8 Magnetic Self-Assembling of Spherical Co Nanoparticles …
183
precursor but varying the surfactants, populations of tunable size have been obtained
characterized by either epsilon [13], fcc [14] or a mixing of hcp/fcc [15] structure.
Another approach for formation of size-controlled spherical Co NPs is the reduction
approach. In this method typically, a reducing agent is injected into a hot solution
of metal-salt precursor either inorganic or organic (CoCl 2 or Co(CH 3 COO) 2 ) in the
presence of surfactants. Inorganic metal-salt chemical reduction first reported by
Murray and Sun [16] allows the formation of uniform size tunable epsilon-Co NPs
[14]. The group of Chaudret, using an organic metal-salt as precursor, Co(η3-C8H13)
(η4-C8H12), that readily decomposes at lower temperature (around 150 °C) under a
pressure of dihydrogen (usually 3 bars), synthesized Co NPs with an fcc/hcp structure
[17]. Using a variant of the inorganic metal-salt reduction where the reducing agent
also plays the role of the solvent (polyol method), Murray et al. published the first
example of hcp-Co NPs. Recently, Mezziane et al. developed a simple organometallic
approach based on the combination of oleylamine and ClCo(PPh 3 ) 3 and evidenced
the formation of pure monodisperse spherical hcp-Co NPs, ferromagnetic at room
temperature [18]. The reduction in reverse micelles, performed at room temperature, also allows the formation of uniform size tunable Co NPs, characterized by
an fcc polycrystalline structure [19–21]. Using the appropriate annealing treatment
and taking advantage of their high thermal stability, Co polycrystals can transform
into hcp single-crystalline NPs [22, 23]. For all these approaches, surfactant is a key
parameter not only to stabilize the NPs against coalescence and oxidation but also
to mediate the NP growth.
The assembly of NPs can be obtained through the evaporation of a colloidal solution. As a result of the balance between van der Waals attractions between the metallic
NP, magnetic interactions between the MNPs and also solvent-mediated interactions
between ligands, uniform colloidal MNPs can assemble into various mesostructures.
The magnitude of these interactions can be tuned by various parameters including
(1) the nature of the MNP, (2) the NP size, (3) the nature of the coating agent, (4)
the nature of the solvent (5) the evaporation rate and (6) if appropriate, the application of an external field during the solvent evaporation. In the absence of external
field, MNPs can self-assemble into 2D and 3D ordered arrays. The first reports of
nanocrystal superlattices were published by Bentzon [24, 25] and coworkers in 1989
with iron oxide NPs used as building blocks. Then after, significant progress has been
made in preparing long-range 2D and 3D superlattices of MNPs. 2D superlattices
composed for example of spherical MNPs (Co [6], Ni [26], Fe [27] and Fe 2 O 3 [28]),
are characterized by a hexagonal packing and can be obtained at the micrometer scale.
Besides, self-organization of MNPs into 3D supercrystals has been, however in a less
extend, evidenced by several groups. Depending on the experimental conditions, two
growth processes are observed (1) onto a substrate (heterogeneous nucleation and
growth) or (2) in solution (homogeneous nucleation and growth). In the first case,
film morphology is obtained whereas in the latter ones, the colloidal crystals obtained
are characterized by well-defined shape. The first example of long-range ordered 3D
supercrystal film of MNPs has been published in 2003 by Lisiecki et al. [6]. These
artificial solids are made of several hundred monolayers of fcc-Co NPs, which order
in an fcc “super” structure. 3D supercrystal films have been also obtained using Ni
183
precursor but varying the surfactants, populations of tunable size have been obtained
characterized by either epsilon [13], fcc [14] or a mixing of hcp/fcc [15] structure.
Another approach for formation of size-controlled spherical Co NPs is the reduction
approach. In this method typically, a reducing agent is injected into a hot solution
of metal-salt precursor either inorganic or organic (CoCl 2 or Co(CH 3 COO) 2 ) in the
presence of surfactants. Inorganic metal-salt chemical reduction first reported by
Murray and Sun [16] allows the formation of uniform size tunable epsilon-Co NPs
[14]. The group of Chaudret, using an organic metal-salt as precursor, Co(η3-C8H13)
(η4-C8H12), that readily decomposes at lower temperature (around 150 °C) under a
pressure of dihydrogen (usually 3 bars), synthesized Co NPs with an fcc/hcp structure
[17]. Using a variant of the inorganic metal-salt reduction where the reducing agent
also plays the role of the solvent (polyol method), Murray et al. published the first
example of hcp-Co NPs. Recently, Mezziane et al. developed a simple organometallic
approach based on the combination of oleylamine and ClCo(PPh 3 ) 3 and evidenced
the formation of pure monodisperse spherical hcp-Co NPs, ferromagnetic at room
temperature [18]. The reduction in reverse micelles, performed at room temperature, also allows the formation of uniform size tunable Co NPs, characterized by
an fcc polycrystalline structure [19–21]. Using the appropriate annealing treatment
and taking advantage of their high thermal stability, Co polycrystals can transform
into hcp single-crystalline NPs [22, 23]. For all these approaches, surfactant is a key
parameter not only to stabilize the NPs against coalescence and oxidation but also
to mediate the NP growth.
The assembly of NPs can be obtained through the evaporation of a colloidal solution. As a result of the balance between van der Waals attractions between the metallic
NP, magnetic interactions between the MNPs and also solvent-mediated interactions
between ligands, uniform colloidal MNPs can assemble into various mesostructures.
The magnitude of these interactions can be tuned by various parameters including
(1) the nature of the MNP, (2) the NP size, (3) the nature of the coating agent, (4)
the nature of the solvent (5) the evaporation rate and (6) if appropriate, the application of an external field during the solvent evaporation. In the absence of external
field, MNPs can self-assemble into 2D and 3D ordered arrays. The first reports of
nanocrystal superlattices were published by Bentzon [24, 25] and coworkers in 1989
with iron oxide NPs used as building blocks. Then after, significant progress has been
made in preparing long-range 2D and 3D superlattices of MNPs. 2D superlattices
composed for example of spherical MNPs (Co [6], Ni [26], Fe [27] and Fe 2 O 3 [28]),
are characterized by a hexagonal packing and can be obtained at the micrometer scale.
Besides, self-organization of MNPs into 3D supercrystals has been, however in a less
extend, evidenced by several groups. Depending on the experimental conditions, two
growth processes are observed (1) onto a substrate (heterogeneous nucleation and
growth) or (2) in solution (homogeneous nucleation and growth). In the first case,
film morphology is obtained whereas in the latter ones, the colloidal crystals obtained
are characterized by well-defined shape. The first example of long-range ordered 3D
supercrystal film of MNPs has been published in 2003 by Lisiecki et al. [6]. These
artificial solids are made of several hundred monolayers of fcc-Co NPs, which order
in an fcc “super” structure. 3D supercrystal films have been also obtained using Ni