Na⁺-Driven Nucleation of NaYF₄:Yb,Er Nanocrystals and Effect of Temperature on Their Structural Transformations and Luminescent Properties

Abstract

Ultrasmall pure hexagonal phase NaYF 4 :Yb,Er is successfully prepared via the solvothermal method. The upconversion (UC) luminescence of hexagonal phase NaYF 4 :Yb,Er nanocrystals is ten times stronger than that of cubic phase nanocrystals with the same size of 6 nm. XRD results reveal that heating above 673 K leads to conversion of the hexagonal phase to the high-temperature cubic phase, whereas the cubic nanocrystals undergo phase transformation from normal cubic at room temperature to hexagonal at 673 K and further to high-temperature cubic above 773 K. The hexagonal nanocrystals exhibit emission enhancement after heat treatment up to 573 K. Further heating above 773 K induces a decreasing trend in emission due to the phase transition to the high-temperature cubic phase. The cubic phase exhibits decreasing luminescence with temperature due to strong cross relaxation and then increasing luminescence above the temperature of 673 K due to the hexagonal phase transformation. The luminescent properties of both the normal cubic phase and high-temperature cubic phase indicate that different crystal fields exist in these two phases due to the rearrangement of ligands around Er 3+ at high temperature.

Controllable crystal form transformation and luminescence properties of up-conversion luminescent material K3Sc0.5Lu0.5F6: Er3+, Yb3+ with cryolite structure

“…The result shows that replacing Sc 3+ with Lu 3+ can effectively affect the transformation of cubic form to monoclinic form. 33,34 With the increase of Lu 3+ doping concentration, the XRD diffraction peaks of the sample shi to lower diffraction angles. This can be explain that the Sc 3+ in the lattice is replaced by a larger Lu 3+ , which leads to the expansion of the unit cell volume, the crystal interplanar spacing (d) becomes larger.…”

“…Therefore, the inuence of the polymorphism transition on the luminescence properties of rare-earth ions has attracted attention from scholars, such as Gao changed the content ratio of ZnO/Na 2 O in the Tb 3+ -Yb 3+ co-doped NaYF 4 nanocrystal-containing zinc uoride hydroxide glass ceramics to achieve the cubic to hexagonal form transition and the enhancement of up-conversion luminescence; 31 Janjua prepared ultrane pure hexagonal NaYF 4 by the solvothermal method: the up-conversion luminescence intensity of hexagonal NaYF 4 : Er 3+ , Yb 3+ is 10 times than cubic nanocrystals of the same size. 32 For cryolite, the most common crystal structures are monoclinic and cubic, and in different crystal form, the upconversion luminescence properties of cryolite materials maybe different. At present, many cryolite materials with different crystal forms were obtained by substitution of monovalent and trivalent cations, but the comparison of upconversion luminescence properties of cryolite materials with polymorphism has not been reported.…”

Precision tuning of rare-earth-doped upconversion nanoparticles via droplet-based microfluidic screening

“…While screening these two parameters, we initially set the RE doping to typical values reported in the literature (molar percentages of 80% Y, 18% Yb, and 2% Er). 33,39 As shown in Fig. 2a and b, the PL intensity of the formed NaYF 4 :Yb,Er nanocrystals was relatively weak when R 1 was lower than 1.2, but rapidly increased to a maximum at R 1 = 1.8.…”

Novel sol-gel SiO2-NaGdF4 transparent nano-glass-ceramics

“…α-NaGdF 4 NCs collapse increases the available energy favouring the crystallisation of hexagonal β-NaGdF 4 NCs, similar to the behaviour found by Bartha et al [25,51], in SG NaYF 4 or LiYF 4 OxGCs doped with Er 3+ -Yb 3+ ions. Another possibility is that αphase acts as precursor crystal of β-NaGdF 4 crystalline phase considering the Ostwald-ripening restricted process in which the NCs β-NaGdF 4 size gradually increases starting from smaller α-NaGdF 4 NCs obtained at lower temperature [52,53].…”