Nanoscale thermometry using point contact thermocouples

Additive Manufacturing of Thermoelectric Microdevices for 4D Thermometry

Thermometry, the process of measuring temperature, is one of the most fundamental tasks not only for understanding the thermodynamics of basic physical, chemical, and biological processes but also for thermal management of microelectronics. However, it is a challenge to acquire microscale temperature fields in both space and time. Here, a 3D printed micro-thermoelectric device that enables direct 4D (3D Space + Time) thermometry at the microscale is reported. The device is composed of freestanding thermocouple probe networks, fabricated by bi-metal 3D printing with an outstanding spatial resolution of a few µm. It shows that the developed 4D thermometry can explore dynamics of Joule heating or evaporative cooling on microscale subjects of interest such as a microelectrode or a water meniscus. The utilization of 3D printing further opens up the possibility to freely realize a wide range of on-chip, freestanding microsensors or microelectronic devices without the design restrictions by manufacturing processes.

Passive dual-probe near-field microscopy

Accurate and simultaneous multiposition near-field measurements are essential to study the time-dependent local dynamics, including heat and carrier transfer. The existing passive long-wavelength infrared (LWIR) scattering-type scanning near-field optical microscopy (s-SNOM) systems with a single probe cannot perform precise near-field measurements of the heat or carrier transporting process at the nanoscale level. Therefore, in this study, we developed a passive LWIR s-SNOM system with two probes. To test the effectiveness of the proposed passive LWIR dual-probe s-SNOM system, each probe was precisely controlled using a shear-force feedback system, and the mechanical interference between the probes was used to monitor the distance between the probes. We achieved simultaneous near-field measurements at two different positions 500 nm apart using the proposed passive LWIR dual-probe s-SNOM system. The simultaneously detected near-field signals from two different points were extracted individually, making this technique an effective nanoscale analysis tool for local carrier dynamics.

A Critical Review on the Junction Temperature Measurement of Light Emitting Diodes

In the new age of illumination, light emitting diodes (LEDs) have been proven to be the most efficient alternative to conventional light sources. Yet, in comparison to other lighting systems, LEDs operate at low temperatures while junction temperature (T_j) is is among the main factors dictating their lifespan, reliability, and performance. This indicates that accurate measurement of LED temperature is of great importance to better understand the thermal effects over a system and improve performance. Over the years, various T_j measurement techniques have been developed, and existing methods have been improved in many ways with technological and scientific advancements. Correspondingly, in order to address the governing phenomena, benefits, drawbacks, possibilities, and applications, a wide range of measurement techniques and systems are covered. This paper comprises a large number of published studies on junction temperature measurement approaches for LEDs, and a summary of the experimental parameters employed in the literature are given as a reference. In addition, some of the corrections noted in non-ideal thermal calibration processes are discussed and presented. Finally, a comparison between methods will provide the readers a better insight into the topic and direction for future research.

Development of Nd (III)-Based Terahertz Absorbers Revealing Temperature Dependent Near-Infrared Luminescence

Molecular vibrations in the solid-state, detectable in the terahertz (THz) region, are the subject of research to further develop THz technologies. To observe such vibrations in terahertz time-domain spectroscopy (THz-TDS) and low-frequency (LF) Raman spectroscopy, two supramolecular assemblies with the formula [Nd^III (phen)₃ (NCX)₃] 0.3EtOH (X = S, 1-S; Se, 1-Se) were designed and prepared. Both compounds show several THz-TDS and LF-Raman peaks in the sub-THz range, with the lowest frequencies of 0.65 and 0.59 THz for 1-S and 1-Se, and 0.75 and 0.61 THz for 1-S and 1-Se, respectively. The peak redshift was observed due to the substitution of SCN⁻ by SeCN⁻. Additionally, temperature-dependent TDS-THz studies showed a thermal blueshift phenomenon, as the peak position shifted to 0.68 THz for 1-S and 0.62 THz for 1-Se at 10 K. Based on ab initio calculations, sub-THz vibrations were ascribed to the swaying of the three thiocyanate/selenocyanate. Moreover, both samples exhibited near-infrared (NIR) emission from Nd (III), and very good thermometric properties in the 300–150 K range, comparable to neodymium (III) oxide-based thermometers and higher than previously reported complexes. Moreover, the temperature dependence of fluorescence and THz spectroscopy analysis showed that the reduction in anharmonic thermal vibrations leads to a significant increase in the intensity and a reduction in the width of the emission and LF absorption peaks. These studies provide the basis for developing new routes to adjust the LF vibrational absorption.

