The Instability of the NbTe2 Surface Structure D - Department of ...

phys. stat. sol. (a) 193, No. 2, 246–250 (2002)
The Instability of the NbTe 2 Surface Structure
D. Cukjati (a), A. Prodan 1 ) (a), N. Jug (a), H. J. P. Van Midden (a),
S. W. Hla (b), H.Böhm (c), F. W. Boswell (d), and J. C. Bennett (e)
(a) Institute Jozef Stefan, Jamova 39, 1000 Ljubljana, Slovenia
(b) Department of Physics and Astronomy, Ohio University, Athens, OH 45701, USA
(c) Institut für Geowissenschaften, Johannes Gutenberg Universität, 55099 Mainz,
Germany
(d) Department of Physics, University of Waterloo, Waterloo, Ontario, N2L 3G1, Canada
(e) Department of Physics, Acadia University, Wolfville, Nova Scotia, B0P 1X0, Canada
(Received October 23, 2001; in revised form May 30, 2002; accepted May 30, 2002)
PACS: 61.14.Hg; 68.35.Bs; 68.37.Ef; 68.90.þg
Low energy electron diffraction from clean NbTe 2 surfaces shows very diffuse reflections. The
effect is attributed to an anisotropic heating of the irradiated Te–Nb–Te surface layer. Diffraction
patterns for electron energies below 90 eV correspond to an overlapped contribution from numerous
domains, belonging to three orientational variants. Electrons of higher energies stabilize the
parent high-temperature CdI 2 structure. A similar effect is observed during scanning tunneling
microscopy, where the expected surface corrugation is usually lost for tunneling currents of a few
ten nA.
1. Introduction
NbTe 2 belongs to the family of layered transition-metal dichalcogenides (MX 2 ), known
for their quasi two-dimensional character [1] and interesting electronic properties [2]. It
crystallizes in a deformed CdI 2 structure with corrugated Te–Nb–Te sandwiches, separated
by van der Waals (vdW) gaps (Fig. 1). The monoclinic structure is built of
deformed octahedra with two non-equivalent Nb and three non-equivalent Te positions.
Nb atoms are shifted towards one of the six corners of the deformed Te octahedra,
which results in an enlarged unit cell with a ¼ 1.939 nm, b ¼ 0.3642 nm, c ¼ 0.9375 nm,
and b ¼ 134 35 0 and in reduced symmetry with the space group (SG) C2/m [3]. The
surface Te atoms form chains at three different heights, aligned along the b direction.
Transmission electron microscopy (TEM) and diffraction revealed a domain structure
with three possible orientational variants [4]. These are separated by ortho, meta, and
para boundaries with two adjacent domains forming 60 , 120 , and 180 , respectively. In
addition, anti-phase boundaries, which displace the corrugated Te chains along the
monoclinic a direction by one octahedral chain, were also observed. Scanning tunneling
(STM) and atomic force microscopy of the corrugated vdW surfaces were performed
[5–7] and it was shown that the strong local field between the tip and the surface
modifies the domain boundaries [7].
There were a few reports published on low energy electron diffraction (LEED) studies
of the MX 2 compounds. It was shown that after Na intercalation into VSe 2 the
corresponding LEED patterns become only fainter and that no superlattice spots were
1 ) Corresponding author; e-mail: albert.prodan@ijs.si
# 2002 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 0031-8965/02/19309-0246 $ 17.50þ.50/0

phys. stat. sol. (a) 193, No. 2 (2002) 247
Fig. 1. Crystal structure of NbTe 2 . Layers formed of deformed coordination octahedra are separated
by vdW gaps
detected in the corresponding LEED patterns [8]. LEED also shows very well charge
density wave (CDW) formation in 1T-TaS 2 and 1T-TaSe 2 . Recently, the dependence of
the CDW phases in these compounds on the function of intercalation [9, 10] and surface
nanostructuring [11, 12] was studied in detail. On the contrary, no LEED studies
of NbTe 2 or any other corrugated MX 2 compound were reported so far.
In the present paper we show that LEED, combined with STM, provides new information
about the stability of the NbTe 2 surface structure.
2. Experimental
NbTe 2 crystals were grown by iodine transport in evacuated quartz tubes [13] between
1123 and 1073 K. Layered single crystals, a few millimeters large, were obtained after
about a week of growth. The samples were mounted onto supporting Ta plates with a
graphite paste and cleaved along their vdW gaps. Freshly cleaved surfaces were examined
under UHV conditions (10 –9 –10 –10 hPa) at room temperature in an Omicron
STM-1, combined with a LEED/Auger spectrometer.
3. Results
LEED patterns from NbTe 2 with electron energies 80, 89, 100, and 113 eV are shown in
Figs. 2a–d. All images show characteristic diffuse reflections and are fully reproducible
in the given energy range, as long as prolonged beam heating does not deteriorate too
much the quality of the examined crystal. Only at energies below about 90 eV the patterns
are composed of overlapped contributions from domains of three orientational
variants. At slightly higher energies the reflections characterizing the deformation of
the NbTe 2 octahedra disappear and patterns characteristic of the parent CdI 2 structure
are detected. At energies above about 150 eV all reflections in the LEED patterns disappear
and an enhanced background covers the entire screen.

