Marat Khafizov, Ohio State University
Paulo Santos, Paul-Drude-Institut fur Festkorperelektronik
Greg Sun, University Massachusetts, Boston
Masashi Yamaguchi, Rensselaer Polytechnic Institute
II2: Phonons in 2D Materials
Monday PM, November 30, 2015
Hynes, Level 1, Room 110
2:30 AM - *II2.01
Thermal Transport in Free-Standing Silicon Membranes: Confinement, Intrinsic and Extrinsic Contributions
Clivia Sotomayor-Torres 1 2 Emigdio Chavez-Angel 1 Francesc Alzina 1 J. Sebastian Reparaz 1 Bartlomiej Graczykowski 1 Alexandros El Sachat 1 Markus Raphael Wagner 1 Marianna Sledzinka 1 Andrey Shchepetov 3 Mika Prunnila 3 Jouni Ahopelto 3
1ICN2 Catalan Institute of Nanoscience and Nanotechnology Bellaterra Spain2ICREA Catalan Institute for Research and Advanced Studies Barcelona Spain3VTT Technical Research Centre of Finland Espoo FinlandShow Abstract
The model system offered by state-of-the-art free-standing ultra-thin silicon membranes  is used to study the contribution of acoustic phonons to the thermal conductivity in silicon with view to its applications as a thermoelectric material. Our understanding so far is based on measured and modelled acoustic phonon dispersion relations , their lifetime , the transport in the diffusive and ballistic regimes , the role of the native oxide , the role of 2-D phononic crystals in the membranes  and direct in-plane thermal conductivity measurements based on laser-Raman thermometry [7-8].
The dispersion relations of membranes of thickness down to 8 nm are measured and simulated and found to be discretised flexural and dilatational modes which deviations from the linear behaviour close to q=0. The group velocity is obtained from these measurements and shown to decrease by a factor larger than 20. The lifetimes of the experimentally accessible first order dilatational mode is observed to decrease over one order of magnitude compared to bulk values. The data are used to model the thermal conductivity using a modified Akhieser mechanism incorporating the discrete and projected acoustic modes  and are compared to measurements based on thermal decay of a hot spot in the membranes. Boundary scattering and normal processes are found to contribute to the thermal transport as well as the native oxide.
We will discuss our results in the light of thermodynamic models, non-equilibrium molecular dynamics calculations and the modified Akhieser approach modified to include the interplay of confinement and real surfaces and interfaces.
 A. Shchepetov et al., Appl. Phys. Lett. 102 192108 (2013).
 J. Cuffe et al., Nano Lett., 12 3569 (2012).
 J. Cuffe et al., Phys. Rev. Lett. 110 095503 (2013).
 J. A. Johnson et al., Phys. Rev. Letts. 110 025901 (2013).
 S. Neogi et al., under review, ACS Nano.
 B. Graczykowski et al., Phys. Rev. B 91 075414 (2015).
 E. Chavez Angel et al., Appl. Phys. Lett. Materials 2 012113 (2014).
 J. S. Reparaz et al., Rev. Scien. Instruments 85 034901 (2014).
 E. Chávez-Ángel et al., Semicond. Scie. Tech. 29 124010 (2014).
3:00 AM - II2.02
Thermal Transport in 2D Materials Measured by Frequency Domain Thermoreflectance
Jia Yang 1 Aaron Schmidt 1
1Boston Univ Boston United StatesShow Abstract
Two-dimensional (2D) materials, such as graphene, molybdenum disulfide (MoS2), and hexagonal boron nitride (h-BN), have attracted extensive research because of their unique electronic and thermal properties. However, when in contact with other materials, the intrinsic properties of these 2D materials are modified by interface interactions. For example, the thermal conductivity of graphene is reduced by ~10 times or more when it is supported or encased by SiO2 and the addition of a monolayer graphene would significantly suppress the heat flow across typical metal-dielectric contacts. Therefore, it is important to understand how interfaces affect both in-plane and cross-plane heat transfer in 2D material systems. Here, we demonstrate simultaneous measurements of thermal conductivity and thermal boundary conductance of graphene, MoS2, and h-BN on various surfaces such as Mica, PET and glass, using frequency domain thermoreflectance. We investigate the effect of surface morphology on heat transfer in 2D materials. Our results have implications for phonon interactions in 2D material systems, and suggest that locally modified surface morphology could be used to optimize thermal transport in devices based on 2D materials.
3:15 AM - II2.03
Diffusive to Ballistic Dynamic Out-of-Plane Heat Transport in Thin Films
Andreas Makris 1 Tobias Haeger 1 Ralf Heiderhoff 1 Thomas Riedl 1
1University of Wuppertal Wuppertal GermanyShow Abstract
Understanding thermal transports of thin films in the nanometer range is essential for both fundamental and applied perspectives of modern electronic devices. While there is extensive experimental work on lateral (in-plane) heat transport in thin films, perpendicular (out-of-plane) thermal transport is far less explored. [1,2].
By means of dynamic Scanning Near-field Thermal Microscopy (SThM), we evidence experimentally for the first time the transition from diffusive to ballistic out-of-plane heat transport through ultra-thin metal-oxide films, prepared by atomic layer deposition. Using the 3omega;-technique , dynamic probe temperature variations (ΔT) are measured for thin films (down to 10 nm) of Al2O3, TiO2, and Al2O3/TiO2 nano-laminates, in reference to Si-substrates.
The properties of diffusive dynamic heat transport will be discussed from the engineering point of view taking equivalent electrical circuits into account. The measured ΔT between probe and substrate saturates with increasing film thickness and can be fitted with an exponential function . We did not find any change in the out-of-plane thermal conductivity for a film thickness larger than 100 nm.
For a film thickness below 100 nm, a description considering the probability of phonon-scattering within the film is found by the phonon Boltzmann transport equation from the ballistic to diffusive transport limits [5,6]. This model will be shown to fail once the Casimir thickness limit of the respective material is reached and a ballistic heat transport of blackbody phonon radiative transfer has to considered .
