance the investigation of an array of emerging biological and energy-related nanophosphor applications.ReferenceYe, X.C., Collins, J., Kang, Y.J., Chen, J., Chen, D., Yodh, A., Murray, C.B. Morphologically Controlled Synthesis of Colloidal Upconversion Nanophosphors and Their Shape-Directed Self-Assembly. Proc Natl Acad Sci USA. In press.
5:00 PM - V2.7
Rare-earth Doped Oxide for NIR Quantum Cutting.
Antoine Guille 1 , Antonio Pereira 1 , Amina Bensalah-Ledoux 1 , Bernad Moine 1
1 , LPCML CNRS/UCBL UMR5620, Villeurbanne France
Show AbstractThe conversion efficiency of silicon based photovoltaic cells is currently around 20%. One major loss mechanism leading to low conversion efficiency is the thermalization of charge carriers generated by absorption of high-energy photons, typically UV and visible. A method to reduce these losses has been proposed and consist in generating two electron-hole pairs per incident photon with an energy larger than twice the band gap of silicon. High-energy incident photons are absorbed by a luminescent conversion layer deposited in front of the solar cell and converted into two NIR photons able to be absorbed by silicon. This can be achieved by using a transparent matrix doped with rare-earth ions. Yb3+ ion is the best choice of activator because it shows one transition emitting around 1µm, where absorption of silicon is the most effective. To sensitize Yb3+ ion, Pr3+ ion seems to be a good choice since quantum-cutting has already been shown for this couple. It consists in a two-step mechanism : Pr3+ (3P0→1G4, 1G4→3H4) → Yb3+(2×{3F7/2→3F5/2}). The upper levels of Pr3+ ion show sharp and inefficient absorption bands since those are 4f→4f transitions. It is thus necessary to introduce a third ion to strongly absorb energy from the UV-blue region of the solar spectrum and transfer it to the Pr3+ ion. Ce3+ ion shows a very efficient 4f→5d transition but the position and the width of its absorption and emission bands are very sensitive to the crystalline environment. In a CYA (CaYAlO4) matrix, the position of Ce3+ absorption and emission bands appears to make it able to sensitize the upper levels of Pr3+ ion. In this work, we present energy transfer between Ce3+, Pr3+, and Yb3+ in a CYA matrix. And we also present the results obtained with a CYA thin film doped with Ce3+ and Pr3+ deposited by PLD.
5:15 PM - V2.8
Bright Red Emission from Eu-doped GaN-based Light-emitting Diodes Grown by Oorganometallic Vapor Phase Epitaxy.
Atsushi Nishikawa 1 , Naoki Furukawa 1 , Dong-gun Lee 1 , Kosuke Kawabata 1 , Takanori Matsuno 1 , Ryo Harada 1 , Yoshikazu Terai 1 , Yasufumi Fujiwara 1
1 , Osaka University, Suita, Osaka, Japan
Show AbstractEu-doped GaN is expected to realize a monolithic device composed of red, green and blue GaN-based light emitting-diodes (LEDs) for full-color display or lighting technology. Recently, we have succeeded in the first demonstration of a Eu-doped GaN-based red LED grown by organometallic vapor phase epitaxy (OMVPE) [1,2]. We have also reported improved luminescence properties of the Eu-doped GaN-based red LED grown at atmospheric pressure [3,4]. In order to improve the luminescence properties, it is inevitable to optimize the LED structure as well as to clarify the properties of the Eu-doped GaN active layer. As the first step, we investigated the light output power of Eu-doped GaN-based LEDs with different active layer thicknesses.The samples were grown on a sapphire (0001) substrate by OMVPE. The group III and V sources were trimethylgallium, trimethylaluminium and ammonia. Eu3+ ions were doped using tris(dipivaroylmethanate) europium [Eu(DPM)3]. The Eu concentration was determined to be 3x1019 cm–3 by secondary ion-mass spectroscopy. The active layer thickness was varied from 300 to 900 nm. Pd/Au and In electrodes were used for ohmic contacts for the p-GaN and n-GaN layers, respectively. The size of the Pd/Au electrode for the p-InGaN was 1 mmφ. The output power of the electroluminescence (EL) was measured with an integrating sphere spectrometer (LMS-100).With increasing the active layer thickness, the light output power monotonically increased. The maximum light output power of 50 μW was obtained for an active layer thickness of 900 nm with an applied current of 20 mA, which is the highest value ever reported. The corresponding external quantum efficiency was 0.12%. The applied voltage for the LED operation also increased with the active layer thickness due to an increase in the resistance of the LED. Therefore, in terms of wall-plug efficiency, the optimized active layer thickness was around 600 nm. These results indicate that the optimization of the LED structure would effectively improve the luminescence properties.[1] A. Nishikawa, T. Kawasaki, N. Furukawa, Y. Terai, and Y. Fujiwara, Appl. Phys. Exp. 2, 071004 (2009). [2] A. Nishikawa, T. Kawasaki, N. Furukawa, Y. Terai, and Y. Fujiwara, Phys. Status Solidi A 207, 1397 (2009). [3] A. Nishikawa, T. Kawasaki, N. Furukawa, Y. Terai, and Y. Fujiwara, Appl. Phys. Lett. 97, 051113 (2010). [4] N. Furukawa, A. Nishikawa, T. Kawasaki, Y. Terai, and Y. Fujiwara, Phys. Status Solidi A, in press.
