NANOTECHNOLOGY

“A RECENT TREND IN MOLECULAR MANUFACTURING”

ABSTRACT:

Most interestingly, we are approaching the world of Fantastic Voyage. Experts in this new field of Nanotechnology promise a world in which very small machines literally circulate within us, pursuing bad bacteria and viruses and dissolving cholesterol and lipids. It sounds great, if a little bit spooky, but it is still a long way away .Nanotechnology is a field of science whose goal is to control individual atoms and molecules to create computer chips and other devices that are thousands of times smaller than current technologies permit. Nanotechnology can best be considered as a 'catch-all' description of activities at the level of atoms and molecules that have applications in the real world. A nanometre is a billionth of a meter, that is, about 1/80,000 of the diameter of a human hair, or 10 times the diameter of a hydrogen atom.

Materials Science research is now entering a new phase where the structure and properties of materials can be investigated,characterized and controlled at the nanoscale. New and sometimes unexpected materials properties appear at the nanoscale, thus bringing new excitement to this research field. In this talk, special emphasis will be given to one-dimensional nanowires because they exhibit unusual physical properties, due to their reduced dimensionality and their enhanced surface/volume ratio. These unusual properties have attracted interest in their potential for applications in novel electronic, optical, magnetic and thermoelectric devices. This paper attempts to fill the need of utility of Diamond and explains about Molecular Manufacturing.

INTRODUCTION:-

Manufactured products are made from atoms. The properties of those products depend on how those atoms are arranged. If we rearrange the atoms in coal we can make diamond. If we rearrange the atoms in sand (and add a few other trace elements) we can make computer chips. If we rearrange the atoms in dirt, water and air we can make potatoes.

Today’s manufacturing methods are very crude at the molecular level. Casting, grinding, milling and even lithography move atoms in great thundering statistical herds. It's like trying to make things out of LEGO blocks with boxing gloves on your hands. Yes, you can push the LEGO blocks into great heaps and pile them up, but you can't really snap them together the way you'd like.

In the future, nanotechnology will let us take off the boxing gloves. We'll be able to snap together the fundamental building blocks of nature easily, inexpensively and in most of the ways permitted by the laws of physics. This will be essential if we are to continue the revolution in computer hardware beyond about the next decade, and will also let us fabricate an entire new generation of products that are cleaner, stronger, lighter, and more precise.

It's worth pointing out that the word "nanotechnology" has become very popular and is used to describe many types of research where the characteristic dimensions are less than about 1,000 nanometers. For example, continued improvements in lithography have resulted in line widths that are less than one micron: this work is often called "nanotechnology." Sub-micron lithography is clearly very valuable (ask anyone who uses a computer!) but it is equally clear that lithography will not let us build semiconductor devices in which individual dopant atoms are located at specific lattice sites. Many of the exponentially improving trends in computer hardware capability have remained steady for the last 50 years. There is fairly widespread belief that these trends are likely to continue for at least another several years, but then lithography starts to reach its fundamental limits.

When it's unclear from the context whether we're using the specific definition of "nanotechnology" (given here) or the broader and more inclusive definition (often used in the literature), we'll use the terms "molecular nanotechnology" or "molecular manufacturing."

Whatever we call it, it should let us

Get essentially every atom in the right place.
Make almost any structure consistent with the laws of physics that we can specify in molecular detail.
Have manufacturing costs not greatly exceeding the cost of the required raw materials and energy.

There are two more concepts commonly associated with nanotechnology:

Positional assembly.
Self Replication.

Clearly, we would be happy with any method that simultaneously achieved the first three objectives. However, this seems difficult without using some form of positional assembly (to get the right molecular parts in the right places) and some form of self replication (to keep the costs down).

