Apr 7, 2014
2D transfers made easy
Researchers at the Kavli Institute of Nanoscience in the Netherlands have come up with the first all-dry technique for transferring 2D materials, like graphene or molybdenum sulphide. The new technique, which is quick, efficient and clean, makes use of viscoelastic stamps. As well as being much simpler than traditional wet transfer techniques, it could also be used to fabricate freely suspended 2D structures thanks to the fact that the samples are not subject to any capillary forces during the process.
2D materials are creating a flurry of interest in labs around the world because they have dramatically different electronic and mechanical properties from their 3D counterparts. This means that they could find use in a host of novel device applications, such as low-power electronics circuits, low-cost or flexible displays, sensors and even flexible electronics that can be coated onto a wide variety of surfaces.
The most well known 2D materials are graphene (which is a sheet of carbon just one atom thick) and the transition metal dichalcogenides – which have the chemical formula MX2, where M is a transition metal (such as Mo or W) and X is a chalcogen (such as S, Se and Te).
For real-world applications, the materials need to be transferred onto substrates to make heterostructures based on the artificial stacking of the 2D layers. Most techniques involve wet chemistry but the problem here is that the chemicals employed often contaminate the 2D materials, which are fragile being so thin, and adversely affect their pristine electronic and physical properties. Moreover, the capillary forces between the chemicals and the material being transferred can cause the 2D structure to simply collapse.
An all-dry technique could be a solution to these problems.
A team led by Herre van der Zant and Gary Steele used a thin layer of a commercially available viscoelastic material called Gelfilm. The researchers transferred 2D crystals, such as graphene and MoS2, onto the film by mechanically shaving off layers from the bulk, or parent 3D, material using the now famous Scotch tape technique (first used to isolate graphene from graphite back in 2004). They selected only the thinnest flakes (by looking at them under an optical microscope) and fixed these onto the XYZ sample stage in the microscope. They then attached a stamp to the sample.
“As the stamp is transparent, we can see the sample through it and can align the flake wherever we want on a 2D substrate surface with sub-micron resolution,” said team member Andres Castellanos-Gomez. “To transfer the flake, we press the stamp against the sample surface and peel it off very slowly.”
The transfer works thanks to viscoelasticity, he told nanotechweb.org. “Our stamps are made of a silicone rubber very similar to the ones found in the 'stretchy sticky hands' toys for kids." This silicone rubber behaves like an elastic solid over short time periods but can flow over longer timescales. "If we contact this rubber with a surface for a long time it will flow until it becomes intimately joint to it. This is why the sticky hands toys can adhere to anything without any glue. We exploited this phenomenon to adhere flakes to our viscoelastic stamps without employing an adhesive. By then slowly peeling the sample off the stamp surface, the viscoelastic material detaches, releasing the flakes that then preferentially stick to a substrate surface instead.”
Infinite combination of material heterostructures
The team has already proved that its technique works by transferring graphene flakes onto hexagonal boron nitride (a 2D material that is a good substrate for graphene). Thanks to optical microscope images, the researchers were able to confirm that nearly half of the graphene flakes lie flat on the h-BN without any bubbles or wrinkles. And the good thing is that whole process takes just 15 minutes or less.
“Our technique could be applied to any kind of exfoliated layered crystal, so allowing for an infinite combination of material heterostructures,” said Castellanos-Gomez. “For example, as well as depositing graphene on h-BN, we have also already managed to ‘sandwich’ a MoS2 bilayer between two h-BN flakes.”
The Kavli team has also succeeded in transferring a single-layer MoS2 crystal onto a SiO2/Si substrate pre-patterned with holes of different diameters. The single-layer MoS2 is freely suspended over the holes, forming “drumheads” – which might be used in mechanical resonator applications. Indeed, the technique might also be employed to transfer 2D crystals onto pre-fabricated devices with trenches and electrodes.
And that is not all. Since the stamping technique is so gentle, it can be used to deposit 2D crystals onto even the most fragile of substrates. For example, the team says that it has succeeded in transferring few-layer MoS2 crystals onto the cantilever of an atomic force microscope without damaging the cantilever at all. “We have also transferred 2D materials onto silicon nitride membranes and holey carbon films, which are typically employed in transmission electron microscopy,” said Castellanos-Gomez.
Details of the new stamping technique can be found in 2D Materials, a new journal from IOP Publishing which publishes its first papers this month.
About the author
Belle Dumé is contributing editor at nanotechweb.org