Heat transport induced by electron transfer: A general temperature quantum calculation

Electron transfer dominates chemical processes in biological, inorganic, and material chemistry. Energetic aspects of such phenomena, in particular, the energy transfer associated with the electron transfer process, have received little attention in the past but are important in designing energy conversion devices. This paper generalizes our earlier work in this direction, which was based on the semiclassical Marcus theory of electron transfer. It provides, within a simple model, a unified framework that includes the deep (nuclear) tunneling limit of electron transfer and the associated heat transfer when the donor and acceptor sites are seated in environments characterized by different local temperatures. The electron transfer induced heat conduction is shown to go through a maximum at some intermediate average temperature where quantum effects are already appreciable, and it approaches zero when the average temperature is very high (the classical limit) or very low (deep tunneling).

Nanoscale Ultrasensitive Temperature Sensing Based on Upconversion Nanoparticles with Lattice Self-Adaptation

Upconversion nanoparticles have recently received increasing attention due to their outstanding performance in temperature sensing at the nanoscale. Although much effort has been devoted to improve their thermal sensitivity, there is no efficient way for achieving significant enhancement. Here, we show that lattice self-adaptation can unlock a new route for remarkably enhancing the thermal sensitivity of upconversion nanoparticles. The thermally sensitive fluorescence intensity ratio (FIR) of the dopant Er³⁺ is used for indicating the temperature variation, while a heterojunction of NaGdF₄/NaYF₄ is prepared as host material to produce a lattice distortion at the interface which is also sensitive to temperature. With the increase of temperature, the FIR of the transitions ²H_11/2/⁴S_3/2 → ⁴I_15/2 increases, accompanied by the self-adapted decrease of interface lattice distortion that leads to the additional increase in FIR. Using core/shell upconversion nanoparticles with lattice self-adaptation, we achieve an enhanced thermal sensitivity three times higher than core-only nanoparticles.

Unprecedented switching endurance affords for high-resolution surface temperature mapping using a spin-crossover film

Temperature measurement at the nanoscale is of paramount importance in the fields of nanoscience and nanotechnology, and calls for the development of versatile, high-resolution thermometry techniques. Here, the working principle and quantitative performance of a cost-effective nanothermometer are experimentally demonstrated, using a molecular spin-crossover thin film as a surface temperature sensor, probed optically. We evidence highly reliable thermometric performance (diffraction-limited sub-µm spatial, µs temporal and 1 °C thermal resolution), which stems to a large extent from the unprecedented quality of the vacuum-deposited thin films of the molecular complex [Fe(HB(1,2,4-triazol-1-yl)₃)₂] used in this work, in terms of fabrication and switching endurance (>10⁷ thermal cycles in ambient air). As such, our results not only afford for a fully-fledged nanothermometry method, but set also a forthcoming stage in spin-crossover research, which has awaited, since the visionary ideas of Olivier Kahn in the 90’s, a real-world, technological application.

Developing novel thermometry techniques for nanoscale temperature measurements are vital for realizing efficient thermal management of nanoscale devices. Here, the authors report thermally stable spin-crossover material-based nanothermometers for high-resolution surface temperature mapping.

A quantum thermometric sensing and analysis system using fluorescent nanodiamonds for the evaluation of living stem cell functions according to intracellular temperature

Intracellular thermometry techniques play an important role in elucidating the relationship between the intracellular temperature and stem cell functions. However, there have been few reports on thermometry techniques that can detect the intracellular temperature of cells during several days of incubation. In this study, we developed a novel quantum thermometric sensing and analysis system (QTAS) using fluorescent nanodiamonds (FNDs). FNDs could label adipose tissue-derived stem cells (ASCs) at high efficiency with 24 h of incubation, and no cytotoxicity was observed in ASCs labeled with less than 500 μg mL^-1 of FNDs. The peak of FNDs was confirmed at approximately 2.87 GHz with the characteristic fluorescence spectra of NV centers that could be optically detected (optically detected magnetic resonance [ODMR]). The ODMR peak clearly shifted to the high-frequency side as the temperature decreased and gave a mean temperature dependence of -(77.6 ± 11.0) kHz °C^-1, thus the intracellular temperature of living ASCs during several days of culturing could be precisely measured using the QTAS. Moreover, the intracellular temperature was found to influence the production of growth factors and the degree of differentiation into adipocytes and osteocytes. These data suggest that the QTAS can be used to investigate the relationship between intracellular temperature and cellular functions.

Operando Nanoscale Sensors in Catalysis: All Eyes on Catalyst Particles

An era of circularity requires robust and flexible catalysts and reactors. We need profound knowledge of catalytic surface reactions on the local scale (i.e., angstrom-nanometer), whereas the reaction conditions, such as reaction temperature and pressure, are set and controlled on the macroscale (i.e., millimeter-meter). Nanosensors operating on all relevant length scales can supply this information in real time during operando working conditions. In this Perspective, we demonstrate the potential of nanoscale sensors, with special emphasis on local molecular sensing with shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) and local temperature sensing with luminescence thermometry, to acquire new insights of the reaction pathways. We also argue that further developments should be focused on local pressure measurements and on expanding the applications of these local sensors in other areas, such as liquid-phase catalysis, electrocatalysis, and photocatalysis. Ideally, a combination of sensors will be applied to monitor catalyst and reactor "health" and serve as feedback to the reactor conditions.

