🔬 materials science

The Hidden Cost of Alloying: Disorder-Driven Transport Collapse in TMDs

This study reveals that while alloying in MoS2xSe2(1-x) continuously tunes optical properties, it causes a severe, disorder-driven collapse in carrier transport at intermediate compositions due to random chalcogen substitution creating scattering barriers and trapping sites, highlighting a critical limitation overlooked by conventional equilibrium probes.

Original authors: Michele Pisarra, Clara Rebanal, Enrique Arévalo Rodríguez, Marc Meléndez, Elena Blundo, Giacomo Amadore, Jonathan J. Finley, Fabián Calleja, Marc G. Cuxart, Jesús Álvarez, Ma-ría José Capitán, Fernand

Published 2026-09-09

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CC BY 4.0

Original authors: Michele Pisarra, Clara Rebanal, Enrique Arévalo Rodríguez, Marc Meléndez, Elena Blundo, Giacomo Amadore, Jonathan J. Finley, Fabián Calleja, Marc G. Cuxart, Jesús Álvarez, Ma-ría José Capitán, Fernando J. Urbanos, Julia García Pérez, Ramón Bernardo Gavito, Daniel Granados, Ji Dai, Massimo Tallarida, Antonello Sindona, Fernando Martín, Ferry Prins, Iolanda Di Bernardo, Amadeo L. Vázquez de Parga

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

Imagine a world built from sheets of atoms, so thin they are essentially two-dimensional, yet capable of carrying electricity and light with remarkable efficiency. These materials, known as transition metal dichalcogenides, have become a cornerstone for the next generation of electronics, promising devices that are faster, smaller, and more energy-efficient than anything currently possible. Scientists have long sought to tune the properties of these sheets, much like a musician tuning a string, to make them absorb or emit light at specific colors. One of the most promising methods for this tuning is alloying, a process where researchers mix two different versions of the material together. By swapping one type of atom for another within the crystal lattice, they can theoretically create a continuous spectrum of colors and electronic behaviors without introducing the defects that usually ruin performance. For years, the assumption has been that if the light coming out of these mixed materials looks smooth and predictable, the electricity flowing through them must be just as reliable.

A team of researchers set out to test this assumption by creating a series of these mixed materials, specifically combining molybdenum disulfide and molybdenum diselenide in varying ratios. They examined samples ranging from a single atomic layer to thicker stacks, using a suite of advanced tools to peer inside the material. They looked at how the atoms were arranged, how the material vibrated, and how it absorbed and emitted light. The results from these standard measurements were exactly what the scientific community expected to see. The energy levels of the electrons shifted smoothly and predictably as the ratio of sulfur to selenium changed. The material behaved like a well-ordered, high-quality semiconductor, with no signs of the chaotic disorder that usually plagues mixed crystals. The light it emitted changed color in a perfectly linear fashion, suggesting that the internal structure was pristine and that the material was ready for use in high-performance devices.

However, when the researchers switched their focus from the light to the movement of the electrical charges themselves, the story changed dramatically. Using a technique that tracks how quickly excited electrons spread out across the surface of the material, they discovered a hidden flaw. While the light suggested a smooth path, the electrons found a landscape littered with invisible obstacles. In the pure materials, the charges flowed freely, but as soon as the two types of atoms were mixed, the ability of the electrons to move collapsed. This drop in mobility was not gradual; it was severe and asymmetric, meaning the material behaved very differently depending on which way the mixing was done. When the researchers added a small amount of sulfur to a selenium-rich material, the electrons encountered high-energy barriers that scattered them like billiard balls, slowing them down. Conversely, when they added selenium to a sulfur-rich material, the electrons fell into deep energy traps, getting stuck in place and halting their movement entirely.

The researchers used computer simulations to understand why this happened. They modeled the atomic landscape and found that even though the average structure looked perfect, the random swapping of atoms created a jagged, uneven terrain of energy levels on a microscopic scale. In some regions, the energy was too high for an electron to pass without a struggle; in others, it was so low that the electron would get stuck. This disorder was invisible to the standard optical tests that measure the average energy of the material, which is why the light looked so clean while the electricity struggled to flow. The study reveals that in these ultra-thin alloys, the ability to tune the color of the light does not guarantee the ability to transport electricity efficiently. The findings suggest that for future devices relying on these materials, simply mixing atoms to get the right color is not enough; engineers must also account for the hidden, chaotic energy landscape that can silently cripple the flow of information.

