Absorption and scattering of single plasmonic nanoparticles
Ha, Ji Won
Master of Arts thesis
In this thesis, I present the absorption and scattering properties of single gold nanoparticles. I performed scanning electron microscopy (SEM) correlated dark-field scattering studies of gold nanorods (AuNRs). I found polarization-dependent scattering of a single AuNR. I studied the dependence of surface plasmon resonance (SPR) linewidth on both the refractive index of a surrounding medium and the adsorption of thiol groups onto the surface of AuNRs. I found that the SPR wavelength (lambdamax) shifts to longer wavelength when increasing the refractive index of the surrounding medium, while the SPR linewidth remains almost constant when increasing the refractive index of the surrounding environment. I also found that SPR wavelength shifts to longer wavelength as thiol groups bind to the surface of a single AuNR and that the SPR linewidth is broadened. I carried out photothermal imaging of gold nanospheres. The successful imaging of gold nanospheres down to 10 nm, which cannot be studied by conventional scattering-based methods, was achieved with a signal to noise (S/N) ratio of ∼35. I studied the size dependence of the photothermal signal of gold nanospheres with diameters ranging from 30 to 250 nm. The experimental results showed a very good agreement with Mie calculations for absorption of nanospheres. I further investigated polarization-dependent photothermal imaging of single AuNRs. It is observed that both gold nanowires and AuNRs show a polarization dependence in photothermal imaging. I utilized polarized photothermal imaging to determine the orientation of AuNRs. By selecting either the longitudinal or the transverse SPR mode, I precisely determined the orientation of individual AuNRs. Correlating SEM with photothermal images, the orientations of AuNRs were accurately measured. Most notably, I determined the orientation of an AuNR by exciting the transverse SPR mode which is not achievable by conventional scattering-based techniques.