Synthesis and Characterization of Metal/Semiconductor Hemispherical Nano-Heterostructure Arrays

• Noah Elbaum, Chemistry, Brown University

• Lars Gundlach, Chemistry and Biochemistry, University of Delaware

Localized surface plasmon excitation, driven by the interaction with incident electromagnetic radiation, makes noble metal nanoparticles highly effective for light-harvesting applications. This phenomenon reaches its maximum amplitude at resonance, occurring when the frequency of the incident photons matches the natural oscillation frequency of the metallic nanoparticles’ free electron cloud. These natural oscillation frequencies are determined by the nanoparticles’ size, shape, and aggregation states. Plasmons can decay non-radiatively through Landau damping, creating hot electron-hole pairs. Hot charge carrier pairs have different lifetimes depending on the local environment of the nanoparticle, and are usually damped by scattering and/or recombination processes in the metal domain. However, when the nanoparticle has a semiconductor shell that the metal is coupled to, the energy and charge from the plasmon can be transferred to the semiconductor domain. To investigate what materials might be benefited by energy or charge transfer, this research investigates cuprous oxide (a p-type semiconductor) as a candidate, due to its band alignment and interface properties to promote charge separation. Arrays of individual hemispherical gold nanoparticles are fabricated on glass substrates using nanosphere lithography, physical vapor deposition, and annealing to control shape. Size-controlled shells of cuprous oxide are then grown via reduction of cupric sulfate in a chemical bath deposition technique. Initial characterization of the arrays is done with SEM and UV-Vis. Then, transient absorption (TA) is utilized on the picosecond and femtosecond timescales to probe the ultrafast charge dynamics. Particular focus is put on a proposed localized surface plasmon polariton created by interaction of the nanoparticles with the probe beam. Ultimately, this research will help advance the accessibility of hot charge carriers for photocatalysis and semiconductor applications.