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.
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