Quanxi Jia

Polymer-Embedded Carbon Nanotube Ribbons for Stretchable Conductors

The creation of stretchable electronics is emerging as one of the most interesting research topics in materials science and technology. [ 1,2 ] Devices that are stretchable, foldable, and deformable into complex curvilinear shapes can enable many new applications that would be impossible to achieve by conventional rigid electronics. Examples of such applications range from fl exible displays, electronic eyeball cameras to stretchable electronic implants and conformable skin sensors. [ 3–6 ]

Carbon nanotube yarn strain sensors

Carbon nanotube (CNT) based sensors are often fabricated by dispersing CNTs into different types of polymer. In this paper, a prototype carbon nanotube (CNT) yarn strain sensor with excellent repeatability and stability for in situ structural health monitoring was developed. The CNT yarn was spun directly from CNT arrays, and its electrical resistance increased linearly with tensile strain, making it an ideal strain sensor. It showed consistent piezoresistive behavior under repetitive straining and unloading, and good resistance stability at temperatures ranging from 77 to 373 K. The sensors can be easily embedded into composite structures with minimal invasiveness and weight penalty. We have also demonstrated their ability to monitor crack initiation and propagation.

High-T/sub c/ coated conductors-performance of meter-long YBCO/IBAD flexible tapes

One meter long tapes based on 50-100 /spl mu/m thick by 1 cm wide nickel alloy substrates have been coated in a continuous process with a textured yttria-stabilized zirconia layer by ion beam-assisted deposition, followed by a 1-2 /spl mu/m thick layer of YBCO by pulsed laser deposition. The best result to date is a tape with a critical current (I/sub c/) at 75 K of 96 A over an 87 cm measurement length. The overall critical current density and engineering current density are 1 MA/cm/sup 2/ and 10 kA/cm/sup 2/, respectively. Using a special probe, individual I-V curves were generated for each centimeter of tape length in order to investigate longitudinal uniformity of the transport properties: the highest and lowest I/sub c/ values fall within a range of /spl plusmn/25%.

High-resolution x-ray diffraction and transmission electron microscopy of multiferroic BiFeO3 films

High-resolution x-ray diffraction and transmission electron microscopy (TEM) have been used to study BiFeO3 thin films grown on the bare and SrRuO3 buffered (001) SrTiO3 substrates. Reciprocal space mapping (RSM) around (002) and (103) reflections revealed that BFO films with a thickness of about 200 nm were almost fully relaxed and had a rhombohedral structure. Cross-sectional, high-resolution TEM showed that the films started to relax at a very early stage of growth, which was consistent with the RSM results. A thin intermediate layer of about 2 nm was observed at the interface, which had a smaller lattice than the overgrown film. Twist distortions about the c axis to release the shear strain introduced by the growth of rhombic (001) BiFeO3 on cubic (001) SrTiO3 were also observed. The results indicate that a strained, coherent BiFeO3 film on (001) SrTiO3 is very difficult to maintain and (111) STO substrates are preferable.

Nanoscale Lithography on Monolayer Graphene Using Hydrogenation and Oxidation

Monolayer graphene is one of the most interesting materials applicable to next-generation electronic devices due to its transport properties. However, realization of graphene devices requires suitable nanoscale lithography as well as a method to open a band gap in monolayer graphene. Nanoscale hydrogenation and oxidation are promising methods to open an energy band gap by modification of surface structures and to fabricate nanostructures such as graphene nanoribbons (GNRs). Until now it has been difficult to fabricate nanoscale devices consisting of both hydrogenated and oxidized graphene because the hydrogenation of graphene requires a complicated process composed of large-scale chemical modification, nanoscale patterning, and etching. We report on nanoscale hydrogenation and oxidation of graphene under normal atmospheric conditions and at room temperature without etching, wet process, or even any gas treatment by controlling just an external bias through atomic force microscope lithography. Both the lithographically defined nanoscale hydrogenation and oxidation have been confirmed by micro-Raman spectroscopy measurements. Patterned hydrogenated and oxidized graphene show insulating behaviors, and their friction values are several times larger than those of graphene. These differences can be used for fabricating electronic or electromechanical devices based on graphene.

