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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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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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zhaokelun1975

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New nanocomposite processing technique creates more powerful capacitors

9X"M-a(n4d4x:cS|Scanning electron micrographs of barium titanate (BaTiO3) nanocomposites with polycarbonate (left, top and bottom) and Viton (right, top and bottom) polymer matrices. The images show the dramatic improvement in film uniformity through the use of phosphonic acid coated BaTiO3 nanoparticles (bottom images) as compared to uncoated nanoparticles (top images). Credit: Image courtesy of Joe PerryA new technique for creating films of barium titanate (BaTiO3) nanoparticles in a polymer matrix could allow fabrication of improved capacitors able to store twice as much energy as existing devices. The improved capacitors could be used in consumer devices such as cellular telephones – and in defense applications requiring both high energy storage and rapid current discharge.
Because of its high dielectric properties, barium titanate has long been of interest for use in capacitors, but until recently materials scientists had been unable to produce good dispersion of the material within a polymer matrix. By using tailored organic phosphonic acids to encapsulate and modify the surface of the nanoparticles, researchers at the Georgia Institute of Technology’s Center for Organic Photonics and Electronics were able to overcome the particle dispersion problem to create uniform nanocomposites.

Our team has developed nanocomposites that have a remarkable combination of high dielectric constant and high dielectric breakdown strength," said Joseph W. Perry, a professor in the Georgia Tech School of Chemistry and Biochemistry and the Center for Organic Photonics and Electronics. "For capacitors and related applications, the amount of energy you can store in a material is related to those two factors."

The new nanocomposite materials have been tested at frequencies of up to one megahertz, and Perry says operation at even higher frequencies may be possible. Though the new materials could have commercial application without further improvement, their most important contribution may be in demonstrating the new encapsulation technique – which could have broad applications in other nanocomposite materials.

"This work opens a door to effectively exploit this type of particle in nanocomposites using the coating technology we have demonstrated," explained Perry. "There are many ways we can envision making advances beyond what we’ve done already."
www.nanost.net
The results were reported in the April 2007 edition (Vol. 19, issue 7) of the journal Advanced Materials. The research was supported by the Office of Naval Research and the National Science Foundation. Georgia Tech has filed a patent application on the nanoparticle encapsulation technique.
Because of their ability to store and rapidly discharge electrical energy, capacitors are used in a variety of consumer products such as computers and cellular telephones. And because of the increasing demands for electrical energy to power vehicles and new equipment, they also have important military applications.

Key to developing thin-film capacitor materials with higher energy storage capacity is the ability to uniformly disperse nanoparticles in as high a density as possible throughout the polymer matrix. However, nanoparticles such as barium titanate tend to form aggregates that reduce the ability of the nanocomposite to resist electrical breakdown. Other research groups have tried to address the dispersal issue with a variety of surface coatings, but those coatings tended to come off during processing – or to create materials compatibility issues.

The Georgia Tech research team decided to address the issue by using organic phosphonic acids to encapsulate the particles. The tailored organic phosphonic acid ligands, designed and synthesized by a research group headed by Seth Marder – a professor in the Georgia Tech School of Chemistry and Biochemistry – provide a robust coating for the particles, which range in size from 30 to 120 nanometers in diameter.

"Phosphonic acids bind very well to barium titanate and to other related metal oxides," Perry said. "The choice of that material and ligands were very effective in allowing us to take the tailored phosphonic acids, put them onto the barium titanate, and then with the correct solution processing, to incorporate them into polymer systems. This allowed us to provide good compatibility with the polymer hosts – and thus very good dispersion as evidenced by a three- to four-fold decrease in the average aggregate size."

gThough large crystals of barium titanate could also provide a high dielectric constant, they generally do not provide adequate resistance to breakdown – and their formation and growth can be complex and require high temperatures. Composites provide the necessary electrical properties, along with the advantages of solution-based processing techniques.

"One of the big benefits of using a polymer nanocomposite approach is that you combine particles of a material that provide desired properties in a matrix that has the benefits of easy processing," Perry explained.

Though the new materials may already offer enough of an advantage to justify commercializing, Perry believes there are additional opportunities for boosting their performance. The research team also wants to scale up production to make larger samples – now produced in two-inch by three-inch films – available to other researchers who may wish to develop additional applications.

