Elementary particles such as electrons or photons are frequent subjects of wave-nature-driven investigations, unlike collective excitations such as phonons. The demonstration of wave-particle crossover, in terms of macroscopic properties, is crucial to the understanding and application of the wave behaviour of matter. We present an unambiguous demonstration of the theoretically predicted crossover from diffuse (particle-like) to specular (wave-like) phonon scattering in epitaxial oxide superlattices, manifested by a minimum in lattice thermal conductivity as a function of interface density. We do so by synthesizing superlattices of electrically insulating perovskite oxides and systematically varying the interface density, with unit-cell precision, using two different epitaxial-growth techniques. These observations open up opportunities for studies on the wave nature of phonons, particularly phonon interference effects, using oxide superlattices as model systems, with extensive applications in thermoelectrics and thermal management.

Graphene-based devices have garnered tremendous attention due to the unique physical properties arising from this purely two-dimensional carbon sheet leading to tremendous efficiency in the transport of thermal carriers (i.e., phonons). However, it is necessary for this two-dimensional material to be able to efficiently transport heat into the surrounding 3D device architecture in order to fully capitalize on its intrinsic transport capabilities. Therefore, the thermal boundary conductance at graphene interfaces is a critical parameter in the realization of graphene electronics and thermal solutions. In this work, we examine the role of chemical functionalization on the thermal boundary conductance across metal/graphene interfaces. Specifically, we metalize graphene that has been plasma functionalized and then measure the thermal boundary conductance at Al/graphene/SiO(2) contacts with time domain thermoreflectance. The addition of adsorbates to the graphene surfaces are shown to influence the cross plane thermal conductance; this behavior is attributed to changes in the bonding between the metal and the graphene, as both the phonon flux and the vibrational mismatch between the materials are each subject to the interfacial bond strength. These results demonstrate plasma-based functionalization of graphene surfaces is a viable approach to manipulate the thermal boundary conductance.

The thermal conductance of interfaces plays a major role in defining the thermal properties of nanostructured materials in which heat transfer is predominantly phonon mediated. Ongoing research has improved the understanding of factors which govern interfacial phonon transport, as well as the ability to predict thermal interface conductance. However, despite this progress, the ability to control interface conductance remains a major challenge. In this manuscript, we present a method to enhance and tune thermal interface conductance at vibrationally mismatched solid-solid interfaces. Enhancement is achieved through the insertion of an interfacial film with mediating vibrational properties, such that the vibrational mismatch at the interface is bridged, and consequently, the total interface conductance is enhanced. This phenomena is explored using non-equilibrium molecular dynamics simulations, where the effects of altering the interfacial film thickness, vibrational spectrum, and the temperature of the system are investigated. A systematic study of these pertinent design parameters explores the ability to enhance and tune phonon transport at both ideal (sharp) and non-ideal (compositionally disordered) interfaces. Results show that interface conductance can be broadly enhanced by up to 53% in comparison to the vibrationally mismatched baseline interface. Additionally, we find that compositional disorder at an interface does not imply a deterministic change in interface conductance, but instead, that the influence of compositional disorder depends on the characteristics of the disordered region itself. These results, in contrast to macroscopic thermal transport theory, imply that it is possible to increase thermal conductance associated with interface scattering by adding more material along the direction of heat flux.

We examine the fundamental phonon mechanisms affecting the interfacial thermal conductance across a single layer of quantum dots (QDs) on a planar substrate. We synthesize a series of Ge x Si 1−x QDs by heteroepitaxial self-assembly on Si surfaces and modify the growth conditions to provide QD layers with different root-meansquare (rms) roughness levels in order to quantify the effects of roughness on thermal transport. We measure the thermal boundary conductance (h K) with time-domain thermoreflectance. The trends in thermal boundary conductance show that the effect of the QDs on h K are more apparent at elevated temperatures, while at low temperatures, the QD patterning does not drastically affect h K. The functional dependence of h K with rms surface roughness reveals a trend that suggests that both vibrational mismatch and changes in the localized phonon transport near the interface contribute to the reduction in h K. We find that QD structures with rms roughnesses greater than 4 nm decrease h K at Si interfaces by a factor of 1.6. We develop an analytical model for phonon transport at rough interfaces based on a diffusive scattering assumption and phonon attenuation that describes the measured trends in h K. This indicates that the observed reduction in thermal conductivity in SiGe quantum dot superlattices is primarily due to the increased physical roughness at the interfaces, which creates additional phonon resistive processes beyond the interfacial vibrational mismatch.

