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Computer modeling of thermal transport at the nanoscale

Demands on engineered thermal transport properties are ever increasing for a wide range of devices and materials-based solutions. However, gaps between the fundamental understanding and technological demands still remain, particularly in our understanding of phonon interactions at the nanoscale. This symposium aims at addressing fundamental issues related to thermal transport, in particular phonon behavior, phonon interactions and manipulation in nanoscale materials.

Scope:

Rapid progress in the synthesis and processing of materials with characteristic length of structures on the nanometer scales has created a demand for greater scientific understanding of thermal transport at the nanoscale. Despite methods for precisely controlling the electronic transport properties are presently available, less attention has been paid to the control of lattice vibration - the phonons. However, as the size of electronic devices turned to be smaller and smaller in the past decade, thermal management has become a bottleneck to the development of nanoelectronic devices because of the rapid decrease in phonon transport lengths. Moreover, phonons play critical role in the functionality of many other classes of devices including thermoelectrics, thermal rectification, etc. In fundamental research over the past few years significant progress has been made in our knowledge of phonon transport across and along arbitrary interfaces, scattering of phonons by crystal defects, collective phonons, and solid acoustic vibrations when these occur in structures with small physical dimensions. Phonon interactions generally strongly depend on the length scale, and phonons in nanoscale material show complex behavior. This symposium aims to uncover the ensemble behavior of scale-dependent phonon behavior and deepen our understanding in the complex mechanisms determining the thermal transport properties of a variety of nanoscale materials. This includes, in particular from atomistic point of view, modeling of phonon transport, phonon-phonon interactions, and robust manipulation with tailored nanoscale materials.

Hot topics to be covered by the symposium:

  • Phonon transport and phonon interactions in complex materials;
  • Electron-phonon coupling in energy materials;
  • Phononics;
  • Coherent phonons, characterization and manipulation;
  • Collective phonon transport;
  • Nanoscale heat transfer around nanoparticles for biomedical use;
  • Heat transport in lipid bilayers;
  • Thermal conductivity of single cells;
  • Thermophoresis and Soret effect in biological media;
  • New methodology to quantify phonon behaviors at the nanoscale;
  • Novel simulation protocols and methods.
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Session 1.1 : Luciano Colombo
09:00
Authors : P. Ferrando-Villalba, J. Ràfols-Ribé, A. Lopeandía, J. Rodríguez-Viejo
Affiliations : Nanomaterials and microsystems group Physics department Universitat Autònoma de Barcelona 08193 Bellaterra, Spain

Resume : Experimental measurements of thermal transport at the nanoscalenare essential to characterize the myriad of novel nanomaterials with potential applications in areas as diverse as thermal management, thermoelectric devices or solar cells. With length scales in the nm regime this low-dimensional architectures have challenged the development of new sensors and methodologies to enable accurate determination of the thermal conductivity. Whether based on optical or electrical signals, in-plane and out-of-plane thermal conductivity measurements are key to a detailed understanding of phonon scattering mechanisms at the nanoscale. In this presentation, we will show recent experimental data on the thermal conductivity of SiGe graded superlattices using the 3ω technique and on the thermal conductance of single porous Si nanowires using suspended structures. Both materials exhibit very low thermal conductivities, close or even below the amorphous limit, due to scattering of phonons in a broad frequency range. By a modification of the 3ω-Völklein method we will also show that the high sensitivity of phonon scattering to the microstructure of a thin film can be used to enable real-time monitoring of the growth dynamics of ultra-thin layers and in-situ measure their in-plane thermal conductivity with high accuracy and resolution. We will show how this technique can provide interesting data on the thermal conductivity of disordered organic layers with potential interest for OLEDs and thermoelectric devices.

U.1.1
09:40
Authors : Sheng Ying Yue, Ming Hu
Affiliations : Aachen Institute for Advanced Study in Computational Engineering Science (AICES), RWTH Aachen University, 52062 Aachen, Germany Institute of Mineral Engineering, Division of Materials Science and Engineering, Faculty of Georesources and Materials Engineering, RWTH Aachen University, 52064 Aachen, Germany

Resume : Many physical properties of metals can be understood in terms of the free electron model, as proven by the Wiedemann-Franz law. According to this model, electronic thermal conductivity can be inferred from the Boltzmann transport equation (BTE). However, the BTE does not perform well for some complex metals, such as Cu. Moreover, the BTE cannot clearly describe the origin of the thermal energy carried by electrons or how this energy is transported in metals. The charge distribution of conduction electrons in metals is known to reflect the electrostatic potential of the ion cores. Based on this premise, we develop a methodology for evaluating electronic thermal conductivity of metals by combining the free electron model and nonequilibrium ab initio molecular dynamics simulations. We confirm that the kinetic energy of thermally excited electrons originates from the energy of the spatial electrostatic potential oscillation, which is induced by the thermal motion of ion cores. This method directly predicts the electronic thermal conductivity of pure metals with a high degree of accuracy, without explicitly addressing any complicated scattering processes of free electrons. Our methodology offers a route to understand the physics of heat transfer by electrons at the atomistic level. The methodology can be further extended to the study of similar electron-involved problems in materials, such as electron-phonon coupling.

U.1.2
10:00
Authors : Daniel O Lindroth, Paul Erhart
Affiliations : Chalmers University of Technology, department of physics; Chalmers University of Technology, department of physics

Resume : The lattice thermal expansion and conductivity in bulk Mo and W-based transition metal dichalcogenides are investigated by means of density functional and Boltzmann transport theory calculations. To this end, a recent van der Waals density functional (vdW-DF-CX) is employed, which is shown to yield excellent agreement with reference data for the structural parameters. The calculated in-plane thermal conductivity compares well with experimental room-temperature values, when phonon-phonon and isotopic scattering are included. To explain the behavior over the entire available temperature range one must, however, include additional (temperature independent) scattering mechanisms that limit the mean free path. Generally, the primary heat carrying modes have mean free paths of 1μm or more, which makes these materials very susceptible to structural defects. The conductivity of Mo- and W-based transition metal dichalcogenides is primarily determined by the chalcogenide species and increases in the order Te-Se-S. While for the tellurides and selenides the transition metal element has a negligible effect, the conductivity of WS2 is notably higher than for MoS2, which may be traced to the much larger phonon band gap of the former. Overall, the present work provides a consistent set of thermal conductivities that reveal chemical trends and constitute the basis for future investigations of thermal transport in van der Waals solids.

U.1.3
10:20 Coffee break    
 
Session 1.2 : Fabrizio Cleri
11:00
Authors : Natalia Bedoya, Egbert Zojer
Affiliations : Institute of Solid State Physics Graz University of Technology Graz, Austria

Resume : Organic semiconductors are attractive for applications as varied as electronics, optoelectronics and thermoelectricity. Despite its importance, thermal transport in organic based semiconductors have received little attention. From a computational point of view one of the main challenges is to correctly describe intramolecular and intermolecular interactions. The latter are dominated by van der Waals (vdW) forces and are crucial to reach a reliable description of the thermal transport properties in these systems. So far, the most accurate description of both interactions is provided by density functional theory (DFT) in combination with methods to account for vdW forces. In this talk we will present a detailed benchmark of different models to describe the vibrational properties, and hence thermal properties, of a set of organic semiconductors. The benchmark includes: DFT, the density functional tight binding method DFTB , and the COMPASS and CHARMM force field potentials. DFT and DFTB were performed including the pairwise and many-body vdW methods of Tkatchenko et al. The accuracy of the DFT-vdW calculations is validated against Raman spectra measurements of organic thin films. As an outcome, we show that DFTB offer the best tradeoff between accuracy and cost.

U.1.4
11:20
Authors : M. Gandolfi (1,2,3,4), F. Medeghini(4), A. Crut(4), T. Stoll(4), F. Rossella(5), S. Hermelin(4), P. Maioli(4), F. Vallée(4), N. Del Fatti(4), G. Ferrini(1,2), C. Giannetti(1,2) and F. Banfi(1,2)
Affiliations : 1 Dipartimento di Matematica e Fisica, Università Cattolica del Sacro Cuore, Brescia I-25121, Italy 2 Interdisciplinary Laboratories for Advanced Materials Physics (I-LAMP), Università Cattolica del Sacro Cuore, Brescia I-25121, Italy 3 Laboratory of Soft Matter and Biophysics, Department of Physics and Astronomy, KU Leuven, Celestijnenlaan 200D, B-3001 Heverlee, Leuven, Belgium 4 FemtoNanoOptics group, Institut Lumière Matière, Université Lyon1, CNRS, Univ Lyon, France 5 NEST, Scuola Normale Superiore and Istituto Nanoscienze-CNR, Piazza S. Silvestro 12, I-56124 Pisa, Italy

Resume : With ever-decreasing device downscaling, understanding thermal transport in nanoscale systems is a key technological issue[1]. In this context, we theoretically address the ultrafast cooling of metal nano-objects embedded in transparent environment as measured in time-resolved optical spectroscopy[2]. In the experiment a “pump” laser pulse impulsively heats a metal nano-object. The thermal relaxation is then accessed exploiting a time-delayed “probe” pulse, monitoring the temperature-dependent relative transmittivity variation. The modeling, based on the Finite-Element Method, couples the two physics involved in the experiment, namely thermal and optical problem. The system thermal dynamics is first computed in the frame of Fourier law and Kapitza-like thermal resistance[3]. The system electromagnetic extinction spectrum is then calculated at various delay times, thus accounting for the temperature-dependent variations of the system dielectric functions. Within this frame we numerically simulate the experiments performed on metal nano-spheres embedded in a liquid environement and metallic nanodisks patterned on a dielectric substrate. By tuning the Kapitza resistance we obtain a good agreement between the experimental and theoretical optical traces, allowing to estimate the Kapitza resistance at metal nano-object-environment interface. [1] Hartland, Chem.Rev. 111,3858(2011) [2] Stoll et al., J.Phys.Chem.C 119,12757(2015) [3] Banfi et al., Appl.Phys.Lett. 100,011902(2012)

U.1.5
11:40
Authors : X. Zianni, K. Termentzidis, D. Lacroix
Affiliations : Dept. of Aircraft Technology, Technological Educational Institution of Sterea Ellada, 34400 Psachna, Greece Université de Lorraine, LEMTA, CNRS, UMR 7563, Faculté des Sciences et Technologies, BP 70239, 54506 Vandoeuvre les Nancy cedex, France

Resume : Heat transfer can be drastically limited in non-uniform materials due to enhanced phonon scattering. The decrease of the thermal conductivity is related to the details of the microstructure. Understanding and modeling this relation would be useful in designing structures for applications such as efficient energy converters. We have examined mesoscopic structures consisting of two phases that differ in the thermal conductivity using the phonon Monte Carlo and Molecular Dynamics techniques. The results have been analyzed in terms of a thermal network model. Phenomenology has been developed extending existing Effective Medium Approximation models that interprets the MC results and provides physics insight. We have systematically explored the effects of the characteristic lengths and the dimensions of the nanoscale non-uniformity. These dependences are introduced as parameters in the phenomenological model. Transition from ballistic to diffusive transport signature in thermal transport is explicitly indicated. We discuss our results and we use the parametric modeling to provide generic conclusions.

