This dissertation examines the effects of surface disorder on phonon dynamics through two different but complementary approaches. First, we use a phonon Monte Carlo (PMC) simulation with random, rough surfaces. PMC is an excellent tool for studying nanostructures of experimentally relevant sizes. We detail our PMC method, including improvements over previous PMC simulations. We investigate why rough silicon nanowires have measured thermal conductivities about two orders of magnitude lower than predicted and comparable to amorphous materials. We show that it can be largely explained through scattering from rough surfaces; extreme roughness causes a qualitative change in how phonons interact with boundaries. During this project, we uncovered the utility of the geometric mean free path (GMFP), which is a concept developed in the study of chaotic billiards. The GMFP is the average distance a particle travels between surface scattering events (in the absence of other scattering mechanisms), and we show that the thermal conductivities obtained from our PMC simulations are a function of the GMFP. Second, we study two-dimensional elastic nanoribbons using finite-difference methods. Elastic materials make good model systems for studying lattice dynamics because elastic materials capture wave behavior, and, in the long-wavelength limit, phonons behave like elastic waves. Our elastic-medium finite-difference time-domain (FDTD) simulation allows us to efficiently model relatively large structures while still treating phonons as waves. We develop a technique to calculate the thermal conductivity of elastic nanoribbons by coupling our FDTD simulation with the Green-Kubo formula. We also employ a time-independent finite-difference (TIFD) method to solve for and study individual modes of our system. We find that rough surfaces can have an outsize impact on phonon dynamics. Surfaces do not simply scatter phonons; rough surfaces can also trap energy and cause all modes throughout the system to localize. The energy trapping and localization coincide with reduced thermal conductivity. We also investigate the effects of Rayleigh waves, a nonbulk mode often ignored in phonon transport simulations. We use TIFD methods to search for signs of wave chaos in nanoribbons. We find an interesting connection between the GMFP and thermal conductivity, which points the way towards future work.

This dissertation examines the effects of surface disorder on phonon dynamics through two different but complementary approaches. First, we use a phonon Monte Carlo (PMC) simulation with random, rough surfaces. PMC is an excellent tool for studying nanostructures of experimentally relevant sizes. We detail our PMC method, including improvements over previous PMC simulations. We investigate why rough silicon nanowires have measured thermal conductivities about two orders of magnitude lower than predicted and comparable to amorphous materials. We show that it can be largely explained through scattering from rough surfaces; extreme roughness causes a qualitative change in how phonons interact with boundaries. During this project, we uncovered the utility of the geometric mean free path (GMFP), which is a concept developed in the study of chaotic billiards. The GMFP is the average distance a particle travels between surface scattering events (in the absence of other scattering mechanisms), and we show that the thermal conductivities obtained from our PMC simulations are a function of the GMFP. Second, we study two-dimensional elastic nanoribbons using finite-difference methods. Elastic materials make good model systems for studying lattice dynamics because elastic materials capture wave behavior, and, in the long-wavelength limit, phonons behave like elastic waves. Our elastic-medium finite-difference time-domain (FDTD) simulation allows us to efficiently model relatively large structures while still treating phonons as waves. We develop a technique to calculate the thermal conductivity of elastic nanoribbons by coupling our FDTD simulation with the Green-Kubo formula. We also employ a time-independent finite-difference (TIFD) method to solve for and study individual modes of our system. We find that rough surfaces can have an outsize impact on phonon dynamics. Surfaces do not simply scatter phonons; rough surfaces can also trap energy and cause all modes throughout the system to localize. The energy trapping and localization coincide with reduced thermal conductivity. We also investigate the effects of Rayleigh waves, a nonbulk mode often ignored in phonon transport simulations. We use TIFD methods to search for signs of wave chaos in nanoribbons. We find an interesting connection between the GMFP and thermal conductivity, which points the way towards future work.

The book is devoted to nanostructures and nanostructured materials containing both amorphous and crystalline phases with a particular focus on their thermal properties. It is the first time that theoreticians and experimentalists from different domains gathered to treat this subject. It contains two distinct parts; the first combines theory and simulations methods with specific examples, while the second part discusses methods to fabricate nanomaterials with crystalline and amorphous phases and experimental techniques to measure the thermal conductivity of such materials. Physical insights are given in the first part of the book, related with the existing theoretical models and the state of art simulations methods (molecular dynamics, ab-initio simulations, kinetic theory of gases). In the second part, engineering advances in the nanofabrication of crystalline/amorphous heterostructures (heavy ion irradiation, electrochemical etching, aging/recrystallization, ball milling, PVD, laser crystallization and magnetron sputtering) and adequate experimental measurement methods are analyzed (Scanning Thermal Microscopy, Raman, thermal wave methods and x-rays neutrons spectroscopy).

