Ultrafast Anisotropic Exciton Dynamics in Nanopatterned MoS₂ Sheets

Abstract

We study the optical properties of an anisotropic ripple-shaped two-dimensional molybdenum disulfide (MoS2) nanosheet deposited by chemical vapor deposition onto a nanopatterned silica (SiO2) substrate. We unveil a giant anisotropic optical response in the linear and nonlinear regime by a combination of optical extinction measurements, ultrafast broadband transient absorption experiments, and finite element method numerical simulations. In steady state optical measurements, such anisotropy appears as a polarization-dependent extinction in correspondence with the characteristic excitonic peaks (A, B, C, and D) of MoS2. Along with spectral changes, ultrafast measurements strikingly exhibit the onset of an anisotropic relaxation dynamic in the region of the C exciton. Numerical simulations indicate that the observed polarization-dependent optical response is dictated by the nanopattering of MoS2, with peculiar features belonging to the out-of-plane component of the dielectric tensor of MoS2 that is made effective through the rippled configuration. Our findings give a rationale to the anisotropic exciton response and show that morphology manipulation represents a valuable option for the tuning of optical and electronic properties in transition-metal dichalcogenides.

Evidence of Plasmon Enhanced Charge Transfer in Large‐Area Hybrid Au–MoS2 Metasurface

“…Figure S3c in the Supporting Information also shows the characteristic linear dichroism of the bare MoS 2 deposited on the rippled silica substrate as previously reported. [31] Note also that the features related to MoS 2 (A, B, C, and D peaks) will be clearly evident in the ultrafast transient absorption measurements that we will discuss later.…”

“…The MoS 2 nanosheet, conformally grown on the silica substrate, was assumed to have ≈3 nm thickness (corresponding to about 4 atomic layers). MoS 2 permittivity was modeled according to the same approach introduced in the previous work, [31] i.e., as a complex tensor with two nondegenerate spectral components, i.e., an in-plane component for y-direction, and a combination of in-plane and out-of-plane component for the x and z directions detailed by the ripple profile. Regarding Au NSTs, the typical cross-section of about 110 nm width and 20 nm thickness reported for other Au NST samples fabricated in similar conditions was assumed.…”

“…For the optical response of the rippled MoS 2 , we adopted a full tensorial, dispersive, complex dielectric permittivity able to reproduce all the main absorption features (A, B, C, and D resonances). [31] Gold NST was instead modeled as an infinitely long stripe with a rectangular cross-section of 20 nm thickness, lateral size of 110 nm (identified so as to match the PR according to the same guidelines reported in ref. [32] ), and dispersive dielectric permittivity obtained from a Drude-Lorentz model fit to the experimental data.…”

“…where, T and T 0 are the FEM numerical transmissions of the probe beam through the hybrid metasurface in the perturbed and unperturbed case; ε λ ( ) MoS 2 in the expressions of the spectral coefficients is the complex dielectric permittivity tensor of MoS 2 , which comprises all the excitonic features (A, B, C, and D). [31]…”

Geometrical Engineering of Giant Optical Dichroism in Rippled MoS2 Nanosheets

“…As an example, for MoS 2 , the in‐plane permittivity (black curves in Figure a,b) is dominated by four resonant peaks in the visible spectrum (attributed to the so‐called A, B, C, and D excitonic resonances), which are almost absent in the out‐of‐plane permittivity component (blue curves in Figure 1a,b), also, being much weaker. [ 28 ] Anyway, the latter does not play any role in the optical response when impinging at normal incidence on a flat MoS 2 nanolayer, which thus operates as an isotropic medium, showing no dependence on light polarization in transmission. However, if we figure out a configuration where the principal axes of the permittivity tensor are rotated by an angle θ with respect to the normal, as an example in a periodic fashion (Figure 1c), a linearly polarized plane wave with TM polarization can interact also with the out‐of‐plane permittivity component, contrary to the TE polarized wave, still feeling the in‐plane component only.…”