Tailoring the Morphology of Carbon Nanotube Arrays: From Spinnable Forests to Undulating Foams

Directly spinning carbon nanotube (CNT) fibers from vertically aligned CNT arrays is a promising way for the application of CNTs in the field of high-performance materials. However, most of the reported CNT arrays are not spinnable. In this work, by controlling catalyst pretreatment conditions, we demonstrate that the degree of spinnability of CNTs is closely related to the morphology of CNT arrays. Shortest catalyst pretreatment time led to CNT arrays with the best spinnability, while prolonged pretreatment resulted in coarsening of catalyst particles and nonspinnable CNTs. By controlling the coalescence of catalyst particles, we further demonstrate the growth of undulating CNT arrays with uniform and tunable waviness. The CNT arrays can be tuned from well-aligned, spinnable forests to uniformly wavy, foam-like films. To the best of our knowledge, this is the first systematical study on the correlation between catalyst pretreatment, CNT morphology, and CNT spinnability.

Microstructure of epitaxial La0.7Ca0.3MnO3 thin films grown on LaAlO3 and SrTiO3

Epitaxial La0.7Ca0.3MnO3 (LCMO) thin films of a thickness ∼170 nm were grown on (001) LaAlO3 (LAO) and (001) SrTiO3 (STO) substrates by pulsed laser deposition. Transmission electron microscopy and associated techniques have been applied to investigate the microstructures introduced by lattice mismatch that are responsible for the observed differences in properties between these two films. Numerous secondary phase rods were observed in both films. For the LCMO/LAO film, Ca-deficient secondary-phase rods originated in the film after a thickness of about 25 nm and were found to be responsible for relieving in-plane compressive stress during the island growth. In the case of STO substrate, however, almost all of secondary-phase rods initiated at the film–substrate interface. The lattice mismatch between LCMO and STO is relaxed into regions of good coherent fit separated by such secondary phases, possibly resulting from interfacial reaction. The two types of substrates lead to the formation of two different c...

Multilevel Data Storage Memory Devices Based on the Controlled Capacitive Coupling of Trapped Electrons

Since the fi rst conceptualization of non-volatile memory devices using a fl oating gate in 1967, [ 1 ] tremendous efforts have been made to develop high-density, low-cost, and non-volatile solidstate memory devices for portable electronics. [ 2–17 ] Among the many kinds of non-volatile memory devices, fl ash memories which use an array of fl oating gate transistors to store information are the most widely used. [ 12–20 ] One of the main limitations of conventional fl ash memory devices is the easy discharge of the stored information from the fl oating gate to the silicon substrates through the thin tunneling oxide. [ 12–20 ] To improve the device reliability, the recent trend for fl ash memory devices is to store information in discrete charge trapping sites such as in silicon nitride [ 17–20 ] or metal nanoparticles. [ 21–29 ] Recently, we reported that ordered arrays of metallic nanoparticles obtained by a micellar route and multilayered metallic nanoparticles can be used as charge storage media for non-volatile memory devices with tailored performances. [ 27–29 ] However, most of the research on nanoparticle-based memory devices has focused on binary data storage in the charge trapping layer. In this work, multiple data storage memory devices based on the controlled capacitive coupling of trapped electrons operated at room temperature have been fabricated by using highly ordered arrays of metal nanoparticles as the charge trapping elements. We present results from metal nanoparticle-based memory devices with controlled nanoparticle charge trapping elements, which undergo gate-voltage-adjustable multilevel memory states. Experimental and theoretical analysis of multilevel data manipulations and visualization of memory states has been done on the nanometer scale. Electrical multilevel data programming and data access of fi ve well-defi ned data levels were

Phase-field model for epitaxial ferroelectric and magnetic nanocomposite thin films

A phase-field model was developed for studying the magnetoelectric coupling effect in epitaxial ferroelectric and magnetic nanocomposite thin films. The model can simultaneously take into account the ferroelectric and magnetic domain structures, the electrostrictive and magnetostrictive effects, substrate constraint, as well as the long-range interactions such as magnetic, electric, and elastic interactions. As an example, the magnetic-field-induced electric polarization in BaTiO3–CoFe2O4 nanocomposite film was analyzed. The effects of the film thickness, morphology of the nanocomposite, and substrate constraint on the degree of magnetoelectric coupling were discussed.

Vertically aligned pearl-like carbon nanotube arrays for fiber spinning.

This work reports the first synthesis of highly aligned, pearl-like carbon nanotube arrays through a chemical vapor deposition process. The unique morphology of these carbon nanotubes makes it possible to spin them into macroscopic fibers with excellent mechanical and electrical properties. In addition, the interesting hollow structure of these nanotubes opens new applications such as nanoreactors.