Is mechanosynthesis feasible? The debate continues.

In my post of December 16th, Is mechanosynthesis feasible? The debate moves up a gear, I published a letter from Philip Moriarty, a nanoscientist from Nottingham University, which offered a detailed critique of a scheme for achieving the first steps towards the mechanosynthesis of diamondoid nanostructures, due to Robert Freitas. The Center for Responsible Nanotechnology‘s Chris Phoenix began a correspondence with Philip responding to the critique. Chris Phoenix and Philip Moriarty have given permission for the whole correspondence to be published here. It is released in a mostly unedited form; however the originals contained some quotations from Dr K. Eric Drexler which he did not wish to be published; these have therefore been removed.

The total correspondence is long and detailed, and amounts to 56 pages in total. It’s broken up into three PDF documents:
Part I
Part II
Part III.

I’m going to refrain from adding any comment of my own for the moment, so readers can form their own judgements, though I’ll probably make some observations on the correspondence in a few days time.

The correspondence between Philip Moriarty and Chris Phoenix, for the time being, ends here. However Philip Moriarty has asked me to include this statement, which he has agreed with Robert Freitas:

“Freitas and Moriarty have recently agreed to continue discussions related to the fundamental science underlying mechanosynthesis and the experimental implementation of the process. These discussions will be carried out in a spirit of collaboration rather than as a debate and, therefore, will not be published on the web. In the event that this collaborative effort produces results that impact (either positively or negatively) on the future of mechanosynthesis, those results will be submitted for publication in a peer-reviewed journal.”

20 Responses to “Is mechanosynthesis feasible? The debate continues.”

  1. [...] 12; Richard Jones @ 10:52 pm Now that the 130 or so people who have downloaded the Moriarty/Phoenix debate about mechanosynthesis so far have had a chance to digest it, here a [...]

  2. [...] y out mechanosynthesis step to bond it to point A. Repassivate if necessary. Much of the debate between Chris Phoenix and Philip Moriarty concerned the constraints that surface phys [...]

  3. [...] one starts thinking through how one might experimentally implement the Drexlerian program a host of practical problems emerge. But one aspect of Drexleré─˘s argument is very import [...]

  4. [...] ge of views. The whole 56 page correspondence can be downloaded as a PDF from this post: “Is mechanosynthesis feasible? The debate continues.” My commentary on the deba [...]

  5. jim moore says:

    First, I want to thank Chris and Philip for spending a great deal of time exchanging views on the wide range of topics covered in the discussion. I also commend both of you for having the courage to agree to publish this exchange, there are points where both of you show some é─˙wartsé─¨. (Of course, there are other parts in the exchange where both of you show a great deal of intelligence, creativity and sharp analysis.)

    My general impression from reading the debate is that both of you bring very different primary goals to the discussion and these different goals limits the productive understanding of each otheré─˘s perspective.

    For Chris, the ultimate goal is to find a way to build a nano-factory, mechanochemistry is just a means to that end, so he keeps a loose definition for the term mechanochemistry. He (rightly, in my opinion) sees immense value in use of a controllable mechanism for positioning é─˙partsé─¨ to be joined together.

    Philip on the other hand is focused on trying to find a near term way do mechanochemistry experimentally (with the term mechanochemisty very precisely defined). For Philip the precise definition is critical, because it guides the experimental procedure.

    I certainly learned a lot more about scanning probe microscopes and carbon vapor deposition. If anyone has ideas on how to make é─˙inverted pyramidé─¨ diamond tips for scanning probe microscopes speak up, we need you (while you are at it, figure out how to funcionalize the very top of the pyramid).

    Finally, I am very glad to see that Philip and Robert (Freitas) will be talking and hopefully working together to demonstrate diamond mechanosynthesis in the lab (if possible).

