Super-Resolution Nanoscale Thermometry Using Upconverting Nanoparticles and STED Imaging for High-Resolution Temperature Mapping

The rapid advancement of modern electronic, optoelectronic, and data storage devices has led to increasingly intricate and densely packed nanoscale features, which operate under high power densities and challenging environmental conditions. These operational stresses frequently result in thermal degradation and failure, making thermal management a critical aspect of device reliability and performance. The precise mapping of temperature at the nanoscale can provide invaluable insights into thermal behavior and failure mechanisms, enabling the design of more robust and efficient devices. However, achieving nanoscale temperature mapping presents significant challenges. Traditional far-field optical temperature mapping techniques are inherently diffraction-limited, restricting their spatial resolution to hundreds of nanometers. This limitation is particularly problematic for applications requiring high spatial resolution, such as measuring interfacial thermal resistances or understanding deviations from classical heat transfer laws at the nanoscale. Near-field optical and contact-based thermometry methods can offer sub-100 nm spatial resolution, but these techniques come with their own set of challenges, such as parasitic heat sinking and the need for ultrahigh vacuum environments, which are not always practical for real-world applications. The emergence of optical super-resolution imaging techniques has revolutionized biological imaging by revealing cellular structures previously unresolvable with conventional methods. These techniques, which include stimulated emission depletion (STED) and single-molecule localization microscopy, achieve sub-diffraction-limited spatial resolution by manipulating the emission states of fluorescent probes. Despite their success in biological contexts, these super-resolution techniques have not been fully leveraged for thermometry, primarily due to the lack of suitable temperature-sensitive probes and the high laser intensities required for depletion. In this context, the study conducted by PhD candidates Ziyang Ye and Benjamin Harrington, under the guidance of Professor Andrea Pickel from the University of Rochester, represents a significant advancement in the field of nanoscale thermometry. The researchers aimed to develop a super-resolution nanothermometry technique that combines the high spatial resolution of STED imaging with the temperature sensitivity of upconverting nanoparticles (UCNPs). By doping UCNPs with high concentrations of Yb3+ and Tm3+, they sought to achieve both efficient STED imaging and reliable temperature-dependent emission signals. The ultimate goal was to create a versatile and practical tool for high-resolution temperature mapping that could be applied across a wide range of scientific and engineering disciplines. The challenges addressed by this study include the need for a probe with both strong temperature-dependent emission and compatibility with STED imaging, the difficulty of achieving high spatial resolution in practical operating environments, and the necessity of developing efficient detection schemes to make temperature mapping feasible within reasonable time frames. By overcoming these challenges, the researchers aimed to demonstrate the potential of their technique to reveal local temperature variations in microstructures and nanostructures, which are often undetectable with conventional thermometry methods.

The researchers designed a custom scanning confocal microscopy and spectroscopy system modified to include STED capabilities to investigate the feasibility of UCNP-based super-resolution thermometry. The system used a continuous-wave (CW) 976-nm fiber Bragg grating-stabilized laser diode for excitation and a CW 808-nm single-mode Fabry-Perot laser diode for depletion. By focusing the laser beams onto the sample with a dry air objective lens, the setup avoided parasitic heat sinking and enabled precise control of laser intensities. Validation of the system using individual hexagonally faceted UCNPs with an average diameter of ~134 nm showed a significant improvement in imaging resolution. Under simultaneous application of the excitation and depletion laser beams, the full width at half maximum (FWHM) of the intensity profile from the resulting image was reduced to 136 nm, compared to 461 nm with only the excitation beam. This demonstrated the system’s capability to achieve sub-diffraction-limited imaging resolution.To assess the temperature dependence of UCNP emission, the researchers acquired emission spectra from individual UCNPs at room temperature (293 K), 350 K, and 400 K. They observed peaks near 455 nm, 480 nm, and 515 nm, assigned to various Tm3+ transitions, along with a 490-nm peak showing a sharp increase in intensity with temperature.  The 490-nm peak’s temperature dependence highlighted its potential for ratiometric thermometry. The ability to identify temperature-dependent emission peaks provided a crucial signal for developing a reliable temperature-dependent metric, essential for high-resolution temperature mapping.

The study demonstrated ratiometric thermometry using the temperature-dependent intensity ratios of the identified emission peaks. Self-assembled UCNP monolayers and multilayers were formed using a liquid-air interfacial self-assembly method and transferred onto silicon substrates. Temperature-dependent ratio maps were obtained with significant reductions in scan time compared to conventional spectroscopy. The results showed excellent consistency between the temperature-dependent luminescence intensity ratios measured in diffraction-limited and super-resolution modalities, validating the feasibility of using UCNPs for practical temperature mapping applications. To showcase the practical application of STED nanothermometry, the researchers applied the technique to NiCr serpentine heater lines fabricated on crystalline quartz substrates. Finite element simulations predicted nonuniform temperature profiles due to current crowding effects, which were experimentally verified. STED measurements revealed a temperature gradient across the heater line that was undetectable with diffraction-limited thermometry. The super-resolution measurements showed a temperature rise at the inner corner of the serpentine heater line ~40 K higher than that at the outer corner, as predicted by the simulations. This demonstrated the technique’s capability to uncover local temperature variations in microstructures and nanostructures, validating its potential for advanced thermal management applications.

