Colloidal β-KYF₄:Yb³⁺,Er³⁺/Tm³⁺nanocrystals: tunable multicolor up-conversion luminescence from UV to NIR regions

Abstract

Pure β-KYF 4 nanocrystals codoped with Er 3+ /Yb 3+ and Tm 3+ /Yb 3+ were successfully synthesized via the thermal decomposition of trifluoroacetate precursors using oleic acid and octadecylene as coordinate solvents. The up-conversion optical properties of the β-KYF 4 nanocrystals with different lanthanide (Ln)-doped ions (Yb 3+ /Er 3+ , Yb 3+ /Tm 3+ ) were investigated. It is found that the colloidal dispersion of β-KYF 4 :Yb,Er/Tm nanocrystals display strong multiple up-conversion emission spanning from the deep UV-to-NIR region under 980 nm excitation. Pure β-KYF 4 nanocrystals codoped with Er 3+ /Yb 3+ and Tm 3+ /Yb 3+ can emit the bright eye-visible blue, green, yellow, and red emissions by adjusting concentration of Yb 3+ sensitizer ion and Er 3+ or Tm 3+ activator ions. Moreover, the UC luminescent colors can be tuned for the four colors from blue to green to yellow and finally to red emission in the average particle size, X-ray wavelength (0.15405 nm), the diffraction angle and full-width at half-maximum, respectively.The morphology and size of Yb 3+ ,Er 3+ /Tm 3+ codoped KYF 4 NCs are analyzed by TEM and HRTEM. Fig.2 shows the TEM images of KYF 4 :20% Yb 3+ /2% Er 3+ and KYF 4 :25% Yb 3+ /0.2% Tm 3+ NCs, respectively. It is shown that as-synthesized KYF 4 :Yb 3+ ,Er 3+ /Tm 3+ NCs contain smaller hexagonal nanorods particles and nanoplates with good monodispersity. From Fig. 2a,b the average grain size for KYF 4 :20% Yb/2% Er and KYF 4 :25% Yb/0.2% Tm NCs are calculated from random 100 nanoparticle to be 16nm, 18nm, respectively , which are in good consistent to XRD analysis result calculated by scherrer's formula. The lattice fringes are obviously observed in HR-TEM images of KYF 4 :Yb,Er/Tm ( Fig. 2c,d ), which indicated that as-synthesized KYF 4 NCs possessed highly crystallinity. The HRTEM images (Fig. 2c) of as-synthesized nanocrystals showed clear lattice fringes with a interplanar spacing of d = 0.33 nm, which corresponds to the (221) lattice plane of the hexagonal phase KYF 4 . In addition, the interplanar distances of 0.21nm was also observed by HR-TEM images in Fig. 2d, corresponding to the (600) lattice planes of the hexagonal phase KYF 4 , which was consistent with the XRD analysis. The corresponding elemental composition of KYF 4 codoped with 20% Yb 3+ /2% Er 3+ and 25% Yb 3+ /0.2% Tm 3+ NCs were obtained by the EDS analysis, as shown in Fig.2f.The EDS result for KYF 4 :20%Yb/2%Er (Fig. 2e) demonstrates that as-synthesizedNCs are mostly consist of K, Y, Yb, Er and F. The EDS result for KYF 4 :25% Yb/0.2% Tm as-synthesized NCs are mostly consist of K, Y, Yb, Tm and F. Based on

Simulating excited-state absorption spectra in upconverting lanthanide doped nanoparticles: KY3F10:Er3+

“…The drastic change in the value of the R/G ratio strongly suggests that the red upconversion fluorescence has its origins in resonant ground state absorption, whilst the green band is probably due to non-resonant excited state absorption. This observed behaviour of β−KYF4:Yb/Er also indicates a possibility for tuning the upconversion fluorescence 'colour' by adjusting the excitation wavelength similar to that observed in β−KYF4:Yb/Er nanoparticles by varying the concentration of Yb/Er [20]. We speculate that strong energy back transfer between Yb 3+ and Er 3+ ion is the reason for the intense red colour of the β−KYF4:Yb/Er upconversion fluorescence as singly doped β−KYF4:Er (2 mol%) fluoresces primarily in the green for 974 nm excitation [38,39].…”

“…Thus, the exact excitation wavelength does not seem to influence the order of the [40,41]. Interestingly, for β−KYF4:Yb/Er, the n value for the combined green upconversion is much larger than 2, suggesting that the process is, at a minimum, a three-photon process, while for the red upconversion, the n value is very close to 2, suggesting a minimum of a two-photon process [20]. The combination of predominantly red fluorescence and a three-photon excitation process involved in green upconversion is rarely reported.…”

“…Unlike NaYF4, only a few studies are reported in the literature on α−KYF4 nanoparticles [13,18,19] and almost none related to β−KYF4 [20].…”

Na(1-x)Lix(Gd0.39Y0.39Yb0.2Er0.02)F4 (0 ≤ x ≤ 1) Solid Solution Microcrystals: Li/Na Ratio-Induced Transition of Crystalline Phase and Morphology and Their Enhanced Upconversion Emission

“…These advantages make Yb 3+ an outstanding sensitizer to donate energy to others in energy transfer upconversion (ETU) process . Thus, Yb 3+ sensitized fluoride nanocrystals (NCs) have been thought to be one of the most efficient upconverters owing to their low energy lattice phonons. − Furthermore, multiple emission in the visible region can be achieved by codoping lanthanide ions, such as Yb 3+ /Er 3+ , Yb 3+ /Tm 3+ , Yb 3+ /Ho 3+ , or Yb 3+ /Er 3+ /Tm 3+ . − Among them, green emissions (525 and 542 nm) and red emission (652 nm) are most usually observed in Yb 3+ /Er 3+ codoped fluoride phosphors …”

A new strategy to directly construct hybrid luminescence–photothermal–magnetism multifunctional nanocomposites for cancer up-conversion imaging and photothermal therapy

“…24,25 In addition, the Yb 3+ ions have a simple energy level scheme, in which the 2 F 7/2 -2 F 5/2 transition is resonant with f-f transitions of a few lanthanide ions, including Er 3+ , Ho 3+ and Tm 3+ ions, that is why these elements constitute the most efficient up-converting activators. [26][27][28] And we all have known that the ultraviolet and visible lights had weak penetration by only a few millimeters, but near-infrared (NIR) have deep one in tissues by one to several centimeters. [29][30][31][32][33][34][35] Excitation with NIR increases their possible application viability because the biological tissues are in the NIR range, where the blood and tissues have the minimal adsorption, thus achieving deep penetration depth of light in the so-called therapeutic window.…”