Researchers at Rice University have developed a new top-down method to make very narrow nanoribbons from graphene (a 2D sheet of carbon just one atom thick). Dubbed meniscus-mask lithography, the simple technique can produce ribbons that are less than 10 nm wide with an aspect ratio of more than 2000. It can be used to produce other narrow nanostructures too, such as metallic nanowires.
Graphene nanoribbons (GNRs) are unique materials that go from being semiconducting to semimetal as their width increases. They could be used in high-performance nanoelectronics devices, such as high-frequency transistors and sensors, and could also be ideal as interconnects in nanoelectronics circuits. Although a number of “planar” techniques exist to make GNRs (such as graphene patterning and shadowing using copolymer masks), most of these methods are quite complicated and not easily scalable.
The new method, developed by a team led by James Tour, goes a long way in overcoming these problems in that it does not require any complicated high-resolution lithography tools. Instead, it simply involves using water found in the atmosphere adsorbed at the edges of a pre-patterned lithography pattern as a mask.
The researchers began by first pre-defining a pattern on the surface of a graphene sheet. They then etched the pattern into the graphene using plasma before depositing a sacrificial material (for example, aluminium) into the same area. The mask is then inverted and the rest of the graphene surface etched. This approach ensures that the graphene inside and outside of the pattern is etched but that a narrow stripe remains on the edge of the pattern, says Tour.
Graphene thin line
“Thanks to the slightly hydrophilic nature of the sacrificial material, a tiny, tens of nanometres wide, meniscus of water forms in the wedge between the mask wall and graphene surface, and this meniscus is enough to ‘shield’ the nanoribbon from further etching,” he explains. “Since the width of this meniscus is unrelated to the pattern shape, or any other lithography-related parameters, but is defined only by the type of materials we used, the resulting nanoribbons will always have the same width.”
Tour adds that the method could be used to make other narrow nanostructures such as metallic nanowires. This means that structures like nanoelectrodes, crosswire architectures, transistor gates and possibly some types of photonic waveguides could be fabricated in this way.
It is not all plain-sailing though. As with any top-down procedure, the technique produces nanostructure edges that are rough and poorly defined at the atomic level. “While this is not a significant issue in the case of metal nanowires, such defects do destroy the fine electronic structure of graphene nanoribbons and adversely affect their performance as high-mobility semiconductors,” Tour told nanotechweb.org. “We have observed this effect in our research and are working towards improving edge quality by studying the chemistry of these edges and applying appropriate ‘healing’ methods.”
The Rice team describes its work in ACS Nano DOI: 10.1021/nn403057t.
About the author
Belle Dumé is contributing editor at nanotechweb.org.