Dukhyun Choi

Fully Rollable Transparent Nanogenerators Based on Graphene Electrodes

[*] Prof. S.-W. Kim, H.-K. Park, J.-S. Seo School of Advanced Materials Science and Engineering SKKU Advanced Institute of Nanotechnology (SAINT) Center for Human Interface Nanotechnology (HINT) Sungkyunkwan University Suwon, 440-746 (Republic of Korea) E-mail: kimsw1@skku.edu Dr. J.-Y. Choi, Dr. D. Choi, Dr. W. M. Choi, H.-J. Shin, Dr. J. Park, S.-M. Yoon, Dr. S. Y. Lee, Dr. J. M. Kim Samsung Advanced Institute of Technology Yongin, Gyeonggi, 446-712 (Republic of Korea) E-mail: jaeyoung88.choi@samsung.com M.-Y. Choi School of Advanced Materials and System Engineering Kumoh National Institute of Technology Gumi, Gyeongbuk, 730-701 (Republic of Korea)

P-Type Polymer-Hybridized High-Performance Piezoelectric Nanogenerators

Enhancing the output power of a nanogenerator is essential in applications as a sustainable power source for wireless sensors and microelectronics. We report here a novel approach that greatly enhances piezoelectric power generation by introducing a p-type polymer layer on a piezoelectric semiconducting thin film. Holes at the film surface greatly reduce the piezoelectric potential screening effect caused by free electrons in a piezoelectric semiconducting material. Furthermore, additional carriers from a conducting polymer and a shift in the Fermi level help in increasing the power output. Poly(3-hexylthiophene) (P3HT) was used as a p-type polymer on piezoelectric semiconducting zinc oxide (ZnO) thin film, and phenyl-C(61)-butyric acid methyl ester (PCBM) was added to P3HT to improve carrier transport. The ZnO/P3HT:PCBM-assembled piezoelectric power generator demonstrated 18-fold enhancement in the output voltage and tripled the current, relative to a power generator with ZnO only at a strain of 0.068%. The overall output power density exceeded 0.88 W/cm(3), and the average power conversion efficiency was up to 18%. This high power generation enabled red, green, and blue light-emitting diodes to turn on after only tens of times bending the generator. This approach offers a breakthrough in realizing a high-performance flexible piezoelectric energy harvester for self-powered electronics.

Self‐Organized Hexagonal‐Nanopore SERS Array

were applied for precisionnanopatterning with the advantage of high resolution withouttheneedforaphysicalmask,sincethepatterncanbechangedatany time by using computer-aided design (CAD) software.However, the disadvantages of these two methods are the longexposure time due to pixel-by-pixel scanning steps, high cost,and substantial maintenance. To overcome these limitations,phase-shift lithography,

Boosted output performance of triboelectric nanogenerator via electric double layer effect

For existing triboelectric nanogenerators (TENGs), it is important to explore unique methods to further enhance the output power under realistic environments to speed up their commercialization. We report here a practical TENG composed of three layers, in which the key layer, an electric double layer, is inserted between a top layer, made of Al/polydimethylsiloxane, and a bottom layer, made of Al. The efficient charge separation in the middle layer, based on Voltas electrophorus, results from sequential contact configuration of the TENG and direct electrical connection of the middle layer to the earth. A sustainable and enhanced output performance of 1.22 mA and 46.8 mW cm−2 under low frequency of 3 Hz is produced, giving over 16-fold enhancement in output power and corresponding to energy conversion efficiency of 22.4%. Finally, a portable power-supplying system, which provides enough d.c. power for charging a smart watch or phone battery, is also successfully developed.

Piezoelectric touch-sensitive flexible hybrid energy harvesting nanoarchitectures

In this work, we report a flexible hybrid nanoarchitecture that can be utilized as both an energy harvester and a touch sensor on a single platform without any cross-talk problems. Based on the electron transport and piezoelectric properties of a zinc oxide (ZnO) nanostructured thin film, a hybrid cell was designed and the total thickness was below 500 nm on a plastic substrate. Piezoelectric touch signals were demonstrated under independent and simultaneous operations with respect to photo-induced charges. Different levels of piezoelectric output signals from different magnitudes of touching pressures suggest new user-interface functions from our hybrid cell. From a signal controller, the decoupled performance of a hybrid cell as an energy harvester and a touch sensor was confirmed. Our hybrid approach does not require additional assembly processes for such multiplex systems of an energy harvester and a touch sensor since we utilize the coupled material properties of ZnO and output signal processing. Furthermore, the hybrid cell can provide a multi-type energy harvester by both solar and mechanical touching energies.

