Wanchul Seung

Nanopatterned Textile-Based Wearable Triboelectric Nanogenerator

Here we report a fully flexible, foldable nanopatterned wearable triboelectric nanogenerator (WTNG) with high power-generating performance and mechanical robustness. Both a silver (Ag)-coated textile and polydimethylsiloxane (PDMS) nanopatterns based on ZnO nanorod arrays on a Ag-coated textile template were used as active triboelectric materials. A high output voltage and current of about 120 V and 65 μA, respectively, were observed from a nanopatterned PDMS-based WTNG, while an output voltage and current of 30 V and 20 μA were obtained by the non-nanopatterned flat PDMS-based WTNG under the same compressive force of 10 kgf. Furthermore, very high voltage and current outputs with an average value of 170 V and 120 μA, respectively, were obtained from a four-layer-stacked WTNG under the same compressive force. Notably it was found there are no significant differences in the output voltages measured from the multilayer-stacked WTNG over 12 000 cycles, confirming the excellent mechanical durability of WTNGs. Finally, we successfully demonstrated the self-powered operation of light-emitting diodes, a liquid crystal display, and a keyless vehicle entry system only with the output power of our WTNG without any help of external power sources.

Active Matrix Electronic Skin Strain Sensor Based on Piezopotential‐Powered Graphene Transistors

Dr. Q. Sun, B. J. Kim, Prof. S.-W. Kim, Prof. J. H. Cho SKKU Advanced Institute of Nanotechnology (SAINT) Sungkyunkwan University Suwon 440-746 , South Korea E-mail: kimsw1@skku.edu; jhcho94@skku.edu W. Seung, Prof. S.-W. Kim School of Advanced Materials Science and Engineering Sungkyunkwan University Suwon 440-746 , South Korea Prof. J. H. Cho School of Chemical Engineering Sungkyunkwan University Suwon 440-746 , South Korea Prof. S. Seo College of BioNano Technology Gachon University Seongnam 461-701 , South Korea

A high performance PZT ribbon-based nanogenerator using graphene transparent electrodes

We report a simple and effective approach for high performance PbZr0.52Ti0.48O3 (PZT) based flexible and semi-transparent NGs that exploit the electrical, mechanical and transparent properties of graphene. PZT NGs are successfully demonstrated for continuous driving of a liquid crystal display screen and a light emitting diode. A good quality PZT film was deposited on Pt/Ti/SiO2/Si wafer by the sol–gel method, exhibiting a typical hysteresis loop in the low voltage region. A graphene film was used in the interdigitated electrode form to improve the PZT/graphene interface under mechanical stress. Further improvement in NGs performance was realized by p-type doping in graphene, resulting in an increase in current density. NGs showed a high output voltage ∼2 V, current density ∼2.2 μA cm−2 and power density ∼88 mW cm−3 at an applying force of 0.9 kgf. This can efficiently run commercially available electronic components in a self-powered mode, without any external electrical supply.

Recent Progress on Flexible Triboelectric Nanogenerators for SelfPowered Electronics.

Recently, smart systems have met with large success. At the origin of the internet of things, they are a key driving force for the development of wireless, sustainable, and independent autonomous smart systems. In this context, autonomy is critical, and despite all the progress that has been made in low-power electronics and batteries, energy harvesters are becoming increasingly important. Thus, harvesting mechanical energy is essential, as it is widespread and abundant in our daily life environment. Among harvesters, flexible triboelectric nanogenerators (TENGs) exhibit good performance, and they are easy to integrate, which makes them perfect candidates for many applications and, therefore, crucial to develop. In this review paper, we first introduce the fundamentals of TENGs, including their four basic operation modes. Then, we discuss the different improvement parameters. We review some progress made in terms of performance and integration that have been possible through the understanding of each operation mode and the development of innovative structures. Finally, we present the latest trends, structures, and materials in view of future improvements and applications.

Graphene Tribotronics for Electronic Skin and Touch Screen Applications

Graphene tribotronics is introduced for touch-sensing applications such as electronic skins and touch screens. The devices are based on a coplanar coupling of triboelectrification and current transport in graphene transistors. The touch sensors are ultrasensitive, fast, and stable. Furthermore, they are transparent and flexible, and can spatially map touch stimuli such as movement of a ball, multi-touch, etc.

Control of Skin Potential by Triboelectrification with Ferroelectric Polymers.

Negatively polarized ferroelectric polymer β-P(VDF-TrFE) shows higher positive triboelectric properties than skin, which could lead to new medical applications. Kelvin force microscope measurements and triboelectric nanogenerator characterizations are performed to demonstrate this new property. In addition, how many negative charges are exchanged by contact electrification between the negatively polarized β-P(VDF-TrFE) and the skin is estimated.

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%.

Fully Stretchable Textile Triboelectric Nanogenerator with Knitted Fabric Structures

Harvesting human-motion energy for power-integrated wearable electronics could be a promising way to extend the battery-operation time of small low-power-consumption electronics such as various sensors. For this purpose, a fully stretchable triboelectric nanogenerator (S-TENG) that has been fabricated with knitted fabrics and has been integrated with the directly available materials and techniques of the textile industry is introduced. This device has been adapted to cloth movement and can generate electricity under compression and stretching. We investigated plain-, double-, and rib-fabric structures and analyzed their potentials for textile-based energy harvesting. The superior stretchable property of the rib-knitted fabric contributed to a dramatic enhancement of the triboelectric power-generation performance owing to the increased contact surface. The present study shows that, under stretching motions of up to 30%, the S-TENG generates a maximum voltage and a current of 23.50 V and 1.05 μA, respectively, depending on the fabric structures. Under compressions at 3.3 Hz, the S-TENG generated a constant average root-mean square power of up to 60 μW. The results of this work show the feasibility of a cloth-integrated and industrial-ready TENG for the harvesting of energy from human biomechanical movements in cloth and garments.