Using quantum mechanics to turn molecules into transistors
(Nanowerk Spotlight) Transistors are the fundamental building blocks of our everyday modern electronics; they are the tiny switches that process the ones and zeroes that make up our digital world. Transistors control the flow of electricity by switching current on or off and by amplifying electrical signals in the circuitry that governs the operation of our computers, cellular phones, iPods and any other electronic device you can think of. The first transistor used in commercial applications was in the Regency TR-1 transistor radio, which went on sale in 1954 for $49.95, that's over $375 in today's dollars. While the first transistors were over 1 centimeter in diameter, the smallest transistors today are just 30 nanometers thick - three million times smaller. This feat would be equivalent to shrinking the 509-meter tall Taipei 101 Tower, currently the tallest building in the world, to the size of a 1.6 millimeter tall grain of rice. The 32nm microprocessor Intel plans to introduce in 2009 will pack a whopping 1.9 billion transistors. However, current microprocessor technology is quickly approaching a physical barrier. Switching the current by raising and lowering the electron energy barrier generates heat, which becomes a huge problem as device densities approach the atomic limit. An intriguing - and technologically daunting - alternative would be to exploit the wave nature of the electron, rather than its particle properties, to control current flow on the nanoscale. Such a device, called the Quantum Interference Effect Transistor (QuIET), has been proposed by researchers in Arizona. This device could be as small as a single benzene molecule, and would produce much less heat than a conventional field effect transistor.
Notwithstanding the incredibly shrinking transistor, nanometer-size transistors work on the same principles as much larger ones: current flows into the base (the gate controller) from one electrode (the collector) and out through another (the emitter). The base switches the current on and off by raising and lowering an electrical potential barrier gate that prevents the flow of electrons. Unfortunately, this type of switching requires a lot of power. As more and more transistors are crammed into the same space, the power density, and with it heat dissipation, becomes an overriding issue. At the current rate of miniaturization, by the end of the decade, you might as well be feeling a rocket nozzle than touching a chip; and soon after 2010, computer chips could feel like the bubbly hot surface of the sun itself.
The growing power density (measured in Watts per square centimeter) of Intel's microchip processor families. (Source: Intel)
"Our proposal to use quantum interference is a novel way to control the flow of electrons through a single-molecule device" Dr. David M. Cardamone tells Nanowerk. "Our work shows that, because they possess exact symmetries one can't achieve in larger systems, molecules are uniquely suited to take advantage of quantum effects needed to build smaller, more efficient electronic devices."
"Our work was originally motivated by curiosity" adds Dr. Charles A. Stafford: "How would quantum wave effects modify electron flow through a single-molecule device? We discovered very early, and somewhat fortuitously, that molecular symmetry can lead to unique device properties which hold promise to overcome important obstacles toward further miniaturization of conventional semiconductor devices."
Stafford, an Associate Professor of Physics at the University of Arizona (UA), and Cardamone, currently a post-doctoral fellow at Simon Fraser University in Canada, together with Dr. Sumit Mazumdar, Professor in UA's Department of Physics, have published a paper in the Sept. 13, 2007 edition of Nanotechnology that describes their proposed QuIET device ("The quantum interference effect transistor"). This paper was preceded in 2006 by an earlier publication in Nano Letters ("Controlling Quantum Transport through a Single Molecule").
Stafford explains how the proposed device promises to solve two fundamental problems in nanoscale electronics:
"The first is the problem of heat production. A field-effect transistor, like the hundreds of millions found in a typical home computer, turns off current flow by building a wall of electric potential energy – turning on and off this wall causes heating. On the other hand, the QuIET guides the electron waves either forward (on) or back the way they came (off) – a much cooler process.
"The second problem that plagues almost all proposed nanoscale devices is their sensitivity to small perturbations. Most nanoscale devices operate in a narrow resonant range, so that each device on a chip would have to be fine tuned to its operating point, possibly requiring atomic-precision control of the on-chip environment of each device. Our proposed device does not operate in a narrow resonance, but in a broad valley between resonances, so it is very robust with respect to small electrical perturbations."
On the molecular scale, electrons behave like waves and can interfere with themselves and each other. In wave theory, when one wave passes through another physicist say the waves interfere. When the crest of one wave passes through, or is superpositioned upon, the crest of another wave, the waves constructively interfere. However, when the crest of one wave passes through, or is superpositioned upon, the trough of another wave, the waves destructively interfere.
The UA scientists realized that a current could be switched off in a circuit if it could be constructed so that the electron waves traveling through it cancel each other out through destructive interference.
The QuIET, which right now is only a theoretical construct, exploits this quantum interference stemming from the symmetry of monocyclic aromatic annulenes such as benzene. Because of the exact symmetry possible in molecular devices, the QuIET possesses a perfect mid-gap transmission node, which serves as the off state for the device. By introducing decoherence or elastic scattering from a third lead the quantum transport through the molecule could be controlled.
Stafford notes that quantum transport through single benzene molecules with two metallic leads connected at para positions has been the subject of extensive experimental and theoretical investigation. "However, a QuIET based on benzene requires the source and drain to be connected at meta positions, as illustrated below:
Artist?s conception of a quantum interference effect transistor based on 1,3-benzenedithiol. The colored spheres represent individual carbon (green), hydrogen (purple), sulfur (yellow), and gold (gold) atoms. In the ?off? state of the device, destructive interference blocks the flow of current between the source (bottom) and drain (right) electrodes. Decoherence introduced by the scanning transmission microscope (STM) tip (upper left) suppresses interference, allowing current flow. Image by Helen M Giesel. (Reprinted with permission from IOP Publishing)
"The most obvious application of the QuIET is as part of a smaller, faster, more efficient microprocessor" says Cardamone. "Also, the QuIET doesn't suffer from the environmental limitations of a semiconductor-based field-effect transistor: It could be useful for computation in an aqueous environment, perhaps even in vivo."
While incredibly intriguing as a concept, the challenges in realizing single-molecule transistors are enormous.
On the practical side, the next challenge is attaching the third lead. Right now, experimentalists can connect two leads to a single small molecule, and are making strides toward connecting a third.
On the theoretical side, the big challenge is to go beyond a mean-field treatment of electron-electron interactions, which are particularly strong in ultrasmall devices, due to their very small capacitance.