Judy Z. Wu

Superconductivity above 130 K in high‐quality mercury‐based cuprate thin films

Superconductivity above 130 K has been achieved in c‐axis‐oriented HgBa2Ca2Cu3O8+δ thin films. At 5 K, the superconducting critical current density (Jc) of the film is 23 MA/cm2 in the absence of magnetic field and above 2 MA/cm2 in 5 T magnetic field with the field along the c axis. A zero field Jc of 0.5 MA/cm2 is maintained to temperatures over 100 K, which indicates that the mercury‐based cuprate thin films are promising for applications of superconducting electronic devices operating at above 100 K.

Novel dye-sensitized solar cell architecture using TiO2-coated vertically aligned carbon nanofiber arrays.

A novel dye-sensitized solar cell (DSSC) architecture based on vertically aligned carbon nanofibers coated with a thin nanoneedle-textured anatase TiO2 film is demonstrated. An encouraging overall conversion efficiency of approximately 1.09% and a rather high open-circuit voltage of approximately 0.64 V have been achieved. The efficient charge separation at the TiO2-CNF junction and the large outer TiO2 surface of this core-shell architecture provide new methods to tune the materials and interfaces in solar cells.

Smoothing of YBa2Cu3O7−δ films by ion cluster beam bombardment

Smoothing high-temperature superconductor (HTS) surfaces, especially HTS thin-film surfaces, is crucial for HTS thin-film device processing. In this letter, we describe a method to planarize the surface of a YBa2Cu3O7−δ HTS film down to a smoothness with a standard deviation of 1 nm or better. The method includes first smoothing the HTS surface by ion cluster beam bombardment, followed by annealing in oxygen ambient to regrow the damaged surface layer. Additional YBCO layers can be grown epitaxially on the treated surface, even without removing the top surface layer, which contained some residual damage after annealing. This method can be integrated into HTS circuit fabrication as a key step of planarization.

Suspending single-wall carbon nanotube thin film infrared bolometers on microchannels

Suspended single-wall carbon nanotube (SWCNT) thin film bolometers have been fabricated on microchannels patterned on Si substrates using electron-beam lithography. The much improved bolometric photoresponse is attributed to the reduced thermal link between SWCNT bolometer and substrate, which can be controlled by tuning the width and spacing of the microchannels. The detectivity D∗ up to 4.5×105 cm Hz1/2/W has been obtained at room temperature, which is at least five times better than that of the unsuspended counterpart and may be further improved via elimination of metallic SWCNTs and improvement of the charge and heat transport across the intertube junctions.

A high-performance lithium-ion battery anode based on the core–shell heterostructure of silicon-coated vertically aligned carbon nanofibers

This study reports a high-performance hybrid lithium-ion anode material using coaxially coated silicon shells on vertically aligned carbon nanofiber (VACNF) cores. The unique “cup-stacking” graphitic microstructure makes VACNFs a good lithium-ion intercalation medium and, more importantly, a robust bush-like conductive core to effectively connect high-capacity silicon shells for lithium-ion storage. The vertical core–shell nanowires remain well separated from each other even after coating with bulk quantities of silicon (equivalent to 1.5 μm thick solid films). This open structure allows the silicon shells to freely expand/contract in the radial direction during lithium-ion insertion/extraction. A high specific capacity of 3000–3650 mA h (gSi)−1, comparable to the maximum value of amorphous silicon, has been achieved. About 89% of the capacity is retained after 100 charge–discharge cycles at the C/1 rate. After long cycling, the electrode material becomes even more stable, showing the invariant lithium-ion storage capacity as the charge–discharge rate is increased by 20 times from C/10 to C/0.5 (or 2C). The ability to obtain high capacity at significantly improved power rates while maintaining the extraordinary cycle stability demonstrates that this novel structure could be a promising anode material for high-performance lithium-ion batteries.

Extraordinary photocurrent harvesting at type-II heterojunction interfaces: toward high detectivity carbon nanotube infrared detectors.

