Near-field infrared nanoscopy of phonon resonant silica and silica-coated gold nanoparticles

Single silica capped and uncapped gold nanoparticles are imaged using apertureless near-field optical microscopy (ANSOM) in the infrared frequency range (λ=9-11 μm). While visible frequency imaging can only allow identification of capped and uncapped particles, IR nearfield imaging also permits spectroscopic resolved imaging through vibrational resonances. Optical phonon resonances of the silica nanoparticles in pure silica and silica capped Au composite single nanoparticles are imaged in amplitude and phase. The phase spectra show larger contrast near the wavelength where phonon resonant near-field interaction is expected even in capped particles with a thin silica coating layer. Experimental results are in good agreement with dielectric function model calculations which include vertical composition of the nanoparticles in the extended dipole approximation.