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Introduction to Diamond Mechanosynthesis (DMS)

 

 

What is Diamond Mechanosynthesis?

Diamond mechanosynthesis (DMS), or molecular positional fabrication, is the formation of covalent chemical bonds using precisely applied mechanical forces to build diamondoid structures. DMS may be automated via computer control, enabling programmable molecular positional fabrication.

Atomically precise fabrication involves holding feedstock atoms or molecules, and a growing nanoscale workpiece, in the proper relative positions and orientations so that when they touch they will join together in the desired manner.

In this process, a mechanosynthetic tool will be brought up to the surface of a workpiece. One or more transfer atoms are added to, or removed from, the workpiece by the tool. Then the tool is withdrawn and recharged. This process is repeated until the workpiece (e.g., a growing nanopart) is completely fabricated to molecular precision with each atom in exactly the right place. Note that the transfer atoms are under positional control* at all times to prevent unwanted side reactions from occurring.

The first experimental demonstration of true mechanosynthesis, establishing covalent bonds using purely mechanical forces on silicon atoms, not carbon atoms, was reported by Oyabu and colleagues in 2003. The first DMS patent was granted in the U.S. on 30 March 2010 to Robert A. Freitas Jr.

* More precisely, it is the handle structure which directly receives the positional control and the applied forces, not the attached transfer atoms. But a side effect of positionally contraining the handle is that the moiety at the tip is also, to some degree, positionally constrained, far more so than, say, a free-gas or solution-phase moiety.

 

 

 

 

 

Unlike macroscale robots, nanoscale manipulators and nanoscale products at intermediate stages of fabrication or assembly will be buffeted by thermal noise. Atoms and molecules are in a constant state of wiggle and jiggle. The higher the temperature, the more vigorous the motion. One technique that can position individual atoms is the scanning probe microscope (SPM), in which a sharp tip is brought down to the surface of a sample, generating a signal that allows the probed surface to be mapped, crudely analogous to a blind person tapping with a cane to sense the path ahead. Some SPMs literally push on the atomic surface and record how hard the surface pushes back, or connect the probe and the surface to a voltage source and measure the current flow when the probe gets close to the surface. A host of other probe-surface interactions can be measured and are used to make different types of SPMs.

Besides mapping, the SPM can also change a surface – for instance, by depositing individual atoms and molecules in a desired pattern. In one well-publicized case in 1989, scientists arranged 35 xenon atoms on a nickel surface to form the letters identifying their employer as “IBM”. But this SPM manipulation required cooling to 4 degrees above absolute zero – hardly ideal conditions for large-scale manufacturing. SPMs also have error rates high enough to require relatively sophisticated error detection and correction methods. While these systems can move around a few atoms or molecules, they can't manufacture large amounts of precisely structured diamond of the kind that might be used to build a molecular robotic arm.

Today’s SPMs also are much too slow. In nature, bacterial ribosomes take at least 25 milliseconds to add a single amino acid to a growing protein under positional control. If a nanofactory production line or molecular assembler is to manufacture a copy of itself (or its own mass) in about a day, and if this requires about a hundred million atom-placement operations, then each such operation must be completed in ~1 millisecond, a somewhat faster operating frequency than the ribosome. Today’s SPMs, by contrast, may take up to an hour to arrange a single atom or molecule. Major advances in SPM speed and accuracy will be required to achieve reliable diamond mechanosynthesis, and such advances are an explicit experimental goal of the Nanofactory Collaboration.

To maintain its proper position, the tooltip handle and other supporting structure (and the workpiece upon which the DMS tool labors) must be extremely stiff. The strength and density of a material depends on the number and strength of the bonds that hold its atoms together, and on the massiveness of the atoms. The element that best fits these criteria is carbon, which is both lightweight and forms stronger bonds than other elements. The carbon-carbon bond is especially strong. Each carbon atom can bond to four neighboring atoms. And carbon atoms can make the stiffest material available: diamond. In diamond, a dense network of strong bonds creates a strong, relatively lightweight and very stiff material.

The working environment for diamond mechanosynthesis is often assumed to be an ultra-high vacuum (UHV), though DMS performed in a noble gas fluid or other chemically inert fluid environment is not inconceivable.

Using computer-automated tooltips performing positionally-controlled DMS in lengthy programmed sequences of reaction steps, we may be able to fabricate simple diamondoid nanomechanical parts such as bearings, gears, struts, springs, diamond logic rods (illustrated at right), and casings to atomic precision. While it is likely that some basic diamondoid structures may be producible using self-assembly techniques from conventional synthetic chemistry, it seems unlikely that multifeatured, highly strained, or complexly interleaved structures can be fabricated without employing some form of positional control.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The Tools of Diamond Mechanosynthesis

It is already possible to synthesize bulk diamond today. In a process somewhat reminiscent of spray painting, we build up layer after layer of diamond by holding a surface in a cloud of reactive hydrogen atoms and hydrocarbon molecules. When these molecules bump into the surface they change it, either by adding, removing, or rearranging atoms. By carefully controlling the pressure, temperature, and the exact composition of the gas in this process -- which is called chemical vapor deposition or CVD -- we can create conditions that favor the growth of diamond on the surface. A typical CVD reactor setup is illustrated at right.

