Patrick K. Schelling University of Central Florida
Jennifer Lukes University of Pennsylvania
Alexander A. Balandin University of California-Riverside
Yulong Ding University of Leeds
National Renewable Energy Laboratory
T1: Biological and Nanofluids
Tuesday AM, April 14, 2009
Room 3002 (Moscone West)
9:30 AM - T1.1
Experimental Investigation of Thermal Properties of Nanofluids.
Dong Liu 1 , Leyuan Yu 1 Show Abstract
1 Mechanical Engineering, University of Houston, Houston, Texas, United States
Nanofluids are novel engineered colloids that have studied extensively in the past decade with the hope that they can be used as advanced heat transfer fluids in a variety of engineering applications. In spite of the tremendous research efforts in nanofluids, it is still under debate if and how the presence of dispersed nanoparticles will alter the thermal transport in the base fluid and lead to enhanced thermal properties. The discrepancies arose largely due to the lack of careful and thorough experimental investigations of thermal transport in nanofluids, which are outnumbered by various exotic analytical/numerical models that have been proposed to explain the observed “anomalies”. In this work, thermal properties of CuO-water and γ-Al2O3-water nanofluids (both thermal conductivity and thermal capacity) are carefully measured to provide a solid experimental database of thermal transport in nanofluids. Several crucial issues in formulating the nanofluids and controlling the experimental conditions during the thermal measurement are critically assessed, which may contribute to the discrepancies in the literature if not handled properly. In particular, several methods that are commonly adopted to stabilize the nanofluids, including sonication, use of surfactant/dispersant, electrostatic repulsion and functionalization of nanoparticles, are examined regarding their impact on the nanoparticle size distribution. The effective nanoparticle//aggregate size is characterized from scanning electron microscope (SEM) and dynamic light scattering (DLS) measurements. Transient hot-wire and 3-omega methods are used to measure the thermal conductivity and thermal capacity of the nanofluids with various particle concentrations over a wide temperature range. Special cares are taken in the experimental design to eliminate the undesired heat transfer modes, particularly, natural convection and conduction due to dissolved gas, at escalated temperatures. The measured thermal properties are compared to predictions from the effective medium models. In the end, recommendations are made to the procedures of preparation and measurement of nanofluids to ensure accurate appraisal of the thermal properties.
9:45 AM - T1.2
Thermal Conductivity Predictions of Water/Carbon Nanotube Composite Materials
John Thomas 1 , Alan McGaughey 1 Show Abstract
1 Mechanical Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania, United States
The effective thermal conductivity of water/single-walled carbon nanotube (CNT) composite systems is predicted using molecular dynamics simulation. Carbon-carbon interactions are modeled using the second-generation REBO potential and water-water interactions are modeled using the TIP4P potential. Interactions between water molecules and carbon atoms are modeled using a new DFT-based potential function we are currently developing. Simulation systems consisting of water inside, water outside, and water both inside and outside CNTs with diameters ranging from 0.83 nm to 1.66 nm are considered. We also consider composite water/graphene systems, which are representative of large-diameter CNTs with negligible surface curvature. We first examine the mechanisms of energy transfer between the phonon modes in the CNT and the vibrational modes in water. Using a direct application of the Fourier Law, we predict the effective thermal conductivity of the composite material and examine how it is affected by the thermophysical properties (e.g. density and pressure) of the water. We conclude by discussing how the length and diameter of the CNT, along with the properties of the surrounding water, can be tuned to construct systems with tailored thermal transport properties.Carbon nanotubes and graphene sheets are promising candidates as next-generation materials for transistors, super capacitors, and desalinization devices. Previous experimental and theoretical investigations have examined the phonon properties and thermal conductivity of isolated CNTs. Others have explored the variation in CNT conductivity due to surface chemisorptions and topological defects. However, little work has been done towards understanding the interaction between a CNT and a confined/surrounding fluid. Knowing how such interactions change CNT transport characteristics is central to predicting the performance of actual CNT-based thermal transport systems.
10:00 AM - T1.3
Thermal Transport Properties of Nanostructures Immobilized on Glass Substrates.
Hugh Richardson 1 , Alexander Govorov 2 , Alyssa Thomas 1 , Michael Carlson 1 Show Abstract
1 Chemistry and Biochemistry, Ohio University, Athens, Ohio, United States, 2 Physics and Astronomy, Ohio University, Athens, Ohio, United States
Understanding heat transfer at the nanoscale is essential to predict and control the thermal energy balance in nanodevices and other nanostructures. As device dimensions continue to be reduced and number densities increase, heat dissipation becomes an increasingly serious problem. We have recently shown that a single isolated metal nanoparticle can generate sufficient heat upon irradiation to induce readily observable phase changes in ice and water. This phenomenon can serve as the basis for a sensitive nanocalorimetry experiment in which the temperature profile around a nanoparticle heat source can be measured as a function of optical energy input. We use confocal Raman and photoluminescence microscopy to measure the water around single gold nanoparticle complexes immobilized on a glass surfaces and embedded in a matrix with known thermodynamic properties. For example, we can use the phase transformation of ice to determine the temperature increase around a gold NP complex but determining the volume of water generated when the gold NP complex is excited with light. The measurement allows us to measure, not only the optical response of the NPs, but also the thermal response as well. The single gold NP complexes are first characterized with atomic force microscopy and then located with photoluminescence microscopy. Once the single NP complexes are located then the temperature profile around the NP complex during optical excitation is determined by measuring the relative amount of water and ice within the excitation volume. The thermal properties of the optically-excited gold clusters are then established by combining theoretical calculations with experimental results. This approach yields a quantitative measure of the amount of heat generation. Our results show for gold NP complexes in ice that at relatively low laser power (less than 10^5 W/cm2) liquid water encases the photo-excited gold particle and the temperature profile agrees with recent theoretical calculations. But at larger laser powers, a vapor cocoon surrounds the excited particle that shuts down thermal transport from the particle to the surrounding ice matrix, causing superheating of the particle. This behavior is also observed for single gold NP complexes in liquid water where an insulating vapor cocoon is formed for laser powers in excess of 10^6 W/cm2. In this case the vapor cocoon expands with laser power displacing water within the excitation volume. The displacement is measured with a decrease in the Raman signal from water. The contribution of convection to thermal transport has been measured by placing glass beads in the water and observing movement of the beads during excitation of the NP complex.
10:15 AM - T1.4
Gold Nanorod Mediated Delivery by Ultrafast Laser Excitation.
Kimberly Hamad-Schifferli 1 2 Show Abstract
1 Biological Engineering, MIT, Cambridge, Massachusetts, United States, 2 Mechanical Engineering, MIT, Cambridge, Massachusetts, United States
The unique material and shape dependent optical properties of gold nanorods have made them attractive for biological applications such as imaging and photothermal therapy.One of the great challenges of drug delivery is the ability to temporally and spatially control drug release. Many have designed carriers that can release single species in a controlled manner at specific rates. However, with complex diseases such as cancer, treatments are beginning to rely on therapies that involve multiple drugs. Combination therapy has proven to be effective, but controlled release of multiple species is still challenging. Because the drugs in a mixture often differ greatly in their chemical properties (molecular weight, size, solubility) and pharmacokinetics, it is a challenge to load, deliver, and release them in a way that maintains desired release rates, concentrations, and order. Therefore, a way to orthogonally control release of each species is highly desirable. We show here a proof of concept demonstration using gold nanorods (NRs) to control the release of multiple species independently. Ultrafast laser irradiation in resonance with their longitudinal surface plasmon resonance (LSP) can heat gold nanorods to high local temperature, melting them. This triggered melting can be exploited for controlled release of biomolecules conjugated to the NRs. LSP is tunable by changing NR aspect ratio (AR), so NRs with different ARs can be excited independently at different wavelengths. If different NRs are conjugated to different molecules, this strategy could be utilized for orthogonal triggered release of multiple species. We exploit this property of nanorods for selective and mutually exclusive release of two distinct DNA oligonucleotides, and show that the released DNA is still functional. Control over the surface chemistry of the NRs is key as the ligand can affect the thermal dissipation of the NRs upon laser excitation. In addition, NR ligands can prevent biofunctionalization and limit stability in physiological buffers. Release is efficient, where 50-80% of the loaded DNA is released selectively. Laser fluence governs the degree of NR melting, therefore the specificity and degree of release are externally tunable. Since conjugation requires only a simple thiol conjugation, it is potentially applicable to a wide range of molecules. This method is expandable, as tuning NR synthesis parameters could extend this approach beyond two species. The chemistry of NR surfaces is customizable, so they can accommodate different physiological environments. Others have also demonstrated active targeting by decorating the NRs with moieties such as antibodies and cell receptor ligands. These characteristics show that triggered release from NRs is a potentially powerful technique for improving drug delivery strategies.
10:30 AM - T1.5
Enhanced Thermal Conductivity by Aggregation in Nanofluids Containing Metal Oxide Nanoparticles and Carbon Nanotubes.
Haiping Hong 1 , Pauline Smith 2 Show Abstract
1 Material and Metallurgical Engineering, South Dakota School of Mines and Technology, Rapid City, South Dakota, United States, 2 , Army Research Lab, Arbedeen Proving ground, Maryland, United States
An significant increase in the thermal conductivity (TC) of heat transfer nanofluids containing metal oxide nanoparticles and carbon nanotubes has been determined with very low percentage loading (around 0.02 wt%) of these two nano materials. These fluids are very stable and the viscosity remains approximately the same as water. A possible explanation for these interesting results is the aggregation of metal oxide particles on the surface of nanotubes by electrostatic attraction and form the aggregation chain along the nanotube. Time dependant magnetic results demonstrate that, under the influence of a strong outside magnetic field, the TC value decreases. Also, the TC value decreases when the pH is shifted from 7 to 11.45.
10:45 AM - T1.6
Relationship Between Wetting and Adhesion and Thermal Conductance of a Range of Hydrophobic to Hydrophilic Aqueous Interfaces.
Natalia Shenogina 1 , Rahul Godawat 2 , Pawel Keblinski 1 , Shekhar Garde 2 Show Abstract
1 Department of Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States, 2 Department of Chemical and Biological Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States
Using extensive molecular dynamics simulations of self-assembled surfactant monolayer-water interfaces we quantify the strength of interfacial thermal coupling at water-solid interfaces over abroad range of surface chemistries from hydrophobic to hydrophilic (-CF3, -CH3, -OCH3, -CONHCH3, -CN, -CONH2 and -OH). We show that the Kapitza thermal conductance is proportional to work of adhesion, i.e., it is directly related to wetting properties of the surface. Excellent agreement with experimental data on similar systems (Ge et al., PRL, 2006 ) highlights convergence of simulations and experiments on these complex nanoscopic systems and reinforces the notion that there is no apparent vapor layer at realistic hydrophobic interfaces that have weak attractive interactions with water. More generally, our results highlight the potential of thermal transport measurements as probes of local molecular environment and bonding at an interface. Z. Ge, D. Cahill, P. Braun, PRL 96, 186101 (2006).
11:30 AM - **T1.7
The Consequence of Nanofluids on Heat Transfer.
Thomas McKrell 1 , Jacopo Buongiorno 1 , Lin-wen Hu 2 Show Abstract
1 Nuclear Science and Engineering, MIT, Cambridge, Massachusetts, United States, 2 Nuclear Reactor Laboratory, MIT, Cambridge, Massachusetts, United States
Nanofluids, stable colloidal suspensions of nanoparticles, have shown great promise in improving the heat transfer characteristics of engineering systems. Heat transfer in engineering systems is complex and includes phenomena such as conduction (thermal conductivity), convection (laminar and turbulent), nucleate boiling heat transfer, and critical heat flux. A number of custom experimental apparatuses including transient hot wire, pool boiling, flow loops, high-speed infrared (IR) and digital imaging have been fabricated, and the associated results for nanofluids will be discussed. Additionally, single phase pressure drop measurements for nanofluids will be presented, because the increased viscosity of nanofluids offers a significant hurdle to the implementation of nanofluids in engineering systems. It will be shown that the effect of nanofluids on pressure drop and conductive and convective heat transfer can be explained by existing theories if a nanofluid’s thermophysical properties are used; these properties are measured for every nanofluid tested. However, the phenomena of nucleate boiling heat transfer and critical heat flux are more challenging to interpret using existing theory because of surface effects. Accordingly, the role of measured contact angle, IR imaging, and morphological data will be discussed.
