Page 76 - Nanotechnology an introduction

electrons or photons). Their applications are mostly either “pure” information processing (e.g., a logic gate) or sensory (e.g., detection of a
magnetic field, or a chemical). “Smart” materials are covered in Section 4.2 (e.g., for drug delivery) or in Section 6.5 (e.g., self-repairing structures).
A further possible classification is according to the number of terminals: typically either two (e.g., a diode) or three (e.g., a transistor), the latter
corresponding to a machine with input in cybernetic terminology. Another one introduces the role of internal states and device history: level 1
devices have no internal states and the output only depends on the input (e.g., a resistor); the output of a level 2 device depends on both input and
its internal state (e.g., a condenser); the output of a level 3 device also depends on the history of its inputs (e.g., a memristor).

One may note that most current activity involving nanodevices is taking place in research laboratories, with the potential for showing dramatic
industrial growth in the future.
The first section of this chapter briefly surveys some general consequences of miniaturization. This is given special consideration due to the
particular importance of digital information processing as an application of nanotechnology. The main alternative is quantum computation, which is
summarized in the next section. Specific devices able to execute information processing operations, based on electron charge or spin or on
photons are then considered. Finally nanomechanical (relays and sensors) and fluidic (mixers) devices are described. The concluding sections of
the chapter deal with sensors and energy-transducing devices.
7.1. Issues of Miniaturization
This section may be read in conjunction with Section 1.5.2.

Surface Relative to the Bulk Interior
An object is delineated by its boundary. Making an object small has an effect on purely physical processes in which it is involved. For example,
3
suppose a spherical object of radius r is heated by internal processes, and the amount of heat is proportional to the volume V = 4π r /3. The loss of
2
heat to the environment will be proportional to the surface area, A = 4π r . Now let the object be divided into n particles. The total surface area is
1/3
2
now n 4π r , hence more heat is lost. This is the basic reason why small mammals have a higher metabolic rate than larger ones—they need to
produce more heat to compensate for its relatively greater loss through the skin in order to keep their bodies at the same steady temperature. It
explains why few small mammals are found in the cold regions of the Earth.
Catalysis and Heterogeneous Chemistry
Reactions take place at surfaces (i.e., the interface between the catalyst and the reaction medium, the latter considered to be three-dimensional).
Indeed in some cases the main catalytic effect is due to the reduction in dimensionality imposed on the system [140]. In general, the greater the
degree of nanostructuring the greater the preponderance of surface per unit mass.

Ballistic Transport
(cf. Section 7.4.1). Usually, carriers cannot move through a medium without encountering some resistance, which is caused by the carrier particles
colliding with (being scattered by) obstacles (which might be their congeners). The characteristic length associated with this scattering is the mean
free path ℓ. If some characteristic size l of the device is less than ℓ, transport will be ballistic and the carrier can move from one end of the device to
the other without encountering any resistance. Similar reasoning can be applied to heat conduction, with resistance normally due to phonon
scattering. Since mean free paths in condensed matter are usually of the order of nanometers long, they are likely to be useful candidates for
defining the nanoscale (cf. Chapter 2).
How Performance Scales with Size
(cf. Section 10.8). Analysis of device performance begins by noting how key parameters scale with device length: area (hence power and thermal
losses) as length squared, volume and mass as length cubed, natural frequency as inverse length, and so forth. Relationships such as these are
used to derive the way a device's performance scales as it is made smaller [73]. This consideration is apart from qualitatively new phenomena that
may intervene at a certain degree of smallness (cf. Chapter 2, Figure 3.1).

Noise in Detectors (Sensors and Dosimeters)
Natural phenomena involving discrete noninteracting entities (e.g., the spacial distribution of photons) can be approximated by the Poisson
distribution (equation 10.15). A fundamental property of this distribution is that its variance equals its mean. The uncertainty (e.g., of the magnitude
of a certain exposure of an object to light) expressed as a standard deviation therefore equals the square root (of exposure).
When objects become very small, the number of information carriers necessarily also becomes small. Small signals are more vulnerable to noise
(i.e., the noise is an increasing proportion of the signal). Repetition of a message (e.g., sending many electrons) is the simplest way of overcoming
noise. A nanoscale device using only one entity (e.g., an electron) to convey one bit of information would, in most circumstances, be associated
with an unacceptably high equivocation in the transmission of information.
An accelerometer (that transduces force into electricity) depends on the inertia of a lump of matter for its function and if the lump becomes too small
the output becomes unreliable. Similarly with photodetectors (that transduce photons into electrons): due to the statistical and quantum nature of
light, the smallest difference between two levels of irradiance that can be detected increases with diminishing size. On the other hand, for many
purposes there is no intrinsic lower limit to the physical embodiment of one bit of information. One bit could be embodied by the presence of a
neutron, for example—indeed distinguishable isotopes have been considered as the basis for ultrahigh density information storage [15].
Information processing and storage is therefore the ideal field of application for nanotechnology. The lower limit of miniaturization is only dependent
on practical considerations of “writing” and “reading” the information. Hence, nanotechnology is particularly suited to information processors.
Utility
Considering the motor car as a transducer of human desire into translational motion, it is obvious that a nanoautomobile would be useless for
transporting anything other than nano-objects. The main contribution of nanotechnology to the automotive industry is in providing miniature sensors
for process monitoring in various parts of the engine and air quality monitoring in the saloon; additives in paint giving good abrasion resistance,
possibly self-cleaning functionality, and perhaps novel aesthetic effects; new ultrastrong and ultralightweight composites incorporating carbon
nanotubes for structural parts; sensors embedded in the chassis and bodywork to monitor structural health; and so forth.