5:30 PM - *
Short Presentations from Poster Authors
Show Abstract
Symposium Organizers
Vokmar Dierolf Lehigh University
WojciechM. Jadwisienczak Ohio University
Yasufumi Fujiwara Osaka University
Tom Gregorkiewicz University of Amsterdam
V4: Insulating Materials: Lasers and Phosphors
Session Chairs
Friday AM, April 29, 2011
Room 3012 (Moscone West)
9:45 AM - V4.1
Process Development for Combustion Synthesis of Oxynitride Phosphors.
Shyan-Lung Chung 1 , Feng-Sheng Chang 2 , Shu-Chi Huang 3
1 Chemical Engineering, National Cheng Kung University, Tainan Taiwan, 2 Chemical Engineering, National Cheng Kung University, Tainan Taiwan, 3 Chemical Engineering, National Cheng Kung University, Tainan Taiwan
Show AbstractDue to concerns on energy and environmental problems, white LED lighting is expected to replace the traditional lighting devices and to become the major lighting technology in the coming decades. The presently used oxide phosphors can not meet the requirements of high efficiency and high thermal stability by the high power white LED applications. In addition, the presently available oxide phosphors all have the drawback of low thermal stability. It causes the color shift when operation temperature increased. It is thus an important research subject to develop phosphor materials with desirable properties such as high efficiency, high thermal stability, chemical resistance and high brightness. Recent literature publications reveal that oxynitride phosphors such as SiAlON may be developed to meet these requirements. However, presently available techniques for the synthesis of oxynitride phosphors all require high temperatures, high nitrogen pressures, and long processing duration time.In this presentation ,we report the development of a new synthesis method for a yellow oxynitride phosphor (i.e.,Mg-α-SiAlON:Eu2+) based on self-propagating high temperature synthesis (SHS) reactions. The reactants used for the synthesis include calcium, silicon, aluminum, silicon nitride and europium oxide. These powders are mixed and pressed into compacts. The reactant compacts are then wrapped up with an igniting agent (e.g., Mg+Fe3O4) . The reactant compact is ignited by electrical heating under a N2 atmosphere of 5 atm. The Mg -α-SiAlON:Eu2+ phosphor can thus be produced under a low pressure within a short duration time. The luminescence properties are compared with the commercial phosphors. The results showed that the luminescence intensity and wavelength of the Mg -α-SiAlON:Eu2+ phosphors are affected by experimental parameters. A typical product has an excitation spectrum in the range of 220-500nm and a single broadband emission in the range of 400-650nm centered at 555nm upon excitation at 380nm. The luminescence intensity is 112% as compared with a corresponding commercial phosphor.
10:00 AM - V4.2
Non Conventional Molecular and Hybrid Precursor Routes to Vanadate and Niobate Compounds as Host Lattices for Lanthanide-doped Luminescent Materials.
