Subhash L. Shinde Sandia National Laboratories
Gyaneshwar P. Srivastava University of Exeter
Jacob Khurgin Johns Hopkins University
Yujie J. Ding Lehigh University
CC1: Phonon Transport in Bulk Materials
Monday AM, November 30, 2009
The Fens (Sheraton - 5th Floor)
9:45 AM -
Opening Remarks by Subhash L. Shinde.
9:50 AM -
Phonon Transport in Bulk Materials -- Comments by Gyaneshwar Srivastava
10:00 AM - **CC1.1
Lifetimes of Phonons in Si from 50 GHz to 15 THz.
David Cahill 1 Show Abstract
1 Department of Materials Science and Engineering, University of Illinois, Urbana, Illinois, United States
One of the great challenges for understanding heat transport in materials is the fact that the thermal transport coefficients are integrals over all of the thermal excitations. In this talk, I will discuss experimental methods that can be used to measure the lifetimes of vibrational modes in Si crystals near room temperature. Using picosecond acoustics, we have shown that the lifetimes of phonons at frequencies near 100 GHz, 5 nsec, are controlled by relaxational damping. Using time-resolved incoherent anti-Stokes Raman scattering, we find that the lifetime of 15 THz optical phonons is 1.6 ps. To probe the lifetimes of phonons with intermediate frequencies near 1 THz, we have developed a novel “thermal conductivity spectroscopy” that enables measurements of the distribution of mean-free-paths of phonons that dominate heat transport. From these measurements we find that the dominate heat carrying modes in SiGe alloys have mean-free-paths on the order of 300 nm, and therefore have lifetimes on the order of 50 ps.
10:30 AM - CC1.2
Phonon Transport in Silicon Nanowire at Low Temperature.
Jean-Savin Heron 1 , Thierry Fournier 1 , Natalio Mingo 2 , Olivier Bourgeois 1 Show Abstract
1 Institut Neel , CNRS, Grenoble France, 2 LITEN, CEA, Grenoble France
We report the measurement of thermal conductance of silicon nanowires at low temperature. It is demonstrated that the roughness at the nanometer scale plays a crucial role for the phonon transport in low dimensional samples. Using e-beam lithography, mechanically suspended nanowires of size 100nm by 200nm and 10μm long have been nanofabricated. Their thermal properties have been measured using the 3-omega method between 0.3K and 6K . The change in the temperature behaviour of the thermal conductance (quadratic temperature dependence of K(T)) is a signature of an intermediate regime lying between the classical Casimir regime and the quantum regime. The dominant phonon wavelength λdom is of the same order of magnitude as the mean roughness amplitude η of the surfaces of the nanowire. The Casimir-Ziman model is used to show that this specific behaviour originates in mesoscopic samples where the dominant phonon wavelength becomes commensurate to the characteristic length of the roughness of the nanowire surfaces . As the temperature is lowered λdom is increasing becoming bigger than η. Then as at high temperature the phonon transport is dominated by diffusion, at low temperature most of the phonons are specularly reflected at the surface. The phonon transport becomes more and more ballistic; the mean free path increases and becomes bigger than the section of the nanowire. In that quasi ballistic regime, at the lowest temperature, it has been also observed that the contact resistance between the nanowire and the pads are limiting the thermal transport when the dominant phonon wave length equals the size of the contact.As a conclusion, three different regimes are observed between 0.3K and 6K. At the lowest temperature (0.3K to 1K) the measured thermal conductance is the contact resistance, at intermediate temperature, the temperature variation of the thermal conductance is the consequence of the ballistic transport of phonon. This is modelize by the temperature dependence of the phonon mean free path. Finally, at high temperature (above 3K) the phonon transport is largely diffusive like in the Casimir regime. O. Bourgeois, Th. Fournier, and J. Chaussy, Measurements of the thermal conductance of silicon nanowires at low temperature, J. Appl. Phys. 101, 016104 (2007) J.-S. Heron, T. Fournier, N. Mingo and O. Bourgeois, Mesoscopic surface effects on the phonon transport in silicon nanowire, Nano Letters 9, 1861 (2009).
10:45 AM - CC1.3
Thermal Conductivity of Silicon-germanium Alloys from First-principles.
Jivtesh Garg 1 , Nicola Bonini 2 , Nicola Marzari 2 Show Abstract
1 Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Thermoelectric materials will become commercially viable for converting heat into electricity and for refrigeration once their figure of merit (ZT) is improved. One key approach to increase performance is to reduce thermal conductivity - e.g. in alloys it is lower than the binary endpoints due to increased scattering induced by strain and disorder. Understanding the thermal conductivity of complex materials is also important in other applications - from reducing hot-spot temperatures in electronic chips to better thermal-insulation materials. Here, we have calculated the thermal conductivity of silicon-germanium alloys using ab-initio density functional perturbation theory. The electronic structure of the alloy is studied with the virtual crystal approximation and the single mode relaxation time approximation; perturbation theory up to the third order provides phonon lifetimes, and disorder effects are taken into account by ensemble averages over configurations with random mass disorder. The contribution of acoustic and optical phonons to the thermal conductivity is also presented, together with the phonon mean free paths. Thermal conductivity of Si/Ge superlattices is also reported along with its dependence on the layer thickness. The effect of disorder in individual layers on the thermal conductivity is presented as well.
11:30 AM - CC1.4
Optimal Matching of Thermal Vibrations into Carbon Nanotubes in Theory and Simulations.
K.g.s.h. Gunawardana 1 , Kieran Mullen 1 Show Abstract
1 physics and astronomy, University of oklahoma, Norman, Oklahoma, United States
Carbon nanotubes (CNTs) are promising candidates to improve the thermal conductivity of nano-composites. The main obstacle to these applications is the extremely high thermal boundary (Kapitza) resistance between the CNTs and their matrix. In this theoretical work our goal is to maximize the heat flux through the CNT by functionalizing their ends. We develop a theoretical continuum model in which we vary the elasticity and density from a soft medium to a hard medium so as to maximize the transmission of thermal vibrations. We calculate the transmission coefficients of longitudinal acoustic waves in one dimension using a scalar wave equation and the heat flux was estimated using the Landauer transport formula. We numerically determined the optimal continuous variation of elasticity and density with position for different geometries. We develop analytical approximations to study the transmission amplitude in the low and high frequency limits. To probe the limitations of the continuum model an atomistic simulation is carried out. In this model a harmonic 1D atomic chain is connected to heat baths at the ends through an interface in which the atomic mass and the force constants matches to those of the optimal variation. We observe the heat flux through the channel varying the number of atoms in the channel and the interface and compare it to the continuum calculations. We also investigate the effect of anharmonic interactions.
11:45 AM - CC1.5
Optical Observations of Strong Thermoelectric Thomson Effect on Self-heated Nanocrystalline Silicon Microwires.
Gokhan Bakan 1 , Niaz Khan 1 , Helena Silva 1 , Ali Gokirmak 1 Show Abstract
1 Electrical and Computer Engineering, University of Connecticut, Storrs, Connecticut, United States
Thermoelectric Thomson heat is predicted and related to the other two thermoelectric phenomena, Seebeck and Peltier effects, by Kelvin relations. This thermoelectric effect results in heat transport due to a charge current flow in a varying temperature profile in a single material. Hence, it results in heating and cooling. Asymmetry in light emission from uniform silicon structures with symmetric thermal boundary conditions is attributed to this effect [1, 2]. We report optical observations of asymmetric heating and melting of uniform nanocrystalline silicon (nc-Si) microwires when large current densities (>10 MA/cm2) are forced through them. The microwires were fabricated on highly doped, n- and p-type thin films of nc-Si. Metal contacts and extensions were also formed to build symmetric and reliable contacts . Videos of the microwires under electrical stress are recorded with a high-speed camera (up to 600 fps) integrated with a high-magnification optical microscope. The captured videos of n-type wires show that the brightest (hottest) spot on the wire consistently shifts towards the end of the wire with lower electrical potential.SEM studies on wires which experience these extreme long duration voltage stresses show fractures at location where the highest light intensity was observed. If the polarity of the voltage is reversed on a wire that is previously stressed, the brightest point appears where the fractures were initially formed and it moves to the other end of the wire during the stress. SEM images of these wires show fractures on both ends. The optical observations on p-type wires are opposite of that of n-type wires – the hottest spot is closer to the end of the wire with higher electrical potential. Hence, the hottest spot is always closer to the end where the majority charge carriers enter. Lattice thermal conductivity of the wire degrades at this end which is attributed to strong interaction between electrical carriers and phonons traveling in opposite directions. Conversely, thermal conductivity at the other end increases, since both electrical carriers and phonons carry heat to the contact pads, which act like heat sinks. Mathematical model of the wires including thermoelectric effects is constructed and computed using the parameters available in the literature. Computational results are inline with experimental observations.  C. H. Mastrangelo, J. H. J. Yeh and R. S. Muller, "Electrical and optical characteristics of vacuum-sealed polysilicon microlamps," IEEE Trans. Elect. Dev., vol. 39, pp. 1363-1375, 1992.  A. Jungen, M. Pfenninger, M. Tonteling, C. Stampfer and C. Hierold, "Electrothermal effects at the microscale and their consequences on system design," J Micromech Microengineering, vol. 16, pp. 1633-1638, 2006.  G. Bakan, A. Cywar, H. Silva and A. Gokirmak, "Melting and crystallization of nanocrystalline silicon microwires through rapid self-heating," Appl. Phys. Lett., 2009.
12:00 PM - CC1.6
Modeling of Phonon Thermal Transport in Semiconductor Heterostructures and Nanowires.
Jie Zou 1 Show Abstract
1 Physics, Eastern Illinois University, Charleston, Illinois, United States
Phonon thermal transport in semiconductor nanostructures has attracted significant attention in recent years. “Phonon engineering” suggests that it is possible to control phonon heat conduction in coated (or layered) semiconductor nanostructures through tuning or engineering phonon dispersion relations in those structures. To elucidate the effect of phonon engineering on heat conduction, we report detailed modeling of lattice thermal conductivity in an AlN/GaN/AlN heterostructure, i.e., a GaN thin film coated with AlN cladding layers, for varied core or cladding layer thicknesses. Thermal conductivity is derived based on the solution of the phonon Boltzmann equation in the relaxation-time approximation. Phonon dispersion relations are obtained in the elastic continuum approximation using a finite-difference numerical method. Quasi-two-dimensional phonon density of states is derived using the actual phonon dispersions. We have demonstrated that partial phonon confinement leads to a higher thermal conductivity in an AlN/GaN/AlN heterostructure than that in a single GaN thin film of a comparable size. Such thermal conductivity can also be tuned higher or lower by adjusting the core or cladding layer thickness. Obtained results have quantitatively shown that it is possible to improve heat conduction in semiconductor nanostructures through phonon engineering. The results are important for thermal management of nanoscale devices. More recently, Guthy et al. reported measurements of thermal conductivity of individual, free-standing GaN nanowires. The observed thermal conductivity is shown to be lower (by a factor of ~5 in some case) than existing theoretical predictions. To explain the observed unusually low measurement, here, we also report on the development of a detailed model for the thermal conductivity of GaN nanowires. Compared to previous models, we include the effects of impurities, boundary scattering, modification of the phonon dispersion relations and phonon group velocity, and also the following important factors: (i) change in the phonon density of states (a quasi-one-dimensional density of states as compared to a three-dimensional Debye density of states) and (ii) change in the non-equilibrium phonon distribution in a nanowire (phonon redistribution due to diffuse boundary scattering). Comparisons with experimental data are also made and analyzed.
CC2: Phonon Transport in Nanostructures I
Monday PM, November 30, 2009
The Fens (Sheraton - 5th Floor)
2:30 PM - **CC2.1
Phonon Engineering with Graphene and Graphene Multi-Layers.
Alexander Balandin 1 , S. Ghosh 1 , D. Nika 1 , E. Pokatilov 1 Show Abstract
1 Department of Electrical Engineering, University of California at Riverside, Riverside, California, United States
Phonon engineering was initially introduced as controlled modification of the phonon spectrum and phonon transport in nanostructures made of conventional semiconductors . The length scale of the structures comparable to the thermal phonon wavelength allowed one to achieve the phonon confinement effects at room temperature (RT). The newly developed synthesis and manufacturing techniques have greatly expanded the list of available materials for phonon engineering and created opportunities for designing devices with the functionality enhanced via tuning of phonon behavior. The “graphene revolution” started by the first micro-mechanical exfoliation and measurements of the single atomic layer graphene by the Manchester, U.K. – Chernogolovka, Russia group  expanded the range of materials available for phonon engineering [3-6]. In this talk I will overview our recent experimental results, which indicate that graphene is an excellent conductor of heat [3-4]. In order to measure the thermal conductivity of the suspended graphene flakes we developed on original non-contact optical technique based on the micro-Raman spectroscopy. The amount of heating 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 position of the G peak as a function of the sample temperature was measured independently to allow one to use the micro-Raman spectrometer as a “thermometer”. Our measurements revealed that the single-layer graphene has an extremely high room-temperature thermal conductivity exceeding ~3080 W/mK . Most of heat was carried out by acoustic phonons rather than electrons. We also determined that the thermal conductivity of graphene depends strongly on the linear dimensions of the graphene flakes and the number of atomic layers, thus allowing for tuning the phonon thermal conductivity over the larger range at RT. We explained the enhanced thermal conductivity of graphene as compared to the basal planes of bulk graphite from the first principles [5-6]. Unlike in bulk graphite the phonon transport in graphene is two-dimensional (2D) all the way down to zero phonon frequency. The extremely high thermal conductivity of graphene coupled with its flat geometry and demonstrated ability for functioning in Si-based devices make graphene promising material for heat removal from nanoelectronic circuits.The work in Balandin group was supported, in part, by AFOSR grant on Phonon Engineering and DARPA – SRC Focus Center Research Program (FCRP) through its Functional Engineered Nano Architectonics (FENA) center and Interconnect Focus Center (IFC).  A. Balandin and K.L. Wang, Phys. Rev. B, 58, 1544 (1998). K.S. Novoselov, et al., Science, 306, 666 (2004); A.A. Balandin, et al., Nano Letters, 8, 902 (2008) S. Ghosh, A.A. Balandin, et al., Appl. Phys. Lett., 92, 151911 (2008).  D.L. Nika, E.P. Pokatilov, A.S. Askerov and A.A. Balandin, Phys. Rev. B, 79, 155413 (2009). D.L. Nika, S. Ghosh, E.P. Pokatilov and A.A. Balandin, Appl. Phys. Lett., 94, 203103 (2009).
3:00 PM - CC2.2
Thermal Energy Transport by Phonons in Nanostructured Materials.
Martin Maldovan 1 , Edwin Thomas 1 Show Abstract
1 , Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
As typical length scales are reduced, coherent interface scattering of phonons in nanostructures becomes increasingly important. In this paper, we concentrate on the coherent regime and propose theoretical methods for calculating the thermal conductivity in nanoscale materials due to phonon transport. We discuss heat capacities, group velocities, and scattering times for phonons at high and low temperatures. Structured nanoscale materials that can control the flow of phonons provide new capabilities for energy and power conversion. For example, by means of the rational structural design, the thermal conductivity can be reduced and the efficiency of thermoelectric materials, which reversibly convert thermal and electrical energy, can be enhanced.
3:15 PM - CC2.3
Computational Studies of Lattice Thermal Conductivities in SiGe Nanostructures.
