Temperature and thermal conductivity are key parameters in applications ranging from microelectronics to energy conversion. As we strive to advance the state of the art, feature sizes continue to shrink, operating temperatures are pushed even higher, and in general devices must function in increasingly extreme regimes. Consequently, many of the traditional methods used to measure temperature or thermal conductivity are no longer applicable. This continued paradigm shift thus necessitates the development of new thermal metrology.

In this dissertation, I develop far-field optical thermometry techniques to measure temperature and thermal conductivity in challenging environments – namely, at temperatures up to 3,000 K and in systems with sub-50 nm features. All of these techniques have the benefit of far-field, non-contact operation, which allows for measurements in scenarios where many conventional methods fail for both practical and fundamental reasons. I begin by describing a Planck spectral fitting method developed to measure the peak temperature of reduced graphene oxide (RGO)-based films that are Joule heated to temperatures as high as 3,000 K. I also describe a “conduction shoulder” fitting method that combines image processing with a nonlinear, multimode heat transfer model in order to experimentally determine the high-temperature thermal conductivity of these same RGO films. Next, I introduce a far-field optical nanothermometry technique based on the temperature-dependent luminescence of individual NaYF4:Yb3+,Er3+ nanoparticles. This technique allows for single-point temperature measurements with spatial resolution of 50 nm or better. I then report apparent self-heating effects at high excitation intensities that indicate an apparent temperature rise > 50 K at an excitation intensity of 2.5 x 105 W cm-2 if interpreted as thermal; however, I demonstrate that these effects are in fact non-thermal in nature. Instead, rate equation modeling shows that these effects are due to both radiative and non-radiative relaxation from high energy excited states in Er3+ that are heavily populated at high excitation intensities. Finally, I demonstrate that individual nanoparticle thermometers can be used to measure nanoscale hotspots that result from both optical heating via a tightly focused laser beam and non- uniform Joule heating caused by a geometric effect known as current crowding.

Detecting depth varying thermal properties is desirable for applications such as cryosurgery, landmine detection and non-destructive testing. This can be accomplished by periodic heating, exploiting the resulting frequency dependent penetration depth. Using this, objects with thermal conductivity distinct from their surroundings (such as material flaws or landmines) can be detected. In cryosurgery, for treatment of atrial fibrillation, it is desirable to track the location of moving boundaries between two phases (i.e. frozen and thawed tissue) with sub-millimeter precision. This spatial resolution is not attainable with conventional imaging.

In this dissertation, the 3ω thermal property measurement technique is used to make new types of measurements on systems changing in real time. Analytical understanding of measurements with multiple frequency excitations is first developed, and verified with experiments. Next, the 3ω technique is used to sense thickness of samples (200μm and 500μm ice). The same sensors are used to detect step changes in thermal properties, including phase change, of water and mouse liver samples in real time. Finally an experimental setup is constructed to track an oil-air interface as it moves. Numerical modeling is used to convert measured 3ω voltages into spatial dimensions. For slow front velocities, the location of the oil-air front determined from the 3ω approach is found to be accurate to within an average error of under 18μm (RMS) for front distances between 12 and 360μm.

Creating technologies to address increasingly diverse challenges ranging from biomedical devices to carbon-free energy solutions requires measuring material properties and system behaviors in increasingly challenging regimes. In the biomedical field, accurate determination of the thermal conductivity (k) of biological tissues is important for cryopreservation, thermal ablation, and cryosurgery, but is hampered by the delicate nature and often-small sizes of tissues. In the electronics and clean energy fields, it is increasingly necessary to reliably model the dissipation of heat from micro and nanoelectronics for thermal management, and the transport of heat through nanostructured materials for energy control and conversion technologies such as batteries and thermoelectrics. However, the classical equations of heat transfer break down at these short length scales, calling into question the validity of various formulations of heat transfer theory and the very concept of thermal conductivity itself. Here, too, is a need for challenging thermal conductivity measurements at micron and nanometer scales. In this thesis, we describe and demonstrate two techniques that combined are capable of measuring the key thermal transport properties in all of these regimes.

We adapt the 3ω method—widely used for rigid, inorganic solids—as a reusable sensor to measure k of soft biological samples, two orders of magnitude thinner than conventional tissue characterization methods. Analytical and numerical studies quantify the error of the commonly used “boundary mismatch approximation” of the bi-directional 3ω geometry, confirm that the generalized slope method is exact in the low-frequency limit, and bound its error for finite frequencies. The bi-directional 3ω measurement device is validated using control experiments to within ± 2% (liquid water, std. dev.) and ± 5% (ice). Measurements of mouse liver cover a temperature range from -69 oC to + 33 oC. The liver results are independent of sample thicknesses from 3 mm down to 100 μm, and agree with available literature for non-mouse liver to within the measurement scatter.

