W. Richard Bowen and Nidal Hilal 4

also increase the effective area of contact, further decreasing the resolution
of images. The presence of a contaminant layer can also adversely affect
any attempt to chemically functionalise probes, preventing formation of
an even layer of the desired coating material. There are various methods
available to successfully clean AFM probes, which will now be described.
Technologically, the simplest method is chemical cleaning by immersing
the probe in an acid peroxide solution – most commonly a mixture
referred to as piranha solution (usually a 7:3 ratio by volume of concentrated
H 2SO 4 and 30% v/v H 2O 2, although the proportions may vary
between laboratories) for a short period of time, which has been verified
as effective in removing organic surface contaminants and increasing the
hydrophilicity of the levers, although inorganic contaminants are not
affected [42, 43]. This mixture is used in the semiconductor industry to
clean photoresist and other contaminants from silicon wafers [44]. Simple
cleaning by rinsing with organic solvents is insufficient to remove all the
organic contaminants [43]. Piranha solution is extremely reactive with any
organic material and can remove contaminants from levers and silicon
surfaces in a very short space, with necessary exposure times to remove
contaminants from AFM probes, which is typically less than 1 min.
However, for this reason it is very corrosive if it comes into contact with
any biological material including skin and must be handled with great
care by the user. In addition if it comes into contact with a relatively large
quantity of organic material, such as by allowing to mix with organic solvents,
the mixture may become explosive, posing a significant danger to
laboratory users, with accounts of such laboratory accidents extant in the
literature [45, 46]. As such, this method is not recommended if other safer
cleaning methods are available and only small volumes should be produced
at a time as and when required. Another commonly used method
to clean AFM probes is to use ultraviolet (UV) light [47, 48]. This works
by converting oxygen to form small amounts of ozone, which is then further
broken down to produce highly reactive singlet oxygen, which in
turn reacts with organic contaminant materials on the surfaces. For greater
effectiveness, such a system is often combined with an independent ozone
source to increase the amount of singlet oxygen radicals produced. Many
laboratories also use plasma ashing or etching processes to remove contaminants
from probes and samples [41, 42, 48–50], which has been demonstrated
to be particularly effective in the removal of thin layers of organic
contamination. Here a process gas, usually oxygen or argon, is ionised
under a partial vacuum in a chamber containing the sample to be cleaned.
.3. effect of probe geometry
1.3 CANTILEvERs ANd PROBEs 7
Because all measurements made using an AFM are based upon the
physical interaction between the probe and the sample, it follows that the