🔬 applied physics

Trion Engineered Multimodal Transistors in Two dimensional Bilayer Semiconductor Lateral Heterostructures

This study demonstrates the development of trion-engineered multimodal transistors in bilayer 2D lateral npn heterostructures, which enable precise electrical and optical control over exciton and trion dynamics to achieve dynamic photoresponsivity tuning and advance practical optoelectronic and quantum technologies.

Original authors: Baisali Kundu, Poulomi Chakrabarty, Avijit Dhara, Roberto Rosati, Chandan Samanta, Suman K. Chakraborty, Srilagna Sahoo, Sajal Dhara, Saroj P. Dash, Ermin Malic, Saurabh Lodha, Prasana K. Sahoo

Published 2026-07-24

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CC BY 4.0

Original authors: Baisali Kundu, Poulomi Chakrabarty, Avijit Dhara, Roberto Rosati, Chandan Samanta, Suman K. Chakraborty, Srilagna Sahoo, Sajal Dhara, Saroj P. Dash, Ermin Malic, Saurabh Lodha, Prasana K. Sahoo

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 where the tiny chips inside your phone and computer are built from materials so thin they are essentially flat sheets, just one atom thick. Scientists call these "two-dimensional" materials, and they are like a magical playground for electronics because they can conduct electricity and react to light in very special ways. In this playground, when light hits the material, it doesn't just create free-floating electrons; it creates tiny, bound pairs of an electron and a "hole" (a missing electron) that stick together like best friends. These pairs are called excitons. Sometimes, these pairs grab an extra friend (an extra electron or hole) and become a trio called a trion. Think of excitons as a couple dancing, and trions as a trio that is a bit heavier and harder to move around.

Why does this matter? Because if we can control how these dancing couples and trios move, we can build smarter, faster, and more efficient devices. We could create sensors that see different colors of light, or computers that process information using light instead of just electricity. However, controlling these tiny dancers is tricky. Usually, scientists have to build complex, stacked structures or use strong external forces to make them behave the way they want. The big question has been: Can we make these materials do something surprising and useful just by changing the color of the light hitting them and tweaking a simple electrical knob?

This paper takes a peek into that question by building a special kind of electronic device using a "lateral heterostructure." Imagine taking two different types of these ultra-thin materials—MoSe₂ and WSe₂—and stitching them together side-by-side to form a long, flat strip. The researchers created a specific pattern: a strip of MoSe₂, followed by a strip of WSe₂, and then another strip of MoSe₂, forming an "n-p-n" sandwich. They then shined different colors of laser light onto the middle section and watched what happened to the electrical current.

What they found was a delightful surprise. By simply changing the color (wavelength) of the laser light and adjusting the electrical voltage, they could make the device switch its behavior like a magic trick. When they used a specific color of light (around 710 nanometers), the device actually became less conductive when illuminated, a phenomenon called "negative photoconductivity." It's as if shining a flashlight on a road made traffic slow down instead of speed up. But when they switched to a slightly different color (around 830 nanometers), the device did the opposite: the light made it conduct electricity much better, known as "positive photoconductivity."

The team discovered that this switch happens because of the different "dancers" involved. When the light is tuned to a specific energy, it creates a lot of those heavy "trion" trios. Because trions are heavy and sluggish, they clog up the flow of electricity, causing the current to drop (the negative effect). However, when the light is tuned to a lower energy, it helps release trapped charges from hidden "traps" in the material, freeing up more dancers to move quickly and boosting the current (the positive effect). The researchers used a mathematical model based on the Saha equation to show that this competition between creating heavy trions and freeing up trapped charges explains exactly why the current goes up or down.

They also confirmed that this isn't just a fluke of one specific metal contact or a weird environmental glitch; they tested the device with different metals and saw the same result. The device is so sensitive it can detect near-infrared light, which is invisible to the human eye, and it works even at room temperature. This study suggests that by carefully designing these flat, side-by-side material strips, we can create electronic switches that are controlled by light color and voltage. This could pave the way for new types of sensors, memory devices, and logic gates that are incredibly efficient and capable of handling complex tasks on a single chip, all by mastering the dance of excitons and trions.

Technical Summary: Trion-Engineered Multimodal Transistors in Two-dimensional Bilayer Semiconductor Lateral Heterostructures

Problem Statement
The integration of two-dimensional (2D) semiconductors into on-chip electronics, photonics, and quantum technologies requires precise control over carrier dynamics, specifically the generation and transport of excitons and trions. While traditional methods for engineering excitons in 2D materials rely on artificially assembled vertical p-n heterostructures or external strain/electric field confinements, these approaches often lack the intrinsic spatial separation of energy landscapes needed for fine-tuned functionality. Furthermore, the microscopic origins of negative photoconductivity (NPC) in 2D transistors remain elusive, with previous attributions often linked to environmental factors or metastable traps rather than intrinsic carrier physics. There is a critical need to understand and manipulate the transition between positive photoconductivity (PPC) and NPC to enable multimodal optoelectronic devices capable of logic operations, memory storage, and versatile sensing within a single configuration.

