of approximately 60 nm and a length of a few micrometers. The red lines in
Figure 5.26 describe the magnetization during the change of the external field.
The experiment was started at an external magnetic field of À1000 mT and changed
gradually up to 400 mT. In Figure 5.26, the arrows 1a and 1b follow this trace. After
reaching the maximum, the magnetic field was again reduced to zero (indicated by
arrow 2). While increasing the magnetic field from À1000 to 400 mT, at approximately 250 mT the magnetization switches from a negative to a positive value. A red
arrow indicates this switching point, A. If the experimental data drawn with red lines
are extrapolated to negative magnetic fields, a field is found where the magnetization
of the composite may change its direction again. This point, obtained by extrapolation, is indicated as B. As the magnetization loop shown in Figure 5.26 is
rectangular, the change in the direction of the magnetization occurs at exactly
defined values of the external magnetic field. Therefore, an iron-filled carbon
nanotube acts as a magnetic switch with clearly defined switching points. The
energy product of this aggregate (remanent magnetization  coercitivity) is quite
large.
5.2.2
Nanotubes and Nanorods from Materials other than Carbon
At a very early stage, Tenne et al. [13] showed that, in general, all compounds that
crystallize in layered structures may form nanotubes and fullerene-like structures.
The first noncarbon nanotubes consisted of MoS 2 and WS 2 , but such structures
were later observed with the selenides of molybdenum and tungsten [14]. Despite
minor differences, these compounds are built according to the same scheme and,
like graphite, they crystallize in layered structures; however, in this case, each layer is
built up of three sublayers consisting either of metal (Me) or nonmetal ions (X):
XÀMeÀX XÀMeÀX XÀMeÀX XÀMeÀX . . .
Within each triple package of layers, there is covalent bonding; in between the
packages the bonding is of the van der Waals type. Hence, the packages can be
shifted against each other, and this is the reason why MoS 2 and WS 2 are, like
graphite, used on a technical basis as solid lubricants. Another potentially important
compound that forms nanotubes and fullerene-like structures is BN. Although this
compound crystallizes in the same structure as graphite, it has no free electrons and
therefore it is an insulator, and the color is white.
An electron micrograph of WS 2 nanotubes with diameters ranging from 15 to
20 nm is shown in Figure 5.27. The insert in Figure 5.27 shows one of the nanotubes
at a higher magnification, such that the four layers of the multiwall nanotube are
clearly visible. When comparing these micrographs with those of carbon nanotubes,
there is a significantly better contrast; this is due to the higher atomic number of
tungsten compared to carbon. Like carbon, these compounds form not only nanotubes, but also fullerene-like structures.
It is of interest to note that these ball-shaped, fullerene-like structures often
consist of many layers. Insofar, they are zero-dimensional in analogy to multiwall
5.2 Nanostructures Related to Compounds with Layered Structures j109
Figure 5.26 describe the magnetization during the change of the external field.
The experiment was started at an external magnetic field of À1000 mT and changed
gradually up to 400 mT. In Figure 5.26, the arrows 1a and 1b follow this trace. After
reaching the maximum, the magnetic field was again reduced to zero (indicated by
arrow 2). While increasing the magnetic field from À1000 to 400 mT, at approximately 250 mT the magnetization switches from a negative to a positive value. A red
arrow indicates this switching point, A. If the experimental data drawn with red lines
are extrapolated to negative magnetic fields, a field is found where the magnetization
of the composite may change its direction again. This point, obtained by extrapolation, is indicated as B. As the magnetization loop shown in Figure 5.26 is
rectangular, the change in the direction of the magnetization occurs at exactly
defined values of the external magnetic field. Therefore, an iron-filled carbon
nanotube acts as a magnetic switch with clearly defined switching points. The
energy product of this aggregate (remanent magnetization  coercitivity) is quite
large.
5.2.2
Nanotubes and Nanorods from Materials other than Carbon
At a very early stage, Tenne et al. [13] showed that, in general, all compounds that
crystallize in layered structures may form nanotubes and fullerene-like structures.
The first noncarbon nanotubes consisted of MoS 2 and WS 2 , but such structures
were later observed with the selenides of molybdenum and tungsten [14]. Despite
minor differences, these compounds are built according to the same scheme and,
like graphite, they crystallize in layered structures; however, in this case, each layer is
built up of three sublayers consisting either of metal (Me) or nonmetal ions (X):
XÀMeÀX XÀMeÀX XÀMeÀX XÀMeÀX . . .
Within each triple package of layers, there is covalent bonding; in between the
packages the bonding is of the van der Waals type. Hence, the packages can be
shifted against each other, and this is the reason why MoS 2 and WS 2 are, like
graphite, used on a technical basis as solid lubricants. Another potentially important
compound that forms nanotubes and fullerene-like structures is BN. Although this
compound crystallizes in the same structure as graphite, it has no free electrons and
therefore it is an insulator, and the color is white.
An electron micrograph of WS 2 nanotubes with diameters ranging from 15 to
20 nm is shown in Figure 5.27. The insert in Figure 5.27 shows one of the nanotubes
at a higher magnification, such that the four layers of the multiwall nanotube are
clearly visible. When comparing these micrographs with those of carbon nanotubes,
there is a significantly better contrast; this is due to the higher atomic number of
tungsten compared to carbon. Like carbon, these compounds form not only nanotubes, but also fullerene-like structures.
It is of interest to note that these ball-shaped, fullerene-like structures often
consist of many layers. Insofar, they are zero-dimensional in analogy to multiwall
5.2 Nanostructures Related to Compounds with Layered Structures j109