Recent experiments provide evidence of intrinsic localized modes (ILMs) in the lattice dynamics of conventional 3D materials. Here evidence that ILMs in uranium metal enhance the thermal conductivity is presented along with speculation on how thermal transport by ILMs might be used to improve a reported design for a solid-state thermal rectifier.

The thermo-optical properties of gold nanoparticles suspended in liquid water droplets have been explored. A water droplet containing suspended gold nanoparticles was irradiated with a laser beam and the efficiency of light-to-heat conversion (η) was determined. Heat generated by the optically stimulated nanoparticles was found to dissipate through several mechanisms. Convection currents were observed visually inside the droplet by the addition of silica beads (˜3-10 micrometers). This was determined to be the dominant mechanism of stirring within the droplet.

We describe two techniques dedicated to observe and study the heating of structured materials like micro and nanowires and multilayered compounds. The techniques are thermally modulated fluorescence and thermoreflectance. Thermally modulated fluorescence allows mapping the heating of devices with a sub-wavelength lateral resolution. Thermoreflectance allows deeper physical investigations and can be directly used to determine the thermal conductivity and diffusivity of layered structures. In particular, we will show that by thermally modulating a surface by a point-like source, we are able to determine such quantities for several geometries, taking into account the nature of the substrate (conductive or not) as well as the interface quality between the layers. The experimental results, measured on aluminum thin films of variable thickness and on vanadium dioxide layers are corroborated by an analytical model that analyzes both the amplitude and the phase of the lateral heat diffusion in the structure

With time resolved ultrafast electron diffraction the cooling process across the interface between a thin film and the underlying substrate was studied after excitation with short laser pulses. From the exponential decay of the surface temperature evolution a thermal boundary conductance of 1430 W/(cm2K) is determined for a 9.7 nm thin Bi(111) film on Si(111). A linear dependence between laser fluence and initial temperature rise was measured for film-thicknesses between 2.5 nm and 34.5 nm. The ratio of initial temperature rise and laser fluence for different film-thicknesses is compared to a model taking multilayer optics into account. The data agree well with this model.

We provide a brief discussion of the Boltzmann equation derived Callaway-Debye relaxation time theory of lattice thermal conductivity of micro- and nano-structured materials (of size greater than 20 nm. Incorporated in the theory is a comprehensive treatment of three-phonon scattering events. Using numerical results from this theory, we present a quantitative investigation of the magnitude and temperature variation of the conductivity of CVD polycrystalline diamond films, suspended GaAs nanostructures, Si nanowires, and AlN micro- and nano-ceramics.

This paper describes a modeling approach to target aspects of heat conduction in metal hydride powders that are essential to metal hydrides as viable H2 storage media, including particle morphology distribution, size distribution, particle packing properties at specified solid fraction, and effective thermal conductivity. An isotropic fracture model is presented that replicates features of particle size and shape distributions observed experimentally. The discrete element method is used to simulate evolution of metal hydride particle contact networks during quasi-static consolidation of decrepitated metal hydride powders. Finally, the effective thermal conductivity of such a powder is modeled assuming that contact conductance is the same for each interparticle contact.

We characterize a temperature sensor made from Erbium ions embedded in an amorphous AlGaN matrix that is accessed remotely by measuring the relative intensities from photoluminescence peaks of Er3+. We use this sensor to measure the nanoscale temperature around an optically excited single gold nanoparticle that has been immobilized on the AlGaN substrate. The maximum temperature increase measured experimentally is 8.3 K. The temperature measurement is diffraction limited by our microscope to 490 nm. This temperature corresponds theoretically to a maximum local temperature increase of 22 K. A straight-forward analysis using energy balance gives the thermal conductivity of the amorphous AlGaN substrate as 4.5 W/m-K.

We report ultrafast measurements of thermal transport in plasma polymerized CHF3 films deposited on standard Si substrates with Al sputtered on top. We characterize the thin films by X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and spectroscopic ellipsometry and measure polymer thicknesses ranging from 33 nm down to 6 nm. Time-domain thermoreflectance (TDTR) provides quantitative data on the polymer thermal response to periodic heating from a pulsed laser source. A pump beam heats the Al layer, which acts as an opto-thermal transducer to the stack (Al-Polymer-Si) and a delayed probe beam measures the change in Al surface reflectance. We extract the polymer thermal conductivity by comparing TDTR data to a thermal diffusion model and find it to increase with decreasing polymer thicknesses below 30 nm.

We have measured the ensemble averaged transverse spin relaxation time T2* (associated with g = 4 resonance) in bulk powders of the organic molecule Alq3, and in samples containing 1-2 molecules confined in nanocavities of dimension ˜ 2 nm. Both T2* times are strongly temperature dependent indicating that they are determined by phonon-mediated spin relaxation. Interestingly, the T2* time in nanocavities is ˜2.5 times longer than in bulk powder over a wide temperature range. The longer T2* in the nanocavity is evidence of weakened electron-phonon interaction. We believe that electron-phonon interaction is suppressed because the cavity confines phonons and discretizes the phonon modes and phonon energies. As a result, the chances of a phonon induced (inelastic) spin relaxation event are reduced owing to the need to conserve energy in the relaxation process. This is a novel “phonon bottleneck effect” that to our knowledge has not been previously reported.

