Issue 41/2015 - USTC invents efficient upconversion luminescent materials

The University of Science and Technology of China recently released an efficient upconversion luminescent material
based on an intelligent phase change lattice cutting strategy.

For many energy conversion materials, their quantum efficiency is often limited by some undesirable processes
that cause energy loss.
For example, the upconversion luminescence effect can absorb two or more low-energy photons and emit higher-energy photons, enabling optical frequency conversion
for biotargeted imaging, detection and therapy, lasers, solar cells, photocatalysis, and many other fields.
This frequency conversion effect relies on the energy transfer process from the absorbing center of the fluorescence upconversion material to the light-emitting center, and the energy transfer process is often subjected to the non-radiative energy relaxation process, which quickly consumes the energy of the excited state of rare earth ions, thereby greatly limiting the quantum efficiency
of upconversion luminescence.
At the same time, this energy relaxation will also produce thermal energy
that is not conducive to the stable operation of the material.

Recently, the Xiong Yujie Experimental Research Group of the University of Science and Technology of China, Jiang Jun Theoretical Research Group, Song Li and Chu Wang Sheng Synchrotron Radiation Characterization Research Group have cooperated to tackle key problems, and have carried out systematic research on such problems and proposed a new strategy
to improve the energy transfer performance of materials.
In this strategy, they cleverly use the thermal energy produced by the unwelcome energy relaxation process mentioned earlier to trigger an "intelligent phase transition" process, that is, the use of energy relaxation thermal energy to drive the rearrangement of atoms in the crystal lattice, forming a highly ordered cubic crystal structure that can no longer effectively undergo energy relaxation, thereby greatly improving its quantum efficiency
.

In terms of experimental methods, the researchers used simple near-infrared light processing methods to complete the photothermal conversion by using the energy relaxation process in the hexagonal phase NaYF4 lattice, triggering an intelligent phase transition process
under the local thermal effect.
First-principles phase transition simulations reveal that this is a novel local phase transition mechanism, that is, the global phase transition is done under the electrostatic potential traction of heat-driven local rearranged atoms
.
What's even more interesting is that this intelligent process automatically stops
once there is no longer a significant local energy relaxation channel in the cubic lattice formed by the phase transition.
Based on this method, the upconversion efficiency before and after the phase transition is increased by more than 700 times
.
In the traditional concept, the industry generally believes that due to the random distribution of sodium ions and rare earth ions in the cubic phase NaYF4 lattice, there is more energy loss caused by non-simple harmonic phonon coupling in the energy transfer process, and its upconversion efficiency is much lower than that of hexagonal phase NaYF4
.
The cubic phase NaYF4 material developed in this work has a highly ordered ion arrangement and exhibits a quantum efficiency of up to 8.
2% during upconversion luminescence, which is even higher than most hexagonal phase NaYF4 materials
currently reported.
(New)