Jung-Il Hong

Lead-free NaNbO3 nanowires for a high output piezoelectric nanogenerator.

Perovskite ferroelectric nanowires have rarely been used for the conversion of tiny mechanical vibrations into electricity, in spite of their large piezoelectricity. Here we present a lead-free NaNbO(3) nanowire-based piezoelectric device as a high output and cost-effective flexible nanogenerator. The device consists of a NaNbO(3) nanowire-poly(dimethylsiloxane) (PDMS) polymer composite and Au/Cr-coated polymer films. High-quality NaNbO(3) nanowires can be grown by hydrothermal method at low temperature and can be poled by an electric field at room temperature. The NaNbO(3) nanowire-PDMS polymer composite device shows an output voltage of 3.2 V and output current of 72 nA (current density of 16 nA/cm(2)) under a compressive strain of 0.23%. These results imply that NaNbO(3) nanowires should be quite useful for large-scale lead-free piezoelectric nanogenerator applications.

Rescaled electrical properties of ZnO/low density polyethylene nanocomposites

ZnO/low density polyethylene (LDPE) nanocomposites were prepared using melt mixing with good dispersion of the ZnO nanoparticles. The electrical properties (dc resistivity and breakdown strength) of the composite with various concentrations of ZnO up to the percolation limit were measured and compared to the corresponding electrical properties of submicron ZnO filled LDPE. It was observed that the nanocomposites exhibited a lower percolation limit and a slower decrease in resistivity with filler concentration compared to the conventional composite. The dielectric breakdown strength was also found to be higher for the nanocomposite at high filler concentration.

Room-temperature, texture-controlled growth of ZnO thin films and their application for growing aligned ZnO nanowire arrays

Texture-controlled growth of ZnO films on substrates of general materials at room temperature by pulsed laser deposition was demonstrated. The texture of the film changed progressively from (001) to (110) to (100) as the laser fluence increased from 2 J cm(-2) up to 45 J cm(-2). Application of the textured films on Si wafers as seed layers for growing aligned ZnO nanowire arrays (grown along the c-axis) with controlled orientation relative to the substrate surface was demonstrated. The individual nanowire forms an epitaxial orientation relationship with the orientation of the grain that nucleated it; therefore the long axis of the nanowire aligns in conformity with the texture of the seed layer.

Doping and Raman Characterization of Boron and Phosphorus Atoms in Germanium Nanowires

Impurity doping is the most important technique to functionalize semiconductor nanowires. The crucial point is how the states of impurity atoms can be detected. The chemical bonding states and electrical activity of boron (B) and phosphorus (P) atoms in germanium nanowires (GeNWs) are clarified by micro-Raman scattering measurements. The observation of B and P local vibrational peaks and the Fano effect clearly demonstrate that the B and P atoms are doped into the crystalline Ge region of GeNWs and electrically activated in the substitutional sites, resulting in the formation of p-type and n-type GeNWs. This method can be a useful technique for the characterization of semiconductor nanowire devices. The B-doped GeNWs showed an increasingly tapered structure with increasing B concentration. To avoid tapering and gain a uniform diameter along the growth direction of the GeNWs, a three step process was found to be useful, namely growth of GeNWs followed by the deposition of an amorphous Ge layer with high B concentration and then annealing.

Magnetism in dopant-free ZnO nanoplates.

It is known that bulk ZnO is a nonmagnetic material. However, the electronic band structure of ZnO is severely distorted when the ZnO is in the shape of a very thin plate with its dimension along the c-axis reduced to a few nanometers while keeping the bulk scale sizes in the other two dimensions. We found that the chemically synthesized ZnO nanoplates exhibit magnetism even at room temperature. First-principles calculations show a growing asymmetry in the spin distribution within the distorted bands formed from Zn (3d) and O (2p) orbitals with the reduction of thickness of the ZnO nanoplates, which is suggested to be responsible for the observed magnetism. In contrast, reducing the dimension along the a- or b-axes of a ZnO crystal does not yield any magnetism for ZnO nanowires that grow along c-axis, suggesting that the internal electric field produced by the large {0001} polar surfaces of the nanoplates may be responsible for the distorted electronic band structures of thin ZnO nanoplates.

