Upconversion (UC) is a promising
option to enhance the efficiency
of solar cells by conversion of sub-bandgap infrared photons to higher
energy photons that can be utilized by the solar cell. The UC quantum
yield is a key parameter for a successful application. Here the UC
luminescence properties of Er3+-doped Gd2O2S are investigated by means of luminescence spectroscopy,
quantum yield measurements, and excited state dynamics experiments.
Excitation into the maximum of the 4I15/2 → 4I13/2 Er3+ absorption band around 1500
nm induces very efficient UC emission from different Er3+ excited states with energies above the silicon bandgap, in particular,
the emission originating from the 4I11/2 state
around 1000 nm. Concentration dependent studies reveal that the highest
UC quantum yield is realized for a 10% Er3+-doping concentration.
The UC luminescence is compared to the well-known Er3+-doped
β-NaYF4 UC material for which the highest UC quantum
yield has been reported for 25% Er3+. The UC internal quantum
yields were measured in this work for Gd2O2S:
10%Er3+ and β-NaYF4: 25%Er3+ to be 12 ± 1% and 8.9 ± 0.7%, respectively, under monochromatic
excitation around 1500 nm at a power of 700 W/m2. The UC
quantum yield reported here for Gd2O2S: 10%Er3+ is the highest value achieved so far under monochromatic
excitation into the 4I13/2 Er3+ level.
Power dependence and lifetime measurements were performed to understand
the mechanisms responsible for the efficient UC luminescence. We show
that the main process yielding 4I11/2 UC emission
is energy transfer UC.
Doping quantum dots (QDs) with lanthanide ions is promising to combine the efficient sharp line emission of lanthanides with the strong and size-tunable absorption of QDs. Incorporating lanthanide ions in II-VI QDs remains challenging, however, here we report successful coupling of CdSe QDs with the lanthanide ion Yb(3+). Our spectroscopic results demonstrate that Yb(3+) ions are first adsorbed on the CdSe surface and subsequently incorporated in the nanocrystalline semiconductor particles by growing a Se shell. Evidence for incorporation is provided by the fine structure of the CdSe QDs absorption in the excitation spectrum of the Yb(3+) emission at 1000 nm and the long lifetime of the Yb(3+) emission after shell overgrowth. Sensitized Yb(3+) infrared emission may find application in optical amplifiers, solar concentrators, and bioimaging. The method described is a promising strategy for incorporating lanthanide ions in other II-VI QDs.
We hereby present
experimental and theoretical insights on the
use of biomineralized magnetite nanoparticles, called magnetosomes,
as heat nanoinductors in the magnetic hyperthermia technique. The
heating efficiency or specific absorption rate of magnetosomes extracted
from Magnetospirillum gryphiswaldense bacteria and
immersed in water and agarose gel, was directly determined from the
hysteresis loops obtained at different frequencies and magnetic field
amplitudes. We demonstrate that heat production of magnetosomes can
be predicted in the framework of the Stoner–Wohlfarth theory
of uniaxial magnetic anisotropy subjected to significant dipolar interactions,
which can be described in terms of an interaction anisotropy superimposed
to that of each particle. Based on these findings, we propose optimal
magnetic field amplitude and frequency values in order to maximize
the heat production while keeping the undesired eddy current effects
below safe and tolerable limits. The efficiency of magnetosomes as
heat generators and their impact on cell viability has been checked
in macrophage cells. Our results clearly indicate that the hyperthermia
treatment causes both cell death and inhibition of cell proliferation.
Specifically, only 36% of the treated macrophages remained alive 2
h after alternating magnetic field exposure, and 24 h later the percentage
fell to 22%.
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