Nicolas Deligne 1 , Michel Devillers 1
1 Chemistry, Universite Catholique de Louvain, Louvain-la-Neuve Belgium
Show AbstractVanadates and niobates constitute widely spread host lattices for Rare Earth (RE)-doped luminescent materials. The present work is devoted to the development of non conventional but easy and widely applicable preparation routes based either on hybrid or on molecular precursors, which are converted into functional materials trough an appropriate thermal treatment. The molecular precursors considered here are stoichiometrically well-defined and water-soluble coordination compounds, namely EDTA and peroxo-bis-N-oxido-EDTA complexes of general formula (NH4)[A(EDTA)].xH2O (A = Y, Pr, Sm, Eu, Gd, Dy, Er, Bi), (NH4)3[V(O)2(EDTA)].H2O and (gu)3[Nb(O2)2(EDTAO2)].2.5H2O (gu = guanidinium). An alternative method was based on hybrid precursors consisting in homogeneous blends between inorganic species and an organic polymer acting as a sacrificial matrix, in this case hydroxypropylmethyl cellulose (HPMC). The molecular precursor route and the HPMC-based method were both used for the preparation of a broad series of simple and mixed vanadates, AVO4 and Y1-xAxVO4, and niobates, ANbO4 and A3NbO7 (A = Y, La, Bi, Pr, Sm, Eu, Gd, Dy, Er, Tm and Lu). Rare Earth-doped vanadates AVO4:Ln and Y1-xAxVO4:Eu were also prepared. Luminescent properties of these doped materials were investigated and a significant effect of the host lattice composition was evidenced. In order to understand the influence of the method on the materials structural features and properties, comparisons were made between these two precursor methods and a conventional solid state reaction method. SEM was used to characterize the particle size and morphology. Zircon-type AVO4 hosts were synthesized at a moderate temperature of 800°C, as evidenced by XRD and Raman analyses. Solid solutions such as Y1-xPrxVO4 and Y1-xGdxVO4 were also obtained, and characterized by the occurrence of linear correlations between chemical compositions and lattice parameters or Raman shifts. A slightly disordered intergranular porosity was evidenced. The nature of the phases obtained was also demonstrated to be affected by the preparation method. The use of HPMC as additive was found to broaden the composition range for which solid solutions between YVO4 and BiVO4 could be obtained.As far as niobates were concerned, these non conventional methods were shown to stabilize scarcely reported distorted tetragonal RENbO4 phases and pyrochlore-type RE3NbO7 compounds. This RENbO4 polymorph is not accessible by conventional preparation routes as the tetragonal form turns to monoclinic when going to 25°C. Finally, in addition to the bulk phases, the same methodology was also proven to be appropriate for the preparation of YVO4:Eu thin films, the use of HPMC in the spin coated solutions appearing to improve significantly the film homogeneity.
10:15 AM - V4.3
Efficient Near-infrared Luminescence and Energy Transfer in Nd-Bi Codoped Zeolites.
Zhenhua Bai 1 , Hong-Tao Sun 2 , Minoru Fujii 1 , Yuki Mori 3 , Yuji Miwa 1 , Minoru Mizuhata 3 , Shinji Hayashi 1
1 Department of Electrical and Electronic Engineering, Kobe University, Kobe Japan, 2 International Center for Young Scientists (ICYS), National Institute for Materials Science (NIMS), Tsukuba Japan, 3 Department of Chemical Science and Engineering, Kobe University, Kobe Japan
Show AbstractRecently, zeolites have attracted considerable attention as hosts of optically active guests for constructing novel materials designed at the nanosize level, because the well-defined and well-organized cavities provided by zeolites host serve as ideal environment to organize the optically active guests well dispersed in their framework. Visible luminescence with very high efficiencies from optically-functionalized zeolites has been reported. In contrast, it is difficult to realize highly efficient near-infrared (NIR) emission in zeolites, because of the existence of coordinated water in cages, causing fast relaxation of its excitation energy through nonradiative vibrational deactivation.On the other hand, a recent study has shown that imaging biological tissue at a central wavelength around 1000 nm yields minimal axial resolution degradation in optical coherence tomography (OCT). In order to achieve a higher depth resolution, a strong emission around 1000 nm is required. Nd3+ is an ideal candidate for this purpose, because it displays sharp emission bands at 1064 nm. However, similar to the situation in other rare-earth ions, optical absorption of Nd3+ is too weak due to the forbidden nature of the intra-4f transitions. To overcome this deficiency, sensitization of the Nd3+ emission by an energy transfer process is an attractive way to obtain efficient emission.In this work, we prepare Nd-Bi codoped FAU-type nanocrystalline zeolites by a method consisting of a simple ion-exchange process and subsequent high temperature annealing under N2 atmospheric condition. The NIR emission of Nd3+ ions is significantly enhanced by the introduction of bismuth in codoped samples, and the lifetime reaches 0.25 ms. Steady state and time-resolved photoluminescence (PL), and PL excitation measurements demonstrate the energy transfer from Bi-related active centers to Nd3+. A breakthrough of this system is represented by the fact that Nd3+ ions can also work under off-resonance excitation conditions in the visible range, so optical pumping using flash lamp or white LED rather than laser becomes feasible using this broadband sensitization. Owing to the peculiar optical properties of this material and the mature fabrication technique of zeolites, we believe this finding must have wide application for biological probes.
