Columbia Engineering researchers have experimentally demonstrated for the first time that it is possible to electrically contact an atomically thin two-dimensional (2D) material only along its one-dimensional (1D) edge, rather than contacting it from the top, which has been the conventional approach. With this new contact architecture, they have developed a new assembly technique for layered materials that prevents contamination at the interfaces, and, using graphene as the model 2D material, show that these two methods in combination result in the cleanest graphene yet realized.
After first encapsulating graphene in boron nitride, the multilayer stack is etched to expose only the very edge of the two-dimensional graphene layer. Electrical contact is then made by metalizing along this one-dimensional edge. Credit: Columbia Engineering; Illustration, Cory Dean
In this work, says Cory Dean, who led the research as a postdoc at Columbia and is now an assistant professor at The City College of New York, the team solved both the contact and contamination problems at once. “One of the greatest assets of 2D materials such as graphene is that being only one atom thick, we have direct access to its electronic properties. At the same time, this can be one of its worst features since this makes the material extremely sensitive to its environment. Any external contamination quickly degrades performance. The need to protect graphene from unwanted disorder, while still allowing electrical access, has been the most significant roadblock preventing development of graphene-based technologies. By making contact only to the 1D edge of graphene, we have developed a fundamentally new way to bridge our 3D world to this fascinating 2D world, without disturbing its inherent properties. This virtually eliminates external contamination and finally allows graphene to show its true potential in electronic devices”
The researchers fully encapsulated the 2D graphene layer in a sandwich of thin insulating boron nitride crystals, employing a new technique in which crystal layers are stacked one-by-one. “Our approach for assembling these heterostructures completely eliminates any contamination between layers,” Dean explains, “which we confirmed by cross-sectioning the devices and imaging them in a transmission electron microscope with atomic resolution.”
Once they created the stack, they etched it to expose the edge of the graphene layer, and then evaporated metal onto the edge to create the electrical contact. By making contact along the edge, the team realized a 1D interface between the 2D active layer and 3D metal electrode. And, even though electrons entered only at the 1D atomic edge of the graphene sheet, the contact resistance was remarkably low, reaching 100 Ohms per micron of contact width—a value smaller than what can be achieved for contacts at the graphene top surface.
With the two new techniques—the contact architecture through the 1D edge and the stacking assembly method that prevents contamination at the interfaces—the team was able to produce what they say is the “cleanest graphene yet realized.” At room temperature, these devices exhibit previously unachievable performance, including electron mobility at least twice as large as any conventional 2D electron system, and sheet resistivity less than 40 Ohms when sufficient charges are added to the sheet by electrostatic “gating.” Amazingly, this 2D sheet resistance corresponds to a “bulk” 3D resistivity smaller than that of any metal at room temperature. At low temperature, electrons travel through the team’s samples without scattering, a phenomenon known as ballistic transport. Ballistic transport, had previously been observed in samples close to one micrometer in size, but this work demonstrates the same behavior in samples as large as 20 micrometers. “So far this is limited purely by device size,” says Dean, “indicating that the true ‘intrinsic’ behavior is even better.”
The team is now working on applying these techniques to develop new hybrid materials by mechanical assembly and edge contact of hybrid materials drawing from the full suite of available 2D layered materials, including graphene, boron nitride, transition metal dichlcogenides (TMDCs), transition metal oxides (TMOs), and topological insulators (TIs). “We are taking advantage of the unprecedented performance we now routinely achieve in graphene-based devices to explore effects and applications related to ballistic electron transport over fantastically large length scales,” Dean adds. “With so much current research focused on developing new devices by integrating layered 2D systems, potential applications are incredible, from vertically structured transistors, tunneling based devices and sensors, photoactive hybrid materials, to flexible and transparent electronics.”
This is an illustration of an encapsulated two-dimensional graphene sheet that is electrically contacted only along its one-dimensional edge. Credit: Columbia Engineering; Illustration, James Hedberg and Cory Dean
Heterostructures based on layering of two-dimensional (2D) materials such as graphene and hexagonal boron nitride represent a new class of electronic devices. Realizing this potential, however, depends critically on the ability to make high-quality electrical contact. Here, we report a contact geometry in which we metalize only the 1D edge of a 2D graphene layer. In addition to outperforming conventional surface contacts, the edge-contact geometry allows a complete separation of the layer assembly and contact metallization processes. In graphene heterostructures, this enables high electronic performance, including low-temperature ballistic transport over distances longer than 15 micrometers, and room-temperature mobility comparable to the theoretical phonon-scattering limit. The edge-contact geometry provides new design possibilities for multilayered structures of complimentary 2D materials.
Brian Wang is a Futurist Thought Leader and a popular Science blogger with 1 million readers per month. His blog Nextbigfuture.com is ranked #1 Science News Blog. It covers many disruptive technology and trends including Space, Robotics, Artificial Intelligence, Medicine, Anti-aging Biotechnology, and Nanotechnology.
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