Jinho Joo

Characteristics and field emission of conducting poly (3,4-ethylenedioxythiophene) nanowires

Conducting poly (3,4-ethylenedioxythiophene) nanowires were synthesized by using an electrochemical polymerization method with a nanoporous template. Scanning and transmission electron microscopy confirmed the formation of conducting polymer nanowires (CPNWs) with an open end. The formation and the electrical properties of the CPNWs formed were dependent on synthetic conditions, such as the doping level, the polymerization time, and the applied current. The measured electrical conductivity of a single strand of CPNW was ∼3.4×10−3 S/cm at room temperature. From the ultraviolet and visible absorbance spectra, we observed a π–π* transition at ∼2.1 eV for the de-doped systems. A field emission cell of CPNW nanotips was fabricated. The turn-on field of the CPNWs was 3.5∼4 V/μm at 10 μA/cm2, and the current density increased up to 100 μA/cm2 at ∼4.5 V/μm. The field enhancement factor of CPNW nanotips was ∼1200, which is comparable to those of carbon nanotubes.

Solution-processed InGaZnO-based thin film transistors for printed electronics applications

This letter reports the utility of using the sol-gel process for exploring the library of multicomponent ZnO-based oxides as an active layer of thin film transistors. We chose InGaZnO as a starting material and modulated the Ga content to examine the potential of this material. Increasing the Ga ratio from 0.1 to 1 brought about a dynamic shift in the electrical behavior from conductor to semiconductor. This exploratory work critically helped us fabricate a device with robust device performance (a mobility of 1∼2 cm2 V−1 s−1 for the 400 °C-sintered samples and 0.2 cm2 V−1 s−1 for the 300 °C-sintered samples).

Characteristics and photoluminescence of nanotubes and nanowires of poly (3-methylthiophene)

We synthesized nanotubes and nanowires of π-conjugated poly (3-methylthiophene) (P3MT) by using nanoporous anodic aluminum oxide (Al2O3) template through electrochemical polymerization method. From scanning electron microscope and transmission electron microscope photographs, we observed the formation of nanotubes with diameters of 100–200 nm and wall thicknesses of 5–10 nm. Relatively long nanotubes and nanowires of P3MT (about 40μm in length) were obtained. To discern the structural and optical properties of the systems, we measured ultraviolet and visible absorbance and Fourier transform-infrared spectroscopy. We observed that the doping level, the π‐π* transition peak, and bipolaron peaks in P3MT nanotubes varied with synthetic temperature. The photoluminescence (PL) spectra of the P3MT nanotubes solution were observed at ∼490nm. For the P3MT nanotubes synthesized at lower temperatures, the PL peaks became sharper and the resistance decreased.

Effect of annealing temperature on microstructural evolution and electrical properties of sol-gel processed ZrO2/Si films

High-permittivity (k) ZrO2/Si(100) films were fabricated by a sol-gel technique and the microstructural evolution with the annealing temperature (Ta) was correlated with the variation of their electrical performance. With increasing Ta, the ZrO2 films crystallized into a tetragonal (t) phase which was maintained until 700 °C at nanoscale thicknesses. Although the formation of the t-ZrO2 phase obviously enhanced the k value of the ZrO2 dielectric layer, the maximum capacitance in accumulation was decreased by the growth of a low-k interfacial layer (IL) between ZrO2 and Si with increasing Ta. On the other hand, the gate leakage current was remarkably depressed with increasing Ta probably due to the combined effects of the increased IL thickness, optical band gap of ZrO2, and density of ZrO2 and decreased remnant organic components.

Effect of silver solubility on microstructure and superconducting properties of YBa2Cu3−xAgxO7−δ superconductors

The effect of silver (Ag) solubility on microstructure and superconducting properties of YBa2Cu3−xAgxO7−δ (0≤x≤0.5) superconductors has been evaluated. The solubility limit of Ag for copper sites in YBCO at ambient conditions (room temperature, atmospheric pressure) was evaluated to be x≂0.06. Analysis revealed the Ag content (x) of YBCO grains to be lower than the nominal Ag content (xn) used for fabrication, probably due to processing‐related problems. Part of the Ag used for fabrication segregates on the YBCO grain boundaries. The segregated Ag particles are believed to pin YBCO grains, resulting in fine‐grain microstructures. For low nominal Ag content, xn<0.2, critical current density (Jc) increased from 98 to 160 A/cm2 as a function of increasing Ag content. These improvements are believed to be due to the increased oxygen content and associated microstructural changes in the YBCO. For higher nominal Ag content, xn≳0.2, however, Jc decreased rapidly with the increasing Ag content due to the precipit...

