Optical and electrical studies of hybrid nanomaterial composites based on noble metal nanostructures

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PhD; 2023; Centre for Nano Science and Engineering

Localized surface plasmon resonance (LSPR) in noble metal nanoparticles like gold (Au) and silver (Ag) results in enhancements in optical absorption and scattering by the nanoparticles, accompanied by a dramatic amplification and localization of electric fields to sub-wavelength regions. These enhancements, especially the confinement of light to such small scales, pave the way for applications like surface-enhanced Raman spectroscopy (SERS), chemical sensing, and subwavelength guiding and can be impactful to quantum technologies, especially on-chip light-driven devices. In this thesis, we discuss in detail the effects of the sub-wavelength confinement of enhanced electric fields by the plasmonic nanocavities on the optical response of two-dimensional materials (2D) like Molybdenum disulphide (MoS2 )and Tungsten diselenide (WSe2) for development of new optical functionalities with scope towards quantum applications. We also briefly discuss the electrical response of Au-Ag nanostructures fabricated using chemical and physical methods. At the heart of this work is fabricating the Au-Ag plasmonic nanocavities with sub-nm separation. We, therefore, engineer an easy-to-fabricate technique to form such nanostructures on demand. Monolayer MoS2 with a thickness of 0.7 nm is used as a spacer layer for the sub-nm cavity, and the design of the cavity is chosen based on numerical simulations. We exploit the enhanced absorption of light by the Au-Ag nanostructures upon resonant illumination and the low thermal conductivity of MoS2 to collectively localize heat at specific locations resulting in the fabrication of metal nanocavities on demand. The Raman signal of MoS2 is seen to increase by 40 times, making it a perfect candidate for SERS applications and other optoelectronic applications. Estimating a temperature rise of ~ 250 ℃ in the system using AI-enabled pattern recognition further corroborates the desired nanostructuring. We focus on the effect of similar plasmonic cavities on the optical response of 2D materials toward applications. We select monolayer WSe2 as our material of choice owing to its tremendous optical and mechanical properties discussed later. We aim to understand the effect of mechanical strain and plasmonic cavity on the optical and mechanical properties of WSe2, and optical characterizations of PL and Raman show a definite effect of the same. A strain of about 0.3% and p-type doping is estimated for WSe2 embedded in an Au-Ag cavity. While conventional electrical measurements establish the nature of doping well, these require a different device fabrication strategy. The non-invasive nature of the PL and the Raman characterizations used here ensures that more complicated electrical device structures can be avoided. After identifying the nature of doping and the amount of strain by the cavity over WSe2, we emphasize the class of interaction between the two entities governing its application scope. Here we describe the interaction type between WSe2 and Au-Ag nanocavity using a custom-built darkfield setup and provide evidence that the coupling between them lies in a strong regime, implying the formation of hybrid states. These have scope for new optical phenomena with non-linearities and device applications, such as low threshold lasers. We explain in detail the features of our plasmonic cavity that help achieve this strong coupling with a monolayer WSe2 and the energy dispersion of the hybrid states, which shows a very large Rabi splitting of 170 meV. We further use the Au-Ag plasmonic cavity to engineer a new geometry to generate efficient single photon emitters in monolayer WSe2. We show that embedding WSe2 in the plasmonic cavity increases its exciton emission by atleast five times. A comparison with bare WSe2 and other control samples shows that our geometry undeniably reduces the line widths of emission with a minimum line width of 120 𝜇eV. While we discuss in detail the implications of excitation wavelength, cavity type, temperature, etc., on these sharp emissions, our results add another device geometry parameter to improve single exciton physics and quantum optics with atomically thin semiconductors. We show that the signal can be improved further by spectrally filtering individual emitters. Finally, we describe preliminary experiments on the electrical and optical properties of Au-Ag nanostructures fabricated using chemical and physical techniques. The anomalous non-metallic features seen in the electrical response may be relevant to recent reports of superconductivity seen in similar Ag-Au nanostructures. A detailed investigation is carried out to study the effect of sample composition and external factors on anomalous response.