Atomic force microscopy resonating mode

A number of methods are available for the characterization and examination of SAMs as well as for the observation of the reactions with the immobilized biomolecules. Only some of these methods are mentioned briefly here. These include surface plasmon resonance (SPR) [46], quartz crystal microbalance (QCM) [47,48], ellipsometry [12,49], contact angle measurement [50], infrared spectroscopy (FT-IR) [51,52], Raman spectroscopy [53], scanning tunneling microscopy (STM) [54], atomic force microscopy (AFM) [55,56], sum frequency spectroscopy. X-ray photoelectron spectroscopy (XPS) [57, 58], surface acoustic wave and acoustic plate mode devices, confocal imaging and optical microscopy, low-angle X-ray reflectometry, electrochemical methods [59] and Raster electron microscopy [60]. 

Atomic force microscopy (AFM) allows the topography of a sample to be scanned by using a very small tip made from silicon nitride. The tip is attached to a cantilever that is characterised by its spring constant, resonance frequency, and a quality factor. The sample rests on a piezoceramic tube which can be moved horizontally x,y motion) and vertically (z motion). Displacement of the cantilever is measured by the position of a laser beam reflected from the mirrored surface on the top side of the cantilever, whereby the reflected laser beam is detected by a photodetector. AFM can be operated in either contact or a noncontact mode. In contact mode the tip travels in close contact with the surface, whereas in noncontact mode the tip hovers 5-10 nm above the surface. 

Atomic Force Microscopy (AFM). We used an Digital Instruments MultiMode Ilia AFM in intermittant mode to avoid damage to the organic thin film. Conventional Si probes with opening angles of 20° and tip radii of less then 10 nm have been employed. The typical resonance frequency of the used cantilevers is 300 kHz and the force constant is about 40 N/m. 

In the so-called piezo-mode of atomic force microscopy an ac voltage is applied to a conductive AFM cantilever while scanning the surface of a piezoelectric material. The tip of the cantilever senses the local deformation of the surface caused by the electric field between the tip and a counter electrode (Fig. 5b, see also Fig. 10). Usually the ac frequency is far below the free resonance frequency of the AFM cantilever [16,17,19,20]. In BaTiOs, an image series based on vertical and torsional cantilever vibration signals of the same surface area allowed the reconstruction of the domain orientation using this mode [20]. 

M. Reinstaedtler, U. Rabe, V. Scherer, J.A. Turner, and W. Arnold, Imaging of flexural and torsional resonance modes of atomic force microscopy cantilevers using optical interferometry. Surface Science, (2003) in print... 

S. Kawai, D. Kobayashi, S. Kitamura, S. Meguro, and H. Kawakatsu, An ultrahigh vacuum dynamic force microscope for high resonance frequency cantilevers. Rev. Sci. Instrum. 76,083703 (20053-S. Nishida, D. Kobayashi, T. Sakurada, T. Nakazawa, Y. Hoshi, and H. Kawakatsu, Photothermal excitation and laser doppler veloclmetry of higher cantilever vibration modes for dynamic atomic force microscopy in liquid. Rev. Sci. Instrum. 79,123703 (2008J. 

Topography The surface topography of the films was analysed by (a) atomic force microscopy (AFM Nanoscope IIIA Multimode SPM, Veeco Instmments) in tapping mode under ambient conditions (cantilever resonant frequencies were in the range 330-350 kHz and the force constant was 42 N/m) and (b) high-resolution scanning electron microscopy (HRSEM FEI, NovaNano SEM 230). 

Yuya et al. [243] extracted the elastic modulus of single electrospun PAN nanofibre dynamically through the natural frequencies of a pair of AFM microcantilevers linked by a nanofibre segment (Fig. 4.24b). The theory of this technique is based on the dynamic relationship between the fibre stiffness (i.e. spring constant) and the resonance frequencies of cantilever vibration mode. On the other hand, Liu et al. [244] used atomic force acoustic microscopy (AFAM) based on ultrasonic frequency oscillations to excite an AFM cantilever when the tip was in contact with a sample. A different approach based on a model of the resonant frequency that is dependent on the bob s free flight was employed to measure the elastic modulus of as-spun nylon 6, 6. A ball was glued to a nanofibre and suspended from a cantilever beam that was attached to a piezoelectric-actuated base [245]. 

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