(PHD, 2022)

Abstract:

Phonon-phonon and electron-phonon interactions underlie many fundamental transport properties like thermal conductivity and electrical mobility, and models of these properties provide information about the underlying microscopic interactions present in the materials. Many of these models use the Boltzmann transport equation where the choice of the expression for the collision integral is the most important and challenging aspect since it should capture all of the relevant interactions. In the past the expressions were semi-empirical, but in recent decades first principles models with no fitting parameters have become more commonplace, leading to discovery of new materials or providing deeper insights into the relevant mechanisms governing transport. This thesis presents first-principles calculations of thermal conductivity in polymer crystals, and charge transport at high electric fields in semiconductors in the Boltzmann transport framework.

Polymers are thermally insulating in their typical amorphous form, but it is known that their thermal conductivity can be enhanced through drawing and aligning of their polymer chains. With perfect chain alignment, the structures can be described as polymer crystals, which tend to contain many atoms per unit cell. However, the conventional understanding of thermal transport in crystals predicts low thermal conductivity for complex, many atom unit cells. It is known from simple models that phonon focusing redirects the heat flow into the polymer chain direction, but the extent to which phonon focusing plays a role in setting the intrinsic upper limits of polymer thermal conductivity has not been assessed from a first principles standpoint. We calculate the ab initio lattice conductivity of polythiophene, a complex molecular crystal with 28 atoms per unit cell, using the temperature dependent effective potential (TDEP) method to obtain finite temperature phonon properties taking into account the large quantum nuclear motion of hydrogen atoms present in polymers. We find a high thermal conductivity due to phonon focusing and stiff branches that overcome the expected low phonon lifetimes. The phonon focusing aligns group velocities along the chain axis throughout the Brillouin zone, even for states with wave vector almost orthogonal to the chain axis.

For charge transport, ab initio calculations focus almost exclusively on low field mobility, but technologically relevant phenomena like negative differential resistance manifest only at high fields far from equilibrium. Further, there are no ab initio calculations of non-equilibrium electronic noise, which differs qualitatively from transport observables at high fields. We report a methodological advance that obtains both the high-field transport properties and the non-equilibrium noise using an ab initio Boltzmann transport approach. Our method extends the collision integral to high fields by making physically motivated approximations to account for the non-linearities at high fields.

Using our method, we calculate the high-field noise and transport properties in GaAs and find that the 1ph level of theory is inadequate. Thus, we implement an approximate form of higher order interactions where electrons are scattered consecutively by two phonons (2ph) and find that these 2ph processes qualitatively alter the energy relaxation of the electron system compared to 1ph scattering, resolving a long-standing discrepancy in the strength of intervalley scattering inferred from different experiments. We also calculate non-equilibrium electronic noise from first principles for the first time. However, we are not able to reproduce experimental trends, and we suggest that 2ph processes beyond our approximation may be necessary to obtain experimental agreement. Our calculation shows how noise provides a new observable against which the accuracy of first-principles methods can be measured.

]]>

(PHD, 2022)

Abstract:

This thesis discusses several topics in extending the capability of conventional quantum many-body methods. The first project focuses on extending quantum chemical methods, namely coupled cluster theory, to the correlated systems in the condensed phase. We consider bulk nickel oxide and manganese oxide, which are two paradigmatic correlated electron materials that pose challenges to traditional density functional theory-based simulation framework. We adapted molecular coupled cluster singles and doubles theory using Gaussian basis sets with translational symmetry and norm-conserving pseudopotential. This allowed us to carry a detailed study on the ground and excited states of the two materials.

The second project investigates numerical optimization techniques for Abelien group symmetric tensor contractions. In many-body quantum simulations, group symmetries in states and operators often lead to block sparse structure in the representing tensors. Exploiting this opportunity can significantly reduce the computation cost and memory footprint in tensor contractions. We consider cyclic group symmetry and introduce an efficient remapping scheme to express the sparse tensor contractions almost fully in terms of dense tensor operations.

The third project is devising a wavefunction-based method for coupled electrons and phonons. We are interested in simulating the interacting electrons and phonons at the same footing using coupled cluster methods. The ground state and excited state of two types of systems are investigated in this work: the Hubbard Holstein model and diamond crystal in ab initio setting.

Finally, the fourth project is to develop a generic framework for tensor network simulation on fermionic systems. Tensor network methods are powerful tools to study strongly correlated physical systems. However, traditionally these methods have been developed with commutative algebraic rules, which are commensurate with bosons but not compatible with anti-symmetric fermions. Our approach encodes the fermion statistics directly in the block sparse tensor backend so the tensors behave just like anti-commuting fermion operators.

