Seok Jin Yoon

Highly Stretchable Piezoelectric‐Pyroelectric Hybrid Nanogenerator

A highly stretchable hybrid nanogenerator has been developed using a micro-patterned piezoelectric polymer P(VDF-TrFE), PDMS-CNT composite, and graphene nanosheets. Mechanical and thermal energies are simultaneously harvested from a single cell of the device. The hybrid nanogenerator exhibits high robustness behavior even after 30% stretching and generates very stable piezoelectric and pyroelectric power outputs due to micro-pattern designing.

High Output Piezo/Triboelectric Hybrid Generator

Recently, piezoelectric and triboelectric energy harvesting devices have been developed to convert mechanical energy into electrical energy. Especially, it is well known that triboelectric nanogenerators have a simple structure and a high output voltage. However, whereas nanostructures improve the output of triboelectric generators, its fabrication process is still complicated and unfavorable in term of the large scale and long-time durability of the device. Here, we demonstrate a hybrid generator which does not use nanostructure but generates much higher output power by a small mechanical force and integrates piezoelectric generator into triboelectric generator, derived from the simultaneous use of piezoelectric and triboelectric mechanisms in one press-and-release cycle. This hybrid generator combines high piezoelectric output current and triboelectric output voltage, which produces peak output voltage of ~370 V, current density of ~12 μA·cm−2, and average power density of ~4.44 mW·cm−2. The output power successfully lit up 600 LED bulbs by the application of a 0.2 N mechanical force and it charged a 10 μF capacitor to 10 V in 25 s. Beyond energy harvesting, this work will provide new opportunities for developing a small, built-in power source in self-powered electronics such as mobile electronics.

All‐Solution‐Processed Flexible Thin Film Piezoelectric Nanogenerator

An all-solution-processed flexible thin film piezoelectric nanogenerator is demonstrated using reactive zinc hydroxo-condensation and a screen-printing method. The highly elastic thin film allows the piezoelectric energy to be generated through the mechanical rolling and muscle stretching of the piezoelectric unit. This flexible all solution-processed nanogenerator is promising for use in future energy harvesters such as wearable human patches and mobile electronics.

Self-activated ultrahigh chemosensitivity of oxide thin film nanostructures for transparent sensors

One of the top design priorities for semiconductor chemical sensors is developing simple, low-cost, sensitive and reliable sensors to be built in handheld devices. However, the need to implement heating elements in sensor devices, and the resulting high power consumption, remains a major obstacle for the realization of miniaturized and integrated chemoresistive thin film sensors based on metal oxides. Here we demonstrate structurally simple but extremely efficient all oxide chemoresistive sensors with ~90% transmittance at visible wavelengths. Highly effective self-activation in anisotropically self-assembled nanocolumnar tungsten oxide thin films on glass substrate with indium-tin oxide electrodes enables ultrahigh response to nitrogen dioxide and volatile organic compounds with detection limits down to parts per trillion levels and power consumption less than 0.2 microwatts. Beyond the sensing performance, high transparency at visible wavelengths creates opportunities for their use in transparent electronic circuitry and optoelectronic devices with avenues for further functional convergence.

Extremely sensitive and selective NO probe based on villi-like WO3 nanostructures for application to exhaled breath analyzers.

Self-assembled WO3 thin film nanostructures with 1-dimensional villi-like nanofingers (VLNF) have been synthesized on the SiO2/Si substrate with Pt interdigitated electrodes using glancing angle deposition (GAD). Room-temperature deposition of WO3 by GAD resulted in anisotropic nanostructures with large aspect ratio and porosity having a relative surface area, which is about 32 times larger than that of a plain WO3 film. A WO3 VLNF sensor shows extremely high response to nitric oxide (NO) at 200 °C in 80% of relative humidity atmosphere, while responses of the sensor to ethanol, acetone, ammonia, and carbon monoxide are negligible. Such high sensitivity and selectivity to NO are attributed to the highly efficient modualtion of potential barriers at narrow necks between individual WO3 VLNF and the intrinsically high sensitivity of WO3 to NO. The theoretical detection limit of the sensor for NO is expected to be as low as 88 parts per trillion (ppt). Since NO is an approved biomarker of chronic airway inflammation in asthma, unprecedentedly high response and selectivity, and ppt-level detection limit to NO under highly humid environment demonstrate the great potential of the WO3 VLNF for use in high performance breath analyzers.

