Detecting Gas Molecules with Low-Voltage Ionization

by Gordy Slack

The ability to detect gases is important, especially those that are poisonous, explosive, or whose presence presages a crisis. But identifying gases, especially in small concentrations, can be difficult and costly. A new approach developed by UC Davis researchers Ramin Sadeghian and Saif Islam, could allow for more affordable, efficient, and portable gas-detecting devices that could revolutionize the industry. 

Ionization, the process of adding or removing an electron from an atom or molecule, has been used to ‘sniff out’ and identify gases at tiny concentrations for more than two decades.  Every kind of gas ionizes at one precise voltage; observing that voltage under certain carefully controlled conditions positively identifies a molecule, says Islam, an Associate Professor of Electrical and Computer Engineering.

Today’s big, power-hungry ionization detectors require voltages in the hundreds to ionize gases and are therefore restricted to specialized industrial uses. The new extremely-fine nanowires developed by the UC Davis group can do the job at voltages at least an order of magnitude lower. “With our device, you will probably ionize nitrogen at 5 volts, argon at 9 volts, helium at 7 volts. It’s extremely selective. No one can misinterpret the results,” says Islam.

The innovation, reported in Nature Materials (January 16, 2011), may well open the door to all kinds of new applications. For example, cancer and lung disease both generate volatile chemicals that healthy cells do not produce. “These can be identified using their unique ionization voltages, and then you can say whether a byproduct of a disease exists in a patient’s breath,” says Islam.

Array of silicon nanowires used as filed‐ionizing elements used in the nanowire-based gas sensor.

Small ionization devices employing this technology could also be built into home fireplaces to scrub chimney emissions of toxic particles. The industrial counterparts of these devices, known as electrostatic precipitators, are used for pollution control in oil and coal-fired industrial power plants to remove toxic solid particulates from the gas exhaust stream at atmospheric pressure. Usual range of ionization voltage in such industrial systems is 5,000 – 15,000 Volts, far above the safe ranges for household electronics. In addition to the reported ultralow-voltage gas ionization at low-pressure, Islam’s group has already developed a gas ionization process that works at atmospheric pressure and can be accomplished at voltages more than two orders of magnitude lower than the commercial ones.

Another application for small gas sensors would be leak detectors that would warn users before small leaks in pipes become dangerous. At such low voltages, the detectors could even be integrated into flexible materials, or “skins” (See Javey story), that could be wrapped around pipes, keeping a constant ‘nose’ on aging gas transmission systems.

This UC Davis discovery was serendipitous. The researchers were trying to grow extremely thin and sharp nanowires out of silicon. To grow the wires very thin, they used gold particles as catalysts to guide the nanowires in one dimension. “You cannot grow nanowires with such a high aspect ratio and unrivaled dimensions using conventional microfabrication methods,” Islam says.  “You have to use catalyst-assisted growth.”

Unfortunately, or so it seemed, using the gold as a catalyst contaminates the silicon, changing its material properties and spoiling the wires as potential transistors or memory devices.

The researchers found the same gold-induced impurities and contamination problems that would disable transistors happened to cause structural changes, known as “immobilized free radicals,” and “mid-gap traps,” that make these nanowires ideal for ionizing gases at very low voltages.

By employing these nanowire fabrication techniques, an ionizing gas-detection device could eventually be contained in a relatively inexpensive desktop-sized or hand-held device instead of the huge and expensive machines in use today.

Before a portable device that can be operated in atmospheric pressures could be commercially marketed, the researchers still must do more studies to correlate ionization voltages with nanowire dimensions, surface properties, pressure, concentration of gas species, noise from background gases, humidity, temperature and other physical conditions, says Islam. But all of that work is doable, with the key low-voltage ionizing insight already in his pocket.