]]>

(PHD, 2022)

Abstract:

Electronic noise, or stochasticity in the current, voltage, and frequency of a carrier signal is caused by microscopic fluctuations in the occupation of quantum electronic states. In the context of scientific instrumentation, understanding the physical origin of these fluctuations is of paramount importance since the associated stochasticity ultimately limits the fidelity of information transmitted through electronically processed-signals. The unifying theme of the work presented in this thesis is the study of electronic fluctuations in semiconductor materials and devices. Our interest in this topic is twofold. First, while the Nyquist law dictates the equivalence of noise and transport properties for systems in thermal equilibrium, this relationship breaks down for systems driven out of equilibrium by external forcing. Simulating non-equilibrium electronic fluctuations can therefore provide new insights into the microscopic processes that control energy and momentum relaxation which would not be available from conventional studies of transport alone. Furthermore, because noise properties are sensitive to the microscopic details of the bandstructure and scattering, *ab initio* simulations of noise observables provide a more rigorous test of the accepted theory of charge transport and carrier scattering in materials. Second, cryogenic low noise amplifiers based on high electron mobility transistors (HEMTs) are widely used in electromagnetic detector chains in applications such as radio astronomy, deep space communications, and quantum computing. The design and optimization of HEMT devices have conventionally relied upon empirical circuit-level models of fluctuations in devices. As the noise performance of modern low-noise amplifiers has saturated to levels five to ten times above the standard quantum limit, these empirical models are unable to resolve the microscopic origin of the limiting excess noise. Identifying the microscopic mechanisms underpinning noise in modern amplifiers is therefore necessary to produce better devices for scientific instrumentation. In this work, we investigate electronic noise in semiconductor materials and devices with a combination of first-principles simulations and Schottky thermometry experiments in transistor amplifiers.

First, we present our work on the development of novel parameter-free simulations of non-equilibrium noise in semiconductor materials. While the *ab initio* theory of low-field electronic transport properties such as carrier mobility is well-established, an equivalent treatment of electronic fluctuations about a non-equilibrium steady state has remained less explored. Starting from the Boltzmann Transport Equation, we develop an *ab initio* method for hot electron noise in semiconductors. In contrast with the typical numerical methods used for electronic noise such as Monte Carlo techniques, no adjustable parameters are required in the present formalism with the electronic band structure and scattering rates calculated from first-principles. Our formalism enables a parameter-free approach to probe the microscopic transport processes that give rise to electronic noise in semiconductors. Next, we apply the developed method to compute the spectral noise power in two materials of technological interest, GaAs and Si. In our first study in GaAs, we show that despite the well-known dominance of optical phonon scattering, the spectral features in AC transport properties and noise originate from a surprising quasi-elasticity in the scattering of warm electrons with the lattice. In our second study, we apply the method to Si which possesses a more complicated multivalley conduction band. This study demonstrates that the widely-accepted one-phonon scattering approximation is insufficient to reproduce the warm electron tensor and that incorporating second-order mechanisms, such as two-phonon scattering, may be critical to obtain an accurate description of noise in such materials.

Finally, we discuss our work on developing deeper understandings of electronic noise in real devices with a focus on transistor amplifiers. While the first-principles work described above is appropriate for evaluating noise in ideal materials, in real semiconductor devices, charge carriers are influenced by mechanisms such as defect scattering, size effects, and reflections at interfaces. Owing to the complexity of these mechanisms, HEMT noise is typically treated with empirical models, where the physical noise sources are reduced to fitting parameters. Existing models of HEMT noise, such as the Pospieszalski model, are unable to resolve the mechanisms that set the noise floor of modern transistor amplifiers. In particular, the magnitude of the contribution of thermal noise from the gate at cryogenic temperatures remains unclear owing to a lack of experimental measurements of thermal resistance under these conditions. We report measurements of gate junction temperature and thermal resistance in a HEMT at cryogenic and room temperatures using a Schottky thermometry method. Based on our findings, we develop a phonon radiation model of heat transfer in the device and estimate that the thermal noise from the gate is several times larger than previously assumed. Our work suggests that self-heating results in a practical lower limit for the microwave noise figure of HEMTs at cryogenic temperatures.

]]>

(PHD, 2022)

Abstract:

Detection and processing of microwave signals is of substantial scientific importance in fields ranging from radio astronomy to quantum computing. An essential component of the signal processing chain is the microwave amplifier, which adds gain to the signal so that it may be processed by subsequent microwave components. However, the amplifier itself adds its own internally generated noise into the measurement chain. As a result, amplifiers which add a minimal amount of noise are crucial to any high precision measurement scheme. A device which is commonly employed for this task is the high-electron-mobility transistor (HEMT) amplifier. Understanding the fundamental limits of the microwave noise performance of HEMT amplifiers is highly desirable. Noise temperatures in these devices as low as 3 times the quantum limit have been observed in the last decade, but the lack of understanding of the origin of the excess noise has hindered further improvements. Noise in HEMTs is attributed to a generator at the output, known as drain noise; and a generator at the input, which is attributed to thermal noise of the gate. At cryogenic temperatures of ~4 K, thermal noise is predicted to be negligible. However, a plateau in noise temperature has been observed at physical temperatures below ~20 K, with a negligible improvement in noise performance upon further cooling.

