NANOSCALE DEVICES: APPLICATIONS AND MODELING - Bio-Inspired and Nanoscale Integrated Computing - page 33

such a large collection of atoms is put together, the general behavior of the
ensemble in everyday life experiences does not directly reflect the atomistic nature
of the system. It looks like a solid, bulky material for which the behavior can be
explained relatively easily: You can describe it in terms of its macroscopic
properties, i.e., weight, dimensions, color, etc. You can also use the laws of
classical physics, such as Newtonian mechanics, to describe its motion if a force is
applied to it.
In fact, atoms behave according to laws discovered much later than Sir Isaac
Newton's classical mechanics. The laws of quantum mechanics, developed mainly
in the first half of the twentieth century, are rather nonintuitive for someone who
is used to everyday phenomena with large objects, but they constitute the best
description we have so far for the world of small particles such as atoms. Let us
continue with the example of the topic. When such a huge collection of atoms is
put together, the overall behavior will be a statistical average of the behavior of
the individual atoms, subject to external forces and the constraints that keep them
together. This average behavior can be explained well with classical physics. The
result is that we do not really need quantum mechanics to be able to handle a
topic! Now, imagine a microelectronic device, such as a transistor, with dimen-
sions on the order of a micrometer. Such a device would contain about one trillion
atoms (a 1 with 12 zeros in front of it). That is still a very large number and so,
even in such a small device, most of the characteristics are determined by the
statistical averages of many interactions and phenomena. Thus, although in order
to properly understand these phenomena and the resulting average behaviors one
needs to apply quantum mechanics, more phenomenological laws can be derived
using statistical analysis that explain the device behavior accurately enough
for most engineering purposes. In fact, quantum mechanics is still somewhat
masked and its effects are only indirectly visible at the microscale. In other words,
you do not need to resort to quantum mechanics every time you want to study or
use a microscale transistor. In this sense, even microdevices could be considered
somewhat classical.
Now let us imagine devices that have dimensions on the order of only a few
nanometers. In matter, atoms are spaced apart by a few angstroms. An angstrom
is 10 times smaller than a nanometer. Thus, in the volume of our nanoscale device,
there would be only a few hundred or thousand atoms. It turns out that in such
small collections of atoms, statistical averages are not always very meaningful. But
the individual character of each atom—its quantum mechanical nature—is much
more visible. Therefore, when working with nanodevices one observes quantum
mechanics very directly and, by the same token, can take advantage of the laws of
quantum mechanics to design devices and systems that are not achievable at larger
scales (that is, where quantum effects have averaged out to give way to more
classical behavior). This direct access to quantum mechanics at the nanoscale
makes a fundamental difference that involves more than manipulating
small objects or squeezing a large number of devices into a small area. A whole
new world of functionalities and possibilities previously unimaginable is opened
up. One is tempted to argue that this is the most important aspect of the nano