Vial holding original white light quantum dots on the left and the enhanced quantum dots on the right. Image: Rosenthal Lab

With
the age of the incandescent light bulb fading rapidly, the holy grail
of the lighting industry is to develop a highly efficient form of
solid-state lighting that produces high quality white light.

 

One
of the few alternative technologies that produce pure white light is
white-light quantum dots. These are ultra-small fluorescent beads of
cadmium selenide that can convert the blue light produced by an LED into
a warm white light with a spectrum similar to that of incandescent
light. (By contrast, compact fluorescent tubes and most white-light LEDs
emit a combination of monochromatic colors that simulate white light).

Seven
years ago, when white-light quantum dots were discovered accidentally
in a Vanderbilt chemistry lab, their efficiency was too low for
commercial applications and several experts predicted that it would be
impossible to raise it to practical levels. Today, however, Vanderbilt
researchers have proven those predictions wrong by reporting that they
have successfully boosted the fluorescent efficiency of these
nanocrystals from an original level of three percent to as high as 45%.

Potential commercial applications

“Forty-five
percent is as high as the efficiency of some commercial phosphors which
suggests that white-light quantum dots can now be used in some special
lighting applications,” said Sandra Rosenthal, the Jack and Pamela Egan Chair of Chemistry, who directed the research which is described online in the Journal of the American Chemical Society. “The fact that we have successfully boosted their efficiency by more than 10 times also means that it should be possible to improve their efficiency even further.”

The
general measure for the overall efficiency of lighting devices is
called luminous efficiency and it measures the amount of visible light
(lumens) a device produces per watt. An incandescent light bulb produces
about 15 lumens/watt, while a fluorescent tubes put out about 100
lumens/watt. White light LEDs currently on the market range from 28 to
93 lumens/watt.

“We
calculate that if you combine our enhanced quantum dots with the most
efficient ultraviolet LED, the hybrid device would have a luminous
efficiency of about 40 lumens/watt,” reported James McBride,
research assistant professor of chemistry who has been involved in the
research from its inception. “There is lots of room to improve the
efficiency of UV LEDS and the improvements would translate directly into
a higher efficiencies in the hybrid.”

An accidental discovery

Quantum
dots were discovered in 1980. They are beads of semiconductor
material—the stuff from which transistors are made—that are so small
that they have unique electronic properties, intermediate between those
of bulk semiconductors and individual molecules. One of their useful
properties is fluorescence that produces distinctive colors determined
by the size of the particles. As the nanocrystal’s size shrinks the
light it emits shifts from red to blue. The Vanderbilt discovery was
that ultra-small quantum dots, containing only 60 to 70 atoms, emit
white instead of monochromatic light.

Scanning electron microscope image of a quantum dot that shows the individual atoms. Image: Rosenthal Lab

“These
quantum dots are so small that almost all of the atoms are on the
surface, so the white-light emission is intrinsically a surface
phenomena,” said Rosenthal.

One
of the first methods various groups used in the attempt to brighten the
nanocrystals was “shelling”—growing a shell around them made of a
different material, like zinc sulfide. Unfortunately, the shells
extinguished the white light effect and the shelled quantum dots
produced only colored light.

Chemists followed their noses

Following
a lead from some research done at the University of North Carolina, the
researchers decided to see if treating the quantum dots with metal
salts would have a brightening effect. They noticed that some of the
salts seemed to produce a small—10 to 20%—but noticeable improvement.

“They
were acetate salts and they smelled a bit like acetic acid,” said
McBride. “We knew that acetic acid binds to the quantum dots so we
decided to give it a try.”

The
decision to follow their nose proved to be fortunate. The acetic acid
treatment bumped up the quantum dots fluorescent efficiency from eight
percent to 20%!

Acetic
acid is a member of the carbocyclic acid family. So the researchers
tried the other members in the family. They found that the simplest and
most acidic member—formic acid, the chemical that ants use to mark their
paths—worked the best, pushing the efficiency as high as 45%.

The
brightness boost had an unexpected side effect. It shifted the peak of
the color spectrum of the quantum dots slightly into the blue. This is
ironic because the major complaint of white-light LEDs is that the light
they produce has an unpleasant blue tint. However, the researchers
maintain that they know how to correct the color-balance of the boosted
light.

The researchers’ next step is to test different methods for encapsulating the enhanced quantum dots.

Bright White Light Emission from Ultrasmall Cadmium Selenide Nanocrystals

2005 story about the original discovery: Quantum dots that produce white light could be the light bulb’s successor

Source: Vanderbilt University