Optical response of mixed Ag-Cu nanocrystals produced by pulsed laser deposition

Abstract

Ag-Cu mixed nanocrystals (NCs) have been embedded in an amorphous Al2O3 matrix by alternate ablation of pure metal and Al2O3 targets. The composition of the NCs has been varied from pure Cu to pure Ag. Structural analysis by high-resolution electron microscopy (HREM) has shown that in the whole range of compositions, the NCs have average in-plane diameters in the range 3–4 nm and are wider in the in-plane direction than their height, the aspect ratio being 1.4–1.6. The optical properties of the resulting nanocomposite films have instead a strong dependence on the NCs composition. A single surface plasmon resonance (SPR) has been observed whose wavelength blueshifts from that of Cu to that of Ag as the Ag content increases, thus allowing the SPR to be tuned over a broad interval (424–572 nm). Although the nucleation and growth mechanisms have been found to depend on the metal that was deposited first, the optical response has no significant sensitivity to the deposition sequence and thus, it is concluded that more controllable results are achieved by depositing the lighter element first. Finally, HREM together with selective area electron diffraction show that the Ag-Cu NCs are most likely formed by metastable solid solutions of Ag and Cu.