Atomic force microscopy (AFM) is a member of the scanning probe microscopy (SPM) mode that uses mechanical, electrical, or magnetic interactions between the sharp needle tip and the sample surface to obtain an image of the sample surface. In recent years, the emergence of high-precision electronic and optical instruments has promoted the development of SPM towards more complex imaging modes and higher time-resolution and spatial-resolution imaging. According to different sample characteristics, the combination of several existing imaging modes to obtain the topography of the sample surface has also attracted widespread attention. In addition, as researchers become more interested in studying the interaction of samples on small timescales, arbitrary waveform generators (AWGs) or femtosecond lasers combined with AFM can achieve time-resolved measurements and imaging.
The weak force of the interaction between the cantilever probe and the sample is converted into a relatively weak electrical signal, and if you want to detect this weak electrical signal, you need to use a lock-in amplifier, signal averaging (BOXCAR) or single-photon counting technology. Among the various advanced imaging modes of AFM, common electrical detection devices are wideband digital lock-in amplifiers and signal averages. Zurich Instruments offers a range of high-precision weak signal detection instruments with different specifications, such as dual-channel lock-in amplifiers (HF2, UHF), BOXCAR equalizers, etc., which can be applied to advanced imaging of AFM.
HF2PLL is used in constant amplitude AFM mode
In this AFM application, the cantilever beam is driven by variable amplitude and variable frequency signals in a resonant state. Due to the interaction between the cantilever beam and the sample, the detector’s signal changes in both amplitude and phase. To keep the cantilever in resonance, a high-speed phase-locked loop (PLL) is used to track the phase shift, with the drive signal applying a 90-degree displacement relative to the sensing signal, and the output amplitude is adjusted by a fast PID controller so that the cantilever beam has readings at a constant amplitude. The frequency shift signal of the phase-locked ring is a direct measurement of the topography, while the dissipation is a direct measurement of an energy applied to the cantilever beam to keep the oscillation constant, so the relationship between electrostatic force/magnetism/chemical/molecular force is applied to the cantilever beam.
Constant amplitude mode can be applied to contact/non-contact AFM, KPFM, high-speed imaging, dynamic force spectroscopy.
HF2PLL Lateral Excitation/Constant Drive AFM Mode
In this AFM application, the cantilever beam is driven by a constant amplitude and variable frequency signal in a resonant state. Due to the interaction between the cantilever beam and the sample, the detector’s signal changes in both amplitude and phase. To maintain the cantilever in the resonant state, a high-speed phase-locked loop (PLL) is used to track the phase shift, and the driving signal is displaced by 90 degrees relative to the sensing signal. The frequency shift of the PLL is a direct measurement of the topography.
Lateral excitation/constant drive mode can be applied to contact/non-contact AFM, KPFM, high-speed imaging, dynamic force spectroscopy.
Dual-frequency resonance tracking atomic force microscopy (DFRT AFM)
As multilayer films and ferrous materials become thinner, the need for resonance-enhanced measurements can be employed to increase sensitivity in order to reduce polarization voltages. Although phase-locked measurements with fixed low-frequency signals are suitable for bulk material applications, the nanomechanical response to mechanical or electrostatic excitation can be greatly improved at the contact resonance of the AFM sensor. With Zurich Instruments dual-channel lock-in amplifiers, dual-model excitation, sideband probing, and PID feedback for amplitude difference can simultaneously perform in-plane and out-of-plane component measurements.
DFRT method description
The PID controller inside the lock-in amplifier is used to adjust the amplitude difference between the f1 and f2 frequencies near the resonant frequency. The magnitude and symbol of the amplitude difference can be used to calculate the error signal so that the PID controls the drive frequency (f1+f2)/2.
In resonance, there is no difference between amplitude A1 and A2, so there is no error signal. If the resonance frequency decreases as shown in the figure above, then A2′-A1′ is negative and the drive frequency decreases.
The grid mode under the DAQ tab in LabOne software directly displays the amplitude and phase image of the sample
The above figure shows the amplitude and phase images of nanocrystalline Sm doped cerium oxide samples measured using the fourth harmonic response in DFRT mode.
The following table shows the various imaging modes and options required for Zurich Instruments instruments in AFM.