Perry and Marder are working with Bernard Kippelen, a professor in the Georgia Tech School of Electrical and Computer Engineering, on the use of these new nanocomposites in organic thin-film transistors in which solution-based techniques are used to fabricate inexpensive electronic components.

"Beyond capacitors, there are many areas where high dielectric materials are important, such as field-effect transistors, displays and other electronic devices," Perry added. "With our material, we can provide a high dielectric layer that can be incorporated into those types of applications."

Source: Georgia Institute of Technology
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6楼2007-04-28 17:32:30
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zhaokelun1975

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Scientists Hand-Make Devices Smaller than 10 Nanometers
by Laura Mgrdichian

Two intersecting nanowires (left panel) were made into a four-terminal nanogap transistor (right panel) using the TEBAL method.A research team from the University of Pennsylvania has used an electron beam to hand-carve ultra-small metal structures and devices, all with dimensions below 10 nanometers, from very thin metal sheets. Their technique could impact the development of many nanotechnologies, including nanoelectronics.

Among the structures created by the two scientists, professor Marija Drndić and graduate student Michael Fischbein, the paper's lead author, are nanodisks, nanorings, nanowires, nanoholes, and multi-terminal nano-transistors.

“Many different approaches have been undertaken to fabricate the small structures needed to probe the phenomena that take place at the nanoscale, but the most widely used and versatile techniques are limited to tens of nanometers,” Drndić said to PhysOrg.com. “Reliably and consistently fabricating devices at the sub-10-nanometer scale from the top down is generally still challenging, but our technique offers a route to this regime.”
“It’s like using a magnifying glass and sun rays to sculpt an ice cube,” says Fischbein. “However, using an electron beam instead of sun rays allows for precision on the atomic scale.”

Among its advantages over other techniques, Drndić and Fischbein say, is the fact that theirs uses the imaging beam of a transmission electron microscope, which means the structures can be imaged with atomic resolution and inspected in real time as they are created. These characteristics result in structures with smooth surfaces (atomically speaking) and high reproducibility – creating nearly identical copies of the same structure.

Additionally, the technique (referred to as TEBAL, for transmission electron beam ablation lithography) allows the contacts between a nanostructure and its leads to be resistance-free, and thus more efficient. Structures made from bottom-up techniques, i.e., assembled from smaller components, typically first need to be placed on a chip and then connected to larger circuitry. TEBAL avoids these steps.

All of the structures created using TEBAL were done so by hand, with a human user guiding the electron beam and watching each structure take shape. “Computer control should offer an even higher degree of precision than we've demonstrated and produce highly intricate patterns over a wide area,” said Fischbein.

TEBAL has many potential applications in nanoscience because it has the ability to produce a wide variety of nanostructures. Running a current through TEBAL-created nanowires could allow them to be used as nanomagnets. Or, the wires could be used to test the effects of wire shape and size on their conductivity, using both regular and superconducting wires. The tiny transistors Drndić and Fischbein created have many uses in molecular electronics. Nanogap-nanohole devices can be used in the manufacture of single-molecule detectors and provide new opportunities for DNA nanopore sequencing.

This research is described in detail in the April 17, 2007, online edition of Nano Letters.

Citation: Michael D. Fischbein and Marija Drndić, “Sub-10 nm Device Fabrication in a Transmission Electron Microscope.” Nano Lett. ASAP Article, DOI: 10.1021/nl0703626

Copyright 2007 PhysOrg.com.
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7楼2007-04-28 17:43:39
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zhaokelun1975

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Nano Structures Can Pose Big Measurement Problems
Materials scientists will tell you that to best understand, characterize and eventually utilize the properties of a specific material, you have to be able to define how the atoms within it are arranged. In the case of common crystals, there are numerous methods, such as X-ray diffraction, by which this can be done.

Not so for nanostructured materials (structures with atomic arrangements at a scale of 1-100 nanometers, or between 5 to 1,000 atoms in size) where the inability to determine atomic order with high precision has been dubbed the “nanostructure problem.”

In a paper published in the April 27 Science, researchers Igor Levin at the National Institute of Standards and Technology and Simon J.L. Billinge at Michigan State University reviewed various classes of nanostructured materials, listed the array of methods currently used to study their atomic makeup and defined the problems inherent with each one.

Overall, the authors state that while many methods exist for probing the atomic structure on the nanoscale, no single technique can provide a unique structural solution.

