Sapphire plays supporting role for nanotubes

Charles Q. Choi

Carbon nanotubes would make ideal connecting wires in advanced circuits if not for the painstaking effort required to line up each tiny, sticky, floppy strand. Now scientists have found that crystalline sapphire can automatically help guide nanotubes into the patterns needed to build transistors and to make flexible electronics. Electrical signals can flow more quickly through carbon nanotubes than through silicon, which in principle could lead to faster computers, explains Chongwu Zhou, an electrical engineer at the University of Southern California. Moreover, nanotubes can be as small as one-fifth the theoretical minimum size of conventional silicon circuitry.

To make nanotube circuits, scientists scatter nanotubes randomly and attach electrodes wherever they can, or else they try to grow nanotubes toward one another and later fabricate electrodes on them. Such efforts, though, are slow and inefficient, leading scientists to wonder if substrates existed that could naturally orient the tubes. After more than a year of experiments on various crystals, Zhou and his colleagues discovered that sapphire could achieve just that. Sapphire crystal is hexagonal, rising from a flat base, and the researchers found that most vertical slices of sapphire apparently expose constituent aluminum and oxygen atoms in layouts that promote the formation of nanotubes in orderly rows.

In the January Nano Letters, Zhou's team reported the creation of transistors with such aligned nanotubes. The researchers coated commercially available artificial sapphire with a cagelike protein called ferritin and flowed hydrocarbon gas over it while baking it. Iron within the protein catalyzed the growth of single-walled nanotubes from carbon supplied by the gas. Once the sapphire was covered with nanotubes, they could place the metal electrodes of the transistors wherever they wanted and remove the unwanted nanotubes with highly ionized oxygen gas.

Past carbon nanotube transistors were typically constructed atop silicon composites common in the electronics industry. The drawback was that the metal electrodes and the silicon interacted to suck up electrical charge, slowing down performance and raising power consumption. Zhou's strategy eliminates the parasitic drain because sapphire is electrically insulating, not semiconducting like silicon. The method is closely related to the so-called silicon-on-sapphire approach that IBM and other chipmakers have used to build specialized high-performance circuitry, "so we can borrow a lot of knowledge from the semiconductor industry," Zhou remarks.

When compared with other carbon nanotube electronics, these findings display the highest density of aligned nanotubes, at up to 40 per micron. Other methods manage only one to five, Zhou states. Nanotube density is crucial, because the more there are between electrodes, the more signals will be conducted. The scientists can control nanotube density by varying how much iron they employ within the ferritin.

The researchers could readily create flexible electronics from their nanotube transistors by baking a plastic film onto the nanotube transistors and then peeling off the strips, which hold on to the transistors. Carbon nanotube flexible electronics could "relatively easily" outperform the silicon-based versions currently used by industry, Zhou says, and he foresees its use in applications such as large flat-panel displays, vehicle windshields and smart cards. He also notes that such aligned nanotubes could act as sensors: attached molecules could send an electrical signal across the nanotubes if they reacted with cancer markers or other compounds.

These findings are "a very important result in resolving one of the most difficult problems related to carbon nanotube manufacture for integrated circuits," says Kang Wang, director of the Center on Functional Engineered Nano Architectonics at the University of California, Los Angeles. He points to one crucial hurdle to overcome: ensuring that all nanotubes made by this technique are semiconducting, because it currently produces a mix of metallic (fully conducting) and semiconducting ones.

Unquiet Ice

Robin E. Bell

Abundant liquid water newly discovered underneath the world's great ice sheets could intensify the destabilizing effects of global warming on the sheets. Then, even without melting, the sheets may slide into the sea and raise sea level catastrophically.

As our P-3 flying research laboratory skimmed above the icy surface of the Weddell Sea, I was glued to the floor. Lying flat on my stomach, I peered through the hatch on the bottom of the plane as seals, penguins and icebergs zoomed in and out of view. From 500 feet up everything appeared in miniature except the giant ice shelves—seemingly endless expanses of ice, as thick as the length of several football fields, that float in the Southern Ocean, fringing the ice sheets that virtually cover the Antarctic landmass. In the mid-1980s all our flights were survey flights: we had 12 hours in the air once we left our base in southern Chile, so we had plenty of time to chat with the pilots about making a forced landing on the ice shelves. It was no idle chatter. More than once we had lost one of our four engines, and in 1987 a giant crack became persistently visible along the edge of the Larsen B ice shelf, off the Antarctic Peninsula—making it abundantly clear that an emergency landing would be no gentle touchdown.

The crack also made us wonder: Could the ocean underlying these massive pieces of ice be warming enough to make them break up, even though they had been stable for more than 10,000 years?

Almost a decade later my colleague Ted Scambos of the National Snow and Ice Data Center in Boulder, Colo., began to notice a change in weather-satellite images of the same ice shelves that I had seen from the P-3. Dark spots, like freckles, began to appear on the monotonously white ice. Subsequent color images showed the dark spots to be areas of brilliant dark blue. Global climate change was warming the Antarctic Peninsula more rapidly than any other place on earth, and parts of the Larsen B ice surface were becoming blue ponds of meltwater. The late glaciologist Gordon de Q. Robin and Johannes Weertman, a glaciologist at Northwestern University, had suggested several decades earlier that surface water could crack open an ice shelf. Scambos realized that the ponding water might do just that, chiseling its way through the ice shelf to the ocean waters below it, making the entire shelf break up. Still, nothing happened.

