Current Applied Physics 3 (2003) 473–476
www.elsevier.com/locate/cap

Formation of metallic NbSe2 nanotubes and nanoﬁbers
T. Tsuneta a,*, T. Toshima a, K. Inagaki a, T. Shibayama a, S. Tanda a, S. Uji b,
M. Ahlskog c, P. Hakonen c, M. Paalanen ca

Department of Applied Physics, Hokkaido University, Sapporo 060-8628, Japanb
National Institute for Materials Science, Tsukuba, Ibaraki 305-0003, Japanc
Low Temperature Laboratory, Helsinki University of Technology, Espoo, Finland
Received 28 April 2003; received in revised form 17 June 2003; accepted 14 July 2003

Abstract
We succeed in synthesizing NbSe2 nanotubes along with nanoﬁbers by chemical vapor transportation. They are stable crystalline
systems and can be synthesized reproducibly in a nearly equilibrium reacting process. We have investigated these nanosize structuresof NbSe2 by transmission electron microscopy and electron diﬀraction. Both of the structures have a similar size of 100–200 nm indiameter. While nanotubes consist of rolled-up NbSe2 layers, nanoﬁbers are a pile of thin ﬂat layers. We propose a mechanism of theformation of NbSe2 nanotubes and nanoﬁbers on the basis of deseleniditive transition from a NbSe3 ﬁber-shaped crystal. We alsomeasured electrical resistance of the nanoﬁbers with conductive atomic force microscopy and demonstrated that the material show
metallic behavior at room temperature.
 2003 Elsevier B.V. All rights reserved.
PACS: 81.07.De; 73.63.Fg; 81.10.Jt
Keywords: Nanotube; Formation mechanism; Superconductor; Charge-density-wave

1. Introduction
The discoveries of fullerenes [1], carbon nanotubes(CNT’s) [2], and M€
obius crystals [3] as topologicalforms of matter have opened a challenging ﬁeld coveringover solid state physics, chemistry and materials science
with wide spectra of possible applications. Topologicaleﬀects, like the Berry phase [4,5], originating from system geometry have attracted much attention both theoretically and experimentally.
On the other hand, CNT is regarded as a prospective
replacement of traditional semiconductor-based devices
for constructing integrated circuits because of its characteristic size of nanometers. The nanometer-scale sizeof CNT oﬀers not only faster operation and lower power
consumption of a circuit, but also nontraditional mesoscopic devices, for instance single electron transistors.
A superconducting or charge-density-wave (CDW)
nanotube creates possibilities for device design for*

Corresponding author.
E-mail address: tsuneta@eng.hokudai.ac.jp (T. Tsuneta).
URL: http://exp-ap.eng.hokudai.ac.jp.

1567-1739/$ - see front matter  2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.cap.2003.07.002

nanotube circuits. However, the nanoscale regime has
not been studied for superconductor and CDW conductor systems, in particular, many aspects of nanoscale
CDW dynamics are still unknown [6]. Moreover, theinterplay between topological eﬀects and downsizedeﬀects of these collective quantum phenomena (CDWand superconductor) are imperious themes because of
their connection to the fundamental understanding ofquantum mechanics as well as the potential for deviceconstruction.
To investigate these eﬀects, we have chosen NbSe2 forconstructing tubular nanostructure. M€
obius crystal of
NbSe3 have been the only other topological material ofCDW compound so far. While NbSe3 has a onedimensional chain structure, which makes it possible toform a M€
obius ring, NbSe2 have a graphite-like layeredstructure, which is responsible for the formation of topological materials such like CNT’s. NbSe2 exhibits
CDW transition at 33 K, and becomes superconductor
below 7.2 K. These properties make the matter apromising candidate of a superconducting and/or CDW
nanotube [7], both of which will be a valuable member
to the known nanotube family: a normal metal or a474T. Tsuneta et al. / Current Applied Physics 3 (2003) 473–476

semiconductor (carbon nanotubes), and insulators (BN[8], MoS2 [9] and WS2 [10] nanotubes). However, NbSe2
nanotubes were so far merely produced under electron
irradiation with extremely high energy [11], while nanotubes of MoS2 and WS2 , whose crystal structures aresimilar to NbSe2 , were synthesized by various chemical
methods. In this study, we present evidence of NbSe2
multi-wall nanotubes with diameters of 30–200 nm
formed by a nearly equilibrium chemical reaction in acontrollable way. Also, we propose a formation mechanism of the NbSe2 nanotube based on transition from aNbSe3 nanoﬁber.

2. Experimental procedures
We abstracted nanosize objects from NbSe2 powder
synthesized by direct chemical vapor transportation(CVT) method through following procedures. We put
high-purity niobium (4N) and selenium (4N) in evacuated (106 Torr) silica ampoules as initial materials. The
ampoules are then heated to the maximum temperatureof 800 C by a rate of 10 C/min inside a furnace thathas a spatial gradient of 1 C/cm. Note that this method
can be applied to synthesize niobium selenides with
diﬀerent composition, such as NbSe3 , by setting appropriate reaction temperature. After an hour of reaction, the ampoules are quenched to room temperature.
Obtained NbSe2 powder is suspended in dichloroethaneor isopropyl alcohol by sonication of few minutes, andthen nanosize particles in the dispersion are deposited
on a substrate.

