Upconverting SrF2:Er3+ Nanoparticles for Optical Temperature Sensors

The hexagonal phase β-NaGdF4:x% Tb3+ (x = 0 to 5%) phosphor crystals with uniform morphology and considerable monodispersity were synthesized via a tri-sodium citrate-assisted hydrothermal route The photoluminescence studies showed the strong emission lines corresponding to 5D4 → 7FJ (J = 6, 5, 4, 3) transitions with a prominent 5D4 → 7F5 green emission transition at 540 nm The as-prepared samples were annealed at different temperatures (200–800 °C), and the effect of thermal treatment on their structural, morphological, surface, photoluminescence and thermoluminescence properties were studied A comparative study of pristine and annealed samples revealed phase alteration to the mixed (ie cubic and hexagonal) phase along with morphology transformation at higher annealing temperatures The photoluminescence emission intensity was remarkably enhanced (∼9 times) at 700 °C owing to the reduction in non-radiative processes from the surface and the volatilization of luminescence quenching (–OH) groups, which were corroborated by surface FTIR studies The systematic study of the optimum concentrations of impurities and thermal treatment were studied to achieve high thermoluminescence (TL) efficiency The best TL dosimetric glow curve was obtained for NaGdF4:3% Tb3+ annealed at 700 °C, which peaked at 230 °C, signifying deeper traps The NaGdF4:3% Tb3+ phosphor showed a linear response over a very wide range of gamma doses from 100 Gy to 20 kGy, revealing that the NaGdF4:Tb3+ phosphor was quite well suited for high gamma dose measurements and their respective applications Fading and reproducibility studies showed low fading and excellent reproducibility characteristics The Tm–Tstop technique in combination with the Initial Rise Method (IRM) was used to identify the number of overlapped TL glow peaks and trap level analysis The kinetic parameters, such as activation energy (E), frequency factor (s), and order of kinetics (b), were estimated by the Glow Curve Deconvolution (GCD) method The Figure of Merit (FOM) value was found to be 22% The trapping parameters resulting from the experimental Tm–Tstop study and theoretical GCD match closely and are in good agreement with each other Thus, along with high luminescence efficiency, the newly explored strong thermoluminescence properties of NaGdF4:Tb3+ make them potential phosphor materials and could provide new avenues into other applications in radiation dosimetry such as agriculture/food sciences, archeological sciences, space dosimetry, etc

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Investigation of the photoluminescence and novel thermoluminescence dosimetric properties of NaGdF4:Tb3+ phosphors

Fabrication of broad-spectrum response photocatalysts in a controllable manner that can efficiently utilize solar light as much as possible remains a top priority target yet a challenging task. In this work, a facile approach of synchronous bottom-up growth was employed to construct UCNP/NMIL(Ti) nanohybrids via NH2-BDC (NH2-MIL-125-ligand) linkage between upconversion NaYbF4:Tm3+ nanoparticles (~13-nm) and NH2-MIL-125. The ligand sharing integration strategy generated abundant oxygen vacancies and coordinatively-unsaturated-metal sites, efficiently enhanced light absorption, energy-transfer upconversion (UC-PL), photo-induced e-/h+ generation, and their separation (CV, EIS, Photocurrent-density). Consequently, UCNP/NMIL(Ti) exhibited excellent acetaldehyde photodegradation activity, with 12.5–77.0-folds higher rate constants (ka) than those of state-of-the-art photocatalysts under visible light and high humidity. Furthermore, the photocatalytic activity of UCNP/NMIL(Ti) only declined from 96.0% to 92.5% after five cycling-runs, demonstrating its remarkable photo-catalytic stability. Considering these attributes, the current strategy provides an avenue to synthesize efficient UCNPs/MOF composites for widespread usage in a variety of commercial applications. 

Ligand-sharing Growth of Upconversion UCNP(NaYbF4:Tm3+)/NMIL(Ti) Nanohybrids with Extended Light Absorbance for Acetaldehyde Photodegradation under High Humidity

