Crystalline AuNP with Strain-Controlled Morphologies and Optical Properties

Metallic nanoparticles (NP) decorated with thin films have been extensively studied and exhibit promising performance in a wide range of applications, including biosensing, electrical devices, photocatalysis, and solar cells. For perovskite thin films, the decoration can produce an enhanced plasmonic response, which indicates a promising future in nano-waveguides, energy harvesting, and chemical sensing applications. The morphologies and chemistries of NPs strongly govern plasmonic performance. The size and density of AuNPs dramatically influence the absorption, surface plasmonic resonance peak intensity, and the second harmonic generation responses. It also alters the so-called epsilon-near-zero (ENZ) effects, where the interplay between real and imaginary parts of permittivity results in the strong enhancement and local confinement of the normal field inside ENZ materials.<br/>Here we present a study of the formation of gold nanoparticles (AuNP) grown by PLD onto perovskite strontium niobate (SNO) thin films deposited on both MgO and SrTiO₃ substrates. The strain-free SNO₃ (a = 4.023 Å) has a -3.0% lattice mismatch with the STO (a = 3.905 Å) and 4.5% with MgO (a = 4.212 Å) which leads to compressive and tensile in-plane residual strains, respectively. We observed that the formation of Au NPs and their morphology depends on the structure of the underlying SNO film. It conveys the surface structure and controls nanoparticle formation. The morphologies of AuNPs were measured using SEM and AFM. The residual strain in the SNO thin films and AuNPs was evaluated by X-ray diffraction (XRD). Finally, the complex permittivity of AuNP-decorated SNO films was measured using Variable Angle Spectroscopic Ellipsometry (VASE).<br/>In summary, AuNP-decorated SNO/MgO and SNO/STO thin films exhibit different optical properties with only gold-decorated SNO/MgO samples showing nano particles’ size-dependent epsilon-near-zero behaviour. These results provide an additional route to control the structure of AuNPs. They can be used for the design and development of strain-engineered gold nanoparticle decorated devices for surface-enhanced Raman spectroscopy (SERS) and photocatalysis.