Introduction to Nanotechnology

6.2. NANOSTRUCTURED CRYSTALS 153
quantum dots and the resulting quantum confinement, and surface states on quantum
dots. Porous silicon also displays electroluminscence, whereby the luminescence is
induced by the application of a small voltage across electrodes mounted on the
silicon, and cathodoluminescence from bombarding electrons.
6.2. NANOSTRUCTURED CRYSTALS
In this section we discuss the properties of crystals made of ordered arrays of
nanoparticles.
6.2.1. Natural Nanocrystals
There are some instances of what might be called “natural nanocrystals.” An
example is the 12-atom boron cluster, which has an icosahedral structure, that is, one
with 20 faces. There are a number of crystalline phases of solid boron containing the
BIZ cluster as a subunit. One such phase with tetragonal symmetry has 50 boron
atoms in the unit cell, comprising four BL2 icosahedra bonded to each other by an
intermediary boron atom that links the clusters. Another phase consists of BI2
icosahedral clusters shown in Fig. 6.22 arranged in a hexagonal array. Of course
there are other analogous nanocrystals such as the hllerene C60 compound, which
forms the lattice shown in Fig. 5.7 (of Chapter 5).
6.2.2. Computational Prediction of Cluster Lattices
Viewing clusters as superatoms raises the intriguing possibility of designing a new
class of solid materials whose constituent units are not atoms or ions, but rather
clusters of atoms. Solids built from such clusters may have new and interesting
properties. There have been some theoretical predictions of the properties of solids
made from clusters such as All& The carbon is added to this cluster so that it has
40 electrons, which is a closed-shell configuration that stabilizes the cluster. This is
necessary for building solids from clusters because clusters that do not have closed
shells could chemically interact with each other to form a larger cluster. Calculations
of the face-centered cubic structure Al12C predict that it would have a very small
band gap, in the order of 0.05 eV, which means that it would be a semiconductor. The
possibility of ionic solids made of KAI13 clusters has been considered. Since the
electron affinity of AlI3 is close to that of CI, it may be possible for this cluster to
form a structure similar to KCl. Figure 6.23 shows a possible body-centered
structure for this material. Its calculated cohesive energy is 5.2eV, which can be
compared with the cohesive energy of KCl, which is 7.19eV. This cluster solid is
quite stable. These calculations indicate that new solids with clusters as their
subunits are possible, and may have new and interesting properties; perhaps even
new high-temperature superconductors could emerge. New ferromagnetic materials
could result from solids made of clusters, which have a net magnetic moment.