Nano/Microscale Thermal Field Distribution: Conducting Thermal Decomposition of Pyrolytic-Type Polymer by Heated AFM Probes

In relevant investigations and applications of the heated atomic force microscope (AFM) probes, the determination of the actual thermal distribution between the probe and the materials under processing or testing is a core issue. Herein, the polyphthalaldehyde (PPA) film material and AFM imaging of the decomposition structures (pyrolytic region of PPA) were utilized to study the temperature distribution in the nano/microscale air gap between heated tips and materials. Different sizes of pyramid decomposition structures were formed on the surface of PPA film by the heated tip, which was hovering at the initial tip–sample contact with the preset temperature from 190 to 220 °C for a heating duration ranging from 0.3 to 120 s. According to the positions of the 188 °C isothermal surface in the steady-state probe temperature fields, precise 3D boundary conditions were obtained. We also established a simplified calculation model of the 3D steady-state thermal field based on the experimental results, and calculated the temperature distribution of the air gap under any preset tip temperature, which revealed the principle of horizontal (<700 nm) and vertical (<250 nm) heat transport. Based on our calculation, we fabricated the programmable nano-microscale pyramid structures on the PPA film, which may be a potential application in scanning thermal microscopy.

Wiedemann-Franz Law for Molecular Hopping Transport

The Wiedemann-Franz (WF) law is a fundamental result in solid-state physics that relates the thermal and electrical conductivity of a metal. It is derived from the predominant transport mechanism in metals: the motion of quasi-free charge-carrying particles. Here, an equivalent WF relationship is developed for molecular systems in which charge carriers are moving not as free particles but instead hop between redox sites. We derive a concise analytical relationship between the electrical and thermal conductivity generated by electron hopping in molecular systems and find that the linear temperature dependence of their ratio as expressed in the standard WF law is replaced by a linear dependence on the nuclear reorganization energy associated with the electron hopping process. The robustness of the molecular WF relation is confirmed by examining the conductance properties of a paradigmatic molecular junction. This result opens a new way to analyze conductivity in molecular systems, with possible applications advancing the design of molecular technologies that derive their function from electrical and/or thermal conductance.

Unexpected Strong Thermally Induced Phonon Energy Shift for Mapping Local Temperature

Measuring temperature in nanoscale is crucial for the research and development of microelectronic devices. Plasmon resonance has been utilized to map local temperature gradient in metallic materials (Al) due to their large coefficients of thermal expansion. However, most semiconductors (including Si and SiC) possess much smaller coefficients of thermal expansion due to their strong covalent bonding in crystal structure, for which the plasmon-based temperature measurement becomes unreliable. Here, we report an unexpected strong, thermally induced phonon energy shift in SiC by spatially resolved vibrational spectroscopy in transmission electron microscopy with in situ heating, demonstrating that this shift can be applied as a useful tool for measuring nanoscale temperature. When a bulk phonon spectrum is used, the spatial resolution of vibrational spectroscopy can be as high as one nanometer. Molecular dynamics simulations reveal that lattice expansion only contributes a small fraction of phonon energy shift and that vibrant motions away from the bonds are predominate factors. This study gains deeper insight into the understanding of dynamic behaviors of the phonon and provides a new avenue to measure local temperature in nanodevices.

Three-Dimensional Nanothermistors for Thermal Probing

Accessing the thermal properties of materials or even full devices is a highly relevant topic in research and development. Along with the ongoing trend toward smaller feature sizes, the demands on appropriate instrumentation to access surface temperatures with high thermal and lateral resolution also increase. Scanning thermal microscopy is one of the most powerful technologies to fulfill this task down to the sub-100 nm regime, which, however, strongly depends on the nanoprobe design. In this study, we introduce a three-dimensional (3D) nanoprobe concept, which acts as a nanothermistor to access surface temperatures. Fabrication of nanobridges is done via 3D nanoprinting using focused electron beams, which allows direct-write fabrication on prestructured, self-sensing cantilever. As individual branch dimensions are in the sub-100 nm regime, mechanical stability is first studied by a combined approach of finite-element simulation and scanning electron microscopy-assisted in situ atomic force microscopy (AFM) measurements. After deriving the design rules for mechanically stable 3D nanobridges with vertical stiffness up to 50 N m^-1, a material tuning approach is introduced to increase mechanical wear resistance at the tip apex for high-quality AFM imaging at high scan speeds. Finally, we demonstrate the electrical response in dependence of temperature and find a negative temperature coefficient of -(0.75 ± 0.2) 10^-3 K^-1 and sensing rates of 30 ± 1 ms K^-1 at noise levels of ±0.5 K, which underlines the potential of our concept.

