Synthesis And Upconversion Luminescence Of NaYF₄: Yb³⁺, Er³⁺ Nanocrystals

Upconversion materials are an important kind of rare earth luminescent materials, turning infrared light into visible light through multi-photon mechanism, which is characterized by absorbing energy lower than the photon light emitting energ, also known as the anti- Stokes materials. In recent years, the upconversion luminescent materials on the nanoscale as a new fluorescent markers in biological macromolecules detection, medical imaging and other aspects are given more and more attention Unlike traditional fluorescent marker, upconversion nanomaterials have many advantages, such as low toxicity, low excitation light energy, narrow emission spectra, big stokes shift, good chemical stability, high and stable luminous intensity and controlling emission wavelength by changing the composition. In addition, excitation light source of rare earth codoped upconversion luminescent nanomaterials is usually a continuous near infrared laser（typically 980nm）, autofluorescence and scattered light interference phenomenon can be avoided under the excitation conditions of biological samples, without damage to biological tissue, the optical wear deeper penetration depth, thereby reducing the background to detect and improve the signal to noise ratio. Therefore, upconversion nanomaterials as fluorescent markers in the analysis of biological macromolecules and clinical testing field have a very good prospect. Oxides, sulfides, fluorides and phosphates are commonly matrix of upconversion nanomaterials, wherein the upconversion materials with the hexagonal phase Na YF₄ as a matrix, have the highest light emission. Recently much synthesis of Na YF₄:Yb,Er in organic phase have been reported, which has excellent morphology and luminescent properties, but poorly water-soluble limitting its application in biological systems. Therefore, preparation of high-performance, hydrophilic Na YF₄:Yb,Er upconversion luminescent nanomaterials is an extension of the premise of their applications in biology and medicine, which has great significance. In this paper, we prepare Na YF₄ upconversion materials with different phases（cubic, hexagonal phases and both） by hydrothermal method. Through the use of X-ray diffraction, field emission scanning electron microscopy and transmission electron microscopy on the different components of Na YF₄:Yb³⁺/Er³⁺ samples were characterized. Na YF₄ matrix have two different crystal phase, one is the cubic phase of α-Na YF₄（metastable high temperature phase）, the other is the hexagonal phase structure of β-Na YF₄（thermodynamic steady low temperature phase）. As can be seen from the upconversion spectrum, Na YF₄ with cubic phase emits strong red light, and Na YF₄ with hexagonal phase emits green light. Intense UC emission of the nanometer-sized phosphors suggest good crystallinity, which is in good agreement with the results of XRD and TEM. A doping level of 20% mol Yb³⁺ and 2mol% Er³⁺ gives the strongest luminescence. A general approach has been developed for the synthesis of the well-disperse hexagonal Na YF₄:Yb³⁺,Er³⁺. Under the excitation of 980 nm, the red emission(⁴F_9/2â†'⁴I_15/2) of the Na YF₄:Yb³⁺,Er³⁺ increases with increasing the Yb³⁺ concentration, while that of the green emission(⁴S_3/2/²H_11/2 â†' ⁴I_15/2) decreases. The red emission enhancement is attributed to the enhanced population of the 4F9/2 level via the energy transfer(⁴G_11/2(Er³⁺) + ²F_7/2(Yb³⁺) â†'⁴F_9/2(Er³⁺) + ²F_5/2(Yb³⁺)), while green emission diminishment is attributed to ⁴S_3/2 + ²F_5/2 â†' ²K_15/2 + ²F_7/2, which depopulates the excited ⁴S_3/2 level at higher Yb³⁺ concentration. So the emission color can be tuned from green to yellow only by changing the Yb³⁺ concentration. We have also confirmed that the up-conversion process in Na YF₄:Yb³⁺, Er³⁺ results from two-photon processes.