Photoluminescence Detection of Polytype Polarization in r-MoS2 Enabled by Asymmetric Dielectric Environments

TL;DR

This work shows that stacking-induced ferroelectric polarization in $r$-MoS$_2$ can be optically read out via photoluminescence when the material is in an asymmetric dielectric environment, achieving strong contrast up to ~400% between domains. The dominant mechanism is polarization-dependent doping that shifts the Fermi level and alters the exciton–trion balance, with spectral features corroborated by KPFM domain maps and temperature-dependent Voigt analysis. Importantly, the pronounced PL contrast persists up to room temperature, enabling non-invasive ferroelectric-domain mapping in fully encapsulated devices and opening pathways for polarization-sensitive optoelectronics in SlideTronics architectures. The methods combine cryogenic optical spectroscopy, selective van der Waals heterostructure fabrication, and AFM/KPFM characterization to establish PL as a practical probe for domain topology in 2D ferroelectric polytypes.

Abstract

The rhombohedral (r) polytypes of transition metal dichalcogenides (TMDs) constitute a novel class of two-dimensional ferroelectric materials, where lateral shifts between parallel layers induce reversible out-of-plane polarization. This emerging field, known as SlideTronics, holds significant potential for next-generation electronic and optoelectronic applications. While extensive studies have investigated the effects of electrical and chemical doping on excitonic signatures in 2H-TMDs, as well as the influence of dielectric environments on their optical properties, the impact of intrinsic polarization in asymmetric environments remains largely unexplored. Here, we demonstrate a striking polarization-dependent photoluminescence (PL) contrast of up to 400\% between ferroelectric domains in bilayer and trilayer rhombohedral molybdenum disulfide (r-MoS2). This pronounced contrast arises from an asymmetric dielectric environment, which induces polarization-dependent shifts in the Fermi energy, leading to a modulation of the exciton-trion population balance. A detailed temperature-dependent line shape analysis of the PL, conducted from 4K to room temperature, reveals domain-specific trends that further reinforce the connection between polarization states and excitonic properties. The persistence of these distinct optical signatures at room temperature establishes PL as a robust and non-invasive probe for ferroelectric domain characterization, particularly in fully encapsulated device architectures where conventional techniques, such as Kelvin probe force microscopy, become impractical.

Figures (4)

• Figure 1: Sample architecture and characterization.a, Schematic cross-section of the device structure, showing bilayer and trilayer r-MoS$_2$ regions encapsulated between a trilayer graphene substrate and h-BN capping layer (8 nm). This asymmetric dielectric environment creates distinct conditions for different polarization states. The interfacial polarization of each interface (and the total interfacial polarization) is marked by short (long) black arrows. b, Optical micrograph under white light illumination distinguishing regions of different layer thickness (solid white outline: trilayer, dashed white outline: bilayer regions). The stacking configurations and resulting polarization domains remain optically indistinguishable. c, Kelvin probe force microscopy (KPFM) map revealing the surface potential distribution across the sample, providing direct visualization of the ferroelectric domains arising from different stacking configurations.
• Figure 2: Stacking-dependent PL spectra and domain identification.a, Low-temperature (4K) PL spectra from bilayer domains showing distinct features for AB (blue) and BA (red) stacking configurations. The AB-stacked regions exhibit markedly enhanced PL intensity, with prominent neutral A exciton and trion peaks. Both configurations show characteristic graphene Raman features (2D) and B exciton transitions. Shaded areas around each spectrum represent the standard deviation from measurements across multiple spots within each domain. The gray-shaded energy range indicates the spectral window used for intensity integration in panel d. Inset: illustration of the charge transfer induced shift in the Fermi energy. b, Corresponding trilayer spectra demonstrating configuration-dependent emission, with ABC stacking (blue) showing significantly enhanced PL compared to CBA (red) and ABA/BAB (green) configurations. The spectral features reflect the complex interplay between stacking order, polarization, and excitonic effects. Shaded areas indicate measurement standard deviation, and gray region marks the integration window used for mapping. c, KPFM surface potential map serving as ground-truth reference for domain identification, with colored overlays matching the spectra in (a,b). d, Spatially-resolved integrated PL intensity map at 4K showing one-to-one correspondence with the KPFM-identified domains, demonstrating PL mapping as a non-invasive probe of ferroelectric configurations.
• Figure 3: Temperature-dependent PL evolution across stacking configurations, from 4K to 295K.a, bilayer AB stacking, b, bilayer BA stacking, d, trilayer ABC stacking, e, trilayer ABA/BAB configuration, and f, trilayer CBA stacking. The spectra reveal systematic thermal broadening of excitonic features and a characteristic red-shift with increasing temperature, while maintaining the relative intensity differences between stacking configurations. c, Room-temperature (295K) integrated PL intensity map demonstrating that the stacking-dependent contrast persists even at elevated temperatures, enabling practical domain identification under ambient conditions.
• Figure 4: Spectral fitting and temperature-dependent analysis.a, PL spectrum from bilayer AB domain at 4K (gray dots) with multi-peak Voigt profile fitting (colored lines). Individual components include neutral A exciton (purple), modified A trion with asymmetric tail (green), B trion (blue) and B neutral exciton (light blue), demonstrating the excellent agreement between the modified trion lineshape model and experimental data. b, Corresponding spectrum and fit for bilayer BA domain at 4K, showing distinct relative intensities of excitonic features. c, Temperature dependence of integrated PL intensity (1.8-2.1 eV) for AB and BA stacking configurations, revealing a more pronounced intensity reduction with increasing temperature in the AB configuration. d, Evolution of linewidth parameters $\gamma$ (Lorentzian) and $\sigma$ (Gaussian) for the neutral A exciton in the AB configuration as a function of temperature. e, Temperature evolution of peak energies for neutral A exciton and trion features (AB configuration) extracted from the fitting procedure, with dashed lines showing fits to the O'Donnell-Chen model.odonnell_temperature_1991