12:00 PM - T1.8
Phase Behavior of Strongly Heated Nanoparticles in Liquid Solution Studied by Molecular Dynamics Simulations.
Sergei Shenogin 1 , Pawel Keblinski 1 2 , Samy Merabia 3 , Laurent Joly 3 , Laurent Lewis 4 , Jean-Louis Barrat 3 Show Abstract
1 Nanotechnology Center, Rensselaer Polytechnic Institute, Troy, New York, United States, 2 Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States, 3 Laboratoire de Physique de la Matiere Condensee et des Nanostructures, Universite de Lyon, Villeurbanne France, 4 Departement de Physique, Universite de Montreal, Montreal, Quebec, Canada
The melting of metal nanoparticles immersed in organic liquid was studied by means of nonequilibrium molecular dynamics (NEMD). The goal was to mimic heating conditions of nanoparticle suspensions subjected to the intensive laser radiation and high temperature gradients. We show that nanoparticles may be heated to temperatures well above the boiling, and even critical point of the liquid without development of the vapor phase. This suppression of vapor formation is attributed to a large Laplace pressure associated with high nanoparticle curvature. Quite surprisingly, metal nanoparticles can be melted with the adjacent fluid being relatively cold. With further increase of the heating power we observe disintegration of the nanoparticles into dispersion of metal atoms. This suggests several possible applications for the effect including cancer therapy, controlled drug release into the living cells and effective design of microelectronic devices with high thermal loads.
12:15 PM - T1.9
Transient Changes of Temperature and Thermal Conductance when H2O Impinges on a Hot Si Surface.
Ji Yong Park 1 , Chang-Ki Min 1 , David Cahill 1 , Steve Granick 1 Show Abstract
1 Department of Materials Science and Engineering and Materials Research Laboratory, University of Illinois, Urbana, Illinois, United States
Heat transfer between a hot surface and a liquid is typically limited by the phenomenon of the critical heat flux. At the critical heat flux, a layer of vapor spreads across the solid surface, thermally isolating the solid from the boiling liquid. We are characterizing the transitions in the heat transfer rates near the critical heat flux using a novel combination of ultrafast pump-probe optical metrologies. The laser source for these experiments is a 100 mW Er:fiber laser oscillator with an output centered at a wavelength of 1.56 μm. A pulsed water jet (velocity 1 m/s; volume 0.6 mm3) impinges on the Ti-coated surface of a Si wafer. We use the IR pump-probe apparatus to measure the time evolution of a) the thermal conductance of the thermal effusivity of interface; and b) the temperature of the Si wafer within 0.1 mm of the interface. Fast and spatially-resolved thermometry is enabled by the strong temperature dependence of two-photon absorption in Si at photon energies below ½ of the direct band gap. The rate of heat transfer makes a gradual transition at temperatures between 120 and 160 °C between the high values characteristic of film boiling to the low values characteristic of the formation of an insulating vapor layer at the interface.
Patrick K. Schelling University of Central Florida
Jennifer Lukes University of Pennsylvania
Alexander A. Balandin University of California-Riverside
Yulong Ding University of Leeds
T3: Interfaces and Superlattices
Wednesday AM, April 15, 2009
Room 3002 (Moscone West)
9:30 AM - **T3.1
Thermal Crosstalk in Field-Effect Transistors
Manu Shamsa 1 2 , Paul Solomon 3 , Keith Jenkins 3 , Alexander Balandin 1 , Wilfried Haensch 3 Show Abstract
1 Department of Electrical Engineering, University of California - Riverside, Riverside, California, United States, 2 Current address:, Intel Corporation, Hillsboro, Oregon, United States, 3 , IBM Thomas J. Watson Research Center, Yorktown Heights, New York, United States
With the continuously decreasing feature size of CMOS and increasing power densities especially in the local on-chip environment, the measurements related to local heating of single devices and the transfer of heat between devices are becoming increasingly important. Furthermore, it is important to be able to characterize the full thermal transient, since the chip thermal activity fluctuations tend to be on the same time scales as typical thermal time constants resulting in peak transient temperatures which may well exceed steady-state average temperatures. I will discuss here our recent results of the resolving thermal transients for CMOS devices. In this work, a high-speed electrical pulse-and-probe sampling technique was used to detect thermally modulated sub-threshold currents. With the measurement technique a temperature resolution of ~50 mK, time resolution of 5 ns and temperature sensitivity of ~0.6 μA/K ,  was achieved. The finite element method was used to solve the heat diffusion equation and to obtain the temperature profiles for the given device structures. The high-resolution experimental technique combined with the simulations allowed us to study the thermal crosstalk between two adjacent devices and probe the local temperature at different locations of the structure. The effects of the interface quality, layer thickness, material selection and the inter-device spacing on the heat diffusion and device performance were investigated.
This work was done at IBM TJ Watson Research Center and UCR. The work at UCR was supported through the SRC/FCRP center on Functional Engineered Nano Architectonics (FENA). M. Shamsa was supported in part by IBM PhD fellowship. He is now with Intel Corporation.
 M. Shamsa, P.M. Solomon, K.A. Jenkins, A.A. Balandin and W. Haensch, "Investigation of thermal crosstalk between SOI FETs by the sub-threshold sensing technique," IEEE Transactions on Electron Devices, vol. 55, pp. 1733 (2008). P.M. Solomon, M. Shamsa, K.A. Jenkins, C.P. D'Emic, A.A. Balandin and W. Haensch, “Measurements of inter-and-intra device transient thermal Transport on SOI FETs,” International Electron Device Meeting Technical Digest, pp. 479 (2007).
10:00 AM - T3.2
Thermal Boundary Resistance of Closely-spaced Si/Ge Interfaces from Lattice Dynamics Calculations.
Eric Landry 1 , Alan McGaughey 1 Show Abstract
1 , Carnegie Mellon University, Pittsburgh, Pennsylvania, United States
The thermal boundary resistance of closely-spaced Si/Ge interfaces is predicted using harmonic lattice dynamics calculations and the scattering boundary method. The atomic interactions are modeled using the Stillinger-Weber potential. The computational domain contains a thin germanium layer sandwiched between two semi-infinite extents of silicon, forming two closely-spaced interfaces. We also consider the opposite situation, where a silicon layer is placed between two large extents of germanium. We examine the effects of interface strain, species mixing, and separation distance on the phonon transmission coefficients and the thermal boundary resistance. Due to the harmonic approximation, these calculations are only valid when the phonon scattering is elastic. To examine the assumption of elastic scattering, we compare the lattice dynamics predictions to those obtained using molecular dynamics simulations and the direct method, which require no assumptions about the nature of the phonon transport. This comparison is made at a temperature of 500 K, a temperature where quantum effects are negligible (MD simulation is a classical approach). We conclude by discussing how the atomic force constants needed in the lattice dynamics calculations can be calculated from density functional theory. This novel approach will allow for the prediction of the thermal boundary resistance for interfaces between semiconductors for which a suitable interatomic potential does not exist.The ability to accurately predict the thermal boundary resistance of closely-spaced semiconductor interfaces is important in modeling thermal transport in electronic devices with high component density and in the design of nanocomposite materials such as superlattices. While the acoustic mismatch and diffuse mismatch models have been successful in predicting the thermal boundary resistance of interfaces at low temperatures (less than ~30 K), they can be an order of magnitude in error at room temperature and above. Furthermore, these methods are limited to the case of isolated interfaces and neglect the atomic-level detail of the interface.
10:15 AM - T3.3
Molecular Dynamics Simulation of the In-plane and Cross-plane Thermal Conductivity of Superlattices with Rough Interfaces.
Konstantinos Termentzidis 1 , Patrice Chantrenne 1 , Pawel Keblinski 2 Show Abstract
1 CETHIL, INSA-LYON, France, Villeurbanne cedex France, 2 Material Science and Engineering, RPI, Troy, New York, United States
The heat transport in superlattices (SL) is greatly influenced by the thermal resistance induced by the interfaces. If the SL period is similar or smaller than to the phonon mean free path (PMFP), collective effects involving multiple interfaces are important. In this context, simulations of smooth interface superlattices showed a minimum of the cross-plane thermal conductivity attributed to the miniband formation, while such minimum disappears for rough interfaces.In our work we use non-equilibrium molecular dynamic (MD) simulation to systematically investigate the role of surface roughness on SL thermal conductivity. A simple SL model is used where interactions between atoms forming the two materials are described by the Lenard Jones potential and differ only by their mass, with a mass ratio equal to two. The rough interface geometry consists of periodic triangles. The influence of the characteristic size of the roughness on the in-plane and cross-plane TC is investigated. The roles of two other length scales: the SL period and the PMFP are also considered. The data are analyzed in term of the scattering mechanism responsible for the observed behaviour. Interestingly, we observe the minimum in-plane thermal conductivity where the characteristic dimension of the surface roughness is of the order of mean free path. This minimum has not been observed for the cross-plane thermal conductivity of SLs with smooth interfaces.
10:30 AM - T3.4
Characterization of Thermal Transport Properties in GaInN based LEDs and Semiconductor Superlattices.
Monalisa Mazumder 1 , Theodorian Borca-Tasciuc 2 Show Abstract
1 Department of Chemical & Biological Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States, 2 Department of Mechanical, Aerospace & Nuclear Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States
Thermal dissipation is often key to successful operation of a wide spectrum of devices ranging from optoelectronics to semiconductor superlattices. Operational Device temperature and associated Joule heating have a direct effect on the performance efficiency and lifetime of GaN based light emitting diodes (LEDs). On the other hand, the unique thermal characteristics of superlattices with significantly reduced thermal conductivity values make them potential candidate for possible phonon engineered solid-state energy conversion devices. Thus investigation of thermal dissipation in these systems is relevant to engineer novel low-dimensional thermo-electric energy conversion devices, microelectronic and optoelectronic devices. Here we report the temperature-dependant anisotropic thermal conductivity of GaInN/ GaN multiple quantum wells based green LEDs and a system of Silicon/ Silicon Carbide superlattices grown on silicon substrates. The thermal conductivity of both the systems are measured by a differential 3ω technique in the temperature range 80-300 K. Fitting the measured temperature rise to a two- dimensional thermal model, we determined the cross-plane and in-plane thermal conductivity of the aforesaid systems. The temperature effect and comparison with bulk thermal conductivity of the structures are subsequently discussed in the light of interface phonon scattering.
10:45 AM - T3.5
Investigation of Phonon Transport in Superlattices by Anharmonic Lattice Dynamics Calculations.
