Ultraviolet, blue and singular green upconversion from Gd2Hf2O7 nanocrystals through dopant manipulation
Graphical abstract
Introduction
Upconversion is a phenomenon wherein low energy photon is converted into high energy photon via multiple photon absorption. It has been found applications in areas of high technological importance such as solar energy, anti-counterfeiting, bioimaging, water disinfection, solid state lighting, security, night vision, optoelectronics, and optical communication, etc. [1,2] Among upconvertible phosphors, rare earth doped nanocrystals (NCs) are considered most efficient photon up-converters compared to nonlinear crystals and other conventional materials such as organic dyes or quantum dots as they enable efficient photon upconversion endowed by physically existing energy levels which are present in abundance [1]. Moreover, rare earth doped upconvertible nanophosphors have also demonstrated good photostability, multicolor emission, wide anti-stokes shift, and low laser power threshold, etc. [3,4].
Most of the reports pertaining to efficient unconvertible NCs are reported with fluoride hosts owing to their lower phonon energy. Though that is a great advantage, they would be highly toxic and unstable compared to oxide crystals. Also, their synthesis in normal laboratory conditions is quite complex and not very environmentally benign [4]. Moreover, there is an issue of inferior performance of fluoride based unconvertible materials at low laser power [4]. We believe that upconversion (UC) potentials can be harnessed with favorable host lattices such as pyrochlore Gd2Hf2O7 (GHO) owing to their moderate phonon energy, chemical/thermal/mechanical stability, ability to accommodate large and several kinds of dopant ions at both A and B sites, and structural flexibility. These features make them good luminescence hosts for both upconversion and downconversion luminescence (DCL and UCL) [5,6]. Owing to importance associated with ultraviolet (UV) light in inactivating virus and bacteria, materials which can emit intense UV light can be a boon to society. At the same time, visible photons have a variety of applications encompassing all spheres of health, energy and environment. Owing to the global material crunch, if a single material can emit both UV and visible light, it not only is viable to global economy but would lead to fascinating and new dimensions in optical research regime. Most of visible UCL from the Er3+-Yb3+ combination has green and red components together, which enables the utilization of the potential of neither green nor red light. Hence, there is an urgent need of generating phosphors which emit singular bright green or red light so cross-contamination can be avoided [[7], [8], [9], [10], [11]].
Here in this work, we generated singular green emission from GHO NCs doped with 1.0mol% Er3+ and sensitized by 20mol% of Yb3+ (GHO-YE) synthesized by a molten salt method [12]. The same lattice could also be harnessed for generating multiple near UV bands and bright blue color by replacing Er3+ by Tm3+ ion as GHO-YT NCs. The details related to the synthesis and instrumentation of the study has been mentioned in the Supplementary Information as S1 and S2.
Section snippets
Results and discussion
Fig. 1a shows XRD patterns of the GHO-YE and GHO-YT NCs. All the peaks correspond to cubic pyrochlore and match with reported XRD pattern of Gd2Hf2O7 [5]. The XRD patterns showed well-defined sharp peaks highlighting the high crystallinity of the GHO-YE and GHO-YT NCs. The lattice parameter and crystallite size obtained from the XRD data were tabulated in Table 1. The lattice parameter was half of the reported value for ideal pyrochlore which agrees well with its defect fluorite structure based
Conclusion
To overcome the limitations of dual color emission in phosphor and to make NIR to UV light emission commercially more viable, here in this work we designed rare earth co-doped Gd2Hf2O7 nanocrystals. A unique design strategy was adopted wherein Yb3+ and Er3+ were directly doped into Gd2Hf2O7 pyrochlore lattice using a molten alt synthesis employing alkali metal nitrate salts. The designed GHO-YE and GHO-YT NCs were characterized using XRD, Raman, FESEM and photoluminescence. We could
Author statement
Santosh K. Gupta: Conceptualization, Data curation, Formal analysis, Visualization, Writing – original draft, Writing – review & editing. Mitzy A Penilla Garcia: Methodology and Data Curation. Yuanbing Mao: Conceptualization, Supervision, Writing – review & editing
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
YM would like to thank the support by the National Science Foundation under CHE (award #1952803) and the IIT startup funds. SKG thanks the United States-India Education Foundation (USIEF) and the Institute of International Education (IIE) for his Fulbright Nehru Postdoctoral Fellowship (Award#2268/FNPDR/2017).
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