Next, we focus the laser spot 1/e^2 radius in TDTR measurements down to single micron length scales to measure quasi-ballistic thermal transport at length scales where Fourier’s law breaks down. We present an in-depth discussion of the instrumentation and provide comprehensive analyses of system sensitivities to all experimental parameters. The system is first validated on sapphire and single crystal silicon control samples. We then measure two nano-grained Si samples (550 nm and 76 nm average grain size) and two SiGe alloys (1% and 9.9% Ge concentration), representing two classes of silicon-based materials with qualitatively different phonon scattering physics. All samples are 5 mm x 5 mm x 0.5 mm or larger. Sub-diffusion measurements are performed on all samples using 1/e^2 laser spot radii down to 1.6 μm. Apparent thermal conductivity suppressions ranging from 18% to 76% are observed at room temperature, indicating that while most of the heat in sapphire, Si, and nano-grained Si is carried by phonons with mean free paths of a couple microns or less, much of the heat in SiGe alloys is still carried by phonons with mean free paths up to a few tens of microns at room temperature. We present a discussion of the microscale origins of this suppressed thermal conductivity and its physical interpretation, addressing some common misconceptions. Our results show that alloying and nanostructuring shift the spectral phonon mean free path distributions in opposite directions. Alloying skews the phonon distribution toward long mean free paths, increasing k suppression at small length scales, while nanostructuring skews the distribution toward short mean free paths, reducing k suppression.

Various modern devices involve highly anisotropic materials. For example, Bi₂Te₃ is used in thermoelectrics, and graphene finds broad applications ranging from microelectronics to optoelectronics. The heat transfer in these materials can deviate significantly from classical isotropic transport theory. Nonlinear thermal devices have also drawn a great deal of attention for such applications as thermal regulation of building envelopes, and thermal protection of delicate components in electrical hardware, spacecraft thermal shielding, and satellite radiators.

In this thesis, heat transfer in nonlinear devices and anisotropic materials, in particular graphene, is investigated using both experimental and theoretical methods. Measurements on graphene sheets encased by silicon dioxide layers show the strong effect of the encasing oxide in disrupting the thermal conductivity of adjacent graphene layers, leading to more than one order of magnitude suppression as compared to the freely-suspended graphene experiment reported in literature. Modeling thermal properties of anisotropic materials reveals an unexpected guideline to engineer heat transport: due to phonon focusing effects, in many cases the heat transfer can be enhanced by reducing a phonon velocity component perpendicular to the transport direction. Finally, a nonlinear thermal diode, based on a new mechanism exploiting asymmetric scattering of ballistic energy carriers by pyramidal reflectors, is demonstrated experimentally. Experiments underline that all thermal rectifiers require nonlinearity in addition to asymmetry.

The study of micro/nanoscale heat transfer is of significant fundamental and practical importance due to the following reasons. First, the physical mechanisms of heat transfer at the micro/nanoscale can be very different and much more complicated than at the macroscale. In addition, micro/nanoscale heat transfer is relevant to diverse applications such as thermal management in nanoelectronics and energy conversion devices including photovoltaics and thermoelectrics. Yet, the experimental studies of micro/nanoscale heat transfer have lagged far behind the theory. Therefore, in this work, we experimentally study both heat radiation and conduction at the micro/nanoscale using microfabricated suspended devices and beam-offset frequency domain thermoreflectance, with three related topics.

First, we experimentally explore near-field radiative heat transfer (NFRHT) in the “dual nanoscale” regime in which both the characteristic dimensions of the emitter and receiver, and the vacuum gap spacing between them, are in the deep subwavelength regime (e.g., 100 nm and below) by using microfabricated suspended devices. Specifically, NFRHT between the two coplanar SiC membranes with thickness comparable to or smaller than their vacuum gap spacing of ~100 nm is studied for the first time. We show that a new physical mechanism introduced by electromagnetic corner and edge modes dominates NFRHT in this dual subwavelength regime and leads to a substantially high heat transfer coefficient of 800 W/m2K at a temperature of 300 K for a membrane thickness of 20 nm. This heat transfer coefficient is 5 times larger than that calculated for a reference scenario of heat transfer between two infinite SiC surfaces separated by a vacuum gap of 100 nm, and 1200 times larger than a reference scenario using classical view factors to calculate the heat transfer between two blackbodies of the same sizes as the membranes. Such a high heat transfer coefficient may have the potential to find applications such as localized radiative cooling, thermal management technologies, and energy conversion devices.