11:00 AM - V4.4
Red-emitting Ca(1-x)SrxS:Eu2+ Phosphors as Light Converters for Plant-growth Applications.
Miroslaw Batentschuk 1 , Qi Xia 1 , Andres Osvet 1 , Juergen Schneider 2 , Albrecht Winnacker 1 , Christoph Brabec 1
1 Materials Science and Engineering, Chair WW6 Materials for Electronics and Energy Technology i-MEET, University of Erlangen-Nuremberg, Erlangen Germany, 2 Materials Research Center, University of Freiburg, Freiburg Germany
Show AbstractLight is the dominant influencing factor for the plant production: solar irradiation serves as the ultimate energy source for photosynthetic activities of higher plants. It is known that green plants have selective utilization of light for their photosynthetic activities: Chlorophyll a and b as the most important photosynthetic pigments show major absorption peaks in the blue (460 nm) and red (640 nm and 660 nm) spectral regions, thereby red photons are especially important for the photosynthesis [1]. Green photons are less active for photosynthetic reactions and are mostly transmitted and reflected by the leaves [2]. An evident way to enhance the quantity of the red photons is to modify the solar spectrum with a “green-to-red” converter, as it was made with (CaSrBa)S:Eu2+ [3]. For tuning of the emission spectra, which is necessary for overlapping with the absorption spectrum of Chlorophyll, the results from the development of converters for LEDs based on CaxSr(1-x)S:Eu2+ can be applicable [4]. Recently we reported the quantum efficiency (QE) of microcrystalline powder of Ca(1-x)SrxS:1mol.% Eu2+ in the temperature range of 20 – 400 K [5]. It was found that it is problematic to maintain the maximal QE during the spectra tuning. In addition, it was not clear what Eu2+ concentration is needed to reach the maximal luminance of the converting layers. In this work, we report on further investigations of the red-emitting Ca(1-x)SrxS:Eu2+, with tunable composition-dependent emission maxima, as a sun light converter for plant photosynthesis applications. The optimal Ca/Sr ratio was found providing the maximal QE and a broad-band emission centered at 650 nm which matches, for instance, the absorption spectrum of spinach chloroplasts. It was shown that the Eu2+ content should be between 0.3 mol.% and 0.7 mol.% for the maximal luminance. Further “color tuning” of the emission can be made by variations of the Eu content, however, a decrease of the luminance should be taken in account, especially if the Eu concentration rises above 1 mol.%. Finally, we investigated the influence of spectral modification of sun light on photosynthetic activities of green plants. A light conversion foil was prepared with embedded Ca0.4Sr0.6S:Eu1 mol% particles. The CO2 assimilation rates of intact spinach leaves were monitored by a sensitive measurement system with controlled light conditions. Comparing to the results from the reference experiments with MgO reflection foil, the converted light conditions lead to enhanced photosynthetic activities by about 30 %. [1] Haeder, P., 1999. Photosynthese Taschenbuch. Thieme, Stuttgart.[2] Taiz, L., Zeiger, E., Plant physiology. 4th Edition. Sunderland, Massachusetts: Sinauer Associates, Inc., 2006. 126-158.[3] Lian, S., 2007.Patent No.: CN 1935937 A. [4] Nazarov, M., Yoon, C., 2006. J. Solid State Chem. 179, 2529-2533.[5] Qi Xia, M. Batentschuk, A. Osvet, J. Schneider, A. Winnacker, 2010. Rad. Measur. 45, 350- 352.
