Chemistry Reference
In-Depth Information
experimental and theoretical chemists and physicists and material scientists. When
performing research in crystalline MOMs one should go beyond the conventional
physics and chemistry frameworks and try to think as an interdisciplinary scientist.
In this topic we shall restrict our studies to solids made out of lowmolecular weight
organic molecules with dimensions typically smaller than 2 nm, because they have
the wonderful tendency to self-assemble in highly ordered structures, so that high-
quality single crystals can be obtained. In this sense they are model systems. The
structure-property relationships can thus be systematically explored in the ideal
situation where disorder can be ignored, an essential issue for the crystal engineer-
ing quest to design materials with predefined properties. However, disorder might
be an essential parameter when e.g., trying to stabilize metastable polymorphs, a
point that will be discussed in Section 5.5.
The paramount advantage of molecular solids over their more classical inorganic
counterparts is that their constituents, the building blocks, are molecules or clusters
that can be designed and synthesized; in other words they can be intentionally
modified. Therefore, we can talk about molecular and crystal engineering and the
goal is to be able to produce materials with predetermined physical properties. We
are not yet at this desired level but the scientific and technical bases are certainly
at hand.
Quantum dots are the engineered counterparts to inorganic materials such as
groups IV, III-V and II-VI semiconductors. These structures are prepared by
complex techniques such as molecular beam epitaxy (MBE), lithography or self-
assembly, much more complex than the conventional chemical synthesis. Quantum
dots are usually termed artificial atoms (0D) with dimensions larger than 20-30
nm, limited by the preparation techniques. Quantum confinement, single electron
transport, Coulomb blockade and related quantum effects are revealed with these
0D structures (Smith, 1996). 2D arrays of such 0D artificial atoms can be achieved
leading to artificial periodic structures.
Neither polymers nor liquid crystals will be studied in this topic, but at some
stages will be recalled for the sake of comparison. Many topics and review articles
devoted to these subjects can be found in the literature and interested readers are re-
ferred to them(e.g., Nalwa, 1997: Vols. 2-4; Pope&Swenberg, 1999; Heeger, 2001).
1.1 Complex simplicity
Let us start this section with the example of the formation of a molecular solid from
the archetypal inert homonuclear linear diatomic nitrogen molecule, N
2
, the most
abundant gas in air. N
2
exhibits very strong intramolecular bonding, with a binding
energy of
c
.10eV(
1000 kJ mol
−
1
), a large value due to the triple bond. Solid-
phase N
2
will help us to introduce some concepts throughout the topic, profiting