  6. jim moore says:

    Philip and Chris,
    If I misunderstood your views and/or incorrectly stated your views it was not intentional, I am working with only a couple of kilos of monkey brains ;-)

    jim

  7. Jim,

    Thanks for your well-balanced appraisal. Ié─˘d like to pick up on two points:

    “My general impression from reading the debate is that both of you bring very different primary goals to the discussion and these different goals limits the productive understanding of each otheré─˘s perspective. ”

    I doné─˘t see the same strong distinction as you do between the discussion of mechanosynthesis in the debate and concepts for assembling micron scale (and larger) objects from smaller nanoscale units. The surface physics and chemistry issues Ié─˘ve raised throughout the discussion of the lowest level mechanosynthesis steps are also central to the higher levels of abstraction. As detailed in my correspondence with Chris, one cannot simply é─˛buryé─˘ the details of the chemistry in a é─˛black boxé─˘ and state baldly that the design is effectively é─˛universalé─˘ for covalently bound systems (i.e. that it ports directly from one materials system to another). As I mention at the end of the final letter, Ié─˘m more than willing to provide a detailed critique of Chrisé─˘ nanofactory paper (and his ideas for directed assembly of é─˛self-assembledé─˘ blocks) from the perspective of a surface physicist (and, indeed, Richard Jones has already focussed on a number of scientific issues at the core of the nanofactory concept) but this would be unfair (on Chris) if there is not a mechanism whereby he can address my comments.

    Moreover, and as highlighted in my correspondence with Chris, if weé─˘re carrying out directed assembled of self-assembled é─˛blocksé─˘ then weé─˘re not doing molecular manufacturing in the CRN or Drexler sense. (Note also that very many groups across the world already currently use self-assembly to first synthesise nanocrystals with diameters in the 2 é─ý 10 nm range and then use a second (é─˛higher levelé─˘) stage of self-assembly to produce nanocrystal solids (for example, see the pioneering work of Alivisatos, Bawendi, Brust, Murray, et al. in the mid-nineties)).

    “Finally, I am very glad to see that Philip and Robert (Freitas) will be talking and hopefully working together to demonstrate diamond mechanosynthesis in the lab (if possible)”

    Ité─˘s important that any future work that I undertake with Rob Freitas not be seen as an endorsement (on my behalf) of the é─˛molecular manufacturingé─˘ vision. My debate with Chris (and my associated more detailed reading of Nanosystems and other molecular manufacturing/ radical nanotechnology literature) has, in fact, é─˛firmed upé─˘ rather than weakened my position as a strong critic of the Drexlerian vision. Nevertheless, Rob Freitas and I are focussed on the same fundamental physical and chemical problems (i.e. directing molecular adsorption and chemical reactions at the single molecule level) and we therefore agree that there is thus considerable scope for useful discussion.

    *If* a detailed, plausible mechanism is discovered whereby it might be possible to implement one or two steps of a mechanosynthetic reaction then, yes, Ié─˘d be willing to attempt this in the lab. However, I am of the opinion that, for example, implementing the 7 step process suggested in my correspondence with Chris is not only far beyond current capability but that reliability will be an issue of paramount importance. (20% success rate in a simulation does not bode well for an experiment). Finally, ité─˘s very important to realise that demonstrating a single cycle of a mechanosynthetic sequence (in a judiciously-chosen materials system) will not é─˛open the flood gatesé─˘ for the implementation of molecular manufacturing. Even é─˙Nanosystemsé─¨ provides no strategy to move from the UHV diamondoid é─˙Stage 1é─¨ to Stages 2, 3 and 4. (My difficulties with the alternative polymer-based route are discussed in my correspondence with Chris).

    Best wishes,

    Philip

  8. jim moore says:

    “Ité─˘s important that any future work that I undertake with Rob Freitas not be seen as an endorsement (on my behalf) of the é─˛molecular manufacturingé─˘ vision. ”

    Philip, I don’t see your willingness to talk and potentially work with Rob Freitas a an endorsement of “molecular manufacturing” but rather as a willingness to explore an idea that you are skeptical about. I agree with you that finding a way to join theory and experiment in a feedback loop is critical to any developmental pathway for radical nanotechnology. But I think ( and you seem to as well) that it is important to be open to new ideas provisionally and let the outcome of well chosen experiments be the deciding factor for rejecting new or old ideas.

  9. “But I think ( and you seem to as well) that it is important to be open to new ideas provisionally and let the outcome of well chosen experiments be the deciding factor for rejecting new or old ideas”

    Absolutely!

    Jim, thanks for your input throughout the debate (both at “Soft Machines” and also on the CRN blog) – I’ve found your impartial comments extremely helpful.

    Best wishes,

    Philip

  10. John B says:

    Fascinating conversation. Thank you both for putting this much effort into the conversation, and then sharing it.