Joseph Turney 1 , Alan McGaughey 1 , Cristina Amon 2 1 Show Abstract
1 Mechanical Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania, United States, 2 Mechanical & Industrial Engineering, University of Toronto, Toronto, Ontario, Canada
The frequencies, heat capacities, group velocities, and relaxation times of phonons in superlattices are predicted using quasi-harmonic and anharmonic lattice dynamics calculations. These phonon properties are used to predict the cross-plane and in-plane thermal conductivity. As a test system, we examine Stillinger-Weber Si/Ge structures. We relate the superlattice period thickness to the dominant regime of phonon transport: coherent or incoherent. Superlattices in which the phonon transport is coherent are handled directly with the lattice dynamics methods. Incoherent phonon transport is handled by using bulk phonon properties in the individual layers and computing, via the scattering boundary method, the mode dependent transmission coefficients for phonon transport across the interfaces. For each superlattice geometry, we also identify the phonon modes that carry the majority of the energy in the cross-plane direction and suggest methods to reduce the mean free paths of these modes.The design of nano-structured materials with specific thermal properties requires efficient numerical methods that provide the details of energy transport on the carrier level. Many numerical methods used to study heat transport, such as the Green-Kubo and direct molecular dynamics methods, are system level approaches and do not provide details of the phonon dynamics. These methods provide little insight into the mechanisms of heat transport in nano-structures; however, such detailed information is directly available within the lattice dynamics framework.
11:30 AM - **T3.6
Raman Thermography: Interface Challenges in GaN-based Electronic Devices.
Martin Kuball 1 Show Abstract
1 , University of Bristol, Bristol United Kingdom
GaN-based electronic devices with their high power densities require efficient thermal management for high device reliability. Typically, GaN for power devices is grown on high thermal conductivity SiC substrates as large scale GaN substrates are not readily available. A nucleation layer at the GaN/SiC interface is required for this heteroepitaxy to accommodate the lattice mismatch between the GaN and the SiC, but this nucleation layer can provide a thermal resistance hindering heat transfer from a GaN device into the heat-sinking high thermal conductivity SiC substrate. We demonstrate that the commonly used AlN nucleation layers, although only tens of nanometer thin, can account for up to 30-50% of the channel temperature in today’s AlGaN/GaN HFET / HEMT devices and therefore result in an unnecessarily high device thermal resistance, and discuss and demonstrate possible solutions. The technique of Raman Thermography used for this study, a technique developed in Bristol for sub-micron spatial and nanosecond time resolution thermal analysis of semiconductor devices and their interfaces, in two and three dimensions, will also be reviewed.
12:00 PM - T3.7
Thermal Conductivity Measurements in Superlattices: Influence of the Interface Sharpness.
Jean-Yves Duquesne 1 , Abdelhak Saci 1 Show Abstract
1 INSP, CNRS / Paris 6 University, Paris France
We have measured the thermal conductivity of short period GaAs/AlAs superlattices exhibiting different interface sharpnesses. The purpose was to evaluate experimentally the relative importance of intrinsic and extrinsic scattering on phonon heat transport. Intrinsic mechanisms are determined by zone-folding effects (group velocity lowering, new gaps opening, mini-Umklapp scattering...), whereas extrinsic mechanisms are due to phonon scattering by interface defects. Theoretical models and molecular dynamic calculations stress that intrinsic mechanisms alone cannot account for the experimental reduction of the thermal conductivity, with respect to the bulk constituents. Therefore, they inferred that both mechanisms must be considered to explain the experimental data, in the short period regime. However, to our knowledge, no direct experiment has been performed in order to probe directly the role of the interfaces.We used the so-called 3ω method to measure the cross-plane thermal conductivity. Our results are presented and discussed.
12:15 PM - T3.8
3ω measurement of thermal conductivities of Thermal Interface Materials in the range 10-300 K
Pierre-Olivier Chapuis 1 , Michael Schmidt 1 , John Cuffe 1 , Mika Prunnila 2 , Jouni Ahopelto 2 , Clivia Sotomayor Torres 1 Show Abstract
1 Phononic and Photonic Nanostructures (P2N) Group, Institut Catala de Nanotecnologia (ICN-CIN2), Bellaterra (Barcelona) Spain, 2 , VTT Micro and Nanoelectronics, VTT, Espoo Finland
Thermal phenomena at nanometer-scale have been explored since more than a decade, as different behaviors are encountered when characteristic dimensions are approaching the energy carrier’s mean free paths or their wavelengths. We have implemented a new 3ω  method setup. Since 20 years, the method has been used for determining the thermal conductivity of bulk materials, thin films or nanocomposites. Recent works [2-4] have refined the first models, showing the sensitivity to anisotropy and improving the accuracy. We report measurements of the thermal conductivity of different kind of samples to be used for microelectronics, including silicon and Thermal Interface Materials such as pastes and composites. We discuss the influence of the geometry and analyze the sensitivity to the thermal conductivity of the samples. We also measured the thermal conductivities as a function of temperature (10-300 K), in order to analyze the evolution of the heat carrier’s mean free path. Finally, we discuss possible improvements of this kind of setups for nonplanar surfaces. D. Cahill, Review of Scientific Instruments 61, 802 (1990) C. Dames and G. Chen, Review of Scientific Instruments 76, 124902 (2005) T. Tong and A. Majumdar, Review of Scientific Instruments 77, 104902 (2006)  S.P. Gurrum, W.P. King, and Y.K. Joshi, Journal of Applied Physics 103, 113517 (2007)
12:30 PM - T3.9
Anharmonic Lifetime of Phonons in Nanophononic Semiconductors.
Steven Hepplestone 1 , Gyaneshwar Srivastava 1 Show Abstract
1 School of Physics, University of Exeter, Exeter, Devon, United Kingdom
Phononic structures are the vibrational analogues of photonic crystals. These structures consist of two or more materials with contrasting physical properties, which result in one or more stop bands in the phonon dispersion relations. With recent fabrication techniques it is possible to grow one-dimensional nanophononic semiconductors . Whilst a considerable amount of work has been performed calculating and measuring these dispersion relations and stop bands [1,2], relatively little progress has been made in calculating the consequential physical properties. In order to calculate physical properties, such as phonon mean free path and thermal conductivity, it is required to calculate the anharmonic lifetime of phonon modes in these structures. We will present the theory of three-phonon interactions in nanophononic semiconductors. 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 .
Numerical results will be presented for nanophononic Si/Ge superlattices. The phonon dispersion relations will be obtained from the application of an enhanced adiabatic bond charge model (EBCM). We show that the lifetime of selected optical phonon modes in ultrathin Si/Ge superlattices is less than the averaged lifetime of phonon modes in Si bulk and Ge bulk, due to availability additional decay routes, the onset of 'mini-Umklapp' processes  and an additional factor which arises due the different densities of Si and Ge. The lifetime of the lowest zone-edge acoustic mode is approximately equal to the average of the results for the two bulk materials, but is greater than that for a fictitious material with the averaged density.
These results show that for phononic structures, the average lifetime of the phonon modes is much less than when compared to a virtual average crystal. Accurate numerical estimates of anharmonic phonon lifetimes in such structures will be directly helpful in explaining the reduced thermal conductivity of such structures.
 Y. Ezzahari et al. Phys. Rev. B 75, 195309 (2007); N. D. Lanzillotti-Kimura et al. Appl. Phys. Lett. 88, 083113 (2006).
 J. O. Vassuer et al. Phys. Rev. B 77, 085415 (2008). J. Baumgartl et al. Phys. Rev. Lett. 99, 205503 (2007).
 S. P. Hepplestone and G. P. Srivastava, Phys. Rev. Lett. 101, 1105502 (2008).
G. P. Srivastava, The Physics of Phonons (Adam Hilger, Bristol; S. P. Hepplestone and G. P. Srivastava, Phys. Rev. B 74, 165420 (2006).
 S. Y. Ren and J. D. Dow, Phys. Rev. B 25, 3750 (1982).
12:45 PM - T3.10
Self-assembled Conductive Networks.
Lin Hu 1 , Pawel Keblinski 1 Show Abstract
1 Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States
Thermal interface materials (TIMs) play a critical role in heat management in energy generation and utilization devices where heat generated needs to be removed to the environment via metal heat sinks. The key challenge is that the TIM has to contact rough surfaces of the two mating materials which often limits the thermal conductivity to effectively ~1 W/m-K. We explore theoretically a novel concept of self-assembled conductive networks that can dramatically increase thermal conductivity of conformal materials, which can be applied to TIMs and other applications. Using molecular dynamics simulation we model assembly of high-volume fraction (10-40%) slurries of solid nanoparticles into conductive networks. In particular we investigate how the fluid-solid interactions and particle volume fraction affect the network formation and viscosity of the slurry. The networks obtained in such simulations are input model structures to Monte-Carlo simulations determining the overall conductivity the solid network-soft phase composite.
T5: Poster Session: Nanoscale Thermal Transport
Wednesday PM, April 15, 2009
Salon Level (Marriott)
9:00 PM - T5.1
Modeling of Heat Conduction in Graphene Flakes of Arbitrary Geometry.
Samia Subrina 1 , Dmitri Kotchetkov 1 , Alexander Balandin 1 Show Abstract
1 Department of Electrical Engineering, University of California - Riverside, Riverside, California, United States
Graphene has already manifested unique electronic and optical properties. It was also recently reported that graphene is an excellent conductor of heat with the room temperature thermal conductivity values on the order of or exceeding those of carbon nanotubes [1-2]. The measurements of thermal conductivity of graphene utilized a non-contact optical technique where the excitation laser initiated a heat wave propagating through graphene flake toward the heat sinks. The data extraction procedure assumed a predominantly diffusive heat transport regime and a plane heat wave. Realistic graphene flakes have variation in their width, and the heat wave front may deviate from the plane wave depending on the geometry of the flake and ratio of the laser-spot diameter and the flake width. In this work we carry out detail numerical study of heat propagation in graphene flakes. The structure parameters, heat sources and boundary conditions were selected to closely correspond to those reported in the experiments [1-3].The thermal conduction in graphene flakes was simulated using the finite element method with the COMSOL software. The generated fine meshes allowed us to study heat conduction with high accuracy and obtain accurate temperature profiles. We focused on the effects of the shape of the flake on the obtained value of thermal conductivity for the fixed amount of the dissipated power in the middle of the suspended graphene. It was found that both the shape of the flake and the selected temperature distribution in the hot spot (liner source vs point source; uniform vs Gaussian) affect the extracted values of thermal conductivity. At the same time, for the flakes with the relatively constant width and the hot spot of the size comparable to the flake width, the thermal conductivity extracted within the simple plane-wave approximation give close values to our simulation results. The developed simulation procedure can be further used for investigation of thermal transport in graphene multi-layers and graphene – heat sink structures. The latter is required in order to study the feasibility of application of graphene multi-layers for the lateral hot-spot removal and other device-level thermal management applications . The work in Balandin group was supported, in part, by DARPA – SRC Focus Center Research Program (FCRP) through its Functional Engineered Nano Architectonics (FENA) center and Interconnect Focus Center (IFC).  A.A. Balandin, et al., "Superior thermal conductivity of single-layer graphene," Nano Letters, 8, 902 (2008) S. Ghosh, I. Calizo, D. Teweldebrhan, E.P. Pokatilov, D.L. Nika, A.A. Balandin, et al., "Extremely high thermal conductivity of graphene: Prospects for thermal management applications in nanoelectronic circuits," Appl. Phys. Lett., 92, 151911 (2008). A.A. Balandin, Thermal conduction in graphene and graphene multilayers, Invited Talk, International Symposium on Graphene Devices, Aizu-Wakamatsu, Japan, 2008.
9:00 PM - T5.2
Thermal Conduction through Diamond – Silicon – Diamond Heterostructures: Effects of the Nano- and Microcrystalline Size and Boundary Resistance.
