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

采用低温扫描隧道显微镜（STM）对层状复合NbSe_2纳米管进行了研究。NbSe_2纳米管是一种有趣的材料，因为它是第一种体相经历密度波和超导等凝聚态的纳米管材料。采用化学气相输运法制备了NbSe_2纳米管。将超声搅拌的NbSe_2纳米管滴在裂解的高取向热解石墨（HOPG）上，制备了用于STM测量的纳米管样品。在室温下，对HOPG衬底上NbSe_2纳米管的形貌进行了STM成像。纳米管的长度估计为300-2000 nm。NbSe_2纳米管的长度几乎都在600 nm以上。通过STM扫描图验证了纳米管的结构。纳米管的直径估计为2 ~ 50 nm，远小于透射电子显微镜下的多壁纳米管。我们认为直径小于10nm的纳米管是单壁纳米管。我们对单壁NbSe_2纳米管的手性（螺旋度）和电子结构之间的关系很感兴趣。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100个纳米管组成。y结是由三根纳米管连接而成的结构，与碳纳米管相似。虽然我们不能在室温下获得原子分辨率的STM图像，但在4.2 K以下可以获得具有原子排列的清晰图像。利用这些图像成功地测定了NbSe_2纳米管的手性。发现不同样品的手性指数不同。我们还发现，所有原子排列确定的纳米管都是所谓的手性型，螺旋度对称性低。这可能对NbSe_2纳米管的生长机理有一定的启示。

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.