Improving lanthanide-based upconversion by overcoming the parity selection rule

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Upconverting materials generate high-energy photons from two or more lower-energy photons. By virtue of this ability, they have the potential to allow solar cells to utilize sub-band gap photons and to enable inherently background-free biological imaging. Lanthanide-based upconverters, which convert near-infrared light to visible light, are particularly well-suited to these applications but remain hindered by poor efficiencies, typically below 1% in nanoparticles. Primarily responsible are their low radiative rates, which stem from the parity-forbidden nature of the constituent transitions. Here, I demonstrate three successful methods of overcoming the parity selection rule, thereby enhancing radiative rates and increasing performance in lanthanide-based upconverting nanoparticles. First, I show how we use mechanical strain to increase orbital mixing and relax the parity selection rule in NaYF4 nanoparticles doped with Yb and Er. Upconversion emission spectra and lifetimes are monitored as the NaYF4:Yb, Er nanoparticles are compressed hydrostatically with a diamond anvil cell. A 1% compressive strain diminishes upconversion emission in the hexagonal-phase nanoparticles but yields nearly a twofold emission enhancement in the cubic-phase particles. I then describe how we use synthetic strain to induce local symmetry distortions. Hexagonal-phase NaYF4:Yb, Er nanoparticles are codoped with Gd and Lu, which have slightly larger and smaller ionic radii than does Y, respectively. Using this approach, a 1.6x upconversion quantum yield enhancement as well as a record efficiency value are achieved. A broad suite of characterization techniques is used to confirm that the particle morphology, global structure, and Yb/Er concentrations are constant throughout the sample series, confirming symmetry distortion to be the source of the quantum yield improvement. Finally, I discuss a method we demonstrate in which energy is shuttled to a fluorescent dye rather than being emitted directly from the upconverter. Here, NaYF4:Yb, Er nanoparticles are decorated with a dye capable of achieving radiative efficiencies above 90%. This sensitization yields size-dependent upconversion quantum yield enhancements as high as 10x. These three broadly implementable approaches for improving upconversion are among the first to successfully overcome the parity selection rule in lanthanides. In conjunction with alternative methods of efficiency enhancement, this work provides a clear path toward realizing the transformative potential of upconversion for photovoltaic and bioimaging applications.


Type of resource text
Form electronic; electronic resource; remote
Extent 1 online resource.
Publication date 2017
Issuance monographic
Language English


Associated with Wisser, Michael D
Associated with Stanford University, Department of Materials Science and Engineering.
Primary advisor Dionne, Jennifer Anne
Primary advisor Salleo, Alberto
Thesis advisor Dionne, Jennifer Anne
Thesis advisor Salleo, Alberto
Thesis advisor Mao, Wendy (Wendy Li-wen)
Advisor Mao, Wendy (Wendy Li-wen)


Genre Theses

Bibliographic information

Statement of responsibility Michael D. Wisser.
Note Submitted to the Department of Materials Science and Engineering.
Thesis Thesis (Ph.D.)--Stanford University, 2017.
Location electronic resource

Access conditions

© 2017 by Michael David Wisser
This work is licensed under a Creative Commons Attribution Non Commercial 3.0 Unported license (CC BY-NC).

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