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Molecules with memories - developing the future of the electronics industry
(Nanowerk Spotlight) Apart from buying a new computer every year it seems you need to upgrade your old machine on a regular basis to keep pace with ever bigger software packages and image files. Apart from the hassle of having to perform major surgery on your computer, these upgrades cost money. But, what if these upgrades were no longer necessary? What if your desktop computer came standard with the ability to store more data than you could ever possibly need and was able to function at unbelievable speeds? This would be too good to be true, right? Besides, who has the space for such a megacomputer.
Well, imagine that this megacomputer could be packaged as a smaller device than current laptops, and cost only a fraction of today’s prices? This sounds like hard core science fiction, but like so many radical science fiction ideas - the real thing might happen sooner than you think. As chip designers are nearing the physical limits of Moore's law (some say that the exponential increase in the cost of semiconductor production will most likely stop the current miniaturization trend before its physical limits are reached), scientists around the globe are working hard on developing the field of molecular electronics.
An interdisciplinary science that includes physics, chemistry, nanotechnology, materials science and even biology, molecular electronics involves using molecular building blocks in the manufacture of electronic components. Driven by a growing interest in alternative concepts, like the integration of molecules as carriers of an electronic function, the electronics industry is poised to take the crucial step of integrating molecular devices into electronic circuits.
Miniaturization of amplification devices in electronic circuits
Miniaturization of amplification devices used in electronic circuits over the last century. From left to the right starting with the vacuum tube over the transistor to current integrated circuits. The last picture on the right side displays a hypothetical single molecule quantum interference transistor for which, based on calculations, interesting transport features have been predicted.9 However, contacting a minute object like a single molecule with three independent electrodes remains an unsolved experimental challenge. (Reprinted with permission from RSC Publishing Group)
A widely reported set of two major nanotechnology achievements by IBM researchers – IBM unveils two major nanotechnology breakthroughs – namely probing an atom's ability to store information, and a single-molecule switch that can operate flawlessly without disrupting the molecule's outer frame. These are significant steps toward building computing elements at the molecular scale that are vastly smaller, faster and use less energy than today's computer chips and memory devices.
Providing a broad overview, and with numerous examples of molecules integrated into electronic circuits, a recent Perspective article in Organic and Biomolecular Chemistry – "Functional Molecules in Electronic Circuits" – illustrates the promising potential of the concept of molecular electronics but also the remaining challenges and limitations. From the point of view of synthetic chemists, the authors set their focus on molecular structures as the origin of electronic function. Particularly, examples of systems providing rectification and switching are considered, as the combination of rectification with hysteretic switching paves the way to future molecule-based memory devices.
The article focuses on rectification and hysteretic switching based on molecular devices, because according to the authors – Nicolas Weibel, Sergio Grundera and Marcel Mayor from the Department of Chemistry at the University of Basel – the control over these two parameters enables the assembly of memory units, which more than likely will be the most interesting and economic application of molecular based electronics. The article also stresses the importance of molecular electronics as a complement to CMOS (complimentary metal-oxide semiconductor) electronics, envisioning the future creation of hybrid CMOS molecular electronics circuits. Hybrid devices, benefiting from molecules as functional units integrated into pre-existing platforms should enable scientific advances and technological achievements.
The idea of a molecular electronic device was suggested in 1974, and since that time, rectification has been achieved in molecular devices ranging from large self-assembled monolayers to individual molecules. Although the integrated molecules in these experiments were confirmed to be responsible for current rectification, the performance was barely comparable to that of currently used semiconductors. More recently, a prototype single molecule diode has been reported ("A single-molecule diode").
According to the authors, at the nano-level, molecular switches are one of the most appealing possibilities of molecular electronics. Switching between two current levels, triggered by an external stimulus such as light or electrolytes, has already been achieved in several prototypes consisting of integrated molecules. Unfortunately, it appears doubtful that a light source or an electrolyte will be integrated into a circuit anytime soon.
Hysteretic switches offer a more promising potential memory device. Hysteresis refers to a system that does not respond instantly to an applied force, but with a delayed reaction. While an immediate switching event upon reduction or oxidation is expected in electrochemically triggered molecular switches, hysteretic switching enables data storage with such devices. By plotting the applied force against the state of the system, loop-like graphs are observed. For applied forces within the loop, the system remembers its previous state. The most advanced hysteretic molecular switches are based on interlocked supermolecules like catenanes or rotaxanes. While catenanes consist of two interlocked cyclic molecules, rotaxanes are assembled from a molecular axis surrounded by a cyclic system. According to the authors, improvements of these rotaxane based memory cells have enabled the fabrication of a 160-kilobit memory unit with a lateral density of 1011 bits per square centimeter.
One of the most significant challenges in the area of nanotechnology shapes up to be overcoming the size gap between electrodes and molecules and the development of concepts for massive parallel integration of molecular devices. The other challenge, according to the Swiss authors, will be the successful cross-disciplinary collaboration between chemists, engineers, nanotechnologists, physicists and other scientists, which will be essential in the successful development of molecular memory devices.
One interesting conclusion the authors provide is that they see the future of molecular electronics not in supplanting but rather in complementing and enriching existing electronic circuits.
"The advent of hybrid devices profiting from molecules as functional units integrated into pre-existing platforms will most likely permit fundamental scientific advances and technological achievements" they write. "Overcoming the size gap between electrodes and molecules as well as bringing in new concepts for massive parallel contacting of molecular devices are still major challenges in the field of nanotechnology. We are convinced that numerous unpredicted solutions and applications will arise from the fascinating and challenging interdisciplinary research in the area of molecular electronics."
By Cathy Garber, Copyright Nanowerk LLC

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