In the last decade, new directions of modern research, broadly defined as ‘‘nano-

scale science and technology’’, have emerged [1, 2]. These new trends involve the

ability to fabricate, characterize, and manipulate artificial structures, whose fea-

tures are controlled at the nanometer level. They embrace areas of research as

diverse as engineering, physics, chemistry, materials science, and molecular biol-

ogy. Research in this direction has been triggered by the recent availability of

revolutionary instruments and approaches that allow the investigation of material

properties with a resolution close to the atomic level. Strongly connected to such

technological advances are the pioneering studies that have revealed new physical

properties of matter at a level intermediate between atomic/molecular and bulk.

Materials science and technology is a field that is evolving at a very fast pace and

is currently giving the most significant contributions to nanoscale research. It is

driven by the desire to fabricate materials with novel or improved properties. Such

properties can be, for instance, strength, electrical and thermal conductivity, optical

response, elasticity, or wear resistance. Research is also evolving toward materials

that are designed to perform more complex and efficient tasks. Examples include

materials that bring about a higher rate of decomposition of pollutants, a selective

and sensitive response toward a given biomolecule, an improved conversion of

light into current, or more efficient energy storage. For such and more complex

tasks to be realized, novel materials have to be based on several components whose

spatial organization is engineered at the molecular level. This class of materials

can be defined as ‘‘nano-composites’’. They are made of assembled nanosized ob-

jects or molecules. Their macroscopic behavior arises from the combination of the

novel properties of the individual building blocks and their mutual interaction.

In electronics, the design and the assembly of functional materials and devices

based on nanoscale building blocks can be seen as the natural, inevitable evolution

of the trend toward miniaturization. The microelectronics industry, for instance, is

fabricating integrated circuits and storage media whose basic units are approaching the size of few tens of nanometers. For computers, ‘‘smaller’’ means higher

computational power at lower cost and with higher portability. However, this race

toward higher performance is driving current silicon-based electronics to the limits

of its capability [3–6]. The design of each new generation of smaller and faster

devices involves more sophisticated and expensive processing steps, and requires

the solution of new sets of problems, such as heat dissipation and device failure.

If the trend toward further miniaturization persists, silicon technology will soon

reach limits at which these problems become insurmountable. In addition to this,

scientists have found that device characteristics in very small components are

strongly altered by quantum mechanical effects. In many cases, these effects will

undermine the classical principles on which most of today’s electronic compo-

nents are based. For these reasons, alternative materials and approaches are cur-

rently being explored for novel electronic components in which the laws of quan-

tum mechanics regulate their functioning in a predictable way. Perhaps in the near

future a new generation of computers will rely on fundamental processing units

that are made only of a few atoms.

Fortunately, the advent of new methods for the controlled production of nano-

scale materials has provided new tools that can be adapted for this purpose. New

terms such as nanotubes, nanowires, and quantum dots are now common jargon

of scientific publications. These objects are among the smallest man-made units

that display physical and chemical properties which make them promising candi-

dates as fundamental building blocks for novel transistors. The advantages envis-

aged here are higher device versatility, faster switching speed, lower power dis-

sipation, and the possibility of packing many more transistors on a single chip.

Prototypes of these new single nano-transistors are nowadays fabricated and

studied in research laboratories and are far from commercialization. How millions

of such components could be arranged and interconnected in complex archi-

tectures and at low cost still remains a formidable problem to be solved.

With a completely different objective, the pharmaceutical and biomedical indus-

tries try to synthesize large supramolecular assemblies and artificial devices that

mimic the complex mechanisms of nature or that can be potentially used for more

efficient diagnoses and better cures for diseases. Examples in this direction are

nanocapsules such as liposomes, embodying drugs that can be selectively released

in living organs, or bioconjugate assemblies of biomolecules and magnetic (or

fluorescent) nanoparticles that may provide faster and more selective analysis of

biotissues. These prototype systems may one day evolve into more complex nano-

machines with highly sophisticated functional features able to carry out compli-

cated tasks at the cellular level in a living body.

This chapter is not meant as a survey of the present state and future develop-

ments of nanoscale science and technology, and the list of examples mentioned

above is far from being complete. Nanoscience and nanotechnology will definitely

have a strong impact on our lives in many disparate areas. We can mention, as the

most significant examples, information technology and the telecommunications

industry, materials science and engineering, medicine. In this introductory chap-

ter, we want to stress the point that any development in nanoscience necessarily

requires an understanding of the physical laws that govern matter at the nanoscale

and of how the interplay of the various physical properties of a nanoscopic system

translates into some novel behavior or into a new physical property. In this sense,

the chapter will serve as an overview of basic physical rules governing nanoscale

materials, with a particular emphasis on quantum dots, including their various

physical realizations and their possible applications. Quantum dots are the ulti-

mate example of a solid in which all dimensions shrink down to a few nanometers.

Moreover, semiconductor quantum dots are probably the most studied nanoscale

systems.

The outline of this chapter is as follows. In Section 2.2 we try to explain with a

few examples why the behavior of nanoscale materials can be remarkably different

from that of bulk materials and from their atomic counterparts, and how quantum

mechanics can help us in rationalizing this. Following this discussion, we give a

definition of a ‘‘quantum dot’’. In Section 2.3 we follow a bottom-up approach and

give the simplified picture of a solid as being a very large molecule, where the en-

ergy levels of the individual atomic components have merged into bands. The

electronic structure of a quantum dot, being intermediate between that of the two

extreme cases of single atoms and bulk material, will then be an easier concept to

grasp. In Section 2.4 we use the model of a free-electron gas and the concept of

quantum confinement to explain what happens to a solid when its dimensions

shrink one by one. This leads us to a more accurate definition of quantum well,

quantum wire, and quantum dot. In Section 2.5 we examine in more detail the

electronic structure of quantum dots, although we try to keep the discussion at a

simple level. Section 2.6 is a brief overview of the most popular methods used to

fabricate quantum dots. Different methods lead to different varieties of quantum

dots, which can be suited for specific applications. In Section 2.7 we discuss the

optical properties of quantum dots. As they are quite unique for this class of

materials, the optical properties are probably the most important reason why the

research on quantum dots has exploded in the last decade. The discussion here

will be focused more on colloidal nanocrystal quantum dots. Electrical and trans-

port properties are nonetheless extremely relevant, as is described in Section 2.8,

since, for instance, the addition or subtraction of a charge from a quantum dot

leads to dramatic modification of its electronic structure and of the way the dot will

handle a further addition or subtraction of a charge. This can be of fundamental

importance for future applications in electronics.