The influence of planar organic linkers on thermal boundary conductance across hybrid interfaces has focused on the organic/inorganic interaction energy rather than on vibrational mechanisms in the molecule. As a result, research into interfacial transport at planar organic monolayer junctions has treated molecular systems as thermally ballistic. We show that thermal conductance in phosphonic acid (PA) molecules is ballistic, and the thermal boundary conductance across metal/PA/sapphire interfaces is driven by the same phononic processes as those across metal/sapphire interfaces without PAs, with one exception. We find a more than 40% reduction in conductance across henicosafluorododecylphosphonic acid (F21PA) interfaces, independent of metal contact, despite similarities in structure, composition, and terminal group to the variety of other PAs studied. Our results suggest diffusive scattering of thermal vibrations in F21PA, demonstrating a clear path toward modification of interfacial thermal transport based on knowledge of ballistic and diffusive scattering in single monolayer molecular interfacial films.

The advances in phonon spectroscopy in homogeneous solids have unveiled extremely useful physics regarding the contribution of phonon energies and mean-free paths to the thermal transport in solids. However, as material systems decrease to length scales less than the phonon mean-free paths, thermal transport can become much more impacted by scattering and transmission across interfaces between two materials than the intrinsic relaxation in the homogeneous solid. To elucidate the fundamental interactions driving this thermally limiting interfacial phonon scattering process, we analytically derive and experimentally measure a thermal boundary conductance accumulation function. We develop a semiclassical theory to calculate the thermal boundary conductance accumulation function across interfaces using the diffuse mismatch model, and validate this derivation by measuring the interface conductance between eight different metals on native oxide/silicon substrates and four different metals on sapphire substrates. Measurements were performed at room temperature using time-domain thermoreflectance and represent the first-reported values for interface conductance across several metal/native oxide/silicon and metal/sapphire interfaces. The various metal films provide a variable bandwidth of phonons incident on the metal/substrate interface. This method of varying phonons' cutoff frequency in the film while keeping the same substrate allows us to mimic the accumulation of thermal boundary conductance and thus provides a direct method to experimentally validate our theory. We show that the accumulation function can be written as the product of a weighted average of the interfacial phonon transmission function and the accumulation of the temperature derivative of the phonon flux incident on the interface; this provides the framework to extract an average, spectrally dependent phonon transmissivity from a series of thermal boundary conductance measurements. Our approach provides a platform for analyzing the spectral phononic contribution to interfacial thermal transport in our experimentally measured data of metal/substrate thermal boundary conductance. Based on the assumptions made in this work and the measurement results on different metals on native oxide/silicon and sapphire substrates, we demonstrate that high-frequency phonons dictate the transport across metal/Si interfaces, especially in low Debye temperature metals with low-cutoff frequencies.

The high mobility exhibited by both supported and suspended graphene, as well as its large in-plane thermal conductivity, has generated much excitement across a variety of applications. As exciting as these properties are, one of the principal issues inhibiting the development of graphene technologies pertains to difficulties in engineering high-quality metal contacts on graphene. As device dimensions decrease, the thermal and electrical resistance at the metal/graphene interface plays a dominant role in degrading overall performance. Here we demonstrate the use of a low energy, electron-beam plasma to functionalize graphene with oxygen, fluorine, and nitrogen groups, as a method to tune the thermal and electrical transport properties across gold-single layer graphene (Au/SLG) interfaces. We find that while oxygen and nitrogen groups improve the thermal boundary conductance (hK) at the interface, their presence impairs electrical transport leading to increased contact resistance (ρC). Conversely, functionalization with fluorine has no impact on hK, yet ρC decreases with increasing coverage densities. These findings indicate exciting possibilities using plasma-based chemical functionalization to tailor the thermal and electrical transport properties of metal/2D material contacts.

We study the interplay between the contributions of electron thermal flux and interface scattering to the Kapitza conductance across metal-metal interfaces through measurements of thermal conductivity of copper-niobium multilayers. Thermal conductivities of copper-niobium multilayer films of period thicknesses ranging from 5.4 to 96.2 nm and sample thicknesses ranging from 962 to 2677 nm are measured by time-domain thermoreflectance over a range of temperatures from 78 to 500 K. The Kapitza conductances between the Cu and Nb interfaces in multilayer films are determined from the thermal conductivities using a series resistor model and are in good agreement with the electron diffuse mismatch model. Our results for the thermal boundary conductance between Cu and Nb are compared to literature values for the thermal boundary conductance across Al-Cu and Pd-Ir interfaces, and demonstrate that the interface conductance in metallic systems is dictated by the temperature derivative of the electron energy flux in the metallic layers, rather than electron mean free path or scattering processes at the interface.