Maria Chan 1 2 , Davide Donadio 3 , Giulia Galli 3 , Gerbrand Ceder 1 Show Abstract
1 Materials Science and Engineering, MIT, Cambridge, Massachusetts, United States, 2 Physics, MIT, Cambridge, Massachusetts, United States, 3 Chemistry, UC Davis, Davis, California, United States
Minimizing the thermal conductivity in silicon-germanium nanostructures is of particular interest for thermoelectric applications. Since proposed by Hicks and Dresselhaus in the 1990's (1), lower dimensional systems such as thin films (2), superlattices (3), nanocomposites (4) and nanowires (5) have been actively investigated for improved thermoelectric properties. Significant reductions have been measured in the lattice thermal conductivities of SiGe nanostructures compared to bulk values (4,6). Given the variations in nanostructure types and sizes, as well as modified phonon spectra and transport properties in the nanoscale, atomistic modeling is necessary in order to understand, compare and optimize the phonon transport in these different SiGe nanostructures.We report on the lattice thermal conductivity, obtained using classical molecular dynamics (MD) computations in the Kubo-Green approach, of a variety of SiGe nanostructures including core-shell nanowires, nanoparticle inclusions and superlattices. The effectiveness of different nanostructure types in the reduction of lattice thermal conductivity is compared. We present MD results suggesting the existence of a minimal thermal conductivity with respect to nanostructure size. For nanoparticle inclusions with an amorphized nanoparticle-matrix interface, we find an additional degree of freedom for the reduction of the thermal conductivity in the relative nanoparticle-matrix crystal orientation. Implications of our computational results for nanostructured thermoelectric materials are discussed.
References:(1) L. D. Hicks and M. S. Dresselhaus, Phys. Rev. B 47, 12727 (1993); Phys. Rev. B 47, 16631 (1993). (2) R. Venkatasubramanian, E. Siivola, T. Colpitts, and B. O’Quinn, Nature 413, 597 (2001).(3) Y. Wu, R. Fan, and P. Yang, Nano Lett. 2, 83 (2002).(4) For a review, see M. S. Dresselhaus et al, MRS Proceedings 2005.(5) A. Abramson et al, J. Microelectromech. Syst. 13, 505 (2004).(6) A. I. Hochbaum et al Nature 451, 163 (2008).
3:30 PM - **CC2.4
Acoustic Transport and Electron-phonon Interactions in Vertically Grown Nanorod Arrays.
Masashi Yamaguchi 1 , Jianxun Liu 1 , Dexian Ye 1 , Toh-Ming Lu 1 Show Abstract
1 Physics, Applied Physics & Astronomy, Rensselaer Polytechnic Institute, Troy, New York, United States
Nanoscale confinement effects on acoustic transport and electron-phonon coupling in nanorod arrays were studied by using GHz-THz acoustic spectroscopy and ultrafast transient reflectivity spectroscopy. We present three experimental results on (1) phonon transport along nanorods in GHz frequency range, (2) electron-phonon coupling in copper nanorod arrays, and (3) mechanical resonant vibrations in submicron scale spirals grown on substrate. We have designed a model structure to study the phonon transport along the long axis of nanorods with the use of acoustic spectroscopy. The structure consists of three layers: substrate, metal transducer and nanorod array layers. Acoustic phonons are excited by photothermal process using femtosecond laser pulses in metal transducer layer deposited on sapphire substrate. Generated acoustic pulses propagate through the metal transducer layer, then they are partially reflected at the interface, and the rest continues to propagate into the nanorod arrays. Acoustic phonons can be detected as an echo signal at the transducer when the pulse returns, or can be detected while the phonons are propagating through the nanorod. Amorphous silicon nanorods were grown by oblique angle deposition technique with substrate rotation. Nanorod arrays were vertically grown on the substrate, and samples of different length in the 100-400 nm range and the diameter of about 50 nm were used for the measurements. We have observed acoustic pulses travelling a round trip and double round trips inside the silicon nanorods. Furthermore, acoustic pulses couple to the bending motion of the nanorods, and the excitation of the bending mode by acoustic phonons was observed. Such coupling of the acoustic phonons and the resonant mechanical mode is an additional energy dissipating channel of acoustic phonons in the nanostructure.Vibrational properties of nanostructure arrays with rather complicated unit structure have been studied. Periodic and random arrays of single-turned silicon submircron spirals were grown using the oblique angle deposition (OAD) technique. Resonant vibrational modes of the submicron spirals were coherently excited by femtosecond laser pulses. Excitation of multiple harmonics of the resonant vibrations has been observed, and the mode patterns of the excited vibrations in the submicron spirals have been calculated.Electron-phonon interaction in copper nanorod arrays has been studied using ultrafast transient reflectivity spectroscopy with both resonant and off-resonant probe to d-band to Fermi-level transition. Slanted nanorod arrays 10 nm - 50 nm in diameter were fabricated by newly developed deposition technique. The use of a variable probe wavelength over the transition energy range suggested the modification of electronic structure in slanted nanorod arrays with relatively large diameter.
4:30 PM - CC2.5
Investigating Phonon Transport using Micro-devices with Fully Integrated Silicon Nanowires.
Karma Sawyer 1 , Renkun Chen 1 , Baoling Huang 1 , Kedar Hippalgaonkar 1 , Bair Budaev 1 , Arun Majumdar 1 2 Show Abstract
1 Mechanical Engineering, University of California, Berkeley, Berkeley, California, United States, 2 , Lawrence Berkeley National Laboratory, Berkeleyc, California, United States
Traditional thermoelectric materials are heavily doped bulk semiconductors such as Bi2Te3 and its alloys. However, despite recent advances in nanostructuring these materials, they are still not used for power generation on a large scale because the raw materials are rare and expensive. Recently roughened silicon nanowires with ultralow thermal conductivity have been discovered, and arrays of these nanowires show promise for use as high-performance, scalable thermoelectric materials. However, the mechanism for transport in the nanowires is still unknown. Here we fabricate silicon nanowires and suspended micro-heaters from a single silicon-on-insulator wafer. In this way, we are able control the key nanowire variables, such as length, diameter and roughness, and experimentally verify the mechanism for phonon transport in the wires.
4:45 PM - CC2.6
Transparent and Thermally Conducting Nanocomposites from Ultra-long Boron Carbide Nanowires and Poly(methyl methacrylate).
Sung-Ryong Kim 1 , Varun Gupta 2 , Seung-Won Yim 1 , Manish Chhowalla 2 Show Abstract
1 Dept. of Polymer Sci. & Eng., Chungju National University, Chungju Korea (the Republic of), 2 Dept. of Materials Sci. & Eng., Rutgers University, Piscataway, New Jersey, United States
The continuous development towards high performance organic devices on large area, flexible platforms holds promise for inexpensive electronics. Integrating organic materials with nanostructures also allows the fabrication of transparent and flexible devices. The realization of high performance integrated devices that are flexible and transparent will require the peripheral electronics components to also meet these criteria. That is, organic electronics will require power dissipation using materials that can be integrated into flexible and transparent platforms. Here we report thermally conducting nanocomposites that are also transparent from direct solution blending of ultra-long boron carbide (nominally B4C) nanowires (NWs) with poly(methyl methacrylate) (PMMA). Introduction of just 0.025 wt% of B4C NWs into PMMA leads to an almost one order of magnitude increase in the thermal conductivity and diffusivity (from 0.176 W/m-K and 1.26 x 10-3 cm2/sec, respectively, for PMMA to 1.11 W/mK and 8.538 x 10-3 cm2/sec, respectively, for the nanocomposite) along with a substantial increase (by ~ 40°C) in the glass transition temperature. The exceptionally low loading of B4C NWs into PMMA at which these properties are achieved in comparison to other filler materials is attributed to the very high aspect ratio (104 – 105) of the ultra-long NWs. Thus, unlike other thermal management composites, the very low loading makes B4C NW/PMMA transparent.
5:00 PM - CC2.7
Frequency-Dependent Monte Carlo Simulations of Phonon Transport in Nanostructured Bulk Silicon.
Qing Hao 1 , Gang Chen 1 , Gaohua Zhu 2 , Giri Joshi 2 , Xiaowei Wang 2 , Zhifeng Ren 2 , Ming-Shan Jeng 3 Show Abstract
1 Department of Mechanical Engineering, Massachusetts Institute of Technology , Cambridge, Massachusetts, United States, 2 Department of Physics, Boston College, Chestnut Hill, Massachusetts, United States, 3 , Industrial Technology Research Institute, Chutung, Hsinchu, Taiwan
In this work, phonon transport in nanostructured bulk silicon is investigated by Monte Carlo (MC) simulations considering the frequency-dependent phonon mean free paths (MFPs). A novel constant virtual wall temperature boundary condition is developed for the studied periodic structures. This allows us to calculate the lattice thermal conductivities of these structures with a minimum computational domain. Our code is first used to compute the in-plane lattice thermal conductivities of periodic porous silicon films. It has been found that the phonon size effects caused by the periodically arranged through-film pores can be remarkable even when the pore size and spacing are much larger than the average phonon MFPs. For silicon nanoparticle composite, the low thermal conductivities calculated by our frequency-dependent simulations agree well with the experimental results. This study shows the potential of achieving a high ZT in silicon by the nanostructuring approach.
Subhash L. Shinde Sandia National Laboratories
Gyaneshwar P. Srivastava University of Exeter
Jacob Khurgin Johns Hopkins University
Yujie J. Ding Lehigh University
CC3: Phonon Transport in Nanostructures II
Tuesday AM, December 01, 2009
The Fens (Sheraton - 5th Floor)
10:00 AM - **CC3.1
Confined Phonons Effects and Thermal Transport in Graphene-based Structures.
Jun Qian 1 , Ke Sun 1 , Mitra Dutta 1 , Michael Stroscio 1 Show Abstract
1 ECE Department, University of Illinois at Chicago, Chicago, Illinois, United States
Recently graphene has been championed as a promising material for high-performance electronics. In order to provide a basis for graphene-based electronics as well as a basis for modeling phonon modes and carrier-phonon interactions in graphene-based structures of finite size, thermal conductivity is modeled for selected graphene-based structures of finite size, phonon engineering in finite-sized graphene-based structures is discussed, confined phonon modes are presented for finite-sized graphene structures, and the role of phonon confinement in carrier-phonon interactions is discussed for confined phonons in graphene sheets of finite size. These results are obtained by with quantum confinement effects included [1,2] by using previous models for graphene-based structures [3-8]. This presentation will include confined LO-phonon modes in finite-sized graphene structures.References Michael A. Stroscio and Mitra Dutta, Phonons in Nanostructures, (Cambridge University Press, Cambridge, 2001), ISBN: 0521792797; also published in a Russia Language Edition by Dauka Press (2006) and in a Chinese Language Edition, Beijing (2005). Vladimir V. Mitin, Viatcheslav V. Kochelap, and Michael A. Stroscio, Quantum Heterostructures for Microelectronics and Optoelectronics, (Cambridge University Press, Cambridge, 1999). ISBN: 0521631777. P. G. Klemens, “Graphite, Graphene and Carbon Nanotubes”, Thermal Conductivity 26, 48-57, in Proceedings of the Twenty-Sixth International Thermal Conductivity Conference (ITCC), August 6-8, 2001 Boston, MA, edited by Ralph B. Dinwiddie (DEStech Publ., Lancaster, PA for ITCC, 2004); ISBN13: 978-1-932078-36-7.  G. Klemens, “Theory of Thermal Conduction in Thin Ceramic Films”, International Journal of Thermophysics, Volume 22, No. 1, 2001. G. Klemens and D. F. Pedraza, “Thermal Conductivity of Graphite in the Basal Plane”, Carbon, Volume 32, No. 4, p. 735-741, 1994. R. Nicklow, N. Wakabayashi, H. G. Smith, “Lattice dynamics of pyrolytic graphite”, Physical Review B, Volume 5, Number 12, 1972. Takahiro Yamamoto, Kazuyuki Watanabe, and Kazuaki Mii, “Empirical-potential Study of Phonon Transport in Graphitic Ribbons,” Phys. Rev., B70, 245402-1-6 (2004). R. Wallace, “The band theory of graphite”, Physical Review, Volume 71, Number 9, 1947.
10:30 AM - CC3.2
Phonon Propagation in Nanoparticles Dispersed in Liquids – A Model for Nanofluid.
Yoshiaki Kogure 1 , Toshio Kosugi 1 , Hideo Kaburaki 2 Show Abstract
1 , Teikyo University of Science and Technology, Uenohara, Yamanashi, Japan, 2 , JAEA, Tokai, Ibaraki, Japan
Strong enhancement of thermal conductivity of liquid by dispersing nanoparticles has been observed. Such a liquid is called nanofuluid. There is possibility of many application of nanofluids as heat conducting materials, but the mechanism of thermal conductivity enhancement is not yet understood. Molecular dynamics simulation on the phonon states and the phonon propagation at the interfaces of nanoparticles and liquid has been performed to investigate the fundamental nature of nanofluid. The simulations are performed for the systems consisted of Lennard-Jones liquid and nanoparticles. Nanoparticles are consisted of 10000 -- 30000 atoms, which have higher melting temperature than liquid. The nanoparticles are immersed in the liquid and stabilized. Atomic vibration is generated at the center of nanoparticle, and the magnitude of the velocity of all atoms is monitored. The power spectra of the vibration of surface atoms are calculated and elastic properties of nanoparticles are evaluated from the lower frequency component of the spectra. The energy transfer from solid to liquid are also calculated to estimate the heat conduction in nanofluid.
10:45 AM - CC3.3
Direct Study of the Thermal Conductivity in Aluminum Nanowires.
Nenad Stojanovic 1 2 , D. Maithripala 1 3 , Jordan Berg 1 2 , Mark Holtz 1 4 Show Abstract
1 Nano Tech Center, Texas Tech University, Lubbock, Texas, United States, 2 Mechanical Engineering, Texas Tech University, Lubbock, Texas, United States, 3 Mechanical Engineering, University of Peradeniya, Peradeniya Sri Lanka, 4 Physics, Texas Tech University, Lubbock, Texas, United States
The electrical conductivity of metallic films is known to decrease considerably from the bulk values with reducing thickness. This is generally attributed to reduced grain size, which scales with thickness below ~ 1 μm, and increased influence of surface scattering. Several studies have established that thermal conductivity also decreases in thin films relative to the bulk values. The thermal conductivity is important to establish in densely packed devices since dissipation of self-heating is critical in maintaining reliable performance. Direct measurement of this quantity is very difficult, since thin films are generally supported by a substrate and heat conduction takes place through the film and this underlying structure. Discriminating between heat conduction through the sample and through the support structure is more difficult in the case of nanowires since the net conductance through the nanowire is prospectively small compared to losses through the substrate. Nanoscale fabrication processes and preparation of devices on these length scales require direct studies of device-related physical properties, such as electrical or thermal conductivity.We report design, fabrication, and measurement of electrical and thermal conductivities for aluminum nanowires produced on a glass support structure. A nanofabricated electrothermal test structure is designed for directly measuring the thermal and electrical conductivity of aluminum nanowires near room temperature. The method combines a microelectrothermal test structure with a finite-element-based data analysis procedure. The test device consists of 30 nm thick gold structure, which serves as resistive heater and two symmetrical resistance temperature detectors on each side 1μm from the heater. The 1 micron long interdigitated aluminum wires are deposited on one side of the heater connecting it with one of the detectors. This non-symmetric structure is designed so that differences in detector temperatures can be directly attributed to the heat flow through the nanowires and the difference in the temperatures yields their thermal conductivity. To interpret the measurements, we develop a detailed thermal model using finite element analysis. Measurements are performed on 100 nm thick nanowires with 75, 100, and 150 nm widths. Separate structure for measuring electrical conductivity is also made on the same substrate using same process.It is found that both the electrical and thermal conductivities decrease as nanowire cross-section decreases. This is attributed to the effects of surface and grain boundary scattering which limits electron transport at this scale. Thermal conductivity is compared to electrical using the Franz-Wiedemann law, and heat transport from the relevant electron, phonon, and combined electron-phonon terms will be discussed including heat transfer across the aluminum-substrate interface. This work is supported by the National Science Foundation and J. F Maddox Foundation.
11:30 AM - CC3.4
Anisotropic Thermal Transport in Thin Si Membranes with Stretched Ge Quantum Dots for Thermoelectrics and Heat Dissipation.