The present study investigates the complex relationship between nanostructures and microscale thermal transport in nanoporous thin films for energy applications. It experimentally and numerically demonstrates that the effective thermal conductivity of nanoporous materials can be tuned by controlling their nanoscale architectures including porosity, pore diameter, wall thickness, nanocrystal size, and crystallinity as well as surface passivation. This study reports measurements of the cross-plane thermal conductivity of nanoporous thin films with various architectures between 25 and 315 K. Physics-based models combining phonon transport theory and effective medium approximations were developed to interpret the experimental data. Ordered mesoporous titania and silicon thin films were prepared based on evaporation-induced self-assembly method. Pure silica zeolite films were produced by either in-situ growth or by spin coating a zeolite nanoparticle suspension followed by crystal growth upon heating. These synthesized thin films were systematically and fully characterized. They featured ordered nanopores with porosity, pore diameter, and film thickness ranging from 30% to 59%, 0.5 to 25 nm, and 120 to 370 nm, respectively. Their dense matrix was amorphous, polycrystalline, or consisted of an aggregate of nanocrystals. The thermal conductivity of all synthesized nanoporous films increased monotonically with temperature within the temperature range considered. At low temperatures, the nanoporous films behaved like amorphous or strongly disordered materials and their thermal conductivity was proportional to T^n with n varied between 1 and 2.3. At high temperatures, the thermal conductivity increased slowly with temperature or reached a plateau due to strong phonon Umklapp scattering and the saturation of phonon modes. The presence of pores in amorphous mesoporous thin films had a purely geometrical effect by reducing the cross-sectional area through which heat can diffuse. By contrast, in crystalline mesoporous thin films the presence of pores also increased phonon scattering. In addition, the film thickness generally did not affect the measured thermal conductivity. Indeed, phonon scattering by pores and by nanocrystal grain boundary dominated over boundary scattering and were identified as the dominant scattering mechanisms for nanoscale energy transport in the synthesized nanoporous films. This study further establishes that the effective thermal conductivity keff of crystalline nanoporous silicon was strongly affected not only by the porosity fv and the system's length Lz but also by the pore interfacial area concentration Ai. A modified effective medium approximation combining kinetic theory and the coherent potential approximation suggested that keff was proportional to (1-1.5fv) and inversely proportional to the sum (Ai/4+1/Lz). This scaling law was in excellent agreement with the thermal conductivity of nanoporous silicon predicted by molecular dynamics simulations for spherical pores as well as for cylindrical pores and vacancy defects. Finally, this study demonstrated, using equilibrium molecular dynamics simulations, that surface passivation added another parameter for reducing the thermal conductivity of nanostructured materials. To do so, there should be strong acoustic vibrational modes coupling between surface and passivation atoms. For example, oxygen passivation reduced the thermal conductivity of nanoporous crystalline silicon. In addition, the effect of passivation reduced with temperature because of increasing contribution of Umklapp scattering. These results could help establish new strategies to control the thermal conductivity of nanoporous materials for a wide range of applications including thermoelectric devices, supercapacitors, dye-sensitized solar cells, and hydrogen storage devices.

In this Brief, authors introduce the advance in theoretical and experimental techniques for determining the thermal conductivity in nanomaterials, and focus on review of their recent theoretical studies on the thermal properties of silicon-based nanomaterials, such as zero-dimensional silicon nanoclusters, one-dimensional silicon nanowires, and graphenelike two-dimensional silicene. The specific subject matters covered include: size effect of thermal stability and phonon thermal transport in spherical silicon nanoclusters, surface effects of phonon thermal transport in silicon nanowires, and defects effects of phonon thermal transport in silicene. The results obtained are supplemented by numerical calculations, presented as tables and figures. The potential applications of these findings in nanoelectrics and thermoelectric energy conversion are also discussed. In this regard, this Brief represents an authoritative, systematic, and detailed description of the current status of phonon thermal transport in silicon-based nanomaterials. This Brief should be a highly valuable reference for young scientists and postgraduate students active in the fields of nanoscale thermal transport and silicon-based nanomaterials.

The monograph is devoted to the investigation of physical processes that govern the phonon transport in bulk and nanoscale single-crystal samples of cubic symmetry. Special emphasis is given to the study of phonon focusing in cubic crystals and its influence on the boundary scattering and lattice thermal conductivity of bulk materials and nanostructures.

