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After earning a degree in physics at Tohoku University in Sendai, Japan, Sumio Iijima moved to Arizona State University as a post-doctoral associate where he initiated high-resolution transmission electron microscopy (HRTEM) (1970-1982). Using the technique, he has brought a new type of information of local atomic structures of crystals into condensed matter physics, solid state chemistry, crystallography, mineralogy and materials science. Ample experiences with the different types of materials including nanostructures of carbon materials have led him to discover carbon nanotubes in later years. In these days the technique has been known as the most powerful one in the research fields of nano-materials science and nanotechnology. In 1982 he returned to Japan and worked for 5 years on a national ERATO project on nano-particles, then joined the NEC fundamental research laboratories. In 1991 he discovered carbon nanotubes that have initiated nano-materials science and nanotechnology and has being attracted world-wide researchers in academia and industry. Following the discovery, he has been honored with numerous awards and prizes that include: Franklin Medal in physics (2001), Agilent Europhysics award (2002), Balzan Prize (2007, Italy-Switzerland), Kavli Prize (Norway, 2008), Prince of Asturias Award (Spain, 2008), Order of Culture (Japan, 2009), He is members of Foreign Associate of the National Academy of Science (USA. 2007), the Norwegian Academy of Science and Letters (2009), member of Japan Academy (2010) and foreign member of Chinese Academy of Sciences (2011).
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ELECTRIC EFFECTS IN NANOTUBE GROWTH
DANIEL T. COLBERT and RICHARDE E. SMALLEY, Rice Quantum Institute and Departments of Chemistry and Physics, MS 100, Rice University, Houston, TX 77251-1892, U.S.A. (Received 3 April 1995; accepted 7 April 1995) Abstract
We present experimental evidence that strongly supports the hypothesis that the electric field of the arc plasma is essential for nanotube growth in the arc by stabilizing the open tip structure against closure. By controlling the temperature and bias voltage applied to a single nanotube mounted on a macroscopic electrode, we find that the nanotube tip closes when heated to a temperature similar to that in the arc unless an electric field is applied. We have also developed a more refined awareness of “open” tips in which adatoms bridge between edge atoms of adjacent layers, thereby lowering the exothermicity in going from the open to the perfect dome-closed tip. Whereas realistic fields appear to be insufficient by themselves to stabilize an open tip with its edges completely exposed, the field-induced energy lowering of a tip having adatom spot-welds can, and indeed in the arc does, make the open tip stable relative to the closed one. Key Words Nanotubes electric field arc plasma 1. INTRODUCTION
As recounted throughout this special issue, significant advances in illuminating various aspects of nanotube growth have been made[1,2] since Iijima’s eventful discovery in 1991; [3] these advances are crucial to gaining control over nanotube synthesis, yield, and properties such as length, number of layers, and helicity. The carbon arc method Iijima used remains the principle method of producing bulk amounts of quality nanotubes, and provides key clues for their growth there and elsewhere. The bounty of nanotubes deposited on the cathode (Ebbesen and Ajayan have found that up to 50% of the deposited carbon is tubular[4]) is particularly puzzling when one confronts the evidence of Ugarte[5] that tubular objects are energetically less stable than spheroidal onions. It is largely accepted that nanotube growth occurs at an appreciable rate only at open tips. With this constraint, the mystery over tube growth in the arc redoubles when one realizes that the cathode temperature (~3000°C) is well above that required to anneal carbon vapor to spheroidal closed shells (fullerenes and onions) with great efficiency. The impetus to close is, just as for spheroidal fullerenes, elimination of the dangling bonds that unavoidably exist in any open structure by incorporation of pentagons into the hexagonal lattice. Thus, a central question in the growth of nanotubes in the arc is: How do they stay open? One of us (RES) suggested over two years ago[6] that the resolution to this question lies in the electric field inherent to the arc plasma. As argued then, neither thermal nor concentration gradients are close to the magnitudes required to influence tip annealing, and trace impurities such as hydrogen, which might keep the tip open, should have almost no chemisorption residence time at 3000°C. The fact that well-formed nanotubes are found only in the cathode deposit, where the electric field concentrates, and never in the soots condensed from the carbon vapor exiting the arcing region, suggest a vital role for the electric field. Furthermore, the field strength at the nanotube tips is very large, due both to the way the plasma concentrates most of the potential drop in a very short distance above the cathode, and to the concentrating effects of the field at the tips of objects as small as nanotubes. The field may be on the order of the strength required to break carbon-carbon bonds, and could thus dramatically effect the tip structure. In the remaining sections of this paper, we describe the experimental results leading to confirmation of the stabilizing role of the electric field in arc nanotube growth. These include: relating the plasma structure to the morphology of the cathode deposit, which revealed that the integral role of nanotubes in sustaining the arc plasma is their field emission of electrons into the plasma; studying the field emission characteristics of isolated, individual arc-grown nanotubes; and the discovery of a novel production of nanotubes that significantly alters the image of the “open” tip that the arc electric field keeps from closing. 2. NANOTUBES AS FIELD EMITTERS