The need for positional assembly implies an interest in molecular robotics, e.g., robotic devices that are molecular both in their size and precision. These molecular scale positional devices are likely to resemble very small versions of their everyday macroscopic counterparts. Positional assembly is frequently used in normal macroscopic manufacturing today, and provides tremendous advantages. The requirement for low cost creates an interest in self replicating manufacturing systems, which are able both to make copies of themselves and to manufacture useful products. If we can design and build one such system the manufacturing costs for more such systems and the products they make (assuming they can make copies of themselves in some reasonably inexpensive environment) will be very low.

Molecular Manufacturing:

Adding Positional Control to Chemical Synthesis:

Manufactured products are made from atoms. The properties of those products depend on how those atoms are arranged. Viewed from the molecular level today's macroscopic manufacturing methods are crude and imprecise. Casting, milling, welding and all the other traditional manufacturing methods spray atoms about in great statistical herds. Even lithography (which already lets us put millions of transistors on a chip no bigger than your fingernail) is fundamentally statistical and random. Exactly how many dopant atoms are in a single transistor and exactly where each individual dopant atom is located is neither specified nor known: if we have roughly the right number in roughly the right place, we can make a working transistor. For today, that is good enough.

The exception is chemistry. Large high purity crystals can have almost every atom in the right place. So, too, can many long polymers. The structures of proteins with hundreds and even thousands of amino acids can be specified down to the last atom. Most dramatically (and fortunately for us!) DNA strands with many tens of millions of bases can be copied with almost perfect accuracy. And it seems that almost any small molecule (with perhaps several dozens of atoms) can be synthesized, if only we have the skill and patience.

Yet the laws of physics and chemistry in principle permit arranging and rearranging the elements in so many combinations and permutations that all of our manufacturing skills and all of our chemical skills barely suffice to scratch the surface of what is possible.

The Utility of Diamond

Almost any manufactured product could be improved, often by several orders of magnitude, if we could precisely control its structure at the molecular level. We often want our products to be light and strong. Diamond is light and strong: the strength-to-weight ratio of diamond is over 50 times that of steel. Yet we do not today have diamond spars in airplanes or diamond hulls for rockets. Today we can't economically make diamond. Even if we could, simple diamond crystals can shatter. We'd have to modify the structure to make it tough and shatter proof: perhaps diamond fibers. While easily done in principle, we can't do this in practice today.

Great strength and lightweight are not the exclusive province of diamond: graphite can be stronger. And if we consider the many ways in which carbon atoms can be arranged and rearranged, then it's obvious that there are a host of other possibilities. Yet all share a common problem: we can't yet economically make them in the exact shapes that we want.

It has greater thermal conductivity, so devices can be more easily cooled. It has a greater breakdown field, hence devices can be smaller. It has higher electron and hole mobility which, when combined with higher electric fields, will result in higher speed. Large pure crystals of silicon can be made relatively easily, but large pure crystals of diamond are scarce. We can etch the silicon surface and add dopants with a precision measured in tenths of microns, while the corresponding steps for diamond are more difficult. Not more difficult in principle: just more difficult today.

How We Make Diamond Today

Today, we can synthesize diamond at low pressure and low temperature by using CVD (Chemical Vapor Deposition) methods. Diamond CVD growth involves highly reactive species (radicals, carbenes, etc.) in a gas over the growing diamond surfaces that bombard and react with that surface at random. Because reaction sites are random, growth of many defect structures occurs (dislocations, etc.) as well as the desired perfect diamond structure.

Two fundamental mechanisms in the growth process include (1) abstraction of hydrogen from the diamond surface leaving behind reactive sites (dangling bonds, radicals) and (2) interaction of carbon species (both reactive (CH2, CH3, etc.) as well as relatively unreactive species (C2H2)) with the surface, thus depositing carbon

Table 1: Some of the outstanding properties of diamond

Extreme mechanical hardness (~90 GPa).

Strongest known material, highest bulk modulus (1.2 x 10¹² N/m²), lowest compressibility (8.3 x 10^-13 m²/ N).

Highest known value of thermal conductivity at room temperature (2 x 10³ W/m/ K).

