Trade.Information

Topic Name: New Insight in Nanotechnology by Uncovering Conductive Property of Carbon-Based Molecules

Category: Organic electronics

Research persons: Hrvoje Petek

Location: Institute for NanoScience and Engineering, University of Pittsburgh, United States

Details

University of Pittsburgh researchers have discovered that certain organic—or carbon-based—molecules exhibit the properties of atoms under certain circumstances and, in turn, conduct electricity as well as metal. Detailed in the April 18 edition of Science, the finding is a breakthrough in developing nanotechnology that provides a new strategy for designing electronic materials, including inexpensive and multifunctional organic conductors that have long been considered the key to smaller, cheaper, and faster technologies.

The Pitt team found that the hollow, soccer-ball-shaped carbon molecules known as fullerenes can hold and transfer an electrical charge much like the most highly conductive atoms, explained project head Hrvoje Petek, a professor of physics and chemistry in Pitt’s School of Arts and Sciences and codirector of Pitt’s Petersen Institute for NanoScience and Engineering. The research was performed by Pitt post-doctoral associates Min Feng and Jin Zhao.

When an electron was introduced into a fullerene molecule, the shape of the electron distribution mimicked that of a hydrogen atom or an atom from the alkali metal group, which includes lithium, sodium, and potassium. Moreover, when two fullerenes were placed next to each other on a copper surface, they showed the electron distribution of their chemical bond and appeared as H2, a hydrogen molecule. The assembly exhibited metal-like conductivity when the team extended it to a wire 1-molecule-wide.

“Our work provides a new perspective on what determines the electronic properties of materials,” Petek said. “The realization that hollow molecules can have metal-like conductivity opens the way to develop novel materials with electronic and chemical properties that can be tailored by shape and size.”

Although the team worked with fullerenes, the team’s results apply to all hollow molecules, Petek added, including carbon nanotubes—rolled, 1-atom-thick sheets of graphite 100,000 times smaller than a human hair.

The team’s research shows promise for the future of electronics based on molecular conductors. These molecule-based devices surpass the semiconductor and metal conductors of today in terms of lower cost, flexibility, and the ability to meld the speed and power of optics and electronics. Plus, unlike such inorganic conductors as silicon, molecule-based electronics can be miniaturized to a 1-dimensional scale (1-molecule-wide), which may enable them to conduct electricity with minimal loss and thus improve the performance of an electronic device.

Traditionally, the problem has been that organic conductors have not conducted electrical current very well, Petek said. The Pitt team’s discovery could enable scientists to finally overcome that problem, he added.

“Metal-like behavior in a molecular material—as we have found—is highly surprising and desirable in the emerging field of molecular electronics,” he said.

“Our work is a unique example of how nanoscale materials can be used as atom-sized building blocks for molecular materials that could replace silicon and copper in electronic devices, luminescent displays, photovoltaic cells, and other technologies.”

Note for Nanotechnology
Nanotechnology refers broadly to a field of applied science and technology whose unifying theme is the control of matter on the atomic and molecular scale, generally 100 nanometers or smaller, and the fabrication of devices with critical dimensions that lie within that size range.

Nanotechnology is a highly multidisciplinary field, drawing from fields such as applied physics, materials science, interface and colloid science, device physics, supramolecular chemistry (which refers to the area of chemistry that focuses on the noncovalent bonding interactions of molecules), self-replicating machines and robotics, chemical engineering, mechanical engineering, biological engineering, and electrical engineering. Much speculation exists as to what may result from these lines of research. Nanotechnology can be seen as an extension of existing sciences into the nanoscale, or as a recasting of existing sciences using a newer, more modern term. Grouping of the sciences under the umbrella of "nanotechnology" has been questioned on the basis that there is little actual boundary-crossing between the different sciences that operate on the nano-scale. Instrumentation is the only area of technology common to all disciplines; on the contrary, for example pharmaceutical and semiconductor industries do not "talk with each other". Corporations that call their products "nanotechnology" typically market them only to a certain industrial cluster.

Two main approaches are used in nanotechnology. In the "bottom-up" approach, materials and devices are built from molecular components which assemble themselves chemically by principles of molecular recognition. In the "top-down" approach, nano-objects are constructed from larger entities without atomic-level control. The impetus for nanotechnology comes from a renewed interest in Interface and Colloid Science, coupled with a new generation of analytical tools such as the atomic force microscope (AFM), and the scanning tunneling microscope (STM). Combined with refined processes such as electron beam lithography and molecular beam epitaxy, these instruments allow the deliberate manipulation of nanostructures, and lead to the observation of novel phenomena.

Examples of nanotechnology in modern use are the manufacture of polymers based on molecular structure, and the design of computer chip layouts based on surface science. Despite the great promise of numerous nanotechnologies such as quantum dots and nanotubes, real commercial applications have mainly used the advantages of colloidal nanoparticles in bulk form, such as suntan lotion, cosmetics, protective coatings, drug delivery, and stain resistant clothing.

A number of physical phenomena become noticeably pronounced as the size of the system decreases. These include statistical mechanical effects, as well as quantum mechanical effects, for example the “quantum size effect” where the electronic properties of solids are altered with great reductions in particle size. This effect does not come into play by going from macro to micro dimensions. However, it becomes dominant when the nanometer size range is reached. Additionally, a number of physical (mechanical, electrical, optical, etc.) properties change when compared to macroscopic systems. One example is the increase in surface area to volume ratio altering mechanical, thermal and catalytic properties of materials. Novel mechanical properties of nanosystems are of interest in the nanomechanics research. The catalytic activity of nanomaterials also opens potential risks in their interaction with biomaterials.