The Seebeck Coefficient of Sputter Deposited Metallic Thin Films: The Role of Process Conditions

Because of their reduced dimensions and mass, thin film thermocouples are a promising candidate for embedded sensors in composite materials, especially for application in lightweight and smart structures. The sensitivity of the thin film thermocouple depends however on the process conditions during deposition. In this work, the influence of the discharge current and residual gas impurities on the Seebeck coefficient is experimentally investigated for sputter deposited copper and constantan thin films. The influence of the layer thickness on the film Seebeck coefficient is also discussed. Our observations indicate that both a decreasing discharge current or an increasing background pressure results in a growing deviation of the film Seebeck coefficient compared to its bulk value. Variations in discharge current or background pressure are linked as they both induce a variation in the ratio between the impurity flux to metal flux towards the growing film. This latter parameter is considered a quantitative measure for the background residual gas incorporation in the film and is known to act as a grain refiner. The observed results emphasize the importance of the domain size on the Seebeck coefficient of metallic thin films.

Understanding contact between platinum nanocontacts at low loads: The effect of reversible plasticity

Metal nanocontacts play a critical role in atomic force microscopy, functional nanostructures, metallic nanoparticles, and nanoscale electromechanical devices. In all cases, knowledge of the area of contact, and its variation with load, is critical for the quantitative prediction of behavior. Often, the contact area is predicted using continuum mechanics models which relate contact size to geometry, material properties, and load. Here we show for platinum nanoprobes that the contact size deviates significantly from these continuum predictions, even at low applied loads and in the absence of irreversible shape change. We use in situ transmission electron microscopy (TEM) with matched molecular dynamics (MD) simulations to investigate the load-dependent size of the contact. Direct measurements of contact radius from MD and TEM exceed the predictions of continuum mechanics by 24%-164%, depending on the model applied. The physical mechanism for this deviation is found to be dislocation activity in the near-surface material, which is fully reversed upon unloading. These findings demonstrate that contact mechanics models are insufficient for predicting contact area in real-world platinum nanostructures, even at ultra-low applied loads.

Assistance of metal nanoparticles in photocatalysis – nothing more than a classical heat source

We show that the number of high energy non-thermal electrons in a metal under CW illumination is very low but much higher than in thermal equilibrium, implying that faster chemical reactions reported previously are extremely likely to originate from a pure thermal effect.

Electron-Transfer-Induced Thermal and Thermoelectric Rectification

Controlling the direction and magnitude of both heat and electronic currents using rectifiers has significant implications for the advancement of molecular circuit design. In order to facilitate the implementation of new transport phenomena in such molecular structures, we examine thermal and thermoelectric rectification effects that are induced by an electron transfer process that occurs across a temperature gradient between molecules. Historically, the only known heat conduction mechanism able to generate thermal rectification in purely molecular environments is phononic heat transport. Here, we show that electron transfer between molecular sites with different local temperatures can also generate a thermal rectification effect and that electron hopping through molecular bridges connecting metal leads at different temperatures gives rise to asymmetric Seebeck effects, that is, thermoelectric rectification, in molecular junctions.

High-performance flexible metal-on-silicon thermocouple

We have demonstrated metal-on-silicon thermocouples with a noticeably high Seebeck coefficient and an excellent temperature-sensing resolution. Fabrication of the thermocouples involved only simple photolithography and metal-liftoff procedures on a silicon substrate. The experimentally measured Seebeck coefficient of our thermocouple was 9.17 × 10-4 V/°K, which is 30 times larger than those reported for standard metal thin-film thermocouples and comparable to the values of alloy-based thin-film thermocouples that require sophisticated and costly fabrication processes. The temperature-voltage measurements between 20 to 80 °C were highly linear with a linearity coefficient of 1, and the experimentally demonstrated temperature-sensing resolution was 0.01 °K which could be further improved up to a theoretical limit of 0.00055 °K. Finally, we applied this approach to demonstrate a flexible metal-on-silicon thermocouple with enhanced thermal sensitivity. The outstanding performance of our thermocouple combined with an extremely thin profile, bending flexibility, and simple, highly-compatible fabrication will proliferate its use in diverse applications such as micro-/nanoscale biometrics, energy management, and nanoscale thermography.