Phosphorus Doped Zn1-xMgxO Nanowire Arrays

We demonstrate the growth of phosphorus doped Zn(1-x)Mg(x)O nanowire (NW) using pulsed laser deposition. For the first time, p-type Zn(0.92)Mg(0.08)O:P NWs are likely obtained in reference to atomic force microscopy based piezoelectric output measurements, X-ray photoelectron spectroscopy, and the transport property between the NWs and a n-type ZnO film. A shallow acceptor level of approximately 140 meV is identified by temperature-dependent photoluminescence. A piezoelectric output of 60 mV on average has been received using the doped NWs. Besides a control on NW aspect ratio and density, band gap engineering has also been achieved by alloying with Mg to a content of x = 0.23. The alloyed NWs with controllable conductivity type have potential application in high-efficiency all-ZnO NWs based LED, high-output ZnO nanogenerator, and other optical or electrical devices.

Ultrasound-induced ordering in poly(3-hexylthiophene): role of molecular and process parameters on morphology and charge transport.

Facile methods for controlling the microstructure of polymeric semiconductors are critical to the success of large area flexible electronics. Here we explore ultrasonic irradiation of solutions of poly(3-hexylthiophene) (P3HT) as a simple route to creating ordered molecular aggregates that result in a one to two order of magnitude improvement in field effect mobility. A detailed investigation of the ultrasound induced phenomenon, including the role of solvent, polymer regioregularity (RR) and film deposition method, is conducted. Absorption spectroscopy reveals that the development of low energy vibronic features is dependent on both the regioregularity as well as the solvent, with the latter especially influential on the intensity and shape of the band. Use of either higher regioregular polymer or ultrasonic irradiation of lower regioregular polymer solutions results in high field effect mobilities that are nearly independent of the dynamics of the film formation process. Surprisingly, no distinct correlation between thin-film morphology and macroscopic charge transport could be ascertained. The relationships between molecular and process parameters are very subtle: modulation of one effects changes in the others, which in turn impact charge transport on the macroscale. Our results provide insight into the degree of control that is required for the development of reproducible, robust materials and processes for advanced flexible electronics based on polymeric materials.

Toward robust nanogenerators using aluminum substrate.

Nanogenerators (NG) have been developed to harvest mechanical energy from environmental sources such as vibration, human motion, or movement of automobiles. We demonstrate a robust and large-area NG based on a cost-effective Al substrate with the capability to be easily integrated in series and parallel for high-output performance. The output voltage and current density of the three-dimensionally integrated NG device reaches up to 3 V and 195 nA under human walking conditions.

Characterization of impurity doping and stress in Si/Ge and Ge/Si core-shell nanowires.

Core-shell nanowires (NWs) composed of silicon (Si) and germanium (Ge) are key structures for realizing high mobility transistor channels, since the site-selective doping and band-offset in core-shell NWs separate the carrier transport region from the impurity doped region, resulting in the suppression of impurity scattering. Four different types of Si/Ge (i-Si/n-Ge, p-Si/i-Ge) and Ge/Si (n-Ge/i-Si, i-Ge/p-Si) core-shell NWs structures were rationally grown. The surface morphology significantly depended on the types of the core-shell NWs. Raman and X-ray diffraction (XRD) measurements clearly characterized the compressive and tensile stress in the core and shell regions. The observation of boron (B) and phosphorus (P) local vibrational peaks and the Fano effect clearly demonstrated that the B and P atoms are selectively doped into the shell and core regions and electrically activated in the substitutional sites, showing the success of site-selective doping.

Preparation and structure investigation of nanoparticle-assembled titanium dioxide microtubes

Nanoparticle-assembled capped TiO2 microtubes were synthesized by means of a recently discovered freeze-drying method. Most tubes are about 1 μm in diameter and have a high aspect ratio. The resulting structure was observed using field emission scanning electron microscopy and transmission electron microscopy. The crystal structure and chemical nature of the nanoparticle constituents and the resulting structures were investigated by x-ray diffraction, selected area energy dispersive x-ray spectroscopy, and Raman spectroscopy. The formation of the observed TiO2 microtubes occurs through the physical rearrangement or self-organization of nanoparticles, most likely driven by capillarity and aided by hydrogen bonding between nanoparticles and the water molecules during freeze-drying.