    Unfortunately, I think you’re both talking past each other, to the closure of the dialogue.

    For instance, Philip states that atomic reactions are different than molecular reactions. Fair enough. However, does the work surface (say, for point of arguement, fully hydrogen-passivated diamond) count as a single molecule? Does the hydrogen extraction tool tip (however that’s formed, working via EM field actions or mechanochemical process) count as a single molecule? If so, you’re performing atomically precise molecular adjustments when you’re de-passivating any given site on the diamond.

    IE – seems to me that you’re talking past each other. At no point that I’m aware of in the Merkle hydrogen metabolism paper are we talking about purely atomic manipulations – but rather molecular reactions. Of course, these molecular reactions end up with a single atom going from here to there, but it’s still all molecular interactions.

    The question this raises in my mind is – are the different steps of diamondoid growth via atomically precise construction different enough to require unique characterization of each interaction? That is, the first carbon will behave in thus-and-so a manner as it’s added to a passivated diamond substrate. Will the second one, bonded in the next site ‘over’ in the crystal lattice will behave significantly differently, or not?

    Please note the ‘significantly’ – if it’s still within the scope of the expected error rate (whatever that ends up being) then is this not a null issue? I freely admit to speculating positively on this point – if it turns out the bonding energies/behavior is radically different, as it may well be, then it makes mechanochemical growth/building of diamondoid – or whatever material – radically different.

    If, on the other hand, you’re able to categorize the bonding properties to a limited subset, however, would this not be overcome?

    Thank you for bringing up the problems with two probes’ “meeting up” and the relocation of the worksite. Wonderful stuff to spend some skullsweat on. I’d have to wonder how this might match up to IBM’s Millipede project’s (AFM data storage) experimental success – http://domino.research.ibm.com/comm/pr.nsf/pages/news.20020611_millipede.html has a 2002 press release.

    Again – thanks, Chris & Philip, for some great information here. I strongly suspect I’ll be back to this material repeatedly for reference.

    -John

  11. John B says:

    http://domino.research.ibm.com/Comm/bios.nsf/pages/millipede.html is an updated link, BTW – one improvement they’ve demonstrated is 10nm ‘dots’ at 20nm pitch – distance between centers.

    -John

  12. Hal Finney says:

    One of the unanswered issues in the discussion is the question of steric hindrance in some of Drexler’s proposed mechanochemical reactions. An example was figure 8.14 in Nanosystems. This shows the removal of a tightly bound hydrogen atom, with two tooltips coming into play at once, each holding an alkynyl carbon radical (with an unpaired electron). First a tooltip approaches at right angles and a radical addition occurs, forming a bond to the carbon atom from which we will extract the H. This is facilitated by a “nonbonded supporting structure” on the opposite side, allowing the use of force in this step. This radical addition weakens the bond to the H, allowing a second tooltip to approach from the direction of the H, bind to it and pull away, cleaving the bond of the H to the carbon which held it originally. Finally, the first alkynyl radical pulls away and breaks the bond it formed in the first step.

    The question of steric hindrance is whether the two tooltips can approach the reactive site at the same time. The tooltips are approaching at right angles, one from the top and one from the side, with their target points a few angstroms apart so the problem would be if the tips and their support structures have an aspect ratio fatter than 1:1.

    Drexler shows a picture of an alkynyl radical in figure 8.3, but it is just the very tip and it is not possible to envision how wide the support structure would be. His proposed manipulator arm in figure 13.14 appears to have roughly a 1:1 (45 degree) aspect ratio, so it would probably be suitable for this reaction.

  13. Philip Moriarty says:

    Apologies for not addressing the comments above until now é─ý ité─˘s been a hectic week.

    - John:

    First I am very glad that you found the debate useful and interesting. Ié─˘m not certain, however, that I entirely agree with you that Chris and I é─˛spoke pasté─˘ each other for the duration of the debate. While there are passages where this is certainly the case, there are other instances where it is clear that we both were focussed on the same issue. To my mind, this is particularly true in the latter stages of the debate re. atom-by-atom vs molecule-by-molecule construction.

