Energy technologies of the 21(st) century require an understanding and precise control over ion t... more Energy technologies of the 21(st) century require an understanding and precise control over ion transport and electrochemistry at all length scales - from single atoms to macroscopic devices. This short review provides a summary of recent studies dedicated to methods of advanced scanning probe microscopy for probing electrochemical transformations in solids at the meso-, nano- and atomic scales. The discussion presents the advantages and limitations of several techniques and a wealth of examples highlighting peculiarities of nanoscale electrochemistry.
Voltage spectroscopies in scanning probe microscopy (SPM) techniques are widely used to investiga... more Voltage spectroscopies in scanning probe microscopy (SPM) techniques are widely used to investigate the electrochemical processes in nanoscale volumes, which are important for current key applications, such as batteries, fuel cells, catalysts, and memristors. The spectroscopic measurements are commonly performed on a grid of multiple points to yield spatially resolved maps of reversible and irreversible electrochemical functionalities. Hence, the spacing between measurement points is an important parameter to be considered, especially for irreversible electrochemical processes. Here, we report nonlocal electrochemical dynamics in chains of Ag particles fabricated by the SPM tip on a silver ion solid electrolyte. When the grid spacing is small compared with the size of the formed Ag particles, anomalous chains of unequally sized particles with double periodicity evolve. This behavior is ascribed to a proximity effect during the tip-induced electrochemical process, specifically, size-...
Enhancement of oxygen ion conductivity in oxides is important for low-temperature (<500 °C) op... more Enhancement of oxygen ion conductivity in oxides is important for low-temperature (<500 °C) operation of solid oxide fuel cells, sensors and other ionotronic devices. While huge ion conductivity has been demonstrated in planar heterostructure films, there has been considerable debate over the origin of the conductivity enhancement, in part because of the difficulties of probing buried ion transport channels. Here we create a practical geometry for device miniaturization, consisting of highly crystalline micrometre-thick vertical nanocolumns of Sm-doped CeO2 embedded in supporting matrices of SrTiO3. The ionic conductivity is higher by one order of magnitude than plain Sm-doped CeO2 films. By using scanning probe microscopy, we show that the fast ion-conducting channels are not exclusively restricted to the interface but also are localized at the Sm-doped CeO2 nanopillars. This work offers a pathway to realize spatially localized fast ion transport in oxides of micrometre thickness.
Flexoelectricity is emerging as a fascinating means for exploring the physical properties of nano... more Flexoelectricity is emerging as a fascinating means for exploring the physical properties of nanoscale materials. Here, we demonstrated the unusual coupling between electronic transport and the mechanical strain gradient in a dielectric epitaxial thin film. Utilizing the nanoscale strain gradient, we showed the unique functionality of flexoelectricity to generate a rectifying diode effect. Furthermore, using conductive atomic force microscopy, we found that the flexoelectric effect can govern the local transport characteristics, including spatial conduction inhomogeneities, in thin-film epitaxy systems. Consideration of the flexoelectric effect will improve understanding of the charge conduction mechanism at the nanoscale and may facilitate the advancement of novel nanoelectronic device design.
ABSTRACT We investigated the ferroelectric (FE) domain nucleation and domain wall motion in epita... more ABSTRACT We investigated the ferroelectric (FE) domain nucleation and domain wall motion in epitaxial PbZr0.4Ti0.6O3 capacitors by using modified piezoresponse force microscopy with the domaintracing method. From time-dependent FE domain evolution images, we observed that defectmediated inhomogeneous nucleation occurred with a stochastic nature. In addition, we found that the number of nuclei N(t) was linearly proportional to log t, where t is the accumulated time of the applied pulse fields. The time-dependence of N(t) suggests a distribution of energy barriers for nucleation, which may determine the stochastic nature of domain nucleation. We also observed that the domain grew with consecutive Barkhausen avalanches and that the growth direction became anisotropic when the domain radius was larger than a critical radius of about 100 nm.
ABSTRACT We determined simultaneously the domain wall speed (v) and the nucleation rate (N) of fe... more ABSTRACT We determined simultaneously the domain wall speed (v) and the nucleation rate (N) of ferroelectric (FE) domain in 100-nm-thick epitaxial PbZr{sub 0.2}Ti{sub 0.8}O{sub 3} capacitors from the successive domain evolution images under various applied electric field (E{sub app}) by piezoresponse force microscopy. It was found that, at a given electric field, the v and N values decreased as the switching process proceeded. The averaged domain wall speed was confirmed to follow the Merz&#39;s law, exp[-(E{sub 0}/E{sub app})], with an activation field E{sub 0} of about 700 kV/cm. Moreover, we found that the nucleation process played a more important role in the FE domain switching at higher fields, while domain wall motion mainly contributed to the switching at lower fields.
