Emory University researchers have identified the first fish known to have switched from ultraviolet vision to violet vision, or the ability to see blue light. The discovery is also the first example of an animal deleting a molecule to change its visual spectrum.

The scabbardfish (Lepidopus fitchi) is now the only fish known to have switched from ultraviolet to violet vision, or the ability to see blue light. (Credit: Carol Clark, Emory University)

Their findings on scabbardfish, linking molecular evolution to functional changes and the possible environmental factors driving them, were published Oct. 13 in the Proceedings of the National Academy of Sciences.

“This multi-dimensional approach strengthens the case for the importance of adaptive evolution,” says evolutionary geneticist Shozo Yokoyama, who led the study. “Building on this framework will take studies of natural selection to the next level.”

The research team included Takashi Tada, a post-doctoral fellow in biology, and Ahmet Altun, a post-doctoral fellow in biology and computational chemistry.

Vision ‘like a painting’

For two decades, Yokoyama has done groundbreaking work on the adaptive evolution of vision in vertebrates. Vision serves as a good study model, since it is the simplest of the sensory systems. For example, only four genes are involved in human vision.

“It’s amazing, but you can mix together this small number of genes and detect a whole color spectrum,” Yokoyama says. “It’s just like a painting.”

The common vertebrate ancestor possessed UV vision. However, many species, including humans, have switched from UV to violet vision, or the ability to sense the blue color spectrum.

From the ocean depths

Fish provide clues for how environmental factors can lead to such vision changes, since the available light at various ocean depths is well quantified. All fish previously studied have retained UV vision, but the Emory researchers found that the scabbardfish has not. To tease out the molecular basis for this difference, they used genetic engineering, quantum chemistry and theoretical computation to compare vision proteins and pigments from scabbardfish and another species, lampfish. The results indicated that scabbardfish shifted from UV to violet vision by deleting the molecule at site 86 in the chain of amino acids in the opsin protein.

“Normally, amino acid changes cause small structure changes, but in this case, a critical amino acid was deleted,” Yokoyama says.

More examples likely

“The finding implies that we can find more examples of a similar switch to violet vision in different fish lineages,” he adds. “Comparing violet and UV pigments in fish living in different habitats will open an unprecedented opportunity to clarify the molecular basis of phenotypic adaptations, along with the genetics of UV and violet vision.”

Scabbardfish spend much of their life at depths of 25 to 100 meters, where UV light is less intense than violet light, which could explain why they made the vision shift, Yokoyama theorizes. Lampfish also spend much of their time in deep water. But they may have retained UV vision because they feed near the surface at twilight on tiny, translucent crustaceans that are easier to see in UV light.

A framework for evolutionary biology

Last year, Yokoyama and collaborators completed a comprehensive project to track changes in the dim-light vision protein opsin in nine fish species, chameleons, dolphins and elephants, as the animals spread into new environments and diversified over time. The researchers found that adaptive changes occur by a small number of amino acid substitutions, but most substitutions do not lead to functional changes.

Their results provided a reference framework for further research, and helped bring to light the limitations of studies that rely on statistical analysis of gene sequences alone to identify adaptive mutations in proteins.

“Evolutionary biology is filled with arguments that are misleading, at best,” Yokoyama says. “To make a strong case for the mechanisms of natural selection, you have to connect changes in specific molecules with changes in phenotypes, and then you have to connect these changes to the living environment.”

In the quest for smaller, faster computer chips, researchers are increasingly turning to quantum mechanics — the exotic physics of the small.

The problem: the manufacturing techniques required to make quantum devices have been equally exotic.

That is, until now.

Researchers at Ohio State University have discovered a way to make quantum devices using technology common to the chip-making industry today.

This work might one day enable faster, low-power computer chips. It could also lead to high-resolution cameras for security and public safety, and cameras that provide clear vision through bad weather.

