Researchers create molecular diode

October 21, 2009

Recently, at Arizona State University’s Biodesign Institute, N.J. Tao and collaborators have found a way to make a key electrical component on a phenomenally tiny scale. Their single-molecule diode is described in this week’s online edition of Nature Chemistry.

In the electronics world, diodes are a versatile and ubiquitous component. Appearing in many shapes and sizes, they are used in an endless array of devices and are essential ingredients for the semiconductor industry. Making components including diodes smaller, cheaper, faster and more efficient has been the holy grail of an exploding electronics field, now probing the nanoscale realm.
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Smaller size means cheaper cost and better performance for electronic devices. The first-generation computer CPU used a few thousand transistors, Tao says noting the steep advance of silicon technology.

“Now even simple, cheap computers use millions of transistors on a single chip.”

But lately, the task of miniaturization has become much more difficult, and the famous dictum known as Moore’s law – which states that the number of silicon-based transistors on a chip doubles every 18 to 24 months – will eventually reach its physical limits.

“Transistor size is reaching a few tens of nanometers, only about 20 times larger than a molecule,” Tao says. “That’s one of the reasons people are excited about this idea of molecular electronics.”

Diodes are critical components for a broad array of applications, from power conversion equipment to radios, logic gates, photodetectors and light-emitting devices. In each case, diodes are components that allow current to flow in one direction around an electrical circuit but not the other. For a molecule to perform this feat, Tao says it must be physically asymmetric, with one end capable of forming a covalent bond with the negatively charged anode and the other with the positive cathode terminal.

The new study compares a symmetric molecule with an asymmetric one, detailing the performance of each in terms of electron transport.

“If you have a symmetric molecule, the current goes both ways, much like an ordinary resistor,” Tao says. This is potentially useful, but the diode is a more important (and difficult) component to replicate.

The idea of surpassing silicon limits with a molecule-based electronic component has been around awhile. “Theoretical chemists Mark Ratner and Ari Aviram proposed the use of molecules for electronics such as diodes back in 1974,” Tao says, adding “people around the world have been trying to accomplish this for more than 30 years.”

Most efforts to date have involved many molecules, Tao says, referring to molecular thin films. Only very recently have serious attempts been made to surmount the obstacles to single-molecule designs. One of the challenges is to bridge a single molecule to at least two electrodes supplying current to it. Another challenge involves the proper orientation of the molecule in the device.

“We are now able to do this – to build a single molecule device with a well-defined orientation,” Tao says.

The technique developed by Tao’s group relies on a property known as AC modulation.

“Basically, we apply a little, periodically varying mechanical perturbation to the molecule,” Tao says. “If there’s a molecule bridged across two electrodes, it responds in one way. If there’s no molecule, we can tell.”

The interdisciplinary project involved Luping Yu, a professor at the University of Chicago, who supplied the molecules for study, as well as theoretical collaborator, Ivan Oleynik, a professor from the University of South Florida. The team used conjugated molecules, in which atoms are stuck together with alternating single and multiple bonds. Such molecules display large electrical conductivity and have asymmetrical ends capable of spontaneously forming covalent bonds with metal electrodes to create a closed circuit.

The project’s results raise the prospect of building single molecule diodes – the smallest devices one can ever build.

“I think it’s exciting because we are able to look at a single molecule and play with it,” Tao says. “We can apply a voltage, a mechanical force, or optical field, measure current and see the response. As quantum physics controls the behaviors of single molecules, this capability allows us to study properties distinct from those of conventional devices.”

Chemists, physicists, materials researchers, computational experts and engineers all play a central role in the emerging field of nanoelectronics, where a zoo of available molecules with different functions provide the raw material for innovation. Tao also is examining the mechanical properties of molecules – more specifically, their ability to oscillate. Binding properties between molecules make them attractive candidates for a new generation of chemical sensors.

“Personally, I am interested in molecular electronics not because of their potential to duplicate today’s silicon applications,” Tao says. Instead, molecular electronics will benefit from unique electronic, mechanical, optical and molecular binding properties that set them apart from conventional semiconductors. This may lead to applications complementing rather than replacing silicon devices.

Richard Harth,">
Biodesign Institute

Lisa Robbins

Assistant Director, Media Relations and Strategic Communications


James Elser joins ranks of elite faculty

October 21, 2009

Editor’s Note: This profile is one in a series that highlights Arizona State University’s 2008 and 2009 Regents’ Professors. The Regents’ Professor honor is the most prestigious faculty award at the university. Click here to view the complete list of awardees.