Despite the potentials and the efforts put in the development of uncooled carbon nanotube infrared detectors during the past two decades, their figure-of-merit detectivity remains orders of magnitude lower than that of conventional semiconductor counterparts due to the lack of efficient exciton dissociation schemes. In this paper, we report an extraordinary photocurrent harvesting configuration at a semiconducting single-walled carbon nanotube (s-SWCNT)/polymer type-II heterojunction interface, which provides highly efficient exciton dissociation through the intrinsic energy offset by designing the s-SWCNT/polymer interface band alignment. This results in significantly enhanced near-infrared detectivity of 2.3 × 10(8) cm·Hz(1/2)/W, comparable to that of the many conventional uncooled infrared detectors. With further optimization, the s-SWCNT/polymer nanohybrid uncooled infrared detectors could be highly competitive for practical applications.

Development of Nanopatterned Fluorine-Doped Tin Oxide Electrodes for Dye-Sensitized Solar Cells with Improved Light Trapping

Transparent conductors (TCs) are an important component of optoelectronic devices and nanoscale engineering of TCs is important for optimization of the device performance through improved light trapping. In this work, patterned periodic arrays of nanopillars and nanolines of pitch size of ~700 nm were created on fluorine-doped tin oxide (FTO) using nanoimprint lithography and reactive ion etching using environmentally friendly gases. The patterned FTO exhibits enhanced light trapping as compared to the unpatterned FTO, which agrees well with simulations based on Finite-Difference Time-Domain method for up to a distance of 4 μm. Dye sensitized solar cells (DSSCs) fabricated on the patterned FTO exhibited improved performance (fill factor and power conversion efficiency), which can be attributed to enhanced light absorption in the range 400-650 nm. Further, electrochemical impedance measurements revealed lower recombination resistance for the patterned FTO/TiO(2) electrode compared to the unpatterned FTO electrode/TiO(2) electrode as a result of better light capturing properties of patterned FTO. The direct fabrication of nanopatterns on TCs developed in the present study is expected to be a viable scheme for achieving improved performance in many other optoelectronic devices.

Cation exchange: A scheme for synthesis of mercury-based high-temperature superconducting epitaxial thin films

A cation-exchange process has been developed for growth of high-quality epitaxial thin films of highly volatile mercury-based high-temperature superconductors. By selecting epitaxial precursor matrices of TlyBa2CaCu2Ox (y=1,2) and annealing them in Hg vapor, c-axis-oriented epitaxial HgBa2CaCu2O6+δ thin films with superior quality were formed through Tl and Hg-cation exchange. At 110 K, these films can carry supercurrent density close to 1 MA/cm2 which is nearly an order of magnitude higher than the best reported previously. This cation-exchange technique also provides a general scheme for synthesis of other volatile compounds with predesigned structure and composition.A cation-exchange process has been developed for growth of high-quality epitaxial thin films of highly volatile mercury-based high-temperature superconductors. By selecting epitaxial precursor matrices of TlyBa2CaCu2Ox (y=1,2) and annealing them in Hg vapor, c-axis-oriented epitaxial HgBa2CaCu2O6+δ thin films with superior quality were formed through Tl and Hg-cation exchange. At 110 K, these films can carry supercurrent density close to 1 MA/cm2 which is nearly an order of magnitude higher than the best reported previously. This cation-exchange technique also provides a general scheme for synthesis of other volatile compounds with predesigned structure and composition.

High critical current density in epitaxial HgBa2CaCu2OX thin films

High quality superconducting HgBa2CaCu2Ox (Hg-1212) thin films have been reproducibly fabricated using cation-exchange method. The thin films have pure Hg-1212 phase and have smooth surface morphology. The superconducting transition temperatures of these films are in the range of 120–124 K. The critical current density Jc is up to 3.2×106 A/cm2 at 77 K and drops only by a factor of 2 at 100 K and self field. At 110 K, a Jc of 7.8×105 A/cm2 has been obtained. X-ray diffraction pole figures show that these films are epitaxially grown on LaAlO3(001) substrates, which is consistent with a χmin of 19% obtained using Rutherford backscattering/channeling analysis.

Growth of HgBa2Ca2Cu3O8+δ thin films on LaAlO3 substrates using fast temperature ramping Hg‐vapor annealing

The fast temperature ramping Hg‐vapor annealing (FTRA) process has been used for growth of superconducting Hg‐based cuprate thin films on (100) LaAlO3 substrates. The film/substrate interface chemical reactions and the formation of a CaHgO2 impurity phase have been effectively reduced with adoption of the FTRA process. A zero‐resistance superconducting transition temperature of 128 K and critical current densities of up to 1.4×106 A/cm2 at 77 K and 2.5×105 A/cm2 at 110 K and zero field have been obtained.