Jean-Numa Gillet 1 , Bahram Djafari-Rouhani 1 , Yan Pennec 1 Show Abstract
1 Physics (IEMN), University of Lille 1, Villeneuve d'Ascq cedex France
The design of nanostructured semiconducting devices with optimal thermal properties and indirect electronic band gap (as those using the Si/Ge IV-IV couple) is currently one of the major challenges for on-chip cooling and thermoelectric (TE) energy conversion in nanoscale silicon-based architectures. These nanodevices should enable continuation of the historical integration pace given by the Moore's law in CMOS microelectronics. Epitaxial self-assembly is a major bottom-up technology to design germanium quantum-dot (QD) arrays in silicon. The Ge QDs stands on or are sandwiched between diamond-cubic (dc) Si thin layers. QD ordering can be three-dimensional (3D) to obtain high-density arrays. 3D Si/Ge supercrystals were recently proposed to obtain crystalline TE materials with an extreme reduction of the thermal conductivity λ that can be as low as 0.04 W/m/K (i.e. twice that of air) for specific size parameters of the QDs [Gillet, Mater. Res. Soc. Symp. Proc. 1172E, T06-02, 2009; Gillet et al., J. Heat Transfer 131, 043206, 2009]. In this paper, we present a theoretical investigation of a novel type of thermal membranous materials. Our suspended nanomaterial is made up of a dc-Si thin membrane covered by self-assembled Ge QDs with facets. The QDs are voluntary stretched according to the in-plane  direction so that their width in the perpendicular  direction is several-fold smaller. Owing to the QD constriction, heat transport is much weaker in the  direction (where the phonons are better confined) than in that . The proposed nanomaterial can be considered for the design of hybrid thermal devices. Indeed, after its insertion between hot and cold reservoirs at both membrane extremities, the nanomaterial behavior can be tuned from a thermal-insulating regime with a low λ to a dissipative regime with a higher λ. The transition between the two behaviors depends on the angle β between the heat flux and in-plane  direction. The membrane is assumed to be cleaved with respect to crystallographic directions given by β. The nanomaterial thermal conductivity is computed from the dispersion-curve diagram in the THz frequencies using both lattice dynamics and incoherent approach of the scattering relaxation times. We obtain a sigmoidal dependence of the λ vs. β curve. In a molecular-scale device, when β is close to 0° for a heat flux in the  direction, λ is as low as 0.7 W/m/K. However, λ increases to 3 W/m/K (i.e. by 4 to 5 folds) when β is close to 90° for a heat flux in the  direction. As a result, our nanomaterial could be used to design hybrid thermal devices that can operate as either (i) TE Peltier coolers and Seebeck generators with a low λ or (ii) dissipative Thermal Interface Materials (TIMs) with a high λ. The effects of the temperature and size parameters of the QDs and membrane will be as well analyzed.
11:45 AM - CC3.5
Thermal Transport in Silicon Nanostructures.
Davide Donadio 1 , Yuping He 1 , Giulia Galli 1 Show Abstract
1 Chemistry, Univ. Calif. Davis, Davis, California, United States
Silicon is one of the best known materials of our age, cheap and readily available, being the basic constituent of semiconductor electronics. It would therefore be highly desirable to broaden its utilization for, e.g. renewable energy applications. Recently, it has been proposed that Silicon may be engineered at the nanoscale to be an efficient thermoelectric material for use in solid state devices. Although a rather inefficient thermoelectric in its bulk form, at the nanoscale Si may become a poor heat conductor, while retaining good electronic conduction properties, and thus exhibit high efficiency in converting heat into electric current. However the fundamental reasons for the reported low heat conduction in Si nanostructures are not yet understood, and different interpretations has so far appeared in the literature. This motivates an extensive study of thermal transport in quasi-one dimensional systems, from Carbon nanotubes  to silicon wires , and in nanoporous silicon , by means of atomistic simulations based on molecular dynamics and lattice dynamics. D. Donadio and G. Galli, Phys. Rev. Lett. 99, 255502 (2007). D. Donadio and G. Galli, Phys. Rev. Lett. 102, 195901 (2009). J-H. Lee, et al. Appl. Phys. Lett, 91, 223110 (2007) and Nano. Lett., 8(11), 3750 (2008).
12:00 PM - CC3.6
Heat Transport in Nanostructured III-V Semiconducting Alloys.
Joseph Feser 1 , Dongyan Xu 1 , Arun Majumdar 1 Show Abstract
1 Mechanical Engineering, Univ. of California, Berkeley, Berkeley, California, United States
By epitaxially embedding nanostructures in a semiconducting matrix, long wavelength phonons are efficiently scattered leading to a dramatic reduction in thermal conductivity, which has significant implications for thermoelectric applications. Here we show that nanocrystals of ErAs embedded in InGaAlAs reduce the conductivity to ~1.8 W/mK, improving the figure-of-merit to ZT~1.5 @ 800K, making them a potentially good n-type medium-temperature thermoelectric. We also show that analogous p-type materials can be made utilizing ErSb nanostructures in InGaSb. We discuss the effect of making these nanostructures very dense (~10% by Vol) as well as ongoing efforts to model and optimize thermal transport in such systems.
CC4: Phonon Interactions I
Tuesday PM, December 01, 2009
The Fens (Sheraton - 5th Floor)
2:30 PM - **CC4.1
Interfaces and Reliability of GaN Electronic Devices.
Martin Kuball 1 , Athikom Manoi 1 , James Pomeroy 1 , Milan Tapajna 1 , Richard Simms 1 , Gernot Riedel 1 , Andrei Sarua 1 , Michael Uren 2 , Trevor Martin 2 , Erik Janzen 3 , Niklas Rorsman 4 , Umesh Mishra 5 Show Abstract
1 Center for Device Thermography and Reliability (CDTR), University of Bristol, Bristol United Kingdom, 2 , QinetiQ Ltd, Malvern United Kingdom, 3 , Linköping University, Linköping Sweden, 4 , Chalmers University, Göteborg Sweden, 5 , University of California Santa Barbara, Santa Barbara, California, United States
Interfaces play a crucial role for GaN power electronics. Acoustic phonon scattering at the heteroepitaxial GaN-on-SiC substrate interface can significantly hinder heat transfer across this interface from the GaN device into the heat sink, resulting in raised device temperatures, reducing device reliability. We will review the in Bristol developed techniques of Raman and time-resolved Raman thermography for sub-micron and nanosecond resolution temperature and stress analysis of semiconductor devices, with particular focus on assessing interfaces and their thermal properties, and will demonstrate implementation of improved interfaces in the GaN-on-SiC material system. The use of these techniques in combination with electroluminescence (EL) and electrical trapping analysis will also be presented to probe and study hot phonons and hot electrons near the device top interfaces of GaN HEMTs, including the relationship between device degradation and hot carriers, and implications for device reliability. Bristol acknowledges financial support from ONR Global, ESA, EPSRC and FP7.
3:00 PM - CC4.2
Theoretical and Simulation-based Predictions of Grain Boundary Kapitza Resistance in Semi-conductors.
Sylvie Aubry 1 , Patrick Schelling 2 , Chris Kimmer 3 , Xiaowang Zhou 4 , Reese Jones 4 Show Abstract
1 , Stanford University, Stanford, California, United States, 2 , University of Central Florida, Orlando, Florida, United States, 3 , University of Louisville, Louisville, Kentucky, United States, 4 , Sandia National Laboratories, Livermore, California, United States
This talk will present recent theoretical and simulation-basedcalculations of the thermal conductivity in GaN as well ascalculations of the Kapitza conductance in silicon grain boundariesusing improved numerical methods such as the direct heat flux, theGreen-Kubo and the lattice dynamics methods.A more accurate molecular dynamics calculation of thermal conductivityusing the direct heat flux method will be explained and applied toGaN. This method will be compared to the Green-Kubo approach. An improved lattice dynamics approach will also be presented andcompared to the direct heat flux method in the case of two differentgrain boundaries, a low scattering and a high scattering grainboundary in silicon. Theoretical predictions based on input from thelattice dynamics method appear to show reasonable agreement with thedirect heat flux method results for the high scattering grain boundarybut disagreement by a factor of about ten for the low scatteringboundary. We show why the apparent discrepancies are large and wedemonstrate how the theoretical predictions and molecular dynamicssimulation results can be compared in a consistent and meaningful way,thereby removing the apparent contradictions.
3:15 PM - CC4.3
Defect and Interface Scattering In Nanophononic Semiconductors.
Steven Hepplestone 1 , Gyaneshwar Srivastava 1 Show Abstract
1 School of Physics, University of Exeter, Exeter, Devon, United Kingdom
Recent technological advancements have lead to the development of new metamaterials. Superlattices are a subgroup of metamaterials, a number of which have recently been shown to have intriguing features such as possessing one or more one-dimensional phononic gaps [1,2,3]. Phononic structures, the vibrational analogues of photonic crystals, consist of two or more materials with contrasting physical properties, which result in one or more stop bands in the phonon dispersion relations. Whilst a considerable amount of work has been performed for calculating and measuring these dispersion relations and stop bands, 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, the scattering mechanisms for phonons need to be understood . For metamaterials, and particularly for phononics, new phonon scattering mechanisms, not applicable to bulk systems, are available, but are poorly understood. In particular, the mixing of atoms between the two different media, and different considerations that need to be made for isotopes, lead to the consider of new and complicated scattering mechanisms for these systems.
We will present a comprehensive theory for phonon scattering due to interface mixing and isotope scattering in nanophononic semiconductors, which can be applied to metamaterials in general. The theory presented is applied to Si/Ge superlattice systems and is compared with the bulk theory . We will discuss the mechanism for the scattering of phonon modes at the interface boundary due to interface mixing and present an expression for this scattering rate. We will also present the corrected form of isotope scattering for such mixed systems showing that the relative ratio of the two systems is critical to calculations, which agrees with . The rate of scattering of phonon modes is estimated from the application of Fermi's Golden Rule and uses realistic phonon dispersion relations which include both the optical and acoustic phonon modes. We show that our expression for the interface mixing scattering term is consistent with the result that the scattering rate is zero for bulk systems. Also, our expression for this term is strongest in thin superlattices as expected and decreases with an increase in period, reducing to zero for structures with a infinite period.
 Y. Ezzahri et al. Phys. Rev. B 75, 195309 (2007).
 A. Huynh et al. Phys. Rev. Lett. 97, 115502 (2006).
 S. P. Hepplestone et al. Phys. Rev. Lett. 101, 1105502 (2008).
 G. P. Srivastava, Physics of Phonons (Adam Hilger, Bristol: 1990).
 P. G. Klemens, Proc. Phys. Soc. London A 68A, 1113 (1955).
 W. Liu et al. J. Appl. Phys. 97, 073710 (2005).
3:30 PM - CC4.4
Estimating Thermal Boundary Conductance in the Limit of Interfacial and Material Disorder.
Thomas Beechem 1 , Patrick Hopkins 1 Show Abstract
1 , Sandia National Laboratories, Albuquerque, New Mexico, United States
Energetic transport across material boundaries is increasingly relevant as devices grow in their complexity multiplying not only the number of interfaces but also their influence on system performance as a whole. Devices designed for such varying applications as thermoelectrics, high power electronics, and thermal interface materials each are significantly influenced by the efficiency in which thermal energy crosses the numerous interfaces that compose these systems. As such, analytical tools that highlight the central physics determining the rate by which thermal energy crosses material interfaces, i.e., thermal boundary conductance (TBC), are useful in their ability to not only quickly estimate performance but to also elucidate the limits by which device designers may leverage this transport. The Diffuse Mismatch Model (DMM) has been widely utilized as one such analytical tool in the estimation of TBC. The model evaluates the transport by counting both the “flux” of phonons reaching a boundary and the ratio of the number of these phonons that pass through the interface. The total number of phonons transmitted through the boundary, in turn, determines the TBC of the system. Implicit in the prediction, however, is the assumption that the interface between the materials is perfect and that the materials on either side of the material are crystalline. The nature of a real, imperfect, interface is one of disorder, however, in which the assumed planar intersection of each material is, in reality, a finite volume of intermixing. Furthermore, the materials on either side of the boundary, especially in the case of metallic films, may exhibit a significant degree of disorder as well. As such, these distinctly non-crystalline characteristics of both the film and the interfacial region highlight the need to consider the ramifications of disorder on the thermal boundary conductance.In response, the current study extends the often employed Diffuse Mismatch Model (DMM) to account for disorder that is frequently present in the materials making up the interface as well as the boundary itself. By applying assumptions regarding the scattering rates and mean free paths of phonons within a disordered solid, the resulting modifications of the spectral density of states induce changes in both the number, and ratio, of forward scattered phonons incident on a surface, and hence predictions of the TBC. Combining these assumptions with an accounting of the distance over which disorder persists, the newly implemented Disorder Diffuse Mismatch Model (δ-DMM) is shown to be more capable of predicting the TBC over a range of temperatures and material systems.
4:15 PM - CC4.5
Phonon Engineering using Endohedral Nano Space in Clathrates.
Katsumi Tanigaki 1 , Jun Tang 1 , Kazumi Sato 1 , Ryotaro Kumashiro 1 Show Abstract
1 WPI/Physics, Tohoku University, Sendai Japan
Phonons as well as electrons and magnons play a very important role for controlling physical properties. Lattice phonons have been the main issues for electron-phonon interactions as well as for phonon scattering in controlling physical properties for long years so far. Recently, intra-cluster phonons are also considered to play an important role [1-3] and even more importantly the latter phonons can provide possible phonon engineering. Thanks to the inner nano spaces in polyhedral network compounds that can accommodate atoms, the atomic anharmonic motions showing large freedom can be generated and these are recently drawing much attention as a new collective mode of phonons. Clathrate compounds have the nano cage structure consisting of IVth group elements with shared faces arranged. Therefore, these materials can accommodate atomic elements to be confined. The endohedral atoms move under the potentials made by the cages and give rise to such rattling phonons featured by anharmonic oscillations. These phonons are greatly different from the conventional lattice phonons and may produce unique electron-phonon interactions. In this talk, we would like to show one of such good examples of electron-phonon interactions as well as a novel phonon scatterings, both of which are considered to leading to new physical properties. In the clathrate compounds, Ba24Si100 and Ba24Ge100, Ba atoms are confined in the polyhedral cage consisting of Si and Ge respectively. These materials are crystallographically identical, however the space inside the cage is different in size because of the framework elements with different covalent bond lengths . This situation leads to the different phonon modes associated with Ba. Actually, quite different electron density maps were observed between these two compounds using MEM/Rietveld analyses. The detailed electronic states have also been studied directly by soft x-ray photoelectron spectroscopy . Apparently, a good causality between the change in the Ba electron density map as a function of temperature and the electronic states was observed. We also describe phonons associated with the motions of Sr and Ba accommodated in the same framework of clathrates.  K. Tanigaki, et al, Nature Materials, 2, 653 (2004).  T. Rachi, K. Tanigaki, R. Kumashiro, J. Winter, H. Kuzmany, Chem. Phys. Letters, 409, 48 (2005).  T. Rachi, K. Tanigaki et al., Phys. Rev. B, 72, 144504 (2005).  T. Rachi , K. Tanigaki et al., J. Physics and Chemistry of Solids 67, 1334 (2006).  J. Tang, T. Rachi, and K. Tanigaki, Phys. Rev. B, 78, 085203 (2008).
4:30 PM - CC4.6
Exploring Electron-Phonon Interactions in Conjugated Polymers as a Fast Actuation Mechanism.
