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

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