In this work, we provide experimental evidence for this ballistic transport for the first time. A mean free path length (l) of 25 nm, 22 nm, and 17 nm, in the Al2O3, TiO2, and Al2O3/TiO2 nano-laminate, is determined, respectively. Thus, for a thickness < l, a frequency independent ΔT is found , which depends on the film material. The out-of-plane thermal conductivity decreases linearly as the film thickness is reduced [8,9].
The decrease of thermal conductivity with decreasing film thickness has motivated the claim of thin films being thermal insulators. In view of our results this concept has to be revised, as thin films are very well able to transport high amounts of thermal energy due to ballistic out-of-plane heat transport.
Marconnet A.M. et al., Journal of Heat Transfer 135, (2013), 061601
Gomes C.J. et al., Journal of Heat Transfer 128, (2006), 1114-1121
Cahill D.G. and Pohl R.O., Phys. Rev. B 35, (1987), 4067-73
Heiderhoff R. et al., Microelectronics Reliability 53, (2013), 1413-1417
Majumdar A., Journal of Heat Transfer 115, (1993), 7-16
Maassen J. and Lundstrom M., J. Appl. Phys. 117, (2015), 135102
Lee S.-M. and Cahill D.G., J. Appl. Phys. 81, (1997), 2590-2595
Yamane T. et al., J. Appl. Phys. 91, (2002), 9772-9776
Feng X.-L. et al.,” Microscale Thermophys. Eng. 7, (2003), 153-161
3:30 AM - II2.04
Silicon Membranes for Thermoelectrics
Andrey Shchepetov 1 M. Ylilammi 1 Mika Prunnila 1 Jouni Ahopelto 1
1VTT Technical Research Centre of Finland Espoo FinlandShow Abstract
Silicon is currently the major semiconductor material for ICT and will probably sustain the Moore&’s law the coming decades in the integrated circuits and memories. As thermoelectric material silicon has suffered from the relatively high thermal conductivity, preventing successful exploitation of silicon in energy harvesting and in thermal management, i.e., like cooling. The recent experimental and theoretical results have shown that in ultra-thin silicon membranes the propagation of phonons can be largely blocked while maintaining the good electrical properties, high conductivity and Seebeck coefficient.
In this presentation we will discuss the potential of silicon membranes as thermoelectric material in the light of the recent findings of controlling the behaviour of phonons in the nanoscale membranes. The simulations show that cooling of several tens of degrees from the room temperature is possible with the membrane devices.
 S. Neogi, J. S. Reparaz, LF C. Pereira, B. Graczykowski, M. R. Wagner, M. Sledzinska, A. Shchepetov, M. Prunnila, J. Ahopelto, C. M. Sotomayor-Torres, and D. Donadio, Tuning Thermal Transport in Ultrathin Silicon Membranes by Surface Nanoscale Engineering, ACS Nano 9 (2015) pp 3820-3828.
4:15 AM - *II2.05
Perspectives on Phonons in Layered Materials
Mildred S. Dresselhaus 1
1Massachusetts Institute of Technology Cambridge United StatesShow Abstract
The unusual properties of phonons in layered materials have attracted much attention in recent years. In the past few years graphene has served as a prototype material for phonon studies despite the unusual properties of the electron-phonon interaction in graphene. Whereas the electronic properties of layered compounds differ greatly between graphene, hexagonal boron nitride, the various metal dichalcogenides, and black phosphorene, the phonon properties are far less diverse. However, the diversity between the different materials&’ phonon properties nevertheless gives opportunities for different applications for the various layered materials, which will be the focus of this talk.
4:45 AM - II2.06
Thermal Conductivity of Vertically Self-Ordered Nanocrystalline Boron Nitride Thin Films for Enhanced Preferential Phonon Transport
Olivier Cometto 1 2 Bo Sun 3 Siu Hon Tsang 1 4 Xi Huang 3 Yee Kan Koh 3 Edwin Hang Tong Teo 1 4
1Nanyang Technological University Singapore Singapore2CINTRA CNRS/NTU/THALES UMI 3288 Singapore Singapore3National University of Singapore Singapore Singapore4Nanyang Technological University Singapore SingaporeShow Abstract
Hexagonal BN (h-BN) displays a preferential phonon transport along the a-axis with reported in-plane thermal conductivity as high as 390 W.m-1.K-1, while its c-axis conductivity remains low, in the range of 2 W.m-1.K-1. In comparison, the thermal conductivity of amorphous BN (a-BN) is in the order of 1 W.m-1.K-1. The anisotropic physical properties of h-BN and in particular of its high in-plane thermal conductivity originate from the orientation of its hexagonal structure, and consequently, the ability to control the orientation of these basal planes would unveil opportunities for applications requiring directional phonon transport.
Here, we present a novel way of controlling the growth of h-BN nanocrystals with preferential vertical orientation (as opposed to h-BN with horizontal alignment) and study its cross plane thermal conductivity. Vertically self-ordered Nanocrystalline Boron Nitride (ordered BN) is a highly ordered material similar to hexagonal BN, with its planar structure perpendicularly oriented to the substrate. The ordered BN thin films were grown using a High Power Impulse Magnetron Sputtering (HiPIMS) system with a lanthanum hexaboride (LaB6) target reactively sputtered in nitrogen gas. Growth parameters were optimized to get the best vertical alignment of the BN planes and the effects of that preferential ordering on the film&’s thermal conductivity where studied using Time-Domain Thermoreflectance (TDTR). Owing to the intrinsic capability of h-BN to facilitate the propagation of phonons along its basal planes, we observed a significant increase in the cross plane thermal conductivity of ordered BN with a 5-fold increase (5 W.m-1.K-1) compared to amorphous BN.
