Basic scientific curiosity paid off in unexpected ways when Rice Univ.
researchers investigating the fundamental physics of nanomaterials discovered a
new technology that could dramatically improve solar energy panels. The
research is described in Science.
“We’re merging the optics of nanoscale antennas with the electronics of
semiconductors,” said lead researcher Naomi Halas, Rice’s Stanley C. Moore
Professor in Electrical and Computer Engineering. “There’s no practical
way to directly detect infrared light with silicon, but we’ve shown that it is
possible if you marry the semiconductor to a nanoantenna. We expect this
technique will be used in new scientific instruments for infrared-light
detection and for higher-efficiency solar cells.”
More than a third of the solar energy on Earth arrives in the form of
infrared light. But silicon—the material that’s used to convert sunlight into
electricity in the vast majority of today’s solar panels—cannot capture
infrared light’s energy. Every semiconductor, including silicon, has a
“bandgap” where light below a certain frequency passes directly
through the material and is unable to generate an electrical current. By
attaching a metal nanoantenna to the silicon, where the tiny antenna is
specially tuned to interact with infrared light, the Rice team showed they
could extend the frequency range for electricity generation into the infrared.
When infrared light hits the antenna, it creates a “plasmon,” a wave
of energy that sloshes through the antenna’s ocean of free electrons. The study
of plasmons is one of Halas’ specialties, and the new paper resulted from basic
research into the physics of plasmons that began in her lab years ago.
It has been known that plasmons decay and give up their energy in two ways;
they either emit a photon of light or they convert the light energy into heat.
The heating process begins when the plasmon transfers its energy to a single
electron—a ‘hot’ electron. Rice graduate student Mark Knight, lead author on
the paper, together with Rice theoretical physicist Peter Nordlander, his
graduate student Heidar Sobhani, and Halas set out to design an experiment to
directly detect the hot electrons resulting from plasmon decay.
Patterning a metallic nanoantenna directly onto a semiconductor to create a
“Schottky barrier,” Knight showed that the infrared light striking
the antenna would result in a hot electron that could jump the barrier, which
creates an electrical current. This works for infrared light at frequencies
that would otherwise pass directly through the device.
“The nanoantenna-diodes we created to detect plasmon-generated hot
electrons are already pretty good at harvesting infrared light and turning it
directly into electricity,” Knight said. “We are eager to see whether
this expansion of light-harvesting to infrared frequencies will directly result
in higher-efficiency solar cells.”