The primary noise mechanism responsible for this plateau is believed to be ohmic heating of the HEMT structure induced by current in the active device channel, a process known as self-heating. At room temperature the ambient thermal noise dominates the amplifier’s overall noise performance, but at the cryogenic temperatures required to achieve low-noise performance the self-heating effect produces thermal noise at the input of the HEMT gate which contributes significantly to the total noise. A potential mechanism to mitigate self-heating is to provide an additional thermal dissipation path for the Joule heating in the channel. However, given the sub-micron length scales and buried gate structure of HEMTs, thermal management is challenging. The primary heat conduction pathway, that of phonons travelling through the bulk HEMT substrate, decreases rapidly in magnitude at cryogenic temperatures. An alternative option is to submerge the HEMT in a cryogenic fluid, thereby presenting an alternate thermal conduction route through the HEMT surface into the fluid. This technique, while commonly employed in cryogenic thermal management of superconducting magnets, has not been investigated for HEMTs.

In this work we explore the use of liquid cryogenic cooling to directly mitigate the effect of HEMT self-heating. We test in particular the effectiveness of cooling using superfluid helium-4, which has the highest known thermal conductivity of any known substance. We report a systematic experimental investigation of the noise performance of a cryogenic packaged two-stage HEMT low-noise amplifier over a wide range of biases in a 4.0 - 5.5 GHz frequency band, with the device immersed in a variety of cryogenic baths including helium-4 vapor, liquid helium-4, superfluid helium-4, and vacuum. We present the details of the experimental apparatus which was constructed to perform microwave noise measurements of the low-noise amplifier when submerged in a liquid cryogen environment. We interpret our results using a small-signal model of the amplifier and compare our findings with the predictions of a phonon radiation model of heat dissipation. We find that liquid cryogenic cooling is unable to mitigate the thermal noise associated with self-heating. Considering this finding, we examine the implications for the lower bounds of cryogenic noise performance in HEMTs by incorporating the effects of self-heating into the existing noise modelling of HEMT amplifiers. Our analysis supports the general design principle for cryogenic HEMTs of maximizing gain at the lowest possible power.

]]>

(PHD, 2021)

Abstract:

The physics of transport of heat-carrying atomic vibrations in amorphous and semi-crystalline solids is a topic of fundamental interest. Diverse tools have been employed to study thermal transport in these materials, including cryogenic thermal conductivity measurements and various inelastic scattering tools. However, unambiguously identifying the damping mechanisms of few THz and smaller frequency excitations remains difficult owing to the lack of the experimental probes in the frequency band. As a result, debate has remained regarding the microscopic origin of weak acoustic damping in amorphous silicon (Si), the unusually high thermal conductivity of ultra-drawn polyethylene, and other topics.

In this thesis, we investigate the transport properties of heat-carrying acoustic excitations in semi-crystalline and amorphous solids using transient grating spectroscopy. This optical method permits the creation of thermal gradients over sub-micron length scales which may be comparable to the attenuation lengths of the excitations. We show how these measurements can be used to constrain the damping mechanisms in the sub-THz range that has been historically inaccessible by typical methods such as inelastic scattering.

First, we report measurements of the bulk thermal conductivity and elastic properties of MoS₂ thin films. Specifically, we use TG to measure the in-plane longitudinal sound velocity and thermal conductivity. We do not observe any size effects of thermal conductivity with grating period, indicating that the propagating distance of heat-carrying acoustic phonons are smaller than the thermal length scale accessible in the experiment. This result is consistent with the mean free paths predicted from ab-initio numerical methods.

Second, we utilize the capability of TG to resolve the microscopic heat transport properties of phonons in highly oriented semi-crystalline polyethylene (PE). Earlier experimental studies have reported thermal conductivities of up to ~ 100 Wm⁻¹ K⁻¹ crystalline polyethylene, orders of magnitude larger than the bulk value of ~ 0.4 Wm⁻¹ K⁻¹. However, the microscopic origin of the high thermal conductivity remains unclear. We address this question by applying TG to highly oriented polyethylene to show that mean free paths on micron length scales are the dominant heat carriers. Using a low-energy anisotropic Debye model to interpret these data, we find evidence of one-dimensional phonon density of states for excitations of frequency less than ~ 2 THz. This transition frequency is consistent with the unique features of ultradrawn PE, in particular the stiff longitudinal branch leading to wavelengths of 8 nm at 2 THz frequency; and fiber diameters < 10 nm observed in prior structural studies of ultradrawn polymers; so that the wavelength does indeed exceed the fiber diameter at the relevant frequencies.

