Dipolar interlayer excitons in transition metal dichalcogenide alloy heterobilayers

This study reports the observation of long-lived dipolar interlayer excitons in a MoS$_{1.4}$Se$_{0.6}$/MoSe$_2$ heterobilayer, demonstrating their potential as a versatile platform for engineering and tuning excitonic interactions in van der Waals materials.

The Gist

Imagine a world made of ultra-thin, atomic sheets of material, like layers of a very delicate sandwich. In this paper, scientists built a special sandwich using two different types of these sheets: one made of a mix of Molybdenum, Sulfur, and Selenium, and the other made of Molybdenum and Selenium. They wrapped this whole thing in a protective "armor" made of hexagonal boron nitride to keep it clean and stable.

Here is what they discovered, explained simply:

The "Long-Distance" Couple

Usually, when you shine light on these materials, an electron (a negative particle) and a "hole" (a positive spot where an electron used to be) get excited and stick together right next to each other. Think of them as a couple holding hands.

But in this specific sandwich, something different happens. Because of how the two layers are stacked, the electron jumps to the top layer, while the hole stays in the bottom layer. They are now stuck in different rooms of the same house.

• The Analogy: Imagine a couple where one partner is on the first floor and the other is on the second floor. They can still "see" each other and are attracted, but they are separated by a floor. This creates a "long-distance relationship" that lasts a long time because they can't easily hug (recombine) and disappear. In physics, this is called an interlayer exciton, and because they are separated, they act like tiny magnets with a permanent north and south pole (a dipole).

The "Blue Shift" Dance

The scientists shone a laser on their sandwich to create many of these long-distance couples. They noticed something interesting: as they turned up the brightness of the laser (creating more couples), the color of the light these couples emitted changed.

• The Analogy: Imagine a crowded dance floor. When there are only a few dancers, they move freely. But as the room gets packed, everyone starts bumping into each other. Because these "couples" have magnetic poles, they actually push each other away (repel). As the crowd gets denser, this pushing force makes the energy of the system go up. In light, higher energy means the color shifts toward the blue end of the spectrum. The scientists saw this "blue shift," which proved that these particles were indeed pushing against each other like magnets.

The "Slow Motion" Glow

Finally, they measured how long these excited couples lasted before they finally came together and stopped glowing.

• The Analogy: Most couples in these materials hug and disappear in a fraction of a second (picoseconds). But these long-distance couples are like a slow-motion movie. They stayed together for nanoseconds—which is a million times longer than usual.
• Why? Because they are separated by a floor (different layers), it's much harder for them to find each other and "kiss" (recombine). The paper found that some of these couples lasted for nearly 50 nanoseconds, which is a very long time in the atomic world. This confirms they are truly separated and "dipolar."

The Big Picture

The main takeaway is that by mixing different ingredients (alloying) in these atomic sheets, the scientists created a new, controllable environment. They proved that they can make these "long-distance" magnetic couples, watch them push against each other, and see them live for a surprisingly long time. This shows that mixing these materials is a great way to build new types of atomic playgrounds where scientists can study how these tiny particles interact with one another.

Technical Summary

Technical Summary: Dipolar Interlayer Excitons in Transition Metal Dichalcogenide Alloy Heterobilayers

Problem and Motivation
Transition metal dichalcogenide (TMD) heterobilayers with type-II band alignment offer a platform for studying excitonic many-body physics due to the formation of spatially indirect interlayer excitons (IXs). These excitons possess permanent electric dipole moments and long recombination lifetimes, facilitating the investigation of phenomena such as exciton transport, condensation, and strongly correlated states. While previous research has explored interlayer excitons in binary TMD heterostructures and those involving transition metal-alloyed compounds, the realization and characterization of dipolar interlayer excitons in heterostructures based specifically on chalcogen-alloyed monolayers remained unexplored. The authors aimed to address this gap by engineering a heterobilayer system where chalcogen alloying could provide additional degrees of freedom for tuning band alignment and excitonic interactions.

Methodology
The study utilized a heterobilayer composed of a MoS$_{1.4}$Se$_{0.6}$ alloy and a MoSe$_2$ monolayer, encapsulated within hexagonal boron nitride (hBN) to ensure optical quality. The MoS$_{1.4}$Se$_{0.6}$ alloy was obtained from bulk crystals and mechanically exfoliated, then deterministically transferred onto MoSe$_2$ monolayers.

• Structural Characterization: Polarization-resolved second-harmonic generation (P-SHG) microscopy was employed to determine the relative crystallographic orientation. The system exhibited a mean twist angle of approximately 3.2° (standard deviation $\sigma = 0.34^\circ$), indicating a near-aligned configuration sufficient to minimize momentum mismatch at band edges.
• Optical Spectroscopy: Low-temperature (78 K) photoluminescence (PL) spectroscopy was conducted under 532 nm excitation to probe excitonic transitions. Photoluminescence excitation (PLE) spectroscopy was used to map the emission intensity as a function of excitation energy.
• Power and Time-Resolved Analysis: PL measurements were performed across a range of excitation powers (20 $\mu$W to 1.8 mW) to analyze intensity scaling and energy shifts. Time-resolved photoluminescence (TRPL) using time-correlated single-photon counting (TCSPC) with 375 nm pulsed excitation (52 ps width) was utilized to measure carrier dynamics.

Key Results

1. Identification of Interlayer Excitons: PL spectra of the heterobilayer revealed a distinct low-energy emission peak at $\sim$1.4 eV, which was absent in the individual MoSe$_2$ and MoS$_{1.4}$Se$_{0.6}$ monolayers. PLE spectroscopy confirmed that this emission is resonantly enhanced when the excitation energy matches the intralayer exciton transitions of both constituent materials, providing strong evidence that the emission originates from interlayer excitons formed via charge transfer.
2. Dipolar Nature and Interactions: The IX emission exhibited a systematic blueshift as excitation power increased. This behavior is attributed to repulsive dipole–dipole interactions between the spatially separated electron-hole pairs, a characteristic signature of dipolar excitons.
3. Sublinear Scaling: The intensity of the IX emission followed a sublinear power-law dependence ($I \propto P^b$) with exponents $b \approx 0.4$ at low powers and $b \approx 0.3$ at higher densities. This scaling indicates nonlinear exciton dynamics, likely driven by exciton–exciton annihilation processes facilitated by the long lifetime of the interlayer excitons.
4. Long-Lived Dynamics: TRPL measurements revealed a biexponential decay profile. A fast component ($\tau_1 \approx 374$ ps) was associated with the redistribution of initially populated bright or weakly localized states. A significantly slower component ($\tau_2 \approx 49$ ns) was observed, consistent with radiative recombination from a reservoir of semi-dark or localized states. This long lifetime supports the spatially indirect nature of the excitons and the reduced electron–hole wavefunction overlap, further influenced by the finite momentum mismatch due to the $\sim$3° twist angle.

Significance and Claims
The paper establishes chalcogen-alloyed TMD heterobilayers as a versatile platform for engineering dipolar excitons. The authors claim that the combination of MoS$_{1.4}$Se$_{0.6}$ and MoSe$_2$ successfully creates a type-II band alignment that promotes the formation of dipolar electron–hole pairs. The study demonstrates that alloying offers a method to tailor band offsets and exciton energies, thereby allowing controlled tuning of exciton localization and many-body effects. The observation of long-lived, dipolar interlayer excitons in this alloy system suggests that such heterostructures can be used to tune excitonic interactions and recombination dynamics in van der Waals materials, providing a foundation for exploring excitonic many-body physics in alloyed 2D systems.