pigment whose color is believed to arise from the coexistence of metal nanoparticles, oxide superlattices and an organic pigment. Gold colloids find wide used
in staining glass, and the nanoparticulate pigment purple of Cassius, obtained by
reducing a gold salt using tin (II) chloride is named for the physician Andreas
Cassius who died in 1673 [3].
What is then new in the area that it should receive such renewed attention.
Firstly, a number of tools, both experimental (exemplified by high-resolution
transmission electronic microscopy and scanning tunneling microscopy) and theoretical (exemplified by density functional theory), that permit a much better understanding of these systems have become available. For the first time, researchers
in the area can ‘‘see’’ what they are doing. Secondly, in close conjunction with the
improved experimental tools are synthetic skills that have been brought to bear
upon this area – monodisperse nanoparticles (particularly of metals and chalcogenides), whose surfaces are stabilized by the use of capping agents, have become
commonplace in recent years. For chalcogenide nanoparticles, key early studies by
the Henglein group in Berlin [4], from Bell laboratories [5], and from the group of
Bawendi in MIT [6] have played a role. In the case of metal nanoparticles, the twophase synthesis of gold nanoparticles by Brust et al. [7] was perhaps the earliest
report of well-capped particles. These authors made use of the well-known ability
of long chain thiols to form close-packed monolayers on gold surfaces [8] to cap
gold nanoparticles. Nanoparticles ensuing from these preparations, unlike the sols
of Davy, Faraday and others, are distinct in that they can be precipitated (or collected through evaporation of solvent) and then redissolved. Colloidal sols such as
clays in rivers, milk and India ink, once precipitated (or flocculated) cannot be redispersed without great effort [9]. Capped nanoparticles therefore resemble large
molecules in their solubility behavior.
Solubility (in the molecular sense, rather than in the sense of forming dispersions and sols) opens up a number of possibilities. The first and perhaps most
important, is that it allows size-selective precipitation [10], permitting monodisperse nanoparticles to be prepared. It is only when particles are monodisperse
that their size-dependent physical properties can be studied in detail [6]. It is also
possible to organize these monodisperse nanoparticles via slow evaporation to yield
superlattices [11–13]. Superlattices of nanocrystals can rightly be described as a
new class of materials, comprising crystals of crystals as opposed to most crystalline solids which are crystals of atoms [14]. In contrast, naturally occurring opals
are crystals of amorphous silica spheres [15].
Oxides, particularly those of the transition metals, display the widest and most
fascinating range of properties of any single class of materials. In recent years,
oxide materials have been at the heart of many dramatic advances in condensed
matter science – layered copper oxides exhibiting high-TC superconductivity
readily spring to mind as a good example [16]. Another example is the finding that
certain perovskite manganese oxides display dramatic (as large as twelve orders of
magnitude) changes in their specific resistivities when subjected to magnetic
fields, opening up the new sub-field of colossal magnetoresistance (CMR) [17].
5.1 Introduction 95
in staining glass, and the nanoparticulate pigment purple of Cassius, obtained by
reducing a gold salt using tin (II) chloride is named for the physician Andreas
Cassius who died in 1673 [3].
What is then new in the area that it should receive such renewed attention.
Firstly, a number of tools, both experimental (exemplified by high-resolution
transmission electronic microscopy and scanning tunneling microscopy) and theoretical (exemplified by density functional theory), that permit a much better understanding of these systems have become available. For the first time, researchers
in the area can ‘‘see’’ what they are doing. Secondly, in close conjunction with the
improved experimental tools are synthetic skills that have been brought to bear
upon this area – monodisperse nanoparticles (particularly of metals and chalcogenides), whose surfaces are stabilized by the use of capping agents, have become
commonplace in recent years. For chalcogenide nanoparticles, key early studies by
the Henglein group in Berlin [4], from Bell laboratories [5], and from the group of
Bawendi in MIT [6] have played a role. In the case of metal nanoparticles, the twophase synthesis of gold nanoparticles by Brust et al. [7] was perhaps the earliest
report of well-capped particles. These authors made use of the well-known ability
of long chain thiols to form close-packed monolayers on gold surfaces [8] to cap
gold nanoparticles. Nanoparticles ensuing from these preparations, unlike the sols
of Davy, Faraday and others, are distinct in that they can be precipitated (or collected through evaporation of solvent) and then redissolved. Colloidal sols such as
clays in rivers, milk and India ink, once precipitated (or flocculated) cannot be redispersed without great effort [9]. Capped nanoparticles therefore resemble large
molecules in their solubility behavior.
Solubility (in the molecular sense, rather than in the sense of forming dispersions and sols) opens up a number of possibilities. The first and perhaps most
important, is that it allows size-selective precipitation [10], permitting monodisperse nanoparticles to be prepared. It is only when particles are monodisperse
that their size-dependent physical properties can be studied in detail [6]. It is also
possible to organize these monodisperse nanoparticles via slow evaporation to yield
superlattices [11–13]. Superlattices of nanocrystals can rightly be described as a
new class of materials, comprising crystals of crystals as opposed to most crystalline solids which are crystals of atoms [14]. In contrast, naturally occurring opals
are crystals of amorphous silica spheres [15].
Oxides, particularly those of the transition metals, display the widest and most
fascinating range of properties of any single class of materials. In recent years,
oxide materials have been at the heart of many dramatic advances in condensed
matter science – layered copper oxides exhibiting high-TC superconductivity
readily spring to mind as a good example [16]. Another example is the finding that
certain perovskite manganese oxides display dramatic (as large as twelve orders of
magnitude) changes in their specific resistivities when subjected to magnetic
fields, opening up the new sub-field of colossal magnetoresistance (CMR) [17].
5.1 Introduction 95