U.1.6
12:20 Lunch    
 
Session 2.1 : Riccardo Rurali
14:00
Authors : Riccardo Rurali-a, Luciano Colombo-ab, Xavier Cartoixa-c, Øivind Wilhelmsen-d, Thuat T Trinh-d, Dick Bedeaux-d, Signe Kjelstrup-d
Affiliations : a-Institut de Cie`ncia de Materials de Barcelona (ICMAB–CSIC) Campus de Bellaterra, 08193 Bellaterra, Barcelona, Spain, b-Dipartimento di Fisica, Universita` di Cagliari, Cittadella Universitaria, I-09042 Monserrato (Ca), Italy, c-Departament d’Enginyeria Electro`nica, Universitat Auto`noma de Barcelona, 08193 Bellaterra, Barcelona, Spain, d-Department of Chemistry, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway.

Resume : Heat transport through a solid–solid junction: the interface as an autonomous thermodynamic system Riccardo Rurali-a, Luciano Colombo-ab, Xavier Cartoixa-c, Øivind Wilhelmsen-d, Thuat T Trinh-d, Dick Bedeaux-d, Signe Kjelstrup-d aInstitut de Cie`ncia de Materials de Barcelona (ICMAB–CSIC) Campus de Bellaterra, 08193 Bellaterra, Barcelona, Spain bDipartimento di Fisica, Universita` di Cagliari, Cittadella Universitaria, I-09042 Monserrato (Ca), Italy cDepartament d’Enginyeria Electro`nica, Universitat Auto`noma de Barcelona, 08193 Bellaterra, Barcelona, Spain dDepartment of Chemistry, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway. E-mail: dick.bedeaux@chem.ntnu.no We performed computational experiments using nonequilibrium molecular dynamics simulations,1 showing that the interface between two solid materials can be described as an autonomous thermodynamic system.2-5 We verify local equilibrium and give support to the Gibbs description6 of the interface also away from global equilibrium. In doing so, we reconcile the common formulation of the thermal boundary resistance as the ratio between the temperature discontinuity at the interface and the heat flux7 with a more rigorous derivation from nonequilibrium thermodynamics.8,9 We also show that thermal boundary resistance of a junction between two pure solid materials can be regarded as an interface property, depending solely on the interface temperature, as implicitly assumed in some widely used continuum models, such as the acoustic mismatch model. Thermal rectification can be understood on the basis of different interface temperatures for the two flow directions. 1) R Rurali, L Colombo, X Cartoixa, Ø Wilhelmsen, TT Trinh, D Bedeaux, S Kjelstrup, Phys.Chem.Chem.Phys. 2016, 18, 13741 2) G. Bakker, Handbuch der Experimentalphysik, Akad. Verlag, Leipzig, Germany, 1928, vol. 6, ch. 10. 3) E. A. Guggenheim, Thermodynamics, North-Holland, Amsterdam, 1985. 4) A. Rosjorde, D. W. Fossmo, D. Bedeaux, S. Kjelstrup and B. Hafskjold, J. Colloid Interface Sci., 2000, 232, 178–185. 5) E. Johannessen and D. Bedeaux, Physica A, 2003, 330, 354–372. 6) J. W. Gibbs, The Scientific Papers of J. W. Gibbs, Dover, New York, 1961. 7) P. L. Kapitza, J. Phys., 1941, 4, 181–210. 8) D. Bedeaux, A. M. Albano and P. Mazur, Physica A, 1976, 82, 438–462. 9) S. Kjelstrup, D. Bedeaux, Non-Equilibrium Thermodynamics of Heterogeneous Systems, World Scientific, Singapore, 2008.

U.2.1
14:40
Authors : Miquel Royo, Carlos Escorihuela-Sayalero, Jorge Íñiguez, Riccardo Rurali
Affiliations : Institut de Ciència de Materials de Barcelona (ICMAB–CSIC), Campus de Bellterra, 08193 Bellaterra, Barcelona, Spain ; Materials Research and Technology Department, Luxembourg Institute of Science and Technology, 41 rue du Brill, L-4422 Belvaux, Luxembourg; Materials Research and Technology Department, Luxembourg Institute of Science and Technology, 41 rue du Brill, L-4422 Belvaux, Luxembourg; Institut de Ciència de Materials de Barcelona (ICMAB–CSIC), Campus de Bellterra, 08193 Bellaterra, Barcelona, Spain

Resume : In ferroelectric materials external electric fields are today routinely used to write, move and erase consecutive regions of different electric polarization, ferroelectric domains, spatially separated by elastic interfaces known as domain walls (DWs). The lattice distortion at these coherent interfaces can induce some amount of scattering to the traveling phonons, a property that shapes them as candidate mobile elements to dynamically manipulate heat fluxes with electrical signals. Indeed, thermal conductivity modulation via DW engineering was demonstrated long time ago at low temperatures [Physica 52, 577 (1971)] and more recently over a broad temperature range, including room temperature [Appl. Phys. Lett. 102, 121903 (2013) & Nano Lett. 15, 1791 (2015)], thus boosting its technological impact. However, little is known about the mechanisms of phonon scattering at different ferroelectric DWs, an information difficult to obtain from experimental measures but necessary in order to rationally design DW thermal transport engineering devices. Therefore, the combined use of theory and numerical simulations becomes imperative. In this contribution we will present the first phonon transport calculations in multidomain ferroelectrics with atomistic precision. Our modeling is based on a second-principles model potential approach [J. Phys. Cond. Matt. 25, 305401 (2013)] and a non-equilibrium Green's function calculation extended to obtain the single-mode contribution of individual phonons to the total transmission function [Phys. Rev. B 91, 174302 (2015)]. We will demonstrate that thermal transport across 180º DWs formed in bulk PbTiO3 is sensitive to the polarization of the heat carrying phonons. The propagation of transverse phonons across the DWs is strongly suppressed by the lattice distortion in the transverse directions whereas longitudinal phonons propagate through samples with multiple DWs without feeling them. All in all 180º DWs behave as longitudinal phonon polarizers of high selectivity whose effect is robust against deviations from the ideal flat shape.

U.2.2
15:00
Authors : J. Jaramillo-Fernandez, E. Chavez-Angel, C. M. Sotomayor-Torres
Affiliations : Department of Material and Nano Physics, KTH Royal Institute of Technology, Electrum 229, S-164 40 Kista, Sweden; Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and The Barcelona Institute of Science and Technology, Campus UAB, Bellaterra, 08193 Barcelona, Spain; Department of Material and Nano Physics, KTH Royal Institute of Technology, Electrum 229, S-164 40 Kista, Sweden, Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and The Barcelona Institute of Science and Technology, Campus UAB, Bellaterra, 08193 Barcelona, Spain, ICREA, Passeig Lluís Companys 23, 08010 Barcelona, Spain

Resume : To extract the thermal conductivity of a micro or nanoscale system, experimental techniques typically require the determination of temperature between two points. Raman spectroscopy provides a contactless non-destructive method to achieve this in samples with challenging geometries, by simultaneously using a focused laser as a heat source and measuring probe. The experimental data of weighted temperature as a function of the power absorbed on the probed volume is subsequently compared to a heat conduction model to extract the thermal conductivity. In the case of bulk materials and thick layers, an analytical formulation based on a steady-state two dimensional heat conduction model is often used. For this derivation, the studied material is considered as a semi-infinite medium with strong light absorption at the surface. However, thermal characterization sometimes is needed for samples that may not be considered as semi-infinite volumes or that have significant light penetration depths at the used wavelength. Then, to employ such approximation in the experiment analysis may introduce considerable measurement errors. In this work, we implemented a finite element based numerical simulation of the heating of a bulk sample by means of a Gaussian laser beam. As a result, the validity ranges of the assumptions involved in the analytical heat conduction model used in Raman thermometry analysis are evaluated. By comparing our numerical model with the simplified analytical solution for the case of laser heating of a semi-infinite solid, we obtain the range of applicability of different conditions, such as sample dimensions and penetration depth of light of the studied material. We found that for a ratio of penetration depth to laser spot radius higher than 0.2 , the assumption of strong shallow absorption used in the simplified heat conduction model is no longer fulfilled, leading to an overestimation higher than 10% of the thermal conductivity. Similarly, we found that the ratio of spot radius to thickness of the studied specimen should be smaller than 0.1 to predict thermal conductivity values below 10% error using the simplified analytical approximation. Work is in progress to extend this study to thin film-on-substrate systems. The simulation here developed appears as a powerful tool for predicting the appropriate sample geometry and range of absorption to analyze Raman thermometry measurements by means of analytical formulas.