    You raise the same issue as Chris did in his second (if I recall correctly) letter. That is, you see an **unfunctionalised** AFM tip as a é─˙molecular toolé─¨ in its own right. As I pointed out to Chris, this very much é─˛goes against the grainé─˘ of the molecular manufacturing vision where **pre-chosen**, well-defined reactive molecules are used to drive the mechanochemistry. The é─˙Nanotech Nowé─¨ glossary gives the following definition of mechanosynthesis:

    é─¨Mechanosynthesis: (where) molecular tools with chemically specific tip structures can be used, sequentially, to modify a work piece and build a wide range of molecular structures.é─¨

    It is clear from Nanosystems that Drexler intended that, as stated in the definition above, é─˙chemically specificé─¨ tip structures be used. Even prior to é─˙Nanosystemsé─¨ Merkle et al. couch their discussion of mechanosynthesis in terms of é─˙molecular tips attached to atomic force microscope (AFM) tipsé─Â.é─¨ [Nanotechnology 2 187 (1991)]. So I am firmly of the opinion that an unfunctionalised AFM or STM tip lies outside the realm of mechanosynthesis. This is an extremely important point because the most basic mechanosynthesis tool (the é─˛hydrogen abstraction toolé─˘ involving an ethynyl radical [see Nanosystems and the Nanotechnology paper referred to above]) has yet to be demonstrated in the lab. (E-field induced hydrogen abstraction is not, for the reasons outlined in my first letter and above, a demonstration of mechanosynthesis). It is very frustrating when work by experimental groups which is not at all related to the Drexlarian é─˛visioné─˘ is é─˛misappropriatedé─˘ as a vindication of the mechanosynthesis approach described in é─˙Nanosystemsé─¨.

    é─˙Of course, these molecular reactions end up with a single atom going from here to there, but ité─˘s still all molecular interactions.é─¨

    I guess that one distinction that needs to be made is between the terms é─˙manipulationé─¨ and é─˙positioningé─¨. Ié─˘m well aware that Merkle, Freitas (and Drexleré─˘s work) focuses on molecular interactions é─ý I have no argument with this. The argument I have is that it is imprecise and misleading to state that é─˙é─˙My proposal is, and always has been, to guide the chemical synthesis of complex structures by mechanically positioning reactive molecules, not by manipulating individual atoms” (Drexler in his Open Letter to Smalley] when the reactions themselves involve the transfer of atoms. What Drexler meant to say is that mechanosynthesis doesné─˘t involve **computer-controlled positioning** of individual atoms. For those of us é─˛at the coalfaceé─˘ who work with single molecule manipulation (and, indeed, speaking in terms of pure chemistry) there is a huge difference between manipulating molecules and manipulating atoms.

    é─˙é─Âare the different steps of diamondoid growth via atomically precise construction different enough to require unique characterization of each interaction?é─¨

    This is not my point. I am not suggesting that each individual addition of an atom or molecule (in, for example, the construction of a diamond lattice) will be sufficiently different to the last so that they must be considered as completely distinct processes. What I mean is that the addition of a carbon atom is radically different to the addition of a carbon dimer in terms of the molecular tool (and detailed chemistry) that is required. (On a é─˛sideé─˘ theme: your comment re. error rates is extremely important and remains an unaddressed thread in my discussion with Chris).

    é─˙Ié─˘d have to wonder how this might match up to IBMé─˘s Millipede projecté─˘s (AFM data storage) experimental successé─Âé─¨

    The following line is taken from a review I wrote in 2001 [Rep. Prog. Phys. 64 297]:

    é─˙To improve the efficiency of SPM-based fabrication, a number of research groups have
    developed innovative methods of parallel feature writing with multi-tip SPM instruments
    (Vettiger et al 2000, Hang and Mirkin 2000, Cooper et al 1999). These methods have succeeded
    in producing features with linewidths of ~15 nm (Hang and Mirkin 2000) and areal densities
    of 100é─ý200 Gb in 1 (Vettiger et al 2000). ***Here the goal (at least thus far) is not the construction
    of a structure é─˛from the bottom upé─˘ (i.e. where the placement of individual atoms/molecules is
    predetermined and carefully controlled) but the development of SPM as a patterning tool that
    can surpass the resolution of current lithographic techniques without sacrificing speed.***é─¨

    Note the highlighted text – the millipede project is wonderful, ground-breaking work but ité─˘s far removed from whaté─˘s required for mechanosynthesis. Note that the process involves indentation in heated polymer films with an array of tips. This array of tips is moved around é─˛collectivelyé─˘ é─ý i.e. there is no control of individual tips (unless there have been further recent developments of which Ié─˘m not aware.) Note also that the process isné─˘t carried out in vacuum. (Furthermore, while modification of polymer films is carried out by the millipede, ité─˘s still not in any way connected to the polymer-based é─˛Stage 1é─˘ of the molecular manufacturing strategy outlined in Chapter 16 of é─˛Nanosystemsé─˘).