3. Results and discussions
3.1. Nanotubes and nanoﬁbers
Fig. 1(a) shows a transmission electron microscope(TEM) image of a NbSe2 nanotube found in deposition.

The 200 nm wide, 1.3 lm long material sticking out of abulk crystal shows an internal structure. The insets show
enlarged views of internal structure from respective
parts of the ﬁber. A transmission electron diﬀraction(TED) pattern (Fig. 1(b)) of the material has a 2 mmsymmetry, and is composed of the spots aligned in rows
vertical to the ﬁber. The crystal structure derived fromthe pattern is that of a hexagonal with a=c ¼ 0:27,
agreeing with that of 2H–NbSe2 (hexagonal, a=c ¼
 and c ¼ 12:552 A). Nevertheless the0:274, a ¼ 3:444 A
material is not any other niobium-selenides including
ﬁber-shaped compounds NbSe3 , Nb2 Se3 , etc. The diffraction spots can be indexed as shown in Fig. 1(b). The
unindexed ones are diﬀractions by a tiny aﬃx.
The material has clear features of a tubular structure
in its TED pattern as follows. In the pattern, eachhexagonal (hk0) spot accompanies diﬀraction spots withnonzero c -component, (hkn) (n ¼ 1; 2; 3 . . .), aligned
horizontally and outwardly. This aspect is crucial to atubular structure according to the previous studies on
CNT [12]. Hence the structure shown in Fig. 1 is aninternal cavity of a width of 60 nm. We estimate thenumber of layers of the material as 120, which is toomuch to obtain an atomic image. Also we should pointout that all coaxial shells of the tube have the chirality ofarmchair, i.e. a Æ1 0 0æ direction is toward the longitudinal axis.
We discovered another kind of nanoﬁbers formed
along with NbSe2 tubes. Fig. 2(a) shows typical lattice
image of a ﬁber without any cavity. The shown lattice corresponds to interlayer distance ofspacing of 6.44 A). It means that the micrograph depicts
NbSe2 (6.276 A
the side-surface of a NbSe2 nanoﬁber consisting ofstacked ribbon-shaped crystal layers. The same structure
is veriﬁed by a TED pattern (Fig. 2(b)) of another ﬁber
containing only (hk0) diﬀractions. The longitudinal axisof most nanoﬁbers is also along a Æ1 0 0æ axis, as in Fig.
2(b).
We believe that the origin of our NbSe2 nanotubes is
scrolling of nanoﬁbers. Actually, we once observed aFig. 1. (a) A TEM micrograph of a 200 nm wide, 1.3 lm long open-end nanotube sticking out of a bulk crystal. An internal cavity of 60 nm in widthis shown in insets, which are enlarged views of respective parts of the material. (b) Each (hk0) diﬀractions in this TED pattern of the NbSe2 nanotube
is accompanied by (hkn) (n ¼ 1; 2; 3 . . .) diﬀractions aligned in an array perpendicular to the longitudinal axis of the material. This aspect is characteristic of a tubular structure.

T. Tsuneta et al. / Current Applied Physics 3 (2003) 473–476

Fig. 2. (a) This TEM image of a nanoﬁber shows lattice spacing of that agree with (0 0 2) plane of 2H–NbSe2 . (b) A TEM image of
6.44A
another NbSe2 nanoﬁber, whose TED pattern (inset) shows only one
set of hexagonal diﬀractions. Both of the materials are ribbon-shaped
NbSe2 nanoﬁbers having the a–b plane as its facet.

Fig. 3. A TEM micrograph of a part of a 110 nm wide, 2 lm long
NbSe2 nanoﬁber that began folding under an electron irradiation. The
left end was made into a scroll (left inset). Its TED pattern (right inset)
agrees with the a –b plane of NbSe2 .

ﬁber irradiated by an electron beam began scrolling andeventually formed a tube-like structure. Here we present
a TEM image of the ‘‘intermediate’’ structure between
nanotubes and ribbon-shaped nanoﬁbers (Fig. 3). The
right part of the 110 nm wide, 2 lm long ﬁber has theshape of a thin ribbon. Its TED pattern (right inset)
, andindicates a hexagonal pattern with d100 ¼ 2:97 A agrees with the a –b plane of NbSe2 . However, the restof the material is folded and the end of the ﬁber isalmost completely scrolled, just like scroll-type CNT’s[13].
3.2. Formation model
Next, we present a model of nanotube formation via
intermediate nanoﬁber (Fig. 4 shows a schematic).
Usually a ﬁber-shaped crystal of NbSe2 cannot be
formed by an equilibrium growth, because the largest