Nothing, that is, until early in the Antarctic summer of 2001-2002. In November 2001 Scambos got a message he remembers vividly from Pedro Skvarca, a glaciologist based at the Argentine Antarctic Institute in Buenos Aires who was trying to conduct fieldwork on Larsen B. Water was everywhere. Deep cracks were forming. Skvarca was finding it impossible to work, impossible to move. Then, in late February 2002, the ponds began disappearing, draining - the water was indeed chiseling its way through the ice shelf. By mid-March remarkable satellite images showed that some 1,300 square miles of Larsen B, a slab bigger than the state of Rhode Island, had fragmented. Nothing remained of it except an armada of ice chunks, ranging from the size of Manhattan to the size of a microwave oven. Our emergency landing site, stable for thousands of years, was gone.

Suddenly the possibility that global warming might cause rapid change in the icy polar world was real. The following August, as if to underscore that possibility, the extent of sea ice on the other side of the globe reached a historic low, and summer melt on the surface of the Greenland ice sheet exceeded anything previously observed. The Greenland meltwaters, too, gushed into cracks and open holes in the ice known as moulins - and then, presumably, plunged to the base of the ice sheet, carrying the summer heat with them. There, instead of mixing with seawater, as it did in the breakup of Larsen B, the water probably mixed with mud, forming a slurry that was smoothing the way across the bedrock - "greasing," or lubricating, the boundary between ice and rock. But by whatever mechanism, the giant Greenland ice sheet was accelerating across its rocky moorings and toward the sea.

More recently, as a part of the investigations of the ongoing International Polar Year (IPY), my colleagues and I have been tracing the outlines of a watery "plumbing" system at the base of the great Antarctic ice sheets as well. Although much of the liquid water greasing the skids of the Antarctic sheets probably does not arrive from the surface, it has the same lubricating effect. And there, too, some of the ice sheets are responding with accelerated slippage and breakup.

Why are those processes so troubling and so vital to understand? A third of the world's population lives within about 300 feet above sea level, and most of the planet's largest cities are situated near the ocean. For every 150 cubic miles of ice that are transferred from land to the sea, the global sea level rises by about a 16th of an inch. That may not sound like a lot, but consider the volume of ice now locked up in the planet's three greatest ice sheets. If the West Antarctic ice sheet were to disappear, sea level would rise almost 19 feet; the ice in the Greenland ice sheet could add 24 feet to that; and the East Antarctic ice sheet could add yet another 170 feet to the level of the world's oceans: more than 213 feet in all. (For a comparison, the Statue of Liberty, from the top of the base to the top of the torch, is about 150 feet tall.) Liquid water plays a crucial and, until quite recently, underappreciated role in the internal movements and seaward flow of ice sheets. Determining how liquid water forms, where it occurs and how climate change can intensify its effects on the world's polar ice are paramount in predicting—and preparing for—the consequences of global warming on sea level.

Rumblings in the Ice

Glaciologists have long been aware that ice sheets do change; investigators simply assumed that such changes were gradual, the kind you infer from carbon 14 dating - not the kind, such as the breakup of the Larsen B ice shelf, that you can mark on an ordinary calendar. In the idealized view, an ice sheet accumulates snow - originating primarily in evaporated seawater - at its center and sheds a roughly equal mass to the ocean at its perimeter by melting and calving icebergs. In Antarctica, for instance, some 90 percent of the ice that reaches the sea is carried there by ice streams, giant conveyor belts of ice as thick as the surrounding sheet (between 3,500 and 6,500 feet) and 60 miles wide, extending more than 500 miles "upstream" from the sea. Ice streams moving through an ice sheet leave crevasses at their sides as they lurch forward. Near the seaward margins of the ice sheet, ice streams typically move between 650 and 3,500 feet a year; the surrounding sheet hardly moves at all.

But long-term ice equilibrium is an idealization; real ice sheets are not permanent fixtures on our planet. For example, ice-core studies suggest the Greenland ice sheet was smaller in the distant past than it is today, particularly during the most recent interglacial period, 120,000 years ago, when global temperatures were warm. In 2007 Eske Willerslev of the University of Copenhagen led an international team to search for evidence of ancient ecosystems, preserved in DNA from the base of the ice sheet. His group's findings revealed a Greenland that was covered with conifers as recently as 400,000 years ago and alive with invertebrates such as beetles and butterflies. In short, when global temperatures have warmed, the Greenland ice sheet has shrunk.

Today the snowfall on top of the Greenland ice cap is actually increasing, presumably because of changing climatic patterns. Yet the mass losses at its edges are big enough to tip the scales to a net decline. The elevation of the edges of the ice sheet is rapidly declining, and satellite measurements of small variations in the force of gravity also confirm that the sheet margins are losing mass. Velocity measurements indicate that the major outlet glaciers - ice streams bounded by mountains—are accelerating rapidly toward the sea, particularly in the south. The rumblings of glacial earthquakes have become increasingly frequent along the ice sheet's outlet glaciers.

Like the Greenland ice sheet, the West Antarctic ice sheet is also losing mass. And like the Greenland ice sheet, it disappeared in the geologically recent past - and, presumably, could do so again. Reed P. Scherer of Northern Illinois University discovered marine micro-fossils at the base of a borehole in the West Antarctic ice sheet that only form in open marine conditions. The age of the fossils showed that open-water life-forms might have lived there as recently as 400,000 years ago. Their presence implies that the West Antarctic ice sheet must have disappeared during that time.

Only the ice sheet in East Antarctica has persisted through the earth's temperature fluctuations of the past 30 million years. That makes it by far the oldest and most stable of the ice sheets. It is also the largest. In many places its ice is more than two miles thick, and its volume is roughly 10 times that of the ice sheet in Greenland. It first formed as Antarctica drew apart from South America some 35 million years ago and global levels of carbon dioxide declined. The East Antarctic ice sheet appears to be growing slightly in the interior, but observers have detected some localized losses in ice mass along the margins.

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