Control of naturally coupled piezoelectric and photovoltaic properties for multi-type energy scavengers†

In this paper, we present a simple, low-cost and flexible hybrid cell that converts individually or simultaneously low-frequency mechanical energy and photon energy into electricity using piezoelectric zinc oxide (ZnO) in conjunction with organic solar cell design. Since the hybrid cell is designed by coupled piezoelectric and photoconductive properties of ZnO, this is a naturally hybrid architecture without crosstalk and an additional assembling process to create multi-type energy scavengers, thus differing from a simple integration of two different energy generators. It is demonstrated that the behavior of a piezoelectric output is controlled from alternating current (AC) type to direct current (DC)-like type by tailoring mechanical straining processes both in the dark and under light illumination. Based on such controllability of output modes, it is shown that the performance of the hybrid cell is synergistically enhanced by integrating the contribution made by a piezoelectric generator with a solar cell under a normal indoor level of illumination. Our approach clearly demonstrates the potential of the hybrid approach for scavenging multi-type energies whenever and wherever they are available. Furthermore, this work establishes the methodology to harvest solar energy and low-frequency mechanical energies such as body movements, making it possible to produce a promising multi-functional power generator that could be embedded in flexible architectures.

Self-Assembled Three-Dimensional Nanocrown Array

Although an ordered nanoplasmonic probe array will have a huge impact on light harvesting, selective frequency response (i.e., nanoantenna), and quantitative molecular/cellular imaging, the realization of such an array is still limited by conventional techniques due to the serial processing or resolution limit by light diffraction. Here, we demonstrate a thermodynamically driven, self-assembled three-dimensional nanocrown array that consists of a core and six satellite gold nanoparticles (GNPs). Our ordered nanoprobe array is fabricated over a large area by thermal dewetting of thin gold film on hexagonally ordered porous anodic alumina (PAA). During thermal dewetting, the structural order of the PAA template dictates the periodic arrangement of gold nanoparticles, rendering the array of gold nanocrown. Because of its tunable size (i.e., 50 nm core and 20 nm satellite GNPs), arrangement, and periodicity, the nanocrown array shows multiple optical resonance frequencies at visible wavelengths as well as angle-dependent optical properties.

Omnidirectionally Stretchable and Transparent Graphene Electrodes

Stretchable and transparent electrodes have been developed for applications in flexible and wearable electronics. For customer-oriented practical applications, the electrical and optical properties of stretchable electrodes should be independent of the directions of the applied stress, and such electrodes are called omnidirectionally stretchable electrodes. Herein, we report a simple and cost-effective approach for the fabrication of omnidirectionally stretchable and transparent graphene electrodes with mechanical durability and performance reliability. The use of a Fresnel lens-patterned electrode allows multilayered graphene sheets to achieve a concentric circular wavy structure, which is capable of sustaining tensile strains in all directions. The as-prepared electrodes exhibit high optical transparency, low sheet resistance, and reliable electrical performances under various deformation (e.g., bending, stretching, folding, and buckling) conditions. Furthermore, computer simulations have also been carried out to investigate the response of a Fresnel lens-patterned structure on the application of mechanical stresses. This study can be significant in a large variety of potential applications, ranging from stretchable devices to electronic components in various wearable integrated systems.

Graphene surface induced specific self-assembly of poly(3-hexylthiophene) for nanohybrid optoelectronics: from first-principles calculation to experimental characterizations

We demonstrate a specific chain alignment of π-conjugated polythiophenes on the graphene monolayer via first-principles calculation and experimental characterizations. The effects of alkyl chain and thiophene backbone in poly(3-hexylthiophene) (P3HT) on the specific binding energy and molecular configuration on the graphene monolayer are independently investigated. Due to specific π–π interaction and van der Waals interaction between P3HT and graphene monolayer, two different configurations (edge-on and face-on) of P3HT are formed on the graphene, while only edge-on configuration of P3HT is found on the indium tin oxide (ITO). These behaviors are verified by using atomic force microscopy (AFM) and transmission electron microscopy (TEM). We also explore the molecular orientation of P3HT chains on the graphene using 2D grazing incidence X-ray diffraction (GIXD) to obtain molecular orientation features over a large area. Our results will provide a strategy to create next-generation polymer–graphene nanohybrid optoelectronic devices.