U.2.3
15:20 Coffee break    
 
Session 2.2 : Ming Hu
16:00
Authors : Samy Merabia(1), Julien Lombard(1), Thierry Biben(1), Haoxue Han(2), F. Mueller-Plathe(2), F. Leroy(2)
Affiliations : 1. Université Lyon1 and CNRS, Lyon France 2. Universitat Darmstadt, Germany

Resume : In this contribution, we will discuss two examples of heat transfer phase change at the nanoscale. The first issue concerns the formation and growth of nanobubbles generated by heated nanoparticles. Nanobubbles have been experimentally generated by laser heated colloidal particles. Understanding their production and growth turns out to have applications in cancer cell therapy [1, 2]. Despite their promising use, the fundamental description of the mechanisms at the origin of nanobubble generation is still missing. Here, we model water dynamics around heated gold nanoparticles [3, 4], using hydrodynamic phase field model. We discuss the major role played by ballistic thermal transport inside the nanobubble [4]. Our simulations demonstrate the existence of a flux threshold to generate long-lived nanobubbles, with relative large nanobubble size. The laser duration is found to have little effect on the maximum nanobubble size, but long laser pulses are preferrable to increase the nanobubble lifetime. The second example of heat transfer phase change discussed here concerns boiling of nanofluids. In this case, we have conducted molecular dynamics simulations to probe boiling at a gold/model nanofluid interface. The nanofluid model consists in massive particles suspended in a solution of liquid Argon atoms, with a typical volume fraction around 0.1. Our simulations show that the presence of nanoparticles may significantly shift the occurrence of boiling crisis, as compared to the pure fluid case. This shift is to a large extent explained by the decrease of the thermal boundary conductance induced by the presence of the nanoparticles. This decrease has been quantified as a function of the nanoparticle-solid and nanoparticle-fluid interactions. Moreover, a spectral analysis of the heat flowing at the interface allowed us to understand the physical origin of the decrease of thermal boundary conductance. The heat flux transmitted from gold to the suspended nanoparticles is rather low as compared to the energy flowing from gold to the suspending fluid. This is the result of the poor coupling between the internal modes of the nanoparticles and the vibrational modes characterizing gold. This study may help in designing fluids with enhanced thermal properties. [1] Lukianova-Hleb, E. and Hu, Y. and Latterini, L. and Tarpani, L. and Lee, S. and Drezek, R.A and Hafner, J.H. and Lapotko, D.O. ACS Nano, 4, pp. 20192123, (2010) [2] Siems, A. and Weber, S.A.L. and Boneberg, J. and Plech, A. New J. Phys., 13, pp. 043018, (2011) [3] Lombard, J. and Biben, T. and Merabia, S.,Phys. Rev. Lett., 112, pp. 106701, (2014) [4] Lombard, J. and Biben, T. and Merabia, S., Nanoscale 8, 14870-14876, (2016)

U.2.4
16:20
Authors : Oleksandr O. Havryliuk, Oleksandr Yu. Semchuk
Affiliations : Chuiko Institute of Surface Chemistry of National Academy of Sciences of Ukraine 17 General Naumov Str., Kyiv, 03164, Ukraine

Resume : Using nano-technologies makes it possible to change in wide range electronic and optical characteristics of semi-conductor nano-crystals. The systems containing silicon nano-crystals in dielectric matrix have a perspective for creation of light-emitting systems compatible with IC-devices technology. Alloying structures of silicon nano crystals with ions of rare-earth metals makes it possible to realize a unique process of practically entire transmission of exiton energy on ions' internal degrees of freedom. For correct alloying such structures it is necessary to know distribution of temperature in process of heating because alloying itself takes place through the heating time. This is important essentially for changing electrical and optical properties of these structures. Semiconductor structures may be partially or fully opaque to radiation at the laser wavelength. Depending on the degree of transparency of different approaches will be useful for modeling the laser heat source. In addition, it must be remembered that all scales should be compared with the wavelength. Different approaches are required to describe the focused radiation for a relatively wide beam. If a material interacting with the incident beam, the geometrical features are comparable with the wavelength, it is necessary to consider further how the beam will interact with these fine structures. Fourier equations gives a possibility to calculate a heat distribution not only in objects-models, but in real studied structures for real experiment environment. It simplifies a process of the experimental planning and enlarges information value of the results obtained. Calculations realized in the given investigation make it possible to forecast a temperature distribution in silicon periodic structures in process of laser annealing. It gives a possibility for more precise alloying such structures that will change their optical and electrical properties in required range.

U.2.5
16:40
Authors : Hugo Aramberri, Riccardo Rurali, Jorge Íñiguez
Affiliations : Institut de Ciència de Materials de Barcelona (ICMAB-CSIC); Institut de Ciència de Materials de Barcelona (ICMAB-CSIC); Materials Research and Technology Department, Luxembourg Institute of Science and Technology (LIST)

Resume : By means of state-of-the-art first principles calculations, we investigate the thermal properties of SiO2 as it undergoes a pressure induced ferroelastic structural phase transition at high pressure. The lattice thermal conductivity is obtained in a wide range of temperatures and pressures by solving the Boltzmann transport equation self-consistently. SiO2 shows a robust peak in the out-of-plane diagonal component of the thermal conductivity tensor close to the phase transition from the stishovite phase to the CaCl2-type phase. The lattice thermal conductivity is almost two orders of magnitude larger close to the critical pressure than at lower or higher pressures in both phases. We attribute the origin of this anomalously high thermal conductivity to a steep decrease in the isotopical scattering rates close to the critical pressure. We thus propose this effect to be exploited to create a pressure-controlled anisotropic thermal transistor.

U.2.6
17:00
Authors : Y Chalopin. S Mayboroda. M Filoche.
Affiliations : Ecole CentraleSupelec, University of Minnesota, Ecole Polytechnique

Resume : Based on a recent theory of wave localization and the development of a mathematical tool (the localization landscape) we propose an approach that predicts the localization of thermal phonons in disordered lattices. An analogy between the Schrödinger equation and the classical equation of motion is introduced to demonstrate that localization of thermal energy in phonon systems arises from atomic disorder through which the localization landscape can be revealed. This approach, illustrated on disordered graphene, provides a powerful framework for engineering heat conduction in nanostructures using wave effects.

U.2.7
 
Poster session : Luciano Colombo, Fabrizio Cleri, Riccardo Rurali, Ming Hu
17:20
Authors : Roman Anufriev, Masahiro Nomura
Affiliations : Institute of Industrial Science, University of Tokyo, Tokyo, 153-8505, Japan

Resume : Two-dimensional phononic crystals (PnCs) can effectively control nanoscale heat conduction at low temperatures and typically consist of periodic arrays of either holes in a membrane (hole-based PnCs) or pillars on top of the membrane (pillar-based PnCs). In such periodic structures, phonon interference flattens phonon dispersion, which causes reduction in the phonon group velocity, modifications in the density of states (DOS), and overall change in thermal conductance. In this work, we simulate heat conduction in both types of PnCs within the purely coherent regime and systematically investigate how various geometrical parameters, lattices and materials affect the thermal conductance. We show that PnCs of both types suppress heat conduction when the period is sufficiently long, and on the contrary, enhance thermal conductance when the period is short (< 60 nm). In pillar-based PnCs, where local resonances play an important role, we show that the resonances increase the DOS and thus surprisingly contribute to enhancement rather than suppression of heat conduction. Thus, the overall suppression of thermal conductance in pillar-based PnCs appears despite (not due to) the local resonances and is caused by periodicity of the structure. The lowest thermal conductance is achieved when the pillars are short and made of materials with a low Poisson’s ratio, and when both pillars and holes have the highest possible radius-to-period ratio. In general, we show that pillar-based PnCs can match the efficiency of the conventional hole-based PnCs and have richer physics which opens new possibilities for heat conduction engineering. Finally, we propose hybrid hole/pillar-based PnCs. These hybrid PnCs combine both types of PnCs and suppresses heat conduction stronger than hole- or pillar-based PnCs, thus can potentially become the second generation of PnCs for heat conduction engineering.

U.3.1
17:20
Authors : J. A. Seijas-Bellido (1,a), M. P. Ljungberg (1,2), C. Escorihuela-Sayalero (3), J. C. Wojdel (1), J. Íñiguez (3), R. Rurali (1)
Affiliations : 1. Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Campus UAB, 08193 Bellaterra, Barcelona, Spain 2. Donostia International Physics Center, Paseo Manuel de Lardizabal 4, E-20018 Donostia-San Sebastián, Spain 3. Luxembourg Institute of Science and Technology, 5 avenue des Hauts-Fourneaux, L-4362 Esch/Alzette, Luxembourg

Resume : The properties of ferroelectric materials have become an intensively studied topic, both theoretically [1] and experimentally [2], attracting the interest of many researchers. In this work, we study the thermal conductivity of the PbTiO3, and also the thermal boundary resistance [3] of the regions disturbed by its domain walls, by means of non-equilibrium molecular dynamics simulations. For these simulations we have modeled the atomic interactions with a potential obtained from first principles [4], accurate but computationally efficient. Our result of the thermal conductivity is in good agreement with previous reports obtained from first-principles [5] and allowed estimating for the first time the interface thermal resistance of a domain wall. [1] Liu Yong, Ni Li-Hong, Xu Gang, Song Chen-Lu, Han Gao-Rong, and Zheng Yao. Phase transition in PbTiO3 under pressure studied by the first-principles method. Physica B: Condensed Matter, 403(2122):3863 – 3866, 2008. [2] S. T. Davitadze, S. N. Kravchun, B. A. Strukov, B. M. Goltzman, V. V. Lemanov, and S. G. Shulman. Specific heat and thermal conductivity of BaTiO 3 polycrystalline thin films. Applied Physics Letters, 80(9), 2002. [3] R. Dettori, C. Melis, X. Cartoixà, R. Rurali & L. Colombo (2016) Thermal boundary resistance in semiconductors by non-equilibrium thermodynamics, Advances in Physics: X, 1:2, 246-261, DOI: 10.1080/23746149.2016.1175317 [4] Jacek C Wojdel, Patrick Hermet, Mathias P Ljungberg, Philippe Ghosez, and Jorge Íñiguez. First-principles model potentials for lattice-dynamical studies: general methodology and example of application to ferroic perovskite oxides. Journal of Physics: Condensed Matter, 25(30):305401, 2013. [5] Anindya Roy. Estimates of the thermal conductivity and the thermoelectric properties of PbTiO 3 from first principles. Phys. Rev. B, 93:100101, Mar 2016.

U.3.2
17:20
Authors : A. Iskandar, A. Gwiazda, Y. Huang, M. Kazan, A. Bruyant, M. Tabbal, G. Lerondel
Affiliations : 1.University of technology of troyes. 2.American University of Beirut.

Resume : In this contribution, we present experimental evidence on the change of the phonon spectrum and vibrational properties of a bulk material through phonon hybridization mechanisms. The phonon spectrum in a finite material is strongly affected by the presence of free surfaces, which is the addition of a contribution from an essentially two-dimensional crystal. The phonon spectrum of a bulk material can hence be altered by an hybridization mechanism between confined phonon modes in nanostructures introduced on the surface of a bulk material and the underlying bulk phonon modes. We measured the heat capacities of bare and surface-structured silicon substrates originating from the same silicon wafer. Then, we deduced important features of the phonon spectra of the samples investigated through a rigorous analysis of the measured heat capacity curves. The results clearly showed that the shape and size of the nanostructures made on the surface of the bulk substrate have a strong effect on the phonon spectrum of the bulk material.

