Dual Emission of Dysprosium-Doped Hafnium Oxide Charecterization as a Function of the Sintering Temperature

Abstract

The effect of the sintering temperature on the luminescence of trivalent dysprosium ion doped hafnium oxide compounds is studied. Temperature treatments of 400°C, 600°C, 800°C, and 1000°C over a period of 12 hours were used to prepare all the samples. Typical optical transitions of Dy³⁺ ions are observed and properly labeled according to the 2S+1LJ multiplets of this ion. Changes in luminescence behavior and the yellow/blue emissions ratio reveal that doping ions can occupy different sites in the host depending on the sintering temperature. The emission efficiency does not depend exclusively on the sintering temperature but also on the occupied site in the host matrix.

Keywords

Phosphors of metal oxides , Hafnium oxide compounds , Hydrothermal-microwave , Crystalline structures

Introduction

Research on the luminescent properties of lanthanides has increased notably during the last decade. Phosphors of metal oxides doped with rare earth (RE) ions have been synthesized by various techniques, such as precipitation method [1], solid state reaction [2], hydrothermal-microwave [3], coprecipitation [4], ultrasonic pyrolytic spray [5,6], sputtering [7], polyethylene glycol [8], sol-gel [9-11], chemical vapor deposition (CVD) [12], atomic layer deposition (ALD) [13,14], solvent evaporation [15-17], metal organic chemical vapor deposition (MOCVD) [18,19], etc. On the other hand, Hafnium oxide is one of the materials that have been investigated for its mechanical, physical, chemical and electrical properties and with an interest due to its high dielectric constant, its high melting point, its wide band gap and excellent thermal stability [6-19]. The broader band gap of HfO2 effectively suppresses luminescence thermal quenching effects, enhancing HfO2 as an ideal material for a wide range of optoelectronic applications [20-23].

There are different crystalline structures in hafnium oxide such as: monoclinic, tetragonal and cubic, corresponding to the space groups P21/a, P42/nmc and Fm-3 m respectively. High temperatures are required to obtain the tetragonal and cubic phase; ~1700°C for the first and ~2200°C for the latter. These phases can be stabilized at temperatures below 1700°C, when the material is doped with different elements such as Magnesium, Barium, Lanthanum, Strontium, Neodymium, Calcium, Ytterbium, Yttrium, Samarium, Dysprosium, among others. [12,17,24-28].

Rare earth ion doped materials are widely used in the development of lasers, optical amplifiers, optical memory devices, medical lasers, flat panel displays, fluorescent lamps and white LED’s, to give a few examples [29]. In this sense, Dysprosium is one of the most popular RE doping ions due to its characteristic transitions. Two of its dominant transitions in the emission spectrum are one in the Yellow (Y) region centered at 579 nm corresponding to the hypersensitive transition ⁴F_9/2→⁶H_13/2 and a Blue (B) one at 485 nm corresponding to ⁴F_7/2 → ⁶H_11/2 transition. These two emissions are very attractive for modern technology industry because of their potential to create an efficient white light source by simple adjustments of the Y/B ratio [30-34].

Obtaining phosphors by means of the solvent evaporation technique is one of the simplest to be carried out in the laboratory, because it is economical and versatile; the precursors can be dissolved in a suitable solvent; the solvent is evaporated at an appropriate temperature; and phosphors with excellent photoluminescent properties can be synthesized; using this technique, various concentrations of lanthanides can be incorporated during synthesis [35]. In this paper, an approach to improve the luminescence efficiency and optimize the emission ratio Y/B of Dy³⁺ doped HfO2 samples by controlling the sintering temperature is presented.

The results show that for a given sintering temperature the blue emission can be optimized, and that the luminescence technique is a very effective one to distinguish between different crystal sites in one compound; these phosphors were synthesized by evaporation of solvents; were characterized by different techniques, such as X-ray diffraction (XRD), photoluminescence (PL), Energy dispersión spectroscopy of X-rays (EDS) and Raman.

Method

For the sample preparation, typically 12 grams of HfCl4 were dissolved in a solution of 20 ml of deionized water and 20 ml of methanol. Dysprosium doping was performed by adding dysprosium chloride (DyCl3) to the solution. The doping concentration for all the cases was 1 atomic percent. All the reagents were used without any further purification and the solution was vigorously stirred using a magnetic stirrer until a homogenous solution was formed. The obtained solution was continuously heated at 250°C for 20 minutes in order to evaporate solvents. The resulting white precipitate was completely dried at 350°C for ten minutes at ambient atmosphere and then cooled gradually to room temperature. The dried powders were sintered in air at 400, 600, 800 and 1000°C for 12 hours. Subsequently 1.2 cm diameter pellets were made by using a stainless-steel die statically pressing the compounds at about 100 MPa. The samples were characterized measuring the X-ray diffraction (XRD) pattern using a Siemens D-5000 diffractometer with Cu-Kα radiation and a Perkin Elmer LS55 fluorescence spectrometer was used to obtain the emission and excitation spectra; the (EDS) measurements were carried out using an Oxford Pentafet Si-Li detector integrated on a Leica-Cambridge Scanning Electron Microscope model Stereoscan 440 (SEM). Raman results were measured with a WITec combined Confocal Raman Imaging and Atomic Force Microscope System, highest sensitivity for 633-nm excitation wavelength.

Conclusion

Summarizing the important facts from the characterization we have: (1) the activation of Dy³⁺ ions in the HfO2 matrix is clear from the intense emission spectra, (2) HfO2 matrix presents two different phases and they are present for sintering temperatures above 400°C, (3) Dy³⁺-doped samples show the existence of a sintering temperature threshold that triggers the dominance of the monoclinic structure and one temperature for the apparition of orthorhombic structure. The luminescent properties of Dy³⁺ ions in a matrix depend in general on the structure of the closest neighbors and the symmetry of the crystalline formation, (4) the XRD and Raman results indicate that the increase in the temperature of the synthesis of these phosphors, the crystallinity increases and the doping with dysprosium does not modify the Raman and the XRD spectra of the phosphors that are not doped.

Based on the addressed facts from the figures, it is possible to conclude that HfO2 matrix represents a convenient host for Dy³⁺ ions, giving the fact that for sinterization temperatures of 600°C the host matrix provides an adequate symmetry that acts as an active sensitizer giving outstanding conditions for the activation of Dy³⁺ ions and allowing a much more effective excitation process. Moreover, HfO2 matrix exhibits the formation of two phases that allows some tailoring of the standard and special transitions, in particular increasing the yellow over the blue intensity. It has been also shown that the luminescence technique is a very powerful one since it allows distinguishing the presence of different phases, no matter one of them is present in a small amount.

Acknowledgment

This work was supported by PAPIIT, UNAM under project IN102510 and IN110617. The authors would like to thank the technical assistance Ing. Cristina Zorrilla from Instituto de Física, UNAM.

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