Quantum strangeness gives rise to new electronics


February 11, 2019

Citing the startling advances in semiconductor technology of the time, Intel co-founder Gordon Moore in 1965 proposed that the number of transistors on a chip would double each year — the accuracy of that observation has been borne out. Still, it’s unlikely Moore could have foreseen the extent of the electronics revolution currently underway.

Today, a new breed of devices that bear unique properties is being developed. As ultra-miniaturization continues apace, researchers have begun to explore the intersection of physical and chemical properties occurring at the molecular scale. Nongjian “NJ” Tao is the director of the Center for Bioelectronics and Biosensors at the Biodesign Institute and is a professor in the Ira A. Fulton Schools of Engineering at Arizona State University. Download Full Image

Advances in this fast-paced domain could improve devices for data storage and information processing and aid in the development of molecular switches, among other innovations.

Arizona State University's Nongjian “NJ” Tao and his collaborators recently described a series of studies into electrical conductance through single molecules. Creating electronics at this infinitesimal scale presents many challenges. In the world of the ultra-tiny, the peculiar properties of the quantum world hold sway. Here, electrons flowing as 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 in not only measuring quantum phenomena in single molecules, but also controlling them. This allows us to understand the basic charge transport in molecular systems and study new device functions,” Tao said.

Tao is the director of the Biodesign Center for Bioelectronics and Biosensors. In research appearing in the journal Nature Materials, Tao and colleagues from Japan, China and the U.K. outline experiments in which a single organic molecule is suspended between a pair of electrodes as a current is passed through the tiny structure.

The researchers explore the charge transport properties through the molecules. They demonstrated that a ghostly wavelike property of electrons — known as quantum interference — can be precisely modulated in two different configurations of the molecule, known as Para and Meta.

It turns out that quantum interference effects can cause substantial variation in the conductance properties of molecule-scale devices. By controlling the quantum interference, the group showed that electrical conductance of a single molecule can be fine-tuned over two orders of magnitude. Precisely and continuously controlling quantum interference is seen as a key ingredient in the future development of wide-ranging 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, quantum cryptography and quantum computing.

For the current study, the molecules — ring-shaped hydrocarbons that can appear in different configurations — were used, as they are among the simplest and most versatile candidates for modeling the behavior of molecular electronics and are ideal for observing quantum interference effects at the nanoscale.

In order to probe the way charge moves through a single molecule, so-called break junction measurements were made. The tests involve the use of a scanning tunneling microscope, or STM. The molecule under study is poised between a gold substrate and the gold 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 versus distance traces were recorded, with the particular molecular properties of the two molecules used for the experiments altering the electron flow through the junction. Molecules in the Para configuration showed higher conductance values than molecules of the Meta form, indicating constructive vs. destructive quantum interference in the molecules.

Using a technique known as electrochemical gating, the researchers were able to continuously control the conductance over two orders of magnitude. In the past, altering quantum interference properties required modifications to the charge-carrying molecule used for the device. The current study marks the first occasion of conductance regulation in a single molecule.

As the authors note, conductance at the molecular scale is sensitively affected by quantum interference involving the electron orbitals of the molecule. Specifically, interference between the highest occupied molecular orbital or HOMO and lowest unoccupied molecular orbital or LUMO appears to be the dominant determinant of conductance in single molecules. Using an electrochemical gate voltage, quantum interference in the molecules could be delicately tuned.

The researchers were able to demonstrate good agreement between theoretical calculations and experimental results, indicating that the HOMO and LUMO contributions to the conductance were additive for Para molecules, resulting in constructive interference, and subtractive for Meta, leading to destructive interference, much as waves in water can combine to form a larger wave or cancel one another out, depending on their phase.

While previous theoretical calculations of charge transport through single molecules had been carried out, experimental verification has had to wait for a number of advances in nanotechnology, scanning probe microscopy and methods to form electrically functional connections of molecules to metal surfaces. Now, with the ability to subtly alter conductance through the manipulation of quantum interference, the field of molecular electronics is open to a broad range of innovations.

Richard Harth

Science writer, Biodesign Institute at ASU

480-727-0378

NASA Space Grant scholars to hold showcase on Tempe campus


February 11, 2019

Since 1988, Arizona State University has participated in the national NASA Space Grant Program that is designed to provide STEM undergraduate and graduate students with the opportunity to actively work on NASA-related research alongside a faculty mentor.

On Wednesday, Feb. 13, the 2019 space grant scholars will be showcasing their research projects at the annual NASA Space Grant Poster Session from 10 a.m. to 2 p.m. in Interdisciplinary Science and Technology Building IV on the Tempe campus. This free event is open to the public.  Chemical engineering major and ASU NASA Space Grant alumna Nikita Kowal at the poster session at the School of Earth and Space Exploration. Photo by ASU Download Full Image

“Being a NASA space grant scholar provides an opportunity for undergraduates to work on real projects and gain real-world experience while they are at ASU,” said Tom Sharp, of the School of Earth and Space Exploration and director of ASU’s NASA Space Grant program.

More than 35 students will be presenting works to the public on research projects involving CubeSats, climate change, high-altitude ballooning, exoplanets, biology and geology. This year, participating students are majoring in degrees from the School of Earth and Space Exploration, the Ira A. Fulton Schools of Engineering, the School of Molecular Sciences and the Department of Physics. The showcase will also include several students from Phoenix College and Glendale Community College as guest presenters.

“The event gives the public the opportunity to see a range of projects that our students do in science and engineering,” Sharp said. “Each scholar is required to lead a project with a reportable outcome and this is their first chance to pull it together and present their findings.”

The poster session will also prepare the scholars for their next hurdle, presenting their projects at the Arizona NASA Space Grant Undergraduate Research Internship Statewide Symposium in Tempe on April 13. Since ASU is already known for its innovation ranking, these students are sure to represent their fields, and the university, with flying colors in this statewide event.

How to apply to be a NASA Space Grant Scholar or Fellow

Students interested in applying to the ASU NASA Space Grant program for the 2019–20 school year may submit an application on the NASA Space Grant website. The deadline for undergraduate and mentor applications is May 10, 2019. The deadline for graduate fellowship applications is April 24, 2019.

“For ASU students interested in becoming a NASA Space Grant Scholar, February’s poster session is a great opportunity to meet with current scholars and find out how to apply for the program,” said Desiree Crawl, senior coordinator for the program. 

Up to 50 ASU students are selected each year for this prestigious program. Each scholar conducts a yearlong research program and presents the results of their research in NASA-focused fields ranging from electronics to small satellites to astrobiology.

“Our scholars work with a faculty mentor throughout their internships. This gives them opportunities, references and a distinct advantage when applying for graduate school or for positions in professional STEM fields,” Sharp said.  

Karin Valentine

Media Relations & Marketing manager, School of Earth and Space Exploration

480-965-9345