U.3.3
17:20
Authors : M. Kazan
Affiliations : Department of Physics, American University of Beirut, P.O. Box 11-0236, Riad El-Solh, Beirut 1107-2020, Lebanon

Resume : Although atomistic simulations and first principles computation of harmonic normal modes and anharmonic forces are nowadays widely used to solve Boltzmann equation for phonons and accurately calculate the lattice thermal conductivity, there is still a need for analytical approximate models leading to reliable solutions to heat transport problems in nanostructures, particularly at low temperatures. The utmost need for such approximate models stems from the fact that most of the atomistic simulation techniques do not account for quantum mechanics effects occurring at low temperatures, and first principles computation requires very small mesh size to account for the excitation of only long wavelength phonons. This paper presents significant advances in the analytical calculation of the low temperature lattice thermal conductivity in cylindrical nanowires. It shows that an accurate prediction of the thermal conductivity in cylindrical nanowires can be obtained at low temperatures when Boltzmann equation in cylindrical coordinates is solved. It also provides an approach to predict from a spatial-dependent Boltzmann equation the rate at which phonons are scattered by the nanowire boundary in the presence of intrinsic scattering mechanisms. The derived formalism clearly shows that the thermal conductivity of a cylindrical nanowire presents specific features that do not appear for the case of bulk materials. It shows that a contribution from an essentially two-dimensional crystal alters the temperature dependence of thermal conductivity and gives rise to distinct size effects on the thermal conductivity. The accuracy of the derived formalism is demonstrated clearly with reference to reported data regarding the size effect on the thermal conductivity of Si nanowire, the dramatic drop in the thermal conductivity for rough Si nanowires, and the deviation of the thermal conductivity of thin cylindrical nanowires from the Debye law at low temperatures.

U.3.4
17:20
Authors : Ali Rajabpour, Saeed Bazrafshan
Affiliations : Mechanical Engineering Department, Imam Khomeini International University, Iran; School of Nano-Science, IPM, Tehran, Iran.

Resume : Disordered or amorphous structure graphene is a type of defective graphene. Recently it has been shown that, disordered graphene can be synthesized by focused ion beam converting ordered hexagonal arrangement of graphene cells to stochastic and disordered polygonal ones. Strain is one of the external parameters that could be used in tuning the thermal and mechanical properties of graphene as well as other nanostructures. In this investigation, we explore the engineering of the thermal transport along amorphous graphene structures through employing the mechanical strains. It is found that the effect of strain on the thermal conductivity of disordered graphene is clearly less considerable as compared with the pristine structure.

U.3.5
17:20
Authors : Riccardo Dettori, Michele Ceriotti, Johannes Hunger, Claudio Melis, Luciano Colombo, Davide Donadio
Affiliations : Riccardo Dettori - Dipartimento di Fisica, Università di Cagliari, Cittadella Universitaria, I-09042 Monserrato (CA), Italy, Michele Ceriotti - Laboratory of Computational Science and Modeling, IMX, École Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland, Johannes Hunger - Max Planck Institute for Polymer research, Ackermannweg 10, 55128 Mainz, Germany, Claudio Melis - Dipartimento di Fisica, Università di Cagliari, Cittadella Universitaria, I-09042 Monserrato (CA), Italy, Luciano Colombo - Dipartimento di Fisica, Università di Cagliari, Cittadella Universitaria, I-09042 Monserrato (CA), Italy, Davide Donadio - Department of Chemistry, University of California Davis, One Shields Avenue, Davis, California 95616, United States

Resume : We introduce a non-equilibrium molecular dynamics simulation approach, based on the generalized Langevin equation, to study vibrational energy relaxation in similar condition to pump-probe spectroscopy experiments. A colored noise thermostat is used to selectively excite a set of vibrational modes, leaving the other modes nearly unperturbed, to mimic the effect of a monochromatic laser pump. Energy relaxation is probed by analyzing the evolution of the system after excitation in the microcanonical ensemble, thus providing direct information about the energy redistribution paths at the molecular level and their time scale. The method is applied to hydrogen bonded molecular liquids, specifically deuterated methanol and water, which show a manifold energy dissipation dynamics due to strong directionality of H-bonds and complexity of their network. The colored thermostat is tuned to excite the stretching mode of the OD, or OH, bond involved in H-bonding. We evaluate how energy is transferred from the excited mode to other modes of the nearby molecules, providing a robust picture of energy relaxation at the molecular scale.

U.3.6
17:20
Authors : Bruno Lorenzi, Riccardo Dettori, Marc T. Dunham, Claudio Melis, Rita Tonini, Luciano Colombo, Kenneth E. Goodson, Dario Narducci
Affiliations : Bruno Lorenzi - Dipartimento di Scienza dei Materiali, Università di Milano-Bicocca, via Cozzi 55, I-20125 Milano, Italy, Riccardo Dettori - Dipartimento di Fisica, Università di Cagliari, Cittadella Universitaria, I-09042 Monserrato (CA), Italy, Marc T. Dunham - Department of Mechanical Engineering, Stanford University, Stanford, California, 94503, USA, Claudio Melis - Dipartimento di Fisica, Università di Cagliari, Cittadella Universitaria, I-09042 Monserrato (CA), Italy, Rita Tonini - Dipartimento di FIM, Università di Modena e Reggio Emilia, via Campi 213, I-41100 Modena, Italy, Luciano Colombo - Dipartimento di Fisica, Università di Cagliari, Cittadella Universitaria, I-09042 Monserrato (CA), Italy, Kenneth E. Goodson - Department of Mechanical Engineering, Stanford University, Stanford, California, 94503, USA, Dario Narducci - Dipartimento di Scienza dei Materiali, Università di Milano-Bicocca, via Cozzi 55, I-20125 Milano, Italy

Resume : Ideal non-metallic thermoelectric materials should show low thermal conductivity ĸ along with low electrical resistivity ρ. Thus, strategies are needed to prevent the propagation of phonons mostly responsible for thermal conduction while only marginally impeding charge carrier di ffusion. Defect engineering may provide tools to this aim, provided that an adequate understanding of the role played by morphological defects may guide the selection of scattering centers mostly e ffective at reducing ĸ with minor impacts on ρ . We present the results of a study on how silicon thermal conductivity is differently affected by multiple morphological defects. A blended approach has been adopted, encompassing simulations and experiments so as to cover a wide range of defect densities. We show that co-presence of morphological defects with very di fferent characteristic sizes e ffectively reduces the thermal conductivity. We also point out that models with no spectral resolution are limited to predict and explain the dependency of ĸ upon the density of morphological defects. However, limits coming from the use of the spectral Matthiessen rule are put forward, especially in the range of very few nanovoids (NV), showing how a fully physically consistent picture of the thermal conductivity calls for non-local e ffects related to lattice disordering induced by NVs over a wider range of porosities.

U.3.7
17:20
Authors : Claudia Caddeo, Claudio Melis, Andrea Ronchi, Claudio Giannetti, Gabriele Ferrini, Francesco Banfi
Affiliations : Università Cattolica del Sacro Cuore and I-LAMP Brescia; Università di Cagliari; Università Cattolica del Sacro Cuore, I-LAMP Brescia, and KU Leuven; Università Cattolica del Sacro Cuore and I-LAMP Brescia; Università Cattolica del Sacro Cuore and I-LAMP Brescia; Università Cattolica del Sacro Cuore and I-LAMP Brescia

Resume : Heat transfer at the meso/nano-scale represents an outstanding challenge, among the most relevant under an applicative standpoint [1]. In this context Thermal Boundary Resistance (TBR) plays a key role. Accessing the TBR between nanosized metals and insulating substrates remains an open issue and the prerequisite to enhance heat dissipation in next generation micro and nano-devices. Unfortunately theoretical predictions deviate from TBR values extracted from TRTR measurements, the go-to technique to inspect TBR at these junctions. The present work, based on multiscale modelling, reconciles the discrepancy for the paradigmatic case of the Al-Sapphire heterojunction. First, we calculate the TBR at the interface between a nanosized Al film and an Al2O3 substrate at an atomistic level. The thermal dynamics occurring in TBTR experiments is then modelled via macro-physics equations upon insertion of the materials parameters obtained from atomistic simulations. Electrons and phonons non-equilibrium and spatio-temporal temperatures inhomogeneities are found to persist up to the nanosecond time scale. The strategy adopted in the literature to extract the TBR from transient reflectivity traces is revised at the light of the present findings. The results are of relevance beyond the specific system, the physical picture being general and readily extendable to other heterojunctions. [1] D. G. Cahill et al., Appl. Phys. Rev.,2014, 1, 011305 [2] C. Caddeo et al., PRB 2017 (submitted)

U.3.8
17:20
Authors : Alessandro Crnjar, Claudio Melis and Luciano Colombo
Affiliations : Dipartimento di Fisica, Università degli Studi di Cagliari, Cittadella Universitaria s.p.8 km 0,7, 09042 Monserrato (CA), Italia - E-mail: claudio.melis@dsf.unica.it

Resume : In this work, we study thermal transport properties of single extended isolated chains of poly-3,4-ethylenedioxythiophene (PEDOT) with the aim of identifying possible anomalous regimes in the heat transport holding for one-dimensional systems. In particular, by investigating the PEDOT length-dependence thermal conductivity by means of the Approach to Equilibrium Molecular Dynamics (AEMD) method[1,2], we observe an ~L^β divergence (β=0.48) indicating that single PEDOT chains obeys to a super diffusive regime defined as an intermediate between a purely ballistic (β=1) and diffusive (β=0) regime[3]. The present results are further analyzed by means of Equilibrium Molecular Dynamics in which the time-dependence of the heat current auto-correlation function has been estimated for a PEDOT chain of 1000-monomers. The results show a ~t^(1-β) time decay giving rise, according to the Green-Kubo formula, to a corresponding ill-defined thermal conductivity[3]. The estimated value of the β exponent is found in perfect agreement with the one estimated with AEMD, further confirming the occurrence of a thermal transport super diffusive regime. Present findings are eventually validated by investigating the decay in time of a localized thermal excitation. [1] E. Lampin et al., J. Appl. Phys. 114, 033525 (2013) [2] C. Melis et al., Eur. Phys. J. B 87, 96 (2014). [3] S. Lepri et al., Lecture Notes in Physics 921, 1-37 (2016)

U.3.9
17:20
Authors : Claudio Genovese, Claudio Melis and Luciano Colombo
Affiliations : Dipartimento di Fisica, Università degli Studi di Cagliari, Cittadella Universitaria s.p.8 km 0,7, 09042 Monserrato (CA), Italia - E-mail: claudio.melis@dsf.unica.it

Resume : Thermoelectric devices are promising and environmentally friendly systems to recover energy from industrial waste heat or natural heat. Even if inorganic materials generally show high efficiencies in thermoelectric devices, these materials are typically expensive and are characterized by brittleness, which makes difficult their application in large areas. Furthermore, organic materials have peculiar advantages as thermoelectric materials, such as cost effectiveness, low thermal conductivity and high flexibility. For these reasons , conjugated polymers, which possess good electrical conductivity, have been actively investigated. Among different conducting polymers, poly(3,4-ethylenedioxythiophene) (PEDOT) and PEDOT: poly(styrenesulfonate) (PSS) are promising candidates due to their water-solubility and high electrical conductivity. Recently, a room temperature ZT value of 0.42 has been reported in PEDOT:PSS which was mainly attributed to the relatively high electrical conductivity and extremely low thermal conductivity k=0.34 W m-1 K-1. Other studies reported k values ranging from 0.34 up to 2.5 W m-1 K-1 consequently affecting the estimates of ZT. A possible explanation of such uncertainty in the k values can be related to the different experimental setups employed (e.g. solvent, temperature). This highlights the possible role played by morphologies in polymers thermal transport. In this work we elucidate this issue by demonstrating how morphology ultimately governs thermal transport properties in polymers. In particular, by means of the Approach to Equilibrium Molecular Dynamics[1], we estimate thermal conductivity of a PEDOT system. Crystalline, amorphous and a series of semi-crystalline structures in between are obtained by varying the degree of polymerization. A dramatic increase up to two orders of magnitude is observed from amorphous to purely crystalline PEDOT. Also the degree of polymerization is found to have a strong effect: by increasing the actual PEDOT chain length from 100 to 1000 monomers, we observe a corresponding one order of magnitude increase in thermal conductivity. [1] E. Lampin et al., J. Appl. Phys. 114, 033525 (2013). [2] C. Melis et al., Eur. Phys. J. B 87, 96 (2014).