We experimentally investigate the role of size effects and boundary scattering on the thermal conductivity of silicon-germanium alloys. The thermal conductivities of a series of epitaxially grown Si(1-x)Ge(x) thin films with varying thicknesses and compositions were measured with time-domain thermoreflectance. The resulting conductivities are found to be 3 to 5 times less than bulk values and vary strongly with film thickness. By examining these measured thermal conductivities in the context of a previously established model, it is shown that long wavelength phonons, known to be the dominant heat carriers in alloy films, are strongly scattered by the film boundaries, thereby inducing the observed reductions in heat transport. These results are then generalized to silicon-germanium systems of various thicknesses and compositions; we find that the thermal conductivities of Si(1-x)Ge(x) superlattices are ultimately limited by finite size effects and sample size rather than periodicity or alloying. This demonstrates the strong influence of sample size in alloyed nanosystems. Therefore, if a comparison is to be made between the thermal conductivities of superlattices and alloys, the total sample thicknesses of each must be considered.

Continued reduction in characteristic dimensions in nanosystems has given rise to increasing importance of material interfaces on the overall system performance. With regard to thermal transport, this increases the need for a better fundamental understanding of the processes affecting interfacial thermal transport, as characterized by the thermal boundary conductance. When thermal boundary conductance is driven by phononic scattering events, accurate predictions of interfacial transport must account for anharmonic phononic coupling as this affects the thermal transmission. In this paper, a new model for phononic thermal boundary conductance is developed that takes into account anharmonic coupling, or inelastic scattering events, at the interface between two materials. Previous models for thermal boundary conductance are first reviewed, including the diffuse mismatch model, which only considers elastic phonon scattering events, and earlier attempts to account for inelastic phonon scattering, namely, the maximum transmission model and the higher harmonic inelastic model. A new model is derived, the anharmonic inelastic model, which provides a more physical consideration of the effects of inelastic scattering on thermal boundary conductance. This is accomplished by considering specific ranges of phonon frequency interactions and phonon number density conservation. Thus, this model considers the contributions of anharmonic, inelastically scattered phonons to thermal boundary conductance. This new anharmonic inelastic model shows improved agreement between the thermal boundary conductance predictions and experimental data at the Pb/diamond and Au/diamond interfaces due to its ability to account for the temperature dependent changing phonon population in diamond, which can couple anharmonically with multiple phonons in Pb and Au. We conclude by discussing phonon scattering selection rules at interfaces and the probability of occurrence of these higher order anharmonic interfacial phonon processes quantified in this work.

Identification of the atomic scale structures of the gold-thiol interfaces of molecular nanowires by inelastic tunneling spectroscopy J. Chem. Phys. 136, 014703 (2012) Interfacial thermal resistance between metallic carbon nanotube and Cu substrate J. Appl. Phys. 110, 124314 (2011) Multiscale modeling of cross-linked epoxy nanocomposites to characterize the effect of particle size on thermal conductivity

We report on the thermal conductivities of microcrystalline [6,6]-phenyl C(61)-butyric acid methyl ester (PCBM) thin films from 135 to 387 K as measured by time domain thermoreflectance. Thermal conductivities are independent of temperature above 180 K and less than 0.030 ± 0.003 W m(-1) K(-1) at room temperature. The longitudinal sound speed is determined via picosecond acoustics and is found to be 30% lower than that in C(60)/C(70) fullerite compacts. Using Einstein's model of thermal conductivity, we find the Einstein characteristic frequency of microcrystalline PCBM is 2.88 × 10(12) rad s(-1). By comparing our data to previous reports on C(60)/C(70) fullerite compacts, we argue that the molecular tails on the fullerene moieties in our PCBM films are responsible for lowering both the apparent sound speeds and characteristic vibrational frequencies below those of fullerene films, thus yielding the exceptionally low observed thermal conductivities.