In another lifetime, ASU Regents’ Professor James Elser might have pursued his childhood passion for a life devoted to faith, as a priest, taking confessions rather than conducting experiments. However, his early desire to discover answers to deep questions took him into a career in science instead. Download Full Image

As a professor in the School of Life Sciences at Arizona State University, Elser has taken fields of study in new directions, both physically and experimentally. In the pursuit of the yet-to-be-known about the evolution, the flow of energy and materials in ecosystems and the impact of human activity in nature, Elser has traveled from the frozen stretches of Antarctica and the alpine lakes of Norway and Colorado to the Mongolian grasslands of China and the hot springs of Yellowstone National Park and Cuatro Cienegas, Mexico.

A scholar and adventurer, Elser examines the balance of the elements carbon, nitrogen and phosphorus in organisms and ecosystems and tries to understand their potential role in ecological and evolutionary dynamics. This focus emerges from Elser’s pioneering world view, known as the theory of “biological stoichiometry.”

“We started our studies examining plankton, nutrient cycling, food quality and element ratios in different zooplankton species,” Elser says. “We came to the realization that some of the fundamental understanding about plankton could be applied to all living things, from fruit flies to tumors.”

In addition to co-writing a book considered a “milestone in ecology” (“Ecological Stoichiometry: the Biology of Elements from Molecules to the Biosphere”) with Robert W. Sterner, his colleague at the University of Minnesota, Elser has penned or co-written more than 150 research publications on topics as wide ranging as ecosystem nutrient limitation, trophic dynamics, biogeochemical cycling, life history evolution, proteomics, cancer and infection. His highly cited work has appeared in venues as diverse as Science, Nature, Nature Reviews Microbiology, Ecology, Limnology and Oceanography, American Naturalist, Genome, Ecology Letters, Ecosystems, and Proceedings of the National Academy of Sciences.

Even more importantly, Elser has communicated his joy of discovery and creative approach to science to others.

As an ASU life sciences professor in the College of Liberal Arts and Science since 1990, Elser has taught more than 12,000 students and directly mentored 33 undergraduates, 10 graduate students, and 11 others in his laboratory. His “Biology 100” course is legendary for “weird days.”

“Weird” was the watchword for Fridays, when Elser would come to class armed with claims from pop culture and pseudo science, such as “Nostradamus predicted the attack on 9/11” or “Therapeutic touch can cure cancer.” He would deconstruct the claims, using the scientific method and look for real evidence behind the “weird.”

Elser melds an inquiry-based and question-driven approach that taps into students’ natural curiosity and desire to answer questions. His teaching style promotes life skills that extend beyond biology, as he challenges students to discover how scientific reasoning can apply in their daily lives and help them make sense of an increasingly complex world where science has major ramifications for political, cultural and personal decision-making.

Elser also is highly active in building integrative research collaborations within ASU and abroad. For example, one project’s study – in partnership with Valeria Souza, a professor with Universidad Nacional Autónoma de México – of a unique series of ancient hot springs and ecosystems in Cuatro Cienegas in Chihuahua, Mexico – has led to the discovery of new species of microbes and fundamental insights into how phosphorus limitation has shaped their evolution and that of the food webs that they support. His work with Souza has greatly increased the national and international profile of this unique landscape, including added support from the governor of Coahuila, the secretary of the environment, and Mexican President Phillipe Calderon.

Creative approaches and partners also have helped Elser garner millions of dollars in research funding from National Science Foundations, National Aeronautics and Space Administration, and National Institutes of Health.

Elser’s commitment to learning and discovery also extends to the wider community in Arizona. As associate director of the Research and Training Initiative Program in the School of Life Sciences, he has led faculty and students to paint houses for the elderly during the Rock and Roll Paint-Thon, held fundraisers for Chrysalis Shelter for Women and Children, Pappas School for Homeless Children, and Camp Kesem (an ASU undergraduate organization that provides experience for children whose family members have cancer), among others projects. He also has spearheaded the creation of credible, institutional relationships with Arizona Science Center and area public schools, and encouraged his colleagues to reach out, communicate and contribute in ways that outlast a funding cycle. In addition, he created the annual SOLS Takes a Hike community outreach event that puts graduate students and faculty in touch with the public through a series of guided hikes conducted in local parks. This event is now part of ASU in the Community, and has touched the minds and hearts of more than 300 children, families, retirees and ASU alumni.

Margaret Coulombe

Director, Executive Communications, Office of the University Provost