Diamondoids May be a TV's Best Friend

Nanometer-scale diamonds from petroleum could find use in displays

PHOTO: Nick Melosh and X.N. Shen

12 June 2007--As if oil weren’t valuable enough on its own, it turns out that there are nanometer-scale diamonds dissolved in every drop of crude. Now scientists at Stanford University and an R&D subsidiary of oil giant Chevron Corp. San Ramon, Calif., have discovered that certain of these nanoscale diamonds emit electrons with a shockingly narrow distribution of energies—the electron analogue of a pure color of light. Pure electron color makes it easier to control the pixels in field-emission displays--a next generation TV technology--and to make fine etchings on microchips, according to Stanford scientists.

Diamondoids are cagelike molecules found in petroleum, which have the basic chemical structure of diamonds, but are coated on the outside in hydrogen molecules. Until a few years ago, high-level diamondoids--those comprised of four or more joined cages--were extremely rare, and nearly impossible to create in the lab, as they require tremendous heat and pressure over a long period of time to develop. However, in 2004 Chevron announced it had isolated gram quantities of them.

These ”higher” nanodiamonds share a diamond’s mechanical and insulating properties, which scientists hope can be imparted somewhat to other materials with which they’re mixed. But their size also lends them some differences from ordinary diamonds. ”The thing about nanoscience and nanotechnology is that what you hope to find are material properties that are better than, or not there in the bulk,” says Zhi-Xun Shen, a professor of physics and applied physics and head of Stanford’s diamondoid research team.

In addition to their unique mechanical and thermal properties, diamonds exhibit a peculiar property at their surface, called negative-electron affinity. Basically, atoms there will emit electrons even when they are cold, as opposed to metals like those used in a TV’s picture tube, which must be hot to ”boil off” electrons. Shen says many researchers have longed to exploit this property of diamonds. However, there have been a number of hindrances, notably that it’s difficult to inject electrons into diamond and get them to move through the highly insulating material once they’re in there.

Diamondoid’s chemical structure and minuscule size gets around those problems. But first you have to get the nanoparticles in a position to spit out electrons. For Shen’s team that meant getting a solution of diamondoids to assemble themselves on a metal substrate in a one-molecule thick layer, a trick they reported in the 11 June issue of Science . Nick Melosh, an assistant professor of materials science and engineering who co-directs the Stanford-Chevron program on diamondoids, did this by combining a four-cage dioamondoid, tetramantane, with a sticklike molecule that would bind at one end to the diamondoid and at the other end to the metal surface.

Once the diamondoids had coated the metal, Shen trained a beam of X-rays on them, and out popped a stream of electrons. ”In fact, we were surprised when we saw the data,” says Shen. And for the moment it defies explanation. ”Basically, it showed that the electrons emitted from the solid came out almost as a single energy.”

Applications for such nano-emitters are a ways off, but Shen, Melosh, and Chevron have some ideas. For one, a pure color of electrons is easier to steer using electric and magnetic fields. That’s what’s done, with great accuracy, in electron beam lithography, a technique used to write nanometer-scale circuit features onto experimental microchips. But perhaps the best mass-market use may be in a type of TV technology called field-emission displays.

Field-emission displays (FEDs) have been in the works for over a decade, because they are expected to be more power efficient than other flat-screen displays while delivering the brightness and clarity of a cathode ray tube. FEDs, are in fact, like thousands of miniaturized CRTs all built on one flat pane of glass. Each minitube makes up a subpixel (red, green, or blue) that together form a single pixel on the television screen. In each subpixel, electrons pour forth from a cathode toward an anode, striking a phosphor layer in between, and causing the phosphor to glow.

In prototype FEDs, the cathodes are the pointed tips of nanometer-scale structures such as carbon nanotubes [see ”Watching the Nanotube,” IEEE Spectrum , September 2003]. However, the problem with nanotubes is that there are many kinds, and there are significant problems in consistently producing large amounts of just one type. Diamondoids, on the other hand, are easier to isolate consistently, says Shen. What’s more, the Stanford team has found a way to make them self-assemble into a single layer, which is just what’s needed to build a display.

FEDs have had a troubled history, being picked up and abandoned by display makers several times. Toshiba Corp., in Tokyo, planned to roll out the first surface-conduction electron-emitter displays (SEDs), based on a technology similar to FEDs, later this year. But the launch has been mired in legal battles over intellectual property rights and hampered by a lack of manufacturing technology. Meanwhile, other companies working on FED TVs have been struggling with carbon nanotube production.

Might diamondoids be the missing piece needed to solve the FED puzzle?  Stanford’s Shen says it’s too early to tell. The substrates he used in his experiments were silver and gold, hardly the stuff of a mass-market TV. The diamondoid layer ”is cheap and scalable, so it is very attractive,” says Shen. But for now diamond vision will have to wait.

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