U.3.10
17:20
Authors : Aleandro Antidormi ¹, Claudio Melis ¹, Enric Canadell ², Luciano Colombo ¹
Affiliations : ¹ Dipartimento di Fisica, Università di Cagliari, Cittadella Universitaria, I-09042 Monserrato (Ca), Italy; ² Institut de Ciència de Materials de Barcelona (ICMAB–CSIC), Campus de Bellaterra, 08193 Bellaterra, Barcelona, Spain

Resume : A promising field which could highly benefit from the intrinsic features of eumalinin molecule is photovoltaics, where such a pigment could be used as photo-active layer in hybrid solar cells, being Si the inorganic counterpart. In these devices, the process of optical to electrical energy conversion takes place at the eumelanin/inorganic interface via the exciton dissociation followed by carrier diffusion and collection at the electrodes. The overall power conversion efficiency of the cell is likely affected by the presence of scattering sources (impurities and vibrational modes): in particular, vibrations could detrimentally impact the dissociative capabilities of the interface and reduce the mean free path of carriers in the organic layer. Hence, an analysis of the thermal properties of the eumelanin/inorganic interface is indeed needed for improving our fundamental understanding of the above processes. In our work we study the thermal transport properties of a silicon[100]/eumelanin interface. By non-equilibrium molecular dynamics techniques, we compute the thermal boundary resistance (TBR) at the interface. Due to the elusive nature of eumelanins and the amorphous character of the corresponding layer, we perform a statistical analysis including the effects of chemical disorder which characterizes our knowledge of eumelanin-like pigments to-date. In particular, we show the particular dependence of the TBR from the compositional structure of the organic layer and the morphological features of the silicon surface (curvature, roughness).

U.3.11
17:20
Authors : Sevil Sarikurt, Cem Sevik
Affiliations : Department of Physics, Faculty of Science, Dokuz Eylul University, Izmir 35390, Turkey; Department of Mechanical Engineering, Faculty of Engineering, Anadolu University, Eskisehir 26555, Turkey

Resume : The newest members of 2D transition metal carbides and nitrides, so-called MXenes, recieve increasing attention due to their novel electronic and thermal properties. Depending on their prominent performances, MXene materials are promising materials for applications in various electronic devices. Previously, the electronic properties and the dynamical stability of some MXene structures have been investigated by different researchers. Only very few studies focus on thermoelectric performance of semiconductor MXene materials. In this study, we investigate the lattice thermal transport properties of semiconductor Ti2CO2 MXene crystal in different functionalized forms using Density Functional Theory and the Phonon Boltzmann Transport Theory. We obtain the thermal conductivity as 40.1 W/(mK) for Model 2 and 17.4 W/(mK) for Model 3 of Ti2CO2 at T=300 K. Because of its low thermal-conductivity, it would be a good candidate for next-generation thermoelectric applications.

U.3.12
17:20
Authors : Georgios Gkantzounis, Timothy Amoah, Marian Florescu
Affiliations : Department of Physics, Advanced Technology Institute, University of Surrey, Guildford, Surrey GU2 7XH, UK

Resume : We introduce and discuss a new class of structurally disordered phononic crystals that efficiently combine the behaviour of periodic and disordered structures. Phononic crystals have shown remarkable applications over the last two decades, while stealthy hyperuniform disordered structures (HUDS), formed by appropriately decorating a 2D stealthy hyperuniform point pattern, have shown remarkable behaviour as photon cavities and waveguides. Here, the underlying mechanisms for the formation of large phononic band gaps in HUDS will be thoroughly presented. As an example, we numerically investigate, through finite element calculations, the formation of band gaps in HUDS made of 500 lead cylinders in an epoxy matrix. In the 2D case (infinite height of the cylinders) the elastic wave for both in-plane and out-of-plane polarization exhibits large phononic band gaps, similar to the periodic ones. Therefore, very high-Q cavity modes (Q~10^14, neglecting dissipative losses) can be easily formed in these structures. We will also present, a new bottom-up approach available for HUDS, to form arbitrary shaped waveguides with 100% transmission through sharp bends, not possible in the periodic structures. Finally, we will discuss 3D elastic wave propagation through finite height slabs of HUDS and the existence of high-Q modes (Q~10^11, neglecting dissipative losses) and efficient waveguiding. Applications of these structures as MEMS, thermoelectric, integrated phonon circuits are anticipated.

U.3.13
17:20
Authors : K.K.Abgaryan, D.L.Reviznikov
Affiliations : K.K.Abgaryan - Dorodnicyn Computing Centre, Federal Research Center "Computer Science and Control", Russian Academy of Sciences, 40 Vavilov st., 119333 Moscow, Russia; D.L.Reviznikov -Dorodnicyn Computing Centre, Federal Research Center "Computer Science and Control", Russian Academy of Sciences, 40 Vavilov st., 119333 Moscow, Russia

Resume : The output characteristics of the semiconductor microwave devices are determined by a multiplicity of factors reflecting both structural and technological features of the heterostructure production. It is extremely important to choose optimal parameters defining the basic electrophysical characteristics of the structure - concentration and mobility of the charge carriers in the two-dimensional electron gas channels (2D electron gas). This work presents a new approach that allows solving the tasks of optimization of nanosized semiconductor heterostructures as the optimal control tasks. The problem of the optimal barrier layer doping of AlGaN/GaN heterostructures is considered.

U.3.14
17:20
Authors : D. Martinez, A. di Pierro, B. Mortazavi, A. Pecchia, L. Medrano, R. Gutierrez, M. Bernal, A. Fina
Affiliations : Dipartimento di Scienza Applicata e Tecnologia, Politecnico di Torino, 15121 Alessandria, Italy; Institute of Structural Mechanics, Bauhaus-Universität Weimar, D-99423 Weimar, Germany; Consiglio Nazionale delle Ricerche, ISMN, 00017 Monterotondo, Italy; Institute for Materials Science, TU Dresden, 01062 Dresden, Germany

Resume : Controlling and improvement of interfacial thermal conductance between graphene nanosheets plays a crucial role with respect to the synthesis of highly thermally conductive nanomaterials and devices[1]. This way, finding practical routes toward the enhancement of interfacial conductance without suppressing the thermal transport is currently among the most challenging issues. In this work, we suggest the chemical functionalization with organic molecules covalently attached to adjacent graphene sheets as a promising approach for the enhancement of thermal conductance between them. In particular, molecules like phenoxypentoxybenzene between graphene sheets were studied, in different possible experimental configurations, including strained,bent molecules and out of plane transmission. To provide physical insight concerning the heat transfer mechanism at interfaces, we conducted molecular mynamics (MD) simulations. To this aim, we employed non-equilibrium MD (NEMD) technique along with the COMPASS force-field for describing the interatomic interactions. Density Functional Tight Binding (DFTB) MD was also carried out using the DFTB+ code [3] to benchmark the LAMMPS NEMD results. Also, phonon properties of the bridging chain were studied via Green's functions formalism [5]. [1] Nature Communications 7 (2016) 11281 [2] Carbon 63 (2013) 460-470 [3] Computational Materials Science 47 (2009) 237-253 [4] http://www.dftb-plus.info/ [5] Numerical Heat Transfer, Part B, 51 (2007) 333-349

U.3.15
17:20
Authors : Emigdio Chavez-Angel, Paulina Komar, Gerhard Jakob
Affiliations : Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and The Barcelona Institute of Science and Technology, Campus UAB, Bellaterra, 08193 Barcelona, Spain; Institut fur Physik, Johannes Gutenberg Universitat Mainz, Staudingerweg 7, 55128 Mainz, Germany, Graduate School Materials Science in Mainz, Staudingerweg 9, 55128 Mainz, Germany; Institut fur Physik, Johannes Gutenberg Universitat Mainz, Staudingerweg 7, 55128 Mainz, Germany, Graduate School Materials Science in Mainz, Staudingerweg 9, 55128 Mainz, Germany.

Resume : In this work we show a phenomenological alloy-like fit of the thermal conductivity of Ad1:Bd2 superlattices with d1 =/ d2. The presented method is a generalization of the Norbury rule of the summation of thermal resistivities in alloy compounds. Namely, we show that this approach can be also extended to describe the thermal properties of crystalline and ordered-system composed by two or more elements, and, has a potentially much wider application range. Using this approximation we estimate that the interface thermal resistance depends on the period and the ratio of materials that form the superlattice structure.

U.3.16
17:20
Authors : M. Á. Sánchez-Martínez, F. Alzina, E. Chávez-Ángel, B. Graczykowski, J.S. Reparaz, C.M. Sotomayor Torres.
Affiliations : Catalan Institute of Nanoscience and Nanotechnology;

Resume : In the study of thermal transport phenomena at the nanoscale, it is usual to introduce a spectrum-averaged phonon mean free path (the gr ay-MFP approximation). However, recent reports have shown that the use of a single-averaged MFP may be inadequate. The analysis of the contribution of heat carriers with different MFP to the total thermal conductivity is key to understand the role that different physical phenomena play in the reduction of thermal conductivity in nanostructured materials. In the MFP reconstruction process, the election of the regularization parameter is of great importance to obtain an adequate suppression function that relates the bulk thermal conductivity and that of the nanostructure. In this work we study the criteria for choosing a value for the regularization parameter and the influence of this election in the final result of the MFP reconstruction. This process is then applied to calculate the thickness-dependant suppression function for thermal conductivity in Silicon membranes. This model is also used to study how different geometrical parameters affect the phonon MFP distribution in nanostructured Silicon membranes.

