Chemical Interface Damping at Metal-Semiconductor Junctions

Nontechnical description This project aims to characterize how light absorbed by particles a few billionths of a meter (nanometers) in size is converted to electrical currents, while the generation of heat is minimized. Such efficient conversion is best achieved with hybrid materials consisting of a strong absorber and a layer that promotes the lifetime and mobility of the electrons responsible for the currents. When shrinking the size of these materials to nanometer dimensions, as needed for miniaturized devices, noble metals like gold and silver become some of the strongest known absorbers. Their absorption, or color, can furthermore be varied by changing the size and shape of the particle. Metal particles by themselves though produce mostly heat when interacting with light. Combining them with semiconductors, which often absorb poorly or only limited colors, takes advantage of the best qualities of both in the form of a hybrid material. This research develops methods to probe the efficiencies and underlying mechanisms for how the initial energy imparted by light is transduced into electrical currents at the interface of a hybrid material between a metal particle and a semiconductor layer. The principal investigator uses techniques that allow him to probe the mobility of electrons from the metal to the semiconductor, using both optical and electrical measurements with a spatial resolution of nanometers and a time resolution of much less than a picosecond, or a millionth of a millionth of a second. In addition, the principal investigator is participating in programs that educate both college and community college students about nanotechnology to inspire and prepare them to pursue potential careers in science, technology, engineering, and math. Technical description Efficient charge separation following photoexcitation is a critical step in semiconductor devices. Plasmon decay generates energetic charges that facilitate this process and only need to overcome the interfacial barrier formed at metal-semiconductor junctions. Two pathways for charge transfer have been identified: i) decay of a plasmon directly into an interfacial electron-hole pair, also known as chemical interface damping; or ii) creation of an electron-hole pair in the metal followed by charge transfer across the metal-semiconductor junction. However, it is not known how their relative contributions depend on materials parameters such as nanoparticle morphology or semiconductor band structure. The goal of this project is to establish a mechanistic understanding for how electron and hole transfer can be maximized from plasmonic metals to semiconductors. The principal investigator is accomplishing this goal by addressing the following objectives: (1) Establish that hole injection occurs directly via plasmon decay for metal-semiconductor junctions with appropriately positioned bands while varying the plasmon energy; (2) characterize the size dependencies of the two charge injection pathways for electrons and holes; (3) visualize charge injection in metal-semiconductor nanostructures by surface photovoltage imaging. This research is accomplished by a combination of time-resolved and frequency-domain spectroscopies, electrical measurements, and hybrid materials fabrication. Quantifying charge separation efficiency of both possible pathways, while separating contributions from electrons and holes, enables strategies that minimize unwanted photothermal heating. Outcomes of these studies are expected to be transformative for plasmonic materials with strongly absorbing and widely tunable surface plasmons for use in opto-electronics. 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.