Nanotechnology explained: Nanowires and nanotubes

nanomanufacturing-2Nanowerk News) Nanowires and nanotubes, slender structures that are  only a few billionths of a meter in diameter but many thousands or millions of  times longer, have become hot materials in recent years. They exist in many  forms — made of metals, semiconductors, insulators and organic compounds — and  are being studied for use in electronics, energy conversion, optics and chemical  sensing, among other fields.

This Scanning Electron Microscope image shows an array of nanowires. (Photo:  Kristian Molhave/Opensource Handbook of Nanoscience and Nanotechnology)

The initial discovery of carbon nanotubes — tiny tubes of pure  carbon, essentially sheets of graphene rolled up unto a cylinder — is generally  credited to a paper published in 1991 by the Japanese physicist Sumio Ijima  (although some forms of carbon nanotubes had been observed earlier). Almost  immediately, there was an explosion of interest in this exotic form of a  commonplace material. Nanowires — solid crystalline fibers, rather than hollow  tubes — gained similar prominence a few years later.
Due to their extreme slenderness, both nanotubes and nanowires  are essentially one-dimensional. “They are quasi-one-dimensional materials,” says MIT associate professor of materials science and engineering Silvija  Gradecak: “Two of their dimensions are on the nanometer scale.” This  one-dimensionality confers distinctive electrical and optical properties.
For one thing, it means that the electrons and photons within  these nanowires experience “quantum confinement effects,” Gradecak says. And  yet, unlike other materials that produce such quantum effects, such as quantum  dots, nanowires’ length makes it possible for them to connect with other  macroscopic devices and the outside world.
The structure of a nanowire is so simple that there’s no room  for defects, and electrons pass through unimpeded, Gradecak explains. This  sidesteps a major problem with typical crystalline semiconductors, such as those  made from a wafer of silicon: There are always defects in those structures, and  those defects interfere with the passage of electrons.
Made of a variety of materials, nanowires can be “grown” on many  different substrates through a vapor deposition process. Tiny beads of molten  gold or other metals are deposited on a surface; the nanowire material, in  vapor, is then absorbed by the molten gold, ultimately growing from the bottom  of that bead as a skinny column of the material. By selecting the size of the  metal bead, it is possible to precisely control the size of the resulting  nanowire.
In addition, materials that don’t ordinarily mix easily can be  grown together in nanowire form. For example, layers of silicon and germanium,  two widely used semiconductors, “are very difficult to grow together in thin  films,” Gradecak says. “But in nanowires, they can be grown without any  problems.” Moreover, the equipment needed for this kind of vapor deposition is  widely used in the semiconductor industry, and can easily be adapted for the  production of nanowires.
While nanowires’ and nanotubes’ diameters are negligible, their  length can extend for hundreds of micrometers, even reaching lengths visible to  the unaided eye. No other known material can produce such extreme  length-to-diameter ratios: millions of times longer than they are wide.
Because of this, the wires have an extremely high ratio of  surface area to volume. That makes them very good as detectors, because all that  surface area can be treated to bind with specific chemical or biological  molecules. The electrical signal generated by that binding can then easily be  transmitted along the wire.
Similarly, nanowires’ shape can be used to produce narrow-beam  lasers or light-emitting diodes (LEDs), Gradecak says. These tiny light sources  might someday find applications within photonic chips, for example — chips in  which information is carried by light, instead of the electric charges that  relay information in today’s electronics.
Compared to solid nanowires, nanotubes have a more complex  structure: essentially one-atom-thick sheets of pure carbon, with the atoms  arranged in a pattern that resembles chicken wire. They behave in many ways as  one-dimensional materials, but are actually hollow tubes, like a long,  nanometer-scale drinking straw.
The properties of carbon nanotubes can vary greatly depending on  how they are rolled up, a property called chirality. (It’s similar to the  difference between forming a paper tube by rolling a sheet of paper lengthwise  versus on the diagonal: The different alignments of fibers in the paper produce  different strength in the resulting tubes.) In the case of carbon nanotubes,  chirality can determine whether the tubes behave as metals or as semiconductors.
But unlike the precise manufacturing control that is possible  with nanowires, so far methods for making nanotubes produce a random mix of  types, which must be sorted to make use of one particular kind. Besides  single-walled nanotubes, they also exist in double-walled and multi-walled  forms.
In addition to their useful electronic and optical properties,  carbon nanotubes are exceptionally strong, and are used as reinforcing fibers in  advanced composite materials. “In any application where one-dimensionality is  important, both carbon nanotubes and nanowires would provide benefits,” Gradecak  says.
  Source: By David L. Chandler,  MIT

