Dual-bias modulation heterodyne Kelvin probe force microscopy in FM mode

The use of a heterodyne detection scheme in Kelvin probe force microscopy (KPFM) is an effective way for enhancing the performance of KPFM. However, this detection scheme generally has difficulty in detecting the first- and second-harmonic electrostatic forces simultaneously. To overcome this problem, we propose dual-bias modulation heterodyne frequency modulation KPFM (DM-hetero-FM KPFM), in which dual AC biases at [Formula: see text] are applied between the tip and the sample. DM-hetero-FM KPFM enables us to measure the contact potential difference and capacitance gradient simultaneously at high frequencies (in the MHz range) beyond the bandwidth of phase-lock loop. Moreover, the present method allows us to perform it in the open-loop mode, which is highly desired for performing KPFM on semiconductors or in liquids at high frequencies.