◄
NANOTECHNOLOGY
__________________________________________________________________________________________________________________________________________________________________________

Nanotechnologies manipulate materials by breaking them down into an 'unimaginably small scale

Nanotechnology is a field of applied science and technology covering a broad range of topics. The main unifying theme is the control of matter on a scale smaller than one micrometre, as well as the fabrication of devices on this same length scale. It is a highly multidisciplinary field, drawing from fields such as colloidal science, device physics, and supramolecular chemistry.

Much speculation exists as to what new science and technology might result from these lines of research. Some view nanotechnology as a marketing term that describes pre-existing lines of research. Despite the apparent simplicity of this definition, nanotechnology actually encompasses diverse lines of inquiry.

 

Nanotechnology cuts across many disciplines, including colloidal science, chemistry, applied physics, biology. It could variously be seen as an extension of existing sciences into the nanoscale, or as a recasting of existing sciences using a newer, more modern term. Two main approaches are used in nanotechnology: one is a "bottom-up" approach where materials and devices are built from molecular components which assemble themselves chemically using principles of molecular recognition; the other being a "top-down" approach where nano-objects are constructed from larger entities without atomic-level control.

The impetus for nanotechnology has stemmed from a renewed interest in colloidal science, coupled with a new generation of analytical tools such as the atomic force microscope (AFM) and the scanning tunneling microscope (STM). Combined with refined processes such as electron beam lithography, these instruments allow the deliberate manipulation of nanostructures, and in turn led to the observation of novel phenomena. Nanotechnology is also an umbrella description of emerging technological developments associated with sub-microscopic dimensions. Despite the great promise of numerous nanotechnologies such as quantum dots and nanotubes, real applications that have moved out of the lab and into the marketplace have mainly utilized the advantages of colloidal nanoparticles in bulk form, such as suntan lotion, cosmetics, protective coatings, and stain resistant clothing.

Usage of the term

Nanotechnology is an umbrella term that is used to describe a variety of techniques to fabricate materials and devices on the nanoscale. The genesis for nanotechnology has its roots in the colloidal science of the late 19th century. These early innovations have been combined with more recent developments in device manufacture. The term has served in some regards as a means to generate new lines of funding from government agencies. One nanometer (nm) is one billionth, or 10-9 of a meter. For comparison, typical carbon-carbon bond lengths, or the spacing between these atoms in a molecule, are in the range .12-.15 nm, and a DNA double-helix has a diameter around 2 nm. On the other hand, the smallest cellular lifeforms, the bacteria of the genus Mycoplasma, are around 200 nm in length.

Nanotechnological techniques include those used for fabrication of nanowires, those used in semiconductor fabrication such as deep ultraviolet lithography, electron beam lithography, focused ion beam machining, nanoimprint lithography, atomic layer deposition, and molecular vapor deposition, and further including molecular self-assembly techniques such as those employing di-block copolymers. However, all of these techniques preceded the nanotech era, and are extensions in the development of scientific advancements rather than techniques which were devised with the sole purpose of creating nanotechnology or which were results of nanotechnology research.

General fields involved with proper characterization of these systems include physics, chemistry, and biology, as well as mechanical and electrical engineering. However, due to the inter- and multidisciplinary nature of nanotechnology, subdisciplines such as physical chemistry, materials science, or biomedical engineering are considered significant or essential components of nanotechnology. The design, synthesis, characterization, and application of materials are dominant concerns of nanotechnologists. The manufacture of polymers based on molecular structure, or the design of computer chip layouts based on surface science are examples of nanotechnology in modern use. Colloidal suspensions also play an essential role in nanotechnology.

Technologies currently branded with the term 'nano' are little related to and fall far short of the most ambitious and transformative technological goals of the sort in molecular manufacturing proposals, but the term still connotes such ideas. Thus there may be a danger that a "nano bubble" will form from the use of the term by scientists and entrepreneurs to garner funding, regardless of (and perhaps despite a lack of) interest in the transformative possibilities of more ambitious and far-sighted work. The above prediction has come to pass, as by 2006 over $400 million has been invested in Nanotechnology, mostly by venture capital, with very meager results. From this perspective, Nanotechnology may be viewed as a collection of wishful predictions, aimed at generating unwarranted excitement among venture capitalists.