    I raise in my prototypical é─˛7 step mechanosynthesis sequenceé─˘ (in my debate with Chris) the requirement that one molecular tip is put down and another picked up. One method to bypass this (which is closer to the é─˛molecular millé─˘ rather than é─˛assembleré─˘ concept) is to use a dual tip STM/ AFM system [see, e.g. Takami et al. J. Phys. Chem. B 108 16353 (2004), Okamato and Chen, Rev. Sci. Instr. 72 4398 (2001), and H. Grube et al. Rev. Sci. Instr. 72 4388 (2001) for examples of dual tip STM systems] with one tip for the hydrogen abstraction tool and another for the dimer placement tool. However, to demonstrate even this basic é─˛molecular millé─˘ technology on a five year timescale is again hopelessly optimistic. (And how will the tips be recharged? And how do we ensure that the error rate is sufficiently low? In addition, as discussed below, the issue of tip structure remains of paramount importance).

    - Hal:

    The diagram of Fig. 8.14 is ridiculously oversimplified. Probe tips doné─˘t have nice sharp edges with facet angles we can pick and choose at will. Thereé─˘s a very interesting paper on high aspect ratio **diamond** tips formed by focussed ion beam methods published a few years ago by Olbrich et al. [J. Vac. Sci. Tech. B 17 1570 (1999)]. (On this point, I think that it was Jim Moore who asked some time ago either here at é─˙Soft Machinesé─¨ or on the CRN blog whether ion beam milling might be used to create an appropriate tip shape.) The tip radius of curvature is typically ~ 30 nm in that paper. We could however use either carbon nanotubes or commercial é─˛supertipsé─˘ [see, for example, http://www.spmtips.com/products/cantilevers/datasheets/hi-res/ to reduce the radius of curvature substantially. Note that even with a 1 nm radius of curvature there is sufficient steric hindrance to rule out the reaction as shown in Fig. 8.14. [Note also an important line in the JVST B paper cited above: é─˙Milling of the tip leaves a 10-20 nm thick nonconducting **amorphous** carbon layer which must be eteched by pure HNO3 to expose the diamond surfaceé─¨ (emphasis mine)].

    Can tips with extremely well-defined facets be formed? Yes é─ý TT Tsongé─˘s group have recently published some amazing work on creating single atom tips [Kuo et al. Nano Lett. 4 2379 (2004)] with well-defined facets. However, these tips are made of metal (and are thus extremely problematic for mechanosynthesis) *and* their facets are defined by surface free energy considerations. Hence, if these tips could be used for mechanosynthesis (and, as stated, metals are problematic) another é─˛design constrainté─˘ which yet again substantially narrows the viable parameter space has been introduced. (Talk of the importance of surface free energies brings us nicely é─˛full circleé─˘ in terms of the debate/discussioné─Â)

    Best wishes,

    Philip

  14. Hal Finney says:

    Philip – As far as Nanosystems figure 8.14 goes, I am sure Drexler did not intend to use high aspect ratio diamond tips made by focused beam methods, carbon nanotubes, or commercial supertips. You should not evaluate the proposed reactions in that chapter in the context of present-day tip-making technology. They are intended to show how a molecular manufacturing system might work in its mature state. If you find that this figure causes you “great difficulty” because you can’t see how to implement this reaction with today’s technology, you are misunderstanding the point of that chapter.

    If you want to argue that will be forever impossible to create tool tips with the desired aspect ratio, then you should say why. If you think the tool tip in figure 8.3 is too fat by itself, or is perhaps chemically unstable, that would be a good criticism. Or if you think it will be impossible to mount that tool tip on a support structure with the necessary thinness while retaining the required strength and stability, that too would be an effective critique. But it’s wrong to claim that this reaction has a fat fingers problem because the only single-atom tips we can make today are of metal, or because Freitas’ tips won’t work. Drexler never claimed that we could make the reactions in chapter 8 work today. IMO they are there to show that his proposed manufacturing system could actually manufacture things. They represent the workings of a hypothetical mature nanotechnology and have to be evaluated in those terms.

