Thermal Annealing and Doping Induced Tailoring of Phase and Upconversion Luminescence of NaYF₄:Yb Er Microcrystals

ABSTRACT

The influence of Mn²⁺ ion concentration (x = 0–20 mol%) as well as the role of thermal-annealing temperature (400–600°C) on the structural as well as luminescence properties of NaYF₄:Yb, Er (Y: 78-x%, Yb: 20%, Er: 2%) microcrystals prepared via a coprecipitation method is investigated. The cubic phase of the as-prepared NaYF₄:Yb, Er (Y: 78%, Yb: 20%, Er: 2%) was found to remain intact upon the addition of the Mn²⁺ ions, but the thermal-annealing elucidates that the phase of the sample depends upon the annealing temperature as well as the Mn²⁺ ion concentration. Among the Mn²⁺ ion co-doped samples, 3 mol% doped samples dominant to have a maximum positive influence on the upconversion luminescence of the sample, and a further increase in concentration leads to the concentration-induced quenching of the upconversion luminescence. Moreover, the enhancement factor of green (${ }^2{\mathrm{H}}_{11/2}{\to }^4{\mathrm{I}}_{15/2}\mathrm{a}\mathrm{n}{\mathrm{d}}^4{\mathrm{S}}_{3/2}{\to }^4{\mathrm{I}}_{15/2}$), as well as red (${ }^4{\mathrm{F}}_{9/2}{\to }^4{\mathrm{I}}_{15/2}$) emission, depend upon the annealing temperature, with a maximum enhancement factor of 5 and 3.12 times for the sample annealed at 400°C, 8.6 and 7.25 times for the sample annealed at 500°C, and 6 and 4 times for the sample annealed at 600°C, as compared to Mn²⁺ ion undoped samples. The maximum emission strength for the green as well as red is observed for the sample annealed at 600°C and co-doped with 3 mol Mn²⁺ ions. The laser power-dependent study on all the samples shows that the upconversion process is a multi-photon process, predominantly a two-photon process.

Graphical abstract

KEYWORDS:

• Upconversion luminescence
• co-precipitation method
• co-doping
• luminescence enhancement
• thermal-annealing

Acknowledgments

SDG also acknowledges the Science and Engineering Research Board, DST, Government of India for the financial assistance through the project DST/EMR/2016/002424 and Board of Research and Nuclear Sciences, DAE, Government of India for the financial assistance through the project BRNS/37(3)/14/15/2016-BRNS/37227.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Supplementary material

Supplemental data for this article can be accessed on the publisher’s website.

Additional information

Funding

We gratefully acknowledge financial support from the joint Manipal Academy of Higher Education and FIST program of the Government of India (DST-FIST program of the Government of India SR/FST/PSI-174/2012). SHN acknowledges the Manipal Academy of Higher Education for the Dr. TMA Pai Ph.D. Scholarship;Science and Engineering Research Board, DST, Government of India [DST/EMR/2016/002424];Board of Research and Nuclear Sciences, DAE, Government of India [BRNS/37(3)/14/15/2016-BRNS/37227];