Noting the amazing advances in semiconductor technology, Intel co-founder Gordon Moore has proposed that the number of chip transistors will double each year, an observation that has arisen since he filed the complaint in 1
Today, a new generation of devices with unique properties is under development. While ultra-miniaturization continues at a steady pace, researchers have begun to explore the intersection of physical and chemical properties occurring at the molecular level.
Advances in this fast-paced domain could improve data storage and information processing devices and help the development of molecular switches, among other innovations.
Nongjian "NJ" Tao and his collaborators have recently described a series of studies on electrical conductance through single molecules. The creation of electronics on this infinitesimal scale presents many challenges. In the world of the very last, the peculiar properties of the quantum world dominate. Here, electrons that flow like the current behave like waves and are subject to a phenomenon known as quantum interference. The ability to manipulate this quantum phenomenon could help open the door to new nanoelectronic devices with unusual properties.
"We are interested not only in measuring quantum phenomena in single molecules, but also in controlling them, which allows us to understand the basic transport charge in molecular systems and study new device functions," says Tao.
Tao is the director of the Biodesign Center for bioelectronics and biosensors. In the research that appears in the journal Nature Materials Tao and colleagues from Japan, China and the United Kingdom boundary experiments in which a single organic molecule is suspended between a pair of electrodes as current is passed through the small structure.
Researchers explore the transport properties of charge through molecules. They demonstrated that a spectral property of electrons – known as quantum interference – can be accurately modulated into two different configurations of the molecule, known as Para and Meta.
It turns out that the effects of quantum interference can cause substantial changes in the conductance properties of molecular-scale devices. By controlling quantum interference, the group demonstrated that the electrical conductance of a single molecule can be precisely regulated over two orders of magnitude. The precise and continuous control of quantum interference is seen as a key ingredient in the future development of a large-scale molecular-scale electronics, operating at high speed and low power.
Such single-molecule devices could potentially act as transistors, wires, rectifiers, switches or logic gates and may find their way into futuristic applications including superconducting quantum interference devices (SQUIDs), quantum cryptography and quantum computing.
For the current study, molecular ring-shaped hydrocarbons that may appear in different configurations – have been used since they are among the simplest and most versatile candidates for modeling molecular electronics behavior and are ideal for observe the effects of quantum interference at the nanometer scale.
To investigate how the charge moves through a single molecule, interrupted breaking junction measurements were performed. The tests involve the use of a tunnel or STM scanning microscope. The molecule under examination is hovering between a golden substrate and a golden tip of the STM device. The tip of the STM is repeatedly brought in and out of contact with the molecule, breaking and reforming the junction while the current passes through each terminal.
Thousands of conductance against traces of distance were recorded, with the particular molecular properties of the two molecules used for experiments that alter the flow of electrons through the junction. The molecules in the "Para" configuration showed higher conductance values than the molecules of the "Meta" form, indicating the constructive vs. destructive quantum interference in the molecules.
Using a technique known as electrochemical gating, researchers were able to continuously monitor conductance over two orders of magnitude. In the past, the alteration of quantum interference properties required changes to the molecule that carried the charge used for the device. The current study marks the first opportunity for conductance regulation in a single molecule.
As the authors note, molecule-scale conductance is significantly influenced by quantum interference involving the electronic orbitals of the molecule. Specifically, the interference between the highest molecular or molecular HOMO or the lowest unoccupied molecular orbital or LUMO seems to be the dominant determinant of conductance in single molecules. Using an electrochemical gate voltage, the quantum interference in the molecules could be gently tuned.
Researchers were able to demonstrate a good agreement between theoretical calculations and experimental results, indicating that the HOMO and LUMO contributions to conductance were additive for the Para molecules, resulting in constructive and subtractive interference for Meta, leading to destructive interference, just like waves in the water can combine to form a bigger wave or cancel each other, depending on their phase.
Whereas previous theoretical calculations of charge transport through the single molecules had been performed, the experimental verification had to wait for a series of advances in nanotechnology, in scanning microscopy and in the methods for forming electro-functional connections of molecules on surfaces metal. Now, with the ability to subtly alter conductance through manipulation of quantum interference, the field of molecular electronics is open to a wide range of innovations.
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Gate control of quantum interference and direct observation of anti-resonances in single-molecule charge transport, DOI: 10.1038 / s41563-018-0280-5, https://www.nature.com/articles/s41563- 018-0280-5