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Nanotechnology Beyond Nanoparticles

RUDRA PRATAP1*

1Centre for Nano Science and Engineering (CeNSE) Indian Institute of Science, Bangalore 5`60012 E-mail : pratap@cense.iisc.ernet.in

Abstract

Research in nanotechnology has seen unprecedented investments, both in time and money, during the last three decades. Yet, the most visible application so far is that of nanoparticles, the most primitive part of nanotechnology. The more sophisticated applications of nanotechnology in the form of nano devices and systems requires a lot more concerted effort than what is currently undertaken by fragmented developments. Nanotechnology needs to learn from the semiconductor industry and adopt an ITRS-likestrategy for developing appropriate technologies along with devices for mass manufacturing and deployment of nano enabled systems. Keywords : nano devices, nanoparticles, nanoscale, self-sensing, nano manufacturing

1. Introduction

Nanotechnology has occupied the centre stage in scientific research during the last three decades. Both in terms of number of research investigations, as evidenced by the number of publications, and the quantum of research funding, no other scientific area seems to compare with nanotechnology. This focused attention has generated incredible amount of probing at this invisible length scale resulting into many new scientific insights and technological possibilities. On the flip side, there has been too much of speculative reporting that has led to unrealistic public expectations. While the pressure on the scientific community to deliver miracles of nanotechnology, be in the field of energy, medicine, electronics, food, or agriculture, has become

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quite intense, the timescale of realistic technology development at nanoscales is simply not cooperating. It is one thing to synthesize nano particles, nano rods, nano tubes and the like, and examine their properties ad nauseam, it is quite another to put them to good use in engineered systems. While the technology for producing nanoscale material components is relatively easy and prevalent, the tools for engineering nano systems is far from what the real applications demand. As a result, making and integrating nano engineered systems is still alluding us. The consolation prize of using nano-x in powder form or a coating is available to all, e.g., silver nanoparticles. The real challenge in nanotechnology is device engineering at a scale where we have neither direct vision nor intuition. All of a sudden, we are confronted with a situation where all our previous experience with engineering and technology is not extendable. The gap between the lab prototypes and their commercial viabilities is at its widest ever.

In this paper, we examine this gap with a few concrete examples and argue that the promised scale of impact of nanotechnology on humanity requires a much bigger scale of engineering effort involving radically different manufacturing with radically different tools. The closest inspiration is the electronics industry which works through an impressive coordinated planning called ITRS (International Technology Roadmap for Semiconductors). For nanotechnology to deliver beyond the use of raw nano materials, an “ITRS-like” initiative is required. A coordinated effort can transform the way nanotechnology is delivering today. In the national context, this need is even more acute. We must weave in technology development, nano manufacturing, and system engineering tightly with nano-technology research. Such coordination requires a network program that links academic research in identified segments of nanotechnology with an aggressive product development program linked with industries. In particular, the huge gap between the lab

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prototype and the product prototype needs to be addressed with a non-traditional thinking.

2. Materials and Synthesis

Development of nanomaterials has received the maximum attention and funding so far. It is no surprise then that the maximum output in scientific literature has been on synthesis and characterization of nano materials. While the excitement of discovering new and fascinating properties of materials at nanoscales is quite understandable, the overemphasis of this singular line of investigation can hardly be justified. Such efforts result in lopsided development that have potential of encouraging disenchantment of the larger stakeholders—the public—in the hyped technology. What is worse is that the hubris of a coterie of top scientists in the western world would have imposed a definition of nanotechnology, largely guided by the greed of cornering major funding, that would require “new properties” to be exploited within the lengthscale of 100 nm in order to qualify as a legitimate work in nanotechnology. Just imagine that if the same definition was used for microtechnology, where will the electronics industry be today. Making new and novel materials is, of course, essential for developing new applications, but nanotechnology is perhaps a singular field where other developments for moving, shaping, manipulating, and manufacturing these materials are even more important. The reason is obvious; the traditional technological tools for these operations do not work.

Nanomaterials in their raw form, such as nano particles, nano rods, nano tubes, have found some applications and already reached the market. These are the low hanging fruits of nanotechnology. There has been a lot of effort on developing low cost techniques for synthesizing nano particles and improving the yield. This effort has certainly led to widespread availability of nano particles for use in paints, ink, cosmetics, detergents, etc. The nano particles that have made it to the market were discovered

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long ago1,2. The nano structures that have fascinated the research community the most—CNTs and graphene—are yet to see any significant market application. The research interest around them, however, far outstrips anything else. A simple search on the Web of Science shows more than 2,33,000 research articles with the word graphene in it and more than 21,500 research papers with the word graphene in their title. CNTs have been around longer and hence appear in the title of more than 47,500 research papers! A more interesting picture emerges when one looks at the distribution of research papers by area. For example, the distribution of papers on graphene is shown in Fig. 1. It is clear from this figure that the number of investigations in applications and engineering is far too less than that in chemistry or material science aspects of graphene. When we know so much about this material and its properties, why is it that we are not able to use it in many products and applications? The answer is that the second part is far harder and requires a lot of more developments in allied fields that are needed for device making, manufacturing, system integration and packaging.