AFM-thermoreflectance for simultaneous measurements of the topography and temperature

To understand the thermal failure mechanisms of electronic devices, it is essential to measure the temperature and characterize the thermal properties of individual nanometer-scale transistors in electronic devices. Previously, scanning thermal microscopy (SThM) has been used to measure the local temperature with nanometer-scale spatial resolutions using a probe with a built-in temperature sensor. However, this type of temperature measurement requires additional equipment to process the temperature-sensing signals and expensive temperature-sensor-integrated probes fabricated by complicated MEMS processes. Here, we present a novel technique which enables the simultaneous measurement of the temperature and topography of nanostructures only with a conventional atomic force microscope (AFM) of the type commonly used for topography measurements and without any modifications of the probe and extra accessories for data acquisition. The underlying principle of the proposed technique is that the local temperature of a specimen is estimated quantitatively from the thermoreflectance of a bare silicon AFM probe that is in contact with a specimen. The temperature obtained by our technique is found to be consistent with a result obtained by SThM measurements.

We propose a novel form of AFM-based thermometry capable of sub-100 nm spatial resolution only with a conventional AFM setup by exploiting the thermoreflectance characteristic of the AFM Si probe.

Feedback control of local hotspot temperature using resistive on-substrate nanoheater/thermometer

This article reports the active control of a local hotspot temperature for accurate nanoscale thermal transport measurement. To this end, we have fabricated resistive on-substrate nanoheater/thermometer (NH/T) devices that have a sensing area of ∼350 nm × 300 nm. Feedback-controlled temporal heating and cooling experiments of the NH/T device confirm that the feedback integral gain plays a dominant role in device's response time for various setpoint temperatures. To further verify the integration of the feedback controller with the NH/T devices, a local tip-induced cooling experiment is performed by scanning a silicon tip over the hotspot area in an atomic force microscope platform. By carefully optimizing the feedback gain and the tip scan speed, we can control the hotspot temperature with the accuracy of ∼±1 K for a broad range of setpoints from 325 K to 355 K. The obtained tip-substrate thermal conductance, including the effects of solid-solid conduction, water meniscus, air conduction, and near-field thermal radiation, is found to be a slightly increasing function of temperature in the range of 127 ± 25 to 179 ± 16 nW/K. Our work demonstrates the reliable controllability of a local hotspot temperature, which will allow the further improvement of various nanoscale thermal metrologies including scanning thermal microscopy and nanoscale thermometry.

Metal nanospheres under intense continuous-wave illumination: A unique case of nonperturbative nonlinear nanophotonics

We show that the standard perturbative (i.e., cubic) description of the thermal nonlinear response of a single metal nanosphere to intense continuous-wave (CW) illumination is sufficient only for a temperature rise of up to 100 degrees above room temperature. Beyond this regime, the slowing down of the temperature rise requires a nonperturbative description of the nonlinear response, even though the permittivity is linearly dependent on the temperature and despite the deep subwavelength effective propagation distances involved. Using experimental data, we show that, generically, the increase of the imaginary part of the metal permittivity dominates the increase of the host permittivity as well as the resonance shift due to the joint changes to the real parts of the metal and host. Thus, the main nonlinear effect is a decrease of the quality factor of the resonance. We further analyze the relative importance of the various contributions to the temperature rise and thermal nonlinearity, compare the nonlinearity of Au and Ag, demonstrate the potential effect of the nanoparticle morphology, and show that although the thermo-optical nonlinearity of the host typically plays a minor role, its thermal conductivity and its temperature dependence is important. Finally, we discuss the differences between CW and ultrafast thermal nonlinearities.

Electrothermal Transistor Effect and Cyclic Electronic Currents in Multithermal Charge Transfer Networks

A theory is developed to describe the coupled transport of energy and charge in networks of electron donor-acceptor sites which are seated in a thermally heterogeneous environment, where the transfer kinetics are dominated by Marcus-type hopping rates. It is found that the coupling of heat and charge transfer in such systems gives rise to exotic transport phenomena which are absent in thermally homogeneous systems and cannot be described by standard thermoelectric relations. Specifically, the directionality and extent of thermal transistor amplification and cyclical electronic currents in a given network can be controlled by tuning the underlying temperature gradient in the system. The application of these findings toward the optimal control of multithermal currents is illustrated on a paradigmatic nanostructure.

To save half contact pads in 2D mapping of local temperatures with a thermocouple array

We have demonstrated the excellent performance of tree-like thin-film micro-thermocouple arrays, where for each array all the leads of one material shared a common contact pad. This simplified configuration may find applications in novel wearable and flexible devices.

Recent advances in hybrid measurement methods based on atomic force microscopy and surface sensitive measurement techniques

This review summaries the recent progress of the combination of optical and non-optical surface sensitive techniques with the atomic force microscopy.

