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小木虫论坛-学术科研互动平台 » 材料区 » 微米和纳米 » 前沿热点 » 纳米材料研究动态系列报道专栏
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wshk1980

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[交流] 纳米材料研究动态系列报道专栏

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美化学家发现金纳米棒自发地将自己组装成一种环状超结构
Science Daily — Rice University chemists have discovered that tiny building blocks known as gold nanorods spontaneously assemble themselves into ring-like superstructures.
链接：http://www.SciEI.com/news/science/Chemistry/Index.html
This finding, which will be published the chemistry journal Angewandte Chemie, could potentially lead to the development of novel nanodevices like highly sensitive optical sensors, superlenses, and even invisible objects for use in the military.

“Finding new ways to assemble nano-objects into superstructures is an important task because at the nanoscale, the properties of those objects depend on the arrangement of individual building blocks,” said principal investigator Eugene Zubarev, the Norman Hackerman-Welch Young Investigator and assistant professor of chemistry at Rice.

Although ring-like assemblies have been observed in spherical nanoparticles and other symmetrical molecules, until now such structures had not been documented with rod-shaped nanostructures.

Like many nanoscale objects, gold nanorods are several billionths of a meter, or 1,000 times smaller than a human hair. Zubarev used hybrid nanorods for this research because attached to their surface are thousands of polymer molecules, which are flexible chainlike structures. The central core of the nanorods is an inorganic crystal, but the polymers attached to the outside are organic species. The combination of the inorganic and organic features resulted in a hybrid structure that proved to be critical to the study.

英文全文：http://www.sciencedaily.com/releases/2007/03/070310145606.htm

[ Last edited by popsheng on 2007-4-28 at 18:12 ]
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1楼 2007-03-22 20:49:37
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zhaokelun1975

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Why nanowires make great photodetectors

A single ZnO nanowire held down by metal contacts. The nanowire segments between metal contacts serve as super-sensitive photodetectors. Credit: UC San DiegoThe geometry of semiconducting nanowires makes them uniquely suited for light detection, according to a new UC San Diego study that highlights the possibility of nanowire light detectors with single-photon sensitivity.

Nanowires are crystalline fibers about one thousandth the width of a human hair, and their inherent properties are expected to enable new photodetector architectures for sensing, imaging, memory storage, intrachip optical communications and other nanoscale applications, according to a new study in an upcoming issue of the journal Nano Letters. The UCSD engineers illustrate why the large surface areas, small volumes and short lengths of nanowires make them extremely sensitive photodetectors – much more sensitive than larger photodetectors made from the same materials.

Schematic of the trapping and photoconduction mechanism in ZnO nanowires. At the top of each box are 'energy band diagrams' ('b' represents the situation in darkness and 'c' under UV illumination). In ZnO nanowires (as compared to some other semiconducting nanowires), the lifetime of the unpaired electrons is further inreased by oxygen molecules desorption from the surface when holes neutralize the oxygen ions. Credit: UC San Diego"These results are encouraging and suggest a bright future for nanowire photodetectors, including single-photon detectors, built from nanowire structures," said Deli Wang, an electrical and computer engineering (ECE) professor from the UCSD Jacobs School of Engineering and corresponding author on the Nano Letters paper.
For a nanowire to serve as a photodetector, photons of light with sufficient energy must hit the nanowire in such a way that electrons are split from their positively charged holes. Electrons must remain free from their holes long enough to zip along the nanowire and generate electric current under an applied electric field -- a sure sign that light has been detected.
The new research demonstrates that the geometry of nanowires – with so much surface area compared to volume – makes them inherently good at trapping holes. Dangling bonds on vast nanowire surfaces trap holes – and when holes are trapped, the time it takes electrons and holes to recombine increases. Delaying the reunion of an electron and its hole increases the number of times that electron travels down the nanowire, which in turn triggers an increase in current and results in "internal photoconductive gain."
"Different kinds of nanowires detect different wavelengths of light. You could make a red-green-blue photodetector on the nanoscale by combining the right three kinds of nanowires," said Cesare Soci, one of two primary authors on the Nano Letters paper and a postdoctoral researcher in the Deli Wang lab at the Jacobs School. The other primary author is Arthur Zhang, a graduate student in the lab of Yu-Hwa Lo, an electrical engineering professor at the Jacobs School.
This work supports recent theoretical work from Peter Asbeck's High Speed Device Group, also at the Jacobs School.
"Our theoretical work showed that light-induced conductivity in nanowires can be increased by more than 10 times over similar bulk structures under the same illumination level. The work from Deli Wang's lab has confirmed some of our calculations and provides further support for the idea that nanowires will be increasingly incorporated into photodetection and photovoltaic applications," said Asbeck.
In the new work, short pulses of ultraviolet light (hundreds of femtoseconds wide) were detected on time scales in the nanosecond range. Moreover, using electronic measurement of photocurrent, the engineers reported internal photoconductive gain (G) as high as 108 – one of the highest ever reported.
"Although nanowire detectors offer both high speed and high gain, the most important figure of merit for the device is the signal-to-noise ratio or the sensitivity," explained Yu-Hwa Lo, an author on the Nano Letters paper and the director of NANO3, the clean nanofabrication facility at Calit2's UCSD campus.
"Because of the unique geometry of nanowires, the active volume that produces dark current, a source of noise, is only one thousandth that of a normal size photodetector. This enables nanowire detectors to achieve very high sensitivity, provided that light can be efficiently coupled into the nanowires. Several methods have been proposed to achieve light coupling efficiency, such as placing the nanowires in an optical resonant cavity. In theory, a nanowire detector can achieve single photon sensitivity, which is the ultimate sensitivity for any photodetector," said Lo.