Finally, we report the measurements of the frequency-resolved mean free path of heat-carrying acoustic excitation in amorphous silicon (aSi), for the first time. The heat-carrying acoustic excitations of amorphous silicon are of interest because their mean free paths approach the micron scale at room temperature. Despite extensive investigation, the origin of the weak acoustic damping in the heat-carrying frequencies remains a topic of debate for decades. A prior study suggested a framework of classifying the vibrations into propagons, diffusons, and locons. Propagons were considered phonon-like, delocalized, propagating vibrations; locons as localized vibrations, and diffusons as delocalized yet non-propagating vibrations. Following the framework, numerous works have predicted mechanism of acoustic damping in aSi, but the predictions have contradicted to observations in experiments. In this work, we obtained measurements of the frequency-dependent mean free path in amorphous silicon thin films from ~0.1-3 THz and over temperatures from 60 - 315 K using picosecond acoustics (PSA) and transient grating spectroscopy. We first describe our PSA experiments to resolve the attenuation of 0.1 THz acoustic excitations in aSi. We then present our table-top approach to resolve MFP of heat-carrying acoustic excitation between ~ 0.1-3 using TG spectroscopy. The mean free paths are independent of temperature and exhibit a Rayleigh scattering trend over most of this frequency range. The observed trend is inconsistent with the predictions of numerical studies based on normal mode analysis, but agrees with diverse measurements on other glasses. The micron-scale MFPs in amorphous Si arise from the absence of Akhiezer and two-level system damping in the sub-THz frequencies, leading to heat-carrying acoustic excitations with room-temperature damping comparable to that of other glasses at cryogenic temperatures. Our results allow us to establish a clear picture for the origin of micron-scale damping in aSi by understanding vibrations as acoustic excitation rather than propagons, diffusons, and locons.

]]>

(PHD, 2021)

Abstract:

Three different computational physics problems are discussed. The first project is solving the semi-classical Boltzmann transport equation (BTE) to compute the thermal conductivity of 1-D superlattices. We consider various spectral scattering models at each interface. This computation requires the inversion of a matrix whose size scales with the number of points used in the discretization of the Brillouin zone. We use spatial symmetries to reduce the size of data points and make the computation manageable. The other two projects involve quantum systems. Simulating quantum systems can potentially require exponential resources because of the exponential scaling of Hilbert space with system size. However, it has been observed that many physical systems, which typically exhibit locality in space or time, require much fewer resources to accurately simulate within some small error tolerance. The second project in the thesis is a two-step factorization of the electronic structure Hamiltonian that allows for efficient implementation on a quantum computer and also systematic truncation of small contributions. By using truncations that only incur errors below chemical accuracy, one is able to reduce the number of terms in the Hamiltonian from *O*(*N*⁴) to *O*(*N*³), where *N* is the number of molecular orbitals in the system. The third project is a tensor network algorithm based on the concept of influence functionals (IFs) to compute long-time dynamics of single-site observables. IFs are high-dimensional objects that describe the influence of the bath on the dynamics of the subsystem of interest over all times, and we are interested in their low-rank approximations. We study two numerical models, the spin-boson model and a model of interacting hard-core bosons in a 1D harmonic trap, and find that the IFs can be efficiently computed and represented using tensor network methods. Consistent with physical intuition, the correlations in the IFs appear to decrease with increased bath sizes, suggesting that the low-rank nature of the IF is due to nontrivial cancellations in the bath.

]]>

(PHD, 2020)

Abstract:

The atomic vibrations and thermal properties of amorphous dielectric solids are of fundamental and practical interest. For applications, amorphous solids are widely used as thermal insulators in thermopile and other detectors where low thermal conductivity directly sets the sensitivity of the detector. Amorphous solids are of fundamental interest themselves because the lack of atomic periodicity complicates theoretical development. As a result, the lower limits of thermal conductivity in solids as well as the nature of the vibrational excitations that carry heat remain active topics of research.

In this thesis, we use numerical and experimental methods to investigate the thermal conduction in amorphous dielectrics. We begin by using molecular dynamics to investigate the thermal conductivity of amorphous nanocomposites. We find that mismatching the vibrational density of states of constituent materials in the composite is an effective route to achieve exceptionally low thermal conductivity in fully dense solids.

We then transition to examining the properties of the atomic vibrations transporting heat in amorphous solids. For decades, normal mode methods have been used extensively to study thermal transport in amorphous solids. These methods naturally assume that normal modes are the fundamental vibrational excitations transporting heat. We examine the predictions from normal mode analysis that are now able to be tested against experiments, and we find that the predictions from these methods do not agree with experimental observations. For instance, normal mode methods predict that the low frequency normal modes are scattered by anharmonic interactions as in single crystalline solids. However, temperature dependent thermal conductivity measurements demonstrate a typical glassy temperature dependence inconsistent with normal modes scattering through anharmonic interactions. These discrepancies suggest that normal modes are not the fundamental heat carriers in amorphous dielectrics.

To identify the actual heat carriers, we draw on fundamental concepts from many- body physics and inelastic scattering theory that dictate that the excitation energies of a many-body interacting system are given by the poles of the single-particle Green’s function. The imaginary part of this function is proportional to the dynamic structure factor that is directly measured in inelastic scattering experiments. Collective excitations of a given energy and wavevector can thus be identified from peaks in the dynamic structure factor; their damping is given by the broadening of the peak. Using these concepts from many-body physics, the physical picture that emerges is that heat is carried in large part by a gas of weakly interacting collective excitations with a cutoff frequency that depends on the atomic structure and composition of the glass.

