Organic Molecules ChallengeSilicon’s Reign as King of SemiconductorsThere is a revolution fomenting in the semiconductor industry.
It may take 30years or more to reach perfection, but when it does the advance may be so greatthat today’s computers will be little more than calculators compared to whatwill come after. The revolution is called molecular electronics, and its goal isto 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 thatmicroscopic patterns of silicon and metal do now. For years these researchersworked in secret, mainly at their blackboards, plotting and planning.
Now theyare beginning to conduct small forays in the laboratory, and their few successesto date lead them to believe they were on the right track. “We have a long way to go before carbon-based electronics replace silicon-basedelectronics, but we can see now that we hope to revolutionize computer designand performance,” said Robert R. Birge, a professor of chemistry, Carnegie-Mellon University, Pittsburgh. “Now it’s only a matter of time, hard work, andsome 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 bypolymers that conduct electricity, such as polyacetylene andpolyphenylenesulfide. Another candidate might be organometallic compounds suchas porphyrins and phthalocyanines which also conduct electricity. Whencrystallized, these flat molecules stack like pancakes, and metal ions in theircenters line up with one another to form a one-dimensional wire. Many organic molecules can exist in two distinct stable states that differ insome measurable property and are interconvertable. These could be switches ofmolecular 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 orangestate and vice versa. Birge and his coworkers have developed high density memorydrives using bacteriorhodopsin. Although the idea of using organic molecules may seem far-fetched, it happensevery day throughout nature.
“Electron transport in photosynthesis one of themost important energy generating systems in nature, is a real-world example ofwhat 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, saidtwo factors are driving this developing revolution, more speed and less space. “Semiconductor chip designers are always trying to cram more electroniccomponents into a smaller space, mostly to make computers faster,” he said. “Andthey’ve been quite good at it so far, but they are going to run into troublequite soon.
“A few years ago, for example, engineers at IBM made history last year when theybuilt a memory chip with enough transistors to store a million bytes ifinformation, the megabyte. It came as no big surprise. Nor did it when they cameout with a 16-megabyte chip. Chip designers have been cramming more transistorsinto less space since Jack Kilby at Texas Instruments and Robert Noyce atFairchild Semiconductor first showed how to put multitudes on electroniccomponents on a slab of silicon. But 16 megabytes may be near the end of the road.
As bits get smaller and losertogether, “crosstalk” between them tends to degrade their performance. If thecomponents were pushed any closer they would short circuit. Physical limits havetriumphed over engineering. That is when chemistry will have its day. Carbon, the element common to allforms of life, will become the element of computers too. “That is when we seeelectronics based on inorganic semiconductors, namely silicon and galliumarsenide, giving way to electronics based on organic compounds,” said Scott E.
Rickert, associate professor of macromolecular science, Case Western ReserveUniversity, Cleveland, and head of the school’s Polymer Microdevice Laboratory. “As a result,” added Rickert, “we could see memory chips store billions of bytesof information and computers that are thousands times faster. The science ofmolecular electronics could revolutionize computer design. “But even if it does not, the research will surely have a major impact on organicchemistry. “Molecular electronics presents very challenging intellectualproblems on organic chemistry, and when people work on challenging problems theyoften come up with remarkable, interesting solutions,” said Jonathan S.
Lindsey,assistant professor of chemistry, Carnegie-Mellon University. “Even if the wholefield falls through, we’ll still have learned a remarkable amount more aboutorganic compounds and their physical interactions than we know now. That’s why Idon’t have any qualms about pursuing this research. “Moreover, many believe that industries will benefit regardless of whether anorganic-based computer chip is ever built. For example, Lindsey is developing anautomated system, as well as the chemistry to go along with it, for synthesizingcomplex organic compounds analogous to the systems now available for peptide andnucleotide synthesis. And Rickert is using technology he developed foe molecularelectronic applications to make gas sensors that are both a thousand timesfaster and more sensitive than conventional sensors.
For now, the molecular electronics revolution is in the formative stage, andmost of the investigations are still basic more than applied. One problem withwhich researchers are beginning to come to grips, though, is determining thekinds if molecules needed to make the transistors and other electroniccomponents that will go into the molecular electronic devices, Some of themolecules are like bacteriorhodopsin in that their two states flip back andforth when exposed to wavelengths of light. These molecules would be theequivalent of an optical switch on which on state is on and the other state isoff. Optical switches have been difficult to make from standard semiconductors.
bacteriorhodopsin is the light-harvested pigment of purple bacteria living insalt marshes outside San Francisco. The compound consists of a pigment coresurrounded by a protein that stabilizes the pigment. Birge has capitalized onthe clear cut distinction between the two states of bacteriorhodopsin to makereadable-write able optical memory devices. Laser disks, are read-only opticalmemory devices, once encoded the data cannot be changed. Birge has been able to form a thin film of bacteriorhodopsin on quartz platesthat can then be used as optical memory disks.
The film consists of a thousandone-molecule thick layers deposited one layer at a time using the Langmuir-Blodgett technique. A quartz plate is dipped into water whose surface is coveredwith bacteriorhodopsin. When the plate is withdrawn at a certain speed, amonolayer of rhodopsin adheres to the plate with all the molecules oriented inthe 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 orangestate. Miniature lasers of the type use din fiber optic communications devicesare used to switch between the two states. Irradiating the disk with a green laser converts the green state to the orangestate, storing a 1. resetting the bit is accomplished by irradiating the samesmall area of the dusk with a red laser. Data stored on the disk are read byusing both lasers.
