In environmental assessment and security, it is necessary to detect hazardous gases at low concentrations. The ball surface acoustic wave (SAW) sensor can have significantly improved sensitivity when the ultramultiple roundtrips (e.g., >100 turns) of the SAW is achieved. However, it is hindered by the thick and coarse organic sensitive film fabricated by the conventional spin-coating method. We propose an ''off-axis spin coating method'' in order to fabricate thin and highly uniform organic sensitive film with high reproducibility. Langasite ball SAW sensors were coated by polyisoprene, polyisobutylene, and polydimethylsiloxane sensitive films and were applied in detecting normal saturated hydrocarbons (pentane, hexane, heptane, octane, and nonane), a nerve gas simulant (dimethyl methylphosphonate: DMMP), and sick-house gases (benzene, toluene, and xylene). With the proposed method, we succeeded in realizing multiple roundtrips and in detecting DMMP at a sub-ppm concentration without preliminary condensation of the sample.
Modern functional materials and devices require thorough testing for safety and reliability. Here, we describe solutions to meet this requirement in the field of scanning probe microscopy, and the most practical approach among them; ultrasonic atomic force microscopy (UAFM). In this review, we focus on evaluation of subsurface defects with scientific and technological importance, such as domains of ferroelectric materials and subsurface delamination of metal electrodes on microdevices. In addition, we show the development and application of the lateral bending (LB) mode and lateral modulation atomic force microscopy (LM-AFM) with applications in nanomaterials including carbon nanotube composites and discuss their future development in combination with UAFM.
Gas or liquid chromatographs (GC/LCs) are frequently used for multiple-compound sensing, but they are too large and heavy to be portable. To provide a handy GC/LC, many groups have proposed micro GC/LCs in which large columns are replaced by small microelectro-mechanical-system (MEMS) columns. However, because of the high packing pressure, a packed MEMS column using an anodically bonded structure has not been realized so far. In this study, we realized such a MEMS column for the first time by introducing a compression jacket to reduce the tensile stress at the silicon-glass boundary. Consequently we succeeded in the separation and detection of lower hydrocarbons using a ball surface acoustic wave sensor.
To improve the precision of dynamic atomic force microscopy (AFM) using cantilever vibration spectra, a simple but effective method for suppressing spurious response (SR) was developed. The dominant origin of SR was identified to be the bending vibration of the cantilever substrate, by the analysis of the frequency of SR. Although a rigid cover pressing the whole surface of the substrate suppressed SR, the utility was insufficient. Then, a method of enhancing the bending rigidity of the substrate by gluing a rigid plate (clamping plate, CP) to the substrate was developed. This chip can be used with an ordinary cantilever holder, so that the reproducibility of SR suppression when attaching and detaching the cantilever chip to the holder was improved. To verify its utility, the evaluation of a microdevice electrode was performed by ultrasonic atomic force microscopy. The delamination at a submicron depth was visualized and the detailed variation of the delamination was evaluated for the first time using clear resonance spectra. The CP method will particularly contribute to improving dynamic-mode AFM, in which resonance spectra with a low quality factor are used, such as noncontact mode AFM in liquid or contact resonance mode AFM. The effect of the CP can be achieved by fabricating a substrate with a thick plate beforehand.
In order to improve the ferroelectric properties of films for ferroelectric memories or relaxor single crystals for actuators, it is necessary to observe domain structures on the nanoscale. We propose an approach to observing three-dimensional (3D) domain structures by ultrasonic atomic force microscopy (UAFM) with a subsurface observation capability. Moreover, it is sometimes necessary to observe the motion of the 3D domain structures under an electric field. We propose for this case a method of applying an electric field using a surface electrode pair (SEP) in the observation plane of UAFM. When applying a field to a lead magnesium niobate–lead titanate [0.65Pb(Mg1/3Nb2/3)O3–0.35PbTiO3: PMN–PT] single crystal, reversible domain switching was observed at low fields, and irreversible switching with domain boundary (DB) motion was observed at high fields. In the latter case, the growth of the domain was observed with the subsurface DB motion. A SEP and UAFM are thus proved useful in evaluating the motion of domain structures in ferroelectrics.
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