A sound card application for AFM cantilever calibration
The knowledge of the applied force in an Atomic Force Microscope (AFM) is important in a variety of experiments from imaging of biological soft samples to single molecule force spectroscopy. In all those applications soft cantilevers are used with a typical peak resonance in the audio frequency range, such as the one reported as example. “Chantilever” is a simple software application that, using the standard PC sound card, acquires the deflection signal of an AFM probe subjected to thermal noise. The software calculate the power spectral density of the signal and it can evaluate the spring constant by applying two different methods: the Sader method and/or the thermal noise method. It also offers the possibility to perform optical lever sensitivity calibration without contact, that is particularly useful for functionalized, tipless and special probes.
The deflection signal is acquired through the input channel of the sound card, at 44100 Hz sampling rate. In the left part of the main window the signal (bottom ) and the spectrum (top) are monitored in real time so that the user can check the signals in the two scopes. Data Acquisition and Fit Pressing “Acquire” button, acquisition starts and the software returns the mean power spectral density. The button “Fit” starts a fitting algorithm: a red curve is superimposed to the data and the Amplitude, the Resonance frequency and the Quality factor of the cantilever are then available as input for successive calibration tabs. (Cantilever NT-MDT CSG10)
Sader In this tab the Length [µm] and Width[µm] of the cantilever are required as inputs together with the Density and Viscosity of the environment (air is the default). The Spring constant is evaluated following the method proposed by Sader [1]. Thermal If the cantilever's dimensions are not known the “Thermal noise method” [2] can be applied to estimate the spring constant. This tab requires as inputs the ambient Temperature [°C] and the optical lever sensitivity [nm/V] InvOLS. The latter is usually measured directly by a force distance curve on an hard surface. InvOLSThis tab allows to calibrate the InvOLS without the need to push the probe tip against an hard surface. Briefly the equipartition theorem and the Sader method are combined to get the InvOLS according to the formula [3]:
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References: [1] J. E. Sader, W. M. James, and Paul Mulvaney Rev. Sci. Instrum. 70, 3967 (1999). [2] S. M. Cook, T. E. Schaffer, K. M. Chynoweth, M. Wigton, R. W. Simmonds and K. M. Lang, Nanotechnology 17, 2135 (2006) [3] M. J. Higgins, R. Proksch, J. E. Sader, M. Polcik, S. Mc Endoo, J. P. Cleveland, and S. P. Jarvis, Rev. Sci. Instrum. 77, 013701 (2006)