MS-1-P-1554 Photo-induced lattice accommodation in Ag/Cu composite nanoparticles

Nanoparticles exhibit specific structural and optical properties which are different from those of the corresponding bulk materials. The lattice softening is one of the specific properties originated from the shallow interatomic potential. On the other hand, localized plasmon in metallic nanoparticles is recently focused on the optical properties. The electric field induced by the localized plasmon may interact with the lattice vibration and enhance the lattice softening.
We confirmed in our previous research that a lattice accommodation takes place in a two-phase nanoparticle which has a lattice misfit. The lattice accommodation is induced not only on the interface between the two phases but also all over the nanoparticle. In such a lattice-accommodated two-phase nanoparticle, if only one phase is resonantly excited by the localized plasmon using well-defined photo-illumination, that is, the electron-phonon interaction is induced in the only one phase, how will the accommodated lattice behave to photo-illumination ?
In the present work, photo-induced lattice accommodation in Ag/Cu composite nanoparticles has been studies in situ by laser-coupled TEM with a double source evaporator, in order to see an electron-phonon interaction in the nanoparticles.
Fig. 1(a) shows a BFI of Ag/Cu composite nanoparticles and the corresponding DFI taken from Ag 111 reflection. Two kinds of morphologies are observed as shown schematically in the figure. One is core-shell structure (type A), and the other is particle-connected structure with a planer interface (type B). The amount of type A is larger than that of type B. Fig. 1(b) shows electron diffraction profiles from the nanoparticles before, during and after photo-illumination with the energy of 2.3 eV. All the diffractions are identified as Debye-Scherrer rings of the fcc silver and copper. The lattice constant of copper with and without photo-illumination are 0.370 and 0.366 nm, respectively. The changes in the lattice constant take place reversibly. The fact that no changes in the lattice constant are induced by photo-illumination in pure silver or copper nanoparticles denies an effect by the thermal expansion.
It was evident that photo-induced lattice accommodation takes place in Ag/Cu composite nanoparticles. Photo-illumination with the energy of 2.3 eV resonantly enhances the localized plasmon with the energy of approximately 2.0 eV in copper nanoparticles. An enhancement of the local electric field in copper nanoparticles may induce the lattice vibration and the subsequent lattice softening in the copper core region. Consequently, it is considered that the lattice of the silver shell accommodated by the copper core is relaxed to increase toward the lattice constant close to that of pure silver.