U.3.17
17:20
Authors : Benoit Latour, Nina Shulumba, Austin Minnich
Affiliations : Division of Engineering and Applied Science, California Institute of Technology, Pasadena, USA

Resume : Designing materials with exceptionally low or high phonon conductance remains an open challenge, contrary to what have been achieved with electrons or photons. For instance, Fresnel coefficients allow to easily compute the angular-dependent reflection/transmission coefficients of light in multilayer systems. Analogous quantities for phonons, which are still to be found, would guide the design of interfacial materials with optimal conductance. In this work, we have investigated how to describe phonon interfacial transport from a modal perspective at the atomic scale. We have characterized how phonons propagate through multilayer materials by developing a modal version of the Atomistic Green's Function. These results give a detailed framework to reinterpret phonon transmission at interfaces in analogy with light propagation. This work provides a foundation to design interfacial materials with extreme values of thermal conductance for thermoelectricity and heat management.

U.3.18
17:20
Authors : Daniel O Lindroth, Paul Erhart
Affiliations : Chalmers University of Technology, department of physics; Chalmers University of Technology, department of physics

Resume : Clathrates exhibit a very low thermal conductivity, which is a key factor for their very good thermoelectric properties and has been attributed to "phonon-glass" conduction behavior. Here, we present a computational analysis of the conduction mechanism using Ba8X16Y30 (X={Al,Ga}, Y={Si,Ge}) as model systems. Contributions to the thermal conductivity from both electrons as well as phonons are computed with Boltzmann transport theory using input from first-principles calculations. The calculations are in good agreement with experimental data and in particular reproduce the experimentally observed ordering of the lattice thermal conductivity between the different systems. We demonstrate that these rather non-intuitive trends can be traced to the presence (or lack thereof) of dispersed higher frequency optical phonon modes, which provide a surprisingly large contribution to the lattice thermal conductivity. The "phonon-glass" behavior manifests itself in our calculations in the form of very short lifetimes, which indicate that already at room temperature the majority of modes is overdamped. In terms of the electronic thermal conductivity, our results provide insight into the applicability of the Wiedemann-Franz law. While the latter is regularly used to separate lattice and electronic contributions to the thermal conductivity in clathrates. Depending on the choice of pre-factor our data show that one can incur an error of up to 50% for the electronic thermal conductivity.

U.3.19
17:20
Authors : Ali Alkurdi, Samy Merabia
Affiliations : Institut Lumière Matière, UMR5306 Université Claude Bernard Lyon 1-CNRS, Université de Lyon 69622 Villeurbanne Cedex, France; Department of Physics, Al-Baath University, Homs, Syria

Resume : Thermal boundary resistance is a critical quantity that controls heat transfer at the nanoscale which is primarily related to interfacial phonon scattering [2]. In microelectronics for instance, there is a strong need to know how energy can be exchanged at separation distance of few nanometers where heat is primarily exchanged by acoustic waves for sub-nanometric gaps [3]. In this communication, we combine lattice dynamics calculations and inputs from first principles ab initio simulations to predict phonon transmission at Si/Ge interface as a function of both phonon frequency and phonon wavevector. This technique allows us to determine the overall thermal transmission coefficient as a function of the scattering direction and frequency. Our results show that, the thermal energy transmission is highly anisotropic while thermal energy reflection is almost isotropic. In addition, we found the existence of a global critical angle of transmission beyond which almost no thermal energy is transmitted. This critical angle around 60 degree. is found to be almost independent on the interaction range between Si and Ge, the interfacial bonding strength, and the temperature above than 30 K. We interpret these results by carrying out a spectral and angular analysis of phonon transmission coefficient and differential thermal boundary conductance (TBC). Besides, our calculations allow us to predict heat transfer mediated by phonons across a nanometer vacuum scale gap and we compare the results with recent experiments [4]. The results are also interpreted by investigating the transmission across the vacuum gap. [1] A. Alkurdi and S. Merabia J. Phys.: Conf. Ser. (2016) [2] D. G. Cahill et al Appl. Phys. Rev. 1 011305 (2014). [3] V. Chiloyan, J. Garg, K. Esfarjani and G. Chen Nature Communications 6 6755 (2015) [4] K. Kloppstech, N. Könne, S.-A. Biehs, A. W. Rodriguez, L. Worbes, D. Hellmann, and A. Kittel. Nature Communications (2017).

U.3.20
17:20
Authors : B Davier, Y Chalopin, P Dollfus, S Volz, J Saint-Martin
Affiliations : Centre de Nanosciences et Nanotechnologies, CNRS, Univ. Paris-Sud, Université Paris-Saclay, Orsay, France & Laboratoire d'Énergétique Moléculaire et Macroscopique, Combustion, CNRS, CentraleSupélec, Université Paris-Saclay, Chatenay Malabry, France; Laboratoire d'Énergétique Moléculaire et Macroscopique, Combustion, CNRS, CentraleSupélec, Université Paris-Saclay, Chatenay Malabry, France; Centre de Nanosciences et Nanotechnologies, CNRS, Univ. Paris-Sud, Université Paris-Saclay, Orsay, France; Laboratoire d'Énergétique Moléculaire et Macroscopique, Combustion, CNRS, CentraleSupélec, Université Paris-Saclay, Chatenay Malabry, France; Centre de Nanosciences et Nanotechnologies, CNRS, Univ. Paris-Sud, Université Paris-Saclay, Orsay, France

Resume : In this work, we have investigated thermal properties of interfaces made of stacked Si and Ge using Molecular Dynamic simulations. The thermal conductance was calculated for several lattice orientations over a wide range of temperatures. The non-equilibrium method based on the temperature difference set between two materials as well as the equilibrium method which relies on the fluctuations of the heat flux are compared. The results of the two methods are discussed as well as their limitations.

U.3.21
17:20
Authors : Pol Torres, Miriam Steinherr, Alvar Torello, Juan Camacho, Xavier Cartoixà, F. Xavier Alvarez, Javier Bafaluy
Affiliations : Physics Department. Universitat Autònoma de Barcelona. (UAB)

Resume : The prediction of thermal transport at the nanoscale has been a question of intense debate in the last decade. Although several abinitio models have been able to predict bulk thermal conductivity of a large number of materials from calculated relaxation times, using the same magnitudes at the nanoscale seems to give wrong predictions. This disagreement may have two reasons: the presence of different orders of magnitude in the mean free paths of the phonons and the role of momentum conserving collisions like normal scattering. The Kinetic Collective Model (KCM) has been proposed recently to predict the thermal transport under the influence of the above characteristics. In this poster we present a new set of analytical equations based on KCM that can be easily used in design time through a finite elements program. The model includes memory and nonlocal effects to describe most of the phenomena observed at the nanoscale. Numerical results and comparison to experimental data are presented for thin films and nanowires. The results show that anomalies found on reduced size samples may have an hydrodynamic basis. Poiseuille flow and effective Fourier flow are shown to be the limit regimes of reduced samples depending on the dominance of normal scattering. In the light of KCM predictions we show that an hydrodynamic description is necessary to understand them. These results open the door to introduce new physics in finite elements programs that will be able to give better predictions for the current electronic devices, where its characteristic scales are of the same order as the hydrodynamic scales predicted by KCM equations.

U.3.22
17:20
Authors : O. Monereo 1, C. Fàbrega 1, O. Casals 1, S. Illera 1, A. Varea 1, M. Schmidt 2, T. Sauerwald 2, A. Schütze 2, A. Cirera 1, J.D. Prades 1
Affiliations : 1 MIND/IN2UB, Departament d’Enginyeries: Electrònica, Universitat de Barcelona, Spain 2 Lab of Measurement Technology, Department of Mechatronics, Saarland University, Germany

Resume : Under electrical bias, small nanoparticles exhibit remarkable local heating effects. As a matter of fact, it is possible reaching temperatures of several hundreds of ºC with just a few microwatts of electrical power [1]. This so called self-heating effect has mostly been observed in small nanosystems, based on just one or a few 1D nanoparticles, preferently nanotubes or nanowires. In this work, we report on local Joule heating effects observed in large arrangements of 1D nanoparticles in which relatively high temperatures could still be achived with unexpectedly low power consumption figures [2]. Combining thermal micrographs with simulations, we found that the reason why such a low power consumption is also possible in large systems is the formation of random percolation networks for current that lead to extremely localized power dissipation spots (the "hot-spots") [3]. Our experiments and models also explain how these effects relate to the macroscopic resistance signal of the system, and how this electrical resistance can be used to monitor heating. Also, the results obtained with our test system (carbon nanofibers) were also confirmed with other nanosystems of reduced dimensionatily (metal oxide nanowires, reduced graphene oxide, CNTs, etc.). Furthermore, thumb rules to design efficient self-heated systems will be provided and discussed. [1] J.D. Prades, R. Jimenez-Diaz, F. Hernandez-Ramirez, S. Barth, A. Cirera, A. Romano-Rodriguez, S. Mathur, J.R. Morante, Ultralow power consumption gas sensors based on self-heated individual nanowires, Appl. Phys. Lett. 93 (2008) 123110. doi:10.1063/1.2988265. [2] O. Monereo, J.D. Prades, A. Cirera, Self-heating effects in large arrangements of randomly oriented carbon nanofibers: application to gas sensors, Sensors Actuators B Chem. 211 (2015) 489–497. doi:10.1016/j.snb.2015.01.095. [3] O. Monereo, S. Illera, A. Varea, M. Schmidt, T. Sauerwald, A. Schütze, A. Cirera, J.D. Prades, Localized self-heating in large arrays of 1D nanostructures, Nanoscale. 8 (2016) 5082–5088. doi:10.1039/C5NR07158E.

U.3.23
17:20
Authors : Hossein Azizi, Sebastian Gurevich, Nikolas Provatas
Affiliations : Department of Physics, Centre for the Physics of Materials, McGill University, Montreal, QC, Canada

Resume : We explore numerically the morphological patterns of thermo-diffusive instabilities in combustion fronts with a continuum fuel source, within a range of Lewis numbers and ignition temperatures, focusing on the cellular regime. For this purpose, we generalize the recent model of Brailovsky et al. to include distinct process kinetics and reactant heterogeneity. The generalized model is derived analytically and validated with other established models in the limit of infinite Lewis number for zero-order and first-order kinetics. Cellular and dendritic instabilities are found at low Lewis numbers. These are studied using a dynamic adaptive mesh refinement technique that allows very large computational domains, thus allowing us to reduce finite-size effects that can affect or even preclude the emergence of these patterns. Our numerical linear stability analysis is consistent with the analytical results of Brailovsky et al. The distinct types of dynamics found in the vicinity of the critical Lewis number, ranging from steady-state cells to continued tip-splitting and cell-merging, are well described within the framework of thermo-diffusive instabilities and are consistent with previous numerical studies. These types of dynamics are classified as “quasi-linear” and characterized by low amplitude cells that may be strongly affected by the mode selection mechanism and growth prescribed by the linear theory. Below this range of Lewis number, highly non-linear effects become prominent and large amplitude, complex cellular and seaweed dendritic morphologies emerge. The cellular patterns simulated in this work are similar to those reported by Malchi et al. in experiments of flame propagation over a bed of nano-aluminum powder burning with a counter flowing oxidizer. These resemble the dendritic fingers observed in this study, in the limit of low-Lewis number. It is noteworthy that the physical dimension of our computational domain is roughly close to their experimental setup.