The National Science Foundation (a major source of funding for nanotechnology in the United States) funded researcher David Berube to study the field of nanotechnology. His findings are published in the monograph “Nano-Hype: The Truth Behind the Nanotechnology Buzz". This published study (with a foreword by Mihail Roco, head of the NNI) concludes that much of what is sold as “nanotechnology” is in fact a recasting of straightforward materials science, which is leading to a “nanotech industry built solely on selling nanotubes, nanowires, and the like” which will “end up with a few suppliers selling low margin products in huge volumes."

Larger to smaller: a materials perspective

A unique aspect of nanotechnology is the vastly increased ratio of surface area to volume present in many nanoscale materials which opens new possibilities in surface-based science, such as catalysis. A number of physical phenomena become noticeably pronounced as the size of the system decreases. These include statistical mechanical effects, as well as quantum mechanical effects, for example the “quantum size effect” where the electronic properties of solids are altered with great reductions in particle size. This effect does not come into play by going from macro to micro dimensions. However, it becomes dominant when the nanometer size range is reached. Additionally, a number of physical properties change when compared to macroscopic systems. One example is the increase in surface area to volume of materials. This catalytic activity also opens potential risks in their interaction with biomaterials.

Nanotechnology can be thought of as extensions of traditional disciplines towards the explicit consideration of these properties. Additionally, traditional disciplines can be re-interpreted as specific applications of nanotechnology. This dynamic reciprocation of ideas and concepts contributes to the modern understanding of the field. Broadly speaking, nanotechnology is the synthesis and application of ideas from science and engineering towards the understanding and production of novel materials and devices. These products generally make copious use of physical properties associated with small scales.

Materials reduced to the nanoscale can suddenly show very different properties compared to what they exhibit on a macroscale, enabling unique applications. For instance, opaque substances become transparent (copper); inert materials become catalysts (platinum); stable materials turn combustible (aluminum); solids turn into liquids at room temperature (gold); insulators become conductors (silicon). Materials such as gold, which is chemically inert at normal scales, can serve as a potent chemical catalyst at nanoscales. Much of the fascination with nanotechnology stems from these unique quantum and surface phenomena that matter exhibits at the nanoscale.

Nanosize powder particles (a few nanometres in diameter, also called nanoparticles) are potentially important in ceramics, powder metallurgy, the achievement of uniform nanoporosity and similar applications. The strong tendency of small particles to form clumps ("agglomerates") is a serious technological problem that impedes such applications. However, a few dispersants such as ammonium citrate (aqueous) and imidazoline or oleyl alcohol (nonaqueous) are promising additives for deagglomeration. (Dispersants are discussed in "Organic Additives And Ceramic Processing," by Daniel J. Shanefield, Kluwer Academic Publ., Boston.)

Another concern is that the volume of an object decreases as the third power of its linear dimensions, but the surface area only decreases as its second power. This somewhat subtle and unavoidable principle has huge ramifications. For example the power of a drill (or any other machine) is proportional to the volume, while the friction of the drill's bearings and gears is proportional to their surface area. For a normal-sized drill, the power of the device is enough to handily overcome any friction. However, scaling its length down by a factor of 1000, for example, decreases its power by 10003 (a factor of a billion) while reducing the friction by only 10002 (a factor of "only" a million). Proportionally it has 1000 times less power per unit friction than the original drill. If the original friction-to-power ratio was, say, 1%, that implies the smaller drill will have 10 times as much friction as power. The drill is useless.

This is why, while super-miniature electronic integrated circuits can be made to function, the same technology cannot be used to make functional mechanical devices in miniature: the friction overtakes the available power at such small scales. So while you may see microphotographs of delicately etched silicon gears, such devices are curiosities only, not actually usable parts. Surface tension increases in the same way, causing very small objects tend to stick together. This could possibly make any kind of "micro factory" impractical: even if robotic arms and hands could be scaled down, anything they pick up will tend to be impossible to put down. All these scaling issues have to be kept in mind while evaluating any kind of nanotechnology.
 
