Sung Kyun Kim

Transparent Flexible Graphene Triboelectric Nanogenerators

S. Kim, Dr. M. K. Gupta, K. Y. Lee, K.-S. Shin, S. K. Kim, K. H. Lee, Prof. S.-W. Kim School of Advanced Materials Science and Engineering Sungkyunkwan University (SKKU) Suwon 440–746 , Republic of Korea E-mail: kimsw1@skku.edu A. Sohn, Prof. D.-W. Kim School of Department of Physics Ewha Womens University Seoul 120–750 , Republic of Korea T. Y. Kim, D. Kim, Prof. S.-W. Kim SKKU Advanced Institute of Nanotechnology (SAINT) Center for Human Interface Nanotechnology (HINT) SKKU-Samsung Graphene Center Sungkyunkwan University (SKKU) Suwon 440–746 , Republic of Korea Dr. H.-J. Shin Samsung Advanced Institute of Technology Yongin 446–712 , Republic of Korea

Ferroelectric Polarization in CH3NH3PbI3 Perovskite

We report on ferroelectric polarization behavior in CH3NH3PbI3 perovskite in the dark and under illumination. Perovskite crystals with three different sizes of 700, 400, and 100 nm were prepared for piezoresponse force microscopy (PFM) measurements. PFM results confirmed the formation of spontaneous polarization in CH3NH3PbI3 in the absence of electric field, where the size dependency to polarization was not significant. Whereas the photoinduced stimulation was not significant without an external electric field, the stimulated polarization by poling was further enhanced under illumination. The retention of ferroelectric polarization was also observed after removal of the electric field, in which larger crystals showed longer retention behavior compared to the smaller sized one. Additionally, we suggest the effect of perovskite crystal size (morphology) on charge collection at the interface of the ferroelectric material even though insignificant size dependency in electric polarization was observed.

Shape memory polymer-based self-healing triboelectric nanogenerator

Recently, triboelectric nanogenerators (TENGs) have received increasing interest due to their large potential for mechanical energy harvesting. Important progress has been achieved in increasing the output power and efficiency while new structures have emerged. In particular, their robustness and endurance have increased, but some critical concerns still remain about the degradation and lifetime of TENGs. How will TENGs age under intensive use in our daily lives? To address this issue, we propose in this paper to use shape memory polymers (SMPs) to extend TENGs’ lifetimes and guarantee their performance. For this purpose we introduce a new smart SMP-based self-healing TENG which has the capacity to be healed and to recover good performance after degradation of its triboelectric layer. We studied the degradation and healing process of the SMP–TENG, and the improvement in its endurance and lifetime, and thus demonstrate the huge potential of self-healing SMP–TENGs.

A Bi-layer TiO2 photoanode for highly durable, flexible dye-sensitized solar cells

Low-temperature processing of dye-sensitized solar cells (DSCs) [B. Oregan, M. Gratzel, Nature, 1991, 353, 737] is crucial to enable their commercialization with low-cost plastic substrates. Much of the previous work in this area has focused on mechanical compression of premade particles on plastic substrates; however, many reported that this technique did not yield sufficient interconnections for high charge carrier transport. Herein, we present bi-layer photoanodes that incorporate microstructured TiO2 sea-urchin-like assemblies, composed of high-aspect-ratio single crystalline nanoribbons, i.e., two-dimensional subunits, which were deposited onto a nanoparticle layer (commercial P25), with a 5.6% conversion efficiency realized. We demonstrate that this Mesoporous Hierarchical Anatase TiO2 (MHAT) nanostructure is beneficial due to its enhanced dye loading as well as enhanced light scattering. Importantly, we also show the benefits of a bi-layer structure where the nanoribbons penetrate into the nanoparticle layer (P25) after cold isostatic pressing (CIP), resulting in improved adhesion between the MHAT top layer and the P25 under layer on the indium tin oxide-coated polyethylene naphthalate (ITO|PEN) substrate, leading to improved mechanical stability and durability, efficient electron transfer pathways, and ultimately, higher solar-to-electric conversion efficiencies.

Ferroelectric Coupling Effect on the Energy‐Band Structure of Hybrid Heterojunctions with Self‐Organized P(VDF‐TrFE) Nanomatrices

Ferroelectric coupling effects on the energy-band structure of hybrid heterojunctions are investigated using hybrid photovoltaic devices with poly(3-hexylthiophene-2,5-diyl) (P3HT)/ZnO and poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)). The self-organized P(VDF-TrFE):P3HT photoactive layer forms a novel architecture consisting of P3HT domains in a P(VDF-TrFE) matrix. The energy-band structure at the interface of the p-n heterojunction is tuned by artificial control of the ferroelectric polarization of the P(VDF-TrFE) material, consequently modulating the photovoltaic performance of the hybrid photovoltaic devices.