Thermal expansion coefficient at room temperature (0.8 x 10^-6 K) is comparable with that of invar.

Broad optical transparency from the deep UV to the far IR region of the electromagnetic spectrum.

Good electrical insulator (room temperature resistivity is ~10¹⁶  cm).

Diamond can be doped to change its resistivity over the range 10-10⁶  cm, so becoming a semiconductor with a wide bad gap of 5.4 eV.

Very resistant to chemical corrosion.

Biologically compatible.

Exhibits low or 'negative' electron affinity.

If we are to synthesize diamonded structures it is plausible that we begin our search for the basic reaction steps involved in this synthesis by looking at existing reactions that occur in the CVD growth of Diamond. The use of a reactive gas in the synthesis process, however, would seem to defeat any hope of making precisely patterned diamonded structures, for the gas will interact with the growing surface at random.

· Present Applications and Future Prospects

How is all this research effort feeding through into the marketplace? A number of areas of application are gradually beginning to appear.

Thermal management - Natural diamond has a thermal conductivity roughly four times superior to that of copper, and it is an electrical insulator: It should therefore come as little surprise to learn that CVD diamond is now being marketed as a heat sink for laser diodes and for small microwave integrated circuits. Reliability can be expected to improve also since, for a given device, junction temperatures will be lower when mounted on diamond.

Cutting tools - CVD diamond is also finding applications as an abrasive and as a coating on cutting tool inserts. CVD diamond-coated drill bits, reamers, countersinks etc. are now commercially available for machining non-ferrous metals, plastics, and composite materials..

Wear Resistant Coatings - In both the previous applications, CVD diamond is performing a task that could have been fulfilled equally well by natural diamond if economics were not a consideration. However, there are many other applications at, or very close to, the market-place where CVD diamond offers wholly new opportunities. Wear resistant coatings are one such use. The ability to protect mechanical parts with an ultra-hard coating, in for example, gearboxes, engines, and transmissions, may allow greatly increased lifetimes of components with reduced lubrication. The phrase 'non-ferrous' is worth emphasizing here since it reminds us of one of the biggest outstanding challenges in the application of diamond film technology - whether as a wear-resistant coating or as a fine abrasive.

Electronic devices - the possibility of doping diamond and so changing it from being an insulator into a semiconductor opens up a whole range of potential electronic applications. However, there are a number of major problems that need to be overcome if diamond-based electronic circuits are to be achieved. Principal among these is the fact that CVD diamond films are polycrystalline and hence contain grain boundaries, twins, stacking faults, and other defects, which all reduce the lifetime and mobilities of carriers. This remains a major limiting factor in the development of diamond devices.

Positional Control is Fundamental

Here, we introduce the fundamental concept of molecular manufacturing: positional control over the site of reactions. To take a specific example we consider site specific hydrogen abstraction from the diamond (111) surface. The ability to remove specific hydrogen atoms from the surface of the diamonded work piece under construction is likely to be a fundamental unit operation in any attempt to make atomically precise diamonded structures.

Hydrogen abstraction during CVD diamond growth typically involves a radical reaction between atomic H from the gas with H bonded to carbon on the surface producing H2. It is unclear how to make this process site specific. However, there are other structures with a high affinity for hydrogen which offer greater possibilities for positional control. In particular, the propynyl radical C3H3 (figure 1) has a great affinity for hydrogen. Further, this radical has the very useful property that it has two ends: one end is a highly reactive radical while the other end is a stable sp3 carbon. Thus, we could synthesize a larger molecule with the propynyl radical at its end. The larger molecule would be held at the tip of a positional device. The positional device would provide control over the orientation and position of this hydrogen abstraction tool (e.g., six degrees and thus control the site of abstraction by controlling the position of the tool.

Figure 1. A site specific hydrogen abstraction tool.

