Optical and Structural Studies of Nanocrystal Assemblies

Abstract

In my thesis I have delved into studying nanoparticle assemblies that can be applied to different types of applications. For example, certain types of nanoparticles can be utilized as quantum light sources or a different set of nanoparticle assembly, can be used to improve battery performance. For this purpose, first I have focused on understanding the charge carrier dynamics of semiconductor nanocrystals or quantum dots (QDs) in the vicinity of metal nanoparticles. Upon photoexcitation QDs become occupied by an electron and hole pair called an exciton. When the excited electron recombines with the hole it results in spontaneous emission (SE) of photons that can be tuned to different wavelengths depending on the size of QDs. As a result, QDs have been widely regarded for their size tunable emission wavelength range (VIS-IR) as well as a narrow emission bandwidth. However, their SE rate may get hampered due to low photonic density of states (PDOS) and the presence of defects in the system. Here I investigate the possibility to improve SE rates by coupling them with metal nanoparticles. Metal nanoparticles exhibit collective oscillations of their free electrons. These oscillations are usually referred to as plasmons. Due to such plasmonic oscillations metal nanoparticles can concentrate the electromagnetic field to a sub wavelength region and thus can increase the PDOS in their vicinity. Hence these nanoparticles can act as antennae that facilitate exciton-plasmon interactions. In the first chapter I focus on designing a bimetallic plasmonic nanostructure (NS) that can be used for coupling with the QDs. Traditionally monometallic nanoparticles are used for this purpose, however I wanted to investigate the plasmon damping effects in exciton-plasmon interaction which is more prominent in bimetallic structure. To enhance the plasmon damping I have designed a high interface Au-Ag nanostructure. This chapter focuses on the optimized colloidal synthetic routes that results in incorporation of tiny (~1 nm sized) Ag nanocluster into an Au matrix. This particular type of NS behaves differently compared to the conventional core-shell or alloyed NS. I have explored two colloidal routes for synthesizing these NSs, one with lesser size control but can be incorporated for a bulk scale synthesis. Later in my work I demonstrate how this large-scale synthesized NS can be used as an alternative cathode material for Zn-Ag batteries. The other method involved controlling the pH of the solution which results in a narrower particle size distribution but is associated with its own challenges for further scaling up. But importantly both the methods yielded NSs with a broad plasmonic resonance, likely due to increased electron scattering at the interface. Interestingly the second method produced two types of NSs, with similar extinction spectra but surprisingly distinct absorptive and scattering sub-types. Light absorbing NSs were chosen for exciton-plasmon coupling studies. Despite sharing structural and compositional similarities, one set displayed complete light absorption (thereby appearing black) while the other exhibited complete scattering (thereby appearing white). This unexpected behavior challenges the current theoretical model and highlights the need for a more comprehensive framework to explain the highly scattering NSs. In the following chapter, I investigate the effect of plasmon damping on the emission characteristics of CdSe/CdS core-shell colloidal quantum dots (CQDs). The study compared the impact of both the bimetallic Au-Ag NSs and monometallic Au NSs. Finite-difference time-domain (FDTD) simulations quantified the electric field enhancement and its influence on the PDOS surrounding the CQDs. It turned out that a hybrid CQD-metal NS assembly can exhibit single photon emission characteristics which is generally expected from a single isolated QD. This study indicated a plasmon-mediated non-radiative excitonic energy transfer among CQDs. This resulted in a multiexciton annihilation process and ultimately one exciton survived to emit only one photon at a time. Remarkably, photoluminescence intensity and emission rates were enhanced with increasing excitation power in CQD-metal NS hybrids compared to pristine CQDs. Moreover, assemblies of CQDs with metal NSs exhibited notably reduced flickering which is essential for practical device applications. A plasmon mediated multiparticle coupling of this type is a fascinating result that paves away a new strategy to improve the performances of QD-based integrated photonic circuits. In the next chapter, I synthesize Ag-rich Au-Ag NSs in gram scale. The aim was to implement them as an alternative cathode material in Zn-Ag batteries compared to traditional Ag electrodes. Here the size distribution of the particles does not play an important role, so a colloidal disruption method was adopted to minimize the synthetic steps. The role of Au was as a scaffolding material that prevented material losses after multiple charge discharge cycling. Further it provided enhanced initial capacity, better capacity retention and higher energy density. The compositional ratio of Au:Ag was tuned, and the optimal electrochemical performance was achieved with a ratio of 20:80. In summary, this study underscores the importance of nanostructuring in advancing rechargeable secondary battery chemistry. Conclusions, summary and future outlook are presented in the final chapter.