Contract value:                     300 000 lei
DURATA PROIECTULUI:       Octombrie 2011 - Septembrie 2013
CONDUCATOR PROIECT:     Institutul National de Cercetare-Dezvoltare pentru Fizica Laserilor, Plasmei si Radiatiei, Magurele, Bucuresti, Romania

Abstract
The aim of this project is the synthesis and study of the upconversion processes in compounds from langasite family doped with rare-earth ions. The Er³⁺ and Tm³⁺ ions will be used as activators and Yb³⁺ as sensitizer. The subject is new and not approached before in the literature. These new upconversion materials could be of interest as luminescent biomarkers. The compounds from the langasite family (La₃Ga₅SiO₁₄-langasite, La₃Ga_5.5Ta_0.5O₁₄-langatate and La₃Ga_5.5Nb_0.5O₁₄-langanite) are partially disordered crystals. The compounds will be obtained as nanopowders using sol-gel method, or as bulk materials using solid state synthesis. For nanopowders, the crystallite size and crystallinity will be controlled by thermal treatments. The obtained materials will be characterized by X-ray diffraction and optical spectroscopy (diffuse reflectance, fluorescence and excitation spectra and kinetics of the metastable levels). Maximal and optimal dopant concentrations will be found. The mechanisms that influence the emission effiency will be elucidated.

OBJECTIVES
1. Synthesis and characterization of crystals from langasite family doped with the pair of rare earths ions Yb-Er for upconversion processes
December 2011:
Activity 1.1 Synthesis of compounds doped with Yb³⁺ and Er³⁺ by sol-gel method and solid- state reaction.
Activity 1.2. Structural characterization of the compounds doped with Yb³⁺ and Er³⁺ by sol-gel method and solid-state reaction.
December 2012:
Activity 1.3. Investigation of emission properties of compounds doped with Yb³⁺ and Er³⁺ by sol-gel method and solid-state reaction.

2. Synthesis and characterization of crystals from langasite family doped with the pair of rare earths ions Yb-Tm for upconversion processes

September 2013:
Activity 2.1. Synthesis of compounds doped with Yb³⁺ and Tm³⁺ by sol-gel method and solid-state reaction.
Activity 2.2. Structural characterization of the compounds doped with Yb³⁺ and Tm³⁺ by sol-gel method and solid-state reaction.
Activity 2.3. Investigation of emission properties of compounds doped with Yb³⁺ and Tm³⁺ by sol-gel method and solid-state reaction

 
RESULTS:
2011:

● Samples from langasite family (langanite, langatate, langasite) doped with the pair of rare earths ions Yb-Er were prepared in order to study the luminescence properties (upconversion processes). The synthesis methods used (sol-gel, solid-phase reaction) are modern and used for the preparation of many oxidic compounds.
● The samples obtained were characterized structurally by X-ray diffraction and electron microscopy. XRD patterns showed that besides the diffraction lines of specific phases, some other lines, are observed (these “extralines” belong mostly to perovskite LaGaO₃ (*) (card PDF-00-041-1103) and β-Ga₂O₃ (+) (card PDF-00-041-1103) phases).
● The analysis of XRD patterns showed that the limit of rare earths’ solubility for langanite and langatate were not exceeded, the dominant phase is LGN, respectively LGT. The XRD patterns for langasite, where the diffraction lines of perovskite are intense, can be interpreted as being due to rare earth concentration exceeding.

