Nanotechnology-Enabled Sensors

266 Chapter 5: Characterization Techniques for Nanomaterials
Fig. 5.44 shows STM images of single-walled carbon nanotubes
(SWCNTs) measured in ultra-high vacuum at 77 K, 98 with bias voltages of
50 and 150 mV, respectively, and a tunneling current of 150 pA. It is seen
that the STM analysis was able to resolve the hexagonal-ring structure of
the walls of the nanotubes, with the size of each hexagons being just a few
Ångstroms. A small portion of a two-dimensional expected honeycombe
lattice is overlayed in the figure to highlight the atomic structure of the
nanotubes.
Fig. 5.44 STM images of (a) a SWCNT exposed at the surface of a rope and (b)
isolated SWCNTs on a Au(111) substrate. The tube axes in both images are indicated
with solid, black arrows, and the zigzag direction are highlighted by dashed
lines. A portion of a two-dimensional graphene layer is overlaid in a to highlight
the atomic structure. Reprinted with permission from the Nature publications. 98
With the STM molecules adsorbed on surfaces and the strength of their
bonds can also be studied. Obviously, whether such molecules are adsorbed
during the growth process of thin films or nanostructures, or whilst
being immobilized onto the surface of another material, or whether they
are gas molecules being adsorbed during a chemical reaction, studying
such adsorptions may have strong relevance for sensing applications. This
issue will be discussed more in Chap. 7.