STM/STS study on the ground state of layered dichalcogenide nanotubes

利用低温扫描隧道显微镜(STM)对层状化合物NbSe_2纳米管进行了研究。NbSe_2纳米管是一种有趣的材料，因为它是第一个体相经历密度波和超导电性等凝聚态的纳米管。采用化学气相输运法制备了NbSe_2纳米管。通过在异丙醇中超声搅拌NbSe_2纳米管，将NbSe_2纳米管滴加到解理的高取向热解石墨(HOPG)上，制备了用于STM测量的纳米管样品，并在室温下拍摄了表面形貌的STM图像。纳米管的长度估计为300-2000纳米。大部分NbSe_2纳米管的长度都在600 nm以上。STM扫描图谱证实了纳米管的结构。纳米管的直径估计为2-50 nm，远小于用透射电子显微镜研究的多壁纳米管的直径。我们认为直径小于10 nm的纳米管是单壁的。我们感兴趣的是单壁NbSe_2纳米管的手性(螺旋度)与电子结构之间的关系。在NbSe_2纳米管中发现了单壁碳纳米管中普遍存在的束状结构。该束由大约100个纳米管组成。虽然在室温下不能获得原子分辨率的扫描隧道显微镜图像，但在4.2K以下获得了原子排列的清晰图像，成功地从这些图像中确定了NbSe_2纳米管的手性。手性指数因样品不同而不同。我们还发现，所有原子排列确定的纳米管都是螺旋度较低的手性类型。这可能对NbSe_2纳米管的生长机理有一定的指导意义。

Layered compound NbSe_2 nanotubes were studied by low temperature scanning tunneling microscope (STM). NbSe_2 nanotube is an interesting material since it is the first nanotube material, of which bulk phase undergoes the condensed state such as the density wave and superconductivity. NbSe_2 nanotubes were synthesized by the chemical vapor transport method. The nanotube sample for STM measurement was prepared by dropping NbSe_2 nanotubes agitated ultrasonically in 2-propanol on cleaved highly oriented pyrolytic graphite (HOPG).Topographic STM images of NbSe_2 nanotubes on the substrate of HOPG were taken at room temperature. The length of nanotubes is estimated as 300-2000 nm. Almost NbSe_2 nanotube has length over 600 nm. The nanotube structure is confirmed by STM scan profiles. Diameter of nanotubes is estimated as 2-50 nm, which is much smaller than that of multi-walled nanotubes studied by transmission electron microscope. We think that nanotubes with diameter of less than 10 nm are single-walled one. We are interested in the relation between the chirality (helicity) and electronic structure in single-walled NbSe_2 nanotubes. The bundle structure, which is popular in single-walled carbon nanotube, is found in NbSe_2 nanotubes. The bundle consists of about hundred nanotubes. The Y-junction, which is the connected structure with three nanotubes, is also found similarly to carbon nanotubes.Although we can not take STM image with atomic resolution at room temperature, clear images with atomic arrangement are obtained below 4.2 K. The chirality of NbSe_2 nanotube is successfully determined from these images. The chiral index is found to vary from sample to sample. We also find that all nanotubes, of which atomic arrangement is determined, are so-called chiral type with low symmetry in helicity. It might give some suggestion to growing mechanism of NbSe_2 nanotubes.