Abstract
Plasmonic metamaterials typically consist of plasmonic nanostructures that are arranged in arrays at a size smaller than the wavelength of interest, and they are known for their unique optical properties that can be precisely controlled with the accurate nanostructure design at a sub-wavelength scale. By exploiting their advantages in localizing and enhancing the electromagnetic field, plasmonic metamaterials have been used in various applications such as optical sensing, plasmon-enhanced spectroscopy and plasmon-assisted chemistry. Recently, hybrid plasmonic nanostructures that combine the plasmonic fields with functional materials have been developed to achieve enhanced optical properties and thus expand therange of possible applications ranging from photo-catalysis and biomedical sensors to optical modulation. In this context, we aim to explore hybrid plasmonic-molecular nanostructures to achieve passive and active functionalities of metamaterials. In this thesis, a hybrid plasmonic nanostructure has been realized by self-assembling a nanoscale layer of poly-L-histidine on gold nanorods forming the metamaterial.
On the one hand, the hybrid nanostructure is used to investigate the effect of environmental humidity on the optical response of nanorods metamaterial. Due to the high refractive index sensitivity, a significant change in the transmission is observed with ΔT/T reaching values of more than 5% when the relative humidity is changed from 11% to 75%. The experimental and simulated results demonstrate that the mechanism behind the phenomenon is associated with the roughness-assisted nanoscale condensation of water on the nanorod surface. Such results further reveal the importance of protecting plasmonic nanostructures from relative humidity variations in many practical applications that work in the ambient environment and also gas sensing applications such as hydrogen and oxygen. The hybrid structure can be used for the development of high-sensitivity relative humidity and dew condensation sensors.
On the other hand, light-emitting plasmonic tunnelling junctions have been successfully built on this hybrid nanostructure where the gold nanorod metamaterial is capped with a monolayer of poly-L-histidine working as the tunnel barrier. The compact electrical excitation of surface plasmons and generation of light emission by inelastically tunnelled electrons has been demonstrated, exhibiting a broadband tunability of the emitted light controlled by the metamaterial and the junction geometry. Besides, the voltage bias-dependent emission performance has been discussed, and typical tunnelling electrical properties have been analysed using current-voltage curves and current mapping. Additionally, by defining the tunnelling junction area, a type of microscale plasmonic tunnelling junction that allows a fast modulation of the tunnelling-excited light emission has been further achieved, which is attractive for the building of ultrafast electrically-driven light sources. Overall, these results demonstrate the advantages of such hybrid plasmonic nanostructures, highlighting their potential for developing optical humidity sensors and nanoscale light sources.
| Date of Award | 1 Aug 2023 |
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| Original language | English |
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| Supervisor | Anatoly Zayats (Supervisor) |