Unifying stability and plasmonic properties in hybrid nanoislands: Au–Ag synergistic effects and application in SERS

In plasmon-based devices, the choice of metal for the nanostructures often leads to an undesirable tradeoff between either the superb performance of Ag or the stability of Au. There is a great interest in combining Au and Ag in a way that preserves both their favourable properties, while also exploiting synergistic effects to boost surface-enhanced Raman scattering (SERS) performance beyond the capabilities of a single metal. To address this issue, a large-area nanofabrication procedure based on galvanic replacement was used to create hybrid Au–Ag nanoislands with a diverse morphology and highly tunable (through visible to NIR) plasmon resonance. Their stability was comprehensively studied and shown to be complex to achieve. However, in the right conditions, Au–Ag nanoislands with silver-like plasmonic properties were fabricated, and shown to be stable over a period of at least 5 months. Moreover, their SERS efficiency considerably outperformed even that of Ag nanoislands. Computational investigation helped explain the advantageous properties and provided insights for future sensor design. Detection of the model molecules was achieved in the nanomolar range and in mild conditions (low laser power), indicating great potential for biomedicine sensing. Beyond that, nanoislands with simultaneous stability and good plasmonic properties can find application in a wide variety of photonic devices.