Helix Shape Power-Dependent Properties of Single Upconversion Nanoparticles.

Area of Science:

• Materials Science
• Nanotechnology
• Photonics

Background:

• Upconversion nanoparticles (UCNPs) offer nonblinking, nonbleaching, and superbright emission.
• Their nonlinear power-dependent properties enable applications in super-resolution microscopy and single-molecule tracking.
• Switchable UCNPs under dual-beam excitations are highly desirable for advanced bioimaging and digital assays.

Purpose of the Study:

• To investigate the unusual double helix shapes observed in Nd³⁺-Yb³⁺-Er³⁺-doped UCNPs.
• To analyze the power-dependent emission spectra, lifetimes, and power-intensity slopes of single UCNPs.
• To elucidate the dynamic roles of Nd³⁺ ions and electron population pathways under varying excitation power densities.

Main Methods:

• Systematic analysis of power-dependent emission spectra and lifetimes of single UCNPs.
• Measurement of power-intensity double-log slopes under 976 nm and 808 nm excitations.
• Investigation of core-shell nanoparticle batches with varying doping concentrations.

Main Results:

• Nd³⁺-Yb³⁺-Er³⁺-doped UCNPs display double helix brightness shapes dependent on excitation power densities (976 nm and 808 nm).
• Nd³⁺ ions exhibit power-dependent dynamic roles, directly emitting upconverted luminescence at high 808 nm power.
• The helix shape phenomenon is universal across different core-shell UCNP batches, irrespective of doping concentrations.

Conclusions:

• The dynamic behavior of Nd³⁺ ions in UCNPs is strongly dependent on excitation power.
• High-power 808 nm excitation allows Nd³⁺ to directly contribute to upconversion, altering traditional energy transfer pathways.
• Understanding power-dependent properties is key for enhancing UCNP emission efficiency and designing novel nonlinear responsive probes for imaging and sensing.