Materials at the nanoscale show properties uniquely different from the bulk scale which when controlled can be utilized for variety of thermal management applications. Different applications require reduction, increase or directional control of thermal conductivity. This thesis focuses on investigating thermal transport in two such application areas, viz., 1) thermoelectric energy conversion and 2) thermal rectification. Using molecular dynamics simulations, several methods for reducing of thermal conductivity in polyaniline and polyacetylene are investigated. The reduction in thermal conductivity leads to improvement in thermoelectric figure of merit. Thermal diodes allow heat transfer in one direction and prevents in the opposite direction. These materials have potential application in phononics, i.e., for performing logic calculations with phonons. Rectification obtained with existing material systems is either too small or too difficult to implement. In this thesis, a more useful scheme is presented that provides higher rectification using a single wall carbon nanotube (SWCNT) that is covalently functionalized near one end with polyacetylene (PA). Although several thermal diodes are discussed in literature, more complex phononic devices like thermal logic gates and thermal transistors have been sparingly investigated. This thesis presents a first design of a thermal AND gate using asymmetric graphene nanoribbon (GNR) and characterizes its performance. / Ph. D.

<div>The rapid advance in modern electronics and photonics is pushing device design to the micro- and nano-scale, and the resulting high power density imposes immense challenges to thermal management. Promising materials like carbon nanotubes (CNTs) and graphene offer high thermal conductivity in the axial (or in-plane) directions, but their thermal transport in the radial (or cross-plane) directions are poor, limiting their applications. Hierarchical structures like pillared graphene, which is composed of many CNT-graphene junctions, have been proposed. However, thermal</div><div>interfacial resistance is a critical issue for thermal management of these systems. In this work, we have systematically explored thermal transport across interfaces,</div><div>particularly in pillared graphene and silicon/heavy-silicon.</div><div><br></div><div><div>First, by recognizing that thermal resistance of the 3D pillared graphene architecture primarily comes from CNT-graphene junctions, a simple network model of thermal transport in pillared graphene structure is developed. Using non-equilibrium molecular dynamics (NEMD), the resistance across an individual CNT-graphene junction with sp2 covalent bonds is found to be around 6 × 10−11 m2K/W, which is significantly lower than typical values reported for planar interfaces between dissimilar materials. Interestingly, when the CNT pillar length is small, the interfacial resistance</div><div>of the sp2 covalent junction is found to decrease as the CNT pillar length decreases, suggesting the presence of coherence effects. The junction resistance Rj is eventually</div><div>used in the network model to estimate the effective thermal conductivity, and the results agree well with direct MD simulation data, demonstrating the effectiveness of our model.</div></div><div><br></div><div><div>Then we identify three different mechanisms which can lead to thermal resistances across the pillared graphene junction: the material mismatch (phonon propagates from CNT to graphene), the non-planar junction (the phonon propagation direction must change), and defects (there are six heptagons at each junction). The NEMD results show that three mechanisms lead to similar resistance at the CNT-graphene junction, each at around 2.5 × 10−11 m2K/W.</div></div><div><br></div><div><div>Further, we have predicted the transmission function of individual phonon mode using the wave packet method at CNT-graphene junction. Intriguing phonon polarization conversion behavior is observed for most incident phonon modes. It is found that the polarization conversion dominates the transmission and is more significant at larger phonon wavelength. We attribute such unique phonon polarization conversion behavior to the dimensional mismatch across CNT-graphene interface. It is found that the transmission functions at the junction cannot be predicted by the conventional acoustic mismatch models due to the existence of dimensional mismatch. Further analysis shows that, the dimensionally mismatched interface, on one hand tends to reduce the transmission and conductance due to defects and the change of phonon propagation direction at the interface, while on the other hand tends to enhance the transmission and conductance due to the new phonon transport channel introduced by polarization conversion.</div></div><div><br></div><div><div>Finally, we address that many recent experiments have shown that the measured thermal boundary conductances (TBCs) significantly exceed those calculated using the Landauer approach. We identify that a key assumption that an interface is a local equilibrium system (different modes of phonons on each side of the interfaces are at the emitted phonon temperature Te), is generally invalid and can contribute to the discrepancy. We show that the measurable temperature for each individual mode is the ”modal equivalent equilibrium temperature” T rather than Te. Also,</div><div>due to the vast range of transmission functions, different phonon modes are out of local thermal equilibrium. Hence, the total conductance cannot be simply calculated as a summation of individual modal conductance. We modify the Landauer approach to include these effects and name it the ”Nonequilibrium Landauer approach”. Our approach has been used on the carbon nanotube (CNT)/graphene and Si/heavy-Si interfaces which are matched interfaces, and it gives 310% increases in TBC as compared to the conventional Landauer approach at CNT-graphene junction and even higher increase for Si/heavy-Si with small mass ratios. A convenient chart is created to estimate the conductance correction based on our approach, and it yields quite accurate results. Our work indicate that the measured high TBCs in experiments can be due to this nonequilibrium effect rather than the other proposed mechanisms, like inelastic phonon transmission and cross-interface electron-phonon coupling.</div></div><div><br></div><div><div>The results obtained in this study will provide a deeper understanding of nanoscale thermal transport across interfaces. This research also provides new perspectives of</div><div>atomic- and nano-scale engineering of materials and structures to enhance performance of thermal management.</div></div>

