Apparent self-heating of individual upconverting nanoparticle thermometers

Area of Science:

• Materials Science
• Nanotechnology
• Spectroscopy

Background:

• Individual luminescent nanoparticles offer sub-diffraction limited spatial resolution for thermometry.
• Potential self-heating effects at high excitation intensities are poorly understood.
• Existing thermal models predict negligible self-heating in nanoparticles.

Purpose of the Study:

• Investigate the unexpected increase in thermometry signal with excitation intensity in NaYF4:Yb3+,Er3+ nanoparticles.
• Determine if the observed signal change represents actual self-heating or an artifact.
• Clarify the underlying mechanisms responsible for the observed phenomenon.

Main Methods:

• Utilized ratiometric luminescence thermometry on individual NaYF4:Yb3+,Er3+ nanoparticles.
• Applied luminescence lifetime thermometry for the first time to individual nanoparticles.
• Systematically varied substrate thermal conductivity, nanoparticle-substrate contact resistance, and nanoparticle size.
• Employed rate equation modeling to analyze nanoparticle energy level dynamics.

Main Results:

• Observed an unexpected increase in the ratiometric thermometry signal with excitation intensity, suggesting a significant apparent temperature rise.
• Luminescence lifetime thermometry confirmed a similar apparent temperature rise.
• The apparent self-heating effect remained constant despite variations in the nanoparticle's thermal environment.
• Rate equation modeling revealed the artifact arises from altered radiative and non-radiative relaxation pathways from higher Er3+ energy levels.

Conclusions:

• The apparent self-heating in NaYF4:Yb3+,Er3+ nanoparticles is an artifact, not a true temperature increase.
• This artifact is caused by changes in electronic relaxation processes under high excitation, not thermal effects.
• Findings are crucial for accurate interpretation of single-particle thermometry data and understanding nanoparticle photophysics.