Published: February 04, 2011 r2011 American Chemical Society 927 dx.doi.org/10.1021/nl1026748 | Nano Lett. 2011, 11, 927–933 LETTER pubs.acs.org/NanoLett Breakdown of Atomic-Sized Metallic Contacts Measured on Nanosecond Scale Shaoyin Guo, Joshua Hihath, and Nongjian Tao* Center for Bioelectronics and Biosensors, Biodesign Institute, and Departments of Electrical Engineering, Arizona State University, Tempe, Arizona 85287, United States b S Supporting Information ABSTRACT: We report on a study of atomic-sized metallic contacts on a time scale of nanoseconds using a combined DC and AC circuit. The approach leads to a time resolution 3-4 orders of magnitude faster than the measurements carried out to date, making it possible to observe fast transient conductance-switching events associated with the break- down, re-formation, and atomic scale structural rearrange- ments of the contact. The study bridges the wide gap in the time scales between the molecular dynamic simulations and real world experiments, and the method may be applied to study nano- and subnanosecond processes in other nanoscale devices, such as molecular junctions. KEYWORDS: Quantum point contact, atomic size contact, electron transport, conductance quantization, molecular junctions, RF STM S tudying electron transport in nanoscale structures and de- vices, including atomic point contacts 1 and molecular junc- tions, 2,3 is critical for understanding many fundamental phenomena and developing potential device applications. Electron trans- port measurements to date are limited to relatively slow time scales (milli- to microseconds). Extending the measurements to faster time scales will open the door to the study of atomic scale structural rearrangement, molecular conformational changes, charge transfer, and many other fast processes in nanostructures. 4 An important example is the mechanical breakdown process of an atomic sized contact created by pulling two electrodes in contact apart. It has been shown that during the last stage of pulling a neck-shaped wire connects the two electrodes, and the diameter of the wire reduces to a single atom before breakdown. 5-7 The conductance of the contact recorded during the breakdown process decreases in a stepwise fashion, and the steps for metals, such as gold, are multiples of the conductance quantum, G 0 = 2e 2 /h, where e is the electron charge and h is Planck's constant. The formation and breakdown of atomic scale contacts have been predicted and modeled by molecular dynamic simula- tions. 8,9 However, the time scales in first-principle simulations are typically limited to picosecond or nanosecond time scales due to computational constraints, in contrast to microsecond to millisecond time scales that typical experiments can measure. Fast transport measurements are needed to bridge the wide time gap between the simulations and real world experiments, but building an electronic circuit with a broad frequency bandwidth to cover fast physical processes has been a difficult task. In this Letter, we report on a study of atomic scale contacts with nano- second time resolution. 10 We overcome the experimental difficulties of measuring fast processes by developing a circuit including a conventional DC current ampli fier and an AC-coupled gigahertz ampli fier 10,11 (Figure 1). The DC ampli fier measures slow processes, which allows us to com- pare the results with previous studies, and the AC-coupled ampli- fier probes fast transient processes that cannot be measured by the DC ampli fier. The separation of the DC and AC components of the signal from an atomic sized contact formed with a scanning tunneling microscope (STM) break junction is achieved by using an LC network consisting of an inductor and a capacitor, commonly referred to as a “bias-T” in microwave terminology. The inductor feeds the low frequency components of the current signal into a DC current amplifier and the capacitor couples the AC component to a radio frequency (rf) ampli fier. Due to the large ratio of the single atom contact resistance (1/G 0 ∼12.9 kΩ) to the standard char- acteristic impedance (50 Ω) used in RF systems, the AC current coupled to the RF circuit is small. To overcome this difficulty, we have built three RF amplification stages to increase the gain of the Received: July 30, 2010 Revised: December 27, 2010