But randomly bombarding a surface with reactive molecules does not offer fine control over the growth process and is more like building a wristwatch with a sand blaster. To achieve atomically precise fabrication, the first challenge is to make sure that all chemical reactions will occur at precisely specified places on the surface. A second problem is how to make the diamond surface reactive at the particular spots where we want to add another atom or molecule. A diamond surface is normally covered with a layer of hydrogen atoms (white atoms in the illustration of the diamond C(110) surface at right). Without this layer, the raw diamond surface would be highly reactive because it would be studded with unused (or “dangling”) bonds from the topmost plane of carbon atoms. While hydrogenation prevents unwanted reactions, it also renders the entire surface inert, making it difficult to add carbon (or anything else) to it.

To overcome this problem, we can use a set of molecular-scale tools that would, in a series of well-defined steps, prepare the surface and create hydrocarbon structures on a layer of diamond, atom by atom and molecule by molecule. A mechanosynthetic tool has two main components -- a chemically active tooltip and a chemically inert handle to which the tooltip is covalently bonded. The handle structure is positionally manipulated using an SPM or similar instrumentality.

At least three basic mechanosynthetic tools that have already received considerable theoretical (and some experimental) study will be required to build atomically precise diamond via positional control:

(1) Hydrogen Abstraction Tools,

(2) Carbon Placement Tools, and

(3) Hydrogen Donation Tools.

 

 

 

 

 

 

 

1 2 3

(1) Hydrogen Abstraction Tools

The first step in the process of mechanosynthetic fabrication of diamond might be to remove a hydrogen atom from each of two specific adjacent spots on the diamond surface, leaving behind two reactive dangling bonds. This could be done using a hydrogen abstraction tool – a still-theoretical molecular structure that has a high chemical affinity for hydrogen at one end but is elsewhere inert.

The tool’s unreactive region serves as a handle or handle attachment point. The tool would be held by a molecular positional device, initially perhaps a scanning probe microscope tip but ultimately a molecular robotic arm, and moved directly over particular hydrogen atoms on the surface. One suitable molecule for a hydrogen abstraction tool is the acetylene radical – two carbon atoms triple bonded together. One carbon would be the handle connection, and would bond to a nanoscale positioning tool through a larger handle structure perhaps consisting of adamantane cages as shown in the illustration at right. The other carbon has a dangling bond where a hydrogen atom would normally be present in a molecule of ordinary acetylene (C2H2). The environment around the tool would be inert (e.g., vacuum or a noble gas such as neon).

The most detailed analysis of the most-studied ethynyl-based hydrogen abstraction tool has been reported by Temelso et al (2006) as one of the many collaborative efforts comprising the Nanofactory Collaboration. Non-ethynyl-based hydrogen abstraction tools have been proposed by others but have received comparatively limited theoretical study to date. A practical method for building this tool were proposed and patented in 2008 by Freitas and Merkle, and an experimental test of this proposal is in the works.

 

 

 

 

 

 

 

 

 

1 2 3

(2) Carbon Placement Tools

Once the abstraction tool has created adjacent reactive spots by selectively removing hydrogen atoms from the diamond surface, the second step is to deposit carbon atoms at the desired sites. In this way a diamond structure is built, molecule by molecule, according to plan.

The first complete tool ever proposed for this carbon deposition function, reported by Merkle and Freitas at a Foresight Conference in 2002, is the DCB6 dimer placement tool. A dimer is a molecule consisting of two of the same atoms or molecules stuck together. In this case, the dimer would be C2 – two carbon atoms connected by a triple bond, with each carbon in the dimer connected to a larger unreactive handle structure.

The dimer placement tool, also held by a molecular positional device, is brought close to the reactive spots along a particular trajectory, causing the two dangling surface bonds to react with the ends of the carbon dimer. The dimer placement tool would then withdraw, breaking the relatively weaker bonds between it and the CC dimer and transferring the carbon dimer from the tool to the surface, as illustrated above. A positionally controlled dimer could be attached almost anywhere on a growing diamondoid workpiece, in principle allowing the construction of a wide variety of useful nanopart shapes.