Vivek Goyal 1 , Dmitri Kotchetkov 1 , Desalegne Teweldebrhan 1 , Samia Subrina 1 , Muhammad Rahman 1 , Qinghui Shao 1 , Suchismita Ghosh 1 , Alexander Balandin 1 Show Abstract
1 Department of Electrical Engineering, University of California - Riverside, Riverside, California, United States
It was recently suggested that the low-field electron drift mobility can be enhanced in silicon nanowires or ultra-thin films embedded within acoustically hard barriers [1-2]. The effect is due to partial suppression of acoustic phonon modes in the structure with the hard boundaries and corresponding reduction of the deformation potential electron – phonon scattering. The mobility enhancement is particularly strong at low temperatures but still substantial at room temperature. Unlike conventionally used strain-engineered mobility enhancement, which uses alloy under-layers with low thermal conductivity, the proposed method utilizes materials with the high thermal conductivity such as diamond or polycrystalline diamond. The latter improves heat removal rather than deteriorates it. Crystalline diamond with the longitudinal – mode acoustic impedance of ~62 x 10E5 g/cm2s is much harder acoustically than silicon with the impedance of ~20 x10E5 g/cm2s. At the same time, crystalline synthetic diamond is better for integration with silicon from the technological point of view despite its degraded thermal conductivity. One has to find a material, which would satisfy a number of requirements for application as the acoustically hard barrier. These requirements include compatibility with the silicon thin-film processing, good thermal conductivity and high-quality interface. Here we describe preliminary results of materials characterization of a set of synthetic polycrystalline diamond – on – silicon structures and diamond – silicon – diamond heterostructures. The thermal conductivity of the heterostructure samples and diamond thin films on silicon was measured by two different techniques: transient planar source “hot-disk” and “laser – flash”. The thickness of the diamond films was varied from 1 to 7 um while the silicon channel layer in some of the heterostructure samples was scaled down to 20 nm. The cross-sectional scanning electron microscopy allowed us to determine the grain sizes in the nano- and microcrystalline diamond films and correlated them with the measured values of the thermal conductivity. The extracted experimental thermal boundary resistance was compared with the theoretical predictions for the Kapitza resistance. Our approach allowed us to separate the effects of the interfaces and grain sizes on the thermal conductivity. The obtained results are essential for selecting the optimum parameters and synthesis technique for the phonon-engineered structures. This work was supported by AFOSR award A9550-08-1-0100 on the Electron and Phonon Engineered Nano- and Heterostructures.  V.A. Fonoberov and A.A. Balandin, "Giant enhancement of the carrier mobility in silicon nanowires with diamond coating," Nano Letters, 6, 2442 (2006).  D.L. Nika, E.P. Pokatilov and A.A. Balandin, “Phonon-engineered mobility enhancement in the acoustically mismatched silicon/diamond transistor channels,” Appl. Phys. Lett., 93, 173111 (2008).
9:00 PM - T5.4
Thermal Conductance Measurements of Metal-CNT Nanolaminates using Micron-sized Suspended Structures.
Ki Sung Suh 1 , Jung Hoon Bak 1 , Myung Rae Cho 1 , Yun Daniel Park 1 Show Abstract
1 Department of Physics & Astronomy, Seoul National University, Seoul, -, Korea (the Republic of)
As carbon nanotubes (CNTs) have a unique structure and remarkable physical properties, CNT composites have attracted much attention from many researchers. Especially thermal properties of CNTs and their composite materials incorporating CNTs have been studied intensively, because CNTs are known to possess very excellent thermal transport properties [1-5]. For example, thermal conductivity of CNT is known to be much larger than that of metals such as Ag, Au, Cu and Al. As Pd has good wetting property on the CNT surface, unlike Al, thermal conductance measurement can be useful to investigate the interaction between Pd and CNT. To study the thermal conductance of metal-CNT nanolaminate, we have fabricated micron-sized suspended structures. By using e-beam lithography and metallization, two thermometers/heaters have been patterned on the GaAs substrates. Thermal links made of metal-only or metal-CNT nanolaminates also have been patterned between the two thermometers. Then GaAs substrate has been selectively etched to form suspended structures. We will show the fabrication methods and preliminary measurement data on the thermal conductance of Pd and Pd-CNT composites as well as Al and Al-CNT composites as a comparison. J.A. Eastman et al., Appl. Phys. Lett. 78, 718 (2001). S.U.S. Choi et al., Appl. Phys. Lett. 79, 2252 (2001). M.J. Biercuk et al., Appl. Phys. Lett. 80, 2767 (2002). R. Ramasubramaniam et al., Appl. Phys. Lett. 80, 4647 (2003). H.Q. Xia et al., Appl. Phys. Lett. 94, 4967 (2003).
9:00 PM - T5.5
Determination of Thermal Parameters of One-dimensional Nanostructures Through a Thermal Transient Method.
Anton Arriagada 1 , Edward Yu 1 , Prabhakar Bandaru 2 Show Abstract
1 Electrical and Computer Engineering, University of California, San Diego, La Jolla, California, United States, 2 Mechanical and Aerospace Engineering, University of California, San Diego, La Jolla, California, United States
We present an improved methodology for a thermal transient method enabling simultaneous measurement of thermal conductivity and specific heat of one-dimensional nanoscale structures. The dispersion of finite duration heat pulse inputs traveling along a square nanowire have been simulated with finite element modeling software to demonstrate our method. However, responses that have sub-Kelvin temperature changes and sub-microsecond transient features can impose a variety of experimental challenges, such as the need for amplification hardware with extremely high speed sampling and resolution, along with rapid settling times. We show, for a given pulse energy, that these challenges can be overcome by applying a duration- and magnitude-optimized input that achieves the onset of steady-state temperature throughout the wire. The temporal response of the nanowire to short (1 nanoseconds), long (10 microseconds), and optimized (5 microseconds) duration heat pulses was then related to its thermal moments to deduced the thermal conductivity (k) and specific heat (C) of the nanowire. Excellent agreement has been obtained between the recovered physical parameters and computational simulations.
9:00 PM - T5.6
The Heat Transfer Analysis of Self-Assembled Mononlayers by Molecular Dynamics.
Lin Hu 1 , Ming Hu 1 , Pawel Keblinski 1 Show Abstract
1 Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States
We use a combination of non-equilibrium molecular dynamics (NEMD) and phonon wave-packet dynamics simulations to study the heat transport through silicon - self assembled molecular layers (SAMs)– gold interface. From NEMD simulation we get overall thermal interfacial conductance of SAM – gold interface of 150 MWm-2K-1. This value is consistent with one group of experiments, and not with the other where much lower G was reported. Our simulations indicate that weaker interfacial bonding can dramatically reduce the interfacial conductance and is perhaps the underlying reason for a large range of values reported for interfacial thermal resistance of SAM interfaces. The phonon wave-packet dynamics simulations are used to investigate the transmission of individual gives us the heat transmission coefficient of the interface. The flow of heat into the chains is mainly limited by the interface conductance.
9:00 PM - T5.7
Experimental Investigation of Thermal Conduction in Suspended Few-Layer Graphene
Suchismita Ghosh 1 2 , Wenzhong Bao 3 , Desalegne Teweldebrhan 1 2 , Irene Calizo 1 2 , Feng Miao 3 , Evghenii Pokatilov 1 2 , Denis Nika 1 2 , Chun Ning Lau 3 , Alexander Balandin 1 2 Show Abstract
1 Electrical Engineering, University of California, Riverside, Riverside, California, United States, 2 Materials Science and Engineering Program, University of California-Riverside, Riverside, California, United States, 3 Physics and Astronomy, University of California-Riverside, Riverside, California, United States
It was recently reported that graphene has very high thermal conductivity values which are on the upper limit or exceeding those reported for carbon nanotubes [1-2]. The theoretical studies of the thermal conduction in graphene, which included detail treatment of the Umklapp scattering, are in agreement with the experiment although the discrepancy in the values of the Gruneisen parameter leads to large uncertainty . The difficulty in comparing the thermal conductivity of graphene with that one of diamond or other bulk materials lies in inherent ambiguity of the thickness of a single atomic plane of atoms, which is used in extracting the thermal conductivity. The conventionally used thickness of 0.35 nm, related to the graphite interlayer spacing, differs from the values obtained from the Young’s modulus and atomistic simulations, which give graphene thicknesses in the range from 0.07 to 0.69 nm. It is important to investigate the thermal conduction in few-layer graphenes and examine the transition to the limiting case of basal planes of graphite. The in-plane room-temperature thermal conductivity of high-quality graphite is about 2000 W/mK, although higher values have been also reported. Here we describe our measurements of the thermal conductivity of the few-layer graphene flakes suspended across trenches in silicon wafers. The difference compared to the first experiments [1-2] is in the use of massive metal heat sinks attached to the examined flakes. A new calibration procedure had to be developed in order to determine the amount of power absorbed by the few-layer graphene flakes. The amount of laser power dissipated in graphene and corresponding local temperature rise were determined from the integrated intensity and spectral position of graphene’s Raman G mode. The preliminary results obtained for graphene flakes with n=2, 3, and 4 atomic layers suggest that the thermal conductivity reduces with the increasing number of layers. The thermal conductivity values are in the range between the maximum, achieved for a single layer graphene, and the data for the basal plane of high-quality bulk graphite. The work in Balandin group was supported, in part, by DARPA – SRC Focus Center Research Program (FCRP) through its Functional Engineered Nano Architectonics (FENA) center and Interconnect Focus Center (IFC).  A.A. Balandin, et al., "Superior thermal conductivity of single-layer graphene," Nano Letters, 8, 902 (2008) S. Ghosh, I. Calizo, D. Teweldebrhan, E.P. Pokatilov, D.L. Nika, A.A. Balandin, W. Bao, F. Miao and C. N. Lau, "Extremely high thermal conductivity of graphene: Prospects for thermal management applications in nanoelectronic circuits," Appl. Phys. Lett., 92, 151911 (2008). A.A. Balandin, Thermal conduction in graphene and graphene multilayers, Invited Talk, International Symposium on Graphene Devices, Aizu-Wakamatsu, Japan, 2008.
9:00 PM - T5.8
Thermal Effects of Colloidal Suspensions of Au Nanoparticles
Michael Carlson 1 , Tyler Barton 1 , Pete Tandler 2 , Hugh Richardson 1 , Alexander Govorov 3 Show Abstract
1 Chemistry & Biochemistry, Ohio University, Athens, Ohio, United States, 2 Math & Sciences, Walsh University, North Canton, Ohio, United States, 3 Physics & Astronomy, Ohio University, Athens, Ohio, United States
Nanoparticle research has gained considerable momentum over the past few years, particularly within the realm of heat generation of metal and semiconducting nanocrystals under optical illumination. Many devices and biomedical applications of nanoparticles (NPs) rely on the NPs’ ability to absorb the incident photons and convert the photon energy into phonon interactions, which are dissipated throughout the surrounding matrix. Our current work focuses on characterizing the thermal effects of colloidal suspensions of Au NPs in water droplets. We measure the dynamic thermal response of colloidal suspensions of Au NPs in μL-sized water droplets optically excited with a 532 nm CW laser and characterize the resulting microscale thermal transport mechanisms. Energy balance calculations allow us to use the photothermal properties of the Au NPs to model the temperature behavior of the droplets. Because Au NPs have very low optical quantum yield (10-6), the amount of heat generated can be quantified by the total optical absorption rate. The energy balance calculations provide two equations, the rate of energy absorbance and rate of heat loss, which can be used to predict the thermal behavior of the suspensions with no adjustable parameters. Combining the experimental transient temperature profiles with these equations, we show that the efficiency for transducing incident resonant light to heat in a colloidal suspension of Au NPs is 1, with an uncertainty of ± 0.03. We also show that the steady-state temperature of the droplet scales directly with the intensity of the incident laser light. In all cases, the modulation of the incident laser light has no effect upon the transduction efficiency. Additional modes of thermal transport were observed and characterized. Convection currents were detected by placing silica beads inside the droplets. Movies of the convection currents were recorded and analyzed. It was seen that before the laser excitation, the silica beads displayed random, fluid movement. However, during the period of laser excitation, the silica beads increased velocity, clearly flowing in convective movements parallel to the path of the laser. As the droplet continued to be irradiated with the laser and temperature of the suspension reached equilibrium, the silica beads returned to the same random, fluid movement originally observed before the excitation. In addition to convection currents, theoretical calculations support the idea of thermal waves propagating throughout the droplet as another mechanism of heat transfer, further explaining the thermal behavior of our experimental results. Dulkeith, E.; Niedereichholz, T.; Klar, T. A.; Feldmann, J.; von Plessen, G.; Gittins, D. I.; Mayya, K. S.; Caruso, F., Plasmon emission in photoexcited gold nanoparticles. Physical Review B 2004, 70, (20).
