Myeongkyu Lee

Growth and electrostrictive properties of Pb(Mg1/3Nb2/3)O3 crystals

Transparent and stoichiometric lead-magnesium-niobate crystals were grown by the Bridgman method. Crystals of centimeter size were obtained without material loss by employing self-seeded and seeded techniques using sealed platinum crucibles. The fastest growth direction was found to be pseudocubic 〈111〉. As-grown crystals showed a broad absorption band centered near λ=600 nm and an OH absorption near λ=2830 nm. The electrostrictive coefficient Q11 was determined to be 1.15×10−2 m4 C−2 from the measured strain and polarization.

Growth of LixTa1−xO3 single crystals and their optical properties

Lithium tantalate (LiTaO 3 ) single crystals having different Li/Ta ratios were grown by using conventional and double crucible Czochralski method. The grown crystals appeared to have mole percent of Li 2 O ranging from 45.5 to 50 mol%. We analyzed the real composition with ICP method and measured the Curie temperatures (T c ) with DTA method. The relationship between T c and the mole percent of Li 2 O in LiTaO 3 crystals was derived. The transmission spectra and refractive indices were measured. The refractive indices measured by prism coupler were analyzed in terms of single-term Sellmeir parameters.

Laser welding of nanoparticulate TiO2 and transparent conducting oxide electrodes for highly efficient dye-sensitized solar cell.

Poor interfacial contact is often encountered in nanoparticulate film-based devices. The dye-sensitized solar cell (DSSC) is a representative case in which a nanoporous TiO(2) electrode needs to be prepared on the transparent conducting oxide (TCO)-coated glass substrate. In this study, we demonstrate that the inter-electrode contact resistance accounts for a considerable portion of the total resistance of a DSSC and its efficiency can be greatly enhanced by welding the interface with a laser. TiO(2) films formed on the TCO-coated glass substrate were irradiated with a pulsed ultraviolet laser beam at 355 nm; this transmits through the TCO and glass but is strongly absorbed by TiO(2). Electron microscopy analysis and impedance measurements showed that a thin continuous TiO(2) layer is formed at the interface as a result of the local melting of TiO(2) nanoparticles and this layer completely bridges the gap between the two electrodes, improving the current flow with a reduced contact resistance. We were able to improve the efficiency by 35-65% with this process. DSSCs fabricated using a homemade TiO(2) paste revealed an efficiency improvement from eta = 3.3% to 5.4%, and an increase from 8.2% to 11.2% was achieved with the TiO(2) electrodes made from a commercial paste.

Laser-induced control of TiO2 porosity for enhanced photovoltaic behavior.

In a dye-sensitized solar cell (DSSC), the morphology of the porous TiO 2 electrode strongly infl uences the cell characteristics because it serves both as a large surface-area support for dye molecules and electron-transporting medium. [ 1–6 ] The electron transport through the TiO 2 network occurs mainly by diffusion because the macroscopic electric fi eld across the cell is screened by the electrolyte. [ 7–9 ] The porous electrode is typically prepared by coating a paste containing TiO 2 nanoparticles (NPs) onto conducting glass with a doctor-blade or by screen printing, followed by annealing at 450–500 ° C. Other types of electrodes consisting of TiO 2 nanotubes, [ 10–12 ] nanotubes fi lled with nanoparticles, [ 13 ] and nanorods [ 14–16 ] have also been widely investigated. These one-dimensional nanostructures can provide larger surface-to-volume ratios and enable faster charge transport with reduced recombination. [ 10 , 15 ] Nevertheless, the maximum effi ciencies obtained so far have not reached those of NP-based cells. Aside from the effi ciency aspect, the typical NP electrodes are more favorable in terms of the process simplicity and scalability. The photocurrent generated by DSSC is directly linked to the number of dye molecules adsorbed on the TiO 2 electrode. Thus, the porosity ( P ) of the electrode signifi cantly infl uences the photo voltaic behavior because the total TiO 2 surface area per unit volume depends on it. The effects of P on the cell performance have been extensively investigated in NP-based DSSCs. Barbe et al. [ 1 ] presented various methods to control the microstructure of TiO 2 electrode and showed that the DSSC effi ciency can be enhanced by adjusting the pore size and porosity. Benkstein et al. [ 2 ] have shown that the network geometry has a strong infl uence on the electron transport dynamics in mesoporous TiO 2 fi lms. P was experimentally varied from 0.52 to 0.71 by changing the amount of binder added to the paste. Electron transport was modeled using simulated mesoporous TiO 2 fi lms and the random-walk approach. The electron pathway through the network was correlated with the fi lm porosity and the coordination number of particles in the fi lm. They have shown that both the experimental measurements and simulations are described very well by percolation theory and developed the following relation between the electron diffusion coeffi cient ( D ) and P .

