| This dissertation presents infrared nano-spectroscopy and nano-imaging studies of graphene plasmons using scattering-type scanning near-field microscope---a unique technique allowing efficient excitation and high-resolution imaging of graphene plasmons. With this technique, we show in real space that common graphene/SiO2/Si back-gated structure support propagating surface plasmons in the infrared frequencies. The observed plasmons are highly confined surface modes with a wavelength around 200 nm that are conveniently tunable by the back gate voltages. In addition, we perform spectroscopic studies on graphene by varying the probing frequencies. The spectroscopy results not only show direct signature of graphene plasmons but also provide evidence of strong coupling between graphene plasmons and SiO2 optical phonons. Furthermore, we investigate the plasmonic properties of bernal-stacking bilayer graphene (BLG) and find that BLG supports gate-tunable infrared plasmons with higher confinement compared to graphene and randomly stacked graphene layers. Moreover, BLG plasmons can be turned off completely in wide gate voltage close to the charge neutrality point. Those unique plasmonic properties are attributed to both interlayer tunneling and bandgap opening inBLG. Finally, we are able to map and characterize grain boundaries inside graphene film fabricated with chemical vapor deposition (CVD) method by launching surface plasmons. We found grain boundaries, as well as other line defects in CVD graphene, trigger distinct plasmonic twin fringes patterns due to plasmon interference. Theoretical modeling and analysis unveil unique electronic properties associated with grain boundaries. |
