26. Synthesis of Aligned Carbon Nanotubes Several applications (such as field-emission-based display), require that carbon nanotubes grow as highly aligned bunches, in highly ordered arrays, or located at specific positions. In that case, the purpose of the process is not mass production but controlled growth and purity, with subsequent control of nanotube morphology, texture, and structure. Generally speaking, the more promising methods for the synthesis of aligned nanotubes are based on CCVD processes, which involve molecular precursors as carbon source, and method of thermal cracking assisted by the catalytic activity of transition metal (Co, Ni, Fe) nanoparticles deposited onto solid supports.
49. SEM images of (a) 6-fold (b) 4-fold and (c) 2-fold symmetry nanobrushes made of an In2O3 core and ZnO nanowire brushes [4.107], and of (d) ZnO nanonails [4.108]
In covalent bonding, the atoms that bond share two electrons, regardless of whether they’re shaken or stirred; this sharing of electrons is what holds the atoms together in a molecule. If the ability of each atom to attract all those nega- tively charged electrons (called electronegativ- ity) is reasonably close (that is, if the difference in electronegativity is no more than 2), then they can form covalent bonds. Because the elec- tronegativity of carbon atoms is 2.5 (roughly in the midrange), they can form strong, stable, covalent bonds with many other types of atoms with higher or lower values.
Buckyballs were discovered through an inter- esting collaboration of researchers from two universities. Richard Smalley at Rice University was studying semiconductor materials. He had a device that shined a laser at a solid sample, vaporized part of it, and analyzed the clusters of atoms that formed in the vapor. Meanwhile, at the University of Sussex, Harry Kroto was attempting to reproduce a material found in deep space that generated specific molecular spectra from carbon atoms. Bob Curl, also from Rice University, was doing similar work. Curl ran into Kroto at a symposium and suggested he drop by Rice University because Smalley’s scientific instrumentation might be helpful in his work. Kroto dropped by Rice, and after seeing the work that Smalley was doing, he became interested in using that equipment to reproduce his carbon molecules. Time spent working with high-end scientific equipment is always at a premium, so Kroto had to wait about a year until the equipment was available. In August of 1985, Smalley, Kroto, Curl, and some graduate students performed a series of experiments producing carbon molecules and clusters. They found that under certain con- ditions, most of the molecules generated con- tained 60 carbon atoms. Voilà: buckyballs.
Source: Nanotechnology For Dummies Researchers found that by adding just a few percentage points of vaporized nickel nanoparticles to the vaporized carbon (using either the arc-discharge or laser-vaporization method to produce the vapor), they could make as many nanotubes as buckyballs — or even more. Here’s why: Carbon atoms dissolve in the metal nanoparticle. When the metal nanoparticle is filled to the brim with carbon atoms, carbon atoms start sweating onto the surface of the particle and bond together, growing a nanotube. When you anchor one end of the growing nanotube to the metal nanoparticle, it can’t close into the sphere shape of a buckyball. This also allows the nanotubes to incorporate many more carbon atoms than a buckyball.
