Daehee Seol

Piezoelectric properties of CH3NH3PbI3 perovskite thin films and their applications in piezoelectric generators

CH3NH3PbI3 (MAPbI3) perovskite thin films were applied to fluorine-doped SnO2 (FTO)/glass and Au/Ti/polyethylene terephthalate (PET) substrates via a two-step process, which involved depositing a CH3NH3I (MAI) solution onto PbI2 films via spin-coating followed by crystallization at temperatures of 100 °C. The 500 nm-thick crystallized MAPbI3 perovskite thin films showed a Curie temperature of ∼328 K, a dielectric permittivity of ∼52, a dielectric loss of ∼0.02 at 1 MHz, and a low leakage current density of ∼10−7 A cm−2 at ±3 V. The polarization–electric field (P–E) hysteresis loop and piezoresponse force microscopy (PFM) results showed that the films had well-developed ferroelectric properties and switchable polarization. Poling at an electrical field of 80 kV cm−1 enhanced the power density of the generator. The values for output voltage and current density of the poled films reached 2.7 V and 140 nA cm−2, respectively, which were 2.7-fold higher than those of the non-poled samples.

Screening effect on photovoltaic performance in ferroelectric CH3NH3PbI3 perovskite thin films

Organic and inorganic hybrid materials of CH3NH3PbX3 with a perovskite crystal structure have been conceived as emerging light absorbing materials for high efficiency photovoltaic devices. Here, we demonstrate the screening effect of polarization states on charge redistribution related to the photovoltaic performance of ferroelectric CH3NH3PbI3 thin films using atomic force microscopy. We show the interplay between polarization and injected charges to have significant effects on charge transfer which potentially influences photovoltaic performance. The obtained results reveal that the direction and the amount of charge transfer can be influenced by the screening of polarization states at the interface. These results could deliver fundamental information regarding the influence of ferroelectricity on CH3NH3PbX3 solar cells.

Determination of ferroelectric contributions to electromechanical response by frequency dependent piezoresponse force microscopy.

Hysteresis loop analysis via piezoresponse force microscopy (PFM) is typically performed to probe the existence of ferroelectricity at the nanoscale. However, such an approach is rather complex in accurately determining the pure contribution of ferroelectricity to the PFM. Here, we suggest a facile method to discriminate the ferroelectric effect from the electromechanical (EM) response through the use of frequency dependent ac amplitude sweep with combination of hysteresis loops in PFM. Our combined study through experimental and theoretical approaches verifies that this method can be used as a new tool to differentiate the ferroelectric effect from the other factors that contribute to the EM response.

Nanoscale mapping of electromechanical response in ionic conductive ceramics with piezoelectric inclusions

Electromechanical (EM) response in ion conductive ceramics with piezoelectric inclusions was spatially explored using strain-based atomic force microscopy. Since the sample is composed of two dominant phases of ionic and piezoelectric phases, it allows us to explore two different EM responses of electrically induced ionic response and piezoresponse over the same surface. Furthermore, EM response of the ionic phase, i.e., electrochemical strain, was quantitatively investigated from the comparison with that of the piezoelectric phase, i.e., piezoresponse. These results could provide additional information on the EM properties, including the electrochemical strain at nanoscale.

Electrostatic-free piezoresponse force microscopy

Contact and non-contact based atomic force microscopy (AFM) approaches have been extensively utilized to explore various nanoscale surface properties. In most AFM-based measurements, a concurrent electrostatic effect between the AFM tip/cantilever and sample surface can occur. This electrostatic effect often hinders accurate measurements. Thus, it is very important to quantify as well as remove the impact of the electrostatic effect on AFM-based measurements. In this study, we examine the impact of the electrostatic effect on the electromechanical (EM) response in piezoresponse force microscopy as a model AFM mode. We quantitatively studied the effects of increasing the external electric field and reducing the spring constant of a cantilever. Further, we explored ways to minimize the electrostatic effect. The results provide broad guidelines for quantitatively analyzing the EM response as well as, eventually, for obtaining the electrostatic-free EM response. The conclusions can be applied to other AFM-based measurements that are subject to a strong electrostatic effect between the AFM tip/cantilever and sample surface, regardless of contact and non-contact modes.