Silicene, the next star material for nanoelectronics
2D-NANOLATTICES is the European project investigating the properties and behaviors of silicene, the graphene’s “cousin”. This new material could make revolutionary progress in nanoelectronic devices and integrated circuits

Author: Athanasios Dimoulas, published in DAE blog on 25/02/2015

This is a guest blog post written by Athanasios Dimoulas, 2D-NANOLATTICES project coordinator, the first European project aimed to study silicene, a 2D semiconductor

I have enjoyed being the coordinator of several ICT projects dealing with advanced nanoelectronic materials and devices; however, coordinating the FET Open 2D-NANOLATTICES project was the most enjoyable and rewarding experience in my professional life. Whenever I get involved in research I am focused on applications. This time, it was a bit more special. Both I and my European collaborators in the project were driven by scientific curiosity when trying to make, for the first time, silicene, a material similar to graphene, which however does not exist in Nature.
Every time we were talking to our colleagues in conferences and other meetings about our new exciting research subject, we were always receiving feedback full of skepticism. Silicene is never going to be as stable as graphene, our colleagues always commented. Indeed, stabilizing silicene was not an easy job. However, about three years ago, our consortium partners from CNRS in Marseilles, in a pioneering work, managed to grow silicene on silver for the first time. They have been leading the research in the field ever since.  It soon became clear that silicene, had a special interaction with silver. It is attracted to it because it can comfortably rest on the surface avoiding reactions with the silver substrate.   At least, this is what we thought. However, when we, at NCSR DEMOKRITOS, looked more carefully at the “inside” of silicene, we realized that there are very gentle interactions between the two materials, therefore, the electronic properties are different than we thought. It looks as if silver has a non-desired effect on silicene which makes it lose its identity.

Being involved, for so many years, in semiconductor research, I learned to appreciate the flexibility of semiconductors which can change their electronic properties when they are affected by external perturbations. This flexibility makes them useful in electronics after all. On the other hand, metals are “boring” materials to my eyes because no matter how hard you try, you cannot change their conductivity or any other electronic and electrical property. We hoped that silicene would behave as a semiconductor just like silicon rather than metal (like silver) but unfortunately it looks as if silicene, in proximity with silver, behaves almost like a metal. This was not good news and brought about a disappointment among us, in 2D NANOLATTICES, about one year ago.
At this turning point, our colleagues at CNR-IMM, in Agrate Italy, members of 2D NANOLATTICES, and their collaborators across the Atlantic from the U. Texas in Austin, came up with the solution. They first prepared silicene on thin silver layers on mica. Then, they found an ingenious way to transfer it without any damage on an insulator substrate, i.e silicon dioxide.  That was it!  Silicon dioxide has no bad influence on silicene at all. Our colleagues at CNR and U. Texas made the first research type silicene field effect transistor in the world, similar to the transistors that can be found in our laptop microprocessors, and their results just appeared in Nature Nanotechnology this month. This is a thrilling new achievement. While in the present day transistors the charge flows through a thick silicon body, in silicene, the current is forced to pass through a single layer of silicon atoms, nearly 100 times thinner. This guarantees better control and less leakage allowing us to shrink the lateral dimensions and fit about 10-50 times more transistors in the same area on the chip. That could lead to dense/low cost electronics, hopefully faster and more energy efficient. 
However, it is a long way before we get to the point to say that we have “silicene inside” our computers. There are a lot of fundamental materials and engineering problems to solve. To have billions of working silicene field effect transistor FET devices integrated in a square centimeter of chip area is a major challenge and can take more than 10 years to get a grasp on it. There is a lot of research and development work ahead of us and we expect a lot of excitement yet to come.

Getting Silicene transistor to work was a real adventure that required concerted effort from all participants in the 2D NANOLATTICES consortium. It is absolutely clear that the spectacular progress in silicene worldwide could not be possible without the generous financial support of EU’s FP7- FET Open program. It is also clear that a lot more public and private investment is required to bring silicene in the next level of development and hopefully near production in several years from now. The first thing to do is to enhance the performance characteristics of the silicene transistors and find ways to make these devices more stable when they are exposed to air.
Right now silicene FETs behave similar to graphene devices showing semimetallic properties. This is not bad of course because this is what we wanted to do in first place: to make a “new graphene” from different atoms and prove that graphene is not the only material that exists in a 2D form.  On the other hand, we always hope that with silicene we can do something more than what graphene is capable of doing in electronics. Also we should not ignore other candidates.
Germanium is in the same group in the periodic table as silicon and is expected to have similar chemical behaviour. Why not trying germanene? In fact, us and other people have also tried this and the first results show that it is equally or even more difficult than making silicene, but is not impossible. First strong evidence of its existence on metals like platinum and gold is already there bringing more excitement in the field. Other elements may be better candidates too. Any ideas?