The ability to direct phonon transport vertically with oriented BN, combined with its low electrical conductivity and low dielectric constant makes it the perfect candidate for top thermal dissipation on electronic devices. Its highly anisotropic heat transfer property could be used to create directional heat guides, where the phonon would propagate along the hexagonal planes straight to a heat sink.
5:00 AM - II2.07
Anisotropic Thermal Conductivity of Exfoliated Black Phosphorus
Hyejin Jang 1 Joshua D Wood 2 Christopher Rogelio Ryder 2 Mark C. Hersam 2 3 4 David Cahill 1
1University of Illinois at Urbana-Champaign Urbana United States2Northwestern University Evanston United States3Northwestern University Evanston United States4Northwestern University Evanston United StatesShow Abstract
Black phosphorus (BP), the stable allotrope of phosphorus at ambient temperature and pressure, has attracted great interest as a novel two-dimensional electronic material. It has a puckered honeycomb structure, which leads to strong in-plane anisotropy in transport properties along the two high-symmetry in-plane directions. We measured the thermal conductivities of BP along the three axes of the orthorhombic crystal at room temperature using conventional time-domain thermoreflectance (TDTR) and beam-offset TDTR. The BP samples are in the form of exfoliated flakes, ~500 nm thick, and encapsulated by a few nm of alumina deposited by atomic-layer deposition. The thermal conductivities are 86±9 W m-1 K-1 in the zigzag direction, 35±5 W m-1 K-1 in the armchair direction, and 3.4±0.4 W m-1 K-1 in the through-plane direction. The longitudinal speed of sound is 5.0±0.3 nm ps-1 in the through-plane direction.
5:15 AM - II2.08
Self-Interaction in 2D-Chiral Architecture: Modification of Thermal Transport without Electron Scattering
Huashan Li 1 Jeffrey C. Grossman 1
1Massachusetts Institute of Technology Cambridge United StatesShow Abstract
Successful thermal management is essential for designing thermoelectric cells, next generation integrated circuits and three-dimensional electronics. While the emerging class of 2D materials shows promise in their unprecedented carrier mobility, their potential tunability in thermal transport remains to be explored. To accelerate this process, multi-scale simulations have been implemented to design novel nanostructures to control both electronic and thermal properties. In particular, a recent theoretical study suggests that appropriate chemical functionalization may lead to efficient graphene-based thermoelectric materials.
Here we consider the prospect of a chiral architecture based on 2D materials, using a prototype system of a graphene nanoribbon (GNR) wrapping around a Si nanowire. The phonon thermal conductivity was calculated by non-equilibrium classical molecular dynamics, while the electronic properties were computed by Density functional theory and a Boltzmann transport approach. The results indicate that the phonon thermal transport may be modified by self-interaction between neighboring turns with the electronic properties nearly unaffected, due to the different characteristic lengths of phonon and electron. On the one hand, the attraction between turns of GNR with its edge passivated by H induces long-range disorder along the spiral axis, which suppresses the thermal contribution from phonons with long wavelengths, and thus leads to anomalous length independent phonon thermal transport in the quasi-1D system. According to our simulations, this may provide substantial improvement in the thermoelectric efficiency compared to planar structures. On the other hand, with appropriate organic ligands, the phonon thermal conductivity may be enhanced compared to the planar architecture, due to the additional transport paths through ligands that are close to or inter-connected to each other. Such effects may be beneficial for producing nanosolenoids for magnetic applications.
 Balandin, A. A. et al., Nat. Mater., 2011, 10, 569.
 Kim, J. et al., Nano Lett., 2015, 15, 2830.
5:30 AM - II2.09
Effect of the Electron Beam Irradiation on Thermal Conductivity of Single Layer Graphene
Hoda Malekpour 1 Pankaj Ramnani 2 Roger Lake 1 Ashok Mulchandani 2 Alexander A. Balandin 1
1University of California - Riverside Riverside United States2University of California - Riverside Riverside United StatesShow Abstract
Thermal properties of graphene reveal some unique features related to the specifics of the two-dimensional (2-D) phonon transport [1-2]. It is expected that phonon scattering due to point defects in 2-D systems may also differ from that in bulk crystals. In this presentation we will report the results of our investigation of thermal conductivity of suspended single layer graphene with defects introduced using the low-energy electron beam irradiation. For this study, the chemical vapor deposition (CVD) grown graphene was suspended over gold transmission electron microscopy (TEM) grid. The Raman D to G peak ratio was used to quantify the amount of defects after each electron beam irradiation step. The samples were exposed to the low energy (20 KeV) electron beam for a certain period of time in several steps, to obtain a certain defect density. For each step, the thermal conductivity was measured using the optothermal Raman technique . For these measurements the gold TEM grid acted as a heat sink. The suspended graphene grains were illuminated at the center with laser light that provided local heating. The temperature rise was determined from the temperature shift of the Raman G peak of SLG. The thermal conductivity was obtained by solving the heat-diffusion equation with the help of COMSOL package. The thermal conductivity decreased with the increasing defect density. At the defect density of 1.5 × 1011 cm-2 we observed a change of the slope followed by the saturation behavior in the thermal conductivity. The non-monotonic decrease can be related to the interplay of the point defect and grain boundary scattering appearing due to defect clustering. The exact mechanism is currently under investigation.
The work at UC Riverside was supported, in part, by the National Science Foundation (NSF) project ECCS-1307671 Two-Dimensional Performance with Three-Dimensional Capacity: Engineering the Thermal Properties of Graphene.
 A.A. Balandin, Nature Materials, 10, 569 - 581 (2011).
 D.L. Nika and A.A. Balandin, Journal of Physics: Condensed Matter, 24, 233203 (2012).