Nanoscale thermal imaging of dissipation in quantum systems

Energy dissipation is a fundamental process governing the dynamics of physical, chemical and biological systems. It is also one of the main characteristics that distinguish quantum from classical phenomena. In particular, in condensed matter physics, scattering mechanisms, loss of quantum information or breakdown of topological protection are deeply rooted in the intricate details of how and where the dissipation occurs. Yet the microscopic behaviour of a system is usually not formulated in terms of dissipation because energy dissipation is not a readily measurable quantity on the micrometre scale. Although nanoscale thermometry has gained much recent interest, existing thermal imaging methods are not sensitive enough for the study of quantum systems and are also unsuitable for the low-temperature operation that is required. Here we report a nano-thermometer based on a superconducting quantum interference device with a diameter of less than 50 nanometres that resides at the apex of a sharp pipette: it provides scanning cryogenic thermal sensing that is four orders of magnitude more sensitive than previous devices-below 1 μK Hz^-1/2. This non-contact, non-invasive thermometry allows thermal imaging of very low intensity, nanoscale energy dissipation down to the fundamental Landauer limit of 40 femtowatts for continuous readout of a single qubit at one gigahertz at 4.2 kelvin. These advances enable the observation of changes in dissipation due to single-electron charging of individual quantum dots in carbon nanotubes. They also reveal a dissipation mechanism attributable to resonant localized states in graphene encapsulated within hexagonal boron nitride, opening the door to direct thermal imaging of nanoscale dissipation processes in quantum matter.

Electron transfer across a thermal gradient

Charge transfer is a fundamental process that underlies a multitude of phenomena in chemistry and biology. Recent advances in observing and manipulating charge and heat transport at the nanoscale, and recently developed techniques for monitoring temperature at high temporal and spatial resolution, imply the need for considering electron transfer across thermal gradients. Here, a theory is developed for the rate of electron transfer and the associated heat transport between donor-acceptor pairs located at sites of different temperatures. To this end, through application of a generalized multidimensional transition state theory, the traditional Arrhenius picture of activation energy as a single point on a free energy surface is replaced with a bithermal property that is derived from statistical weighting over all configurations where the reactant and product states are equienergetic. The flow of energy associated with the electron transfer process is also examined, leading to relations between the rate of heat exchange among the donor and acceptor sites as functions of the temperature difference and the electronic driving bias. In particular, we find that an open electron transfer channel contributes to enhanced heat transport between sites even when they are in electronic equilibrium. The presented results provide a unified theory for charge transport and the associated heat conduction between sites at different temperatures.

Wide-Range Thermometry at Micro/Nano Length Scales with In2O3 Octahedrons as Optical Probes

We report the temperature-dependent photoluminescence and Raman spectra of In2O3 octahedrons synthesized by an evaporation-condensation process. The luminescence obtained here is due to the defect-related deep level emission, which shows highly temperature-dependent behavior in 83-573 K range. Both the position as well as the intensity varies with temperature. Similarly, Raman spectroscopy in 83-303 K range shows temperature-dependent variation in peak intensity but no change in the peak position. Interestingly, the variation of intensity for different peaks is consistent with Placzek theory which invokes the possibility of temperature sensing. We demonstrate the reversibility of peak intensity with temperature for consecutive cycles and excellent stability of the octahedrons toward cryogenic temperature sensing. Overall, both the temperature-dependent photoluminescence and Raman spectra can be explored to determine temperature in the cryogenic range at micro/nano length scales. As an example, we evaluate the temperature-dependent Raman spectra of WO3 that undergoes a phase transition around 210 K and temperature-dependent luminescence of Rhodamine 6G (Rh6G) where intensity varies with temperature.

Nanothermometer Based on Resonant Tunneling Diodes: From Cryogenic to Room Temperatures

Sensor miniaturization together with broadening temperature sensing range are fundamental challenges in nanothermometry. By exploiting a large temperature-dependent screening effect observed in a resonant tunneling diode in sequence with a GaInNAs/GaAs quantum well, we present a low dimensional, wide range, and high sensitive nanothermometer. This sensor shows a large threshold voltage shift of the bistable switching of more than 4.5 V for a temperature raise from 4.5 to 295 K, with a linear voltage-temperature response of 19.2 mV K(-1), and a temperature uncertainty in the millikelvin (mK) range. Also, when we monitor the electroluminescence emission spectrum, an optical read-out control of the thermometer is provided. The combination of electrical and optical read-outs together with the sensor architecture excel the device as a thermometer with the capability of noninvasive temperature sensing, high local resolution, and sensitivity.

Optical heating and temperature determination of core-shell gold nanoparticles and single-walled carbon nanotube microparticles

The real-time temperature measurement of nanostructured materials is particularly attractive in view of increasing needs of local temperature probing with high sensitivity and resolution in nanoelectronics, integrated photonics, and biomedicine. Light-induced heating and Raman scattering of single-walled carbon nanotubes with adsorbed gold nanoparticles decorating silica microparticles are reported, by both green and near IR lasers. The plasmonic shell is used as nanoheater, while the single-walled carbon nanotubes are Raman active and serve as a thermometer. Stokes and Anti-Stokes Raman spectra of single-walled carbon nanotubes serve to estimate the effective light-induced temperature rise on the metal nanoparticles. The temperature rise is constant with time, indicating stability of the adsorption density. The effective temperatures derived from Stokes and Anti-Stokes intensities are correlated with those measured in a heating stage. The resolution of the thermal experiments in our study was found to be 5-40 K.

