Recent research by Zhiping Xu's group in the Department of Engineering Mechanics and Center for Nano and Micro Mechanics elucidates the mechanism of heat dissipation in molecular junctions and identifies the critical power to maintain their thermal stabilities. The study provides guidelines for the design of molecular-, nano-electronics and scanning thermal probing techniques.

Beyond the foreseen limits of conventional silicon integrated circuits, molecular electronics has gained wide attention for its promise of transcending Moore’s law and pushing the limit of downsizing devices. Single or multiple molecules could serve as active electronic components, displaying advantages over conventional semiconductors including their reduced size, flexibility, convenience in fabrication, wide material selection and so on. The thermal equilibrium of molecular junction based devices during operation is established by balancing electronic heating in the junction and heat dissipation into its contacts and environment. In practice, the power density of heat generation increases dramatically as the dimension of devices is reduced, which may lead to thermally induced instability such as large-amplitude thermal vibration and even detachment of molecules from the contacts. The understanding of heat dissipation and transport processes in molecular junctions thus becomes of critical importance in designing real-world molecular devices and paves the way for continuously growing development of aforementioned applications.

In recent work published in the journal Nature Communciations, Yanlei Wang and Zhiping Xu investigate heat dissipation and conduction processes through molecular junctions by performing molecular dynamics (MD) simulations. The critical power of heat generation in the junction to maintain its thermal stability is identified. For example, the temperature of junction will rise above a threshold of 50 K with power higher than ~1 μW. This value of critical power can be elevated, for example, by using junctions consisting of more molecules.

The heat dissipation process in a molecular jucntion is defined by the interfacial thermal resistance between molecules and the contact, which is also known as the Kapitza resistance. Wang and Xu’s study further concludes that the resistance between benzene molecules and diamond contacts is on the order of 109 KW-1, displaying explicit power- and temperature-dependences, and can increase by a facor of two as the junction is stretched. These findings are explained by using a simple resistor chain model, analyzed by vibrational spectra analysis, and are discussed with respect to their key applications in molecular electronics and scanning probe techniques, providing practical design guidelines for relevant applications by offering fundamental understandings of the thermal energy transfer process.

The energy transfer processes across nanoscale interfaces have great implications in nanoscale devices, hybrid nanostructures, and nanocomposites. The work in Xu’s group has been focused on understanding interfaces between carbon nanostructures and their contacts or the environment, as well as developing interfacial engineering solutions to tune thermal and electronic couplings therein. For example, intercalating gas molecules between graphene and the substrate is found to be able to establish an electronically insulating but thermally transparent interface, which is promising for nanoelectronic applications.

These works were supported by the National Natural Science Foundation of China and Tsinghua University Initiative Scientific Research Program. The computer simulations were performed on the Explorer 100 cluster system of Tsinghua National Laboratory for Information Science and Technology.