J. Phys. IV France 131 (2005) 235–237
C EDP Sciences, Les Ulis
DOI: 10.1051/jp4:2005131060

Imaging of NbSe2 nanotube by STM
K. Ichimura1 , K. Tamura1 , M. Ogata1 , K. Nomura1 , T. Toshima2 and S. Tanda2
12

Division of Physics, Hokkaido University, Sapporo 060-0810, Japan
Division of Applied Physics, Hokkaido University, Sapporo 060-8628, Japan

Abstract. NbSe2 nanotubes were studied by scanning tunneling microscopy (STM). Topographic imagesof NbSe2 nanotubes on the substrate of HOPG were taken at room temperature. The length of nanotubes isestimated as 300-2000 nm. Diameter of nanotubes is found to be several nm, which is much smaller than thatof multi-walled nanotubes, suggesting single-walled one. The bundle structure, which consists of several ten
nanotubes, and Y-junction were found similarly to carbon nanotubes.

1. INTRODUCTION
Nanotube materials as well as carbon nanotube have attracted much attention because of their topologicalstructure. The electronic state can be controlled by the chirality. Recently, NbSe2 nanotube wassynthesized independently by two groups [1, 2]. Tsuneta et al. [1] reported that the nanotube is multiwalled and the stacking is determined as 2H from electron diffraction. On the other hand, the nanotube
synthesized by Nath et al. [2] has 6 Ha stacking. It should be emphasized that NbSe2 nanotube is theﬁrst nanotube material, of which bulk phase exhibits the condensed state. The bulk phase of 2H-NbSe2
undergoes the incommensurate charge density wave (CDW) at 33 K, and moreover, the superconductivity
at 7.2 K. Our interest is how the condensed state, which is characterized with the coherence length, survives
in nano-scaled electronic system.
The tunneling spectroscopy using STM, i.e. scanning tunneling spectroscopy (STS) is useful method
to investigate the condensed state such as the superconductivity, charge and spin density wave becauseof non-contacting tip conﬁguration.
The goal of our study is elucidating the ground state of nano-scaled topological material by STS at
low temperature. As the ﬁrst state of our study on NbSe2 nanotube, in this paper, we report STM imagesof NbSe2 nanotubes at room temperature and discuss about the diameter of tubes.
2. EXPERIMENTAL
NbSe2 nanotubes were synthesized by the chemical vapor transport (CVT) method. After purifying,
the nanotube sample for STM measurement was prepared by dropping NbSe2 nanotubes agitatedultrasonically in 2-propanol on cleaved highly oriented pyrolytic graphite (HOPG). We used low
temperature STM equipment, in which temperature is variable from room temperature down to 1.2 K.
A mechanically sharpened Pt-Ir alloy wire was used as an STM tip, which is attached to a tube typepiezoelectric scanner.
3. RESULTS AND DISCUSSION
Figure 1 shows a typical STM image of NbSe2 nanotube on HOPG substrate. The ﬁber-like structures arealmost straight and have length of about 300-2000 nm. We conclude that the linear structure corresponds
to NbSe2 nanotube from dimension and linearity of the structure. At present, we cannot obtain the atomic
resolution to determine the chirality of nanotubes. The scan proﬁle across the linear structure, which

Article published by EDP Sciences and available at http://www.edpsciences.org/jp4 or http://dx.doi.org/10.1051/jp4:2005131060

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Figure 1. (left) Typical STM image of NBsE2 nanotobe on HOPG. Scan area is 200 × 200 nm2 . The tube diameteris estimated as abut 10 nm from the scan proﬁle.

shows a peak, suggests that the structure is nanotube. Diameter of NbSe2 nanotubes observed at present
work is ranging from 1 to 20 nm. In much case, the tube diameter is distributed from 4 to 10 nm. The
obtained value in the present work is much smaller than that reported by TEM study [1] on multiwalled NbSe2 nanotubes. We speculate that thick tube in the present STM study is multi-walled and thinnanotube is rather single-walled. The present result of diameter of several nm is almost consistent withthe theoretical calculation [3] for single-walled NbSe2 nanotubes.
The bundle structure of NbSe2 nanotube was found similarity to carbon nanotube as shown in Fig. 2.
The bundle with diameter of about 70 nm consists of several ten nanotubes with diameter of a few
nm, which are spaced closely and arranged in parallel. It is noteworthy that NbSe2 nanotube has muchsimilarity with carbon nanotube in structure.
We also note the Y-junction of NbSe2 nanotube. The Y-junction was often found in carbon nanotubes[4]. The Y-junction of NbSe2 nanotube suggests the possibility of an application to nano-scaled devices
in the CDW and superconducting state.
We are interested in the CDW and superconductivity in nano-sized sample. We are trying to study
STS on NbSe2 nanotubes at low temperature.

Figure 2. (right) STM image of bundle structure of NbSe2 nanotbes. Scanarea is 87.4 × 87.4 nm2 .

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Acknowledgments
This work has been partially supported by the 21COE program on “Topological Science and Technology” from the
Ministry of Education, Culture, Sport, Science and Technology of Japan.

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Paalanen, Curr. Appl. Phys. 3, 473 (2003).
[2] M. Nath, S. Kar, A. K. Raychaudhuri, C. N. R. Rao, Chem. Phys. Lett. 368, 690 (2003).
[3] V.V. Ivanovskaya, A.N. Enyashin, N.I. Medvedeva, A. L. Ivanovskii, Phys. Stat. Sol. B238, R1(2003).
[4] L.P. Biro, Z.E. Horvath , G.I. Mark, Z. Osvath, A.A. Koos, A.M. Benito, W. Maser, Ph. Lambinc,Diamond Relat. Mater. 13, 241 (2004).