II1: Phonon Transport 1 Thermal Transport, Magnetic Effects
Monday AM, November 30, 2015
Hynes, Level 1, Room 110
9:00 AM - *II1.01
Phonon-Driven Thermal-Transport Properties from First Principles: Capabilities of a Community Code and Its Applications
Aleksandr V. Chernatynskiy 2 1 Simon Robert Phillpot 1
1Univ of Florida Gainesville United States2Missouri University of Science and Technology Rolla United StatesShow Abstract
Only recently has the prediction of the thermal conductivity on the basis of the first principles calculations become possible. Implementation of the methodology, however, is technically challenging and requires a lengthy development process. We thus introduce the Phonon Transport Simulator (PhonTS), a Fortran90, fully parallel code to perform such calculations. PhonTS possesses a large array of options and returns the thermal conductivity tensor together with related quantities, such as spectral thermal conductivity, phonon lifetimes, mean free paths and Grüneisen parameters. It can also determine the Raman and infra-red response of materials. First principles calculations are implemented via convenient interfaces to widely-used third-party codes, such as VASP, while many classical potentials are included in PhonTS itself. The code is carefully validated against data published in the literature from various thermal conductivity computational techniques and against experimental data. Examples of insights obtained from PhonTS are given, including fluorite-structured materials and Mg2X (X=C, Si, Ge, Sn and Pb) family.
9:30 AM - II1.02
Thermal Transport in Colloidal Nanocrystal-Based Materials
Yuanyu Ma 1 Minglu Liu 1 Robert Y. Wang 1
1Arizona State University Tempe United StatesShow Abstract
Colloidal nanocrystals consist of an inorganic crystalline core with short ligand molecules bound to its surface. These nanocrystals can be made with excellent control over size, shape, and composition, thereby enabling the exploration of relationships between microstructure and thermal transport. Furthermore, these nanomaterials are made via inexpensive solution-phase processing and can be easily transformed into nanoparticle-in-matrix composites. The excellent microstructural control and inexpensive processing of these materials make them an attractive candidate for thermal properties exploration.
In the first part of this talk, we discuss thermal transport in PbS nanocrystals that are assembled into thin films. We systematically study the effect of nanocrystal diameter (3 - 9 nm) and surface chemistry (oleic acid, ethanethiol, ethylamine, and tetrabutylammonium iodide). Our measurements find that these colloidal nanocrystal films have very low thermal conductivities ranging from 0.1 - 0.5 W/m-K. We find that larger nanocrystals and shorter ligands lead to higher thermal conductivities. Importantly, we find that surface chemistry can have a larger impact on thermal conductivity than nanocrystal size.
In the second part of this talk, we use a modular process to create nanoparticle-in-matrix composites and then study the corresponding composite thermal conductivity. We create the composites by mixing soluble metal-chalcogenide precursors with colloidal nanocrystals in the solution-phase. We then thermally transform the metal-chalcogenide precursors into a polycrystalline matrix that encapsulates the nanocrystals. Specifically, we focus on polycrystalline In2Se3 matrices with varying volume fractions of embedded CdSe nanocrystals. We find that the thermal conductivity of these composites is very low over the entire nanocrystal volume fraction range, on the order of 10-1 W/m-K. We also find that the presence of CdSe nanocrystals strongly effects the formation of the In2Se3 matrix (i.e. grain orientation and size as well as ternary phase formation). These structural changes lead to competing effects on thermal transport relative to the cases of 0% and 100% nanocrystal volume fractions.
9:45 AM - II1.03
Optimization Technique to Solving the Boltzmann Transport Equation to Obtain Thermal Conductivity Suppression Functions
Vazrik Chiloyan 1 Lingping Zeng 1 Samuel Huberman 1 Gang Chen 1
1MIT Cambridge United StatesShow Abstract
There is a recent growth in the demand and availability of analytical solutions to the Boltzmann transport equation. With experiments that probe small length scales, such as transient grating experiments, or fast time scales, such as the pump-probe thermo reflectance experiments, there is a clear need to understand the deviation of thermal transport from the diffusive limit. However, the BTE is notoriously difficult to solve and analytical solutions exist only for very special geometries and considerations. In this study we present an optimization technique to approximate the temperature profile from the BTE that is then used to extract suppression functions analytically which are accurate both at the diffusive limit and the ballistic limit. The utilization of the expected temperature behavior at the diffusive and ballistic limits allows one to solve the BTE with great accuracy and minimal computation effort over the entire range of possible length scales. The analytical suppression functions we obtain can then be used to study various materials and understand non-diffusive transport. We utilize these suppression functions to understand the size effects that come into effect at small length scales and to provide more accurate mean free path reconstruction. This work is supported by Solid State Solar-Thermal Energy Conversion Center (S3TEC), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, under Award Number: DE-SC0001299/DE-FG02-09ER46577.
10:00 AM - II1.04
The Roles of Ionic Radius and Mass of Doped Trivalent Cations on the Thermal Transport in CeO2
Xianming Bai 1 Aleksandr V. Chernatynskiy 2
1Idaho National Laboratory Idaho Falls United States2University of Florida Gainesville United StatesShow Abstract
Fluorite-based oxides (e.g. CeO2, ZrO2 and UO2) are of technological importance to many applications such as fuel cells, thermal barrier coating materials, and nuclear fuels. The thermal transport property in these oxides is an important performance metrics for their applications. Impurities in these oxides, either doped or produced by reaction, can affect the phonon transport behavior in the oxides significantly. Here we use five different trivalent cations doped CeO2 as model systems to study the roles of ionic radius and ionic mass of doped cations on the thermal transport in oxides. As these cations have the same charge state, they induce the same amount of oxygen vacancies. Therefore, the effects of ionic radius and ionic mass on thermal transport can be compared for different doped cations. Using non-equilibrium molecular dynamics simulations, the thermal conductivities of these doped systems are calculated for a wide range of concentrations of doped ions. We found that the bulk thermal conductivity decreases as the ionic radius increases while such a correlation does not exist for ionic mass. When these doped cations segregate to grain boundaries, they increase the grain boundary thermal resistance (Kapitza resistance) and the increment also correlates strongly with the ionic radius rather than ionic mass. Finally, using these atomistic results as inputs for an analytical model, the effect of segregation of doped cations to grain boundaries on the thermal transport in nanocrystals and polycrystals is studied for different doped ions and at different grain sizes. Our results show that segregation of cations to grain boundaries increases the thermal conductivity of nanocrystals significantly and the smaller ionic radius increases the thermal conductivity more.