Andre Botelho 1 , Xi Lin 1 Show Abstract
1 Mechanical Eng, Boston University, Brookline, Massachusetts, United States
We perform ab-initio Hartree-Fock (HF) calculations to demonstrate conformation changes associated with multiple solitons, polarons, and bipolarons due to electron-phonon interactions from charge injection on trans-polyacetylene (t-PA) as a prototypical conjugated polymer. A comparison between the Su-Schrieffer-Heeger (SSH) and extended Hubbard models demonstrates a pi-band reaction to the soliton states due to electron-electron interactions. The combined efects of the soliton and the pi-band cause in-plane bending localized within the soliton region. With multiple charges, each subsequent soliton and anti-soliton bends the chain in the opposite directions, creating a wave-like pattern while maintaining a straight axis on average. Conformation changes are an intrinsic actuation mechanism that may be explored to overcome the current strain rate problem with conjugated polymer actuators; the electron-phonon coupling could allow for the defects to travel at the speed of sound.
4:45 PM - CC4.7
Measurement of the Surface Scattering of Thermal Phonons in Silicon Nanostructures.
John Sullivan 1 , T. Friedmann 1 , E. Piekos 1 , S. Shinde 1 , J. Wendt 1 Show Abstract
1 , Sandia National Laboratories, Albuquerque, New Mexico, United States
One of the biggest surprises that has emerged from recent measurements of the thermal conductivity of silicon nanowires has been the observed remarkable reduction in thermal conductivity as the dimensions or surface roughness of the nanowires are reduced to the few nm to few tens of nm length scales [Hochbaum, et al., Nature 451, 163 (2008) and Boukai, et al., Nature 451, 168 (2008)]. For these nanoscale structures, phonon mean free paths exceed the sample dimensions, and surface scattering dominates the thermal conductance up to temperatures well above room temperature. The nature of the phonon surface scattering, i.e. the probability of diffuse versus specular scattering, is of paramount importance in determining the phonon mean free path and, hence, the thermal conductance of the nanowire. To specifically address the question of surface scattering of thermal phonons, we have created a unique differential thermal conductance measurement structure to identify the diffuse and specular scattering probability in silicon nanostructures. The structure is formed from a silicon-on-insulator wafer and consists of three suspended silicon platforms with a center platform connected to its neighbors on each side by thin silicon nanoligaments that are formed by nanomachining. The ligaments are single crystal with a blade-like cross-section, with width, length, and depth of 100 nm, 1000 nm, and 2000 nm, respectively. The key aspect of the structure is that one nanoligament is straight while the other is bent. Phonons with a mean free path exceeding 1000 nm may travel without boundary scattering in the straight ligament, while these phonons would necessarily scatter from the surfaces of the bent ligament. The determination of specular versus diffuse scattering is made by measuring the differential heat flux across the two ligaments, using the center silicon platform as a common heater and measuring the differential temperature rise on the two outer platforms. With this differential approach, bulk effects common to both ligaments are cancelled and phonon-boundary interactions are highlighted. The measured differential heat flux is compared to Monte Carlo simulations of phonon transport, using the specular versus diffuse scattering probability as a fitting parameter. The measurements and simulations are performed as a function of ambient temperature, enabling tuning of the average phonon mean free path and determination of the temperature-dependent surface scattering process. This work was supported by the DOE Office of Basic Energy Sciences, Division of Materials Science and Engineering and by a Laboratory Directed Research and Development project at Sandia National Laboratories. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the US Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.
5:00 PM - CC4.8
Thermal Conductance of Metal-Graphite Interfaces.
Kimberlee Collins 1 , Aaron Schmidt 1 , Austin Minnich 1 , Gang Chen 1 Show Abstract
1 Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
The thermal interface conductance between metals and carbon nanotubes (CNTs), graphenes and other graphite-based materials is a subject of both fundamental and practical interest. Existing work suggests that the interfacial thermal resistance may be the limiting factor in measuring the thermal conductivity of the materials, and in the thermal management of carbon-based microelectronic devices. The interface between metal and the c-axis direction of highly ordered pyrolytic graphite (HOPG) provides insights on the thermal interfacial transfer between metal and the sidewalls of MWCNT and graphenes. We present measurements of thermal interface conductance between HOPG in the c-axis direction and several metals in the temperature range 77-300 K. Measurements are taken using transient thermoreflectance methods. Our work adds a valuable tool for interpreting thermal measurements on MWCNTs, and for resolving theory with experiment. AcknowledgementsThis work was supported by NSF Grant No. CBET-0506830, and DoD and NSF Graduate Research Fellowships for A. J. Schmidt and A. Minnich.
Subhash L. Shinde Sandia National Laboratories
Gyaneshwar P. Srivastava University of Exeter
Jacob Khurgin Johns Hopkins University
Yujie J. Ding Lehigh University
CC5: Phonon Interactions II
Wednesday AM, December 02, 2009
The Fens (Sheraton - 5th Floor)
10:00 AM - **CC5.1
Time Resolved Thermal Transport Measurements Across a Si-Si Twist Boundary.
David Hurley 1 , Matthew Fig 2 , Subhash Shinde 3 , Sylvie Aubry 4 Show Abstract
1 Physics, Idaho National Laboratory, Idaho Falls, Idaho, United States, 2 Basic Fuel Properties and Modeling, Idaho National Laboratory, Idaho Falls, Idaho, United States, 3 , Sandia National Laboratories, Albuquerque, New Mexico, United States, 4 Department of Mechanical Engineering, Stanford University , Stanford, California, United States
Lateral looking, time-resolved thermal wave microscopy was used to measure thermal transport in uncoated semiconductor bicrystals. The basis for this approach is to eliminate the obscuring influence of the diffusing electron-hole plasma by promoting fast carrier recombination. Preliminary results using this approach for a Si bicrystal will be presented. The sigma 29 bicrystal interface studied is perpendicular to the exposed sample surface. The sample was mechanically polished to introduce a thin, highly damaged layer to cause fast carrier recombination. Also, great care was taken to avoid inadvertently etching material near the region where the boundary intersects the exposed surface. Preferential etching of this region can cause a change in topography which, in turn, can result in an erroneous signal due to beam deflection. We measured the thermal wave phase profile as the probe beam is scanned across the bicrystal interface. The value of the Kapitza resistance associated with the interface was estimated by comparing experimental results to the results of an empirical finite element model. We also compared our experimental results with results from a molecular dynamics (MD) study. The experimentally measured Kapitza resistance is larger than that predicted using MD simulation. High resolution transmission electron microscopy was employed to reveal the atomic nature of the interface. The microscopy results will be used to reassess assumptions made in the MD simulation.
10:30 AM - CC5.2
Calculation of el-ph Interactions in Bulk Materials Using Maximally Localized Wannier Functions.
Matteo Salvetti 1 , Nicola Bonini 1 , Matteo Calandra 2 , David Parks 1 , Nicola Marzari 1 Show Abstract
1 , MIT, Cambridge, Massachusetts, United States, 2 , Institut de Minéralogie et de Physique des Milieux Condensés , Paris France
Phonon-mediated el-el interactions are the microscopic basis of low temperature superconductivity. The Eliashberg-Migdal theory formulates the problem in terms of el-ph matrix elements and corresponding el-ph linewidths. In principle, ab-initio DFT codes based on pseudo-potentials and plane-waves  can provide an accurate prediction of the el-ph linewidth spectrum over the entire Brillouin zone in bulk materials. In practice, fully converged calculations are often unattainable because of the required dense samplings of the Fermi surfaces and limitations in the available CPU time. The reformulation of the el-ph matrix problem using maximally localized Wannier functions [2,3] restricts the time-consuming part of the calculation to the determination of the phonon frequencies via linear perturbation DFT and allows the evaluation of the el-ph linewidths with k- and q-point samplings of millions of points. Here, we present results on the superconducting critical temperature Tc of Al and Nb crystals under various mechanically-strained configurations and show that even in such simple bulk systems the use of the Wannier basis approach is necessary to ensure accurate and reasonably fast calculations.REFERENCES S. Baroni, A. Dal Corso, S. de Gironcoli, P. Giannozzi, C. Cavazzoni, G. Ballabio, S. Scandolo, G. Chiarotti, P. Focher, A. Pasquarello, K. Laasonen, A. Trave, R. Car, N. Marzari, A. Kokalj, http://www.pwscf.org/.  F. Giustino, M. L. Cohen and S. G. Louie, Phys. Rev. B 76, 165108 (2007). A. A. Mostofi, J. R. Yates, Y.-S. Lee, I. Souza, D. Vanderbilt and N. MarzariComput. Phys. Commun. 178, 685 (2008).
10:45 AM - CC5.3
Monte Carlo Simulation of Phonon Transport in a Surface Scattering Measurement Structure.
Edward Piekos 1 , John Sullivan 1 , Subhash Shinde 1 , Thomas Friedmann 1 Show Abstract
1 , Sandia National Laboratories, Albuquerque, New Mexico, United States
A microfabricated structure, currently in production at Sandia, has been designed that will employ a differential measurement of heat flowing through straight and bent ligaments of otherwise identical description to probe phonon-surface interaction. In this structure, phonons traversing the straight ligament will, on average, suffer fewer surface collisions than those traversing the bent ligament. This difference will increase as surface scattering increases in importance relative to bulk scattering, which can be manipulated via the ambient temperature. Most importantly, the difference is also a function of the tangential momentum change associated with surface scattering events. If scattering events tend to destroy tangential momentum (diffuse scattering) the flux difference between the straight and bent ligaments is smaller than if they tend to preserve tangential momentum (specular scattering). This structure therefore provides a means for determining the character of phonon-surface interaction.
Simulation provides a key tool in the design and operation of the differential scattering structure. Because its geometry and operating principle place this device outside the applicable range of continuum-based simulation tools, as well as the practical range of molecular dynamics, a phonon Monte Carlo technique is employed. Simulations of both ligaments are performed at a range of temperatures and with varying degrees of specularity on the surfaces. It is shown that, down to at least 200K, an effective thermal conductivity can be defined. This observation allows continuum tools to be used to simulate operation of the device in its entirety by specifying differing thermal conductivities for the bent and straight ligaments. The variation of the difference between these effective thermal conductivities is shown to be roughly quadratic in the degree of specularity, with fully specular ligaments showing a roughly six-fold larger difference than their fully diffuse counterparts between 200K and 400K.Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the US Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.
11:30 AM - CC5.4
Phonon Scattering at Interfaces to Disordered Materials: A Path to Efficient Transparent Thermoelectrics?
Bruce White 1 Show Abstract
1 , Binghamton University, Binghamton, New York, United States
The supply of energy is one of the great challenges for society in the twenty first century. The development of efficient transparent thermoelectric materials would enable a variety of unique energy scavenging opportunities ranging from energy scavenging windows in buildings and homes to components of photovoltaic systems designed to scavenge energy from the large fraction of photons that generate waste heat. Here, we utilize phonon wavepacket molecular dynamics simulations to explore the feasibility of meeting these challenges with thermoelectric nanostructures that take advantage of phonon scattering at the surface of the nanostructure to guide heat carrying phonons into a cladding layer, formed from low thermal conductivity material. Phonon wavepacket molecular dynamics simulations have been carried out on an artificial system comprised of a single crystal with little to no intrinsic scattering interfaced to a single crystal with a large degree of diffuse Rayleigh scattering due to artificially induced mass fluctuations. The results indicate that as the dimensionality of the system is changed from 1D to 3D, through changes in the length scales associated with periodic boundary conditions, the amount of energy transferred from the single crystal to the mass disordered crystal increases drastically, with the 1D case being better described by the Acoustic Mismatch Model and the 3D case being better described by the Diffuse Mismatch Model. These results indicate that controlled surface scattering of phonons could enable thermoelectric materials in which the electronic properties are optimized independently of the thermal properties by creating composite materials comprised of thermoelectric nanostructures with a relatively low phonon density of states (high speed of sound) embedded in low thermal conductivity polymers with a high phonon density of states (low speed of sound). Experimental efforts to realize such a system using ZnO nanowires and nanotubes cladded in polyimide will be discussed.
11:45 AM - CC5.5
Phonon Interface Transmissivity Determined from Transient Thermoreflectance Spectroscopy.
Austin Minnich 1 , Kimberlee Collins 1 , Gang Chen 1 Show Abstract
1 Mechanical Engineering, MIT, Cambridge, Massachusetts, United States
Transient ThermoReflectance Spectroscopy (TTRS) is a commonly used technique to investigate ultrafast processes. In these experiments, a pump laser pulse is used to excite a sample while a probe pulse measures the change in reflectivity due to the pump pulse as a function of the time delay between the pulses. The change in reflectivity is compared to the prediction of a model, and by fitting the model to the experimental data quantitative results can be obtained. For small fluences, where the system is not perturbed too far from equilibrium, the change in reflectivity is proportional to the electron temperature, and so the change in reflectivity can be interpreted as a temperature decay curve. Assuming a standard heat diffusion model, the thermal conductivity and interface conductance can be easily obtained by comparing the model’s predictions and the experimental results. However, for times smaller than the phonon relaxation time, the heat transport is quasi-ballistic and Fourier’s law is no longer valid, leading to incorrect results. In this work, we solve the phonon Boltzmann equation for a laser pulse incident on a layered structure and use the results to analyze ballistic phonon transport. This treatment also allows us to determine the phonon transmissivity through an interface, a quantity which has mostly been studied only theoretically. Acknowledgments: this work is supported by MITEI Seed funding. A.M. also acknowledges support from NDSEG and NSF Fellowships.
12:00 PM - CC5.6
Temperature and Composition Dependence of Thermal Interface Resistance Between Metal Films and Single and Multi Wall Carbon Nanotube Arrays.
Matthew Panzer 1 , Hai Duong 2 , Senyo Senyo Dogbe 3 , Jun Okawa 4 , Junichiro Shiomi 4 , Jeremy Rowlette 1 , John Reifenberg 1 , Shigeo Maruyama 4 , Brian Wardle 2 , Ken Goodson 1 Show Abstract
1 Mechanical Engineering, Stanford University, Stanford, California, United States, 2 Aeronautics and Astronautics, Massachusetts Institute of Technology, Boston, Massachusetts, United States, 3 , Molecular Nanosystems, Sunnyvale , California, United States, 4 Mechanical Engineering, University of Tokyo, Tokyo Japan
Owing to their outstanding thermal properties, aligned arrays of carbon nanotubes (CNTs) are promising for use in advanced composites and interfaces requiring high thermal performance. In spite of the high conductivity of individual carbon nanotubes, the interface resistance of the CNT-substrate contacts often significantly reduces the thermal performance of CNT films and their composites below its potential value. The complicated structure of these interfaces at the nanoscale strongly affects the interface resistances and is highly dependent on the interaction, adhesion, and wettability of the CNT and substrate materials, which is typically a metallic layer. The nanoscale features in the contact further complicate the interface resistance by modifying the phonon transport physics, for which there are no established models. Further data are needed for the interface resistances between metal films and its temperature dependence to improve upon the thermal performance of CNT arrays and develop models for the thermal transport in nanostructured contacts. This work uses ultra fast transient optical thermometry (picosecond and nanosecond) to measure 1) the interface resistance of the base contact in the temperature range of 50-500 K of aligned multi-walled nanotube arrays (MWNT) grown on metallic films (W, Ti, Pd) deposited on a fused silica substrate, and 2) the interface resistance between metallic films (Al, Pd, Pt, Ni, Ti, Cr) and the tips of aligned SWNT arrays grown on Si.The MWNT are grown using chemical vapor disposition (CVD) to the lengths of ~20 um and with CNT densities of ~1% using iron catalyst deposited on alumina-coated (2 nm) metallic layers (W, Ti, Pd) with thicknesses in the range of 15-25 nm on fused silica substrates. A picosecond time domain thermoreflectance thermometry system (TDTR) measures the interface resistance by heating the metallic film through the transparent substrate using 10 ps pulses from a modelocked Nd:YVO4 laser. The reflected intensity of a variably delayed probe beam derived from the pump beam measures the relative temperature change of the metal film, which is fit to a rigorous solution of the heat diffusion equation for a multi-layer stack to extract the MWNT base interface resistance.The SWNT films are grown using alcohol catalytic CVD to a length of 11.5 um and densities of 3% using a Co-Ni catalyst deposited on a Si substrate. Fifty nanometers of either Al, Pd, Pt, Ni, Ti, or Cr is evaporated on separate SWNT samples. Fabricating samples out of the same SWNT reduces variations in the thermal property data caused by variations in SWNT quality, volume fraction, and length. The combined application of nanosecond thermoreflectance and picosecond TDTR extract the metal-CNT interface resistance and SWNT film thermal properties.
