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The following words of legendary theoretical Physicist Richard P. Feynman who is acclaimed for his contribution towards developing the IDEA of Nanotechnology in 1960s still need to be understood again and again. In his famous lecture, 'There's Plenty of Room at the Bottom- An Invitation to Enter a New Field of Physics’ he says, “I would like to describe a field, in which little has been done, but in which an enormous amount can be done in principle. This field is not quite the same as the others in that it will not tell us much of fundamental physics ...(in the sense of, `What are the strange particles?')."
He further says, “On the contrary it is more like solid-state physics in the sense that it might tell us much of great interest about the strange phenomena that occur in complex situations. Furthermore, a point that is most important is that it would have an enormous number of technical applications.” We have to keep in mind that that it was much later in 1974 that Noria Taniguchi used the term ‘nanotechnology’ while measuring precise machining tolerances
Continuing the quest of new probabilities to discover the concealed secrets of Nanotechnology, a group of eight Japanese scientists lead by Tsuyoshi Sekitan of Tokyo University has embarked successfully towards realizing yet another possibility of Stretchability which will significantly expand the applications scope of electronics. This research will be path breaking particularly for large-area electronic displays, sensors and actuators. In a research communication “Stretchable active-matrix organic light-emitting diode display using printable elastic conductors” published in Nature online in May 2009.
Why Single Wall Carbon Nanotube?
Nanotubes were discovered by Sumio Ijima at NEC Fundamental Research Laboratory of Japan in 1991. Because of their simple and well defined structure, such single walled nanotubes serve as a model system both for theoretical calculations and for key experiments. Nanotubes exhibit unique quantum wire properties that derive from tube’s nanometer diameters in combination with special electronic structure of graphite.
Low resistance conductors are crucial for the development of ultra-low-cost electronic systems such as radio frequency identification tags. Low resistance conductors are required to enable the fabrication of high-Q inductors, capacitors, tuned circuits, and interconnect. The fabrication of these circuits by printing will enable a dramatic reduction in cost, through the elimination of lithography, vacuum processing, and the need for high-cost substrates.
Carbon nanotubes have been regarded since their discovery as potential molecular quantum wires. In the case of multi walled nanotubes, many tubes are arranged in coaxial manner. Here electrical properties of each tube vary from tube to tube. Single wall nanotubes have been important because of their high yields and structural uniformity. Because of structural symmetry and stiffness of SWNTs, their molecular wave functions may extend over the entire tube. According to the research completed around 1998 by group led by Tans J sander SWNTs indeed act as a genuine quantum wire.
Unlike for conventional devices, stretchable electronics can cover arbitrary surfaces and movable parts. However, a large hurdle is the manufacture of large-area highly stretchable electrical wirings with high conductivity. In this research scientists have tried to address the how the process of highly precision oriented manufacturing of printable elastic conductors comprising single-walled carbon nanotubes (SWNTs) is uniformly dispersed in a fluorinated rubber. In this mechanism, electrical conduction happens through well separated, discrete electronic states that are quantum mechanically coherent over long distance i.e. approximately 140 nanometer.
Looking back in time, an 1998 assessment of an important strategy for realizing flexible electronics directs us towards use of solution-processable materials that can be directly printed and integrated into high-performance electronic components on plastic. This study was communicated in 1998 by a group of scientists led by Jeong Ho Cho.
Although examples of functional inks based on metallic, semiconducting and insulating materials have been developed, enhanced printability but performance is still a challenge. Printable high-capacitance dielectrics that serve as gate insulators in organic thin-film transistors are a particular priority.
Solid polymer electrolytes (a salt dissolved in a polymer matrix) have been investigated for this purpose, but they suffer from slow polarization response, limiting transistor speed to less than 100 Hz. The significance of this research lies in developing new approach towards emerging class of polymer electrolytes known as ‘ion gels’. These ion gels can serve as printable, high-capacitance gate insulators in organic thin-film transistors. The specific capacitance exceeds that of conventional ceramic or polymeric gate dielectrics, enabling transistor operation at low voltages with kilohertz switching frequencies.
The current 2009 progress has been on the front of using an ionic liquid and jet-milling. Here scientists are trying to produce long and fine SWNT bundles that can form well-developed conducting networks in the rubber. Conductivity of more than 100 S cm-1 and stretchability of more than 100% are obtained. Making full use of this extraordinary conductivity, we constructed a rubber-like stretchable active-matrix display comprising integrated printed elastic conductors, organic transistors and organic light-emitting diodes. The display could be stretched by 30–50% and spread over a hemisphere without any mechanical or electrical damage.
The early projections about resolution were made by Zhenan Bao in 2004. He says, “Although the advent of organic electronics promises the development of such futuristic applications as electronic paper, the limited resolution with which these materials can be patterned is hampering the progress.”
