During dynamic atomic force microscopy (AFM), the deflection of a scanning cantilever generates multiple frequency terms due to the nonlinear nature of AFM tip-sample interactions. Even though each frequency term is reasonably expected to encode information about the sample, only the fundamental frequency term is typically decoded to provide topographic mapping of the measured surface. One of main reasons for discarding higher harmonic signals is their low signal-to-noise ratio. Here, we introduce a new design concept for multi-harmonic AFM, exploiting intentional nonlinear internal resonance for the enhancement of higher harmonics. The nonlinear internal resonance, triggered by the non-smooth tip-sample dynamic interactions, results in nonlinear energy transfers from the directly excited fundamental bending mode to the higher-frequency mode and, hence, enhancement of the higher harmonic of the measured response. It is verified through detailed theoretical and experimental study that this AFM design can robustly incorporate the required internal resonance and enable high-frequency AFM measurements. Measurements on an inhomogeneous polymer specimen demonstrate the efficacy of the proposed design, namely that the higher harmonic of the measured response is capable of enhanced simultaneous topography imaging and compositional mapping, exhibiting less crosstalk with an abrupt height change.
Systematic physical and electrochemical characterizations revealed unique positive multifunction of a polymeric salen-type nickel(II) complex, poly[Ni(CH 3salen)], as an additive for conventional cathodes in lithiumion batteries. Due to its promising electrochemical and mechanical properties, combined with its unique threedimensional weblike electron-network structure, the redoxactive-organometallic polymer can eliminate conductive carbon and replace a significant portion of the poly(vinylidene fluoride) (PVdF) binder that has been used in conventional LiFePO 4 cathodes. By replacing such electrochemically inactive components (i.e., carbon and PVdF), LiFePO 4 cathodes with poly[Ni(CH 3 -salen)] deliver improved energy density compared with the conventional LiFePO 4 cathode. Facile electron transfer via large-area contact at polymer/LiFePO 4 interfaces significantly accelerates charge-transfer reactions and consequently improves the rate capability of the cathodes. In addition, unlike PVdF, poly[Ni(CH 3 -salen)] retains steady Young's modulus values after immersing in an electrolyte solvent, which enhances the mechanical integrity of the cathodes during the cycling of battery cells and thereby improves their cycle life. The unique multifunction of the poly[Ni(CH 3 -salen)] will be of broad interest for its application in next-generation energy-storage devices.
Atomic force microscopy (AFM) has been widely utilized to gain insight into various material and structural functionalities on the nanometer scale, leading to numerous discoveries and technologies. Despite the phenomenal success in applying AFM to the simultaneous characterization of topological and functional properties of materials, it has continuously suffered from the crosstalk between the observables, causing undesirable artifacts and complicated interpretations. Here, we introduce a two-field AFM probe, namely an inner-paddled cantilever integrating two discrete pathways such that they respond independently to the variations in surface topography and material functionality. Hence, the proposed design allows reliable and potentially quantitative determination of functional properties. In this paper, the efficacy of the proposed design has been demonstrated via piezoresponse force microscopy of periodically poled lithium niobate and collagen, although it can also be applied to other AFM methods such as AFM-based infrared spectroscopy and electrochemical strain microscopy.
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