Fast multifrequency measurement of nonlinear conductance

We demonstrate fast acquisition of nonlinear current-voltage characteristics (IVC) at every pixel of a high-resolution conductive AFM image [1]. The technique exploits phase-coherent multifrequency lock-in measurement to acquire IVCs on a 512x512 grid in under 9 minutes (trace and retrace), and the inverse Fourier transform to perform the analysis in real time. Our approach overcomes the high resistance of the nanometer-scale tip-surface junction and the stray capacitance of the measurement leads which impose speed limitations (tens of seconds) on the traditional methods of measuring IVCs, making maps of variation over a surface impractical. The measurement technique allows for easy cancellation of parasitic displacement current due to the measurement leads and for separation of the galvanic and displacement currents in the junction. This ultrafast (milliseconds) acquisition of IVCs enables the AFM to reveal nanometer-scale variations in the electrical transport properties of organic photovoltaic and semiconducting thin films.

[1] R. Borgani, M. Gilzad Kohan, A. Vomiero, and D. B. Haviland, arXiv:1809.07671 [cond-mat.mes-hall].