Probing the Local Dielectric Function by Near Field Optical Microscopy Operating in the Visible Spectral Range

TL;DR

This paper introduces a dual s-SNOM technique to image and quantify local dielectric function variations in nanoscale materials, validated through multiple microscopy and spectroscopy methods.

Contribution

The study presents a novel dual s-SNOM method for local dielectric function imaging and extraction, validated by complementary techniques.

Findings

Successfully imaged dielectric variations in WS₂ monolayer

Validated dielectric measurements with spectroscopic ellipsometry

Identified charge transfer differences in nanoscale regions

Abstract

The optoelectronic properties of nanoscale systems such as carbon nanotubes (CNTs), graphene nanoribbons and transition metal dichalcogenides (TMDCs) are determined by their dielectric function. This complex, frequency dependent function is affected by excitonic resonances, charge transfer effects, doping, sample stress and strain, and surface roughness. Knowledge of the dielectric function grants access to a material's transmissive and absorptive characteristics. Here we introduce the dual scanning near field optical microscope (dual s-SNOM) for imaging local dielectric variations and extracting dielectric function values using a mathematical inversion method. To demonstrate our approach, we studied a monolayer of WS$_2$ on bulk Au and identified two areas with differing levels of charge transfer. Our measurements are corroborated by atomic force microscopy (AFM), Kelvin force probe microscopy (KPFM), photoluminescence (PL) intensity mapping, and tip enhanced photoluminescence (TEPL). We extracted local dielectric variations from s-SNOM images and confirmed the reliability of the obtained values with spectroscopic imaging ellipsometry (SIE) measurements.