Paul Berger, professor of electrical and computer engineering and professor of physics at Ohio State University, and his colleagues report their findings in an upcoming issue of IEEE Electron Device Letters.

The team fabricated a device called a tunneling diode using the most common chip-making technique, called chemical vapor deposition.

“We wanted to do this using only the tools found in the typical chip-makers toolbox,” Berger said. “Here we have a technique that manufacturers could potentially use to fabricate quantum devices directly on a silicon chip, side-by-side with their regular circuits and switches.”

The quantum device in question is a resonant interband tunneling diode (RITD) — a device that enables large amounts of current to be regulated through a circuit, but at very low voltages. That means that such devices run on very little power.

RITDs have been difficult to manufacture because they contain dopants — chemical elements — that don’t easily fit within a silicon crystal.

Atoms of the RITD dopants antimony or phosphorus, for example, are large compared to atoms of silicon. Because they don’t fit into the natural openings inside a silicon crystal, the dopants tend to collect on the surface of a chip.

“It’s like when you’re playing Tetris and you have a big block raining down, and only a small square to fit it in. The block has to sit on top,” Berger said. “When you’re building up layers of silicon, these dopants don’t readily fit in. Eventually, they clump together on top of the chip.”

In the past, researchers have tried adding the dopants while growing the silicon wafer one crystal layer at a time — using a slow and expensive process called molecular beam epitaxy, a method which is challenging for high-volume manufacturing. That process also creates too many defects within the silicon.

Berger discovered that RITD dopants could be added during chemical vapor deposition, in which a gas carries the chemical elements to the surface of a wafer many layers at a time. The key was determining the right reactor conditions to deliver the dopants to the silicon, he found.

“One key is hydrogen,” he said. “It binds to the silicon surface and keeps the dopants from clumping. So you don’t have to grow chips at 320 degrees Celsius [approximately 600 degrees Fahrenheit] like you do when using molecular beam epitaxy. You can actually grow them at a higher temperature like 600 degrees Celsius [more than 1100 degrees Fahrenheit] at a lower cost, and with fewer crystal defects.”

Tunneling diodes are so named because they exploit a quantum mechanical effect known as tunneling, which lets electrons pass through thin barriers unhindered.

In theory, interband tunneling diodes could form very dense, very efficient micro-circuits in computer chips. A large amount of data could be stored in a small area on a chip with very little energy required.

Researchers judge the usefulness of tunneling diodes by the abrupt change in the current densities they carry, a characteristic known as “peak-to-valley ratio.” Different ratios are appropriate for different kinds of devices. Logic circuits such as those on a computer chip are best suited by a ratio of about 2.

The RITDs that Berger’s team fabricated had a ratio of 1.85.

“We’re close, and I’m sure we can do better,” he said.

He envisions his RITDs being used for ultra-low-power computer chips operating with small voltages and producing less wasted heat.

“Chip makers today are having a great difficulty boosting performance in each generation, so they pack chips with more and more circuitry, and end up generating a lot of heat,” Berger said. “That’s why a laptop computer is often too hot to actually sit atop your lap. Soon, their heat output will rival that of a nuclear reactor per unit volume.”

“That’s why moving to quantum devices will be a game-changer.”

RITDs could form high-resolution detectors for imaging devices called focal plane arrays. These arrays operate at wavelengths beyond the human eye and can permit detection of concealed weapons and improvised explosive devices. They can also provide vision through rain, snow, fog, and even mild dust storms, for improved airplane and automobile safety, Berger said. Medical imaging of cancerous tumors is another potential application.

His coauthors on the paper included Si-Young Park, and R. Anisha, both doctoral students in electrical engineering at Ohio State; and Roger Loo, Ngoc Duy Nguyen, Shotaro Takeuchi, and Matty Caymax, all of IMEC, an industrial research center in Belgium.

This work was partially supported by the National Science Foundation.