Superhydrophobic and antireflective nanograss-coated glass for high performance solar cells

We present a facile method for producing superhydrophobic nanograss-coated (SNGC) glass surfaces that possess both reduced reflectivity and self-cleaning properties at the air/glass interface. The refractive index of a CaF2 nanograss (NG) layer on a glass substrate, deposited by glancing angle vapor deposition, is 1.04 at 500 nm, which is the second-lowest value ever reported so far. The fluorinated NG layer gives rise to a high water contact angle (>150°) and very efficient cleaning out of dust with water drops. Using the dual functionalities of the SNGC glass, we demonstrate superhydrophobic and antireflective organic photovoltaic cells with excellent power conversion efficiency.

Effects of CuO and ZnO Additives on Sintering Temperature and Piezoelectric Properties of 0.41Pb(Ni1/3Nb2/3)O3-0. 36PbTiO3-0.23PbZrO3 Ceramics

ZnO-added 0.41PNN–0.36PT–0.23PZ ceramics cannot be sintered below 950°C. However, when CuO is added, the liquid phase forms and specimens can be sintered even at 850°C. The dielectric constant (eT3/eo), piezoelectric constant (d33) and electromechanical coupling factor (kp) increase with the addition of CuO and this is due to the enhancement of the density of the specimens. When CuO is added to 0.41PNN–0.36PT–0.23PZ ceramics, improvements of density, eT3/eo, d33 and kp are also observed. However, the d33 and kp values of the CuO-added 0.41PNN–0.36PT–0.23PZ ceramics are less than those of the CuO- and ZnO-added specimens. Therefore, ZnO is effective in improving the piezoelectric properties of 0.41PNN–0.36PT–0.23PZ ceramics. Good dielectric and piezoelectric properties of d33=575(pC/N), kp=0.55 and eT3/eo= 3900 are obtained for 0.41PNN–0.36PT–0.23PZ+3.0 mol% ZnO with 1 mol% CuO sintered at 900°C for 2 h.

Low-Temperature Sintering and Piezoelectric Properties of ZnO-Added 0.41Pb(Ni1/3 Nb2/3)O3–0.36PbTiO3–0.23PbZrO3 Ceramics

The structure of 0.41(PbNi1/3Nb2/3)O3–0.36PbTiO3–0.23PbZrO3 (0.41PNN–0.36PT–0.23PZ) ceramics changed from pseudocubic to tetragonal when ZnO was added. For the specimens sintered above 1000°C, bulk density slightly decreased with the addition of ZnO but it significantly increased for the specimens sintered below 1000°C. Curie temperature (Tc) and maximum dielectric constant increased when ZnO was added. Coercive electric field (Ec) and mechanical quality factor (Qm) increased with the addition of ZnO but piezoelectric constant (d33), electromechanical coupling factor (kp) and dielectric constant (eT3/e0) decreased with the addition of ZnO for the specimens sintered above 1,000°C. However, for the specimens sintered below 1000°C, the addition of ZnO significantly improved the piezoelectric and dielectric constants, which could be due to the improvement of bulk density. The good dielectric and piezoelectric properties of d33=525 (pC/N), kp=0.53 and eT3/eo = 3400 were obtained for the specimen with 3 mol% ZnO sintered at 950°C for 1 h.

High-temperature thermoelectric properties of nanostructured Ca3Co4O9 thin films

We prepared nanostructured Ca3Co4O9 (CCO) thin films by promoting localized epitaxial growth on polycrystalline Al2O3 substrates. The thermoelectric properties of the CCO films were studied in the temperature range 300 to 1023 K. We confirmed that localized epitaxial growth occurred on the seed grains that dominate the (006) plane. The nanostructured CCO thin films were found to have a maximum Seebeck coefficient of 206 μV/K and a power factor (at 920 K) of 0.514 mW/mK2. Moreover, the presence of nanostructure was found to reduce the thermal conductivity, and thus, should enhance the overall performance of CCO films in thermoelectric devices.

Effects of non-stoichiometry and chemical inhomogeneity on the order-disorder phase formation in the complex perovskite compounds, Ba(Ni1/3Nb2/3)O3and Ba(Zn1/3Nb2/3)O3

The order-disorder phase formation of the complex perovskite compounds, Ba(Ni1/3Nb2/3)O3 and Ba(Zn1/3Nb2/3)O3, was investigated by various heat treatments. Heat-treated samples were characterized using X-ray diffraction, SEM, and EDS, interactively. On the basis of lattice parameter measurements, the defect concentration in the complex perovskite compounds was thought to control ordered or disordered phase formation. The chemical inhomogeneity in the calcined powder was responsible for the formation of a liquid phase in the non-equilibrium state. These compounds are postulated to melt incongruently above temperature at which the ordered phase forms. Irrespective of extended heat treatments, second phases were observed but could not be identified by XRD due to lack of information. The second phase in BNN was unreacted NiO with a small amount of barium and niobium, and the second phase in BZN was a pyrochlore-like compound of which the composition was Ba∶Nb = 1∶1 with a small amount of zinc.