Biomedical Engineering Reference
In-Depth Information
AFM-topography
EFM-before charging
EFM-after charging
7
6
after
charging
5
4
3
2
1
0
-1
before
charging
-2
1.0
0.5
1.5
2.0
2.5
500 nm
A
B
C
D
Distance [µm]
2.0
AFM
0.2
1.6
Semicond
Metallic
SWNT diameter
1.2
300 nm
0.1
0.8
EFM
0.4
0.0
0.0
300 nm
0
2468 0
Force / length (N/m)
12
14
Fig. 7.17. Electrical measurements of carbon nanotubes. Top: charge injection into nanotubes. A:
topography of isolated MWNT, and its EFM image before charging (B). After charge injection by
the AFM probe (at the arrowed location), the EFM contrast reverses(C). D: a line profile through the
EFM images at the point indicated by the white line. Bottom: measurement of the effect of
mechanical compression on charging behaviour of SWNTs. Left: AFM and EFM (measured by
lifting) images of a SWNT after charge injection by the AFM tip. Right: plot showing charge density
º
(red triangles) as a function of compression of the semiconducting CNT by the probe (indicated by
the diameter,
d
, which decreases as the CNT is compressed, green circles). The black squares show
the behaviour of a metallic SWNT, illustrating how compression with the AFM changes the
behaviour of the semiconducting SWNT to match the metallic behaviour. Adapted with permission
from [550] and [558]. Lower figures copyright (2008) by the American Physical Society. (A colour
version of this illustration can be found in the plate section.)
AFM has unique advantages for the electrical characterization of carbon nanotubes,
specifically the ability to combine high-resolution imaging with electrical characterization
both by imaging and spectroscopy of electrical properties, at the single-particle level. An
example of the use of AFM techniques in the characterization of carbon nanotube
electrical properties in shown in Figure 7.17. In the upper portion of the figure, the effect
of charging on the EFM signal from CNTs is given. The leftmost image shows a standard
IC-AFM height image of an isolated MWNT. EFM was carried out in a lifting mode using
conducting probes, and the initial result is shown in the second image, which shows a
negative frequency shift of the cantilever over the CNT, indicating attractive force
between the probe and CNT. Charge was then injected into the nanotube using the AFM
probe, by approaching the tube with a bias applied between the sample and the probe.
