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NANOSCALE MECHANOSYNTHESIS
Abstract
Nanotechnology and molecular engineering has gained great deal of importance in the recent years. The main obstruction to molecular manufacturing today is the lack of an experimental procedure for routinely and precisely building objects, atom by atom, at the molecular scale. The key to this is molecular positional assembly, or mechanosynthesis “ the formation of covalent chemical bonds using precisely applied mechanical forces. This paper investigates a specific mechanical dimer placement tool for diamond mechanosynthesis. The tool is stable in isolation, and capable of depositing carbon dimers on diamond surface at room temperature. A preliminary proposal for a four-step experimental process is also presented by which the dimer placement tool, along with its associated macroscale handle structure, could be fabricated using presently-available bulk-chemistry techniques. A practical dimer placement tool built will allow the fabrication of improved mechanosynthesis tools, thus opening up the entire field of molecularly-precise material fabrication and, indeed, molecular machine manufacturing to the practical field of mechanical engineering.
CONTENTS
1. ABSTRACT¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦01
2. INTRODUCTION¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦...03
3. BASIC MOLECULAR STRUCTURE¦¦...¦¦¦¦¦¦¦¦¦¦¦¦...03
4. CARBENE TOOL AND HORIZONTAL DIMER PLACEMENT TOOL...04
5. POSISTIONALLY CONTROLLED DIMER DEPOSISTION¦¦¦¦¦..05
6. BUILDING THE FIRST TOOL¦¦¦......¦¦¦¦¦¦¦¦¦¦¦¦¦..07
7. FOUR STEP MANUFACTURING PROCESS...¦¦¦¦¦¦¦¦¦¦¦.07
8. IMPROVING DESIGN¦¦¦.¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦..11
9. CONCLUSION¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦12
10. REFERENCES¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦.12
INTRODUCTION
The revolutionary Feynman vision of a powerful and general nanotechnology, based on nanomachines that build with atom-by-atom control, promises great opportunities in the field of engineering. The Feynman vision projects the development of nanomachines able to build nanomachines and other products with atom-by-atom control (a process termed molecular manufacturing). The individual control of molecular composition and placement is often cited as a goal of nanotechnology. To date, many nanotechnology efforts have been content to achieve nanoscale, but not atomic-precision, or to build large quantities of small identical molecules. This paper takes a step closer in realizing the ideal of the Feynman vision of nano-molecular manufacturing.
Molecular manufacturing relies on two counterintuitive ideas: first that mechanical operations can reliably be carried out at the nanoscale and second, that handling of individual molecules can be scaled up to produce useful quantities of product. On closer examination, these ideas appear to be supportable; and indicate that molecular manufacturing is practical. The basic objective is as shown in the picture. Starting with a flat diamond surface, a mechanosynthetic tool is brought in and positioned precisely over the workplace, it is lowered down to deposit, say, a carbon dimer on the diamond surface and lifted the discharged tool away, then the same is repeated with successive dimers, resulting in the positional assembly of diamond. Diamond has already found many applications in mechanical and electromechanical microdevices, sensors, and electronics. To build nanomachines, one must be able to fabricate diamond, atom by atom.
BASIC MOLECULAR STRUCTURE
The smallest unit cage of diamond is a molecule of adamantane, which has 10 carbon atoms and 16 hydrogen atoms arranged in a tricyclic cage structure. The process is carried out in C(110) surface. For obtaining the C(110) diamond surface the basic adamantane cage is rotated counterclockwise and the cages are attached side by side giving one row of C(110) surface. Stacking multiple rows gives an extended diamond C(110) surface. The C(110) surface is selected because of its simplicity and also the fact that the C(110) surface doesnâ„¢t reconstruct either at high temperatures, or when dehydrogenated, unlike the other two diamond surfaces.
A bulk-produced C(110) surface is entirely passivated by hydrogen atoms, eliminating any flaccid bonds. Working with hydrogenated surfaces would require removal of H atoms, creating flaccid bonds to accept additional carbon atoms. This is carried out by heating the entire C(110) surface at a temperature above 1400 K, in vacuum. This removes the hydrogen, leaving a clean carbon surface. Hydrogen abstraction tools, for the selective removal of hydrogen atoms from a diamond surface can be developed. Besides removing hydrogen, carbon atoms might be added to a growing diamond surface in precisely chosen locations. One of the methods is to pick and place molecular building blocks such as adamantane cages. But control of simultaneous multiple bond formations between adjacent blocks are relatively difficult, and the minimum feature size is about one block.
THE CARBENE TOOL AND HORIZONTAL DIMER PLACEMENT TOOL
Drexler proposed single-carbon additions using a carbene tool, as shown in this schematic. Computational studies of this tool on the diamond surface found that to avoid a significant energy barrier, the tip must approach from the side. Also, the carbene tool must be rotated 90 degrees around a vertical axis, during normal operation. A horizontal dimer placement tool deposits twice the atoms per operation as a carbene tool, and needs no approach trajectories or twisting motions to complete the mechanosynthetic operation. In a good dimer placement tool, the dimer is bonded relatively weakly to the tooltip. A new tooltip family that uses a lonsdaleite or hexagonal diamond structure for the base is available. The best is the Dicarbon Bridge or DCB pattern where each group IV atoms (yellow in Fig) are bound to two central carbon bridges.
