Research Details

In a development that brings the promise of mass production to nanoscale devices, Lawrence Berkeley National Laboratory scientists have transformed carbon nanotubes into conveyor belts capable of ferrying atom-sized particles to microscopic worksites.

By applying a small electrical current to a carbon nanotube, they moved indium particles along the tube like auto parts on an assembly line. Their research lays the groundwork for the high-throughput construction of atomic-scale optical, electronic, and mechanical devices that will power the burgeoning field of nanotechnology.

“We’re not transporting atoms one at a time anymore — it’s more like a hose,” says Chris Regan of Berkeley Lab’s Materials Sciences Division, who co-authored the article along with fellow Materials Sciences researchers Shaul Aloni, Ulrich Dahmen, Robert Ritchie, and Alex Zettl. Aloni, Regan, and Zettl are also scientists in the University of California at Berkeley’s Department of Physics, where much of the work was conducted.

The ability to shuttle a stream of particles to precise locations fills a void that has stymied the efficient assembly of nanostructures. For years, scientists have been able to simultaneously deliver millions of atoms to millions of sites simply by mixing chemicals. Although this fast technique has grown quite sophisticated, it remains far too blunt to build atomic-scale devices. On the other end of the spectrum is the ability to manipulate individual atoms, a feat that came of age in 1990 when IBM researchers spelled out the company logo by positioning 35 xenon atoms with a scanning tunneling microscope. Although precise, this technique is painstakingly slow, with no way to swiftly deliver atoms to the work area.

“It’s either all at once, or excruciatingly serial,” says Regan. “So we combined incredibly precise localization with something that has higher throughput.”

This middle ground is made possible by carbon nanotubes, which are hollow cylinders of pure carbon about ten thousand times smaller than the diameter of a human hair. Since their discovery in the sooty residue of vaporized carbon rods, these incredibly strong and versatile macromolecules have been engineered into frictionless bearings, telescoping rods, and the world's smallest room-temperature diodes. Now, they’re poised to change the way these and other devices are constructed.

As described in their Nature article, the research team thermally evaporated indium metal onto a bundle of carbon nanotubes. The amount of evaporated metal is so small it populates the tubes’ surfaces as isolated indium crystals, instead of uniformly coating them. The bundle is then placed inside a transmission electron microscope, where a tungsten tip mounted on the end of a nanomanipulator approaches one nanotube. After physical contact is made between the tip and the free end of the nanotube, voltage is applied between the tip and the other end of the nanotube, creating a circuit. This sends an electrical current through the nanotube, which generates thermal energy that heats the indium particles.

Next, if the voltage and thermal energy is carefully controlled, something strange occurs. Real-time video of the nanotube’s surface captures an indium particle as it disappears, while the particle to its right grows. Several seconds later, that newly enlarged particle also disappears, replaced by another even further to the right. Like squeezing the last bits of toothpaste from a tube, particles to the left become smaller while those to the right grow.

In this manner, the thermally driven indium atoms inchworm along the nanotube, momentarily occupying a reservoir where a particle is located, and then moving to the next, until all of the indium piles up at the end of the nanotube. In the future, this nano-sized conveyor belt could be aimed anywhere scientists want to deliver mass atom-by-atom — the makings of a formidable nanoassembly tool. Moreover, if the voltage is slightly increased, the indium’s temperature increases, and the metal moves from left to right more quickly.

“It’s the equivalent of turning a knob with my hand and taking macroscale control of nanoscale mass transport,” Regan says. “And it’s reversible: we can change the current’s polarity and drive the indium back to its original position.”

In other words, indium can be repeatedly moved back and forth along the nanotube without losing a single atom. Nothing is lost in transit. This conservation of mass occurs because the atoms don’t evaporate from the system during their journey — an advantage in any process meant to deliver valuable material to a worksite. Instead, the atoms hug the nanotube’s surface as they move, tethered by a process called surface diffusion.

“In order to build a structure we have to be able transport material to the construction site, and we’re developing a better way to do that,” Regan says. “Our nanoscale mass delivery system is simple and reversible. It requires only a nanotube, a voltage source, and something to transport.”