Abstraction of hydrogen from isobutene using an ethynyl abstraction tool supports the idea that the barrier to this reaction is zero. The reaction will proceed rapidly and, because of the large exothermicity, irreversibly. Calculated barriers for abstraction from several other molecules were also small, suggesting that this hydrogen abstraction tool could be used to abstract hydrogen from a wide range of different molecules. Molecular dynamics simulations provide evidence that the abstraction reaction will select the correct hydrogen atom in the face of thermal noise at room temperature, as well as providing further support for the basic mechanism.

Other Molecular Tools

If we are to grow diamond, we must also have carbon deposition tools. Drexler has suggested the use of positional controlled carbenes (figure 2) and alkynes (figure 3) and proposed reaction pathways and surface structures where these tools would apply. In both cases, the tools are positioned at a precise point on the growing diamondoid structure and are used to deposit one or more carbon atoms at a desired location. These deposition reactions parallel proposals in the CVD literature except for the addition of positional control (e.g., at least one portion of the moiety must be part of an extended "handle" which can be held by a positional device). These are only two examples from the wide range of tools that are capable of depositing carbon on a surface.

Figure 2. A positional controlled carbine

Figure 3. A positional controlled strained cycloalkyne

The broad range of possible tools coupled with the great power of ab initio computational chemistry should let us define and verify a complete set of molecular tools capable of synthesizing essentially any diamonded structure. The work by Musgrave and Sonnet et. al. are first steps toward this objective. Modern ab initio methods can produce results that are sufficiently accurate for this type of analysis. Further research in this area is feasible and should be pursued.

The Context of Tool Use

For such tools to be usable in a system context we must satisfy certain constraints. First and foremost, we must have a device capable of positioning the tool to within something like an atomic diameter. On the diamond surface, the distance between adjacent hydrogen’s is about 2.5 Angstroms. Thus, positional accuracy of 1 to 2 angstroms for the hydrogen abstraction tool is required to prevent abstraction of the wrong hydrogen. Second, because the tools can be highly reactive, we require an inert environment. A simple inert environment is vacuum. Compressed helium or some other inert gas would also work. Third, because it is the relative tool-work piece position that must be controlled, the work piece under construction must be relatively rigid (e.g., not subject to vibration motions that would exceed about an angstrom). Fourth and last, we must have some way of generating the sometimes highly reactive tools (e.g., we need to define a precursor to the hydrogen abstraction tool, as well as precursors to the other tools).

Positional Devices and Molecular "Arms"

Work with SPMs (Scanning Probe Microscopes) clearly shows that it is possible to achieve positional accuracies of a small fraction of an angstrom. Small (~0.1 microns) diamonded "arms" or positional devices with similar positional accuracy are in principle quite feasible. The field of robotics provides a broad range of designs for positional devices which are largely scale independent. Shrinking these designs to submicron size is conceptually straight forwards. A factor of crucial importance in the design of molecular-scale positional devices is the accuracy with which the tip can be positioned, particularly in the face of thermal noise. To control this source of error, it is essential that the robotic arm be very stiff, and so the use of stiff materials is desirable. The Young's modulus for diamond is about 10^12 Pascal’s (very stiff), and back-of-the-envelope calculations show that a hollow cylinder of such material that is perhaps 100 nanometers long and 30 nanometers in diameter should have a positional accuracy at the tip, in the face of thermal noise at room temperature, of a small fraction of an atomic diameter. A more detailed design and analysis of a jointed tubular robotic arm taking into account the bending and rocking motions of joints in the arm further supports this conclusion.

Other Requirements for Tool Use

Creating an inert environment also presents no fundamental problems: high quality vacuums are common in laboratories today. If our objective is to have a very small very high quality vacuum, then a relatively thin wall of diamonded material could be used as a barrier to keep a volume which was a modest fraction of a cubic micron free of any contaminants. If the volume were initially constructed free of contaminants then such a barrier would keep the inside free of any contaminants with high probability.

