Synthesizing Au/NiO Nano-Heterostructures for Effective Hot Charge Carrier Harvest

• Rohan Goyal, Chemical Engineering, University of Massachusetts - Amherst

• Lars Gundlach, Department of Chemistry and Biochemistry, University of Delaware

Energy contained in the optical excitation of plasmons in materials such as gold metal nanoparticles and metal/semiconductor nano-heterostructures (MSNHs) can be converted directly or indirectly into other forms of excitation. An example of such conversion is the formation of hot charge carriers within a metal nanoparticle or exciton in the semiconductor domain of MSNHs. Despite numerous investigations of hot charge carrier dynamics, there is little insight into the first step conversion of plasmons into charge carriers or excitons. In this study, we report an ordered array of hemispherical MSNHs that are adapted experimentally to serve as a model system to study the dynamics of hot charge carriers. MSNHs offer considerable control over significant parameters such as size and material composition, while being compatible with time resolved measurements down to 10 femtoseconds. Here, we present a method to synthesize a highly ordered array of Au/NiO MSNHs, and data from a time-resolved, ultrafast measurement of the charge carrier dynamics within the domains of the heterostructures. The Au/NiO array was fabricated by assembling a monolayer of closely packed colloidal polystyrene spheres, followed by deposition of Au and Ni to yield a nano-triangular Au/Ni film. After liftoff of polystyrene, the sample was annealed at 500 °C for 30 minutes to experimentally yield a nanopyramid array of Au/NiO nano-heterostructures. The synthesized Au/NiO array were morphologically and crystallographically characterized. Also, data about the hot charge carrier dynamics were obtained by employing transient absorption spectroscopy (TAS). The information extracted from these measurements will improve our understanding of charge carrier dynamics at early time delay, and how to efficiently harvest the energy or charge carriers for various potential applications such as solar energy conversion, photocatalysis, and optoelectronics.