by August 11, 2001 0 comments



From amongst the possible new ways that can be used to deliver faster microelectronics, molecular electronics holds a lot of promise. Carbon nanotubes (NTs) are a prime candidate here because of their unique properties. They are very thin (as little as 1 nanometer in diameter), and their length can be controlled up to many microns. They are also extremely strong mechanically, very stable thermally and chemically, conduct heat very well, and most importantly, depending on the orientation of the carbon hexagons with respect to the tube’s axis, can be either metals or semiconductors. 

Metallic NTs can carry extremely large current densities, while semiconducting NTs can be electrically switched on and off by the field generated by a gate electrode to produce field-effect transistors (FETs). Semiconducting NTs have band-gaps (denoted by Eg) that are inversely proportional to their diameter. 

There are two types of nanotubes: single-walled nanotubes (SWNTs), which have one shell of carbon atoms; and multi-walled nanotubes (MWNTs), which consist of multiple, nested carbon tubes. Unfortunately, no methods exist for reliably preparing only metallic or semiconducting nanotubes either by selective synthesis or through post-synthesis separation. This lack of control, compounded by the single-walled nanotubes’ tendency to bundle together to form ropes, has been the main roadblock towards any nanotube-based electronic technology.

Our work demonstrates a reliable method for permanently modifying individual MWNTs and SWNT ropes to tailor their properties. Carbon NTs can withstand remarkable current densities (102—103 times higher than normal metals). At high enough currents, however, NTs ultimately fail. We found that we can control this breakdown to remove individual shells one at a time from MWNTs, or selectively destroy metallic tubes in an SWNT bundle.

Modifying MWNTs

The different shells of an MWNT have different electrical properties. This opens the possibility of selecting and using only that MWNT shell that has the desired properties.

MWNTs break down in air at a certain threshold power, through the rapid oxidation of the outer-most carbon shell. During breakdown, the current flowing through the MWNT shows a step-like behavior with steps of surprising constancy, corresponding to the breakdown of individual shells. By controlling the shell-by-shell removal process, we can generate tubes with the desired outer-shell characteristics, metallic or semiconducting, and most importantly, by selecting the diameter of the outer shell, we can obtain the desired band-gap Eg (as discussed before,
Eg is inversely proportional to the diameter).

Modifying SWNTs

If an SWNT rope is used to fabricate a FET, the metallic tubes in it cannot be switched by the gate field and will dominate the transport properties of the device. We have solved this by using selective breakdown. Unlike in MWNTs, in a thin rope each SWNT can connect independently to the external electrodes. Thus, an MWNT rope may be modeled as independent, parallel conductors with total conductance G(Vg) = Gm + Gs(Vg), where Gm is the contribution of the metallic NTs and Gs is the gate-dependent conductance of the semiconducting NTs. In addition, multiple SWNTs within a rope are in contact with air, a potentially oxidizing environment, so many nanotubes can fail at once, unlike the uniform, shell-by-shell failure observed in MWNTs. Finally, the SWNTs within a small rope do not electrostatically shield each other as effectively as the concentric shells of an MWNT. A gate electrode can be used to effectively deplete the electrical carriers (electrons or holes) in the semiconducting SWNTs within the rope, turning the semiconducting tubes into insulators. In this state, current-induced oxidation can be directed solely at the metallic SWNTs within the rope.

We have taken advantage of these properties to selectively destroy the metallic nanotubes in SWNT ropes, while preserving the semiconducting SWNTs. Also, since rope formation doesn’t inhibit fabrication of FETs, concentrated solutions of nanotubes, which allow dense arrays of FETs to be fabricated, can be used. To make these arrays, SWNT ropes are deposited on an oxidized silicon wafer, and then an array of source, drain, and side-gate electrodes is fabricated lithographically on top of the ropes. The concentration of the tubes is pre-adjusted so that on the average there is one rope bridging the source and drain. No special arrangement or orientation of the nanotubes is required. The back gate (the silicon wafer itself) is used to deplete the semiconducting tubes, followed by the application of a stress voltage to destroy the metallic tubes in the ropes, thus producing carbon NT field-effect transistors (NT-FETs). This sidesteps the need for selective nanotube synthesis and assembly and has allowed us to fabricate dense arrays of nanotube
FETs. 

Carbon Origins

What are carbon nanotubes?

Carbon nanotubes are fullerene-related structures that consist of graphene cylinders closed at either end with caps containing pentagonal rings. They were discovered in 1991 by the Japanese electron microscopist Sumio Iijima who was studying the material deposited on the cathode during the arc-evaporation synthesis of fullerenes. He found that the central core of the cathodic deposit contained a variety of closed graphitic structures including nanoparticles and nanotubes, of a type that had never previously been observed. A short time later, Thomas Ebbesen and Pulickel Ajayan, from Iijima’s lab, showed how nanotubes could be produced in bulk quantities by varying the arc-evaporation conditions. 

Single-layer nanotubes and nanotube ropes.

A major event in the development of carbon nanotubes was the synthesis in 1993 of single-layer nanotubes. The standard arc-evaporation method produces only multi-layered tubes. It was found that adding metals such as cobalt to the graphite electrodes resulted in extremely fine tubes with single-layer walls.

An alternative method of preparing single-walled nanotubes was described in 1996. This involved the laser-vaporization of graphite, and resulted in a high yield of single-walled tubes with unusually uniform diameters. These highly uniform tubes had a greater tendency to form aligned bundles than those prepared using arc-evaporation. These bundles were christened nanotube “ropes”>

Extracted from www.rdg.ac.uk/~scsharip/tubes.htm 

This is an important step toward the realization of electronic equipment such as switches and interconnects using carbon nanotubes as key elements. In short, we have developed a technique based on nanotube modification by electrical breakdown that allows the fabrication of NT-FETs from ropes of SWNTs containing both metallic and semiconducting nanotubes without the need to separate the two types. Using this technique, we demonstrated an approach that allows the fabrication of dense arrays of NT-FETs using ropes of SWNTs without the need for pre-alignment or orientation of the nanotubes on the electrodes. We have also demonstrated shell-by-shell breakdown of MWNTs, learned to control it, and used it to determine the nature of the individual shells of an MWNT. Finally, by taking advantage of the dependence of the band-gap of semiconducting shells on their diameter, we produced FETs with desired band-gaps.

Phaedon Avouris is manager, Nanometric Scale Science and Technology at IBM’s T J Watson Research Center

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