In a development that holds much promise for the future of solar cells made
from nanocrystals, and the use of solar energy to produce clean and renewable
liquid transportation fuels, researchers with the U.S.
Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab)
have reported a technique by which the electrical conductivity of nanorod crystals
of the semiconductor cadmium-selenide was increased 100,000 times.
Image (a) is a transmission electron micrograph of a cadmium-selenide nanocrystal before gold tip growth in solution and image (b) is after. Image (c) is a scanning electron micrograph of a single nanocrystal two-terminal device.
“The key to our success is the fabrication of gold electrical contacts
on the ends of cadmium-selenide rods via direct solution phase-growth of the
gold tips,” says Paul Alivisatos, interim-Director of Berkeley Lab, who
led this research. “Solution-grown contacts provide an intimate, abrupt
nanocrystal-metal contact free of surfactant, which means that unlike previous
techniques for adding metal contacts, ours preserves the intrinsic semiconductor
character of the starting nanocrystal.”
Alivisatos is a chemist who holds joint appointments with Berkeley Lab’s
Materials Sciences Division, and with the University of California-Berkeley
where he is the Larry and Diane Bock professor of Nanotechnology. He is an internationally-recognized
authority on nanocrystal growth and the corresponding author of a paper published
in the on-line edition of Nano Letters ("Enhanced Semiconductor Nanocrystal
Conductance via Solution Grown Contacts").
Co-authoring the paper with Alivisatos were Matthew Sheldon and Paul-Emile
Trudeau, members of Alivisatos’ research group; Taleb Mokari, of Berkeley
Lab’s Molecular Foundry; and Lin-Wang Wang, in Berkeley Lab’s Computational
Research Division.
With the world demand for energy projected to more than double by 2050 and
more than triple by the end of the 21st century, it is imperative that sustainable
and carbon-neutral energy technologies be developed. The use of sunlight to
generate electricity as well as to split water molecules for the production
of fuels is envisioned as an ideal energy source, and nanocrystals could be
pivotal to the success of this vision. Electrical conductance in semiconductor
nanocrystals is a critical element for both solar electricity and solar fuel
technologies.
“Standard contacting procedures that deposit metal onto semiconductor
nanocrystals directly, such as those used in commercial wafer-scale chip fabrication,
cause alloying and chemical reactions at the metal-semiconductor interface,”
says Sheldon, who was the lead author on the Nano Letters paper. “This
means that the finished electrical device is actually made of a different material
than the starting nanocrystal.”
Sheldon notes that while chemical treatments, such as etching off surfactant,
have been shown to enhance the conductivity of thin film nanocrystal solids,
these treatments will often alter the semiconductor’s electrical properties,
for example switching the material from n-type to p-type or altering the density
of surface states. Furthermore, he says, previous studies have not explained
why electrical conductance was enhanced, other than acknowledging the removal
of surfactant coverage.
In this new study, Sheldon, Alivisatos and their co-authors used single nanostructure
electrical measurements to make systematic comparisons between cadmium-selenide
nanorods with and without gold tips. The solution-grown tipping process started
with the addition of gold salt to a solution of toluene and cadmium-selenide
nanorods, which resulted in gold metal being selectively deposited on the nanorod
tips. A silicon wafer test chip was then dipped in this nanorod solution. After
submersion, the evaporation of the toulene solvent oriented individual cadmium-selenide
nanorods across pre-defined gold electrodes, which were fabricated through electron
beam lithography. The results were gold-tipped cadmium-selenide heterostructure
devices whose electrical conductance was characterized in a two-terminal geometry
as a function of source-drain voltage and temperature.
Says Alivisatos, “Our study shows that the superior performance of gold-tipped
cadmium-selenide heterostructures results from a lower Schottky barrier and
that solution grown contacts do not alter the chemical composition of the semiconductor.
Further, our work demonstrates the increasing sophistication of high-quality
electrical devices that can be achieved through self-assembly and verifies this
process as an excellent route to the next generation of electronic and optoelectronic
devices utilizing colloidal nanocrystals.”
Adds Sheldon, “We believe our approach is an ideal strategy for making
future devices from nanocrystals because it preserves the semiconductor character
of the nanocrystal as synthesized with the precise control of their synthesis
developed over the past decades.”
Sheldon says the next step in this work will be to determine if the dramatic
improvements in electrical behavior can translate to improvements in nanocrystal-based
energy production. Initially, the group plans to investigate the use of solution
grown contacts in photovoltaic applications.