Sandia researchers explore how energy and charge move on ultrafast time scales between gold nano-islands and a single layer of 4-mercaptobenzoic acid (4MBA) molecules attached to their surface, a process at the core of emerging technologies like plasmonic sensors and photocatalysts, which have clear national-security implications for chemical detection and on-demand energy conversion.
By using femtosecond UV-visible pump-probe spectroscopy to excite electrons in the gold and then watching how the system relaxes, researchers identified three key “clock ticks” of relaxation: electron–electron collisions (tens to hundreds of femtoseconds), electron–phonon coupling (a few picoseconds) and phonon–phonon thermalization (hundreds of picoseconds).
Compared to bare gold, the gold covered with the 4MBA monolayer exhibited noticeably longer electron–phonon and phonon–phonon lifetimes. First-principles calculations attribute the slower electron cooling to strong coupling between gold’s low-frequency acoustic vibrations and specific vibrational modes of the 4MBA molecules, which reduces the overlap between the metal’s electronic states and its acoustic phonons. The delayed phonon thermalization is likely due to the self-assembled monolayer’s poor heat-conducting properties, which impede efficient energy flow from gold into the surroundings.
These findings demonstrate that molecular layers can be used to control how quickly charge and heat move in plasmonic metals. This ability opens pathways for more effective plasmon-based technologies such as sensors, catalysts, and nanoscale devices designed to manage heat more effectively.
Sandia researchers linked to work
- Bandaranayake, S.
- Ramasesha, K.
- Schrader, P.
- Shivanna, M.
- Stavila, V.
Sponsored by
The Division of Chemical Sciences, Geosciences and Biosciences; Office of Basic Energy Sciences (BES); U.S. Department of Energy (DOE)
Associated Publications
Bandaranayake, S., Schrader, P., Chen, G., Tan, L.Z., Shivanna, M., Stavila, V., Luna, J.R., and Ramasesha, K., (2026). “Phonon-mediated ultrafast dynamics in self-assembled monolayers of 4-mercaptobenzoic acid on gold”, Phys. Chem. Chem. Phys. 28, 8531-8540. https://doi.org/10.1039/D5CP04826E
August 24, 2026