Extreme thermal anisotropy in high-aspect-ratio titanium nitride nanostructures for efficient photothermal heating

High optical absorptivity or a large absorption cross-section is necessary to fully utilize the irradiation of light for photothermal heating. Recently, titanium nitride (TiN) nanostructures have been demonstrated to be robust optical absorbers in the optical range owing to their nonradiative decay processes enhanced by broad plasmon resonances. Because the photothermally generated heat dissipates to the surroundings, suppressing heat transfer from TiN nanostructures is crucial for maximizing the photothermal temperature increase. In the current work, compared to the planar TiN film, high-aspect-ratio TiN nanostructures with subwavelength periodicities have been demonstrated to enhance the photothermal temperature increase by a 100-fold using nanotube samples. The reason is attributed to the extremely anisotropic effective thermal conductivities. Our work has revealed that high-aspect-ratio TiN nanostructures are effective in improving photothermal heating, and they can be used in various applications, such as solar heating, chemical reactions, and microfluidics.