Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session D6: Computational Challenges in Describing Mechanical Phenomena at the Nanoscale |
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Sponsoring Units: DCOMP Chair: Jeff Grossman, University of California, Berkeley Room: Colorado Convention Center 207 |
Monday, March 5, 2007 2:30PM - 3:06PM |
D6.00001: Energy Transfer and Resonance Enhancement at the Nanoscale. Invited Speaker: Classical molecular dynamics (MD) simulations of carbon nanotubes are used to elucidate important phenomena in the transfer of (lattice) vibrational energy between nanoscale objects. We study in particular the transfer of energy between specific vibrational modes. The calculations show efficient transfer of energy between modes that are in resonance and the time scale over which energy is transfered is set by the weak van der Waal's coupling between nanotubes. These observations provide the mechanistic basis for a new theoretical framework for describing energy transfer at the nanoscale. The insight gained from this theoretical picture is used to propose several novel nanomechanical devices with applications in chemical sensing and wireless communications that function by the exchange of vibrational energy. The operation and feasibility of these devices is demonstrated by further MD simulations. [Preview Abstract] |
Monday, March 5, 2007 3:06PM - 3:42PM |
D6.00002: Interfacial proximity effects on nanostructure thermal transport Invited Speaker: Thermal transport across interfaces is a key consideration in the development of advanced nanostructured materials for thermal management and thermoelectric energy conversion. Presented here is recent work from molecular dynamics simulation that illustrates how interfacial spacing can be tailored to modify thermal transport in such materials. Specifically, two examples are discussed: thermal transport between single wall carbon nanotubes and thermal conduction in superlattices. A four order of magnitude decrease in nanotube-nanotube thermal resistance is observed as nanotube spacing decreases, and a clear thermal conductivity minimum is observed in lattice matched superlattices. Mechanical phenomena emerging from the simulations include length dependence of nanotube Young's modulus and the importance of interfacial strain in maintaining coherent lattice waves in superlattices. [Preview Abstract] |
Monday, March 5, 2007 3:42PM - 4:18PM |
D6.00003: Heat flow at solid-liquid interfaces: confrontation between experiment and simulation Invited Speaker: Heat transport in nanostructures and nanostructured materials provides a novel paradigm for direct comparisons between the results of experiment and simulation. Time-resolved, pump-probe optical techniques enable measurements of the evolution of temperature on time scales from ps to ns. Our pump-probe experiments take two basic forms: measurements of heat transport across planar interfaces using time-domain thermoreflectance and measurements of heat flow from a metal or semiconductor nanostructure into its surroundings using transient absorption. The systems that we are studying are directly accessible to simulation by classical molecular dynamics on the same time and length scales that are encountered in the experiments. Working with our collaborators, P. Keblinski and his colleagues at RPI, we have made quantitative comparisons between experiment and simulation for heat transport from carbon nanotubes and fullerene molecules into a surrounding fluid; and heat transport across hydrophilic and hydrophobic interfaces with water. Any such comparison must take into account i) non-idealities in the experiments; ii) uncertainties in the potentials and atomic geometries in the computational model; and iii) the fact that classical simulations may include high frequency vibrational modes that are not thermally excited in the experiments. Despite the fact that transport at solid-liquid interfaces is more difficult to measure than more commonly studied solid-solid interfaces, we argue that solid-liquid interfaces provide a more reliable system for quantitative comparisons between experiment and simulation. [Preview Abstract] |
Monday, March 5, 2007 4:18PM - 4:54PM |
D6.00004: First-principles simulations of failure mechanisms, mechanical strength and electromechanical response Invited Speaker: Mechanical failure in response to external strain occurs at time scales that are usually much longer than those that are accessible to simulation. In particular, when processes leading to failure are highly activated, straight-forward molecular dynamics simulations will lead to qualitatively wrong results for both failure modes and mechanical strength. Nevertheless, judiciously chosen simulations can identify potential failure mechanisms, which can then be accurately mapped out and assessed, resulting in correct predictions of the initial modes of failure, breakage mechanisms, and strength. As an example, we will discuss simulations of the breakage of carbon nanotubes, which have been shown by first-principles calculations to be the strongest materials known. Very recently, the predictions of their stress-induced transformations and temperature-dependent failure have been spectacularly confirmed by experiments, which corroborated the key theoretical results. Nevertheless, the predicted ultimate strength is still substantially higher than the observed one, probably due to the presence of defects in as-grown samples. The simulations have also suggested avenues for forming nanotube-based electronic devices solely by mechanical transformations. In some molecular devices, e.g., those based on rotaxane, electric field and current-induced atomic transformations are the basis of device operation. For such structures, it is necessary to use open boundary conditions to account for current flow through the molecule and for the buildup of charge at the molecular interfaces. We will describe the key steps of this approach, based on linear-scaling non-equilibrium Green's function methodology, and its first applications. In collaboration with M. Buongiorno Nardelli, W. Lu, S. Wang and Q. Zhao. [Preview Abstract] |
Monday, March 5, 2007 4:54PM - 5:30PM |
D6.00005: Lattice thermal transport through atomically defined systems in a quantum mechanical description. Invited Speaker: There are different theoretical approaches to describe lattice thermal transport through nano-sized solid structures. From those approaches, atomistic calculations represent the smallest level of description, and provide a straight route towards fully understanding the phonon transport process across nanomaterials and interfaces. Within the atomistic descriptions themselves, there are several categories: 1-``classical,'' such as molecular dynamics, 2-``semi-classical,'' such as the Boltzmann-Peierls equation, and 3-``quantum-mechanical,'' such as Green's functions techniques. In this talk we will focus on quantum mechanical effects on nanoscale thermal transport, with specific examples in nanowires, nanotubes, and molecular junctions. Thus, we will discuss specific theoretical techniques from categories 2 and 3 above. We will start from the simplest of these approaches [1], which gives a good account of experimental measurements in semiconductor nanowires. Then we will discuss the more complex problem of thermal conduction in single walled carbon nanotubes, graphene, and graphite. We will see how the character of the 3-phonon scattering process in these systems results in long phonon mean free paths and thermal conductivities [2]. Subsequent experimental results have confirmed findings from the theoretical study [3]. Then, we will discuss a newer technique, based on non-equilibrium Green's functions, that allows to study the quantum mechanical many-body problem of interacting phonons flowing through generic, atomically described, anharmonic structures [4]. This technique is applied to investigate a simple model molecular junction. We will show some strictly quantum mechanical effects that take place in the anharmonic scattering process. Finally, we will present new results on first-principles calculations of phonon conduction across nitrogen impurities in carbon nanotubes [5]. \newline \newline [1] N. Mingo, Phys. Rev. B 68, 113308 (2003); N. Mingo and D. A. Broido, Phys. Rev. Lett. 93, 246106 (2004). \newline [2] N. Mingo and D. A. Broido, Nano Letters 5, 1221-1225 (2005); N. Mingo and D. A. Broido, Phys. Rev. Lett. 95, 096105 (2005). \newline [3] C. Yu, L. Shi, Z. Yao, D. Li, A. Majumdar, Nano. Lett., Vol. 5, 1842-1846 (2005); E. Pop, D. Mann, Q. Wang, K. E. Goodson and H. Dai, Nano Letters, 6, 96 (2006). \newline [4] N. Mingo, Phys. Rev. B, 74, 125402 (2006). \newline [5] N. Mingo, D. A. Stewart, D. A. Broido, and D. Srivastava, Nanoscale phonon transport from First-Principles (to be published). [Preview Abstract] |
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