The authors conclude their paper by calling for a coordinated effort by researchers to develop a coherent strategy for a comprehensive solution of the “nanostructure problem” using inputs from multiple experimental methods and theory.

Citation: S.J.L. Billinge and I. Levin. The problem with determining atomic structure at the nanoscale. Science, 316: 5823, April 27, 2007.

Source: National Institute of Standards and Technology
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8楼2007-04-28 17:44:30
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zhaokelun1975

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Placing single nanowires: NIST makes the connection

Schematic of NIST single nanowire manipulation system. Credit: NISTResearchers at the National Institute of Standards and Technology have devised a system for manipulating and precisely positioning individual nanowires on semiconductor wafers. Their technique, described in a recent paper, allows them to fabricate sophisticated test structures to explore the properties of nanowires, using only optical microscopy and conventional photolithographic processing in lieu of advanced (and expensive) tools such as focused ion or electron beams.

Nanowires and nanotubes are being studied intensively as essential elements for future nanoscale electronics, but some fundamentals remain to be worked out—among them, how to put wires only a handful of atoms in diameter where you want them. The smallest-diameter nanowires today are built in a “bottom-up” fashion, assembled atom-by-atom through a chemical growth process such as chemical vapor deposition.

Scanning electron microscope image shows a single silicon nanowire positioned in an etched trench using NIST's nanowire manipulation technique. The trench helps keep the nanowire in position during the fabrication of the rest of the test structure, which measures metal/nanowire contact resistance. The scale bar is 20 micrometers long. Credit: NISTThis is essentially a bulk process; it produces haystacks of jumbled nanowires of varying lengths and diameters. “The normal research approach,” explains NIST electronics engineer Curt Richter, “is to throw a whole bunch of these down on the test surface, hunt around with a microscope until you find a good-looking wire in about the right place, and use lithography to attach electrical contacts to it.”
To achieve better control, the NIST engineers modified a standard probe station used to test individual components in microelectronic circuits. The station includes a high-resolution optical microscope and a system for precisely positioning work surfaces under a pair of customized titanium probes with tips less than 100 nanometers in diameter.

In a two-step process, silicon nanowires suspended in a drop of water are deposited on a special staging wafer patterned with a grid of tiny posts, and dried. Resting on the tops of the posts, selected nanowires can be picked up by the two probe tips, which they cling to by static electricity. The test structure wafer is positioned under the probes, the nanowire is oriented by moving either the probe tips or the wafer, and then placed on the wafer in the desired position.

Although not at all suited to mass production, the technique’s fine level of control allows NIST engineers to place single nanowires wherever they want to create elaborate structures for testing nanowire properties. They’ve demonstrated this by building a multiple-electrical-contact test structure for measuring the resistance of a nanowire independent of contact resistance, and a simple electromechanical “switch” suitable for measuring the flexibility of nanowires. They’ve used the technique successfully with nanowires greater than about 60 nm in diameter, and say sharper probe tips and high-resolution microscopes could push the limit lower.

Citation: Q. Li, S. Koo, C.A. Richter, M.D. Edelstein, J.E. Bonevich, J.J. Kopanski, J.S. Suehle and E.M. Vogel. Precise alignment of single nanowires and fabrication of nanoelectromechanical switch and other test structures. IEEE Transactions on Nanotechnology. V.6, No.2. March 2007.

Source: National Institute of Standards and Technology
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9楼2007-04-28 17:45:41
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zhaokelun1975

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Optoelectronic tweezers push nanowires around
In efforts that can improve studies of biological objects and the construction of nanotech materials, researchers at the University of California-Berkeley have invented "optoelectronic tweezers," a new way of controlling nanometer-scale objects. The research will be presented at the upcoming CLEO/QELS meeting in Baltimore.

In the design, the researchers reflect light from a digitally controlled array of mirrors, sending the light through a magnifying lens, and then into a sandwich of semiconductor planes, creating (at the interface between two of the planes) as many as 15,000 traps that can be addressed separately. In each of the traps, objects such as biological cells can be studied

Optoelectronic tweezers, which use optical energy to create powerful electric forces in carefully prescribed places, differ from ordinary optical tweezers, which use optical energy to create mechanical forces that can push things around, helping to make the technique potentially easier for laboratories to implement.

