Abstract
Kelvin probe force microscopy (KPFM) has provided deep insights into the local electronic, ionic and electrochemical functionalities in a broad range of materials and devices. In classical KPFM, which utilizes heterodyne detection and closed loop bias feedback, the cantilever response is down-sampled to a single measurement of the contact potential difference (CPD) per pixel. This level of detail, however, is insufficient for materials and devices involving bias and time dependent electrochemical events; or at solid-liquid interfaces, where non-linear or lossy dielectrics are present. Here, we demonstrate direct recovery of the bias dependence of the electrostatic force at high temporal resolution using General acquisition Mode (G-Mode) KPFM. G-Mode KPFM utilizes high speed detection, compression, and storage of the raw cantilever deflection signal in its entirety at high sampling rates. We show how G-Mode KPFM can be used to capture nanoscale CPD and capacitance information with a temporal resolution much faster than the cantilever bandwidth, determined by the modulation frequency of the AC voltage. In this way, G-Mode KPFM offers a new paradigm to study dynamic electric phenomena in electroactive interfaces as well as a promising route to extend KPFM to the solid-liquid interface.
Highlights
Progress in high-resolution imaging techniques such as Kelvin probe force microscopy (KPFM)[1], a variant of atomic force microscopy (AFM)[2], has allowed imaging of static electronic and electrochemical surface properties[3,4,5]
Pump-probe KPFM34 has been demonstrated to simultaneously detect the time averaged contact potential difference (CPD) and nanosecond changes in surface charges. This approach has been used to spatially map “speed bumps” in organic field-effect transition devices[35]. This technique still relies on single frequency heterodyne detection and bias feedback, and is subject to the standard assumptions required for KPFM operation, albeit with the added benefit of temporal dynamic information
The General Acquisition Mode (G-Mode) approach has been applied to tapping mode AFM36, piezoresponse force microscopy (PFM)[37], Magnetic force microscopy (MFM)[39] and dual harmonic-KPFM38
Summary
Probe Force Microscopy: Mapping dynamic electric phenomena in real received: 16 March 2016 accepted: 22 June 2016. Using physics based analysis; we show that G-Mode KPFM allows the parabolic bias dependence of the electrostatic force to be recovered for each period cycle of the AC voltage, leading to spatial and temporal dependence of the CPD and capacitance information channels We illustrate that this methodology can allow fast KPFM measurements with a temporal resolution substantially faster than the cantilever bandwidth, determined by the modulation frequency of the AC voltage alone (e.g. 66 μs in this work). The parabolas were fit to a second order polynomial to determine the exact CPDs, Fig. 3(d), which measured the bias pulse to be 880 mV, underestimating the 1 V pulse by 22% This deviation is a result of the frequency dependent gain of the cantilever transfer function, known in dual harmonic-KPFM47, with its removal described elsewhere[23,38] as well as in the Supplementary information. G-Mode KPFM is universally implementable on all AFM platforms and can potentially provide new knowledge on local electrochemical landscapes
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