Organic Molecules Challenge Silicon's Reign as King of Semiconductors
There is a revolution fomenting in the semiconductor industry. It may take 30 years or more to reach perfection, but when it does the advance may be so great that today's computers will be little more than calculators compared to what will come after. The revolution is called molecular electronics, and its goal is to depose silicon as king of the computer chip and put carbon in its place.
The perpetrators are a few clever chemists trying to use pigment, proteins, polymers, and other organic molecules to carry out the same task that microscopic patterns of silicon and metal do now. For years these researchers worked in secret, mainly at their blackboards, plotting and planning. Now they are beginning to conduct small forays in the laboratory, and their few successes to date lead them to believe they were on the right track.
"We have a long way to go before carbon-based electronics replace silicon-based electronics, but we can see now that we hope to revolutionize computer design and performance," said Robert R. Birge, a professor of chemistry, Carnegie-Mellon University, Pittsburgh. "Now it's only a matter of time, hard work, and some luck before molecular electronics start having a noticeable impact."
Molecular electronics is so named because it uses molecules to act as the "wires" and "switches" of computer chips. Wires, may someday be replaced by polymers that conduct electricity, such as polyacetylene and polyphenylenesulfide. Another candidate might be organometallic compounds such as porphyrins and phthalocyanines which also conduct electricity. When crystallized, these flat molecules stack like pancakes, and metal ions in their centers line up with one another to form a one-dimensional wire.
Many organic molecules can exist in two distinct stable states that differ in some measurable property and are interconvertable. These could be switches of molecular electronics. For example, bacteriorhodpsin, a bacterial pigment, exists in two optical states: one state absorbs green light, the other orange. Shinning green light on the green-absorbing state converts it into the orange state and vice versa. Birge and his coworkers have developed high density memory drives using bacteriorhodopsin.
Although the idea of using organic molecules may seem far-fetched, it happens every day throughout nature. "Electron transport in photosynthesis one of the most important energy generating systems in nature, is a real-world example of what we're trying to do," said Phil Seiden, manager of molecular science, IBM, Yorkstown Heights, N.Y.
Birge, who heads the Center for Molecular Electronics at Carnegie-Mellon, said two factors are driving this developing revolution, more speed and less space. "Semiconductor chip designers are always trying to cram more electronic components into a smaller space, mostly to make computers faster," he said. "And they've been quite good at it so far, but they are going to run into trouble quite soon."
A few years ago, for example, engineers at IBM made history last year when they built a memory chip with enough transistors to store a million bytes if information, the megabyte. It came as no big surprise. Nor did it when they came out with a 16-megabyte chip. Chip designers have been cramming more transistors into less space since Jack Kilby at Texas Instruments and Robert Noyce at Fairchild Semiconductor first showed how to put multitudes on electronic components on a slab of silicon.
But 16 megabytes may be near the end of the road. As bits get smaller and loser together, "crosstalk" between them tends to degrade their performance. If the components were pushed any closer they would short circuit. Physical limits have triumphed over engineering.
That is when chemistry will have its day. Carbon, the element common to all forms of life, will become the element of computers too. "That is when we see electronics based on inorganic semiconductors, namely silicon and gallium arsenide, giving way to electronics based on organic compounds," said Scott E. Rickert, associate professor of macromolecular science, Case Western Reserve University, Cleveland, and head of the school's Polymer Microdevice Laboratory.
"As a result," added Rickert, "we could see memory chips store billions of bytes of information and computers that are thousands times faster. The science of molecular electronics could revolutionize computer design."
But even if it does not, the research will surely have a major impact on organic chemistry. "Molecular electronics presents very challenging intellectual problems on organic chemistry, and when people work on challenging problems they often come up with remarkable, interesting solutions," said Jonathan S. Lindsey, assistant professor of chemistry, Carnegie-Mellon University. "Even if the whole field falls through, we'll still have learned a remarkable amount more about organic compounds and their physical interactions than we know now. That's why I don't have any qualms about pursuing this research."
Moreover, many believe that industries will benefit regardless of whether an organic-based computer chip is ever built. For example, Lindsey is developing an automated system, as well as the chemistry to go along with it, for synthesizing complex organic compounds analogous to the systems now available for peptide and nucleotide synthesis. And Rickert is using technology he developed foe molecular electronic applications to make gas sensors that are both a thousand times faster and more sensitive than conventional sensors.