12:15 PM - CC5.7
Experimental Investigation of Heater Size-Effect on Substrate Thermal Conductivity and Interface Thermal Resistance.
Monalisa Mazumder 1 , Wei Jiang 2 , Karthik Chinnathambi 3 , Ganapathiraman Ramanath 2 , Theodorian Borca-Tasciuc 4 Show Abstract
1 Department of Chemical and Biological Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States, 2 Department of Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States, 3 Focus Center - New York, Rensselaer Polytechnic Institute, Troy, New York, United States, 4 Department of Mechanical, Aerospace and Nuclear Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States
Future nano-electronics and nano-interconnects will have local hot spots with dimensions comparable to the mean free path of the heat carriers. As the device characteristic length is reduced, the effect of heater-size manifests itself in the thermal transport inside the nano-structure and across the interfaces - fundamental understanding of which is crucial for the thermal management of these devices. In the present work, we have studied the interface thermal resistance across Au-SiO2/Si nano-interface as well as the size effect of the nanoscale heater on the thermal conductivity of the substrate as a function of temperature in the range of 80 – 300K. A series of gold nano-heaters of varying width were fabricated by ebeam lithography onto SiO2/Si substrate. The thermal-measurement technique employs joule heating thermometry where the rise in temperature across the nano-heater/substrate interface was measured to estimate the interface thermal resistance by both DC and 3ω AC technique. As the feature size of the heater becomes comparable or smaller than the phonon mean free path in the substrate, the rate of heat transfer between the heater and the substrate decreases due to reduced scattering interaction of the phonon with the heater leading to less effective removal of thermal energy from the heater. This consequently reduces the effective thermal conductivity of the substrate as compared to the bulk value at any given temperature. Here, we experimentally demonstrate the aforesaid effect and analyze the result in the light of non-local heat conduction.
12:30 PM - CC5.8
Thermal Boundary Resistance in Phase Change Memory Materials.
John Reifenberg 1 , Kuo-Wei Chang 2 , Matthew Panzer 1 , SangBum Kim 3 , Jeremy Rowlette 1 , H. -S. Philip Wong 3 , Kenneth Goodson 1 Show Abstract
1 Mechanical Engineering, Stanford University, Stanford, California, United States, 2 , Intel Corporation, Santa Clara, California, United States, 3 Electrical Engineering, Stanford University, Stanford, California, United States
Phase change memory (PCM) data storage technology offers the scalability, cycling characteristics, and speed to meet the ever-increasing demand for high density data storage. Joule heating initiates reversible phase transitions between the amorphous and FCC crystalline phases of a phase change material such as Ge2Sb2Te5 (GST). The resulting resistance contrast of ~ 3 orders of magnitude between the phases specifies the memory state. Write current reduction remains a challenge for reducing the energy load and decreasing the size of programming transistors. Typical programming pulses contain ~1000 times more energy than is required for the phase transitions. Heat conduction through the bottom electrode, commonly made of TiN, dissipates most of the excess energy. Understanding the conduction physics in thin TiN and GST films, and interface transport is essential for improved PCM device engineering and simulation.
This study investigates the heat conduction physics in FCC GST, at GST/TiN interfaces, and at Cr/TiN interfaces over the temperature range 300K < T < 550K. A picosecond time-domain thermoreflectance system measures the spatial distribution of thermal properties at the nanometer length scale. We fit data from two Cr/TiN/GST/TiN/Si samples with different GST thicknesses. Strongly different sensitivities to the intrinsic GST thermal conductivity, Cr/TiN thermal boundary resistance (TBR), and GST/TiN TBR uniquely extract these properties.
The intrinsic thermal conductivity of FCC GST is ~0.5 W/m/K over the entire temperature range. The GST/TiN thermal boundary resistance (TBR) decreases with temperature from ~17 m2K/GW to ~10 m2K/GW. These values are comparable or greater than the intrinsic thermal resistance of the TiN bottom electrode/heat sink. Upon cooling from 550K to 300K the GST/TiN TBR shows a permanent reduction to ~12 m2K/GW. Changing interface quality in the annealed PVD films may decrease the TBR. The Cr/TiN interface resistance decreases with temperature from ~6 m2K/GW to ~3 m2K/GW. The evaporated Cr/TiN interface exhibits no change after annealing and returning to room temperature. The results correlate well with existing data showing the TBR increases with dissimilarity between Debye temperatures. Theoretical predictions from the diffuse mismatch model and the acoustic mismatch model for weakly bonded interfaces underpredict the GST/TiN TBR.
This work provides the first thermal interface data essential for understanding PCM device performance and reducing write current. Interface engineering to reduce the effect of annealing and/or increase the mismatch between GST and the electrode material can significantly improve device performance. The relatively poor agreement between the data and existing TBR models highlights the need for continued experimental and theoretical investigation of interfacial transport, particularly at high temperatures.
CC6: Phonon Dynamics
Wednesday PM, December 02, 2009
The Fens (Sheraton - 5th Floor)
2:30 PM - **CC6.1
Studies of Electron-phonon and Phonon-phonon Interactions in GaN and InN by Ultrafast Raman Spectroscopy.
Kong-Thon Tsen 1 Show Abstract
1 Physics, Arizona State University, Tempe, Arizona, United States
Subpicosecond time-resolved Raman spectroscopy has been employed to interrogate electron-phonon interactions and phonon dynamics in GaN and InN. Electron-longitudinal optical phonons scattering rate as well as the decaying dynamics of longitudinal optical phonons have been directly measured. Our results indicate that hot-phonon effects can play an important role in the electron relaxation and transport in these technically important nitride-based semiconductors. The carrier-dependence of the lifetime of longitudinal optical phonons has also been measured. The results suggest that more theoretical work is needed to account for the lifetime dependence of longitudinal optical phonon on photoexcited carrier density.
3:00 PM - CC6.2
Investigation of Anti-Stokes Photoluminescence in GaN Single Crystal.
Yujie Ding 1 , Suvranta Tripathy 1 , Jacob Khurgin 2 Show Abstract
1 Department of Electrical and Computer Engineering, Lehigh University, Bethlehem, Pennsylvania, United States, 2 Electrical and Computer Engineering, The Johns Hopkins University, Baltimore, Maryland, United States
We have investigated anti-Stokes photoluminescence from n-type free-standing GaN at room temperature. Such a process is caused by phonon-assisted absorption. When the excitation photon energy is sufficiently below the donor-acceptor transition energy, however, two-photon absorption becomes the dominant mechanism for anti-Stokes photoluminescence. By measuring the dependences of the photoluminescence spectra on temperature, excitation power, and excitation photon energy, we have demonstrated that donor-acceptor pair transition plays an important role in the anti-Stokes photoluminescence. Our study could result in efficient laser cooling of semiconductors.
3:15 PM - CC6.3
Lifetime and Anharmonicity of Librational Modes in Semiconductors.
Koun Shirai 1 , Haruhiko Dekura 1 , Hiroshi Katayama-Yoshida 2 Show Abstract
1 , ISIR, Osaka University, Ibaraki, Osaka, Japan, 2 Engineering Science, Osaka University, Toyonaka, Osaka, Japan
Librational modes are rigid-rotation mode of molecular units and usually have low frequencies. Because of low-frequency property, librational modes have sometimes decisive roles on phase transitions. In typical tetrahedrally-coordinated semiconductors, there is no librational mode, although zone-boundary TA modes have to some extent similarity to librational mode. However, the advent of fulleranes and clathrate compounds in the semiconducting materials, the role of librational modes has become important to understand their physical properties. The authors have been studying specific properties of the librational mode of semiconductor boron crystals, which has icosahedral units as the building block. The librational mode of α-boron has interesting properties. The line width of the librational mode appearing in the Raman spectrum is exceptionally narrow. The pressure dependence of the libration frequency is vanishingly small. These abnormal behaviors along with other properties used to have the researchers in this field call it the ghost peak. The abnormal properties of the ghost peak have been clarified by the present authors from the standpoint of lattice dynamics [1,2]. These abnormal behaviors are specific to boron crystals. However, the authors became aware that there are many common properties between boron and other non-tetrahedrally-coordinated semiconductors (and even tetrahedrally-coordinated semiconductors).In this paper, we discuss several interesting issues of librational modes in semiconductors, by citing heavily our studies on boron, because this example has been well studied. The following topics are discussed in this paper.(i) frequency of librational mode: Usually, the frequency of the librational mode is very low, that is, less than 100 cm-1. This is reasonable, because the angle bending forces that determine the frequency are very weak. Boron is not exception in this sense, but the frequency of the librational mode is as high as 520 cm-1. The reason is the geometrical effect of icosahedron.(ii) pressure dependence of librational mode: It is a common feature that the pressure dependence of librational modes is almost zero. Nonetheless, it does not imply that anharmonicity of angle bending forces is very small. This pressure dependence eventually causes softening of the librational mode .(iii) lifetime of librational mode: Usually, the line width of the librational modes is very broad. The boron case is exceptional. We like to discuss an ingenious mechanism of such a wide range variation of the lifetime of the librational modes. K. Shirai and H. Katayama-Yoshida, Phys. Soc. Jpn. 67, 3801 (1998). K. Shirai and H. Katayama-Yoshida, Physica B 263/264, 791 (1999). K. Shirai, H. Dekura, and A. Yanase, Phys. Soc. Jpn., (in press).
3:30 PM - CC6.4
Dynamical Stability of Cubic YH3 under Pressure.
Duck Young Kim 1 2 3 , Ralph Scheicher 1 , Rajeev Ahuja 1 2 Show Abstract
1 Physics and Materials Science, Condensed Matter Theory, Uppsala University, Uppsala Sweden, 2 Materials Science and Engineering , Applied Material Physics, Royal Institute of Technology (KTH), Stockholm Sweden, 3 physics, Cavendish Laboratory, University of Cambridge , Cambridge United Kingdom
Yttrium can form a hydride able to absorb about 300 mol.% hydrogen, known to form a hcp-structure at ambient pressure. This system undergoes an insulator-metal (IM) transition upon hydrogen uptake, which has led to suggestions for possible applications as a switchable mirror. Due to the recent development of diamond anvil cell technique, has enabled scientists to achieve extreme pressure relatively easily, which opens another path to research of YHx materials. Similarly to thin film experiments, high-pressure studies have observed an IM transition in cubic YH3 under pressure near 25 GPa, whose mechanism has remained unclear. It was observed in experiment that lower pressure hcp-YH3 phase turns into fcc phase sluggishly starting at 10 GPa. Raman and infrared studies found that fcc-structured YH3 can exist at 10 GPa and is clearly stabilized around 25 GPa. It has been argued that another intermediate phase or a coexisting hcp-fcc phase could appear in the pressure range 10-25 GPa.In this abstract, we report on the behavior of structural and electronic properties of YH3 under pressure using first principles calculations. We show that YH3 undergoes a structural transformation to fcc- structure under pressure. In our calculations, the fcc phase is energetically stable above 20 GPa and dynamically stable above 17.7 GPa since the peak at the imaginary frequencies of the phonon density of states which account for the structural instability disappears at high pressure. Especially, the phonon momentum driving structural instability is closely related with Fermi surface nesting, which enhances electron-phonon coupling in this system. Furthermore, our GW calculations confirm the metallic nature of the fcc-YH3.
4:15 PM - **CC6.5
Coupled Plasmon-Phonon Modes in GaN.
Brian Ridley 1 , Angela Dyson 2 Show Abstract
1 Electronic Engineering, University of Essex, Colchester, Essex, United Kingdom, 2 Physics, University of Hull, Hull, Yorkshire, United Kingdom
B. K. Ridley* and A. Dyson***Department of Electronic Engineering, University of Essex, Colchester, UK.**Department of Physics, University of Hull, Hull, UK.An enduring problem in the engineering of high-power semiconductor devices is how to mitigate the effect of heating. Heating means the proliferation of phonons, and phonons, interacting with electrons directly affect the electronic performance of the device. Nowhere is this more evident than the role of hot polar-optical phonons in reducing the drift velocity in the channel of an HFET and hence reducing its performance at high frequencies. The task of describing hot-phonon effects is complicated by the coupling to plasma modes. We present a theory of coupled plasmon-phonon modes in GaN, how they interact with electrons and how their lifetime becomes density-dependent. Raman scattering in bulk material shows a reduction of lifetime with increasing density and we offer an explanation for this in terms of the frequency dependence of the anharmonic decay mechanism. A more substantial reduction in lifetime is deduced from noise measurements of GaN HFET channels. We believe this may be due to the large enhancement of the group velocity of the modes as a result of coupling, leading to diffusion out of the channel. We describe its dependence on electron temperature.
4:45 PM - CC6.6
Effects of Motions of Encapsulated Atoms on Phonons in Clathrate Compounds.
Kazumi Sato 1 , Jun Tang 1 , Alfonso Miguel 2 , Eiichi Matsuoka 1 , Katsumi Tanigaki 1 Show Abstract
1 WPI/Department of physics, Tohoku University, Sendai, Miyagi, Japan, 2 Laboratoire de Physique de la Matie`re Condense'e et Nanostructures, University Lyon, Lyon France
There are many materials which have a cage structure and atoms or molecules can be accommodated into these inner spaces. Because of their large inner spaces, the accommodated atoms or molecules can anharmonically move. Such motions have recently collect much scientific attention and called as rattling. The acoustic phonons are strongly scattered by these motions, hence the thermal conductivity of these materials can be suppressed, being different from those of crystals and moreover similar to those of glasses. Such unique phonon scattering processes have intensively been investigated. Clathrates are one of these families, and will become hopeful for thermoelectricity owing to both their high electric and low thermal conductivities. In clathrates, metal atoms (e.g. Ba and Sr) are encapsulated into the cage structure composed of silicon or germanium. Some of these compounds show glass-like thermal conductivity. The glass-like thermal conductivity observed experimentally has been explained in terms of the anharmonic motions of the accommodated atoms. In addition, the specific low energy excitation modes are thought to play an important role in heat capacity. Both physical properties described here may well be explained in the framework of two-level-system (TLS), which has been used for providing the origin of similar properties in many glasses. The anharmonic motions of the encapsulated atoms may lead to a good general grasp for the electric and thermal properties in these systems. In this meeting, we will present how the motions of the accommodated atoms affect the physical properties, such as thermal and electric conductivities, heat capacity, Seebeck coefficient, in clathrate compounds, especially focusing on the low temperature properties of heat capacity.  J. Tang, R. Kumashiro, J. Ju, Z. Li, M. A. Avila, K. Suekuni, T. Takabatake, F. Guo, K.Kobayashi, K. Tanigaki,Chemical Physics Letters,472, 2, 6?64, 2009. J. Tang, T. Rachi, R. Kumashiro, M. A. Avila, K. Suekuni, T. Takabatake, FZ. Guo, K. Kobayashi, K. Aoki and K. Tanigaki, Phys. Rev. B, 78, 085203-085206 (2008).
5:00 PM - CC6.7
Effects of Strain, Composition, and Local Atomic Configuration on Phonon Characteristics in Si1-xGex.
