Sideband Kelvin Probe Force Microscopy
Tis method uses two lock-in amplifiers (Figure 2B) to lock on the sideband frequencies. Figure 2B shows that the second lock-in amplifier modulates the leſt sideband, while the third modulates the right sideband. Te surface potential is obtained by averaging the DC voltage feedback from both sideband frequencies [1].
Electrostatic Force Microscopy (EFM) Amplitude Sensitivity Te KPFM signal-to-noise ratio reflects the magnitude of
the electrostatic interaction, or EFM amplitude sensitivity. Te higher the EFM amplitude sensitivity, the higher the KPFM signal-to-noise ratio response can be. Te EFM amplitude sensitivity can be defined as ΔEFM amplitude / Δ tip bias. When the cantilever’s resonance frequency equals the electrical driving frequency, the EFM amplitude can be defined as:
XV V VQ k
=+ δ
δ() Figure 1: Contact potential difference between an AFM tip and sample.
Te sideband frequencies of the mechanical oscillation and the local interaction of the tip apex allow sideband KPFM to accurately measure the surface potential with an increased resolution compared to off-resonance KPFM. When performing sideband KPFM, instead of using a frequency
of 17 kHz, the KPFM mode works with the sideband’s frequencies, usually 1–5 kHz off from the cantilever’s mechanical resonance.
c z
CPDDC AC (3)
where X is the amplitude of the cantilever displacement, Q is the cantilever’s Q factor, and k is the spring constant of the cantilever.
Tereby, the EFM amplitude is directly proportional to the VAC and inversely proportional to the cantilever’s spring constant. Another factor that affects the EFM amplitude sensitivity is the laser displacement on the photo detector (Δa) [4]. It is defined as:
∆a = 3
L L Zx
∆ C (4)
Figure 2: Sideband KPFM feedback loop: a) off-resonance KPFM loop; b) sideband KPFM loop. 2021 May •
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