9:00 PM - T5.9
The Influence of Microwave and Ion Irradiation on Electrical and Thermal Transport Properties of Carbon Nanotube Films.
Daniel McLeod 1 , Michael McCarthy 1 , Yunki Gwak 1 , Assel Aitkaliyeva 1 , Hae-Kwon Jeong 1 , Lin Shao 1 , Choongho Yu 1 Show Abstract
1 , Texas AM Univ, College station, Texas, United States
Single walled carbon nanotubes (SWCNTs) are intensively studied for many potential applications. For example, CNTs are very promising for applications that require high electrical and thermal transport. However, when they are bundled in a mat or chunk form, the properties are quite different from what is expected, which becomes a bottleneck of realizing practical applications. It is now timely to investigate the reasons behind such differences in transport properties. For this study, a 300 - 700 nm thick CNT films were synthesized and irradiated by microwaves and helium ions. Using 300W microwaves for 30 - 40 seconds reduces the resistivity of the films by up to 100%. When films are exposed for up to 10 minutes, a significant reduction in the defect mode frequency has been found using Raman Spectroscopy. Additionally, in-situ experimental results to observe the influence of ion bombardment on the film may shed lights on revealing destructive and self-healing mechanisms in CNT composites.
Patrick K. Schelling University of Central Florida
Jennifer Lukes University of Pennsylvania
Alexander A. Balandin University of California-Riverside
Yulong Ding University of Leeds
T6: Thermoelectric Materials
Thursday AM, April 16, 2009
Room 3002 (Moscone West)
9:30 AM - **T6.1
Thermal Conductivity Reduction and ZT Enhancement in 2-D and Nano-dot Superlattices
Rama Venkatasubramanian 1 , Xianfan Xu 2 , Azzam Mansour 3 , Phil Barletta 1 , Chris Caylor 1 Show Abstract
1 , RTI International, Research Triangle Park, North Carolina, United States, 2 , Purdue University, West Lafayette, Indiana, United States, 3 , Naval Surface Warfare Center, West Bathesda, Maryland, United States
We will describe our recent studies and results in superlattice structural characterization including by X-ray absorption fine structure spectroscopy and X-ray diffraction, coherent opical phonon property measurements using ultra-fast time resolved optical measurements, thermal conductivity reduction by 3-omega method and ZT enhancement in a couple of p-type Bi2Te3/Sb2Te3 superlattice material systems. These studies have allowed us to grow thicker films (~15 microns) to extract larger cooling differentials from devices. We will also describe delta-doped n-type nanoscale thin-film materials in the Bi2Te3-materials system to complement the advances in p-type materials, to enable us to produce high-performance devices. The work in low-temperature Bi2Te3-based superlattice thin-films have inspired us to develop 2-D and nano-dot superlattices in the mid-temperature PbTe-based systems and high-temperature SiGe-based systems. The thermal conductivity reduction as a function of superlattice period as well as the coverage of nano-dots in a nano-dot superlattice are intriguing. Work is in progress to characterize both the in-plane and cross-plane electronic transport in such nanoscale materials to see if we can realize the benefits of lower thermal conductivity in obtaining enhanced ZT. The 2-D and the nano-dot superlattice systems would be compared and contrasted, based on experimental data.Acknowledgements: Work supported by DARPA/ONR through U.S. Navy Contract No. N00014-04-C-0042 and DARPA/ARO through U.S. Army Contract No. W911NF-08-C-0058.
10:00 AM - T6.2
Size Effects on the Thermal Properties of Self-assembled Ge Quantum Dots in Single-crystal Silicon.
Jean-Numa Gillet 1 Show Abstract
1 Aerospace Engineering Sciences, University of Colorado at Boulder, Boulder, Colorado, United States
Superlattices with a low thermal conductivity have been used to design thermoelectric materials. Like nanowires, superlattices however affect heat transfer in only one main direction, and are also susceptible to cracking owing to lattice mismatches. It is challenging to obtain a thermoelectric figure of merit ZT higher than unity with the superlattices. Self-assembly (SA) is a bottom-up technology that can be used to fabricate ultradense arrays of Ge quantum dots (QDs) in Si for numerous promising applications in electronics and photonics. An example is quantum computing where accurate QD positioning is required. A high ZT is expected in these Ge QDs arrays obtained by SA in Si especially because these are single crystals and can be easily electrically doped. In this work, we first demonstrate that high-density 3-D arrays of self-assembled diamond-cubic (DC) Ge QDs in a DC Si matrix can exhibit low values of thermal conductivity. This property can be exploited to design 3-D single-crystal thermoelectric materials, which are CMOS-compatible. To study the thermal behavior of these effectively 3-D “phononic crystal” nanocomposites, we create an atomistic model of a supercell consisting of several Si DC unit cells (Gillet et al., J. Heat Transfer, 03/2009 issue). Inside each supercell, we substitute Si atoms with Ge atoms to form an inner cubic QD. With a proper choice of the filling ratio, the thermal conductivity has been shown to reduce to a value lower than 0.2 W/m/K (a factor of at least 750 reduction compared to bulk Si). Such a result is realized by ensuring minimum group velocities in the supercell dispersion diagram. To date we have not considered multiple scattering of the particle-like phonons between the QDs. Further reduction in thermal conductivity is expected when these collective effects are incorporated, and this is the first objective of the current work. The second objective is concerned with analyzing the size dependence of the thermal conductivity versus the nanocomposite volumic composition. We define N and M as the numbers of unit cells in one spatial direction in a supercell and in the QD inside this supercell, respectively. To determine the supercell size effects on the thermal conductivity, we assign the ratio M/N to the constant value of 1/3. The Ge volumic fraction in the nanocomposites with this ratio is therefore given by f = 3.7 %. From preliminary results with this value of f, we obtain an exponential-like decrease of the thermal conductivity when the size parameter N is increased. When N is increased from 3 to 6, the thermal-conductivity upper limit is decreased from 21.3 to 8.4 W/m/K. Ongoing work focuses on studying that relationship as N is increased further.
10:15 AM - T6.3
Lattice Thermal Conductivity of Nanostructured Thermoelectric Materials Based on PbTe.
Yee Kan Koh 1 , David Cahill 1 Show Abstract
1 Materials Science and Engineering, University of Illinois at Urbana Champaign, Urbana, Illinois, United States
We report the through-thickness lattice thermal conductivity Λl of (PbTe)1-x/(PbSe)x nanodot superlattices (NDSLs) over a wide range of periods 5≤h≤50 nm, compositions 0.15≤x≤0.25, growth temperatures 550≤Tg≤620 K and growth rates 1≤R≤4 μm hr-1. The NDSLs are grown at MIT Lincoln Laboratory*. All of our measurements approach Λl of bulk homogenous PbTe1-xSex alloys with the same average composition. For 5≤h≤ 50 nm, Λl is independent of h; a result we attribute to short mean-free-paths of phonons in PbTe and small acoustic impedance mismatch between PbTe/PbSe. We alloyed the PbTe layers of four NDSLs with SnTe up to a mole fraction y=18%; Λl is reduced by <25%. To provide a baseline for comparisons, and to help guide future work, we calculate the thermal conductivity of a homogeneous PbTe-based alloy using a Debye-Callaway model that includes phonon scattering by point defects. We find that phonon scattering by point defects in most PbTe alloys is controlled by variations in bond strengths and lengths rather than variations in the atomic masses. Our work provides guidelines for future work on nanostructured thermoelectrics based on PbTe.* We acknowledge C. J. Vineis, T. C. Harman, S. D. Calawa and M. P. Walsh of MIT Lincoln Laboratory for providing us the NDSL samples.
10:30 AM - **T6.4
The Role of Defects on Thermal and Thermoelectric Transport in Nanowires.
Li Shi 1 Show Abstract
1 Mechanical Engineering, UT Austin, Austin, Texas, United States
Thermal and thermoelectric properties distinctly different from the bulk behaviors have been experimentally observed in nanowire structures in recent years. The modified properties have often been attributed to surface scattering of heat and charge carriers, and in some cases to quantum confinement of these carriers. The role of defects in the nanowire structure has often been overlooked. The capability that we have developed recently for characterizing the structure-thermoelectric relationships of individual nanostructures has enabled us to examine the role of defects on the transport properties of individual nanowires. We found that nanoscale defects such as local voids and strains can significantly suppress the thermal conductivity and electrical conductivity. The measurement results are analyzed with transport models of nano-constrictions.
11:30 AM - **T6.5
Exploring and Expanding the Limits of Heat Transport.
Arun Majumdar 1 2 Show Abstract
1 Mechanical Engineering and Materials Science and Engineering, University of California-Berkeley, Berkeley, California, United States, 2 Materials Sciences Division & Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States
Research in thermal transport phenomena has encountered some seemingly insurmountable limits, which have created barriers for design of new devices and systems. Examples include alloy limit of thermal conductivity in crystalline materials, amorphous limit of thermal conductivity, critical heat flux in boiling heat transfer, etc. This talk will focus on understanding the fundamental nature of these limits and ask the questions: What are the physical limits of heat transport? How far are we from those limits? How can we reach the limits? Can we circumvent these limits by a different approach? Invariably, these relate to insights about the length scales of the phenomena, which often happen to be in the nanoscale regime. This talk will demonstrate how these limits can be overcome by using nanostructures that are cleverly designed to exploit unique features that cannot be accessed by macroscopic structures. Experiments have shown that the thermal conductivity can be reduced below the alloy limit by a factor of 5-6, the amorphous limit can be broken as well, and the critical heat flux in boiling can be increased by a factor of 2. Can these be pushed further? What are the true physical limits and how far are we from those limits? And what new technologies can open up by breaking these limits. The talk will conclude by addressing these issues.
12:00 PM - T6.6
Are Nonlinear Vibrational Modes the Key to Controlling Heat Flow in Materials?
Kevin Moore 1 , Michael Manley 1 Show Abstract
1 Chemistry and Matrials Science, Lawrence Livermore Nationa Labs, Livermore, California, United States
Electric diodes have transformed the world, altering the way we live our life and conduct our business through the simple act of checking the movement of electrons. They are what make technology, such as computers and cell phones work. Imagine what possibilities are feasible through the ability to control heat. This is precisely what a thermal rectifier, a device that controls the rate of heat flow in certain directions, promises to accomplish with thermal vibrations rather than electrical current. Such as device would transport and utilize thermal energy in a more efficient manner, fostering development of more energy-efficient devices and buildings. In short, thermal rectification would take technology in entirely new directions. The underlying physical phenomena that govern such behavior must be identified and understood. Normally, heat movement follows ordinary perturbative wave theory, meaning it behaves in a linear fashion. Sometimes heat flow does not follow ordinary perturbative wave theory  and this behavior is believed by some to be due to nonlinear vibrational modes of the atomic lattice. Nonlinear modes come in two forms: solitons and intrinsic localized modes (ILMs), which are alternatively knows as discrete breathers. Solitons are a vibrational packet of energy that is spatially confined, yet mobile in an atomic lattice. Intrinsic localized modes are spatially-constrained. These are nanometer-sized hot spots vibrating in a nonlinear fashion due to the anharmonic nature of the crystal. The first experimental detection of solid-state thermal rectification was achieved on carbon and boron nitride nanotubes . Experimental results could not be explained using ordinary perturbative wave theory, so the authors invoked solitons. Intrinsic localized modes have been observed in solid body-centered cubic 4He , metallic α-U , and ionic NaI . The fact that ILMs occur in three very different anharmonic crystals suggests that nonlinear vibrational modes are a characteristic of materials at high temperature. What is more, the thermal conductivity of polycrystalline α-U increases at the temperature where ILMs form , suggesting they influence heat flow. Here we show the behavior of heat flow along the three primary crystal axes in orthorhombic α-U at temperatures above and below where the ILMs become activated. These ILMs, which become activated above ~200C, are polarized along the crystallographic b-axis, allowing us to more effectively isolate the effects of ILMs from conventional sources of heat flow.References C.W. Chang et al., Science 314, 1121 (2006). T. Markovich et al., Phys. Rev. Lett. 96, 125501 (2006); M.E. Manley et al., Phys. Rev. Lett. 96, 125501 (2006); J.R. Minke, Phys. Rev. Focus 17, story 11. M.E. Manley et al., Phys. Rev. Lett. (In review); arXiv:0810.2823. M.E. Manley et al. Phys. Rev. B 77, 214305 (2008).
