and erasing pulses. Switching speed changes from several hundred nanoseconds to a
few hundred microseconds, depending on the voltage pulse condition, the thickness
of the Ag-PEO film, and the fabrication conditions of the devices. Even at this early
stage, the SPE-based atomic switch exhibits high programing speeds (<1 μs) and
long retention times (>1 week), demonstrating as appropriate capabilities for nonvolatile memories.
2.2 Switching Mechanism
To understand the observed switching behavior, the thermal and transport properties
of the Ag-PEO film were investigated as a function of the Ag salt concentration
[11]. The pure PEO film shows a low conductivity of ~10
À9 S cm
À1 and a glass
transition temperature T g ¼ À55
C. With increasing Ag salt concentration, conductivity rises by two orders of magnitude up to 3 wt% and then decreases slightly
for higher concentrations. T g increases monotonically and reaches À25
C at 6 wt%.
The electronic transport number is estimated to be less than 3 Â 10
À3 and is almost
independent of the salt concentration. In contrast, the cation transport number
increases up to 7 Â 10
À2 at 2 wt%, and then decreases for higher Ag salt concentrations. This behavior can be explained by initial increase of carrier ion numbers
with introducing Ag salts and subsequent decreased number of cations due to
formation of ionic aggregates such as ion pairs and triple ions [12].
The standard redox potentials for Ag in aqueous solutions are given as Ag
2+ /Ag
+
(E
ϕ,1
¼ 1.98 V) and Ag
+
/Ag (E
ϕ,2
¼ 0.8 V) [13]. Since water is absorbed from the
atmosphere into the PEO matrix [14], the residual water can involve redox reactions
at the interface and the general trend of redox potentials holds in our devices.
According to the standard redox potentials, oxidation from Ag to Ag
+ is thermodynamically the most favorable. Cyclic voltammetry measurements were performed
for Ag/Ag-PEO/Pt devices with different salt concentrations to investigate redox
reactions at the Ag/PEO interface. At 3 wt%, current peaks were observed at around
Æ1 V, which can be attributed to oxidation from Ag to Ag
+ and reduction from Ag
+
to Ag respectively. The redox current was reduced for higher salt concentrations,
although the oxidation and reduction peaks were still observed. This current reduction is considered to come from a decrease in both the mobility and number of free
carriers, due to the formation of ionic aggregates.
Based on the results obtained, we inferred the switching mechanism that is
illustrated schematically in Fig. 3. When a positive bias voltage is applied to the
Ag electrode at the first sweep cycle (Fig. 3a), Ag
+ ions oxidized at the Ag/PEO
interface and initially dissolved in the PEO matrix transport to the Pt electrode. At
the same time, ClO 4
À ions move toward the Ag electrode. Then, a metal filament is
formed between the Ag and Pt electrodes by heterogeneous nucleation of Ag on Pt
followed by the Ag nucleus growth, which turns on the device (Fig. 3b). When a
negative bias voltage is subsequently applied, the oxidation reaction takes place at
surfaces of the metal filament, and the thinnest part of the metal filament is dissolved,
Solid-Polymer-Electrolyte-Based Atomic Switches
143
few hundred microseconds, depending on the voltage pulse condition, the thickness
of the Ag-PEO film, and the fabrication conditions of the devices. Even at this early
stage, the SPE-based atomic switch exhibits high programing speeds (<1 μs) and
long retention times (>1 week), demonstrating as appropriate capabilities for nonvolatile memories.
2.2 Switching Mechanism
To understand the observed switching behavior, the thermal and transport properties
of the Ag-PEO film were investigated as a function of the Ag salt concentration
[11]. The pure PEO film shows a low conductivity of ~10
À9 S cm
À1 and a glass
transition temperature T g ¼ À55
C. With increasing Ag salt concentration, conductivity rises by two orders of magnitude up to 3 wt% and then decreases slightly
for higher concentrations. T g increases monotonically and reaches À25
C at 6 wt%.
The electronic transport number is estimated to be less than 3 Â 10
À3 and is almost
independent of the salt concentration. In contrast, the cation transport number
increases up to 7 Â 10
À2 at 2 wt%, and then decreases for higher Ag salt concentrations. This behavior can be explained by initial increase of carrier ion numbers
with introducing Ag salts and subsequent decreased number of cations due to
formation of ionic aggregates such as ion pairs and triple ions [12].
The standard redox potentials for Ag in aqueous solutions are given as Ag
2+ /Ag
+
(E
ϕ,1
¼ 1.98 V) and Ag
+
/Ag (E
ϕ,2
¼ 0.8 V) [13]. Since water is absorbed from the
atmosphere into the PEO matrix [14], the residual water can involve redox reactions
at the interface and the general trend of redox potentials holds in our devices.
According to the standard redox potentials, oxidation from Ag to Ag
+ is thermodynamically the most favorable. Cyclic voltammetry measurements were performed
for Ag/Ag-PEO/Pt devices with different salt concentrations to investigate redox
reactions at the Ag/PEO interface. At 3 wt%, current peaks were observed at around
Æ1 V, which can be attributed to oxidation from Ag to Ag
+ and reduction from Ag
+
to Ag respectively. The redox current was reduced for higher salt concentrations,
although the oxidation and reduction peaks were still observed. This current reduction is considered to come from a decrease in both the mobility and number of free
carriers, due to the formation of ionic aggregates.
Based on the results obtained, we inferred the switching mechanism that is
illustrated schematically in Fig. 3. When a positive bias voltage is applied to the
Ag electrode at the first sweep cycle (Fig. 3a), Ag
+ ions oxidized at the Ag/PEO
interface and initially dissolved in the PEO matrix transport to the Pt electrode. At
the same time, ClO 4
À ions move toward the Ag electrode. Then, a metal filament is
formed between the Ag and Pt electrodes by heterogeneous nucleation of Ag on Pt
followed by the Ag nucleus growth, which turns on the device (Fig. 3b). When a
negative bias voltage is subsequently applied, the oxidation reaction takes place at
surfaces of the metal filament, and the thinnest part of the metal filament is dissolved,
Solid-Polymer-Electrolyte-Based Atomic Switches
143