For now, the molecular electronics revolution is in the formative stage, and most of the investigations are still basic more than applied. One problem with which researchers are beginning to come to grips, though, is determining the kinds if molecules needed to make the transistors and other electronic components that will go into the molecular electronic devices, Some of the molecules are like bacteriorhodopsin in that their two states flip back and forth when exposed to wavelengths of light. These molecules would be the equivalent of an optical switch on which on state is on and the other state is off. Optical switches have been difficult to make from standard semiconductors.
bacteriorhodopsin is the light-harvested pigment of purple bacteria living in salt marshes outside San Francisco. The compound consists of a pigment core surrounded by a protein that stabilizes the pigment. Birge has capitalized on the clear cut distinction between the two states of bacteriorhodopsin to make readable-write able optical memory devices. Laser disks, are read-only optical memory devices, once encoded the data cannot be changed.
Birge has been able to form a thin film of bacteriorhodopsin on quartz plates that can then be used as optical memory disks. The film consists of a thousand one-molecule thick layers deposited one layer at a time using the Langmuir-Blodgett technique. A quartz plate is dipped into water whose surface is covered with bacteriorhodopsin. When the plate is withdrawn at a certain speed, a monolayer of rhodopsin adheres to the plate with all the molecules oriented in the same direction. Repeating this process deposits a second layer, then a third, and so on.
Information is stored by assigning 0 to the green state and 1 to the orange state. Miniature lasers of the type use din fiber optic communications devices are used to switch between the two states.
Irradiating the disk with a green laser converts the green state to the orange state, storing a 1. resetting the bit is accomplished by irradiating the same small area of the dusk with a red laser. Data stored on the disk are read by using both lasers. The disk would be scanned with the red laser and any bit with a value 1 would be reset using the green laser.
This is analogous to the way in which both magnetic and electrical memories are read today, but with one important difference: "Because the two states take only five picoseconds (five trillionths of a second) to flip back and forth, information storage and retrieval are much faster than anything you could ever do magnetically or electrically," explained Birge.
In theory, each pigment molecule could store one bit of information. In practice, however approximately 100,000 molecules are sued. The laser beam as a diameter if approximately 10 molecules and penetrates through the 1,000 molecule think layer. Although this reduces the amount of information that can be stored on each disk, it does provide fidelity though redundancy.
"We can have half the molecules or more in a disk fall apart and there would still be enough excited by the laser at each spot to provide accurate data storage," said Birge. And even using 100,000 molecules per data bit, an old 5.25 inch floppy disk could store well over 500 megabytes of data.
One drawback to this system is that the bacteriorhodopsin's two states are only stable at liquid nitrogen temperatures, -192¡C. But Birge does not see this as anything more than a short term problem. "We're now using genetic engineering to modify the protein part of the molecule so that it will stabilize the two states at room temperature," he said. "Based in outstanding work, we don't think this will be a problem."
Faster, higher-density disk storage is a laudable goal, but the big stakes are in improving on semiconductor components. Birge, for example, is developing a random access chip using the bacteriorhodopsin system. Instead of having millions of transistors wired together on a slab of silicon, there would be millions of tiny lasers pointed at a film of bacteriorhodopsin. "These RAM chips would actually be a little bigger than what we have," he said, "but they would still be 1,000 times faster because the molecular components work so much faster than ones made of semiconductor materials."
Recently, Theodre O. Poehler, director of research, John Hopkin's Applied Physics Laboratory, Laurel, Md., and Richard S. Potember, a senior chemist there, built a working four-byte RAM chip using molecular charge-transfer system. Four bytes may seem crude compared to the million-byte chip built by IBM, but the first semiconductor chip, built by Texas Instruments' Kilby in 1959, was also crude compared to today's chips.
Poehler and Potember's system also uses laser light to activate the molecular switches, but the chemistry is much different than Birge's. In the Carnegie-Mellon system, light causes an electron on the bacteriorhodopsin to move into a higher energy level within the same molecule. This changes its absorption spectrum. In the Hopkin's system, light causes an electron to transfer between two different molecules, one called an electron donor, the other an electron acceptor. This is known as a charge-transfer reaction, and the researchers in several laboratories are designing devices using this type of molecular switch.