According to Berkeley's Aaron Ohta, the optoelectronic approach uses much less power than optical tweezers and doesn't need to be as carefully focused. In recent months the Berkeley group has had some success in using their locally controlled electric fields to manipulate the positions of tiny nanorods (100 nanometers in diameter and 1-50 microns long). The rods are suspended in a thin layer of water by sound waves and then transferred to the tweezer apparatus. Ohta says that the lateral-field optoelectronic device will possibly be used to place rods for the sake of building 3-D circuitry or for positioning oblong-shaped cells or cell protrusions with micron-level precision.

Source: Optical Society of America
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10楼2007-04-28 17:46:33
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zhaokelun1975

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New nanocomposites may mean more durable tooth fillings

A calcium phosphate nanocomposite filling in a tooth. The nanocomposite can "smartly" release decay-fighting agents to buffer against acids produced by bacteria, and rebuild the lost tooth minerals by releasing ions into the mineral-deficient area of the tooth. Credit: NISTThe mouth is a tough environment—which is why dentists do not give lifetime guarantees. Despite their best efforts, a filling may eventually crack under the stress of biting, chewing and teeth grinding, or secondary decay may develop where the filling binds to the tooth. Fully 70 percent of all dental procedures involve replacements to existing repairs, at a cost of $5 billion per year in the United States alone.

Now, however, scientists at the American Dental Association’s Paffenbarger Research Center, a joint research program at the National Institute of Standards and Technology, have shown that nanotechnology has the potential to lessen that toll by producing tooth restorations that are both stronger than any decay-fighting fillings available today, and more effective at preventing secondary decay. They report their findings in a recent issue of The Journal of Dental Research.

The researchers’ new technique solves a problem with the standard composite resin filling, a natural-looking restoration that is the method of choice when appearance is an issue. A dentist creates the filling by mixing the pure liquid resin with a powder that contains coloring, reinforcement and other materials, packing the resulting paste into the cavity, and illuminating the tooth with a light that causes the paste to polymerize and harden. For decay-fighting composite fillings, the problem arises from an additive that is included in the powder to provide a steady release of calcium and phosphate ions.

These ions are essential to the long-term success of the filling because they not only strengthen the crystal structure of the tooth itself, but buffer it against the decay-causing acid produced by bacteria in the mouth. Yet the available ion-releasing compounds are structurally quite weak, to the point where they weaken the filling as a whole．

To get around this conundrum, the Paffenbarger researchers have devised a spray-drying technique that yields particles of several such compounds, one of which being dicalcium phosphate anhydrous, or DCPA, that are about 50 nanometers across—20 times smaller than the 1-micrometer particles in a conventional DCPA powder.

Because these nanoscale particles have a much higher surface to volume ratio, they are much more effective at releasing ions, which means that much less of the material is required to produce the same effect. That, in turn, leaves more room in the resin for reinforcing fibers that strengthen the final filling. To exploit that opportunity, the Paffenbarger researchers also have developed nanoscale silica-fused fibers that produce a composite resin nearly twice as strong as the currently available commercial variety.

Citation: H.H.K. Xu, M.D. Weir, L. Sun, S. Takagi and L.C. Chow. Effects of calcium phosphate nanoparticles on Ca-PO4 composite, J Dent Res 86(4):378-383m 2007.

Source: National Institute of Standards and Technology
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11楼2007-04-28 17:47:23
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zhaokelun1975

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Nanoscale 'Coaxial Cables' for Solar Energy Harvesting

Figure 1: A cross-section of a conventional coaxial cableScientists have designed a new type of nanowire – a tiny coaxial cable – that could vastly improve a few key renewable energy technologies, particularly solar cells, and could even impact other cutting-edge, developing technologies, such as quantum computing and nanoelectronics.

The nanowire, developed by researchers from the National Renewable Energy Laboratory (NREL) and Lawrence Berkeley National Laboratory, may solve several problems currently associated with renewable energy applications.