11:15 AM - V4.5
Photostimulable Fluorescent Nanoparticles for Biological Imaging.
Andres Osvet 1 , Moritz Milde 2 , Sabine Rupp 2 , Sofia Dembski 2 , Nils Lundt 1 , Carsten Gellermann 2 , Miroslaw Batentschuk 1 , Albrecht Winnacker 1 , Christoph Brabec 1
1 Materials Science and Engineering, University of Erlangen-Nuremberg, Chair WW6 Materials for Electronics and Energy Technology i-MEET, Erlangen Germany, 2 ISC, Fraunhofer Institute, Wuerzburg Germany
Show AbstractThe importance of the fluorescent markers as an imaging tool for biomedical research requires new light emitting materials, primarily those having their excitation and emission bands in the red or near-infrared spectral regions. The second problem to be solved is the time delay between excitation and emission light-pulses. The latter is especially desirable due to the autofluorescence of cell tissue and the significant scattering of the exciting light during detection of the marker emission. The autofluorescence and the light scattering result in poor signal-to-noise ratio in biological experiments. To overcome these difficulties, Ca-Zn-Mg-Silicate nanomarkers with a long-lasting afterglow were developed and tested [1]. However, these markers had the disadvantage that their emission is distributed over hours and, therefore, the signal-to-noise ratio is rather poor, especially at the end of the emission decay.We suggest as a solution to this problem the usage of phosphors which show photo-stimulated luminescence (PSL). Such PSL phosphors are known as microcrystalline powders for digital imaging applications [2]. First, a photostimulable phosphor will be “charged” by x-ray- or UV- or visible photons. After some time (minutes, hours or even days) the pulse-emission can be released by means of a red or infra-red light pulse. This results in a significant increase of the signal-to-noise ratio in biological imaging systems. Furthermore, such PSL markers open promising chances to trace in vivo diffusion processes. The main challenge is to create nanoparticles (NPs) with a narrow size distribution and to provide suitable crystal defects which are necessary for the long time storage of e- - h+ pairs in the NPs. We report on nanoparticles with dimensions between 60 nm and 500 nm showing an intensive photo-stimulated luminescence. PSL microcrystalline powder with CaS:Eu,Sm composition were prepared via a solid-state reaction. After milling by a special air-stream mill and by segregation from the mill charge, particles less than 500 nm in size were obtained. Alternatively, we have synthesized SiO2/Zn2SiO4:Mn2+ core/shell spherical monodisperse NPs via a modified Pechini sol-gel process. The structure and the storage capacity of the synthesized NPs, their stimulation as well as emission spectra were investigated and will be discussed in the talk. Some possibilities to increase the storage capacity of Zn2SiO4:Mn2+ by co-doping with i.e. Dy3+,and to modify the spectra of PSL phosphors based on Zn2SiO4 by co-doping with Eu2+/Eu3+ and Sm3+ will be discussed as well. [1] Q. le Masne, C. Chanéac, J. Seguin, F. Pellé, S. Maítrejean, J-P. Jolivet, D. Gourier, M. Bessodes. PNAS 104 (2007) 9266 - 9271.[2] A. Winnacker. Phys. Med. IX, 2-3, (1993) 95 – 101.
11:30 AM - V4.6
Altering Luminescent Properties of BaHfO3:Eu by Co-Dopants and Fabrication Parameters.
Eugeniusz Zych 1 , Anna Dobrowolska 1
1 Chemistry, University of Wroclaw, Wroclaw Poland
Show AbstractIn our presentation an analysis of the spectroscopic properties of BaHfO3:Eu powders will be presented. A special attention will be given to the location of the activator in the host material and possibility to steer the Eu3+ ions into a specific position – either Ba2+ or Hf(IV). Spectroscopic consequences of modifying the composition and/or technological parameters will be presented and discussed. The role of co-dopants, Y3+ and La3+ ions, in a preferential channeling of the activator into either Ba2+ sites in the presence of Y3+ or to Hf(IV) positions when La3+ accompanied the Eu3+ ions in the host will be showed and discussed. A possibility to reduce the dopant into Eu2+ state and spectroscopic consequences of that will be presented too. We will show that both co-dopants and fabrication parameters may strongly affect the luminescence color which varies from red to white.