Methodology
The study utilizes bilayer (2L) lateral heterostructures (LHS) composed of an n-p-n multi-junction architecture: MoSe₂-WSe₂-MoSe₂. These devices were synthesized via a one-pot water-assisted chemical vapor deposition (CVD) process, allowing for the in-situ switching of carrier gases to grow distinct MoSe₂ and WSe₂ domains with sharp interfaces.

Device Fabrication: The researchers fabricated field-effect transistors (FETs) with a narrow WSe₂ channel (~2 µm) sandwiched between two MoSe₂ regions. A thick hexagonal boron nitride (hBN) layer was partially transferred to insulate specific domains for electrical characterization.
Characterization: The structural quality was verified using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and Raman spectroscopy. Photoluminescence (PL) measurements were conducted at both room temperature and cryogenic temperatures (4 K) to resolve excitonic and trionic states.
Optoelectronic Testing: The devices were subjected to tunable laser excitation (690 nm to 850 nm) focused on the central WSe₂ domain. Photoresponsivity was measured under varying source-drain bias ( $V_d$ ) and gate voltage ( $V_g$ ) to map the transition between PPC and NPC.
Theoretical Modeling: A microscopic model based on the Saha equation was developed to describe the interplay between gate-controlled doping, optical excitation, and the formation of excitons and trions. This model coupled charge conservation laws with the binding energy of trions to simulate the observed photocurrent behaviors.

Key Contributions

Intrinsic Exciton Engineering: The paper demonstrates that bilayer lateral n-p-n heterostructures possess intrinsically spatially separated energy landscapes that facilitate preferential exciton generation and manipulation without external confinement.
Multimodal Photoresponse Control: The study achieves a dynamic, reversible tuning of channel photoresponsivity from positive (PPC) to negative (NPC) by adjusting electrical bias ( $V_g$ ) and incident photon energy ( $\lambda_p$ ).
Identification of Trion-Driven NPC: The research identifies trions (charged excitons) as the primary mechanism for NPC in these devices. The formation of trions, which have higher effective mass and lower mobility than free carriers, reduces the overall photocurrent.
Trap State Characterization: Cryogenic PL measurements revealed intrinsic trap states in WSe₂ and trion states in bilayer MoSe₂, explaining the device's sensitivity to near-infrared photons and the specific wavelength ranges where PPC or NPC dominates.

Results

Structural and Optical Properties: HAADF-STEM confirmed sharp, defect-free interfaces between 2L MoSe₂ and 2L WSe₂. PL spectra at 4 K resolved neutral excitons ( $X^0$ ) and trions ( $X^-$ ) in MoSe₂ (separated by ~23 meV) and distinct trap-related emission bands in WSe₂.
Photoresponse Switching:
- In the n-p-n configuration, illumination at 830 nm (below the WSe₂ resonance) resulted in PPC, attributed to the detrapping of carriers from low-lying trap states in WSe₂.
- Illumination at 710 nm (above resonance) induced NPC. Theoretical modeling and experimental data suggest that high-energy photons generate excitons in WSe₂, which diffuse to the MoSe₂ interface and bind with free electrons to form immobile trions, thereby reducing conductivity.
- The transition between PPC and NPC is tunable via gate voltage. Positive $V_g$ enhances electron density in MoSe₂, promoting trion formation and NPC, while negative $V_g$ suppresses trion formation, favoring PPC.
Performance Metrics: The devices exhibited a high signal-to-noise ratio (up to $10^6$ ) and a maximum specific detectivity ( $D^*$ ) of $1.92 \times 10^{10}$ Jones. The photoresponse showed distinct power-dependence behaviors: linear for 710 nm excitation (trion-dominated) and saturating for 830 nm excitation (trap-state dominated).
Reproducibility: Consistent NPC-PPC switching was observed across multiple devices with different metal contacts (Ti/Au and Ti/Pt/Au), confirming the effect is intrinsic to the lateral heterostructure rather than contact-dependent.

Significance and Claims
The paper claims to provide a fundamental understanding of the microscopic origins of NPC and PPC in 2D lateral heterostructures, specifically attributing NPC to trion formation rather than environmental artifacts. By leveraging the unique geometry of n-p-n lateral junctions, the authors demonstrate a new degree of freedom for controlling excitonic characteristics through electrical bias and photon energy.

The findings are presented as a pathway toward developing practical, room-temperature exciton-based transistors, sensors, and multimodal optoelectronic devices. The ability to electrically manipulate trion generation and transport suggests potential applications in on-chip logic gates, multi-bit memory, and neuromorphic computing. The study emphasizes that these devices offer a robust platform for resolving complex carrier dynamics (excitons, trions, and traps) that are typically difficult to distinguish using optical probing alone at room temperature.

Technical Summary: Trion-Engineered Multimodal Transistors in Two-dimensional Bilayer Semiconductor Lateral Heterostructures

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