Dynamical conductance measurement of single molecular junctions with both high-speed and high-precision

Abstract

Real-time and rapid monitoring of the electron transport in nanoscale structures is critical for understanding many fundamental phenomena. However, it is not possible to rapidly record the dynamical current that varied across several orders of magnitude by using a typical linear low-noise current-to-voltage converter due to its fixed gain. In addition, it faces a great challenge in carrying out a dynamical small current measurement by using a commercial source-monitor unit device with both high-precision and high-speed because a high-precision measurement normally requests long integration time, while high-speed sampling demands short integration time. To overcome these challenges, we designed a measurement system with an integrated logarithmic amplifier, which can convert the current/conductance signal (varied across eight orders of magnitude) into an output voltage signal (varied within only one order of magnitude). We successfully applied it for the dynamical conductance measurement of single molecular break junctions in which the current rapidly changed from milliampere (mA) to picoampere (pA) within tens of milliseconds under a fixed bias voltage. It is demonstrated that the intrinsic conductance can be determined accurately independent of the applied bias and the real-time dynamical conductance traces can be precisely recorded with a high-speed sampling ratio. This logarithmic amplifier design and home-made measurement system provide a way to realize a fast measurement (30 kHz) for a rapidly varied current (mA–pA), making it suitable for the characterization of single-molecule junctions during the break process, and show potential for a wide application far beyond molecule electronics.