In scanning force microscopy, probe-sample interaction forces can be classified into two categories: conservative forces and dissipative forces. The conservative forces change the effective spring constant of the cantilever which induce the change of its resonance frequency. On the other hand, the dissipative forces act as dampers and change the effective damping factor of the cantilever which changes the cantilever vibration energy. The sensitivity of noncontact atomic force microscopy (NC-AFM) to dissipative forces is much higher compared to conservative forces. This indicates that the use of dissipative forces should improve such sensitivity in various surface properties measurement techniques such as Kelvin probe force microscopy (KFM) and electrostatic force spectroscopy (EFS).
In the present paper, we propose a novel surface potential measurement technique using dissipative force modulation (DM) method with an ac bias voltage of doubled harmonic frequency (2ω) of the oscillating cantilever (2ω DM method) [1]. The effect of the stray capacitance between a cantilever and a sample on electrostatic force spectroscopy/Kelvin probe force microscopy measurement is almost completely removed in 2ω DM method, since the distance dependence of the modulated electrostatic force increases from 1/ z to 1/ z2. 2ω DM method has an advantage of high force sensitivity due to the high Q factor of the cantilever in vacuum. We demonstrate quantitative surface potential measurement by using dissipative tip-sample interactions. Finally, we discuss the minimum detectable contact potential difference in 2ω DM-KFM.
It is noteworthy that 2ω DM method proposed here can be applied not only to NC-AFM but also widely applied to several AFM such as tapping-mode AFM, phase-modulation AFM [2], and so on.
References
[1] H. Nomura, K. Kawasaki, T. Chiakamoto, Y. Naitoh, M. Kageshima, Y. Sugawara,
Appl. Phys. Lett., 90, 033118 (2007).
[2] N. Kobayashi, Y. J. Li, Y. Naitoh, M. Kageshima, and Y. Sugawara,
Jpn. J. Appl. Phys., 45, L793 (2006).
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