Note for Carbon Nanotube
Carbon nanotubes (CNTs) are allotropes of carbon. This results in a nanostructure where the length-to-diameter ratio exceeds 1,000,000. Such 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 buckyballs. Whereas buckyballs are spherical in shape, a nanotube is cylindrical, with at least one end typically 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 are 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 great possibility for producing strong, unlimited-length wires through high-pressure nanotube linking. Carbon Nanotubes are said to have the strength of diamonds and research is being made into weaving them into clothes to create stab and bulletproof clothing. The Nanotubes would effectively stop the bullet from penetrating the body but the force and velocity of the bullet would be likely to cause broken bones and internal bleeding.

Carbon nanotubes are the strongest and stiffest materials on earth, in terms of tensile strength and elastic modulus respectively. This strength results from the covalent sp² bonds formed between the individual carbon atoms. In 2000, a multi-walled carbon nanotube was tested to have a tensile strength of 63 gigapascals (GPa). Since carbon nanotubes have a low density for a solid of 1.3-1.4 g/cm³, its specific strength of up to 48,000 kN·m/kg is the best of known materials, compared to high-carbon steel's 154 kN·m/kg.

Under excessive tensile strain, the tubes will undergo plastic deformation, which means the deformation is permanent. This deformation begins at strains of approximately 5% and can increase the maximum strain the tube undergoes before fracture by releasing strain energy.

CNTs are not nearly as strong under compression. Because of their hollow structure and high aspect ratio, they tend to undergo buckling when placed under compressive, torsional or bending stress.

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.

Note for Conveyor Belt
A belt conveyor consists of two or more pulleys, with a continuous loop of material - the conveyor belt - that rotates about them. One or both of the pulleys are powered, moving the belt and the material on the belt forward. The powered pulley is called the drive pulley while the unpowered pulley is called the idler. There are two main industrial classes of belt conveyors; Those in general material handling such as those moving boxes along inside a factory and bulk material handling such as those used to transport industrial and agricultural materials, such as grain, coal, ores, etc. generally in outdoor locations. Generally companies providing general material handling type belt conveyors do not provide the conveyors for bulk material handling. In addition there are a number of commercial applications of belt conveyors such as those in grocery stores.

The belt consists of one or more layers of material they can be made out of rubber. Many belts in general material handling have two layers. An under layer of material to provide linear strength and shape called a carcass and an over layer called the cover. The carcass is often a cotton or plastic web or mesh. The cover is often various rubber or plastic compounds specified by use of the belt. Covers can be made from more exotic materials for unusual applications such as silicone for heat or gum rubber when traction is essential.

Material flowing over the belt may be weighed in transit using a beltweigher. Belts with regularly spaced partitions, known as elevator belts, are used for transporting loose materials up steep inclines. Belt Conveyors are used in self-unloading bulk freighters and in live bottom trucks. Conveyor technology is also used in conveyor transport such as moving sidewalks or escalators, as well as on many manufacturing assembly lines. Stores often have conveyor belts at the check-out counter to move shopping items. Ski areas also use conveyor belts to transport skiers up the hill. A wide variety of related conveying machines are available, different as regards principle of operation, means and direction of conveyance, including screw conveyors, vibrating conveyors, pneumatic conveyors, the moving floor system, which uses reciprocating slats to move cargo, and roller conveyor system, which uses a series of powered rollers to convey boxes or pallets.

The longest belt in the world is in Western Sahara. It is 100 km long, from the phosphate mines of Bu Craa to the coast south of El-Aaiun. The longest single belt conveyor runs from Meghalaya in India to Sylhet in Bangladesh. It is 17 miles long and conveys limestone and shale. The Conveyor belt was manufactured in about 300 meter lengths and was joined together and installed on the conveyor at site. The job was carried out by NILOS India Pvt. Ltd. in Chennai India.

Conveyors are used as components in automated distribution and warehousing. In combination with computer controlled pallet handling equipment this allows for more efficient retail, wholesale, and manufacturing distribution. It is considered a labor saving system that allows large volumes to move rapidly through a process, allowing companies to ship or receive higher volumes with smaller storage space and with less labor expense.

