Microscopic engineering of material properties is crucial to advancing the technological needs in the age when development is pushing towards the ends of the Moore’s Law. The desire to engineer materials with properties best suited for intended applications has driven development of various modulation methods. Intercalation and the use of van der Waals materials emerged as a versatile property manipulation tool due to their unique atomic structure.

In the first part of this dissertation, we developed a novel platform based on lithium ion batteries with ultrafast optical spectroscopy and electrochemical control in black phosphorus. Our investigation revealed a strong dependence in phonon-ion interactions. The thermal conductivity of black phosphorus is reversibly tunable from 2.45 to 3.86 W�m−1�K−1, 62.67 to 85.80 W�m−1�K−1, and 21.66 to 27.58 W�m−1�K−1 in the three principal directions. The phenomenon is attributed to phonon scattering introduced by intercalation. At the fully discharged state, thermal conductivity is reduced up to six times compared to the pristine crystal.

Next, we modified the intercalation system to achieve reversible complementary doping. We produce a p-type and n-type transistor through lithium ion intercalation and copper zero-valent chemical intercalation. Interestingly, we found that intercalation can suppress the neutral impurity scattering from the environment, leading to enhancement of electron mobility in the system.

We apply these findings in 2D systems to overcome ongoing challenges of multiplexable sensing in gas sensors. Due to the high noise levels in thin film sensors, alternate current analysis is seldom employed. We use bulk tin selenide crystal as sensing material in our sensors. The sensor-analyte interaction was investigated using electrochemical impedance spectroscopy coupled with an equivalent circuit model. We demonstrate that frequency-dependent phase signal signatures can detect methanol and ethanol within 10% accuracy and water vapor within 15% accuracy in gas mixtures.

In addition, we investigated the electrical transport behavior of microshutters though investigation of the electric field effects surrounding the Next-Generation Microshutter Array. We find that that design modification to a bi-electrode design can significantly improve the electrostatic torque actuation, enabling reliable actuation at a reduced voltage difference of 70 V.

Advanced materials with extreme thermal conductivity are critically important for various technological applications including energy conversion, storage, and thermal management. Low thermal conductivity is needed for thermal insulation and thermoelectric energy harvesting, while high thermal conductivity is desirable for efficient heat spreading in electronics. However, practical application deployments are usually limited by the materials availability in nature. Moreover, understanding the fundamental origins for extreme thermal conductivity still remains challenging. My PhD research focuses on finding new thermal materials and unveiling fundamental phonon transport mechanisms in extreme thermal conductivity matters to push the frontier of thermal science.

My dissertation is composed of three topics. The first topic is focused on developing and investigating a new group of ultrahigh conductivity materials. High-quality boron phosphide (BP) and boron arsenide (BAs) crystal are synthesized and measured with thermal conductivities of 460 and 1300 W/mK, respectively. In particular, our result shows that BAs is the best thermal conductor among common bulk metals and semiconductors. To better understand the fundamental origin of such an ultrahigh thermal conductivity, advanced phonon spectroscopy and temperature dependent characterizations are performed. Our measurements, in conjunction with atomistic theory, reveal that, unlike the commonly accepted rule for most materials near room temperature, high-order anharmonicity through the four-phonon process is significant in BA because of its unique band structure. Our result underscores the promise of using BP and BAs for thermal management and develops microscopic understanding of the phonon transport mechanisms.

The second topic of my thesis is to investigate phonon transport in ultralow thermal conductivity material with a focus on tin selenide (SnSe). SnSe is a recently discovered material for high performance thermoelectricity. However, the thermal properties of intrinsic SnSe remain elusive in literature. To understand the dominant phonon transport mechanisms for the extremely low thermal conductivity of SnSe, temperature-dependent sound velocity, lattice expansion, and Gr�neisen parameter was measured. The measurement result shows that high-order anharmonicity introduces strong phonon renormalization and the ultralow thermal conductivity.

The third topic of the thesis is to investigate in-situ dynamic tuning of thermal conductivity in layered materials. A novel device platform based on lithium ion battery is developed to characterize the interactions between ions and phonons of layered materials. We observe a highly reversible modulation and anisotropy of thermal conductivity from phonon scattering introduced by ionic intercalation in the interspacing layers. This study provides a unique approach to explore the fundamental energy transport involving lattices and ions and open up new opportunities in thermal engineering.

