Ligand-Length Asymmetry Controls Nanoscale Heat Flow in Gold Janus Nanorods

Plasmonic nanoparticles play a crucial role in enabling technologies used in medical, catalytic, and imaging applications. Introducing particle heterogeneity in Janus nanoparticles provides a route for molecular-level control of heat transport across nanoscale length scales. Here, we show that the temperature field generated by gold Janus nanorods in water can be tuned by engineering the length of one of the ligand layers. Using non-equilibrium molecular dynamics simulations, we investigate nanorods coated via self-assembly with a hydrophobic octanethiolate monolayer and a hydrophilic hydroxy-terminated alkanethiolate monolayer of varying lengths (3 to 13 carbon atoms). Increasing the hydrophilic ligand length, together with changes in the ligand packing and grafting density, decreases the temperature of the water next to the nanoparticle by approximately 15 K at comparable heat rates and reduces the temperature difference between hydrophobic and hydrophilic regions significantly, by a factor of three, from 25 to 7 K. This effect arises from a systematic increase in the Kapitza resistance of the hydrophilic region, while the resistance of the hydrophobic region remains nearly unchanged, demonstrating that ligand modification introduces local changes in heat transport while the opposite side of the Janus nanorod shows no systematic variation within the statistical uncertainty. The temperature contrast persists near the hydrophobic–hydrophilic–water three-phase boundary, indicating weak lateral heat transfer along the ligand coating. Our findings establish ligand-length asymmetry as a molecular strategy for designing materials to control anisotropic heat flow and nanoscale temperature contrast in Janus nanorods.