Biomedical Engineering Reference
In-Depth Information
a
b
c
500 nm
50
0.0
-0.2
-0.4
-0.6
-0.8
-1.0
-1.2
1200
40
30
20
10
0
A
A
B
C
B
800
C
dark
365 nm
245 nm
400
0
-3
-2
-1
0
1
2
3
-3
-2
-1
0
0
400
800
1200
Voltage (V)
Voltage (V)
nm
Fig. 7.16. Electrical measurements of nanowires. Top: characterization of a device consisting of
vertically aligned germanium nanowires in an aluminium oxide substrate. The surface was polished
and the aluminium selectively etched away to expose the tops of the nanowires which can be seen in
the topography image (a). Conducting AFM with a platinum-coated probe was used to measure the
current passing thought the nanowires, while potential differences of 20 V (b) or 40 V (c) was
applied between the probe and the other ends of the wires. All the wires seen in the topographic
image are conductive, and some that are not obvious in the topography can be seen in the current
images. Bottom: electrical measurements on ZnO nanorods as a function of distance along the rods,
and of light flux. Left: AFM height image of a ZnO rod showing the locations where
I-V
curves were
measured. Middle:
I-V
curves at different places - the response is uniform, indicating no defects.
Right:
I-V
curves measured on an individual nanorod during dark conditions, and during illumination
at two different wavelengths, showing the frequency-dependence of the device. Adapted with
permission from [565] and [563].
area of nanoscience. All these measurements require that electronics contacts can be
made to the nanosized components, typically with placement resolution on the order of
1 nm. This is a major experimental challenge.
Conducting AFM techniques are ideally suited to overcome this difficulty of electrical
measurements of carbon nanotubes, one of the most important materials for nanoelec-
tronics [550-554]. Directly measuring the properties of individual CNTs is made much
simpler by the ability of AFM to position an electrode (i.e. a conducting AFM probe) at
any point along the tube desired. AFM can also be used to deliberately introduce defects
into pristine CNTs in order to measure the effect on their electrical properties [553,
555-558]. Other nanostructures that have been probed electrically include quantum dots
[559, 560], many types of nanowire [561-565], nanowire-based transistors [566], and even
electrically active biological nanostructures such as single metal-containing proteins
[567-570], and other types of nanostructures [571, 572].