Ferroelectric-like hysteresis loop originated from non-ferroelectric effects

Piezoresponse force microscopy (PFM) has provided advanced nanoscale understanding and analysis of ferroelectric and piezoelectric properties. In PFM-based studies, electromechanical strain induced by the converse piezoelectric effect is probed and analyzed as a PFM response. However, electromechanical strain can also arise from several non-piezoelectric origins that may lead to a misinterpretation of the observed response. Among them, electrostatic interaction can significantly affect the PFM response. Nonetheless, previous studies explored solely the influence of electrostatic interaction on the PFM response under the situation accompanied with polarization switching. Here, we show the influence of the electrostatic interaction in the absence of polarization switching by using unipolar voltage sweep. The obtained results reveal that the electromechanical neutralization between piezoresponse of polarization and electrostatic interaction plays a crucial role in the observed ferroelectric-like hysteresis l...

Humidity effect of domain wall roughening behavior in ferroelectric copolymer thin films

We have demonstrated that domain switching in ferroelectric copolymer films can be significantly affected by humidity. With increasing relative humidity (RH), we observed larger domains with highly irregular boundaries as a result of lateral spreading of the tip-induced electric field that originates from water adsorption. Fractal dimension study of irregular domains reveals that the fractal dimension is higher in cases where the RH is higher. The results show that the RH is one of the major switching parameters in ferroelectric copolymers, and therefore could allow clear understanding with regard to domain switching behavior in the ferroelectric copolymer films under ambient conditions.

Evenly transferred single-layered graphene membrane assisted by strong substrate adhesion

We explored the transfer of a single-layered graphene membrane assisted by substrate adhesion. A relatively larger adhesion force was measured on the SiO2 substrate compared with its van der Waals contribution, which is expected to result from the additional contribution of the chemical bonding force. Density functional theory calculations verified that the strong adhesion force was indeed accompanied by chemical bonding. The transfer of single-layered graphene and subsequent deposition of the dielectric layer were best performed on the SiO2 substrate exhibiting a larger adhesion force. This study suggests the selection and/or modification of the underlying substrate for proper transfer of graphene as well as other 2D materials similar to graphene.

Probing of multiple magnetic responses in magnetic inductors using atomic force microscopy.

Even though nanoscale analysis of magnetic properties is of significant interest, probing methods are relatively less developed compared to the significance of the technique, which has multiple potential applications. Here, we demonstrate an approach for probing various magnetic properties associated with eddy current, coil current and magnetic domains in magnetic inductors using multidimensional magnetic force microscopy (MMFM). The MMFM images provide combined magnetic responses from the three different origins, however, each contribution to the MMFM response can be differentiated through analysis based on the bias dependence of the response. In particular, the bias dependent MMFM images show locally different eddy current behavior with values dependent on the type of materials that comprise the MI. This approach for probing magnetic responses can be further extended to the analysis of local physical features.

Dynamic mechanical control of local vacancies in NiO thin films

The manipulation of local ionic behavior via external stimuli in oxide systems is of great interest because it can help in directly tuning material properties. Among external stimuli, mechanical force has attracted intriguing attention as novel stimulus for ionic modulation. Even though effectiveness of mechanical force on local ionic modulation has been validated in terms of static effect, its real-time i.e., dynamic, behavior under an application of the force is barely investigated in spite of its crucial impact on device performance such as force or pressure sensors. In this study, we explore dynamic ionic behavior modulated by mechanical force in NiO thin films using electrochemical strain microscopy (ESM). Ionically mediated ESM hysteresis loops were significantly varied under an application of mechanical force. Based on these results, we were able to investigate relative relationship between the force and voltage effects on ionic motion and, further, control effectively ionic behavior through combination of mechanical and electrical stimuli. Our results can provide comprehensive information on the effect of mechanical forces on ionic dynamics in ionic systems.