2d Materials Synthesis Via Nano-Confinement Creates Tailored Monolayers and Janus Structures

The quest for precisely-structured two-dimensional materials has taken a significant step forward, as researchers demonstrate a new method for both creating and integrating these materials with atomic precision. Ce Bian, Yifan Zhao, and Roger Guzman, alongside colleagues at their institutions, have developed a technique using nano-confinement to direct the growth of transition metal dichalcogenides, such as molybdenum disulphide and niobium diselenide. This innovative approach not only allows for the creation of tailored material structures, ranging from isolated layers to large-scale films and patterned rings, but also simultaneously produces exceptionally clean interfaces between different two-dimensional materials. Crucially, the team successfully synthesised Janus monolayers with atomic accuracy and preserved the air-sensitive properties of niobium diselenide, enhancing its superconductivity, establishing a versatile platform for advanced applications in materials science and nanotechnology.

Van der Waals Heterostructures of 2D Materials

Research into two-dimensional (2D) materials, including transition metal dichalcogenides, continues to advance, driven by the potential for creating novel devices. A central theme involves constructing heterostructures by stacking different 2D materials, a process known as Van der Waals engineering, to tailor their properties. Graphene frequently serves as a crucial component, either as part of the heterostructure or as an encapsulating layer. Scientists are also actively exploring Janus materials, which possess distinct atomic arrangements on each layer, and investigating their unique electronic, optical, and catalytic characteristics.

Improving the stability of these 2D materials in everyday environments is a key challenge, and researchers are employing encapsulation techniques using materials like hexagonal boron nitride or graphene to protect them. Understanding and controlling how charges move and how excitons behave within these materials and heterostructures is crucial for many applications. This research aims to uncover and exploit novel properties arising from 2D materials, such as superconductivity, unique optical responses, and enhanced catalytic activity. Scientists are utilizing chemical vapor deposition and mechanical exfoliation to grow and obtain pristine 2D materials.

They characterize these materials using techniques like Raman spectroscopy and optical spectroscopy to understand their structure and properties. This work supports the development of applications in areas like superconductivity, optoelectronics, and spintronics. Recent advances include research into Ising superconductivity and the design of heterostructures for efficient charge separation in solar cells. The field is rapidly evolving, with a focus on creating and controlling heterostructures, understanding fundamental properties, and overcoming challenges related to stability and scalability.

Monolayer Growth via Nano-Confinement Demonstrated

Scientists have developed a precise method for growing atomically-thin layers of two-dimensional materials, specifically transition metal dichalcogenides. This innovative technique utilizes nano-confinement, employing graphene or hexagonal boron nitride as capping layers to control the growth process and create tailored shapes, ranging from isolated domains to large-scale continuous films and even intrinsically-patterned rings. Experiments demonstrate that 98% of the niobium diselenide crystals grown under these confined conditions are monolayers, a significant improvement over conventional growth methods. Cross-sectional scanning transmission electron microscopy confirms the formation of these monolayers, clearly identifying both the niobium diselenide and the graphene capping layer.

Raman spectroscopy further characterizes the nano-confined niobium diselenide, confirming its monolayer structure. Importantly, the nano-confinement technique simultaneously achieves in-situ encapsulation, protecting air-sensitive materials and markedly enhancing their stability. Measurements reveal that nano-confined niobium diselenide monolayers maintain sharp Raman signals even after 60 days of air exposure, while conventionally grown monolayers quickly degrade. This exceptional air stability broadens the application potential of these materials, enabling extensive characterization and reliable device integration. Theoretical calculations demonstrate that the binding energy of niobium diselenide monolayers is enhanced under nano-confinement, contributing to the observed stability and precise monolayer formation.

Capped Growth Yields Clean Janus Monolayers

Researchers have demonstrated a novel approach to synthesizing two-dimensional materials by employing graphene or hexagonal boron nitride as a protective capping layer during growth. This nano-confined environment directs the growth kinetics, enabling the precise formation of single-layer materials with controlled morphologies, ranging from isolated domains to large-scale, continuous films and even intrinsically patterned structures. Notably, the team successfully synthesized Janus monolayers, a configuration with distinct atomic arrangements on each layer, through precise chalcogen substitution protected by the van der Waals capping layer. The method simultaneously creates exceptionally clean interfaces between the 2D materials and the capping layer, effectively preserving the quality of air-sensitive materials like niobium diselenide.

This preservation results in enhanced superconductivity, with the synthesized niobium diselenide exhibiting a significantly higher onset temperature compared to samples produced using conventional chemical vapor deposition techniques, approaching the performance of mechanically exfoliated materials. The authors acknowledge that introducing controlled defects into the capping layers will be necessary to achieve wafer-scale synthesis and fully realize the potential for scalable fabrication of 2D circuits. Future work will focus on integrating established wafer-scale growth technologies for the capping layers with this nano-confined growth method, paving the way for novel quantum structures and functional quantum devices, alongside the confined growth of other elemental 2D materials.