M. Hossain 1 , Harley Johnson 1 Show Abstract
1 , University of Illinois, Urbana, Illinois, United States
The composition and strain dependence of phonon modes in important binary alloys such as Si1-xGex have not been widely studied computationally, despite their importance for phonon engineering applications. Here we report the results of ab initio calculations of composition and strain dependence of Raman modes over the full composition range of Si1-xGex. Using the supercell force-constant approach, the three Raman active optical modes of vibration -- Si-Si, Si-Ge, Ge-Ge -- are investigated under both hydrostatic and biaxial applied strain, taking into account the effects of disorder and disorder induced strain. Atomic randomness is found to affect the composition dependence of the optical phonons, but the strain-shift coefficients are approximately insensitive to the local atomic configuration. Under strain, the shift-coefficients for all the modes change nonlinearly with Ge content in the Si1-xGex alloy. The mode dependent Gruneisen parameter is computed and found to be in excellent agreement with experimental data for the pure elemental materials, but to vary nonlinearly with composition for all three modes, thus challenging the virtual crystal approximation or the rule of mixtures approach for the phonon characteristics of the alloys. Finally, by calculating electrical conductivity using the same first-principles electronic structure formulation, we present the Seebeck coefficient as an example of the importance of properly accounting for alloy and strain effects on the phonon properties.
CC7: Poster Session
Wednesday PM, December 02, 2009
Exhibit Hall D (Hynes)
9:00 PM - CC7.1
Phonon Effect on the Performance of Cr-doped Material for Broadband Photonic Application.
Jimmy Wang 1 Show Abstract
1 Department of Photonics, National Sun Yat-Sen University, Kaohsiung Taiwan
The vibrational–electronic interaction of chromium ion (Cr) with host lattice leads to its broadband emission. Furthermore, various oxidation states and site variations of chromium in doped materials can potentially further extend the broadband characteristics of emissions. The broadband emission can potentially be exploited in many ways. For instance, a Cr-doped broadband (amplified) spontaneous emission can improve spatial resolution many times in an optical coherence tomography (OCT) instrumentation and a wide gain bandwidth can be exploited in wide wavelength tunability of laser and amplifier.Based upon the recent research results, two critical tasks were proposed and have to be resolved before the possible demonstration of effectiveness of ultra-wide-bandwidth applications: namely eliminating both pump and signal excited state absorption by engineering the spectroscopy of Cr in materials and fabricating controllable nano-Cr:crystal by novel processing development.In this study, a preliminary investigation of phonon engineering effect on the following performances will be studied. The effect of phonon confinement of nano Cr:doped crystalline on the absorption spectrum, excited absorption spectrum and emission spectrum will be analyzed. The vibronic effect of ligand field attribute from various hosts on the bandwidth of emission will be also investigated. The goal is to study the phonon engineering toward developing a Cr:doped material that has better ASE/Lasing/Gain performances for broadband photonic application.
9:00 PM - CC7.10
Evaluating Polymer Brushes as a Strategy for Enhancing the Dominance of Interfacial Thermal Conductivity at Organic / Inorganic Interfaces.
Mark Losego 1 , Ian Blitz 1 , Marc Ghossoub 2 , Sanjiv Sinha 2 , David Cahill 1 , Paul Braun 1 Show Abstract
1 Materials Science and Engineering, University of Illinois Urbana Champaign, Champaign, Illinois, United States, 2 Mechanical Science and Engineering, University of Illinois Urbana Champaign, Champaign, Illinois, United States
Large anharmonicity in the vibrational states of polymers are possible near room temperature due to glass transitions and other thermally activated phase changes. For this reason, inorganic / polymer / inorganic heterostructures have attracted interest as possible thermal rectification devices. Such heterostructures would be composed of two inorganic materials with offset phonon frequencies and a polymer whose temperature dependent vibrational states span these two extremes. Alignment of the vibrational states at each interface would depend on the direction of the temperature gradient resulting in high thermal conductivity in the aligned state and low thermal conductivity in the misaligned state. The low thermal conductivity of polymer films (~0.3 W/mK) necessitates an ultrathin polymer layer for the effects of interfacial thermal conductivity to be dominant (interfacial thermal conductivity, G ~ 200 MW/m2K, implying a Kapitza length of ~1.5 nm). However, thermal conductivity measurements on drawn polymer fibers and self-assembled monolayers have indicated that the thermal conductivity of aligned polymer chains can be orders of magnitude higher than disordered bulk polymers. We hypothesize that similar enhancements in thermal conductivity may be possible in well aligned polymer brushes. In this work, we use time-domain thermal reflectance measurements to evaluate the thermal conductivity of poly(methylmethacrylate) polymer brushes grown on silicon surfaces as a function of brush thickness and initiator density. We compare these results to spun-cast polymer films of similar molecular weight.
9:00 PM - CC7.11
Could Porosity Induce Gaps in the Phonon Density of States of Nanoporous Silicon?
Juan Carlos Noyola 2 , Alexander Valladares 2 , R. Valladares 2 , Ariel Valladares 1 Show Abstract
2 Departamento de Física, Facultad de Ciencias, Universidad Nacional Autónoma de México, Apartado Postal 70-542, Mexico D.F., 04510, Mexico, 1 Materia Condensada, Instituto de Investigaciones en Materiales, Universidad Nacional Autónoma de México, Apartado Postal 70-360, Mexico D.F., 04510, Mexico
Nanoporous silicon periodic supercells with 1000 atoms and 50 % porosity were constructed using the Tersoff potential and our novel approach . The approach consists in constructing a crystalline diamond-like supercell with a density (volume) close to the real value, then halving the density by doubling the volume, and subjecting the resulting periodic supercell to Tersoff-based molecular dynamics processes at a temperature of 300 K, followed by geometry relaxation . As in the ab initio approach  the resulting samples are also essentially amorphous and display pores along some of the crystallographic directions. We have now constructed a porous supercell with 1000 atoms and 80 % porosity using the same approach and have calculated its vibrational density of states. The present results are compared to those reported in Ref. 1 and they manifest that the soft acoustic phonons are displaced towards lower energy in the 80 % porosity sample whereas the optical modes are displaced towards higher energies. The pseudo gap, existing in the 50 % porous samples, is depleted even more in the 80 % sample indicating a tendency towards the creation of a real phonon gap for higher porosity materials. We shall present and analyze atomic structures, Radial Distribution functions, plane angles and vibrational densities of states for both samples. Some conjectures that point to the possible engineering of porous materials to produce predetermined phonon properties will be discussed. J. C. Noyola, Alexander Valladares, R. M. Valladares and Ariel A. Valladares, Symposium PP, Mater. Res. Soc. Symp. Proc. (2008). Accepted for publication. Ariel A. Valladares, Alexander Valladares and R. M. Valladares, Mater. Res. Soc. Symp. Proc. 988E, 97 (2007).
9:00 PM - CC7.2
Band Structures of Phononic Crystals Composed of Viscous and Visco-elastic Materials.
Jerome Vasseur 1 , Caroline Lespagnol 1 , Bahram Djafari-Rouhani 1 , Pierre Deymier 2 Show Abstract
1 , IEMN/EPHONI, Villeneuve d'Ascq France, 2 DMSE, University of Arizona, Tucson, Arizona, United States
Over the past decade a large body of work has been accumulated on the properties of phononic crystals constituted of elastic materials. Among others, these properties include sonic insulation, sound waveguiding and abnormal acoustic waves refraction. The constitutive materials range from fluids to solids. However, little attention has been paid to the effect of the viscosity of the fluids or the viscoelasticity of solid constituents on the band structure of phononic crystals. We present a method based on the plane waves expansion (PWE) approach to calculate the band structure of phononic crystals constituted of materials with complex and frequency dependent elastic coefficients. This method allows the determination of the complex wave vectors at a specific frequency. We apply this method to two classes of phononic crystals. In the first class, one of the constituent is a viscous fluid and the other an elastic solid. Here we show significant effects of viscosity on the band structure of the phononic crystal in the form of frequency shifts of the pass bands as well as in the presence of additional bands. We discuss the nature of these bands in terms of the real and imaginary parts of the wave vector. The imaginary part which relates to attenuation enables us to separate measurable and non-measurable modes. The second class of phononic crystals is composed of viscoelastic solids. We limit our study of the effects of viscoelasticity to the standard linear solid (SLS) model. Similarly to the fluid system we describe the effect of viscoelasticity on the band structure of phononic crystals. Finally we consider the implications of the viscous and viscoelastic band structures on some of the potential applications of phononic crystals.
9:00 PM - CC7.3
Band Structure and Wave Guiding in a Phononic Crystal Made up of Cylindrical Dots on a Slab.
Yan Pennec 1 , Bahram Djafari Rouhani 1 , Hocine Larabi 1 , Abdellatif Akjouj 1 , Jean-Numa Gillet 1 , Jerome Vasseur 1 , Guillaume Thabet 2 Show Abstract
1 IEMN, UFR de Physique, University of Lille 1, Villeneuve d'Ascq France, 2 , SGI Silicon Graphics S.A., Versailles France
We present a theoretical analysis of the phonon transport and guiding of acoustic waves in a phononic crystal made up of a square array of cylindrical dots, which is deposited on a thin homogeneous plate. With appropriate choice of the geometrical parameters, this structure can display several gaps, one of them being well below the Bragg gap. With the help of the Finite Difference Time Domain method, we calculate the transmission coefficient vs. the frequency, and demonstrate a good agreement with the dispersion curves. We show the possibility of guided modes inside an extended linear defect created either by removing one row of cylinders or by changing the height or the materials constituting the dots in a row. The wavelengths of the waves transmitted in the low frequency gap are more than ten times larger than the width of the waveguide. We discuss the number and localization of the confined branches appearing in each band gap and the transmittivity of each branch as well as the conversion in the polarization of the transmitted waves that can occur more or less significantly.
9:00 PM - CC7.4
Combining Molecular Dynamics and Kinetic Modeling of Phonon Transport in Defective Carbon Nanostructures.
Andrey Knizhnik 1 , Inna Iskandarova 1 , Dmitry Krasikov 1 , Alexander Eletskii 1 , Boris Potapkin 1 , Vinayak Tilak 2 , Kamala Raghavan 3 Show Abstract
1 , Kintech Lab Ltd, Moscow Russian Federation, 2 , GE Global Research, Niskayuna, New York, United States, 3 , GE Global Research, Bangalore India
Low dimensional carbon nanostructures, such as carbon nanotubes, graphene, possess unique physical properties, in particular, high thermal conductivity. These unique physical properties depend strongly on the structural and compositional characteristics of nanostructures. Atomistic methods, such as equilibrium and non-equilibrium molecular dynamics, are widely used to investigate dependence of thermal conductivity on the structure of the nanomaterial, in particular on the number and nature of defects. However, atomistic methods have severe drawbacks in considering real nanostructures due to the spatial and temporal limitation. At the same time, available macroscopic methods, such as kinetic Boltzmann equation, can overcome size limitations of atomistic methods, but cannot predict accurately scattering properties of structural and compositional defects. In this work we present an approach for combination of molecular dynamics methods for calculation of thermal conductivity with macroscopic description based on Boltzmann transport. The approach is based on determination of parameters of macroscopic model from the results of atomistic simulations of intermediate-size systems. We applied this approach for investigation of influence of point defects on thermal transport in super micron-scale carbon nanotubes and graphene. While such systems cannot be considered now directly with molecular dynamics methods, we were able to describe thermal properties of these systems using combined approach. Based on the results of the proposed approach we investigate the effect of different types of defects on thermal conductivity and determine the critical fraction of point defects in the carbon nanostructures as a function of their length.
9:00 PM - CC7.5
Phonon of DNA by Numerical Model using First-principles Potentials.
Shigeki Saito 1 2 , Hiroshi Mizuseki 2 , Yoshiyuki Kawazoe 2 Show Abstract
1 Computational Materials Science Center, National Institute for Materials Science, Tsukuba, Ibaraki, Japan, 2 Institute for Materials Research, Tohoku University, Sendai, Miyagi, Japan
Deoxyribonucleic acid (DNA) contains the biological information required for replication, transcription, and recombination. The polymorphic structure of DNA determines in part its mechanical and thermal properties, which are in turn related to the phonon and vibrational modes revealed by optical scattering and absorption. The vibrational dynamics of DNA have been studied spectroscopically at low frequencies. In these analyses, large-scale molecular calculations of normal modes using generic force fields were performed. These calculations have suggested sequence and conformational effects on DNA thermodynamics and unlocalized modes that correspond to long-range deformations resembling standing waves in the double helix. However, the use of energy surfaces is limited by the empirical formula and the defined parameters. Molecular orbital calculations have been reported previously for individual bases and DNA fragments. In the present study, first-principles calculations of phonon modes in an infinite DNA model were performed. The potential energy surface (PES) enabled accurate mode assignments using periodic density functional theory (DFT). First-principles force constants estimated from the PES are generally more reliable for evaluating phonon modes in complicated molecules except when a dependence on long-range Coulombic interactions occurs. Mode analysis based on first-principles calculations is needed for theoretical investigations of the absorption spectra, structural stability, thermodynamic properties, and chemical reactions of polymorphic DNA under various conditions. DNA phonon modes were determined based on first-principles potentials using a model with a fixed axial length. The numerical model of DNA was optimized for a fixed length of DNA with a periodicity of 10 A–T base pairs. The calculated bandgap (1.88 eV) resembled that of a semiconductor according to the generalized gradient approximation of the Perdew–Wang 1991 functions. The spectral form of the phonon density exhibited two large distributions due to hydrogen single bonds and other interatomic bonds. High-frequency modes greater than 2756 cm–1 represent the stretching and bending motions of single hydrogen bonds with slight shifts in the bonded atoms. The in-plane modes correspond to higher frequencies than those of the out-of-plane and other complex modes, which in both cases were single adenine (A) and thymine (T) bases. Macroscopic modes were generally divided into nucleobases and the phosphoester backbone in the mid-infrared range, and several far-infrared modes included the amplitudes of full atoms. The phonon contribution of internal energy was estimated using the density distribution of these phonon modes. These results strongly suggest that this method, which is based largely on first-principles calculations of nucleobase arrangements, can be applied to spectral analyses and biochemical reaction models and used to determine the thermodynamic properties of DNA.
9:00 PM - CC7.6
Thermal Conductivity of a Model 1-D Nanostructured Material: Organically Modified Layered Clays.
Ian Blitz 1 , Mark Losego 2 , Richard Vaia 3 , David Cahill 2 , Paul Braun 2 1 Show Abstract
1 Chemistry, University of Illinois Urbana-Champaign, Champaign, Illinois, United States, 2 Materials Science and Engineering, University of Illinois Urbana-Champaign, Champaign, Illinois, United States, 3 , U.S. Air Force Research Laboratory, Wright-Patterson Air Force Base, Ohio, United States
Acoustically mismatched superlattice nanostructures represent a way of reducing the effective bulk thermal conductivity of a material below classical limits. Here, we present a system which may provide a model for phonon-mediated heat conduction in technologically relevant layered nanostructured solids, such as thermoelectric materials. In this work, we show the effect of layer thickness and layer phase on the cross-plane thermal conductivity of organically modified montmorillonite clays as measured by time-domain thermoreflectance techniques. The charge balancing sodium ions on the surface of the clay platelets were replaced with primary alkylammonium ions via ion exchange, providing an acoustically mismatched superlattice system with tunable spacing dependent on the alkyl ammonium chain length. We also explored the effect of using a phase change material as the low modulus component of the superlattice on the cross-plane thermal conductivity as a function of temperature as the alkylammonium layers show glass transitions between 50 and 150 degrees Celsius. Results will be discussed.
9:00 PM - CC7.7
Thermal Conductivity of Gd2Zr2O7 Films and the Interfacial Effect.
Jun-Gu Kang 1 , Ho-Soon Yang 1 Show Abstract
1 Department of physics, Pusan National University, Busan Korea (the Republic of)
Gadolinium zirconium oxide (Gd2Zr2O7) films, a promising candidate for thermal barrier coatings and buffer layers in superconductors, were prepared using an RF-magnetron sputtering method. High-resolution TEM and Rutherford backscattering revealed no significant changes in structure at the interface or in element composition as the film thickness varies. Time-domain thermoreflectance (TDTR) and 3ω techniques were used to investigate the thermal conductivity of the Gd2Zr2O7 films, and the resultant measured values were compared. In 3ω measurement, the frequency applied to the metal line was limited to generate a heat penetration depth in the film much larger than the film thickness. Therefore, the measured thermal conductivity is the effective thermal conductivity and includes the interfacial thermal resistance between the film and substrate. The film thickness was larger than the estimated heat penetration depth at the frequency used in the TDTR measurements. The discrepancy in the resultant thermal conductivity values measured by the two methods can be described as an interfacial effect. This work draws attention to the importance of the interfacial effect on thermal conductivity and emphasizes the importance of the interfacial effect in nanoscale systems.