ABSTRACT The influence of Mn2+ ion concentration (x = 0–20 mol%) as well as the role of thermal-annealing temperature (400–600°C) on the structural as well as luminescence properties of NaYF4:Yb, Er (Y: 78-x%, Yb: 20%, Er: 2%) microcrystals prepared via a coprecipitation method is investigated. The cubic phase of the as-prepared NaYF4:Yb, Er (Y: 78%, Yb: 20%, Er: 2%) was found to remain intact upon the addition of the Mn2+ ions, but the thermal-annealing elucidates that the phase of the sample depends upon the annealing temperature as well as the Mn2+ ion concentration. Among the Mn2+ ion co-doped samples, 3 mol% doped samples dominant to have a maximum positive influence on the upconversion luminescence of the sample, and a further increase in concentration leads to the concentration-induced quenching of the upconversion luminescence. Moreover, the enhancement factor of green ( ), as well as red ( ) emission, depend upon the annealing temperature, with a maximum enhancement factor of 5 and 3.12 times for the sample annealed at 400°C, 8.6 and 7.25 times for the sample annealed at 500°C, and 6 and 4 times for the sample annealed at 600°C, as compared to Mn2+ ion undoped samples. The maximum emission strength for the green as well as red is observed for the sample annealed at 600°C and co-doped with 3 mol Mn2+ ions. The laser power-dependent study on all the samples shows that the upconversion process is a multi-photon process, predominantly a two-photon process. Graphical abstract

Thermal Annealing and Doping Induced Tailoring of Phase and Upconversion Luminescence of NaYF4:Yb Er Microcrystals

808 nm triggered multifunctional UCNPs@PDA nanocomposites for temperature sensing and photothermal conversion

SrF2 based upconversion nanoparticles (UCNPs) were synthesized by hydrothermal method using ethylenediaminetetraacetic acid (EDTA) as surfactant, by doping with single-lanthanide-activator Er ion and sensitizer-activator Yb3+−Er3+ ion-pair. The excited state absorption (ESA) and energy transfer upconversion (ETU) properties were investigated respectively by synthesizing SrF2:2%Er and SrF2:20%Yb,2%Er nanoparticles. EDTA aided hydrothermal synthesis resulted in nano-rod, nano-plate and nano-sphere morphological particles. The hydrothermally grown SrF2:Yb,Er UCNPs are formed as nano-spheres in size ~ 30 to 60 nm. It gives a high specific surface area of 47.17 to 23.58 m2 g−1. The nano-spherical SrF2:Yb,Er resulted an intense red upconversion emission at 660 nm, under the near-infrared (NIR) 980 nm excitation, due to its high specific surface area and ETU mechanism. The post-calcined SrF2:Yb3+,Er3+ gives better red-to-green emission intensity ratio compared to the as-prepared counterparts. SrF2:Er3+ gives very weak red emission due to ESA mechanism. The optical energy gap of SrF2:Yb,Er UCNP was calculated using the barycentre of green emission at 524 and 542 nm from the thermally coupled energy levels of 2H11/2 and 4S3/2. It gives an optical energy gap of 766 cm−1 and the estimated thermal sensitivity is 1.22% K−1 at 300 K. The result is in agreement with other published reports. We also demonstrated the NIR-to-NIR (980-to-850 nm) upconversion emission in the SrF2:Yb3+, Er3+ nanoparticle.

EDTA functionalization of SrF2:Yb,Er nanoparticles by hydrothermal synthesis: Intense red upconversion, NIR-to-NIR emission and luminescence nanothermometry characteristics

Polycrystalline Cu(In,Ga)Se2 (CIGSe) thin-film solar cells exhibit gradual onset in their external quantum efficiency (EQE) spectra whose shape can be affected by various CIGSe material properties. Apart from influences on the charge-carrier collection, a broadening of the EQE onset leads to enhanced radiative losses in open-circuit voltage (Voc). In the present work, Gaussian broadening of parameters describing the EQE onset of thin-film solar cells, represented by the standard deviation, 𝜎total, was evaluated to study the impacts of the effective band-gap energy, the electron diffusion length, and the Ga/In gradient in the CIGSe absorber. It is shown that 𝜎total can be disentangled into contributions of these material properties, in addition to a residual component 𝜎residual. Effectively, 𝜎total depends only on a contribution related to the Ga/In gradient as well as on 𝜎residual. The present work highlights the connection of this compositional gradient, the microstructure in the polycrystalline CIGSe absorber, and the luminescence emission with the residual component 𝜎residual. It is demonstrated that a flat band-gap with no compositional gradient in the bulk of the CIGSe absorber is essential to obtain the lowest 𝜎total values and thus result in lower recombination losses and gains in Voc. 

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Effects of material properties of band‐gap‐graded Cu(In,Ga)Se <sub>2</sub> thin films on the onset of the quantum efficiency spectra of corresponding solar cells

Sinju Thomas

Papers

EDTA functionalization of SrF2:Yb,Er nanoparticles by hydrothermal synthesis: Intense red upconversion, NIR-to-NIR emission and luminescence nanothermometry characteristics

Effects of material properties of band‐gap‐graded Cu(In,Ga)Se <sub>2</sub> thin films on the onset of the quantum efficiency spectra of corresponding solar cells