Assessment of Self‐Assembled Monolayers as High‐Performance Thermal Interface Materials

Abstract

gaps is of critical importance for their performance. Interfacial thermal resistance arises from the mismatch between vibrational modes in the contact materials [4] and weak interactions between them at the interface. [5] The resistance could be reduced by introducing thermal interface materials (TIMs), which fill the gap between materials and enhance the interfacial thermal coupling. A characteristic length scale [6] for interfacial thermal transport could be defined as l I = κ/G, where κ is the thermal conductivity of materials on two sides of the interface, and G is the interfacial thermal conductance (ITC). The interfacial thermal resistance R = 1/G could be neglected for thermal transport over a distance that is much longer than l I . Accordingly, we can define the thermal transparency as λ = L/l I , where L is the thickness of the interface or the TIM. On the other hand, soft TIMs with structural flexibility are beneficial for geometrical compatibility between rough surfaces and structural robustness under external perturbations. One can measure the structural flexibility of a TIM by Y −1 , where Y is the Young's modulus. A combination of high thermal transparency and structural flexibility makes a TIM versatile in practical applications. In recent studies, polymers and nanostructures such as carbon nanotubes and graphene sheets have been explored as potential TIMs with these merits. [6,7] It has been reported that polymer chains exhibit high thermal conductivity along the chain direction if the orientational order is preserved as in the crystalline phase (≈130 W m −1 K −1 for polyethylene), which hold great promises in thermal management applications. [8] Linear molecules such as alkane chains could also arrange themselves into self-assembled monolayers (SAMs) on various substrates, [9] which feature certain orientational order and entropy-controlled soft elasticity, and have been proposed for applications that range from molecular electronics, [1] thermoelectric materials, [10] thermal management, [9a,11] to interfacial functionalization for enhanced thermal transport. [5,12] Thermal and mechanical properties of SAMs have been explored both experimentally and theoretically as potential TIMs. [4,5,13] Zheng et al. [14] showed that silane-based SAMs with varying end groups enhance the ITC between sapphire and poly styrene by a factor of 7, through functionalizing the sapphire surface with SAM. Acharya et al. [15] calculated the thermal conductance of silicon-SAM-water interfaces through Thermal interface materials (TIMs) are highly desirable for efficient thermal transfer or dissipation in a wide range of material and device applications. The self-assembled monolayers (SAMs) are promising candidates for these applications due to their high thermal transparency and structural flexibility. In this work, the performance of SAMs as practical TIMs is assessed by performing atomistic simulations. The mixed nature of diffusive and ballistic thermal transport is identified across the monolayers from signatures ...

Molecular Structure Effect of a Self-Assembled Monolayer on Thermal Resistance across an Interface

“…In the past few years, efforts have been made to find out strategies to reduce ITR between the solid and fluid. Among these strategies, one of the most promising approaches is to anchor a self-assembled monolayer to the solid surface [4]. With this approach, researchers have focused on modifying the interface with the alkanethiol SAMs, X-(CH 2 )n-SH, where X represents -COOH, -OH, -CH 3 , -NH 2 , and so on.…”

“…These studies have investigated the effects of various characteristics of SAMs on the interfacial thermal conductance from two different points of view: (1) considering accurate information about SAM, including SAM coverage [4,9,10], length of SAMs [4,[11][12][13], distinct functional group [9,[13][14][15][16][17], heterogeneous fin structure [18], vibrational spectral overlapping [9], nanoscale roughness [19][20][21], and so on. Huang et al [16] investigated interfacial thermal conductance (ITC) across the interface of water and gold tailored with -CH 3 SAM, -OHSAM, and -COOHSAM.…”

Remarkable Reduction of Interfacial Thermal Resistance in Nanophononic Heterostructures

“…[17][18][19][20][21][22][23][24][25][26] In principle, there always exists contact thermal resistance when phonons transport across the interface between two dissimilar materials. A large number of studies have been done to understand the interfacial thermal conductance of various types of interface, including mass-mismatched solidsolid interface, [27,28] hard-soft materials interface, [29,30] solid-liquid interface, [31] metal-nonmetal interface considering electron-phonon interaction, [32,33] and solid-gas interface. [34] Despite these researches have improved the understanding of phonons transport across these interfaces, however, the interfacial thermal resistance between nanophononic metamaterials (phononic crystals) is still unexplored.…”

Thermal resistance of an interfacial molecular layer by first-principles molecular dynamics

“…We have also plotted in Fig. 6 (blue point) another calculation performed with interatomic potentials 10 for an alkane containing 11 C covalently bonded to diamond reservoirs. This configuration consists of an interface linked to the reservoirs by C-C bonds only (-CH 2 -C-: ∼88.5 kcal/mol) 25 , thereby mitigating the qualitative character of interatomic potentials compared to the case of heterogeneous bonding.…”

“…Nowadays, (nanoscale) interfaces are essential components for materials and devices used in thermoelectric energy conversion, electrochemical energy storage and nanoelectronics. Thermal transport through interfaces with nanoscale gaps is of crucial importance for their performance 10 . However, interfaces can act as the heat transport bottlenecks since the characteristic dimensions of nanodevices approach electron and/or phonon mean free paths 11,12 .…”