  15. Hal Finney says:

    Philip – You also asked about Drexler’s AFM tip array concept in Nanosystems 15.4, in particular his statement that, “On a large scale, scanning the primary bead provides a low-resolution image of the array of flat-side beads.” As you point out, the reality is more complex, and it is more useful to think of the smaller flat-side beads imaging the primary bead rather than vice versa. However, I think that Drexler’s statement is merely attempting to help the reader picture the process of scanning the primary bead over the flat plate. Just as scanning any broad tip over a surface produces a low resolution image of the surface, the same thing happens here. You characterize the result as “an extremely complicated, convoluted image comprising a number of overlapping images of the primary bead surface as the imaging centre changes from molecular tip to molecular tip.” I think this is another way of saying the same thing Drexler did, that the resulting “complicated, convoluted” image can be thought of as a low resolution image (full of artifacts) of the flat-side beads. So you and Drexler are substantially in agreement here, just using different words to describe the big picture of what is happening. If you go on and read Drexler’s subsequent sentences in section 15.4.5, I think you will find that his more detailed description of what happens is accurate and reasonable.

  16. Philip Moriarty says:

    Hal,

    Unfortunately, I’m going to have to disagree with each of the points you raise in Comments 11 and 12 above.

    “If you want to argue that will be forever impossible to create tool tips with the desired aspect ratio, then you should say why.”

    I thought that my previous post (Comment #10 above) dealt with this. However, I’ll try to make myself a little clearer. My issue with Fig. 8.14 is that there are fundamental physical limits associated with controlling the atomic and geometric structure of **any** type of tip. **Regardless of the tip-making technology** (whether it’s available now or in 30 years’ time), the surface physics and chemistry will constrain the detailed structure of the tip. You seem to think that it’s just a matter of enhancing the technology to produce tips with *arbitrary* radii of curvature and facets (e.g. we simply choose whether we want {111} faces or {211} faces or {322} faces) – the point of my post was to highlight that the same surface physics issues crop up time and time again with regard to constraining the ‘nanoscopic’ geometry of the tip. (There are many parallels with this point and the Freitas et al. model discussed with Chris Phoenix – surface physics will constrain the shape of the tip).

    For example, Drexler first suggests a very plausible materials system (hydrogen-passivated diamond) based on sound reasoning re. surface free energies and the elimination of high dangling bond (radical) densities. In some key cases, he then remarkably puts aside detailed considerations of the underlying physics/chemistry and goes on to suggest mechanosynthetic reactions on the basis of completely unconvincing ‘stick’ diagrams (such as Fig. 8.14). You comment: “Drexler never claimed that we could make the reactions in chapter 8 work today. IMO they are there to show that his proposed manufacturing system could actually manufacture things.” As stated above, it’s not a question of what we can do today. It’s a question of the constraints imposed by the fundamental physics and chemistry. How does the diagram in Fig. 8.14 help to show that his proposed manufacturing system could actually manufacture things when the viability of the reaction has not been considered in **any type** of depth with regard to the structural properties of the support? The examples of current capability in tip manufacture I raised were to highlight just how essential it is to consider the precise physicochemical properties of the system before ‘dreaming up’ hypothetical reactions based on highly idealised tips.

    My difficulty with Fig. 8.14 (and “Nanosystems” and, indeed, Drexler’s ‘backward chaining’ approach in general) remains: it is pointless putting forward an entire hypothetical molecular manufacturing process based around mechanosynthesis when the **detail** of the lowest level machine language steps has not been worked out. Richard (Jones) makes an extremely important point in a recent post (http://www.softmachines.org/wordpress/index.php?p=71) on this issue: “if you let the science dictate where you go (and I doné─˘t think you have any choice but to do this), your path will probably take you somewhere interesting and useful, but ité─˘s probably not going to be the destination you set out towards.” To my mind, this approach is the polar opposite of that expounded by Drexler et al.