12:15 PM - T6.7
A Diminished Thermal Conductivity of Si/SiGe Multilayers, Established through Heating Current Frequency Variation.
Alex Dooraghi 1 , Anton Arriagada 1 , Norbert Elsner 2 , Prabhakar Bandaru 1 Show Abstract
1 Materials Science program, Mechanical Engineering department, UC, San Diego, La Jolla, California, United States, 2 , Hi-Z Inc., San Diego, California, United States
The determination of thermal conductivity (κ) of thermoelectrics is necessary for the characterization of the figure of merit ZT(= S2 σ/κ), where σ is the electrical conductivity, and S, the Seebeck coefficient, which underlies the heat conversion efficiency. We will report on the design of a thermal conductivity measurement system for nanostructured devices comprised of Si/SiGe multilayers, based on the 3-omega method. We have measured 20 nm (10nm Si0.8Ge0.2 and 10nm Si) period Si0.8Ge0.2/Si multilayer films of varying thicknesses (0.4 μm, 1.0μm, 5.6 μm) on Silicon substrates. As the penetration of the thermal wave is inversely proportional to the frequency, spectral measurements covering decades of frequency were used to finely probe the substrate, and the overlying Si and SiGe thin film layers. Both in-phase and out-of phase measurements of the 3 omega voltages yielded comparable values of the thermal conductivity in the range of 3-5 W/mK, much lower than the reported bulk numbers. Consistent values of the thermal conductivity, through measurements using both the conventional “slope” method and the “offset” method yield confidence in the measurements. These results provides proof of the potential of multilayered media to be used for reduced thermal conductance applications such as heat insulation, thermoelectrics etc.
12:30 PM - T6.8
Observation of Strong Phonon-drag Effect in Pulse Voltage Stressed Silicon Micro-bridges.
Ali Gokirmak 1 , Gokhan Bakan 1 , Cicek Boztug 1 , Adam Cywar 1 , Helena Silva 1 Show Abstract
1 Electrical and Computer Engineering , University of Connecticut, Storrs, Connecticut, United States
The predominant heat carriers are phonons in insulators and semiconductors and electrons in metals. Wieldmann-Franz (W-F) Law is reported to be accurate for most metals in a large temperature range. In the W-F framework, electrons are treated as a fluid with a specific heat and the thermal conductivity is proportional to the product of electrical conductivity and temperature. At low current densities, the thermal conduction is predominantly diffusive. However, a significant convective heat transport due to electron flow (drift) is expected at high current densities. In our experiments we electrically stress micro-bridges made of a n+ doped ([P] ~ 5x1020cm-3 ) nanocrystalline-Si (nc-Si) film of ~ 100 nm over a thick layer of SiO2. Low-pressure-chemical-vapor-deposition (LPCVD) is used for deposition of the nc-Si film. Optical lithography and reactive ion etching are used to define 200~400 nm wide, 1-5 μm long wires with large contact areas at each end. The underlying oxide is partially etched to release the wires, forming the micro-bridges between the contact regions.10-40 V stresses are applied to the wires for ~1 μs duration while the current through the wire is monitored, revealing the changes in the material during the pulse, including the solid-liquid phase change when Si becomes a metal with 75 μΩ.cm resistivity. The current density through the wires can be as high as 108A/cm2 when the wire is in liquid state. If the applied voltage and the time are controlled, the wires can resolidify without breaking as observed by scanning electron microscopy (SEM) after stress. SEM images reveal consistent partial melting of the wires at the ground terminal side for moderate voltage stresses (~30 V). This asymmetric melting along the length of the wire becomes very significant for wires of ~3 μm length. We have also observed asymmetric light emission from the ground-terminal side of the wires if the voltage is ramped slowly.Melting would be expected on the positive-terminal end if this asymmetry was due to the convective electronic heat transport, since the incoming electrons would cool the ground terminal end. The observed polarity suggests that the backscattering of the phonons by the incoming electrons - phonon-drag - significantly suppresses the phonon diffusion, hence thermal conductivity at the ground-terminal end.Phonon-drag and electronic convective heat transport result in asymmetries in opposite directions and one or the other will dominate at a given temperature. The two contributions are typically modeled as a single temperature dependent term known as Thomson heat (effect). Thomson coefficient (β) is related to the temperature dependence of Seebeck coefficient (S) (β=T∂S/∂T). We have extrapolated the available Seebeck data for Si films to high temperatures and included the Thomson term in electro-thermal simulations of the electrically stressed wires. Numerical results are in qualitative agreement with our experimental observations.
T7: Electrons and Phonons
Thursday PM, April 16, 2009
Room 3002 (Moscone West)
2:30 PM - **T7.1
Ultrafast Structural Transformation in Metals under Femtosecond Laser Irradiation: Microscopic Mechanisms and the Effect of Transient Changes in the Electron-phonon Coupling.
Leonid Zhigilei 1 , Zhibin Lin 1 2 , Derek Thomas 1 3 Show Abstract
1 Materials Science and Engineering, University of Virginia, Charlottesville, Virginia, United States, 2 Physics, Texas A&M University, College Station, Texas, United States, 3 Materials Engineering, University of Tokyo, Tokyo Japan
Active expansion of femtosecond laser techniques into the area of micro- and nano-scale material processing calls for a better theoretical understanding of the connections between the basic mechanisms of laser interaction with materials and the final microstructure of materials treated by laser irradiation. The highly nonequilibrium character of laser-induced processes makes the quantitative theoretical/computational description of the laser-materials interactions challenging. In particular, due to the small heat capacity of the electrons, laser excitation can transiently bring the electron temperature to very high values, comparable to the Fermi temperature. At such high electron temperatures, the temperature dependence of thermophysical material properties is directly affected by the thermal excitation of the lower band electrons, leading to large deviations from the commonly used approximations of linear temperature dependence of the electron heat capacity and a constant electron-phonon coupling .Practical implications of the modification of the electron temperature dependences of the thermophysical properties under conditions of the strong electronic excitation are investigated in atomistic simulations performed with a model combining the classical molecular dynamic method with the two-temperature model . The simulations performed for one-component metal targets (Au, Ni, Cr) provide information on the kinetics of the laser-induced melting and resolidification , generation of crystal defects , as well as the mechanisms of material ejection from the targets. Recent simulations performed for targets composed of 30 nm Au or Ag films deposited on a bulk Cu substrate predict that the higher strength of the electron-phonon coupling in Cu, as compared to Au and Ag, results in a preferential sub-surface heating and melting of the Cu substrate. The large difference in the atomic mobility in the transiently melted and crystalline regions of the target makes it possible to connect the final concentration profiles in the resolidified targets to the history of the laser-induced melting process, thus allowing for experimental verification of the computational predictions. Z. Lin, L. V. Zhigilei, V. Celli, Electron-phonon coupling and electron heat capacity of metals under conditions of strong electron-phonon nonequilibrium, Phys. Rev. B 77, 075133 (2008). D. S. Ivanov, L. V. Zhigilei, Combined atomistic-continuum modeling of short pulse laser melting and disintegration of metal films, Phys. Rev. B 68, 064114 (2003). Z. Lin, L. V. Zhigilei, Time-resolved diffraction profiles and atomic dynamics in short pulse laser induced structural transformations: Molecular dynamics study, Phys. Rev. B 73, 184113 (2006). Z. Lin, R. A. Johnson, L. V. Zhigilei, Computational study of the generation of crystal defects in a bcc metal target irradiated by short laser pulses, Phys. Rev. B 77, 214108 (2008).
3:00 PM - T7.2
Enhanced Molecular Dynamics for Simulating Thermal and Charge Transport Phenomena.
Reese Jones 1 Show Abstract
1 , sandia natl lab, Livermore, California, United States
Molecular dynamics (MD) has become a common tool to explore and quantify the thermal and mechanical properties of nanostructures like carbon nanotubes (CNTs). In fact it is quite feasible to simulate a single wall CNT a few micrometers long for a few nanoseconds with a cluster machine. MD is used to explicitly represent phonon modes and scatterers of phonons such as defects at these length and time-scales. However, it has no representation of the mobile electrons and the charge and energy transport processes that they mediate. Since it is not currently possible to simulate electron-mediated transport at device scales with first principles methods, we have turned to so-called hydrodynamic models of electron transport, see e.g. [Rudan1986,Lai1996], which stem from the first three moments of the Boltzmann equation describing the incoherent transport of electrons and are commonly used in device modeling. These models have validity where density approximations and diffusive models become accurate representations the underlying behavior.We take hydrodynamic models, such as the two temperature model (TTM), as a framework to couple MD simulation of the phonon processes with finite element-based solvers for the remaining partial differential equations (PDEs). Specifically, in the TTM two temperatures are used to represent the kinetic energy of the phonons and electrons. The coupling scheme uses the continuum PDE framework together with localized thermostats and interscale operators to provide a concurrent simulation of thermal and electrical transport. Others, e.g. [Ivanov2003,Xu2004] have used this methodology to model fast laser processes. We have extended it via the more complex hydrodynamics models, such as drift-diffusion, to applications involving high current nanodevices. We also have used novel first principles calculations to determine the electron-phonon coupling coefficients. Some example simulations include: the thermal breakdown of Joule heated semiconductor nanowires, the thermoelectric performance of Si/Ge superlattices and the laser heating of CNTs.
3:15 PM - T7.3
Investigation of the Effects of Photon Energy on Electron-phonon Coupling in Short-pulsed Laser Heated Metallic Films.
Justin Smoyer 1 , John Duda 1 , Pamela Norris 1 , Patrick Hopkins 2 Show Abstract
1 Mechanical and Aerospace Engineering, University of Virginia, Charlottesville, Virginia, United States, 2 Engineering Sciences Center, Sandia National Laboratories, Albuquerque, New Mexico, United States
This work investigates the changes in the electron-phonon coupling factor of noble metals after short-pulsed laser heating with varying incident photon energy. Recent work has shown that laser heating of thin metal films with photon energies at and around the ITT threshold cause repopulation of the electron bands, affecting the electron density of states and subsequently the electron heat capacity and electron-phonon coupling factor. The transient thermoreflectance technique is used to monitor the electron-phonon coupling factor in noble metal thin films for photon energies above and below the ITT threshold for various electron temperatures. The results are compared to calculations of the electron-phonon coupling factor using phonon energy dependent calculations.
3:30 PM - T7.4
Thermal Transport in Diamond Containing Metal Matrix Composites.
