Title: Small Alkaline-Earth-based Core/Shell Nanoparticles for Efficient Upconversion

Abstract

The optical efficiency of lanthanide-based upconversion is intricately related to the crystalline host lattice. Different crystal fields interacting with the electron clouds of the lanthanides can significantly affect transition probabilities between the energy levels. Here, we investigate six distinct alkaline-earth rare-earth fluoride host materials (M_1-xLn_xF_2+x, MLnF) for infrared-to-visible upconversion, focusing on nanoparticles of CaYF, CaLuF, SrYF, SrLuF, BaYF, and BaLuF doped with Yb³⁺ and Er³⁺. We first synthesize ~5 nm upconverting cores of each material via a thermal decomposition method. Then we introduce a dropwise hot-injection method to grow optically inert MYF shell layers around the active cores. Five distinct shell thicknesses are considered for each host material, resulting in 36 unique, monodisperse upconverting nanomaterials each with size below ~15 nm. The upconversion quantum yield (UCQY) is measured for all core/shell nanoparticles as a function of shell thickness and compared with hexagonal (β-phase) NaGdF₄, a traditional upconverting host lattice. While the UCQY of core nanoparticles is below the detection limit (<10-5%), it increases by 4 to 5 orders of magnitude as the shell thickness approaches 4-6 nm. The UCQY values of our cubic MLnF nanoparticles meet or exceed the β-NaGdF₄ reference sample. Across all core/shell samples, SrLuF nanoparticles are themore » most efficient, with UCQY values of 0.53% at 80 W/cm² for cubic nanoparticles with ~11 nm edge length. This efficiency is 5 times higher than our β-NaGdF₄ reference material with comparable core size and shell thickness. Our work demonstrates efficient and bright upconversion in ultrasmall alkaline-earth-based nanoparticles, with applications spanning biological imaging and optical sensing.« less

Authors:

Fischer, Stefan  ^[1];

• Search DOE PAGES for author "Fischer, Stefan"
• Search DOE PAGES for ORCID "0000-0003-4110-6576"

Mehlenbacher, Randy D. ^[1]; Lay, Alice  ^[1];

• Search DOE PAGES for author "Lay, Alice"
• Search DOE PAGES for ORCID "0000-0001-5179-7150"

Siefe, Chris ^[1]; Alivisatos, A. Paul  ^[2];

• Search DOE PAGES for author "Alivisatos, A. Paul"
• Search DOE PAGES for ORCID "0000-0001-6895-9048"

Dionne, Jennifer A. ^[1]

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+ Show Author Affiliations

1. Stanford Univ., CA (United States)
2. Univ. of California, Berkeley, CA (United States); Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States); Kavli Energy NanoScience Inst., Berkeley, CA (United States)

Publication Date:

Mon May 06 00:00:00 EDT 2019

Research Org.:

Energy Frontier Research Centers (EFRC) (United States). Photonics at Thermodynamic Limits (PTL); Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)

Sponsoring Org.:

USDOE Office of Science (SC), Basic Energy Sciences (BES)

OSTI Identifier:

1545151

Grant/Contract Number:  

AC02-05CH11231

Resource Type:

Accepted Manuscript

Journal Name:

Nano Letters

Additional Journal Information:

Journal Volume: 19; Journal Issue: 6; Journal ID: ISSN 1530-6984

Publisher:

American Chemical Society

Country of Publication:

United States

Language:

English

Subject:

36 MATERIALS SCIENCE