Get in touch with us:

www.2d-nanolattices.eu | http://www.mdm.imm.cnr.it | http://piim.univ-amu.fr/

Follow us on Twitter to get all the news on Future and Emerging Technologies - @fet_eu!

To know more about this interesting project, have a look at this article: 2D-NANOLATTICES: The ultimate in semiconductor miniaturisation!

References:

®P. Vogt, P. De Padova, C. Quaresima, J. Avila, E. Frantzeskakis, M. C. Asensio, A. Resta, B. Ealet, and G. Le Lay, “Silicene: Compelling Experimental Evidence for Graphene like Two-Dimensional Silicon’, Phys. Rev. Lett. 108 (2012) 155501.

®D. Tsoutsou, E. Xenogiannopoulou, E. Golias , P. Tsipas, A. Dimoulas, ‘Evidence for hybrid surface metallic band in (4 × 4) silicene on Ag(111)’, Appl. Phys. Lett. 103 (2013) 231604

®A. Dimoulas, ‘Review Article: Silicene and germanene: Silicon and germanium in the "flatland"’, Microelec. Eng. 131, 68 (2015).

®L. Tao, E. Cinquanta, D. Chiappe, C. Grazianetti, M. Fanciulli, M. Dubey, A. Molle and D. Akinwande, ‘Silicene field-effect transistors operating at room temperature’, at Nature Nanotech. published online, February 2, 2015| DOI: 10.1038/ NNANO.2014.325

Simple to complex: a molecular perspective

Modern synthetic chemistry has reached the point where it is possible to prepare small molecules to almost any structure. These methods are used today to produce a wide variety of useful chemicals such as pharmaceuticals or commercial polymers. The ability of this is to extend the control to the next, seeking methods to assemble these single molecules into supramolecular assemblies consisting of many molecules arranged in a well defined manner.

These approaches utilize the concepts of molecular self-assembly and/or supramolecular chemistry to automatically arrange themselves into some useful conformation through a bottom-up approach. The concept of molecular recognition is especially important: molecules can be designed so that a specific conformation or arrangement is favored due to non-covalent intermolecular forces. The Watson-Crick basepairing rules are a direct result of this, as is the specificity of an enzyme being targeted to a single substrate, or the specific folding of the protein itself. Thus, two or more components can be designed to be complementary and mutually attractive so that they make a more complex and useful whole.

Such bottom-up approaches should, broadly speaking, be able to produce devices in parallel and much cheaper than top-down methods, but could potentially be overwhelmed as the size and complexity of the desired assembly increases. However, the bottom-up approach is viewed by many thoughtful scientists as being mostly wishful thinking. Most useful structures require complex and thermodynamically unlikely arrangements of atoms. The basic laws of probability and entropy make it very unlikely that atoms will "self-assemble" in useful configurations, or can be easily and economically nudged to do so. About the only example of this is crystal-growing, for which Nanotechnology cannot take any credit, it having been around for millenia.

Molecular Nanotechnology: a long-term view

Advanced nanotechnology, sometimes called molecular manufacturing, is a term given to the concept of engineered nanosystems (nanoscale machines) operating on the molecular scale. By the countless examples found in biology it is currently known that billions of years of evolutionary feedback can produce sophisticated, stochastically optimized biological machines, and it is hoped that developments in nanotechnology will make possible their construction by some shorter means, perhaps using biomimetic principles. However, K Eric Drexler and other researchers have proposed that advanced nanotechnology, although perhaps initially implemented by biomimetic means, ultimately could be based on mechanical engineering principles (see also mechanosynthesis)

When the term "nanotechnology" was independently coined and popularized by Eric Drexler, who at the time was unaware of an earlier usage by Norio Taniguchi, it referred to a future manufacturing technology based on molecular machine systems. The premise was that molecular-scale biological analogies of traditional machine components demonstrated that molecular machines were possible, and that a manufacturing technology based on the mechanical functionality of these components (such as gears, bearings, motors, and structural members) would enable programmable, positional assembly to atomic specification (see the original reference PNAS-1981). The physics and engineering performance of exemplar designs were analyzed in the textbook Nanosystems.