ABSTRACT We report on the multiferroic properties of single-phase (PbZr0.57Ti0.43O3)(0.8)(PbFe0.6... more ABSTRACT We report on the multiferroic properties of single-phase (PbZr0.57Ti0.43O3)(0.8)(PbFe0.67W0.33O3)(0.2) solid-solution (PZT-PFW) epitaxial thin films. The epitaxial PZT-PFW thin films are deposited using a pulsed laser deposition method. An optimized growth condition has been indentified to achieve single-phase epitaxial thin films. Our films clearly show ferroelectric and weak ferromagnetic properties at room temperature. The synthesis of single-phase solid-solution materials using conventional ferroelectrics and relaxor ferroelectrics might open a way to realize a room-temperature multiferroic material.
ABSTRACT We deposited NiO thin films with SrRuO(3) bottom electrodes on SrTiO(3) (001) substrates... more ABSTRACT We deposited NiO thin films with SrRuO(3) bottom electrodes on SrTiO(3) (001) substrates by using pulsed laser deposition. The growth temperature and the oxygen pressure were varied in order to obtain NiO films with different structural and electrical properties. We investigated the I-V characteristics of the Pt/NiO/SRO structures and observed a strong dependence of bipolar resistance switching on the growth conditions of the NiO thin films. Stable bipolar memory resistance switching was observed only in the devices with NiO films deposited at 400 degrees C and 10 mTorr of O(2). The off-state I-V curve of bipolar switching showed a linear fitting to the Schottky effect, indicating its origin in the NiO/SRO interface. Our results suggest that the growth conditions of NiO may affect the bipolar switching behavior through the film&#39;s resistance, the film&#39;s crystallinity, or the status of the grain boundaries.
We investigated the scaling behavior of ferroelectric (FE) hysteresis loops as a function of the ... more We investigated the scaling behavior of ferroelectric (FE) hysteresis loops as a function of the applied field amplitude (Eâ) in a high-quality epitaxial PbZr{sub 0.2}Ti{sub 0.8}Oâ (PZT) thin film. We observed that the areas of the polarization-electric field hysteresis loops (A) followed the scaling law A â Eâ{sup α}, with the exponent α = 0.45 {+-} 0.01. This result is
We report on epitaxial growth of single-phase [Pb(Zr0.57Ti0.43)O3]0.8[Pb(Fe2/3W1/3)O3]0.2 (PZT-PF... more We report on epitaxial growth of single-phase [Pb(Zr0.57Ti0.43)O3]0.8[Pb(Fe2/3W1/3)O3]0.2 (PZT-PFW) solid-solution thin films using pulsed laser deposition. X-ray diffraction measurements reveal that the films have a tetragonal structure. The films exhibit ferroelectric properties and weak ferromagnetic responses at room temperature. Magnetoelectric effects were investigated; the nonlinear magnetoelectric coefficient was measured and found to be comparable to those of multiferroic hexagonal manganites, but at least two orders of magnitude smaller than that for polycrystalline PZT-PFW films.
ABSTRACT We find that resistance switching (RS) phenomena change reversibly between bipolar RS (B... more ABSTRACT We find that resistance switching (RS) phenomena change reversibly between bipolar RS (BRS) and unipolar RS (URS) in a Pt/SrTiOx/Pt cell. For an asymmetric electrode configuration of Ti/SrTiOx/Pt cells whose top and bottom interfaces are Ohmic and Schottky-like rectifying, we determine that BRS only occurs when a positive voltage is applied to the bottom Pt electrode at the forming process. During the set process of BRS in a Pt/SrTiOx/Pt cell, O2 bubbles develop on the top Pt electrode. From the experimental results for a single sample in which both BRS and URS occur, O2− ion movement and consequent interfacial resistance modification might play an important role in BRS but not URS.
ABSTRACT Recent developments in ferroelectric (FE) domain imaging techniques have established an ... more ABSTRACT Recent developments in ferroelectric (FE) domain imaging techniques have established an understanding of intriguing polarization switching dynamics. In particular, nanoscale studies of FE domain switching phenomena using piezoresponse force microscopy (PFM) can provide important microscopic details on nucleation and subsequent growth of domains, complementing conventional electrical measurements that only give macroscopic information. This review covers recent nanoscale PFM studies of domain switching dynamics in FE thin films. Recent nanoscale PFM-based studies have demonstrated that quenched defects inside the FE thin films play important roles in domain switching processes, including defect-mediated inhomogeneous nucleation, pinning-dominated nonlinear dynamics of domain walls, and many other intriguing phenomena.Highlights►We review recent piezoresponse force microscopy studies of ferroelectric domains. ►Piezoresponse force microscopy has provided nanoscale details of domain switching. ►Quenched defects in ferroelectric thin films act as nucleation centers. ►Defects also act as pinning sites for propagating ferroelectric domain walls.