We present a theory of three-phonon interactions in nanophononic semiconductors at 300˜K. The intrinsic lifetime of phonon modes is estimated from the application of Fermi's Golden Rule, based on realistic phonon dispersion relations and a quasi-continuum model for the cubic anharmonicity. We show that the lifetime of phonon modes in the Si(0.543˜nm)/- Ge(0.543˜nm)[100] superlattice is shorter than the average of results for bulk Si and Ge. This is explained in terms of the availability of additional decay routes and an additional Dual Mass factor which arises due the different densities of Si and Ge.

Understanding thermal transport between carbon nanotubes (CNTs) and dielectric substrates is important both for nanoscale thermal management and CNT device applications. We investi-gate thermal transport between a (10,10) CNT and an SiO2 substrate through non-equilibrium classical molecular dynamics (MD) simulations. The thermal boundary conductance (TBC) is computed by setting up a temperature pulse in the CNT and monitoring its relaxation. The TBC is found to scale nearly linearly with temperature between 200�600 K, where a quantum correction is applied to the CNT heat capacity through its phonon density of states. However, the TBC ap-pears most sensitive to the strength the CNT-substrate interaction, which linearly modulates it between 0.05�0.30 WK-1m-1, in the range suggested by recent experimental data.

The Boltzmann transport equation (BTE) had been successfully used to predict phonon transport in semiconductors including silicon and germanium. However, in a composite system, the method requires external inputs to include accurate boundary conditions at internal interfaces. The atomistic Green's function (AGF) method is particularly useful for addressing interfacial heat transfer problems. In this paper, phonon transmission functions derived using the AGF method are incorporated in a non-gray BTE calculation of phonon transport in a relaxation time approximation. A Landauer-type heat flux is computed at the interface using the transmission function and the lattice temperatures on either side of the interface to compute distribution functions. The formulation is applied to a Si/Ge interface and the dependence of the effective thermal conductivity of the composite medium is investigated as a function of domain length.

Superlattices with an ultra-low thermal conductivity were extensively studied to design thermoelectric materials. However, since they are made up of superposed materials showing lattice mismatches, they often show cracks and dislocations. Therefore, it is challenging to fabricate superlattices with a thermoelectric figure of merit ZT higher than unity. Moreover, like nanowires, they decrease heat transport in only one main direction. Self-assembly from epitaxial layers on a Si substrate is a major bottom-up technology to fabricate 3D Ge quantum-dot (QD) arrays in Si, which have been used for 3D quantum-device applications. Using the model of the atomic-scale 3D phononic crystal, we showed that 3D high-density arrays of self-assembled Ge QDs in Si can also show an ultra-low thermal conductivity in 3D, which can be several orders of magnitude lower than that of bulk Si. As a result, they can be considered to design novel 3D thermoelectric devices showing CMOS compatibility. In an example QD crystal, the thermal conductivity can be decreased below only 0.2 W/m/K. The main objective of this paper is to show the size dependence of the thermal conductivity versus the supercell lattice parameter d. For a constant QD-crystal filling ratio x = 12.5 at%, a decrease of the thermal conductivity is observed for an increasing d. This analysis enables us to predict that the optimal d-value will be of the order of 11 nm for the given filling ratio. At this optimum, the thermal conductivity decreases to the global minimum value of 0.9 W/m/K. The presented results are a first step towards the optimal design of thermoelectric devices with a maximal ZT obtained by global optimization of the size parameters.

We present an improved methodology for a thermal transient method enabling simultaneous measurement of thermal conductivity and specific heat of nanoscale structures with one-dimensional heat flow. The temporal response of a sample to finite duration heat pulse inputs for both short (1 ns) and long (5μs) pulses is analyzed and exploited to deduce the thermal properties. Excellent agreement has been obtained between the recovered physical parameters and computational simulations.

We report on the measurement of the thermal conductivity of Si/Si0.8Ge0.2 multilayers on Si substrates through a variation of the 3? method. We exploit the frequency dependent variation of the thermal wave, through invoking the thermal penetration depth (TPD), which is inversely proportional to the frequency. Consequently, spectral measurements covering decades of frequency were used to finely probe the substrate and the overlying Si and Si0.8Ge0.2 thin film layers. Both in-phase and out-of phase measurements yielded comparable values of the thermal conductivity in the range of 3-5 W/mK, much lower than the reported bulk values. Our results provide proof of the potential of multilayered media to be used for reduced thermal conductance applications such as thermoelectrics, heat insulation etc.