Nonvolatile two-color holographic recording in Tb-doped LiNbO3

We have found that Tb-doped near-stoichiometric LiNbO3 crystals have three different types of energy levels: ultraviolet (UV) absorption centers just above the valence band, shallow electron traps slightly below the conduction band, and deep traps located about 1.9 eV (λ≈650 nm) below it. Using this shallow trap as an intermediate energy state, two-color holographic recording has been carried out at λ=852 nm with a UV gating light at 313 nm. A few % of diffraction efficiency was achieved, and the measured two-color sensitivity was 0.01–0.02 cm/J. No reduction of diffraction efficiency was observed during 4 h continuous readout, and the bit-error-rate of constructed digital hologram was less than 10−5.

Three-dimensional optical memory using photoluminescence change in Sm-doped sodium borate glass

The feasibility of three-dimensional (3D) optical memory has been demonstrated by utilizing the photoluminescence (PL) spectrum change in a Sm-doped fluoride glass [K. Miura, J. Qiu, S. Fujiwara, S. Sakasuchi, and K. Hirao, Appl. Phys. Lett. 80 2263 (2002)]. We here report on a femtosecond laser-induced PL change in a Sm-doped sodium borate glass that is easier to synthesize and its potential application to 3D memory. Irradiation with a femtosecond pulsed laser (800 nm, 1 kHz, 100 fs) induced a PL peak near 682 nm, resulting from the photoreduction of the Sm ions. A multilayer pattern (bit size=1μm,layer separation=8μm) formed by femtosecond laser irradiation was read out by a reflection-type fluorescent confocal microscope, which detected the emission at 682 nm as a signal. High-contrast pattern images were obtained without crosstalk.

Fabrication and properties of As-doped ZnO films grown on GaAs(0 0 1) substrates by radio frequency (rf) magnetron sputtering

ZnO thin films were grown on GaAs(0 0 1) substrate to study the feasibility of making As-doped textured ZnO films for possible optical device application using radio frequency (rf) magnetron sputtering. It was demonstrated that highly c-axis oriented ZnO crystal with uniformly doped As could be grown using this deposition technique. Crystallinity was shown to improve with higher processing temperature. Photoluminescence spectroscopy, supplemented by cathodo-luminescence imaging, showed that the ZnO films have good optical quality with strong near band emission peak at 3.3 eV and spatially homogeneous luminescence indicating possibility of producing As-doped ZnO films with good crystallinity and optical properties using the technique used herein.

Parallelized laser-direct patterning of nanocrystalline metal thin films by use of a pulsed laser-induced thermo-elastic force

Thin film patterning by the conventional lithographic technique requires a number of steps including the deposition, development, and removal of the photoresist layer. Here we demonstrate that metal thin films evaporated on glass can be directly patterned by a spatially modulated pulsed Nd-YAG laser beam (wavelength = 1064 nm, pulse width = 6 ns) incident from the backside of the substrate. This method utilizes a pulsed laser-induced thermo-elastic force exerted on the film which plays a role in detaching it from the substrate. High-fidelity patterns at the micrometer scale have been fabricated over a few square centimeters by a single pulse with pulse energy of 850 mJ. This is attributed to the fact that deposited metal films are polycrystalline with nano-sized grains, and thus localized etching of the material is possible with shearing along the weakly bonded grain boundary regions. We have also developed a nano-block model to simulate the laser-direct patterning of nanocrystalline thin films. Experimental results could be well described with this simulation model. The patterning process presented here provides a simple photoresist-free route to fabricate metal thin film patterns on transparent substrates.

Parallel laser printing of nanoparticulate silver thin film patterns for electronics

This report discusses a parallel laser printing process for fabricating metallization patterns, which utilizes a pulsed laser-induced thermoelastic force exerting on nanoparticles. Silver thin films solution deposited on a glass substrate were transferred onto diverse receiver substrates such as Si, glass, and plastics by a spatially modulated Nd:YAG pulsed laser beam (1064nm, 6ns pulse width). High-fidelity patterns at the sub-10μm scales could be printed over several cm2 by a single pulse with 850mJ of energy. The fabrication of organic thin film transistors is demonstrated using printed source and drain Ag electrodes.

PHOTOREFRACTIVE PROPERTIES OF TUNGSTEN BRONZE FERROELECTRIC LEAD BARIUM NIOBATE (PB1-XBAXNB2O6) CRYSTALS

Lead barium niobate (Pb1−xBaxNb2O6) is a very promising material for photorefractive applications in that it has a high ferroelectric-to-paraelectric transition temperature (300–500 °C depending on composition) and simultaneously can have large electro-optic coefficients, because of the nearly vertical morphotropic phase boundary (MPB) near 1−x=0.63. Pb1−xBaxNb2O6 crystals have been grown by the vertical Bridgman method for near-MPB compositions (0.5<1−x<0.6), and their photorefractive properties were investigated by the two wave mixing experiments. Material properties such as composition and absorption coefficient varied along the growth direction because of the incongruent charge composition and the nonuniform distribution of impurities. A maximum diffraction efficiency of 50% at λ=514.5 nm was observed in a 3.2 mm thick nominally undoped as-grown crystal. The photorefractive sensitivity increased as the wavelength decreased. The gain coefficient also increased with decreasing wavelength, changing from ...