U.3.24
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Session 4.1 : Luciano Colombo
09:00
Authors : Giuliano Benenti
Affiliations : Center for Nonlinear and Complex Systems, Dipartimento di Scienza e Alta Tecnologia, Università degli Studi dell'’Insubria, via Valleggio 11, 22100 Como, Italy Istituto Nazionale di Fisica Nucleare, Sezione di Milano, via Celoria 16, 20133 Milano, Italy

Resume : The possibility of manipulating heat currents represents a fascinating challenge for the future, especially in view of the need for sustainable supplies of energy and limiting the environmental impact of fossil-fuel combustion. An effective control of heat currents requires the development of a new class of nanoscale thermal devices, analogous to electric diodes and transistors, such as thermal rectifier and amplifiers. However along these lines there are severe difficulties both of theoretical and experimental nature. In particular it turns out that manipulation of the heat current is much more difficult than the manipulation of the electric current. A deeper understanding of the properties of heat transport is needed. We discuss thermal rectification from the perspective of nonequilibrium statistical mechanics and dynamical systems theory. After preliminary considerations on the dynamical foundations of the phenomenological Fourier law, we discuss the basic ingredients needed to control the phononic heat flow and design thermal diodes. Finally, we discuss preliminary results on heat rectification in systems with a ballistic spacer.

U.4.1
09:40
Authors : Emanuele Panizon, Roberto Guerra, Erio Tosatti
Affiliations : SISSA - Trieste; SISSA - Trieste, CNR-IOM; SISSA - Trieste, CNR-IOM, ICTP - Trieste

Resume : The textbook thermophoretic force which acts on a body in a fluid is proportional to the local temperature gradient; the same could be assumed to hold for a diffusive nanometer sized object physisorbed on a 2D layer such as graphene. By means of a Non-Equilibrium Molecular Dynamics (NEMD) study of a test system - a gold nanocluster adsorbed on free - standing graphene clamped between two different temperatures - we find a phoretic force which for relatively large submicron lengths L is parallel to, but roughly independent of, the gradient magnitude. This signals a nonconventional thermophoresis that is ballistic in character. Analysis shows that the phoretic force is dominated by flexural phonons, whose flow is indeed known to be ballistic and distance-independent up to the relatively long scattering lengths that precede the standard diffusive regime. Interestingly, ordinary harmonic phonons only carry pseudomomentum and could not exert a force. Yet, the monolayer supports a specific anharmonic coupling between corrugation and 2D density which endow the flexural phonons with some real momentum, part of which is transmitted to the adsorbate through scattering. The resulting distance-independent thermophoretic force is not unlikely to possess practical applications.

U.4.2
10:00
Authors : S. Illera, L. Colombo, M. Pruneda, P. Ordejon
Affiliations : Institut Català de Nanociència i Nanotecnologia (ICN2) and Institut de Ciència de Materials de Barcelona (ICMAB), CSIC and BIST, Campus de la UAB, 08193 Bellaterra (Barcelona), Spain ; Dipartamento di Fisica, Università di Cagliari Cittadella Universitaria, 09042 Monserrato (Ca), Italy; Institut Català de Nanociència i Nanotecnologia (ICN2), CSIC and BIST, Campus de la UAB, 08193 Bellaterra (Barcelona), Spain ; Institut Català de Nanociència i Nanotecnologia (ICN2), CSIC and BIST, Campus de la UAB, 08193 Bellaterra (Barcelona), Spain

Resume : From the fundamental science point of view, the heat transport properties are strongly related to the material's structure and composition down to the atomic scale. When the dimensionality is reduced or nanometric scales are reached, novel thermal properties emerge. Concerning 2D materials, graphene (GN) has shown exceptional electrical and thermal properties stimulating the interest in similar isomorphic materials, such boron nitride (BN). Regarding the BN nanoribons (BNNRs), it has been recently shown that the thermal conductivity is strongly dependent on the transport direction in contrast to the GNRs [1]. Here, we present the study from first-principles of the thermal transport properties of BN using the methodology of "Approach to Equilibrium Molecular Dynamics" (AEMD) [2] from a pure Density Functional Theory (DFT) approach as implemented in the SIESTA program [3]. The SIESTA MD module was modified to perform AEMD simulations, allowing the determination of the thermal conductivity from a purely first-principles description, without assumptions on the interatomic potentials or fits to experimental information. This is the first time that both approaches (DFT AEMD) are used in combination opening the possibility to determine accurately the thermal conductivity in structures whose force fields are not available or sufficiently accurate (defective materials, doped systems or surfaces with adsorbates). The anisotropy of the thermal conductivity of BN was studied in order to demonstrate the capabilities of the proposed methodology, and to test the validity of the recent force field studies [1], quantifying the variation of the thermal conductivity as a function of the transport direction. [1] Chen, Lee, Liu, and Chang, International Journal of Thermal Sciences 94,72-78 (2015) [2[ Melis, Dettori, Vandermeulen, and Colombo, Eur. Phys. J. B 87:96 (2014). [3] Soler, Artacho, Gale, García, Junquera, Ordejón, and Sánchez-Portal, J. Phys.: Condens. Matter. 14, 2745 (2002).

U.4.3
10:20 Coffee break    
 
Session 4.2 : Fabrizio Cleri
11:00
Authors : E. Lampin a), H. Zaoui a), P. L. Palla a), G. Ori b), A. Bouzid c), M. Boero b), C. Massobrio b) and F. Cleri a)
Affiliations : a) IEMN, Lille, France; b) IPCMS, Strasbourg, France; c) EPFL, Lausanne, Switzerland

Resume : Modeling the atomic structure of materials is crucial to achieve a precise understanding of the effects of scale, dimensionality, surface, interfaces, disorder... and the effect of their concurrent and mutual interactions their thermal response. Among the various modeling approaches, molecular dynamics (MD) ensures a complete description of all scattering mechanisms experienced by phonons via the anharmonicity of the energy landscape. We have recently proposed a method to study and quantify thermal transport by MD, the approach-to-equilibrium molecular dynamics (AEMD). This method, based on the creation and analysis of heat transients, is faster than standard ones. In this talk, we will present the latest achievements of the approach on real-size nanostructures. We will also present an application of the same methodology used in classical MD to first-principles MD, in the case of g-GeTe4. It appears that the transient and the approach-to-equilibrium steps do comply with the heat equation despite the limited size of the model (<200 atoms), allowing to determine the thermal conductivity of the materials via the decay time.

U.4.4
11:20
Authors : Claudia Caddeo, Claudio Melis, Maria Ilenia Saba, Alessio Filippetti, Alessandro Mattoni
Affiliations : CNR-IOM; Università di Cagliari; CNR-IOM; CNR-IOM; CNR-IOM

Resume : Hybrid perovskites (HYP) have attracted enormous interest in photovoltaics with 22% power conversion efficiency achieved in MAPI solar cells. Such materials combine the superior optoelectronic and semiconducting properties of the PbI with the possibility to be easily synthesized from solution. Despite the great technological opportunities, HYP have several drawbacks, such as the thermal instability of the material; MAPI is in fact characterized by a low sublimation temperature and by degradation reactions easily activated with small thermal budgets [1]. Heat control in MAPI is accordingly technologically relevant and it requires a precise determination of the thermal conductivity κ. By using approach-to-equilibrium molecular dynamics we provide an accurate estimate of κ in MAPI as a function of sample size and temperature, in agreement with experimental works. We show that κ is intrinsically low, due to the low sound velocity of PbI lattice. Furthermore, by selectively analyzing the effect of different molecular degrees of freedom, we clarify the role of the molecular substructure by showing that the internal modes above 150 cm−1 (in addition to rotations) are effective in reducing the thermal conductivity of the hybrid perovskites. This analysis suggests strategies to tailor the thermal conductivity by modifying the internal structure of the organic cations [2]. [1] M. Grätzel et al., Chem. Mater., 2014, 26, 6160–6164 [2] C. Caddeo et al., PCCP, 2016, 18 (35), 24318-24324

U.4.5
11:40
Authors : Daryoush Shiri and Andreas Isacsson
Affiliations : Department of Physics, Chalmers University of Technology, SE-412 96, Göteborg, Sweden

Resume : Using molecular dynamics we study how the heat flow in a single layer graphene nanoribbon can be controlled by geometrical design. We observed that by using asymmetrical features in a nanoribbon e.g. sharp or graded pinned interface arrays, the heat flux can be asymmetric in response to a symmetrically applied temperature gradient. The heat rectification efficiency, which is defined as the ratio of heat flux difference to the right going flux, can reach up to 14 % and 53 % at equilibrium temperatures of 300 K and 100 K, respectively. The role of equilibrium temperature and temperature difference between the ends of the device were investigated and the mechanism of rectification is explained based on the calculated vibrational density of states of hot and cold sections of the device. Close agreement exists between our observations and previous theoretical modelling of 1D and 2D anharmonic chains of oscillators with weak links [1,2]. Chief among our results is the possibility of adjusting the polarity (direction) of rectification by size and periodicity of the pinned nano-patterns. Importantly we propose a method to adjust the geometrical features on the fly using external stimuli like strain. This adds more flexibility to thermal devices and removes the necessity to perform complex nanolithography for each new geometry or pattern. (1) M. Terraneo, M. Peyrard, & G. Casati, Controlling the energy flow in nonlinear lattices: A model for a thermal rectifier. Phys. Rev. Lett. 88, 094302 (2002). (2) J. Lan, & B. Li, Thermal rectifying effect in two-dimensional anharmonic lattices, Phys. Rev. B, 74, 214305 (2006)

U.4.6
12:20 Lunch    
 
Session 5.1 : Riccardo Rurali
14:00
Authors : Davide Donadio
Affiliations : University of California Davis

Resume : Heat is a form of energy of which we still have relatively poor control: overheating during operation is a serious issue for electronic devices, and in any energy conversion process a large amount of thermal energy is wasted in the environment. Better control of thermal energy, starting from the microscopic scale, would allow us to target efficiently major technological issues, such as heat management in information and communication technology and in photovoltaics, as well as thermoelectric energy harvesting. The optimization of materials, devices and processes with respect to heat management stems from a better understanding of phonon transport at the molecular and nanoscale. In this talk I will illustrate cases in which predictive molecular simulations shed light on thermal energy transport in nanostructured materials and in molecular systems, thus suggesting viable optimization pathways for thermoelectric material and devices. Finally I will address the case of energy relaxation in molecular liquids, and discuss how heat dissipation entangles to molecular energy relaxation in pump-probe experiments.