Heterogeneous nanostructures involve nanoscale interfaces with different materials components, such as matrix and fillers in composites, stacking planer structures in electronics, and aggregates of nanomaterials. Thermal transport in heterogenous nanostructures is critical to the safety and performance of various applications ranging from high temperature turbines, microelectronics, solar cells, thermoelectrics, buildings’ thermal management and so on. However, it remains challenging to achieve rational control of the thermal properties in heterogeneous nanostructures due to limitations in current characterization techniques and fundamental understandings of interface thermal transport. My PhD research focuses on developing new thermal measurement techniques and investigating fundamental interface transport mechanisms through the combination of experiments and modeling, to provide rational control over heterogeneous nanostructures for better addressing practical heat management and energy conversion problems using nanoengineering. The study of thermal transport in heterogeneous nanostructures in my dissertation spans from technical development of new tools, experimental measurements at nanoscale interfaces and porous structures, and atomistic modelling of fundamental transport physics to practical device applications. First, I have developed a new metrology based on asymmetric beam time-domain thermoreflectance (AB-TDTR) that enables accurate measurements over three-dimensional thermal transport. Through the design of an asymmetric laser beam with controlled elliptical ratio and spot size, the experimental signals can be exploited to be dominantly sensitive to measure anisotropic thermal conductivity along the cross-plane or any specific in-plane directions. I have further applied this new approach to investigate anisotropic transport phenomena that enables unique applications. Second, I have explored the effects of crystal orientations and dipole-dipole interactions on interface thermal transport. In particular, for the first time, we have observed a record-high anisotropy ratio of 3.25 in the thermal boundary resistance across a prototype two-dimensional material, i.e., black phosphorus. Moreover, my study has resulted in the first observation of strong effects from long-range molecular dipole-dipole interactions on interface thermal transport. In addition, I have also investigated the heterogeneous integration of our recently developed new high thermal conductivity materials with prototyped high-power semiconductor, i.e., gallium nitride. Our in-situ measurement demonstrated substantially reduced hot-spot temperatures in devices using boron arsenide cooling substrates, beyond the best state-of-the-art HEMTs using diamond or silicon carbide. Lastly, I have investigated thermal transport in porous and mesoporous structures, including super-hydrophobic polymer aerogel, transparent mesoporous silica, and flexible tin selenide nanosheet films for applications in buildings, windows, and thermoelectric energy conversion.

Advanced materials with extreme thermal conductivity are critically important for various technological applications including energy conversion, storage, and thermal management. High thermal conductivity is desirable for efficient heat spreading in electronics, and low thermal conductivity is needed for thermal insulation and thermoelectric energy harvesting. However, practical application deployments are usually limited by the materials availability and understanding the fundamental origins for extreme thermal conductivity remains challenging. My PhD research focuses on applying and developing first-principles computations to understand the microscopic thermal transport mechanisms of the emerging materials and to discover new materials with ultrahigh and ultralow thermal conductivity. My dissertation is composed of three themes. The first theme is focused on understanding the fundamental origins and transport mechanisms for a group of high thermal conductivity semiconductors that were discovered recently by our group. In particular, boron phosphide (BP) and boron arsenide (BAs) crystals have been synthesized and measured with thermal conductivities of 460 and 1300 W/mK respectively, representing the best thermal conductor among common bulk metals and semiconductors. I have conducted ab initio calculations based on density functional theory to investigate phonon anharmonicity, size-dependent transport from diffusive to ballistic regime, as well as the effect from defect scattering. Our study shows that, unlike the commonly accepted rule for most materials near room temperature, high-order anharmonicity through the four-phonon process is significant in BA because of its unique band structure. In addition, I have performed multiscale Monte Carlo simulations to solve phonon Boltzmann transport equations to compute heat dissipation in three-dimensional practical measurement samples and electronic devices, which quantitively determines temperature distributed resulted by non-equilibrium phonon transport and underscores the promise of our developed BP and BAs for the next generation of thermal management technologies. The second theme of my thesis is to theoretical search for new ultra-high thermal conductivity materials, with the aim to push the limit of existing materials database. We have calculated the thermal conductivity of several B-C-X ternary compounds and found the R3m-BNC2 has ultrahigh thermal conductivity at ~2200 W/mK, which is comparable with the existing highest thermal conductivity materials, diamond. We also calculate the thermal conductivity of single-layer boron compounds in III-V group, and find high thermal conductivity of single-layer h-BAs at around 400 W/K. My computational studies enable atomistic understanding through their phonon band structures, scattering spaces, lifetimes, etc. The third theme of my thesis is to investigate phonon transport in ultralow thermal conductivity materials with a focus on tin selenide (SnSe). SnSe is a recently discovered high performance thermoelectric material, but its intrinsic low thermal conductivity remains debating in recent literature. In collaboration with my labmates, we combine phonon theory and experiments to investigate phonon softening physics. In particular, my calculated phonon frequencies of SnSe under varying temperatures indicate strong phonon renormalization due to higher-order anharmonicity. The comparison of my theory results with experiments indicates that the widely used harmonic model fails to descript the phonon renormalization and thus thermal conductivity of SnSe. Instead, I have developed self-consistent phonon theory to capture the higher order interactions and provided very good agreement with the experimentally measured ultralow thermal conductivity and thermophysical properties of SnSe.

