Near-infrared luminescence properties of small-sized homogeneous/heterogeneous core/shell structured NaGdF4:Nd3+ nanoparticles
Authors: Ma Wen-Jun  , You Fang-Tian  , Peng Hong-Shang  , Huang Shi-Hua  ,   Key Laboratory of Luminescence and Optical Information, Ministry of Education, Institute of Optoelectronic Technology, Beijing Jiaotong University, Beijing 100044, China
Published: December 2020

In recent years, considerable researches have focused on the upconversion phosphor nanoparticles in the application of biomedical imaging, which emit visible light. Nevertheless, these kinds of nanoparticles limit the light penetration depth and imaging quality. The Nd3+ doped nanoparticles excited and emitted in a spectral range of 700-1100~nm can overcome those shortcomings. Furthermore, considering the applications of rare earth nanoparticles in biomedical imaging, smaller particle size is needed. However, the luminescence efficiencies of nano-structured materials are lower due to the inherent drawback of high sensitivity of Nd3+ ions to the surface defects. So, it is of vital importance for introducing a shell with low phonon energy to be overgrown on the surface of nanoparticles. According to the ratio of core material to the shell, core/shell structured nanoparticles are separated into homogeneous and homogeneousnanoparticles. And the shell material may influence the luminescence performance. In few reports there have been made the comparisons of luminescence performance of Nd3+ between heterogeneous and homogeneous core/shell nanoparticles. In the present work, small-sized hexagonal NaGdF4:3%Nd3+ nanoparticles with an average size of sub-5~nm are synthesized by a coprecipitation method. To overcome the nanosize-induced surface defects and improve the luminous performance, the NaGdF4:3%Nd3+ nanoparticles are coated with homogeneous and heterogeneous shells, respectively. Core/shell structured nanoparticles with different values of shell thickness are synthesized by using the core/shell ratios of 1:2, 1:4 and 1:6. The luminescence properties of the prepared nanoparticles are characterized by photoluminescence spectra and fluorescence lifetimes. Under 808~nm excitation, the NaGdF4:3%Nd3+ nanoparticles exhibit nearinfrared emissions with sharp bands at ~866 nm, ~893 nm, ~1060 nm, which can be assigned to the transitions of 4F3/2 to 4I9/2, 4F2/3 to 4I11/2, respectively. The locations of emission peaks of the core/shell nanoparticles are in accordance with the those of cores while the fluorescence intensity increases significantly. In addition, the average lifetimes of Nd3+ ions at 866 nm of core/shell nanoparticles are longer than those of the cores, which indicates that the undoped shell can minimize the occurrence of unwanted surfac-related deactivations. Notably, comparing with the homogeneous NaGdF4:3%Nd3+@NaGdF4 nanoparticles, the fluorescence intensity of heterogeneous NaGdF4:3%Nd3+@NaYF4 nanoparticles is enhanced and their lifetimes become longer. It is due to the low stability of hexagonal NaYF4, which suppresses the nucleation of the shell precursor and makes the shell able to be fully coated on the core. The decrease of electron charge density on the surface of core/shell nanoparticles is also beneficial to shell growth and crystallization. The high crystallinity of heterogeneous core/shell structured nanoparticles can eliminate negative influence of surface effect more efficiently. In addition, the phonon energy of NaYF4 is lower than that of NaGdF4, which leads to low possibility of non-radiative cross-relaxation between Nd3+ ions, thereby improving the luminescence efficiency in the near in frared emission.