The custom-built STED microscope was validated using single UCNPs to demonstrate its room temperature imaging performance. A CW 976-nm Gaussian excitation laser and a CW 808-nm doughnut-shaped depletion laser were applied to image single UCNPs on a borosilicate glass substrate. The FWHM of the intensity profile from the resulting STED image was 136 nm, a notable improvement compared to 461 nm obtained with only the excitation beam. This indicated that the STED image’s FWHM represented only an upper bound on the imaging resolution, affirming the potential for even lower FWHM values with smaller UCNPs and validating the system’s high-resolution imaging capabilities. Emission spectra from single UCNPs were acquired at various temperatures, revealing temperature-dependent peaks at 455 nm, 480 nm, 490 nm, and 515 nm. The 490-nm peak, in particular, showed a sharp increase in intensity between room temperature and 400 K.  The 490-nm peak’s temperature dependence provided a critical signal for ratiometric thermometry. The results confirmed that temperature-dependent emission peaks from highly doped UCNPs could be leveraged for high-resolution temperature mapping. STED imaging of individual UCNPs was performed at various temperatures using bandpass filters for the 490-nm and 515-nm emission peaks. Images recorded with and without the 808-nm doughnut-shaped depletion beam demonstrated notable imaging resolution enhancement. The FWHM values for all temperatures and both wavelength bands ranged from 171 to 179 nm, corresponding to an estimated imaging resolution of 119 nm or better. This confirmed the technique’s ability to maintain high imaging resolution at elevated temperatures and across different emission peaks.

The researchers performed ratiometric thermometry using the 490-nm and 515-nm emission peaks. Single-UCNP spectra were acquired with and without the depletion beam at various temperatures to assess the temperature dependence of the luminescence intensity ratio. The temperature-dependent ratios obtained from diffraction-limited and STED measurements were consistent, demonstrating that the depletion beam did not affect the temperature dependence of the ratio. This validated the feasibility of using STED nanothermometry for practical temperature mapping applications.

STED nanothermometry was applied to NiCr serpentine heater lines to detect temperature gradients caused by current crowding effects. The researchers recorded temperature-dependent ratios at various points along the heater line, both with and without applied current. Under no applied current, there was no meaningful difference between diffraction-limited and STED measurements. However, with an applied current, STED measurements detected a temperature gradient, with a ~40 K higher temperature rise at the inner corner of the heater line compared to the outer corner. This confirmed the technique’s ability to reveal temperature heterogeneities undetectable with conventional thermometry.

This study marks a significant advancement in the field of nanoscale thermometry, addressing the critical need for high-resolution temperature mapping in various scientific and engineering domains. By demonstrating a super-resolution nanothermometry technique based on highly doped upconverting nanoparticles (UCNPs) and stimulated emission depletion (STED) imaging, the researchers have introduced a powerful tool for investigating thermal behavior at the nanoscale. This technique overcomes the diffraction limit of traditional far-field optical thermometry, achieving spatial resolutions better than 120 nm. Such high-resolution thermal mapping is essential for understanding and improving the thermal management of modern electronic, optoelectronic, and data storage devices, which operate under increasingly challenging conditions.

The ability to map temperatures with nanoscale resolution can help identify hotspots and thermal gradients in electronic and optoelectronic devices, enabling better thermal management and, consequently, enhanced performance and reliability. Nanoscale thermometry can provide insights into interfacial thermal resistances across individual grains or material phases with nanoscale dimensions. This can aid in the development of materials with superior thermal properties for various applications, including thermal barrier coatings and thermoelectric materials. High-resolution temperature maps can provide direct evidence to verify or refute predictions of deviations from classical heat transfer laws at the nanoscale. This can lead to more accurate models and simulations, improving the design of thermal management systems. The technique can be applied to study temperature-dependent processes in chemical reactions, biological systems, phase transitions, and lithium-ion batteries. Understanding the role of temperature in these processes can lead to optimized reaction conditions, improved biological assays, and better battery performance. Nanoscale thermometry can be used to investigate thermal effects in plasmonic and quantum devices, guiding the design of devices with improved thermal stability and performance.  The method’s compatibility with different sample forms, material types, and operating environments (including ambient air, liquid, and vacuum) makes it versatile and widely applicable. This broad compatibility can facilitate its adoption in various research and industrial settings.

In summary, the experiments conducted by Professor Andrea Pickel and her team validated the feasibility and practicality of UCNP-based STED nanothermometry for high-resolution temperature mapping. The findings demonstrated the technique’s ability to achieve sub-diffraction-limited spatial resolution, maintain high imaging performance at elevated temperatures, and uncover local temperature variations in complex microstructures, highlighting its potential for a wide range of scientific and engineering applications.