    Regarding Comment #12:

    “I think this is another way of saying the same thing Drexler did, that the resulting é─˙complicated, convolutedé─¨ image can be thought of as a low resolution image (full of artifacts) of the flat-side beads.”

    Hal, I can’t find any mention of the tip artefact issue in 15.4.5. I’ve read the entire section a number of times and the discussion is largely based on producing well-defined tip shapes rather than accounting for ‘tip imaging’ artefacts. My argument is that if the geometry of the beads is as shown in Fig. 15.3, then it is incorrect to state that “on a large scale, scanning the primary bead provides a low-resolution image of the array of flat side beads”. One will instead produce a sum of different **high resolution** images of the surface of the **primary bead**. This is a substantially different statement and is most definitely not “another way of saying the same thing Drexler did”. A sum of high resolution images of the surface of the primary bead is most definitely not the same as a low resolution image of the array of flat-side beads.

    “Just as scanning any broad tip over a surface produces a low resolution image of the surface, the same thing happens here.” But that’s not what’s happening in Fig. 15.3 : we don’t have a broad tip – we have a broad ‘primary bead’ imaged by a *sharp* molecular tip on the ‘flat side bead’.

  17. John B says:

    My turn to apologize on the turnaround time. *wry grin* All I can really say is that life *is* a four-letter word!

    Quoth Philip: “Ié─˘m not certain, however, that I entirely agree with you that Chris and I é─˛spoke pasté─˘ each other for the duration of the debate.” -snip-

    Fair criticism. The main disappointment I had with the debate was the acrimony, rather than rational disagreement, between the two sides. My interpretation on reading the posted comments leads me to think that the segments where you were ‘talking past each other’ led to increased emotional heat in the areas where you were both operating off the same set of definitions.

    Philip again – “You raise the same issue as Chris did in his second (if I recall correctly) letter. That is, you see an **unfunctionalised** AFM tip as a é─˙molecular toolé─¨ in its own right.” -snip-

    Actually, no. Sorry if I left you with that impression.

    I see an unfunctionalized AFM tip – a physical probe with no planned chemically active component – as a good precursor that can work as a technological springboard to handle some of the positioning problems already expected for mechanochemistry. That is – given a ‘Milipede’ type capability, it may well be possible to set up one or more reference points at the tens of nanometer scale to help limit down the displacement caused by switching tool tips.

    IE – when you start working on the hydrogenated diamond substrate, begin with setting a control stage with 10s of nanometers precision anchored off of a milipede-like divot system. While this does not get you down into the tenths of a nanometer precision (or is it even smaller?) you’d need for a mechanochemical reaction, it would give you 1/100th of the way there when you return with a fresh, characterized tool tip.

    Philip again – “It is clear from Nanosystems that Drexler intended that, as stated in the definition above, é─˙chemically specificé─¨ tip structures be used. Even prior to é─˙Nanosystemsé─¨ Merkle et al. couch their discussion of mechanosynthesis in terms of é─˙molecular tips attached to atomic force microscope (AFM) tipsé─Â.é─¨ [Nanotechnology 2 187 (1991)]. So I am firmly of the opinion that an unfunctionalised AFM or STM tip lies outside the realm of mechanosynthesis.” -snip-

    I’m not disagreeing. However, is it possible that the AFM/’milipede’ capability is useful as a ‘coarse focus’ mechanism prior to mechanochemical precision, as above?

    -snip-
    I opined, é─˙Of course, these molecular reactions end up with a single atom going from here to there, but ité─˘s still all molecular interactions.é─¨

    Philip replied, “I guess that one distinction that needs to be made is between the terms é─˙manipulationé─¨ and é─˙positioningé─¨. Ié─˘m well aware that Merkle, Freitas (and Drexleré─˘s work) focuses on molecular interactions é─ý I have no argument with this. The argument I have is that it is imprecise and misleading to state that é─˙é─˙My proposal is, and always has been, to guide the chemical synthesis of complex structures by mechanically positioning reactive molecules, not by manipulating individual atomsé─¨ (Drexler in his Open Letter to Smalley] when the reactions themselves involve the transfer of atoms. What Drexler meant to say is that mechanosynthesis doesné─˘t involve **computer-controlled positioning** of individual atoms. For those of us é─˛at the coalfaceé─˘ who work with single molecule manipulation (and, indeed, speaking in terms of pure chemistry) there is a huge difference between manipulating molecules and manipulating atoms.” -snip-

    OK – that’s certainly fair. It’s my impression that you have less qualms about positioning molecules than atoms – is that correct?