Energy technologies of the 21(st) century require an understanding and precise control over ion t... more Energy technologies of the 21(st) century require an understanding and precise control over ion transport and electrochemistry at all length scales - from single atoms to macroscopic devices. This short review provides a summary of recent studies dedicated to methods of advanced scanning probe microscopy for probing electrochemical transformations in solids at the meso-, nano- and atomic scales. The discussion presents the advantages and limitations of several techniques and a wealth of examples highlighting peculiarities of nanoscale electrochemistry.
Voltage spectroscopies in scanning probe microscopy (SPM) techniques are widely used to investiga... more Voltage spectroscopies in scanning probe microscopy (SPM) techniques are widely used to investigate the electrochemical processes in nanoscale volumes, which are important for current key applications, such as batteries, fuel cells, catalysts, and memristors. The spectroscopic measurements are commonly performed on a grid of multiple points to yield spatially resolved maps of reversible and irreversible electrochemical functionalities. Hence, the spacing between measurement points is an important parameter to be considered, especially for irreversible electrochemical processes. Here, we report nonlocal electrochemical dynamics in chains of Ag particles fabricated by the SPM tip on a silver ion solid electrolyte. When the grid spacing is small compared with the size of the formed Ag particles, anomalous chains of unequally sized particles with double periodicity evolve. This behavior is ascribed to a proximity effect during the tip-induced electrochemical process, specifically, size-...
Enhancement of oxygen ion conductivity in oxides is important for low-temperature (<500 °C) op... more Enhancement of oxygen ion conductivity in oxides is important for low-temperature (<500 °C) operation of solid oxide fuel cells, sensors and other ionotronic devices. While huge ion conductivity has been demonstrated in planar heterostructure films, there has been considerable debate over the origin of the conductivity enhancement, in part because of the difficulties of probing buried ion transport channels. Here we create a practical geometry for device miniaturization, consisting of highly crystalline micrometre-thick vertical nanocolumns of Sm-doped CeO2 embedded in supporting matrices of SrTiO3. The ionic conductivity is higher by one order of magnitude than plain Sm-doped CeO2 films. By using scanning probe microscopy, we show that the fast ion-conducting channels are not exclusively restricted to the interface but also are localized at the Sm-doped CeO2 nanopillars. This work offers a pathway to realize spatially localized fast ion transport in oxides of micrometre thickness.
Flexoelectricity is emerging as a fascinating means for exploring the physical properties of nano... more Flexoelectricity is emerging as a fascinating means for exploring the physical properties of nanoscale materials. Here, we demonstrated the unusual coupling between electronic transport and the mechanical strain gradient in a dielectric epitaxial thin film. Utilizing the nanoscale strain gradient, we showed the unique functionality of flexoelectricity to generate a rectifying diode effect. Furthermore, using conductive atomic force microscopy, we found that the flexoelectric effect can govern the local transport characteristics, including spatial conduction inhomogeneities, in thin-film epitaxy systems. Consideration of the flexoelectric effect will improve understanding of the charge conduction mechanism at the nanoscale and may facilitate the advancement of novel nanoelectronic device design.
ABSTRACT We investigated the ferroelectric (FE) domain nucleation and domain wall motion in epita... more ABSTRACT We investigated the ferroelectric (FE) domain nucleation and domain wall motion in epitaxial PbZr0.4Ti0.6O3 capacitors by using modified piezoresponse force microscopy with the domaintracing method. From time-dependent FE domain evolution images, we observed that defectmediated inhomogeneous nucleation occurred with a stochastic nature. In addition, we found that the number of nuclei N(t) was linearly proportional to log t, where t is the accumulated time of the applied pulse fields. The time-dependence of N(t) suggests a distribution of energy barriers for nucleation, which may determine the stochastic nature of domain nucleation. We also observed that the domain grew with consecutive Barkhausen avalanches and that the growth direction became anisotropic when the domain radius was larger than a critical radius of about 100 nm.
ABSTRACT We determined simultaneously the domain wall speed (v) and the nucleation rate (N) of fe... more ABSTRACT We determined simultaneously the domain wall speed (v) and the nucleation rate (N) of ferroelectric (FE) domain in 100-nm-thick epitaxial PbZr{sub 0.2}Ti{sub 0.8}O{sub 3} capacitors from the successive domain evolution images under various applied electric field (E{sub app}) by piezoresponse force microscopy. It was found that, at a given electric field, the v and N values decreased as the switching process proceeded. The averaged domain wall speed was confirmed to follow the Merz&#39;s law, exp[-(E{sub 0}/E{sub app})], with an activation field E{sub 0} of about 700 kV/cm. Moreover, we found that the nucleation process played a more important role in the FE domain switching at higher fields, while domain wall motion mainly contributed to the switching at lower fields.