We test this picture using numerical and experimental inelastic scattering measurements on amorphous silicon, a commonly studied amorphous solid. We observe collective excitations up to 10 THz, well into the thermal spectrum, and far higher than previous inelastic scattering measurements on other glasses. Our numerical and experimental evidence also confirms that the collective excitations are damped by structural disorder rather than anharmonic interactions and that they dominate the thermal conduction in amorphous silicon. Subsequent analysis shows that these high frequency acoustic excitations are supported in amorphous silicon due to a large sound velocity and monatomic composition, suggesting that other monatomic amorphous solids with large sound velocities may also support these thermal excitations.

Overall, our results provide strong evidence that the heat carriers in amorphous dielectrics are collective excitations rather than normal modes. This change in physical picture advances our understanding of atomic dynamics in glasses and also provides a foundation for realizing dielectric solids with ultralow thermal conductivity.

]]>

(PHD, 2019)

Abstract:

Polymers are widely used in applications due to their diverse and controllable properties in many physical domains. However, polymers have not historically been used in applications for which a high thermal conductivity is required as bulk polymers are typically thermal insulators. However, research in recent decades on a handful of highly oriented or semi-crystalline polymers has shown the potential for dramatically increased uniaxial thermal conductivity by factors exceeding 100. This dramatic increase in thermal conductivity is because heat is conducted by atomic vibrations along the covalently bonded polymer backbone rather than across chains by weak van der Waals bonds as in unoriented polymers. While it is known that polymers can be processed to yield these properties, much remains unknown about the microscopic transport properties of atomic vibrations in these materials and the true upper limits to thermal conductivity. In this thesis, we address these knowledge gaps by using a combination of simulations and experiments to investigate thermal conduction in semi-crystalline and crystalline polymers.

First, we present molecular dynamics simulations of a perfect polymer crystal, polynorbornene. While polymer crystals studied typically exhibit substantially enhanced thermal conductivities above those of the amorphous form, polynorbornene exhibits a glass-like thermal conductivity of less than 1 Wm^{-1}K^{-1} even as a perfect crystal. This unusual behavior occurs despite the polymer satisfying many of the conventional criteria for high thermal conductivity. Using our simulations, we show that the origin of this unusual behavior is excessively anharmonic bonds and a complex unit cell.

Second, we move to experimental studies of thermal transport in polymers. A key requirement to perform materials science is a method to routinely and easily characterize the property of interest in diverse samples. For polymers, this property is typically the in-plane thermal conductivity. This property turns out to be surprisingly difficult to measure using conventional thermal characterization methods. In this work, we adapt transient grating spectroscopy (TG), a well-known method in the chemistry community, to perform in-plane thermal conductivity measurements of polymer films. TG can resolve the in-plane thermal anisotropy of a sample without any physical contact and at tunable length scales, a substantial advance in capability over all prior characterization methods. We extend the application of TG to probe sub-µm length scales, and we successfully apply the technique to numerous poor quality polymer samples as well as thin films.

Finally, we exploit the capability of TG to probe thermal conduction over sub-µm length scales to provide the first experimentally resolved microscopic transport properties of atomic vibrations in semi-crystalline polyethylene (PE). Despite the intense interest over decades in PE due to its high intrinsic thermal conductivity, no experimental measurement has yet been able to directly probe the heat-carrying phonons, leading to many questions about the relevant scattering mechanisms and absolute upper limits of thermal conductivity in real samples. Using TG, we present the first observation of quasi-ballistic thermal transport at sub-µm length scales, from which we obtain the phonon mean free path spectra of a semi-crystalline PE sample. Further, we pair these results with Small-Angle X-ray Scattering measurements to show that thermal phonons propagate ballistically within and across nanocrystalline domains, contrary to the conventional viewpoint. These results provide an unprecedented microscopic view of thermal transport in polymer crystals that was previously experimentally inaccessible.

]]>

(PHD, 2019)

Abstract:

Materials that control the absorption and emission of thermal radiation have attracted renewed interest for energy applications. Materials of interest include those with static optical properties that vary with photon wavelength in a desired manner as well as those with dynamic properties that can be actively tuned by external stimuli. The research in this thesis focuses on creating materials in both categories.

First, we examine selective absorbers for solar thermal energy conversion with high absorptivity in solar wavelengths and low emissivity in infrared wavelengths. Achieving stagnation temperatures exceeding 200 °C with unconcentrated sunlight, pertinent to technologies like industrial process heat, air conditioning, and electricity generation, requires better spectrally selective absorbers with ultra-low thermal emittance. Current state-of-art surfaces are based on ceramic-metal mixtures and patterned metal or metal-dielectric structures. Semiconductor based selective surfaces with near zero absorption below the bandgap offer the potential for lower thermal emittance than that achieved with such surfaces that employ metals in the primary absorbing medium. In this thesis, we report a semiconductor-based multilayer selective absorber that exploits the sharp drop in optical absorption at the band gap energy to achieve a measured absorptance of 76% at solar wavelengths and a low emittance of approximately 5% at thermal wavelengths. In field tests, we obtain a peak temperature of 225 °C, comparable to that achieved with state-of-the-art selective surfaces. With straightforward optimization to improve solar absorption, our work shows the potential for unconcentrated solar thermal systems to reach stag- nation temperatures exceeding 300 °C, higher than any available selective surface. Our surface would eliminate the need for solar concentrators for mid-temperature solar applications such as supplying process heat.

