Nanotechnology scales weigh atoms with carbon nanotubes
(Nanowerk Spotlight) Shrinking device size to nanometer dimensions presents many fascinating opportunities such as manipulating nanotechnology objects with nanotools, measuring mass in attogram (10-18 gram) ranges, sensing forces at femtonewton (10-15 newton) scales, and inducing gigahertz (109 hertz) motion, among other new possibilities waiting to be discovered. The two principal components common to most electromechanical systems irrespective of scale are a mechanical element and transducers. The mechanical element either deflects or vibrates in response to an applied force. Depending on their type, the mechanical elements can be used to sense static or time-varying forces. The transducers in microelectromechanical systems (MEMS) and nanoelectromechanical systems (NEMS) convert mechanical energy into electrical or optical signals and vice versa. To give an example, MEMS are used as accelerometers in modern automobile airbags where they sense deceleration and, if the force is beyond a programmed threshold, initiate the inflation of the airbag (for more on this, see Nanoelectromechanical systems start to take shape).
NEMS devices have two particular attributes – minuscule mass and high quality factor Q – that provides them with unprecedented potential for mass sensing down to zeptogram (10-21 gram, zg) resolution. A Spanish team has now demonstrated an ultrasensitive carbon nanotube(CNT) based mass sensor in which they measured chromium atoms with a mass resolution of only 1.4 zg. For comparison, the best sensitivity achieved before was 7 zg using resonators microfabricated in silicon.
Dr. Adrian Bachtold, whose Quantum Nanoelectronics group at the Centre Investigacions Nanociencia Nanotecnologia (CSIC-ICN, Research Center of Nanoscience and Nanotechnology) Barcelona in Spain developed this ultra-low mass sensor, describes the device to Nanowerk: "The sensor consists of a device based on a carbon nanotube that is suspended and clamped at the extremities. The nanotube acts as guitar string: when actuated, it oscillates at specific frequencies. When atoms or molecules are deposited onto the nanotube, the mass of the oscillating nanotube increases and the frequency decreases. In other words, the reduction of the velocity of the nanotube motion is directly related to the mass of the deposited atoms or molecules."
Scanning electron microscopy image of the nanotube resonator. (Image: Dr. Bachtold)
Contrary to silicon often used in NEMS, carbon nanotubes are chemically inert and do not suffer from the surface roughness inherent to lithographically patterned NEMS. This is very important in order to obtain high Q resonance quality factors. CNTs are also the stiffest material known and have low density, so that frequencies are expected to be very high, above 1GHz. One of the goals of Bachtold's group is to fabricate nanotube resonators with resonance frequencies and Q factors as high as possible.
The novelty of the Spanish team's work is the use of a carbon nanotube as the oscillating element. The mass of a nanotube is ultralow, typically a few attograms, so even a tiny amount of atoms deposited onto the nanotube makes up a significant fraction of the total mass.
Although CNT resonators have proven to be excellent mass sensors in laboratory set-ups, and might one day even provide compact alternatives to current mass spectrometers, the way they are currently fabricated lacks the ability to precisely control positioning that would allow large-scale, industrial type applications.
Bachtold's team fabricated their nanotube resonators by growing single-walled CNTs by chemical-vapor deposition on a highly doped silicon substrate and then used electron-beam lithography to connect a CNT nanotube to two chromium/gold electrodes. Wet etching and subsequent annealing was performed to suspend the CNT and remove impurities.
An necessary feature required for real world applications of CNT 'scales' is the ability to be reset. According to Bachtold, his team has done that successfully and repeatedly without any loss in sensitivity.
"This process is accomplished by applying a few microamperes of current through the nanotube for several minutes" he says. "Adsorbed atoms get removed via heating and/or electromigration. As a result, the resonance frequency is reset to its initial value, and the nanotube resonator is ready for new sensing measurements."
The team is now focusing on improving their measurement setup and they hope to achieve in a near future a resolution of 0.001 zg (or 1 yoctogram: 10-24 gram) – the mass of one atomic nucleus.
"Such a mass resolution would open new perspectives for mass spectrometry" says Bachtold. "It would be possible to weigh large molecules with subatomic precision. Individual atoms or molecules could also be placed on the nanotube in order to probe the variation of their mass. Chemical reactions in organical or biological molecules could then be monitored in real time, as well as nuclear reactions in individual atoms."
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