Break junctions (BJs) provide a robust method for studying single molecules in ambient conditions and pave the way for building a new class of nanoelectronic devices controlled via single-molecule properties. There are three types of break junctions:
Mechanically controllable (MCBJs)
Scanning tunneling microscope (STM-BJs)
, which are quite common thanks to their ease of fabrication and long-term operational stability. A mechanically controllable break junction consists of a photolithographically defined metal constriction suspended on top of a flexible substrate. Once the mechanical structure is prepared, the molecules are brought in, in a solution or in a vacuum, and their presence is confirmed by comparing conductance measurements with and without them. The distance between the two junction electrodes – the junction width – can be varied mechanically and fine-tuned using a piezo stage. Molecular transport can be further controlled electrostatically through a metallic gate electrode, or optically using a transparent glass substrate.
One of the biggest challenges in MCBJ measurements is how to establish the presence of a single molecule. To capture a molecule, the junction needs to break while immersed in the gaseous or liquid molecular environment. A single molecule has a well-defined number of quantized conduction channels that can be probed via charge transport, so the measurement starts by passing a current I through a junction that is opened and closed continuously, driven by an AC voltage excitation V that results in a conductance G = I/V through the junction.
When the junction is closed, conduction takes place over a few of the junction's atoms and exhibits a quantized conductance that is a multiple of conductance quanta G0 = 2e2/h. When the junction breaks and a gap is formed between the electrodes, the conductance drops exponentially with the junction width due to tunneling, so that G << G0. In this regime, a conductance plateau appears whenever a single molecule attaches between the electrodes – even when the junction width increases further.
Once this conductance plateau is identified, a control loop is activated to keep the junction width constant for additional spectroscopic investigations. In this PID control loop, the feedback mechanism is based on the piezo voltage output that is proportional to the junction width.
The entire process is stochastic and noisy due to the variations in the molecule's binding to the metallic leads as the junction is opened and closed. In these conditions, lock-in amplifiers are used to increase the signal-to-noise ratio through signal modulation and filtering. With a lock-in amplifier such as the Zurich Instruments MFLI, the junction's I-V characteristics can be measured along with higher-order conductance terms to identify the vibrational spectra of a single molecule. For the best measurement results, the dynamic range of the current input needs to cover more than five orders of magnitude to capture all junction configurations.
The Benefits of Choosing Zurich Instruments
The MFLI-Lock-in Amplifier helps you simplify your MCBJ setup and get results
Measure the junction current with a high signal-to-noise ratio over a wide dynamic range spanning from pA to mA using a dedicated low-noise current input.
Take advantage of measurement and analysis tools coming all in one box: keep the junction width and conductance level constant thanks to the MF-PID option, simultaneously capture DC, AC and the second derivative of the conductance with the MF-MD option, and digitize all the data you need for further analysis.
Reduce the amount of time spent on data post-processing thanks to the Scope, Plotter, Spectroscope, and Sweeper tools offered by the LabOne® instrument control software.
Automate your measurement workflow with the APIs for Python, C, MATLAB®, LabVIEW™ and .NET.