Nonlinear Plasmonic Sensing with Nanographene

Plasmons provide excellent sensitivity to detect analyte molecules through their strong interaction with the dielectric environment. Plasmonic sensors based on noble metals are, however, limited by the spectral broadening of these excitations. Here we identify a new mechanism that reveals the presence of individual molecules through the radical changes that they produce in the plasmons of graphene nanoislands. An elementary charge or a weak permanent dipole carried by the molecule are shown to be sufficient to trigger observable modifications in the linear absorption spectra and the nonlinear response of the nanoislands. In particular, a strong second-harmonic signal, forbidden by symmetry in the unexposed graphene nanostructure, emerges due to a redistribution of conduction electrons produced by interaction with the molecule. These results pave the way toward ultrasensitive nonlinear detection of dipolar molecules and molecular radicals that is made possible by the extraordinary optoelectronic properties of graphene.

Figure 1

Linear and nonlinear optical sensing with nanographene. (a) Illustration of an armchair-edged graphene nanohexagon (GNH) interacting with a charge- or dipole-carrying analyte (the white arrow indicates a permanent dipole moment). The molecule induces asymmetry in the nanohexagon conduction electron distribution, which can be detected by measuring either changes in the optical absorption spectrum or the onset of a second-harmonic signal. (b),(c) Linear absorption cross section (b) and second-harmonic polarizability (c) of the GNH in the absence (blue curves) or presence (red curves) of a nearby charge-carrying molecule. The analyte produces new resonance features and spectral shifts in the linear response. Additionally, it enables a large second-harmonic response from the nanohexagon, otherwise prevented by symmetry in its absence. Calculations in (b) and (c) correspond to an undoped GNH of 1 nm side length under the conditions of Figs. 2 and 4 for the linear and SHG responses, respectively.

Figure 2

Sensing of charge-carrying analytes through linear optical absorption by nanographenes. We show the linear absorption cross section of single GNHs exposed to an individual singly charged analyte ($Q^{\text{ext}}=e$) placed in the graphene plane at different distances $d_x$ from the carbon edge [see legend in (c)]. Doped [net charge $Q=-4e$, panel (b)] and undoped [$Q=0$, panels (a) and (c)] nanohexagons are considered with different side lengths $L=1\text{and}2\text{nm}$ (see labels), containing 114 and 414 carbon atoms, respectively. The incident electric field ${\mathbf{E}}^{\mathrm{inc}}$ is oriented as shown in the inset of (b). Conduction charge distributions are plotted above each corresponding panel for $d_x=0.5\mathrm{nm}$. Optically induced charge distributions are shown in (a) for three dominant resonances (blue and red indicate charges of opposite signs). Spectra for unexposed graphene correspond to $d_x\to \infty$.

Figure 3

Sensing of dipolar molecules through the linear optical absorption of nanographenes. We show the linear absorption cross section of individual GNHs exposed to a single analyte carrying a permanent dipole [$p^{\text{ext}}=50\mathrm{D}$ in (a)–(c); $p^{\text{ext}}=5\mathrm{D}$ in (d)–(f); aligned along $x$ in all cases] placed at different distances $d_z$ above the hexagon center [see inset to (d)]. Doped [net charge $Q=-4e$, panels (b) and (e)] and undoped ($Q=0$, rest of the panels) nanohexagons are considered with different side lengths $L$, as indicated by labels. Conduction charge distributions are plotted above (a)–(c) for $d_z=0.5\mathrm{nm}$ and $p^{\text{ext}}=50\mathrm{D}$. Optically induced charge distributions are shown in (a) for three dominant resonances. Unexposed graphene is represented by $d_z\to \infty$.

Figure 4

Nonlinear sensing of charged and dipolar analytes. We show second-harmonic polarizability spectra of single GNHs in the presence of an individual singly charged (a)–(c) or dipole-carrying (d)–(f) molecule. Results for charged (dipolar) analytes are obtained using the same parameters as in Figs. 2 [Figs. 3].