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Elemental boron arouses great interest from both scientific and technological areas of research because it has unique chemical and physical properties and its theoretical tubular structures may have higher electrical conductivity than carbon nanotubes.[1–7] High conductivity and chemical stability of boron or boride nanostructures have made it an attractive candidate for future applications in ideal cold-cathode materials, high-temperature semiconductor devices, or fieldeffect transistors.[8–14] In particular, for the application of field emission (FE), it is especially useful to synthesize large, vertical arrays of boron nanowires (BNWs) with the desired surface work function and FE behavior. So far, to our knowledge, while both amorphous [15–20] and crystalline boron nanowires [21, 22] have been fabricated by magnetron sputtering, laser ablation, or chemical vapor methods, vertical arrays of single-crystal boron nanowires over a large area have not been synthesized in a one-step process. In addition, little attention [23–25] has been paid to the measurements of the physical properties of an individual boron nanowire. In this Communication, we report the successful synthesis of high-density, vertically aligned single-crystal boron nanowire arrays with a nanowire diameter of approximately 20–40 nm by a thermal carbon-reduction method. Moreover, we have measured the FE behavior and surface work function of a single boron nanowire, which is critical to evaluate the possibility of using boron nanowires as field-emission materials. For the purpose of better understanding the field-emission mechanism of a boron nanowire, the field-emission properties …