Transferring semiconductor monolayers offers material benefits

The technique is claimed to be much faster than existing methods and can transfer the atomic scale thin films from one substrate to others, without causing any cracks.

At issue are molybdenum sulphide (MoS₂) thin films that are only one atom thick, first developed by Dr. Linyou Cao, an assistant professor of materials science and engineering at NC State. MoS₂ is an inexpensive semiconductor material with electronic and optical properties similar to materials already used in the semiconductor industry.

‘The ultimate goal is to use these atomic-layer semiconducting thin films to create devices that are extremely flexible, but to do that we need to transfer the thin films from the substrate we used to make it to a flexible substrate,’ said Cao, who is senior author of a paper on the new transfer technique. ‘You can’t make the thin film on a flexible substrate because flexible substrates can’t withstand the high temperatures you need to make the thin film.’

Cao’s team makes MoS₂ films that are an atom thick and up to 5cm in diameter. The researchers needed to find a way to move that thin film without wrinkling or cracking it, which is challenging due to the film’s extreme delicacy.

‘To put that challenge in perspective, an atom-thick thin film that is 5cm wide is equivalent to a piece of paper that is as wide as a large city,’ Cao said in a statement. ‘Our goal is to transfer that big, thin paper from one city to another without causing any damage or wrinkles.’

Existing techniques for transferring such thin films from a substrate rely on chemical etching, but the chemicals involved in that process can damage or contaminate the film. Cao’s team has developed a technique that takes advantage of the MoS₂’s physical properties to transfer the thin film using only room-temperature water, a tissue and a pair of tweezers.

MoS₂ is hydrophobic but the sapphire substrate the thin film is grown on is hydrophilic. Cao’s new transfer technique works by applying a drop of water to the thin film and then poking the edge of the film so that the water can begin to penetrate between the MoS₂ and the sapphire.

Once it has begun to penetrate, the water pushes into the gap, floating the thin film on top. The researchers use a tissue to soak up the water and then lift the thin film with tweezers and place it on a flexible substrate. The whole process takes a couple of minutes whereas chemical etching takes hours.

Cao said: ‘The water breaks the adhesion between the substrate and the thin film - but its important to remove the water before moving the film. Otherwise, capillary action would case the film to buckle or fold when you pick it up.

‘This new transfer technique gets us one step closer to using MoS₂ to create flexible computers. We are currently in the process of developing devices that use this technology.’

A version of the paper, Surface Energy-Assisted Perfect Transfer of Centimeter-Scale Monolayer and Fewlayer MoS₂ Films onto Arbitrary Substrates, has been published in ACS Nano.