Chemistry Reference
In-Depth Information
deposited thin films in plasma processing are often characterized by macroradicals
at the surface or in thin films [70].
An unpaired electron is connected with a magnetic spin moment. This magnetic
moment generates energy splitting into two energy levels in an external magnetic
field. Transition between these two levels can be induced by external electromagnetic
radiation of suitable frequency. At an external magnetic field
H
0
the frequency ν
for which the external radiation is in resonance with the paramagnetic substance is
given by
1
h
(
ν
=
g
μ
0
H
0
)
,
(7.24)
where
h
the Planck constant
g
the Landé factor
μ
0
the Bohr magneton
The ESR spectroscopy measures the absorption of electromagnetic wave energy at
the resonance using a monochromatic radiation (e.g., microwaves) and a scan of the
external magnetic field. The position (
g
-value) and shape of the absorption spectrum
are determined by the nature of the radical, whereas the line intensity depends on the
radical concentration.
The impact of radicals in a thin film is different; they can generate dielectric losses
and according to their reactivity with water or oxygen they cause aging effects. In par-
ticular, radicals near the surface influence the chemical interaction with surrounding
media [71].
ESR studies on radical generation and recombination in thin plasma polymers
deposited in a c-C
4
F
8
rf discharge show radical densities of
≈
10
20
cm
−
3
. Storage on
air reduces the radical number with a half time of 24 h [71].
The formation of radicals on conventional synthetic polymers is studied by [72-
74]. Powdered TEFLON treated by Ar and oxygen plasmas is studied by ESR
technique. Midchain (-CF
2
-CF-CF
2
-), endchain (-CF
2
-CF
2
) radicals and peroxy
radicals in O
2
plasma were identified. Peroxy radical formation was also observed on
plasma-treated polyethylene surfaces after exposure to air.
The treatment of natural fibers like wool in a gas flow of a gliding arc discharge
plasma at atmospheric pressure leads to the formation of radicals on the wool fiber
surface as shown by ESR techniques [75].
Studies of radicals on surfaces are possible also by chemical methods, for
example, by application of the stable radical DPPH (2,2-Diphenyl-1-pikrylhydrazyl).
Furthermore, the reaction of nitric oxide or iodine in combination with XPS
measurements can also be applied for radical detection [76].