2012:
● Langasite nanopowders doped with Er³⁺ and Yb³⁺ were synthesized by a modified Pechini route, for the first time. Ceramic samples (LGT, LGS, LGN doped with Er³⁺ and Yb³⁺) with good phase purity and low concentration of color centers were obtained by solid - state reaction in air.
● The luminescence of the samples was excited in IR, with a laser diode at 973 nm; both green and red luminescence were obtained.
● Under UV pumping (at 365 nm), practically only green luminescence ((²H_11/2, ⁴S_3/2) - ⁴I_15/2) was observed.
● The optimal concentration was established for Yb³⁺ as 4 at.%, for higher concentrations occurring parasite phases.
● The lifetimes of metastabile level increases with temperature treatment due to amorphous-crystalline phase transition and the adsorbed impurities.
● The observed strong luminescence recommends the langasite nanocrystals doped with erbium and ytterbium for upconversion applications.
● The results were published in 3 ISI journals and presented at 6 international conferences.

2013:
● The LGT and LGN ceramic samples doped with Tm and Yb were synthesized by solid-state reaction from high purity oxides (La₂O₃ /Nb₂O₅, Ga₂O₃, Tm₂O₃, Yb₂O₃, Ta₂O₅) for 35 h at 1350ºC.
● The purity of the langatate phase was checked by X ray diffraction. Besides the diffraction lines of LGT phase, some other lines, but with very low intensities, were observed. The “extralines” belong to perovskite LaGaO₃ and β-Ga₂O₃ phases.
● The samples were characterized by optical spectroscopy (luminescence, absorption, decay measurements). The luminescence was excited at 973 nm with various pump powers. When excited in IR, the sample emits UV (360 nm), blue (465 nm), red (650 nm) and infrared (800 nm, 1200 nm and 1700 nm) luminescence.
● The strong blue luminescence of this samples recommends these materials as upconversion blue phosphors.
● The results were presented at International Conference "MODERN LASER APPLICATIONS" Third Edition, INDLAS 2013, 20-24 May 2013 Bran, Romania.
● The doped nanopowders La₃Ga₅SiO₁₄:Yb³⁺(3%):Tm³⁺(1%) were prepared by a Pechini sol-gel method. The nanopowders were annealed in air, at various temperatures (750^oC, 800^oC, 900^oC and 1000^oC) for 5 hours.
● The purity of the langasite phase was checked by X ray diffraction using the Rigaku MiniFlexII (Cu, Kα) diffractometer. . Besides the diffraction lines of LGS phase (card PDF-01-070-7027), some other lines (observed in the XRD spectrum of the sample annealed at 1000^oC), are observed. These “extralines”, denoted with *, belong to the perovskite LaGaO₃ (card PDF-00-041-1103). The nanocrystallite size (the coherence domain) is about 38 nm.
● The samples were characterized by optical spectroscopy (luminescence, absorption, decay measurements). The luminescence of the LGS:Yb:Tm powders was excited in IR, at 973 nm with a diode laser in blue at 476.5 nm with Argon laser. The transition ¹G₄ → ³H₆ is responsible for the blue luminescence while the transition ¹G₄ → ³F₄ is responsible for the red one.
● For decay measurements, the luminescence was excited with the Quantel Rainbow OPO. All the measurements were performed at room temperature.
● The results were presented at the 13th International Balkan Workshop on Applied Physics and Materials Science, 4-6 July 2013, Constanta, Romania.

DISSEMINATION

PAPERS PUBLISHED IN PEER-REVIEW JOURNALS
1. A. M. Voiculescu, S. Georgescu, S. Nastase, C. Matei, D. Berger, C. Matei, A. Stefan, and O. Toma, “Upconversion luminescence of Er³⁺/Yb³⁺ co-doped nanolangasite synthesized by a modified Pechini route,” J. Sol-Gel Sci. Technol. 64 (3), 667-672 (2012).
2. S. Georgescu, A. M. Voiculescu, C. Matei, A. G. Stefan, and O. Toma, “Violet and and near-ultraviolet in La₃Ga_5.5Ta_0.5O₁₄ codoped with Er³⁺ and Yb³⁺,” Physica B 413 , 55-58 (2013).
3. A. M. Voiculescu, S. Georgescu, C. Matei, A. G. Stefan, L. Gheorghe, A. Achim, and F. Voicu, “Infrared-excited red, green, violet and UV luminescence from langasite crystal doped with erbium and ytterbium,” Rom. J. Phys. 58 (1-2), 136-142 (2013).
4. S. Georgescu, A. M. Voiculescu, S. Nastase, A. Zanfir, C. Matei, D. Berger, C. Matei, A. Stefan, and O. Toma, “Luminescence of Eu-doped langasite nanopowders synthesized by a modified Pechini route,” Journal of Luminescence 145, 690-696 (January 2014).

