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Fast Interlayer Energy Transfer from the Lower Bandgap MoS2 to the Higher Bandgap WS2

This study reports an ultrafast (~33 fs) interlayer energy transfer from lower-bandgap MoS2 to higher-bandgap WS2 at 300 K, driven by a resonant overlap between MoS2 B and WS2 A excitonic levels, which challenges the traditional donor-to-acceptor bandgap paradigm and reveals a process faster than excitonic intervalley scattering.

Original authors: Gayatri, Mehdi Arfaoui, Debashish Das, Tomasz Kazimierczuk, Sabrine Ayari, Natalia Zawadzka, Takashi Taniguchi, Kenji Watanabe, Adam Babinski, Saroj K. Nayak, Maciej R. Molas, Arka Karmakar

Published 2026-02-23

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Gayatri, Mehdi Arfaoui, Debashish Das, Tomasz Kazimierczuk, Sabrine Ayari, Natalia Zawadzka, Takashi Taniguchi, Kenji Watanabe, Adam Babinski, Saroj K. Nayak, Maciej R. Molas, Arka Karmakar

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Big Idea: Breaking the Rules of Energy Flow

Imagine you have two buckets of water. One bucket is sitting on a high shelf (High Energy), and the other is on the floor (Low Energy). In the world of physics, water naturally flows down from the high shelf to the floor. It takes a lot of effort to make it flow up.

Usually, in materials science, Energy Transfer works the same way. If you have a material with a "high energy" bandgap (the high shelf) and one with a "low energy" bandgap (the floor), energy naturally flows from the high one to the low one.

This paper discovered a magic trick: The scientists found a way to make energy flow UP the hill. They made energy jump from a "low energy" material (MoS₂) to a "high energy" material (WS₂). Even better, they did it so fast that it happened before the energy could even decide to stay put or scatter around.

The Characters: The Materials

Think of the materials as two different types of dancers on a stage:

MoS₂ (The Donor): This is the "lower energy" dancer. In a single layer (1 layer thick), it's very energetic and direct. But if you stack more layers on top of it, it gets "lazy" and indirect (its energy gets stuck in a different part of the stage).
WS₂ (The Acceptor): This is the "higher energy" dancer. It needs a big push to get going.
hBN (The Barrier): This is a thin sheet of "insulating glass" placed between them. It stops them from touching physically (which would cause a short circuit) but lets them "talk" to each other through invisible waves.

The Magic Trick: The "Resonant" Handshake

Normally, MoS₂ is too "low energy" to give anything to WS₂. It's like trying to push a heavy boulder up a hill with a gentle breeze.

However, the scientists noticed something special. The "B-level" energy of MoS₂ and the "A-level" energy of WS₂ are almost perfectly matched.

The Analogy: Imagine two tuning forks. If you strike one, it vibrates. If the second fork is tuned to the exact same frequency, it will start vibrating too, even if it's not touching the first one. This is called Resonance.

In this experiment, the "B-level" of MoS₂ and the "A-level" of WS₂ were tuned so perfectly that the energy didn't need to climb a hill. It just "slid" across the gap because the frequencies matched.

The Experiment: Changing the Layers

The scientists wanted to see how this magic handshake worked. They built a sandwich:

Top: A single layer of WS₂.
Middle: A thin layer of insulating glass (hBN).
Bottom: MoS₂, but they changed the thickness. Sometimes it was 1 layer, sometimes 2, 4, or 5 layers.

What happened?

1 Layer of MoS₂: The handshake was perfect! The energy jumped from MoS₂ to WS₂ instantly. The WS₂ glowed 3 times brighter than it would have on its own.
More Layers (2, 4, 5): As they added more layers to the MoS₂, the material changed its nature. It became "indirect." The energy got stuck in a different part of the atom, like a runner getting stuck in mud. The "handshake" broke. The energy couldn't jump up to WS₂ anymore, and the glow got dimmer (or even disappeared).

The Speed: Faster Than a Blink

The most exciting part of this paper is how fast this happened.

The scientists measured the time it took for the energy to jump. It took about 33 femtoseconds.

What is a femtosecond? It is one-quadrillionth of a second.
The Analogy: If a femtosecond were a second, then a second would be about 31 million years.

To put it in perspective: The energy jumped from one material to the other faster than the electrons inside the material could even scatter around or get confused. It was faster than the "intervalley scattering" (a fancy way of saying the electrons running around the room).

It's like a relay race where the runner passes the baton to the next runner before the first runner even realizes they are running.

Why Does This Matter?

New Rules: It proves that energy doesn't always have to flow from high to low. If the "tuning" is right, it can go the other way.
Super-Fast Electronics: Because this happens in 33 femtoseconds, it suggests we could build future computers or solar cells that process information incredibly fast, beating the speed limits of current technology.
Efficiency: It shows that we can design materials to capture energy that would otherwise be wasted, potentially making better solar panels or light-emitting devices.

Summary

The scientists built a tiny, atom-thin sandwich. They discovered that if the ingredients are just right (specifically, a single layer of MoS₂), the energy can magically jump uphill from one material to another. They did it so fast that it broke the usual rules of how electrons behave, opening the door for a new generation of super-fast, ultra-efficient electronic devices.

1. Problem Statement

Energy transfer (ET) in van der Waals (vdW) heterostructures is traditionally understood as a thermodynamically favorable process occurring from a higher bandgap material (donor) to a lower bandgap material (acceptor). While rare instances of "reverse" ET (from lower-to-higher bandgap) have been theorized based on spectral overlap, experimental verification remains scarce due to:

The difficulty in spectrally resolving emissions when bands overlap significantly.
The ultrafast timescales involved, which often compete with or are obscured by carrier thermalization and intervalley scattering.
A lack of understanding regarding how bandgap tuning (e.g., layer thickness) affects these unconventional transfer pathways.