Another view, put forth by Carlo Montemagno, is that future nanosystems will be hybrids of silicon technology and biological molecular machines, and his group's research is directed toward this end.

The seminal experiment proving that positional molecular assembly is possible was performed by Ho and Lee at Cornell University in 1999. They used a scanning tunneling microscope to move an individual carbon monoxide molecule (CO) to an individual iron atom (Fe) sitting on a flat silver crystal, and chemically bound the CO to the Fe by applying a voltage.

Though biology clearly demonstrates that molecular machine systems are possible, non-biological molecular machines are today only in their infancy. Leaders in research on non-biological molecular machines are Dr. Alex Zettl and his colleagues at Lawrence Berkeley Laboratories and UC Berkeley. They have constructed at least three distinct molecular devices whose motion is controlled from the desktop with changing voltage: a nanotube nanomotor, a molecular actuator, and a nanoelectromechanical relaxation oscillator.

Manufacturing in the context of productive nanosystems is not related to, and should be clearly distinguished from, the conventional technologies used to manufacture nanomaterials such as carbon nanotubes and nanoparticles.

There exists the potential to design and fabricate artificial structures analogous to natural cells and even organisms. Note that these are just blue-sky "potentials", and fall closer to the disciplines of Applied Biology and gene-splicing than to Nanotechnology.

Current research

As nanotechnology is a very broad term, there are many disparate but sometimes overlapping subfields that could fall under its umbrella. The following avenues of research could be considered subfields of nanotechnology. Note that these categories are fairly nebulous and a single subfield may overlap many of them, especially as the field of nanotechnology continues to mature.

Nanomaterials

This includes subfields which develop or study materials having unique properties arising from their nanoscale dimensions.

Bottom-up approaches

- Colloid science has given rise to many materials which may be useful in nanotechnology, such as carbon nanotubes and other fullerenes, and various nanoparticles and nanorods.
- Nanoscale materials can also be used for bulk applications; most present commercial applications of nanotechnology are of this flavor.
- Headway has been made in using these materials for medical applications; see Nanomedicine.

These seek to arrange smaller components into more complex assemblies.

  • DNA Nanotechnology utilizes the specificity of Watson-Crick basepairing to construct well-defined structures out of DNA and other nucleic acids.

  • More generally, molecular self-assembly seeks to use concepts of supramolecular chemistry, and molecular recognition in particular, to cause single-molecule components to automatically arrange themselves into some useful conformation.

Top-down approaches

These seek to create smaller devices by using larger ones to direct their assembly.

  • Many technologies descended from conventional solid-state silicon methods for fabricating microprocessors are now capable of creating features smaller than 100 nm, falling under the definition of nanotechnology. Giant magnetoresistance-based hard drives already on the market fit this description, as do atomic layer deposition (ALD) techniques.

  • Solid-state techniques can also be used to create devices known as nanoelectromechanical systems or NEMS, which are related to microelectromechanical systems or MEMS.

  • Atomic force microscope tips can be used as a nanoscale "write head" to deposit a chemical on a surface in a desired pattern in a process called dip pen nanolithography. This fits into the larger subfield of nanolithography.

Functional approaches

These seek to develop components of a desired functionality without regard to how they might be assembled.

  • Molecular electronics seeks to develop molecules with useful electronic properties. These could then be used as single-molecule components in a nanoelectronic device. For an example see rotaxane.

  • Synthetic chemical methods can also be used to create synthetic molecular motors, such as in a so-called nanocar.

Speculative

These subfields seek to anticipate what inventions nanotechnology might yield, or attempt to propose an agenda along which inquiry might progress. These often take a big-picture view of nanotechnology, with more emphasis on its societal implications than the details of how such inventions could actually be created.

  • Molecular nanotechnology is a proposed approach which involves manipulating single molecules in finely controlled, deterministic ways. This is more theoretical than the other subfields and is beyond current capabilities.

  • Nanorobotics centers on self-sufficient machines of some functionality operating at the nanoscale.

  • Programmable matter based on artificial atoms seeks to design materials whose properties can be easily and reversibly externally controlled.