In semiconductor devices, thermal energy is carried by phonons, the quantized excitation of atomic vibrations. These phonons scatter with impurities, electrons, grain boundaries, and other phonons. At a sufficiently large scale, phonon dynamics can be approximated as a Brownian random walk, leading to ordinary diffusion described by the heat equation. However, such approximations fail at the scale of the phonon mean free path. In this regime, a proper wave description encoding phonon scattering is required. For sufficiently short thermal systems, the thermal conductivity becomes extrinsic and exhibits linear scaling with system size. This scale is known as the ballistic transport regime. As the system size grows beyond this scale, the thermal conductivity asymptotes into the intrinsic, ordinary diffusive regime. However, there are special circumstances where this transition does not occur. In this Thesis, we demonstrate the anomalous scaling of thermal conductivity. The source of this anomaly is the Anderson localization of thermal phonons. Anderson localization is the spatial trapping of waves due to extreme levels of elastic disorder. The hallmark of Anderson localization is an exponential decay law of conductance with increasing system size. Since thermal transport is a broadband process, this exponential suppression leads to a thermal conductivity maximum as a function of system size. Our numerical study of GaAs/AlAs superlattices with ErAs nanoparticles exhibits this thermal conductivity maximum, yielding quantitative agreement to experiments. We then generalize our elastic model to allow for the incorporation of finite-temperature effects. The inclusion of phonon-phonon scattering decoheres phonons, resulting in phonon delocalization. Counterintuitively, the additional inelastic scattering increases conductance for originally localized phonons. This localization to diffusive transition as a function of temperature is captured in our model at low temperatures (~20K).

With the advancement of nanofabrication techniques, the sizes of semiconductor electronic and optoelectronic devices keep decreasing while the operating speeds keep increasing. High-speed operation leads to more heat generation and puts more thermal stress on the devices. Since the heat conduction in semiconductors is dominated by the lattice (i.e., phonons), understanding phonon transport in nanostructures is essential to addressing and alleviating the thermal-stress problem in these modern devices. In addition to the increased thermal stress, the advanced techniques that have allowed for the shrinking of the devices routinely rely on heterostructuring, doping, alloying, and the growth of intentionally strained layers to achieve the desired electronic and optical properties. These introduce impediments to phonon transport such as boundaries, interfaces, point defects (alloy atoms or dopants), and strain. Phonon transport is strongly affected by this nanoscale disorder. This dissertation examines how different types of disorder interact with phonons and degrade phonon transport. First, we study thermal transport in graphene nanoribbons (GNRs). GNRs are quasi-one-dimensional (quasi-1D) systems where the edges (boundaries) play an important role in reducing thermal conductivity. Additionally, the thermal transport in GNRs is anisotropic and depend on the GNR's chirality (GNR orientation and edge termination). We use phonon Monte Carlo (PMC) with full phonon dispersions to describe two highly-symmetric types of GNRs: the armchair GNR (AGNR) and the zigzag GNR (ZGNR). PMC tracks phonon in real space and we can explicitly include non-trivial edge structures. Moreover, the relatively low computational burden of PMC allows us to simulate samples up to 100 $\mu$m in length and predict an upper limit for thermal conductivity in graphene. We then investigate the thermal conductivity in III-V superlattices (SLs). SLs consist of alternating thin layers of different materials and III-V SLs are widely used in nanoscale thermoelectric and optoelectronic devices. The key feature in SLs is that it contains many interfaces, which dictates thermal transport. As III-V SLs are often fabricated using well-controlled techniques and have high-quality interfaces, we develop a model with only one free parameter---the effective rms roughness of the interfaces---to describe its twofold influence: reducing the in-plane layer thermal conductivity and introducing thermal boundary resistance (TBR) in the cross-plane direction. Both the calculated in-plane and cross-plane thermal conductivity of SLs agree with a number of different experiments. Finally, we study thermal conductivity of ternary III-V alloys. In modern optoelectronic devices, ternary III-V alloys are used more often than binary compounds because one can use composition engineering to achieve different effective masses, electron/hole barrier heights, and strain levels. Ternary alloys are usually treated under the virtual crystal approximation (VCA) where cation atoms are assumed to be randomly distributed and possess an averaged mass. This assumption is challenged by a discrepancy between different experiments, as well as the discrepancy between experiments and calculations. We use molecular dynamics (MD) to study the ternary alloy system as both atom masses and atom locations are explicitly tracked in MD. We discover that the thermal conductivity is determined by a competition between mass-difference scattering and the short-range ordering of the cations.