Pulsed Force Kelvin Probe Force Microscopy through Integration of Lock-In Detection

Abstract

Kelvin probe force microscopy measures surface potential and delivers insights into nanoscale electronic properties, including work function, doping levels, and localized charges. Recently developed pulsed force Kelvin probe force microscopy (PF-KPFM) provides sub-10 nm spatial resolution under ambient conditions, but its original implementation is hampered by instrument complexity and limited operational speed. Here, we introduce a solution for overcoming these two limitations: a lock-in amplifier-based PF-KPFM. Our method involves phase-synchronized switching of a field effect transistor to mediate the Coulombic force between the probe and the sample. We validate its efficacy on two-dimensional material MXene and aged perovskite photovoltaic films. Lock-in-based PF-KPFM successfully identifies the contact potential difference (CPD) of stacked flakes and finds that the CPDs of monoflake MXene are different from those of their multiflake counterparts, which are otherwise similar in value. In perovskite films, we uncover electrical degradation that remains elusive with surface topography.

The local potential variation mapping including thermoelectromotive force in nanocomposite materials under non-thermal equilibrium

“…2(b), in spite of no application of ΔT, small image contrasts were observed at the tops of Si NWs. This contrast in V vac images likely comes from the work function difference [47][48][49][50] between PEDOT:PSS and Si NW. Let's discuss the spatial resolution of these images.…”

Pulsed Force Kelvin Probe Force Microscopy─A New Type of Kelvin Probe Force Microscopy under Ambient Conditions

“…In an effort to decrease the entry barrier of PF-KPFM and further facilitate its adoption, we have developed a lock-in-based PF-KPFM that has the same complexity as the AM-KPFM. 41 Tapping mode AFM is used in the regular AM-or FM-KPFM, meaning the cantilever oscillations persist at all times and are thus suitable for lock-in detection. However, for the digitizer-based PF-KPFM, the Coulombic force-induced oscillations are segmented between each tip−sample contact moment during the pulsed force mode AFM operation.…”

“…To establish an electrical connection between the sample surface and the AFM sample holder, all samples were glued to regular conductive AFM specimen disks (Ted Pella) and then electrically connected to the disk with adhesive copper tape or silver paint (Leitsilber 200 silver paint, Ted Pella). 37,41 A platinum-coated AFM cantilever (HQ:NSC15/Pt, Mikro-Masch) is used. In pulsed force or peak force tapping mode, the sample stage performs a sinusoidal z-axis motion at a predefined frequency (1.975 kHz) which is far lower than the cantilever free space resonance frequency (∼250 to 300 kHz for HQ:NSC15/Pt, Mikromasch).…”

“…The analog V DC from the lock-in amplifier output is applied to the tip to maintain the demodulation amplitude on the set point value and coregistered by the AFM controller as the tip scans. 41 Accurate CPD values from individual positions are obtained by sweeping the external bias voltage V DC and registering the amplitude of the Coulombic force-induced cantilever oscillations from the lock-in demodulation (Figure 5e). The CPD value of the sample surface is determined by the intersection between the two branches of the V-shape pattern.…”

“…Further analysis of the CPD values of different thicknesses revealed that 1 layer flake of MXene with approximately 8 nm in thickness has a CPD value closer to the Si substrate while all 2, 3, 4, and 5 layered stacks pose similar CPD, indicating full electrical shielding of the substrate cannot be achieved with 1 layer flake with 8 nm thickness. 41 Anisotropic Ferroelectric Behavior of BaTiO 3 . Barium titanate (BaTiO 3 ) and other ferroelectric oxides exhibit spontaneous polarization below the Curie temperature, resulting in the formation of anisotropic ferroelectric domains.…”