Principles of STM and AFM

Other uses have been developed, as for instance the use of AFM tip as a local voltmeter. In that case, AFM becomes an Electrostatic 

Force Microscope (EFM). Measurements can be performed under the application of a voltage between the sample and the tip. In the dc-EFM mode, a constant voltage is applied to the sample the AFM is operating in non contact mode with the cantilever oscillating near its resonant frequency. In the ac-EFM mode, the voltage is oscillating so that the cantilever always oscillates at its resonance frequency. 

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