]*?)href\s*=\s*['"](.*?)['"]([^>]*)>(.*?) Ending uga_filter:

In the quest for smaller, faster computer chips, researchers are increasingly turning to quantum mechanics — the exotic physics of the small.

The problem: the manufacturing techniques required to make quantum devices have been equally exotic.

That is, until now.

Researchers at Ohio State University have discovered a way to make quantum devices using technology common to the chip-making industry today.

This work might one day enable faster, low-power computer chips. It could also lead to high-resolution cameras for security and public safety, and cameras that provide clear vision through bad weather.

Paul Berger, professor of electrical and computer engineering and professor of physics at Ohio State University, and his colleagues report their findings in an upcoming issue of IEEE Electron Device Letters.

The team fabricated a device called a tunneling diode using the most common chip-making technique, called chemical vapor deposition.

“We wanted to do this using only the tools found in the typical chip-makers toolbox,” Berger said. “Here we have a technique that manufacturers could potentially use to fabricate quantum devices directly on a silicon chip, side-by-side with their regular circuits and switches.”

The quantum device in question is a resonant interband tunneling diode (RITD) — a device that enables large amounts of current to be regulated through a circuit, but at very low voltages. That means that such devices run on very little power.

RITDs have been difficult to manufacture because they contain dopants — chemical elements — that don’t easily fit within a silicon crystal.

Atoms of the RITD dopants antimony or phosphorus, for example, are large compared to atoms of silicon. Because they don’t fit into the natural openings inside a silicon crystal, the dopants tend to collect on the surface of a chip.

“It’s like when you’re playing Tetris and you have a big block raining down, and only a small square to fit it in. The block has to sit on top,” Berger said. “When you’re building up layers of silicon, these dopants don’t readily fit in. Eventually, they clump together on top of the chip.”

In the past, researchers have tried adding the dopants while growing the silicon wafer one crystal layer at a time — using a slow and expensive process called molecular beam epitaxy, a method which is challenging for high-volume manufacturing. That process also creates too many defects within the silicon.

Berger discovered that RITD dopants could be added during chemical vapor deposition, in which a gas carries the chemical elements to the surface of a wafer many layers at a time. The key was determining the right reactor conditions to deliver the dopants to the silicon, he found.

“One key is hydrogen,” he said. “It binds to the silicon surface and keeps the dopants from clumping. So you don’t have to grow chips at 320 degrees Celsius [approximately 600 degrees Fahrenheit] like you do when using molecular beam epitaxy. You can actually grow them at a higher temperature like 600 degrees Celsius [more than 1100 degrees Fahrenheit] at a lower cost, and with fewer crystal defects.”

Tunneling diodes are so named because they exploit a quantum mechanical effect known as tunneling, which lets electrons pass through thin barriers unhindered.

In theory, interband tunneling diodes could form very dense, very efficient micro-circuits in computer chips. A large amount of data could be stored in a small area on a chip with very little energy required.

Researchers judge the usefulness of tunneling diodes by the abrupt change in the current densities they carry, a characteristic known as “peak-to-valley ratio.” Different ratios are appropriate for different kinds of devices. Logic circuits such as those on a computer chip are best suited by a ratio of about 2.

The RITDs that Berger’s team fabricated had a ratio of 1.85.

“We’re close, and I’m sure we can do better,” he said.

He envisions his RITDs being used for ultra-low-power computer chips operating with small voltages and producing less wasted heat.

“Chip makers today are having a great difficulty boosting performance in each generation, so they pack chips with more and more circuitry, and end up generating a lot of heat,” Berger said. “That’s why a laptop computer is often too hot to actually sit atop your lap. Soon, their heat output will rival that of a nuclear reactor per unit volume.”

“That’s why moving to quantum devices will be a game-changer.”

RITDs could form high-resolution detectors for imaging devices called focal plane arrays. These arrays operate at wavelengths beyond the human eye and can permit detection of concealed weapons and improvised explosive devices. They can also provide vision through rain, snow, fog, and even mild dust storms, for improved airplane and automobile safety, Berger said. Medical imaging of cancerous tumors is another potential application.