248 D. Cukjati et al.: The Instability of the NbTe 2 Surface Structure
Fig. 2. Series of LEED patterns from the NbTe 2
(001) surface for electron energies a) 80, b) 89,
c) 100, and d) 113 eV. The additional reflections in
a) correspond to three overlapped orientational
variants
A similar effect was observed in the LEED
patterns of NbTe 2 if the intensity of the electron
beam was varied between zero and its
maximum value of about 2 mA, while the energy
of the electrons was kept constant at
about 90 eV. However, this dependence was
difficult to follow, mainly because the already
poor LEED patterns are further deteriorated
with reduced beam intensity.
STM images of the NbTe 2 vdW surfaces
show in addition to surface defects, domain
boundaries, and antiphase boundaries [7] a
characteristic dependence on the tunneling current.
At low tunneling currents the two uppermost
Te chains are usually not resolved
(Fig. 3a). All three different Te chain types become
clearly distinguished at tunneling currents
of a few nA (Fig. 3b), while the corrugation
is almost lost for tunneling currents larger
by one order of magnitude (Fig. 3c).
4. Discussion
For the energies used in the described LEED
experiments, the penetration depth of the impinging
electrons fits approximately to the
thickness of one Te–Nb–Te layer [14]. Thus,
the origin of the very diffuse nature of the
NbTe 2 LEED reflections can be either in the
fine domain structure of the surface layer or in
an enhanced thermal disorder. The contribution
of both can be estimated.
In case the broadening of the reflections is a
consequence of the domain structure, the average
sizes of the domains can be estimated by
comparing the diameters of the diffuse reflections
with the inter-reflection distances [15]. If
the much smaller instrumental contribution is
ignored, the broadening corresponds in the present case to average domain sizes of about
10nm. On the contrary, the sizes estimated from TEM and STM measurements were

Both LEED and STM studies reveal an unstable surface structure of NbTe 2 . It can be
modified by varying the energy of the impinging electrons or the intensity of the elecphys.
stat. sol. (a) 193, No. 2 (2002) 249
Fig. 3. Dependence of the NbTe 2 STM image on
the tunneling conditions. All images were recorded
with the constant height mode and with a Pt–Rh
tip: a) (20 nm) 2 , V g ¼ –0.1 mV, I t ¼ 0.8 nA, b)
(10 nm) 2 , V g ¼þ0.1 mV, I t ¼ 2 nA, and c) (10 nm) 2 ,
V g ¼þ0.1 mV, I t ¼ 40 nA
a)
b)
c)
found to be at least an order of magnitude larger.
In addition, a similar broadening of LEED
reflections is observed in other transition-metal
dichalcogenides, which do not show any domain
structure. Thus, the broadening of the reflections
is to be attributed to an enhanced thermal
disorder, caused by strongly anisotropic local
heating of the top Te–Nb–Te layer(s).
The diffraction patterns appear for the first
time for electron energies between 55 and
80 eV. The extinction of the somewhat weaker
reflections above about 90 eV is likewise to be
attributed to beam heating, which triggers a
transition of the top Te–Nb–Te layer(s) into
the parent high-temperature CdI 2 polytype (SG
P 3 m1). Further experiments with the intensity
of the primary beam varied instead of the electron
energy, support this conclusion. On the
other hand, entire bulk samples could not be
heated high enough to achieve the transition,
which was an indication that the surface regions
are more inclined to undergo the described
transition. The enhanced background and the
disappearance of all reflections above about
150 eV are a result of multiple inelastic electron
scattering, which reduces the intensities of the
reflections and enhances the background.
Finally, the STM images recorded at various
tunneling currents show that the structure of
the top Te–Nb–Te triple layer is in a similar
way dependent on the strong local field
between the tunneling tip and the surface,
which can likewise modify the observed surface
corrugation.
5. Conclusions

250 D. Cukjati et al.: The Instability of the NbTe 2 Surface Structure
tron beam in LEED experiments and by using appropriate tunneling currents in STM
measurements. Both types of experiments indicate that at least the top Te–Nb–Te triple
layer can be switched between the collapsed NbTe 2 and its more symmetrical CdI 2
parent structure. The transition is of a displacive type and fully reversible as long as the
quality of the crystal examined is not deteriorated by prolonged beam heating.
Acknowledgements Financial support of the Ministry of Education, Science, and Sport
of the Republic of Slovenia (D.C., A.P., N.J., H.J.P.v.M.), of the Deutsche Forschungsgemeinschaft
(H.B.), and of the Natural Sciences and Engineering Research Council of
Canada (F.W.B., J.C.B.) is acknowledged. A German–Slovenian joint research program
(H.B., A.P.) and a NATO Expert Visit Grant (A.P., J.C.B.) were helpful in facilitating
the collaboration.
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