Although printing is an emerging approach for low-cost, large-area manufacturing of electronic circuits, it has to be taken into consideration the disadvantages it has in terms of poor resolution, large overlap capacitances, and film thickness limitations. These deficiencies may result in slow circuit speeds and high operating voltages. In 2007 Yong-Young Noh has demonstrated that ‘a self-aligned printing approach’ allows downscaling of printed organic thin-film transistors to channel lengths of 100–400 nm. The use of a ‘cross linkable polymer gate dielectric’ with 30–50 nm thickness ensures that basic scaling requirements are fulfilled and that operating voltages are below 5 V. This enhancement in efficiency is shown by the device architecture which minimizes contact resistance effects, enabling clean scaling of transistor current with channel length.
The work of another scientist in 2009 has helped to reach a level of paradigm shift in respect with continuing efforts to develop semiconducting inks. These type of inks are based on carbon nanotubes have mobilities that are comparable with those of polycrystalline silicon, and could one day match the performance of single-crystal silicon. Thanks to work done by Takao Someya in this regard, a host of applications based on this inexpensive approach to electronics are expected to emerge rapidly once the commercial feasibility of this application is established.
Replacement of conventional metallic emitter in increased Use of carbon nanotubes removes the need for ultrahigh vacuum in these devices. This saves energy because nanotube field emit at room temperature and no heating is required. This capability has been achieved by scientific community and this marked very significant step towards the real commercial products based on carbon nanotubes.
Many potential applications have been proposed for carbon nanotubes, including conductive and high-strength composites; energy storage and energy conversion devices; sensors; field emission displays and radiation sources; hydrogen storage media; and nanometer-sized semiconductor devices, probes, and interconnects. Some of these applications are now realized in products. Others are demonstrated in early to advanced devices, and one, hydrogen storage, is clouded by controversy. Nanotube cost, polydispersity in nanotube type, and limitations in processing and assembly methods are important barriers for some applications of single-walled nanotubes.
Nanotechnology as we know is enabling technology that will pave the way for novelty in every stream of technology. Research in this technology began with developing an understanding of materials with novel characteristics at the nano-scale. Attempts to achieve control over conductivity, opacity, strength, ductility, reactivity, etc. in different combinations of matter, are among the earliest of research forays in this field.
Henceforth, while there are challenging goals about nanotechnology in front of science-technology experts as answering to the call of emerging expectations of scientific and business community. There do not appear to be any fundamental barriers for achieving it. A proper marriage of Physics, Chemistry and Electrical Engineering may be up to the task. Electronics may begin to go the way of Biology and use the carbon atom as its backbone. And realistically there are some good signs in this direction.
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References:
1) Stretchable active-matrix organic light-emitting diode display using printable elastic conductors, Tsuyoshi Sekitani, Hiroyoshi Nakajima, Hiroki Maeda, Takanori Fukushima, Takuzo Aida, Kenji Hata & Takao Someya, Nature Materials 8, 494 - 499 (2009) Published online: 10 May 2009 doi:10.1038/nmat2459
2) Plastic-Compatible Low Resistance Printable Gold Nanoparticle Conductors for Flexible Electronics; Daniel Huang, Frank Liao, Steven Molesa, David Redinger, and Vivek Subramanian, J. Electrochem. Soc. 150, G412 (2003), DOI:10.1149/1.1582466
3) Printable ion-gel gate dielectrics for low-voltage polymer thin-film transistors on plastic; Jeong Ho Cho, Jiyoul Lee, Yu Xia, BongSoo Kim, Yiyong He, Michael J. Renn, Timothy P. Lodge & C. Daniel Frisbie; Nature Materials 7, 900 - 906 (2008) Published online: 19 October 2008 doi:10.1038/nmat2291
4) Downscaling of self-aligned, all-printed polymer thin-film transistors; Yong-Young Noh, Ni Zhao, Mario Caironi & Henning Sirringhaus ; Nature Nanotechnology 2, 784 789 (2007); Published online: 18 November 2007 doi:10.1038/nnano.2007.365;
5) Conducting polymers: Fine printing; Zhenan Bao ; Nature Materials 3, 137 - 138 (2004), doi:10.1038/nmat1079 Zhenan Bao is at the Department of Chemical Engineering, Stanford University, Stanford, California, USA.
6) Carbon Nanotubes--the Route Toward Applications, Ray H. Baughman, Anvar A. Zakhidov, Walt A. de Heer; Science 2 August 2002: Vol. 297. no. 5582, pp. 787 – 792 DOI: 10.1126/science.1060928
7) Carbon nanotubes as molecular quantum wires, C Dekker, Physics Today, 1999 - www-inst.eecs.berkeley.edu