We describe quantum dots, semiconducting nanocrystals roughly 5nm in size, in Chapter 8, where we show how they fit into the electrical world. They also have applications in the biological world as fluorescent tags. Quantum dots are nanometer-scale nanocrystals composed of a few hundred to a few thousand semiconductor atoms made out of bio-inert materials — meaning they are nonintrusive and nontoxic to the body. Additionally, unlike fluores- cent dyes (which tend to decompose and lose their ability to fluoresce), quantum dots maintain their integrity withstanding more cycles of excitation and light emission before they start to fade. Changing their size or composi- tion allows scientists to cater their optical properties — which means they can fluoresce in a multitude of colors. This effect is called quantum confine- ment (hence the name quantum dots) — they have quantized, discrete energy levels that are directly related to their size. Interestingly enough, quantum dots can even be tuned to fluoresce in differ- ent colors with the same wavelength of light. In other words, we can choose quantum dot sizes where the frequency of light required to make one group of dots fluoresce is an even multiple of the frequency required to make another group of dots fluoresce; both dots then fluoresce with the same wavelength of light. This allows for multiple tags to be tracked while using a single light source. A. Paul Alivisatos and his company (Quantum Dot Corporation) have used these concepts in their Qdot product — a quantum dot surrounded by an inorganic shell that amplifies its optical properties while protecting the dot from its environment. The Qdot can have a variety of attachments to its shell, allowing it to attach to specific cell walls — or even penetrate a cell and light it up from the inside. In the summer of 2003, Quantum Dot Corporation joined forces with Matsushita Electronic Industrial Co. (Panasonic) and Sumitomo Corporation Biosciences to develop advanced optical and image-processing technologies that utilize the Qdot. Products under this agreement are expected to generate revenue of more than $100 million per year for Quantum Dot Corporation by 2007. (Tiny product, big bucks.) An example of quantum dots in action involves targeting and imaging cancer cells. Researchers at Emory University, Georgia Tech, and Cambridge Research and Instrumentation have used quantum dots to identify tumors in mice. These quantum dots were made of cadmium selenide-zinc sulphide, each 5nm in diameter. They were coated with polymers to prevent both the body from attacking the quantum dot and to keep the dots themselves from leaking toxic cadmium and selenium ions. (Eek! Toxic! Read on. . . .) Finally, they attached antibodies to the outer shell that was first targeted and then attached themselves to a prostate tumor cell surface. The scientists injected the quantum dots into the circulatory system and the dots accumulated at the tumor, which could then be detected by fluorescence imaging. As an added benefit, these quantum dots have a large surface area allowing for a dual role of both diagnostics and therapy — the surface is big enough to attach both diagnostic and therapeutic antibodies to the surface. I know what you’re thinking — “Rich, what’s with all this about toxicity and eek-ing? We can’t have our imaging material causing more problems.” And, you’re right. The scientists have also thought of this and have done experiments mirroring the body’s environment to make sure quantum dots and their coatings are stable over a broad range of pH and salt conditions — even hydrochloric acid. And they passed with (ahem) flying colors. Carbon nanotubes also have this fluorescence quality. R. Bruce Weisman and his group at Rice University have determined that semiconducting carbon nanotubes fluoresce in the near-infrared spectrum and can be fine-tuned to different wavelengths by varying the nanotube diameters. The near-infrared spectrum is particularly important for biomedical applications — but nothing in the human body fluoresces in this region of the spectrum; in effect, human tissue is fairly transparent. It’s especially handy that nanotubes also maintain their fluorescent properties inside living cells, with no adverse effects to the cell. (For more on carbon nanotubes, see Chapter 4.) Down the road, such carbon nanotube technology may be used along the same lines as the quantum dots — you could end up wrapping the tubes with a specific protein, allowing them to target cells (such as tumors). Along these lines, a proposal by Michael Strano and his team at the University of Illinois- Urbana/Champaign (involving a glucose-detection optical sensor) looks espe- cially promising. Here, nanotubes are wrapped in a glucose oxidase and placed inside a small, porous capillary (200 microns across by 1cm in length). The capillary pores are only big enough for glucose to penetrate; once through, the glucose promptly reacts with the oxidase solution — changing the fluorescence properties of the nanotubes. This capillary is subsequently inserted just underneath the skin, but within range of being able to detect the near-infrared flouresence. Imagine a patient with diabetes wearing a watch that periodically checks the fluorescence/glucose and sounds an alert if levels are too low or too high — all without needles Unlike quantum dots, nanotubes don’t contain heavy metals, so they don’t raise any toxicity issues. Additionally, nanotubes can be fine-tuned to very narrow wavelengths, providing fluorescence in a greater number of wave- lengths, giving us greater flexibility (in other words, more colors to our palette). Such properties may give nanotubes the advantage among products marketed as laboratory imaging markers — we’ll have to see whether they can squeak by to take the lead from quantum dots.
