In traditional electronics – LCR circuits, for example – the Ls and the Cs are
invariably oxide materials. In the area of integrated semiconductor devices, gate
dielectrics [18], dielectrics in dynamic random access memories [19], ferroelectrics
in non-volatile memories [20], and decoupling capacitors [21] are all oxide materials. Oxides are also at the heart of many fuel cell [22] and secondary battery materials [23].
Given the importance of oxide materials, it is only natural to ask whether recent understanding regarding the synthesis, properties and applications of capped
metal and semiconductor nanoparticles can be extended to oxides.
This review does not concern itself with the vast literature of nanophase materials, which are monolithic materials comprising crystalline grains whose dimensions are in the nanometer range. Such materials are typically prepared using
sol–gel and related techniques. This is an older, more mature area that continues
to attract a great deal of interest [24].
5.2
Magnetite Particles in Nature
While it was known that many organisms respond to the earth’s magnetic field,
Blakemore’s demonstration in 1975 [25] that certain bacteria found in marine
marsh muds tended to rapidly navigate along a specific direction – the local geomagnetic north – was seminal. In addition, Blakemore used transmission electron
microscopy to demonstrate that these bacteria contained iron-rich crystals about
100 nm in diameter. Several crystals were found to align themselves into chains
along their major axis [Figure 5.1]. Frankel, Blakemore and Wolfe [26] then found
that an unclassified magnetotactic spirillum (denoted MS-1) when cultured in solutions containing ferric salts, precipitated uniform single crystals that were shown,
by Mo ¨ssbauer spectroscopy, to be mostly spinel Fe 3 O 4 . Since these early studies,
the importance of magnetotaxis and magnetoreception in a number of organisms,
from bacteria through higher vertebrates, continue to be the subject of investigation [27, 28]. It is interesting that the effects of magnetic fields on organisms –
described as ‘‘a romping ground for quakes and charlatans, dating at least to the
French Mesmerists in the late 18th century’’ [27] should finally find some legitimacy through the discovery that magnetic nanoparticles play a role in magnetotaxis.
Microbial magnetite (and the ferrimagnetic sulfide biomineral greigite g-Fe 3 S 4 ,
found in magnetotactic bacteria that grow in marine, sulfidic environments) are
fine examples of the control that Nature is able to exert over inorganic crystallizations. The magnetic particles are invariably at the optimal single domain size
for the specific mineral. What this means is that each magnetic particle contains
only a single magnetic domain. If the particle were smaller or larger, it would not
be as effective a magnet. In addition, within the magnetosome, the magnetite (or
greigite) crystals are aligned in such a manner that the assembly develops a per5 Oxide Nanoparticles
96
invariably oxide materials. In the area of integrated semiconductor devices, gate
dielectrics [18], dielectrics in dynamic random access memories [19], ferroelectrics
in non-volatile memories [20], and decoupling capacitors [21] are all oxide materials. Oxides are also at the heart of many fuel cell [22] and secondary battery materials [23].
Given the importance of oxide materials, it is only natural to ask whether recent understanding regarding the synthesis, properties and applications of capped
metal and semiconductor nanoparticles can be extended to oxides.
This review does not concern itself with the vast literature of nanophase materials, which are monolithic materials comprising crystalline grains whose dimensions are in the nanometer range. Such materials are typically prepared using
sol–gel and related techniques. This is an older, more mature area that continues
to attract a great deal of interest [24].
5.2
Magnetite Particles in Nature
While it was known that many organisms respond to the earth’s magnetic field,
Blakemore’s demonstration in 1975 [25] that certain bacteria found in marine
marsh muds tended to rapidly navigate along a specific direction – the local geomagnetic north – was seminal. In addition, Blakemore used transmission electron
microscopy to demonstrate that these bacteria contained iron-rich crystals about
100 nm in diameter. Several crystals were found to align themselves into chains
along their major axis [Figure 5.1]. Frankel, Blakemore and Wolfe [26] then found
that an unclassified magnetotactic spirillum (denoted MS-1) when cultured in solutions containing ferric salts, precipitated uniform single crystals that were shown,
by Mo ¨ssbauer spectroscopy, to be mostly spinel Fe 3 O 4 . Since these early studies,
the importance of magnetotaxis and magnetoreception in a number of organisms,
from bacteria through higher vertebrates, continue to be the subject of investigation [27, 28]. It is interesting that the effects of magnetic fields on organisms –
described as ‘‘a romping ground for quakes and charlatans, dating at least to the
French Mesmerists in the late 18th century’’ [27] should finally find some legitimacy through the discovery that magnetic nanoparticles play a role in magnetotaxis.
Microbial magnetite (and the ferrimagnetic sulfide biomineral greigite g-Fe 3 S 4 ,
found in magnetotactic bacteria that grow in marine, sulfidic environments) are
fine examples of the control that Nature is able to exert over inorganic crystallizations. The magnetic particles are invariably at the optimal single domain size
for the specific mineral. What this means is that each magnetic particle contains
only a single magnetic domain. If the particle were smaller or larger, it would not
be as effective a magnet. In addition, within the magnetosome, the magnetite (or
greigite) crystals are aligned in such a manner that the assembly develops a per5 Oxide Nanoparticles
96