Getting lead atoms into carbon nanotubes.

A year ago, stuffing a buckytube with atoms to make an ultrathin wire seemed a highly speculative proposition. Now researchers have found that these microscopic, sealed carbon tubes, when heated in the presence of lead, can open up to suck in molten material.

"The nanotubes thus act as molds for the fabrication of... wires, some of which are less than 2 nanometers in diameter," P.M. Ajayan and Sumio lijima of the NEC Fundamental Research Laboratory in Tsukuba, Japan, report in the Jan. 28 NATURE.

This advance follows in rapid succession Iijima's discovery of buckytubes in 1991 (SN: 11/16/91, p. 310), the finding by his colleagues at NEC of a method for mass-producing these layered tubules (SN: 7/18/92, p.36), and the computer simulation-based prediction by Jeremy Q. Broughton and Mark R. Pederson of the Naval Research Laboratory in Washington, D.C., that these cylindrical, all-carbon molecules would act as molecular drinking straws (SN: 1//14/92, p.327).

"It's nice corroboration, so I'm rather excited by it:' Broughton says. "I think it's just the beginning of a whole new technology."

Ajayan and Iijima proceeded by depositing lead particles, from 1 to 15 nanometers in diameter, on the surfaces of carbon tubules. Then they heated the samples in air for about 30 minutes, keeping the temperature constant at approximately 400[degrees] C, which is above lead's melting point.

The researchers discovered that after the heating step, a small fraction of the tubes contained solid material. They also observed that the ends of the partially filled carbon tubes - originally capped -- were now open. However, when the samples were heated in a vacuum instead of in air, tube tips remained closed and little filling occurred.

"This suggests that opening of the tips involves a chemical reaction between the metal, oxygen, and the carbon that makes up the tubes:' the researchers note. "The outer and capped portions of the inner tubes are apparently attacked and destroyed during the heating in air, following which molten material is drawn inside by strong capillary forces."

"Clearly, it's a very delicate thing, because the lead is able to eat away the endcaps but not the rest of the tubes:' Broughton says.

Rodney S. Ruoff and his colleagues at SRI International in Menlo Park, Calif., also have been studying the possibility of introducing various elements into nanotubes. They plan to take a closer look at the effect of oxygen on the reactivity of tube ends. "This could allow preferential etching of the ends to make nanostraws:' Ruoff says.

The NEC researchers are as yet unsure whether the filling itself is pure lead or a solid compound formed by the reaction of lead with carbon and oxygen. Moreover, "the solidification of molten material in such constrained geometries could conceivably be very different from that in the bulk, and new phases might be formed," they say

Meanwhile, physicists Jae-Yel Yi and Jerzy Bernhole of North Carolina State University in Raleigh used a supercomputer simulation to show that boron or nitrogen atoms can be inserted directly into the carbon-atom sheets that make up buckytube walls. This finding suggests that buckytubes can be engineered to have certain characteristics, much as the addition of trace amounts of various elements to a semiconductor alters its electrical properties. Yi and Bernholc report their results in the Jan. 15 PHYSICAL REVIEW B.

However, the step from partially filled buckytubes to "quantum" wires so narrow that electrons must, in effect, pass down them in single file remains a large one. To start with, researchers have to deal with such problems as the lack of uniformity in buckytube sizes and the potential difficulty of separating filled from empty tubes.