Self-connected and habitually tilted piezoelectric nanorod array

Huge Piezoelectric Response of LaN-based Superlattices

We construct LaN-based artificial superlattices to investigate the ferroelectricity and piezoelectricity using the volume matching conditions of the parent components that soften the elastic constant C₃₃ and increase the piezoelectric modulus d₃₃. The proposed superlattice consists of LaN and YN (or LaN and ScN) buckled monolayers alternately arranged along the crystallographic c-direction. The structure of polar wurtzite (w-LaYN/w-LaScN) is both mechanically and dynamically stable, and the computed energy barrier makes the ferroelectric polarization switching possible. We show that the epitaxial strain can modify the spontaneous ferroelectric polarization as well as d₃₃. The LaN/YN superlattice exhibits a huge piezoelectric response in the unstrained state, due to their small c/a value and extremely soft C₃₃. In addition, the epitaxial strain is revealed as effective control of the nature (indirect and direct) and value of the electronic band gap.

Mimicking of five human senses using nanostructured ZnO single material

We demonstrated a single-material multifunctional sensor using ZnO nanorods, which is capable of mimicking the five human senses. A simple method for the growth of ZnO nanorods and their application as a multifunctional sensor was studied. High-density ZnO nanorods were synthesized by the carbothermal transport method on a graphene catalyst. To optimize the sensing mechanisms, we investigated ZnO nanostructure growth under two oxygen flow rates. ZnO nanorods as sensors for smell, light, sound, touch, and taste, the five human senses, showed stable electrical signals under standard ambient conditions.

Water-Soluble Epitaxial NaCl Thin Film for Fabrication of Flexible Devices

We studied growth mechanisms of water-soluble NaCl thin films on single crystal substrates. Epitaxial growth of NaCl(100) on Si(100) and domain-matched growth of NaCl(111) on c-sapphire were obtained at thicknesses below 100 nm even at room temperature from low lattice mismatches in both cases. NaCl thin film, which demonstrates high solubility selectivity for water, was successfully applied as a water-soluble sacrificial layer for fabrication of several functional materials, such as WO₃ nano-helix and Sn doped In₂O₃ nano-branches.

Copper Micro-Labyrinth with Graphene Skin: New Transparent Flexible Electrodes with Ultimate Low Sheet Resistivity and Superior Stability

We have developed self-assembled copper (Cu) micro-labyrinth (ML) with graphene skin for transparent flexible electrodes of optoelectronic devices. The Cu ML is simply formed by heating a thin Cu film with a 100-nm thickness on a SiO₂/Si substrate at 950 °C under hydrogen ambient to block the oxidation. Moreover, the Cu ML can have graphene skin at the surface by inserting carbo-hydroxyl molecules (C_xH_y) during heating due to the catalytic decomposition of C–H bonds on the Cu surface. The Cu ML with graphene skin (Cu ML-G) has superior sheet resistivity below 5 Ω/sq and mechanical flexibility without cracks at the bending radius of 0.1 cm. Although the transmittance of Cu ML-G is a little lower (70%~80%) than that of conventional metallic nanowires electrodes (such as Ag, ~90% at the visible wavelength), it has good thermal stability in conductivity without any damage at 200 °C due to a micro-sized pattern and graphene skin which prohibits the surface migration of Cu atoms.

Lithium-doped zinc oxide nanowires-polymer composite for high performance flexible piezoelectric nanogenerator

We present a method to develop high performance flexible piezoelectric nanogenerators (NGs) by employing Li-doped ZnO nanowires (NWs). We synthesized Li-doped ZnO NWs and adopted them to replace intrinsic ZnO NWs with a relatively low piezoelectric coefficient. When we exploited the ferroelectric phase transition induced in Li-doped ZnO NWs, the performance of the NGs was significantly improved and the NG fabrication process was greatly simplified. In addition, our approach can be easily expanded for large-scale NG fabrication. Consequently, the NGs fabricated by our simple method exhibit the excelling output voltage and current, which are stable and reproducible during periodic bending/releasing measurement over extended cycles. In addition, output voltage and current up to ∼ 180 V and ∼ 50 μA, respectively, were obtained in the large-scale NG. The approach introduced here extends the performance limits of ZnO-based NGs and their potentials in practical applications.

Chemical vapor deposition of graphene on a "peeled-off" epitaxial Cu(111) foil: a simple approach to improved properties

We present a simple approach to improving the quality of CVD grown graphene, exploiting a Cu(111) foil catalyst. The catalyst is epitaxially grown by evaporation on a single crystal sapphire substrate, thickened by electroplating, and peeled off. The exposed surface is atomically flat, easily reduced, and exclusively of (111) orientation. Graphene grown on this catalyst under atmospheric CVD conditions and without wet chemical prereduction produces single crystal domain sizes of several hundred micrometers in samples that are many centimeters in size. The graphene produced in this way can easily be transferred to other substrates using well-established techniques. We report mobilities extracted using field-effect (as high as 29 000 cm(2) V(-1) s(-1)) and Hall bar measurement (up to 10 100 cm(2) V(-1) s(-1)).

Hemispherically aggregated BaTiO3 nanoparticle composite thin film for high-performance flexible piezoelectric nanogenerator

We report high-performance flexible nanogenerators (NGs) based on a composite thin film, composed of hemispherically aggregated BaTiO3 nanoparticles (NPs) and poly(vinylidene fluoride-co-hexafluoropropene) P(VDF-HFP). The hemispherical BTO-P(VDF-HFP) clusters were realized by a solvent evaporation method, which greatly enhanced piezoelectric power generation. The flexible NGs exhibit high electrical output up to ∼75 V and ∼15 μA at the applied force normal to the surface, indicating the important role of hemispherical BTO clusters. Besides, the durability and reproducibility of the NGs were tested by cyclic measurement under bending stage, generating the output of ∼5 V and ∼750 nA. The approach we introduce here is simple, cost-effective, and well-suited for large-scale high-performance flexible NG fabrication.

ZnO nanowire-based antireflective coatings with double-nanotextured surfaces

High-aspect-ratio nanotextured surfaces with different morphologies (straight, core-shell type, and core-branch type nanowires) are prepared by a hydrothermal method of ZnO nanowires, followed by means of RF sputtering for core-shell type nanowires and e-beam evaporation for branch-type nanowires. The structural analysis showed that the MgO has highly preferred orientation along the ⟨111⟩ and ⟨200⟩ direction, respectively, and the crystalline continuity between the ZnO and MgO layers were also showed. Compared with ZnO nanowires, the MgO/ZnO samples drastically suppress broad and omnidirection reflection, which ascribes to the refractive-index modulation along the lateral direction of nanowires growth as well as the vertical direction. It was also shown that morphology could have a substantial influence on the antireflection property. These results suggest that double-nanotextured surface is one of the promising structures for antireflective surfaces without fine control in nanowire morphology.