The disk would be scanned with the red laser and any bit witha value 1 would be reset using the green laser. This is analogous to the way in which both magnetic and electrical memories areread today, but with one important difference: “Because the two states take onlyfive picoseconds (five trillionths of a second) to flip back and forth,information storage and retrieval are much faster than anything you could everdo 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 diameterif approximately 10 molecules and penetrates through the 1,000 molecule thinklayer. Although this reduces the amount of information that can be stored oneach disk, it does provide fidelity though redundancy. “We can have half the molecules or more in a disk fall apart and there wouldstill be enough excited by the laser at each spot to provide accurate datastorage,” said Birge. And even using 100,000 molecules per data bit, an old 5. 25inch floppy disk could store well over 500 megabytes of data. One drawback to this system is that the bacteriorhodopsin’s two states are onlystable at liquid nitrogen temperatures, -192C.
But Birge does not see this asanything more than a short term problem. “We’re now using genetic engineering tomodify the protein part of the molecule so that it will stabilize the two statesat room temperature,” he said. “Based in outstanding work, we don’t think thiswill be a problem. “Faster, higher-density disk storage is a laudable goal, but the big stakes arein improving on semiconductor components. Birge, for example, is developing arandom access chip using the bacteriorhodopsin system. Instead of havingmillions of transistors wired together on a slab of silicon, there would bemillions of tiny lasers pointed at a film of bacteriorhodopsin.
“These RAM chipswould actually be a little bigger than what we have,” he said, “but they wouldstill be 1,000 times faster because the molecular components work so much fasterthan ones made of semiconductor materials. “Recently, Theodre O. Poehler, director of research, John Hopkin’s AppliedPhysics Laboratory, Laurel, Md. , and Richard S. Potember, a senior chemist there,built a working four-byte RAM chip using molecular charge-transfer system. Fourbytes may seem crude compared to the million-byte chip built by IBM, but thefirst semiconductor chip, built by Texas Instruments’ Kilby in 1959, was alsocrude compared to today’s chips.
Poehler and Potember’s system also uses laser light to activate the molecularswitches, but the chemistry is much different than Birge’s. In the Carnegie-Mellon system, light causes an electron on the bacteriorhodopsin to move into ahigher energy level within the same molecule. This changes its absorptionspectrum. In the Hopkin’s system, light causes an electron to transfer betweentwo different molecules, one called an electron donor, the other an electronacceptor.
This is known as a charge-transfer reaction, and the researchers inseveral laboratories are designing devices using this type of molecular switch. In their system, Poehler and Potember use compounds formed form either copper orsilver- the electron donor-and the tetracyaboquinodimethane (TCNQ) or variousderivatives-the electron acceptor. The researchers first deposit the metal ontosome substrate-it could be either a silicon or plastic slab. Next, they deposita 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 istransferred from copper to TCNQ, forming a positive metal ion and a negativeTCNQ ion.
Irradiating this complex with light from an argon laser causes thereverse reaction to occur, forming neutral metal and neutral TCNQ. Two measurable changed accompany this reaction. One is that the laser-lit areachanges color from blue to a pale yellow if the metal is copper or from violetif 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 asoptical memory storage devices. Poehler said that they have already builtseveral such devices and are now testing their performance.
They work at roomtemperature. The other change that occurs, however, is more like those that take place onstandard microelectronics switches. When an electric field id applied to theorganometallic film, it becomes conducting in the irradiated area, just as asemiconductor does when an electric field is applied to it. Erasing a data or closing the switch is accomplished using any low-intensitylaser, including carbon dioxide, neodymium yttrium aluminum garnet, or galliumarsenide devices.
The tiny amount of heat generated by the laser beam causes themetal and TCNQ to return to their equilibrium, non-conducting state. Turning offthe applied voltage also returns the system to its non-conducting state. The Hoptkins researchers found they could tailor the on/off behavior of thissystem by changing the electron acceptor. Using relative weak electron acceptors,such as dimethoxy-TCNQ, produced organometallic films with a very sharp on/offbehavior. But of a strong electron acceptor such as tetrafluoro-TCNQ is used,the film remains conductive even when the applied field is removed.
This effectcan last from several minutes to several days; the stronger the electronacceptor, the longer the memory effect. Poehler and his colleagues are now working to optimize the electrical andoptical behavior of these materials. They have found, for example, that filmsmade with copper last longer than those made of silver. In addition, they aretesting various substrates and coatings to further stabilize these systems.
“Weknow the system works,” Poehler said. “Now we’re trying to develop it into asystem that will work in microelectronics applications. “At Case Wester Rickert is also trying to make good organic chemistry and turn itinto something workable in microelectronics. He and his coworkers have foundthat using Langmuir-Blodgett techniques they can make polymer films actuallylook like and behave like metal foils.
“The polymer molecules are arranged in avery regular, ordered array, as if they were crystalline,” said Rickert. These foils, made from polymers such as polyvinylstearate, behave much as metaloxide films do in standard semiconductor devices. but transistors made with theorganic foils are 20 percent faster than their inorganic counterparts, andrequire much less energy to make and process. Early in 1986, Rickert made adiscovery about these films that could have a major impact on the chemicalindustry long before any aspect of molecular electronics. “the electricalbehavior of these foils is very sensitive to environmental changes such astemperature, 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 partsper billion concentrations of nitrogen oxides, carbon dioxide, oxygen, andammonia. Moreover, it can measure all four simultaneously. Response times for the new “supersniffer,” as Rickert calls the sensor, are inthe 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 andbiochemical detectors. In spite of such successes, molecular electronics researchers point out thatMEDs will never replace totally those made of silicon and other inorganicsemiconductors. “Molecular electronics will never make silicon technologyobsolete,” 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 withsilicon and the combination of the two technologies should revolutionizecomputer design and function.”
