Quest to enhance up-conversion efficiency: a comparison of anhydrous vs. hydrous synthesis of NaGdF4: Yb3+ and Tm3+ nanoparticles

Abstract

A major challenge in the field of up-converting (UC) nanomaterials is to enhance their efficiencies. The-OH defects on the surface of the nanoparticles are thought to be the main cause of luminescence quenching, but there are no comparative studies in the literature showing the impact of anhydrous versus hydrous synthesis on up-conversion efficiency. In this article, we present the synthesis of up-converting NaGdF4: Yb +3 , Tm +3 nanoparticles by two different methods: Thermal decomposition of single source metal-organic anhydrous precursors [NaLn(TFA)4(diglyme)] (Ln 2 = Gd, Tm, Yb; TFA = trifluoroacetate) and room temperature co-precipitation using hydrated inorganic salts Ln(NO3)3ꞏ5H2O (Ln = Gd, Tm, Yb), NaNO3 and NH4F in ethylene glycol. After a detailed study on the influence of solvents and the percentage of lanthanide dopant on the crystal phase of the up-converting nanoparticles (NPs) and their complete characterization, a comparative up-conversion study was carried out which revealed that the uniform nanospheres (av. size ~13 nm) obtained from the anhydrous SSP had significantly higher up-conversion efficiency than agglomerated nanorods (~197 nm in length and ~95 nm in width) produced from hydrated inorganic salts. An enhanced up-conversion quantum yield of 1.8% for the anhydrous sample validates the anhydrous precursor approach as a strategy to obtain small but highly emitting upconverting particles without requiring a silica or undoped matrix surface passivation layer.

BaYF5:Yb3+,Tm3+ Upconverting Nanoparticles with Improved Population of the Visible and Near-Infrared Emitting States: Implications for Bioimaging

“…When focusing on the NIR 3 H 4 → 3 H 6 transition, the observed quantum yield varies from 0.06 to 0.88% in the studied power density range, following an apparent power function trend. These values are comparable to other Yb 3+ - and Tm 3+ -doped systems under the same excitation conditions, despite the smaller nanoparticle size and the absence of additional nanoparticle shell or functionality, which speaks of its improved efficiency over other hosts. ,− The blue 1 G 4 → 3 H 6 emission has an observed quantum yield of 3.4 × 10 –6 –1.8 × 10 –4 . At the lowest 0.3 W/cm 2 , this is substantially less efficient than other studied compositions; however, at the highest studied power of 4.8 W/cm 2 , these nanoparticles show an improved quantum yield. , By preventing the population of the 1 D 2 excited state in the BaYF 5 host, higher excitation powers are substantially contributing to the population of the 1 G 4 state, which explains the variability in quantum yield, further proving the previously discussed mechanisms.…”

“…At the lowest 0.3 W/cm 2 , this is substantially less efficient than other studied compositions; however, at the highest studied power of 4.8 W/cm 2 , these nanoparticles show an improved quantum yield. 5,38 By preventing the population of the 1 D 2 excited state in the BaYF 5 host, higher excitation powers are substantially contributing to the population of the 1 G 4 state, which explains the variability in quantum yield, further proving the previously discussed mechanisms.…”

Laser-Induced Heating in GdVO4: Yb3+/Er3+ Nanocrystals for Thermometry

“…For example, emission quenching due to the concentration of Ln, the surface of the NPs, the thermal effects, or the presence of OH can significantly reduce the intensity of the emission. Laser power density can also affect the intensity of the luminescence by interfering with the population of electrons in the excited state of the Ln ions. − In addition to radiative relaxations, some of the energy is converted to heat by nonradiative relaxation pathways. − In the upconversion process, the quantum yield is always low compared to other excitation processes because of the important nonradiative relaxation . Therefore, this heat can be significant and then affects the temperature measurement.…”

“…42−45 In the upconversion process, the quantum yield is always low compared to other excitation processes because of the important nonradiative relaxation. 46 Therefore, this heat can be significant and then affects the temperature measurement. In a nonlinear optical process, the heating depends on the power density of the laser.…”

Incorporation of Upconverting LiYF4:Yb3+, Tm3+ Nanoparticles with High Quantum Yield in TiO2 Metallogels for Near Infrared-Driven Photocatalytic Dye Degradation

“…We have thus focused on the synthesis of smaller UCNPs with intense emission without any intermediate passivation layer such as silica or an undoped matrix. Our synthesis requires thus appropriately designed anhydrous molecular precursors as a synthetic strategy to minimize −OH functionality on the surface as well as in the volume of the NPs. − Indeed, it has been shown recently that, like surface hydroxyl groups, the internal hydroxyl impurity too has a detrimental effect on the upconversion efficiency . The NaGdF 4 -based UCNPs prepared using this strategy showed a much higher upconversion intensity in comparison to NPs of a similar composition produced from hydrated inorganic salts .…”