Page 276 - Book Hosokawa Nanoparticle Technology Handbook

4.6 SELF-ASSEMBLY FUNDAMENTALS
3. The nanoparticles are dispersed by preventing Nanorheological properties for the high concentra-
aggregation. tion of nanoparticle suspensions more than 10%
strongly affect the final structure of thin films. The
4. The surface of nanoparticles is modified to
processes of dispersing, coating, and drying are
avoid aggregation.
responsible for the formation of highly ordered struc-
ture of nanoparticles. The surface modification of
There are a lot of trade-off relationships between
keeping dispersion and desired function. nanoparticles and the surface treatment of a substrate
The applications of thin films of self-ordered characterize nanointerfaces responsible for the forma-
nanoparticles are a light emitted diode [2] of semi- tion of nanoparticles aggregation. The highly ordered
conductor nanoparticles, a solar cell [3], and an opti- structures of nanoparticles are due to the balance
cal switching [4]. A magnetic memory media [5] with between field conditions determined by processes and
high-density memory is also studied by using mag- nanointerfaces characterized by molecules modified
netic nanoparticles such as FePt. FED displays on nanoparticle surface. In materials nanotechnology,
expected as next generation need phosphor nanoparti- the control of dispersion and aggregation of nanopar-
cles such as Y O :Eu. Furthermore, there are also a lot ticles is a key technology. The geometrical shape of
3
2
of applications of nanocomposites consisting of aggregates must be also controlled for the nanomate-
nanoparticles dispersed in polymers. The anticipated rials design. The relationships among colloidal struc-
characteristics of nanocomposites are; improving the tures, thin film structures of nanoparticles, and
thermal resistance, high optical reflection, high stiff- functions are schematically shown in Fig. 4.6.1.
ness of mechanical strength [6], and improving tribol- The aggregation dynamics of fine particles in a sus-
ogy characteristics such as friction and wear. There pension under steady state are well studied in col-
are also bio-medical applications such as DDS and loidal science, but there are few studies about both the
biosensors using antigen-antibody reactions with size distribution and the shape of aggregates. The
antibody surface modification on nanoparticles. The aggregation of fine particles is a typical example of
thin films of nanoparticles above mentioned can be self-organization, and therefore the prediction of both
fabricated by conventional coating–drying processes. the size and the shape of aggregates becomes a key
In general, there are two methods to fabricate thin issue. For instance, the structure of aggregates of
films of nanoparticles. They are wet and dry nanoparticles in nanocomposites affects the physical
processes summarized in Table 4.6.1. Here we focus properties such as optical transparency, mechanical
on coating and drying. strength, and electrical conductivity. In general, the
balance between dispersion and aggregation must be
controlled to obtain high quality functions. In another
1. Self-organization process of nanoparticles in colloidal
word, the control of isolation and interaction among
suspensions nanoparticles is expected for the design of nanocom-
Coating and drying processes are employed to fabri- posites.
cate thin films of nanoparticles. Coating process is There are two methods of self-ordering. One is
categorized in two methods [1] which are a continu- self-assembly realized in the vicinity of equilibrium,
ous coating such as die coating and a discontinuous and the other is self-organization [7] operated at non-
coating such as ink jet and microcontact stamp. The equilibrium far from equilibrium. The typical exam-
continuous coating is usually employed for nanocom- ple of self-assembly is epitaxial growth in a single
posites, memory media, and photoluminescence crystal growth. The self-organization is typically
device. The discrete coating is used for patterning of observed in the nucleation or the generation of
thin films such as electrode lines and organic LED. nanoparticles in a bulk. The structure formation

Table 4.6.1
Comparison between wet and dry processes of thin films of nanoparticles.

Method Process Characteristics Merit Demerit Applications
Wet Coating Surface modification Control of Purity Magnetic tapes
Ink jet of nanoparticles and a dispersion phosphor display
SAM substrate solar cells
color filters
Dry Evaporation Charge control of High purity Aggregation Hard disk
CVD nanoparticles quantum dot laser
Sputtering SWNT
MBE

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