Fig. 1. The distribution of research papers on graphene by field from the Web of Science. [2].

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3. Technology Development

Technology development for nanotech products is by far the most challenging and most capital intensive endeavor. Despite two decades of intensive research in nanotechnology, we are not yet there. Anything beyond nanoparticles or nanomaterials is still eluding us in terms of reproducibility, mass manufacturing and intergrability into larger systems. The reason is, technology development, unlike basic science, requires much coordinated effort across various disciplines with varied expertise. It is simply not within the capacity of a single researcher or lab, or for that matter, even a small group of multidisciplinary teams. One only needs to look at the developments in nanoelectronics and the effort being put into bringing 22 nm node technologies and beyond. It takes a large number of experts from academia and industries even to come up with the requirements that the ITRS puts up for development of these technologies spanning across fields of materials, tools, processes and manufacturing. Fig. 2 provides a glimpse of how transistor technology has developed over decades and where it is headed2. For those working in the area of nanoelectronics, each picture in Fig. 2 contains a concerted effort

Fig. 2. Development of transistor technology over the decades has happened with much concerted effort in technology development with the help of ITRS. Figure courtesy Dr. Garry Patten, IBM Inc.

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of thousands of professionals and researchers working towards a single goal. Going forward, the picture gets even more complex because neither the tools nor the technologies are likely to be extrapolations of the previous ones.

Technology development, thus, is not an individual sport. In the case of nanotechnology, the game is not limited to a team sport of a single kind; it requires many teams from many disciplines. In particular, the pervasiveness of electronics in terms of information processing and memory has to undergo a radical change of accommodating sensors and perhaps actuators at the same scale using the same technology in order to transform today’s “pervasive information” society into a “pervasive intelligence” society. Integration of sensing elements, whether mechanical, electrical, optical, or chemical, with transistor technology is much harder than we previously thought. This is why even the microelectromechanical systems (MEMS) have not yet seen the large scale integration with information processing blocks, let alone the nanoelectromechanical systems (NEMS). The process integration of logic and memory elements with MEMS devices is turning out to be a much harder and expensive affair even though both use the same CMOS technology. The new MEMS accelerometers from Analog Devices are a perfect example of his difficulty which has forced the company to move away from its initial pioneering development of a single chip process to two chip process that provides more cost effective manufacturing. It is, therefore, not difficult to imagine the enormity of the task of technology development required for combining sensors with chemical processing units for healthcare applications—perhaps the most coveted application domain of nanotechnology—with logic and memory units. The task of combining multimodal sensing with corresponding varied signal processing for mechanical, optical, and chemical sensors along with complicated integration of input ports is an engineering nightmare. The “more than Moore” scaling path envisages precisely this kind of technological integration for

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changing the increasingly difficult path of performance and density doubling of traditional Moore’s law to the path functional scaling. This kind of development will require even wider roadmap than what ITRS has been doing so far and including disparate constituencies such as that of biologists and mechanical engineers is certainly more challenging.

4. Devices and Systems

Nano devices hold the key to proliferation of nanotechnology to wide areas of applications such as ITC, healthcare, agriculture, and environment. It is important to realize that nano devices are unlikely to be deployed just by themselves in any application. These devices would need to be integrated into larger systems with several other components, perhaps orders of magnitude larger in size, in order to deliver the intended functionality. If the overall system is going to be large, then why struggle so much to develop nano devices? Well, apart from the fact that there are going to be some niche applications for extremely small scale systems such as nano robots and nano drug capsules, the nano devices embedded in larger micro or meso scale systems, or perhaps even much bigger ones, are intended to address unmet needs as well as provide much more functionality in a given volume. Thus, reducing an entire pathology lab to the size of a laptop or, even better, to the size of a credit card would require billions of nano devices with varied functionality. As of now, the development of such devices suffers from problems of variability as a direct result of process variability and precision limitations of feature defining tools. It is not clear yet whether the answer lies in tightening process controls and engineering the manufacturing tools better, or devising an inherently variability tolerant system with enough redundancy and real time adaptation such as those that nature seems to use in the design of biological systems. Nature’s nano manufacturing units for making proteins and other building blocks of cells, tissues, and organs seem to employ a dichotomous practice of precise design

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specs in the form of genetic codes and generously fault tolerant acceptance of manufactured parts. The manufacturing units employed inside cells with appropriate quality control checks along the assembly line accept remarkably large tolerances and yet the nano assemblies produce systems from such nanoengineered parts that work remarkably well. This design paradigm for the integrated system is still not within our reach. Our experience and intuition is based on tightening tolerances in proportion to the size of the system. That, there is a completely different paradigm of system design that accepts variability of parts as a design feature, is yet to be exploited in the design of systems with nano devices.