Vikas Sinha 2 , Sabyasachi Ganguli 3 , Robert Wheeler 2 , Jonathan Spowart 1 Show Abstract
2 , UES, Inc., Dayton, Ohio, United States, 3 , UDRI, Dayton, Ohio, United States, 1 Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson AFB, Ohio, United States
Several research groups around the globe have examined and developed the diamond containing metal matrix composites for more than the past decade. These composites also are commercially available in different chemistry-thermal property combinations. This class of materials is attractive for use as a substrate/ packaging material for electronic devices because of their relatively high thermal conductivity and a coefficient of thermal expansion (CTE), which can be tailored to match that of semiconductors. The matching of CTE is important to minimize the thermal stresses at the device/ package interfaces and thereby, to improve the component life/ reliability. In the current research, diamond-copper matrix composite was examined. The thermal conductivity was measured via laser flash techniques. To better understand the thermal transport coupling between constituents (i.e. Cu and diamond), the Cu/diamond interface was examined in more detail. A qualitative idea about the heat transport across the Cu/ diamond interface was obtained via infra-red microscope observations. An attempt also was made to quantitatively measure the thermal conductance of interfaces via laser pump probe methods. To aid in improved understanding of interfacial thermal transport, microstructural characterization of the interface region was also carried out. The characterization included high resolution transmission electron microcopy to examine the coherency and nanostructure of Cu, diamond and interface phase. The chemistry changes near the interface were evaluated via energy dispersive spectroscopy (EDS) and/ or electron energy loss spectroscopy (EELS) techniques in a transmission electron microscope.
4:00 PM - **T7.5
Nanoscale Thermal Transport during Ultrafast Melting and Crystal Growth of Metals and Semiconductors.
David Cahill 1 , Wai-Lun Chan 1 , Wen-Pin Hsieh 1 , Robert Averback 1 Show Abstract
1 , University of Illinois, Urbana, Illinois, United States
The transformation between a crystal and liquid is one of the most fundamental phase transitions in the science of materials. We are probing the ultimate limits for the rates of reversible melting and crystallization of Ag and Si using excitation by femtosecond laser pulses and measurements of the melt-depth by optical third-harmonic generation (THG). For a (100) oriented crystal and circularly polarized light, third-harmonic light is generated by the crystalline phase but not by the me< optical absorption in the melt causes the THG to fall-off exponentially with melt depth. We find that various aspects of ultrafast energy exchange and thermal transport at the nanoscale play critical roles in determining the velocity of the liquid/crystal interface during both melting and crystallization. For Ag, the critical factors are, at short times, suppression of heat conduction by electron-scattering from d-band excitations and, at long times, suppression of heat conduction by weak electron-phonon coupling; the melt velocity reaches 350 m/s. A thin layer of Si melts homogeneously on a 1 ps time-scale but, because of the relatively large latent heat and presumably because of the relatively low thermal conductance of the liquid/crystal interface, we have not observed the superheated melt to propagate further into the crystal.
4:30 PM - **T7.6
Role of Hot Phonons in the Performance of GaN High Power FET's.
Jacob Khurgin 1 Show Abstract
1 Department of Electrical and Computer Engineering, Johns Hopkins University, Baltimore, Maryland, United States
Wide bandgap Nitride Semiconductors are characterized by combination of strong electron-LO-phonon interaction and slow decay of LO phonons that causes build-up of LO phonon populations and subsequent deterioration of the performance of the nitride electronic devices. In this talk the magnitude of the hot phonon effect will be theoretically investigated and experimentally confirmed using high order Anti-Stokes Raman measurements of working GaN device. Ways to mitigate the hot phonon build up and to capture and recycle the LO phonon energy will be explored.
5:00 PM - T7.7
Thermal Transport in Atomistic and Mesoscale Simulations with Implicit Degrees of Freedom.
Ya Zhou 1 , Alejandro Strachan 1 Show Abstract
1 School of Materials Engineering, Purdue University, West Lafayette, Indiana, United States
Degrees of freedom (DoFs) not explicitly described in molecular dynamics (MD) simulations of metals and coarse grain simulations play a key role in thermal transport. The thermal role of these DoFs (valence electrons in MD and atoms internal to the mesoparticles in coarse grain simulations) can be described with a recently proposed method denoted dynamics with implicit degrees of freedom (DID) [Strachan and Holian, Phys. Rev. Lett 94, 014301 (2005)]. DID describes the thermal role of the DoFs internal to the mesoparticles using finite local thermostats and their corresponding specific heat. In the case of metals DID can be used with a two-temperature model and we use non-equilibrium MD simulations to show that the thermal conductivity of Al can be characterized with good accuracy (within 10% of experimental value).We also apply DID to study thermal transport in a molecular crystal and we characterize how the specific heat of the implicit DoFs affects thermal transport. We find that the contribution of implicit DoFs to overall thermal conductivity increases very sharply with temperature during melting when mass diffusion becomes increasingly important; under these conditions the thermal conductivity increases linearly with internal specific heat. We find that for molecular liquids with large internal specific heats increasing temperature can lead to increased thermal conductivity due to enhanced mobility of molecules at higher temperatures.
5:15 PM - T7.8
Computation of Optical Phonon Decay Times using Molecular Dynamics.
Neil Zuckerman 1 , Jennifer Lukes 1 Show Abstract
1 Mechanical Engineering, University of Pennsylvania, Philadelphia , Pennsylvania, United States
The lifetimes of localized hotspots caused by Joule heating in electronics are computed using Molecular Dynamics (MD), to investigate the influence of local nanoparticles on these lifetimes. The hotspots are primarily composed of high frequency mechanical vibrations categorized as optical phonons. The authors' previously-developed atomistic MD models are used to simulate atomic motions, including optical vibrations imposed on a thermal background. Through Fourier analysis the energy content of the optical modes is seen to decay over time. The model can quantify the effect of local material inclusions or other structures (nanoparticles) upon the rate at which optical mode energy is transferred into faster-propagating vibrations, causing a hotspot to dissipate.
5:30 PM - T7.9
The Near Field Correlation Spectrum of a Metallic Film.
Karl Joulain 1 , Carsten Henkel 2 Show Abstract
1 Laboratoire d'études thermiques, Université de Poitiers, Poitiers France, 2 Institut für Physik, Universität Potsadm, Potsdam Germany
Metallic surfaces exhibit surface waves called surface plasmon polaritons. These waves propagate along the interface and decay exponentially from both sides of the interface. In the near field above a heated metal, these modes are thermally excited so that they can dominate the emitted radiation field. Its energy density is enhanced compared to the far field and its coherence length increased, being related to the propagation length of the surface polariton.In the case of a film, surface waves from both interfaces hybridize to give two coupled modes. One of these modes is mainly concentrated in the film and has a short propagation length, the other one lies outside the film and propagates further. From their dispersion relation,we show that these two modes dominate the film's local density of states in the near field. We show that they are well separated in frequency when the film is thinner than an wavelength. Furthermore, at some frequencies, two k-vectors are excited. We show that in this case, the spatial coherence function of the thermal field above the film shows a beatnote due to the interference between the two modes.
Patrick K. Schelling University of Central Florida
Jennifer Lukes University of Pennsylvania
Alexander A. Balandin University of California-Riverside
Yulong Ding University of Leeds
T8: Low-Dimensional Systems
Friday AM, April 17, 2009
Room 3002 (Moscone West)
9:00 AM - **T8.1
Thermal Transport across Nanoscale Interfaces.
Nitin Shukla 1 , Hao-Hsiang Liao 1 , Harikrishna Harikrishna 1 , Scott Huxtable 1 Show Abstract
1 Mechanical Engineering, Virginia Tech, Blacksburg, Virginia, United States
Heat transport in nanostructured materials is often controlled by interfacial thermal resistance between dissimilar materials. In this talk I will discuss some of our recent experimental work on characterizing interface thermal conductance in a variety of materials systems under varying conditions. In particular I will discuss the influence of amorphous and crystalline structures at metal/non-metal interfaces, the role of surface coatings at solid-liquid interfaces, the effect of nanoparticles embedded in a host matrix, as well as the effect of elevated pressure on interface conductance.
9:30 AM - T8.2
Thermal Conductance at Heterointerfaces in Nanowires by Molecular-dynamics Simulation.
Patrick Schelling 1 , Ashton Skye 1 Show Abstract
1 AMPAC and Department of Physics, University of Central Florida, Orlando, Florida, United States
Experiment and simulation have shown previously very low thermal conductance in semiconductor nanowires. Wave-packet results have shown previously suggested that interfacial conductance in nanowire heterostructures might be quite low due to the discrete nature of the vibrational density of states in small-diameter nanowires. However, there has not yet been a clear theoretical picture of how conductance depends on nanowire diameter. We present molecular-dynamics simulation results that show fairly high conductance is possible when nanowire surfaces are smooth, consistent with previous observations that surface roughness dominates phonon scattering. For interfaces, we find that conductance scales with the length of the simulation cell due to partially ballistic transport, requiring finite-size scaling analysis to achieve meaningful results. Our results suggest that for perfect interfaces, interfacial conductance in nanowire structures decreases with decreasing diameter, in agreement with wave packet results. However, the effect of reduced dimensions is not as dramatic as previously thought.
9:45 AM - T8.3
Modeling of Heat Transport along Horizontal Carbon Nanofiber Bridging Two Electrodes.
Toshishige Yamada 1 , Tsutomu Saito 1 , Drazen Fabris 1 , Patrick Wilhite 1 , Cary Yang 1 Show Abstract
1 Center for Nanostructures, Santa Clara University, Santa Clara, California, United States
Carbon nanofibers (CNFs) are promising candidates for future interconnect materials. When conducting large electric current, Joule heat generation and the resulting breakdown constitute a critical determining factor for system reliability. We have prepared a horizontal CNF bridging two gold (Au) electrodes with and without tungsten (W) deposition on the contacts . The maximum current density Jmax just before breakdown is measured and the breakdown location as a function of reciprocal CNF length L is recorded. Results depend on whether CNF is suspended (no contact to the oxide substrate) or supported by the substrate (partially or completely), or whether W deposition affects the CNF-electrode contact. In suspended cases, Jmax depends linearly on 1/L (Jmax MA/cm2 = 5.4/L μm-1), confirming that the highest temperature is reached at the mid-point of the CNF. In supported cases, Jmax is consistently larger (up to a factor of four) than in suspended cases. Breakdown locations are determined from scanning electron microscope (SEM) images. There are three cases. (1) When the CNF is fully suspended, breakdown occurs at or near the mid-point with and without W deposition. (2) When the CNF is partially supported, without W deposition, breakdown occurs near the middle of the unsupported or suspended segment. (3) For case (2) but with W deposition, breakdown occurs very close to the substrate-supported region. This finding is consistent with the much higher heat dissipation capability of W-deposited electrodes compared to that of the oxide substrate. These measurements and observations can be explained with a heat transport model taking into account the difference in heat dissipation efficiency a at W-deposited electrode contact (extremely high a), at electrode contact without W (intermediate a), at substrate contact (intermediate a), and in suspended segment (negligible a). The model shows that Jmax saturates to an a-dependent minimum for long CNFs, while Jmax increases monotonically with 1/L for shorter ones, approaching asymptotically the 1/L line discussed above. The model also predicts larger Jmax saturated values in supported cases than suspended cases, consistent with experimental results. The breakdown location can be determined by calculating the maximum temperature point in the CNF, using the model with the location-dependent a. Observed breakdown locations (1) – (3) above are satisfactorily explained with the model. 1T. Saito, T. Yamada, D. Fabris, H. Kitsuki, P. Wilhite, M. Suzuki, and C. Y. Yang, "Improved contact for thermal and electrical transport in carbon nanofiber interconnects," Appl. Phys. Lett. 93, 102108 (2008).
10:00 AM - T8.4
Thermal Transport Across Graphene Nano-junctions using First-principles.
