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Molecular electronics—where individual molecules are employed to perform the same function as microelectronic devices like diodes—offers a wealth of opportunities for miniaturization of electronics for a plethora of applications. It will also deliver significant cost savings. Scientists began to take a serious interest in this technology in the 1990s, with the aim of creating electronic components made from molecules that are smaller and faster than wafer-based options. Three decades later, after years of slow progress, research into this technology is finally coming to fruition, with a few niche products based on these underlying principles now becoming available—including a device for guitar distortion that reached the market in 2017—and clearly demonstrating its potential.
The recent technological breakthroughs in molecular electronics have opened up possibilities for manufacturers to reimagine the development of products from the ground up. They now have the ability to create custom components for a specific application rather than making do with existing ones. One can also develop devices by breaking large silicon wafers down into smaller pieces—an approach that would drastically reduce the size and cost of parts.
Researchers at the University of Liverpool, U.K., are focused on the fundamental phenomena behind this technology, looking at how molecules behave at a quantum level and the chemical complexity involved. The knowledge acquired during these investigations will enable these quantum phenomena to be exploited to achieve the ultimate performance, delivering a combination of novel functionality, better charge transport and reduced power consumption.
Motionless light collection
One aspect of the Liverpool research is the study of light emission from minute single-entity electronic devices in the 1- to 2-nm range. The hope is that by understanding what happens in one molecule, it will then be possible to scale up and understand what happens in the collection of molecules that make an OLED, similar to those used in phones. However, extracting good data was initially challenging due to neighboring effects, leaving the researchers reliant on sample averaging from measurements of hundreds of thousands of molecules. The ability to measure a single molecule would provide far more accurate data, allowing the fundamental laws behind light emission from these molecules to be established.
The experimental setup at Liverpool is based on a single molecule positioned between two electrodes: one anchored to stop it from moving and the other linked to a piezo positioner for small adjustments. The junction can then be fine-tuned to the correct dimensions, where the molecule is perfectly extended and not stretched or compressed. Once the molecule is in position, the bias—the voltage between the two electrodes and a third electrode acting as an electrostatic gate, forming a molecular transistor—can be altered and the light emitted by the molecule examined. Maintaining a molecular junction like this for more than a few seconds is challenging, requiring a very precise and stable positioner to extend this timeframe and allow more photons to be collected. The N-216 NEXLINE actuator from Physik Instrumente (PI) proved the answer, enabling a position to be held for several minutes with no significant drift and very low piezo creep.
The N-216 NEXLINE actuator employs the PiezoWalk principle, combining regular, sheer and compression actuators that work together seamlessly to provide sub-nanometer precision and, most importantly, incredible stability. Previously, it was only possible to collect low counts at the emission maxima, but the N216 NEXLINE actuator can maintain a stable molecular junction for minutes—rather than just a few seconds—at room temperature, allowing light emission to be collected for a longer period of time and a whole spectrum to be recorded.
Achieving compliance
European regulations are very stringent on the composition and materials used in components, which makes careful choice of the right tools essential. All PI products are RoHS-compliant, making the N-216 NEXLINE actuator an excellent option for Liverpool’s molecular electronics application. This RoHS-compliant, unique PiezoWalk system has proven to be the perfect solution to precisely hold and manipulate single molecules in the team’s research in the exciting field of molecular electronics.
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