High resolution direct measurement of temperature distribution in silicon nanophotonics devices

Following the miniaturization of photonic devices and the increase in data rates, the issues of self heating and heat removal in active nanophotonic devices should be considered and studied in more details. In this paper we use the approach of Scanning Thermal Microscopy (SThM) to obtain an image of the temperature field of a silicon micro ring resonator with sub-micron spatial resolution. The temperature rise in the device is a result of self heating which is caused by free carrier absorption in the doped silicon. The temperature is measured locally and directly using a temperature sensitive AFM probe. We show that this local temperature measurement is feasible in the photonic device despite the perturbation that is introduced by the probe. Using the above method we observed a significant self heating of about 10 degrees within the device.

Ratiometric temperature imaging using environment-insensitive luminescence of Mn-doped core-shell nanocrystals

We report a ratiometric temperature imaging method based on Mn luminescence from Mn-doped CdS-ZnS nanocrystals (NCs) with controlled doping location, which is designed to exhibit strong temperature dependence of the spectral lineshape while being insensitive to the surrounding chemical environment. Ratiometric thermometry on the Mn luminescence spectrum was performed by using Mn-doped CdS-ZnS core-shell NCs that have a large local lattice strain on the Mn site, which results in the enhanced temperature dependence of the bandwidth and peak position. The Mn luminescence spectral lineshape is highly robust with respect to the change in the polarity, phase and pH of the surrounding medium and aggregation of the NCs, showing great potential in temperature imaging under chemically heterogeneous environment. The temperature sensitivity (ΔIR/IR = 0.5%/K at 293 K, IR = intensity ratio at two different wavelengths) is highly linear in a wide range of temperatures from cryogenic to above-ambient temperatures. We demonstrate the surface temperature imaging of a cryo-cooling device showing a temperature variation of >200 K by imaging the luminescence of the NC film formed by simple spin coating, taking advantage of the environment-insensitive luminescence.

Thermometry at the nanoscale

Non-invasive precise thermometers working at the nanoscale with high spatial resolution, where the conventional methods are ineffective, have emerged over the last couple of years as a very active field of research. This has been strongly stimulated by the numerous challenging requests arising from nanotechnology and biomedicine. This critical review offers a general overview of recent examples of luminescent and non-luminescent thermometers working at nanometric scale. Luminescent thermometers encompass organic dyes, QDs and Ln(3+)ions as thermal probes, as well as more complex thermometric systems formed by polymer and organic-inorganic hybrid matrices encapsulating these emitting centres. Non-luminescent thermometers comprise of scanning thermal microscopy, nanolithography thermometry, carbon nanotube thermometry and biomaterials thermometry. Emphasis has been put on ratiometric examples reporting spatial resolution lower than 1 micron, as, for instance, intracellular thermometers based on organic dyes, thermoresponsive polymers, mesoporous silica NPs, QDs, and Ln(3+)-based up-converting NPs and β-diketonate complexes. Finally, we discuss the challenges and opportunities in the development for highly sensitive ratiometric thermometers operating at the physiological temperature range with submicron spatial resolution.

An extremely simple thermocouple made of a single layer of metal

A novel temperature sensor consisting of a single layer of metal (Ni, Pd, W, or Pt) is constructed. Its configuration challenges a long-established concept and may lead to development of a new category of devices. Reliable two-dimensional mapping of local temperatures is demonstrated using an array of these sensors. These single-metal thermocouples (SMTCs) can be readily applied on flexible substrates or at high temperatures.

Ultra-high vacuum scanning thermal microscopy for nanometer resolution quantitative thermometry

Understanding energy dissipation at the nanoscale requires the ability to probe temperature fields with nanometer resolution. Here, we describe an ultra-high vacuum (UHV)-based scanning thermal microscope (SThM) technique that is capable of quantitatively mapping temperature fields with ∼15 mK temperature resolution and ∼10 nm spatial resolution. In this technique, a custom fabricated atomic force microscope (AFM) cantilever, with a nanoscale Au-Cr thermocouple integrated into the tip of the probe, is used to measure temperature fields of surfaces. Operation in an UHV environment eliminates parasitic heat transport between the tip and the sample enabling quantitative measurement of temperature fields on metal and dielectric surfaces with nanoscale resolution. We demonstrate the capabilities of this technique by directly imaging thermal fields in the vicinity of a 200 nm wide, self-heated, Pt line. Our measurements are in excellent agreement with computational results-unambiguously demonstrating the quantitative capabilities of the technique. UHV-SThM techniques will play an important role in the study of energy dissipation in nanometer-sized electronic and photonic devices and the study of phonon and electron transport at the nanoscale.

