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Nanometer size metallic clusters and sharp features of metal surfaces support collective excitations of many electrons, known as plasmons. They extend many nanometers along surfaces and on the order of nanometer outside. Plasmons interact strongly very electromagnetic fields, and absorb and reflect light. Colored glass in old cathedrals in Europe have color due to plasmons in imbedded metal particles. Metallic structures giving rise to plasmons are used in photocatalysis and photochemistry, detection and sensing, information processing, etc. It is particularly intriguing for chemists to understand how extended plasmons give rise to local chemical events.
There are three types of states in metal surfaces and clusters: bulk, surface and plasmons. Plasmon states are localized away from atoms, and couple to atomic vibrations weekly. Thus, electron-vibrational relaxation of plasmonic excitations is relatively slow.
Z. Guo, W.-Z. Liang, O. V. Prezhdo, “Ab initio study of phonon-induced dephasing of plasmon excitations in silver quantum dots”, Phys. Rev. B, 81, 125415 (2010).
Plasmonic excitations are particularly strong on sharp features of metal surfaces and clusters. These places have unsaturated chemical bonds and are chemically active, facilitating catalytic events. Chemically undercoordinated metal atoms can undergo slow fluctuations, creating additional unsaturated bonds, making them even more chemically active, and supporting longer lived electronic excitations.
W. Chu, W. A. Saidi, O. V. Prezhdo, “Long-Lived Hot Electron in a Metallic Particle for Plasmonics and Catalysis: Ab Initio Nonadiabatic Molecular Dynamics with Machine Learning”, ACS Nano, 14, 10608 (2020).
Strong interfacial interactions at metal/semiconductor or metal/molecule interfaces can give rise to hybrid plasmon/charge-transfer excitations. This creates an opportunity for plasmonic excitation to generate chemically active charge carriers and avoid rapid charge relaxation and electron-hole annihilation typical of metals.
Z. S. Zhang, L. H. Liu, W. H. Fang, R. Long, M. V. Tokina, O. V. Prezhdo, “Plasmon-mediated electron injection from Au nanorods into MoS2: traditional versus photoexcitation mechanism”, Chem, 4, 1112-1127 (2018).
R. Long, O. V. Prezhdo “Instantaneous generation of charge-separated state on TiO2 surface sensitized with plasmonic nanoparticles”, J. Am. Chem. Soc., 136, 4343 (2014).
Plasmonic excitations can extend by a nanometer into vacuum, providing efficient channels for charge and energy transfer.
C. Xu, H. W. Young, J. He, R. Long, A. R. Cadore, I. Paradisanos, A. K. Ott, G. Soavi, S. Tongay, G. Cerullo, A. C. Ferrari, O. V. Prezhdo, Z. H. Loh, “Weak Distance Dependence of Hot-Electron-Transfer Rates at the Interface between Monolayer MoS2 and Gold”, ACS Nano, 15, 819-828 (2021).
Electrons in doped or non-stoichiometric semiconductors can scatter strongly with hot electrons in metals, generated by plasmon excitations, giving rise to efficient energy flow across metal/semiconductor interfaces. The phenomenon can be utilized to construct thermal diodes for efficient heat dissipation.
J. A. Tomko, et al., “Long-Lived Modulation of Plasmonic Absorption by Ballistic Thermal Injection”, Nature Nanotech., 16, 47-51 (2021).
T. F. Lu, Y. S. Wang, J. A. Tomko, P. E. Hopkins, H. X. Zhang, O. V. Prezhdo, “Control of Charge Carrier Dynamics in Plasmonic Au Films by TiOx Substrate Stoichiometry”, J. Phys. Chem. Lett., 11, 1419-1427 (2020).
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