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Research Papers: Micro/Nanoscale Heat Transfer

# Equilibrium Molecular Dynamics Study of Lattice Thermal Conductivity/Conductance of Au-SAM-Au Junctions

[+] Author and Article Information
Tengfei Luo

Department of Mechanical Engineering, Michigan State University, 2555 Engineering Building, East Lansing, MI 48824luotengf@msu.edu

John R. Lloyd1

ASFC 1.316.C RRMC, Rapid Response Manufacturing Center, University of Texas Pan American, Edinburg, TX 78539lloyd@egr.msu.edu

1

Corresponding author.

J. Heat Transfer 132(3), 032401 (Dec 22, 2009) (10 pages) doi:10.1115/1.4000047 History: Received November 13, 2008; Revised August 14, 2009; Published December 22, 2009; Online December 22, 2009

## Abstract

In this paper, equilibrium molecular dynamics simulations were performed on Au-SAM (self-assembly monolayer)-Au junctions. The SAM consisted of alkanedithiol $(–S–(CH2)n–S–)$ molecules. The out-of-plane ($z$-direction) thermal conductance and in-plane ($x$- and $y$-direction) thermal conductivities were calculated. The simulation finite size effect, gold substrate thickness effect, temperature effect, normal pressure effect, molecule chain length effect, and molecule coverage effect on thermal conductivity/conductance were studied. Vibration power spectra of gold atoms in the substrate and sulfur atoms in the SAM were calculated, and vibration coupling of these two parts was analyzed. The calculated thermal conductance values of Au-SAM-Au junctions are in the range of experimental data on metal-nonmetal junctions. The temperature dependence of thermal conductance has a similar trend to experimental observations. It is concluded that the Au-SAM interface resistance dominates thermal energy transport across the junction, while the substrate is the dominant media in which in-plane thermal energy transport happens.

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## Figures

Figure 1

Simulated Au-SAM-Au systems of different sizes: (a) a 1×1 system of 328 atoms (4 alkanedithiol molecules and 288 gold substrate atoms), (b) a 2×2 system of 1312 atoms (16 alkanedithiol molecules and 1152 gold substrate atoms), and (c) a 3×3 system of 2952 atoms (36 alkanedithiol molecules and 2592 gold substrate atoms)

Figure 2

Procedures of preparing the Au-SAM-Au simulation system: (a) one gold substrate is optimized by Morse potential, (b) alkanedithiol molecules are implanted on the substrate and the whole system is relaxed using the potentials specified in Table 1, and (c) the other optimized substrate is imposed on top of the alkanedithiol molecules

Figure 3

Absorption sites and tilt directions of SAM molecules on Au(111) surface (37). Open circles are gold atoms on the surface of substrate. Filled circles are sulfur heads which are absorbed on the substrate surface. Arrows represent tilt directions of SAM molecules. Dashed lines form the boundary of the simulation system and dotted lines represent SAM lattice constants.

Figure 4

A typical normalized HCAC function

Figure 5

Thermal conductivity profiles obtained from the calculations: (a) profile that has a flat area, (b) profile that has an overall fluctuation minimized area, (c) profile whose value fluctuates about 0, and (d) profile with a first overall peak

Figure 6

Temperature dependence of in-plane thermal conductivities of systems with free boundary condition in z-direction

Figure 7

Temperature dependence of thermal conductivities and conductance of systems with PBC in z-direction: (a) thermal conductivities in x-, y-, and z-directions, and (b) thermal conductance of the Au-SAM-Au junction

Figure 8

Out-of-plane thermal conductance vs. simulation cell thicknesses

Figure 9

Temperature dependence of Au-SAM-Au junction thermal conductance with different alkanedithiol chain lengths

Figure 10

Experimental data of temperature dependence of Au-SAM-GaAs junction thermal conductance with different alkanedithiol chain lengths (42)

Figure 11

Normalized vibrational power spectra of Au substrate, sulfur heads, and carbon atoms

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