ABSTRACT We report on the multiferroic properties of single-phase (PbZr0.57Ti0.43O3)(0.8)(PbFe0.6... more ABSTRACT We report on the multiferroic properties of single-phase (PbZr0.57Ti0.43O3)(0.8)(PbFe0.67W0.33O3)(0.2) solid-solution (PZT-PFW) epitaxial thin films. The epitaxial PZT-PFW thin films are deposited using a pulsed laser deposition method. An optimized growth condition has been indentified to achieve single-phase epitaxial thin films. Our films clearly show ferroelectric and weak ferromagnetic properties at room temperature. The synthesis of single-phase solid-solution materials using conventional ferroelectrics and relaxor ferroelectrics might open a way to realize a room-temperature multiferroic material.
ABSTRACT We deposited NiO thin films with SrRuO(3) bottom electrodes on SrTiO(3) (001) substrates... more ABSTRACT We deposited NiO thin films with SrRuO(3) bottom electrodes on SrTiO(3) (001) substrates by using pulsed laser deposition. The growth temperature and the oxygen pressure were varied in order to obtain NiO films with different structural and electrical properties. We investigated the I-V characteristics of the Pt/NiO/SRO structures and observed a strong dependence of bipolar resistance switching on the growth conditions of the NiO thin films. Stable bipolar memory resistance switching was observed only in the devices with NiO films deposited at 400 degrees C and 10 mTorr of O(2). The off-state I-V curve of bipolar switching showed a linear fitting to the Schottky effect, indicating its origin in the NiO/SRO interface. Our results suggest that the growth conditions of NiO may affect the bipolar switching behavior through the film&#39;s resistance, the film&#39;s crystallinity, or the status of the grain boundaries.
We investigated the scaling behavior of ferroelectric (FE) hysteresis loops as a function of the ... more We investigated the scaling behavior of ferroelectric (FE) hysteresis loops as a function of the applied field amplitude (Eâ) in a high-quality epitaxial PbZr{sub 0.2}Ti{sub 0.8}Oâ (PZT) thin film. We observed that the areas of the polarization-electric field hysteresis loops (A) followed the scaling law A â Eâ{sup α}, with the exponent α = 0.45 {+-} 0.01. This result is
We report on epitaxial growth of single-phase [Pb(Zr0.57Ti0.43)O3]0.8[Pb(Fe2/3W1/3)O3]0.2 (PZT-PF... more We report on epitaxial growth of single-phase [Pb(Zr0.57Ti0.43)O3]0.8[Pb(Fe2/3W1/3)O3]0.2 (PZT-PFW) solid-solution thin films using pulsed laser deposition. X-ray diffraction measurements reveal that the films have a tetragonal structure. The films exhibit ferroelectric properties and weak ferromagnetic responses at room temperature. Magnetoelectric effects were investigated; the nonlinear magnetoelectric coefficient was measured and found to be comparable to those of multiferroic hexagonal manganites, but at least two orders of magnitude smaller than that for polycrystalline PZT-PFW films.
ABSTRACT We find that resistance switching (RS) phenomena change reversibly between bipolar RS (B... more ABSTRACT We find that resistance switching (RS) phenomena change reversibly between bipolar RS (BRS) and unipolar RS (URS) in a Pt/SrTiOx/Pt cell. For an asymmetric electrode configuration of Ti/SrTiOx/Pt cells whose top and bottom interfaces are Ohmic and Schottky-like rectifying, we determine that BRS only occurs when a positive voltage is applied to the bottom Pt electrode at the forming process. During the set process of BRS in a Pt/SrTiOx/Pt cell, O2 bubbles develop on the top Pt electrode. From the experimental results for a single sample in which both BRS and URS occur, O2− ion movement and consequent interfacial resistance modification might play an important role in BRS but not URS.
ABSTRACT Recent developments in ferroelectric (FE) domain imaging techniques have established an ... more ABSTRACT Recent developments in ferroelectric (FE) domain imaging techniques have established an understanding of intriguing polarization switching dynamics. In particular, nanoscale studies of FE domain switching phenomena using piezoresponse force microscopy (PFM) can provide important microscopic details on nucleation and subsequent growth of domains, complementing conventional electrical measurements that only give macroscopic information. This review covers recent nanoscale PFM studies of domain switching dynamics in FE thin films. Recent nanoscale PFM-based studies have demonstrated that quenched defects inside the FE thin films play important roles in domain switching processes, including defect-mediated inhomogeneous nucleation, pinning-dominated nonlinear dynamics of domain walls, and many other intriguing phenomena.Highlights►We review recent piezoresponse force microscopy studies of ferroelectric domains. ►Piezoresponse force microscopy has provided nanoscale details of domain switching. ►Quenched defects in ferroelectric thin films act as nucleation centers. ►Defects also act as pinning sites for propagating ferroelectric domain walls.
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Papers by Sang Mo Yang