Theory of Photoinjection of Hot Plasmonic Carriers from Metal Nanostructures into Semiconductors and Surface Molecules

Abstract

We investigate theoretically the effects of generation and injection of plasmonic carriers from an optically excited metal nanocrystal to a semiconductor contact or to surface molecules. The energy distributions of optically excited hot carriers are dramatically different in metal nanocrystals with large and small sizes. In large nanocrystals, the majority of hot carriers has very small excitation energies, and the excited-carrier distribution resembles the case of a plasmon wave in bulk. For nanocrystal sizes smaller than 20 nm, the carrier distribution extends to larger energies and occupies the whole region E F < ε < E F + ℏω. The physical reason for the above behaviors is nonconservation of momentum in a nanocrystal. Because of the above properties, nanocrystals of small sizes are most suitable for designing opto-electronic and photosynthetic devices based on injection of plasmonic electrons and holes. For gold nanocrystals, the optimal sizes for efficient generation of hot carriers with overbarrier energies are in the range of 10−20 nm. Another important factor is the polarization of the exciting light. For efficient excitation of carriers with high energies, the electric-field polarization vector should be perpendicular to a prism-like nanoantenna (slab or platelet). We also show the relation between our theory for injection from plasmonic nanocrystals and the Fowler theory of injection from a bulk metal. Along with a prism geometry (or platelet geometry), we consider cubes. The results can be applied to design both purely solid-state opto-electronic devices and systems for photocatalysis and solar-energy conversion.

Harvesting the loss: surface plasmon-based hot electron photodetection

“…Plasmonic metallic nanoparticles have an absorption cross-section much larger than the physical size of the particles [67], yielding much more efficient hot electron generation than bulk metal. Plasmonic nanoparticles also produce hot carriers at a higher average energy compared to absorption in bulk metals [30,31,68,69]. Lastly, the momentum distribution of hot carriers can be modified by engineering the modes of the plasmonic structures.…”

“…This broad hot electron energy distribution sets an impediment for realizing high efficiency in the internal photoemission process, as many carriers will have an energy that is below the Schottky barrier [28]. In contrast to bulk metals, it has been shown that the hot carrier energy distribution in plasmonic nanoparticles can potentially be narrowband [30,31,69,71,72] and is strongly dependent on the particle size [30,69] and geometry [71,73,74] ( Figure 1D). For instance, hot carriers are more efficiently generated in small plasmonic nanoparticles compared to large ones [30].…”

“…In contrast to bulk metals, it has been shown that the hot carrier energy distribution in plasmonic nanoparticles can potentially be narrowband [30,31,69,71,72] and is strongly dependent on the particle size [30,69] and geometry [71,73,74] ( Figure 1D). For instance, hot carriers are more efficiently generated in small plasmonic nanoparticles compared to large ones [30]. Nanoparticle geometry, including shape [71] and aspect ratio [75], also plays an important role in determining the hot carrier generation and injection due to carrier confinement and surface scattering [71].…”

“…For hot carrier distribution, although much work has been devoted to providing theoretical predictions of hot carrier distribution in plasmonic nanostructures [30,31,69,71,72,158], more experimental work is needed to validate these models. In addition, a better understanding of the hot carrier relaxation timescale [75,117,[159][160][161] in various materials as well as the transport dynamics in nanostructures would provide more valuable information for designing plasmonic devices with efficient hot carrier transport to the Schottky interface.…”

Molecular interfaces for plasmonic hot electron photovoltaics

“…18,[23][24][25] To this end, metal-semiconductor (MS) Schottky junctions have been employed, 11,12,26 that take advantage of the built-in field in the vicinity of the metal nanostructure to separate the photogenerated carriers. The use of metalinsulator-metal architectures was also successfully employed for hot-electron photodetection.…”