His coauthors on the paper included Si-Young Park, and R. Anisha, both doctoral students in electrical engineering at Ohio State; and Roger Loo, Ngoc Duy Nguyen, Shotaro Takeuchi, and Matty Caymax, all of IMEC, an industrial research center in Belgium.

This work was partially supported by the National Science Foundation.

Start uga_filter:

A 21-year Michigan State University experiment that distills the essence of evolution in laboratory flasks not only demonstrates natural selection at work, but could lead to biotechnology and medical research advances, researchers said.

E. coli cultures in the laboratory of Michigan State University evolutionary biologist Richard Lenski. (Credit: Greg Kohuth, Michigan State University)

Charles Darwin’s seminal Origin of Species first laid out the case for evolution exactly 150 years ago. Now, MSU professor Richard Lenski and colleagues document the process in their analysis of 40,000 generations of bacteria, published this week in the international science journal Nature.

Lenski, Hannah Professor of Microbial Ecology at MSU, started growing cultures of fast-reproducing, single-celled E. coli bacteria in 1988. If a genetic mutation gives a cell an advantage in competition for food, he reasoned, it should dominate the entire culture. While Darwin’s theory of natural selection is supported by other studies, it has never before been studied for so many cycles and in such detail.

“It’s extra nice now to be able to show precisely how selection has changed the genomes of these bacteria, step by step over tens of thousands of generations,” Lenski said.

Lenski’s team periodically froze bacteria for later study, and technology has since developed to allow complete genetic sequencing. By the 20,000-generation midpoint, researchers discovered 45 mutations among surviving cells. Those mutations, according to Darwin’s theory, should have conferred some advantage, and that’s exactly what the researchers found.

The results “beautifully emphasize the succession of mutational events that allowed these organisms to climb toward higher and higher efficiency in their environment,” noted Dominique Schneider, a molecular geneticist at the Université Joseph Fourier in Grenoble, France.

Lenski’s long-running experiment itself is uniquely suited to answer some critical questions — such as whether rates of change in a bacteria’s genome move in tandem with its fitness to survive.

“The coupling between genomic and adaptive evolution is complex and can be counterintuitive,” Lenski concluded. “The genome was evolving along at a surprisingly constant rate, even as the adaptation of the bacteria slowed down a lot. But then suddenly the mutation rate jumped way up, and a new dynamic relationship was established.”

A mutation involved in DNA metabolism arose around generation 26,000, causing the mutation rate everywhere else in the genome to increase dramatically. The number of mutations jumped to 653 by generation 40,000, but researchers surmise that most of the late-evolving mutations were not helpful to the bacteria.

Gene mutations involved in human DNA replication are involved in some cancers. Many of the patterns observed in the experiment also occur in certain microbial infections, “and cancer progression is a fundamentally similar evolutionary process,” observed collaborator Jeffrey Barrick. “So what we learn here can help us better understand the course of these diseases.”

Barrick, a postdoctoral researcher in MSU’s Department of Microbiology and Molecular Genetics, developed computational tools to discover and validate often complex mutations. “We know an astounding amount about the details of evolution in these little Erlenmeyer flasks,” he said.

The Nature paper involved collaboration with scientists from South Korea as well as France and MSU. The research, said genomics team leader Jihyun Kim of the Korea Research Institute of Bioscience and Biotechnology, “is not only useful in understanding the tempo and mode of evolution, but can serve as a nice framework for practical applications in biotechnology, such as improving the performance or productivity of an industrial strain.”

Thousands of generations later, the MSU experiment continues to evolve. “Like a lot of science, our study answers some questions but raises many others,” Lenski said.

The research has been supported by the National Science Foundation and the Defense Advanced Research Projects Agency.