Second, we theoretically propose and experimentally implement a thermal switch for near-field radiative transfer. In the field of active thermal materials for manipulating heat flow in a controllable and reversible manner, numerous approaches to perform thermal switching have been reported. However, they typically suffer from various limitations, including small switching ratio or requiring large temperature differentials. We report the experimental implementation of a scheme to electrostatically control near-field radiative transfer in a graphene field effect heterostructure. We measure a maximum heat flux modulation of 4 ± 3% and an absolute heat flux modulation rate of 24 ± 7 mWm^{−2} per V bias. Employing gate dielectrics with lower surface warp and higher dielectric breakdown strength as well as reducing conductive losses would enable modulations up to 100%, substantially exceeding the switching ratios achievable by other methods. Our work paves the way for electrostatic control of near-field radiative transfer using two-dimensional materials.

]]>

(PHD, 2019)

Abstract:

Control of heat flow in both near and far field through thermal radiation is of fundamental interest for applications in thermal management and energy conversion.

One challenge is how we can realize high contrast control of heat flow with high temporal frequencies and without moving parts. We try to resolve this problem and propose two schemes in the near field: one based on electrical tuning of silicon and the other based on optical pumping of doped silicon slabs. Both methods rely on the change of free carriers, leading to tuning of the plasma frequency, resulting in modulation of near-field thermal radiation. Calculations based on fluctuational electrodynamics show that the electric method gives 10% tuning range. On the other hand, heat transfer coefficient between two silicon films can be tuned from near zero to 600 Wm^{-2}K^{-1} with a gap distance of 100 nm at room temperature with the optical pumping method.

In the far field, we predict and demonstrate two spectrally selective absorbers based on semiconductors, by utilizing their band gap properties and dedicated photonic structure design. The germanium photonic crystals have around 95% absorption from 500 nm to 1000 µm and over 0.9 over the entire visible and near infrared spectrum. The effective absorptivity is as high as 0.91. The black silicon achieves 100% absorption for light with wavelength under 1 µm. The effective absorptivity is as high as 0.96. Field test shows that black silicon is able to maintain at 130 degrees Celcius under unconcentrated condition.

Another interesting topic is to achieve over 100 Wm^{-2} electricity-free cooling power density with simple fabrication method by passive radiative cooling under direction sunlight. We theoretically predicted three schemes for achieving this goal and experimentally demonstrate that a polymer-coated fused silica mirror, as a near-ideal black-body in the mid-infrared and near-ideal reflector in the solar spectrum, achieves radiative cooling below ambient air temperature under direct sunlight (8.2 °C) and at night (8.4 °C). Its performance exceeds that of a multi-layer thin film stack fabricated using vacuum deposition methods by nearly 3 °C. Furthermore, we estimate the cooler has an average net cooling power of about 127 Wm^{-2} during daytime at ambient temperature, more than twice that reported previously, even considering the significant influence of external conduction and convection. Our work demonstrates that abundant materials and straight-forward fabrication can be used to achieve daytime radiative cooling, advancing applications such as dry cooling of thermal power plants.

]]>

(PHD, 2018)

Abstract:

Materials that simultaneously possess ultralow thermal conductivity, high stiffness, and damage tolerance are highly desirable for engineering applications. However, this combination of properties has never been demonstrated in a single material because thermal and mechanical properties are coupled in most fully dense and porous solids. A new class of lattice materials with nanoscale features, called nanolattices, can fill this void in the material property space by virtue of their architecture and nanoscale dimensions. Extensive work on nanolattice mechanical properties report their excellent stiffness-to-density ratio and recoverability from large compressive strains. In contrast, the framework for studying their thermal properties has not been established. Our work develops the computational and experimental tools necessary to study heat conduction in nanoarchitected materials and applies those tools to prove the viability of octet-truss nanolattices as multifunctional thermal insulators.

We implement significant improvements to a phonon Monte Carlo method to solve the Boltzmann transport equation (BTE) in highly complex geometries like the octet-truss. No prior works solve the BTE in a domain as intricate as a nanolattice, so we create a geometry representation scheme that can model any arbitrary 3-D body. Our enhanced variance-reduced Monte Carlo code incorporates this scheme, allowing us to predict the thermal conductivity of nanolattices and analyze the phonon transport behavior in them. Results suggest that hollow-beam silicon nanolattices indeed reach ultralow thermal conductivities. Based on Monte Carlo and finite element simulations, we develop a predictive thermal conductivity model that accounts for both diffusive and radiative phonon transport in nanolattices.