Tools and techniques

Typical AFM setup. A microfabricated cantilever with a sharp tip is deflected by features on a sample surface, much like in a phonograph but on a much smaller scale. A laser beam reflects off the backside of the cantilever into a set of photodetectors, allowing the deflection to be measured and assembled into an image of the surface.

Nanoscience and nanotechnology only became possible in the 1910's with the development of the first tools to measure and make nanostructures. But the actual development started with the discovery of electrons and neutrons which showed scientists that matter can really exist on a much smaller scale than what we normally think of as small, and/or what they thought was possible at the time. It was at this time when curiosity for nanostructures had originated.

The atomic force microscope (AFM) and the Scanning Tunneling Microscope (STM) are two early versions of scanning probes that launched nanotechnology. There are other types of scanning probe microscopy, all based on the idea of the STM, that make it possible to see structures at the nanoscale. The tip of scanning probes can also be used to manipulate nanostructures (a process called positional assembly). However, this is a very slow process. This led to the development of various techniques of nanolithography such as dip pen nanolithography, electron beam lithography or nanoimprint lithography. Lithography is a top-down fabrication technique where a bulk material is reduced in size to nanoscale pattern.

The top-down approach anticipates nanodevices that must be built piece by piece in stages, much as manufactured items are currently made. Scanning probe microscopy is an important technique both for characterization and synthesis of nanomaterials. Atomic force microscopes and scanning tunneling microscopes can be used to look at surfaces and to move atoms around. By designing different tips for these microscopes, they can be used for carving out structures on surfaces and to help guide self-assembling structures. Atoms can be moved around on a surface with scanning probe microscopy techniques, but it is cumbersome, expensive and very time-consuming. For these reasons, it is not feasible to construct nanoscaled devices atom by atom. Assembling a billion transistor microchip at the rate of about one transistor an hour is inefficient.

One hope is that these techniques may eventually be used to make primitive nanomachines, which in turn can be used to make more sophisticated nanomachines. But the whole nanomachine concept is wild speculation, as we are unable to even conceptually design human scale machines that can independently make other machines. If we can't make them on a convenient scale, what are the chances they can be made on a nano scale? Also nanomachines have the very substantial hurdles of friction and surface-tension.

In contrast, bottom-up techniques build or grow larger structures atom by atom or molecule by molecule. These techniques include chemical synthesis, self-assembly and positional assembly. Another variation of the bottom-up approach is molecular beam epitaxy or MBE. Researchers at Bell Telephone Laboratories like John R. Arthur. Alfred Y. Cho, and Art C. Gossard developed and implemented MBE as a research tool in the late 1960s and 1970s. Samples made by MBE were key to to the discovery of the fractional quantum Hall effect for which the 1998 Nobel Prize in Physics was awarded. MBE allows scientists to lay down atomically-precise layers of atoms and, in the process, build up complex structures. Important for research on semiconductors, MBE is also widely used to make samples and devices for the newly emerging field of spintronics.

Newer techniques such as Dual Polarisation Interferometry are enabling scientists to measure quantitatively the molecular interactions that take place at the nano-scale.

Societal implications

Potential risks of nanotechnology can broadly be grouped into three areas:

  • the risk to health and environment from nanoparticles and nanomaterials;

  • the risk posed by molecular manufacturing (or advanced nanotechnology);

  • societal risks.

Nanoethics concerns the ethical and social issues associated with developments in nanotechnology, a science which encompass several fields of science and engineering, including biology, chemistry, computing, and materials science. Nanotechnology refers to the manipulation of very small-scale matter – a nanometer is one billionth of a meter, and nanotechnology is generally used to mean work on matter at 100 nanometers and smaller.

Social risks related to nanotechnology development include the possibility of military applications of nanotechnology (such as implants and other means for soldier enhancement) as well as enhanced surveillance capabilities through nano-sensors. However those applications still belong to science-fiction and will not be possible in the next decades. Significant environmental, health, and safety issues might arise with development in nanotechnology since some negative effects of nanoparticles in our environment might be overlooked. However nature itself creates all kinds of nanoobjects, so probable dangers are not due to the nanoscale alone, but due to the fact that toxic materials become more harmful when ingested or inhaled as nanoparticles.