Tian-Chiuan Wu

Effect of compressed TiO2 nanoparticle thin film thickness on the performance of dye-sensitized solar cells

In this study, dye-sensitized solar cells (DSSCs) were fabricated using nanocrystalline titanium dioxide (TiO2) nanoparticles as photoanode. Photoanode thin films were prepared by doctor blading method with 420 kg/cm2 of mechanical compression process and heat treatment in the air at 500°C for 30 min. The optimal thickness of the TiO2 NP photoanode is 26.6 μm with an efficiency of 9.01% under AM 1.5G illumination at 100 mW/cm2. The efficiency is around two times higher than that of conventional DSSCs with an uncompressed photoanode. The open-circuit voltage of DSSCs decreases as the thickness increases. One DSSC (sample D) has the highest conversion efficiency while it has the maximum short-circuit current density. The results indicate that the short-circuit current density is a compromise between two conflict factors: enlargement of the surface area by increasing photoanode thickness and extension of the electron diffusion length to the electrode as the thickness increases.

Anisotropic pinning effect on a nb thin film with triangular arrays of pinning sites

A superconducting Nb thin film with triangular arrays of pinning sites has been fabricated to investigate the commensurate vortex lattices. Vortex-vortex interactions in this pattern film make the observed anisotropy in the pinning properties. Periodic minima appear in the magnetoresistance curves at equal field intervals that can be associated with matching effect. The transport properties at near critical temperatures show anisotropic pinning properties related to the configuration of pinning centers. The critical current is depressed when the driving force is applied along the short diagonal in the rhombic unit cell of pinning array. It implies that the pinning potential along the short diagonal creates a moving channel of vortices in triangular arrays of defects. The pinned and interstitial vortices can be movable in an incommensurate vortex row and a straight unhindered one-dimensional path.

n-ZnO nanorods/p+-Si (111) heterojunction light emitting diodes

In this study, we report the effects of thermal annealing in nitrogen ambient on the optical and electrical properties of zinc oxide (ZnO) nanorod (NR) arrays for the application in light emission diodes (LED). The single-crystalline ZnO NR array was synthesized on p+-Si (111) substrate without seed layer using simple, low-cost, and low-temperature hydrothermal method. The substrate surface was functionalized by hydrofluoric acid and self-assembled monolayer of octadecyltrimethoxysilane ((CH3 (CH2)17Si(OCH3)3). ZnO NRs were characterized by field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), and micro-photoluminescence (micro-PL). The results of FESEM and XRD indicate that single crystalline ZnO NRs with (002) preferred orientation along the substrate surface is successfully grown on functionalized p+-Si (111) substrate. The current–voltage and electroluminescence (EL) characteristics of the LED show that the most suitable annealing temperature ranges from 400°C to 600°C. Both PL and EL spectra show broadband emissions, ultraviolet and visible (green-yellow) light. The white-like light emission is able to be observed by naked eyes.

Novel pinning phenomena in Nb thin films with square pinning arrays

Electron-beam lithography and reaction dry etching techniques have been used to fabricate superconductor thin films with regular square hole arrays to explore the vortex pinning behavior. Magnetotransport measurements were carried out by a four-probe technique. Periodic critical current matching peaks are observed. We found that the height of the third matching peak is close to that of the fourth matching peak for film with holes of diameter of 200nm at certain temperatures, and it appears that the third matching peak is missing. Similar peculiar phenomena are observed for films with holes of different diameters, but the missing peaks are at the fourth or fifth matching field for larger holes. We made molecular dynamic simulations to study the ground state vortex distributions and the dynamics of the vortex motion. The simulation results offer good explanations to the features we found in the experiments.

Pinning Effects Induced by Periodic Arrays of Structural Corrugations in Nb Thin Films

Artificial vortex pinning arrays have been fabricated by electron-beam lithography in conjunction with reactive ion etching in Nb superconducting thin films in order to investigate flux pinning. Based on the structural corrugation in the Nb thin film, the square lattice of defects that act as pinning centers was created. A strong periodic pinning effect, as the vortex lattice matches the array of defects, can be observed in the evolution of resistivity and critical current with the applied magnetic fields in the mixed state of the Nb film, thus, revealing a synchronized pinning effect due to structural corrugation. This result is in agreement with the resistivity–temperature curves at matching fields and half-integer matching fields in which a crossover occurs for temperatures exceeding a certain threshold value. The phenomenon is related to the presence of the pinning structure at matching fields.