Molecular Dynamics (MD) simulations of nano-scale flows typically utilize fixed lattice crystal interactions between the fluid and stationary wall molecules. This approach cannot properly model thermal interactions at the wall-fluid interface. In order to properly simulate the flow and heat transfer in nano-scale channels, an interactive thermal wall model is developed. Using this model, the Fourier’s law of heat conduction is verified in a 3.24 nm height channel, where linear temperature profiles with constant thermal conductivity is obtained. The thermal conductivity is verified using the predictions of Green-Kubo theory. MD simulations at different wall wettability ( εωf /ε ) and crystal bonding stiffness values (K) have shown temperature jumps at the liquid/solid interface, corresponding to the well known Kapitza resistance. Using systematic studies, the thermal resistance length at the interface is characterized as a function of the surface wettability, thermal oscillation frequency, wall temperature and thermal gradient. An empirical model for the thermal resistance length, which could be used as the jump-coefficient of a Navier boundary condition, is developed. Temperature distributions in the nano-channels are predicted using analytical solution of the continuum heat conduction equation subjected to the new temperature jump condition, and validated using the MD results. Momentum and heat transfer in shear driven nanochannel flows are also investigated. Work done by the viscous stresses heats the fluid, which is dissipated through the channel walls, maintained at isothermal conditions. Spatial variations in the fluid density, kinematic viscosity, shear- and energy dissipation rates are presented. The energy dissipation rate is almost a constant for εωf /ε < 0.6, which results in parabolic temperature profiles in the domain with temperature jumps due to the Kapitza resistance at the liquid/solid interfaces. Using the energy dissipation rates predicted by MD simulations and the continuum energy equation subjected to the temperature jump boundary conditions developed in this study, the analytical solutions are obtained for the temperature profiles, which agree well with the MD results.

The mechanisms of thermal transport in defect-free silicon nanostructures are examined using a combination of lattice dynamics (LD) calculations and the Boltzmann transport equation (BTE). To begin, the thermal conductivity reduction in thin films is examined using a hierarchical method that first predicts phonon transport properties using LD calculations, and then solves the phonon BTE using the lattice Boltzmann method. This approach, which considers all of the phonons in the first Brillouin-zone, is used to assess the suitability of common assumptions used to reduce the computational effort. Specifically, we assess the validity of: (i) neglecting the contributions of optical modes, (ii) the isotropic approximation, (iii) assuming an averaged bulk mean-free path (i.e., the Gray approximation), and (iv) using the Matthiessen rule to combine the effect of different scattering mechanisms. Because the frequency-dependent contributions to thermal conductivity change as the film thickness is reduced, assumptions that are valid for bulk are not necessarily valid for thin films. Using knowledge gained from this study, an analytical model for the length-dependence of thin film thermal conductivity is presented and compared to the predictions of the LD-based calculations. The model contains no fitting parameters and only requires the bulk lattice constant, bulk thermal conductivity, and an acoustic phonon speed as inputs. By including the mode-dependence of the phonon lifetimes resulting from phonon-phonon and phonon-boundary scattering, the model predictions capture the approach to the bulk thermal conductivity better than predictions made using Gray models based on a single lifetime. Both the model and the LD-based method are used to assess a procedure commonly used to extract bulk thermal conductivities from length-dependent molecular dynamics simulation data. Because the mode-dependence of thermal conductivity is not included in the derivation of this extrapolation procedure, using it can result in significant error. Finally, phonon transport across a silicon/vacuum-gap/silicon structure is modelled using lattice dynamics and Landauer theory. The phonons transmit thermal energy across the vacuum gap via atomic interactions between the leads. Because the incident phonons do not encounter a classically impenetrable potential barrier, this mechanism is not a tunneling phenomenon. The heat flux due to phonon transport can be 4 orders of magnitude larger than that due to photon transport predicted from near-field radiation theory.

