Integrated Tapping Mode Kelvin Probe Force Microscopy with Photoinduced Force Microscopy for Correlative Chemical and Surface Potential Mapping

Abstract

the sample through the measurement of Coulomb force and typically operates in the tapping mode of atomic force microscopy (AFM). [10] The tapping mode AFM operation avoids excess tip-sample contact and is advantageous for preserving the integrity of the sample surface, compared with the contact mode AFM. Besides, AFM imaging in the tapping mode can operate at a higher scan rate than the pulsed force mode. [11][12][13] Although the recent development of KPFM in the pulsed force mode considerably improves the spatial resolution on mapping surface potential under ambient conditions, its operational speed is limited by the subresonance tapping frequency of the pulsed force mode. [14] In comparison, a regular tapping mode operates with a cantilever resonant frequency of hundreds of kilohertz, whereas the pulsed force mode can operate only at a few kilohertz. The upper limit of the potential operational speed of KPFM in the tapping mode should be faster than that of the pulsed force KPFM. The limitations of the current popular tapping mode KPFM are associated with its operational mechanism. [15] Although the original design of the Kelvin probe method by Lord Kelvin for macroscopic measurement did not involve applying an AC voltage, [16] the tapping mode KPFM variants for nanoscopic measurement implement an external AC voltage between the tip and the sample to modulate the electrical force. A DC bias voltage is used under a negative feedback loop to minimize the electrical force-induced cantilever oscillation amplitude at the AC voltage frequency or the frequency shift of the cantilever oscillation at the tapping frequency. [17] Under ambient conditions, the amplitude of the AC drive voltage is often within the range of 1-5 V, posing a risk to voltage-sensitive materials, wherein tip-induced band-bending and charge transfer may generate significant artifacts in the images. [18,19] Furthermore, irreversible sample or tip damage from the large amplitude AC voltage may occur during KPFM measurements if the tip and sample come into unwanted transient contact (e.g., due to external vibrations and noises), even if the AFM is operated in the noncontact or attractive regime of the tapping mode. One way to reduce external noise is to use a low-temperature and ultrahigh vacuum system. However, such systems introduce additional experimental complexity. Therefore, a dual-pass lift Kelvin probe force microscopy (KPFM) is a popular technique for mapping the surface potential at the nanoscale through measurement of the Coulombic force between an atomic force microscopy (AFM) tip and sample. The lateral resolution of conventional KPFM variants is limited to between ≈35 and 100 nm in ambient conditions due to the long-range nature of the Coulombic force. In this article, a novel way of generating the Coulombic force in tapping mode KPFM without the need for an external AC driving voltage is presented. A field-effect transistor (FET) is used to directly switch the electrical connectivity of the tip and sample on and off periodically. The...

NaHCO3‐induced porous PbI2 enabling efficient and stable perovskite solar cells

“…Kelvin probe force microscopy (KPFM), which can directly measure the contact potential difference (CPD) using a nondestructive method, was used to investigate the impact of Na + doping on work function (W F ) of perovskite surface. 41 The KPFM images of pristine and NaHCO 3 -treated perovskite are shown in Figure S10. We obtained a distribution plot of CPDs from KPFM images and used Gaussian distribution to fit the histogram displayed in Figure 3E.…”

“…Na + has been verified to induce p‐type doping in perovskites through both density functional theory and experimental studies, which directly impact Fermi level position of perovskite films. Kelvin probe force microscopy (KPFM), which can directly measure the contact potential difference (CPD) using a nondestructive method, was used to investigate the impact of Na + doping on work function ( W F ) of perovskite surface 41 . The KPFM images of pristine and NaHCO 3 ‐treated perovskite are shown in Figure S10.…”

Photo induced force microscopy: chemical spectroscopy beyond the diffraction limit

“…Jakob et al 104 designed a system that can obtain KPFM and PiFM simultaneously as an alternative to the more complex PFIR-KPFM systems. In the study, the system was used to characterise a perovskite thin film formed from formamidinium (FA), methylammonium (MA) and Cs (Cs 0.05 (FA 0.93 MA 0.07 ) 0.95 -PbI 3 ).…”

“…PIFM has been combined with two other systems in recent years, TERS, 102 SNOM 103 and Kelvin probe force microscopy (KPFM). 104…”

“…PIFM has been combined with two other systems in recent years, TERS, 102 SNOM 103 and Kelvin probe force microscopy (KPFM). 104 Meyer et al 102 integrated a typical TERS system with a PiFM to image 2D MoS 2 monolayer flakes. Signal optimisation is a crucial aspect of any TERS experiment but since PiFM can map the electromagnetic field of various materials and light, it can be used in tandem with the TERS to assist in the focusing and alignment of the laser.…”