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Graphene nanoribbon growth, simplified

Graphene nanoribbon growth, simplified

5 April 2024

The synthesis of graphene nanoribbons inside layers of boron nitride makes it straightforward to grow them defect-free for use in high-performance electronics.

Ever since Andre Geim, Konstantin Novoselov, and their collaborators used Scotch tape in 2004 to isolate graphene, the single sheet of carbon atoms has been touted as a nearly ideal electronic material (see Physics Today, December 2010, page 14). But although its lattice conducts electric current better than any other material, graphene in pristine form is a semimetal and has no bandgap. Later researchers modified the material by cutting the sheets into ultrathin strips, called graphene nanoribbons (GNRs). Thanks to confinement effects, the GNRs then boasted tunable bandgaps. The modification gave the strips all the advantages of pristine graphene but also the behavior of a semiconductor—an ideal platform for digital logic and electronic switching applications.

In practice, however, field-effect transistors (FETs) made from GNRs are often riddled with disorder, including lattice defects, charged impurities, and contaminants adsorbed on their surfaces and edges. That disorder lowers the mobility of charges and wreaks havoc on device performance. To reduce the influence of disorder, the most promising technique is to encapsulate the graphene ribbons between stacks of hexagonal boron nitride, an atomically flat insulator. When encapsulated between such sheets, each ribbon could retain the ultrahigh electron mobilities and ultralong electron mean free paths of graphene. But conventionally, the procedure involves mechanical assembly—a low-yield technique that introduces contamination and strain into the material. That renders it unsuitable for advanced electronics applications.

Zhiwen Shi of Shanghai Jiao Tong University in China and collaborators have now developed a new approach to grow the GNRs inside the BN stacks. As the figure illustrates, their growth is catalyzed by depositing iron nanoparticles on multilayered BN flakes that reside on a silicon dioxide substrate. The system is then heated, which prompts the nanoparticles to migrate toward BN step edges.

A diagram showing the nanoribbon growth process, and a micrograph of the encapsulation region.
Growing a graphene nanoribbon inside a hexagonal boron nitride crystal. (a) An iron nanoparticle attached to a step edge of the BN catalyzes the graphene nanoribbon’s growth (black) from methane gas. (b) Transmission electron microscopy reveals the ultrathin encapsulation region. Credit: Adapted from B. Lyu et al., Nature (2024), doi:10.1038/s41586-024-07243-0

At 850 °C, methane gas is then introduced, which initiates carbon growth within one of the interlayer gaps. The leading edge of the GNR is pushed into that space as extra carbon rows are added to the trailing edge on the iron nanoparticle. Conveniently, after the growth, the catalytic nanoparticle on each GNR requires no removal. It simply becomes attached to one of the two terminals when later configured into an FET.

The embedded GNRs turn out perfectly straight. Limited by the size of the BN single crystal, they grow up to 250 µm long, and at less than 5 nm wide, they are extremely narrow. The aspect ratio of the longest ribbons is about 105—at least two orders of magnitude larger than GNRs synthesized by any other method. What’s more, those GNRs exhibit uniform zigzag-edge chirality—the most appealing structure for spin-polarized edge states.

To incorporate the GNRs and their surrounding BN sheaths into an FET, Shi and his colleagues etch both components into shorter sections and demonstrate that the electrodes on the device can be connected to the GNR terminals. The presence of the BN around the nanoribbons eliminates any oxidation, environmental contamination, strain, or photoresist damage on the graphene. In essence, the encapsulation protects the nanoribbons and allows their superior features to shine in high-performance FETs. (B. Lyu et al., Nature, 2024, doi:10.1038/s41586-024-07243-0.)

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