The mechanisms of thermal transport in defect-free silicon nanostructures are examined using a combination of lattice dynamics (LD) calculations and the Boltzmann transport equation (BTE). To begin, the thermal conductivity reduction in thin films is examined using a hierarchical method that first predicts phonon transport properties using LD calculations, and then solves the phonon BTE using the lattice Boltzmann method. This approach, which considers all of the phonons in the first Brillouin-zone, is used to assess the suitability of common assumptions used to reduce the computational effort. Specifically, we assess the validity of: (i) neglecting the contributions of optical modes, (ii) the isotropic approximation, (iii) assuming an averaged bulk mean-free path (i.e., the Gray approximation), and (iv) using the Matthiessen rule to combine the effect of different scattering mechanisms. Because the frequency-dependent contributions to thermal conductivity change as the film thickness is reduced, assumptions that are valid for bulk are not necessarily valid for thin films. Using knowledge gained from this study, an analytical model for the length-dependence of thin film thermal conductivity is presented and compared to the predictions of the LD-based calculations. The model contains no fitting parameters and only requires the bulk lattice constant, bulk thermal conductivity, and an acoustic phonon speed as inputs. By including the mode-dependence of the phonon lifetimes resulting from phonon-phonon and phonon-boundary scattering, the model predictions capture the approach to the bulk thermal conductivity better than predictions made using Gray models based on a single lifetime. Both the model and the LD-based method are used to assess a procedure commonly used to extract bulk thermal conductivities from length-dependent molecular dynamics simulation data. Because the mode-dependence of thermal conductivity is not included in the derivation of this extrapolation procedure, using it can result in significant error. Finally, phonon transport across a silicon/vacuum-gap/silicon structure is modelled using lattice dynamics and Landauer theory. The phonons transmit thermal energy across the vacuum gap via atomic interactions between the leads. Because the incident phonons do not encounter a classically impenetrable potential barrier, this mechanism is not a tunneling phenomenon. The heat flux due to phonon transport can be 4 orders of magnitude larger than that due to photon transport predicted from near-field radiation theory.

<p dir="ltr">Knowledge of transport processes plays a critical role in the development and application of materials in many technologies. As manufacturing technologies continue to push the geometries of materials to smaller scales, traditional means of predicting and measuring transport properties begin to fail. Micro and nanoscopic effects tend to alter transport phenomena in materials, leading to new physics and different properties from the bulk state. In particular, the dynamics of thermal transport of a material varies greatly in both spatial and temporal senses. Different energy carriers have intrinsically different mechanisms of thermal transport; depending on the time and lengths scales in question, the contribution to the overall thermal transport by one carrier may be vastly different than others. To characterize and understand the dynamics of thermal transport at these small scales, novel ultrafast experimental techniques and theories are crucially needed. This work will discuss the efforts made to develop a framework to measure and differentiate the dynamics of transport processes of a material due to different energy carriers using ultrafast optical techniques. This dissertation is organized as follows.</p><p dir="ltr">Chapter 1 gives a background in the theory of thermal transport. This will serve as the foundation for the physical models that are used to extract thermal properties from experimental works. A brief review of the advances in ultrafast experimental and theoretical works will also be given. This will assist in placing this work in the context of ongoing work in the thermal transport community. Chapter 2 illustrates the experimental setups and physical models used to measure the effective thermal transport properties of thin film materials. Steady-state optical measurements are used to quantify the effective, in-plane, anisotropic, thermal conductivity of a 2D material. Time resolved, ultrafast optical measurements are used to quantify the effective, out-of-plane, thermal conductivity of a material. Chapters 3 and 4 demonstrate the capabilities of an ultrafast spatiotemporal scanning pump-probe system, where the high temporal and nanometric resolution measurements directly probe the electron contribution to thermal transport in metals as well as the ambipolar diffusion of carriers in semiconductors. Lastly, Chapter 5 summarizes this dissertation and provides a discussion on the use of the developed experimental capabilities to probe transport of emerging materials.</p>