Thermal imaging of nanostructures by quantitative optical phase analysis

We introduce an optical microscopy technique aimed at characterizing the heat generation arising from nanostructures, in a comprehensive and quantitative manner. Namely, the technique permits (i) mapping the temperature distribution around the source of heat, (ii) mapping the heat power density delivered by the source, and (iii) retrieving the absolute absorption cross section of light-absorbing structures. The technique is based on the measure of the thermal-induced refractive index variation of the medium surrounding the source of heat. The measurement is achieved using an association of a regular CCD camera along with a modified Hartmann diffraction grating. Such a simple association makes this technique straightforward to implement on any conventional microscope with its native broadband illumination conditions. We illustrate this technique on gold nanoparticles illuminated at their plasmonic resonance. The spatial resolution of this technique is diffraction limited, and temperature variations weaker than 1 K can be detected.

Quantitative thermometry of nanoscale hot spots

A method is described to quantify thermal conductance and temperature distributions with nanoscale resolution using scanning thermal microscopy. In the first step, the thermal resistance of the tip-surface contact is measured for each point of a surface. In the second step, the local temperature is determined from the difference between the measured heat flux for heat sources switched on and off. The method is demonstrated using self-heating of silicon nanowires. While a homogeneous nanowire shows a bell-shaped temperature profile, a nanowire diode exhibits a hot spot centered near the junction between two doped segments.

Thermoelectric voltage at a nanometer-scale heated tip point contact

We report thermoelectric voltage measurements between the platinum-coated tip of a heated atomic force microscope (AFM) cantilever and a gold-coated substrate. The cantilevers have an integrated heater-thermometer element made from doped single crystal silicon, and a platinum tip. The voltage can be measured at the tip, independent from the cantilever heating. We used the thermocouple junction between the platinum tip and the gold substrate to measure thermoelectric voltage during heating. Experiments used either sample-side or tip-side heating, over the temperature range 25-275 °C. The tip-substrate contact is ∼4 nm in diameter and its average measured Seebeck coefficient is 3.4 μV K(-1). The thermoelectric voltage is used to determine tip-substrate interface temperature when the substrate is either glass or quartz. When the non-dimensional cantilever heater temperature is 1, the tip-substrate interface temperature is 0.593 on glass and 0.125 on quartz. Thermal contact resistance between the tip and the substrate heavily influences the tip-substrate interface temperature. Measurements agree well with modeling when the tip-substrate interface contact resistance is 10(8) K W(-1).

Quantitative measurement with scanning thermal microscope by preventing the distortion due to the heat transfer through the air

Because of its high spatial resolution, scanning thermal microscopy (SThM) has been developed quite actively and applied in such diverse areas as microelectronics, optoelectronics, polymers, and carbon nanotubes for more than a decade since the 1990s. However, despite its long history and diverse areas of application, surprisingly, no quantitative profiling method has been established yet. This is mostly due to the nonlocal nature of measurement by conventional SThM: the signal measured by SThM is induced not only from the local heat flux through the tip-sample thermal contact but also (and mostly) from the heat flux through the air gap between the sample and the SThM probe. In this study, a rigorous but simple and practical theory for quantitative SThM for local measurement is established and verified experimentally using high-performance SThM probes. The development of quantitative SThM will make possible new breakthroughs in diverse fields of nanothermal science and engineering.

Quantitative heat dissipation characteristics in current-carrying GaN nanowires probed by combining scanning thermal microscopy and spatially resolved Raman spectroscopy

Using an approach combining scanning thermal microscopy (SThM) and spatially revolved Raman spectroscopy, we have investigated quantitatively the heat dissipation characteristics in substrate-supported and suspended (with asymmetric type of contacts) current-carrying GaN nanowires with diameters of ∼40-60 nm, where the phonon confinement is expected to play an important role in thermal transport. In particular, this approach allows direct measurements of nanowire-substrate/electrode interface thermal resistances and the nanowire thermal conductivity. On the basis of these results, the nanowire-substrate thermal transfer was suggested to be the main heat dissipation route, counting for ∼80-93% of the total dissipated heat, whereas the nanowire-electrode interface plays a minor role. The relative significance of nanowire-substrate/electrode interfaces in dissipating heat was further demonstrated in suspended nanowire devices. The measured nanowire thermal conductivity (∼40-60 W/mK) is lower than that in bulk GaN, possibly due to the phonon confinement and boundary scattering effects. Besides providing quantitative insight into heat dissipation characteristics, our results also reveal aspects, particularly the topography-related thermal signals and the relative significance of various tip-sample thermal transfer processes, that are important to advancing the applications of SThM technique in nanoscale thermal characterizations.