Investigating the Ferroelectric Potential Landscape of 3R-MoS$_2$ through Optical Measurements

TL;DR

The paper addresses identifying and mapping ferroelectric domain structures in 3R-MoS$_2$ through room-temperature optical measurements. It combines near-resonant Raman and photoluminescence with reference KPFM maps to link optical signatures to ferroelectric stacking and layer count. The main contribution is demonstrating that domain-dependent signatures in $E^1_{2g}$ and $A_{1g}$ Raman modes, together with shifts in A and B excitons and a low-energy PL feature, enable fast, substrate-free optical mapping of ferroelectric landscapes in 3R-MoS$_2$ under ambient conditions. This approach offers a practical, non-invasive route to characterize sliding ferroelectrics in vdW materials and could inform design of optoelectronic devices leveraging ferroelectric domain control.

Abstract

In recent years, sliding ferroelectricity has emerged as a topic of significant interest due to its possible application in non-volatile, reconfigurable storage devices. This phenomenon is unique to two-dimensional van der Waals materials, where out-of-plane ferroelectric polarization switching is induced by relative in-plane sliding of adjacent layers. The intrinsic stacking order influences the resulting polarization, creating distinct polarization regions separated by domain walls. These regions and the domain walls can be manipulated using an applied vertical electric field, enabling a switchable system that retains the environmental robustness of van der Waals materials under ambient conditions. This study investigates 3R-MoS$_2$ using various optical measurement techniques at room temperature. The spatially resolved optical measurements reveal apparent signal changes corresponding to different ferroelectric stacking orders and variations in layer count. Our findings demonstrate that fast optical mapping at room temperature is a reliable method for probing ferroelectric potential steps in 3R-stacked MoS$_2$ samples, thereby facilitating the identification of the ferroelectric configuration. This approach does not require a conductive substrate or an electrical contact to the sample, making it more versatile than traditional atomic force probe techniques.

Figures (7)

• Figure 1: Basic characterization of the 3R-stacked MoS$_2$ sample on top of a hBN backing, the scale bar indicates 5µ m (a) Optical microscope image with dashed black lines, indicating the borders of the layer regions. The corresponding number marks the layer count of each region. (b) AFM image showing the topography of the flake and the layer outlines (dashed black lines). (c) KPFM map showing the ferroelectric potential landscape across the flake. Regions of equal potential and ferroelectric configuration are identified using specific colors for the layer count, followed by the effective dipole direction (u for up, d for down) and an individual number (e.g., 2d1). The asterisk added to some indicates that these regions have a different optical contrast and amount of adsorbates between hBN and MoS$_2$. (d) Potential profile across a domain edge for a single step in the bilayer region (blue) and a double step in the trilayer region (red). The origins of the profiles are marked in (c) with white lines
• Figure 2: Intensity distributions of (a) the E$^1_{2g}$ Mode (at 383cm^-1) and (b) the A$_{1g}$ Mode (at 405,8cm^-1) extracted from a 633nm exitation near-resonant Raman scan. The white scale bar indicates 5µ m and the domain and layer outlines are marked according to the previously introduced scheme (see Figure \ref{['fig:panel1']}(c)).
• Figure 3: Changes of spatially averaged 633nm excitation Raman signal between neighboring domains of opposing configurations with the same layer count. (a) Comparing opposing domains in the bilayer region. The domains in the bottom frame are marked with an asterisk to indicate that their location on the sample has a different adhesion to the hBN than the rest. (b) Comparing opposing domains in the trilayer and tetralayer regions. The trilayer spectra include the null domain specific to regions of odd layer count. For the identification of the second-order Raman modes, several papers on near-resonant Raman spectroscopy on 2H-MoS$_2$ were referenced Lee2015Livneh2015Lu2017Carvalho2017.
• Figure 4: Change of the spatially averaged 633nm excitation Raman signal of the down domains across the bi-, tri-, and tetralayer regions. The spectrum of a trilayer null domain is included for comparison. The arrows visualise the spectral changes with increasing layer count. Note the increase in intensity of the Raman features around 450cm^-1 but decrease in the response of the E$^1_{2g}$ and the A$_{1g}$ modes as they do not increase in intensity along the overall signal. The domain marked with an asterisk is located in a region with different adhesion to the hBN than the rest.
• Figure 5: (Room temperature PL with 532nm excitation to investigate the ferroelectricity-influenced behavior of the A and the B excitons (at 1855meV and 1990meV). (a) Comparing spatially averaged and normalized spectra of down and up domains in the bilayer region of the 3R-MoS$_2$ flake. The down domain has a reduced half-width of the A exciton with and the B exciton is redshifted by about 10meV. (b) Spatial map of the energetic distance of the A and B excitons based on fitted data. Peak distance changes with domain orientation across all layer counts in the same fashion. Outlines of the ferroelectric domains are marked in the previously introduced scheme. The white scale bar indicates 5µ m.