Understanding thermal transport across length scales lays the foundation to developing high-performance electronic devices. Although many experiments and models of the past few decades have explored the physics of heat transfer at nanoscale, there are still open questions regarding the impact of periodic nanostructuring and coherent phonon effects, as well as the interaction of strain and thermal transport. Thermomechanical effects, as well as strains applied in flexible electronic devices, impact the thermal transport. In the simplest kinetic theory models, thermal conductivity is proportional to the phonon group velocity, heat capacity, and scattering times. Periodic porous nanostructures impact the phonon dispersion relationship (group velocity) and the boundaries of the pores increase the scattering times. Strain, on the other hand, affects the crystal structure of the lattice and slightly increases the thermal conductivity of the material under compression. Intriguingly, applying strain combined with the periodic porous structures is expected to influence both the dispersion relation and scattering rates and yield the ability to tune thermal transport actively. But often these interrelated effects are simplified in models.<br><br>This work evaluates the combination of structure and strain on thermal conductivity by revisiting some of the essential methods used to predict thermal transport for a single layer of graphene with a periodic porous lattice structure with and without applied strain. First, we use the highest fidelity method of Non-Equilibrium Molecular Dynamics (NEMD) simulations to estimate the thermal conductivity which considers the impact of the lattice structure, strain state, and phononic band structure together. Next, the impact of the geometry of the slots within the lattice is interrogated with Boltzmann Transport Equation (BTE) models under a Relaxation Time Approximation. A Monte Carlo based Boltzmann Transport Equation (BTE) solver is also used to estimate the thermal conductivity of phononic crystals with varying pore geometry. Dispersion relations calculated from continuum mechanics are used as input here. This method which utilizes a simplified pore geometry only partially accounts for the effects of scattering on the pore boundaries. Finally, a continuum level model is also used to predict the thermal conductivity and its variations under applied strain. As acoustic phonon branches tend to carry the most heat within the lattice, these continuum models and other simple kinetic theories only consider their group velocities to estimate their impact on phonon thermal conductivity. As such, they do not take into account the details of phonon transport across all wavelengths.<br><br>By comparing the results from these different methods, each of which has different assumptions and simplifications, the current work aims to understand the effects of changes to the dispersion relationship based on strain and the periodic nanostructures on the thermal conductivity. We evaluate the accuracy of the kinetic theory, ray tracing, and BTE models in comparison to the MD results to offer a perspective of the reliability of each method of thermal conductivity estimation. In addition, the effect of strain on each phononic crystal with different pore geometry is also predicted in terms of change to their in-plane thermal anisotropy values. To summarize, this deeper understanding of the nanoscale thermal transport and the interrelated effects of geometry, strain, and phonon band structure on thermal conductivity can aid in developing lattices specifically designed to achieve the required dynamic thermal response for future nano-scale thermoelectric applications.

Plasmonic properties and the related novel applications are studied on various types of metallic nano-structures in one, two, or three dimensions. For 1D nanostructure, the motion of free electrons in a metal-film with nanoscale thickness is confined in its normal dimension and free in the other two. Describing the free-electron motion at metal-dielectric surfaces, surface plasmon polariton (SPP) is an elementary excitation of such motions and is well known. When further perforated with periodic array of holes, periodicity will introduce degeneracy, incur energy-level splitting, and facilitate the coupling between free-space photon and SPP. We applied this concept to achieve a plasmonic perfect absorber. The experimentally observed reflection dip splitting is qualitatively explained by a perturbation theory based on the above concept. If confined in 2D, the nanostructures become nanowires that intrigue a broad range of research interests. We performed various studies on the resonance and propagation of metal nanowires with different materials, cross-sectional shapes and form factors, in passive or active medium, in support of corresponding experimental works. Finite- Difference Time-Domain (FDTD) simulations show that simulated results agrees well with experiments and makes fundamental mode analysis possible. Confined in 3D, the electron motions in a single metal nanoparticle (NP) leads to localized surface plasmon resonance (LSPR) that enables another novel and important application: plasmon-heating. By exciting the LSPR of a gold particle embedded in liquid, the excited plasmon will decay into heat in the particle and will heat up the surrounding liquid eventually. With sufficient exciting optical intensity, the heat transfer from NP to liquid will undergo an explosive process and make a vapor envelop: nanobubble. We characterized the size, pressure and temperature of the nanobubble by a simple model relying on Mie calculations and continuous medium assumption. A novel effective medium method is also developed to replace the role of Mie calculations. The characterized temperature is in excellent agreement with that by Raman scattering. If fabricated in an ordered cluster, NPs exhibit double-resonance features and the double Fano-resonant structure is demonstrated to most enhance the four-wave mixing efficiency.