Technical Summary: The Hidden Cost of Alloying in TMDs

Problem Statement
Transition metal dichalcogenide (TMD) alloys, such as MoS $_{2x}$ Se $_{2(1-x)}$ , are widely utilized for bandgap engineering in optoelectronics and layered semiconductor technologies. The prevailing assumption is that isovalent alloying allows for continuous, disorder-tolerant tuning of optical transitions without significantly degrading electronic quality. While vibrational spectroscopies and optical measurements suggest smooth spectral tunability, the implications of random chalcogen substitution on charge and energy transport remain poorly understood. Specifically, it is unclear whether the energetic disorder introduced by alloying creates invisible bottlenecks that decouple bandgap tunability from carrier transport efficiency.

Methodology
The authors employed a multi-modal approach combining experimental characterization, time-resolved transport measurements, and first-principles simulations to investigate MoS $_{2x}$ Se $_{2(1-x)}$ alloys across varying stoichiometries and thicknesses (1 to 10 layers).

Structural and Chemical Characterization: Synchrotron-based photoemission spectroscopy (XPS) and Raman spectroscopy were used to verify stoichiometry and homogeneity. Scanning tunneling microscopy (STM), X-ray diffraction (XRD), and scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDX) confirmed the absence of phase segregation, grain boundaries, or Janus structures, establishing the samples as homogeneous alloys.
Electronic Structure Probing: Photoluminescence (PL) spectroscopy tracked excitonic transitions (A and I excitons) as a function of composition and layer number. Angle-resolved photoemission spectroscopy (ARPES) mapped valence band dispersions to observe changes in band edges and spin-orbit splitting.
Transport Measurements: Transient scattering microscopy was utilized to perform time-resolved spatial mapping of photoexcited carriers. This technique measured the spatial variance of carrier distributions to extract initial diffusivity values before trapping mechanisms dominated.
Theoretical Modeling: Density Functional Theory (DFT) calculations were performed to analyze atom-projected density of states (DOS) and optical absorption. Additionally, numerical simulations modeled carrier transport as Brownian walkers interacting with a stochastic energetic landscape generated by random atomic substitution.

Key Results

Adiabatic Evolution of Electronic Structure:
- Optical and Band Structure: Both PL and ARPES data revealed that the electronic structure evolves smoothly and adiabatically with composition and thickness. The optical bandgap shows negligible bowing ( $\beta \approx 0$ ) across the composition range.
- Exciton Behavior: The A exciton (direct gap) energy is dominated by composition, shifting by ~~270 meV from MoS $_2$ to MoSe $_2$ , with negligible thickness dependence. Conversely, the I exciton (indirect gap) exhibits strong thickness dependence (~~200 meV shift from 2 to 5 layers) but weaker composition dependence. This is attributed to the orbital character: A excitons involve in-plane orbitals ( $d$ , $p_x$ , $p_y$ ) localized within layers, while I excitons involve out-of-plane orbitals ( $d$ , $p_z$ ) sensitive to interlayer coupling.
- Valence Band: The valence band maximum at the $\Gamma$ point remains stationary, while the K-point maximum shifts systematically with composition, altering the direct-indirect gap separation. Spin-orbit splitting at the K point decreases with increasing Se content.
Disorder-Driven Transport Collapse:
- Asymmetric Diffusivity Suppression: In stark contrast to the smooth spectral evolution, carrier diffusivity exhibits a pronounced and asymmetric collapse at intermediate compositions.
- Directional Dependence: The nature of the disorder depends on the alloying direction.
  - MoSe $_2$ -rich (S-doping): Substituting Se with S creates high-energy sites that act as scattering barriers, increasing scattering events and reducing mobility.
  - MoS $_2$ -rich (Se-doping): Substituting S with Se creates low-energy sites that act as deep trapping centers, effectively halting carrier diffusion.
- Decoupling: This transport degradation occurs despite the absence of discontinuities in the average band structure or valence-band dispersion measured by PL and ARPES.

Significance and Claims
The paper establishes that while chemical composition and thickness provide powerful control over the average electronic structure of TMD alloys, carrier transport is governed by a fundamentally different principle: the emergence of disorder-induced energetic landscapes.

Invisible Bottleneck: The study demonstrates that energetic disorder creates real-space fluctuations (scattering barriers or deep traps) that are invisible to conventional equilibrium spectroscopic probes (PL, ARPES) but catastrophic for transport.
Metric of Quality: Transport measurements are proposed as a more stringent metric of electronic quality than bandgap tunability alone, as they are the first to reveal the collapse of performance due to disorder.
Design Rules: The findings highlight intrinsic limitations in using TMD alloys for devices requiring both spectral control and efficient carrier propagation. The results suggest that for applications relying on carrier diffusion, the direction of alloying (S-to-Se vs. Se-to-S) is critical, as the resulting energetic landscape differs significantly in its impact on carrier localization versus scattering.

The authors conclude that the "hidden cost" of alloying is the generation of a complex energetic landscape that can severely degrade transport properties even in samples that appear electronically homogeneous and spectrally tunable.

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