The next topic is related to the thermal conductivity measurements of polycrystalline SiC thin films grown by LPCVD. The effects of post-deposition thermal annealing on the film microstructures and thus the thermal conductivity of SiC are studied using microfabricated suspended devices. By annealing the films at 1100 oC for at least 2 hours, the grain size increases from 5.5 nm to 6.6 nm and the porosity decreases from 6.5% to practically fully dense, with the combined effect of increasing the measured thermal conductivity near room temperature by 34%, from 5.8 W/m-K to 7.8 W/m-K. The thermal conductivity measurements also show a very good agreement (better than 3%) with a simple model based on kinetic theory combined with a Maxwell-Garnett porosity correction. Such information is important for the thermal management of various SiC thin film applications ranging from MEMS to optoelectronic devices.

Finally, in order to facilitate further thermal conductivity measurements, a scannable, non-contact and relatively simple technique called beam-offset frequency domain thermoreflectance (BO-FDTR) is explored and discussed. Specifically, we extend previous BO-FDTR, which had been limited to transversely isotropic materials, to measure the thermal conductivity tensor for a more general case of materials lacking in-plane symmetry (i.e., transversely anisotropic materials). Extensive sensitivity analysis is performed to determine appropriate heating frequencies and beam offsets for the measurements. We demonstrate this new technique by measuring the thermal conductivity tensor of a model transversely anisotropic material x-cut quartz (<110> α-SiO2) along with control measurements on two transversely isotropic materials, sapphire and highly oriented pyrolytic graphite (HOPG). All of the measurements are in very good agreement with literature values. In addition, when isotropic directions are present, the measurements show excellent self-consistency in correctly identifying them, with residual errors less than 4%. This extended BO-FDTR technique shown in this work may facilitate future heat transfer study on a broader class of materials with arbitrary in-plane anisotropy.

Mapping temperature with nanoscale spatial resolution is important for thermal transport studies and for device applications. Current state of the art techniques include scanning thermal microscopy, which requires physical contact, and optical thermoreflectance, which has diffraction limited spatial resolution. Scanning electron microscopy (SEM) is routinely used for topographic imaging due to the high spatial resolution and simple sample preparation, but the secondary electron signal has not been used as a temperature sensor.

In this work, the temperature dependence of SEM signals from a sample’s surface is investigated. The charge flowing through the SEM sample and the emitted secondary electron currents are measured at different temperatures to quantify the temperature dependence of secondary electron emission. I have measured several group III-V and IV semiconductors and have found 100s of ppm/K of thermal response coefficients. I have attempted to utilize the temperature coefficient to map temperature gradients in an SEM image by creating temperature profiles using a microfabricated structure and by implementing a double heater scheme. Technical challenges and practical limitations will require further future work on the effort of mapping temperature and demonstrating higher spatial resolution. This work generalizes the temperature response coefficient of secondary electron emission across various materials and investigates the challenges involved in mapping temperature. SEM thermometry is a novel means of measuring temperature and has the potential for high resolution temperature mapping which can open new research directions such as phonon mean free path spectroscopy.

Scanning electron microscopy (SEM) is widely used to produce images of nanostructures by scanning the surface of the sample with a focused electron beam. The electron beam interaction with the sample produces various electron and x-ray signals at different depths of the sample. Secondary electrons (SE) only escape from the top few nanometers of the surface of a sample due to their low energy and short mean free path characteristic in solid matter. Past studies have reported small temperature effects on SE yield in different materials. Furthermore, during the electron-substrate interaction, heat is also generated and this makes it possible to apply the e-beam as a high-quality mobile heat source for generating nanoscale thermal hotspots.

In this work, I first describe my studies of the e-beam heating effect from the SEM on suspended silicon nitride (SiNx) thin films with thickness ranging from 200 to 500 nm using micro-fabricated calorimeter devices inside a standard SEM. The results validate the absorption energy profiles calculated using Monte Carlo (MC) techniques by CASINO in Chapter 2. Then, the secondary electron yields from polycrystalline silicon as a function of temperature using SEM imaging have been studied with the assistance of CASINO in Chapter 3 and 4. This study guides us to a path of sub 100 nm spatial resolution thermometry technique using widely available hardware, overall this approach demonstrated and validated the SEM thermometry technique using SE intensity change at ~100 nm scale resolution, which is far finer than the previous demonstration by Khan et al. (JAP, 2018) which had averaged over much larger, ~500 m by 500 m, area. These results will help develop the application of the e-beam as an advanced nanoscale mobile heating source and secondary electron signal as temperature signal for future thermal metrologies at the micro and nanoscale (Chapter 5).

