5 research outputs found
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Improving Quantum Yield of Upconverting Nanoparticles in Aqueous Media via Emission Sensitization
We
demonstrate a facile method to improve upconversion quantum
yields in Yb,Er-based nanoparticles via emission dye-sensitization.
Using the commercially available dye ATTO 542, chosen for its high
radiative rate and significant spectral overlap with the green emission
of Er<sup>3+</sup>, we decorate the surfaces of sub-25 nm hexagonal-phase
NaÂ(Y/Gd/Lu)<sub>0.8</sub>F<sub>4</sub>:Yb<sub>0.18</sub>Er<sub>0.02</sub> upconverting nanoparticles with varying dye concentrations. Upconversion
photoluminescence and absorption spectroscopy provide experimental
confirmation of energy transfer to and emission from the dye molecules.
Upconversion quantum yield is observed to increase with dye sensitization,
with the highest enhancement measured for the smallest particles investigated
(10.9 nm in diameter); specifically, these dye-decorated particles
are more than 2× brighter than are unmodified, organic-soluble
nanoparticles and more than 10× brighter than are water-soluble
nanoparticles. We also observe 3× lifetime reductions with dye
adsorption, confirming the quantum yield enhancement to result from
the high radiative rate of the dye. The approach detailed in this
work is widely implementable, renders the nanoparticles water-soluble,
and most significantly improves sub-15 nm nanoparticles, making our
method especially attractive for biological imaging applications
Strain-Induced Modification of Optical Selection Rules in Lanthanide-Based Upconverting Nanoparticles
NaYF<sub>4</sub>:Yb<sup>3+</sup>,Er<sup>3+</sup> nanoparticle upconverters
are hindered by low quantum efficiencies arising in large part from
the parity-forbidden nature of their optical transitions and the nonoptimal
spatial separations between lanthanide ions. Here, we use pressure-induced
lattice distortion to systematically modify both parameters. Although
hexagonal-phase nanoparticles exhibit a monotonic decrease in upconversion
emission, cubic-phase particles experience a nearly 2-fold increase
in efficiency. In-situ X-ray diffraction indicates that these emission
changes require only a 1% reduction in lattice constant. Our work
highlights the intricate relationship between upconversion efficiency
and lattice geometry and provides a promising approach to modifying
the quantum efficiency of any lanthanide upconverter
Enhancing Quantum Yield via Local Symmetry Distortion in Lanthanide-Based Upconverting Nanoparticles
Lanthanide-based
upconverting nanoparticles exhibit significant
promise for solar energy generation, biological imaging, and security
technologies but have not seen widespread adoption due to the prohibitively
low efficiencies of current materials. Weak transition dipole moments
between 4f orbitals hinder both photon absorption and emission. Here,
we introduce a novel way to increase the radiative transition rates
in Yb,Er-based upconverting nanoparticles based on local symmetry
distortion. Beginning from a host matrix of the well-studied hexagonal
(β)-phase NaYF<sub>4</sub>, we incrementally remove Y<sup>3+</sup> ions and cosubstitute for them a 1:1 mixture of Gd<sup>3+</sup> and
Lu<sup>3+</sup>. These two ions act to expand and contract the lattice,
respectively, inducing local-level distortion while maintaining the
average host structure. We synthesize a range of β-NaY<sub>0.8–2<i>x</i></sub>Gd<sub><i>x</i></sub>Lu<sub><i>x</i></sub>F<sub>4</sub>:Yb<sub>0.18</sub>Er<sub>0.02</sub> nanoparticles
and experimentally confirm that particle size, phase, global structure,
and Yb<sup>3+</sup> and Er<sup>3+</sup> concentrations remain constant
as <i>x</i> is varied. Upconversion quantum yield is probed
as the degree of cosubstitution is varied from <i>x</i> =
0 to <i>x</i> = 0.24. We achieve a maximum quantum yield
value of 0.074% under 63 W/cm<sup>2</sup> of excitation power density,
representing a 1.6× enhancement over the unmodified particles
and the highest measured value for near-infrared-to-visible upconversion
in sub-25 nm unshelled nanoparticles. We also investigate upconversion
emission at the single-particle level and report record improvements
in emission intensity for sub-50 nm particles. Radiative rate enhancements
are confirmed by measuring excited-state lifetimes. The approach described
herein can be used in combination with more established methods of
efficiency improvement, such as adding passivating shells or coupling
to plasmonic nanoattenas, to further boost the upconversion quantum
yield
Strain-Induced Modification of Optical Selection Rules in Lanthanide-Based Upconverting Nanoparticles
NaYF<sub>4</sub>:Yb<sup>3+</sup>,Er<sup>3+</sup> nanoparticle upconverters
are hindered by low quantum efficiencies arising in large part from
the parity-forbidden nature of their optical transitions and the nonoptimal
spatial separations between lanthanide ions. Here, we use pressure-induced
lattice distortion to systematically modify both parameters. Although
hexagonal-phase nanoparticles exhibit a monotonic decrease in upconversion
emission, cubic-phase particles experience a nearly 2-fold increase
in efficiency. In-situ X-ray diffraction indicates that these emission
changes require only a 1% reduction in lattice constant. Our work
highlights the intricate relationship between upconversion efficiency
and lattice geometry and provides a promising approach to modifying
the quantum efficiency of any lanthanide upconverter
Upconverting Nanoparticles as Optical Sensors of Nano- to Micro-Newton Forces
Mechanical forces affect a myriad
of processes, from bone growth
to material fracture to touch-responsive robotics. While nano- to
micro-Newton forces are prevalent at the microscopic scale, few methods
have the nanoscopic size and signal stability to measure them in vivo
or in situ. Here, we develop an optical force-sensing platform based
on sub-25 nm NaYF<sub>4</sub> nanoparticles (NPs) doped with Yb<sup>3+</sup>, Er<sup>3+</sup>, and Mn<sup>2+</sup>. The lanthanides Yb<sup>3+</sup> and Er<sup>3+</sup> enable both photoluminescence and upconversion,
while the energetically coupled <i>d</i>-metal Mn<sup>2+</sup> adds force tunability through its crystal field sensitivity. Using
a diamond anvil cell to exert up to 3.5 GPa pressure or ∼10
μN force per particle, we track stress-induced spectral responses.
The red (660 nm) to green (520, 540 nm) emission ratio varies linearly
with pressure, yielding an observed color change from orange to red
for α-NaYF<sub>4</sub> and from yellow–green to green
for <i>d</i>-metal optimized β-NaYF<sub>4</sub> when
illuminated in the near infrared. Consistent readouts are recorded
over multiple pressure cycles and hours of illumination. With the
nanoscopic size, a dynamic range of 100 nN to 10 μN, and photostability,
these nanoparticles lay the foundation for visualizing dynamic mechanical
processes, such as stress propagation in materials and force signaling
in organisms