Keivan Esfarjani 1 , Natalio Mingo 2 1 Show Abstract
1 Electrical Engineering, UC Santa Cruz, Santa Cruz, California, United States, 2 , CEA, Grenoble France
Transmission and thermal conductance of a junction made of bulk graphene and a nanoribbon have been calculated using force constants determined from tight-binding and first-principles methods. This forms an abrupt junction. The transmission, which is in principle exact in the limit where anharmonicity can be neglected, will be compared to the results for an abrupt T-junction, which are based on elasticity considerations. Results for different junction geometries of ranging from a single vacancy to a single atom will be presented.We will also discuss the calculation of the total and transport scattering cross sections by individual impurities. Analytical formulas in the low frequency limit will be given.
10:15 AM - T8.5
Molecular Dynamics Simulations of Transverse Thermal Transport of Covalently Bonded Carbon Nanotubes
Vikas Varshney 1 2 , Soumya Patnaik 1 , Ajit Roy 1 , Barry Farmer 1 Show Abstract
1 Air Force Research Laboratory, Wright Patterson Air Force Base, Dayton, Ohio, United States, 2 , Universal Technology Corporation, Dayton, Ohio, United States
It is well known from literature that carbon nanotubes possess excellent thermal transport characteristics along their axial (longitudinal) direction(1). However, the thermal transport is significantly hindered across carbon nanotubes (by two orders of magnitude) in transverse direction wherever they are in contact (perpendicular to their axis) due to short range van der Waals interactions. This has been repeatedly confirmed in CNT nanocomposites literature that even at concentrations that exceed their geometric percolation threshold, the measured thermal conductivity values does not show a characteristic jump as they show in electrical conductivity measurements(2). In other words, touching of nanotubes in a nanocomposite material does not suggest thermal percolation. This is often attributed to the high thermal interface resistance, also known as Kapitza resistance(3) and it is quite significant at the nanotube-nanotube transverse interface. Here, one should ask a simple but important question: How would the interfacial thermal resistance of nanotubes vary in transverse direction if one is able to connect the nanotubes through covalent bonds (as compared to just van der Waals interactions)? To address this question, we have performed atomistic non-equilibrium molecular dynamics simulations of covalently connected (10, 0) single-wall carbon nanotubes submerged in epoxy matrix surroundings, focusing on their transverse heat transport. Here, we have explored the effect of the length of covalently bonded connectors; and the concentration of covalent bonds, to investigate the thermal interface resistance in the transverse direction of nanotubes. Our results suggest that the formation of covalent bond between nanotubes significantly decreases thermal interface resistance (when compared to just van der Waals interactions). The results also suggest that as the concentration of the covalent bond increases, the thermal interface resistance decreases for all studied cases, saturating towards higher concentration.1. J. Che, T. Cagin and W. A. Goddard III, Nanotechnology, 11, 65, (2000).2. X.-L. Xie, Y.-W. Mai, X.-P. Zhou, Materials Science and Engineering, R49, 89–112, (2005); F. H. Gojny, M. H. G. Wichmann, B. Fiedler, I. A. Kinloch, W. Bauhofer, A. H. Windle, K. Schulte , Polymer, 47, 2036–2045, (2006); W. Bauhofer, J. Z. Kovacs Composites Science and Technology, doi:10.1016/j.compscitech.2008.06.018.3. P. L. Kapitza, J. Phys (USSR), 4, 181, (1941).
11:00 AM - **T8.6
Thermal Rectification by Ballistic Phonons in Asymmetric Nanostructures.
John Miller 1 , Wanyoung Jang 1 , Chris Dames 1 Show Abstract
1 Mechanical Engineering, University of California, Riverside, Riverside, California, United States
A thermal rectifier transports heat more favorably in one direction than in the reverse direction. One approach to thermal rectification is asymmetric scattering of phonons and/or electrons, similar to suggestions in the literature for a sawtooth nanowire [Saha, Shi, & Prasher, IMECE 2006] or a 2-dimensional electron gas with triangular scatterers [Song et al., PRL 80, 3831]. To model the asymmetric heat transport in such nanostructures, we have used phonon ray-tracing and a Landauer-Buttiker method, focusing on nanostructure sizes that are small compared to the bulk mean free path. At low thermal bias, the phonon distribution function is very close to the usual isotropic equilibrium (Bose-Einstein) distribution, and there is no thermal rectification. In contrast, at large temperature gradients, the anisotropy in the phonon distribution function becomes significant, and the resulting heat flux vs. temperature curve (analogous to the I-V curve of an electrical diode) reveals large thermal rectification. Possible experiments to observe this novel phenomenon will also be discussed.
11:30 AM - **T8.7
Theory of Thermal Conductivity of Micro- and Nano-structured Materials.
Gyaneshwar Srivastava 1 Show Abstract
1 School of Physics, University of Exeter, Exeter United Kingdom
We start with a brief discussion of different theories of lattice thermal conductivity. We then present a theory of thermal conductivity of micro- and nano-structured materials, based on Callaway's model relaxation time approach and a detailed account of relevant phonon scattering processes, including three-phonon Normal and Umklapp interactions. The theory is applied to present a quantitative investigation of the magnitude and temperature variation of the conductivity for CVD diamond films , suspended GaAs nanostructures , Si nanowires, and AlN micro- and nano-ceramics [3-5]. Experimentally observed results are explained and in some cases predictions are made.We quantify the enhancement of the conductivity of diamond with isotopic purity. Experimentally observed dip in low temperature conductivity curve for HFCVD diamond films  is explained to arise from a crossover from the Rayleigh-like scattering of long wavelength phonons to the geometrical scattering of short wavelength phonons. Our investigations suggest that the severe reduction in the low-temperature conductivity of suspended GaAs undoped nanobeams fabricated by Fon et al  is mainly due to diffuse surface scattering of phonons. Additional reduction in the conductivity of doped nanobeams is due to a joint effect of phonon scatterings from point impurities and donor electrons. The theory is also applied to provide a quantitative study of the enhancement of the thermal conductivity of AlN ceramics by nano-scale processing. We predict that the room temperature conductivity of commercially available microcrystalline AlN  can increase by more than 150% due to insertion of nano-sized AlN particles and of Yttria (Y2O3) and CaO as sintering additives. We also predict that the room temperature conductivity of AlN nanoceramics  can increase by more than 100% due to inclusion of Y2O3 as sintering additive. However, the conductivity enhancement resulting from these processing techniques is insufficient to push AlN ceramics into the 'high thermal conductivity' category to which crystalline AlN belongs.Finally, we present a brief discussion of how the theory might be modified for a quantitative analysis of phonon transport in films and wires. D. T. Morelli, C. Uher, and C. J. Robinson, Appl. Phys. Lett. 62, 1085 (1993); S. Barman and G. P. Srivastava, Phys. Rev. B 71, 073301 (2006); ibid J. Appl. Phys. 101, 123507 (2007). W. Fon, K. C. Schwab, J. M. Worlock, and M. L. Roukes, Phys. Rev. B 66, 045302 (2002); S. Barman and G. P. Srivastava, Phys. Rev. B 73, 205308 (2006). J. Qiu, Y. Hotta, K. Watari, and K. Mitsuishi, J. Am. Ceram. Soc. 89, 377 (2006). M. L. Panchula and J. Y. Ying, J. Am. Ceram. Soc. 86, 1121 (2003). A. AlShaikhi and G. P. Srivastava, J. Appl. Phys. 103, 083554 (2008).
12:00 PM - T8.8
Understanding the Thermo-Mechanical Coupling between AFM Tip and Sample
Anna Morozovska 2 , Eugine Eliseev 3 , Maxim Nikiforov 1 , Sergei Kalinin 1 Show Abstract
2 , Inst Semicond Phys, Kiev Ukraine, 3 , Natl Acad Sci Ukraine, Kiev Ukraine, 1 , ORNL, Knoxville, Tennessee, United States
The progress in local thermal analysis (LTA) in the last decade allowed studies of thermo mechanical properties of the materials at sub-100 nm level. All experimental techniques providing sub-100 nm resolution involve the contact of heated tip and the sample, AFM with heated probe, heated tip during nanoindentation etc. In order to understand thermomechanical properties frequency response of mechanical response on thermal gradient needs to be known.In this work we discuss Hertzian and Maugis contact mechanics and its effect on mechanical displacement caused by local heating, as well as temperature distributions for the case of Newton law boundary conditions and for the case of mixed boundary conditions.This work provides quantitative basis for recently developed methods of measuring temperature dependencies of mechanical properties such as acoustic atomic force microscopy (AFAM) with heated tip and localized measurements of thermal expansion coefficient. Theoretical results will be illustrated by experimental data of measuring mechanical properties of poly-ethylene terephthalate (PET) and poly-ethylene terephthalate glycol (PETG) using AFAM with heated tip and localized measurements of thermal expansion coefficient.This Research at Oak Ridge National Laboratory's Center for Nanophase Materials Sciences was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.
12:15 PM - T8.9
Interfacial Heat Conduction Using Boltzmann Transport Simulation with Atomistic Transmission Functions.
Zhen Huang 1 , Dhruv Singh 1 , Jayathi Murthy 1 , Timothy Fisher 1 Show Abstract
1 , Purdue University, West Lafayette, Indiana, United States
The BTE (Boltzmann Transport Equation) had been successfully used to predict ballistic phonon transport in bulk semiconductors including silicon and germanium. However, in a composite system the method requires external inputs to include accurate boundary conditions at internal interfaces. The AGF (Atomistic Green's Function) method is particular adept at addressing interfacial heat transfer problems. The method computes phonon transmission functions through interfaces using the harmonic approximation for phonon dynamics. In this work, a non-gray BTE model is developed and applied to investigate the thermal conductivity of a 2D composite domain. The interfaces of dissimilar materials are treated as a boundary. The frequency-dependent transmission function from the AGF method is provides the input to the heat flux cross the interface. The energy intensity crossing the plane contains the interfacial temperature information. The results are compared to reported BTE results under the diffuse scattering limit. Also, transport through Si/Ge superlattices is studied and compared to the thermal conductivity of the constituent materials. The effect of superlattice periodicity highlighted in the results. The results reveal that the superlattice structure can exhibit markedly distinct thermal behavior as compared its bulk materials.
12:30 PM - T8.10
Generation and Study of Nanoengineered Heat Pipe.
Tao Deng 1 , Kripa Varanasi 1 , Boris Russ 1 , Hendrik De Bock 1 , Pramod Chamarthy 1 , Brian Rush 1 , Gary Mandrusiak 1 , Shakti Chauhan 1 , Stanton Weaver 1 , Frank Gerner 2 Show Abstract
1 , GE Global Research Center, Niskayuna, New York, United States, 2 , University of Cincinnati, Cincinnati, Ohio, United States
Thermal management of nanoelectronic devices becomes more important and difficult as such devices use more nanosized components with increased power densities. The core challenge in the thermal management is the capability to remove heat from the device while maintaining acceptable component operating temperatures. Conventionally, bulk materials, such as copper, are used as heat sinkers to manage the heat generated by the active devices. With the rising demand for the heat dissipation, conventional approaches will not be able to meet the thermal requirement and the need for alternative approaches thus is increasing. Heat pipes are devices to facilitate heat transport that have seen increased usage to address this challenge. Heat pipes take advantage of two-phase heat transfer in the transportation of heat: heat is extracted from the heat-generating devices when liquid phase evaporates into vapor phase and heat is ejected as vapor phase condenses into liquid phase. A heat pipe consists of a hollow casing with an internal wick structure, an evaporator section, a condenser section, and a vapor channel. This presentation will discuss the benefits of introducing nanostructures to common heat pipes and focus on the experimental design and fabrication of nanoengineered heat pipes. Possible usage of such nanoengineered heat pipes in applications that involve extremely high external acceleration will also be discussed. In such applications, nanostructures are particularly needed to generate enough capillary force to transport the liquid from condenser region to evaporator region for high performance functional heat pipes.