Andrea D. Pickel

ConspectusThe growing demand for nanoscale temperature measurement capabilities is motivated by diverse applications such as thermal management of microelectronics and batteries, design of plasmonic systems, mechanistic studies of catalysis, and unraveling intracellular processes. Upconverting nanoparticles (UCNPs) are lanthanide-doped inorganic probes that are popular luminescent thermometers, with advantages including well-understood temperature-dependent behavior, broadly tunable excitation and emission wavelengths, and exceptional thermal and chemical stability. Like other optical thermometry techniques, luminescence thermometry provides the desirable capability of remotely collecting the temperature-dependent signal from the far field. Conventional implementations of luminescence thermometry also share a major limitation of other optical thermometry techniques, namely, their diffraction limited spatial resolution. However, in contrast with other optical thermometry techniques, luminescence thermometry also creates an opportunity to leverage certain unique strategies for circumventing the diffraction limit.In this Account, we discuss our contributions to initiating or building on three major strategies for achieving UCNP thermometry beyond the diffraction limit. Some of these concepts originate from or have direct parallels in the realm of biological imaging, where optical imaging with spatial resolution below the diffraction limit has been a longstanding goal; conversely, others have no direct bioimaging analogy. Exciting an isolated single UCNP with a diffraction limited laser beam enables thermometry with subdiffraction limited spatial resolution governed by the UCNP size, although this approach is inherently restricted to measurements at a single spatial point. We begin by describing our efforts to extend single-UCNP measurements to smaller UCNP sizes and understand how their temperature-dependent emission can be influenced by external factors such as the excitation laser intensity or the surrounding optical environment, the latter of which is exemplified by an investigation of how single-UCNP emission is altered when the UCNPs are placed on various metallic substrates. Next, we show how the principles underlying single-UCNP thermometry can be expanded to sample multiple temperature points within a subdiffraction region by combining different UCNP compositions with spectrally orthogonal temperature-dependent luminescence. As a practical demonstration, we resolve a nearly 20 K temperature difference over a sub-110 nm distance originating from the steep temperature gradient near a laser-heated Ag nanodisk. Finally, we discuss our adaptation of UCNP-based stimulated emission depletion (STED) super-resolution imaging for super-resolution nanothermometry, combining temperature-dependent STED spectroscopy, self-assembled UCNP monolayer formation, and a detection scheme that enables practical scan times. STED nanothermometry can reveal a temperature gradient on a Joule-heated microstructure that is undetectable with analogous diffraction limited measurements, showcasing the power of this approach. We conclude with our perspective on the outlook for UCNP thermometry methods that circumvent the diffraction limit, highlighting both current research needs to further improve the measurement capabilities and strategies that could facilitate broader adoption of these emerging techniques.

High Resolution Image Download MS PowerPoint Slide Recording the temperature-dependent luminescence emitted by an isolated single nanoparticle offers one strategy for performing far-field optical thermometry with spatial resolution below the diffraction limit. However, such measurements are inherently restricted to probing the temperature at a single spatial point. Here, we demonstrate an approach to sampling temperature gradients at multiple points within a subdiffraction region by simultaneously collecting the emission from different nanoparticle species with spectrally orthogonal temperature-dependent luminescence. Taking advantage of the narrow spectral bands and wavelength tunability of lanthanide-doped upconverting nanoparticle (UCNP) emission, we use a single laser to excite both NaYF 4:Yb 3+,Er 3+ and NaYF 4:Yb 3+,Tm 3+ UCNPs and concurrently acquire their spectrally distinct temperature-dependent luminescence. The emission spectra and temperature response obtained from tandem UCNP pairs consisting of one NaYF 4:Yb 3+,Er 3+ and one NaYF 4:Yb 3+,Tm 3+ UCNP are in excellent agreement with corresponding measurements using isolated individual UCNPs of each composition. To demonstrate the utility of this approach, we use a tandem pair of UCNPs located ∼108 nm from each other to probe the sharp temperature gradient resulting from laser heating of an isolated silver nanodisk. While the diffraction-limited emission spots of the UCNPs overlap nearly completely, we can distinguish a temperature difference of ∼19 K between their two locations. This capability is particularly applicable to scenarios that would benefit from multiple temperature data points, but where the majority of the sample surface must remain accessible for other purposes, such as in the case of plasmonic and photothermal catalysis.

Microscale heating platforms capable of generating localized temperature rises can find applications in wide‐ranging areas including nanomaterials synthesis and microscale thermometry. Here, commercially available optical calibration samples called Ronchi rulings, which consist of an array of chrome lines on a float glass substrate, are demonstrated to serve as reconfigurable Joule heaters. Electrical connections are formed by wire bonding onto the chrome to Joule heat individual lines and monitor their temperature rises using electrical resistance thermometry. Tests across multiple heater lines demonstrate a negative temperature coefficient of resistance with an average value of −6.93 × 10 −4 ± 8.18 × 10 −5 K −1 . Under Joule heating, temperature rises exceeding 100 K are measured. To characterize the temperature gradient across the chrome line and glass, a noncontact optical thermometry technique based on the temperature‐dependent luminescence of upconverting nanoparticles (UCNPs) is used, producing temperature measurements that match finite element simulations. A 1:1 area ratio between the chrome lines and glass offers a high probability of finding UCNPs across both materials. The temperature rise on chrome determined from luminescence thermometry, electrical resistance thermometry, and simulations are also consistent. Furthermore, over 50% of the peak temperature rise is maintained along the neighboring glass region.

temperature measurements of all three components in a single cell. With application of a discharge current of 65 mA in a commercial primary coin cell (CR2032), a maximum temperature difference of 7.9 °C was measured between the cell separator and external packaging. It is envisioned that this technique can be extended to larger format lithium-ion battery cells, revealing non-uniform internal temperature distribution within the cells to better understand critical thermal processes.