Figure 2: A cross-section of the nanoscale coaxial cable, in which nitrogen, phosphorus, and gallium atoms are shown in blue, yellow, and magenta, respectively. White spheres represent hydrogen atoms, which help render the surface of the wire chemically non-reactive.One overarching problem is that current semiconducting materials with the potential for use in renewable energy devices lack one key characteristic. When electrons in these materials are excited by light and jump to higher energy levels (leaving vacancies, known as “holes,” in the lower levels), both the electrons and the holes typically move around in the same region. Thus, they tend to recombine. This is desirable for certain applications, such as light-emitting devices, where electron-hole recombination produces light, but is not ideal for renewable energy devices. A better scenario is the separation of the excited electrons from the holes such that, in the case of solar cells, for example, the electrons can be drawn off and used for electricity.
“Our nanowires were designed to provide this feature, along with a superior electrical conductivity,” said NREL materials scientist Yong Zhang, the study's corresponding researcher, to PhysOrg.com. “Both of these properties are critical in order for renewable energy devices to reach their ultimate efficiency limits.”
Conventional coaxial cables consist of a central copper wire symmetrically surrounded by a braided copper conductor, with an insulating spacer material between the two. The braid serves as a return route for electrons that have already passed down the core wire; it can equally be viewed as a channel for holes moving in the opposite direction. The insulator separates the charge passing through the wire and braid.
Mimicking this structure, the group designed a nanoscale version consisting of a central wire, the “core,” surrounded by a shell (the shell is not cylindrical like conventional cables, but rather is hexagonal). The researchers used two semiconducting materials: gallium nitride (GaN) and gallium phosphide (GaP). They made two samples, one with a GaN core and GaP shell, and another with a GaP core and GaN shell. Both wires are approximately four nanometers in diameter (according to Zhang, this particular size was chosen by considering the computational effort needed to analyze the wires' properties, because larger wires, while easier to make, require considerably more computing power and time to model. Similar success, he says, could be achieved with nanowires up to 10-15 nanometers in diameter). In neither sample is an insulating spacer required. This phenomenon is the result of the specific semiconducting behaviors of GaN and GaP.

GaN and GaP, like all semiconductors, are classified by “band gap” – how much energy is required for electrons in the material to jump from the top of the “valence band,” a range of energies for which they don't participate in conduction, to the bottom of the “conduction band,” a range for which they do participate. When GaN and GaP are combined into a wire, the structure as a whole assumes its own band gap, which is very different from that of either component but much more appropriate for solar energy applications.

Besides providing efficient charge separation, the design may be able to remedy several shortcomings of solar-energy applications. For example, they could help widen the coverage of the solar spectrum and minimize energy loss associated with electron-hole recombination.

“We can tailor the properties of these cables to address the specific problems associated with each application,” said Zhang. “Beyond renewable energy applications, they could have exciting uses ranging from quantum computing to nanoelectronics.”

This research is described in detail in the April 5, 2007, online edition of Nano Letters.

Citation: Yong Zhang, Lin-Wang Wang, and Angelo Mascarenhas, “'Quantum Coaxial Cables' for Solar Energy Harvesting.” Nano Lett. ASAP Article, DOI: 10.1021/nl070066t

Copyright 2007 PhysOrg.com.
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12楼2007-04-28 17:48:16
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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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zhaokelun1975

木虫 (著名写手)
能够称出单个纳米颗粒重量的方法（Nanobalances: weight for it）


Weighing of biomolecules, single cells and single nanoparticles in
fluid
Nature 446: 7139
26 April 2007

纳米尺度的机械共鸣器可用来以极高的分辨率测量粒子的质量，测量精度可以达
到zeptogram，即10-21g级。这种让人吃惊的分辨率在如医学诊断或环境监测等
很多实际应用中一直是不可能做到的，因为流体的存在会对该体系赖以工作的机
械振动产生阻尼。现在，来自麻省理工和“创新微技术公司”圣巴巴拉实验室及
Affinity生物传感器公司的一个小组用一种非常聪明的方式绕开了这一问题——
将流体放在共鸣器内。他们的这种真空包装的共鸣器（要测量的粒子溶液被放在
微型流体通道中）能够称出单个纳米颗粒、单个细菌和蛋白质单层的重量，分辨
率达到亚毫微微克（10–15 g）级。（Letter p. 1066）
This illustration shows an artistic depiction of the concept that enables measuring the mass of a single bacterium and single nanoparticles in fluid with a very high resolution. A hollow resonator, represented by a hollow, fluid-filled guitar, vibrates while small particles, represented here by a bacterium, flow through it. As the particles flow through the resonator, they change the frequency (tone) of the vibration. Credit: Thomas Burg
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