Can you imagine electronic circuits being synthesized by pure chemical routes instead of the conventional top-down lithographic process? Irrespective of our imagination, scientists have already made electronic components using molecules from chemical species. In the picture to the left, one of the earliest computers can be seen, which was as big as the size of a house; the image to the right shows a modern computer, which is the smallest computer in contemporary times, the size of which is comparable to a matchbox. This huge miniaturization has been possible because of the integration of a large number of electronic components on a single silicon chip, which is again a gift of the advancements in nanotechnology (Nanotechnology deal with the engineering of systems having sizes in range of ~1-100 nm). The advancements in the miniaturization techniques using nanotechnology has made it possible to upgrade the performance of huge variants of electronic devices such as computers, cell phones, handheld PDAs almost on a day-to-day basis. The increasing numbers of electronic components on an electronic chip not only miniaturizes computers but also enhances their computing power. Our present semiconductor based solid-state microelectronics follows one of the most famous axioms in technology: ‘Moore's law’. He had predicted that the number of transistors that could be fabricated on a silicon integrated circuit would double every two years, and therefore the computing speed of such a circuit would also double every two years. However, if you look at processor speeds from the 1970’s to 2009 and then again, in 2015, you may think that the law has reached its limit or is approaching the limit. This can be attributed to the fact that the lithography-based miniaturization of semiconductor electronic components is not a never-ending process; instead, it has certain limits beyond which miniaturization of the components is not feasible. This limitation arises due to some physical problems like technical hurdles encountered during fabrication of smaller electronic components on a semiconductor chip, interconnecting the huge number of components, dissipation of heat from so many closed packed devices and the effect of stray signals etc. Therefore, it is appropriate to look for alternative approaches to avoid these problems and further progress in this field. Scientists today are trying to solve this problem using molecular chemistry. How does a chemist improve the performance of a computer circuit? You would be surprised to read this, but it is the ‘molecule’ that can not only solve the problem of further miniaturization of electronic chips but can also improve the performance as well as can reduce the cost manifolds. The branch of the science and engineering that deals with engaging a single and/or ensemble of molecules to fabricate electronic components is known as Molecular Electronics. In this field, it is possible to build individual electronic components using molecules that can perform functions identical or analogous to those of switches, transistors, conductors, diodes and other key electronics components. Molecular electronics can play an important role in overcoming the limits of semiconductor technology, and make the electronic circuitry way smaller, faster and more importantly, cheaper. It is an interdisciplinary area that spans physics, chemistry, materials science, biology, electronics, and computer science. The idea that a single molecule could be embedded between electrodes and perform the basic functions of digital electronics, such as rectification, amplification and storage, was put forward in the mid-1970s by two scientists named Ratner and Aviram, who were working for IBM. Due to the remarkable development of modern nanotechnology, the characterizations and measurement of characteristics of single molecular electronic devices are not out of reach. Though molecular electronics is comparatively a younger research field, huge efforts have been made in exploring experimental methods to fabricate molecular electronics devices. Recently, a scientist of Indian origin, Dr. Latha Venkataraman from the Department of Applied Physics and Department of Chemistry, Colombia University, has explored a new technique to fabricate single molecule based diode (an electronic component associated with computer circuit) and interestingly the new molecular diode can perform ~50 times faster than all previous existing designs. Her research group is the first one to develop such real world application based molecular electronic components, which may change the entire scenario of electronic chips, their performance, and finally the cost of the electronic products. Their work was published in the journal Nature Nanotechnology in the paper entitled, "Single-Molecule Diodes with High On-Off Ratios through Environmental Control," on May 25th of this year.
To understand their work, let us discuss the basic functions of a diode. In electronics, a diode works as an electronic component that can allow current to pass in one direction, but blocks current flow in the other direction. Basically, it works similar to a valve. To make a similar diode using a single molecule, the molecule must be asymmetric in structure so that electricity can flow in one direction rather than in both directions. In order to fabricate these single molecular diodes, their group has developed a molecule with asymmetric structure, which allows current to flow in only one direction. Although such asymmetric molecules show diode-like properties, but they are not effective to be used in real life application. Therefore, they further modified the design of the asymmetric molecule to fabricate a real life application based single molecular diode. In this innovative design, instead of using asymmetric molecules, they have created an asymmetry in the environment around the molecular junction. This asymmetric environment was created by simply surrounding the active molecules with an ionic solution, and using gold metal electrodes of different sizes to contact the molecule. Because this new technique is so easily implemented, it can be applied to nanoscale devices of all types, including those that are made with graphene electrodes.
One of the important advantages of molecular electronics is size miniaturization; therefore a molecular semiconductor made using this technique will be 3000 times smaller than a semiconductor transistor sized at 20 nm. The molecules not only provides thermodynamically favorable electron conduction, they can also work as current conduction wires due to their pi- conjugation electron. In addition, other advantages of the molecular electronics is that electronic devices are now faster, smarter, smaller and more of a cost effective design as compared to metal-oxide-semiconductor (CMOS) technologies which is leading the modern integrated electronic circuitry. Moreover, molecules are identical and can be fabricated defect-free in enormous numbers. Some molecules can self-assemble and can thus create large arrays of identical devices. Almost all existing inorganic semiconductors have their organic molecular counterparts to design the existing electronic circuitry. Molecules can also be designed to make p-type as well as n-type conductors to make acceptor and donor molecules respectively. Organic charge-transfer crystals and conducting polymers yield organic equivalents of a variety of inorganic electronic systems: semiconductors, metals, superconductors, batteries, etc. Some of the molecules show considerable electrical and electronic properties in bulk, in the likes of conducting polymers, organic light emitting polymers, piezoelectric polymers etc.
As we have seen, a revolution in the electronic industry had occurred during the time (~1950) of the death of vacuum tube based electronic components and the rise of integrated circuits (IC). Similarly, in near future, there is a huge possibility of revolution to occur in the entire electronic and computer industry through the elevation of molecular electronics. Although an ample amount of research is going on in the field of molecular electronics, some of the basic problems of the integration of the electronic circuitry using molecular electronics, such as proper understanding of current flow behavior between the molecule/electrode junctions have to be addressed. In addition, another important challenge in the field is controlling the precise geometry at the molecule–metal contacts, which can determine the performance of single molecule devices. Scientists all over the globe are extensively trying to overcome the limitation in the path of advancement in molecular electronics and hopefully, we will soon have a workable faster, cheaper and smarter molecular electronic devices in the near future.
Image References
1. http://news.bbc.co.uk/2/hi/technology/7458479.stm
2. http://www.cottoncandyusb.com/
3. http://www.int.kit.edu/1263.php
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Dr. Abhijit Chandra Roy
Mobile No. 09935394366
E-mail. abhijitroysoft@gmail.com
