reduced surface could be reversibly re-oxidized by applying a positive tip voltage. In
Fig. 9 the recorded images for these experiment are presented.
The modified areas have been studied by ST spectroscopy to identify the changes
in the electronic structure and conclude on the nature of induced modifications. As it
can be seen from the I-V plot in Fig. 9 the reduced state shows purely metallic
behaviour with liner relation between tunnelling current and applied voltage. After
re-oxidation the metallic atoms are oxidized and the bandgap of the oxide was
restored. Thus, we have confirmed that Ta-ions can be reduced at the surface of
TaOx and reversible re-oxidized without inducing irreversible changes on the
surface morphology.
Similar experiments have been performed by initially applying positive voltage to
the STM tip. In this case the surface has been again modified but instead expected
oxidation a clearly pronounced reduction of the selected area was detected. These
reduction could be reversibly removed by applying a negative tip voltage. The STM
images can be seen in Fig. 10.
The observed changes were explained by removing oxygen (2O
2–
– 4e
–
¼ O 2 )
from the Ta 2 O 5-x , thus causing an effective reduction of the surface. An additional
peak was observed close to the unoccupied states, indicating defect states in the band
gap. Calculations on the hexagonal Ta 2 O 5 structure revealed that occupied states
mainly correspond to O 2p levels and unoccupied states to Ta 5d and 6s levels.
Creating oxygen vacancies in the structure leads to an extra peak in LDOS close to
the unoccupied states due to electron localization on Ta 5d and 6s states. The extra
peak found in our experimental dI/dV data is consistent with this computational
LDOS analysis on Ta 2 O 5 . The characteristic peak close to the unoccupied states and
smaller band gap on LRS confirm resistive switching by an oxygen vacancy
mechanism [29].
In these large scale experiments we were able also to distinguish that the
predominance of one of the ionic species i.e. Ta
x+ -cations or O
2– -anions in the
redox reactions is strongly influenced by the stoichiometry of the oxide. Thus, more
strongly reduced Ta 2 O 5-x shows cation-type switching (as in Fig. 9), whereas less
reduced Ta 2 O 5-y (y < x) allows only anion-type switching. Therefore, it can be
concluded that the level of non-stoichiometry also influences significantly the ionic
transference numbers. These conclusions were supported by theoretical calculations
showing the same trend.
Redox reactions, effects of polarity of the applied voltage and the influence of
oxygen molecules have also been studied with purely electronic oxides e.g. SrRuO 3
that however, allow STM-tip induced ionic redox reactions. The electrical and
structural properties of SrRuO 3 are very sensitive to the oxygen non-stoichiometry
in the material and distinguishing between these effects is very challenging. The
complex processes that were observed during anodic or respectively cathodic polarization were able to be resolved only by a combination of STM imaging, spectroscopy as well supported by PEEM and AFM analysis.
Under ultra-high vacuum conditions applying of positive tip bias resulted in
irreversible modifications (reduction) of the surface. Two processes, different in
nature, were able to explain these initial observations—(1) purely structural changes
Nanoscale Electrochemical Studies: How Can We Use the Atomic Switch
89
Fig. 9 the recorded images for these experiment are presented.
The modified areas have been studied by ST spectroscopy to identify the changes
in the electronic structure and conclude on the nature of induced modifications. As it
can be seen from the I-V plot in Fig. 9 the reduced state shows purely metallic
behaviour with liner relation between tunnelling current and applied voltage. After
re-oxidation the metallic atoms are oxidized and the bandgap of the oxide was
restored. Thus, we have confirmed that Ta-ions can be reduced at the surface of
TaOx and reversible re-oxidized without inducing irreversible changes on the
surface morphology.
Similar experiments have been performed by initially applying positive voltage to
the STM tip. In this case the surface has been again modified but instead expected
oxidation a clearly pronounced reduction of the selected area was detected. These
reduction could be reversibly removed by applying a negative tip voltage. The STM
images can be seen in Fig. 10.
The observed changes were explained by removing oxygen (2O
2–
– 4e
–
¼ O 2 )
from the Ta 2 O 5-x , thus causing an effective reduction of the surface. An additional
peak was observed close to the unoccupied states, indicating defect states in the band
gap. Calculations on the hexagonal Ta 2 O 5 structure revealed that occupied states
mainly correspond to O 2p levels and unoccupied states to Ta 5d and 6s levels.
Creating oxygen vacancies in the structure leads to an extra peak in LDOS close to
the unoccupied states due to electron localization on Ta 5d and 6s states. The extra
peak found in our experimental dI/dV data is consistent with this computational
LDOS analysis on Ta 2 O 5 . The characteristic peak close to the unoccupied states and
smaller band gap on LRS confirm resistive switching by an oxygen vacancy
mechanism [29].
In these large scale experiments we were able also to distinguish that the
predominance of one of the ionic species i.e. Ta
x+ -cations or O
2– -anions in the
redox reactions is strongly influenced by the stoichiometry of the oxide. Thus, more
strongly reduced Ta 2 O 5-x shows cation-type switching (as in Fig. 9), whereas less
reduced Ta 2 O 5-y (y < x) allows only anion-type switching. Therefore, it can be
concluded that the level of non-stoichiometry also influences significantly the ionic
transference numbers. These conclusions were supported by theoretical calculations
showing the same trend.
Redox reactions, effects of polarity of the applied voltage and the influence of
oxygen molecules have also been studied with purely electronic oxides e.g. SrRuO 3
that however, allow STM-tip induced ionic redox reactions. The electrical and
structural properties of SrRuO 3 are very sensitive to the oxygen non-stoichiometry
in the material and distinguishing between these effects is very challenging. The
complex processes that were observed during anodic or respectively cathodic polarization were able to be resolved only by a combination of STM imaging, spectroscopy as well supported by PEEM and AFM analysis.
Under ultra-high vacuum conditions applying of positive tip bias resulted in
irreversible modifications (reduction) of the surface. Two processes, different in
nature, were able to explain these initial observations—(1) purely structural changes
Nanoscale Electrochemical Studies: How Can We Use the Atomic Switch
89