9:00 PM - CC7.8
The Interfacial Effect on Thermal Conductivity of Diamond-like Carbon Films using a Thermo-reflectance Method.
Jong Wook Kim 1 , Kyung Chun Kim 1 , Young Ha Jun 2 , Ho-Soon Yang 3 Show Abstract
1 School of Mechanical Engineering, Pusan National University, Busan Korea (the Republic of), 2 , J&L Tech. Ltd., Shiheung, Kyunggi-Do, Korea (the Republic of), 3 Department of Physics, Pusan National University, Busan Korea (the Republic of)
Diamond-like carbon (DLC) has been of interest as a promising coating for protection and insulating layer in microelectromechanical systems due to high hardness, wear resistance, transparency in IR range, chemical inertness, and biocompatibility. As the systems become miniature, the interfacial effect becomes more important in determining the physical properties of the system. The interfacial effect on thermal transport is studied for the hydrogenated amorphous carbon(a-C:H) films deposited on Al2O3 substrates. DLC thin films with thicknesses of 200, 600, 1200, and 1800 nm are prepared using an ion gun method. A thermo-reflectance method is used to study thermal conductivity and specific heat of the DLC films, where an AC modulated current is driven in a metal strip deposited on the films to generate heat and the reflectance of probe laser beam incident on the sample is monitored to measure a temperature oscillation of the sample. Thermal conductivity and specific heat of DLC films are obtained using a nonlinear least square fit to the one-dimensional thermal diffusion solution of multi-layered system. The thermal resistance of the DLC-Al2O3 interface can be extracted from the thermal conductivity of the DLC films of various thicknesses between of 200 and 1800 nm. The film thickness dependence of the measured thermal conductivity makes the intrinsic thermal conductivity of DLC films and interfacial thermal resistance estimated.
9:00 PM - CC7.9
Monte Carlo Study of Phonon Relaxation in III-V Compounds Accounting for all the Different Individual 3-phonon Processes.
Hani Hamzeh 1 , Eric Tea 1 , Frederic Aniel 1 Show Abstract
1 , Institut d'Electronique Fondamentale, UMR 8622 CNRS, Université Paris Sud, 91405 Orsay Cedex France
Novel thermal management solutions in nanoscale devices, and high efficiency hot carrier solar cells, using phonon engineering have been attracting significant attention in recent years. A better understanding of phonon-phonon interactions and phonon relaxation in semiconductors is required to address these new concepts. We investigate phonons dynamics using the Monte Carlo (MC) stochastic method to solve the Boltzmann Transport Equation (BTE) in III-V compounds, addressing specifically bulk GaAs. Past research using the relaxation time approximation for solving the BTE by the MC method, has neglected the explicit individual contributions of the different three-phonon scattering processes, and though dispersion has been accounted for, the optical polarization branches were neglected. We present a solution scheme of the BTE using MC technique, accounting for all the different individual types of N- and U-processes, which respect energy and momentum conservation laws, using a generalized Ridley’s scheme of three-phonon interactions, with the Grüneisen’s constant as the only semi-adjustable parameter. Polarization branches with real non-linear dispersion relations for transverse or longitudinal optical and acoustical phonons are considered in our calculations.
Subhash L. Shinde Sandia National Laboratories
Gyaneshwar P. Srivastava University of Exeter
Jacob Khurgin Johns Hopkins University
Yujie J. Ding Lehigh University
Thursday AM, December 03, 2009
The Fens (Sheraton - 5th Floor)
10:00 AM - **CC8.1
Emission Detection and Use of THz Coherent Acoustic Waves.
Bernard Perrin 1 , Agnes Huynh 1 Show Abstract
1 INSP, University Pierre & Marie Curie, Paris France
The realization of very high frequency emitters and detectors has been challenging in phonons physics since a long time. In the seventies and the eighties experiments performed on ballistic propagation of heat pulses relied on phonons sources which have the same characteristics as candle light: the emitted phonons are incoherent with a broad energy distribution and no directivity. At the time phonons were detected with bolometers which are only energy-sensitive. More recently it has been shown that absorption of femtosecond laser pulses on semiconductor superlattices produces emission of well collimated coherent acoustic phonon beams with a very narrow energy spectrum . In these experiments phonons were still detected with bolometers. Since then, we have shown that superlattices can also be used as phase-sensitive phonon detectors . We will describe a series of picosecond acoustics experiments performed on GaAs/AlAs superlattices demonstrating that superlattices are indeed very good monochromatic phonons generators and detectors in the terahertz range. These new THz acoustic waves transducers have been used to study propagation of THz acoustic waves through thick wafers and to measure sound velocity dispersion and sound absorption on a large temperature range. We will discuss the future of such experiments and how the sensitivity and efficiency of such transducers could be further improved. P. Hawker, A. J. Kent, L. J. Challis, A. Bartels, T. Dekorsy, H. Kurtz, K. Khöler, Appl. Phys. Lett. 77, 3209 (2000). A. Huynh, B. Perrin, N. Lanzillotti-Kimura, B. Jusserand, A. Fainstein and A. Lemaître, Phys. Rev. B 78, 233302 (2008).
10:30 AM - CC8.2
Acoustic Metamaterials: Negative, Positive and Zero Refraction and Super-lensing in Phononic Crystals.
Peirre Deymier 1 , Jaim Bucay 1 , Bassam Merheb 1 , Krishna Muralidharan 1 , Alexey Sukhovich 3 , John Page 3 , Jerome Vasseur 2 , Anne Christine Hladky-Hennion 2 , Yan Pennec 2 , Bahram Jafari-Rouhani 2 , Bertrand Dubus 2 , Eleonore Roussel 2 Show Abstract
1 , The University of Arizona, Tucson, Arizona, United States, 3 , University of Manitoba, Winnipeg, Manitoba, Canada, 2 , Institut d'Electronique, de Micro-electronique et de Nanotechnologie, Lille France
We first show experimentally and theoretically that super-resolution can be achieved while imaging with a flat lens consisting of a phononic crystal exhibiting negative refraction. The phononic crystal is composed of a triangular lattice of steel rods in a methanol matrix. The theoretical study uses the Finite Difference Time Domain (FDTD) method. Band structure calculations show that the phenomenon of super-resolution is related to the coupling between the incident evanescent waves and a bound slab mode of the phononic crystal lens. This coupling leads to amplification of evanescent waves by the slab mode. Super-resolution is only observed when the source is located very near to the lens, and is very sensitive to the location of the source parallel to the lens surface as well as to site disorder in the phononic crystal lattice. Other effects on super-resolution such as lens length, thickness, and frequency are reported. The extension this work to solid/solid phononic crystals exhibiting negative refraction of longitudinal waves will also be discussed.We will also report on the properties of a phononic crystal consisting of a square array of cylindrical Polyvinylchloride inclusions in air. This phononic crystal exhibits positive, negative, or zero refraction depending on the angle of the incident sound beam. These properties are analyzed theoretically using the FDTD method and are demonstrated experimentally. For all three cases of refraction, the transmitted beam undergoes splitting upon exiting the crystal. Band structures and equifrequency surfaces (EFSs) calculated with the Plane Wave Expansion method show that the observed properties result from the unique geometry of the phononic crystal’s EFS as compared to that of the incident media.
10:45 AM - CC8.3
Investigating Coherent Zone-Folded Acoustic Phonons in Si/SiGe Superlattices by a Pump-Probe Transient Thermoreflectance Technique.
Helene Michel 1 , Gilles Pernot 2 , Jean-Michel Rampnoux 2 , Stefan Dilhaire 2 , Younes Ezzahri 1 , Ali Shakouri 1 Show Abstract
1 Electrical engineering, University of California Santa Cruz, Santa Cruz, California, United States, 2 CPMOH, Universite de Bordeaux 1, Talence France
Folded acoustic phonons in semiconductor superlattices (SLs) have been extensively studied during the three past decades. Most of the studies have focused on GaAs/AlAs and GaAs/AlxGa1-xAs superlattices. More recently Si/SixGe1-x SLs have received considerable attention due to the achievement of high-quality growth, and their electrical and thermal properties promising for the design of CMOS compatible thermoelectric and phononic devices. Here we present a systematic study of coherent phonons in superlattices with two different periods. The SL structures have 3 μm Si/Si0.7Ge0.3 active layer, grown on top of 2 μm thick buffer layer on a silicon substrate. On the top of the SL, a Si0.9Ge0.1 cap layer with a thickness about 500 nm was grown. With the use of different thicknesses aluminum film transducer on the surface, various phonon distributions are generated. In addition, partial transmission of the femtosecond laser pulse through the thin aluminum transducer and the interaction with the superlattice phonon modes, allow us to study time-dependent Brillouin and stimulated Raman scattering in the structure. Three samples are analyzed. Sample A and B have a 30 nm SL period, while sample C has a 15 nm SL period. Samples A and C have a 15 nm thick Al film transducer, whereas sample B’s Al film is about 22 nm.The SL structure generates an artificial periodicity of the elastic properties along the growth direction. Thus, SL’s phonons dispersion curves exhibits folding of the phonon branches into a mini-Brillouin zone (mini-BZ). One of the most fundamental acoustic properties of a SL is the Bragg reflection of long-wavelength phonons. Femtosecond pump-probe experiments have allowed observing several classes of coherent phonons in SL structures which have distinct generation mechanisms, as impulsive stimulated Raman scattering (ISRS), Brillouin oscillation, and coherent longitudinal acoustic Bragg reflection. In an ISRS process there is a direct excitation of folded phonons in the SL. Light couples to zone folded acoustic modes of the SL, and both forward and back scattering modes could be observed. In sample A, the Al film is thin enough to have residual light reaching the SL, and produce an ISRS excitation, while in sample B, all the light energy is absorbed within the Al film, and only coherent longitudinal acoustic phonon Bragg reflection was detected. Moreover, a comparison between samples B and C shows the effect of the SL period on the frequency of the longitudinal acoustic phonon reflected due to the Bragg condition.
11:30 AM - **CC8.4
Terahertz Phonon Amplification and Sasing in Semiconductor Superlattice Structures.
Anthony Kent 1 , Paul Walker 1 , Ryan Beardsley 1 , Andrey Akimov 1 , Mohamed Henini 1 , Boris Glavin 2 Show Abstract
1 School of Physics and Astronomy, University of Nottingham, Nottingham United Kingdom, 2 , V E Lashkarev Institute of Semiconductor Physics, Kiev Ukraine
Sound amplification by the stimulated emission of (acoustic) radiation, or saser, is the acoustic equivalent of laser. A terahertz saser would be a source of coherent acoustic waves with nanometre wavelengths which could have numerous applications including: the probing and imaging of nanoscale structures, and the control of light and electrical current on ultra-fast timescales. Like photons, phonons obey bosonic statistics and the process of stimulated emission is theoretically possible. It has been demonstrated that the stimulated emission of phonons by electrons in semiconductors can give rise to phonon amplification and sasing.Semiconductor heterostuctures with nanoscale feature sizes present many opportunities to engineer the electronic states to provide population inversion under electrical or optical pumping, as in semiconductor lasers, but also, due to the nanometre wavelength of the phonons, the phonon states and their coupling with the electrons can be engineered to facilitate stimulated emission. In this paper we describe the fabrication and measurement of electrically and optically pumped saser devices based on Aluminium Arsenide/Gallium Arsenide superlattice structures. Such superlattices form the essential elements of a saser: the acoustic gain medium and the acoustic cavity. We show that the devices coherently amplify sound, exhibit a superlinear increase and an increase of directionality of the acoustic emission above a pumping threshold.
12:00 PM - CC8.5
Hypersonic Phonon Bandgaps in Porous Silicon Multilayers.
Gazi Aliev 1 , Bernhard Goller 1 , Paul Snow 1 Show Abstract
1 Dept of Physics, University of Bath, Bath, Somerset, United Kingdom
Porous silicon is produced by the electrochemical etching of crystalline silicon wafers, with the resultant pore morphology depending on the doping of the silicon wafer. Using heavily boron-doped silicon produces samples that have pore sizes of 10-20nm and a volume fraction of voids (porosity) between 30 and 85%. The porosity is determined by the etching current used and is continuously variable across this range. The mechanical properties of the porous layers vary smoothly with porosity and we have measured the longitudinal velocity of sound – and hence the acoustic impedance – in porous silicon as a function of porosity. Recently, there has been much interest in wave propagation in artificially periodic materials. In optical media, photonic crystals have been used to control the propagation of light. The acoustic equivalents are often called phononic crystals where the realization has ranged from stop bands for audible sound in a periodic artistic sculptures to acoustic crystals at ~100MHz for radio frequency communications. Meanwhile, working at the nanoscale, phonon modes have been controlled in solid-state systems where the added periodicity in a semiconductor superlattice is analysed as a zone-folding of the original phonon dispersion relation. Here, we report on the potential for hypersonic (>1 GHz) acoustic devices in nanostructured porous silicon. We will present recent measurements of an acoustic band gap in porous silicon multilayer samples etched from single crystalline silicon wafers. This is by a direct measurement of the transmission of acoustic waves between two matched transducers operating at 1GHz with a bandwidth of ~0.5GHz. The production technique for the acoustic multilayer mirrors is compatible with standard silicon fabrication technology and is readily scalable to produce acoustic bandgaps – and hence devices – at frequencies of ~40 GHz.
12:15 PM - CC8.6
Hypersonic Rugate Filters Using Spatially Modulated Nanoporous Silicon.
Leigh-Anne Thomas 1 , Gazi Aliev 1 , Paul Snow 1 Show Abstract
1 Physics Dept, University of Bath, Bath United Kingdom
Porous silicon is produced using electro-chemical etching techniques on heavily boron-doped crystalline silicon wafers. Resulting samples have pore sizes of 10-20nm with their nanostructure depending on the wafer doping level, with volume fractions (porosity) in the range 30-85%. The applied etching current density controls the porosity within the silicon matrix as an etch front progresses into the silicon away from the wafer surface. Control of the etching current with time determines the porosity profile with depth. The phononic properties of single-porosity layers have been studied at GHz frequencies and the longitudinal velocity of sound (and hence acoustic impedance) is thus known as a function of porosity. It is therefore possible to fabricate 1D hypersonic phononic structures with tunable features that are dependent upon the acoustic impedance profile. This may allow rapid device production within one modulated material rather than from multiple layer deposition as in current production techniques. The impedance profiles (analogous to refractive-index profiles in optics) of fabricated samples determine device properties. For example, Bragg mirrors are fabricated using square-wave porosity profiles; Rugate filters can be fabricated using sinusoidal porosity profiles. This gives a phononic bandgap with a controllable central frequency, reflectivity and bandwidth. Apodisation techniques can be applied to sample designs to give more desirable device properties. Additionally, impedance-matching layers (using quintic polynomials) can also be added to filters or used as anti-reflection coatings. Using a superposition of rugate filters and phononic bandgaps within the same sample it is possible to produce acoustic transmission and notch filters that do not exhibit higher order harmonics. Future opportunities to combine hypersonic rugate filters with photonic bandgaps could enable acousto-optical interactions to be studied within these structures.We present simulated results for Rugate filter modelling based on the transfer matrix method, for Rugate filters with different characteristic features and determine the optimum parameters necessary for fabricating such devices using porous silicon material.
12:30 PM - CC8.7
Theoretical Study of Phononic Bangaps in Multiple Lattices and Directions.
