[논문 리뷰] Polytype-Dependent Upconversion Photoluminescence in 3R-MoS2

Omri Meron, Idan Kizel외 6명|arXiv (Cornell University)|2026. 02. 03.

2D Materials and Applications인용 수 0

한 줄 요약

이 연구는 rhombohedral 3R-MoS2에서 업컨버전 형광(UPL)을 보고하고, Gamma-전도 망과 엑시톤-엑시톤 소멸 채널(K-Γ 및 Q-Γ)에 의해 stacking order가 UPL 강도에 미치는 강한 폴리타입 의존성을 보여준다.

ABSTRACT

Ferroelectric van der Waals materials offer switchable polarization states, yet optical readout of their stacking configurations remains challenging. Building on the resonant exciton-exciton annihilation (EEA) mechanism in 2H-phase TMDs, we report the first observation of upconversion photoluminescence (UPL) in rhombohedral MoS2 and demonstrate that this many-body process is strongly polytype-dependent. Using low-temperature spectroscopy, we observe anti-Stokes emission with superlinear power dependence. Beyond serving as a layer-number sensor, UPL provides a sensitive probe of stacking order. Trilayer ABA and BAB polytypes, indistinguishable by surface potential measurements and second harmonic generation, exhibit markedly different UPL intensities, and this persists in thicker samples. First-principles calculations attribute this polytype dependence to modulation of the Gamma-point conduction manifold, which controls energy-matching conditions for the annihilation process. Power-dependent spectroscopy further disentangles two distinct annihilation channels originating from different dark exciton valleys, identified through their contrasting intensity scaling and opposite density-induced energy shifts. Crucially, the annihilation process doubles the energy separation of nearly degenerate dark excitons while converting their weak emission into bright signal, providing experimental access to valley-specific responses that are obscured in direct dark-exciton spectroscopy. Our findings demonstrate that ferroelectric configurations provide a new degree of freedom for controlling nonlinear optical processes, with implications for all-optical ferroelectric readout and electrically switchable wavelength conversion in two-dimensional materials.

연구 동기 및 목표

• Investigate whether upconversion photoluminescence (UPL) can reveal stacking order in rhombohedral MoS2 (3R polytypes).
• Determine how different 3R stacking sequences (ABC/CBA vs ABA/BAB) affect UPL efficiency and underlying many-body processes.
• Identify the electronic-structure features that govern UPL and its polytype sensitivity across thicknesses.
• Disentangle distinct exciton-exciton annihilation (EEA) channels associated with different dark exciton valleys.
• Explore the persistence of polytype-resolved UPL in thicker flakes where multiple polytypes coexist.

제안 방법

• Perform low-temperature (4 K) photoluminescence spectroscopy with 532 nm CW excitation.
• Use Kelvin probe force microscopy (KPFM) to map surface potential and distinguish stacking domains.
• Carry out first-principles calculations (DFT with PBE-D3 for structure, HSE for band structure) to analyze the Gamma-conduction manifold.
• Model EEA feeding into bright Gamma-Gamma states using a Fermi’s golden rule-based framework with a Lorentzian energy-mismatch term and Voigt emission lineshape.
• Decompose UPL spectra into two components (P1 and P2) via spectral fitting to extract layer-, thickness-, and power-dependent behavior.
• Analyze thickness dependence from 3 to 10 layers and thickness-dependent resonance conditions for EEA.

실험 결과

연구 질문

• RQ1 Does UPL in 3R-MoS2 vary with stacking polytype, enabling optical discrimination of otherwise KPFM-degenerate ABA vs BAB domains?
• RQ2 What is the role of the Gamma-point conduction manifold in mediating EEA-driven upconversion for different polytypes?
• RQ3 Can UPL reveal valley-specific dark exciton dynamics (K-Γ vs Q-Γ) and their dependence on thickness and stacking order?
• RQ4 Does polytype-resolved UPL persist in thicker flakes with coexisting polytypes?

주요 결과

• UPL is observed in rhombohedral 3R-MoS2 and is strongly dependent on stacking polytype (ABC/CBA vs ABA/BAB).
• First-principles calculations show a Gamma-conduction cluster whose energy spacing relative to 2E_dark differs between ABA and BAB, explaining the UPL intensity hierarchy.
• UPL spectra can be captured by a two-channel EEA model feeding bright gamma-gamma states, with a Lorentzian (K-Γ) and a Fano (Q-Γ) component.
• Two distinct EEA channels are identified: K-Γ and Q-Γ dark exciton pathways, evidenced by their contrasting power laws and density-induced energy shifts.
• In thicker flakes, UPL persists and shows domain-dependent intensity, indicating polytype variations modulate UPL even at fixed thickness.
• UPL provides a non-invasive optical readout of ferroelectric stacking order and suggests routes for polarization-multiplexed photonics and wavelength conversion.