475

facet should have six-fold rotation symmetry. However,
we found the possibility of the transition from a NbSe3
ﬁber into a NbSe2 ﬁber by deselenidation. NbSe3 has achain-like structure along the b-axis and always makes aﬁber-shaped crystal. A deseleniditive transition fromNbSe3 chains to a NbSe2 layer would be achieved
through union of an array of parallel chains into a plane(the conﬁguration of Nb atoms along a chain is keptunchanged) because the Nb–Nb distance along a NbSe3) is very close to that within a NbSe2chain (3.478 A
plane (3.444 A) and the intra-chain bonding is strong
compared to the inter-chain bonding. As a result, aNbSe3 ﬁber made of parallel chains becomes a NbSe2
ﬁber made of stacked strips.
This model is consistent with experimental results.
We observed that the a-axis of most NbSe2 ﬁbers isalong the longitudinal direction. This fact agrees withthe atomic conﬁguration of our model, in which thea-axis of NbSe2 , the direction of Nb–Nb bonds, replacesthe b-axis of NbSe3 that is along the length of a ﬁber aswell as Nb–Nb bonds. Although we supposed presenceof NbSe3 in ﬁrst place, it is reasonable that NbSe3
are synthesized ﬁrst during heating process, becausetemperature reaches the optimum reaction temperature for NbSe3 (740 C) earlier than that for NbSe2(800 C).
In expansion of this model, the deselenidation canalso cause curvature to a NbSe2 ﬁber. The deselenidation would propagate from the surface to inner layers.
In the transient stage of the propagation, a few NbSe2layers cling to surface of NbSe3 . On this occasion, discord between Nb–Nb distance within a NbSe2 layer andthat across NbSe3 chains eventuate in a stress thatmakes NbSe2 layers scrolled lengthwise (see Fig. 4(d) for
an illustration). A similar mechanism is known forkaolinite minerals [14] that have a cylindrical structure
originated from discord in in-plane atomic distance between adjacent layers.
3.3. Electric properties
Preliminary studies on electric properties of theNbSe2 nanoﬁbers were performed in room temperature.
Standard electron beam (e-beam) lithography and focused ion beam (FIB) deposition techniques were used
to fabricate electrodes on the ﬁbers. By e-beam lithography, four ﬁngers of gold ﬁlm with adhesive layer oftitanium were deposited on the ﬁbers. However, we havenot succeeded by e-beam lithography to make electricalconduction between the electrode and the sample todate. This is probably due to a thin insulating layer on
the surface of the ﬁbers because we could make a good
contact by FIB deposition of tungsten with high acceleration voltage. Since there is a crucial disadvantage ofthe FIB-deposited tungsten due to its superconducting

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T. Tsuneta et al. / Current Applied Physics 3 (2003) 473–476

Fig. 4. Our model of deselenidative transition from a NbSe3 ﬁber to a NbSe2 nanotube. The outermost layer, which we suppose to transit into NbSe2ﬁrst, is colored diﬀerently. (a) A NbSe3 ﬁber composed of one-dimensional molecular chains along the b-axis. The yellow Se atoms are to be removed
by deselenidation. (b) The voids produce inter-chain attraction. (c) The chains unite into a two-dimensional layer of NbSe2 . (d) Because of discord
between inter-chain distance of NbSe3 and in-plane atomic distance of NbSe2 , the NbSe2 layer at surface shrinks dragging the inner NbSe3 layers.
Consequently, a scrolling NbSe2 nanoﬁber is formed.

nature, e-beam lithography should be developed for
low-temperature measurement. From the measurementof the FIB deposited ﬁber, in-plane resistivity was calculated as 7 · 107 X m.
In addition to these methods for making electrodes,
we used the tip of the AFM as an electrode directly
placed on the sample. The conductive tip is made ofsilicon single crystal with platinum coat. The NbSe2ﬁbers were deposited on an indium ﬁlm of a thickness of
200 nm. The performance of this system was tested withmultiwall CNT’s, and conﬁrmed that the resistanceswere in agreement with literature values. By applying
the load of 200 nN at the AFM tip end to penetrate thesurface insulating layer, a current of 5 · 109 A ﬂowed at
1 · 103 V bias, corresponding to the resistance of 200kX. A crude estimation of the resistivity ranges 103 –102 X m. This value is relatively in good agreementwith that perpendicular to the c-axis, 4 · 105 X m.

4. Summary
We have presented evidence of the formation ofNbSe2 nanotubes and nanoﬁbers by chemical vapor
transportation method. And we have proposed a modelof the formation of nanotubes and nanoﬁbers via transition from a NbSe3 nanoﬁber. Also, we conﬁrmed
metallic conductivity of the nanostructures in room
temperature.
It is an important problem whether the NbSe2
nanotube can be a superconductor, because superconductivity is aﬀected by topology of the system. Electronbeam holography with cryogenic system will reveal nature of the electron properties of such the ‘‘topologicalmaterials’’.

Acknowledgements
The authors are grateful to K. Yamaya, Y. Okajima,N. Hatakenaka and M. Hayashi. We also acknowledge
S. Iijima and K. Suenaga for useful discussions, and H.
Takahashi and K. Sugawara for providing experimental
apparatus. This work is supported by Grant-in-Aid for
Scientiﬁc Research (B), Grant-in-Aid for Exploratory
Research, and Grant-in-Aid for Encouragement ofYoung Scientists of the Ministry of Education, Culture,Sports, Science and Technology in Japan.

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