Showing papers by "Yi Cui published in 2008"

High-performance lithium battery anodes using silicon nanowires

Abstract: There is great interest in developing rechargeable lithium batteries with higher energy capacity and longer cycle life for applications in portable electronic devices, electric vehicles and implantable medical devices. Silicon is an attractive anode material for lithium batteries because it has a low discharge potential and the highest known theoretical charge capacity (4,200 mAh g(-1); ref. 2). Although this is more than ten times higher than existing graphite anodes and much larger than various nitride and oxide materials, silicon anodes have limited applications because silicon's volume changes by 400% upon insertion and extraction of lithium which results in pulverization and capacity fading. Here, we show that silicon nanowire battery electrodes circumvent these issues as they can accommodate large strain without pulverization, provide good electronic contact and conduction, and display short lithium insertion distances. We achieved the theoretical charge capacity for silicon anodes and maintained a discharge capacity close to 75% of this maximum, with little fading during cycling.

Solution-processed metal nanowire mesh transparent electrodes.

Abstract: Transparent conductive electrodes are important components of thin-film solar cells, light-emitting diodes, and many display technologies. Doped metal oxides are commonly used, but their optical transparency is limited for films with a low sheet resistance. Furthermore, they are prone to cracking when deposited on flexible substrates, are costly, and require a high-temperature step for the best performance. We demonstrate solution-processed transparent electrodes consisting of random meshes of metal nanowires that exhibit an optical transparency equivalent to or better than that of metal-oxide thin films for the same sheet resistance. Organic solar cells deposited on these electrodes show a performance equivalent to that of devices based on a conventional metal-oxide transparent electrode.

High Capacity Li Ion Battery Anodes Using Ge Nanowires

Abstract: Ge nanowire electrodes fabricated by using vapor−liquid−solid growth on metallic current collector substrates were found to have good performance during cycling with Li. An initial discharge capacity of 1141 mA·h/g was found to be stable over 20 cycles at the C/20 rate. High power rates were also observed up to 2C with Coulombic efficiency > 99%. Structural characterization revealed that the Ge nanowires remain intact and connected to the current collector after cycling. Nanowires connected directly to the current collector have facile strain relaxation and material durability, short Li diffusion distances, and good electronic conduction. Thus, Ge nanowire anodes are promising candidates for the development of high-energy-density lithium batteries.

Spinel LiMn2O4 nanorods as lithium ion battery cathodes

Abstract: Spinel LiMn2O4 is a low-cost, environmentally friendly, and highly abundant material for Li-ion battery cathodes. Here, we report the hydrothermal synthesis of single-crystalline beta-MnO2 nanorods and their chemical conversion into free-standing single-crystalline LiMn2O4 nanorods using a simple solid-state reaction. The LiMn2O4 nanorods have an average diameter of 130 nm and length of 1.2 microm. Galvanostatic battery testing showed that LiMn2O4 nanorods have a high charge storage capacity at high power rates compared with commercially available powders. More than 85% of the initial charge storage capacity was maintained for over 100 cycles. The structural transformation studies showed that the Li ions intercalated into the cubic phase of the LiMn2O4 with a small change of lattice parameter, followed by the coexistence of two nearly identical cubic phases in the potential range of 3.5 to 4.3 V.

Wafer-scale silicon nanopillars and nanocones by Langmuir-Blodgett assembly and etching

Abstract: We have developed a method combining Langmuir–Blodgett assembly and reactive ion etching to fabricate nanopillars with uniform coverage over an entire 4 inch wafer. We demonstrated precise control over the diameter and separation between the nanopillars ranging from 60 to 600 nm. We can also change the shape of the pillars from having vertical to tapered sidewalls with sharp tips exhibiting a radius of curvature of 5 nm. This method opens up many possible opportunities in nanoimprinting, solar cells, batteries, and scanning probes.

Formation of chiral branched nanowires by the Eshelby Twist.

Abstract: Manipulating the morphology of inorganic nanostructures, such as their chirality and branching structure, has been actively pursued as a means of controlling their electrical, optical and mechanical properties. Notable examples of chiral inorganic nanostructures include carbon nanotubes, gold multishell nanowires, mesoporous nanowires and helical nanowires. Branched nanostructures have also been studied and been shown to have interesting properties for energy harvesting and nanoelectronics. Combining both chiral and branching motifs into nanostructures might provide new materials properties. Here we show a chiral branched PbSe nanowire structure, which is formed by a vapour-liquid-solid branching from a central nanowire with an axial screw dislocation. The chirality is caused by the elastic strain of the axial screw dislocation, which produces a corresponding Eshelby Twist in the nanowires. In addition to opening up new opportunities for tailoring the properties of nanomaterials, these chiral branched nanowires also provide a direct visualization of the Eshelby Twist.

