UC Berkeley Chemists Find Reliable Way To Grow Quantum Rods And Pack Them Into Microscopic Solar Cells And Leds

A report this week by chemists at the University of California, Berkeley, details how to make quantum rods of a reliable size and get them to pack together. The quantum rods can be used as active elements in light-emitting diodes (LEDs) and solar cells.

"This is the first time anyone has gotten control of semiconductor rod growth," said Paul Alivisatos, a professor of chemistry at UC Berkeley and a member of the Materials Sciences Division of Lawrence Berkeley National Laboratory. "These quantum rods can be used as components in any number of devices. One of our long-term projects is to make an effective and low-cost photovoltaic device."

These crystals, more properly known as nanocrystals because of their nanometer or billionths-of-a-meter size, are chemically pure clusters of from 100 to 100,000 atoms. Because of their small size, they exhibit unusual properties predicted by quantum mechanics.

These properties include emitting a single color of light when zapped by a laser, with the color depending on the size of the nanocrystal. A two-nanometer quantum dot flashes green; a five-nanometer dot emits red. This property makes them ideal as markers or tracers, like the dyes now used to stain cells or the tracers used to follow processes in living cells.

Alivisatos is part of UC Berkeley's Health Sciences Initiative, a research effort that draws scientists from both the physical and biological sciences into the search for solutions to today's major health problems.

A pioneer in the realm of nanocrystals, Alivisatos co-founded a company last year - Quantum Dot Corp. - to develop nanocrystals into biological markers for scientists and doctors alike.

"There is a need for looking at many channels of information at once so that biologists can follow many different proteins as they move around a cell," said Alivisatos. "The advantage of quantum dots is that you can label each protein with a different quantum dot, shine a light on them and get all colors emitted simultaneously - one input but different outputs."

Alivisatos also has been experimenting with quantum dots and rods as photovoltaic devices or solar cells. Instead of emitting colorful light when illuminated by a laser or white light, they would produce electricity.

Three years ago he and UC Berkeley physicist Paul McEuen created a single-electron transistor using nanocrystals. In that electronic circuit, a single nanocrystal served as a tunable bridge between two leads of a transistor.

Now Alivisatos and his UC Berkeley colleagues have found a way to reliably stretch quantum dots into quantum rods with their own unique properties.

In a paper in the March 2 issue of Nature, they describe the chemical manipulations necessary to grow rods of a given dimension, up to 10 times longer than wide. The rods are made of cadmium selenide, a semiconducting material from which Alivisatos also makes quantum dots. The rods range in size up to about 10 nanometers (a millionth of a centimeter) long and one nanometer thick.

"Once we can do shape control, we can control the properties and get homogeneous formation," he said. "As this field has developed, research has centered around how to make and control very small crystals and their fundamental properties."

Given the right chemistry, nanorods even line up neatly, side by side, into strips up to 25 nanometers long. This suggests that quantum rods could be grown into large plates that emit light bright enough to serve as light-emitting diodes (LED), which are found today in many consumer electronics and appliances.

Similarly, large quantities of quantum rods could be grown to make solar cells. Alivisatos says the rods are 20 times better than dots in converting light to electricity.

The rods themselves could be useful as biological tracers too, Alivisatos said. Unlike quantum dots, the rods emit polarized light, which could provide information about the orientation as well as location of the protein to which they are attached.

Alivisatos has been experimenting with nanocrystals since 1985, when the idea was new that clusters of several hundred atoms could exhibit unique quantum properties not seen in larger crystals.

Experiments by Alivisatos and colleague Shimon Weiss of the Materials Sciences Division of LBNL led eventually to biological applications for nanocrystals of different sizes, where each emits a different color of light when hit with a laser.

The idea of using these light-emitting quantum dots as biological tracers was the nucleus of the start-up Quantum Dot Corp., which he and Weiss founded with several others in February of 1999. Their patent for the synthesis process was issued in January of this year.

"Right now we can get quantum dots that emit visible light at five to 10 independent colors, but we can definitely extend this further into the red," Alivisatos said.

The dots are coated in shells of cadmium sulfide and glass, turning them into glass beads that can easily be stuck onto biomolecules, such as proteins or DNA. Unlike organic dyes, the separate colors do not bleed together, and they last much longer than dyes, which fade.

Alivisatos hopes the quantum dots will find use in off-the-shelf medical assays as well as in so-called "labs on a chip."

Coauthors with Alivisatos on the Nature paper are Xiaogang Peng, now at the University of Arkansas, Fayetteville, Department of Chemistry and Biochemistry; UC Berkeley graduate students Liberato Manna, Juanita Wickham, Erik Scher and Andreas Kadavanich; and postdoctoral associate Weidong Yang.

The work was funded by the Department of Energy and the National Renewable Energy Laboratory.