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Flexible, light solar cells could provide new opportunities

QDOTS imagesCAKXSY1K 8David L. Chandler, MIT News Office
MIT researchers develop a new approach using graphene sheets coated with nanowires.
MIT researchers have produced a new kind of photovoltaic cell based on sheets of flexible graphene coated with a layer of nanowires. The approach could lead to low-cost, transparent and flexible solar cells that could be deployed on windows, roofs or other surfaces.

The new approach is detailed in a report published in the journal Nano Letters, co-authored by MIT postdocs Hyesung Park and Sehoon Chang, associate professor of materials science and engineering Silvija Gradečak, and eight other MIT researchers.Flexible, light solar cells could provide new opportunities

While most of today’s solar cells are made of silicon, these remain expensive because the silicon is generally highly purified and then made into crystals that are sliced thin. Many researchers are exploring alternatives, such as nanostructured or hybrid solar cells; indium tin oxide (ITO) is used as a transparent electrode in these new solar cells.

“Currently, ITO is the material of choice for transparent electrodes,” Gradečak says, such as in the touch screens now used on smartphones. But the indium used in that compound is expensive, while graphene is made from ubiquitous carbon.

The new material, Gradečak says, may be an alternative to ITO. In addition to its lower cost, it provides other advantages, including flexibility, low weight, mechanical strength and chemical robustness.

Building semiconducting nanostructures directly on a pristine graphene surface without impairing its electrical and structural properties has been challenging due to graphene’s stable and inert structure, Gradečak explains. So her team used a series of polymer coatings to modify its properties, allowing them to bond a layer of zinc oxide nanowires to it, and then an overlay of a material that responds to light waves — either lead-sulfide quantum dots or a type of polymer called P3HT.

Despite these modifications, Gradečak says, graphene’s innate properties remain intact, providing significant advantages in the resulting hybrid material.

“We’ve demonstrated that devices based on graphene have a comparable efficiency to ITO,” she says — in the case of the quantum-dot overlay, an overall power conversion efficiency of 4.2 percent — less than the efficiency of general purpose silicon cells, but competitive for specialized applications. “We’re the first to demonstrate graphene-nanowire solar cells without sacrificing device performance.”

In addition, unlike the high-temperature growth of other semiconductors, a solution-based process to deposit zinc oxide nanowires on graphene electrodes can be done entirely at temperatures below 175 degrees Celsius, says Chang, a postdoc in MIT’s Department of Materials Science and Engineering (DMSE) and a lead author of the paper. Silicon solar cells are typically processed at significantly higher temperatures.

The manufacturing process is highly scalable, adds Park, the other lead author and a postdoc in DMSE and in MIT’s Department of Electrical Engineering and Computer Science. The graphene is synthesized through a process called chemical vapor deposition and then coated with the polymer layers. “The size is not a limiting factor, and graphene can be transferred onto various target substrates such as glass or plastic,” Park says.

Gradečak cautions that while the scalability for solar cells hasn’t been demonstrated yet — she and her colleagues have only made proof-of-concept devices a half-inch in size — she doesn’t foresee any obstacles to making larger sizes. “I believe within a couple of years we could see [commercial] devices” based on this technology, she says.

László Forró, a professor at the Ecole Polytechnique Fédérale de Lausanne, in Switzerland, who was not associated with this research, says that the idea of using graphene as a transparent electrode was “in the air already,” but had not actually been realized.

“In my opinion this work is a real breakthrough,” Forró says. “Excellent work in every respect.”

He cautions that “the road is still long to get into real applications, there are many problems to be solved,” but adds that “the quality of the research team around this project … guarantees the success.”

The work also involved MIT professors Moungi Bawendi, Mildred Dresselhaus, Vladimir Bulovic and Jing Kong; graduate students Joel Jean and Jayce Cheng; postdoc Paulo Araujo; and affiliate Mingsheng Wang. It was supported by the Eni-MIT Alliance Solar Frontiers Program, and used facilities provided by the MIT Center for Materials Science Engineering, which is supported by the National Science Foundation.