Controlling the growth mode of Au depositing on Au nanobipyramids via ligand coverage for SERS enhancement

Gold deposition on Au nanoparticles is a common method to control the shape and further modify the properties of nanoparticles as their properties have a strong correlation with their nanostructures. For Au nanobipyramid (Au NBP), it has advantages such as the enhancement of electric field and a higher tunability in plasmon wavelength than the Au nanorod and thus owns a greater potential in shape control. In this paper, we demonstrate a scheme of depositing Au on the surface of Au NBP with the presence of a type of ligand 2-mercaptobenzoimidazole-5-carboxylic acid (MBIA) to synthesize Au NBP@Au dimers. The growth mode of Au depositing on Au NBP can be controlled by the coverage of MBIA. As the coverage is low, with a concentration of MBIA below 0.4 mM, the rough core-shell nanostructure is synthesized; However, as the coverage is high, with a concentration of MBIA over 0.8 mM, gold deposition may form islands on the surface of Au NBP. The SERS performance of Au depositing on Au NBP can also be enhanced by growth mode. For the rough-surface core-shell growth mode, the enhancement is more significant as the EF is improved from 3.5 × 10⁵ to 1.06 × 10⁶ than the islands-growing growth mode due to the coupling between core and shell. And our results show that with multiple types of nanosturctures easy to obtained by changing modified ligand coverage, the controlled growth has a great potential in the dimer design and SERS enhancement using Au NBP.