An interesting extension of the silane chemistry discussed above is the
sol-gel processing method used to prepare highly porous silica networks.
As an example, let’s consider ethoxysilane (Figure 10.19), which can easily
be hydrolyzed to the (R-O) 3 Si–OH form. Since silanes are highly reactive
to OH groups, (R-O) 3 Si–OH molecules can self-react in a condensation
process leading to the silica network containing nanopores.
10.5 POLED AMORPHOUS FILMS
The methods that we have discussed thus far have involved formation of
films that are self-assembled by various types of interactions, whether
electrostatics, the hydrophobic effect, or covalent bonds. These types of
film are intrinsically ordered to some degree based on the interactions
used to build them. However, in some situations, thicker (but still
nanoscale) films need to be quickly assembled or it is desirable to fabricate a film from materials that do not intrinsically order well. This can be
done through a process called poling, where an external potential such as
an applied voltage is used to order molecules, which are then locked in
place by reducing the temperature and/or chemical crosslinking.
10.5.1 Spin-coating
To produce a poled film, the material of interest first needs to be coated
on to the substrate. This is often done using a process called spin-coating,
where a droplet of a solution containing the material in a volatile solvent
is placed on the substrate, which is then rotated at high speeds. The
(apparent) centrifugal force of the substrate rotating pulls the solution
outward, resisted by the surface tension of the liquid, creating a uniform
film with a thickness that depends on the solvent and rotational speed of
the spin-coater. A schematic of spin-coating is shown in Figure 10.20.
Spin-coating is extensively used in the semiconductor industry for
applying photoresists for lithography (see Section 10.6.1).
10.5.2 Poling
Unlike films produced using self-assembly methods, spin-coated films are
not intrinsically ordered. If acentric order is desired, for example, for a
nonlinear optical material, the film must be poled to orient the molecules in
a desired direction. If the film is composed of polar molecules, an electric
field can be used for poling. The film is heated to near its glass transition
POLED AMORPHOUS FILMS 411
sol-gel processing method used to prepare highly porous silica networks.
As an example, let’s consider ethoxysilane (Figure 10.19), which can easily
be hydrolyzed to the (R-O) 3 Si–OH form. Since silanes are highly reactive
to OH groups, (R-O) 3 Si–OH molecules can self-react in a condensation
process leading to the silica network containing nanopores.
10.5 POLED AMORPHOUS FILMS
The methods that we have discussed thus far have involved formation of
films that are self-assembled by various types of interactions, whether
electrostatics, the hydrophobic effect, or covalent bonds. These types of
film are intrinsically ordered to some degree based on the interactions
used to build them. However, in some situations, thicker (but still
nanoscale) films need to be quickly assembled or it is desirable to fabricate a film from materials that do not intrinsically order well. This can be
done through a process called poling, where an external potential such as
an applied voltage is used to order molecules, which are then locked in
place by reducing the temperature and/or chemical crosslinking.
10.5.1 Spin-coating
To produce a poled film, the material of interest first needs to be coated
on to the substrate. This is often done using a process called spin-coating,
where a droplet of a solution containing the material in a volatile solvent
is placed on the substrate, which is then rotated at high speeds. The
(apparent) centrifugal force of the substrate rotating pulls the solution
outward, resisted by the surface tension of the liquid, creating a uniform
film with a thickness that depends on the solvent and rotational speed of
the spin-coater. A schematic of spin-coating is shown in Figure 10.20.
Spin-coating is extensively used in the semiconductor industry for
applying photoresists for lithography (see Section 10.6.1).
10.5.2 Poling
Unlike films produced using self-assembly methods, spin-coated films are
not intrinsically ordered. If acentric order is desired, for example, for a
nonlinear optical material, the film must be poled to orient the molecules in
a desired direction. If the film is composed of polar molecules, an electric
field can be used for poling. The film is heated to near its glass transition
POLED AMORPHOUS FILMS 411