Schematic illustration of the 55-µm long silicon nitride beam (green) flanked by two gold electrodes (yellow). Artwork by Christoph Hohmann, Nanosystems Initiative Munich (NIM).
The measurement of very low concentrations of various agents plays an important role in medicine, pharmacology and food technology. So-called “nanomechanical resonators”—vibrating nanostrings—represent promising candidates for suitable detectors, because their oscillating motion is extremely sensitive to the binding of substances of interest. In recent years scientists have refined these techniques to the point where single atoms can now be detected. These analyses, however, have their shortcomings. They tend to be time-consuming, require expensive instrumentation and frequently operate only at temperatures near absolute zero. Recently, a group of physicists at the LMU developed a compact sensor architecture on the nanometer scale, which is easy to handle and works at room temperature.
The group is led by Dr. Eva Weig, who is also a member of the Nanosystems Initiative Munich (NIM). The new work builds on their initial demonstration of an efficient electrical interface for nanomechanical resonators which was published in Nature in 2009. They now describe a fully integrated nanomechanical sensor platform that permits robust and sensitive detection of tiny displacements.
The most important part of the nanosensor is a thin beam of highly stressed silicon nitride, about 50 ?m in length and 200 nm wide, suspended between two silica supports. The large pre-stress on this “nano guitar string” allows one to drive its resonant motion with low excitation energy and gives rise to a high mechanical quality factor. The beam is flanked on each side by slightly elevated, parallel gold electrodes. An electric voltage is applied to the two gold electrodes, which act as a capacitor. The resulting electric field couples to the resonator. In the preceding 2009 Nature publication, this effect was employed to control and drive the vibration of the beam. In the new work, it is utilized to sense its motion. The measurement scheme is based on a simple effect: when the nanobeam oscillates up and down within the electric field, the capacitance between the two electrodes varies slightly. In order to pick up this tiny signal, the scientists devised an elegant extension of the existing setup. They incorporated a so-called microwave cavity into the design, which allows them to detect even the thermal motion of the suspended nanobeam.
The microwave cavity can be described as an electrical circuit formed by an inductor and a capacitor, which is connected to the gold electrodes. It is powered by a microwave signal and transmits the combined response of nanobeam and microwave cavity. This effectively allows one to employ the microwave cavity as an amplifier to enhance the signal generated by the moving nanoresonator. The measurement scheme combines two major advantages. Besides considerably enhancing the sensitivity, the microwave cavity can be easily connected to a whole set of nanobeams, which dramatically simplifies operation.
“This will enable the development of highly integrated sensors in the future,” says Thomas Faust, who is first author of the publication. In addition, the scientists have also demonstrated a back-action of the microwave cavity field on the oscillation of the nanomechanical resonator. In this way it is possible to directly drive the resonator motion into self-oscillation and to narrow the width of the peak down to only a few Hz. This offers a means of further enhancing the sensitivity of any future sensor.
Furthermore, this latest version of the device is much easier to utilize than other existing solutions. “You only need to connect two cables and, in principle, you can obtain the read-out from thousands of resonators at the touch of a button.” explains Eva Weig. Because the system is simple to operate and is not susceptible to external influences, the new method should be suitable for use even under the non-ideal conditions found outside physics labs. (bige, NIM)
The work has been funded by the German Research Foundation (DFG) and the FET-Open project QNEMS of the European Commission.