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

用低温扫描隧道显微镜（STM）研究了层状化合物NbSe_2纳米管。NbSe_2纳米管是第一种体相具有密度波、超导等凝聚态性质的纳米管材料，是一种很有研究价值的材料。采用化学气相传输法合成了NbSe_2纳米管。将在异丙醇中超声搅拌的NbSe_2纳米管滴加到劈开的高取向热解石墨（HOPG）上，制备了用于STM测量的纳米管样品，并在室温下拍摄了HOPG基底上NbSe_2纳米管的STM形貌图像。纳米管的长度估计为300-2000 nm。NbSe_2纳米管的长度大多在600 nm以上。通过STM扫描轮廓证实了纳米管结构。纳米管的直径估计为2-50 nm，这比透射电子显微镜研究的多壁纳米管的直径小得多。我们认为直径小于10 nm的纳米管是单壁纳米管。我们对单壁NbSe_2纳米管中的手性（螺旋度）与电子结构之间的关系感兴趣。在NbSe_2纳米管中发现了单壁碳纳米管中常见的束状结构。该束由大约一百个纳米管组成。虽然在室温下不能得到原子级分辨率的STM图像，但在4.2K以下得到了清晰的原子级排列的STM图像。从这些图像中成功地确定了NbSe_2纳米管的手性。发现手性指数随样品的不同而不同。我们还发现，所有的纳米管，其原子排列是确定的，是所谓的手征型低对称性的螺旋。这对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.