COMMUNICATIONS AT INTERNATIONAL CONFERENCES
1. A. M. Voiculescu, S. Georgescu, S. Nastase, C. Matei, A. Stefan, “Synthesis and characterization of langasite nanopowders doped with erbium and ytterbium”, The 8^th General Conference of Balkan
Physical Union, 5-7 July 2012, Constanţa, Romania (presentation SP-P11).
2. A. M. Voiculescu, S. Georgescu, O. Toma, S. Nastase, “Properties of luminescence for nanopowders compounds from langasite family doped with erbium and ytterbium”, International Conference
on Nanosciences + Technology, July 23-27 2012, Paris, France (presentation PO3-21, Europhysics Conf. Abstract, Vol. 36D, ISBN No 2-914771-77-0, pag. 97)
3. A. M. Voiculescu, S. Georgescu, S. Nastase, O. Toma, C. Matei, A. Stefan, “Upconversion processes in langasite nanopowders doped with with erbium and ytterbium”, 8^th International Conference
of f-elements, August 26-31, 2012, Udine, Italy (presentation OPT 32P)
4. A. M. Voiculescu, S. Georgescu, L. Gheorghe, A. Achim, C. Matei, A. Stefan, “Infrared-excited red, green, violet, and UV luminescence from langasite crystal doped with ebium and ytterbium”,
The 3^rd Internat. Conf. on the Physics of Optical Materials and Devices, ICOM 2102, 3-6 September 2012, Belgrad, Serbia (Book of Abstracts ISBN: 978-86-7306-116-0, pag. 145).
5. S. Georgescu, A. M. Voiculescu, C. Matei, A. G. Stefan, O. Toma, “Upconversion luminescence in La₃Ga_5.5Ta_0.5O₁₄ codoped with Er³⁺ and Yb³⁺”, Internat. Conf. on Optics
“Micro-to-Nanophotonics III”, September 3-6 2012, Bucharest, Romania (presentation I.I.6).
6. A. M. Voiculescu, S. Georgescu, C. Matei, S Nastase, “Upconversion luminescence of nanolangasite doped with Er³⁺ and Yb³⁺”, Physics Conf. TIM-12, Timisoara, 27-30 noiembrie 2012 (oral presentation).
7. A. M. Voiculescu, S. Georgescu, C. Matei, A. Stefan, and R. Birjega, “Upconversion luminescence properties of La₃Ga_5.5Ta_0.5O₁₄ doped with Yb and Tm,” International Conference
"MODERN LASER APPLICATIONS" Third Edition, INDLAS 2013, 20-24 May 2013 Bran, Romania (presentation P5).
8. A. M. Voiculescu, S. Georgescu, C. Matei, A. Stefan, O. Toma, F. Voicu, and R. Birjega, “Synthesis and characterization of La₃Ga_5.5Ta_0.5O₁₄ doped with Yb and Tm for upconversion luminescence”, Materials, Methods & Technologies, 15th Int. Symposium, 10-14 June 2013, Sunny Beach, Bulgaria, presentation - P66.
9. A.M. Voiculescu, S. Georgescu, C. Matei, A. Stefan, and S. Nastase, “Upconversion processes in Langasite nanopowders doped with Thulium and Ytterbium,” The 13th International Balkan Workshop on Applied Physics, 4-6 July 2013, Constanta, Romania, presentation S1-P02, Book of Abstracts p. 47.