Above: As the system evolves in time, hot carriers cool via various pathways including phonon emission and Auger-like electron-to-hole energy transfer.

 

(A) TDDFT calculation of the normal Raman spectra of pyridine in the gas phase at an incident wavelength of 514.5 nm based on static polarizability derivatives.

(B)

(B) Transient absorption spectra of aminated and thiolated Au NP samples, at a series of time delays after photoexcitation.

The spatial confinement of matter in nanometer-sized particles leads to unique physical properties, drastically different from those of a bulk material. These new properties show a strong dependence on nanoparticles’ size, shape, composition and environment, allowing their fine control to meet technological requirements for biomedical applications, photocatalysis, optoelectronics, and many more. In particular, understanding and modeling how the vibrational response of a semiconductor or metallic particle change as its size decreases appears as a challenging goal for both fundamental and technological interest.

The dynamics of a nanoparticle following its optical excitation is due to the excited electrons thermalization that further relax through electron-phonon coupling and is experimentally observed using ultrafast transient absorption spectroscopy. The vibrational motion of a particle exhibits discrete normal frequencies, whose characteristics are signatures of the nanoparticle mechanical and physical properties.

(C) Surface enhanced Raman scattering enhancement factor of gold FON calculated with FDTD.

(C) Oscillation period of the acoustic mode as a function of the LSPR for five samples of gold bipyramids. Inset: simulated displacement
of a bipyramid during acoustic mode oscillation (not to scale).

This mechanism is theoretically described in the Schatz group by using and combining various computational methods. Successful results have been obtained through different approaches, based on partial differential equations resolution (FDTD and FEM) as well as by applying an atomistic (TD)DFT formalism. This work, conducted in fruitful collaboration with experimentalists, is a promising way to obtain information on the fundamental processes that occur at the nanoscale following absorption of photons.

 

Recent Publications:

Jensen, Lasse; Zhao, Lin L.; Schatz, George C.,“Size-Dependence of the Enhanced Raman Scattering of Pyridine Adsorbed on Ag(n) (n = 2−8, 20) Clusters” J. Phys. Chem. C, 2007, 111, 4756–4764.

Hannah, Daniel C.; Brown, Kristen E.; Young, Ryan M.; Wasielewski, Michael R; Schatz, George C.; Co, Dick T.; Schaller, Richard D., “Direct Measurement of Lattice Dynamics and Optical Phonon Excitation in Semiconductor Nanocrystals Using Femtosecond Stimulated Raman Spectroscopy”. Phys. Rev. Lett. 2013, 111, 107401.

Mullin, Jonathan; Valley, Nicholas; Blaber, Martin G.; Schatz, George C., “Combined Quantum Mechanics (TDDFT) and Classical Electrodynamics (Mie Theory) Methods for Calculating Surface Enhanced Raman and Hyper-Raman Spectra”. J. Phys. Chem. A 2102, 116, 9574-9581.

Aruda, Kenneth O.; Tagliazucchi, Mario; Sweeney, Christina M.; Hannah, Daniel C.; Schatz, George C.; Weiss, Emily A., “Identification of parameters through which surface chemistry determines the lifetimes of hot electrons in small Au nanoparticles”. P.N.A.S., 2013, 110, 4212-4217.

Kirschner, Matthew S.; Lethiec, Clotilde M.; Lin, X-M.; Schatz, George C.; Chen Lin X.; and Schaller, Richard D., “Size Dependent Coherent Phonon Plasmon Modulations and Dephasing in Gold Bipyramids and Nanojavelins”, in preparation.

Lethiec, Clotilde M.; Madison, Lindsey R.; and Schatz, George C., “Dependence of Plasmon Energies on the acoustic normal modes of Agn (n=20, 84 and 120) clusters”, in preparation.