Defects in arc-grown nanotubes place limitations on their utility. Since defects appear to arise predominantly due to sintering of adjacent nanotubes in the high temperature of the arc, it seemed sensible to try to reduce the extent of sintering by cooling the cathode better[2]. The most vivid assay for the extent of sintering is the oxidative heat purification treatment of Ebbesen and coworkers [7], in which amorphous carbon and shorter nanoparticles are etched away before nanotubes are substantially shortened. Since, as we proposed, most of the nanoparticle impurities originated as broken fragments of sintered nanotubes, the amount of remaining material reflects the degree of sintering. Our examinations of oxygen-purified deposits led to construction of a model of nanotube growth in the arc in which the nanotubes play an active role in sustaining the arc plasma, rather than simply being a passive product[2]. Imaging unpurified nanotube-rich arc deposit from the top by scanning electron microscopy (SEM) revealed a roughly hexagonal lattice of 50-micron diameter circles spaced ~50 microns apart. After oxidative treatment the circular regions were seen to have etched away, leaving a hole. More strikingly, when the deposit was etched after being cleaved vertically to expose the inside of the deposit, SEM imaging showed that columns the diameter of the circles had been etched all the way from the top to the bottom of the deposit, leaving only the intervening material. Prior SEM images of the column material (zone 1) showed that the nanotubes there were highly aligned in the direction of the electric field (also the direction of deposit growth), whereas nanotubes in the surrounding region (zone 2) lay in tangles, unaligned with the field [2]. Since zone 1 nanotubes tend to be in much greater contact with one another, they are far more susceptible to sintering than those in zone 2, resulting in the observed preferential oxidative etch of zone 1. These observations consummated in a growth model that confers on the millions of aligned zone 1 nanotubes the role of field emitters, a role they play so effectively that they are the dominant source of electron injection into the plasma. In response, the plasma structure, in which current flow becomes concentrated above zone 1, enhances and sustains the growth of the field emission source–that is, zone 1 nanotubes. A convection cell is set up in order to allow the inert helium gas, which is swept down by collisions with carbon ions toward zone 1, to return to the plasma. The helium flow carries unreacted carbon feedstock out of zone 1, where it can add to the growing zone 2 nanotubes. In the model, it is the size and spacing of these convection cells in the plasma that determine the spacing of the zone 1 columns in a hexagonal lattice. 3. FIELD EMISSION FROM AN ATOMIC WIRE
Realization of the critical importance played by emission in our arc growth model added impetus to investigations already underway to characterize nanotube field emission behavior in a more controlled manner. We had begun working with individual nanotubes in the hope of using them as seed crystals for controlled, continuous growth (this remains an active goal). This required developing techniques for harvesting nanotubes from arc deposits, and attaching them with good mechanical and electrical connection to macroscopic manipulators[2,8,9]. The resulting nano-electrode was then placed in a vacuum chamber in which the nanotube tip could be heated by application of Ar+-laser light (514.5 nm) while the potential bias was controlled relative to an opposing electrode, and if desired, reactive gases could be introduced. Two classes of emission behavior were found. An inactivated state, in which the emission current increased upon laser heating at a fixed potential bias, was consistent with well understood thermionic field emission models. Figure la displays the emission current as the laser beam is blocked and unblocked, revealing a 300-fold thermal enhancement upon heating. Etching the nanotube tip with oxygen while the tube was laser heated to 1500°C and held at –75 V bias produced an activated state with exactly the opposite behavior, shown in Fig. 2b; the emission current increased by nearly two orders of magnitude when the laser beam was blocked! Once we eliminated the possibility that species chemisorbed on the tip might be responsible for this behavior, the explanation had to invoke a structure built only of carbon whose sharpness would concentrate the field, thus enhancing the emission current. As a result of these studies[9], a dramatic and unexpected picture has emerged of the nanotube as field emitter, in which the emitting source is an atomic wire composed of a single chain of carbon atoms that has been unraveled from the tip by the force of the...