We report on the thermal conductivities of amorphous Stillinger-Weber and Lennard-Jones superlattices as determined by non-equilibrium molecular dynamics simulations. Thermal conductivities decrease with increasing interface density, demonstrating that interfaces contribute a non-negligible thermal resistance. Interestingly, Kapitza resistances at interfaces between amorphous materials are lower than those at interfaces between the corresponding crystalline materials. We find that Kapitza resistances within the Stillinger-Webber based Si/Ge amorphous superlattices are not a function of interface density, counter to what has been observed in crystalline superlattices. Furthermore, the widely used thermal circuit model is able to correctly predict the interfacial resistance within the Stillinger-Weber based amorphous superlattices. However, we show that the applicability of this widely used thermal circuit model is invalid for Lennard-Jones based amorphous superlattices, suggesting that the assumptions made in the model do not hold for these systems.

13 The advances in phonon spectroscopy in homogeneous solids have unveiled extremely useful physics 14 regarding the contribution of phonon energies and mean free paths to the thermal transport in solids. How15 ever, as material systems decrease to length scales less than the phonon mean free paths, thermal transport 16 can become much more impacted by scattering and transmission across interfaces between two materials 17 than the intrinsic relaxation in the homogeneous solid. To elucidate the fundamental interactions driv18 ing this thermally-limiting interfacial phonon scattering process, we analytically derive and experimentally 19 measure a thermal boundary conductance accumulation function. We develop a semi-classical theory to 20 calculate the thermal boundary conductance accumulation function across interfaces using the diffuse mis21 match model (DMM), and validate this derivation by measuring the interface conductance between nine 22 different metals on native oxide/silicon substrates and five different metals on sapphire substrates. Mea23 surements were performed at room temperature using time-domain thermoreflectance and represent the 24 first-reported values for interface conductance across several metal/native oxide/silicon and metal/sapphire 25 interfaces. The various metal films provide a variable bandwidth of phonons incident on the metal/substrate 26 interface. This method of varying phonons cutoff frequency in the film while keeping the same substrate 27 allows us to mimic the accumulation of thermal boundary conductance and thus provides a direct method to 28 experimentally validate our theory. We show that the accumulation function can be written as the product 29 of an average interfacial phonon transmission function and the accumulation of the temperature derivative 30 of the phonon flux incident on the interface; this provides the framework to extract an average, spectrally 31 dependent phonon transmissivity from a series of thermal boundary conductance measurements. Based on 32 the assumptions made in this work and the measurement results on different metals on native oxide/silicon 33 and sapphire substrates, we demonstrate that thermal transport across solid interfaces may not necessarily 34 be dictated by phonon mismatch of materials and interfacial transmission, but is rather directly correlated 35 to the temperature derivative of phonon flux incident on the interface. 36

Energy processes and vibrations in biological macromolecules such as proteins ultimately dictate biological, chemical, and physical functions in living materials. These energetic vibrations in the ribbon-like motifs of proteins interact on self-similar structures and fractal-like objects over a range of length scales of the protein (a few angstroms to the size of the protein itself, a few nanometers). In fact, the fractal geometries of protein molecules create a complex network of vibrations; therefore, proteins represent an ideal material system to study the underlying mechanisms driving vibrational thermal transport in a dense, fractal network. However, experimental studies of thermal energy transport in proteins have been limited to dispersive protein suspensions, which limits the knowledge that can be extracted about how vibrational energy is transferred in a pure protein solid. We overcome this by synthesizing solid, water-insoluble protein films for thermal conductivity measurements via time-domain thermoreflectance. We measure the thermal conductivity of bovine serum albumin and myoglobin solid films over a range of temperatures from 77 to 296 K. These temperature trends indicate that anharmonic coupling of vibrations in the protein is contributing to thermal conductivity. This first-ever observation of anharmonic-like trends in the thermal conductivity of a fully dense protein forms the basis of validation of seminal theories of vibrational energy-transfer processes in fractal objects.

The thermal boundary conductance across solid-solid interfaces can be affected by the physical properties of the solid boundary. Atomic composition, disorder, and bonding between materials can result in large deviations in the phonon scattering mechanisms contributing to thermal boundary conductance. Theoretical and computational studies have suggested that the mixing of atoms around an interface can lead to an increase in thermal boundary conductance by creating a region with an average vibrational spectra of the two materials forming the interface. In this paper, we experimentally demonstrate that ion irradiation and subsequent modification of atoms at solid surfaces can increase the thermal boundary conductance across solid interfaces due to a change in the acoustic impedance of the surface. We measure the thermal boundary conductance between thin aluminum films and silicon substrates with native silicon dioxide layers that have been subjected to proton irradiation and post-irradiation surface cleaning procedures. The thermal boundary conductance across the Al/native oxide/Si interfacial region increases with an increase in proton dose. Supported with statistical simulations, we hypothesize that ion beam mixing of the native oxide and silicon substrate within similar to 2.2 nm of the silicon surface results in the observed increase in thermal boundary conductance. This ion mixing leads to the spatial gradation of the silicon native oxide into the silicon substrate, which alters the acoustic impedance and vibrational characteristics at the interface of the aluminum film and native oxide/silicon substrate. We confirm this assertion with picosecond acoustic analyses. Our results demonstrate that under specific conditions, a "more disordered and defected" interfacial region can have a lower resistance than a more "perfect" interface.

