Following up on the webinar dedicated to Kelvin probe force microscopy (KPFM) techniques, I'd like to post a short summary in this blog as well as answers to most questions raised during this event.
The recording of this webinar can be found on the Zurich Instruments YouTube channel here.
Key Take-Home Messages
Advantages of single-pass techniques
In contrast to Tip-Lift technique, where the tip is lifted off the surface after a first topography scan, single pass techniques offer better lateral resolution as the tip is brought closer to the surface, can image faster as it passes only once at every line and most importantly can measure the true height, free from electrostatic artefact since even at zero bias voltage some electrostatic offset in the topography can appear when the surface potential is not compensated
Force vs force gradient techniques
Amplitude-Modulation technique makes use of the measured amplitude for the regulation and is sensitive to the pure static Electrostatic Force which is compensated by bringing this amplitude to zero. In contrast, Frequency-Modulation technique relies on the phase coherence between the mechanical and electrostatic drive, that comes from the frequency detuning of the mechanical resonator from this electrostatic modulation, which makes this technique sensitive to the Force Gradient instead of the Force. In FM mode, the static contribution from the cantilever is cancelled out and the dominant contribution to the CPD signal comes from the apex of the tip at short tip-sample separation, while in AM mode, every part of the cantilever and tip contributes to the overall signal.
Better time resolution with accurate control of delays
As we saw in the 2nd part of this webinar, electrical pump-probe technique can be used in KPFM, by sweeping the delay between the pump and the probe while modulating the probe pulse train at the same AC electric drive as standard KPFM modes, thus being detectable by standard lock-in technique. The time resolution is therefore determined by the pulse delay and width, controlled by the Arbitrary Waveform Generator in the UHFLI and not by the detection bandwidth of the photodetector.
Here are the supportive slides used during this webinar. For the tutorial sections on AM-KPFM, Heterodyne KPFM and Pump-Probe KPFM, please refer to the video recording.
If you want to reproduce some of the measurements on your own Zurich Instruments MFLI or UHFLI, you can start by using the settings file here:
- Tutorial AM-KPFM.xml setting file
- Tutorial FM-KPFM.xml setting file
- Tutorial KPFM Time Resolved.xml setting file along with the AWG sequencer file (SeqC)
Right-click on the links and choose 'save as...' to download the files; alternatively, you can open the links and copy their content in files renamed with the extension .xml (LabOne settings) or .seqc (AWG sequencer). These files can then be uploaded on the instrument by a simple drag&drop in the Configuration tab when LabOne is running.
Q&A on KPFM and Time-Resolved Measurements Methods
Single-pass KPFM modes
Are these methods also applicable with other SPM company such as Bruker?
Yes, these KPFM methods can work with any third-party microscopy, provided you have access to the raw vertical deflection (cantilever oscillation) and can apply a bias voltage to the tip or sample. For FM-KPFM modes, you also need the tapping mode reference frequency which can come from the shaker mechanical drive. In case of Bruker, these signals are available via the Signal Access Module (SAM) but most other manufacturer have their own breakout board or BNC connector from the SPM Controller directly. Please refer to their corresponding User Manual.
What is the effect of Z-Limit on FM-KPFM ?
In FM-KPFM, the electrostatic force is sensed mostly through the tip which has a stronger Z-dependence compared to the contribution from the cantilever. Please refer to the graph and paper mentioned on slide 15 in the presentation above and this will also depend a bit on the cantilever oscillation amplitude. It is always a good practice to start with a Force Spectroscopy (Force as a function of distance) in order to determine best operating parameters (such as setpoint, amplitude, AC drive, etc.) and you can then also determine such Z-dependence.
Can Zurich Instruments' lock-in be used for Pulsed Force KPFM as described in ACS Nano 2020, 14, 4, 4839?
This is indeed a new method which doesn’t require AC drive but rely on Peak Force tapping or some Z-modulation of the tip-sample distance. If the MFLI or HF2LI only takes care of the electrical loop, leaving the Peak Force tapping to the original controller, it can drive some TTL pulses, 90° phase-locked with repect to the peak force modulation frequency and the resulting electrostatic force measured via the demodulators at the same frequency. The PID slope and setpoint offset can also be optimized for the feedback.
How do your CPD values in the different modes (AM, heterodyne, true FM) depend on cantilever amplitude?
In principle, the Contact Potential Difference (CPD) should not depend on tip amplitude, other than the local averaging. Overall Signal to Noise Ratio (SNR) decreases with increasing Amplitude but at the expense of more local sensitivity: the larger the amplitude, the greater the averaging effect. There is therefore a tradeoff between the local sensitivity to get better lateral resolution, which is achieved with small tip amplitude, and overall accuracy or the actual CPD resolution, which is achieved with larger tip swings. How it depends on each KPFM mode also depends on the Force vs Force Gradient sensitivity, since in AM-KPFM due to large background capacitive force, it is possible to reduce the amplitude and still get some decent SNR while for FM-KFPM techniques, it is often required to increase the amplitude in order to reach acceptable SNR.
Why is CPD measured at different values for various modes, how could you measure some 'absolute CPD'?
By definition CPD is a difference of work function between the tip and the sample. It would therefore require some preliminary tip work function characterization before claiming any absolute numbers. And the problem with AFM tips is that it can change during scanning as well! Between various modes, and assuming the tip remains the same, we should however measure similar value but this also depends how the overall force sensed by the tip, cone and cantilever is constructed, including the overall sensitivity. It is therefore not surprising that AM-KPFM and FM-KPFM leads to different CPD measurements and can also depend on distance.
Electrical pump-probe methods
What is the ultimate time resolution you can achieve with such techniques?
The ultimate temporal resolution of the electrical pump-probe techniques is determined by the minimum width of the probe pulse and thus the sampling time of the pulse generator. Using UHF-AWG with a sampling rate of 1.8 GSa/s, we can achieve a time resolution of ~2 ns considering a minimum of 4 samples per pulse.
How do you deal with impedance mismatch, attenuation or standing waves in RF cables when dealing with real experiments?
Impedance matching is required in distributed electrical circuits where the dimension of a circuit like the cable length is comparable with the wavelength of the electric signal. For medium temporal resolutions, impedance mismatch is not a problem but when it comes to high temporal resolutions corresponding to high frequency components, it is crucial to take the reflection into consideration. Shortening the cables as well as using impedance-matching circuits can help in this regard.
Why did you use 2 outputs to generate the signals? Is it possible to output both pump and probe pulses from single output? What is the benefit of using 2 outputs?
The UHF-AWG can provide the two pump and probe pulse trains in a single output channel or in two different output channels. In some applications, the pump is applied to the sample and the probe is applied to the tip and therefore, it is required to generate the two signals in two different channels. However, some other applications need pump and probe to be applied in one point. In such cases, the two AWG channels can be sent through a single output channel.
How flexible is it to adjust the measurement parameters such as measurement speed and temporal resolution?
These measurement parameters can be easily adjusted in the sequence program of the AWG by simply modifying a value such as pulse width, period, number of repetition, etc.
Acknowledgements: Big thanks to many contributors to this webinar, starting with Mehdi Alem with all his AWG expertise, Kivanç Esat for the AFM preparation and content support as well as Tino Wagner, Thilo Glatzel and the Nanosurf support team for valuable feedback on the Nanosurf FlexAFM use with our instruments.