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J. Richardi et al.
of the solvent, (5) the evaporation rate and (6) if appropriate, the application of an
external field during the solvent evaporation. On the one hand, simulations based
on a flory-type solvation theory using Hansen solubility colloidal parameters allow
to predict the cobalt NP size. On the other hand, Monte Carlo simulations and free
energy theories are able to predict the size and type of patterns appearing during the
evaporation of a solution of magnetic NPs under a magnetic field.
8.1 Introduction
Self-assemblies of inorganic nanoparticles (NPs) into micrometer-scale ordered
arrays and original mesostructures constitute a new generation of advanced materials [1–3]. Depending on the deposition strategy, involving solvent evaporation of a
colloidal solution, various types of assemblies can be obtained. In the case of spontaneous self-assembling of magnetic NPs (MNPs), compact hexagonal 2D arrays [4,
5] and 3D superlattices called supercrystals [6] can form. Mesostructures of hexagonally ordered columns, labyrinths and void structures can result from assisted selfassembling, induced by the application of an external magnetic field [7]. In highly
ordered superlattices, individual NPs act as “artificial atoms” and occupy the lattice
sites to form repetitive, periodic “artificial planes". From a fundamental point of
view, these artificial solids constitute good models for investigating crystallization
behavior. In addition, their properties are determined by both individual NPs and
their collective interactions. Resulting from collective interactions between neighboring NPs, they exhibit new mechanical, [8] transport, [9] optical, vibrational, [10]
chemical (stability against oxidation and coalescence) and magnetic properties [11].
Fundamentally, one of the prerequisites to create assembly into well-defined
superstructures is the use of NPs with uniform size and shape. The necessary conditions to form such populations are (1) a short nucleation step followed by (2) a slower
growth step of the nuclei. Focusing on cobalt, size-controlled colloidal synthesis of
spherical metallic NPs, remains, to this day, challenging. Such uniform MNPs have
to be characterized by a high stability against oxidation and coalescence but not
only. Regarding to the applications, high anisotropy of NPs is mainly required, it is
then crucial to obtain preferentially highly crystallized hcp-Co NPs, rather than the
two other possible structures, i.e., epsilon and fcc ones. The existing strategies to
address these challenges are based on decomposition of organometallic precursors
and metal-salt reduction, including the micellar method. In the case of decomposition
of organometallic precursor, a precursor (e.g., Co 2 (CO) 8 ) is rapidly decomposed at
high temperatures in presence of surfactants and the NP size is tuned by tailoring the
reaction time, reaction temperature, precursor injection time, surfactant to precursor
ratio and chemistry of reagents and surfactants. For Co, this approach has been initiated by Dinega and Bawendi who obtained epsilon-Co NPs with however a rather high
size polydispersity [12]. The first example of uniform size and shape tunable Co NPs
using this approach has been reported by Puntes et al. [13] who obtained epsilon-Co
NPs coexisting however with hcp-Co nanodisks. Thereafter, keeping unchanged the
J. Richardi et al.
of the solvent, (5) the evaporation rate and (6) if appropriate, the application of an
external field during the solvent evaporation. On the one hand, simulations based
on a flory-type solvation theory using Hansen solubility colloidal parameters allow
to predict the cobalt NP size. On the other hand, Monte Carlo simulations and free
energy theories are able to predict the size and type of patterns appearing during the
evaporation of a solution of magnetic NPs under a magnetic field.
8.1 Introduction
Self-assemblies of inorganic nanoparticles (NPs) into micrometer-scale ordered
arrays and original mesostructures constitute a new generation of advanced materials [1–3]. Depending on the deposition strategy, involving solvent evaporation of a
colloidal solution, various types of assemblies can be obtained. In the case of spontaneous self-assembling of magnetic NPs (MNPs), compact hexagonal 2D arrays [4,
5] and 3D superlattices called supercrystals [6] can form. Mesostructures of hexagonally ordered columns, labyrinths and void structures can result from assisted selfassembling, induced by the application of an external magnetic field [7]. In highly
ordered superlattices, individual NPs act as “artificial atoms” and occupy the lattice
sites to form repetitive, periodic “artificial planes". From a fundamental point of
view, these artificial solids constitute good models for investigating crystallization
behavior. In addition, their properties are determined by both individual NPs and
their collective interactions. Resulting from collective interactions between neighboring NPs, they exhibit new mechanical, [8] transport, [9] optical, vibrational, [10]
chemical (stability against oxidation and coalescence) and magnetic properties [11].
Fundamentally, one of the prerequisites to create assembly into well-defined
superstructures is the use of NPs with uniform size and shape. The necessary conditions to form such populations are (1) a short nucleation step followed by (2) a slower
growth step of the nuclei. Focusing on cobalt, size-controlled colloidal synthesis of
spherical metallic NPs, remains, to this day, challenging. Such uniform MNPs have
to be characterized by a high stability against oxidation and coalescence but not
only. Regarding to the applications, high anisotropy of NPs is mainly required, it is
then crucial to obtain preferentially highly crystallized hcp-Co NPs, rather than the
two other possible structures, i.e., epsilon and fcc ones. The existing strategies to
address these challenges are based on decomposition of organometallic precursors
and metal-salt reduction, including the micellar method. In the case of decomposition
of organometallic precursor, a precursor (e.g., Co 2 (CO) 8 ) is rapidly decomposed at
high temperatures in presence of surfactants and the NP size is tuned by tailoring the
reaction time, reaction temperature, precursor injection time, surfactant to precursor
ratio and chemistry of reagents and surfactants. For Co, this approach has been initiated by Dinega and Bawendi who obtained epsilon-Co NPs with however a rather high
size polydispersity [12]. The first example of uniform size and shape tunable Co NPs
using this approach has been reported by Puntes et al. [13] who obtained epsilon-Co
NPs coexisting however with hcp-Co nanodisks. Thereafter, keeping unchanged the