Armchair-enriched batches of nanotubes show their colors in an array of varying types. The vial at left is a mix of nanotubes straight from the furnace, suspended in liquid. The vials at right show nanotubes after separation through ultracentrifugation. Excitons absorb light in particular frequencies that depend on the diameter of the tube; the mix of colors not absorbed are what the eye sees. Image: Erik Hároz/Rice University |
Rice University researchers have figured out
what gives armchair nanotubes their unique bright colors: hydrogen-like objects
called excitons.
Their findings appear in the online edition of the Journal of the American Chemical Society.
Armchair carbon nanotubes—so named for the “U”-shaped
configuration of the atoms at their uncapped tips—are 1D metals and have no
band gap. This means electrons flow from one end to the other with little resistivity,
the very property that may someday make armchair quantum wires possible.
The Rice researchers show armchair nanotubes absorb light like
semiconductors. An electron is promoted from an immobile state to a conducting
state by absorbing photons and leaving behind a positively charged
“hole,” said Rice physicist Junichiro Kono. The new electron-hole
pair forms an exciton, which has a neutral charge.
“The excitons are created by the absorption of a particular wavelength
of light,” said graduate student and lead author Erik Hároz. “What
your eye sees is the light that’s left over; the nanotubes take a portion of
the visible spectrum out.” The diameter of the nanotube determines which
parts of the visible spectrum are absorbed; this absorption accounts for the
rainbow of colors seen among different batches of nanotubes.
Scientists have realized that gold and silver nanoparticles could be
manipulated to reflect brilliant hues—a property that let artisans who had no
notions of “nano” create stained glass windows for medieval
cathedrals. Depending on their size, the particles absorbed and emitted light
of particular colors due to a phenomenon known as plasma resonance.
In more recent times, researchers noticed semiconducting nanoparticles, also
known as quantum dots, show colors determined by their size-dependent band
gaps.
But plasma resonance happens at wavelengths outside the visible spectrum in
metallic carbon nanotubes. And armchair nanotubes don’t have band gaps.
Kono’s laboratory ultimately determined that excitons are the source of
color in batches of pure armchair nanotubes suspended in solution.
The results seem counterintuitive, Kono said, because excitons are
characteristic of semiconductors, not metals. Kono is a professor of electrical
and computer engineering and of physics and astronomy.
While armchair nanotubes don’t have band gaps, they do have a unique
electronic structure that favors particular wavelengths for light absorption,
he said.
“In armchair nanotubes, the conduction and valence bands touch each
other,” Kono said. “The one-dimensionality, combined with its unique
energy dispersion, makes it a metal. But the bands develop what’s called a van
Hove singularity,” which appears as a peak in the density of states in a
1D solid. “So there are lots of electronic states concentrated around this
singularity.”
Exciton resonance tends to occur around these singularities when hit with
light, and the stronger the resonance, the more distinguished the color.
“It’s an unusual quality of these particular 1D materials that these
excitons can actually exist,” Hároz said. “In most metals, that’s not
possible; there’s not enough Coulomb interaction between the electron and the
hole for an exciton to be stable.”
The new paper follows on the heels of work by Kono and his team to create
batches of pure single-walled carbon nanotubes through ultracentrifugation. In
that process, nanotubes were spun in a mix of solutions with different
densities up to 250,000 times the force of gravity. The tubes naturally
gravitated toward separated solutions that matched their own densities to
create a colorful “nano parfait.”
As a byproduct of their current work, the researchers proved their ability
to produce purified armchair nanotubes from a variety of synthesis techniques.
They now hope to extend their investigation of the optical properties of
armchairs beyond visible light. “Ultimately, we’d like to make one
collective spectrum that includes frequency ranges all the way from ultraviolet
to terahertz,” Hároz said. “From that, we can know, optically, almost
everything about these nanotubes.”
