modification/doping of the thin film electrolytes was necessary, because the intrinsic
electronic conductivity was sufficient for electron tunnelling.
These results have demonstrated the potential of the Atomic Switch (and STM in
general) as powerful tool for studies of electrochemical processes with an ultimate
lateral, mass and charge resolution.
4.2.2 Redox Processes on Oxides Studied by Atomic Switch
The concept of the Atomic Switch has been further developed and applied on oxides
such as Ta 2 O 5 , HfO 2 and TiO 2 . These samples have been adapted for Atomic Switch
studies by annealing in vacuum in order to slightly increase their electronic conductivity. As discussed in the previous section the electronic conductivity is not
influencing the kinetic measurements of the ionic redox reactions.
Applying different voltage polarities to the STM tip, it has been possible to
distinguish between two different partial redox reactions, indicating that different
ionic species are involved in the electrode reaction process. The system used for
these experiments was Ta 2 O 5-x thin film deposited on 300 nm metallic Ta. As in the
case of RbAg 4 I 5 we firstly applied negative voltage to the tip and have observed
formation of metallic nuclei at the oxide surface. We were also able to detect the
formation of quantum point contacts for each of the experiments. Figure 8 shows
STM images recorded after three consecutive switching events.
Using negative tip voltage we formed clusters of metallic Ta as also confirmed
from the STS sweeps (sweep 3) performed immediately after the cluster formation. It
shows a linear relation between voltage and current, thus evidencing for a metallic
phase. The formed nucleus is however prone to re-oxidation and if one waits for
~15 min the metallic nucleus is covered by oxide scale (sweep 1 black curve)
showing restored band gap. This surface oxide can be easily reduced back to metallic
state (sweep 1 red curve).
Applying a positive bias to the tip results in a completely different electrical
responses and images at the Ta 2 O 5-x surface. No formation of quantum point contact
was observed at these conditions. The form and morphology of the modified areas
were much broader and semiconducting instead of metallic properties were detected
as evident from the STS sweeps, indicating that no metallic phase has been formed.
Based on these analysis it has been concluded that during the cathodic process
(negative tip bias) the main reacting species are metal cations that are reduced to
form metallic nuclei, whereas during the anodic process (positive tip bias) oxygen
ions have been oxidized and oxygen has been released. Thus, depending on the
applied voltage polarity different ionic species are undergoing electrochemical
reaction at the interface.
Similar experiments were performed with other oxides such as HfO 2-x and
TiO 2-x [32].
These conclusions were further supported and additional details on these processes were provided by using STM on larger surface areas on the oxide surfaces.
Applying a negative voltage to the tip caused a reduction of the selected area. The
Nanoscale Electrochemical Studies: How Can We Use the Atomic Switch
87
electronic conductivity was sufficient for electron tunnelling.
These results have demonstrated the potential of the Atomic Switch (and STM in
general) as powerful tool for studies of electrochemical processes with an ultimate
lateral, mass and charge resolution.
4.2.2 Redox Processes on Oxides Studied by Atomic Switch
The concept of the Atomic Switch has been further developed and applied on oxides
such as Ta 2 O 5 , HfO 2 and TiO 2 . These samples have been adapted for Atomic Switch
studies by annealing in vacuum in order to slightly increase their electronic conductivity. As discussed in the previous section the electronic conductivity is not
influencing the kinetic measurements of the ionic redox reactions.
Applying different voltage polarities to the STM tip, it has been possible to
distinguish between two different partial redox reactions, indicating that different
ionic species are involved in the electrode reaction process. The system used for
these experiments was Ta 2 O 5-x thin film deposited on 300 nm metallic Ta. As in the
case of RbAg 4 I 5 we firstly applied negative voltage to the tip and have observed
formation of metallic nuclei at the oxide surface. We were also able to detect the
formation of quantum point contacts for each of the experiments. Figure 8 shows
STM images recorded after three consecutive switching events.
Using negative tip voltage we formed clusters of metallic Ta as also confirmed
from the STS sweeps (sweep 3) performed immediately after the cluster formation. It
shows a linear relation between voltage and current, thus evidencing for a metallic
phase. The formed nucleus is however prone to re-oxidation and if one waits for
~15 min the metallic nucleus is covered by oxide scale (sweep 1 black curve)
showing restored band gap. This surface oxide can be easily reduced back to metallic
state (sweep 1 red curve).
Applying a positive bias to the tip results in a completely different electrical
responses and images at the Ta 2 O 5-x surface. No formation of quantum point contact
was observed at these conditions. The form and morphology of the modified areas
were much broader and semiconducting instead of metallic properties were detected
as evident from the STS sweeps, indicating that no metallic phase has been formed.
Based on these analysis it has been concluded that during the cathodic process
(negative tip bias) the main reacting species are metal cations that are reduced to
form metallic nuclei, whereas during the anodic process (positive tip bias) oxygen
ions have been oxidized and oxygen has been released. Thus, depending on the
applied voltage polarity different ionic species are undergoing electrochemical
reaction at the interface.
Similar experiments were performed with other oxides such as HfO 2-x and
TiO 2-x [32].
These conclusions were further supported and additional details on these processes were provided by using STM on larger surface areas on the oxide surfaces.
Applying a negative voltage to the tip caused a reduction of the selected area. The
Nanoscale Electrochemical Studies: How Can We Use the Atomic Switch
87