Direct Interfacial Charge Separation in Plasmonic Heterostructures Revealed by Single-Particle Spectroscopy

Non-Technical Description This project is developing methods to understand how metal nanoparticles, 1000 times smaller than the width of a hair, capture and convert light into usable energy when contacting metal oxide semiconductors. Although metal nanoparticles efficiently absorb light, most of the absorbed energy is converted into heat. On the other hand, metal oxide semiconductors can store light energy for much longer times than metals making them useful for applications such as photodetection. However, metal oxide semiconductors do not absorb as strongly or often only at specific wavelengths, while metal nanoparticle can be designed to strongly interact with light of any color. This project overcomes these limitations by combining the high absorption of metal nanoparticles with the longer lifetimes of the absorbed light energy in metal oxide semiconductors. The principal investigator uses techniques that allow him to study how the light energy absorbed by a metal nanoparticle is transferred to an adjacent metal oxide semiconductor layer. These experiments are carried out for one nanoparticle at a time to resolve heterogeneities that arise from materials synthesis. In addition, the PI is continuing his longstanding participation in Rice University’s Civic Scientist Program and Research Experience for Teachers, allowing him to educate K-12 students about nanotechnology and inspire them to pursue scientific careers as well as to provide teachers with experience to in turn help students in those pursuits. Technical Description The goal of this project is to understand and maximize plasmon decay into charge separated states between a metal nanoparticle and an adjacent metal oxide semiconductor via direct charge transfer following plasmon excitation. The principal investigator will accomplish this goal by addressing the following objectives: 1) Design and fabricate plasmonic metal–semiconductor heterostructures and establish a correlation with interface induced plasmon decay via changes to the homogeneous plasmon linewidth; 2) Quantitatively determine charge injection into semiconductors surrounding plasmonic nanostructures using single particle ultrafast spectroscopy and correlate with efficiencies obtained from plasmon damping; 3) Apply Stokes and anti-Stokes emission spectroscopy to independently follow interfacial charge transfer through emission quenching under both one- and multi-photon excitation conditions. These proposed studies will elucidate the mechanism of interfacial charge transfer in plasmonic heterostructures and the underlying material parameters that determine efficiencies with a focus on excess energy as determined by the plasmon resonance and the relative band alignment including Schottky barrier height. Such detailed mechanistic information would be impossible to obtain without single-particle techniques due to the heterogeneity of plasmonic nanoparticle sizes and local environments. The proposed studies will potentially have a transformative impact on developing efficient photovoltaic devices based on plasmonic metal-semiconductor heterostructures taking advantage of a wide wavelength sensitivity, large absorption cross section, and long hot carrier lifetime. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.