Symmetry of the nanocrystal affects its surface charge distribution and the free electrons and thus determines their localized surface plasmon resonance. Planar nanocrystals with low symmetry contain extra resonance modes comparing to the polyhedral nanocrystals with cubic symmetry, including both in-plane and out-plane dipole plasmon modes.
Among the planar nanocrystals, planar branched nanocrystals are rich in tips and edges, therefore, they exhibit unique localized surface plasmon resonance properties and offer promising prospects in optical applications. Yet the formation of such nanocrystal requires growth of the branches to be restricted in-plane. Therefore it remains a major challenge.
Prof. ZENG Jie's group at University of Science & Technology of China of Chinese Academy of Sciences made a breakthrough in fabricating Pd@AuCu core-shell planar tetrapods with adjustable in-plane dipole resonance from visible to near-infrared region. The nanocrystal also exhibited splendid performance in Raman scattering. This work was published on Nano Letters.
Prof. ZENG and his research team fabricated Pd@AuCu core-shell planar tetrapods through preferential overgrowth of AuCu branches on Pd cubic seeds with Oh symmetry. They adopted an integration of kinetic control and lattice mismatch to induce the formation of the branches.
Meanwhile, the features of Pd cubic seed determine the directions that the AuCu branches grow and eventually form a planar structure. The size of the product can be controlled in the range of 33 to 70 nm simply by varying the amount of Pd cubic seeds. Through adjustment of sizes, the peak position of in-plane dipole resonance can be shifted from visible to near-infrared region. These planar tetrapods structures exhibited excellent surface-enhanced Raman scattering (SERS) performance with an enhancement factor up to 9.0 × 103 for 70 nm Pd@AuCu planar tetrapods.
The surface plasmon resonance can break the diffraction limit and therefore is vital to achieving transmitting and processing nano-scale optical information. It is wildly applied in many fields including high density data storage and photoetching technology.
This work was supported by Collaborative Innovation Center of Suzhou Nano Science and Technology, MOST of China, NSFC, Strategic Priority Research Program B of the Chinese Academy of Sciences under Grant, and Fundamental Research Funds for the Central Universities.
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