Breaking the Time Barrier in Kelvin Probe Force Microscopy: Fast Free Force Reconstruction Using the G-Mode Platform

Abstract

Atomic force microscopy (AFM) offers unparalleled insight into structure and material functionality across nanometer length scales. However, the spatial resolution afforded by the AFM tip is counterpoised by slow detection speeds compared to other common microscopy techniques (e.g., optical, scanning electron microscopy, etc.). In this work, we develop an ultrafast AFM imaging approach allowing direct reconstruction of the tip-sample forces with ∼3 order of magnitude higher time resolution than is achievable using standard AFM detection methods. Fast free force recovery (FR) overcomes the widely viewed temporal bottleneck in AFM, that is, the mechanical bandwidth of the cantilever, enabling time-resolved imaging at sub-bandwidth speeds. We demonstrate quantitative recovery of electrostatic forces with ∼10 μs temporal resolution, free from influences of the cantilever ring-down. We further apply the FR method to Kelvin probe force microscopy (KPFM) measurements. FR-KPFM is an open loop imaging approach (i.e., no bias feedback), allowing ultrafast surface potential measurements (e.g., <20 μs) to be performed at regular KPFM scan speeds. FR-KPFM is demonstrated for exploration of ion migration in organometallic halide perovskite materials and shown to allow spatiotemporal imaging of positively charged ion migration under applied electric field, as well as subsequent formation of accumulated charges at the perovskite/electrode interface. In this work, we demonstrate quantitative FR-KPFM measurements-however, we fully expect the FR approach to be valid for all modes of noncontact AFM operation, including noninvasive probing of ultrafast electrical and magnetic dynamics.

Substrate-Dependent Photoconductivity Dynamics in a High-Efficiency Hybrid Perovskite Alloy

“…Prior scanning probe microscopy (SPM) studies of lead-halide perovskite solar-cell materials have used Kelvin probe force microscopy to observe the dependence of the surface potential and surface photovoltage on time, electric field, and light intensity in order to draw conclusions about the spatial distribution of charges and ions. [51][52][53][54][55][56][57][58][59][60][61][62] In studies of organic solar cell materials, frequency-shift measurements have been used to follow the time evolution of photo-induced capacitance and correlate the photocapacitance risetime with device performance. [63][64][65][66][67] Sample-induced dissipation has been used to monitor local dopant concentration in silicon 44 and GaAs; 36 probing quantizied energy levels in quantum dots; 38 examine photo-induced damage in organic solar cell materials; 41,42 quantify local dielectric fluctuations in insulating polymers; 43,[46][47][48][49]68 and probe dielectric fluctuations and intra-carrier interactions in semiconducting small molecules.…”

Intermodulation spectroscopy as an alternative to pump-probe for the measurement of fast dynamics at the nanometer scale

“…The product of Eq.s (8) and (9) introduces terms in Eq. (6) forF TS at frequencies that are integer linear combinations of ω D and ω E , the so-called intermodulation products (IMPs), or frequency-mixing products:…”

Quantitative comparison of closed-loop and dual harmonic Kelvin probe force microscopy techniques

“…Here we adopt an approach where we measured X gain directly at each tip-sample separation and applied bias by recording A ω with excitation at ω and 2ω, consecutively. 7,13,44 Other approaches based on half-harmonic excitation, 13 resonance tracking, 45 band excitation, 7,13,14,46 or G-mode 19,20,47,48 may also be used to determine X gain . Figure 4(a) shows the thermal spectra at a tip-sample separation of ∼3 µm with excitation A ω applied at 12.5 kHz.…”

“…The larger bandwidth of DH-KPFM observed here may increase the speed at which accurate measurements can be acquired and aid in the study of charging dynamics in materials and electrical devices. 47,[49][50][51]…”