10:15 AM - II1.05
Thermal Annealing Effect on the Electrical and Thermal Properties of Organic Semiconductor Thin Film
Xinyu Wang 1 Boyu Peng 1 Paddy K. L. Chan 1
1The University of Hong Kong Hong Kong Hong KongShow Abstract
With the strong needs for high-performance organic electronics, improving the electrical and thermal properties of organic semiconductors draws a lot of attentions. Due to the intrinsic amorphous nature of the material, the crystallinity, grain size, roughness, dislocation of organic semiconductor thin films significantly rely on the growth of organic semiconductors. Up to now, different techniques, including self-assembled monolayers (SAMs), elevated substrate deposition temperature, can be used to modify the properties of organic semiconductors. Here we particularly focus on investigating the post processing thermal annealing effect on the electrical and thermal properties of a small molecule organic semiconductor, dinaphtho[2,3-b:2&’,3&’-f]thieno[3,2-b]thiophene (DNTT). DNTT thin films are firstly deposited by thermal evaporation at room temperature (24 °C) and then annealed for 1 hour at different temperatures (24 °C to 140 °C) under nitrogen environment. From atomic force microscopy (AFM) and high resolution transmission electron microscope (HRTEM) images, it can be noticed that DNTT molecules will start to aggregate together and thus the crystal size and roughness will increase after thermal annealing. For the sample annealed at 140 °C, the selected area electron diffraction (SAED) pattern clearly indicates a single crystal diffraction pattern while the film annealed at 24 °C shows a polycrystal diffraction ring pattern. We further measure the cross plane space-charge-limited current (SCLC) mobility of DNTT thin films and the results indicates that the mobility firstly goes up and then drops down while the annealing temperature increases. At 80 °C, the mobility obtains a maximum value of 1.11×10-3 cm2/V-s which gives a threefold increase in comparison with room temperature, 3.6×10-4 cm2/V-s and this trend is correlated to crystallinity improvement of DNTT thin films. Other than electrical characterizations, we apply 3-omega; method to measure thermal conductivity of DNTT thin films. We find that thermal conductivity shows a much weak dependence on thermal annealing temperature and thermal conductivity at 80 oC just has a 17% enhancement in comparison with 24 °C. At higher temperature (ge;100 °C), thermal conductivity also shows a decrease trend. We also attribute the increase of thermal conductivity to the crystallinity improvement after annealing. As shown in HRTEM images, because thermal annealing generates larger crystal size, DNTT with larger crystal size can have a higher phonon mean free path, which enhances thermal conductivity. However, when the annealing temperature is higher than 100 °C, due to the serious aggregation of DNTT grains, larger roughness leads to the stronger charge and phonon scattering, which results in the decrease of SCLC mobility and thermal conductivity. Our findings show that thermal annealing could be an efficient and facile method to improve the electrical and thermal properties of organic semiconductors.
11:00 AM - *II1.06
Magnetic Field Dependence of Phonon Heat Transport
Joseph P. Heremans 1 Stephen R Boona 1 Nikolas Antolin 1 Hyungyu Jin 1 Oscar Restrepo 1 Roberto C Myers 1 Wolfgang Windl 1
1Ohio State Univ Columbus United StatesShow Abstract
Recently, we have shown  that the lattice thermal conductivity of InSb is sensitive to an external magnetic field and that this effect arises from the diamagnetic moment induced in the samples by a high magnetic field. Our density-functional theory calculations showed that the atomic displacements that constitute a phonon result in a local distortion of the valence band structure. Since the valence band determines the diamagnetic susceptibility of a solid, this means that there is a local variation of the diamagnetic moment in the presence of an external applied magnetic field. While the spatial variation of the moment is small, its spatial derivative, which determines the magnetic force, is not. This force still results in only a small perturbation of the phonon frequencies, but has a much bigger influence on the Grüneisen parameters when the correct temperature range is chosen. These parameters determine the probability of phonon-phonon interactions. Experimentally, the effect is measured on a specially prepared sample in which the contribution of phonon-phonon scattering is amplified compared to that of all other phonon properties. An external field of 7 T is measured to affect the phonon-phonon Umklapp processes by 12% at 4K. The effect can be modeled in the absence of any adjustable parameter from band structure and phonon dispersion calculations, and the model explains the experimental results. In this talk, the consequences of this observation will also be discussed. For example, the presence of a phonon magnetoresistance implies the existence of a phonon Hall effect. Preliminary attempts at observing and calculating this new effect will be presented.
 Hyungyu Jin, Oscar D. Restrepo, Nikolas Antolin, Stephen R. Boona, Wolfgang Windl, Roberto C. Myers and Joseph P. Heremans, Nature Materials 14, 601-606 (2015) (DOI:10.1038/nmat4247)
11:30 AM - *II1.07
Time-Resolved Magneto-Optical Kerr Effect for Studies of Phonon Thermal Transport
David Cahill 1 Jun Liu 1 Judith Kimling 1 Johannes Kimling 1
1Univ of Illinois Urbana United StatesShow Abstract
In a conventional time-domain thermoreflectance (TDTR) measurement, the sample is first coated with an optically opaque metal film, typically Al, that acts as the optical transducer in the experiment. The transducer layer converts the incident pump optical pulses to heat and reports changes in temperature of the surface of the sample through changes in the intensity of the reflected probe. In TDTR, the transducer must be optically opaque to suppress contributions to the changes in the intensity of the reflected probe that would otherwise come from the temperature dependence of the optical constants of the materials under study. We have found that the rotation of the polarization of the probe created by the magneto-optical Kerr effect (TR-MOKE) of a thin ferromagnetic film provides robust alternative measurement of temperature that is both ultrafast and high signal-to-noise even when the magnetic layer is <10 nm thick. We will discuss how the reduction in thickness of the transducer layer in a TR#8209;MOKE experiment by an order of magnitude in comparison to TDTR enables improved measurements of i) lateral heat flow in thin films and crystals; ii) the thermal conductance of interfaces with low thermal conductivity materials and iii) the additional thermal resistance of metal/dielectric interfaces that is generated by the mismatch in the spectrum of phonons that carry heat across the interface versus the spectrum of phonons that carry heat in the crystal.