Finally, generation of "activated" tools from relatively stable precursors can be done by a variety of methods. Because we are assuming an environment in which we have positional control we can use particularly simple precursors. We illustrate this by considering a precursor to the hydrogen abstraction tool (see figure 4). This precursor has two handles, and X is chosen so that the X-C bond is weaker than the C-C bond. X might be Si or Ge. If we pull on the two handles with sufficient force, something will break. Because the X-C bond was deliberately selected to be weaker than the other bonds in the structure, it will break. This gives us the activated hydrogen abstraction tool

Figure 4. A possible precursor to the hydrogen abstraction tool of figure 1.

A related question is: how can we get the hydrogen off the tip of the abstraction tool? A simple answer is: don't. Throw the tool away after one use. In a system design using this approach, it would be necessary to provide a continuous stream of precursors. These would be activated, used once, and then discarded.

More generally the activation of relatively stable precursors can be done by using any of several forms of energy: mechanical, optical, chemical or other. While the use of mechanical means to provide the activation energy for chemical reactions is relatively novel, in an environment where positional control is already available it is quite natural.

Figure 5. Selective transport of desired molecules across a diamondoid barrier.

Control Signals

Finally, we will need a source of control signals for our molecular arm. One general approach would be to use a molecular computer. We will not consider a particular design for a molecular computer here, it is sufficient to note that many proposals for molecular computation have been considered in the literature and it is generally expected that some type of very small computational device will be feasible in a few decades.

Self replication and nanotechnology

A crucial objective of nanotechnology is the ability to make products inexpensively. While the ability to make a few very small, very precise molecular machines very expensively would clearly be a major scientific achievement, it would not fundamentally change how we make most products.

Just as the early pioneers of flight took inspiration by watching birds soar effortlessly through the air, so we can take inspiration from nature as we develop molecular manufacturing systems. Of course, "inspired by" does not mean "copied from." Airplanes are very different from birds: a 747 bears only the smallest resemblance to a duck even though both fly. The artificial self replicating systems that have been envisioned for molecular manufacturing bear about the same degree of similarity to their biological counterparts as a car might bear to a horse.

The machines that people make tend to be inflexible and brittle in response to changes in their environments. By contrast, living biological systems are wonderfully flexible and adaptable. Horses can pick their way along a narrow trail or jump over shrubs; they get "parts" (from their food) in the same flexible way they get energy; and they have a remarkable self-repair ability.

In the same way, the artificial self-replicating systems that are being proposed for molecular manufacturing are inflexible and brittle. It's difficult enough to design a system able to self replicate in a controlled environment, let alone designing one that can approach the marvelous adaptability that hundreds of millions of years of evolution have given to living systems. Designing a system that uses a single source of energy is both much easier to do and produces a much more efficient system: the horse pays for its ability to eat potatoes when grass isn't available by being less efficient at both. For artificial systems where we wish to decrease design complexity and increase efficiency, we'll design the system so that it can handle one source of energy, and handle that one source very well.

Conclusion

The long term goal of molecular manufacturing is to build exactly what we want at low cost. Many if not most of the things that we'll want to build are complex (like a molecular Cray computer), and seem difficult if not impossible to synthesize with currently available methods. Adding programmed positional control to the existing methods used in synthesis should let us make a truly broad range of macroscopic molecular structures. To add this kind of positional control, however, requires that we design and build what amount to very small robotic manipulators. This general approach, used by trees for a very long time, should let us develop a low cost general purpose molecular manufacturing technology.

While we have focused in this article on diamonded structures and molecular computers based on semiconductors such as diamond, it will probably be easier to first make systems that rely on materials that are simpler to synthesize but whose material properties are not as good as diamond. Many challenges must be met and it will be many years before we develop molecular manufacturing; but the goal is worthwhile, achievable, and offers great rewards both financial and scientific.

. Artificial systems able to make a wide range of non-biological products (like diamond) under programmatic control are likely to be more brittle and less adaptable in their response to changes in their environment than biological systems. At the same time, they should be simpler and easier to design. The complexity of such systems need not be excessive by present engineering standards

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