Week 3: Measuring the Conductance of Single-Molecule Organic Semiconductors
Rohan V -
Hi all, it’s Rohan. Last week, I discussed the science behind organic semiconductors, and how a molecule containing only carbon and hydrogen can conduct electricity. Now, having covered much of the foundational science behind my work, I’ll be able to delve into more specifics regarding the tasks I am performing each week.
A central goal for me this past week has been more deeply understanding how scientists can measure the conductance of singular molecules, especially taking into consideration their very small scale. To investigate, I spent time reading some foundational papers in single-molecule electronics. In the remainder of this post, I will detail the experimental setup researchers use to collect data on organic semiconductors to test if they are a good candidate for use in electronic devices.
Most quantum researchers, including my current site placement, use the mechanically-controlled break junction (MCBJ) technique to measure single-molecule conductance. In this technique, the molecule that will be analyzed is dissolved in a solvent and poured between two gold electrodes. The gold electrodes are initially connected to one another, but a ‘push rod’ slowly pushes them apart until they finally break at the midpoint. At the point when the gold atoms fully break apart from one another, a molecule can bind to ‘bridge the gap’ between the gold. However, as the gold continues to get pushed apart, the molecule falls out of the junction. After a specified amount of time, the push rod reverses its movement, descending and allowing the gold electrodes to bind to each other once again; this allows the experiment to be repeated. A diagram of this setup is shown here.
During this process, an electric potential difference is applied across the junction, inducing a current (the flow of charge from one end of the junction to the other). By continuously measuring the current passing through the junction, researchers can calculate the conductance of the molecule: how efficiently it conducts electricity.
Due to the small scale of these experiments, the results are significantly affected by random fluctuations in the surroundings (humidity of the air, ambient temperature, etc). To remedy this source of error, researchers conduct the experiment thousands of times (breaking and reforming the gold thousands of times in a row) and collect data of the conductance of the junction as a function of the distance the gold has been pushed apart.
This method of experimentation begs the question: how can one analyze such a large quantity of data efficiently? My work in this project centers around developing more efficient ways to analyze and interpret the bulky datasets that contain the results of these experiments. In my next post, I’ll explain some of the commonly used methods of data analysis in single-molecule electronics, as well as some of their shortcomings.