12:00 PM - II1.08
Comparing InGaZnO Thin-Film Thermal Conductivity for Different Crystallinity and Porosity
Boya Cui 1 2 Donald B Buchholz 3 Robert P. H. Chang 1 3 Li Zeng 1 3 Michael J. Bedzyk 1 3 Denis Keane 4 Xinge Yu 5 Jeremy Smith 5 Tobin J. Marks 5 3 Antonio Facchetti 6 5 Jiajun Luo 2 Matthew Grayson 1 2
1Northwestern University Evanston United States2Northwestern University Evanston United States3Northwestern University Evanston United States4Argonne National Lab Lemont United States5Northwestern University Evanston United States6Polyera Corp. Skokie United StatesShow Abstract
Indium gallium zinc oxide (IGZO) thin films have been attracting attention due to their outstanding electrical properties which enable applications such as display backplanes, smart windows, and low-cost flexible electronics. On the other hand, thermal properties of IGZO thin films are just as important in designing efficient thermal management, optimizing IGZO-based thermoelectric applications and understanding fundamental phonon behavior, but data until now has been lacking. Since IGZO can be grown by various techniques leading to different degrees of porosity over a range of morphologies from crystalline to amorphous, we report the temperature-dependent thermal conductivity of IGZO thin films representing all these morphologies, and observe that the thermal conductivity in certain cases changes over year-long time scales indicating morphological instability.
IGZO films with different morphologies were grown by both physical deposition and chemical synthesis on sapphire substrates. Pulsed laser deposition (PLD), sputtering, and combustion solution process were conducted to fabricate amorphous IGZO (a-IGZO) films, and additionally semi-crystalline (semi-c) and crystalline (c-) films were obtained by post-annealing after PLD. The crystallinity and film quality was confirmed by X-ray diffraction (XRD) and grazing incidence wide-angle X-ray scattering (GIWAXS).
The thin-film thermal conductivity k was measured with the 3omega; method from 18 K to room temperature 300 K. Dependences are described below:
(1) Porosity: All the dense films (by PLD, sputtering and spray-combustion) follow the same empirical power law of k ~T0.6 throughout the measured temperature range showing consistency with the Cahill-Pohl model, while the porous spin-combustion k decreases significantly below 60 K. Pores acting as voids in phonon transport may play a more important role in suppressing low-frequency phonons at low temperatures, or residual chemical impurities may scatter long-wavelength phonons at low temperatures.
(2) Crystallinity: For PLD-grown films, the amorphous and crystalline films have similar low thermal conductivity. The disordered structure in a-IGZO and acoustic impedance variation in the layered structure in c-IGZO might both scatter phonons similarly. The semi-c film exhibits the highest thermal conductivity among the three. This may be caused by higher in-plane thermal conductivity for tilted grains.
(3) Shelf-life: After being stored in vacuum for months, the thermal conductivities of a- and c-IGZO films remain the same, while that of semi-c IGZO thin film reduces significantly at all temperatures but still follows the law k ~T0.6. Consequently, the semi-c structure may be meta-stable between amorphous and crystalline phases, and it may have changed its morphology in ambient conditions. Storage in vacuum may also induce more oxygen vacancies, altering the mesoscopic structures and affecting phonon transport.
12:15 PM - II1.09
Modeling Thermal Conductivity Reduction in Composites with Embedded Nanofibers
Vineet Unni 1 Joseph Patrick Feser 1
1Univ of Delaware Newark United StatesShow Abstract
Introducing nanoscale inhomogeneity into semiconductor alloys is a known route to enhance the scattering of long-wavelength phonons and to subsequently reduce thermal conductivity. While extensive modeling exists for layered and nanoparticulate media, less focus has been directed toward fibrous nanostructures. Using a continuum mechanical treatment, we develop an analytic model for the phonon scattering cross-section of a cylindrically shaped elastic discontinuity in an isotropic medium, where incident phonons can have arbitrary polarization, angle of incidence, and wavelength. A Boltzmann transport theory framework is used to simulate the anisotropic thermal conductivity tensor of composites with fiber networks composed of aligned fibers, fiber mats, and randomly oriented fibers. We find that for fixed fiber volume fraction, there is an optimal fiber radius that produces lowest thermal conductivity, and that the optimal fiber radius can achieve lower thermal conductivity than a similarly optimized composite using spherical scatterers. Specific results are given for the case of silicide fibers embedded in silicon-germanium alloys. We also investigate the breakdown of the continuum treatment of phonon scattering cross-section using an atomistic model.