POSITIONALLY CONTROLLED CARBON DEPOSITION
The dimer placement tool positions the carbon dimer, bonding it to a precisely chosen lattice location, and during withdrawal leaves behind the C2 dimer. There are three stationary points of interest. Both active and discharged tooltips are minima on the potential energy surface as perceived from the potential energy curve. The undesired rearrangement in the middle is just a transition state, where X of DCB6-X is a Group IV element.
The required positional accuracy for inserting a dimer, while avoiding the nearest transition state leading to an undesired defect is 0.7 Angstrom for the leftmost dimer atom and 0.5 Angstrom for the second-from-left dimer atom. 0.5 Angstrom positional accuracy is very near state of the art “ both for positioning resolution and for positioning repeatability. In an actual tool for diamond mechanosynthesis, the DCB6 tooltip molecule would be attached to a much larger diamond structure. If the extended tool is positioned over a C(110) surface and heated to 20 K, there™s hardly any motion. At room temperature or 300 K, the dimer closest to the diamond surface starts moving. At 900 K, the motions become more severe. A scatter plot of dimer motions shows that thermal uncertainty increases according to the relative stiffness of dimer holding bonds of Si and Ge and Tin.
The extended tool is about twice as wide in Y, along the dimer axis, as in X, perpendicular to the dimer axis. Thatâ„¢s why at room temperature the thermal uncertainty of the dimer is plus-or-minus 0.5 Angstrom in X but only 0.2 Angstrom in Y. Adding a stiffening crossbeam reduces the room-temperature thermal uncertainty of dimer positioning in the X direction by half temperatures. The 0.5 Angstrom nanopositioner accuracy available today approaches what is needed for mechanosynthesis on diamond C (110).
The deposition of a dimer on the diamond surface by a tooltip is tested on a tooltip. Starting with a 46-atom Germanium tooltip brought to a 200-atom slab of diamond C(110) surface. The tip is raised or lowered in 0.2 Angstrom steps and allowed to run for 200 femtoseconds at constant temperature. The Germanium tooltip is finally retracted from the C(110) surface at 300 K. At 1.6 Angstroms up, the dimer detaches from the tooltip and drops into the global minimum on the C(110) surface. The complete deposition/retraction cycle works accurately. This stepwise simulation doesnâ„¢t track continuous events, but is a series of snapshots of the system in slightly different positions to help analyze which bonds will form and break at specific points during the cycle. When a Silicon tooltip is retracted from the surface at 300 K, the dimer detaches in a partly uncontrollable manner. But at liquid nitrogen temperature the dimer neatly detaches and drops into the global minimum on the C(110) surface.
BUILDING THE FIRST TOOL
A preliminary proposal for building a two-part tool is as follows. The first part is the DCB6 tooltip molecule. The second part is the handle structure, though the actual handle is much larger, 0.1-10 microns in diameter “ big enough for a MEMS manipulator to grab. At the apex of the handle structure, the tooltip molecule is covalently bonded to the handle structure, forming a complete tool.
A FOUR STEP MANUFACTURING PROCESS
The manufacture of the complete positional diamond mechanosynthesis tool requires four distinct steps: synthesizing a capped tooltip molecule, attaching it to a deposition surface, attaching a handle to it, and then separating the tool. These four steps are discussed in detail.
STEP 1: SYNTHESIS OF CAPPED TOOL TIP MOLECULE
A capping group is temporarily added to the two dangling bonds of the carbon dimer, passivating the sagging bonds and chemically stabilizing the tooltip molecule for a solution-phase chemical synthesis environment. At left, there is a DCB6Ge tooltip molecule capped with two iodine atoms, passivating the reactive double-bonded dimer. This capped molecule is what is to be synthesized. Large samples were studied and Iodine seems to satisfy the requirements. Over 20,000 adamantane-based compounds are already known. Adamantane derivatives and polyadamantanes are readily synthesized from scratch. Iceane, is the twinned 30-atom core structure of tooltip molecule, and was first synthesized 30 years ago.
Adamantanes also occur widely in nature. Chemists have extracted dozens of polyadamantane molecules of various sizes and arrangements from natural petroleum.
STEP 2: ATTACHIHG TOOL TIP IN THE PREFERRED ORIENTATION
The purpose is to grow isolated micron-scale crystals over tooltip molecule nucleation sites, rather than a continuous diamond film. So the deposition surface should minimize the number of natural nucleation sites. The selection criteria are satisfied by carbide exclusion materials like Germanium, Gold, Copper, and Sapphire. Graphene sheets, like Graphite or Carbon Nanotubes, could also be used with nonhydrogenic processes. Having chosen a deposition surface, at least two tooltip attachment methods can be identified.
