of magnetically hard and soft phases interacting via magnetic exchange coupling,
have been made by carefully annealing the mixed nanocrystal array consisting of
FeaPt and Fe 3 O 4 [133]. The easy magnetic axis of nanocrystals can be aligned by
applying a magnetic field during evaporation of the colloid on a substrate to obtain
films with high magnetic anisotropy [211–213].
Thus, ferromagnetic films with parallel anisotropy have been made of superparamagnetic g-Fe 2 O 3 nanocrystals [211]. By the use of substrate–nanocrystal interactions, films of the same g-Fe 2 O 3 nanocrystals can be made to exhibit perpendicular anisotropy [212] (see Figure 4.31). The film properties such as coercivity
and anisotropy can be tuned by altering the size of the nanocrystals, the film
thickness or by suitably doping the nanocrystals with magnetic ions [211–213].
4.4.3
Nanocomputing
Ordered arrays of nanocrystals, in principle, could be thought of as arrays of SETs,
where the electrostatic interaction between neighboring SETs acts as a wireless
communication means. It has been suggested by Korotkov [214] and Lent [215]
that simple logical operations can be performed on a circuitry consisting of arrays
of SETs in the form of chains or cells with suitable insulating spacers. An electric
field applied in one direction polarizes the strings into either the 0 or the 1 state.
Lent’s scheme, called quantum cellular automata uses, instead, a square cell consisting of five nanocrystals to denote the state of polarization. Preliminary experiments to evaluate the schemes are currently being pursued.
The realization that a self-assembly driven fabrication process is not capable of
producing defect-free structures, has fuelled a search for algorithms that can compute even with defective circuitry. Heath and co-workers [216] have developed Teramac, a computer that works despite a high concentration of defects in its bank of
microprocessors. A more radical solution called amorphous computing aims to
‘‘engineer pre-specified, coherent behavior from the cooperation of large numbers
of unreliable parts interconnected in unknown, irregular and time varying ways’’
[217–219].
4.5
Conclusions
Nanocrystals of metal and semiconductors with diameters in the range 1 to 50 nm
form a class of materials with unusual properties which are size-dependent. Excellent electrical conductivity that primarily characterizes a metallic state, becomes
a rare entity in small nanocrystals (< 2 nm) due to quantum confinement of the
electronic states. Similarly, magnetic metals lose much of the coercivity with diminishing size. On the other hand, chemical properties such as reactivity may
show up better at smaller sizes due to a greater number of surface bonding sites
and other electronic effects. Considering the importance of nanocrystals in tech4 Mesoscopic Assembly and Other Properties of Metal and Semiconductor Nanocrystals
86
have been made by carefully annealing the mixed nanocrystal array consisting of
FeaPt and Fe 3 O 4 [133]. The easy magnetic axis of nanocrystals can be aligned by
applying a magnetic field during evaporation of the colloid on a substrate to obtain
films with high magnetic anisotropy [211–213].
Thus, ferromagnetic films with parallel anisotropy have been made of superparamagnetic g-Fe 2 O 3 nanocrystals [211]. By the use of substrate–nanocrystal interactions, films of the same g-Fe 2 O 3 nanocrystals can be made to exhibit perpendicular anisotropy [212] (see Figure 4.31). The film properties such as coercivity
and anisotropy can be tuned by altering the size of the nanocrystals, the film
thickness or by suitably doping the nanocrystals with magnetic ions [211–213].
4.4.3
Nanocomputing
Ordered arrays of nanocrystals, in principle, could be thought of as arrays of SETs,
where the electrostatic interaction between neighboring SETs acts as a wireless
communication means. It has been suggested by Korotkov [214] and Lent [215]
that simple logical operations can be performed on a circuitry consisting of arrays
of SETs in the form of chains or cells with suitable insulating spacers. An electric
field applied in one direction polarizes the strings into either the 0 or the 1 state.
Lent’s scheme, called quantum cellular automata uses, instead, a square cell consisting of five nanocrystals to denote the state of polarization. Preliminary experiments to evaluate the schemes are currently being pursued.
The realization that a self-assembly driven fabrication process is not capable of
producing defect-free structures, has fuelled a search for algorithms that can compute even with defective circuitry. Heath and co-workers [216] have developed Teramac, a computer that works despite a high concentration of defects in its bank of
microprocessors. A more radical solution called amorphous computing aims to
‘‘engineer pre-specified, coherent behavior from the cooperation of large numbers
of unreliable parts interconnected in unknown, irregular and time varying ways’’
[217–219].
4.5
Conclusions
Nanocrystals of metal and semiconductors with diameters in the range 1 to 50 nm
form a class of materials with unusual properties which are size-dependent. Excellent electrical conductivity that primarily characterizes a metallic state, becomes
a rare entity in small nanocrystals (< 2 nm) due to quantum confinement of the
electronic states. Similarly, magnetic metals lose much of the coercivity with diminishing size. On the other hand, chemical properties such as reactivity may
show up better at smaller sizes due to a greater number of surface bonding sites
and other electronic effects. Considering the importance of nanocrystals in tech4 Mesoscopic Assembly and Other Properties of Metal and Semiconductor Nanocrystals
86