Linear Arrays
A growing number of electronics researchers are fascinated by the
potential of single-walled carbon nanotubes—one-atom thick sheets of
graphite rolled into seamless cylinders. These tiny objects have attractive
electrical properties and physical features. But incorporating single-walled carbon nanotubes into scalable ICs has proven to be a challenge,
largely because of difficulties in manipulating and positioning the tiny
objects and in achieving sufficient current outputs.
Engineers at the University of Illinois, Lehigh University, and Purdue
University studied this problem and developed an approach that uses
dense arrays of aligned and linear nanotubes as a thin-film semiconductor material. According to John Rogers, a University of Illinois professor of
materials science and engineering, the nanotube arrays can be transferred to plastic and other unusual substrates.
To create the arrays, the researchers begin with a wafer of single-crystal quartz, upon which they deposit thin strips of iron nanoparticles. The
iron acts as a catalyst for the growth of carbon nanotubes by chemical
vapor deposition. As the nanotubes grow past the iron strips, they lock
onto the quartz crystal, which then aligns their growth ().
According to William Dichtel, a project researcher, the new memory is
based on a series of perpendicular nanowires, similar to a tic-tac-toe
board, with 400 bottom wires and another 400 top wires crossing the bottom (). Sitting at each crossing of the tic-tac-toe structure, and serving as the storage elements, are approximately 300 bistable rotaxane
molecules. These molecules can be switched between two different
states. Each crossbar junction can be addressed individually by controlling the voltages applied to the appropriate top and bottom wires, forming
a bit at each nanowire crossing.
A rotaxane is a molecule with a dumbbell-shaped component, consisting of a rod section and two stoppers, encircled by a ring. It has the potential to be a molecular abacus. The molecule can act as a switch by inducing the ring to slide from one side of the rod to the other. "It's not just a
simple storage of charge, like most of today's memory devices use," Dichtel said. "There are actually molecules which respond to applied voltage
and the molecules switch from one state to another."
The bistable rotaxane molecules used in the crossbar memory can be
switched at very modest voltages from an "off" (low conductivity) state
to an "on" (high conductivity) state. The stoppers for the rotaxane molecules are designed to allow the molecules to be organized into layers
that are a single molecule thick, after which they're incorporated into
the memory device. The 160-kbit molecular memory was fabricated at a
density of 100 billion bits per square centimeter—a density predicted by
Ditchel for commercial memory devices by approximately 2020.
"One of the most exciting features of this research is that it moves
beyond the testing of molecular electronic components in individual, nonscalable device formats and demonstrates a large, integrated array of
working molecular devices," Dichtel said.
The resulting linear arrays consist of
hundreds of thousands of nanotubes,
each approximately 1 nm in diameter,
and up to 300 µm long. The nanotubes
are spaced approximately 100 nm apart.
The arrays function as an effective thin-film semiconductor material in which a
charge moves independently through
each of the nanotubes. In this configuration, the nanotubes can be integrated
into electronic devices in a straightforward fashion by conventional chip-processing techniques.
"Our approach has been not to figure
out how to grow electronically identical
tubes everywhere, or to position or control the location of any individual tube,
Rogers said, "but to instead make a
thin form that consists of very well
aligned arrays of individual tubes." The
researchers used the technique to build
and test a variety of transistors and logic gates, as well as to compare the properties of nanotube arrays versus individual nanotubes.
Rogers notes that nanotube arrays
aren't likely to replace silicon, but could
be added to a silicon chip and exploited
for special purposes. Higher-speed
operation, higher power capacity, and
linear behavior are a few options for
enhanced functionality, and they can
also be used for applications that silicon can't easily support, such as flexible devices.
Nano Now
Mamikunian believes that nanotechnology, and the new and enhanced products the field promises to generate, will
open an almost endless array of fresh
opportunities for the electronics industry. Furthermore, the field's benefits
won't only be limited to major industrialized nations. "A lot of developing countries are looking at nanotechnology as a way to leapfrog from where they are
today to become more of a player in the
larger state of commercialization and
manufacturing," he said.
According to Mamikunian, enough
progress is being made in nanotechnology research that the field is on the
verge of becoming mainstream. "Over
the next five years or so you'll begin to
see this transformation where the
terms ‘nano' and ‘nanotechnology' will
fade away and it will be just ‘technology,'" he said.
Maynard agrees with Mamikunian,
adding, "One thing is certain, and that is
that the next generation of electronics
has got to rely on our ability to engineer
at the nanoscale."