Drew Goettler 1 , Yasser Soliman 2 , Mehmet Su 2 , Roy Olsson 3 , Zayd Leseman 1 2 , Ihab El-Kady 3 Show Abstract
1 Mechanical Engineering, University of New Mexico, Albuquerque, New Mexico, United States, 2 Electrical and Computer Engineering, University of New Mexico, Albuquerque, New Mexico, United States, 3 , Sandia National Laboratories, Albuquerque, New Mexico, United States
Results are presented from three-dimensional finite difference time domain (FDTD) simulations of phononic bandgap crystals in multiple lattice structures (simple cubic lattice, hexagonal lattice, and honeycomb structure) and directions (Γ-X and Γ-M). The model employs the full elastic wave equation in an in-homogenous media. The media is a cermet topology of silicon dioxide with Tungsten rods placed in a lattice pattern. Based on the simulations, the ratio of the Tungsten inclusion radius and lattice constant has a significant effect on the acoustic waves propagating through the crystal. Suppression of a wide range of frequencies, up to 80% of the midgap frequency, is observed.The ability to manipulate and understand the propagation of elastic waves in solid media has many applications including: communications, ultrasonic application, non-destructive evaluation (NDE) and phonon manipulation. Recent experimental and theoretical work initiated by the authors clearly demonstrates that heterogeneous structures can be created that manipulate and precisely control the propagation of elastic waves. Considerable agreement was found between the model of a semi-infinite phononic crystal (2 infinite axes) and experiments. Specifically, there was strong agreement, between experiment and theory, of the location of the bandgap and the bandgap width. The FDTD model also predicted some of the fine features seen in the experimental results.Due to the model’s previous success in modeling the complex behavior of the simple cubic phononic crystal a purely theoretical study is undertaken in which inclusions with other spacings are examined. Specifically, numerical studies are conducted for inclusions placed on a simple cubic lattice, hexagonal lattice, and a honeycomb structure. The cermet topology of the phononic crystals consists of silicon dioxide with tungsten rods (inclusions) placed at the aforementioned spacings. Results presented elucidate that the ratio of the tungsten rods’ radius and lattice constant have a significant effect on the propagation of elastic waves through the crystal.
12:45 PM - CC8.8
Release Holes Size Effects in Acoustic Bandgap Crystals.
Yasser Soliman 2 , Drew Goettler 1 , Mehmet Su 2 , Ihab El-Kady 3 , Zayd Leseman 1 , Roy Olsson 3 Show Abstract
2 Electrical and Computer Engineering, University of New Mexico, Albuquerque, New Mexico, United States, 1 Mechanical Engineering, University of New Mexico, Albuquerque, New Mexico, United States, 3 , Sandia National Laboratories, Albuquerque, New Mexico, United States
In this work we have experimentally investigated the effects of release hole size on acoustic transmission through micro-machined Acoustic Bandgap Crystals (ABGCs). The results confirm previous theoretical studies and determine the range of release hole sizes for which the band gap is minimally compromised. The experiments were performed on micro fabricated 2D ABGC plates comprised of high acoustic impedance tungsten rods arranged in a square lattice inside a low acoustic impedance silicon dioxide host medium. The lattice constant and diameter of the W rods in this experiment were 45 µm and 28.8 µm corresponding to a band-gap centered at 67 MHz. Crystals were characterized with 15, 12.5, 10, and 7.5 µm diameter release holes, required to undercut the crystal and suspend it from the substrate, placed in the center of the W rods. As the air hole diameter decreases, the band gap frequency width increases. However, as the diameter of the release holes approach 5 µm, the fabrication yield significantly decreases. From these experiments an optimum release hole diameter of 7.5 µm was found that maximizes phononic band gap performance and manufacturability.Release holes are necessary in order to physically isolate the ABGCs from the substrate on which they were fabricated in a reasonable amount of time. Surface micro-machined devices are fabricated through the deposition and etching of different layers of thin films on top of the substrate. After the process is complete, the last step is usually etching one of the layers deposited earlier in the process, called a release or sacrificial layer, to suspend the membrane containing the device to isolate it from the substrate. Strategically placed holes in the ABGCs allow for the crystal to be released faster than if the device were to be release only from the sides. Release holes are fabricated in the centers of the tungsten inclusions. The size of the release holes has a very pronounced effect on the width of the band-gaps, which is the main function of an ABGC. In this work, we describe, in detail, the fabrication steps of the ABGCs and show experimental as well as theoretical results.
CC9: Device Fabrication and Control
Thursday PM, December 03, 2009
The Fens (Sheraton - 5th Floor)
2:30 PM - **CC9.1
Electronic Control in Semiconductor Nanostructures using Surface Acoustic Phonons.
Paulo Santos 1 Show Abstract
1 , Paul Drude Institute, Berlin Germany
The piezoelectric potential of a surface acoustic wave (SAW) on a semiconductor induces a type-II potential modulation of the band edges, which captures and transports photo-excited electrons and holes. The photons can be recovered by forcing the recombination of the transported carriers. The inter-conversion between photons and carriers open the way for indirect photon control and storage by acting on carriers during the acoustic transport.In this talk, I review recent results on the control and manipulation of carriers and spins by SAWs in (Al, Ga)As nanostructures as well as applications of acoustic transport in single-photon detectors and emitters for quantum information processing. Recently, a novel concept has been proposed for efficient photon detectors based on the SAW transport of photoexcited carriers to microscopic detection areas, where they can be detected with high sensitivity by single-electron transistors. The feasibility of the concept has been accessed by investigating the absolute acoustic transport efficiency in a GaAs channel using a lateral p-i-n junction to collect the transported carriers, which are then detected using electrometers. The 200 μm-long GaAs transport channel is embedded in a microcavity to ensure that 70% of the incoming photons are absorbed. We show that 60% of the incoming photons are detected, implying a transport and detection efficiency of approx. 85%. This result demonstrates that very few carriers are lost during transport and subsequent detection at the contacts.SAWs can also be used for the controlled transfer of carriers between a high mobility QW and quantum wires (QWRs) and (QDs) embedded in it. We have recently demonstrated the fabrication of such coupled structures via the molecular-beam epitaxy growth of an (Al,Ga)As QW on (311)-GaAs substrates pre-patterned with etched mesas. The particular growth mode on these substrates leads to the formation of QWRs at the edges of the mesa.  Dot-like structures can be formed by the growth on μm-sized triangular mesas. Their optical properties were investigated by optically generating carriers in the QW and transporting them to the dot using a SAW. The remotely excited, spectrally resolved photoluminescence from the dots consists of a series of sharp lines, which indicate the radiative recombination in quantized electronic states (QDs) attributed to potential fluctuations along the mesa edges. The quantized nature was established by two-photon correlations studies of the emission from a single triangular dot using a Hanbury-Brown and Twist (HBT) setup, thus demonstrating single-photon sources based on acoustic transport.Work supported by the EU-ACDET2 project and by the NanoQuit consortium (BMBF, Germany) V. I. Talyanskii et al., Semicond. Sci. and Technol. 22, 209 (2007). P. D. Batista et al., Appl. Phys. Lett. 93, 262108 (2008). R. Nötzel et al., Appl. Phys. Lett. 68, 1132 (1996). O. D. D. Couto, Jr. et al., submitted.
3:00 PM - **CC9.2
Stress-controlled Phonon-impurity Resonances in Terahertz Silicon Lasers.
Sergey Pavlov 1 Show Abstract
1 , German Aerospace Center, Berlin Germany
Optically pumped terahertz silicon lasers utilize transitions between shallow donor states at low lattice temperatures. Population inversion in these lasers is built-up due to selective relaxation routes of optically excited electrons into impurity ground state. Each relaxation step of the electron occurs under assistance of intervalley and intravalley phonons with energies approaching the particular energy gaps between interacting excited donor states. These impurity phonon interactions determine, at the end, the lifetimes of the laser levels, and, therefore, efficiency of intracenter silicon lasers. Deformation of silicon crystal is a classical example of controllable influence on energy spectrum of shallow donor levels due to specific splitting and shifts of conduction band valleys. Using moderate (up to 3MPa) external uniaxial deformation of a crystal, one can radically modify the impurity spectra while the phononic spectra remain almost unchanged. We have demonstrated significant improvement of efficiency for intracenter silicon lasers followed by changes of lifetime for the upper and the lower laser levels due to moving the impurity levels either into or out of resonance with corresponding intervalley phonon frequencies. Due to nonlinearity of the stress-induced energy shifts of donor states involved in laser action, frequency tunability under external stress can be achieved for some laser mechanisms.
3:30 PM - CC9.3
Engineered Thermal Conductivity in Polymer-Metallic-Ceramic Nanolaminate Optical Coatings.
Adam Waite 1 2 , Jamie Gengler 1 2 3 , John Jones 1 , Chris Muratore 1 2 , Andrey Voevodin 1 Show Abstract
1 AFRL/RXBT, Air Force Research Labs, Wpafb, Ohio, United States, 2 , UTC, Dayton, Ohio, United States, 3 , Spectral Energies, LLC, Dayton, Ohio, United States
Multilayered polymer-metal-ceramic nanolaminate coatings were grown by room temperature plasma enhanced chemical vapor deposition (PECVD) and magnetron sputtering processes in a dual chamber PVD-CVD system to examine flexible optical coatings with tailored, through-thickness thermal conductivity. Highly cross-linked fluoropolymer films with refractive indices as low as 1.38 and decomposition temperatures over 150°C were grown by PECVD from an octafluorocyclobutane gas precursor. High refractive index (2.31) ceramic layers were deposited by sputtering of a TiO2 target. Thin (5-50 nm) metal interlayers with thicknesses on the order of phonon mean free paths were also integrated into the coating materials. The thickness and position of the layers with high and low refractive index layers could by adjusted to develop optical coatings with desired functionality for different wavelengths of incident light, while metal layers, such as silver and titanium were integrated to distribute heat and eliminate decomposition of the polymer films during heating by incident light. The architecture of the as-deposited films was examined using electron microscopy, and elipsometry was used to characterize the optical properties of films. The through-thickness values of thermal conductivity of films with and without the integrated metal layers were compared by time domain thermoreflectance (TDTR), a laser pump probe technique. After heating, the film architectures were re-examined with electron microscopy to see the effects of metal layers on polymer film stability after exposure to incident radiation.
4:15 PM - CC9.4
Engineering of High Frequency Acoustics in Nanoporous Alumina.
Yan Pennec 1 , Akihiro Sato 2 , Bahram Djafari-Rouhani 1 , George Fytas 3 2 , Wolfgang Knoll 2 , Martin Steinhart 4 , Jean-Numa Gillet 1 Show Abstract
1 Physics (IEMN), University of Lille 1, Villeneuve d'Ascq cedex France, 2 , Max Planck Institute for Polymer Research, Mainz Germany, 3 Dept. of Materials Science and Technology, FORTH, Heraklion Greece, 4 , Max Planck Institute for Microstructure Physics, Halle Germany
The phononic band diagram of periodic composite media can be engineered by the solid or liquid nature of the components, the contrast between their densities and elastic constants, their volume fractions as well as by the symmetry and morphology of the lattice that they constitute. Up to now, only few attempts have been reported to adapt concepts for functional phononic bandgap structures to the GHz frequency range, despite the potential of such systems for a plethora of applications. We show that self-ordered anodic aluminium oxide (AAO) containing aligned cylindrical nanopores with pore diameters of a few tens of nm and pore depths up to several 100 μm is a promising material platform for the still-challenging manipulation of sound propagation in the hypersonic (GHz frequency) range. AAO can be infiltrated with a broad range of fluids, thus yielding customized composite systems with high elastic contrast. Their tailor-made acoustic properties embracing their anisotropic elastic properties parallel and perpendicular to the pore axes and phonon localization were explored using FDTD simulations and compared to experimental studies by high resolution Brillouin spectroscopy. Reversible changes in density and elastic properties of the materials hosted in the nanopores of the rigid AAO scaffold induced by phase transitions allow switching the acoustic properties of this tunable AAO-based hypersonic phononic systems. In turn, probing phonons by BLS spectroscopy may reveal valuable information about anisotropic elastic properties in nanomaterials and the nature of interfaces in nanocomposites. A peculiar advantage of hypersonic phononic systems is the possibility to engineer interactions between phonons and visible wavelength photons, thus enabling the design of miniaturized acousto-optical devices.
4:30 PM - CC9.5
Modeling High Power Thermoelectric Waves from Exothermic Reactions around Nanoscale Thermal Conduits.
Joel Abrahamson 1 , Wonjoon Choi 2 , Michael Strano 1 Show Abstract
1 Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Coupling an exothermic chemical reaction with a nanotube or nanowire possessing high axial thermal conductivity creates a self-propagating reaction wave along its length. Such waves have been realized using 7-nm cyclotrimethylene-trinitramine (TNA) shells around multi-walled carbon nanotubes, and they propagate as fast as 2 m/s, more than 10,000 times faster than in bulk TNA. The wave produces a concomitant thermopower electrical pulse of high specific power, as large as 20 kW/kg. The long-term stability of the energy source and the high power density mean that these materials could be useful for micro- or nanoscale power sources for sensors, actuators, transmitters, and other micro-devices.The physics of a thermopower wave are distinct from conventional, static thermopower because of its anisotropic propagation; it traverses the system faster than the region behind the wave can cool, producing a high power pulse. Carrier velocity may be further increased by phonon drag from the motion of the thermal wave.A modeling framework to simulate thermopower generation is presented. It helps to explain the effects of various conduit material parameters (e.g. electrical and thermal conductivity) on the thermopower wave. The thermopower performance of various chemical fuels is also explored. Understanding the physics of these systems will point the way towards controlling and designing thermopower wave materials with desired properties (e.g. electrical pulse duration, voltage, performance at different temperatures).
4:45 PM - CC9.6
Thermomutability in Carbon Nanostructures and Protein Materials.
Zhiping Xu 1 , Markus Buehler 1 Show Abstract
1 Civil and Environmental Engineering, MIT, Cambridge, Massachusetts, United States
Carbon nanotubes are superior materials for thermal management and phononic devices because of their unique structure and ultra-high thermal conductivity. Interest has arisen recently to fabricate adaptive thermomutable materials or phononic devices, where the heat transfer performance or behavior can be controlled externally, for instance, through mechanical cues or electromagnetic fields. Here we present results of thermomutability in both carbon based nanostructures as well as biological protein materials. Using molecular dynamics (MD) simulations, we investigate the thermal conductivity of single-walled carbon nanotubes under a wide range of strains. We find that three mechanisms are responsible for the reversible thermomutabability phenomenon. At large tensile strain, the softening of group speed of phonon modes results in a monotonically decreasing of thermal conductivity; and this result is consistent with conventional theory of heat conduction in solids. However, at compressive strain the one dimensional nature of carbon nanotube results in transverse buckling, which releases the compressive strain stored in bond shortening. This unique feature in low dimensional materials gives rise to unexpected phenomena that cannot be explained by the heat conduction theory for solids, which would predict an increase in the thermal conductivity under compression. At low strain of –2%, the nanotube buckles into a spiral with radius of curvature. As a result the scattering reduces the thermal conductivity by 10%. As the compression increases to –5%, localized buckling with flattened cross-sections (~ 1 nm) occurs in the carbon nanotube and forms a thermal barrier with Kapitza length of 6 nm, which extremely limits the performance of carbon nanotube as thermal path. Our results reported here unveil the effects of strain on the thermal conductivity of low dimensional materials, and lay the groundwork for future design of thermomutable materials and phononic devices, where strain cues provide a convenient and efficient way to tuning their performance and behaviors. We conclude the presentation with a study of thermal properties of protein domains, the fundamental building blocks of a vast array of biological materials involved in critical functions of life. We show that the change in the conformation of backbones under mechanical load contributes mainly to the kinetic energy transfer in protein materials, and has significant effects on the ability to migrate heat. Our comparable study on thermal transfer within various secondary structures and amino acids sequences elucidate the molecular basis of energy transfer in living systems and provide a new class of thermomutable materials based on protein materials which is straightforward to design and to synthesize through self-assembly approaches.
5:00 PM -