As of 2006, the DCB6 dimer placement tool remains the most studied of any mechanosynthetic tooltip to date, having had more than 150,000 CPU-hours of computation invested thus far in its analysis as one of the earliest collaborative efforts comprising the Nanofactory Collaboration, using two Beowulf clusters at Zyvex. The DCB6 tooltip motif is the only tooltip motif that has been successfully simulated for its intended function on a full 200-atom diamond surface. On 30 March 2010, U.S. Patent No. 7,687,146 was issued on a method for manufacturing the DCB6 tool -- the first patent ever issued on diamond mechanosynthesis. Other dimer (and related carbon transfer) tooltip motifs that have received less study but are also expected to perform well have been proposed by Drexler (1992), Merkle (1997), Merkle and Freitas (2003), Allis and Drexler (2005), Freitas, Allis and Merkle (2006), Freitas and Merkle (2008), and others, including most usefully the GermylMethylene (GM) tool for adding methyl groups to diamond, as first described by Freitas and Merkle in 2008.

 

 

 

 

 

 

 

 

 

1 2 3

(3) Hydrogen Donation Tools

After an atomically precise structure has been fabricated by a succession of hydrogen abstractions and carbon depositions, the fabricated structure must be passivated to prevent additional unplanned reactions.

While the hydrogen abstraction tool is intended to make an inert structure reactive by creating a dangling bond, the hydrogen donation tool does the opposite. It makes a reactive structure inert by terminating a dangling bond. Such a tool would be used to stabilize reactive surfaces and help prevent the surface atoms from rearranging in unexpected and undesired ways. The key requirement for a hydrogen donation tool is that it include a weakly attached hydrogen atom. Many molecules fit that description, but the bond between hydrogen and germanium (or tin) is especially weak. A Ge-based (or Sn-based) hydrogen donation tool should be effective.

The most detailed analysis of the most-studied substituted-adamantane-based hydrogen donation tool was reported by Temelso et al (2007) as one of the collaborative efforts comprising the Nanofactory Collaboration. Alternative hydrogen donation tool motifs have been proposed by others but have received comparatively limited theoretical study to date.

 

 

 

 

 

 

Exemplar Mechanosynthetic Reaction Sequence

 

Mechanosynthetic Tools Used in this Reaction Sequence:
HAbst Tool
HDon Tool
GermylMethylene (GM) Tool
GeRad Tool

Here we describe a typical mechanosynthetic reaction sequence using the four atomically precise tooltips shown in the table above. Sequences like this have been verified using advanced ab initio computational chemistry calculations but not experimentally. This particular sequence can be used to add a CH3 to essentially any selected carbon atom on a hydrocarbon workpiece. In the illustration below, the workpiece is represented by a cluster of atoms at the bottom of the frame representing a small piece of C(100)-H(2x1) diamond surface. Carbon atoms are black, hydrogen atoms are white, and germanium atoms are yellow.

 

This reaction sequence directly employs three tools during the fabrication process: the Hydrogen Abstraction (HAbst) tool, the GermylMethylene (GM) tool, and the Hydrogen Donation (HDon) tool. Execution of the sequence produces a spent HAbst tool and two GeRad tools (the Germanium Radical is a fourth tooltype) in the process, which must be refreshed prior to repeating the sequence at a second site. Reactions for refreshing these tools have also been proposed and verified computationally, as well as reactions for synthesizing all tools and reactions for synthesizing a wide range of useful hydrocarbons, including diamond, graphite, fullerenes, and more.

The reaction sequence illustrated above proceeds as follows:
         (A) An HAbst tool approaches a specific hydrogen atom.
         (B) The HAbst tool withdraws, carrying off the abstracted hydrogen atom.
         (C) A GM tool with its CH2 group approaches the radical carbon atom on the workpiece.
         (D) The GM tool with its CH2 group bonds to the workpiece carbon atom.
         (E) The CH2 remains bonded to the workpiece carbon atom as the GM tool is pulled away, converting the tool to a GeRad handle by detaching from (breaking the bond with) the CH2.
         (F) An HDon tool approaches the newly added CH2 group.
         (G) The hydrogen atom leaves the HDon tool and bonds to the highly reactive CH2 group, producing a stable CH3 group on the workpiece; as the tool withdraws, the transfer of the hydrogen atom converts the HDon tool to a GeRad handle.

 

 

 

 

 

 

 

Why Only Diamond?

These several molecular tools, plus a few others, should enable us to make a wide range of atomically precise stiff structures composed of hydrogen and carbon -- e.g., diamond.

Admittedly, this is a far less ambitious initial goal than attempting to use all 90+ natural chemical elements in the periodic table. But in exchange for narrowing our focus to this more limited class of structures, we make it much easier to analyze in detail those structures that can be fabricated and the synthetic reactions needed to make them. Diamond and its shatterproof variants fall within this category, as do the fullerenes (sheets of carbon atoms rolled into spheres, tubes, and other shapes). These materials can compose all of the parts needed for basic nanomechanical devices such as struts, bearings (illustration at right), gears, rods, housings, and robotic arms.

