within the SrRuO 3 or (2) redox reactions, related to change of stoichiometry. Both
possibilities are related to loss of oxygen ions. To resolve this issue small amount of
oxygen was introduced in the STM chamber. Without applied voltage no effect of O 2
has been observed. However, by applying a negative tip voltage it was possible to
verify that the redox reactions related to change in stoichiometry but not decomposition (change of structure) induced from the STM tip were responsible for the
observed behaviour. In presence of molecular oxygen the loss of oxygen ions within
the oxide lattice could be completely reversed by applying negative tip bias, thus
forming O
2– that is incorporated into the SrRuO 3-x to restore the initial stoichiometry
of SrRuO 3 [29]. Structural changes (being completely irreversible) were in addition
safely excluded by PEEM/AFM analysis.
In contrary the cathodic changes caused by initial negative tip bias were relieved,
trigged by internal self-induced process. SrRuO 3 is typically p-type conducting and
of this reason enrichment with oxygen vacancies (positive relative charge in the
sub-lattice) leads first to conditions of p ~ n and further can lead to change to n-type
conductivity. Applying negative tip voltage attracts the oxygen vacancies causing
these changes. However, no oxygen ions are effectively lost and after removing the
applied bias the vacancy concentration can relax and the initial profile is restored [29].
Thus, it was demonstrated that STM studies can be very powerful tool for
inducing and studying electrode reactions on oxide interfaces, using materials of
different composition and electronic/ionic properties.
5 Conclusions
The Atomic Switch approach is an alternative and powerful way to study electrochemical surface reactions with highest lateral, mass and charge resolution. In
combination with classical STM modes it allows precise, site-invariant selection of
the reaction location, avoiding statistical signal deviations averaged over properties
of larger electrode surfaces. Instead of current, it uses the switching time (short
circuiting the tunnel gap) as a critical kinetic parameter and is insensitive to the
electronic partial conductivity of the materials. The variation of the tip-sample
distance provides the unique opportunity to define and/or change the transfer
coefficient of the charge transfer reactions, by keeping all other parameters constant.
The Atomic Switch can be used not only on electronically conducive materials
but also on ionic solid electrolytes and even macroscopically insulating oxides after
appropriate doping and/or treatment. The presented examples on difference classes
of materials such as RbAg 4 I 5 , Ta 2 O 5 and SrRuO 3 have demonstrated the ability of
this method not only to allow to determine the kinetic parameters of a particular
redox reaction but also to distinguish between different reacting species and rate
limiting steps. This technique will be further developed for even atomically resolved
experiments and expand the variety of studied materials and reactions.
92
I. Valov et al.
possibilities are related to loss of oxygen ions. To resolve this issue small amount of
oxygen was introduced in the STM chamber. Without applied voltage no effect of O 2
has been observed. However, by applying a negative tip voltage it was possible to
verify that the redox reactions related to change in stoichiometry but not decomposition (change of structure) induced from the STM tip were responsible for the
observed behaviour. In presence of molecular oxygen the loss of oxygen ions within
the oxide lattice could be completely reversed by applying negative tip bias, thus
forming O
2– that is incorporated into the SrRuO 3-x to restore the initial stoichiometry
of SrRuO 3 [29]. Structural changes (being completely irreversible) were in addition
safely excluded by PEEM/AFM analysis.
In contrary the cathodic changes caused by initial negative tip bias were relieved,
trigged by internal self-induced process. SrRuO 3 is typically p-type conducting and
of this reason enrichment with oxygen vacancies (positive relative charge in the
sub-lattice) leads first to conditions of p ~ n and further can lead to change to n-type
conductivity. Applying negative tip voltage attracts the oxygen vacancies causing
these changes. However, no oxygen ions are effectively lost and after removing the
applied bias the vacancy concentration can relax and the initial profile is restored [29].
Thus, it was demonstrated that STM studies can be very powerful tool for
inducing and studying electrode reactions on oxide interfaces, using materials of
different composition and electronic/ionic properties.
5 Conclusions
The Atomic Switch approach is an alternative and powerful way to study electrochemical surface reactions with highest lateral, mass and charge resolution. In
combination with classical STM modes it allows precise, site-invariant selection of
the reaction location, avoiding statistical signal deviations averaged over properties
of larger electrode surfaces. Instead of current, it uses the switching time (short
circuiting the tunnel gap) as a critical kinetic parameter and is insensitive to the
electronic partial conductivity of the materials. The variation of the tip-sample
distance provides the unique opportunity to define and/or change the transfer
coefficient of the charge transfer reactions, by keeping all other parameters constant.
The Atomic Switch can be used not only on electronically conducive materials
but also on ionic solid electrolytes and even macroscopically insulating oxides after
appropriate doping and/or treatment. The presented examples on difference classes
of materials such as RbAg 4 I 5 , Ta 2 O 5 and SrRuO 3 have demonstrated the ability of
this method not only to allow to determine the kinetic parameters of a particular
redox reaction but also to distinguish between different reacting species and rate
limiting steps. This technique will be further developed for even atomically resolved
experiments and expand the variety of studied materials and reactions.
92
I. Valov et al.