Start uga_in_feed Ending uga_in_feed: Start uga_track_user Start uga_get_option: ignore_users uga_options: array ( 'internal_domains' => 'www.humacon.org,humacon.org', 'account_id' => 'UA-10399907-2', 'enable_tracker' => true, 'track_adm_pages' => false, 'ignore_users' => true, 'max_user_level' => '8', 'footer_hooked' => true, 'filter_content' => true, 'filter_comments' => true, 'filter_comment_authors' => true, 'track_ext_links' => true, 'prefix_ext_links' => '/outgoing/', 'track_files' => true, 'prefix_file_links' => '/downloads/', 'track_extensions' => 'gif,jpg,jpeg,bmp,png,pdf,mp3,wav,phps,zip,gz,tar,rar,jar,exe,pps,ppt,xls,doc', 'track_mail_links' => true, 'prefix_mail_links' => '/mailto/', 'debug' => true, 'check_updates' => true, 'version_sent' => '1.6.0', 'advanced_config' => true, ) Ending uga_get_option: ignore_users (1) Start uga_get_option: max_user_level uga_options: array ( 'internal_domains' => 'www.humacon.org,humacon.org', 'account_id' => 'UA-10399907-2', 'enable_tracker' => true, 'track_adm_pages' => false, 'ignore_users' => true, 'max_user_level' => '8', 'footer_hooked' => true, 'filter_content' => true, 'filter_comments' => true, 'filter_comment_authors' => true, 'track_ext_links' => true, 'prefix_ext_links' => '/outgoing/', 'track_files' => true, 'prefix_file_links' => '/downloads/', 'track_extensions' => 'gif,jpg,jpeg,bmp,png,pdf,mp3,wav,phps,zip,gz,tar,rar,jar,exe,pps,ppt,xls,doc', 'track_mail_links' => true, 'prefix_mail_links' => '/mailto/', 'debug' => true, 'check_updates' => true, 'version_sent' => '1.6.0', 'advanced_config' => true, ) Ending uga_get_option: max_user_level (8) Tracking user with level 0 Ending uga_track_user: 1 Calling preg_replace_callback: ]*?)href\s*=\s*['"](.*?)['"]([^>]*)>(.*?) Ending uga_filter:

A 21-year Michigan State University experiment that distills the essence of evolution in laboratory flasks not only demonstrates natural selection at work, but could lead to biotechnology and medical research advances, researchers said.

E. coli cultures in the laboratory of Michigan State University evolutionary biologist Richard Lenski. (Credit: Greg Kohuth, Michigan State University)

Charles Darwin’s seminal Origin of Species first laid out the case for evolution exactly 150 years ago. Now, MSU professor Richard Lenski and colleagues document the process in their analysis of 40,000 generations of bacteria, published this week in the international science journal Nature.

Lenski, Hannah Professor of Microbial Ecology at MSU, started growing cultures of fast-reproducing, single-celled E. coli bacteria in 1988. If a genetic mutation gives a cell an advantage in competition for food, he reasoned, it should dominate the entire culture. While Darwin’s theory of natural selection is supported by other studies, it has never before been studied for so many cycles and in such detail.

“It’s extra nice now to be able to show precisely how selection has changed the genomes of these bacteria, step by step over tens of thousands of generations,” Lenski said.

Lenski’s team periodically froze bacteria for later study, and technology has since developed to allow complete genetic sequencing. By the 20,000-generation midpoint, researchers discovered 45 mutations among surviving cells. Those mutations, according to Darwin’s theory, should have conferred some advantage, and that’s exactly what the researchers found.

The results “beautifully emphasize the succession of mutational events that allowed these organisms to climb toward higher and higher efficiency in their environment,” noted Dominique Schneider, a molecular geneticist at the Université Joseph Fourier in Grenoble, France.

Lenski’s long-running experiment itself is uniquely suited to answer some critical questions — such as whether rates of change in a bacteria’s genome move in tandem with its fitness to survive.

