Ultralow-Tensile Strain Enables Exciton Funneling and Energy Transfer to Boost MoSe2 Photoluminescence Quantum Yield

Light and matter interact in ways that often seem counterintuitive, especially when we shrink materials down to the thickness of a single atom. In the world of two-dimensional semiconductors, scientists have long sought a way to make these ultra-thin sheets emit light more brightly and efficiently. This quest is central to developing faster, more energy-efficient electronics and new types of quantum devices. The behavior of light in these materials depends heavily on how electrons and their partners, called excitons, move and interact. Researchers can influence this behavior by stretching or squeezing the material, a technique known as strain engineering. Just as a guitar string changes pitch when tightened, the electronic properties of these atomic sheets change when their atomic bonds are stretched. Another key factor is how energy moves between different layers when they are stacked together. When one layer absorbs light, it can pass that energy to a neighboring layer without moving any electric charge, a process that acts like a silent handoff of a baton. The challenge has been to find a simple, controllable way to combine these effects to boost the brightness of the light emitted.

A team of researchers has now demonstrated that applying a tiny amount of stretch to a stack of these atomic layers can dramatically increase their ability to emit light. They constructed a sandwich-like structure using two different types of single-atom-thick crystals, separated by a thin insulating layer of boron nitride. This entire stack was placed on top of a microscopic bubble made of the same insulating material, which had been grown on a sapphire surface. The bubble acts as a gentle, curved ramp that stretches the materials sitting on top of it. The researchers found that this setup, which applies a very small amount of stretching force, caused the light emitted by the stack to become roughly eight times brighter than it was in an unstretched version of the same material.

The experiment began with the careful creation of a microscopic bubble on a sapphire chip. Using a specialized growth technique, the team produced a bubble of hexagonal boron nitride that stood about 0.12 micrometers high with a base diameter of 6.25 micrometers. They then used a precise, dry-transfer method to pick up a single layer of molybdenum diselenide and place it over the bubble, followed by a thin spacer layer and a single layer of rhenium disulfide on top. The spacer was about 12 nanometers thick, a distance chosen specifically to stop electric charges from jumping between the layers while still allowing energy to pass through. When they examined the sample, they saw that the layers on top of the bubble were under a very slight tension, stretching the atomic bonds by approximately 0.1 percent.

To understand what was happening, the researchers first looked at how the material responded to light without the top layer in place. They shone a laser on the sample and observed that the light coming from the molybdenum diselenide was significantly brighter at the very top of the bubble compared to the flat area nearby. This happened because the stretching created a gentle slope in the energy landscape of the material. Excitons, which are pairs of electrons and holes that carry energy, naturally drift toward the area of highest stretch, much like water flowing downhill to the lowest point. This accumulation of energy at the peak of the bubble increased the local population of excitons, making the light emission stronger.

When the researchers added the top layer of rhenium disulfide to complete the stack, the effect became even more pronounced. The rhenium disulfide absorbed the incoming laser light and transferred that energy to the molybdenum diselenide below it. In the flat, unstretched part of the sample, this transfer resulted in a modest increase in brightness. However, on the bubble, where the material was stretched, the brightness jumped by a factor of roughly seven to eight compared to the single layer alone. The stretching did two things simultaneously: it helped funnel the excitons to the center of the bubble, and it increased the strength of the interaction between the two layers, making the energy transfer more efficient.

The team was careful to rule out other possible explanations for this dramatic increase in brightness. One common concern in such experiments is that the curved shape of the bubble might act like a lens or a mirror, trapping light and making it appear brighter through an optical interference effect. To test this, the researchers used computer simulations to model how light would behave in the stack. These calculations showed that the bubble's shape would only cause a negligible change in how much light was absorbed or reflected. This confirmed that the massive boost in brightness was not an optical trick but a genuine change in how the material handled energy.

Further investigation into the speed of the light emission provided additional clues. By measuring how quickly the excited states decayed after being hit by a laser pulse, the researchers observed that the stretched material on the bubble recombined its energy faster than the unstretched material. This faster decay indicated that the stretched environment was helping the excitons release their energy as light more efficiently. Theoretical calculations supported these findings, showing that the stretching reduced the energy gap between the material's electronic states and increased the strength of the dipole moments, which are the fundamental drivers of light emission.

This work demonstrates that a very small amount of mechanical strain, applied in a controlled way, can be a powerful tool for enhancing the performance of two-dimensional materials. The researchers showed that by simply placing a stack of atomic layers on a tiny bubble, they could create a localized region where light emission is significantly boosted. The results suggest that this approach could be scaled up or refined to create more efficient light-emitting devices. Because the effect relies on the interaction between layers, the researchers noted that using materials that absorb more light to begin with could lead to even greater gains. The study provides a clear, practical path for engineers to tune the optical properties of these materials without complex fabrication, opening new possibilities for future optoelectronic applications.