T c was observed as the applied gate voltage was modulated. T c was enhanced from
8.33 to 8.39 K when the gate voltage was reduced from 2.5 to À2.5 V. These
changes are almost three orders of magnitude larger than the values obtained using
solid dielectric [54], indicating that the EDLT can modulate a much higher carrier
density. In particular, modulation of T c in Nb film using an EDLT composed of an
ionic liquid was recently reported [55]. The reported direction of the T c variation
with respect to the gate voltage agrees with the result, but with a slightly smaller
difference in T c .
Superconducting T c of the BCS type superconductors is dependent on carrier
density on the basis of Matthias’ law shown in Eq. (3). Given that DOS curve and
Fermi level of Nb film in the EDLT are similar to that without electrolyte, a positive
gate voltage application and the resultant EDL charging causes decrease in D(0),
leading to decrease in T c , and vice versa. The T c tuning function shown in Fig. 10 can
be explained on the basis of the EDL mechanism.
Since chemical doping has been a sole mean to dope high density electronic
carrier in superconductors and potential superconductors, development of high T c
superconductor has been strictly limited by thermodynamic solubility of dopants so
far. The electrostatic carrier doping using EDL in the vicinity of solid/solid interface
should be applicable to potential superconductors which cannot sufficiently doped
through chemical routes although redox reaction of mobile ions including decomposition of electrolyte (redox window) limits the extent of the carrier doping. The
exploring room-temperature superconductivity in the vicinity of solid/solid electrolyte interface is underway.
5 Conclusions
Recent progress in nanoionic devices for various physical property tuning and
enhancement has been reviewed. Nanoionic devices enabled electrochemical carrier
doping using redox reaction and electrostatic carrier doping using EDL which are
promising approaches to overcome the limitations in conventional material synthesis
and semiconductor technology.
Nanoionic devices are novel approaches to enable various functions although
they are on the basis of conventional electrochemical processes. Note that the
approach applied here is not necessarily limited to the reduction of ionic transport
distance due to usage of thin films. In the atomic switch as the first nanoionic device,
quantized and high-speed conductance switching was achieved by generation and
annihilation of silver nanofilament at a tunnel gap between a probe tip of scanning
probe microscope (STM) and a counter platinum electrode. It was not electron-ion
mixed conductor but the gap that was miniaturized to achieve the function. This
indicates possible extension of the approach toward various directions.
A distinct advantage of nanoionic devices over electronic devices is their high
density of electronic carriers achieved by use of EDLs and redox reactions. On the
other hand, regarding conventional material synthesis, distinct advantages are in situ
Nanoionic Devices for Physical Property Tuning and Enhancement
171
8.33 to 8.39 K when the gate voltage was reduced from 2.5 to À2.5 V. These
changes are almost three orders of magnitude larger than the values obtained using
solid dielectric [54], indicating that the EDLT can modulate a much higher carrier
density. In particular, modulation of T c in Nb film using an EDLT composed of an
ionic liquid was recently reported [55]. The reported direction of the T c variation
with respect to the gate voltage agrees with the result, but with a slightly smaller
difference in T c .
Superconducting T c of the BCS type superconductors is dependent on carrier
density on the basis of Matthias’ law shown in Eq. (3). Given that DOS curve and
Fermi level of Nb film in the EDLT are similar to that without electrolyte, a positive
gate voltage application and the resultant EDL charging causes decrease in D(0),
leading to decrease in T c , and vice versa. The T c tuning function shown in Fig. 10 can
be explained on the basis of the EDL mechanism.
Since chemical doping has been a sole mean to dope high density electronic
carrier in superconductors and potential superconductors, development of high T c
superconductor has been strictly limited by thermodynamic solubility of dopants so
far. The electrostatic carrier doping using EDL in the vicinity of solid/solid interface
should be applicable to potential superconductors which cannot sufficiently doped
through chemical routes although redox reaction of mobile ions including decomposition of electrolyte (redox window) limits the extent of the carrier doping. The
exploring room-temperature superconductivity in the vicinity of solid/solid electrolyte interface is underway.
5 Conclusions
Recent progress in nanoionic devices for various physical property tuning and
enhancement has been reviewed. Nanoionic devices enabled electrochemical carrier
doping using redox reaction and electrostatic carrier doping using EDL which are
promising approaches to overcome the limitations in conventional material synthesis
and semiconductor technology.
Nanoionic devices are novel approaches to enable various functions although
they are on the basis of conventional electrochemical processes. Note that the
approach applied here is not necessarily limited to the reduction of ionic transport
distance due to usage of thin films. In the atomic switch as the first nanoionic device,
quantized and high-speed conductance switching was achieved by generation and
annihilation of silver nanofilament at a tunnel gap between a probe tip of scanning
probe microscope (STM) and a counter platinum electrode. It was not electron-ion
mixed conductor but the gap that was miniaturized to achieve the function. This
indicates possible extension of the approach toward various directions.
A distinct advantage of nanoionic devices over electronic devices is their high
density of electronic carriers achieved by use of EDLs and redox reactions. On the
other hand, regarding conventional material synthesis, distinct advantages are in situ
Nanoionic Devices for Physical Property Tuning and Enhancement
171