Imagine electronic circuits where the components are made not of silicon, but of a seamless sheet of atoms just one layer thick. This is the revolutionary promise of lateral heterojunctions within monolayer semiconductors.
The world of materials science is undergoing a quiet revolution, moving from three-dimensional blocks to two-dimensional sheets. At the forefront of this revolution are lateral heterojunctions—atomically thin circuits where different semiconductors are seamlessly stitched together within a single molecular layer.
Circuits built in a single layer of atoms, enabling unprecedented miniaturization.
Lower power consumption compared to traditional silicon-based electronics.
Enables applications in wearable technology and transparent displays.
These structures are not just scientific curiosities; they form the foundation for developing next-generation electronics that are thinner, more flexible, and more energy-efficient than anything possible with silicon today 1 . By enabling precise band engineering within a 2D plane, they open up new realms in materials science, device physics, and engineering 3 .
To appreciate the breakthrough of lateral heterojunctions, one must first understand the materials that make them possible. Transition metal dichalcogenides, or TMDs, are a family of compounds with properties that make them ideal for this purpose. Common examples include MoS₂ (molybdenum disulfide), MoSe₂ (molybdenum diselenide), WS₂ (tungsten disulfide), and WSe₂ (tungsten diselenide) .
When these materials are thinned down to a single molecular layer, they exhibit remarkable electronic and optical properties distinct from their bulk forms. For instance, while bulk MoS₂ has an indirect bandgap of about 1.2 eV, monolayer MoS₂ has a direct bandgap of 1.85 eV, making it highly efficient for absorbing and emitting light 1 6 .
What makes lateral heterostructures particularly intriguing is the type-II band alignment that naturally forms at the junction between different TMDs. In this arrangement, the conduction band minimum resides in one material (e.g., MoSe₂) while the valence band maximum resides in the other (e.g., WSe₂) 5 .
This creates a built-in electric field that efficiently separates electrons and holes, making these junctions exceptionally good for photovoltaics and photodetection 6 .
Creating seamless junctions between different semiconductors within a single atomic layer requires remarkable precision. Researchers have developed two primary approaches to achieve this, both yielding high-quality results.
Begin with a pristine MoSe₂ monolayer crystal synthesized on a substrate.
Use electron beam lithography to protect certain areas with SiO₂ while leaving others exposed.
Transform exposed areas to MoS₂ using pulsed laser vaporization of sulfur at ~700°C.
Reveal perfectly defined heterojunctions with the same crystal orientation throughout.
This approach effectively uses the original crystal as a template, maintaining the same crystal orientation throughout the entire structure 1 .
Grow the first material (e.g., WS₂) using chemical vapor deposition, forming triangular domains.
Change chalcogen source without breaking vacuum (e.g., selenium instead of sulfur).
Epitaxially extend the new material (e.g., WSe₂) from the edges of the original triangles.
The success of this method hinges on the small lattice mismatch (approximately 4%) between materials like WS₂ and WSe₂, which enables coherent epitaxial growth .
Raman and photoluminescence mapping clearly show the resulting compositional modulation, with distinct optical signatures from the inner and outer regions of the triangular domains .
| Feature | Selective Conversion Method | Lateral Epitaxial Growth Method |
|---|---|---|
| Process Principle | Patterned conversion of existing monolayer | Sequential growth of different materials |
| Key Advantage | Enables complex, predefined patterns | Creates naturally seamless junctions |
| Materials Demonstrated | MoSe₂ to MoS₂, WSe₂ to WS₂ | MoS₂–MoSe₂, WS₂–WSe₂ |
| Interface Sharpness | ~5 nanometer transition width | Atomically sharp interfaces |
| Pattern Flexibility | High (using lithography) | Limited to growth-front determined patterns |
While both methods produce impressive results, the selective conversion approach stands out for its potential for high-density integration. The 2015 experiment published in Nature Communications represents a crucial step toward practical applications 1 .
The researchers began with high-quality monolayer MoSe₂ crystals with lateral sizes ranging from 10 to 100 micrometers, synthesized using a vapor phase transport method 1 .
Temperature control proved critical to the process. Below 600°C, the conversion was incomplete, leading to intermediate alloys rather than full conversion to MoS₂. Above 800°C, the crystals began to decompose, indicating the narrow window for optimal results 1 .
The success of the conversion process was unequivocally demonstrated through multiple characterization techniques. Raman mapping showed clear spatial separation between the MoSe₂ regions (showing a characteristic peak at 238 cm⁻¹) and the converted MoS₂ regions (showing a peak at 403 cm⁻¹) 1 .
Perhaps the most compelling evidence came from atomic-resolution Z-contrast STEM imaging, which revealed that both the pristine MoSe₂ and converted MoS₂ regions maintained the same honeycomb lattice structure 1 .
The interface between the two materials showed a transition region of approximately 5 nanometers, identified as a MoSₓSe₁ₓ ternary alloy with a composition gradient 1 .
| Material | Raman Peaks | Photoluminescence |
|---|---|---|
| MoSe₂ | E₁₂g mode at 238 cm⁻¹ | ~1.55 eV |
| MoS₂ | E₁₂g mode at 403 cm⁻¹ | ~1.85 eV |
| WSe₂ | A₁g mode at 256 cm⁻¹ | ~1.65 eV |
| WS₂ | A₁g at 419.3 cm⁻¹, E₁₂g at 355.4 cm⁻¹ | ~1.65 eV |
The researchers took their analysis a step further by fabricating devices to measure electrical transport across the heterojunctions 1 . Electrodes were strategically placed on both the MoSe₂ and converted MoS₂ regions of the same crystal, confirming that both materials remained semiconducting and that the junction between them displayed the rectifying behavior characteristic of a diode.
Creating and studying lateral heterojunctions requires specialized materials and instruments. Below is a breakdown of the key components in the researcher's toolkit.
Provide high-quality monolayer semiconductors
Mechanically exfoliated flakes, CVD-grown MoSe₂, WSe₂, MoS₂, WS₂ crystals
Enable formation of heterojunctions
Tube furnaces for CVD, pulsed laser vaporization systems, precision temperature controllers
Define regions for selective conversion
Electron beam lithography systems, photolithography equipment, SiO₂ deposition tools
Verify structure and composition
Raman spectroscopy, photoluminescence mapping, atomic force microscopy (AFM)
Reveal atomic structure and interface quality
High-resolution STEM, HAADF-STEM, TEM, electron diffraction
Create functional electronic devices
Electron beam evaporators for electrodes, transfer systems for van der Waals assembly
Lateral heterojunctions represent more than just a laboratory curiosity—they offer a tangible path toward ultrathin, flexible, and transparent electronics that could transform our relationship with technology.
Perhaps the most exciting development is the recent discovery of unique quantum phenomena at these 1D interfaces. Studies have revealed a new type of dipolar interfacial exciton that resides exclusively at the boundary between two monolayer semiconductors 5 .
These excitons possess remarkable properties, including large in-plane dipole moments and narrow discrete emission states, suggesting potential applications in quantum information processing and nonlinear optics 5 .
The ability to pattern different semiconductors within a single atomic layer enables the miniaturization of electronic circuits to their fundamental limits while introducing novel functionalities 1 . The journey from three-dimensional blocks to two-dimensional sheets represents not just a change in dimension, but a fundamental rethinking of what's possible in electronics.
The work of stitching together different semiconductors within a single atomic layer, once confined to theoretical speculation, has now become an experimental reality—and it promises to reshape our technological landscape in the decades to come.