Plasmonic nanostructures can now be used in several fields such as energy harvesting, sensing, light guiding, imaging gadgets, and biomedical applications. One aspect of these nanostructures stands out; their capacity to focus electromagnetic energy. It’s an effect that is further amplified in nanostructures with nanometer scale gaps owing to the excitation of the plasmonic hotspots. It has been found that the size between the gaps of the plasmonic nanoparticles is central for localization as well as the intensity of the electric field in the hotspots.
Lithographic methods proposed for surface pattering and fabrication necessary for characterizing the properties of hierarchical trimers (structures with three nanoparticles) limited the inter-particle spacing to approximately more than 10 nm and, accordingly, the intensity of field confinement. These spacings were found to be much larger than those achieved through self-assembly methods. Smaller gaps are desirable for strong inter-particle plasmon modes coupling, which leads to strong field localization.
However, only selected self-assembly methods are applicable for reproducible production of large scale trimers necessary for a number of applications. Therefore, Australian researchers led by professor Udo Bach investigated the plasmonic attributes as well as the lensing effect of the hierarchical gold trimer nanolenses. They adopted an electrostatic self-assembly method owing to its simplicity and required control. They also ascertained the ability of the trimers to confine light through cathodoluminescence. Their research work is now published in ACS Nano.
The authors obtained 30 nm particles from citrate stabilized gold particles. The particles were then modified with single-stranded DNA in a bid to convey a negative charge on their surface. The approach of hierarchical assembly of trimers was based on protocol used for assembly of dimers. The method was a three step assembly process that relied on electrostatic interactions between charged substrates and particles with alternating positive and negative charges. Glass substrates and silicon dioxide coated silicon substrates were functionalized to confer a positive charge.
In the first step, a (3-aminopropyl)-triethyoxysilane modified substrate with positive surface charges was immersed into a colloidal solution of gold nanoparticles with negative charges. In the second step, the produced substrates were immersed in a solution of positively charged 20 nm satellites. Finally, in the last step the authors completed trimer formation by immersing the gold nanoparticle dimer substrates in a solution of positively charged 50 nm satellites.
The wet chemical electrostatically driven gold nanoparticles assembly process allowed the authors to realize nanolenses with below 2 nm inter-particles spacing at high density on a macroscopic scale. They achieved a trimer yield of over 60%. With the characterization of the plasmonic behavior through UV-vis, cathodoluminescence and electron energy loss spectroscopy, they were able to pinpoint lower energy resonance that was pivotal for the strong lensing effect towards the smallest particle gap. In the paper, the authors demonstrated that the resonance could be excited effectively when focusing an electron beam on the smallest particle.
Julian A. Lloyd1,2, Soon Hock Ng1,2, Amelia C. Y. Liu3,4, Ye Zhu1, Wei Chao1, Toon Coenen5, Joanne Etheridge1,3, Daniel E. Gómez2,6,7, and Udo Bach1,2,6. Plasmonic Nanolenses: Electrostatic Self Assembly of Hierarchical Nanoparticle Trimers and Their Response to Optical and Electron Beam Stimuli. ACS Nano, volume 11 (2017), pages 1604−1612.Show Affiliations
- Department of Materials Science and Engineering, Monash University , Clayton, Victoria 3800, Australia.
- Melbourne Centre for Nanofabrication , Wellington Road 151, Clayton, Victoria 3168, Australia.
- Monash Centre for Electron Microscopy, Monash University , Clayton, Victoria 3800, Australia.
- School of Physics, Monash University , Clayton, Victoria 3800, Australia.
- DELMIC BV , Thijsseweg 11, 2629 JA, Delft, The Netherlands.
- Commonwealth Scientific and Industrial Research Organisation , Manufacturing, Research Way, Clayton, Victoria 3168, Australia.
- School of Applied Science, RMIT University , Melbourne, Victoria 3000, Australia.
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