Plumbing Carbon Nanotubes
Scientists have determined how to connect carbon nanotubes together like water pipes, a feat that may lead to a whole new group of bottom-up-engineered nanostructures and devices.
The researchers, from Japan's National Institute of Advanced Industrial Science and Technology, were able to "plumb" together nanotubes with similar or equal diameters using a technique they developed. They expect that their method could be used in the future to seamlessly join carbon nanotubes regardless of their diameters.
"Our method could allow longer carbon nanotubes to be created, and even nanotubes with multiple branches," the study's corresponding scientist, Chuanhong Jin, said to PhysOrg.com. "Such structures could have many applications, such as field-effect transistors or current lead-wires."
The work is described in a paper in Nature Nanotechnology.
Working through the eyes of a transmission electron microscope, which allowed them to watch the process as it occurred, Jin and his colleagues first split a single carbon nanotube by bridging it across two electrodes and applying a high current. This caused the middle section of the nanotube to become gradually narrower until it eventually split, resulting in two nanotubes with equal diameters and closed, or capped, ends.
The capped ends were moved near each other and the voltage across the electrodes was slowly raised from zero. At certain threshold values of voltage and current, the two nanotubes suddenly joined again. This process was so quick that Jin and his colleagues are as yet unsure of how it occurs.
The researchers found that they could repeat this split/join process on the same nanotube several times; so far, up to seven times.
The group also attempted to join carbon nanotubes with different diameters, but were not successful. In each case, at a certain threshold of voltage and current, an obvious deformation occurred on the cap of the larger nanotube. The nanotubes would then detach, pulling away from each other, and the cap structures of both nanotubes seemed to change, causing a shrinkage in length. Attempts to reposition and attach the nanotubes produced the same results.
"It seems intrinsically difficult to join two carbon nanotubes with entirely different diameters," says Jin.
The difficulties seem to arise from the nanotubes' "chiralities"-whether the carbon atoms are bonded in chains that run straight down the tube or chains that twist around it. Two nanotubes made from the same mother tube have the same chirality, but nanotubes with different diameters rarely do. This mismatch caused problems at the atomic level when the scientists attempted to force the tubes to merge.
But the scientists came up with a fix: inserting tungsten atoms between the two nanotubes to catalyze the joining process. Tungsten has long been known to help carbon atoms "graphitize," or arrange themselves into ordered structures, as are found in one crystal form of carbon, graphite. By moving the particle back and forth during the annealing process, the nanotubes joined seamlessly.
Nanotechnology with Carbon Nanotubes
pipes, bearings and springs are a few common ways that engineers have made use of the geometric shape known as a 'cylinder.' The utility of this shape is apparent in architecture, plumbing and mechanical devices. Carbon nanotubes are molecular cylinders that are rapidly extending our ability to fabricate nanoscale devices by providing molecular probes, pipes, wires, bearings and springs.
Their strength as structural supports comes from their sturdy molecular structure, which looks like what one would get if onecould roll a two dimensional sheet of graphite into a three dimensional cylinder. The limit to how long they can be is unknown, thus aerospace scientists are seriously considering using them as cables extending into space, an idea that is not possible with traditional ropes since they would break under their own weight. Furthermore, carbon nanotubes can easily be cut into sections as small as a few nanometers [1]. One of the first important applications of carbon nanotubes has been in the fabrication of sharp, strong and functionalized AFM probe tips [2].
The hollow nature of nanotubes allow them to function as pipes for transporting and molding atoms and molecules. Furthermore,the tubes come in insulating, semiconducting and conducting form, meaning that they can also be used as molecular wires and circuits [3]. Whats more, capillary induced filling of the nanotubes with other materials further extends the diversity of nanowires that can be fabricated [4]. The electronic properties of carbon nanotubes are directly related to their shape, making them an important Nano-Electromechanical System (NEMS). For example, the feasibility of a nanotube-based random access memory device with a memory density around 100 gigabytes/cm2 and an operation frequency around 100 gigahertz has recently been developed at Harvard University [5].
In addition to their high aspect ratio (meaning long and thin) and particle transport capabilities, carbon nanotubes can alsofunction as durable bearings and springs. Nanotubes can be fabricated in two forms: single-wall nanotubes (SWNT) or multi-wall nanotubes (MWNT). While a SWNT consists of only a single cylinder, a MWNT consists of several (between 2 and 30) concentric tubes, each with a specific diameter. Physicists at the University of California, Berkeley have recently demonstrated that a MWNT can act as a molecular bearing when one of the inner tubes rotates, or as a molecular spring when an inner tube is pulled out, causing the MWNT to stretch in a way similar to a telescope .
The stable arangement of atoms into cylindrical nanocrystals is not limited to carbon. The inert nature of boron nitride and tungsten disulfide nanotubes makes them particularly durable molecular components for NEMS.
Inorganic Nanotubes
The stability of the cylindrical form is not limited to carbon nanotubes. In 1992, scientists at the Weizmann Institute discovered crystalline molecular cylinders (nanotubes) composed of Tungsten and Sulfur [1] (figure). Thus began the subject of inorganic fullerene-like materials and nanotubes. Any of the numerous elements and compounds known to form stable two dimensional sheets (as well as several other elements) can be used for theoretical simulations. Predictions made by these simulations are continually being confirmed as nanotube engineers find new ways to roll those sheets into tubes.
Several methods exist for the synthesis of inorganic nanotubes. Each method may result in a different type, size and yield. The 'type' of tube refers to its atomic structure and chirality, 'Size' means the diameter and length of the tube and 'yield' refers to the purity of the product. A common method for synthesis of both inorganic and organic nanotubes is exposure to high temperatures by laser heating or an arc discharge [3]. Another technique is to substitute the atoms in an already fabricated tube by a substitution reaction .Template assisted synthesis is perhaps the most promising method, since it allows more precise control of the nanotube type
As the diversity of available nanotubes increases, nanoscientists are gaining access to a wide variety of molecular columns, pipes, bearings and springs. The mechanical properties of these tubes are controllable by electronic means, making them ideal components for Nano-Electromechanical Systems, otherwise known as nanomachines. Two of the best understood inorganic nanotubes.
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