In a major advance, researchers at the Indian Institute of Science (IISc) have devised a method to grow high-quality 2D magnetic materials (2D-MMs) over centimetre-scale wafers. Earlier approaches in the field were limited to growing micrometre-sized flakes. This advance paves the way for their integration into next-generation electronics and spintronics materials used in hard drives and sensors.
2D-MMs can retain their magnetism at the single atomic layer scale. Existing processes like mechanical exfoliation, which involves peeling layers from bulk 2D-MM materials, work well only for research purposes, not for technological integration. For scalable fabrication, these materials need to be grown over a large scale. This is typically done using Vapour Deposition methods in which vapours of (precursor) materials are passed over a hot surface, where they atomically rearrange and form a thin layer of material. This surface, called the substrate, acts like a base that helps the material grow efficiently. However, wafer-scale production using these methods is difficult for 2D-MMs because even tiny imperfections during growth can disrupt the material's atomic structure and magnetic properties.
In a study published in Advanced Materials, Akshay Singh, Assistant Professor in the Department of Physics at IISc, and his team devised a technique to grow chromium chloride (CrCl3) as a representative and physics-rich 2D-MM with interesting magnetic ordering. They used a novel technique, termed Physical Vapour Transport Deposition (PVTD), in which a material is heated until it vapourises, then transported and redeposited onto a surface to form highly ordered, well-aligned crystals.
"Researchers think of synthesis like magic; they would struggle for many months to years, and suddenly, they start getting beautiful growth," says Singh.
To enable this "magical" synthesis, the team focused on four key innovations: reducing unwanted radiative heating, increasing carrier gas flow rate far beyond conventional limits, dynamically controlling material supply during growth, and rigorously eliminating oxygen and moisture from the system.
Vivek Kumar, first author and PhD student at the Department of Physics, first started working on this technique in 2023, along with Abhishek Jangid, PhD student and second author. Early attempts to grow the 2D magnetic CrCl3 yielded poor results, prompting him to redesign the setup. First, he darkened the chamber inside which the furnace was placed, and made customised filters to prevent oxygen and moisture from damaging the synthesised material. Gas leakage was tackled using couplings at tube ends, inspired by a research group in Germany.
Still, the team observed surface etching due to light exposure. They traced the excess light to radiative emissions from resistive heating elements in the furnace. A simple solution was devised to cover the growth tube with aluminium foil.
The researchers also explored how different substrates influence production. After testing silicon dioxide and sapphire substrates, the team found that synthetic mica delivered the best results. Mica is made up of layers held together by weak forces that can stack up loosely and have no unfulfilled bonds at the surface, which makes it highly compatible with CrCl3. Mica's crystalline nature also provides a well-ordered surface for growth. "It is like Lego; if the wall where Lego pieces have to be fitted is not structured, the material you are going to grow will not be structured," Kumar explains.
Kumar also proposed using very high carrier gas flow rates – an unconventional approach – which resulted in coalesced and significantly smoother films with minimal surface roughness.
The team was able to grow the material in different patterns, and transfer the grown films onto other substrates – an essential step for integrating these materials into electronic platforms.
To delve into the underlying mechanisms, the team combined experiments with large-scale simulations based on density functional theory calculations and machine-learning molecular dynamics, by collaborating with Ananth Govind Rajan, Associate Professor at the Department of Chemical Engineering. "The simulations showed that F‑mica enables easier diffusion and ordered chain formation, which are the key to 2D CrCl3 growth, while also explaining the material's sensitivity to oxygen and moisture," says Govind Rajan.
"We demonstrated [growing] 2D magnetic materials, but this workflow or process can be applied to any air-sensitive or light-sensitive material," adds Singh.
