Figure 1

Schematic diagram of FM- and HAM-KPFMs. In FM-KPFM, an AC bias voltage of V_ACcos (ω_mt) was applied between the tip and the sample, the ω_m component of the frequency shift Δf_m is measured with the PLL circuit and the lock-in amplifier. In HAM-KPFM, an AC bias voltage of V_ACcos (ω₂ - ω₁) t was applied between the tip and the sample, the ω₂ component of the cantilever deflection is measured with a lock-in amplifier.