Sep 30, 2003
Nanotube helium sensors could bring atom beam microscope
Scientists from the University of Cambridge, UK, have come up with a high-efficiency technique for detecting neutral atoms such as helium. The researchers used multiwalled carbon nanotubes under a positive bias to field-ionize passing gas atoms.
“Our research has focused on the detection of helium, which we use for surface structural studies, although it is clear that the new detector will be appropriate for all gaseous species,” Donald MacLaren told nanotechweb.org. “Conventional helium detectors are limited to efficiencies of a few thousandths of a per cent, which restricts our experiments to situations with relatively strong signal levels. When completed, the new detector could boost efficiency by several orders of magnitude, making a host of atom-optical experiments that have previously been limited by poor signal quality now feasible.”
To carry out the technique, MacLaren and colleagues grew a densely packed array of multiwalled carbon nanotubes onto a stainless-steel wire. The nanotubes were perpendicular to the substrate, up to 2 µm long and about 50 nm in diameter. The team placed the detector in an ultra-high vacuum chamber and then added helium to a pressure of 4 x 10-5 mbar. Applying a positive voltage to the nanotubes caused field ionization of passing helium atoms. The resulting helium ions then accelerated towards a counter-electrode about 20 mm away, which detected the ion current.
According to MacLaren, previous attempts to use the field ionization effect for gas detection have been limited by the extremely small size of the ionization region around any single tip. The easiest way to improve detection is to increase the number of tips, and advances in the controlled growth of carbon nanotubes have now made such a detector practical. The scientists believe that the design is simple, cheap and readily scalable to much larger sizes.
“The most exciting aspect is the fascinating possibility of seeing the spatial distributions of atomic beams in two dimensions - just as optical, electron and ion beams can currently be observed,” said MacLaren. “The work suggests the possibility of fabricating a spatially resolved neutral atom detector that is sensitive even to helium - the most difficult element to detect in low concentrations.”
The detector could have applications “wherever gases need to be detected in very low concentrations - ranging from space-based applications to molecular beam research.” The Cambridge team’s goal is to use the detector in a scanning helium microscope - a novel microscope that will image samples with the low energy, inert matter waves of a helium beam. The microscope should offer “unprecedented sensitivity and the ability to image delicate organic specimens without degradation.”
Now, the scientists say they have moved from using random, densely packed nanotube "forests" to well-defined arrays of isolated nanotubes. “The improvements promise an increase in detection efficiency and we are working towards incorporating the array into a spatially resolved detector,” added MacLaren.
The researchers reported their work in Nano Letters.
About the author
Liz Kalaugher is editor of nanotechweb.org.