We also devise custom modifications to the 3ω method to experimentally measure the thermal conductivity of additively manufactured nanolattices. Since the serial fabrication process of nanolattices makes it costly to cover large areas, we design a specialized 3ω sample that minimizes the required structure size while maintaining good experimental sensitivity. We derive a new thermal model to account for conductive losses through the heater line in our novel sample geometry. 3ω measurements and compression tests of hollow-beam alumina nanolattices show that they combine ultralow thermal conductivity with excellent mechanical stiffness and resilience, which proves that nanolattices occupy a previously unreachable region in material property space. Our work provides motivation to further investigate and improve the thermal properties of architected materials.

]]>

(PHD, 2017)

Abstract:

Recent progress in nanosciences challenges the conventional understanding of Fourier’s law for heat conduction and Planck’s law for thermal radiation, calling for theoretical and experimental advancement to improve our understanding at these length scales. Advances in both theoretical and experimental progress at these length scale have been made in the past two decades, but there are still many challenges and possibilities in further understanding how heat conducts or radiates at these length scales.

The first half of this thesis focuses on topics in nanoscale thermal radiation. First, we will discuss an effort to modify thermal emission using a hyperbolic metamaterial (HMM). Recent efforts in utilizing different metamaterial designs to modify thermal emission has led to greater control over the spectral and directional properties of thermal radiation, and the HMM is one such metamaterial. HMM is typically made up of sub-wavelength alternating layers of metal and dielectric that result in an anisotropic permittivity. Here we demonstrate that an annular, transparent HMM lens enables selective controlling of the plasmonic resonance such that a nanowire emitter, surrounded by an HMM, appears dark to incoming radiation from an adjacent nanowire emitter unless the second emitter is surrounded by an identical lens.

While many metamaterial schemes exist to modify thermal emission, these schemes are ultimately limited by the maximum possible emission of a blackbody. In an effort to further increase radiative thermal emission, we made another effort to explore the possibility of removing the enhanced but trapped thermal radiation energy density at sub-wavelength distances. Here, we propose and numerically demonstrate an active scheme that exploits the monochromatic nature of near-field thermal radiation to drive a transition in a laser gain medium, which, when coupled with external optical pumping, allows the resonant surface mode to be emitted into the far-field. We compare this proposed active radiative cooling (ARC) approach to the better-understood laser cooling of solids (LCS) technique, which achieves cooling by extracting phonons instead of thermal radiation. We show that LCS and ARC can be described with the same mathematical formalism and find that ARC can achieve higher efficiency and extracted power over a wide range of conditions.

In the second half of thesis, we switch our attention to nanoscale heat conduction where phonons are the dominant heat carriers. Phonons require a medium to travel, unlike thermal radiation, and thus experience much stronger interaction with the medium. Typical assumptions of many scattering events of phonons at the larger length scales break down at the nanoscale when phonon transport can no longer be accurately described by diffusion theory. Here, we present a numerical modeling effort using the Boltzmann Transport Equation to accurately model nanoscale phonon transport of a recent experiment. We show a calculated trend of pump beam size dependence on thermal conductivity similar to results from the time-domain thermal reflectance (TDTR) experiment. We also identify the radial suppression function that describes the suppression in heat flux, compared to Fourier’s law, that occurs due to quasiballistic transport and demonstrate good agreement with experimental data.

While time-domain thermal reflectance (TDTR) experiment is widely used to characterize thermal transport, it is not ideal for in-plane thermal measurements compared to the transient grating (TG) techniques which utilize interference of two beams to create a in-plane grating pattern for thermal measurements. In the last part of my thesis, we highlight details of an experimental effort to develop the ultra-fast transient grating (TG) technique capable of measuring fast thermal decays. We will then highlight the results of thermal and acoustic measurements of molybdenum disulphide that can be obtained from this technique. Our results are in good agreement with other measurements and calculations.

With nanosciences paving way for the future of technology, understanding thermal management at the nanoscale is crucial for device performance and reducing energy waste. We believe that these results in thermal radiation and conduction will benefit thermal management at the nanoscale.

]]>

(PHD, 2017)

Abstract:

The thermal transport properties of thin semiconductor membranes play an important role in the performance of many technologies like micro-electronics and solid-state energy conversion. The dominant resistance to heat flow in thin membranes is offered by the scattering of thermal phonons at the membrane boundaries. In this dissertation, we examine the nature of microscopic phonon boundary scattering processes and their effect on the thermal conductivity of the thin membranes using a pump-probe experimental technique and computationally efficient solutions of the phonon Boltzmann transport equation (BTE).

First, we investigate the boundary scattering-limited thermal transport in nanostructures using an efficient variance-reduced Monte Carlo (MC) solution of the BTE to elucidate the impact of specular and diffuse phonon boundary scattering events on the thermal conductivity of the nanostructures. To directly measure the relative frequency of these two boundary scattering events, called the phonon specularity parameter, we design, implement and characterize a non-contact laser-based pump-probe experiment called the transient grating (TG) to perform phonon mode-dependent measurements of the specularity parameter in suspended free-standing thin silicon membranes. We describe the phenomenon of quasiballistic heat conduction, which enables the phonon mode-dependent measurements of the specularity parameter, and derive a transfer function based on the BTE with ab-initio phonon properties as inputs, to connect the specularity parameter with the experimentally measured thermal conductivity of the thin membranes.

