Biomedical Engineering Reference
In-Depth Information
[(CO)
4
Fe(
μ
- PPh
2
)Pd(
μ
- Cl)]
2
[37]
and
CoPt(CO)
4
(dppe)Me
[38] ,
respectively,
as
single- source precursors.
FePd and CoPt alloys also exist as disordered fcc structures having small coerciv-
ity and soft magnetic properties, or as L1
0
ordered fct lattices having high coerciv-
ity, large magnetic anisotropy, and hard magnetic properties. The production of
fct FePd and CoPt nanoparticles exhibiting high room-temperature coercivities is
of major interest in the further study of hard ferromagnets, and also possibly of
high-density data storage materials [3, 39]. Both alloys undergo a fcc-to-fct transi-
tion upon thermal annealing (between 500- 700 ° C for FePd and between 600 -
850 °C for CoPt) [40]. A fct-to-fcc phase transition resulting in a material without
ferromagnetic properties occurs at high temperatures for both compositions
[41, 42] .
Recently reported FePd and CoPt nanoparticle syntheses have relied on rapid
solution-based synthetic strategies used for the preparation of FePt and other
metal alloy nanoparticles [9, 43]. Typical methods utilize a solvo-thermal process
where dual-source precursors are dissolved in a high-boiling solvent and reduced
to metal atoms by a strong reducing agent (e.g., sodium borohydride) or by a polyol
process in the presence of an organic surface-passivating agents to form solubi-
lized fcc nanoparticles. FePd nanoparticles have been prepared using solution-
based strategies [44 - 46] , electron - beam evaporation [47] , and microwave radiation
[48]. CoPt nanoparticles have been prepared using solution-based strategies [45,
49- 52] , core - shell nanoparticles [53] , electrodeposition [54] , and ion - implantation
[55, 56]. In most preparations, the fcc nanoparticles are formed fi rst and then are
annealed, typically on wafer supports, to convert the superparamagnetic fcc mate-
rial to the ferromagnetic fct phase. Although these annealing steps result in
various degrees of interparticle coalescence, they are necessary to alter the mag-
netic properties.
A unique approach to the formation of supported magnetic binary alloy nanopar-
ticles is the thermal decomposition of metal- atom - containing polymers. For this,
Liu
et al
. [57] pyrolyzed a metallized polymer precursor comprised of polyferroce-
nylsilane with pendant Co clusters under a reducing atmosphere, and this resulted
in the formation of magnetic CoFe alloy nanoparticles. Those nanoparticles
formed with unprotected surfaces underwent surface oxidation in air; however, a
portion of the nanoparticles formed within a silicon carbide matrix that was a
reaction byproduct of polymer pyrolysis on a silicon substrate, suffi ciently protect-
ing the nanoparticles from air oxidation. The magnetic properties of the substrate-
supported CoFe nanoparticles were seen to be wholly dependent on particle size.
More recently, Li,
et al
. [17, 58] described a similar polymer-pyrolysis synthesis
of FePt nanoparticles from two different types of polyferroplatinyne polymers
(Figure 13.7). The initial metallopolyyne precursor,
P1
, containing
n - tert
-
butylphosphino ligands bound to the Pt atom, underwent reductive decomposition
to give fct FePt nanoparticles and unwanted metal phosphides (i.e. Fe
2
P and PtP
2
).
In order to overcome the phosphide formation, an alternative poylferroplatinyne
precursor was developed,
P2
, where phosphine ligands were replaced with a
bipyridine- type ligand.