In Attachment Method A, capped tooltip molecules are directed as a beam in a scanning pattern across the deposition surface, as in Image A. Upon striking the surface, some fraction of the tooltip molecule ions, partially fragment with the release of the capping group, producing hanging bonds at the C2 dimer which can then insert into the deposition surface in the desired orientation, as in mage, creating a low density of preferred diamond nucleation sites. There are also many unwanted outcomes, where only one atom of the capping pair is released, binding the tooltip molecule to the surface with just one bond through the C2 dimer. If beam energy is too high, impact bonding through the tooltip molecule base could occur, with migration or release of 1 or 2 hydrogen atoms, as in D or E. The harm from unwanted attachments is avoided by inspection or by post-process surface editing.
In Attachment Method B, tooltip molecules are bonded to the deposition surface by non-impact dispersal and weak physisorption onto the deposition surface. This is followed by tooltip molecule decapping in vacuum via targeted energy input, producing hanging bonds at the C2 dimer which bond into the deposition surface, thus affixing the tooltip molecule to the surface in the desired tip-down orientation and again creating a low density of preferred diamond nucleation sites. STM-mediated positionally-controlled single-molecule dissociation of an iodine atom from individual molecules of diiodobenzene physisorbed on copper surface was demonstrated experimentally in 2000.
STEP 3: ATTACHING HANDLE STRUCTURE TO TOOL TIP MOLECULE
In Handle Attachment Method A, new diamond is grown onto the tooltip molecule base using standard Chemical Vapor Deposition or CVD.
The adamantane or diamonded nanocrystal base of each bound tooltip molecule serves as a nucleation seed from which a large diamond crystal will grow outward, in preference to growth on areas of the deposition surface where tooltip nucleation seed molecules are absent. CVD should proceed until enough bulk diamond crystal has grown, outward and around each of the tooltip molecule seeds, which the tooltip and its newly grown handle can be securely grasped by a nano-scale manipulator mechanism. Deposition is halted before adjacent growing crystals can merge into a single film.
In Handle Attachment Method B, a dehydrogenated diamond shard is brought down vertically onto a surface upon which tooltip molecules are already tethered. The shard™s flaccid bonds attach it to the tooltip base. Retraction of the shard breaks the dimer bonds to the surface and pulls off the tooltip molecule, yielding a tool for diamond mechanosynthesis with an active C2 dimer exposed at the tip. This method produces an inferior tool because the position, orientation, number, and stiffness of attached tooltip molecules are poorly controlled. Method B is much easier from an experimental standpoint, so it may be possible to manufacture early, though less capable, mechanosynthetic tools in this manner “ and without as much positional accuracy.
STEP 4: SEPARATE FINISHED TOOL FROM DEPOSITION SURFACE
The covalent bonds between the tooltip, through the C2 dimer, and the deposition surface will mechanically break when pulled. This yields either the desired mechanosynthetic tool having a naked carbon dimer attached, most probable according to preliminary analysis, or else a tool with no dimer attached, essentially a discharged tool requiring recharge. The tool will be securely grasped by, or bonded to, a conventional tip, a robotic end-effector, or other similarly rigid and well-controlled microscale manipulation device.
Once the completed mechanosynthetic tool is detached from the deposition surface, the exposed C2 dimer radical is extremely chemically active. A typical UHV (Ultra High Vacuum) on the order of 1nano-torr should give on an average, more than 1000 seconds after tool detachment before tooltip poisoning is likely to occur due to impingement of stray atoms, ions, and molecules.
IMPROVING DESIGN
These steps are only a preliminary foundation on which one has to build a strong fortress. The simplest thing which has been accomplished is a single plane of dimers, spelling out the logo of the corporate sponsor, on diamond. A bigger challenge is to build something useful “ like a better tool, which will allow conduct more precise operations, and to use one tool to replicate a second tool, under human guidance. Larger diamond nanostructures are stable during construction, but careful simulations must be done to verify this expectation. Another key improvement is to design tools that can be used, then recharged and returned to service. In one scheme, the spent tool is recharged with acetylene, and then the hydrogen is removed using an abstraction process, restoring an active carbon dimer at the tip.
CONCLUSION
Next generation of durable, rechargeable mechanosynthetic tools, would be designed to minimize the thermal positional uncertainty and provide good dimer placement accuracy. Using such next-generation tools, simple linear structures like this diamond Logic Rod and after that, more complicated diamond-based molecular machine components can be built. The methods described here can get research to the starting gate of molecular manufacturing. One will be able to build diamond objects out of carbon atoms, dimer by dimer. The future steps are to build a better tool, then to build recyclable tools, and so forth, down the list. Each of these steps should yield commercially useful outputs, improving the likelihood for research funding.
REFERENCES
1) rfreitas.com
2) reserch.unc.in
3) molecularassembler.com