Later on, as our analytical and experimental abilities in DMS mature and as more tooltip motifs are proposed and analyzed, a handful of additional elements can be added, such as dopant atoms to fabricate diamond electronic devices and silicon replacing carbon as a structural cage atom in some applications.

These and related structures, perhaps still composed primarily of carbon and hydrogen but now in combination with atoms of nitrogen, oxygen, silicon, and a few other chemical elements, will fill out our ability to manufacture a broader range of the entire class of “diamondoid” materials. This will enable a much greater diversity of fabricated products, such as bearings in a wider range of sizes that use other atoms (beyond hydrogen and carbon) with different covalent atomic radii, as illustrated at right.

 

 

 

 

 

How Can We Build These Tools?

The first proposal of a practical process for building a DCB6Ge mechanosynthetic tooltip, by Freitas, was filed as a provisional patent application in February 2004 and as a full utility patent by Zyvex in February 2005 – the first mechanosynthesis patent ever filed. Read an early version of the patent application here or here.

The workability of Freitas’ proposed process has already received valuable and welcome critique from the scientific community, and Freitas believes that some version of the process may be sufficiently viable to serve as a vital stepping-stone to more sophisticated DMS approaches.

Methods for building three additional DMS tooltips experimentally using only currently available laboratory techniques were proposed by Freitas and Merkle in 2008. By September 2007 when the patent was filed, calibration runs had begun on newly-acquired scanning probe equipment that was expected to be used by our experimentalist participants in an attempt to build the first DMS tooltip using one of the proposed DMS reaction sequences.

Other practical proposals for building the first DMS tooltips, using existing technology, are eagerly sought by the Nanofactory Collaboration.

Once the first DMS tools are built, they can be used to build the next generation of more precise, more easily rechargeable, and generally much improved mechanosynthetic tools (illustration at right). The end result of this iterative development process will be a mature set of efficient, positionally controlled mechanosynthetic tools that can reliably build atomically precise diamondoid structures -- including more DMS tools.

 

 

 

 

 

 

DMS Tools on the Assembly Line

In a factory production line, individual DMS tooltips can be affixed to rigid moving support structures and guided through repeated contact events with workpieces, recharging stations, and other similarly-affixed apposed tooltips. These molecular mills can thus perform repetitive fabrication steps using simple, efficient mechanisms. Mills can, in principle, be operated at high speeds -- with positionally constrained mechanosynthetic encounters possibly occurring at up to megahertz frequencies.

Using computer-automated tooltips performing positionally-controlled DMS in lengthy programmed sequences of reaction steps, we may be able to fabricate simple diamondoid nanomechanical parts such as bearings, gears, struts, springs, logic rods and casings to atomic precision.

Early tools would progress from single DMS tools manipulated by SPM-like mechanisms, to more complex multitip tools and jigs which the simple tools could fabricate, one at a time. These ancillary tools would then be employed to create a progression of more capable tools and mechanisms, a line of development ending in production lines conceptually similar to those illustrated (only schematically) at right.

 

 

 

 

Additional Resources

Annotated Bibliography on Diamond Mechanosynthesis (DMS)

Landmark "minimal toolset" paper on DMS by Freitas and Merkle in 2008

List of remaining Technical Challenges to achieve Diamond Mechanosynthesis

First Patent ever filed on Diamond Mechanosynthesis; U.S. Patent 7,687,146 was issued on 30 March 2010

Second Patent ever filed on Diamond Mechanosynthesis

Technical Book: Diamond Surfaces and Diamond Mechanosynthesis (in preparation)

Library of Mechanosynthetic Tool Designs (under construction)

 

 


Written contents of this page © 2006-10 Robert A. Freitas Jr. and Ralph C. Merkle

Image credits: Nanofactory, Assembly Line -- © John Burch, Lizard Fire Studios. Molecule Tooltip -- © Forrest Bishop. DMS Tool Sequence, DMS Tooltip on Handle, Diamond Logic Rod, Hydrogenated C(110) Surface, 3-Tooltip Stick Figures, and Large DMS Tool -- Robert A. Freitas Jr. DCB6Ge Tooltip -- Ralph Merkle. H-Abstraction Animation and H-Donation Tool -- Berhane Temelso. Scanning Probe Microscope diagram -- Antoine Dagan, CNRS Intl. Mag, Spring 2006, p. 20. IBM in atoms -- IBM Corporation. CVD Reactor -- Gareth Fuge, May 2001. Two Diamond Bearings -- designer Ralph Merkle, image created from atom coordinate files by Robert Freitas. Multi-Element Bearing -- designers K. Eric Drexler and Ralph Merkle. Molecular Mill -- K. Eric Drexler. Copyright applies to all images.


Last modified on 4 June 2011

since 14 June 2006