Chemically-doped graphene with improved surface plasmon characteristics: an optical near-field study

Abstract

One of the most fascinating and important merits of graphene plasmonics is their tunability over a wide range. While chemical doping has proven to be a facile and effective way to create graphene plasmons, most of the previous studies focused on the macroscopic behaviors of the plasmons in chemically-doped graphene and little was known about their nanoscale responses and related mechanisms. Here, to the best of our knowledge, we present the first experimental near-field optical study on chemically-doped graphene with improved surface plasmon characteristics. By using a scattering-type scanning near-field optical microscope (s-SNOM), we managed to show that the graphene plasmons can be tuned and improved using a facile chemical doping method. Specifically, the plasmon interference patterns near the edge of the monolayer graphene were substantially enhanced via nitric acid (HNO3) exposure. The plasmon-related characteristics can be deduced by analyzing such plasmonic fringes, which exhibited a longer plasmon wavelength and reduced plasmon damping rate. In addition, the local carrier density and therefore the Fermi energy level (EF) of graphene can be obtained from the plasmonic nano-imaging, which indicated that the enhanced plasmon oscillation originated from the injection of free holes into graphene by HNO3. These findings were further corroborated by theoretical calculations using density functional theory (DFT). We believe that our findings provide a clear nanoscale picture on improving graphene plasmonics by chemical doping, which will be helpful for optimizing graphene plasmonics and for elucidating the mechanisms of two-dimensional light confinement by atomically thick materials.

Tailoring of electromagnetic field localizations by two-dimensional graphene nanostructures

“…Therefore, one can manipulate the plasmonics in the graphene by charge carrier-doping approaches, such as electrostatic gating or chemical doping. Our previous study revealed that the graphene plasmon strengths and wavelengths were enhanced by HNO 3 doping, which was due to the injection of free holes and the reduction of the plasmon damping rates 39 .…”

“…For the disk with L =90 nm, the electromagnetic field was tightly confined to a bright spot ( Figure 6c ). These behaviors originated from the increase of the graphene plasmon wavelengths upon hole injections 39 , whereby the interference nodes within a specific graphene disk are reduced. The dependence of the interference patterns on chemical doping suggests that one can tailor the electromagnetic fields of the graphene nanostructure with chemical doping without modifying the geometry.…”

A Nano‐Imaging Study of Graphene Edge Plasmons with Chirality‐Dependent Dispersions

“…As shown in our previous study, this can introduce slight hole-doping to the pristine graphene flake. [9] Specifically, according to the relation…”

“…Obviously, as the carrier concentration increases, the plasmon amplitude increases correspondingly, which is consistent with previous results. [9,10] Furthermore, the plasmon amplitude of the zigzag edge upon chemical doping is stronger than that of the doped www.advopticalmat.de armchair edge. Another interesting issue is that it is known that the electron-phonon interactions will be stronger at the armchair edge, [33] which can impact the 1D edge plasmon behaviors therein.…”

“…[ 1–4 ] The GPs exhibit outstanding electromagnetic field confinements [ 5 ] and substantially low loss, [ 6 ] and can be actively controlled via external stimuli. [ 7–10 ] These virtues make the GPs highly attractive for tunable nanophotonics devices. In addition to the sheet plasmon modes which propagate within the 2D plane, there exists another type of plasmon modes, that is, 1D edge plasmon modes propagating along the graphene edge.…”