Seol Hee Oh

Piezoelectric power generation of vertically aligned lead-free (K,Na)NbO3 nanorod arrays

We report piezoelectric power generation at room temperature, as well as at elevated temperature, using vertically aligned lead-free (K,Na)NbO3 nanorod arrays on a conducting substrate. Ferroelectric (K0.6Na0.4)NbO3 nanorod arrays having a high piezoelectric coefficient of ca. 180 pm V−1 were epitaxially grown on a conducting SrRuO3/SrTiO3 substrate via the facile hydrothermal method at low temperature (ca. 190 °C). After electric poling, the (K,Na)NbO3 nanorod array-based nanogenerator generated a stable high power density of ∼101 μW cm−3 at 20 °C, almost 16 times greater than a BaTiO3 nanowire-based vibrational energy harvester (ca. 6.27 μW cm−3). The (K,Na)NbO3 nanorod array-based nanogenerator exhibited little frequency dependence on external force over a relatively low-frequency range (≤10 Hz) and showed long-term durability even at 100 °C, with a power density of 29 μW cm−3. This work demonstrates the potential of eco-friendly nanogenerators based on (K,Na)NbO3 nanorod arrays for high-output power generation under various environmental conditions.

Efficient Carrier Separation and Intriguing Switching of Bound Charges in Inorganic–Organic Lead Halide Solar Cells

We fabricated a mesoporous perovskite solar cell with a ∼14% conversion efficiency, and we investigated its beneficial grain boundary properties of the perovskite solar cells through the use of scanning probe microscopy. The CH3NH3Pb(I0.88,Br0.12)3 showed a significant potential barrier bending at the grain boundary and induced passivation. The potential difference value in the x = 0.00 sample is ∼50 mV, and the distribution of the positive potential is lower than that of the x = 0.12 sample. We also investigated the polarization and hysteretic properties of the perovskite thin films by measuring the local piezoresponse. Specifically, the charged grain boundaries play a beneficial role in electron-hole depairing and in suppressing recombination in order to realize high-efficiency perovskite solar cells.

Reduced leakage currents and possible charge carriers tuning in Mg-doped Ga0.6Fe1.4O3 thin films

Ga0.6Fe1.4O3 is predicted to be magnetoelectric with non zero magnetization at room temperature. However, in thin films, electric properties are overshadowed by strong leakage currents. In this Letter, we show that Mgdoping in Ga0.6Fe1.4O3thin filmsgrown by pulsed laser deposition allows decreasing the leakage current density by four orders of magnitude and might simultaneously allow tuning the carriers’ nature. These results suggest the possibility to develop a new class of material exhibiting room temperature magnetization, tunable transport properties, and magnetoelectric properties.

Room-temperature polarization switching and antiferromagnetic coupling in epitaxial (Ga,Fe)2O3/SrRuO3 heterostructures

Room-temperature reversible remnant polarization of gallium ferrite thin-films is reported as a multiferroic material with non-zero order parameters of polarization and magnetization. With the addition of Fe ions in Ga sites, Ga0.6Fe1.4O3 (GFO) thin films have been considered as potentially promising of multiferroicity. The b-axis oriented epitaxial GFO films were grown on SrRuO3(111)/SrTiO3(111). The six-fold symmetric in-plane epitaxy of the GFO films was confirmed using X-ray diffraction. The magnetic moment of the films was measured as a function of temperature and external magnetic field, which shows a room-temperature non-zero magnetization. Macroscopic and microscopic methods have been applied to demonstrate the polarization switching of the films. The remnant polarization is measured as 0.05 μC/cm2. Reduction of leaky behaviors of the GFO films owing to the conducting oxide of SrRuO3 will pave a way to take advantage of the room-temperature non-zero multi-orders for future non-volatile memory devi...