11:45 AM - V4.7
Synthesis of Rare Earth Ion Co-doped Core-shell Nanostructures for Enhanced Luminescence Efficiency.
James Dorman 1 , Ju Choi 1 , Jane Chang 1
1 , UCLA, Los Angeles, California, United States
Show AbstractThe development of rare-earth ion (RE) doped phosphors allows for the conversion of an absorbed wavelength of light to one that is more suited for the desired application. Additionally, these RE phosphors have long lifetimes, on the order of ms, which offer potential in many energy conversion applications. Currently, RE phosphors are used in fiber optics amplifiers, modulated hybrid laser, optical interconnects and switches, optical displays, site-specific bioanalysis, broad absorption solar cells and various other lighting applications. Typically the RE phosphors are on the microscale, which removes the majority surface effects that can dominate the luminescence. However, reduction in particle size results in the absorbed energy being emitted as a photon instead of non-radiative loss within the crystal structure as phonons. High efficiency phosphors need to take advantage of both aspects of the microscale and nanoscale particles. In order to synthesize high efficiency phosphors, trivalent RE ions are being doped into a core-shell metal oxide host lattice. The role of the core-shell structure reduces the effect of the surface quenching sites by increasing the distance between active ions and the surface hydroxyl groups. Secondly, the luminescent fingerprint can be further controlled with proper doping of the shell structure by either increasing the absorption spectrum or adding additional emission peaks. Primarily, this work focuses on the emission of visible photons through upconversion, Y2O3:Er3+, Yb3+ nanophosphors, or from downconversion, LaPO4:Eu3+ . By spatially controlling the position of the RE ions within the nanostructure, increased luminescence is observed due to energy transfer between the dopant ions. Additionally, the shell layers increase lifetimes and PL intensity due to surface passivation and energy transfer within the core-shell structure. Furthermore, the core-shell structure showed an increase in Y2O3:Er3+ luminescence intensity was increased with the addition of Y2O3, Y2O3:Yb3+and Yb2O3 shell layers. When the lifetime of the 565 nm transition was investigated, the luminescence decay was ~10 μs longer with the aforementioned shell structures at 77 K while it increased from 27 μs to 45 μs at room temperature, resulting in a 10 % and 55 % increase in phosphor quantum efficiency, respectively. Similar results were observed when studying the upconversion luminescence with respect to the shell structure, requiring 0.5 more photons to populate the 4S3/2, 2H11/2 without the active Yb2O3 shell layer. Based on the measured spectroscopic properties, it was found that the changed in crystal field surrounding the Er3+ ions causes a blue shift in the PL spectra and the quality factor (χ=Ω4/Ω6) has been improved through shell addition. Similar work is being performed on the phosphate materials, which will be highlighted at the meeting.
12:00 PM - V4.8
Imaging Upconversion from NaYF4:Er:Yb Nanoparticles on Au and Ag Plasmonic Substrates.
Steve Smith 1 , P. Stanely May 2 , Lanlan Zhong 1 , QuocAnh Luu 2
1 , SDSM&T, Rapid City, South Dakota, United States, 2 Chemistry, University of South Dakota, Vermillion, South Dakota, United States
Show AbstractNear-infrared-to-visible upconversion materials have many promising applications, including use in luminescent solar concentrators, in next-generation displays, and as biological labels. NaYF4 nano-particles doped with Yb and Er exhibit efficient upconversion and are easily deployed in these applications. It is known that a rough metal surface may increase the yield of fluorescence of a nearby fluorophore, by local field enhancement due to plasmonic resonances, and by modification of the radiative rate(s) of the fluorophore. Thus, properly chosen metallic nanostructures can potentially increase the upconversion efficiency of lanthanide-doped nanoparticles, yet the optimal design of these nanostructures is still an active area of research. In our experiments, we use a spectroscopic imaging system to study the upconversion efficiency of NaYF4: Er3+/ Yb3+ through spatially-resolved upconversion spectra, using a custom-built scanning confocal microscope system with infra-red excitation, and wide-field fluorescence imaging. We present spectrally-resolved upconversion images of NaYF4:Yb3+/Er3+ nanoparticles on plasmonic substrates, including silver nanowires and patterned substrates of gold and silver, which show localized regions (about 1 micron) of relatively stronger intensity and modified upconversion spectra, and compare these to wide-field fluorescence images of samples with and without plasmonic substrates.
