clusters, were formed between the electrodes (Fig. 6e and f) after forming process.
These results show that the gap between electrodes plays a crucial role in the filament
formation in the PEO-based device.
The effects of the salt inclusion in the polymer matrix are found in the comparison
of the observations between Ag/Ag-PEO/Pt and Ag/PEO/Pt devices. Since pure
PEO is a highly crystalline polymer [20], redox reactions at the Ag/PEO interface
and Ag
+ ion mobility are significantly hindered in the Ag/PEO/Pt device. This
results in almost no filament formation and only precipitations on the Pt electrode
for a gap of 2 μm (Fig. 6d). With decreasing gap, precipitated Ag atoms tend to grow
along sites where Ag
+ ions transport, inducing unidirectional filament growth, as
seen in Fig. 5e and f. The observed filaments consist of small Ag clusters rather than
being connected continuously. This situation changes drastically in the Ag/Ag-PEO/
Pt device. The salt inclusion of 3 wt% reduces the crystalline degree of the PEO
matrix, giving rise to an increased ionic conductivity of more than two orders of
magnitude [11]. The oxidation rate at the Ag/Ag-PEO interfaces is also enhanced
due to an increased amorphous phase. Therefore, both the Ag
+ ions dissolved from
the Ag electrode and the pre-existing Ag
+ ions can contribute to ion transport. Once
precipitations take place on the Pt electrode, the Ag nucleus grow by incorporating
Ag
+ ions from the surrounding area. As a result, the precipitated atoms form
dendritic morphologies with tree-like branching structures, as seen in Fig. 5a–c.
If the I CC is increased, the filament morphologies are retained in both the Ag/AgPEO/Pt and Ag/PEO/Pt devices, but the density and size of the filament structures
are increased. The I CC level also determines whether the switching behavior is
volatile or nonvolatile. The results obtained indicate that the filament formation
and switching behavior depend strongly on several kinetic factors, such as the redox
reaction at the electrode/polymer interfaces, ion mobility and electric field strength in
the polymer matrix, and reduction sites for precipitations. The different filament
formations, between unidirectional and dendritic growth, can be controlled by tuning
specific parameters, which can improve the stability and performance of SPE-based
atomic switches.
4 Highly Reproducible Conductance Quantization
Besides having bi-stable switching ability, one of the characteristic features of
atomic switches is conductance quantization. This quantized conductance was first
realized in a Ag 2 S-based atomic switch [21, 22], and was subsequently demonstrated
in a Cu 2 S-based atomic switch [23] and a Ta 2 O 5 -based atomic switch [24]. We
found that it is also possible to observe highly reproducible conductance quantization in Ag/PEO/Pt devices. In this section, we first present experimental results on
the quantized conductance of Ag/PEO/Pt devices, and then discuss, using firstprinciples density functional theory (DFT) simulations, how the structural variation
of atomic point contacts influences the stability of the quantized conductance. Most
of this section is reproduced with permissions from Ref. 25. Copyright 2017 John
Wiley & Sons, and from Ref. 26. Copyright 2017 IOP Publishing.
Solid-Polymer-Electrolyte-Based Atomic Switches
149
These results show that the gap between electrodes plays a crucial role in the filament
formation in the PEO-based device.
The effects of the salt inclusion in the polymer matrix are found in the comparison
of the observations between Ag/Ag-PEO/Pt and Ag/PEO/Pt devices. Since pure
PEO is a highly crystalline polymer [20], redox reactions at the Ag/PEO interface
and Ag
+ ion mobility are significantly hindered in the Ag/PEO/Pt device. This
results in almost no filament formation and only precipitations on the Pt electrode
for a gap of 2 μm (Fig. 6d). With decreasing gap, precipitated Ag atoms tend to grow
along sites where Ag
+ ions transport, inducing unidirectional filament growth, as
seen in Fig. 5e and f. The observed filaments consist of small Ag clusters rather than
being connected continuously. This situation changes drastically in the Ag/Ag-PEO/
Pt device. The salt inclusion of 3 wt% reduces the crystalline degree of the PEO
matrix, giving rise to an increased ionic conductivity of more than two orders of
magnitude [11]. The oxidation rate at the Ag/Ag-PEO interfaces is also enhanced
due to an increased amorphous phase. Therefore, both the Ag
+ ions dissolved from
the Ag electrode and the pre-existing Ag
+ ions can contribute to ion transport. Once
precipitations take place on the Pt electrode, the Ag nucleus grow by incorporating
Ag
+ ions from the surrounding area. As a result, the precipitated atoms form
dendritic morphologies with tree-like branching structures, as seen in Fig. 5a–c.
If the I CC is increased, the filament morphologies are retained in both the Ag/AgPEO/Pt and Ag/PEO/Pt devices, but the density and size of the filament structures
are increased. The I CC level also determines whether the switching behavior is
volatile or nonvolatile. The results obtained indicate that the filament formation
and switching behavior depend strongly on several kinetic factors, such as the redox
reaction at the electrode/polymer interfaces, ion mobility and electric field strength in
the polymer matrix, and reduction sites for precipitations. The different filament
formations, between unidirectional and dendritic growth, can be controlled by tuning
specific parameters, which can improve the stability and performance of SPE-based
atomic switches.
4 Highly Reproducible Conductance Quantization
Besides having bi-stable switching ability, one of the characteristic features of
atomic switches is conductance quantization. This quantized conductance was first
realized in a Ag 2 S-based atomic switch [21, 22], and was subsequently demonstrated
in a Cu 2 S-based atomic switch [23] and a Ta 2 O 5 -based atomic switch [24]. We
found that it is also possible to observe highly reproducible conductance quantization in Ag/PEO/Pt devices. In this section, we first present experimental results on
the quantized conductance of Ag/PEO/Pt devices, and then discuss, using firstprinciples density functional theory (DFT) simulations, how the structural variation
of atomic point contacts influences the stability of the quantized conductance. Most
of this section is reproduced with permissions from Ref. 25. Copyright 2017 John
Wiley & Sons, and from Ref. 26. Copyright 2017 IOP Publishing.
Solid-Polymer-Electrolyte-Based Atomic Switches
149