The tenth U.S.–Japan Joint Seminar on Nanoscale Transport Phenomena was held in San Diego, California, from July 16–19, 2023. The goals of the joint seminar series, established in 1993, are to encourage research and international exchange between US and Japan researchers in the nanoscale thermal transport community, foster US-Japan collaborations, and expose new junior scientists to leading-edge research in an interdisciplinary and international environment. The research topics were organized into 8 topical sessions, including (1) and (2) Conduction; (3) radiation and photonics; (4) and (5) Applications/Devices; (6) Fluids/Phase change; (7) Magnetism/Phonons; and (8) Thermal transport. The joint seminar opened with a plenary session and additionally featured an expert industry panel which discussed the industrial applications of thermal transport phenomena. An evening poster session provided graduate students and postdoctoral scholars with the opportunity to present their latest research results. A total of 99 researchers participated, with 51 from the United States and 48 from Japan. Of these participants, 47 were faculty, 9 held positions at national laboratories, industry, or government, and 43 were students or postdocs. The meeting was organized by Renkun Chen, Gota Kikugawa, Austin J. Minnich, and Junichiro Shiomi. Around 16 of the participants served as session chairs. The summaries of the various sessions prepared by the organizers and session chairs are presented in this report.

The emission intensity of individual upconverting nanoparticles (UCNPs) on metal surfaces is determined by an interplay between quenching and reflection effects, while the ratiometric thermometry signal is unaffected by the underlying material.

From engineering improved device performance to unraveling the breakdown of classical heat transfer laws, far-field optical temperature mapping with nanoscale spatial resolution would benefit diverse areas. However, these attributes are traditionally in opposition because conventional far-field optical temperature mapping techniques are inherently diffraction limited. Optical super-resolution imaging techniques revolutionized biological imaging, but such approaches have yet to be applied to thermometry. Here, we demonstrate a super-resolution nanothermometry technique based on highly doped upconverting nanoparticles (UCNPs) that enable stimulated emission depletion (STED) super-resolution imaging. We identify a ratiometric thermometry signal and maintain imaging resolution better than ~120 nm for the relevant spectral bands. We also form self-assembled UCNP monolayers and multilayers and implement a detection scheme with scan times >0.25 μm 2 /min. We further show that STED nanothermometry reveals a temperature gradient across a joule-heated microstructure that is undetectable with diffraction limited thermometry, indicating the potential of this technique to uncover local temperature variation in wide-ranging practical applications.

As the field of luminescence thermometry has matured, practical applications of luminescence thermometry techniques have grown in both frequency and scope. Due to the biocompatibility of most luminescent thermometers, many of these applications fall within the realm of biology. However, luminescence thermometry is increasingly employed beyond the biological realm, with expanding applications in areas such as thermal characterization of microelectronics, catalysis, and plasmonics. Here, we review the motivations, methodologies, and advances linked to nonbiological applications of luminescence thermometry. We begin with a brief overview of luminescence thermometry probes and techniques, focusing on those most commonly used for nonbiological applications. We then address measurement capabilities that are particularly relevant for these applications and provide a detailed survey of results across various application categories. Throughout the review, we highlight measurement challenges and requirements that are distinct from those of biological applications. Finally, we discuss emerging areas and future directions that present opportunities for continued research.

Heat transfer in gases in the continuum regime follows Fourier's law and is well understood. However, it has been long understood that in the subcontinuum, rarefied gas regime Fourier's law is no longer valid and various models have been proposed to describe heat transfer in these systems. These models have very limited experimental exploration for spherical geometries due to the difficulties involved. Optically levitated nanoparticles are presented as the ideal experimental system to study heat transfer in rarefied gases due to their isolation from their environment. Nanodiamonds with nitrogen-vacancy centers are used to measure temperature. As the pressure decreases so does the heat transfer to the rarefied gas and the nanodiamond temperature increases by over 200 K. These experiments demonstrate the utility of optically levitated nanoparticles to study heat transfer in any gas across a wide range of pressures. In the future, these measurements can be combined with models to empirically determine the energy accommodation coefficient of any gas.

Abstract Operando thermometry can help resolve open questions about the importance of thermal contributions to plasmonic photocatalysis, but identifying high‐fidelity thermometers with the requisite chemical inertness, thermal stability, and spatial resolution remains challenging. Here, it is demonstrated that a single near‐infrared laser can simultaneously excite upconverting nanoparticles (UCNPs) that serve as luminescent thermometers and photocatalyze the dimerization of 4‐nitrothiophenol (4‐NTP), which is employed as a model reaction. Due to its large anti‐Stokes shift, the UCNP thermometry signal naturally separates from the 4‐NTP Raman signal, which is used to monitor the chemical reaction, in the spectral domain. The surface temperature rise of plasmonic substrates under varying illumination intensity is systematically correlated with the reaction progress. Temperature rises exceeding 40 K are recorded at the maximum intensity used, yet lower intensities combined with external heating to achieve the same temperature rise are shown to catalyze the reaction less effectively. Furthermore, measurements performed using equivalent external heating and an intensity too low to photocatalyze the reaction display no evidence of the reaction occurring. By providing high‐fidelity operando surface temperature measurements, this method offers a valuable tool for elucidating thermal contributions to plasmonic photocatalysis.