Rubber Conveyor Belts are commonly used to convey items with irregular bottom surfaces, small items that would fall in between rollers, or bags of product that would sag between rollers. Belt conveyors are generally fairly similar in construction consisting of a metal frame with rollers at either end of a flat metal bed. The belt is looped around each of the rollers and when one of the rollers is powered (by an electrical motor) the belting slides across the solid metal frame bed, moving the product. In heavy use applications the beds which the belting is pulled over are replaced with rollers. The rollers allow weight to be conveyed as they reduce the amount of friction generated from the heavier loading on the belting. Belt conveyors can now be manufactured with curved sections which use tapered rollers and curved belting to convey products around a corner. These conveyor systems and are commonly used in postal sorting offices and airport baggage handling systems.

Note for Transmission Electron Microscopy
Transmission electron microscopy (TEM) is a microscopy technique whereby a beam of electrons is transmitted through an ultra thin specimen, interacting with the specimen as it passes through it. An image is formed from the electrons transmitted through the specimen, magnified and focused by an objective lens and appears on an imaging screen, a fluorescent screen in most TEMs, plus a monitor, or on a layer of photographic film, or to be detected by a sensor such as a CCD camera. The first practical transmission electron microscope was built by Albert Prebus and James Hillier at the University of Toronto in 1938 using concepts developed earlier by Max Knoll and Ernst Ruska.

Theoretically the maximum resolution that one can obtain with a light microscope has been limited by the wavelength of the photons that are being used to probe the sample and the numerical aperture of the system. Early twentieth century scientists theorized ways of getting around the limitations of the relatively large wavelength of visible light (wavelengths of 400–700 nanometers) by using electrons. Like all matter, electrons have both wave and particle properties (as theorized by Louis-Victor de Broglie), and their wave-like properties mean that a beam of electrons can be made to behave like a beam of electromagnetic radiation. Electrons are usually generated in an electron microscope by a process known as thermionic emission from a filament, usually tungsten, in the same manner as a light bulb, or by field emission. The electrons are then accelerated by an electric potential (measured in V, or volts) and focused by electrostatic and electromagnetic lenses onto the sample. The beam interacts variously with the sample due to differences in density or chemistry. The beam that is transmitted through the sample contains information about these differences, and this information in the beam of electrons is used to form an image of the sample.

Just as details of a light microscope sample can be enhanced by the use of stains, staining can be used to enhance differences in a sample for electron microscopy. Compounds of heavy metals such as osmium, lead or uranium can be used to selectively deposit heavy atoms in areas of the sample and to enhance structural detail by the dense nuclei of the heavy atoms scattering the electrons out of the optical path. The electrons that remain in the beam can be detected using a photographic film, or fluorescent screen among other technologies. So areas where electrons have been scattered in the sample can appear dark on the screen, or on a positive image due to this scattering.

The TEM is used heavily in both material science/metallurgy and the biological sciences. In both cases the specimens must be very thin and able to withstand the high vacuum present inside the instrument.

For biological specimens, the maximum specimen thickness is roughly 1 micrometre. To withstand the instrument vacuum, biological specimens are typically held at liquid nitrogen temperatures after embedding in vitreous ice, or fixated using a negative staining material such as uranyl acetate or by plastic embedding. Typical biological applications include tomographic reconstructions of small cells or thin sections of larger cells and 3-D reconstructions of individual molecules via Single Particle Reconstruction.

In material science/metallurgy the specimens tend to be naturally resistant to vacuum, but must be prepared as a thin foil, or etched so some portion of the specimen is thin enough for the beam to penetrate. Preparation techniques to obtain an electron transparent region include ion beam milling and wedge polishing. The focused ion beam (FIB) is a relatively new technique to prepare thin samples for TEM examination from larger specimens. Because the FIB can be used to micro-machine samples very precisely, it is possible to mill very thin membranes from a specific area of a sample, such as a semiconductor or metal. Materials that have dimensions small enough to be electron transparent, such as powders or nanotubes, can be quickly produced by the deposition of a dilute sample containing the specimen onto support grids. The suspension is normally a volatile solvent, such as ethanol, ensuring that the solvent rapidly evaporates allowing a sample that can be rapidly analysed.

Berkeley Lab is a U.S. Department of Energy national laboratory located in Berkeley, California. It conducts unclassified scientific research and is managed by the University of California.

In figure 1, Someday, nanoscale conveyor belts could expedite the atom-by-atom construction of the world’s smallest devices

In figure 2, A glimpse into the factory of the future. Four images, each taken 60 seconds apart, portray the rightward march of indium atoms along a carbon nantoube subjected to about two volts