Depth-profiling of Yb3+ sensitizer ions in NaYF4 upconversion nanoparticles

Enhancing the efficiency of upconversion nanoparticles (UCNPs)

and therefore their brightness is the critical goal for this emerging

material to meet growing demands in many potential applications

including sensing, imaging, solar energy conversion and photonics.

The distribution of the photon sensitizer and activator ions that

form a network of energy transfer systems within each single

UCNP is vital for understanding and optimizing their optical properties.

Here we employ synchrotron-based X-ray Photoelectron

Spectroscopy (XPS) to characterize the depth distribution of Yb3+

sensitizer ions in host NaYF4 nanoparticles and systematically correlate

the structure with the optical properties for a range of

UCNPs with different sizes and doping concentrations. We find a

radial gradient distribution of Yb3+ from the core to the surface of

the NaYF4 nanoparticles, regardless of their size or the sensitizer’s

concentration. Energy dispersive X-ray Spectroscopy (EDX) was

also used to further confirm the distribution of the sensitizer ions

in the host matrix. These results have profound implications for the

upconversion optical property variations.