Application Description

Dual-frequency resonance tracking (DFRT) is a contact mode atomic force microscopy (AFM) technique used to measure weak electrical or mechanical responses from a sample. Traditional resonance tracking techniques rely on a phase-locked loop (PLL) to keep the phase constant, but such an approach fails with ferroelectric or other materials that exhibit a phase reversal depending on the domain orientation. The advantage of DFRT lies in the ability to measure phase reversal while using solely the resonant amplitude for feedback. Applications related to the DFRT technique include piezoresponse force microscopy (PFM), electrochemical strain microscopy (ESM), which is sensitive to the ionic-current-induced strain, and scanning thermo-ionic microscopy (STIM) – which measures the strain induced by thermal oscillations.

DFRT is particularly relevant for thin-film characterization of ferroelectric and multiferroic materials, where resonance-enhanced measurements enable the measurement of weaker signals and the use of lower polarization voltages to avoid film breakdown. While lock-in measurements at a fixed low frequency is the norm for bulk materials, the nano-mechanical response to a mechanical or an electric excitation can be greatly enhanced by turning to a contact resonance technique.

Measurement Strategies

The first step is to identify the contact resonance (CR) by sweeping the output frequency that is either electrically or mechanically driven when the AFM tip is in contact with the sample. It is then possible to generate an amplitude-modulated signal (on the signal output) that gives rise to two sideband amplitudes A₁ and A₂ on either side of the CR. In the figure, the red curve illustrates the difference A₂ - A₁ as a function of the drive frequency: this exhibits a monotonic behavior around the resonance with good gain sensitivity and is thus used for feedback. A PID controller – internal to Zurich Instruments lock-in amplifiers and optimized with the PID Advisor – regulates the difference A₂' - A₁' between the sideband amplitudes measured at frequencies f_c+/-f_m. This amplitude difference is used as the error signal for the PID controller and acts upon the center frequency f_c. If the resonance frequency changes due to tip-sample interactions, the measured amplitude difference A₂' - A₁ varies and the drive frequency is shifted as a result, as shown in the figure. At resonance, A₁ and A₂ coincide and the chosen setpoint is thus zero.

For multiferroic measurements and related PFM modes, the driving signal output is directed to the bias voltage. The same measurement principle applies when the signal output goes to a shaker piezo mechanically coupled to the sample, which leads to the observation of a nano-mechanical response.