Memory effect behavior with respect to the crystal grain size in the organic-inorganic hybrid perovskite nonvolatile resistive random access memory

The crystal grain size of CH3NH3PbI3 (MAPbI3) organic-inorganic hybrid perovskite (OHP) film was controllable in the range from ~60 nm to ~600 nm by non-solvents inter-diffusion controlled crystallization process in dripping crystallization method for the formation of perovskite film. The MAPbI3 OHP non-volatile resistive random access memory with ~60 nm crystal grain size exhibited >0.1 TB/in2 storage capacity, >600 cycles endurance, >104 s data retention time, ~0.7 V set, and ~−0.61 V re-set bias voltage.

Incorporation of cobalt ions into magnetoelectric gallium ferrite epitaxial films: tuning of conductivity and magnetization

Thin films of Ga0.6Fe1.4O3 show ferrimagnetism with a transition temperature at around 360 K but suffer from large charge conduction. Substituting Fe2+ with non-magnetic Mg2+ ions reduces the charge conduction but also lowers the magnetic transition temperature. Doping Ga0.6Fe1.4O3 thin films with magnetic Co2+ ions leads to a similar reduction in the charge conduction, which is significant by two orders of magnitude, and, on the other hand, does not lead to any modification of the ferrimagnetic transition. The remnant magnetization of the leakage currents free Co-doped Ga0.6Fe1.4O3 thin films is of 53 emu cm−3 at 300 K. These films, therefore, are promising materials with potential uses in magnetoelectric and multiferroic devices.

Structural properties and phase formation of epitaxial PbVO3 thin films grown on LaAlO3(0 0 1) by pulsed laser deposition

We report epitaxial synthesis of PbVO3 (PVO) thin films on LaAlO3 (LAO) (0?0?1) substrates under reduction atmosphere from a Pb2V2O7 sintered target over a wide range of temperatures using laser ablation. Polar magnetic PVO has a non-centrosymmetric structure (tetragonality c/a?=?1.23). The effects of substrate temperature on structural properties and chemical composition of PVO thin films were studied by x-ray diffraction and energy-dispersive spectrometry. The c-axis of the PVO films was abnormally elongated as much as 5?7% on the well-lattice-matched LAO substrates. Using Raman scattering spectroscopy, single phase formation of PVO with the shift of phonon modes was confirmed and epitaxial growth of the PVO films was also confirmed with polarization-dependent measurements. The strained and epitaxial growth of the PVO films will lead to a new class of multiferroic materials with future electronic device applications.

Hetero-epitaxial growth and large piezoelectric effects in (0 0 1) and (1 1 1) oriented PbTiO3–LaNiO3multilayers

Hetero-epitaxial thin films were obtained in PbTiO3/LaNiO3 thin films by using laser ablation for sequential deposition of each thin film. Structural variations were built to characterize the differences in local ferroelectric properties. For this study, 50 nm-thick-PTO thin films with (0 0 1) and (1 1 1) orientations were investigated. Piezoresponse force microscopy (PFM) was used to investigate the local ferroelectricity in the thin films. High piezoelectric responses were obtained in the thin films, with values of approximately 75 and 83 pm V−1 in the (0 0 1) and (1 1 1)-oriented thin films, respectively. It is notable that remarkable piezoelectric properties were obtained in the (1 1 1)-oriented thin films. The (1 1 1)-oriented thin films have the potential to be used as multi-bit memory because of the reduction of degradation in ferroelectricity and various directional piezoelectricity. We compared the functionalities of the (0 0 1)- and (1 1 1)-oriented thin films and obtained different performances in the thin films.

Strain-induced non-linear optical characteristics of pyroelectric PbVO 3 epitaxial thin films

Highly strained PbVO3 films have potential applications in pyroelectric sensors and optoelectronic devices. To understand how strain influences the nonlinear optical characteristic of PbVO3, highly distorted tetragonal PbVO3 epitaxial thin films were grown on LaAlO3 and SrTiO3 substrates. Using Raman scattering spectroscopy, theoretical and experimental studies on the phonon spectra yield insights on the strained structures of V-O apical bondings. Effects of applied strain on the linear optical properties of PbVO3 thin films were experimentally characterized by spectroscopic ellipsometry. Second-order nonlinear susceptibility tensor components of PbVO3/SrTiO3 thin films were determined to be almost twice as large as those of PbVO3/LaAlO3 thin films.