Materials reduced to the nanoscale can suddenly show very different properties compared to what they exhibit on a macroscale, enabling unique applications. For instance, opaque substances become transparent (copper); inert materials become catalysts (platinum); stable materials turn combustible (aluminum); solids turn into liquids at room temperature (gold); insulators become conductors (silicon). A material such as gold, which is chemically inert at normal scales, can serve as a potent chemical catalyst at nanoscales. Much of the fascination with nanotechnology stems from these unique quantum and surface phenomena that matter exhibits at the nanoscale.

Note for Fullerenes
Fullerenes are a family of carbon allotropes, molecules composed entirely of carbon, in the form of a hollow sphere, ellipsoid, tube, or plane . Spherical fullerenes are also called buckyballs, and cylindrical ones are called carbon nanotubes or buckytubes. Graphene is an example of a planar fullerene sheet. Fullerenes are similar in structure to graphite, which is composed of a sheet of linked hexagonal rings, but may also contain pentagonal (or sometimes heptagonal) rings that would prevent a sheet from being planar.

Fullerenes were discovered in 1985 by Robert Curl, Harold Kroto and Richard Smalley at the University of Sussex and Rice University, and are named after Richard Buckminster Fuller.

In molecular beam experiments, discrete peaks were observed corresponding to molecules with the exact mass of sixty or seventy or more carbon atoms. In 1985, Harold Kroto (then of the University of Sussex, now of Florida State University), James R. Heath, Sean O'Brien, Robert Curl and Richard Smalley, from Rice University, discovered C60, and shortly thereafter came to discover the fullerenes. Kroto, Curl, and Smalley were awarded the 1996 Nobel Prize in Chemistry for their roles in the discovery of this class of compounds. C60 and other fullerenes were later noticed occurring outside the laboratory (e.g., in normal candle soot). By 1991, it was relatively easy to produce gram-sized samples of fullerene powder using the techniques of Donald Huffman and Wolfgang Krätschmer. Fullerene purification remains a challenge to chemists and to a large extent determines fullerene prices. So-called endohedral fullerenes have ions or small molecules incorporated inside the cage atoms. Fullerene is an unusual reactant in many organic reactions such as the Bingel reaction discovered in 1993.

Minute quantities of the Buckminsterfullerenes, in the form of C60, C70, C76, and C84 molecules, are produced in nature, hidden in soot and formed by lightning discharges in the atmosphere. Recently, Buckminsterfullerenes were found in a family of minerals known as Shungites in Karelia, Russia.

The existence of C60 was predicted in 1970 by Eiji Osawa of Toyohashi University of Technology. He noticed that the structure of a corannulene molecule was a subset of a soccer-ball shape, and he made the hypothesis that a full ball shape could also exist. His idea was reported in Japanese magazines, but did not reach Europe or America.

Note for Carbon Nanotube
Carbon nanotubes (CNTs) are allotropes of carbon with a nanostructure that can have a length-to-diameter ratio greater than 1,000,000. These cylindrical carbon molecules have novel properties that make them potentially useful in many applications in nanotechnology, electronics, optics and other fields of materials science. They exhibit extraordinary strength and unique electrical properties, and are efficient conductors of heat. Inorganic nanotubes have also been synthesized.

Nanotubes are members of the fullerene structural family, which also includes the spherical buckyballs. The cylindrical nanotube usually has at least one end capped with a hemisphere of the buckyball structure. Their name is derived from their size, since the diameter of a nanotube is in the order of a few nanometers (approximately 1/50,000th of the width of a human hair), while they can be up to several millimeters in length. Nanotubes are categorized as single-walled nanotubes (SWNTs) and multi-walled nanotubes (MWNTs).

The nature of the bonding of a nanotube is described by applied quantum chemistry, specifically, orbital hybridization. The chemical bonding of nanotubes is composed entirely of sp2 bonds, similar to those of graphite. This bonding structure, which is stronger than the sp3 bonds found in diamond, provides the molecules with their unique strength. Nanotubes naturally align themselves into "ropes" held together by Van der Waals forces. Under high pressure, nanotubes can merge together, trading some sp² bonds for sp³ bonds, giving the possibility of producing strong, unlimited-length wires through high-pressure nanotube linking.

The strength and flexibility of carbon nanotubes makes them of potential use in controlling other nanoscale structures, which suggests they will have an important role in nanotechnology engineering. The highest tensile strength an individual multi-walled carbon nanotube has been tested to be is 63 GPa. Bulk nanotube materials may never achieve a tensile strength similar to that of individual tubes, but such composites may nevertheless yield strengths sufficient for many applications. Carbon nanotubes have already been used as composite fibers in polymers to improve the mechanical, thermal and electrical properties of the bulk product. A 2006 study published in Nature determined that some carbon nanotubes are present in damascus steel, possibly helping to account for the legendary strength of the (almost ancient) swords made of it.

Because of the great mechanical properties of the carbon nanotubule, a variety of structures has been proposed ranging from everyday items like clothes and sports gear to combat jackets and space elevators. However, the space elevator will require further efforts in refining carbon nanotube technology, as the practical tensile strength of carbon nanotubes can still be greatly improved.

For perspective, outstanding breakthroughs have already been made. Pioneering work lead by Ray H. Baughman at the NanoTech Institute has shown that single and multi-walled nanotubes can produce materials with toughness un-matched in the man-made and natural worlds.