Flux pinning force in Nb thin films with periodic vortex pinning arrays

Flux pinning force has been investigated in superconducting Nb thin films with square arrays of artificial flux pinning centers. These pinning centers due to the structural corrugation in the Nb thin film were created. In the mixed state, the magnetoresistance minima and critical current density maxima to be separated at constant field intervals corresponding to the matching fields were found. The temperature dependence of critical current density can be fitted to an expression (1−T/TC0)n near superconducting transition temperature. The n values at applied field equal matching fields (H=H1,H2,H3…) are 5/2, in contrast to 3/2 at half integer matching fields. It indicates that the dynamics of interstitial vortices at higher matching fields is similar to those at the first matching field. It can be believed that the collective flux pinning force stabilized by the vortices on pinning centers indirectly prevents interstitial vortices from flowing.

Structure characterization and magnetic properties of Ln0.7Pb0.3MnO3 (Ln=La, Nd, and Pr) manganites

Abstract Ln 0.7Pb0.3MnO3 (Ln=La, Nd, Pr) oxides have been synthesized by using the conventional ceramic method. The magnetic measurements show that Tc values alter considerably by substituting different rare earth ions. The zero field cooled (ZFC) magnetization curve of La manganite shows almost the same magnitude with the field cooled (FC); however, the curve of Nd manganite drops to zero at low temperatures. The magnetization is hardly saturated by the external field for Pr and Nd manganites due to the spin-glass nature. These facts are in good agreement with the lower ionic radii of the Nd ions and the corresponding large distortion of their perovskite structures.

Structural and magnetic properties in La0.7Pb0.3Mn1-XCoxO3 systems

Abstract In this study we present the results of an extensive structural investigation of the La 0.7 Pb 0.3 Mn 1− X Co X O 3 (0≦ x ≦1) system. Powder X-ray diffraction shows a single-phase rhombohedral distorted perovskite structure for all samples. There is a gradual decrease in the lattice parameter as the Co content increases. In addition, we have systematically investigated the magnetic properties by using a SQUID magnetometer. The magnetoresistances are also measured and a giant negative magnetoresistance of over 50% can be obtained.

Temperature dependence of vortex configuration by honeycomb hole arrays in a superconducting Nb film

A honeycomb array of submicrometer holes in a Nb superconducting thin film has been fabricated to investigate the flux pinning effect. It is found that the minima positions reveal two regimes characterized by the matching fields and the fractional ones. It is believed that the complex behavior may come from more than one vortex being captured per pinning site. Furthermore, the temperature dependence of the saturation number of vortices per pinning site together with vortex-vortex interaction gives the complex vortex configurations.

Study of One-Side-Flat Edge on Vortex in Submicro-Scaled Permalloy Disks

By introducing a one-side-flat edge to a submicroscaled Permalloy disk, the isotropy of the disk is broken to achieve the control on vortex nucleation and annihilation. Series one-side-flat magnetic disk arrays with different cutting percentages of disk are fabricated to observe the influence from asymmetry on the vortex configuration in submicroscaled magnetic disks. In the excised angles were varied from 15° to 90° in 500-nm-diameter and 40-nm-thick NiFe disk arrays, it is observed the dependences of vortex nucleation and annihilation fields on the asymmetry. Linear relations of vortex nucleation and annihilation fields to aspect ratio were found that would be useful for controlling nucleation and annihilation fields on purpose. Micromagnetism simulations for Permalloy disk array with a diameter of 500 nm and different excise dangles were executed to compare with experimental data. The relations of vortex existence field to the aspect ratio were found that could be taken into account for designing single vortex element applications. In addition to magnetic field parallel to excised flat edge, the constant perpendicular field (Hy) applied to sample could change vortex nucleation and annihilation field of Py disk array. Finally, It was found that a higher asymmetry effect could depress the unpleasantly interactions and lead to a narrow range of switching field distribution, which was an important index for designing high-density magnetic devices.