Nanotechnology-Enabled Sensors

Successive ion layer adsorption and reaction (SILAR)
4.6 Liquid Phase Techniques 175
The distinguishing characteristic of SILAR is the use of alternating
aqueous solutions (a metal salt solution, followed by a hydrolyzing or sulfidizing
solution). In principle, this allows molecule-by-molecule growth
of the compound film via the sequential addition of individual atomic layers.
Liquid phase deposition (LPD)
LPD refers to the formation of oxide thin films from an aqueous solution
of a metal–fluoro complex [MFn] m−n which can be hydrolyzed by adding
water to a scavenging agent such as boric acid (H3BO3) or aluminum
metal. In the LPD process, it is possible to produce thin metal oxide films
films directly on a substrate that is immersed in a treatment solution for
deposition. 57
In this process, metal oxide thin films can be formed via a two step reaction.
In the first step, which is an equilibrium reaction, a metal-fluoro
complex ion (MFx (x-2n)- ) reacts with water to form the desired metal oxide
plus F − and H + ions. However, the F − and H + ions quickly react with the
metal oxide and reform the original metal-fluoro complex:
MFx (x-2n)- + nH2O → MOn + x F − + 2nH + . (5.10)
The equilibrium reaction can be shifted towards the right-hand side of
the chemical equation through the addition of a scavenging agent such as
H3BO3. This acid readily reacts with F − to form HBF4 and 3H2O:
H3BO3 + 4H + + 4F − → HBF4 + 3H2O. (5.11)
The advantage of producing HBF4 is that it does not interact with MOn,
and as a result, the MOn will be eventually formed on the substrate. 58,59
The block diagram of the process is shown in Fig. 4.32.