Lead selenide nanowires integrated in a device. Image: Univ. of Pennsylvania |
The advancements of our
electronic age rests on our ability to control how electric charge moves, from
point A to point B, through circuitry. Doing so requires particular precision,
for applications ranging from computers, image sensors and solar cells, and
that task falls to semiconductors.
Now, a research team at
the Univ. of Pennsylvania’s schools of Engineering
and Applied Science and Arts and Sciences has shown how to control the
characteristics of semiconductor nanowires made of a promising material: lead
selenide.
Led by Cherie Kagan,
professor in the departments of Electrical and Systems Engineering, Materials
Science and Engineering and Chemistry and co-director of Pennergy, Penn’s
center focused on developing alternative energy technologies, the team’s
research was primarily conducted by David Kim, a graduate student in the
Materials Science and Engineering program.
The team’s work was
published online in the journal ACS Nano.
The key contribution of
the team’s work has to do with controlling the conductive properties of lead
selenide nanowires in circuitry. Semiconductors come in two types, n and p,
referring to the negative or positive charge they can carry. The ones that move
electrons, which have a negative charge, are called “n-type.” Their “p-type”
counterparts don’t move protons but rather the absence of an electron—a “hole”—which is the
equivalent of moving a positive charge.
Before they are
integrated into circuitry, the semiconductor nanowire must be “wired up” into a
device. Metal electrodes must be placed on both ends to allow electricity to
flow in and out; however, the “wiring” may influence the observed electrical
characteristics of the nanowires, whether the device appears to be n-type or p-type. Contamination, even from air, can also influence the
device type. Through rigorous air-free synthesis, purification and analysis,
they kept the nanowires clean, allowing them to discover the unique properties
of these lead selenide nanomaterials.
Researchers designed
experiments allowing them to separate the influence of the metal “wiring” on
the motion of electrons and holes from that of the behavior intrinsic to the
lead selenide nanowires. By controlling the exposure of the semiconductor
nanowire device to oxygen or the chemical hydrazine, they were able to change
the conductive properties between p-type
and n-type. Altering the
duration and concentration of the exposure, the nanowire device type could be
flipped back and forth.
“If you expose the
surfaces of these structures, which are unique to nanoscale materials, you can
make them p-type, you can make
them n-type, and you can make
them somewhere in between, where it can conduct both electrons and holes,”
Kagan said. “This is what we call ‘ambipolar.’”
Devices combining one n-type and one p-type semiconductor are used in many
high-tech applications, ranging from the circuits of everyday electronics, to
solar cells and thermoelectrics, which can convert heat into electricity.
“Thinking about how we
can build these things and take advantage of the characteristics of nanoscale
materials is really what this new understanding allows,” Kagan said.
Figuring out the
characteristics of nanoscale materials and their behavior in device structures
are the first steps in looking forward to their applications.
These lead selenide
nanowires are attractive because they may be synthesized by low-cost methods in
large quantities.
“Compared to the big
machinery you need to make other semiconductor devices, it’s significantly
cheaper,” Kagan said. “It doesn’t look much more complicated than the hoods
people would recognize from when they had to take chemistry lab.”
In addition to the low
cost, the manufacturing process for lead selenide nanowires is relatively easy
and consistent.
“You don’t have to go to
high temperatures to get mass quantities of these high-quality lead selenide
nanowires,” Kim said. “The techniques we use are high yield and high purity; we
can use all of them.”
And because the conductive
qualities of the lead selenide nanowires can be changed while they are situated
in a device, they have a wider range of functionality, unlike traditional
silicon semiconductors, which must first be “doped” with other elements to make
them ”p” or “n.”
The Penn team’s work is a
step toward integrating these nanomaterials in a range of electronic and
optoelectronic devices, such as photo sensors.