Metal-insulator-metal optical nanoantenna with equivalent-circuit analysis.

2010 WILEY-VCH Verlag Gmb Nanoplasmonic optical antennas have been a rapidly emerging science and technology due to the potential applications of high-speed electronics, solar cells, optoelectronics, plasmon resonance energy transfer (PRET), oligonucleotides on a nanoplasmonic carrier-based optical switch (ONCOS), and surface-enhanced Raman scattering (SERS). In order to accomplish the effective device design of optical nanoantennas, it is critical to simulate the effects of each component of integrated device. However, the simulation of an optical antenna is a complex and time-consuming operation and it is important to reduce this step. Here, we demonstrate the effective design and simple simulation of metal–insulator–metal (MIM) optical nanoantennas by applying an equivalent nanocircuit model and compare with optical experiments. Since complicated MIM optical nanoantennas are converted into the effective impedance components such as nanoresistor, nanoinductors, and nanocapacitors, the simulation of far-field optical response can be easily obtained by solving the equivalent nanocircuit models. Moreover, for modified nanoantennas, new optical responses can be determined by changing the matched impedance value. Although this analysis requires the geometric factor from measurement results in case of complex nanostructures, it is an effective method to analyze far-field optical properties. We expect that this will have an impact on nanoplasmonic antenna analysis. Since it has been reported that plasmonics can manipulate light waves at the nanometer scale, various metallic nanostructures with tunable plasmon resonances have been widely used as optical nanoantennas that can overcome the diffraction limit and collect a specific wavelength of light within the nanometer scale. As the ability of computational analyses and nanofabrication techniques has been growing fast, advanced optical plasmonic systems have been achieved. However, it is still challenging to design an optimum nanostructure because electromagnetic field (EM) simulation and nanofabrication require repeatedly solving Maxwell’s equations for all designed structures in order to quantify and compare them. Recently, as an alternative way to surpass this limit, a simple and effective optical analysis using equivalent electric components such as resistor, capacitor, and inductor has been proposed to obtain far-field optical properties without solving Maxwell’s equations for complex geometries. In addition, optical properties obtained by this circuit model have been demonstrated for simple nanostructures such as nanorods and nanovoids in a metallic film. Here, we present the fundamental equivalent-circuit analysis, design, fabrication, and optical and SERS characterizations of MIM optical nanoantennas. Once the complex plasmonic nanoantenna is converted into the equivalent-circuit system, we can obtain the optical response from our nanostructures by solving the electrical model within a second. More importantly, when one of the parameters is changed, the new solution can be very easily obtained by modifying the value of the matched electric component instead of solving the whole optical system, which is required in conventional computational analysis using EM simulation. Moreover, since complicated optical antennas can be decomposed into a parallel and a serial circuit of simple, equivalent-nanocircuit components, we can quantify how each component or layer can contribute to the final optical properties and is changed as a function of wavelength and experimental parameters. For example, when only the height of dielectric layer (i.e. aluminum oxide) is changed in MIM optical nanoantennas, a new solution for the optical response of the newly designed optical nanoantenna was easily obtained by modifying the capacitance value, which represents the dielectric layer. In addition, we could also observe how the impedance of each layer changed with the varying height of the dielectric layer. As shown in Figure 1, an optical nanoantenna comprises hexagonally ordered gold, alumina, and aluminum layers. This nanostructure is fabricated by gold deposition (15 nm) on top of a self-ordered anodic aluminum oxide (AAO), which is fabricated by anodization of aluminum. The top (Z1) and bottom (Z3) metal layers are converted into the matched impedances by the combination of the equivalent resistors, capacitors, and inductors and the dielectric alumina layer in the middle is represented by a capacitor (Z2). Since this dielectric alumina layer plays a key role in controlling the plasmonic-coupling effect between the ordered gold and aluminum nanostructures, the height of this layer was used as a variable parameter to tune the optical properties (i.e., extinction profile) in this study. Firstly, the top gold layer consists of a hexagonally arrayed nanovoid film with nanoholes (Fig. 1a, top yellow part). Since