Modeling and analysis of post plasmonic effects in optically excited metal nanosystems

• Main Author: Thomas, Rijil
• Other Authors: Soh Cheong Boon
• Format: Theses and Dissertations
• Language: English
• Published: 2018
• Subjects: DRNTU::Engineering::Electrical and electronic engineering::Optics, optoelectronics, photonics
• Online Access: http://hdl.handle.net/10356/73529
• Institution: Nanyang Technological University

Plasmonic nanoparticles such as gold (Au) and silver (Ag) with their heterogeneous nanocomposite structures have aggrandized in multitude of applications including biomedical imaging (optical, photoacoustics (PA), dark field, etc.), sensing techniques including wavelength shift Local Surface Plasmon Resonance (LSPR) bands and Surface Enhanced Raman Spectroscopy (SERS), photothermal therapy, and drug delivery. Their usage is made plausible by optical excitation of suitable wavelength and subsequently triggering LSPR. To manipulate LSPR, this field of nanotechnology requires appropriate modification and design of nanostructures in terms of size, shape, surface functionalization, coating, interparticle coupling, etc. These modifications are inevitable to achieve better performance and sensitivity via one or more of these objectives: 1) higher absorption, 2) field enhancement and 3) thermodynamic stability. Though the unique capabilities of coatings/shell on nanomaterials are quite promising, post effects of optical excitation intuitively can be claimed to disturb the core-shell nanostructures. There are few possible intermediate phenomena like electric field enhancement, thermal expansion, heat generation, gas evolution, boiling, ablation, breakdown, and plasma formation which can cause this. This thesis investigates the cause of perturbations in such nanostructures via few of the above effects (thermal expansion & electric field enhancement). These mechanisms occur under specific conditions of temperature, irradiation timing, laser intensity, nanoparticle shape, environment matrix, pulse duration, beam diameter, etc. In addition to the investigation on structural damage, the thesis also investigates the formation of plasmonic coupling via optical excitation as a stimulus to build nanostructures and the effect of a conductive interface in such coupled systems. There have been studies reported about melting and reshaping of metal nanoparticles, damage caused by plasmonic precursors embedded in a dielectric via heat or field enhancement), etc. However, an understanding of the stability of a standalone hetero-nanostructure under laser excitation has not been reported yet. Hence this work provides an understanding of plasmonic excitation and subsequent transfer of energy affecting the stability of a hetero-nanostructure which depends on size and shape of the plasmonic material. This will aid the modeling and designing of nanostructures, especially for biomedical applications. In this thesis, we have explored several mechanisms which induce morphological or structural change on composite hetero - nanostructures with a dielectric coating on gold nanoparticles. We used finite element method (FEM), MATLAB and theoretical analysis to numerically compute and predict the intensity range where a breakdown is expected. First, we explored structural damage on smaller nanoparticles like silica coated spherical Au nanoparticles (AuNP@SiO2). Using a nanosecond pulsed laser we have shown the possibility of structural breakdown which was characterized using Transmission Electron Microscopy (TEM) images and Scanning Electron Microscopy images (SEM). An intensive literature and theoretical model backup the fact that structural failure in these core-shell nanospherical structures happens due to the differential thermal expansion of core-shell pair and that rupture or deformation happens throughout the silica coating. Following this, we extended our study towards silica coated Ag nanoprism (AgNPr@SiO2) which exhibits a different breakdown mechanism. Due to the triangular geometry and high scattering of Ag than that of Au, a low input intensity could create a field enhancement near the corners causing damage to be initiated at the corners. Numerical models were used to analyze the localization of intensity near corners and estimate the range of values for the breakdown. Thus, a more controlled breakdown is achieved and with a low laser fluence. These nanosystems are anticipated to have a significant impact on biomedical applications, providing futuristic possibilities for non-invasive methods as suitable agent carriers. These agents can be injected non-invasively and be remotely activated to respond to external stimuli at convenient sites inside a real-life sample. Finally, we have investigated the effect of optical excitation on plasmonic clusters connected covalently by a conductive inter-particle interface. For this, we have selected spherical gold nanoparticles (AuNPs) and gold nanoprisms (AuNPrs) and modified their surface with a synthetically modified diacetylene monomer. Upon exposure to ultra-violet light (254 nm), the diacetylene undergoes photopolymerization to polydiacetylene resulting in the formation of interparticle assembly and initiate clustering. We anticipate that the interface comprising of conductive polymers allows interparticle plasmonic exchange upon suitable light excitation. Theoretical simulation and experimental findings suggested that upon suitable optical excitation, plasmonic nanosystems including core-shell analogs as well as interparticle clusters undergo various changes. The resultant transformation in these nanosystems varies for different structures depending on factors such as size, shape, coating, and nature of laser used for excitation. The study presented in this thesis will provide insight into plasmon interactions based structural instabilities and changes in plasmonic nanomaterials during interaction with electromagnetic (EM) waves.