Scanning electron microscope image of a gas sensor segment fabricated of a semiconducting nanowire of gallium nitride. The nanowire of less than 500 nm across is coated with nanoclusters of titanium dioxide, which alters the current in the nanowire in the presence of a volatile organic compound and ultraviolet light. Image: NIST |
A team of researchers from the National Institute of
Standards and Technology (NIST), George Mason Univ., and the Univ. of Maryland
has made nano-sized sensors that detect volatile organic compounds—harmful
pollutants released from paints, cleaners, pesticides, and other products—that
offer several advantages over today’s commercial gas sensors, including
low-power room-temperature operation and the ability to detect one or several
compounds over a wide range of concentrations.
The recently published work is proof of concept for a gas
sensor made of a single nanowire and metal oxide nanoclusters chosen to react
to a specific organic compound. This work is the most recent of several efforts
at NIST that take advantage of the unique properties of nanowires and metal
oxide elements for sensing dangerous substances.
Modern commercial gas sensors are made of thin, conductive
films of metal oxides. When a volatile organic compound like benzene interacts
with titanium dioxide, for example, a reaction alters the current running
through the film, triggering an alarm. While thin-film sensors are effective,
many must operate at temperatures of 200 C (392 F) or higher. Frequent heating
can degrade the materials that make up the films and contacts, causing
reliability problems. In addition, most thin-film sensors work within a narrow
range: one might catch a small amount of toluene in the air, but fail to sniff
out a massive release of the gas. The range of the new nanowire sensors runs from
just 50 ppb up to 1 part per 100, or 1% of the air in a room.
These new sensors, built using the same fabrication
processes that are commonly used for silicon computer chips, operate using the
same basic principle, but on a much smaller scale: the gallium nitride wires are
less than 500 nm across and less than 10 micrometers in length. Despite their
microscopic size, the nanowires and titanium dioxide nanoclusters they’re
coated with have a high surface-to-volume ratio that makes them exquisitely
sensitive.
“The electrical current flowing through our nanosensors
is in the microamps range, while traditional sensors require milliamps,”
explains NIST’s Abhishek Motayed. “So we’re sensing with a lot less power
and energy. The nanosensors also offer greater reliability and smaller size.
They’re so small that you can put them anywhere.” Ultraviolet light,
rather than heat, promotes the titanium dioxide to react in the presence of a
volatile organic compound.
Further, each nanowire is a defect-free single crystal,
rather than the conglomeration of crystal grains in thin-film sensors, so
they’re less prone to degradation. In reliability tests over the last year, the
nano-sized sensors have not experienced failures. While the team’s current
experimental sensors are tuned to detect benzene as well as the similar
volatile organic compounds toluene, ethylbenzene, and xylene, their goal is to
build a device that includes an array of nanowires and various metal oxide
nanoclusters for analyzing mixtures of compounds. They plan to collaborate with
other NIST teams to combine their ultraviolet light approach with heat-induced
nanowire sensing technologies.