Nanowire Battery Methods and Arrangements

Abstract: A variety of methods and apparatus are implemented in connection with a battery. According to one such arrangement, an apparatus is provided for use in a battery in which ions are moved. The apparatus comprises a substrate and a plurality of growth-rooted nanowires. The growth-rooted nanowires extend from the substrate to interact with the ions.

Large anisotropy of electrical properties in layer-structured In2Se3 nanowires.

Abstract: Layer-structured indium selenide (In2Se3) nanowires (NWs) have large anisotropy in both shape and bonding. In2Se3 NWs show two types of growth directions: [11−20] along the layers and [0001] perpendicular to the layers. We have developed a powerful technique combining high-resolution transmission electron microscopy (HRTEM) investigation with single NW electrical transport measurement, which allows us to correlate directly the electrical properties and structure of the same individual NWs. The NW devices were made directly on a 50 nm thick SiNx membrane TEM window for electrical measurements and HRTEM study. NWs with the [11−20] growth direction exhibit metallic behavior while the NWs grown along the [0001] direction show n-type semiconductive behavior. Excitingly, the conductivity anisotropy reaches 103−106 at room temperature, which is 1−3 orders magnitude higher than the bulk ratio.

Void Formation Induced Electrical Switching in Phase-Change Nanowires

Abstract: Solid-state structural transformation coupled with an electronic property change is an important mechanism for nonvolatile information storage technologies, such as phase-change memories. Here we exploit phase-change GeTe single-nanowire devices combined with ex situ and in situ transmission electron microscopy to correlate directly nanoscale structural transformations with electrical switching and discover surprising results. Instead of crystalline-amorphous transformation, the dominant switching mechanism during multiple cycling appears to be the opening and closing of voids in the nanowires due to material migration, which offers a new mechanism for memory. During switching, composition change and the formation of banded structural defects are observed in addition to the expected crystal-amorphous transformation. Our method and results are important to phase-change memories specifically, but also to any device whose operation relies on a small scale structural transformation.

Semitransparent Organic Photovoltaic Cells with Laminated Top Electrode

Abstract: We demonstrate semitransparent small molecular weight organic photovoltaic cells using a laminated silver nanowire mesh as a transparent, conductive cathode layer. The lamination process does not damage the underlying solar cell and results in a transparent electrode with low sheet resistance and high optical transmittance without impacting photocurrent collection. The resulting semitransparent phthalocyanine/fullerene organic solar cell has a power conversion efficiency that is 57% of that of a device with a conventional metal cathode due to differences in optical absorption.

InGaN Thin Films Grown by ENABLE and MBE Techniques on Silicon Substrates

Abstract: The prospect of developing electronic and optoelectronic devices, including solar cells, that utilize the wide range of energy gaps of InGaN has led to a considerable research interest in the electronic and optical properties of InN and In-rich nitride alloys. Recently, significant progress has been achieved in the growth and doping of InGaN over the entire composition range. In this paper we present structural, optical, and electrical characterization results from InGaN films grown on Si (111) wafers. The films were grown over a large composition range by both molecular beam epitaxy (MBE) and the newly developed “energetic neutral atomic-beam lithography & epitaxy” (ENABLE) techniques. ENABLE utilizes a collimated beam of ∼2 eV nitrogen atoms as the active species which are reacted with thermally evaporated Ga and In metals. The technique provides a larger N atom flux compared to MBE and reduces the need for high substrate temperatures, making isothermal growth over the entire InGaN alloy composition range possible. Electrical characteristics of the junctions between n- and p-type InGaN films and n- and p-type Si substrates were measured and compared with theoretical predictions based on the band edge alignment between those two materials. The predicted existence of a low resistance tunnel junction between p-type Si and n-type InGaN was experimentally confirmed.

Nanowire batteries for next generation electronics

Abstract: The scaling of electronic devices also requires the evolution of high energy density power sources. By using nanowires, high charge storage materials, which otherwise have mechanical breakage problems due to large structure transformations and volume changes, can be adopted as electrode materials. High power operation can also be possible due to the short lithium insertion distances in the nanowires. We have studied Si and Ge nanowires and demonstrated charge storage capacities several times higher than the graphite anodes used in existing battery technology. LiMn2O4 nanorod cathodes were found to show much higher power rates than commercial powders. Detailed morphology and structure characterization have shown that these improvements are attributed to facile strain relaxation, good electronic contact and conduction, and short Li insertion distances in the nanowire battery electrode. We also developed a Langmuir-Blodgett assembly technique to produce nanowire pillars as battery electrodes, which opens up the possibility for the fabrication of on-chip battery power sources.

Doped elongated semiconductor articles, growing such articles, devices including such articles and fabricating such devices

Abstract: A method for depositing one or more semiconductor nanowires on a substrate, the method comprising providing a substrate and depositing the one or more semiconductor nanowires on the surface of the substrate.