This study addresses the specific challenge of quantifying the ET rate from a lower bandgap material (Monolayer MoS₂) to a higher bandgap material (Monolayer WS₂) and investigating how the transition from a direct to an indirect bandgap in MoS₂ influences this process.

2. Methodology

The researchers employed a combination of advanced experimental spectroscopy and theoretical modeling:

Sample Fabrication:
- Constructed van der Waals heterostructures (HS) consisting of $n$ -layer MoS₂ (where $n = 1, 2, 4, 5$ ) as the bottom layer and 1-layer WS₂ as the top layer.
- Separated the TMDC layers with a thin (~5.5 nm) hexagonal boron nitride (hBN) interlayer to completely suppress interlayer charge transfer, isolating the dipole-dipole (Förster) energy transfer mechanism.
Experimental Techniques:
- Differential Reflection Contrast (RC): Used to derive absorption spectra and confirm the near-resonant overlap between MoS₂ B-exciton and WS₂ A-exciton levels.
- Photoluminescence Excitation (PLE) Spectroscopy: Mapped the WS₂ emission intensity as a function of excitation energy across different MoS₂ layer thicknesses at Room Temperature (RT, 300 K) and Low Temperature (LT, 5 K).
- Time-Resolved Photoluminescence (TR-PL): Measured carrier dynamics using a streak camera to determine exciton lifetimes and rise/decay times.
Theoretical Modeling:
- Density Functional Theory (DFT): Calculated transition dipole moments (TDM) to assess oscillator strengths.
- Förster Resonance Energy Transfer (FRET) Theory: Applied Fermi's Golden Rule to calculate ET rates based on thermal averages and dipole-dipole coupling.
- Coupled Rate Equations: Modeled the population dynamics of bright and dark excitons across K, K', $\Lambda$ , and $\Gamma$ valleys to simulate the time-dependent PL behavior.

3. Key Contributions

Demonstration of Reverse ET: The paper provides robust experimental evidence of energy transfer from a lower bandgap material (MoS₂) to a higher bandgap material (WS₂), driven by a near-resonant overlap between the MoS₂ B-exciton and WS₂ A-exciton levels.
Bandgap Tuning Effect: The study systematically demonstrates that the efficiency of this reverse ET is highly sensitive to the MoS₂ layer thickness. As MoS₂ transitions from a direct bandgap (1L) to an indirect bandgap (multi-layer), the ET efficiency drops drastically.
Ultrafast Timescale Determination: The authors successfully extracted an ET timescale of ~33 fs at 300 K, which is faster than the reported intervalley scattering time (K↔K') of MoS₂ and comparable to ultrafast charge transfer rates.
Mechanism Elucidation: The work clarifies that the process is mediated by long-range dipolar coupling (Förster mechanism) rather than charge transfer or Dexter transfer, confirmed by the thick hBN spacer and the specific dependence on excitonic resonance.

4. Key Results

Spectral Overlap: Absorption measurements confirmed a significant overlap between the MoS₂ B-exciton absorption and the WS₂ A-exciton emission, creating the necessary condition for resonant energy transfer.
Layer-Dependent PL Enhancement:
- 1L MoS₂/WS₂: Under resonant excitation (2.4 eV), the WS₂ PL intensity was enhanced by ~3× compared to isolated WS₂.
- 2L MoS₂/WS₂: Enhancement dropped to ~1.25×.
- 4L & 5L MoS₂/WS₂: The WS₂ emission was quenched (enhancement factor < 1).
- Explanation: In 1L MoS₂, photoexcited carriers remain in the K-valley long enough to transfer energy. In multi-layer MoS₂, the indirect bandgap allows rapid scattering of carriers to the $\Lambda$ or $\Gamma$ valleys, depleting the K-valley population available for ET.
Temperature Dependence: At 5 K, the enhancement was minimal (~1.3× for 1L) compared to RT. This is attributed to reduced phonon-assisted scattering at low temperatures, which limits the return of "hot" carriers from indirect valleys back to the K-valley where ET occurs.
Dynamics:
- TR-PL showed a shortened lifetime for WS₂ in the heterostructure (~4 ps) compared to isolated WS₂, indicating rapid energy influx.
- Theoretical modeling yielded an ET timescale of ~50 fs at 5 K and ~33 fs at 300 K.
Competing Processes: The calculated ET timescale (~~33 fs) is faster than the MoS₂ intervalley scattering time (~~110 fs for A-exciton, few ps for B-exciton), proving that energy transfer dominates over intralayer relaxation in this specific resonant configuration.

5. Significance

Fundamental Physics: This work challenges the conventional thermodynamic view of energy transfer, proving that strong spectral overlap can drive ultrafast energy flow "uphill" in energy. It highlights the critical role of valley dynamics and phonon-assisted scattering in governing these processes.
Device Applications: The discovery of such fast (~33 fs) interlayer energy transfer opens new avenues for designing high-speed optoelectronic devices, such as ultrafast photodetectors, light-emitting diodes (LEDs), and excitonic circuits where energy routing is controlled by layer thickness and resonance conditions rather than just band alignment.
Methodological Benchmark: The study establishes a rigorous framework for distinguishing between charge transfer and energy transfer in vdW heterostructures and provides a benchmark for ultrafast timescale measurements in 2D materials.

In conclusion, this paper successfully identifies and quantifies an unconventional, ultrafast energy transfer mechanism in MoS₂/WS₂ heterostructures, demonstrating that by tuning the bandgap nature of the donor material, one can control the efficiency of energy flow in next-generation 2D optoelectronic systems.