As discussed above, there is a great deal of interest in assembled noble metal nanostructures from the point of view of nanotechnology. Most real devices are expected to be based on assemblies of nanoparticles, thus making it important to characterize the interactions between nanoparticles in their assembled state and understand their effects on the optical and non-radiative properties of the nanostructure. This is the goal of our research, as described in the remaining chapters of this thesis. Of special interest is the plasmon coupling between nanoparticles, which is not only fundamentally interesting but also important for applications such as biosensing, Raman enhancement, and plasmonic waveguiding.
Before entering into the nature of coupling between noble metal nanoparticles, we take a detailed look at isolated nanoparticles in Chapter 2. We provide a detailed comparative study of the absorption and scattering cross-sections of three important plasmonic nanostructures, viz., gold nanospheres, gold nanorods, and silica-gold nanoshells. While the radiative aspects of the plasmon resonance (e.g. scattering) are important in imaging and sensing applications, the non-radiative part (absorption) is important in photothermal applications. We show that the relative contribution of scattering versus absorption can be increased by increasing the nanostructure size in all three nanostructure types. For bio-applications where nanoparticle size is an important consideration, we present a comparison of size-normalized scattering and absorption coefficients, showing that the nanorod shape offers the most enhanced optical coefficients at small size.
We look at coupled gold nanospheres in the form of colloidal aggregates in Chapter 3, where we study their plasmon resonance using UV-vis spectroscopy and their non-radiative electron relaxation dynamics using ultrafast pump-probe transient absorption spectroscopy. The aggregate solution shows a broad plasmon absorption band due to contributions from a heterogeneous mix of assemblies (with different inter-particle spacings or number of particles). However, we are able to use an optical probe of a specific wavelength to selectively interrogate only those aggregate subsets that absorb at the probe wavelength. As we probe at longer wavelengths, we interrogate the electron- relaxation dynamics in aggregates with progressively stronger coupling, observing an increase in the measured rate of hot electron relaxation. Based on these results we propose the role of inter-colloid electronic coupling and interfacial electronic scattering in determing electron dynamics in nanoparticle assemblies.
How does the particle shape affect the nature of plasmon coupling? We address this question in Chapter 4, where we describe the assembly of anisotropic particles i.e. colloidal gold nanorods, which offer two modes by which they can be assembled. The side-by-side assembly of gold nanorods in solution results in a blue-shift of the longitudinal plasmon absorption band, which is in contrast to the end-to-end assembly of nanorods seen in the past to result in a red-shift of the longitudinal plasmon resonance mode. This orientation dependence of the assembly-induced plasmon shift is confirmed by electrodynamic simulations of nanorod dimers, based on which we conclude simple polarization “selection” rules of plasmon coupling. When the plasmon oscillations are polarized along the inter-particle interaction axis, the coupling between the plasmons is bonding in nature, resulting in a red-shift (end-to-end assembly), in contrast to the blue-
shift that results from the anti-bonding interaction when the polarization is perpendicular to the inter-particle axis (side-by-side assembly).
In Chapter 5, we study the distance dependence of the near-field plasmon coupling in gold nanodisc pairs fabricated by electron-beam lithography (by Wenyu Huang, another group member) with systematically varying inter-particle separations, accompanied by an electrodynamic study of particle pairs. In agreement with past literature, we find that the plasmon resonance wavelength maximum red-shifts almost exponentially with decreasing inter-particle separation and also that this exponential trend becomes independent of the nanodisc diameter when the inter-particle gap is normalized by the diameter. Further, we find from our simulations that the fractional shift (∆λ/λ0) of
the plasmon resonance wavelength decays over an inter-particle separation s, which is roughly 0.2 in units of the particle dimension D, independent of the particle size, the shape (nanodisc pairs or nanosphere pairs), type of metal (gold or silver), and the surrounding medium. This universal scaling behavior is expressed as ∆λ/λ0 ~ a.exp-s/0.2D,
giving us a “plasmon ruler” equation which can be used to calculate the inter-particle separation in a nanoparticle pair from its plasmon resonance shift. We successfully test this with Alivisatos and coworkers’ experimentally determined plasmon resonances of gold nanosphere pairs assembled by DNA linkers of varying number of base pairs, thus making our equation applicable to the determination of nanoscale distances in macro/ biomolecular systems using the particle-pair plasmon ruler. A quasistatic dipolar- coupling model is used to explain the universal size-scaling behavior based on the magnitude of the (inter-particle) coupling strength relative to the (intraparticle) polarizability.
The universal scaling behavior also extends to the metal nanoshells, as described in Chapter 6. Mie theory simulations show that the plasmon resonance red-shifts near- exponentially with decreasing thickness of the metal shell in units of the core size, according to a trend universally independent of the nanoshell size, type of shell metal, the core dielectric, and surrounding medium. We are able to extend the universal size-scaling from the particle-pair system to the nanoshell, because in analogy to the particle-pair, the nanoshell plasmon resonance results from plasmons on the inner and outer shell surfaces coupling over a distance defined by the shell thickness. Our model is consistent with the plasmon hybridization model developed by Prodan, Nordlander and Halas. At the same time, it gives a simple dependence of the nanoshell plasmon resonance frequency on both the shell thickness and nanoshell size unlike earlier models and is thus very useful in estimating the resonance wavelength maximum of a nanoshell of given dimensions.
Chapter 7 describes how the plasmonic sensitivity of assembled nanostructures (e.g. metal nanoshell and particle-pair) is enhanced in correlation with increasing plasmon coupling strength. Mie theory results show that the sensitivity of a metal nanoshell increases near-exponentially with decreasing shell thickness (in units of the core size) according to the same universal scaling behavior as seen for the plasmon resonance. A general physical rule that emerges from these observations is that the plasmon sensitivity is strongly enhanced in those nanostructures, which offer a high polarizability of the electrons to the metal-medium interface. Thus, nanostructures with sharp curvatures, high aspect ratios, or narrow inter-particle junctions are most suitable for refractive index-based sensing applications.
We extend the universal scaling model to nanostructures of complex geometry in Chapter 8 by means of electrodynamic simulations. The fractional plasmon shift in a trimer of nanospheres decays with the inter-particle separation (in units of particle diameter) with the same universal scaling behavior seen in dimers. This is the first step towards extending the universal size-scaling model to 1-D, 2-D, and 3-D assemblies with large number of interacting particles. In addition, the plasmon shift in a pair of elongated nanoparticles assembled head-to-tail decays according to the universal scaling law, when the inter-particle separation is scaled by the long-axis dimension, which is the dimension in the direction of inter-particle coupling. We also see that an increase in the particle aspect ratio and end-curvature results in an increase in the coupling-induced plasmonic shifts, without affecting the universal size scaling.
In Chapter 9, we describe the investigation of the non-radiative electron relaxation dynamics in gold nanoparticles conjugated to self-assembled monolayers of thiolated DNA ligands, using femtosecond laser studies. We find that the femtosecond pulse (400 nm) excitation of the thiolated DNA-modified nanoparticles leads to desorption of the thiol ligands from the nanoparticle surface, as seen from the blue shift of the plasmon resonance, followed by nanoparticle aggregation. We attribute this to the nonradiative relaxation of the hot electron energy, on the electron-phonon relaxation timescale, into surface gold-sulfur bond vibrations leading to the surface desorption. This additional pathway for non-radiative cooling of the electrons in the conjugates is manifested in a faster rate of electron-phonon relaxation in the thiolated DNA-modified nanoparticles compared to that in unmodified nanoparticles, at progressively higher laser pulse energies.