Introduction

The current revolution in nanoscience was brought about by the concomitant development of several advances in technology. One of them has been the progressive ability to fabricate smaller and smaller structures, and another has been the continual improvement in the precision with which such structures are made. One widely used method for the fabrication of nanostructures is lithography, which makes use of a radiation-sensitive layer to form well-defined patterns on a surface. Molecular-beam epitaxy, the growth of one crystalline material on the surface of another, is a second technique that has become perfected. There are also chemical methods, and the utili2ation of self-assembly, the spontaneous aggregation of molecular groups.

Since the early 1970s the continual advancement in technology has followed Moore's law (Moore 1975) whereby the number of transistors incorporated on a memory chip doubles every year and a half. This has resulted from continual improvements in design factors such as intercoimectivity efficiency, as well as from continual decreases in size. Economic considerations driving this revolution are the need for more and greater information storage capacity, and the need for faster and broader information dispersal through communication networks. Another major

Introduction to Nanotechnology, by Charles P. Poole Jr. and Frank J. Owens. ISBN 0-471-07935-9. Copyright © 2003 John Wiley & Sons, Inc.

factor responsible for the nanotechnology revolution has been the improvement of old and the introduction of new instrumentation systems for evaluating and characterizing nanostructures. Many of these systems are very large and expensive, in the million-dollar price range, often requiring specialists to operate them. The aim of the present chapter is to explain the principles behind the operation of some of these systems, and to delineate their capabilities.

In the following sections we describe instruments for determining the positions of atoms in materials, instruments for observing and characterizing the surfaces of structures, and various spectroscopic devices for obtaining information of the properties of nanostructures [see e.g., Whan (1986)].

3.2. STRUCTURE 3.2.1. Atomic Structures

To understand a nanomaterial we must, first, learn about its structure, meaning that we must determine the types of atoms that constitute its building blocks and how these atoms are arranged relative to each other. Most nanostructures are crystalline, meaning that their thousands of atoms have a regular arrangement in space on what is called a crystal lattice, as explained in Section 2.1.2 (of Chapter 2). This lattice can be described by assigning the positions of the atoms in a unit cell, so the overall lattice arises from the continual replication of this unit cell throughout space. Figure 2.1 presents sketches of the unit cells of the four crystal systems in two dimensions, and the characteristics of the parameters a, b, and 9 for these systems are listed in the four top rows of Table 3.1. There are 17 possible types of crystal structures called space groups, meaning 17 possible arrangements of atoms in unit cells in two dimensions, and these are divided between the four crystal systems in the manner indicated in column 4 of the table. Of particular interest is the most efficient way to arrange identical atoms on a surface, and this corresponds to the hexagonal system shown in Fig. 2.4a.

In three dimensions the situation is much more complicated, and some particular cases were described in Chapter 2. There are now three lattice constants a, b, and c, for the three dimensions x, y, z, with the respective angles a, /?, and y between them (a is between b and c, etc.). There are seven crystal systems in tore dimensions with a total of 230 space groups divided among the systems in the manner indicated in column 4 of Table 3.1. The objective of a crystal structure analysis is to distinguish the symmetry and space group, to determine the values of the lattice constants and angles, and to identify the positions of the atoms in the unit cell.

Certain special cases of crystal structures are important for nanocrystals, such as those involving simple cubic (SC), body-centered cubic (BCC), and face-centered cubic (FCC) unit cells, as shown in Fig. 2.3. Another important structural arrangement is formed by stacking planar hexagonal layers in the manner sketched in Fig. 2.4b, which for a monatomic (single-atom) crystal provides the highest density or closest-packed arrangement of identical spheres. If the third layer is placed directly above the first layer, the fourth directly above the second, and so on, in an A-B-A-B----type sequence, the hexagonal close-packed (HCP) structure results.

Table 3.1. Crystal systems, and associated number of space groups, in two and three dimensions*

Dimension

System

Conditions

Space Groups

0 0

Post a comment