The engineers also show that molecular oxygen absorbed at the surface of zinc oxide (ZnO) nanowires capture free electrons present in n-type ZnO nanowires and make them especially good at keeping holes and electrons apart. The oxygen mechanism the authors outline explains much of the enhanced sensitivity reported in ZnO nanowire photodetectors.

The engineers fabricated and characterized UV photodetectors made from ZnO nanowires with diameters of 150 to 300 nanometers and lengths ranging from 10 to 15 micrometers. The researchers studied the photoconductivity of zinc oxide nanowires over a broad time range and under both air and vacuum.

Analytical studies performed by Peter Asbeck and ECE graduate student Lingquan Wang and published in the proceedings of IEEE-NANO 2006 support the mechanism outlined in the Nano Letters paper.

According to Wang, this work also highlights how moving to the nanoscale can sometimes throw intuitions out the window.

"The surface trap states that help to make nanowires such sensitive light detectors are the very same surface features that engineers desperately avoid when manufacturing semiconductors for computer transistors, where they hamper performance," Wang said.

Reference: ZnO Nanowire UV Photodetectors with High Internal Gain, C. Soci, A. Zhang, B. Xiang, S. A. Dayeh, D. P. R. Aplin, J. Park, X. Y. Bao, Y. H. Lo, and D. Wang, Nano Letters 7(4), 1003-1009 (2007), DOI: 10.1021/nl070111x

Source: University of California - San Diego
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13楼2007-04-28 17:49:14
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wshk1980

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Aberration-Corrected Imaging of Active Sites on Industrial Catalyst
Nanoparticle

Lionel Cervera Gontard, Lan-Yun Chang, Crispin J. D. Hetherington, Angus I. Kirkland,
Dogan Ozkaya, and Rafal E. Dunin-Borkowski

Angew. Chem. Int. Ed. 2007, 46, 1 – 4

http://www3.interscience.wiley.c ... /114199478/PDFSTART
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2楼2007-03-24 14:00:24
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wshk1980

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3楼2007-03-29 16:27:40
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zhaokelun1975

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IBM将摩尔定律推进到三维时代


赛迪网2007年4月27日讯    日前，IBM宣布在制造环境中实现了一种突破性的芯片堆叠技术，此举为制造三维芯片扫清了障碍，摩尔定律也将因此而突破原来预期的极限。这种被称为“穿透硅通道（through-silicon vias）”的技术可以大大缩小不同芯片组件之间的距离，从而设计出速度更快、体积更小和能耗更低的系统。

    IBM的这项突破实现了从二维芯片设计到三维芯片堆叠的转变，将传统上并排安装在硅圆片上的芯片和内存设备以堆叠的方式相互叠加在一起，最终实现了一种紧凑的组件层状结构，大大减小了芯片的体积，并提高了数据在芯片上各个功能区之间的传输速度。

    IBM半导体研发中心副总裁Lisa Su表示：“这一突破性的进展是IBM开展十多年探索研究的成果。我们可以将三维芯片从实验室走向制造生产环节，来支持各种各样的应用。”

    这种IBM新方法是依靠新的穿透硅通道技术而非长金属电线来连接目前的二维芯片，这实际上是在硅圆片上蚀刻出来的垂直连接通道，并在其中注满金属。这些通道可以使多个芯片堆叠在一起，同时支持芯片之间更大信息量的传输。

    这项工艺将信息在芯片上传输的距离缩短了1000倍，与二维芯片相比可以增加最多100倍的信息通道或路径。

    IBM已经在自己的生产线上运行使用这种穿透硅通道技术的芯片，并将在2007年下半年开始为客户提供使用这种方法制造的芯片样本，同时在2008年投入生产。这种穿透硅通道技术最早将被用于无线通信芯片领域，这些芯片将被安装在无线LAN和蜂窝应用所使用的功率放大器之中。另外，三维技术也将应用于更广泛的芯片应用领域，包括目前那些运行在IBM高性能服务器和超级计算机中的芯片，这些服务器和超级计算机支持着全球的商业活动、政府和科学研究工作。
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5楼2007-04-28 17:06:23
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