Preparation of plasmonic HfN nanoparticle arrays for hot-electron photochemistry

Plasmonic nanostructures feature extraordinary absorption cross sections, highly tunable optical responses, and capability of concentrating the energy of the light in subwavelength volumes. Upon excitation of such structures,... [ view full abstract ]

Plasmonic nanostructures feature extraordinary absorption cross sections, highly tunable optical responses, and capability of concentrating the energy of the light in subwavelength volumes. Upon excitation of such structures, all the plasmon energy is dumped in a single electron, which becomes “hot” for a brief period of time. Recent advances have demonstrated the vast potential of these hot electrons to drive chemical reduction reactions, such as hydrogen generation or organic molecule transformations. 

However, although our understanding of the electron transfer mechanisms, electron energy distributions, time scale of mechanisms, and chemical substrate specificity is steadily progressing, the overall reaction efficiencies remain well below that of application requirements (<1 %). One major factor contributing to these low efficiencies is the extremely short lifetime of the hot electron: traditional plasmonic materials such as gold and silver feature hot electron lifetimes of only 1 - 2 ps, which results in ultrafast recombination of hot electrons and holes and loss of the energy as heat.

Will tuning of the hot electron lifetime offer us control over photochemical reaction yield and/or chemical specificity? To answer this question, we focus on the development of nanostructured materials that feature higher hot electron lifetimes. Especially interesting materials are zirconium nitride and hafnium nitride, which feature bulk hot electron lifetimes up to three orders of magnitude longer than that in gold due to a large phonon bandgap. Furthermore, favorable material properties such as high melting point, hardness, chemical inertness, and high free electron density make these materials excellent candidates to be used in photochemical applications. However, neither the nanostructuring of these materials, nor the plasmonic properties of such devices have been investigated thus far. Here we report for the first time on hafnium nitride nanoparticle arrays that were prepared by an electron-beam lithography synthesis. These results pave the way towards development of real-world hot-electron photochemical devices with plasmonic hafnium nitride in which hot electrons are efficiently used.