The enhancement of modern electronic devices' performance relies on the increased density of electronic components and the miniaturization of device sizes, in line with Moore's Law. However, this improvement often leads to concentrated heat generation within a small area, which can damage the device and limit its performance. Consequently, efficient thermal management has become critically important. My Ph.D. research focuses on the engineering of electron and phonon transport in semiconductors to fine-tune material properties, thereby reducing heat generation and improving thermal management.

My thesis is structured into five main sections. The first section investigates the vacuum field emission properties of tin selenide (SnSe) nanostructures, specifically in the form of 'nano flowers' (NFs), as an alternative electron transport mechanism that could circumvent the heat generation associated with Joule heating in solid-state devices. The ideal field emission material requires a low work function, high atomic bond energy, and sharp morphological features. SnSe NFs possess these necessary properties. Field emission measurements reveal that SnSe NFs exhibit a high electron emission current density, significant field enhancement factor, and robust emission stability, highlighting their potential for vacuum field emission applications.

The second section delves into phonon transport at the interface between boron arsenide (BAs) and gallium nitride (GaN) to enhance heat dissipation. While GaN is a wide bandgap material favored in high-power and high-voltage devices, such devices demand efficient thermal management for reliable operation. BAs has been identified as a material with superior thermal conductivity, making it a promising candidate for thermal management applications. This study integrates BAs into GaN-based electronic devices to investigate phonon transport across the GaN-BAs interface. The findings indicate a high thermal conductance at this interface, attributable to the good lattice vibration matching between BAs and GaN. Additionally, GaN-on-BAs devices show a substantially reduced hotspot temperature compared to integrated GaN devices with other high thermal conductivity substrates such as diamond or silicon carbide, achieving enhanced heat dissipation and device performance.

The third section examines the controlled doping of van der Waals materials, focusing on black phosphorus (BP). Doping can alter the electron or hole concentration in semiconductors, modifying charge transport properties. However, introducing foreign materials as dopants can also scatter phonons and decrease thermal conductivity. Therefore, precise control over doping is essential to balance high electrical performance with efficient thermal management. Especially, rational doping control remains challenging for van der Waals structures. This research demonstrates controlled doping of BP, utilizing an electrochemical approach to manage the ion intercalation of lithium and copper. This control transforms pristine black phosphorus-based resistors into n-type and p-type phosphorus-based field-effect transistors, as well as p-n junction diodes, depending on the doping parameters. Furthermore, lithium doping has been shown to enhance charge mobility by reducing neutral impurity scattering. This study proposes a novel method for tailoring the properties of van der Waals materials and provides a new platform for engineering the electron and phonon transport properties of 2D materials.

The fourth section demonstrates dynamic tuning thermal conductivity of tin selenide (SnSe). The manipulation of electricity via logic circuits has been one of the cornerstones of modern technologies. However, manipulating heat through logic units, though equally crucial, has been a longstanding challenge. A primary obstacle is to develop a simple yet effective approach to dynamically tune the thermal conductivity of materials across a broad range, which is vital for optimizing thermal management, enhancing energy harvesting efficiency, and developing thermal logic devices. This study presents a breakthrough in dynamically tuning thermal conductivity via electrical gate-tuning, achieved by modulating electron-phonon interactions in a SnSe thermal transistor. The thermal transistor attained a thermal conductivity tuning. Our analysis based on ab initio theory reveals that this modulation in thermal conductivity is primarily stems from the alignment of the SnSe chemical potential with its defect energy level, which activates the electrons of the defect states to scatter phonons. These findings present a novel method for controlling heat flow, opening up new avenues for application in various thermal technologies.

Finally, the fifth section presents thermal properties of defect doped lead selenide (PbSe). PbSe is extensive studied due to its promising thermoelectric properties. Experimental study on the thermal properties of PbSe showed sharpen longitudinal acoustic phonon mode linewidth at high temperature due to reduction of scattering phase space. This study examines the effect of vacancy-induced dislocations on the phonon dynamics of Sb-doped PbSe using inelastic neutron scattering method and diffusivity measurement. The results show sharpening phonon linewidth and enhancement of thermal diffusivity in vacancy-induced dislocations doped PbSe at 300 K. The results show an unexpected impact of defects on the thermal properties of PbSe.