Biomedical Engineering Reference
In-Depth Information
contaminated fi lms [15]. Volatile and thermally stable cobalt (I) hydride complexes
HCo [P(OR)
3
]
4
(where R = methyl, ethyl,
iso
- propyl, or
n
- propyl) were synthesized
and used as precursors in CVD, and smooth and dense cobalt thin fi lms with
variable grain size were fabricated on Si at temperatures as low as 300 ° C with-
out employing hydrogen [16]. In addition, a highly conformal pure Co fi lm was
deposited on either SiO
2
-coated or trench-patterned wafers by using Co
2
(CO)
8
or C
12
H
10
O
6
(Co)
2
(dicobalt hexacarbonyl
tert
-butylacetylene) as the Co precursor,
which would be quite promising for the production of advanced electronic devices
[17]. Recently, pulsed-spray evaporation CVD has been adopted to grow metallic
cobalt and Co
2
C fi lms on solid substrates. For this, alcoholic solutions of cobalt
acetylacetonate were loaded as a liquid feedstock, in which alcohol acted as both
solvent and reducing agent. Different types of alcohols, pulse width and frequency,
and starting concentrations of the loading precursor determined the growth rate,
size, crystallinity, and thickness of metallic fi lms [18] .
16.3.3.3
Liquid - Phase Chemical Precipitation
16.3.3.3.1
Microemulsion Approach
Microemulsions are thermodynamically
stable, isotropic dispersions of oil and water with a thin fi lm of surfactant mole-
cules adsorbed at the water/oil interface. Surfactants are amphiphilic molecules
which have a polar hydrophilic (water-loving) “head” and a nonpolar hydrophobic
(water- hating) “ tail ” . By varying the hydrophile - lipophile balance ( HLB ) value of
the surfactants and the continuous/dispersed phase, both oil-in-water (direct or
water-borne) microemulsions and water-in-oil microemulsions can be produced.
The latter type, which is also referred to as an “inversed microemulsion”, is gener-
ally exploited in the preparation of inorganic oxide, semiconductor and metal
nanoparticles, and so on.
Typically, surfactants may be classifi ed as: (i) anionic, such as sodium bis(2-
ethylhexyl)sulfosuccinate, usually called AOT; (ii) cationic, such as tetra-alkyl qua-
ternary ammoniums; and (iii) nonionic, such as
tert
- octylphenoxy - polyethoxyethanol
(Triton X-100). The inverse micelle is explained in terms of the water-surfactant
molar ratio
w
[H
2
O]/[surfactant]), which determines the droplet size. Due to its
small dispersion sizes (which usually are 5 - 20 nm in diameter), the microemul-
sion will be either transparent or translucent. In contrast to the colloidal approach,
the micellar reagent may act as a physical boundary rather than as a true surface-
capping agent. Besides its variable size, cylindrical, lamellar and hexagonal shapes
of droplets can be obtained by adding cosurfactants together with normal surfac-
tants; thus, the fi nal nanostructure was dependent on the micellar morphology
[7c, 19] .
Pileni and coworkers reported the synthesis of monodispersed cobalt nanopar-
ticles by using AOT as surfactant and Co (AOT)
2
as the starting materials. The
diameter of the droplets was controlled by the volume of water, and varied from
0.5 to 18 nm [20]. The uniform cobalt nanoparticles could be self-organized into
either 2-D [21, 22] or 3-D superlattices [23] by extracting cobalt nanoparticle with
lauric acid or triphosphine oxide from reverse micelles, and then re-dispersed into
