Strain-Induced Plasmon Confinement in Polycrystalline Graphene

Abstract

Terahertz spectroscopy is a perfect tool to investigate the electronic intraband conductivity of graphene, but a phenomenological model (Drude-Smith) is often needed to describe disorder. By studying the THz response of isotropically strained polycrystalline graphene and using a fully atomistic computational approach to fit the results, we demonstrate here the connection between the Drude-Smith parameters and the microscopic behavior. Importantly, we clearly show that the strain-induced changes in the conductivity originate mainly from the increased separation between the single-crystal grains, leading to enchanced localization of the plasmon excitations. Only at the lowest strain values explored, a behavior consistent with the deformation of the individual grains can instead be observed.

Figure 1

Spectroscopic evidence of the Drude-Smith behavior observed in the far-infrared response of strained polycrystalline graphene. (a) Experimental (black to gray dotted traces) and calculated (blue to cyan solid lines) relative transmittance spectra, defined as the ratio between the transmittance of the investigated sample (graphene on polymer membrane) and a reference sample (bare polymer membrane). The spectral region mostly affected by the Drude-Smith strain-dependent response is that between 0.2 and 3 THz. The inset shows a small effect mostly due to membrane thickness reduction. (b, c) Real and imaginary parts of graphene conductivity giving rise to the calculated spectra in (a). The suppression of Re(σ) at low frequencies is a fingerprint of Drude-Smith behavior. (d, e) Strain dependence of the Drude-Smith model parameters c (quantifying the carrier backscattering) and τ_DS (effective scattering time). Large strain imply stronger backscattering and smaller effective scattering time (more on this in the main text).

Figure 2

Polycrystalline graphene sheet composed of 6 grains at rest (a) and under the effect of biaxial strain (b). ωFQ values for the real (c) and imaginary (d) part of σ^2d as a function of the external frequency. Drude-Smith parameters c (e) and τ_DS (f) as a function of the applied strain. Charge density plots for the system at rest (g) and at 10% strain (h).

Figure 3

(a) Graphical representation of the elongation of a single graphene ring under the effect of an increasing level of isotropic strain. (b) Drude-Smith parameter τ_DS as a function of the applied strain by imposing a stretching of C–C bonds only.