Electrical characteristics of pentacene-based thin film transistor with conducting poly(3,4-ethylenedioxythiophene) electrodes

This is a report on the fabrication and electrical characteristics of an all-organic-based thin film transistor that uses conducting poly(3,4-ethylenedioxythiophene) (PEDOT) as electrodes. The conducting PEDOT layers as source, drain, and gate electrodes were patterned by using photolithography. The poly(vinyl cinnamate) (PVCN) was spin coated and cross-linked as a gate insulator. The pentacene as an active layer was vapor deposited onto the PVCN layer. In order to compare the characteristics of the pentacene-based organic thin film transistor (OTFT) with PEDOT electrodes, we fabricated another pentacene-based OTFT using a Si-based pattern with Au electrodes. The electrical characteristics of the devices, such as charge carrier mobility (μ), threshold voltage (Vth), and on/off current ratio (Ion∕off), were measured from its current-voltage (I-V) characteristic curves. The μ, Vth, and Ion∕off of the pentacene-based OTFT with PEDOT electrodes were ∼2.3×10−3cm2∕Vs, 4V, and ∼100, respectively. We evaluated th...

Multiseeding with (100)/(100) grain junctions in top-seeded melt growth processed YBCO superconductors

Multiseeding with (100)/(100) grain junctions of top-seeded melt growth (TSMG) processed YBCO superconductors was studied. Multiple seeding shortened the processing time for the fabrication of TSMG-processed YBCO superconductors. The relationship among the number of seeds, the levitation forces and the trapped magnetic fields of the TSMG-processed YBCO samples is reported. The characteristic of the (100)/(100) grain junction is discussed in terms of a wetting angle of a melt.

Role of Ag additions in the microstructural development, strain tolerance, and critical current density of Ag-sheathed BSCCO superconducting tapes

Ag-sheathed tapes of Bi--Sr--Ca--Cu--O (BSCCO) and BSCCO--Ag superconducting cores were made by a powder-in-tube tecnique and subjected to repeated cycles of pressing and heat treatments. These thermomechanical treatments resulted in enhanced texturing and grain growth that improved the critical current density ([ital J][sub [ital c]]). Additions of Ag to the BSCCO core further increased texturing and brought an additional improvement in [ital J][sub [ital c]]. Strain tolerance of the tapes was evaluated by measuring [ital J][sub [ital c]] before and after application of a predetermined level of tensile strain in a uniaxial mode. The fraction of [ital J][sub [ital c]] retained after the strain application was higher in the BSCCO--Ag composite tapes than in the monolithic BSCCO. For a 1.2% applied strain, 90% of the initial [ital J][sub [ital c]] was retained in the BSCCO--Ag composite tape, compared to only 40% in the monolithic BSCCO tapes. The higher strain tolerance of the BSCCO--Ag tapes may be related to the improved mechanical properties (strength, flexibility, and fracture toughness) and grain connectivity due to the Ag addition.

Role of silver addition on mechanical and superconducting properties of high-Tc superconductors

Abstract The effect of silver additions on the mechanical and superconducting properties of sintered bulk YBa 2 Cu 3 O δ (YBCO), Bi 2 Sr 1.7 CaCu 2 O δ (BSCCO-2212), and Bi 1.8 Pb 0.4 Sr 2.2 Ca 2 Cu 3 O δ (BSCCO-2223) has been evaluated. Strength and fracture toughness of YBCO and BSCCO bars increased with increasing Ag content up to 30 vol.% Ag. Addition of 30 vol. % Ag to YBCO increased strength from 87 to 136 MPa and fracture toughness from 1.82 to 3.9 MPa√m. Addition of 30 vol.% Ag to 2212 and 2223 increased strength from 58 to 107 and 41 to 90 MPa, respectively. Corresponding increases in fracture toughness were from 1.89 to 2.79 and 1.09 to 1.94 MPa√m, respectively. These improvements in strength and fracture toughness are believed to be due to the presence of Ag particles that may induce compressive stresses in the superconducting matrix and resist crack propagation by pinning the propagating cracks. The values of strength and fracture toughness of BSCCO-30 vol.% Ag specimens are comparable to those of monolithic BSCCO obtained by sinter forging, hot pressing, and hot isostatic pressing. On the other hand, the hardness of YBCO and BSCCO decreased with increasing Ag contents because of the lower hardness of Ag. Addition of Ag showed no adverse effects on superconducting properties ( J c and T c ) of YBCO or BSCCO superconductors.

Hydrothermally Grown In-doped ZnO Nanorods on p-GaN Films for Color-tunable Heterojunction Light-emitting-diodes

The incorporation of doping elements in ZnO nanostructures plays an important role in adjusting the optical and electrical properties in optoelectronic devices. In the present study, we fabricated 1-D ZnO nanorods (NRs) doped with different In contents (0% ~ 5%) on p-GaN films using a facile hydrothermal method, and investigated the effect of the In doping on the morphology and electronic structure of the NRs and the electrical and optical performances of the n-ZnO NRs/p-GaN heterojunction light emitting diodes (LEDs). As the In content increased, the size (diameter and length) of the NRs increased, and the electrical performance of the LEDs improved. From the electroluminescence (EL) spectra, it was found that the broad green-yellow-orange emission band significantly increased with increasing In content due to the increased defect states (oxygen vacancies) in the ZnO NRs, and consequently, the superposition of the emission bands centered at 415 nm and 570 nm led to the generation of white-light. These results suggest that In doping is an effective way to tailor the morphology and the optical, electronic, and electrical properties of ZnO NRs, as well as the EL emission property of heterojunction LEDs.