Binary oxide materials exhibit a wide range of technologically relevant behaviors (i.e. magnetism, superconductivity, and ferroelectricity). For this reason, there is much interest in oxides due to their potential in applications ranging from batteries and solar cells to power electronics and more. Unfortunately, oxide materials are hindered by experimental and physical challenges such as sample growth, control of the bandgap, and doping —processes that are well understood and realized in traditional semiconductors (i.e., Si, Ge). Band structure engineering is one method that modifies semiconductor properties providing a means to control the performance of a wide range of electronic materials to meet device requirements.

Highly mismatched alloys are semiconductor alloys formed through isoelectronic substitution of anions with very different ion size and electronegativity, which allows drastic band structure modification with dilute alloy content. In this work, ZnO and Ga2O3 were alloyed with S to study the drastic band structure modification with alloy content.

Alloys from ZnO and ZnS were synthesized by radio-frequency magnetron sputtering and pulsed-laser deposition over the entire alloying range. Sputtering is a favorable deposition technique for an industrial production line due to better process integration with currently used methods. Pulsed-laser deposition is suitable for the growth of materials with large miscibility gaps arising from the large differences in atomic size and electronegativity due to the potential for both stoichiometric transfer of target materials to the substrate and the use of non-equilibrium growth conditions. Using X-ray diffraction, the ZnO1-xSx films were found to be highly textured with a columnar-like structure that remains throughout the entire composition range (determined by transmission electron microscopy). The optical absorption edge of these alloys decreases rapidly with small amount of added sulfur (x ~ 0.02) and continues to red shift to a minimum of 2.6eV at x=0.45. At higher sulfur concentrations (x > 0.45), the absorption edge shows a continuous blue shift. The strong reduction in the bandgap for O-rich alloys is the result of the upward shift of the valence-band edge with x as observed by X-ray photoelectron spectroscopy. As a result, the room temperature bandgap of ZnO1-xSx alloys can be tuned from 3.7 eV to 2.6 eV. The observed large bowing in the composition dependence of the energy bandgap arises from the anticrossing interactions between (1) the valence-band of ZnO and the localized sulfur level at 0.30 eV above the ZnO valence-band maximum for O-rich alloys and (2) the conduction-band of ZnS and the localized oxygen level at 0.20 eV below the ZnS conduction-band minimum for the S-rich alloys. The ability to tune the bandgap and knowledge of the location of the valence and conduction-band can be advantageous in applications, such as heterojunction solar cells, where band alignment is crucial.

Stoichiometric gallium oxide sulfide Ga2(O1-xSx)3 thin-film alloys were synthesized by pulsed-laser deposition with x≤0.35. One challenge has been synthesizing Ga2(O,S)3 crystalline films, as these samples have been determined to be amorphous through X-ray diffraction and transmission electron microscopy measurements. Despite the amorphous structure, the films have a well-defined, room-temperature optical bandgap tunable from 5.0 eV down to 3.0 eV. Similar to the amorphous GaN1-xAsx system, the band structure behavior of amorphous Ga2(O,S)3 alloys is in agreement with the predictions of the band anticrossing model. In the case for amorphous Ga2(O,S)3 alloys, the addition of sulfur at a merely 0.013 ratio shows a reduction in bandgap of about 1 eV suggesting that the localized sulfur level is located roughly 1 eV above the valence band of Ga2O3, a value that is comparable to the sulfur level location found in ZnO1-xSx alloys. The optical absorption data are interpreted using a modified valence-band anticrossing model that is applicable for highly mismatched alloys. The model provides a quantitative method to more accurately determine the bandgap as well as insight to how the band edges are changing with composition.

With escalating demands for freshwater and cooling, sustainable technologies are crucial to address these challenges on a global scale. This thesis presents a two-part investigation into solar-powered desalination and an innovative thermodynamic cycle that integrates cooling with desalination. The first part examines a techno-economic analysis of solar desalination systems across multiple configurations, focusing on how factors like energy storage, brine management, and desalination technology affect levelized cost of water production. This study identifies strategic drivers for economic viability, highlighting the potential for modular, inland desalination powered by renewable energy sources.

Building on these cost-reduction drivers, the second part introduces a novel dual-function thermodynamic cycle that combines cooling and high-salinity desalination using dimethyl ether (DME) working fluid as both a refrigerant and a water-extracting solvent. This cycle couples DME-based refrigeration with solvent-driven water extraction and multi-effect distillation to enhance energy efficiency, offering a promising approach for reducing costs associated with separate cooling and desalination systems. Together, these studies showcase how advanced technologies with high water recovery and high energy efficiency can pave the way for resource-efficient and cost-effective solutions in water and cooling applications.