U.5.1
14:40
Authors : Pol Torres (1), Alvar Torello (1), Juan Camacho (1), Amirkoushyar Ziabari (2), Javier Bafaluy (1), Xavier Cartoixà (1), Ali Shakouri (2), F. Xavier Alvarez (1)
Affiliations : (1) Physics Department, Universitat Autònoma de Barelona, 08193 Bellaterra, Barcelona Spain; (2) Birck Nanotechnology Center, Purdue University, West Lafayette, Indiana 47907, United States

Resume : Recent experimental techniques such as time domain or frequency domain thermoreflectance (TDTR and FDTR)  have shown that thermal transport phenomena at ultrafast heating rates or small scale (submicron) geometries cannot be understood using a Fourier heat transport model. Several explanations have been proposed such as introducing frequency-dependent or anisotropic thermal conductivity or an interface thermal boundary resistance depending on the geometry and time scales. In these experiments two main phonon transport phenomena are relevant. On one side, phonon mean free paths in semiconductors can span several orders of magnitude, and consequently the diffusive to ballistic transition occurs at different length scales for different modes. On the other side, the non-resistive character of normal scattering can impact thermal transport in a way that is different from the rest of the scattering mechanisms.  Recently, the Kinetic Collective Model (KCM) has been proposed to describe thermal transport at multiple length scales by including nonlocality effects and the role of normal scattering in a more accurate way. The model, based on the Boltzmann Transport Equation, can be combined with ab-initio calculations in a parameter-free approach. KCM formalism has been able to predict the thermal conductivity of semiconductor bulk, wires and films without using any fitting parameters. Based on KCM, thermal transport can be split into two different regimes: the standard kinetic regime where each mode has its own velocity and mean free path, and a collective regime where all the phonons move together. In this work, we will show that the collective regime is the natural framework to describe the recently proposed hydrodynamic behaviour. We will also present the methodology to obtain the parameters needed to describe this new transport regime from ab-initio calculations. Next, we will use the same equations to model the non-Fourier behaviour in ultrafast or nanoscale experiments and show that some non-diffusive behaviour can be interpreted as the appearance of a collective interaction of phonons. Moreover, the simplicity of KCM offers a way to introduce non-Fourier behaviour in commercial finite element programs. This can be used to predict the thermal behaviour of nanoscale semiconductor chips where the characteristic sizes are of the same order as the hydrodynamic characteristic scale. Some limitations due to the fractal nature of superdiffusive thermal transport of long tail phonons will be discussed.

U.5.2
15:00
Authors : Laura de Sousa Oliveira, P. Alex Greaney
Affiliations : University of California, Riverside

Resume : The Green–Kubo method is a popular equilibrium molecular dynamics approach for computing thermal conductivity. This approach is based on the premise of the fluctuation dissipation theorem that the same information can be derived from the lifetime of small fluctuations as from the system’s response to equilibrium perturbations. For thermal transport, the lattice conductivity is related to the integral of the autocorrelation of the instantaneous heat flux. The autocorrelation function requires a long averaging time to reduce remnant noise and integrating the noise in the tail of the autocorrelation function often becomes conflated with physically important slow relaxation processes. We present a method to quantify the uncertainty on transport properties computed using the Green-Kubo formulation on-the-fly based on recognizing that the integrated noise is a random walk, with a growing envelope of uncertainty. By characterizing the noise we can choose integration conditions to best trade off systematic truncation error with unbiased integration noise, to minimize uncertainty for a given allocation of computational resources.

U.5.3
15:20 Coffee break    
 
Session 5.2 : Ming Hu
16:00
Authors : Herve Ness, Lorenzo Stella, Lev Kantorovich, Chris Lorenz
Affiliations : King's College London, Department of Physics, Strand Campus, Strand, London WC2R 2LS, UK; Atomistic Simulation Centre, School of Mathematics and Physics, Queen’s University Belfast, University Road,Belfast BT7 1NN, Northern Ireland, UK; King's College London, Department of Physics, Strand Campus, Strand, London WC2R 2LS, UK; King's College London, Department of Physics, Strand Campus, Strand, London WC2R 2LS, UK

Resume : The generalised Langevin equation (GLE) method which we have developed is used to calculate the dissipative dynamics of classical systems described at the atomic level via molecular dynamics simulations. The GLE scheme goes beyond the commonly used bilinear coupling between the central system and the bath, and permits us to have a realistic description of both the dissipative central system and its surrounding bath. We have developed a simulation protocol which allows us to obtain the vibrational properties of a realistic bath and then convey these properties into an extended Langevin dynamics by mapping these vibrational properties onto a set of auxillary variables. We have used this methodology for systems with one and two temperature baths. In the case of the two bath systems, our methodology provides a realistic description of not only both the dissipative central system and its surrounding baths, but also of heat transport through the central system. In this talk, I will present an overview of our methodology and the results of the method we applied to model heat transport in realistic systems.

U.5.4
16:20
Authors : Bjorn Vermeersch, Jesús Carrete, Natalio Mingo
Affiliations : CEA-Grenoble, TU Wien, CEA-Grenoble

Resume : Heat generated in contemporary electronic devices must often find its way towards ambient through multi-layered substrates. Thermal transport in such structures becomes quasiballistic because characteristic phonon mean free paths are comparable to the (sub)micron-scale thickness of the individual layers. Advanced computational methods going beyond the classical Fourier diffusion framework become therefore necessary to obtain accurate assessments of the thermal performance. In this presentation, we analyse multilayer substrates for GaN-based high electron mobility transistors (HEMTs) and light emitting diodes (LEDs) with our recently released almaBTE software [1]. This open-source BTE solver enables efficient variance-reduced Monte Carlo simulations of multilayer structures with ab initio phonon dispersions and scattering rates that rigorously preserve crystal anisotropies. The obtained temperature profiles and spectral flow maps (showing heat flux resolved by phonon frequency) reveal intricate interplay amongst the various layers that remains hidden in conventional diffusive predictions, underlining the rich physics contained within the quasiballistic transport. As a particular consequence, the effective total resistance of the substrates no longer follows from simple summation of the nominal contributions of the composing material layers and interfaces. [1] www.almabte.eu

U.5.5
16:40
Authors : Francesca Costanzo1, Bernd Ensing1,2, Miguel Pruneda1 and Pablo Ordejón1
Affiliations : 1Catalan Institute of Nanoscience and Nanotechnology - ICN2, CSIC and BIST, Campus de Bellaterra, Spain 2 University of Amsterdam, The Netherlands

Resume : Ionic Liquids are one of the preferred options used by the industry for the storage of thermal energy in solar energy plants. Improving their thermophysical properties is an important goal to achieve more efficient heat storage and transportation media. A promising approach for improving these properties is to introduce nanoparticles dispersed in the ionic liquid or the molten salt, the so-called nanofluids. However, how thermophysical properties such as the heat capacity, self-diffusion, or heat conductivity depend on the microstructure of the nanofluids is still rather unknown. Molecular simulation, therefore, can play a major role in this research, as producing reliable experimental data for these systems is difficult and expensive. We have calculated by classical molecular dynamic simulations, thermal properties of disk-like graphene nanoflakes dispersed in organic solvent. In my contribution, I will discuss how the heat capacity and the thermal conductivity depend on the shape, the size and the density of the dispersed carbon nanoflakes. While thermal conductivity is well simulated by our classical model, the insertion of quantum corrections (QM) is necessary to calculate the heat capacity in good agreement with experiments. With our classical model including QM corrections, we are able to shed light and gather basic understanding on the dependence of thermal transport properties on the nature of solute-solute and solute-solvent interaction. Work supported by the MaX Center of Excellence in HPC Applications (http://www.max-centre.eu/), financed by EU Grant H2020-676598.

U.5.6
17:00
Authors : A. Glensk, B. Grabowski, T. Hickel, J. Neugebauer, P. Neibecker, J. Neuhaus, M. Leitner, K. Hradil, W. Petry
Affiliations : Max-Planck-Institut für Eisenforschung GmbH, Max-Planck-Str. 1, 40237 Düsseldorf, Germany and the Heinz Maier-Leibnitz Zentrum (MLZ), Technische Universität München, Lichtenbergstr. 1, 85748 Garching, Germany

Resume : Engineering thermal transport properties requires a detailed understanding of the underlying phonon-phonon interactions in the materials. These interactions cause anharmonic lattice vibrations in the crystal and result in a broadening of the phonon linewidth (inverse of phonon lifetime), which is a key ingredient for the description of thermal transport properties. The success of established ab initio approaches based on density functional perturbation theory (DFPT) was so far mainly restricted to low temperatures and/or the Gamma point. Whereas lifting these constraints, significant discrepancies to experiment (~100%) have been uncovered at low and elevated temperatures [1]. Applying our recently developed local anharmonic (LA) method for the asymmetric displacement of atoms out of their equilibrium position [2], we show how phonon lifetimes can be accurately captured from DFT over the whole temperature range up to melting for low computational costs in comparison to DFPT. We evaluate the performance of this approach by comparison with experimental phonon lifetimes of Al at low and high temperatures as obtained with inelastic neutron scattering. [1] X. Tang et al., PRB 84, 054303 (2011). [2] A. Glensk et al., PRL 114, 195901 (2015).

U.5.7

No abstract for this day

No abstract for this day

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Symposium organizers
Fabrizio CLERIInstitut d'Electronique, Microelectronique et Nanotechnologie

Université de Lille I, 59652 Villeneuve d'Ascq, France

fabrizio.cleri@univ-lille1.fr
Luciano COLOMBOUniversity of Cagliari

Department of Physics, Cittadella Universitaria, 09042 Monserrato (Ca), Italy

luciano.colombo@unica.it
Ming HURWTH Aachen University

Institute of Mineral Engineering, Division of Mat. Sci. & Eng., Mauerstrasse 5, 52064 Aachen, Germany

hum@ghi.rwth-aachen.de
Riccardo RURALIICMAB-CSIC

Campus UAB, 08193 Bellaterra Barcelona, Spain

rrurali@icmab.es