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devices very directly embrace the properties of quantum physics to serve their
function. In this section, we briefly describe various nanoscale device technologies,
referring to the specific chapters where topics are discussed in more detail.
It should be noted that this introductory material is not intended to be a
comprehensive list of nanoscale devices; such information can be found in later
chapters. In fact, this section only describes a mere fraction of the devices that are
being explored at the nanometer scale. Instead, the purpose of this section is to
give an intuitive understanding for several common aspects of nanoscale devices.
1.4.1. Molecular Devices
In general, there are a huge variety of molecules and structures that can be
explored (for example DNA, proteins, rotaxanes, nanotubes, and more) [10]. In
some sense, atoms and molecules are just highly complicated toy blocks: there are
an infinite number of ways to assemble molecules into something useful, limited
only by the creativity of future research.
Molecular structures can be used to create very tiny switches, ranging from 1
to 10 nm in size. One possible approach is to control how easily electrons can flow
through the molecule, very much like a transistor, but with different underlying
physics (e.g., [11]). Another possible approach is to control how light is absorbed
or scattered by the molecules (e.g., [12]). These interactions with molecules can be
controlled in many different ways, for example, by applying a nearby voltage or by
changing the structure of molecules. Molecular switches and molecular computing
are discussed further in Chapter 11.
A big challenge with molecular switches—and many nanoscale devices—is to
effectively fabricate and interconnect them to perform complex logic functions. In
an attempt to circumvent these problems, one proposed molecular device is the
NanoCell [13]. The NanoCell tolerates defects and variability that occur during
self-assembly fabrication. To provide reliability, the NanoCell depends on post-
fabrication ''training'' to create the desired logic function. This approach is
interesting for two reasons. First, the logic function of the NanoCell can (ideally)
be reconfigured instead of permanently fixed; second, it allows the use of larger
and fewer wires to connect between different cells. The function implemented by a
single cell would be equivalent to using many transistors, thus simplifying the
arrangement of large-scale computations.
1.4.2. Nanotubes
One interesting class of molecular devices is nanotubes, particularly carbon
nanotubes. Recall that pure carbon has two common crystalline forms: diamond,
where carbon atoms form a three-dimensional structure; and graphite, where
carbon atoms form flat sheets that can easily slide and peel from each other. A
single-wall carbon nanotube (Fig. 1.8) can be visualized as a single sheet of
graphite rolled into a tube (though it is not created in this way), with a diameter of
only a few nanometers.
 
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