© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 6093 www.advmat.de www.MaterialsViews.com wileyonlinelibrary.com COMMUNICATION Molecular Bridge Enables Anomalous Enhancement in Thermal Transport across Hard-Soft Material Interfaces Fangyuan Sun, Teng Zhang, Matthew M. Jobbins, Zhi Guo, Xueqiang Zhang, Zhongli Zheng, Dawei Tang, Sylwia Ptasinska, and Tengfei Luo* F. Sun, [+] T. Zhang, [+] Z. Guo, T. Luo Department of Aerospace and Mechanical Engineering University of Notre Dame Notre Dame, IN 46556, USA E-mail: tluo@nd.edu F. Sun, D. Tang Institute of Engineering Thermophysics Chinese Academy of Sciences Beijing 100190, China F. Sun University of Chinese Academy of Sciences Beijing 100049, China M. M. Jobbins, X. Zhang Department of Chemistry University of Notre Dame Notre Dame, IN 46556, USA Z. Guo, X. Zhang, S. Ptasinska Department of Physics University of Notre Dame Notre Dame, IN 46556, USA S. Ptasinska Radiation Laboratory University of Notre Dame Notre Dame, IN 46556, USA Z. Zheng Department of Chemical and Biomolecular Engineering University of Notre Dame Notre Dame, IN 46556, USA T. Luo Center for Sustainable Energy at Notre Dame Notre Dame, IN 46556, USA DOI: 10.1002/adma.201400954 simulations reveal that the SAMs chemically absorbed on the gold surface act as media to bridge the vibrational mismatch (acoustic mismatch) between gold and polymer and thus enable efficient resonance-like thermal transport. Such a strategy can be generalized to any hard-soft material interfaces to provide a design principle that can be used to synthesize nanocomposites for heat transfer applications. In heat transfer-critical applications such as composite poly- meric materials, [1,2] thermal management of electronics, [3,9] nanofluids [4–6] and nanoparticle-assisted hyperthermia therapy, [7,8,10–13] one common feature is the existence of inter- faces between hard and soft materials. The role of interfaces also becomes increasingly important in thermal transport as the characteristic dimensions of structures approaching nanoscale where the interfacial thermal resistance is of similar magnitude as that of the material. [14,15] It is well-known that interfacial thermal transport is largely dictated by the phonon spectra match (acoustic match) of the materials constituting the interface. [16] The phonon spectra of hard and soft materials usually have large mismatch due to dis- tinct compositions and bond natures. Binding strength at the interface is another critical factor that influences thermal trans- port. [17–22] The interfaces between hard and soft materials are usually dominated by weak van der Waals (vdW) interactions and thus present large interfacial thermal resistance. [1] Forming covalent bonds at the interface can significantly enhance inter- facial thermal transport. [23–25] However, such a strategy is usu- ally not viable for hard-soft interfaces since in real applications they are usually formed through physical means instead of chemical reactions. Functionalizing hard material surfaces to improve hydro- philicity and thus interfacial binding energy has been an effec- tive strategy to enhance interfacial thermal transport between liquid and solid. For example, Ge et al. [17] found that properly functionalizing metal surfaces can enhance thermal conduct- ance between metal and water by up to 3 times due to the improved hydrophilicity. The same observation was made by Shenogina et al. [26] using MD simulations. Harikrishna et al. [20] found that as the hydrophilicity of gold surface increases, the interface adhesion energy increases and the thermal conduct- ance increase linearly. Enhancing interface adhesion energy to increase interfacial thermal conductance has also been real- ized in solid-solid interfaces. [27,28] For example, O'Brien et al. [22] demonstrated that a nanomolecular monolayer at metal/dielec- tric interfaces, which form covalent bonds with both surfaces, can significantly enhance the interfacial thermal conductance. Their analyses also suggest that the changes in the vibrational density of states near the interface due to the monolayer can Thermal resistance at hard-soft material interfaces has been one of the most important bottlenecks for the improvement of thermal transport properties of nanocomposites which are critical for a wide range of applications like thermal interface materials, nanofluids and nanoparticle-assisted therapeu- tics. [1–8] Conventional strategies have focused on improving adhesion of the interface to increase thermal conductance. Here we demonstrate a significant enhancement of thermal transport across the hard-soft material interfaces consisting of gold and amorphous polyethylene (PE) by functionalizing the gold surfaces with self-assembled monolayers (SAM). Our tran- sient thermoreflectance (TTR) measurements show remarkable increases by as much as 7 times in the thermal conductance of gold-hexadecane (HD) and gold-paraffin wax (PW) interfaces. Surprisingly, such significant increases are realized despite the observed decrease in the interface adhesion energy when the gold surface is functionalized. Our molecular dynamics (MD) [+] These authors contributed equally to this work Adv. Mater. 2014, 26, 6093–6099