    If you interpret Drexler’s work as using molecular interactions which end up with atomic precision manipulation, is that a better fit to work ‘at the coalface’?

    Philip, “… the addition of a carbon atom is radically different to the addition of a carbon dimer in terms of the molecular tool (and detailed chemistry) that is required.” -snip-

    OK – so, pardon the boneheadedness on my part, but where’s the problem? Drexler, Frietas, Merkle and others have stated that they’re planning on different tools (and hence reactions) to produce different actions. While I agree with you that there’s no solid design for these tools at this time, isn’t categorizing steps towards one format of mechanochemistry a useful function? That is, “If you want to build hydrogenated diamond structures, you’ll need these kinds of tools.” ??

    Further, you’re the first person I’ve seen bring up the positioning problem for multiple tools. Thank you for that most useful addition to the discussion.

    Further still, I don’t believe any of them have committed to saying that they’ve designed the toolset. The hydrocarbon metabolism paper is a first step towards that, as is the recent work demonstrating the 80% error rate on the one tool tested, but this is still progress. Do I think that it’ll work out exactly as listed? Heck no – you’re right, science takes you however it does. My personal opinion is that it’s good to have specific goals to strive for, however.

    Philip – “Note the highlighted text – the millipede project is wonderful, ground-breaking work but ité─˘s far removed from whaté─˘s required for mechanosynthesis.” -snip-

    No arguement with your statements. I hope I clarified why I brought up the Milipede project above.

    Philip – “(Furthermore, while modification of polymer films is carried out by the millipede, ité─˘s still not in any way connected to the polymer-based é─˛Stage 1é─˘ of the molecular manufacturing strategy outlined in Chapter 16 of é─˛Nanosystemsé─˘).”

    Understood.

    Philip – “I raise in my prototypical é─˛7 step mechanosynthesis sequenceé─˘” -snip many good points that DO need to be addressed-

    I’ve been unable to find a copy of it online, but does the ‘Oxidation sharpening of silicon tips’ paper by T.S. Ravi and R.B. Marcus from J. Vac. Sci. Technol. B, Vol 9 pp 2733-2737 (reference from the milipede project white paper) provide any hints along this line? Perhaps using a ‘sharpened’ silicon tip as a substrate to a diamond film deposition via plasma? (If I’m way out of the park here, let me know – as I said, I have no copy of this to refer to, nor am I anything resembling an authority on any of this. Consider me brainstorming as a nosy amateur…)

    -John

  18. Sud says:

    http://www.google.com/search?sourceid=mozclient&ie=utf-8&oe=utf-8&q=inverted+pyramid+nanotechnology

    only 649 results (on Feb 11 2004) Maybe some one with time can read each one… ;-)

  19. Sud says:

    inverted pyramid nanotechnology diamond -deborah
    on google.com on Feb 11 2005 produces 111 results.
    Hope I ain’t diluting the discussion ;-)

  20. Philip Moriarty says:

    Sud,

    My discussion with Chris Phoenix focussed on a very specific **atomically sharp** inverted pyramid (proposed by Freitas et al.) formed by chemical vapour deposition on a diamond surface (to be subsequently ‘picked up’ and used for mechanosynthesis applications). See the .pdf documents downloadable at the top of this page. You need to narrow your search parameters! It’s not at all surprising that you’ll find lots of sites if you search using the terms you’ve suggested. Indeed, Google hasn’t done a particularly “intelligent” search because the fourth item on the web page to which you link is a paper on III-V semiconductor heterostructures where the inverted pyramids have been formed by anisotropic wet etching of a *GaAs(111)B* substrate (i.e. not diamond!). I obviously haven’t scanned the remainder of the web pages found by Google (!) but it’s clear from this one example that you need to be very careful to ensure that the results of a Google search are appropriately filtered. I think that, with appropriate and intelligent filtering (or a better choice of search terms), you’ll find there are a very small number of sites related to Freitas’ ‘inverted pyramid’ model (much, much fewer than 649 and closer to single figures…).

    Best wishes,

    Philip