Page 75 - Nanotechnology an introduction

Chapter Contents

7.1 Issues of Miniaturization 127
7.2 Digital Information Processing 129
7.3 Quantum Computing 133
7.4 Electronic Devices 135
7.4.1 Ballistic Transport 135
7.4.2 Depletion Layers 135
7.4.3 Single-Electron Devices 136
7.4.4 Molecular Electronic Devices 140
7.4.5 Quantum Dot Cellular Automata 141
7.5 Trends in the Miniaturization of Electronics 142
7.6 Spintronics (Magnetic Devices) 145
7.6.1 Ultrasensitive Magnetic Sensors 145
7.6.2 Other Memory Devices 146
7.6.3 Spin-Dependent Transistors 147
7.6.4 Single Spin Logic 147
7.7 Photonic Devices 148
7.8 Mechanical Devices 151
7.9 Fluidic Devices 153
7.9.1 Mixers and Reactors 156
7.9.2 Chemical and Biochemical Sensors 156

7.9.3 Energy Conversion Devices 157
7.10 Summary 158
7.11 Further Reading 159
Consideration of what happens to things when we reduce their size reveals two kinds of behavior. In one group of phenomena there is a discontinuous change of properties at a certain size. This change can be very reasonably taken to
demarcate the nanoscale. The nanoscale therefore reveals itself as property-dependent. In the other group of phenomena the properties change gradually without any discontinuous (qualitative) change occurring. In this case the upper
boundary of the nanoscale is somewhat arbitrary, but one hundred nanometers seems reasonable, and it is immaterial whether this boundary is taken to be approximate or exact. The Hegelian concept of quantitative change becoming
qualitative if great enough can be used to justify the application of the term “nanotechnology”.
Keywords: molecules, surfaces, nucleation, creativity, electromagnetic properties, mechanical properties, quantum behavior, function
A device, fundamentally, is a synonym for an information processor or transducer. Other synonyms are machine or automaton. The most advanced
devices are able to adapt to their environment. Adaptation implies sensing (gathering information), processing (information), and actuating
(transducing information into physical action) functions. In the analysis of living organisms, or in robotics, or more generally in cybernetics these
different functions are often considered as separate units connected via information channels. In the ultraminiature realm of nanotechnology
separation might not be realizable, nor indeed is it necessary, except possibly as an aid to conceptual thinking. Analogous situations are
encountered in integrated circuits. A traditional a.c. to d.c. converter is a little circuit with a number of clearly separable components. In an
integrated device, a piece of pyroelectric material suffices to fulfill the same function. Sensorial nanomaterials [103] already exist (sometimes they
are called “smart” materials), combining the sensing, processing and actuating functions without clearly separating them physically. An excellent
example of such an adaptive material, actually belonging to the realm of chemistry rather than of nanotechnology, is viscostatic motor oil, containing
some randomly coiled long chain hydrocarbons. If the temperature rises the chains unfurl, thereby increasing the viscosity so as to compensate for
the generic viscosity-decreasing effects of temperature. Adaptation, of course, confers on the device a certain independence of being, hence it
deserves to be called an entity rather than a mere object.

Apart from this ultimate integration of the macroscopically inseparable functions involved in adaptation, it is especially appropriate to miniaturize
the hardware associated with information processing, because there is no definite lower bound to the physical embodiment of one bit of
information, considered as its ultimate, irreducible quantum. A single electron or photon can perfectly well carry one bit, even though it lacks
extension. This is the basis of the special strength of the connexion between nanotechnology and information technology.
No clue as to the extent of a device is given by the fundamental definition. A single neuron or logic gate is as much a device as a brain or a
computer. In this chapter we shall, however, interpret “device” as essentially a rather basic part of an information processor, up to the complexity of
a logic gate. A nanodevice is a device with at least one overall dimension in the nanoscale, or comprising one or more nanoscale components
essential to its operation. A nanosystem (cf. Chapter 10) is a system of nanodevices, or any system, the nanoscale features of which are essential
to its function. Figure 7.1 summarizes the main functional categories of devices.

Figure 7.1 The main functional categories of devices. Energy transducers might also have been included as a separate category.

In this chapter, logic gates and data storage devices are also roughly classified according to the nature of their internal information carriers (e.g.,