Reliable Piezoelectricity in Bilayer WSe2 for Piezoelectric Nanogenerators

Recently, piezoelectricity has been observed in 2D atomically thin materials, such as hexagonal-boron nitride, graphene, and transition metal dichalcogenides (TMDs). Specifically, exfoliated monolayer MoS2 exhibits a high piezoelectricity that is comparable to that of traditional piezoelectric materials. However, monolayer TMD materials are not regarded as suitable for actual piezoelectric devices due to their insufficient mechanical durability for sustained operation while Bernal-stacked bilayer TMD materials lose noncentrosymmetry and consequently piezoelectricity. Here, it is shown that WSe2 bilayers fabricated via turbostratic stacking have reliable piezoelectric properties that cannot be obtained from a mechanically exfoliated WSe2 bilayer with Bernal stacking. Turbostratic stacking refers to the transfer of each chemical vapor deposition (CVD)-grown WSe2 monolayer to allow for an increase in degrees of freedom in the bilayer symmetry, leading to noncentrosymmetry in the bilayers. In contrast, CVD-grown WSe2 bilayers exhibit very weak piezoelectricity because of the energetics and crystallographic orientation. The flexible piezoelectric WSe2 bilayers exhibit a prominent mechanical durability of up to 0.95% of strain as well as reliable energy harvesting performance, which is adequate to drive a small liquid crystal display without external energy sources, in contrast to monolayer WSe2 for which the device performance becomes degraded above a strain of 0.63%.

Rewritable ghost floating gates by tunnelling triboelectrification for two-dimensional electronics

Gates can electrostatically control charges inside two-dimensional materials. However, integrating independent gates typically requires depositing and patterning suitable insulators and conductors. Moreover, after manufacturing, gates are unchangeable. Here we introduce tunnelling triboelectrification for localizing electric charges in very close proximity of two-dimensional materials. As representative materials, we use chemical vapour deposition graphene deposited on a SiO2/Si substrate. The triboelectric charges, generated by friction with a Pt-coated atomic force microscope tip and injected through defects, are trapped at the air–SiO2 interface underneath graphene and act as ghost floating gates. Tunnelling triboelectrification uniquely permits to create, modify and destroy p and n regions at will with the spatial resolution of atomic force microscopes. As a proof of concept, we draw rewritable p/n+ and p/p+ junctions with resolutions as small as 200 nm. Our results open the way to time-variant two-dimensional electronics where conductors, p and n regions can be defined on demand.

Point‐Defect‐Passivated MoS2 Nanosheet‐Based High Performance Piezoelectric Nanogenerator

In this work, a sulfur (S) vacancy passivated monolayer MoS2 piezoelectric nanogenerator (PNG) is demonstrated, and its properties before and after S treatment are compared to investigate the effect of passivating S vacancy. The S vacancies are effectively passivated by using the S treatment process on the pristine MoS2 surface. The S vacancy site has a tendency to covalently bond with S functional groups; therefore, by capturing free electrons, a S atom will form a chemisorbed bond with the S vacancy site of MoS2 . S treatment reduces the charge-carrier density of the monolayer MoS2 surface, thus the screening effect of piezoelectric polarization charges by free carrier is significantly prevented. As a result, the output peak current and voltage of the S-treated monolayer MoS2 nanosheet PNG are increased by more than 3 times (100 pA) and 2 times (22 mV), respectively. Further, the S treatment increases the maximum power by almost 10 times. The results suggest that S treatment can reduce free-charge carrier by sulfur S passivation and efficiently prevent the screening effect. Thus, the piezoelectric output peaks of current, voltage, and maximum power are dramatically increased, as compared with the pristine MoS2 .

Application of ferroelectric materials for improving output power of energy harvesters

In terms of advances in technology, especially electronic devices for human use, there are needs for miniaturization, low power, and flexibility. However, there are problems that can be caused by these changes in terms of battery life and size. In order to compensate for these problems, research on energy harvesting using environmental energy (mechanical energy, thermal energy, solar energy etc.) has attracted attention. Ferroelectric materials which have switchable dipole moment are promising for energy harvesting fields because of its special properties such as strong dipole moment, piezoelectricity, pyroelectricity. The strong dipole moment in ferroelectric materials can increase internal potential and output power of energy harvesters. In this review, we will provide an overview of the recent research on various energy harvesting fields using ferroelectrics. A brief introduction to energy harvesting and the properties of the ferroelectric material are described, and applications to energy harvesters to improve output power are described as well.

Triboelectric Series of 2D Layered Materials

Recently, as applications based on triboelectricity have expanded, understanding the triboelectric charging behavior of various materials has become essential. This study investigates the triboelectric charging behaviors of various 2D layered materials, including MoS2 , MoSe2 , WS2 , WSe2 , graphene, and graphene oxide in a triboelectric series using the concept of a triboelectric nanogenerator, and confirms the position of 2D materials in the triboelectric series. It is also demonstrated that the results are obviously related to the effective work functions. The charging polarity indicates the similar behavior regardless of the synthetic method and film thickness ranging from a few hundred nanometers (for chemically exfoliated and restacked films) to a few nanometers (for chemical vapor deposited films). Further, the triboelectric charging characteristics could be successfully modified via chemical doping. This study provides new insights to utilize 2D materials in triboelectric devices, allowing thin and flexible device fabrication.