12:30 PM - II1.10
Thermal Conductivity in Multiferroic BiFeO3/CoFe2O4 Nanocomposite Films
Chen Zhang 1 Bolin Liao 2 Shuchi Ojha 1 Vazrik Chiloyan 2 Gang Chen 2 Caroline A Ross 1
1Massachusetts Institute of Technology Cambridge United States2Massachusetts Institute of Technology Cambridge United StatesShow Abstract
Thermal conductivity of thin films is sensitive to the presence of interfaces and grain boundaries. Self-assembled oxide nanocomposite films, in which one phase grows as columns within another phase and both phases are epitaxial with the substrate, provide a model system with well-defined vertical interfaces that allow the contribution of interfaces to thermal properties to be quantified. A spinel/perovskite system consisting of pillars of ferrimagnetic CoFe2O4 (CFO) in a matrix of ferroelectric BiFeO3 (BFO) was grown on single crystal SrTiO3 by pulsed laser deposition. The pillars were ~100 nm tall and ~30 nm in diameter, spaced ~100 nm apart. The pillars have a square or rectangular cross-section and the interface between the BFO and CFO consists of (110) planes. The thermal properties of the BFO/CFO nanocomposites were investigated using time-domain thermoreflectance (TDTR). The nanocomposites exhibited decreased thermal conductivity, 2.1 - 2.5 W/mK, compared to single phase CFO (3.3 W/mK) and BFO (5.0 W/mK) films. The volume fraction of CFO and the strain state of the film were varied by changing the deposition conditions and substrate type and the effects on the thermal conductivity in BFO/CFO nanocomposites will be discussed. The dependence of thermal conductivity on the presence of magnetic domain walls was also investigated by comparing CFO films after ac and dc demagnetization, but the magnetic domain walls had small effects compared to the physical interfaces. Based on the systematic study, we interpret the reduced thermal conductivity of BFO/CFO nanocomposites by evaluating the effects of phase boundaries, strain, and domain walls on phonon transport. Understanding of the way phonons transport in multiferroic nanocomposites is useful for developing multifunctional thermoelectric energy conversion devices and manipulating phonons via external means. This work is supported by S3TEC, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Basic Energy Sciences, under Award NO. DE-FG02-09ER46577.
12:45 PM - II1.11
On The Solenoidal Heat Flux in Quasi-Ballistic Thermal Conduction
Ashok Ramu 1 John E. Bowers 1
1University of California Santa Barbara Santa Barbara United StatesShow Abstract
The Boltzmann transport equation for phonons is recast directly in terms of the heat-flux by means of iteration followed by truncation at the second order in the spherical harmonic expansion of the distribution function. This procedure displays the heat-flux in an explicitly coordinate-invariant form, and leads to a natural decomposition into two components, namely the solenoidal component in addition to the usual irrotational component. The solenoidal heat-flux is explicitly shown to arise in a right-circular cylinder when the heating breaks the azimuthal symmetry. These findings are important in the context of phonon resonators that utilize the strong quasi-ballistic thermal transport reported recently in silicon membranes at room temperature.
Marat Khafizov, Ohio State University
Paulo Santos, Paul-Drude-Institut fur Festkorperelektronik
Greg Sun, University Massachusetts, Boston
Masashi Yamaguchi, Rensselaer Polytechnic Institute
II4: Phonon Mean-Free-Path and Ballistic Transport
Tuesday PM, December 01, 2015
Hynes, Level 1, Room 110
2:30 AM - *II4.01
Phonon Transport: the Ballistic, Coherent, and Hydrodynamic Regimes
Gang Chen 1 Maria Luckyanova 1 Lingping Zeng 1 Sangyeop Lee 1 Bolin Liao 1 Jiawei Zhou 1 Vazrik Chiloyan 1 Samuel Huberman 1
1MIT Cambridge United StatesShow Abstract
In this talk I will discuss the ballistic, coherent, and hydrodynamic regimes of heat conduction in nanostructures. We develop phonon mean free path spectroscopy techniques by exploring quasi-ballistic heat transfer around nanoscale heat sources. Two-dimensional and one-dimensional metallic nanostructures serve as nanoscale heat sources with minimal substrate electron-hole generation in femtosecond, time-domain thermoreflectance pump-probe experiments that measure size dependent thermal conductivity. Algorithms are developed to extract phonon mean free path distributions based on the experimental results, first-principles simulations, and solutions of the Boltzmann equation. In superlattice structures, ballistic phonon transport across the entire thickness of the superlattices implies that phase coherence is maintained throughout the structures. We observed such coherent transport in GaAs/AlAs superlattices with fixed period thicknesses and a varying number of periods. This interpretation is supported by first-principles, Green&’s functions, and molecular dynamics simulations. Accessing the coherent heat conduction regime opens a new venue for phonon engineering. We will show further thermal conductivity reduction and signs of phonon localization in GaAs/AlAs superlattices with ErAs nanodots at the interfaces. Finally, we will discuss the phonon hydrodynamic transport regime in graphene, recently discovered via first-principle simulations. In this regime, phonons under a temperature gradient drift with an average velocity, a behavior similar to fluid flow under a pressure gradient in a pipe. Conditions for observing this phonon hydrodynamic regime will be discussed.
This material is based upon work supported as part of the “Solid State Solar-Thermal Energy Conversion Center (S3TEC), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number: DE-SC0001299/DE-FG02-09ER46577.
3:00 AM - II4.02
Phonon Ballistic Transport and Frequency Dependent Thermal Conductivity in Al0.1Ga0.9N Thin Film Semiconductors
Yee Rui Koh 1 2 MohammadAli Shirazi 1 2 Amr Mohammed 1 2 Michael James Manfra 1 2 3 Ali Shakouri 1 2
1Purdue University West Lafayette United States2Purdue University West Lafayette United States3Purdue University West Lafayette United StatesShow Abstract
AlxGa1-xN proves to be technologically important material for LED industry as well as high electron mobility transistors (HEMTs). Because of high power applications, it is important to study submicron thermal transport in thin film AlxGa1-xN structures. Previous literature focused on films that are 100&’s nm in thickness and this limited the study of long mean-free-path ballistic phonons.   In this study, a high quality 1.2mu;m Al0.1Ga0.9N thin film sample was grown by using plasma-assisted molecular beam epitaxy (PAMBE). We report on measurements of the thermal conductivity of Al0.1Ga0.9N thin film by using time-domain thermoreflectance (TDTR) over the laser modulation frequency 0.8< f <10 MHz and temperature range 100 < T < 500 K. Al0.1Ga0.9N achieves highest thermal conductivity at a relative high temperature ~300K. Based on the temperature dependence, we analyze the role of the different scattering mechanisms. The measurements show pronounced boundary scattering at low temperature (100K and 150K). This is also reflected in flat plateau in the thermal conductivity versus modulation frequency. We also found a unique frequency-dependent thermal conductivity at 200K, where both Umklapp (U-type) and boundaries scattering contribute to the thermal transport. Thermal conductivity of Al0.1Ga0.9N drops up to 40% at 10MHz compare to the bulk thermal conductivity at room temperature. Similarities and differences with other semiconductor alloys, e.g. SiGe, InGaAs  will be highlighted. Preliminary study of the superdiffusive transport parameters and Lévy fractal dimension for phonon random walk will be also presented.
