Technological devices are becoming smaller and more feature-rich in part because scientists are exploring new materials and new ways to use existing materials. One area being investigated by materials scientists is the function of flat two-dimensional (2D) crystals. These crystals are just one molecule thick and can be arranged in layers to create van der Waals heterostructures (vdWs). “Many interesting phenomena have been seen in 2D vdWs, and new kinds of electronic and optical components have been proposed as a result,” professor Shigeo Maruyama at the University of Tokyo said. “However, we wondered whether it’s possible to create spatially compact one-dimensional (1D) vdWs and what kinds of useful and unique properties these nanotube structures may have.” A visual comparison of 2D (above) and 1D (below) van der Waals heterostructures. Courtesy of Shigeo Maruyama et al. Maruyama and associate professor Rong Xiang, also at the University of Tokyo, with their team created pure carbon nanotubes and placed the nanotubes in a high-temperature atmosphere containing boron nitride. The boron nitride bound to the surface of the nanotube to form a uniform and continuous layer, or crystal. A similar process was used to add a third layer to the tube in the form of molybdenum disulfide (MoS2). When tube structures encapsulate one another like this, the result is called a coaxial structure, as multiple 1D shapes share an axis of orientation. According to the Tokyo researchers, they are the first to grow crystals of various materials uniformly on the surface of carbon nanotubes. “At that time, the yield of this structure was still extremely low,” Xiang said. “I spent one full day at the controls of a transmission electron microscope probing the sample. In the afternoon when I was almost giving up, I found one of our coaxial nanotubes. Then a few minutes later, I found a second one! With two observations, I became fully confident that MoS2-based 1D vdWs can exist.” Microscopic images of the 1D vdW taken by various instruments. Courtesy of Shigeo Maruyama et al. The research suggests that materials in the current 2D library could be rolled into their 1D counterparts, and a number of function-designable 1D heterostructures could be realized. One-dimensional (1D) van der Waals heterostructures are a new class of material, whose properties have not yet been studied. However, Maruyama, Xiang, and their team are hopeful that these structures could find use in applications ranging from flexible electronics, lasers, and solar energy conversion to electrocatalytic water splitting and photoelectric devices. The research was published in Science (www.doi.org/10.1126/science.aaz2570).