It may be illustrative to discuss a couple of simple examples here. Since sensing is at the heart of intelligence of any system—natural or man-made—it is the nano enabled sensing technology that is likely to have the highest impact as far as achieving the goals of ubiquitous intelligence is concerned. Even the application with perhaps the most revolutionary impact on human society—nano manufacturing—will depend on nano sensors just the way quality checks on protein manufacturing inside our cells depends on sensing sugar tags in their structure. Nano sensors can use many transduction techniques available at the macro scale and yet take the resolution and sensitivity to such levels that are simply inaccessible at larger scales. For a more definitive discussion, let us take the example of mass sensing. There is quite a bit of interest in exploiting oscillations of extremely small scale resonators to detect tiny masses—to the tune of zepto grams (10-21) or even smaller. The basic idea used here is macroscopic. When any elastic structure oscillates, its resonant frequency depends on its stiffness and mass (the canonical model is a system consisting of a spring of stiffness k and a particle of mass m that has a resonant frequency = (k/m)1/2). If the mass of the oscillating structure changes by a small amount ∆m, the resonant frequency decreases correspondingly by ∆, and if one can measure the change in frequency reliably and accurately, then the change in mass can also

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be detected accurately. The basic idea is remarkably simple but using it for sensing mass of single molecules is a different matter altogether5,6. MEMS or NEMS resonators designed for mass sensing have resonant frequencies in MHz, sometimes hundreds of MHz, and the change in their frequency due to the addition of intended molecules (i.e., additional mass) is hundred of Hz. Thus, one needs reliable measurement of frequency shift in ppm levels. Such measurements necessitate the use of very high Q resonators. Naturally, the quest leads to understanding the physics of oscillations of such structures at extremely small length scales and their interaction with the environment. There are several energy dissipation mechanisms at play and their relative dominance at various frequencies needs to be understood in order to design high Q resonators7. Even if one has high Q resonators, getting the signal out is a major challenge.

If one were to exploit the idea of frequency shift for mass sensing and design a mass spectrometer, what would the system look like? A schematic of such a system is shown in Fig. 38. The various parts of the spectrometer, making a total height of about 1.5 meters, are designed to get the intended molecule from a volumetric sample injected by the electrospray needle at atmospheric pressure on the top of the system to the nanoelectromechanical resonator shown in Fig. 3(b) in a ultra high vacuum environment of about 10-8 torr pressure at 40 K temperature. The extreme controls on the environment are essential for high Q operation and signal noise reduction. Thus, although the essential sensor is a really small NEMS device, the rest of the system is really huge. It is reminiscent of mainframe computers of yore that used to fill up a whole room to provide processing power and memory that now comes with high-end cell phones. The ambition in the current example is also very similar; Fig. 3(c) depicts the kind of encapsulated array of such mass-spectrometers we would like to see. The simile drawn here with mainframe

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computers is deliberate to indicate that the effort required in development is also very similar.

Fig. 3. A NEMS based mass spectrometer (a) a schematic diagram of a current lab system where the NEMS resonator shown in (b) as a doubly clamped beam is at the bottom of the system (sample stage). (c) A futuristic NEMS based mass spectrometer employing an array of mass sensing units with integrated electronics. (Picture courtesy Dr. Akshay Naik [8] based on his work reported in [6] Naik et al. (2009)).

The most desirable development in sensing at nanoscales would perhaps be a mechanism that is built into a NEMS sensing element for self-sensing. That is, when the sensor responds to the desired stimulus, the response is directly converted into a signal by the sensing element without the help of additional structures such as those required by optical detection (an external light source, photosensitive diodes, etc.). For mechanical stimuli, for example, displacement or strain sensors with self-sensing capabilities and integrated signal processing could enable a host of applications in smart home appliances, health monitoring systems, automobiles, aerospace vehicles, and environment monitoring systems. For self-sensing, piezoresistive materials have a great potential. However,

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the most favored piezoresistive material at the microscale—semicondutors—are not so great at nanoscales because of their poor signal-to-noise ratio as shown by Li et al.9. Metals, on the other hand, are great for SNR at nanoscales but poor in their gauze factor. It is, however, possible, as shown by Mohanasundaram et al.10, that the gauge factor of gold metallic films as thin as a few tens of nanometers could be enhanced by two orders of magnitude by a nanoscale process of local inhomogenization using controlled electromigration. As a result, metals become extremely competitive with other known piezoresistors (see Fig. 4.) in terms of their gauge factor and become a clear winner for integration in self-sensing NEMS devices due to their inherent low resistance. It is this kind of development which is likely to lead to easily deployable compact NEMS sensors. It is conceivable that self-sensing NEMS devices will make nanoscale strain gauges ubiquitous and atomic force microcopy integrable in a large number of other systems. But, for such things to happen,

Fig. 4. Large enhancement in the effective gauge factor of gold thin films obtained by local nanoscale inhomogenization using controlled electro-migration; reported in [11].

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sensor technology, signal processing for extremely low signals, SNR engineering, system scaling and packaging must all develop simultaneously keeping in pace with each other.

5. Conclusions

The fast paced developments in nanotechnology, despite massive investments, are too fragmented to result into large-scale manufacturing and deployment of nanoscale devices and systems. Applications of nanotechnology beyond the use of nanoparticles require parallel technology developments in several areas that call for a planned and concerted effort. It is argued that an ITRS-like effort is required for the development of systems employing nanoscale devices and other appropriate peripherals.

References

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