| dc.description.abstract | In this thesis, a variety of novel emerging systems were chosen to explore new and enhanced optical functionalities in coupled nano-systems. The coupling of novel nanomaterials with complimentary properties paves the way for enhanced performance and functionality for future device applications. Near-field interactions between nano-systems can lead to a variety of interesting mechanisms, such as, modification of the emission properties of a fluorescent species, and nonradiative energy transfer (NRET) via dipole-dipole coupling. A coupled nano-system of quantum dots (QDs) and chiral Ag nanohelices is investigated with a view towards optical antennas. The chiral Ag nanohelices are nanoscale analogues of traditional helical antennas. It is shown that there is a strong interaction between the QDs and the Ag nanohelices, demonstrating an interaction efficiency of (82 ± 2)%. The far-field emission pattern from the QDs when coupled to the Ag nanohelices is found to exhibit greater directionality than the emission pattern from the QDs on a planar substrate. The far-filed emission pattern is also shown to fit with an ordinary end-fire emission pattern, a characteristic radiation pattern exhibited by traditional antennas. Similarly, the coupling between the QDs and the chiral Ag nanohelices leads to circular polarisation of the QD emission, with the emission polarised in the same handedness as the nanohelices, with a maximum value of (17.7 ± 3.0)% circularly polarised emission. A composite structure of Ag nanoparticle (NP) decorated graphene oxide (GO) is studied to investigate the influence of the composite substrate on the emission and Raman scattering signals of three organic dyes; Rhodamine 6G (R6G), Rhodamine B (RhB), and Sulforhodamine 101 (SR101). The interactions between each of the dyes and the Ag NPs, GO, and the Ag NP decorated GO (AgGO) were studied to investigate the relationship between fluorescence quenching and surface enhanced Raman scattering (SERS) enhancements. The SERS enhancements of R6G were found to be influenced most strongly by the fluorescence quenching by the GO, while SR101 was more strongly influenced by the field enhancement associated with the Ag NPs. RhB was shown to have the weakest SERS enhancements and also the weakest coupling to the Ag NP decorated GO substrate. Colloidal Ag NPs and lithographically defined arrays of Ag NPs were used to demonstrate plasmon mediated NRET from QDs to quantum wells (QWs). This is the first experimental demonstration of plasmon mediated NRET from QDs to QWs. Plasmon mediated NRET efficiencies as large as ~25% are observed. The colloidal Ag NPs were used to study the distance dependence of the plasmon mediated NRET, where it was found to follow the same d(-4) dependence as the direct NRET from the QD to the QW. In the case of the plasmon mediated NRET, there is evidence of an increased interaction distance, indicating that the process follows a Förster-type NRET model, with the coupled QD-Ag NP acting as an enhanced donor dipole. The lithographically defined arrays of Ag NPs display plasmon mediated NRET efficiencies of ~17% and demonstrate the tunability of the interaction, going from a situation of overall quenching to an enhancement of the QW emission, simply by changing the geometry of the Ag NPs. Coupled systems of QDs and MoS2 devices are studied to investigate the influence of the MoS2 film quality on the photocurrent enhancements due to NRET from the QDs. The MoS2 film quality is found to be critically important in order to achieve large photocurrent enhancements. Multiple devices with varying film quality are studied, including pristine monolayers, mixed monolayer/bilayer, and polycrystalline bilayer devices. NRET efficiencies of over 90% are measured on each device, however, photocurrent enhancements of ~14 fold are found for pristine monolayer devices, with modest enhancements of ~2.5 fold on mixed layer devices. The polycrystalline bilayer and bulk-like thickness devices show no enhancement of the photocurrent. A spectral dependence study of NRET and photocurrent enhancements in coupled QD- monolayer MoS2 devices is presented. This is the first demonstration of the spectral dependence of NRET in these coupled systems and also the first demonstration of the spectral dependence of the photocurrent enhancements. Three spectrally separated QDs with peak emission wavelengths of 450 nm, 530 nm, and 630 nm, were chosen for the study in order to identify the optimal spectral location for the sensitizing species in a hybrid QD-sensitized MoS2 device. The largest photocurrent enhancements and NRET efficiencies were found for the 630 nm QD-monolayer MoS2 system. Good agreement is found between the spectral overlap, NRET efficiency and photocurrent enhancement for each device, which indicates that the NRET drives the photocurrent enhancement in each hybrid device. | en |