12:15 PM - V4.9
Novel Luminescent Materials: Shaping the Future of Lighting with High Brightness pc-LEDs.
Hailing Cui 1 , Alexander Baumgartner 1 , Kirstin Petersen 1 , Dominik Eisert 1
1 Backend Technology, OSRAM Opto Semiconductors GmbH, Regensburg Germany
Show AbstractToday high-brightness Light-Emitting Diodes (LEDs) with phosphor conversion (pc), which could be adjusted from cool bluish white to warm white with good color rendering, are strongly pushed into the application field of general illumination. In terms of brightness and light quality, phosphors become one of the key factors for future success of LED lighting. Recently, new LED-phosphors from different material classes based on Ce3+ and Eu2+ dopants have been developed by Osram / Osram Opto Semiconductors. Besides strong absorption in the blue / NUV light region and suitable emission spectrum, the phosphors have also the following properties: I) high quantum efficiency, II) super long-life stability, III) lower thermal-quenching. Several latest achievements at Osram / Osram Opto Semiconductors based on phosphor optimization for Solid-State-Lighting LEDs will be summarized.
12:30 PM - V4.10
Investigation of Surface Loss in Nano-scale Luminescent Materials under VUV Excitation.
Kristopher Olsen 1 , Rusty Mann 1 , Christopher Waite 1 , Anthony Diaz 1
1 Chemistry, Central Washington University, Ellensburg, Washington, United States
Show AbstractLuminescent materials with particle sizes less than 1 μm have become a major focus of study, as such materials often exhibit unique properties relative to their bulk counterparts. However, as the particle size is decreased a significant reduction in efficiency is observed because excitation energy is lost to the particle surface. Studies of host-to-activator transfer efficiency can help to clarify the nature of these surface loss processes. Our research group has shown that excitation and absorption spectra can be used to estimate host-to-activator transfer efficiencies under vacuum ultraviolet (VUV) excitation. Such data can then be analyzed using accepted kinetic models, resulting in quantitative information about electron transport and trapping, as well as surface losses, for a given host/activator combination. We have been conducting such studies on YBO3 and Y2O3 doped with Eu3+ and Tb3+. Our results indicate that measurements of host-to-activator transfer efficiency can indeed be used to quantify surface loss, particularly in doped YBO3, where a clear trend in surface loss versus particle size is observed as the particle size is decreased below about 500 nm. A discussion of the measurement and analysis of transfer efficiency will be presented, along with our most recent results.
12:45 PM - V4.11
Colloidal Fluoride Nanocrystals with Near-infrared to Near-infrared and Visible Energy Upconversion for Biomedical and Photonic Applications.
Tymish Ohulchanskyy 1 , Guanying Chen 1 , Paras Prasad 1
1 Institute for Lasers, Photonics and Biophotonics, State University of New York at Buffalo, Buffalo, New York, United States
Show AbstractThis talk will present our progress in design and synthesis of the fluoride nanocrystals doped with rare-earth ions (Er3+, Yb3+, Gd3+, Tm3+) and their biomedical and photonic applications. Utilization of the highly efficient near-infrared (NIR) to NIR and visible energy upconversion in rare-earth ions doped nanoparticles allowed us to demonstrate targeted photoluminescence imaging of the cancer cells in vitro, high contrast photoluminescence imaging in vivo. An incorporation of the paramagnetic Gd3+ ions into photoluminescent nanocrystals resulted in a dual modality (optical and magnetic resonance imaging) nanoprobes for biomedical applications. We also report the colloidal rare-earth ions doped fluoride nanocrystals which are capable of efficient conversion of the telecommunication wavelength (~1.5 μm) to the visible, NIR and short-wavelength infrared (SWIR) ranges, thus providing a promising tool for photonic applications. We have shown that the concentration of the dopants and structure of the nanoparticles can be changed to optimize the efficiency of upconversion photoluminescence and magnetic resonance imaging. With the same aim, size of the prepared nanophosphors was demonstrated to be tuned in a controlled way from 10 nm to 100 nm. The design of nanophosphor/semiconductor/metal hybrid nanomaterials for photonics will be also discussed.