Lanthanide-doped upconversion (UC) phosphors absorb low-energy infrared light and convert it into higher-energy visible light. Despite over 10 years of development, it has not been possible to synthesize nanocrystals (NCs) with UC efficiencies on a par with what can be achieved in bulk materials. To guide the design and realization of more efficient UC NCs, a better understanding is necessary of the loss pathways competing with UC. Here we study the excited-state dynamics of the workhorse UC material β-NaYF4 co-doped with Yb3+ and Er3+. For each of the energy levels involved in infrared-to-visible UC, we measure and model the competition between spontaneous emission, energy transfer between lanthanide ions, and other decay processes. An important quenching pathway is energy transfer to high-energy vibrations of solvent and/or ligand molecules surrounding the NCs, as evidenced by the effect of energy resonances between electronic transitions of the lanthanide ions and vibrations of the solvent molecules. We present a microscopic quantitative model for the quenching dynamics in UC NCs. It takes into account cross-relaxation at high lanthanide-doping concentration as well as Förster resonance energy transfer from lanthanide excited states to vibrational modes of molecules surrounding the UC NCs. Our model thereby provides insight in the inert-shell thickness required to prevent solvent quenching in NCs. Overall, the strongest contribution to reduced UC efficiencies in core–shell NCs comes from quenching of the near-infrared energy levels (Er3+: 4I11/2 and Yb3+: 2F5/2), which is likely due to vibrational coupling to OH– defects incorporated in the NCs during synthesis.
The rapid development of nanomaterials with unique size-tunable properties forms the basis for a variety of new applications, including temperature sensing. Luminescent nanoparticles (NPs) have demonstrated potential as sensitive nanothermometers, especially in biological systems. Their small size offers the possibility of mapping temperature profiles with high spatial resolution. The temperature range is however limited, which prevents use in high-temperature applications such as, for example, nanoelectronics, thermal barrier coatings, and chemical reactors. In this work, we extend the temperature range for nanothermometry beyond 900 K using silica-coated NaYF4 nanoparticles doped with the lanthanide ions Yb3+ and Er3+. Monodisperse ∼20 nm NaYF4:Yb,Er nanocrystals were coated with a ∼10 nm silica shell. Upon excitation with infrared radiation, bright green upconversion (UC) emission is observed. From the intensity ratio between 2H11/2 and 4S3/2 UC emission lines at 520 and 550 nm, respectively, the temperature can be determined up to at least 900 K with an accuracy of 1–5 K for silica-coated NPs. For bare NaYF4:Yb,Er NPs, the particles degrade above 600 K. Repeated thermal cycling experiments demonstrate the high durability and reproducibility of the silica-coated nanocrystals as temperature probes without any loss of performance. The present results open avenues for the development of a new class of highly stable nanoprobes by applying a silica coating around a wide variety of lanthanide-doped NPs.
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.
Bandshape luminescence thermometry during in situ temperature measurements has been reported by preparing three catalytically relevant systems, which show temperature-dependent luminescence. One of these systems was further investigated as a showcase for application. Microcrystalline NaYF4 doped with Er3+ and Yb3+ was mixed with a commercial zeolite H-ZSM-5 to investigate the Methanol-to-Hydrocarbons (MTH) reaction, while monitoring the reaction products with online gas chromatography. Due to the exothermic nature of the MTH reaction, a front of increased temperature migrating down the fixed reactor bed was visualized, showing the potential for various applications of luminescence thermometry for in situ measurements in catalytic systems.
The temperature-sensitive luminescence of nanoparticles enables their application as remote thermometers. The size of these nanothermometers makes them ideal to map temperatures with a high spatial resolution. However, high spatial resolution mapping of temperatures >373 K has remained challenging. Here, we realize nanothermometry with high spatial resolutions at elevated temperatures using chemically stable upconversion nanoparticles and confocal microscopy. We test this method on a microelectromechanical heater and study the temperature homogeneity. Our experiments reveal distortions in the luminescence spectra that are intrinsic to high-resolution measurements of samples with nanoscale photonic inhomogeneities. In particular, the spectra are affected by the high-power excitation as well as by scattering and reflection of the emitted light. The latter effect has an increasing impact at elevated temperatures. We present a procedure to correct these distortions. As a result, we extend the range of high-resolution nanothermometry beyond 500 K with a precision of 1–4 K. This work will improve the accuracy of nanothermometry not only in micro- and nanoelectronics but also in other fields with photonically inhomogeneous substrates.
Ultrathin two-dimensional (2D) nanomaterials have attracted intense research efforts due to their extraordinary optoelectronic properties. However, the nucleation and growth mechanisms of 2D colloidal nanosheets are still poorly understood. Here, we follow the formation of ultrathin colloidal Cu2–x S nanosheets by in situ small-angle X-ray scattering. While thermal decomposition of copper–dodecanethiolates produces spheroidal Cu2–xS nanocrystals, the addition of chloride to the reaction mixture results in 2 nm thick Cu 2–x S nanosheets with well-defined shape and size. Our results show that chloride stabilizes stacks of lamellar copper–thiolate supramolecular complexes, so that they remain intact beyond the onset of Cu2–x S nucleation at 230 °C, leading to 2D-constrained stack-templated nucleation and growth. The face-to-face stacking of the nanosheets reinforces the 2D constraints imposed by the lamellar soft template, since it prevents internanosheet mass transport and nanosheet coalescence, thereby inhibiting growth in the thickness direction and allowing only for lateral growth. Our work thus provides novel insights into soft-templating formation mechanisms of ultrathin colloidal nanosheets, which may be exploited for other metal sulfide compositions.
In this work we present 3 showcases that luminescence thermometry is a promising and versatile technique for temperature monitoring in various microfluidic devices.
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