A new method provides precise, layer-by-layer control over the growth of tungsten disulfide (WS₂), a two-dimensional material with significant potential for next-generation electronics. The process, which uses a carbon-assisted chemical vapor deposition (CVD) technique, overcomes persistent challenges in manufacturing high-quality, atomically thin semiconductors, enabling the production of bilayer WS₂ sheets with high yields and repeatability. This enhanced control over the material’s thickness is critical for developing advanced electronic and optoelectronic devices that surpass the limitations of silicon-based technology.
The breakthrough addresses a key bottleneck in the practical application of transition metal dichalcogenides (TMDs), a class of 2D materials to which WS₂ belongs. While single-layer TMDs offer high charge carrier mobility and multilayers provide a high density of states, bilayer structures strike a crucial balance between these properties. However, reliably producing uniform bilayer crystals has been difficult, limiting their use in high-performance electronics. The new carbon-assisted process facilitates the vertical, layer-by-layer stacking of WS₂, creating high-quality bilayer crystals with superior electronic properties, including carrier mobility as high as 58 cm²·V⁻¹·s⁻¹.
Mechanism of Controlled Growth
The synthesis technique relies on introducing carbon into the chemical vapor deposition process alongside tungsten trioxide (WO₃) and a sodium chloride promoter. Researchers found that carbon plays a dual role as both a reducing agent and a catalyst. It preferentially reacts with the tungsten precursor to form intermediate tungsten suboxide products (WO₃₋ₓ) with a lower valence state, which are more easily sulfurized to create the initial WS₂ monolayer.
Once the first layer of WS₂ is formed, the resulting oxycarbide gas generated during the reaction has a low surface adsorption energy. This allows it to deposit onto the surface of the as-grown WS₂, creating nucleation sites for the next layer to form. This mechanism encourages vertical stacking rather than lateral expansion, leading to controlled, layer-by-layer growth. Without the presence of carbon in the precursor mix, the process typically yields only single-layer triangular sheets of WS₂.
Advancing 2D Material Synthesis
The development of controllable synthesis methods is a major focus in materials science, aimed at producing 2D semiconductors for advanced technological applications. Various techniques, including metal-organic chemical vapor deposition (MOCVD) and solid-source CVD, are continuously being refined to grow highly crystalline TMDs. A separate but related hybrid MOCVD method combines liquid-phase metal precursor deposition with vapor-phase chalcogen delivery to achieve similar control, allowing for tunable morphologies from single-crystal domains to continuous monolayer films on different substrates like sapphire and silicon dioxide. Another approach has demonstrated the direct epitaxial growth of continuous WS₂ films on graphene substrates using tungsten hexacarbonyl and elemental sulfur as precursors. These parallel efforts highlight the intense demand for reliable methods to fabricate pristine 2D materials.
Properties of the Synthesized Material
Structural and Electronic Quality
Structural analysis confirms the high crystal quality of the WS₂ bilayers produced with the carbon-assisted method. Atomic force microscopy reveals continuous, uniform height steps from the substrate to the first monolayer and then to the second, confirming the layer-by-layer stacking characteristic. The thickness of the upper layer measures approximately 0.8 nanometers, consistent with a single layer of WS₂.
The electronic performance of the resulting material is among the best reported for TMDs produced via CVD. Electrical transport measurements of field-effect transistors fabricated from the bilayer WS₂ show a high carrier mobility and a small subthreshold swing of 148 millivolts per decade. These properties are essential for creating energy-efficient, high-performance transistors that can operate at low voltages, a key requirement for next-generation computing hardware. The absence of dangling bonds and the atomically smooth surfaces of these 2D materials give them excellent electrostatic control, making them prime candidates to replace silicon in future devices.
Implications for Future Electronics
As Moore’s Law approaches its physical limits for silicon-based transistors, 2D materials like WS₂ represent a promising path forward. Their unique properties—including ultrathin profiles, tunable band gaps, and high mobility—make them suitable for a wide range of applications, from flexible electronics and transparent displays to advanced sensors and quantum computing components. The ability to grow these materials with precise control over their atomic structure is fundamental to harnessing their full potential.
This carbon-assisted growth technique opens a new avenue for the scalable and repeatable synthesis of high-quality bilayer TMDs. By providing a clearer understanding of the atomic-scale growth mechanism, the research offers a robust methodology for fabricating wafer-scale 2D materials. This progress is a critical step toward the bottom-up fabrication of complex devices based on van der Waals heterostructures, where different 2D materials are stacked to create novel functionalities. Further refinement of such growth processes will accelerate the transition of these materials from laboratory curiosities to foundational components of commercial technologies.