 W. Liu and A. A. Balandin. J. Appl. Phys, Vol. 97, 073710 (2005)
 B. C. Daly, H. J. Maris, A. V. Nurmikko, M. Kuball and J. Han, J. Appl. Phys. 92,3820 (2002)
 Y. K. Koh and D. Cahill, Phys. Rev. B, 76, 075207 (2007)
3:15 AM - II4.03
Ballistic to Diffusive Heat Transfer in Molecular Building Blocks of Inorganic/Organic Multilayers
Ashutosh Giri 1 Janne-Petteri Niemela 2 Tommi Tynell 2 John Gaskins 1 Brian Francis Donovan 1 maarit Karppinen 2 Patrick Edward Hopkins 1
1Univ of Virginia Charlottesville United States2Aalto University Aalto FinlandShow Abstract
Nanomaterial interfaces and concomitant thermal resistances are generally considered as atomic-scale planes that scatter the fundamental energy carriers. Given that the nanoscale structural and chemical properties of solid interfaces can strongly influence this thermal boundary conductance, the ballistic and diffusive nature of phonon transport along with the corresponding phonon wavelengths can affect how energy is scattered and transmitted across interfacial region between two materials. In hybrid composites comprised of atomic layer building blocks of inorganic and organic constituents, the varying interaction between the phononic spectrum in the inorganic crystals and vibrionic modes in the molecular films can provide a new way to manipulate the energy exchange between the fundamental vibrational energy carriers across interfaces. Here, we demonstrate nearly perfect transmission of phonon energy across interfaces comprised of single monolayer of hydroquinone molecules. In multilayer composites of atomic- and molecular-layer-grown zinc oxide and hydroquinone, the ballistic energy transfer of phonons in the zinc oxide is limited by scattering at the zinc oxide/hydroquinone interface, and then significant portion of the phonons with wavelengths larger than the thickness of the molecular monolayer ballistically transmit into the subsequent zinc oxide layer. The transmission of energy across the molecular interface transitions from quasi-ballistic to diffusive for hydroquinone interfaces greater than or equal to three molecules thick. This marks a ballistic to diffusive crossover size for which molecular interfacial thermal conductance transitions from being limited by the inorganic phonon flux to intrinsic vibrational resistance within the linked molecules.
4:00 AM - *II4.04
Nonlocal Thermal Conductivity and Superdiffusive Heat Transport
Ali Shakouri 1 Bjorn Vermeersch 1 Amir Koushyar Ziabari 1 Amr Mohammed 1 Yeerui Koh 1 Je-Hyeong Bahk 1
1Purdue Univ West Lafayette United StatesShow Abstract
Many micro/nanoscale heat transport experiments are interpreted using phenomenologically adjusted Fourier theory. It was recently shown that this can severely misrepresent the internal processes. When temperature gradient is over micron distances, the energy dynamics are much better described as truncated superdiffusive Lévy flights instead of conventional Brownian motion. All essential physics of the nondiffusive transport are captured by the fractal dimension and exponential decay length of the stochastic process. We determine these two new material parameters experimentally for several semiconductor alloys using transient laser thermoreflectometry. Finally, nonlocal relation between the heat flux and the temperature gradient is quantified and the thermal conductivity kernel is described as a function of truncated Lévy parameters. Implications for the design of high power electronic and optoelectronic devices will be discussed.
4:30 AM - *II4.05
Studying Micro/Nanoscale Thermal Transport with Laser-Induced Transient Gratings
Alexei Maznev 1
1MIT Cambridge United StatesShow Abstract
In recent years, significant progress has been made in studying phonon-mediated thermal transport in nanostructured materials. However, many questions remain unanswered and some issues are still hotly debated. Are “phononic crystal” effects observed in the thermal conductivity of nanostructures at room temperature? What are the limits of the diffuse boundary scattering (Casimir) model? Can thermal conductivity of nanostructures be obtained from ab-initio calculations without fitting parameters as has been recently demonstrated for many bulk materials? Answering these and other questions poses a challenge both for the theory and for experimental techniques. Measuring thermal transport properties with high accuracy is hard enough even with bulk materials, and becomes much more challenging with nanostructured samples. In this talk, I will present an overview of the laser-induced transient grating (TG) technique in which a spatially sinusoidal temperature “grating” is created in the sample, and its decay measured by a laser probe provides information about the thermal transport. The TG method is noncontact and yields intrinsically high absolute accuracy as it is based on measuring the dynamics of the TG decay rather than absolute values of the temperature and heat flux. In practical terms, it is particularly well suited for measuring the thermal conductivity of thin films and near-surface layers of bulk samples. The factors of particular significance for studying the fundamentals of phonon transport are that the TG technique avoids heat transfer across interfaces (i.e. all thermal transport takes place inside the material of interest), and the simple geometry with the sinusoidal temperature profile, which greatly simplifies theoretical analysis. We will review the experimental methodology of the optically heterodyned TG technique and discuss its contribution to the progress in understanding micro/nanoscale thermal transport by phonons made in recent years. In particular, we will be discussing the direct observation of no