In their system, Poehler and Potember use compounds formed form either copper or silver- the electron donor-and the tetracyaboquinodimethane (TCNQ) or various derivatives-the electron acceptor. The researchers first deposit the metal onto some substrate-it could be either a silicon or plastic slab. Next, they deposit a solution of the organic electron acceptor onto the metal and heat it gently, causing a reaction to occur and evaporating the solvent.
In the equilibrium state between these two molecular components, an electron is transferred from copper to TCNQ, forming a positive metal ion and a negative TCNQ ion. Irradiating this complex with light from an argon laser causes the reverse reaction to occur, forming neutral metal and neutral TCNQ.
Two measurable changed accompany this reaction. One is that the laser-lit area changes color from blue to a pale yellow if the metal is copper or from violet if it is silver. This change is easily detected using the same or another laser. Thus, metal TCNQ films, like those made from bacteriorhodopsin, could serve as optical memory storage devices. Poehler said that they have already built several such devices and are now testing their performance. They work at room temperature.
The other change that occurs, however, is more like those that take place on standard microelectronics switches. When an electric field id applied to the organometallic film, it becomes conducting in the irradiated area, just as a semiconductor does when an electric field is applied to it.
Erasing a data or closing the switch is accomplished using any low-intensity laser, including carbon dioxide, neodymium yttrium aluminum garnet, or gallium arsenide devices. The tiny amount of heat generated by the laser beam causes the metal and TCNQ to return to their equilibrium, non-conducting state. Turning off the applied voltage also returns the system to its non-conducting state.
The Hoptkins researchers found they could tailor the on/off behavior of this system by changing the electron acceptor. Using relative weak electron acceptors, such as dimethoxy-TCNQ, produced organometallic films with a very sharp on/off behavior. But of a strong electron acceptor such as tetrafluoro-TCNQ is used, the film remains conductive even when the applied field is removed. This effect can last from several minutes to several days; the stronger the electron acceptor, the longer the memory effect.
Poehler and his colleagues are now working to optimize the electrical and optical behavior of these materials. They have found, for example, that films made with copper last longer than those made of silver. In addition, they are testing various substrates and coatings to further stabilize these systems. "We know the system works," Poehler said. "Now we're trying to develop it into a system that will work in microelectronics applications."
At Case Wester Rickert is also trying to make good organic chemistry and turn it into something workable in microelectronics. He and his coworkers have found that using Langmuir-Blodgett techniques they can make polymer films actually look like and behave like metal foils. "The polymer molecules are arranged in a very regular, ordered array, as if they were crystalline," said Rickert.
These foils, made from polymers such as polyvinylstearate, behave much as metal oxide films do in standard semiconductor devices. but transistors made with the organic foils are 20 percent faster than their inorganic counterparts, and require much less energy to make and process. Early in 1986, Rickert made a discovery about these films that could have a major impact on the chemical industry long before any aspect of molecular electronics. "the electrical behavior of these foils is very sensitive to environmental changes such as temperature, pressure, humidity and chemical composition," he said. "As a result, they make very good chemical sensors, better than any sensor yet developed."
He has been able to develop an integrated sensor that to date can measure parts per billion concentrations of nitrogen oxides, carbon dioxide, oxygen, and ammonia. Moreover, it can measure all four simultaneously.
Response times for the new "supersniffer," as Rickert calls the sensor, are in the millisecond range, compared to tens of seconds for standard gas sensors, Recovery times are faster too; under five seconds compared to minutes or hours. The Case Western team is now using polymer foils as electrochemical and biochemical detectors.
In spite of such successes, molecular electronics researchers point out that MEDs will never replace totally those made of silicon and other inorganic semiconductors. "Molecular electronics will never make silicon technology obsolete," said Carnegie-Mellon's Birge. "The lasers we will need, for example, will probably be built from gallium arsenide crystals on silicon wafers.
"But molecular electronic devices will replace many of those now made with silicon and the combination of the two technologies should revolutionize computer design and function."
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