5.24 Chemical Message
In 1988, it became relevant to look for the detection of free radicals in tissues,
thinking that the diseases associable to radicals could be thus controlled. The
above-reported research strongly indicates that the tissue manipulation could artifactually generate radicals, simply due to mechanical break of the same tissues. It
has been not excluded that the artifacts can also depend on the chemical treatment
of tissues, like chemotherapy, but they are not generated by a chemical interaction
effect.
5.25 The Case of Functional Defects in Semiconductor
Oxides: ZnO
The semiconducting properties of several oxides, e.g., ZnO, have been under
investigation for a long time [16]. A marked variation of the number of electrons in
the conduction band has been brought about by modifying the surrounding atmosphere. This behavior is relevant to the use as chemical sensor. ZnO belongs to the
non-stoichiometric oxide family, due to its excess of metal. Interstitial zinc atoms or
anionic O
2− vacancies could act as donor centers as their electrons have sufficient
thermal energy to enter the conduction band. Temperature and vacuum are both
able to change the stoichiometry of ZnO, modifying the original defects.
Chemisorption of oxidizing or reducing gases at the oxide surface also induces
defect modification as the chemisorbed molecules interact with the electrons of the
conduction band. Thus, an understanding of the nature of the defects and of their
reactivity is a fundamental requirement to understand the electron transfer mechanism at the solid gas interface and to drive the application of the oxide in sensor
devices.
5.26 Discussion of the Case
The main defects in ZnO are interstitial Zn i centers and oxygen vacancies. Both are
shallow defects and are able to ionize, transferring their electrons to the conduction
band. For this reason, any change in the ionizable centers generates change in the
conductivity of the material. Electron spin resonance spectra recorded under inert
atmosphere show two groups of signals (Fig. 5.18): the first one with g values
g A = 1.955 and g B = 1.958 and the second with g C at 2.008 and 2.002. Signal A
cannot be observed at room temperature, while it becomes strong at low temperature. Thermal treatment under vacuum leads to an increase of signal B, while A
decreases. These results suggest that the signal A is attributable to zinc centers
singly ionized, Zn
+
, whose spin–orbit coupling contribution obliges to lower the
5.24 Chemical Message
109
In 1988, it became relevant to look for the detection of free radicals in tissues,
thinking that the diseases associable to radicals could be thus controlled. The
above-reported research strongly indicates that the tissue manipulation could artifactually generate radicals, simply due to mechanical break of the same tissues. It
has been not excluded that the artifacts can also depend on the chemical treatment
of tissues, like chemotherapy, but they are not generated by a chemical interaction
effect.
5.25 The Case of Functional Defects in Semiconductor
Oxides: ZnO
The semiconducting properties of several oxides, e.g., ZnO, have been under
investigation for a long time [16]. A marked variation of the number of electrons in
the conduction band has been brought about by modifying the surrounding atmosphere. This behavior is relevant to the use as chemical sensor. ZnO belongs to the
non-stoichiometric oxide family, due to its excess of metal. Interstitial zinc atoms or
anionic O
2− vacancies could act as donor centers as their electrons have sufficient
thermal energy to enter the conduction band. Temperature and vacuum are both
able to change the stoichiometry of ZnO, modifying the original defects.
Chemisorption of oxidizing or reducing gases at the oxide surface also induces
defect modification as the chemisorbed molecules interact with the electrons of the
conduction band. Thus, an understanding of the nature of the defects and of their
reactivity is a fundamental requirement to understand the electron transfer mechanism at the solid gas interface and to drive the application of the oxide in sensor
devices.
5.26 Discussion of the Case
The main defects in ZnO are interstitial Zn i centers and oxygen vacancies. Both are
shallow defects and are able to ionize, transferring their electrons to the conduction
band. For this reason, any change in the ionizable centers generates change in the
conductivity of the material. Electron spin resonance spectra recorded under inert
atmosphere show two groups of signals (Fig. 5.18): the first one with g values
g A = 1.955 and g B = 1.958 and the second with g C at 2.008 and 2.002. Signal A
cannot be observed at room temperature, while it becomes strong at low temperature. Thermal treatment under vacuum leads to an increase of signal B, while A
decreases. These results suggest that the signal A is attributable to zinc centers
singly ionized, Zn
+
, whose spin–orbit coupling contribution obliges to lower the
5.24 Chemical Message
109