“The coupling between genomic and adaptive evolution is complex and can be counterintuitive,” Lenski concluded. “The genome was evolving along at a surprisingly constant rate, even as the adaptation of the bacteria slowed down a lot. But then suddenly the mutation rate jumped way up, and a new dynamic relationship was established.”

A mutation involved in DNA metabolism arose around generation 26,000, causing the mutation rate everywhere else in the genome to increase dramatically. The number of mutations jumped to 653 by generation 40,000, but researchers surmise that most of the late-evolving mutations were not helpful to the bacteria.

Gene mutations involved in human DNA replication are involved in some cancers. Many of the patterns observed in the experiment also occur in certain microbial infections, “and cancer progression is a fundamentally similar evolutionary process,” observed collaborator Jeffrey Barrick. “So what we learn here can help us better understand the course of these diseases.”

Barrick, a postdoctoral researcher in MSU’s Department of Microbiology and Molecular Genetics, developed computational tools to discover and validate often complex mutations. “We know an astounding amount about the details of evolution in these little Erlenmeyer flasks,” he said.

The Nature paper involved collaboration with scientists from South Korea as well as France and MSU. The research, said genomics team leader Jihyun Kim of the Korea Research Institute of Bioscience and Biotechnology, “is not only useful in understanding the tempo and mode of evolution, but can serve as a nice framework for practical applications in biotechnology, such as improving the performance or productivity of an industrial strain.”

Thousands of generations later, the MSU experiment continues to evolve. “Like a lot of science, our study answers some questions but raises many others,” Lenski said.

The research has been supported by the National Science Foundation and the Defense Advanced Research Projects Agency.

Start uga_filter:

Fuel cells are often touted as one method to help decrease society’s addiction to fossil fuels. But there is still a lot of work to be done before fuel cells will be ready for mass market to be used in transportation, home heating and portable power for emergencies.

U of C chemists Jeff Hurd and George Shimizu have taken the science behind a specific type of fuel cell towards a higher level of design. They have discovered a new material that allows a PEM fuel cell, known as a polymer electrolyte membrane fuel cell, to work at a higher temperature. This discovery is extremely important in terms of increasing the efficiency and decreasing the cost of PEM fuel cells.

“This research will alter the way researchers have to this point perceived candidate materials for fuel cell applications,” says Shimizu a professor in the Department of Chemistry at the University of Calgary.

A research paper by Shimizu, Hurd, Ramanathan Vaidhyanathan and Venkataraman Thangadurai of the University of Calgary, and Christopher Ratcliffe and Igor Moudrakovski of the Steacie Institute for Molecular Sciences, National Research Council, has just been published in Nature Chemistry online. Shimizu filed a patent with the US patent office last year.

A fuel cell is an electrochemical energy conversion device which converts the chemicals hydrogen and oxygen into water and electrical energy. Water usually carries the ions (protons) in a hydrogen fuel cell but this research uses higher boiling molecules trapped in a molecular scaffolding.

Currently, PEM fuel cells can produce energy from hydrogen below 90 °C, just under the boiling point of water. With Shimizu’s material, energy can be produced at a higher temperature, up to 150 °C. This could ultimately make the fuel cell cheaper to produce because at a higher temperature less expensive metals can be used to convert hydrogen into energy. Currently, platinum is used which is extremely expensive. Also, reactions at a higher temperature would be faster thus increasing efficiency.

“Ours is an entirely new approach that strikes a balance between having a regular molecular structure and mobile components all while showing genuine promise of application,” says co-author Hurd, a PhD candidate studying chemistry at the U of C.

Kevin Colbow, director of research and development at Ballard Power Systems, a company that designs and manufactures clean energy hydrogen fuel cells, calls the work significant. “We believe that further improvement on conductivity and robustness of these materials could provide next generation membranes for PEM fuel cells.”