Finally, we present the methodology adopted to invert the BTE transfer function to extract the phonon specularity parameter from the thermal conductivity measurements in the TG experiment, while rigorously accounting for the experimental uncertainties. We find that the observed magnitudes and trends of the thermal conductivity of the thin membranes cannot be explained by the 50-year old Ziman’s model for the phonon specularity parameter and the Fuchs-Sondheimer theory of phonon boundary scattering. We also find that the partially specular boundary scattering picture of phonon boundary interactions works well for one of the membranes, enabling a direct measurement of the mode-dependent phonon specularity parameter for the first time in an experiment. We discuss the possibility of phonon mode conversion at the boundaries of a few membranes for which the partially specular phonon boundary scattering picture fails to explain the observed thermal conductivity trends. Considering the importance of understanding phonon boundary scattering to engineer and improve nanoscale device performance, we expect that the new experimental and computational tools developed in this work will advance a variety of nanoscale energy applications and further our understanding of nanoscale heat transport.

]]>

(PHD, 2016)

Abstract:

Heat is one of the most fundamental forms of energy, and the ability to control heat plays a critical role in most current and future energy applications. Recently, interface engineering between heterogeneous solids has provided new approaches to manipulate heat transport at the scales of the energy carriers in solids, *i.e.* phonons which are quantized lattice vibrations. For example, nanocrystalline materials, which are polycrystalline materials with nanoscale grain sizes, are promising thermoelectric (TE) materials that have achieved substantially improved figure of merits compared to their bulk counterparts. This enhancement is typically attributed to a reduction in lattice thermal conductivity by phonon scattering at grain boundaries. On the other hand, inefficient heat dissipation across interfaces has been a long-standing problem that shortens the lifetime of electronics such as light-emitting diodes.

Despite the importance of interfaces, we still lack a comprehensive understanding of interfacial thermal phonon transport. For instance, the Fresnel coefficients enable the straightforward mathematical description of light as it moves between media of differing dielectric constants. Similarly, interfacial phonon transport can also be characterized by transmission coefficients that vary over the broad phonon spectrum in an analogous manner to Fresnel coefficients for light. However, despite decades of work, the spectral profile of these coefficients and how the profile is influenced by the atomic structure of actual interfaces remains unclear. As a result, the basic phenomenon of interfacial heat transport remains among the most poorly understood transport processes.

To elucidate this process, in this thesis we investigate interfacial thermal phonon transport using both modeling and experiment. The first portion of the thesis examines the impact of frequency-dependent grain boundary scattering in nanocrystalline silicon and silicon-germanium alloys using a novel computational method. We find that the grain boundary may not be as effective as commonly considered in scattering certain phonons, with a substantial amount of heat being carried by low frequency phonons with mean free paths longer than the grain size. Our result will help guide the design of more efficient TEs.

The second part of the thesis focuses on studying heat conduction using the Boltzmann transport equation (BTE), which is the governing equation of energy transport at length scales comparable to phonon mean free paths. The BTE is an integro-differential equation of time, real space, and phase space. Due to its high dimensionality, it is extremely challenging to solve. Here, we develop analytical methods to solve the frequency-dependent BTE, which allow us to obtain simple, closed-form solutions to complex multidimensional problems that have previously been possible to solve only with computationally expensive numerical simulations. We demonstrate that the solution leads to a more accurate measurement of phonon MFP spectra in thermal transient grating experiments.

Finally, we report the first measurements of thermal phonon transmission coefficients at a metal-semiconductor interface using ab-initio phonon transport modeling based on the BTE we develop in the second part and a thermal characterization technique, time-domain thermoreflectance. With our approach, we are able to directly link the atomic structure of an interface to the spectral content of the heat crossing it for the first time. Our work realizes the long-standing goal of directly measuring thermal phonon transmission coefficients and demonstrates a general route to study microscopic processes governing interfacial heat conduction.

]]>

(BS, 2013)

Abstract:

Ultralow thermal conductivity materials play an important role in many applications such as space exploration and thermal insulation. One of the primary challenges in studying heat conduction in these materials is performing basic thermal conductivity measurements, because even a small amount of steady heating results in enormous temperature increases. While methods do exist to measure the thermal conductivities of macroscopic materials, these techniques are difficult to apply to the microscopic samples that are the only samples available for some materials.

In this thesis, we describe an optical, non-contact experimental system for measuring ultralow thermal conductivities. The system we designed and built is a modified pump-and-probe system. With this system, we will be able to measure a sample as small as tens of microns without having physical contact between the tester and the sample. Different from a traditional pump-and-probe method, we used a pump laser with a very low repetition rate of a few hundred Hz, which can efficiently reduce the steady heating, and at the same time the transient heating is still enough to give a measurement result.

In the following chapters, we will first discuss the background of this thesis, including a brief introduction to the manufacture of the nano-lattice and the principles of the pump-and-probe method. Then we will go through some calculations done during the design process of the apparatus. Finally, we will present the test results from the experiments and compare them against the simulation.

]]>