Optical Anisotropy and Excitons in MoS2 Interfaces for Sensitive Surface Plasmon Resonance Biosensors

Abstract

The use of ultra-thin spacer layers above metal has become a popular approach to the enhancement of optical sensitivity and immobilization efficiency of label-free SPR sensors. At the same time, the giant optical anisotropy inherent to transition metal dichalcogenides may significantly affect characteristics of the studied sensors. Here, we present a systematic study of the optical sensitivity of an SPR biosensor platform with auxiliary layers of MoS₂. By performing the analysis in a broad spectral range, we reveal the effect of exciton-driven dielectric response of MoS₂ and its anisotropy on the sensitivity characteristics. The excitons are responsible for the decrease in the optimal thickness of MoS₂. Furthermore, despite the anisotropy being at record height, it affects the sensitivity only slightly, although the effect becomes stronger in the near-infrared spectral range, where it may lead to considerable change in the optimal design of the biosensor.

Keywords: MoS2; sensitivity enhancement; surface plasmon resonance; transition metal dichalcogenides.

Figure 1

(a) Schematic view of the studied SPR biosensor operation; (b) Anisotropic optical properties of MoS₂ nanosheets [32,36]. “A, B, C₁, C₂” label the excitonic absorption peaks; (c) Reflectance as a function of the angle of incidence at a wavelength of 633 nm and varying number $$N_{{\mathrm{MoS}}_2}$$ of MoS₂ layers. Black dashed curve represents the reference biosensor without the MoS₂ cover ($$N_{{\mathrm{MoS}}_2}=0$$ ). SPR curves are plotted assuming that the analyte layer is pure water; (d) The sensitivity of SPR biosensor as a function of the thickness of the MoS₂ layer. In calculations using isotropic optical properties of MoS₂, we set $\epsilon ={\epsilon }_{\parallel }$.

Figure 2

Effective refractive indices of the anisotropic MoS₂ layer, governing: (a) phase accumulation and field attenuation upon propagation of a p-polarized wave (n_eff = β/β₀) and (b) reflection and transmission of a p-polarized wave at interfaces with other materials, calculated from Equation (5).

Figure 3

Heatmaps of the angular sensitivity of the SPR biosensor as a function of the operating wavelength and the number of MoS₂ atomic layers covering gold. The heatmaps were calculated assuming: (a) anisotropic optical properties of MoS₂; (b) isotropic dielectric function $\epsilon ={\epsilon }_{\parallel }$ of MoS₂. Solid green line shows the dependence of the optimal thickness on the wavelength. (c) Maximum angular sensitivity of the biosensor versus the operating wavelength. The sensitivity of the sensor without MoS₂ cover (blue curve) is added for reference.

Figure 4

(a) The phase of the reflected p-polarized wave as a function of the angle of incidence, plotted at a variable number of atomic layers of MoS₂. (b) The phase sensitivity and the minimum reflectance versus the thickness of MoS₂ cover, showing the singularity of sensitivity at a zero-reflection point. (c) Dependence of the biosensor signal on the refractive index change. The operating wavelength in all panels was set to 633 nm.

Figure 5

Heatmaps of phase sensitivity as functions of the number of atomic layers in MoS₂ film and the operating wavelength. Calculations were performed using: (a) full anisotropic dielectric tensor and (b) isotropic dielectric permittivity for the MoS₂ layer.