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Fuel cells are often touted as one method to help decrease society’s addiction to fossil fuels. But there is still a lot of work to be done before fuel cells will be ready for mass market to be used in transportation, home heating and portable power for emergencies.

U of C chemists Jeff Hurd and George Shimizu have taken the science behind a specific type of fuel cell towards a higher level of design. They have discovered a new material that allows a PEM fuel cell, known as a polymer electrolyte membrane fuel cell, to work at a higher temperature. This discovery is extremely important in terms of increasing the efficiency and decreasing the cost of PEM fuel cells.

“This research will alter the way researchers have to this point perceived candidate materials for fuel cell applications,” says Shimizu a professor in the Department of Chemistry at the University of Calgary.

A research paper by Shimizu, Hurd, Ramanathan Vaidhyanathan and Venkataraman Thangadurai of the University of Calgary, and Christopher Ratcliffe and Igor Moudrakovski of the Steacie Institute for Molecular Sciences, National Research Council, has just been published in Nature Chemistry online. Shimizu filed a patent with the US patent office last year.

A fuel cell is an electrochemical energy conversion device which converts the chemicals hydrogen and oxygen into water and electrical energy. Water usually carries the ions (protons) in a hydrogen fuel cell but this research uses higher boiling molecules trapped in a molecular scaffolding.

Currently, PEM fuel cells can produce energy from hydrogen below 90 °C, just under the boiling point of water. With Shimizu’s material, energy can be produced at a higher temperature, up to 150 °C. This could ultimately make the fuel cell cheaper to produce because at a higher temperature less expensive metals can be used to convert hydrogen into energy. Currently, platinum is used which is extremely expensive. Also, reactions at a higher temperature would be faster thus increasing efficiency.

“Ours is an entirely new approach that strikes a balance between having a regular molecular structure and mobile components all while showing genuine promise of application,” says co-author Hurd, a PhD candidate studying chemistry at the U of C.

Kevin Colbow, director of research and development at Ballard Power Systems, a company that designs and manufactures clean energy hydrogen fuel cells, calls the work significant. “We believe that further improvement on conductivity and robustness of these materials could provide next generation membranes for PEM fuel cells.”

Start uga_filter:

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

This is a schematic for molecular diode. The symmetric molecule (top) allows for two-way current. The asymmetrical molecule (bottom) permits current in one direction only and acts as a single-molecule diode. (Credit: Biodesign Institute at Arizona State University)

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.

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 gotten much harder, and the famous dictum known as Moore’s law—which states that the number of silicon-based transistors on a chip doubles every 18-24 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 explains, 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 observes. This is potentially useful, but the diode is a more important (and difficult) component to replicate (Fig 1).

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 like diodes back in 1974,” Tao says, adding “people around world have been trying to accomplish this for over 30 years.”

Most efforts to date have involved many molecules, Tao notes, 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. 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 Professor Luping Yu, at the University of Chicago, who supplied the molecules for study, as well as theoretical collaborator, Professor Ivan Oleynik 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 is also examining the mechanical properties of molecules, for example, 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.

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

This is a schematic for molecular diode. The symmetric molecule (top) allows for two-way current. The asymmetrical molecule (bottom) permits current in one direction only and acts as a single-molecule diode. (Credit: Biodesign Institute at Arizona State University)

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.

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 gotten much harder, and the famous dictum known as Moore’s law—which states that the number of silicon-based transistors on a chip doubles every 18-24 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 explains, 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 observes. This is potentially useful, but the diode is a more important (and difficult) component to replicate (Fig 1).

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 like diodes back in 1974,” Tao says, adding “people around world have been trying to accomplish this for over 30 years.”

Most efforts to date have involved many molecules, Tao notes, 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. 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 Professor Luping Yu, at the University of Chicago, who supplied the molecules for study, as well as theoretical collaborator, Professor Ivan Oleynik 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 is also examining the mechanical properties of molecules, for example, 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.

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