The thermal conductivities of 1μm copper-niobium multilayer films of different period thicknesses are measured by time–domain thermoreflectance at room temperature. The values for thermal conductivity are then used to calculate the thermal conductance between Cu/Nb interfaces using a series resistors model. Results show that Cu/Nb interface conductance increases with the decrease in period thickness reaching a value as high as 20 GWm−2K−1 for a 1×1 Cu/Nb multilayer. At shorter period thicknesses, ballistic electron transport dominates the thermal transport across this interface resulting in high interface conductance. The results are well described by a model that accounts for both ballistic and diffusive transport of electrons. This model assumes that an electron on one side of a metal-metal multilayer may not scatter at the interface but rather move ballistically on to the adjacent material and scatter in the adjacent material.

Thermal boundary conductance is of critical importance to gallium nitride (GaN)-based device performance. While the GaN-substrate interface has been well studied, insufficient attention has been paid to the metal contacts in the device. In this work, we measure the thermal boundary conductance across interfaces of Au, Al, and Au-Ti contact layers and GaN. We show that in these basic systems, metal-GaN interfaces can impose a thermal resistance similar to that of GaN-substrate interfaces. We also show that these thermal resistances decrease with increasing operating temperature and can be greatly affected by inclusion of a thin adhesion layers.

Extending the free-electron theory of electronic transport in simple amorphous metals, the Eliashberg function r2F(w) and the electron-phonon coupling constant j. are calculated for such materials. A formula relating i. to the temperature coefficient of the resistivity at higher temperatures ( T 2 8,/2) is derived. The validity of this formula is tested for amorphous Ga.

With the characteristic lengths of electronic and thermal devices approaching the mean free paths of the pertinent energy carriers, thermal transport across these devices must be characterized and understood, especially across interfaces. Thermal interface conductance can be strongly affected by the strength of the bond between the solids comprising the interface and the presence of an impurity mass between them. In this work, we investigate the effects of impurity masses and mechanical adhesion at molecular junctions on phonon transmission via non-equilibrium Green's functions (NEGF) formalisms. Using NEGF, we derived closed form solutions to the phonon transmission across an interface with an impurity mass and variable bonding. We find that the interface spring constant that yields the maximum transmission for all frequencies is the harmonic mean of the spring constants on either side of the interface, while for a mass impurity, the arithmetic average of the masses on either side of the interface yields the maximum transmission. However, the maximum transmission for each case is not equal. For the interface mass case, the maximum transmission is the transmission predicted by a frequency dependent form of the acoustic mismatch model, which we will refer to as the phonon mismatch model (PMM), which is valid for specular phonon scattering outside the continuum limit. However, in the interface spring case, the maximum transmission can be higher or lower than the transmission predicted by the PMM.

We report on the thermal conductivities of poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate), [6,6]-phenyl C-61-butyric acid methyl ester (PCBM), poly(3-hexylthiophene-2,5-diyl) (P3HT), and P3HT:PCBM blend thin films as measured by time domain thermoreflectance. Thermal conductivities vary from 0.031 +/- 0.005 to 0.227 +/- 0.014 W m(-1) K-1 near room temperature and exhibit minimal temperature dependence across the range from 319 to 396 K. Thermal conductivities of blend films follow a rule of mixtures, and no percolation threshold is found. Thermal annealing of blend films has a variable effect on thermal conductivity. Finally, the thermal conductivities of P3HT films do not vary with changes in film thickness from 77 to 200 nm. (C) 2013 AIP Publishing LLC.

Thermal boundary resistance dominates the overall resistance of nanosystems. This effect can be utilized to improve the figure of merit of thermoelectric materials. It is also a concern for thermal failures in microelectronic devices. The interfacial resistance depends sensitively on many interrelated structural details including material properties of the two layers, the system dimensions, the interfacial morphology, and the defect concentrations near the interface. The lack of an analytical understanding of these dependencies has been a major hurdle for a science-based design of optimum systems on a nanoscale. Here we have combined an analytical model with extensive, highly converged direct-method molecular dynamics simulations to derive analytical relationships between interfacial thermal boundary resistance and structural features. We discover that thermal boundary resistance linearly decreases with total interfacial area that can be modified by interfacial roughening. This finding is further elucidated using wave-packet analysis and local density of state calculations. DOI: 10.1103/PhysRevB.87.094303