University of Utah,
Keywords: cantilever, co-resoncance, coupling, sensitivity enhancement
Summary:Dynamic cantilever sensors are used for many different applications, for example in material’s research to study magnetic properties of small particles and thin films, in biology for real-time observation of cell processes and as balances or as gas sensors to detect trace analytes in vapor (‘artificial nose’). In contrast to static cantilever sensors where the static bending is used as a measurement signal, in the dynamic mode amplitude, oscillation frequency and phase shift are observed. For this case, the cantilever’s sensitivity is mainly determined by the beam’s stiffness, i.e. its spring constant, and effective mass. Both quantities need to be decreased in order to increase sensitivity. This can be achieved by reducing the cantilever’s dimensions, especially its thickness and width, leading to the use of nanocantilevers (at least in two out of three dimensions). However, this approach is limited as the commonly employed laser-based detection methods require a certain thickness and width of the cantilever for reliable oscillation detection. Therefore, methods need to be devised which allow the use of highly sensitive nanocantilevers but at the same time maintain the ease of oscillation detection. Our recently developed co-resonant sensor concept addresses this challenge by coupling of a micro- and a nanocantilever and the matching of their eigenfrequencies. This introduces a strong interplay between the two cantilevers. Consequently, any interaction applied at the highly sensitive nanocantilever influences the oscillatory state of the coupled system as a whole and the change of the oscillation can be detected with standard laser-based methods at the microcantilever. Furthermore, the parameters of the coupled system are a mix of parameters from both cantilevers. Hence, a large fraction of the high sensitivity of the nanocantilever is accessible with this method without requiring advanced methods to determine the nanocantilever’s oscillatory state. Hence, the nanocantilever has the potential for further miniaturization without any restrictions applied by a possible detection method. The immense potential of this concept was demonstrated in first proof-of-principle experiments in cantilever magnetometry and magnetic force microscopy for the study of magnetic properties of samples. In case of cantilever magnetometry, the sample was a carbon nanotube filled with few individual Co2FeGa Heusler nanoparticles (diameter ~35 nm) and we were, to the knowledge of the authors for the first time, able to directly observe magnetic switching of these individual nanoparticles at room temperature and with simple laser-deflection detection. These experiments strongly indicate the potential for highly sensitive investigations of novel nanomaterials without the need for low temperatures or advanced setups. The sensor concept is not limited to material’s research but may instead be employed as a platform concept which can be tailored for other applications, for example in gas sensors, magnetic field sensors or mass detection. First numerical considerations indicate a likewise sensitivity increase as in the study of novel nanomaterials.